Articles de revues : « Chemical clocks »

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McEwen, J. S., P. Gaspard, T. V. de Bocarme et N. Kruse. « Nanometric chemical clocks ». Proceedings of the National Academy of Sciences 106, n^o 9 (17 février 2009) : 3006–10. http://dx.doi.org/10.1073/pnas.0811941106.

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Okamoto-Uchida, Yoshimi, Akari Nishimura, Junko Izawa, Atsuhiko Hattori, Nobuo Suzuki et Jun Hirayama. « The Use of Chemical Compounds to Identify the Regulatory Mechanisms of Vertebrate Circadian Clocks ». Current Drug Targets 21, n^o 5 (20 avril 2020) : 425–32. http://dx.doi.org/10.2174/1389450120666190926143120.

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Circadian clocks are intrinsic, time-tracking processes that confer a survival advantage on an organism. Under natural conditions, they follow approximately a 24-h day, modulated by environmental time cues, such as light, to maximize an organism’s physiological efficiency. The exact timing of this rhythm is established by cell-autonomous oscillators called cellular clocks, which are controlled by transcription–translation negative feedback loops. Studies of cell-based systems and wholeanimal models have utilized a pharmacological approach in which chemical compounds are used to identify molecular mechanisms capable of establishing and maintaining cellular clocks, such as posttranslational modifications of cellular clock regulators, chromatin remodeling of cellular clock target genes’ promoters, and stability control of cellular clock components. In addition, studies with chemical compounds have contributed to the characterization of light-signaling pathways and their impact on the cellular clock. Here, the use of chemical compounds to study the molecular, cellular, and behavioral aspects of the vertebrate circadian clock system is described.

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Wilhelm, Stefan, et Otto S. Wolfbeis. « Opto-chemical micro-capillary clocks ». Microchimica Acta 171, n^o 3-4 (24 septembre 2010) : 211–16. http://dx.doi.org/10.1007/s00604-010-0456-4.

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Harms, A. A., et O. E. Hileman. « Chemical clocks, feedback, and nonlinear behavior ». American Journal of Physics 53, n^o 6 (juin 1985) : 578. http://dx.doi.org/10.1119/1.14242.

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Andrieux, David, et Pierre Gaspard. « Fluctuation theorem and mesoscopic chemical clocks ». Journal of Chemical Physics 128, n^o 15 (21 avril 2008) : 154506. http://dx.doi.org/10.1063/1.2894475.

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Uehara, Takahiro N., Yoshiyuki Mizutani, Keiko Kuwata, Tsuyoshi Hirota, Ayato Sato, Junya Mizoi, Saori Takao et al. « Casein kinase 1 family regulates PRR5 and TOC1 in the Arabidopsis circadian clock ». Proceedings of the National Academy of Sciences 116, n^o 23 (16 mai 2019) : 11528–36. http://dx.doi.org/10.1073/pnas.1903357116.

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The circadian clock provides organisms with the ability to adapt to daily and seasonal cycles. Eukaryotic clocks mostly rely on lineage-specific transcriptional-translational feedback loops (TTFLs). Posttranslational modifications are also crucial for clock functions in fungi and animals, but the posttranslational modifications that affect the plant clock are less understood. Here, using chemical biology strategies, we show that the Arabidopsis CASEIN KINASE 1 LIKE (CKL) family is involved in posttranslational modification in the plant clock. Chemical screening demonstrated that an animal CDC7/CDK9 inhibitor, PHA767491, lengthens the Arabidopsis circadian period. Affinity proteomics using a chemical probe revealed that PHA767491 binds to and inhibits multiple CKL proteins, rather than CDC7/CDK9 homologs. Simultaneous knockdown of Arabidopsis CKL-encoding genes lengthened the circadian period. CKL4 phosphorylated transcriptional repressors PSEUDO-RESPONSE REGULATOR 5 (PRR5) and TIMING OF CAB EXPRESSION 1 (TOC1) in the TTFL. PHA767491 treatment resulted in accumulation of PRR5 and TOC1, accompanied by decreasing expression of PRR5- and TOC1-target genes. A prr5 toc1 double mutant was hyposensitive to PHA767491-induced period lengthening. Together, our results reveal posttranslational modification of transcriptional repressors in plant clock TTFL by CK1 family proteins, which also modulate nonplant circadian clocks.

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Gaspard, Pierre. « The correlation time of mesoscopic chemical clocks ». Journal of Chemical Physics 117, n^o 19 (15 novembre 2002) : 8905–16. http://dx.doi.org/10.1063/1.1513461.

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Espinoza-Rojas, Francisca, Julio Chanamé, Paula Jofré et Laia Casamiquela. « The Consistency of Chemical Clocks among Coeval Stars ». Astrophysical Journal 920, n^o 2 (1 octobre 2021) : 94. http://dx.doi.org/10.3847/1538-4357/ac15fd.

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Moya, A., L. M. Sarro, E. Delgado-Mena, W. J. Chaplin, V. Adibekyan et S. Blanco-Cuaresma. « Stellar dating using chemical clocks and Bayesian inference ». Astronomy & ; Astrophysics 660 (avril 2022) : A15. http://dx.doi.org/10.1051/0004-6361/202141125.

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Context. Dating stars is a major challenge with a deep impact on many astrophysical fields. One of the most promising techniques for this is using chemical abundances. Recent space- and ground-based facilities have improved the quantity of stars with accurate observations. This has opened the door for using Bayesian inference tools to maximise the information we can extract from them. Aims. Our aim is to present accurate and reliable stellar age estimates of FGK stars using chemical abundances and stellar parameters. Methods. We used one of the most flexible Bayesian inference techniques (hierarchical Bayesian models) to exceed current possibilities in the use of chemical abundances for stellar dating. Our model is a data-driven model. We used a training set that has been presented in the literature with ages estimated with isochrones and accurate stellar abundances and general characteristics. The core of the model is a prescription of certain abundance ratios as linear combinations of stellar properties including age. We gathered four different testing sets to assess the accuracy, precision, and limits of our model. We also trained a model using chemical abundances alone. Results. We found that our age estimates and those coming from asteroseismology, other accurate sources, and also with ten Gaia benchmark stars agree well. The mean absolute difference of our estimates compared with those used as reference is 0.9 Ga, with a mean difference of 0.01 Ga. When using open clusters, we reached a very good agreement for Hyades, NGC 2632, Ruprecht 147, and IC 4651. We also found outliers that are a reflection of chemical peculiarities and/or stars at the limit of the validity ranges of the training set. The model that only uses chemical abundances shows slightly worse mean absolute difference (1.18 Ga) and mean difference (−0.12 Ga).

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Panzarasa, Guido, et Eric R. Dufresne. « Temporal Control of Soft Materials with Chemical Clocks ». CHIMIA International Journal for Chemistry 74, n^o 7 (12 août 2020) : 612. http://dx.doi.org/10.2533/chimia.2020.612.

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Allen, V., F. F. S. van der Tak et C. Walsh. « Complex cyanides as chemical clocks in hot cores ». Astronomy & ; Astrophysics 616 (août 2018) : A67. http://dx.doi.org/10.1051/0004-6361/201732553.

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Context. In the high-mass star-forming region G35.20−0.74N, small scale (~800 AU) chemical segregation has been observed in which complex organic molecules containing the CN group are located in a small location (toward continuum peak B3) within an apparently coherently rotating structure. Aims. We aim to determine the physical origin of the large abundance difference (~4 orders of magnitude) in complex cyanides within G35.20−0.74 B, and we explore variations in age, gas/dust temperature, and gas density. Methods. We performed gas-grain astrochemical modeling experiments with exponentially increasing (coupled) gas and dust temperature rising from 10 to 500 K at constant H2 densities of 107 cm−3, 108 cm−3, and 109 cm−3. We tested the effect of varying the initial ice composition, cosmic-ray ionization rate (1.3 × 10−17 s−1, 1 × 10−16 s−1, and 6 × 10−16 s−1), warm-up time (over 50, 200, and 1000 kyr), and initial (10, 15, and 25 K) and final temperatures (300 and 500 K). Results. Varying the initial ice compositions within the observed and expected ranges does not noticeably affect the modeled abundances indicating that the chemical make-up of hot cores is determined in the warm-up stage. Complex cyanides vinyl and ethyl cyanide (CH2CHCN and C2H5CN, respectively) cannot be produced in abundances (vs. H2) greater than 5 ×10−10 for CH2CHCN and 2 ×10−10 for C2H5CN with a fast warm-up time (52 kyr), while the lower limit for the observed abundance of C2H5CN toward source B3 is 3.4 ×10−10. Complex cyanide abundances are reduced at higher initial temperatures and increased at higher cosmic-ray ionization rates. Reaction-diffusion competition is necessary to reproduce observed abundances of oxygen-bearing species in our model. Conclusions. Within the context of this model, reproducing the observed abundances toward G35.20−0.74 Core B3 requires a fast warm-up at a high cosmic-ray ionization rate (~1 × 10−16 s−1) at a high gas density (>109 cm−3). The abundances observed at the other positions in G35.20-0.74N also require a fast warm-up but allow lower gas densities (~108 cm−3) and cosmic-ray ionization rates (~1 × 10−17 s−1). In general, we find that the abundance of ethyl cyanide in particular is maximized in models with a low initial temperature, a high cosmic-ray ionization rate, a long warm-up time (>200 kyr), and a lower gas density (tested down to 107 cm−3). G35.20−0.74 source B3 only needs to be ~2000 years older than B1/B2 for the observed chemical difference to be present, which maintains the possibility that G35.20−0.74 B contains a Keplerian disk.

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Johnson-Buck, Alexander, et William M. Shih. « Single-Molecule Clocks Controlled by Serial Chemical Reactions ». Nano Letters 17, n^o 12 (2 novembre 2017) : 7940–44. http://dx.doi.org/10.1021/acs.nanolett.7b04336.

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Rosenblum, Michael, et Arkady Pikovsky. « Synchronization : From pendulum clocks to chaotic lasers and chemical oscillators ». Contemporary Physics 44, n^o 5 (septembre 2003) : 401–16. http://dx.doi.org/10.1080/00107510310001603129.

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d'Eysmond, Thomas, Alessandro De Simone et Felix Naef. « Analysis of precision in chemical oscillators : implications for circadian clocks ». Physical Biology 10, n^o 5 (16 septembre 2013) : 056005. http://dx.doi.org/10.1088/1478-3975/10/5/056005.

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Buckle, J. V., et G. A. Fuller. « Sulphur-bearing species as chemical clocks for low mass protostars ? » Astronomy & ; Astrophysics 399, n^o 2 (février 2003) : 567–81. http://dx.doi.org/10.1051/0004-6361:20021816.

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Marsland, Robert, Wenping Cui et Jordan M. Horowitz. « The thermodynamic uncertainty relation in biochemical oscillations ». Journal of The Royal Society Interface 16, n^o 154 (mai 2019) : 20190098. http://dx.doi.org/10.1098/rsif.2019.0098.

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Living systems regulate many aspects of their behaviour through periodic oscillations of molecular concentrations, which function as ‘biochemical clocks.’ The chemical reactions that drive these clocks are intrinsically stochastic at the molecular level, so that the duration of a full oscillation cycle is subject to random fluctuations. Their success in carrying out their biological function is thought to depend on the degree to which these fluctuations in the cycle period can be suppressed. Biochemical oscillators also require a constant supply of free energy in order to break detailed balance and maintain their cyclic dynamics. For a given free energy budget, the recently discovered ‘thermodynamic uncertainty relation’ yields the magnitude of period fluctuations in the most precise conceivable free-running clock. In this paper, we show that computational models of real biochemical clocks severely underperform this optimum, with fluctuations several orders of magnitude larger than the theoretical minimum. We argue that this suboptimal performance is due to the small number of internal states per molecule in these models, combined with the high level of thermodynamic force required to maintain the system in the oscillatory phase. We introduce a new model with a tunable number of internal states per molecule and confirm that it approaches the optimal precision as this number increases.

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Mena, E. Delgado, A. Moya, V. Adibekyan, M. Tsantaki, J. I. González Hernández, G. Israelian, G. R. Davies et al. « Abundance to age ratios in the HARPS-GTO sample with Gaia DR2 ». Astronomy & ; Astrophysics 624 (avril 2019) : A78. http://dx.doi.org/10.1051/0004-6361/201834783.

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Aims. The purpose of this work is to evaluate how several elements produced by different nucleosynthesis processes behave with stellar age and provide empirical relations to derive stellar ages from chemical abundances. Methods. We derived different sets of ages using Padova and Yonsei–Yale isochrones and HIPPARCOS and Gaia parallaxes for a sample of more than 1000 FGK dwarf stars for which he have high-resolution (R ~ 115 000) and high-quality spectra from the HARPS-GTO program. We analyzed the temporal evolution of different abundance ratios to find the best chemical clocks. We applied multivariable linear regressions to our sample of stars with a small uncertainty on age to obtain empirical relations of age as a function of stellar parameters and different chemical clocks. Results. We find that [α/Fe] ratio (average of Mg, Si, and Ti), [O/Fe] and [Zn/Fe] are good age proxies with a lower dispersion than the age-metallicity dispersion. Several abundance ratios present a significant correlation with age for chemically separated thin disk stars (i.e., low-α) but in the case of the chemically defined thick disk stars (i.e., high-α) only the elements Mg, Si, Ca, and Ti II show a clear correlation with age. We find that the thick disk stars are more enriched in light-s elements than thin disk stars of similar age. The maximum enrichment of s-process elements in the thin disk occurs in the youngest stars which in turn have solar metallicity. The slopes of the [X/Fe]-age relations are quite constant for O, Mg, Si, Ti, Zn, Sr, and Eu regardless of the metallicity. However, this is not the case for Al, Ca, Cu and most of the s-process elements, which display very different trends depending on the metallicity. This demonstrates the limitations of using simple linear relations based on certain abundance ratios to obtain ages for stars of different metallicities. Finally, we show that by using 3D relations with a chemical clock and two stellar parameters (either Teff, [Fe/H] or stellar mass) we can explain up to 89% of age variance in a star. A similar result is obtained when using 2D relations with a chemical clock and one stellar parameter, explaining up to a 87% of the variance. Conclusions. The complete understanding of how the chemical elements were produced and evolved in the Galaxy requires the knowledge of stellar ages and precise chemical abundances. We show how the temporal evolution of some chemical species change with metallicity, with remarkable variations at super-solar metallicities, which will help to better constrain the yields of different nucleosynthesis processes along the history of the Galaxy.

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Wakelam, V., P. Caselli, C. Ceccarelli, E. Herbst et A. Castets. « Resetting chemical clocks of hot cores based on S-bearing molecules ». Astronomy & ; Astrophysics 422, n^o 1 (juillet 2004) : 159–69. http://dx.doi.org/10.1051/0004-6361:20047186.

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Uehara, Takahiro N., Takashi Nonoyama, Kyomi Taki, Keiko Kuwata, Ayato Sato, Kazuhiro J. Fujimoto, Tsuyoshi Hirota et al. « Phosphorylation of RNA Polymerase II by CDKC;2 Maintains the Arabidopsis Circadian Clock Period ». Plant and Cell Physiology 63, n^o 4 (27 janvier 2022) : 450–62. http://dx.doi.org/10.1093/pcp/pcac011.

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Abstract The circadian clock is an internal timekeeping system that governs about 24 h biological rhythms of a broad range of developmental and metabolic activities. The clocks in eukaryotes are thought to rely on lineage-specific transcriptional–translational feedback loops. However, the mechanisms underlying the basic transcriptional regulation events for clock function have not yet been fully explored. Here, through a combination of chemical biology and genetic approaches, we demonstrate that phosphorylation of RNA polymerase II by CYCLIN DEPENDENT KINASE C; 2 (CDKC;2) is required for maintaining the circadian period in Arabidopsis. Chemical screening identified BML-259, the inhibitor of mammalian CDK2/CDK5, as a compound lengthening the circadian period of Arabidopsis. Short-term BML-259 treatment resulted in decreased expression of most clock-associated genes. Development of a chemical probe followed by affinity proteomics revealed that BML-259 binds to CDKC;2. Loss-of-function mutations of cdkc;2 caused a long period phenotype. In vitro experiments demonstrated that the CDKC;2 immunocomplex phosphorylates the C-terminal domain of RNA polymerase II, and BML-259 inhibits this phosphorylation. Collectively, this study suggests that transcriptional activity maintained by CDKC;2 is required for proper period length, which is an essential feature of the circadian clock in Arabidopsis.

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Tian, Yonggang, et Dekui Zhang. « Biological Clock and Inflammatory Bowel Disease Review : From the Standpoint of the Intestinal Barrier ». Gastroenterology Research and Practice 2022 (14 mars 2022) : 1–11. http://dx.doi.org/10.1155/2022/2939921.

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Inflammatory bowel disease is a group of chronic, recurrent, nonspecific inflammatory diseases of the intestine that severely affect the quality of life of patients. The pathogenesis of this disease is caused by complex and interactive neural networks composed of factors such as genetic susceptibility, external environment, immune disorders, and intestinal barrier dysfunction. It is well known that there is a strong link between environmental stressors (also known as circadian clocks) that can influence circadian changes and inflammatory bowel disease. Among them, the biological clock is involved in the pathogenesis of inflammatory bowel disease by affecting the function of the intestinal barrier. Therefore, this review is aimed at systematically summarizing the latest research progress on the role of the circadian clock in the pathogenesis of inflammatory bowel disease by affecting intestinal barrier functions (intestinal mechanical barrier, intestinal immune barrier, intestinal microecological barrier, and intestinal chemical barrier) and the potential clinical value of clock genes in the management of inflammatory bowel disease, for the application of circadian clock therapy in the management of inflammatory bowel disease and then the benefit to the majority of patients.

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Prabhu, Gurpur Rakesh D., Henryk A. Witek et Pawel L. Urban. « Chemical clocks, oscillations, and other temporal effects in analytical chemistry : oddity or viable approach ? » Analyst 143, n^o 15 (2018) : 3514–25. http://dx.doi.org/10.1039/c7an01926b.

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Steinbock, Oliver, et Petteri Kettunen. « Chemical clocks on the basis of rotating waves. Measuring irrational numbers from period ratios ». Chemical Physics Letters 251, n^o 5-6 (mars 1996) : 305–8. http://dx.doi.org/10.1016/0009-2614(96)00099-1.

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Chen, Z., S. H. Yoo, Y. S. Park, K. H. Kim, S. Wei, E. Buhr, Z. Y. Ye, H. L. Pan et J. S. Takahashi. « Identification of diverse modulators of central and peripheral circadian clocks by high-throughput chemical screening ». Proceedings of the National Academy of Sciences 109, n^o 1 (19 décembre 2011) : 101–6. http://dx.doi.org/10.1073/pnas.1118034108.

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Wang, Xuelei, Mao Ye, Fei Lu, Yunkai Mao, Hao Tian et Jianli Li. « Recent Progress on Micro-Fabricated Alkali Metal Vapor Cells ». Biosensors 12, n^o 3 (6 mars 2022) : 165. http://dx.doi.org/10.3390/bios12030165.

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Alkali vapor cells are the core components of atomic sensing instruments such as atomic gyroscopes, atomic magnetometers, atomic clocks, etc. Emerging integrated atomic sensing devices require high-performance miniaturized alkali vapor cells, especially micro-fabricated vapor cells. In this review, bonding methods for vapor cells of this kind are summarized in detail, including anodic bonding, sacrificial micro-channel bonding, and metal thermocompression bonding. Compared with traditional through-lighting schemes, researchers have developed novel methods for micro-fabricated vapor cells under both single- and double-beam schemes. In addition, emerging packaging methods for alkali metals in micro-fabricated vapor cells can be categorized as physical or chemical approaches. Physical methods include liquid transfer and wax pack filling. Chemical methods include the reaction of barium azide with rubidium chloride, ultraviolet light decomposition (of rubidium azide), and the high-temperature electrolysis of rubidium-rich glass. Finally, the application trend of micro-fabricated alkali vapor cells in the field of micro-scale gyroscopes, micro-scale atomic clocks, and especially micro-scale biomagnetometers is reviewed. Currently, the sensing industry has become a major driving force for the miniaturization of atomic sensing devices, and in the near future, the micro-fabricated alkali vapor cell technology of atomic sensing devices may experience extensive developments.

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Dulieu, O., et S. Willitsch. « Ion Coulomb crystals : from quantum technology to chemistry close to the absolute zero point ». Europhysics News 48, n^o 2 (mars 2017) : 17–20. http://dx.doi.org/10.1051/epn/2017203.

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Ion Coulomb crystals are ordered structures of atomic or molecular ions stored in ion traps at temperatures close to the absolute zero point. These unusual “crystals” form the basis of extremely accurate clocks, provide an environment for precise studies of chemical reactions and enable advanced implementations of the technology for a quantum computer. In this article, we discuss the techniques for generating atomic and molecular Coulomb crystals and highlight some of their applications.

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Mackenzie, Dana. « Time Gets More Precise with Transportable Optical Lattice Clocks ». Engineering 6, n^o 11 (novembre 2020) : 1210–11. http://dx.doi.org/10.1016/j.eng.2020.08.008.

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Skúladóttir, Á., C. J. Hansen, S. Salvadori et A. Choplin. « Neutron-capture elements in dwarf galaxies ». Astronomy & ; Astrophysics 631 (novembre 2019) : A171. http://dx.doi.org/10.1051/0004-6361/201936125.

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The heavy elements (Z > 30) are created in neutron (n)-capture processes that are predicted to happen at vastly different nucleosynthetic sites. To study these processes in an environment different from the Milky Way, we targeted the n-capture elements in red giant branch stars in the Sculptor dwarf spheroidal galaxy. Using ESO VLT/FLAMES spectra, we measured the chemical abundances of Y, Ba, La, Nd, and Eu in 98 stars covering the metalliticy range −2.4 < [Fe/H] < −0.9. This is the first paper in a series about the n-capture elements in dwarf galaxies, and here we focus on the relative and absolute timescales of the slow (s)- and rapid (r)-processes in Sculptor. From the abundances of the s-process element Ba and the r-process element Eu, it is clear that the r-process enrichment occurred throughout the entire chemical evolution history of Sculptor. Furthermore, there is no evidence for the r-process to be significantly delayed in time relative to core-collapse supernovae. Neutron star mergers are therefore unlikely the dominant (or only) nucleosynthetic site of the r-process. However, the products of the s-process only become apparent at [Fe/H] ≈ −2 in Sculptor, and the s-process becomes the dominant source of Ba at [Fe/H] ≳ −2. We tested the use of [Y/Mg] and [Ba/Mg] as chemical clocks in Sculptor. Similarly to what is observed in the Milky Way, [Y/Mg] and [Ba/Mg] increase towards younger ages. However, there is an offset in the trends, where the abundance ratios of [Y/Mg] in Sculptor are significantly lower than those of the Milky Way at any given age. This is most likely caused by metallicity dependence of yields from the s-process, as well as by a different relative contribution of the s-process to core-collapse supernovae in these galaxies. Comparisons of our results with data of the Milky Way and the Fornax dwarf spheroidal galaxy furthermore show that these chemical clocks depend on both metallicity and environment.

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Malloy, Jaclyn N., Jiffin K. Paulose, Ye Li et Vincent M. Cassone. « Circadian rhythms of gastrointestinal function are regulated by both central and peripheral oscillators ». American Journal of Physiology-Gastrointestinal and Liver Physiology 303, n^o 4 (15 août 2012) : G461—G473. http://dx.doi.org/10.1152/ajpgi.00369.2011.

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Circadian clocks are responsible for daily rhythms in a wide array of processes, including gastrointestinal (GI) function. These are vital for normal digestive rhythms and overall health. Previous studies demonstrated circadian clocks within the cells of GI tissue. The present study examines the roles played by the suprachiasmatic nuclei (SCN), master circadian pacemaker for overt circadian rhythms, and the sympathetic nervous system in regulation of circadian GI rhythms in the mouse Mus musculus . Surgical ablation of the SCN abolishes circadian locomotor, feeding, and stool output rhythms when animals are presented with food ad libitum, while restricted feeding reestablishes these rhythms temporarily. In intact mice, chemical sympathectomy with 6-hydroxydopamine has no effect on feeding and locomotor rhythmicity in light-dark cycles or constant darkness but attenuates stool weight and stool number rhythms. Again, however, restricted feeding reestablishes rhythms in locomotor activity, feeding, and stool output rhythms. Ex vivo, intestinal tissue from PER2::LUC transgenic mice expresses circadian rhythms of luciferase bioluminescence. Chemical sympathectomy has little effect on these rhythms, but timed administration of the β-adrenergic agonist isoproterenol causes a phase-dependent shift in PERIOD2 expression rhythms. Collectively, the data suggest that the SCN are required to maintain feeding, locomotor, and stool output rhythms during ad libitum conditions, acting at least in part through daily activation of sympathetic activity. Even so, this input is not necessary for entrainment to timed feeding, which may be the province of oscillators within the intestines themselves or other components of the GI system.

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Osypova, Alina, Matthias Dübner et Guido Panzarasa. « Oscillating Reactions Meet Polymers at Interfaces ». Materials 13, n^o 13 (2 juillet 2020) : 2957. http://dx.doi.org/10.3390/ma13132957.

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Chemo-mechanical phenomena, including oscillations and peristaltic motions, are widespread in nature—just think of heartbeats—thanks to the ability of living organisms to convert directly chemical energy into mechanical work. Their imitation with artificial systems is still an open challenge. Chemical clocks and oscillators (such as the popular Belousov–Zhabotinsky (BZ) reaction) are reaction networks characterized by the emergence of peculiar spatiotemporal dynamics. Their application to polymers at interfaces (grafted chains, layer-by-layer assemblies, and polymer brushes) offers great opportunities for developing novel smart biomimetic materials. Despite the wide field of potential applications, limited research has been carried out so far. Here, we aim to showcase the state-of-the-art of this fascinating field of investigation, highlighting the potential for future developments and providing a personal outlook.

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Tiemei, Chen. « Book Review : Age determination of young rocks and artifacts : physical and chemical clocks in Quaternary geology and archaeology ». Holocene 9, n^o 3 (avril 1999) : 379. http://dx.doi.org/10.1177/095968369900900315.

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Casali, G., L. Spina, L. Magrini, A. I. Karakas, C. Kobayashi, A. R. Casey, S. Feltzing et al. « The Gaia-ESO survey : the non-universality of the age–chemical-clocks–metallicity relations in the Galactic disc ». Astronomy & ; Astrophysics 639 (juillet 2020) : A127. http://dx.doi.org/10.1051/0004-6361/202038055.

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Context. In the era of large spectroscopic surveys, massive databases of high-quality spectra coupled with the products of the Gaia satellite provide tools to outline a new picture of our Galaxy. In this framework, an important piece of information is provided by our ability to infer stellar ages, and consequently to sketch a Galactic timeline. Aims. We aim to provide empirical relations between stellar ages and abundance ratios for a sample of stars with very similar stellar parameters to those of the Sun, namely the so-called solar-like stars. We investigate the dependence on metallicity, and we apply our relations to independent samples, that is, the Gaia-ESO samples of open clusters and of field stars. Methods. We analyse high-resolution and high-signal-to-noise-ratio HARPS spectra of a sample of solar-like stars to obtain precise determinations of their atmospheric parameters and abundances for 25 elements and/or ions belonging to the main nucleosynthesis channels through differential spectral analysis, and of their ages through isochrone fitting. Results. We investigate the relations between stellar ages and several abundance ratios. For the abundance ratios with a steeper dependence on age, we perform multivariate linear regressions, in which we include the dependence on metallicity, [Fe/H]. We apply our best relations to a sample of open clusters located from the inner to the outer regions of the Galactic disc. Using our relations, we are able to recover the literature ages only for clusters located at RGC > 7 kpc. The values that we obtain for the ages of the inner-disc clusters are much greater than the literature ones. In these clusters, the content of neutron capture elements, such as Y and Zr, is indeed lower than expected from chemical evolution models, and consequently their [Y/Mg] and [Y/Al] are lower than in clusters of the same age located in the solar neighbourhood. With our chemical evolution model and a set of empirical yields, we suggest that a strong dependence on the star formation history and metallicity-dependent stellar yields of s-process elements can substantially modify the slope of the [s/α]–[Fe/H]–age relation in different regions of the Galaxy. Conclusions. Our results point towards a non-universal relation [s/α]–[Fe/H]–age, indicating the existence of relations with different slopes and intercepts at different Galactocentric distances or for different star formation histories. Therefore, relations between ages and abundance ratios obtained from samples of stars located in a limited region of the Galaxy cannot be translated into general relations valid for the whole disc. A better understanding of the s-process at high metallicity is necessary to fully understand the origin of these variations.

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Amaike, Kazuma, Tsuyoshi Oshima, Nicola Stephanie Skoulding, Yoshifumi Toyama, Tsuyoshi Hirota et Kenichiro Itami. « Small Molecules Modulating Mammalian Biological Clocks : Exciting New Opportunities for Synthetic Chemistry ». Chem 6, n^o 9 (septembre 2020) : 2186–98. http://dx.doi.org/10.1016/j.chempr.2020.08.011.

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Breen, Shari L., et Simon P. Ellingsen. « Masers as evolutionary tracers of high-mass star formation ». Proceedings of the International Astronomical Union 8, S287 (janvier 2012) : 156–60. http://dx.doi.org/10.1017/s1743921312006837.

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AbstractDetermining an evolutionary clock for high-mass star formation is an important step towards realising a unified theory of star formation, as it will enable qualitative studies of the associated high-mass stars to be executed. We have carried out detailed studies of a large number of sources suspected of undergoing high-mass star formation and have found that common maser transitions offer the best opportunity to determine an evolutionary scheme for these objects. We have investigated the relative evolutionary phases of massive star formation associated with the presence or absence of combinations of water, methanol and main-line hydroxyl masers. The locations of the different maser species have been compared with the positions of 1.2 mm dust clumps, radio continuum, GLIMPSE point sources and Extended Green Objects. Comparison between the characteristics of coincident sources has revealed strong evidence for an evolutionary sequence for the different maser species in high-mass star formation regions. We present our proposed sequence for the presence of the common maser species associated with young high-mass stars and highlight recent advances. We discuss future investigations that will be made in this area by comparing data from the Methanol Multibeam (MMB) Survey with chemical clocks from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey.

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Johnson, William C. « Chronometric dating in archaeology ; Age determination of young rocks and artifacts : Physical and chemical clocks in quaternary geology and archaeology ». Geoarchaeology 14, n^o 7 (octobre 1999) : 719–24. http://dx.doi.org/10.1002/(sici)1520-6548(199910)14:7<719 ::aid-gea7>3.0.co;2-z.

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Guo, Ping, Hongling Meng, Lin Dan et Jianye Zhao. « Wafer-Level Filling of MEMS Vapor Cells Based on Chemical Reaction and Evaporation ». Micromachines 13, n^o 2 (29 janvier 2022) : 217. http://dx.doi.org/10.3390/mi13020217.

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Micro-electro-mechanical system (MEMS) vapor cells are key components for sensors such as chip-scale atomic clocks (CSACs) and magnetometers (CSAMs). Many approaches have been proposed to fabricate MEMS vapor cells. In this article, we propose a new method to fabricate wafer-level filling of MEMS vapor cells based on chemical reaction and evaporation. The Cs metals are firstly obtained through the chemical reaction between cesium chloride and barium azide in a reservoir baseplate. Then, the Cs metals are evaporated to the preform through the microchannel plate and condensed on the inner glass surface of the preform. Lastly, the MEMS vapor cells are filled with buffer gas, sealed by anodic bonding, and mechanically diced into three dimensions: 5 mm × 5 mm × 1.2 mm, 4 mm × 4 mm × 1.2 mm, and 3 mm × 3 mm × 1.2 mm. The full width at half maximum (FWHM) linewidth of the coherent population trapping (CPT) signal of the MEMS vapor cells is found to be 4.33 kHz. The intrinsic linewidth is about 1638 Hz. Based on the CPT signal, the frequency stability is 4.41 × 10−12@1000 s. The results demonstrate that the presented method of the wafer-level filling of MEMS vapor cells fulfills the requirements of sensors such as CSACs.

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Titarenko, A., A. Recio-Blanco, P. de Laverny, M. Hayden, G. Guiglion et C. Worley. « [Y/Mg] stellar dating calibration ». Proceedings of the International Astronomical Union 12, S330 (avril 2017) : 267–68. http://dx.doi.org/10.1017/s1743921317006780.

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AbstractGaia DR1 has opened a new era of stellar age dating, that is crucial for many astrophysical objectives. In addition, the Gaia based isochrone fitting ages can be compared to other chemical clocks like the [Y/Mg] one (Nissen et al. 2015). In our work we have used ESO archived data of the AMBRE project (de Laverny et al. 2013) for UVES spectra, in order to evaluate the age [Y/Mg] abundance correlation for turn off stars. 310 turn off stars of the UVES-archive (setups 564 and 580) are included in the TGAS database. Isochrone fitting ages were derived. We have applied the GAUGUIN procedure for those stars to derive the Mg and the Y abundances. As the result we present the [Y/Mg] vs stellar age dependence for ~40 TO-stars.

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Zhang, Guo-Yin, Jin-Long Xu, A. I. Vasyunin, D. A. Semenov, Jun-Jie Wang, Sami Dib, Tie Liu et al. « Physical properties and chemical composition of the cores in the California molecular cloud ». Astronomy & ; Astrophysics 620 (décembre 2018) : A163. http://dx.doi.org/10.1051/0004-6361/201833622.

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Aims. We aim to reveal the physical properties and chemical composition of the cores in the California molecular cloud (CMC), so as to better understand the initial conditions of star formation. Methods. We made a high-resolution column density map (18.2′′) with Herschel data, and extracted a complete sample of the cores in the CMC with the fellwalker algorithm. We performed new single-pointing observations of molecular lines near 90 GHz with the IRAM 30m telescope along the main filament of the CMC. In addition, we also performed a numerical modeling of chemical evolution for the cores under the physical conditions. Results. We extracted 300 cores, of which 33 are protostellar and 267 are starless cores. About 51% (137 of 267) of the starless cores are prestellar cores. Three cores have the potential to evolve into high-mass stars. The prestellar core mass function (CMF) can be well fit by a log-normal form. The high-mass end of the prestellar CMF shows a power-law form with an index α = −0.9 ± 0.1 that is shallower than that of the Galactic field stellar mass function. Combining the mass transformation efficiency (ε) from the prestellar core to the star of 15 ± 1% and the core formation efficiency (CFE) of 5.5%, we suggest an overall star formation efficiency of about 1% in the CMC. In the single-pointing observations with the IRAM 30m telescope, we find that 6 cores show blue-skewed profile, while 4 cores show red-skewed profile. [HCO+]/[HNC] and [HCO+]/[N2H+] in protostellar cores are higher than those in prestellar cores; this can be used as chemical clocks. The best-fit chemical age of the cores with line observations is ~5 × 104 yr.

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Sales-Silva, J. V., S. Daflon, K. Cunha, D. Souto, V. V. Smith, C. Chiappini, J. Donor et al. « Exploring the S-process History in the Galactic Disk : Cerium Abundances and Gradients in Open Clusters from the OCCAM/APOGEE Sample ». Astrophysical Journal 926, n^o 2 (1 février 2022) : 154. http://dx.doi.org/10.3847/1538-4357/ac4254.

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Abstract The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce ii lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/α-element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]–age plane (where X can be H, Fe, or the α-elements O, Mg, Si, or Ca). These metallicity-dependent relations result in [Ce/Fe] and [Ce/α] ratios with ages that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d[Ce/H]/d R GC being negative, while d[Ce/Fe]/d R GC is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by ∼+0.009 dex kpc−1 Gyr−1. Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.

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Gedela, Ravi. « A ROLE OF ENVIRONMENTAL FACTORS ON CALCIUM SIGNALING GENES IN NEUROSPORA CRASSA ». JOURNAL OF ADVANCES IN BIOTECHNOLOGY 5, n^o 3 (30 décembre 2015) : 699–710. http://dx.doi.org/10.24297/jbt.v5i3.4833.

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Â Neurospora crassa possesses a complex of Ca2+_signaling system consisting of 48 Ca2+-signaling proteins.Â The Ca+2-signallingÂ protein plays an important role in a range of processes such as a Ca2+ stress tolerance, hyphal tip branching growth, cytoskeletal organization, cell cycle progression, circadian clocks, sporulation, sexual development, and ultraviolet (uv) survival.Â The environmental factors, broadly defined to include chemical, physical, nutritional, and behavioral factors...etc.Â In this article, we are reporting here a role of physic-chemical environmental factors pH, glucose and ultraviolet (UV) affect on âˆ†NCU06366, and âˆ†NCU05225 Ca2+ -signaling knockout mutants in N. crassa.Â The verified result showed that, âˆ†NCU06366 and âˆ†NCU05225 Ca2+ -signaling knockout mutants slower growth rate at pH (7.6), and glucose starvation against to the control wild type respectively.Â In addition to that the found results showed, ultraviolet (UV) survival, there is no UV radiation affects on âˆ†NCU06366 and âˆ†NCU05225 Ca2+-signaling knockout mutants as evaluate to the positive and the negative controls in N.crassa.Â Along with that, In-silico analysis Multiple sequence analysis and Phylogenetics tree for conserve domain of NCU05225 (NADH dehydrogenase) and NCU06366 (Ca2+/H+ anti-porter) Ca2+-signaling genes encodes proteins in N.crassa, showed high sequence similarity and 68-100% and 89% homology Â to the other class of fungi respectively.Â It indicates that, NCU05225 (Mitochondrial NADH dehydrogenase) and NCU06366 (Ca2+/H+ exchangers) Ca2+-signaling gene encoding conserve domain widespread in other class of fungi as well.Â Â

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Unosson, Måns, Marco Brancaccio, Michael Hastings, Adam M. Johansen et Bärbel Finkenstädt. « A spatio-temporal model to reveal oscillator phenotypes in molecular clocks : Parameter estimation elucidates circadian gene transcription dynamics in single-cells ». PLOS Computational Biology 17, n^o 12 (17 décembre 2021) : e1009698. http://dx.doi.org/10.1371/journal.pcbi.1009698.

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We propose a stochastic distributed delay model together with a Markov random field prior and a measurement model for bioluminescence-reporting to analyse spatio-temporal gene expression in intact networks of cells. The model describes the oscillating time evolution of molecular mRNA counts through a negative transcriptional-translational feedback loop encoded in a chemical Langevin equation with a probabilistic delay distribution. The model is extended spatially by means of a multiplicative random effects model with a first order Markov random field prior distribution. Our methodology effectively separates intrinsic molecular noise, measurement noise, and extrinsic noise and phenotypic variation driving cell heterogeneity, while being amenable to parameter identification and inference. Based on the single-cell model we propose a novel computational stability analysis that allows us to infer two key characteristics, namely the robustness of the oscillations, i.e. whether the reaction network exhibits sustained or damped oscillations, and the profile of the regulation, i.e. whether the inhibition occurs over time in a more distributed versus a more direct manner, which affects the cells’ ability to phase-shift to new schedules. We show how insight into the spatio-temporal characteristics of the circadian feedback loop in the suprachiasmatic nucleus (SCN) can be gained by applying the methodology to bioluminescence-reported expression of the circadian core clock gene Cry1 across mouse SCN tissue. We find that while (almost) all SCN neurons exhibit robust cell-autonomous oscillations, the parameters that are associated with the regulatory transcription profile give rise to a spatial division of the tissue between the central region whose oscillations are resilient to perturbation in the sense that they maintain a high degree of synchronicity, and the dorsal region which appears to phase shift in a more diversified way as a response to large perturbations and thus could be more amenable to entrainment.

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Gerasimov, Roman, Adam J. Burgasser, Derek Homeier, Luigi R. Bedin, Jon M. Rees, Michele Scalco, Jay Anderson et Maurizio Salaris. « The HST Large Program on ω Centauri. V. Exploring the Ultracool Dwarf Population with Stellar Atmosphere and Evolutionary Modeling ». Astrophysical Journal 930, n^o 1 (1 mai 2022) : 24. http://dx.doi.org/10.3847/1538-4357/ac61e5.

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Abstract Brown dwarfs can serve as both clocks and chemical tracers of the evolutionary history of the Milky Way due to their continuous cooling and high sensitivity of spectra to composition. We focus on brown dwarfs in globular clusters that host some of the oldest coeval populations in the galaxy. Currently, no brown dwarfs in globular clusters have been confirmed, but they are expected to be uncovered with advanced observational facilities such as the James Webb Space Telescope (JWST). In this paper we present a new set of stellar models specifically designed to investigate low-mass stars and brown dwarfs in ω Centauri—the largest known globular cluster. The parameters of our models were derived from iterative fits to Hubble Space Telescope photometry of the main-sequence members of the cluster. Despite the complex distribution of abundances and the presence of multiple main sequences in ω Centauri, we find that the modal color–magnitude distribution can be represented by a single stellar population with parameters determined in this study. The observed luminosity function is well represented by two distinct stellar populations having solar and enhanced helium mass fractions and a common initial mass function, in agreement with previous studies. Our analysis confirms that the abundances of individual chemical elements play a key role in determining the physical properties of low-mass cluster members. We use our models to draw predictions of brown dwarf colors and magnitudes in anticipated JWST NIRCam data, confirming that the beginning of the substellar sequence should be detected in ω Centauri in forthcoming observations.

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Stávek, Jiří. « A New Interpretation of Contributions Presented at the Solvay Conference 1911. Can We Falsify the “Geocentric” Foundations of Quantum Mechanics in the Solar System ? » European Journal of Applied Physics 3, n^o 6 (15 décembre 2021) : 61–65. http://dx.doi.org/10.24018/ejphysics.2021.3.6.135.

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We have studied the contributions and presentations published in the Proceedings of the Solvay Conference 1911. Based on the lecture of Ernest Solvay on the “gravito-matérialitique” we can distinguish two features of the Earth´s gravitational field – 1. “gravité réelle” described by the Newton´s gravitational law and 2. “gravité potentielle” acting as an agent of the self-organization on quantum particles and creating structures described by the Planck constant hEARTH. From the discussions followed after the presentations of Walther Nernst and Albert Einstein we interpreted the Nernst- Lindemann Formula for the specific heat of solids using the comment of Heike Kamerlingh Onnes (the discoverer of the superconductivity) as two transverse and one longitudinal oscillations of phonon in the surroundings at temperature T. In order to falsify this “geocentric” model of foundations of quantum mechanics in the spirit of Karl Popper we propose to initiate the CURE Project (China – USA – Russia – European Union) (cure = to solve a problem) in order to build quantum laboratories on different orbits around the Earth, on the surface of the Moon and Mars, and in the Lagrange points of the system the Earth – Moon and the Earth – Sun to get new experimental data for the specific heat of solids, the critical temperatures of superconductors, chemical and physical self-organized reactions (Liesegang rings, Belousov- Zhabotinsky waves, chemical clocks, Bose-Einstein condensates, de Broglie waves, etc.). There is space enough for all participants on this CURE Project to collect new valuable data describing this “hidden variable” presented by Ernest Solvay in his forgotten lecture in 1911.

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Breen, S. L., et S. P. Ellingsen. « An evolutionary sequence for high-mass star formation ». Proceedings of the International Astronomical Union 8, S292 (août 2012) : 39. http://dx.doi.org/10.1017/s1743921313000203.

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AbstractDetermining an evolutionary clock for high-mass star formation is an important step towards realizing a unified theory of star formation, as it will enable qualitative studies of the associated high-mass stars to be executed. Our recent studies have shown that masers have great potential to accurately trace the evolution of these regions. We have investigated the relative evolutionary phases associated with the presence of combinations of water, methanol and hydroxyl masers. Comparison between the characteristics of coincident sources has revealed strong evidence for an evolutionary sequence for the different maser species, a result that we now aim to corroborate through comparisons with chemical clocks.Using our new, large samples of methanol masers at 6.7 GHz (MMB survey; Green et al. (2009)) and 12.2 GHz (Breen et al. 2012), 22 GHz water masers (Breen & Ellingsen 2012), OH masers together with complementary data, we find strong evidence that it is not only the presence or absence of the different maser species that indicates the evolutionary stage of the associated high-mass star formation region (see e.g. Breen et al. (2010)), but that the properties of those masers can give even finer evolutionary details. Most notably, the intensity and velocity range of detected maser emission increases as the star forming region evolves (Breen et al. 2011).Subsequent work we have undertaken (Ellingsen et al. 2011) has shown that the presence of rare 37.7 GHz methanol masers may signal the end of the methanol maser phase. They show that 37.7 GHz methanol masers are associated only with the most luminous 6.7 and 12.2 GHz methanol masers, which combined with the rarity of these objects is consistent with them being a short lived phase towards the end of the 6.7 GHz methanol maser lifetime.An independent confirmation of our maser evolutionary timeline can be gained through comparisons with chemical clocks. MALT90 is a legacy survey of 1000s of dense star forming cores at 90GHz, simultaneously observing 16 molecular lines with the Mopra radio telescope (see e.g. Foster et al. 2011). It provides the perfect dataset to test the maser evolutionary timeline due to the targeted lines and the fact that at least one-quarter of the MALT90 sources correspond to maser sites, providing a large enough sample for meaningful analysis. From our preliminary analysis, we find that star formation regions showing similar maser properties also show similar thermal line properties; as would be expected if our evolutionary scenario were accurate.

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Chernysheva, Daria, Ludmila Pudova, Yuri Popov, Nina Smirnova, Olga Maslova, Mathieu Allix, Aydar Rakhmatullin, Nikolay Leontyev, Andrey Nikolaev et Igor Leontyev. « Non-Isothermal Decomposition as Efficient and Simple Synthesis Method of NiO/C Nanoparticles for Asymmetric Supercapacitors ». Nanomaterials 11, n^o 1 (13 janvier 2021) : 187. http://dx.doi.org/10.3390/nano11010187.

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A series of NiO/C nanocomposites with NiO concentrations ranging from 10 to 90 wt% was synthesized using a simple and efficient two-step method based on non-isothermal decomposition of Nickel(II) bis(acetylacetonate). X-ray diffraction (XRD) measurements of these NiO/C nanocomposites demonstrate the presence of β-NiO. NiO/C nanocomposites are composed of spherical particles distributed over the carbon support surface. The average diameter of nickel oxide spheres increases with the NiO content and are estimated as 36, 50 and 205 nm for nanocomposites with 10, 50 and 80 wt% NiO concentrations, respectively. In turn, each NiO sphere contains several nickel oxide nanoparticles, whose average sizes are 7–8 nm. According to the tests performed using a three-electrode cell, specific capacitance (SC) of NiO/C nanocomposites increases from 200 to 400 F/g as the NiO content achieves a maximum of 60 wt% concentration, after which the SC decreases. The study of the NiO/C composite showing the highest SC in three- and two-electrode cells reveals that its SC remains almost unchanged while increasing the current density, and the sample demonstrates excellent cycling stability properties. Finally, NiO/C (60% NiO) composites are shown to be promising materials for charging quartz clocks with a power rating of 1.5 V (30 min).

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Raddi, Roberto, Santiago Torres, Alberto Rebassa-Mansergas, Jesús Maldonado, María E. Camisassa, Detlev Koester, Nicola Pietro Gentile Fusillo et al. « Kinematic properties of white dwarfs ». Astronomy & ; Astrophysics 658 (27 janvier 2022) : A22. http://dx.doi.org/10.1051/0004-6361/202141837.

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Context. Kinematic and chemical tagging of stellar populations have both revealed much information on the past and recent history of the Milky Way, including its formation history, merger events, and mixing of populations across the Galactic disk and halo. Aims. We present the first detailed 3D kinematic analysis of a sample of 3133 white dwarfs that used Gaia astrometry plus radial velocities, which were measured either by Gaia or by ground-based spectroscopic observations. The sample includes either isolated white dwarfs that have direct radial velocity measurements, or white dwarfs that belong to common proper motion pairs that contain nondegenerate companions with available radial velocities. A subset of common proper motion pairs also have metal abundances that have been measured by large-scale spectroscopic surveys or by our own follow-up observations. Methods. We used the white dwarfs as astrophysical clocks by determining their masses and total ages through interpolation with dedicated evolutionary models. We also used the nondegenerate companions in common proper motions to chemically tag the population. Combining accurate radial velocities with Gaia astrometry and proper motions, we derived the velocity components of our sample in the Galactic rest frame and their Galactic orbital parameters. Results. The sample is mostly located within ∼300 pc from the Sun. It predominantly contains (90–95%) thin-disk stars with almost circular Galactic orbits, while the remaining 5–10% of stars have more eccentric trajectories and belong to the thick disk. We identified seven isolated white dwarfs and two common proper motion pairs as halo members. We determined the age – velocity dispersion relation for the thin-disk members, which agrees with previous results that were achieved from different white dwarf samples without published radial velocities. The age – velocity dispersion relation shows signatures of dynamical heating and saturation after 4–6 Gyr. We observed a mild anticorrelation between [Fe/H] and the radial component of the average velocity dispersion, showing that dynamical mixing of populations takes place in the Galactic disk, as was detected through the analysis of other samples of FGK stars. Conclusions. We have shown that a white dwarf sample with accurate 3D kinematics and well-measured chemical compositions enables a wider understanding of their population in the solar neighborhood and its connection with the Galactic chemodynamics. The legacy of existing spectroscopic surveys will be boosted by the availability of upcoming larger samples of white dwarfs and common proper motion pairs with more uniform high-quality data.

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Castro, M., F. Baudin, O. Benomar, R. Samadi, T. Morel, C. Barban, J. D. do Nascimento et al. « Modeling of two CoRoT solar analogues constrained by seismic and spectroscopic analysis ». Monthly Notices of the Royal Astronomical Society 505, n^o 2 (8 juin 2021) : 2151–58. http://dx.doi.org/10.1093/mnras/stab1410.

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ABSTRACT Solar analogues are important stars to study for understanding the properties of the Sun. Combined with seismic and spectroscopic analysis, evolutionary modelling becomes a powerful method to characterize stellar intrinsic parameters, such as mass, radius, metallicity and age. However, these characteristics, relevant for other aspects of astrophysics or exoplanetary system physics, for example, are difficult to obtain with high precision and/or accuracy. The goal of this study is to characterize the two solar analogues, HD 42618 and HD 43587, observed by CoRoT. In particular, we aim to infer their precise mass, radius and age, using evolutionary modelling constrained by spectroscopic, photometric and seismic analysis. These stars show evidence of being older than the Sun but with a relatively large lithium abundance. We present the seismic analysis of HD 42618, and the modelling of the two solar analogues, HD 42618 and HD 43587 using the cestam stellar evolution code. Models were computed to reproduce the spectroscopic (effective temperature and metallicity) and seismic (mode frequency) data, and the luminosity of the stars, based on Gaia parallaxes. We infer very similar values of mass and radius for both stars compared with the literature, within the uncertainties, and we reproduce correctly the seismic constraints. The modelling shows that HD 42618 is slightly less massive and older than the Sun, and that HD 43587 is more massive and older than the Sun, in agreement with previous results. The use of chemical clocks improves the reliability of our age estimates.

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Pagani, L., P. Lesaffre, E. Roueff, M. Jorfi, P. Honvault, T. González-Lezana et A. Faure. « H 2 , H 3 + and the age of molecular clouds and prestellar cores ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 370, n^o 1978 (13 novembre 2012) : 5200–5212. http://dx.doi.org/10.1098/rsta.2012.0027.

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Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H 3 + into H 2 D + to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H 2 that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H 2 phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO + and N 2 D + argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H 2 remains comparable to para-H 2 in abundance outside the dense cores.

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Sabatini, G., S. Bovino, A. Giannetti, T. Grassi, J. Brand, E. Schisano, F. Wyrowski, S. Leurini et K. M. Menten. « Establishing the evolutionary timescales of the massive star formation process through chemistry ». Astronomy & ; Astrophysics 652 (août 2021) : A71. http://dx.doi.org/10.1051/0004-6361/202140469.

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Context. Understanding the details of the formation process of massive (i.e. M ≳ 8–10 M⊙) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging. Aims. We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process. Methods. We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH3CCH), acetonitrile (CH3CN), formaldehyde (H2CO), and methanol (CH3OH). Results. We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 103 to 105 AU during the warm-up phase. We succeed in reproducing the observed column densities of CH3CCH and CH3CN, but H2CO and CH3OH agree less with the observed values. The total (massive) star formation time is found to be ~5.2 × 105 yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: ~5 × 104 yr for 70-μm weak, ~1.2 × 105 yr for mid-IR weak, ~2.4 × 105 yr for mid-IR bright, and ~1.1 × 105 yr for HII-region phases. Conclusions. With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales.

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Bechara, J., A. Borbon, C. Jambert, A. Colomb et P. E. Perros. « Evidence of the impact of deep convection on reactive volatile organic compounds in the upper tropical troposphere during the AMMA experiment in West Africa ». Atmospheric Chemistry and Physics Discussions 9, n^o 5 (29 septembre 2009) : 20309–46. http://dx.doi.org/10.5194/acpd-9-20309-2009.

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Abstract. A large dataset of reactive trace gases was collected for the first time over West Africa during the African Monsoon Multidisciplinary Analysis (AMMA) field experiment in August 2006. Volatile Organic Compounds (VOC from C5–C9) were measured onboard the two French aircrafts the ATR-42 and the Falcon-20 by a new instrument AMOVOC. The goal of this study is (i) to characterize VOC distribution in the tropical region of West Africa (ii) to determine the impact of deep convection on VOC distribution and chemistry in the tropical upper troposphere (UT) and (iii) to characterize its spatial and temporal extensions. Experimental strategy consisted in sampling at altitudes between 0 and 12 km downwind of Mesoscale Convective Systems (MCS) and at cloud base. Biogenic and anthropogenic VOC distribution in West Africa is clearly affected by North to South emission gradient. Isoprene, the most abundant VOC, is at maximum level over the forest (1.26 ppb) while benzene reaches its maximum over the urban areas (0.11 ppb). First, a multiple physical and chemical tracers approach using CO, O3 and relative humidity was implemented to distinguish between convective and non-convective air masses. Then, additional tools based on VOC observations (tracer ratios, proxy of emissions and photochemical clocks) were adapted to characterize deep convection on a chemical, spatial and temporal basis. VOC vertical profiles show a "C-shaped" trend indicating that VOC-rich air masses are transported from the surface to the UT by deep convective systems. VOC mixing ratios in convective outflow are up to two times higher than background levels even for reactive and short-lived VOC (e.g. isoprene up to 0.19 ppb at 12 km-altitude) and are dependent on surface emission type. As a consequence, UT air mass reactivity increases from 0.52 s−1 in non-convective conditions to 0.95 s−1 in convective conditions. Fractions of boundary layer air contained in convective outflow are estimated to be 40±15%. Vertical transport timescale is calculated to be 25±10 min. These results characterize deep convection occurring over West Africa and provide relevant information for tropical convection parameterization in regional/global models.

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Bechara, J., A. Borbon, C. Jambert, A. Colomb et P. E. Perros. « Evidence of the impact of deep convection on reactive Volatile Organic Compounds in the upper tropical troposphere during the AMMA experiment in West Africa ». Atmospheric Chemistry and Physics 10, n^o 21 (4 novembre 2010) : 10321–34. http://dx.doi.org/10.5194/acp-10-10321-2010.

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Abstract. A large dataset of reactive trace gases was collected for the first time over West Africa during the African Monsoon Multidisciplinary Analysis (AMMA) field experiment in August 2006. Volatile Organic Compounds (VOC from C5–C9) were measured onboard the two French aircrafts the ATR-42 and the Falcon-20 by a new instrument AMOVOC (Airborne Measurement Of Volatile Organic Compounds). The goal of this study is (i) to characterize VOC distribution in the tropical region of West Africa (ii) to determine the impact of deep convection on VOC distribution and chemistry in the tropical upper troposphere (UT) and (iii) to characterize its spatial and temporal extensions. Experimental strategy consisted in sampling at altitudes between 0 and 12 km downwind of Mesoscale Convective Systems (MCS) and at cloud base. Biogenic and anthropogenic VOC distribution in West Africa is clearly affected by North to South emission gradient. Isoprene, the most abundant VOC, is at maximum level over the forest (1.26 ppb) while benzene reaches its maximum over the urban areas (0.11 ppb). First, a multiple physical and chemical tracers approach using CO, O3 and relative humidity was implemented to distinguish between convective and non-convective air masses. Then, additional tools based on VOC observations (tracer ratios, proxy of emissions and photochemical clocks) were adapted to characterize deep convection on a chemical, spatial and temporal basis. VOC vertical profiles show a "C-shaped" trend indicating that VOC-rich air masses are transported from the surface to the UT by deep convective systems. VOC mixing ratios in convective outflow are up to two times higher than background levels even for reactive and short-lived VOC (e.g. isoprene up to 0.19 ppb at 12 km-altitude) and are dependent on surface emission type. As a consequence, UT air mass reactivity increases from 0.52 s−1 in non-convective conditions to 0.95 s−1 in convective conditions. Fractions of boundary layer air contained in convective outflow are estimated to be 40 ± 15%. Vertical transport timescale is calculated to be 25 ± 10 min between 0 to 12 km altitude. These results characterize deep convection occurring over West Africa and provide relevant information for tropical convection parameterization in regional/global models.

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