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Pakmor, Rüdiger, Christine M. Simpson, Freeke van de Voort, et al. "Formation and fate of low-metallicity stars in TNG50." Monthly Notices of the Royal Astronomical Society 512, no. 3 (2022): 3602–15. http://dx.doi.org/10.1093/mnras/stac717.
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ABSTRACT Low-metallicity stars give rise to unique spectacular transients and are of immense interest for understanding stellar evolution. Their importance has only grown further with the recent detections of mergers of stellar mass black holes that likely originate mainly from low-metallicity progenitor systems. Moreover, the formation of low-metallicity stars is intricately linked to galaxy evolution, in particular to early enrichment and to later accretion and mixing of lower metallicity gas. Because low-metallicity stars are difficult to observe directly, cosmological simulations are crucial for understanding their formation. Here, we quantify the rates and locations of low-metallicity star formation using the high-resolution TNG50 magnetohydrodynamical cosmological simulation, and we examine where low-metallicity stars end up at z = 0. We find that $20{{\ \rm per\ cent}}$ of stars with $Z_*\lt 0.1\, \mathrm{Z_\odot }$ form after z = 2, and that such stars are still forming in galaxies of all masses at z = 0 today. Moreover, most low-metallicity stars at z = 0 reside in massive galaxies. We analyse the radial distribution of low-metallicity star formation and discuss the curious case of seven galaxies in TNG50 that form stars from primordial gas even at z = 0.
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Sabach, Efrat. "Jsolated Stars of Low Metallicity." Galaxies 6, no. 3 (2018): 89. http://dx.doi.org/10.3390/galaxies6030089.
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We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies for stars with low metallicities of Z = 0 . 001 and Z = 0 . 004 , and compare with the evolution with solar metallicity, Z = 0 . 02 . We further study the possibility for late asymptomatic giant branch (AGB)—planet interaction and its possible effects on the properties of the planetary nebula (PN). We find for all metallicities that only with a reduced mass-loss rate an interaction with a low mass companion might take place during the AGB phase of the star. The interaction will most likely shape an elliptical PN. The maximum post-AGB luminosities obtained, both for solar metallicity and low metallicities, reach high values corresponding to the enigmatic finding of the PN luminosity function.
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Bouret, J. C., T. Lanz, F. Martins, et al. "Massive stars at low metallicity." Astronomy & Astrophysics 555 (June 18, 2013): A1. http://dx.doi.org/10.1051/0004-6361/201220798.
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Hirschi, R. "Very low-metallicity massive stars:." Astronomy & Astrophysics 461, no. 2 (2006): 571–83. http://dx.doi.org/10.1051/0004-6361:20065356.
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Hirschi, Raphael, Cristina Chiappini, Georges Meynet, André Maeder, and Sylvia Ekström. "Stellar Evolution at Low Metallicity." Proceedings of the International Astronomical Union 3, S250 (2007): 217–30. http://dx.doi.org/10.1017/s1743921308020528.
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AbstractMassive stars played a key role in the early evolution of the Universe. They formed with the first halos and started the re-ionisation. It is therefore very important to understand their evolution. In this review, we first recall the effect of metallicity (Z) on the evolution of massive stars. We then describe the strong impact of rotation induced mixing and mass loss at very low Z. The strong mixing leads to a significant production of primary 14N, 13C and 22Ne. Mass loss during the red supergiant stage allows the production of Wolf-Rayet stars, type Ib,c supernovae and possibly gamma-ray bursts (GRBs) down to almost Z = 0 for stars more massive than 60 M⊙. Galactic chemical evolution models calculated with models of rotating stars better reproduce the early evolution of N/O, C/O and 12C/13C. Finally, the impact of magnetic fields is discussed in the context of GRBs.
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Garcia, Miriam, Artemio Herrero, Francisco Najarro, et al. "Low-metallicity (sub-SMC) massive stars." Proceedings of the International Astronomical Union 12, S329 (2016): 313–21. http://dx.doi.org/10.1017/s1743921317003088.
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AbstractThe double distance and metallicity frontier marked by the SMC has been finally broken with the aid of powerful multi-object spectrographs installed at 8-10m class telescopes. VLT, GTC and Keck have enabled studies of massive stars in dwarf irregular galaxies of the Local Group with poorer metal-content than the SMC. The community is working to test the predictions of evolutionary models in the low-metallicity regime, set the new standard for the metal-poor high-redshift Universe, and test the extrapolation of the physics of massive stars to environments of decreasing metallicity. In this paper, we review current knowledge on this topic.
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Karakas, Amanda I., Maria Lugaro, and Simon W. Campbell. "The slow-neutron capture process in low-metallicity asymptotic giant branch stars." Proceedings of the International Astronomical Union 5, S265 (2009): 57–60. http://dx.doi.org/10.1017/s1743921310000165.
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AbstractElements heavier than iron are produced in asymptotic giant branch (AGB) stars via the slow neutron capture process (s process). Recent observations of s-process-enriched Carbon Enhanced Metal-Poor (CEMP) stars have provided an unprecedented wealth of observational constraints on the operation of the s-process in low-metallicity AGB stars. We present new preliminary full network calculations of low-metallicity AGB stars, including a comparison to the composition of a few s-process rich CEMP stars. We also discuss the possibility of using halo planetary nebulae as further probes of low-metallicity AGB nucleosynthesis.
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Bomans, Dominik J., and Kerstin Weis. "Massive variable stars at very low metallicity?" Proceedings of the International Astronomical Union 6, S272 (2010): 265–70. http://dx.doi.org/10.1017/s1743921311010519.
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AbstractObservational contraints on the evolution and instabilities of massive stars at very low metallicities are limited. Most of the information come from HST observations of one target, I Zw 18. Recent distance estimates of I Zw 18 put it at 17 Mpc, moving detailed studies of single stars clearly beyond the range of current ground based telescopes. Since massive stars with metallcities of 1/10 of solar and below are our best proxies for massive stars in (proto-) galaxies around the time of reionization, finding them and studying their evolution and instabilities is of premium importance for our understanding of galaxy formation, feedback, and the IGM reionization. Here we present pilot study results of variable stars in two more nearby extremely low metallicity galaxies, UGC 5340 and UGCA 292, and comment on the possibilities of more detailed studies of variable massive stars with new ground-based instrumentation.
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Yang, Ming, Alceste Z. Bonanos, Biwei Jiang, et al. "Evolved massive stars at low-metallicity." Astronomy & Astrophysics 647 (March 2021): A167. http://dx.doi.org/10.1051/0004-6361/202039596.
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We present a case study in which we used a novel method to identify red supergiant (RSG) candidates in NGC 6822 based on their 1.6 μm H-bump. We collected 32 bands of photometric data for NGC 6822 ranging from the optical to the mid-infrared, derived from Gaia, PS1, LGGS, VHS, UKIRT, IRSF, HAWK-I, Spitzer, and WISE. Using the theoretical spectra from MARCS, we demonstrate that there is a prominent difference around 1.6 μm (H-bump) between targets with high and low surface gravity (HSG and LSG). Taking advantage of this feature, we identify efficient color–color diagrams of rzH (r − z vs. z − H) and rzK (r − z vs. z − K) to separate HSG (mostly foreground dwarfs) and LSG targets (mainly background red giant stars, asymptotic giant branch stars, and RSGs) from crossmatching of optical and near-infrared (NIR) data. Moreover, synthetic photometry from ATLAS9 gives similar results. We further separated RSG candidates from the remaining LSG candidates as determined by the H-bump method by using semi-empirical criteria on NIR color–magnitude diagrams, where both the theoretic cuts and morphology of the RSG population are considered. This separation produced 323 RSG candidates. The simulation of foreground stars with Besançon models also indicates that our selection criteria are largely free from the contamination of Galactic giants. In addition to the H-bump method, we used the traditional BVR method (B − V vs. V − R) as a comparison and/or supplement by applying a slightly aggressive cut to select as many RSG candidates as possible (358 targets). Furthermore, the Gaia astrometric solution was used to constrain the sample, where 181 and 193 targets were selected with the H-bump and BVR method, respectively. The percentages of selected targets in the two methods are similar at ∼60%, indicating a comparable accuracy of the two methods. In total, there are 234 RSG candidates after combining targets from the two methods, and 140 (∼60%) of them are in common. The final RSG candidates are in the expected locations on the mid-infrared color–magnitude diagram with [3.6]−[4.5] ≲ 0 and J − [8.0] ≈ 1.0. The spatial distribution is also coincident with the far-ultraviolet-selected star formation regions, suggesting that the selection is reasonable and reliable. We indicate that our method can also be used to identify other LSG targets, such as red giants and asymptotic giant branch stars, and it can also be applied to most of the nearby galaxies by using recent large-scale ground-based surveys. Future ground- and space-based facilities may promote its application beyond the Local Group.
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Bonifacio, P. "Low Metallicity Stars in our Galaxy." EAS Publications Series 24 (2007): 251–61. http://dx.doi.org/10.1051/eas:2007034.
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Yang, Ming, Alceste Z. Bonanos, Bi-Wei Jiang, et al. "Evolved massive stars at low-metallicity." Astronomy & Astrophysics 629 (September 2019): A91. http://dx.doi.org/10.1051/0004-6361/201935916.
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We present a clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog for the Small Magellanic Cloud (SMC) with 45 466 targets in total, with the purpose of building an anchor for future studies, especially for the massive star populations at low-metallicity. The catalog contains data in 50 different bands including 21 optical and 29 infrared bands, retrieved from SEIP, VMC, IRSF, AKARI, HERITAGE, Gaia, SkyMapper, NSC, Massey (2002, ApJS, 141, 81), and GALEX, ranging from the ultraviolet to the far-infrared. Additionally, radial velocities and spectral classifications were collected from the literature, and infrared and optical variability statistics were retrieved from WISE, SAGE-Var, VMC, IRSF, Gaia, NSC, and OGLE. The catalog was essentially built upon a 1″ crossmatching and a 3″ deblending between the Spitzer Enhanced Imaging Products (SEIP) source list and Gaia Data Release 2 (DR2) photometric data. Further constraints on the proper motions and parallaxes from Gaia DR2 allowed us to remove the foreground contamination. We estimate that about 99.5% of the targets in our catalog are most likely genuine members of the SMC. Using the evolutionary tracks and synthetic photometry from MESA Isochrones & Stellar Tracks and the theoretical J − KS color cuts, we identified 1405 red supergiant (RSG), 217 yellow supergiant, and 1369 blue supergiant candidates in the SMC in five different color-magnitude diagrams (CMDs), where attention should also be paid to the incompleteness of our sample. We ranked the candidates based on the intersection of different CMDs. A comparison between the models and observational data shows that the lower limit of initial mass for the RSG population may be as low as 7 or even 6 M⊙ and that the RSG is well separated from the asymptotic giant branch (AGB) population even at faint magnitude, making RSGs a unique population connecting the evolved massive and intermediate stars, since stars with initial mass around 6 to 8 M⊙ are thought to go through a second dredge-up to become AGB stars. We encourage the interested reader to further exploit the potential of our catalog.
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Yang, Ming, Alceste Z. Bonanos, Bi-Wei Jiang, et al. "Evolved massive stars at low metallicity." Astronomy & Astrophysics 639 (July 2020): A116. http://dx.doi.org/10.1051/0004-6361/201937168.
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We present the most comprehensive red supergiant (RSG) sample for the Small Magellanic Cloud (SMC) to date, including 1239 RSG candidates. The initial sample was derived based on a source catalog for the SMC with conservative ranking. Additional spectroscopic RSGs were retrieved from the literature, and RSG candidates were selected based on the inspection of Gaia and 2MASS color-magnitude diagrams (CMDs). We estimate that there are in total ∼1800 or more RSGs in the SMC. We purify the sample by studying the infrared CMDs and the variability of the objects, though there is still an ambiguity between asymptotic giant branch stars (AGBs) and RSGs at the red end of our sample. One heavily obscured target was identified based on multiple near-IR and mid-IR (MIR) CMDs. The investigation of color-color diagrams shows that there are fewer RSGs candidates (∼4%) showing PAH emission features compared to the Milky Way and LMC (∼15%). The MIR variability of RSG sample increases with luminosity. We separate the RSG sample into two subsamples (risky and safe), and identify one M5e AGB star in the risky subsample based on simultaneous inspection of variabilities, luminosities, and colors. The degeneracy of mass loss rate (MLR), variability, and luminosity of the RSG sample is discussed, indicating that most of the targets with high variability are also the bright ones with high MLR. Some targets show excessive dust emission, which may be related to previous episodic mass loss events. We also roughly estimate the total gas and dust budget produced by entire RSG population as ∼1.9−1.1+2.4 × 10−6 M⊙ yr−1 in the most conservative case, according to the derived MLR from IRAC1–IRAC4 color. Based on the MIST models, we derive a linear relation between Teff and observed J − KS color with reddening correction for the RSG sample. By using a constant bolometric correction and this relation, the Geneva evolutionary model is compared with our RSG sample, showing a good agreement and a lower initial mass limit of ∼7 M⊙ for the RSG population. Finally, we compare the RSG sample in the SMC and the LMC. Despite the incompleteness of LMC sample in the faint end, the result indicates that the LMC sample always shows redder color (except for the IRAC1–IRAC2 and WISE1–WISE2 colors due to CO absorption) and higher variability than the SMC sample, which is likely due to a positive relation between MLR, variability and the metallicity.
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Martayan, Christophe, Dietrich Baade, Juan Zorec, Yves Frémat, Juan Fabregat, and Sylvia Ekström. "Massive Oe/Be stars at low metallicity: candidate progenitors of long GRBs?" Proceedings of the International Astronomical Union 6, S272 (2010): 300–301. http://dx.doi.org/10.1017/s1743921311010660.
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AbstractAt low metallicity B-type stars rotate faster than at higher metallicity, typically in the SMC. As a consequence, a larger number of fast rotators is expected in the SMC than in the Galaxy, in particular more Be/Oe stars. With the ESO-WFI in its slitless mode, we examined the SMC open clusters and found an occurence of Be stars 3 to 5 times larger than in the Galaxy. The evolution of the angular rotational velocity seems to be the main key on the understanding of the specific behaviour and stellar evolution of such stars at different metallicities. With the results of this WFI study and using observational clues on the SMC WR stars and massive stars, as well as the theoretical indications of long gamma-ray burst progenitors, we identify the low metallicity massive Be and Oe stars as potential LGRB progenitors. Therefore the expected rates and numbers of LGRB are calculated and compared to the observed ones, leading to a good probability that low metallicity Be/Oe stars are actually LGRB progenitors.
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Heap, Sara R., and Thierry Lanz. "Effects of Rotation in Low-Metallicity Stars." Symposium - International Astronomical Union 215 (2004): 220–21. http://dx.doi.org/10.1017/s0074180900195580.
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We have carried out a spectral analysis of 17 O-type stars in the Small Magellanic Cloud (SMC). We find several lines of evidence that rotation plays an important role in massive stars in this low-metallicity galaxy (Z=0.2 Z⊙): (1) strong enrichment of nitrogen in half the program stars due to rotational mixing of processed material in the core up to the surface; (2) reduction of the effective gravity by the centrifugal force; (3) apparent anomalies in cluster isochrones that imply some stars are rotating at critical velocities.
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Iwamoto, N., T. Kajino, G. J. Mathews, and M. Y. Fujimoto. "Nucleosynthesis in low-mass, low-metallicity AGB stars." Nuclear Physics A 719 (May 2003): C57—C60. http://dx.doi.org/10.1016/s0375-9474(03)00958-8.
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Kubátová, B., D. Szécsi, A. A. C. Sander, et al. "Low-metallicity massive single stars with rotation." Astronomy & Astrophysics 623 (February 25, 2019): A8. http://dx.doi.org/10.1051/0004-6361/201834360.
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Context. Metal-poor massive stars are assumed to be progenitors of certain supernovae, gamma-ray bursts, and compact object mergers that might contribute to the early epochs of the Universe with their strong ionizing radiation. However, this assumption remains mainly theoretical because individual spectroscopic observations of such objects have rarely been carried out below the metallicity of the Small Magellanic Cloud. Aims. Here we explore the predictions of the state-of-the-art theories of stellar evolution combined with those of stellar atmospheres about a certain type of metal-poor (0.02 Z⊙) hot massive stars, the chemically homogeneously evolving stars that we call Transparent Wind Ultraviolet INtense (TWUIN) stars. Methods. We computed synthetic spectra corresponding to a broad range in masses (20−130 M⊙) and covering several evolutionary phases from the zero-age main-sequence up to the core helium-burning stage. We investigated the influence of mass loss and wind clumping on spectral appearance and classified the spectra according to the Morgan-Keenan (MK) system. Results. We find that TWUIN stars show almost no emission lines during most of their core hydrogen-burning lifetimes. Most metal lines are completely absent, including nitrogen. During their core helium-burning stage, lines switch to emission, and even some metal lines (oxygen and carbon, but still almost no nitrogen) are detected. Mass loss and clumping play a significant role in line formation in later evolutionary phases, particularly during core helium-burning. Most of our spectra are classified as an early-O type giant or supergiant, and we find Wolf–Rayet stars of type WO in the core helium-burning phase. Conclusions. An extremely hot, early-O type star observed in a low-metallicity galaxy could be the result of chemically homogeneous evolution and might therefore be the progenitor of a long-duration gamma-ray burst or a type Ic supernova. TWUIN stars may play an important role in reionizing the Universe because they are hot without showing prominent emission lines during most of their lifetime.
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Thielemann, Friedrich-K., Khalil Farouqi, Stephan Rosswog, and Karl-Ludwig Kratz. "r-Process Contributions to Low-Metallicity Stars." EPJ Web of Conferences 260 (2022): 09002. http://dx.doi.org/10.1051/epjconf/202226009002.
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Various nucleosynthesis studies have pointed out that the rapid neutron capture r-process elements in very metal-poor (VMP) halo stars might have different origins. It has been known that an r-process can either be obtained in neutron-rich low Ye conditions or in high entropy environments [see e.g. 1–5], an overview over many investigations has appeared recently [6]. In the present article we analyze with statistical methods the observational abundance patterns from trans-Fe elements up to the actinides and come to the conclusion that four to five categories of astrophysical events must have contributed. These include the ejection of Fe and trans-Fe elements Sr, Y, Zr (continuing possibly beyond to slightly higher mass numbers) in category 0 events (hereafter "C0"), Fe and weak r-process contributions (including Eu in moderate to slightly larger but varying amounts) in CI and CII events, strong r-process abundance patterns with no or negligible (in comparison to solar) Fe production in CIIIa and CIIIb events, where category CIIIb shows a tendency for an actinide boost behavior. When comparing these categories with presently existing nucleosynthesis predictions, we suggest to identify them (despite remaining uncertainties) with regular core-collapse supernovae, quark deconfinement supernovae, magneto-rotational supernovae, neutron star mergers, and outflows from black hole accretion tori.
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Szécsi, Dorottya, Norbert Langer, Sung-Chul Yoon, et al. "Low-metallicity massive single stars with rotation." Astronomy & Astrophysics 581 (August 25, 2015): A15. http://dx.doi.org/10.1051/0004-6361/201526617.
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Meynet, Georges, Nolan R. Walborn, Ian Hunter, et al. "Evolution of Massive Stars at Low Metallicity." Proceedings of the International Astronomical Union 3, S250 (2007): 571–76. http://dx.doi.org/10.1017/s1743921308020954.
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AbstractThis paper reports the contributions made on the occasion of the Special Session entitled “Evolution of Massive Stars at Low Metallicity” which was held on Sunday, December 9, 2007 in Kauai (USA).
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Tolstoy, Eline, Giuseppina Battaglia, and Andrew Cole. "Stars at Low Metallicity in Dwarf Galaxies." Proceedings of the International Astronomical Union 4, S255 (2008): 310–17. http://dx.doi.org/10.1017/s174392130802499x.
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AbstractDwarf galaxies offer an opportunity to understand the properties of low metallicity star formation both today and at the earliest times at the epoch of the formation of the first stars. Here we concentrate on two galaxies in the Local Group: the dwarf irregular galaxy Leo A, which has been the recent target of deep HST/ACS imaging (Cole et al. 2007) and the Sculptor dwarf spheroidal, which has been the target of significant wide field spectroscopy with VLT/FLAMES (Battaglia 2007).
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Iwamoto, Nobuyuki, Toshitaka Kajino, Grant J. Mathews, Masayuki Y. Fujimoto, and Wako Aoki. "s-Process Nucleosynthesis in Low-Metallicity Stars." Journal of Nuclear Science and Technology 39, sup2 (2002): 554–57. http://dx.doi.org/10.1080/00223131.2002.10875161.
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D'Orazi, V., S. Randich, E. Flaccomio, F. Palla, G. G. Sacco, and R. Pallavicini. "Metallicity of low-mass stars in Orion." Astronomy & Astrophysics 501, no. 3 (2009): 973–83. http://dx.doi.org/10.1051/0004-6361/200811241.
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Pols, O. R., R. G. Izzard, M. Lugaro, and S. E. de Mink. "Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars." Proceedings of the International Astronomical Union 4, S252 (2008): 383–89. http://dx.doi.org/10.1017/s1743921308023260.
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AbstractWe present the results of binary population simulations of carbon-enhanced metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers of the origin of CEMP stars, and conclude that the observed paucity of very nitrogen-rich stars puts strong constraints on possible modifications of the initial mass function at low metallicity. The large number fraction of CEMP stars may instead require much more efficient dredge-up from low-metallicity asymptotic giant branch stars.
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Heap, Sara. "Role of massive stars in the evolution of primitive galaxies." Proceedings of the International Astronomical Union 10, H16 (2012): 370. http://dx.doi.org/10.1017/s1743921314011429.
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AbstractAn important factor controlling galaxy evolution is feedback from massive stars. It is believed that the nature and intensity of stellar feedback changes as a function of galaxy mass and metallicity. At low mass and metallicity, feedback from massive stars is mainly in the form of photoionizing radiation. At higher mass and metallicity, it is in stellar winds. I Zw 18 is a local blue, compact dwarf galaxy that meets the requirements for a primitive galaxy: low halo mass <109M⊙, strong photoionizing radiation, no galactic outflow, and very low metallicity, log(O/H)+12=7.2. We will describe the properties of massive stars and their role in the evolution of I Zw 18, based on analysis of ultraviolet images and spectra obtained with HST.
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Sharina, M. E., L. N. Makarova, and D. I. Makarov. "Population gradients in dwarf spheroidal galaxies KKs 3 and ESO 269-66." Proceedings of the International Astronomical Union 14, S344 (2018): 420–21. http://dx.doi.org/10.1017/s1743921318005550.
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AbstractWe compare the properties of stellar populations for globular clusters (GCs) and field stars in two dwarf spheroidal galaxies (dSphs): ESO269-66, a close neighbour of NGC5128, and KKs3, one of the few isolated dSphs within 10 Mpc. We analyse the surface density profiles of low and high metallicity (blue and red) stars in two galaxies using the Sersic law. We argue that 1) the density profiles of red stars are steeper than those of blue stars, which evidences in favour of the metallicity and age gradients in dSphs; 2) globular clusters in KKs3 and ESO 269-66 contain 4 and 40 percent of all stars with [Fe / H] ~ 1.6 dex and the age of 12 Gyr, correspondingly. Therefore, GCs are relics of the first powerful star-forming bursts in the central regions of the galaxies. KKs 3 has lost a smaller percentage of old low-metallicity stars than ESO269-66, probably, thanks to its isolation.
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Matsuura, M., G. C. Sloan, J. Bernard-Salas, et al. "Carbon-rich AGB stars in our Galaxy and nearby galaxies as possible sources of PAHs." Proceedings of the International Astronomical Union 4, S251 (2008): 197–200. http://dx.doi.org/10.1017/s1743921308021558.
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AbstractWe have obtained infrared spectra of carbon-rich AGB stars in three nearby galaxies – the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate gas compositions and mass-loss rate of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. C2H2are detectable from AGB stars, and possibly PAHs are subsequently formed from C2H2. Thus, it is worth investigating chemical processes at low metallicity. These stars were observed using the Infrared Spectrometer (irs) onboard theSpitzer Space Telescopewhich covers 5–35 μm region, and the Infrared Spectrometer And Array Camera (isaac) on the Very Large Telescope which covers the 2.9–4.1 μm region. HCN, CH and C2H2molecular bands, as well as SiC and MgS dust features are identified in the spectra. The equivalent width of C2H2molecular bands is larger at lower metallicity, thus PAHs might be abundant in AGB stars at low metallicity. We find no evidence that mass-loss rates depend on metallicity. Chemistry of carbon stars is affected by carbon production during the AGB phase rather than the metallicities. We argue that lower detection rate of PAHs from the interstellar medium of lower metal galaxies is caused by destruction of PAHs in the ISM by stronger UV radiation field.
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Aoki, Wako, Timothy C. Beers, Takuma Suda, Satoshi Honda, and Young Sun Lee. "Very Low-Mass Stars with Extremely Low Metallicity in the Milky Way's Halo." Proceedings of the International Astronomical Union 11, S317 (2015): 45–50. http://dx.doi.org/10.1017/s174392131500959x.
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AbstractLarge surveys and follow-up spectroscopic studies in the past few decades have been providing chemical abundance data for a growing number of very metal-poor ([Fe/H] <−2) stars. Most of them are red giants or main-sequence turn-off stars having masses near 0.8 solar masses. Lower mass stars with extremely low metallicity ([Fe/H] <−3) are yet to be explored. Our high-resolution spectroscopic study for very metal-poor stars found with SDSS has identified four cool main-sequence stars with [Fe/H] <−2.5 among 137 objects (Aoki et al. 2013). The effective temperatures of these stars are 4500–5000 K, corresponding to a mass of around 0.5 solar masses. Our standard analysis of the high-resolution spectra based on 1D-LTE model atmospheres has obtained self-consistent chemical abundances for these objects, assuming small values of micro-turbulent velocities compared with giants and turn-off stars. The low temperature of the atmospheres of these objects enables us to measure their detailed chemical abundances. Interestingly, two of the four stars have extreme chemical-abundance patterns: one has the largest excesses of heavy neutron-capture elements associated with the r-process abundance pattern known to date (Aoki et al. 2010), and the other exhibits low abundances of the α-elements and odd-Z elements, suggested to be signatures of the yields of very massive stars (> 100 solar masses; Aoki et al. 2014). Although the sample size is still small, these results indicate the potential of very low-mass stars as probes to study the early stages of the Milky Way's halo formation.
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Sahlholdt, Christian L., Sofia Feltzing, and Diane K. Feuillet. "Characterizing epochs of star formation across the Milky Way disc using age–metallicity distributions of GALAH stars." Monthly Notices of the Royal Astronomical Society 510, no. 4 (2021): 4669–88. http://dx.doi.org/10.1093/mnras/stab3681.
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ABSTRACT We provide a detailed map of the ages and metallicities of turn-off stars in the Milky Way disc based on data from GALAH DR3 and Gaia EDR3. From this map, we identify previously undetected features in the age–metallicity distribution of disc stars and interpret these results as indicating a three-phase formation history of the Milky Way. In the first phase, inner disc stars form along a single age–metallicity sequence and are today kinematically hot. The end of this phase is marked by a local minimum in the inner disc age distribution 10 Gyr ago. At this time, we find the stellar populations to transition from high to low alpha-element abundances and from high to low vertical velocity dispersion. In the second phase, stars form across the disc with outwardly decreasing metallicity. In this phase, inner disc stars form at supersolar metallicities in a continuation of the early age–metallicity relation, while outer disc stars begin forming at metallicities at least 0.5 dex lower. Finally, the third phase is associated with a recent burst of star formation across the local disc marked by a local minimum in the age–metallicity distribution 4–6 Gyr ago. Future quantitative comparisons between the observed age–metallicity distribution and those of simulated galaxies could help constrain the processes driving each of the star formation phases.
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Tanaka, Kei E. I., Jonathan C. Tan, Yichen Zhang, and Takashi Hosokawa. "Multiple Feedback in Low-Metallicity Massive Star Formation." Proceedings of the International Astronomical Union 14, S344 (2018): 190–94. http://dx.doi.org/10.1017/s1743921318005549.
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AbstractWe theoretically investigate the impact of feedback and its metallicity dependence in massive star formation from prestellar cores at all metallicity range. We include the feedback by MHD disk winds, radiation pressure, and photoevaporation solving the evolution of protostars and accretion flows self-consistently. Interestingly, we find that the feedback does not set the upper mass limit of stellar birth mass at any metallicity. At the solar metallicity, the MHD disk wind is the dominant feedback to set the star formation efficiencies (SFEs) from the prestellar cores similar to low-mass star formation. The SFE is found to be lower at lower surface density environment. The photoevaporation becomes significant at the low metallicity of Z < 10−2 Z⊙. Considering this efficient photoevaporation, we conclude that the IMF slope is steeper, i.e., massive stars are rarer at the extremely metal-poor environment of 10−5 − 10−3Z⊙. Our study raises a question on the common assumption of the universal IMF with a truncated at 100M⊙. Since the total feedback strength in the cluster/galaxy scale is sensitive to the number fraction of massive stars, the re-evaluations of IMF at various environments are necessary.
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Whitten, D. D., V. M. Placco, T. C. Beers, et al. "J-PLUS: Identification of low-metallicity stars with artificial neural networks using SPHINX." Astronomy & Astrophysics 622 (February 2019): A182. http://dx.doi.org/10.1051/0004-6361/201833368.
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Context. We present a new methodology for the estimation of stellar atmospheric parameters from narrow- and intermediate-band photometry of the Javalambre Photometric Local Universe Survey (J-PLUS), and propose a method for target pre-selection of low-metallicity stars for follow-up spectroscopic studies. Photometric metallicity estimates for stars in the globular cluster M15 are determined using this method. Aims. By development of a neural-network-based photometry pipeline, we aim to produce estimates of effective temperature, Teff, and metallicity, [Fe/H], for a large subset of stars in the J-PLUS footprint. Methods. The Stellar Photometric Index Network Explorer, SPHINX, was developed to produce estimates of Teff and [Fe/H], after training on a combination of J-PLUS photometric inputs and synthetic magnitudes computed for medium-resolution (R ~ 2000) spectra of the Sloan Digital Sky Survey. This methodology was applied to J-PLUS photometry of the globular cluster M15. Results. Effective temperature estimates made with J-PLUS Early Data Release photometry exhibit low scatter, σ(Teff) = 91 K, over the temperature range 4500 < Teff (K) < 8500. For stars from the J-PLUS First Data Release with 4500 < Teff (K) < 6200, 85 ± 3% of stars known to have [Fe/H] < −2.0 are recovered by SPHINX. A mean metallicity of [Fe/H] = − 2.32 ± 0.01, with a residual spread of 0.3 dex, is determined for M15 using J-PLUS photometry of 664 likely cluster members. Conclusions. We confirm the performance of SPHINX within the ranges specified, and verify its utility as a stand-alone tool for photometric estimation of effective temperature and metallicity, and for pre-selection of metal-poor spectroscopic targets.
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Alonso-Santiago, J., A. Marco, I. Negueruela, et al. "NGC 3105: a young open cluster with low metallicity." Astronomy & Astrophysics 616 (August 2018): A124. http://dx.doi.org/10.1051/0004-6361/201833073.
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Context. NGC 3105 is a young open cluster hosting blue, yellow, and red supergiants. This rare combination makes it an excellent laboratory for constraining evolutionary models of high-mass stars. It has been poorly studied, and the fundamental parameters such as its age or distance are not well defined. Aims. We intend to characterise in an accurate way the cluster and its evolved stars, for which we derive for the first time atmospheric parameters and chemical abundances. Methods. We performed a complete analysis combining UBVR photometry with spectroscopy. We obtained spectra with classification purposes for 14 blue stars and high-resolution spectroscopy for an in-depth analysis of the six other evolved stars. Results. We identify 126 B-type likely members within a radius of 2.7 ± 0.6 arcmin, which implies an initial mass, Mcl ≈ 4100 M⊙. We find a distance of 7.2 ± 0.7 kpc for NGC 3105, placing it at RGC = 10.0 ± 1.2 kpc. Isochrone fitting supports an age of 28 ± 6 Ma, implying masses around 9.5 M⊙ for the supergiants. A high fraction of Be stars (≈25%) is found at the top of the main sequence down to spectral type b3. From the spectral analysis we estimate for the cluster an average νrad = +46.9 ± 0.9 km s−1 and a low metallicity, [Fe/H] = −0.29 ± 0.22. We also have determined, for the first time, chemical abundances for Li, O, Na, Mg, Si, Ca, Ti, Ni, Rb, Y, and Ba for the evolved stars. The chemical composition of the cluster is consistent with that of the Galactic thin disc. An overabundance of Ba is found, supporting the enhanced s-process. Conclusions. NGC 3105 has a low metallicity for its Galactocentric distance, comparable to typical LMC stars. It is a valuable spiral tracer in a very distant region of the Carina–Sagittarius spiral arm, a poorly known part of the Galaxy. As one of the few Galactic clusters containing blue, yellow, and red supergiants, it is massive enough to serve as a test bed for theoretical evolutionary models close to the boundary between intermediate- and high-mass stars.
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Hallakoun, Na’ama, and Dan Maoz. "A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way." Monthly Notices of the Royal Astronomical Society 507, no. 1 (2021): 398–413. http://dx.doi.org/10.1093/mnras/stab2145.
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ABSTRACT We use Gaia DR2 to measure the initial mass function (IMF) of stars within 250 pc and masses in the range of 0.2 &lt; m/M⊙ &lt; 1.0, separated according to kinematics and metallicity, as determined from Gaia transverse velocity, vT, and location on the Hertzsprung–Russell diagram (HRD). The predominant thin-disc population (vT &lt; 40 km s−1) has an IMF similar to traditional (e.g. Kroupa) stellar IMFs, with star numbers per mass interval dN/dm described by a broken power law, m−α, and index $\alpha _\textrm {high}=2.03^{+0.14}_{-0.05}$ above m ∼ 0.5, shallowing to $\alpha _\textrm {low}=1.34^{+0.11}_{-0.22}$ at m ≲ 0.5. Thick-disc stars (60 km s−1 &lt;vT &lt; 150 km s−1) and stars belonging to the ‘high-metallicity’ or ‘red-sequence’ halo (vT &gt; 100 km s−1 or vT &gt; 200 km s−1, and located above the isochrone on the HRD with a metallicity [M/H] &gt; −0.6) have a somewhat steeper high-mass slope, $\alpha _\textrm {high}=2.35^{+0.97}_{-0.19}$ (and a similar low-mass slope $\alpha _\textrm {low}=1.14^{+0.42}_{-0.50}$). Halo stars from the ‘blue sequence’, which are characterized by low metallicity ([M/H] &lt; −0.6), however, have a distinct, bottom-heavy IMF, well described by a single power law with $\alpha =1.82^{+0.17}_{-0.14}$ over most of the mass range probed. The IMF of the low-metallicity halo is reminiscent of the Salpeter-like IMF that has been measured in massive early-type galaxies, a stellar population that, like Milky Way halo stars, has a high ratio of α elements to iron, [α/Fe]. Blue-sequence stars are likely the debris from accretion by the Milky Way, ∼10 Gyr ago, of the Gaia-Enceladus dwarf galaxy, or similar events. These results hint at a distinct mode of star formation common to two ancient stellar populations – elliptical galaxies and galaxies possibly accreted early-on by ours.
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Lattanzio, John C. "Carbon dredge-up in low-mass stars and solar metallicity stars." Astrophysical Journal 344 (September 1989): L25. http://dx.doi.org/10.1086/185522.
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Maldonado, J., E. Villaver, C. Eiroa, and G. Micela. "Connecting substellar and stellar formation: the role of the host star’s metallicity." Astronomy & Astrophysics 624 (April 2019): A94. http://dx.doi.org/10.1051/0004-6361/201833827.
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Context. Most of our current understanding of the planet formation mechanism is based on the planet metallicity correlation derived mostly from solar-type stars harbouring gas-giant planets. Aims. To achieve a more extensive grasp on the substellar formation process, we aim to analyse in terms of their metallicity a diverse sample of stars (in terms of mass and spectral type) covering the whole range of possible outcomes of the planet formation process (from planetesimals to brown dwarfs and low-mass binaries). Methods. Our methodology is based on the use of high-precision stellar parameters derived by our own group in previous works from high-resolution spectra by using the iron ionisation and equilibrium conditions. All values were derived in an homogeneous way, except for the M dwarfs where a methodology based on the use of pseudo equivalent widths of spectral features was used. Results. Our results show that as the mass of the substellar companion increases the metallicity of the host star tends to lower values. The same trend is maintained when analysing stars with low-mass stellar companions and a tendency towards a wide range of host star’s metallicity is found for systems with low-mass planets. We also confirm that more massive planets tend to orbit around more massive stars. Conclusions. The core-accretion formation mechanism for planet formation achieves its maximum efficiency for planets with masses in the range 0.2–2 MJup. Substellar objects with higher masses have higher probabilities of being formed as stars. Low-mass planets and planetesimals might be formed by core-accretion even around low-metallicity stars.
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Wang, Shu, Xiaodian Chen, Jianxing Zhang, and Licai Deng. "Double-mode RR Lyrae star — robust distance and metallicity indicators." Proceedings of the International Astronomical Union 18, S376 (2022): 275–80. http://dx.doi.org/10.1017/s1743921323003186.
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AbstractRR Lyrae (RR Lyr) stars are a well-known and useful distance indicator for old stellar populations such as globular clusters and dwarf galaxies. Fundamental-mode RR Lyr (RRab) stars are commonly used to measure distances, and the accuracy of the determined distance is strongly constrained by metallicity. Here, we investigate the metallicity dependence in the period–luminosity (PL) relation of double-mode RR Lyr (RRd) stars. We find and establish a linear relation between metallicity and period or period ratio for RRd stars. This relation can predict the metallicity as accurately as the low-resolution spectra. Based on this relation, we establish a metallicity-independent PL relation for RRd stars. Combining the distance of the Large Magellanic Cloud and Gaia parallaxes, we calibrate the zero point of the derived PL relation to an error of 0.022 mag. Using RRd stars, we measure the distances of globular clusters and dwarf galaxies with an accuracy of 2-3% and 1-2%, respectively. In the future, RRd stars could anchor galaxy distances to an accuracy of 1.0% and become an independent distance ladder in the Local Group.
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Szécsi, Dorottya, and Norbert Langer. "The Life and Death of Massive Stars in the Starburst Galaxy I Zw 18." Proceedings of the International Astronomical Union 11, A29B (2015): 215–16. http://dx.doi.org/10.1017/s1743921316004968.
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AbstractMassive stars at low metallicity are strong candidates for two of the most energetic explosions in the Universe: long duration gamma-ray bursts and superluminous supernovae. But what is the reason these explosions prefer low metallicity environments? To answer this question, we investigate how massive stellar evolution proceeds in low metallicity environments.
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Placco, Vinicius M., Timothy C. Beers, Rafael M. Santucci, et al. "Spectroscopic Validation of Low-metallicity Stars from RAVE." Astronomical Journal 155, no. 6 (2018): 256. http://dx.doi.org/10.3847/1538-3881/aac20c.
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Engelbrecht, Chris, Refilwe Kgoadi, and Fabio Frescura. "A search for low-metallicity pulsating B stars." EPJ Web of Conferences 152 (2017): 01019. http://dx.doi.org/10.1051/epjconf/201715201019.
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Ventura, P., F. D'Antona, and I. Mazzitelli. "Yields from low metallicity, intermediate mass AGB stars:." Astronomy & Astrophysics 393, no. 1 (2002): 215–23. http://dx.doi.org/10.1051/0004-6361:20021001.
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Robert, Carmelle. "Ultraviolet spectral libraries of massive stars at low metallicity." Symposium - International Astronomical Union 193 (1999): 616–17. http://dx.doi.org/10.1017/s0074180900206463.
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Three new ultraviolet spectral libraries of massive, hot stars using high and medium resolution spectra of objects located in the solar neighbourhood and the Magellanic Clouds are presented. Massive stars display unique wind signatures which are relatively easy to study in the ultraviolet. These libraries are crucial tools when investigating the massive stellar population of distant star-forming galaxies.
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Plante, S., M. Sauvage, and D. Kunth. "Mid-Ir Emission of Low-Metallicity Galaxies." Highlights of Astronomy 11, no. 1 (1998): 113–14. http://dx.doi.org/10.1017/s153929960002013x.
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NGC 595 is a giant Hɪɪ region located in the western part of the spiral galaxy M 33. It is the second in importance in this galaxy, after NGC 604. At 0.84 Mpc, HST is able to resolve its stellar content. Malumuth et al. (1996) obtained HST UV, U, B and V images of this region and derived an ionizing luminosity of 5 × 1050 phots-1 and an average reddening EB-V = 0.36±0.28 mag. The stars are mostly concentrated in the central part of the region, where little emission of gas is seen (the ionized gas lies more in a shell around the stars, figure 1a). Wilson & Scoville (1993) showed the molecular gas to be situated in the south-east part of the region, just outside of the bright knot of stars. Viallefond et al. (1986) found a reddening gradient in the north-east/south-west direction by observing the Hi gas, which was confirmed by Malumuth et al. (1996) with stellar photometry.We obtained ISO images for NGC 595 in the 5.0 to 8.5 μm range. The emission in this spectral range is dominated by the so-called PAH bands. Current interpretation of these has them originating from stochastically heated molecules. Two of these bands are located in the range observed, at 6.2 μm and 7.7 μm. Stochastic heating implies that the in-band flux is directly proportional to the number of photons absorbed by the molecules. For typical HII regions, Cohen et al. (1989) found 0.58 for the I6.2/I7.7 in-band ratio. However many processes, ionization, dehydrogenation, can modify this ratio. Furthermore, an underlying continuum is present though its exact origin is unknown.
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Oey, M. S., M. C. Jecmen, A. N. Sawant, et al. "Massive-Star Feedback at Low Metallicity." Proceedings of the International Astronomical Union 18, S377 (2022): 14–21. http://dx.doi.org/10.1017/s1743921323001072.
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AbstractEarly cosmic epochs are characterized by low metallicity and high specific star-formation rates. These conditions are dominated by massive-star feedback that may be dramatically different than the traditional model dominated by hot, thermal superwinds driven by supernova explosions. Instead, metal-poor feedback from massive stars may be radiation-dominated, with weak mechanical feedback, possibly aiding the escape of Lyα and Lyman continuum radiation. I will describe our understanding that is emerging from observations of starburst galaxies in the local universe.
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Stancliffe, Richard J., George C. Angelou, and John C. Lattanzio. "Lithium production by thermohaline mixing in low-mass, low-metallicity asymptotic giant branch stars." Proceedings of the International Astronomical Union 5, S268 (2009): 405–10. http://dx.doi.org/10.1017/s1743921310004539.
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AbstractWe examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have evolved models of 1, 1.5 and 2M⊙ and of metallicity Z = 10−4 from the pre-main sequence to the end of the thermal pulsing asymptotic giant branch with thermohaline mixing applied throughout the simulations. We find that the small amount of 3He that remains after the first giant branch is enough to drive thermohaline mixing on the AGB and that the mixing is most efficient in the early thermal pulses, with the efficiency dropping from pulse to pulse. We note a surprising increase in the 7Li abundance, with log10ϵ(7Li) reaching values of over 2.5 in the 1.5M⊙ model. It is thus possible to get stars which are both C- and Li-rich at the same time. We compare our models to measurements of carbon and lithium in carbon-enhanced metal-poor stars which have not yet reached the giant branch. These models can simultaneously reproduced the observed C and Li abundances of carbon-enhanced metal-poor turn-off stars that are Li-rich.
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Bouret, Jean-Claude, Thierry M. Lanz, Sara R. Heap, et al. "Quantitative Analysis of O-Type Stars Properties, at Low Metallicity." Symposium - International Astronomical Union 212 (2003): 156–57. http://dx.doi.org/10.1017/s0074180900211790.
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We have investigated the properties of main-sequence O-type stars in the SMC. Mass-loss rates, luminosities and Teff are much smaller for these stars than for Galactic ones, resulting in a steeper wind-momentum relation.
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Lanfranchi, Gustavo A., and Francesca Matteucci. "Stellar metallicity distributions in local dwarf spheroidal galaxies: a comparison between model and observations." Proceedings of the International Astronomical Union 5, S262 (2009): 370–71. http://dx.doi.org/10.1017/s1743921310003273.
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AbstractWe analyze the main populations of 6 local Dwarf Spheroidal Galaxies by comparing the observed stellar metallicity distributions with the predictions of chemical evolution models. The predicted metallicity distributions of stars generally exhibit a low range of metallicities, a peak below the one of the MW disc in the solar neighborhood (but similar to the Halo), and a sharp decrease at higher metallicities in agreement with observations. The position of the peak is related to the low star formation rates adopted whereas the sharp decrease is a consequence of the occurrence of strong galactic winds. In the low metallicity tail of the distributions we predict a low number of stars in agreement with observations.
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Witzke, V., T. Reinhold, A. I. Shapiro, N. A. Krivova, and S. K. Solanki. "Effect of metallicity on the detectability of rotational periods in solar-like stars." Astronomy & Astrophysics 634 (February 2020): L9. http://dx.doi.org/10.1051/0004-6361/201936608.
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The accurate determination of stellar rotation periods is important for estimating stellar ages and for understanding stellar activity and evolution. While rotation periods can be determined for about thirty thousand stars in the Kepler field, there are over one hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars that are identical to our Sun with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature and photometric variability, and with metallicity between M/H = −0.35 and M/H = 0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we find a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87 solar-like Kepler stars for the first time.
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Beers, Timothy C., Deokkeun An, Jennifer A. Johnson, et al. "Metallicity Mapping with gri Photometry: The Virgo Overdensity and the Halos of the Galaxy." Proceedings of the International Astronomical Union 5, S262 (2009): 127–30. http://dx.doi.org/10.1017/s1743921310002644.
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AbstractWe describe the methodology required for estimation of photometric estimates of metallicity based on the SDSS gri passbands, which can be used to probe the properties of main-sequence stars beyond ~10 kpc, complementing studies of nearby stars from more metallicity-sensitive color indices that involve the u passband. As a first application of this approach, we determine photometric metal abundance estimates for individual main-sequence stars in the Virgo Overdensity, which covers almost 1000 deg2 on the sky, based on a calibration of the metallicity sensitivity of stellar isochrones in the gri filter passbands using field stars with well-determined spectroscopic metal abundances. Despite the low precision of the method for individual stars, internal errors of σ[Fe/H]~0.1 dex can be achieved for bulk stellar populations. The global metal abundance of the Virgo Overdensity determined in this way is 〈[Fe/H]〉 = −2.0±0.1 (internal) ±0.5 (systematic), from photometric measurements of 0.7 million stars with heliocentric distances from ~10 kpc to ~20 kpc. A preliminary metallicity map, based on results for 2.9 million stars in the northern SDSS DR-7 footprint, exhibits a shift to lower metallicities as one proceeds from the inner- to the outer-halo population, consistent with recent interpretation of the kinematics of local samples of stars with spectroscopically available metallicity estimates and full space motions.
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Bonifacio, Piercarlo. "The first galactic stars and chemical enrichment in the halo." Proceedings of the International Astronomical Union 5, S265 (2009): 81–89. http://dx.doi.org/10.1017/s1743921310000268.
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AbstractThe cosmic microwave background and the cosmic expansion can be interpreted as evidence that the Universe underwent an extremely hot and dense phase about 14 Gyr ago. The nucleosynthesis computations tell us that the Universe emerged from this state with a very simple chemical composition: H, 2H, 3He, 4He, and traces of 7Li. All other nuclei where synthesised at later times. Our stellar evolution models tell us that, if a low-mass star with this composition had been created (a “zero-metal” star) at that time, it would still be shining on the Main Sequence today. Over the last 40 years there have been many efforts to detect such primordial stars but none has so-far been found. The lowest metallicity stars known have a metal content, Z, which is of the order of 10−4Z⊙. These are also the lowest metallicity objects known in the Universe. This seems to support the theories of star formation which predict that only high mass stars could form with a primordial composition and require a minimum metallicity to allow the formation of low-mass stars. Yet, since absence of evidence is not evidence of absence, we cannot exclude the existence of such low-mass zero-metal stars, at present. If we have not found the first Galactic stars, as a by product of our searches we have found their direct descendants, stars of extremely low metallicity (Z ≤ 10−3Z⊙). The chemical composition of such stars contains indirect information on the nature of the stars responsible for the nucleosynthesis of the metals. Such a fossil record allows us a glimpse of the Galaxy at a look-back time equivalent to redshift z = 10, or larger. The last ten years have been full of exciting discoveries in this field, which I will try to review in this contribution.
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Ramachandran, V., W. R. Hamann, L. M. Oskinova, et al. "Testing massive star evolution, star formation history, and feedback at low metallicity." Astronomy & Astrophysics 625 (May 2019): A104. http://dx.doi.org/10.1051/0004-6361/201935365.
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Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung–Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below ∼30 M⊙ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than ∼30 M⊙ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales.
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Telford, O. Grace, John Chisholm, Kristen B. W. McQuinn, and Danielle A. Berg. "Far-ultraviolet Spectra of Main-sequence O Stars at Extremely Low Metallicity." Astrophysical Journal 922, no. 2 (2021): 191. http://dx.doi.org/10.3847/1538-4357/ac1ce2.
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Abstract Metal-poor massive stars dominate the light we observe from star-forming dwarf galaxies and may have produced the bulk of energetic photons that reionized the universe at high redshift. Yet, the rarity of observations of individual O stars below the 20% solar metallicity (Z ⊙) of the Small Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of metal-poor stellar populations. We present new Hubble Space Telescope far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P (3% Z ⊙), Sextans A (6% Z ⊙), and WLM (14% Z ⊙). We quantify equivalent widths of photospheric metal lines and strengths of wind-sensitive features, confirming that both correlate with metallicity. We infer the stars’ fundamental properties by modeling their FUV through near-infrared spectral energy distributions and identify stars in the SMC with similar properties to each of our targets. Comparing to the FUV spectra of the SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and (2) the most metal-poor star in Leo P is driving a much weaker stellar wind than its SMC counterparts. We measure projected rotation speeds and find that the two most metal-poor stars have high v sin ( i ) ≥ 290 km s−1, and estimate just a 3%–6% probability of finding two fast rotators if the metal-poor stars are drawn from the same v sin ( i ) distribution observed for O dwarfs in the SMC. These observations suggest that models should include the impact of rotation and weak winds on ionizing flux to accurately interpret observations of metal-poor galaxies in both the near and distant universe.