Journal articles on the topic 'Molecular clouds'
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1
Sullivan, Colin H., L. M. Fissel, P. K. King, C.-Y. Chen, Z.-Y. Li, and J. D. Soler. "Characterizing the magnetic fields of nearby molecular clouds using submillimeter polarization observations." Monthly Notices of the Royal Astronomical Society 503, no. 4 (March 16, 2021): 5006–24. http://dx.doi.org/10.1093/mnras/stab596.
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ABSTRACT Of all the factors that influence star formation, magnetic fields are perhaps the least well understood. The goal of this paper is to characterize the 3D magnetic field properties of nearby molecular clouds through various methods of statistically analysing maps of polarized dust emission. Our study focuses on nine clouds, with data taken from the Planck Sky Survey as well as data from the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry observations of Vela C. We compare the distributions of polarization fraction (p), dispersion in polarization angles ($\mathcal {S}$), and hydrogen column density (NH) for each of our targeted clouds. To broaden the scope of our analysis, we compare the distributions of our clouds’ polarization observables with measurements from synthetic polarization maps generated from numerical simulations. We also use the distribution of polarization fraction measurements to estimate the inclination angle of each cloud’s cloud-scale magnetic field. We obtain a range of inclination angles associated with our clouds, varying from 16○ to 69○. We establish inverse correlations between p and both $\mathcal {S}$ and NH in almost every cloud, but we are unable to establish a statistically robust $\mathcal {S}$ versus NH trend. By comparing the results of these different statistical analysis techniques, we are able to propose a more comprehensive view of each cloud’s 3D magnetic field properties. These detailed cloud analyses will be useful in the continued studies of cloud-scale magnetic fields and the ways in which they affect star formation within these molecular clouds.
2
Clube, S. V. M. "Molecular clouds: comet factories?" International Astronomical Union Colloquium 83 (1985): 19–30. http://dx.doi.org/10.1017/s0252921100083779.
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AbstractRecent discoveries seem to indicate a catastrophic history of terrestrial evolution, explicable in terms of Oort cloud disturbance by molecular clouds in the Galactic disc. The problem of Oort cloud replenishment thus assumes considerable significance and reasons are given for supposing comet exchange takes place during actual penetration of molecular clouds. The number density of comets in molecular clouds, thereby implied, seems to suggest primary condensations of ≤103km in a dense precursor state of spiral arms. If chemical and/or isotopic signatures of comets should indicate an extra-Solar System source, the theory of terrestrial catastrophism may place new constraints on our understanding of the origin of molecular clouds.
3
Bot, Caroline, Mónica Rubio, François Boulanger, Marcus Albrecht, Frank Bertoldi, Alberto D. Bolatto, and Adam K. Leroy. "Tracing the cold molecular gas reservoir through dust emission in the SMC." Proceedings of the International Astronomical Union 4, S256 (July 2008): 148–53. http://dx.doi.org/10.1017/s174392130802838x.
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AbstractThe amount of molecular gas is a key for understanding the future star formation in a galaxy. However, this quantity is difficult to infer as the cold H2 is almost impossible to observe and, especially at low metallicities, CO only traces part of the clouds, keeping large envelopes of H2 hidden from observations. In this context, millimeter dust emission tracing the cold and dense regions can be used as a tracer to unveil the total molecular gas masses. I present studies of a sample of giant molecular clouds in the Small Magellanic Cloud. These clouds have been observed in the millimeter and sub-millimeter continuum of dust emission: with SIMBA/SEST at 1.2 mm and the new LABOCA bolometer on APEX at 870 μm. Combining these with radio data for each cloud, the spectral energy distribution of dust emission are obtained and gas masses are inferred. The molecular cloud masses are found to be systematically larger than the virial masses deduced from CO emission. Therefore, the molecular gas mass in the SMC has been underestimated by CO observations, even through the dynamical masses. This result confirms what was previously observed by Bot et al. (2007). We discuss possible interpretations of the mass discrepancy observed: in the giant molecular clouds of the SMC, part of cloud's support against gravity could be given by a magnetic field. Alternatively, the inclusion of surface terms in the virial theorem for turbulent clouds could reproduce the observed results and the giant molecular clouds could be transient structures.
4
Blitz, Leo. "Molecular Clouds at High z." Symposium - International Astronomical Union 144 (1991): 41–51. http://dx.doi.org/10.1017/s0074180900088896.
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The evidence for the existence of molecular clouds at large distances from the Galactic plane is reviewed. The molecular clouds at high Galactic latitudes are shown to be largely confined to the Galactic plane. There is evidence for one giant molecular cloud as much as four scale heights from the Galactic plane, but given the sample size from which the cloud is drawn, it is reasonable to suppose that it is part of the tail of the thin disk population. There is weak evidence that one star-forming molecular cloud may have originated in the Galactic halo. On the basis of kinematic evidence however, it is shown that there are three molecular clouds identified at high galactic latitude that, if not at high z, are likely to have resulted from interaction with gas in the halo. Understanding how these clouds have formed is likely to be an important key to understanding how the halo interacts with the disk gas.
5
Li, Pak Shing, and Richard I. Klein. "Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds." Monthly Notices of the Royal Astronomical Society 485, no. 4 (March 27, 2019): 4509–28. http://dx.doi.org/10.1093/mnras/stz653.
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Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.
6
Wong, Tony, Annie Hughes, Jürgen Ott, Jorge L. Pineda, and Erik Muller. "The Molecular Cloud Population of the Large Magellanic Cloud." Proceedings of the International Astronomical Union 8, S292 (August 2012): 71–74. http://dx.doi.org/10.1017/s1743921313000495.
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AbstractWe have mapped an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) at 11 pc resolution in the CO(1-0) line as part of the Magellanic Mopra Assessment (MAGMA). We identify clouds as regions of connected CO emission and determine their sizes, line widths, and fluxes. We find that GMCs are not preferentially located in regions of high Hi line width or velocity gradient, and that there is no clear Hi column density threshold for CO detection. The luminosity function of CO clouds is steeper than dN/dL ∝ L−2, suggesting a substantial fraction of mass in low-mass clouds. The correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. The virial parameter (the ratio of a cloud's kinetic to gravitational energy) shows a wide range of values and exhibits no clear trends with the likelihood of hosting young stellar object (YSO) candidates, suggesting that this parameter is a poor reflection of the evolutionary state of a cloud. More massive GMCs are more likely to harbor a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming GMCs as the principal sites of massive star formation.
7
Körtgen, Bastian. "The turbulence driving parameter of molecular clouds in disc galaxies." Monthly Notices of the Royal Astronomical Society 497, no. 1 (July 24, 2020): 1263–74. http://dx.doi.org/10.1093/mnras/staa2028.
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ABSTRACT Supersonic turbulence plays a pivotal role during the formation of molecular clouds and stars in galaxies. However, little is known about how the fraction of compressive and solenoidal modes in the velocity field evolves over time and how it depends on properties of the molecular cloud or the galactic environment. In this work, we carry out magnetohydrodynamical simulations of disc galaxies and study the time evolution of the turbulence driving parameter for an ensemble of clouds. We find that the time-averaged turbulence driving parameter is insensitive to the position of the cloud within the galaxy. The ensemble-averaged driving parameter is found to be rather compressive with b ∼ 0.5–0.7, indicating almost time-independent global star formation properties. However, each individual cloud shows a highly fluctuating driving parameter, which would strongly affect the cloud’s star formation rate. We find that the mode of turbulence driving can rapidly change within only a few Myr, both from solenoidal to compressive and vice versa. We attribute these changes to cloud collisions and to tidal interactions with clouds or overdensities in the environment. Last, we find no significant differences in the average driving parameter between hydrodynamic and initially strongly magnetized galaxies. However, the magnetic field tends to reduce the overall fluctuation of the driving parameter. The average driving and its uncertainty are seen to be in agreement with recent constraints on the turbulence driving mode for solar neighbourhood clouds.
8
Braine, J., E. Rosolowsky, P. Gratier, E. Corbelli, and K. F. Schuster. "Properties and rotation of molecular clouds in M 33." Astronomy & Astrophysics 612 (April 2018): A51. http://dx.doi.org/10.1051/0004-6361/201732405.
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The sample of 566 molecular clouds identified in the CO(2–1) IRAM survey covering the disk of M 33 is explored in detail. The clouds were found using CPROPS and were subsequently catalogued in terms of their star-forming properties as non-star-forming (A), with embedded star formation (B), or with exposed star formation (C, e.g., presence of Hα emission). We find that the size-linewidth relation among the M 33 clouds is quite weak but, when comparing with clouds in other nearby galaxies, the linewidth scales with average metallicity. The linewidth and particularly the line brightness decrease with galactocentric distance. The large number of clouds makes it possible to calculate well-sampled cloud mass spectra and mass spectra of subsamples. As noted earlier, but considerably better defined here, the mass spectrum steepens (i.e., higher fraction of small clouds) with galactocentric distance. A new finding is that the mass spectrum of A clouds is much steeper than that of the star-forming clouds. Further dividing the sample, this difference is strong at both large and small galactocentric distances and the A vs. C difference is a stronger effect than the inner vs. outer disk difference in mass spectra. Velocity gradients are identified in the clouds using standard techniques. The gradients are weak and are dominated by prograde rotation; the effect is stronger for the high signal-to-noise clouds. A discussion of the uncertainties is presented. The angular momenta are low but compatible with at least some simulations. Finally, the cloud velocity gradients are compared with the gradient of disk rotation. The cloud and galactic gradients are similar; the cloud rotation periods are much longer than cloud lifetimes and comparable to the galactic rotation period. The rotational kinetic energy is 1–2% of the gravitational potential energy and the cloud edge velocity is well below the escape velocity, such that cloud-scale rotation probably has little influence on the evolution of molecular clouds.
9
Yan, Qing-Zeng, Ji Yang, Yang Su, Yan Sun, Xin Zhou, Ye Xu, Hongchi Wang, Shaobo Zhang, and Zhiwei Chen. "Dependence of Molecular Cloud Samples on Angular Resolution, Sensitivity, and Algorithms." Astronomical Journal 164, no. 2 (July 18, 2022): 55. http://dx.doi.org/10.3847/1538-3881/ac77ea.
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Abstract In this work, we investigate the observational and algorithmic effects on molecular cloud samples identified from position–position–velocity (PPV) space. By smoothing and cutting off the high quality data of the Milky Way Imaging Scroll Painting (MWISP) survey, we extract various molecular cloud samples from those altered data with the DBSCAN (density-based spatial clustering of applications with noise) algorithm. Those molecular cloud samples are subsequently used to gauge the significance of sensitivity, angular/velocity resolution, and DBSCAN parameters. Two additional surveys, the FCRAO Outer Galaxy Survey and the CfA-Chile 1.2 m complete CO (CfA-Chile) survey, are used to verify the MWISP results. We found that molecular cloud catalogs are not unique and that the catalog boundary and therefore the sample size show strong variation with angular resolution and sensitivity. At low angular resolution (large beam sizes), molecular clouds merge together in PPV space, while a low sensitivity (high cutoffs) misses small faint molecular clouds and takes bright parts of large molecular clouds as single ones. At high angular resolution and sensitivity, giant molecular clouds (GMCs) are resolved into individual clouds, and their diffuse components are also revealed. Consequently, GMCs are more appropriately interpreted as clusters or aggregates of molecular clouds, i.e., GMCs represent molecular cloud samples themselves.
10
Kutner, Marc L., and Kathryn N. Mead. "Outer-Galaxy molecular clouds." Symposium - International Astronomical Union 106 (1985): 209–10. http://dx.doi.org/10.1017/s0074180900242460.
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Since our original report of CO emission from outside the solar circle in the first quadrant (Kutner and Mead, 1981) we have extended the observations in two ways: (1) We have improved latitude and longitude coverage. Preliminary results on the latitude distribution were reported by Kutner (1983). (2) We have extended our cloud mapping, giving us at least partial CO maps of 55 clouds, along with 13CO, C18O, CO (2–1), and 2-mm H2CO observations of some clouds.
11
Duarte-Cabral, A., D. Colombo, J. S. Urquhart, A. Ginsburg, D. Russeil, F. Schuller, L. D. Anderson, et al. "The SEDIGISM survey: molecular clouds in the inner Galaxy." Monthly Notices of the Royal Astronomical Society 500, no. 3 (September 11, 2020): 3027–49. http://dx.doi.org/10.1093/mnras/staa2480.
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ABSTRACT We use the 13CO (2–1) emission from the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the Spectral Clustering for Interstellar Molecular Emission Segmentation (scimes) algorithm. This work compiles a cloud catalogue with a total of 10 663 molecular clouds, 10 300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well-resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density, and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases (such as completeness and survey limitations), and thus require further follow up work in order to be confirmed.
12
Johansson, Lars E. B., Arto Heikkilä, and Hans Olofsson. "Molecular Line Observations in the Magellanic Clouds." Symposium - International Astronomical Union 190 (1999): 116–17. http://dx.doi.org/10.1017/s0074180900117553.
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We have observed spectral line emission from a sample of clouds in the Magellanic Clouds to investigate the effects of metallicity and FUV radiation on the physical and chemical properties of the interstellar medium. The clouds were identified by CO surveys and selected to cover a wide range of environments. We present molecular line data for five clouds in the LMC and one cloud in the SMC.
13
Rubio, Monica. "Molecular gas in the Small Magellanic Cloud." Symposium - International Astronomical Union 148 (1991): 429–30. http://dx.doi.org/10.1017/s007418090020106x.
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We summarize the results of observations of molecular gas from the Small Magellanic Cloud (SMC) made with low angular resolution (8'.8). These observations show that the CO emission is weak (TA˜ 0.04K) and that the CO luminosities of the Clouds are low compared to those of Galactic molecular clouds. The factor to convert the CO luminosity to molecular hydrogen column density for the SMC is ˜20 and three times larger than those derived for clouds in our Galaxy and in the Large Magellanic Cloud (LMC) respectively. In addition, we present preliminary results of high resolution (40″) observations of SMC molecular clouds made with the SEST telescope.
14
Lazarian, A. "Magnetic Field Generation within Molecular Clouds." Symposium - International Astronomical Union 157 (1993): 429–30. http://dx.doi.org/10.1017/s0074180900174571.
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Magnetic field generation in molecular (atomic) clouds at the early stages of galactic evolution is considered. It is shown that if there is no internal motions immersed the cloud, battery mechanisms (Lazarian 1992a) can account for the generation of thin magnetic shells around clouds insides in plasma with temperature gradients. If turbulent motions are present, the dynamo can be essential. The operation of α — ω, α2 and turbulent dynamos within molecular clouds is discussed. It is shown that the turbulent dynamo leads to generation of magnetic fields in the trace behind the cloud. These magnetic fields within the molecular clouds and in their vicinity are important for the solution of the galactic seed field problem (see Lazarian 1992b) and the formation of structures in clumpy molecular complexes.
15
Fujii, Michiko S., and Simon Portegies Zwart. "Formation of young massive clusters from turbulent molecular clouds." Proceedings of the International Astronomical Union 12, S316 (August 2015): 25–30. http://dx.doi.org/10.1017/s1743921316000545.
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AbstractWe simulate the formation and evolution of young star clusters from turbulent molecular clouds using smoothed-particle hydrodynamics and direct N-body methods. We find that the shape of the cluster mass function that originates from an individual molecular cloud is consistent with a Schechter function with power-law slopes of β = −1.73. The superposition of mass functions turn out to have a power-law slope of < −2. The mass of the most massive cluster formed from a single molecular cloud with mass Mg scales with 6.1 M0.51g. The molecular clouds that tend to form massive clusters are much denser than those typical found in the Milky Way. The velocity dispersion of such molecular clouds reaches 20km s−1 and it is consistent with the relative velocity of the molecular clouds observed near NGC 3603 and Westerlund 2, for which a triggered star formation by cloud-cloud collisions is suggested.
16
Alves, João, Marco Lombardi, and Charles Lada. "Insights on molecular cloud structure." Proceedings of the International Astronomical Union 6, S270 (May 2010): 99–102. http://dx.doi.org/10.1017/s1743921311000238.
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AbstractStars form in the densest regions of clouds of cold molecular hydrogen. Measuring structure in these clouds is far from trivial as 99% of the mass of a molecular cloud is inaccessible to direct observation. Over the last decade we have been developing an alternative, more robust density tracer technique based on dust extinction measurements towards background starlight. The new technique does not suffer from the complications plaguing the more conventional molecular line and dust emission techniques, and when used with these can provide unique views on cloud chemistry and dust grain properties in molecular clouds. In this brief communication we summarize the main results achieved so far using this technique.
17
Rosolowsky, E. "Giant Molecular Clouds in M31. I. Molecular Cloud Properties." Astrophysical Journal 654, no. 1 (January 2007): 240–51. http://dx.doi.org/10.1086/509249.
18
Yamaguchi, Nobuyuki, Norikazu Mizuno, Hiro Saito, Ken'ichi Matsunaga, Akira Mizuno, Hideo Ogawa, and Yasuo Fukui. "A Study of Dense Molecular Gas and Star Formation toward the Vela Molecular Ridge with NANTEN." Publications of the Astronomical Society of Japan 51, no. 6 (December 1, 1999): 775–90. http://dx.doi.org/10.1093/pasj/51.6.775.
Abstract:
Abstract New observations of the J=1−0 12CO, 13CO, and C18O emission lines have been extensively made toward the Vela Molecular Ridge (VMR) with NANTEN. The most prominent cloud is the giant molecular cloud, corresponding to the VMR-C region (Vela C). The present C18O distribution has been identified as 29 clouds. Among them, the most massive one is included in Vela C, having a total mass of ∼ 4.4 × 104M⊙. The rest of them are smaller C18O clouds of 102-103M⊙. Star formation in the region is almost exclusively occurring in the C18O clouds. The luminosities of the associated protostellar IRAS sources range from 5 L⊙ to 1.1 × 104L⊙, and the luminosity distribution is found to be well represented by the relation dNstar/dLIR ∞ L-1.65±0.14IR. We find that the ratios of the total luminosity of the sources associated with given C18O clouds to the cloud masses are significantly enhanced for those clouds associated with H II regions by an order of magnitude. This is interpreted as meaning that the formation of massive stars is enhanced due to the effects of H II regions, as is consistent with the preceding work. We have also newly found molecular outflow toward IRAS 08588–4347 as well as five possible candidates for outflows.
19
Rubio, Mónica. "Giant Molecular Clouds and Cluster Formation." Symposium - International Astronomical Union 207 (2002): 499–504. http://dx.doi.org/10.1017/s0074180900224303.
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We will review the present knowledge of molecular cloud properties and its relation to star formation. We will discuss the evidence for cluster formation associated with giant molecular clouds, and will concentrate on recent results in our Galaxy and the Magellanic Clouds.
20
Priestley, F. D., and A. P. Whitworth. "Molecular line signatures of cloud–cloud collisions." Monthly Notices of the Royal Astronomical Society 506, no. 1 (June 24, 2021): 775–80. http://dx.doi.org/10.1093/mnras/stab1777.
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ABSTRACT Collisions between interstellar gas clouds are potentially an important mechanism for triggering star formation. This is because they are able to rapidly generate large masses of dense gas. Observationally, cloud collisions are often identified in position–velocity (PV) space through bridging features between intensity peaks, usually of CO emission. Using a combination of hydrodynamical simulations, time-dependent chemistry, and radiative transfer, we produce synthetic molecular line observations of overlapping clouds that are genuinely colliding, and overlapping clouds that are just chance superpositions. Molecules tracing denser material than CO, such as NH3 and HCN, reach peak intensity ratios of 0.5 and 0.2 with respect to CO in the ‘bridging feature’ region of PV space for genuinely colliding clouds. For overlapping clouds that are just chance superpositions, the peak NH3 and HCN intensities are co-located with the CO intensity peaks. This represents a way of confirming cloud collisions observationally and distinguishing them from chance alignments of unrelated material.
21
Nogueras-Lara, F., R. Schödel, N. Neumayer, and M. Schultheis. "Distance to three molecular clouds in the central molecular zone." Astronomy & Astrophysics 647 (March 2021): L6. http://dx.doi.org/10.1051/0004-6361/202140554.
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Context. The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial for inferring its orbital structure and dynamics and for understanding star formation in the clouds. Aims. Recent work has suggested that the G0.253+0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to crosscheck those results, computing the absolute and relative distances to the Brick as well as to another two molecular clouds (the 50 km s−1 and the 20 km s−1 clouds), and discuss their CMZ membership. Methods. We used the colour magnitude diagrams Ks versus H − Ks to compare stars detected towards the target clouds with stars detected towards three reference regions in the nuclear stellar disc (NSD) and the Galactic bulge. We used red clump (RC) stars to estimate the distance to each region. Results. We found that all the clouds present a double RC feature. Such a double RC has been reported in previous work for the NSD, but not for the bulge adjacent to it. We exclude the possibility that the different RC features are located at significantly different distances. The obtained absolute and relative distances are compatible with the Galactic centre distance (∼8 kpc).
22
Pound, Marc W., and Alyssa A. Goodman. "The Ursa Major Molecular Clouds." Symposium - International Astronomical Union 170 (1997): 33–35. http://dx.doi.org/10.1017/s0074180900234037.
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The Ursa Major molecular cloud complex lies in the direction of an expanding HI shell known as the North Celestial Pole loop. The NCP loop, which is centered at (l, b) ∼ (138°,30°) and easily seen in IRAS 100 μm emission, is some 60 pc across and 150 pc distant (Meyerdierks et. al 1991). At 100 μm, the Ursa Major clouds appear in projection as “finger” (l ∼ 140, b ∼ 38) which “hangs down” towards the center of the loop and the plane of the Galaxy. Distance estimates to the molecular clouds (Penprase 1993) are consistent with that of the NCP loop, indicating that the clouds are physically associated with the loop.
23
Yan, Qing-Zeng, Ji Yang, Yang Su, Yan Sun, Ye Xu, Hongchi Wang, Xin Zhou, and Chen Wang. "Improved Measurements of Molecular Cloud Distances Based on Global Search." Astrophysical Journal 922, no. 1 (November 1, 2021): 8. http://dx.doi.org/10.3847/1538-4357/ac214f.
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Abstract The principle of the background-eliminated extinction-parallax (BEEP) method is examining the extinction difference between on- and off-cloud regions to reveal the extinction jump caused by molecular clouds, thereby revealing the distance in complex dust environments. The BEEP method requires high-quality images of molecular clouds and high-precision stellar parallaxes and extinction data, which can be provided by the Milky Way Imaging Scroll Painting (MWISP) CO survey and the Gaia DR2 catalog, as well as supplementary A V extinction data. In this work, the BEEP method is further improved (BEEP-II) to measure molecular cloud distances in a global search manner. Applying the BEEP-II method to three regions mapped by the MWISP CO survey, we collectively measured 238 distances for 234 molecular clouds. Compared with previous BEEP results, the BEEP-II method measures distances efficiently, particularly for those molecular clouds with large angular size or in complicated environments, making it suitable for distance measurements of molecular clouds in large samples.
24
Lehmann, Andrew, and Mark Wardle. "Diffusion of cosmic-ray electrons in the Galactic centre molecular cloud G0.13–0.13." Proceedings of the International Astronomical Union 9, S303 (October 2013): 434–38. http://dx.doi.org/10.1017/s1743921314001082.
Abstract:
AbstractThe Galactic center (GC) molecular cloud G0.13–0.13 exhibits a shell morphology in CS J = (1 − 0), with ∼ 105 solar masses and expansion speed ∼ 20 km s−1, yielding a total kinetic energy ∼ 1051 erg. Its morphology is also suggestive of an interaction with the nonthermal filaments of the GC arc. 74 MHz emission indicates the presence of a substantial population of low energy electrons permeating the cloud, which could either be produced by the interaction with the arc or accelerated in the shock waves responsible for the cloud's expansion. These scenarios are explored using time dependent diffusion models.With these diffusion models, we determine the penetration of low-energy cosmic-ray electrons accelerated into G0.13–0.13 and calculate the spatial distribution of the cosmic-ray ionization and heating rates. We show that the 6.4 keV Fe Kα line emission associated with the electron population provides an observational diagnostic to distinguish these two acceleration scenarios.We discuss the implications of our results for understanding the distinct character of clouds in the central molecular zone compared to clouds in the Galactic disk, and how GC nonthermal filaments interact with molecular clouds.
25
Krieger, Nico, Jürgen Ott, Fabian Walter, J. M. Diederik Kruijssen, and Henrik Beuther. "Temperature Evolution of Molecular Clouds in the Central Molecular Zone." Proceedings of the International Astronomical Union 11, S322 (July 2016): 160–61. http://dx.doi.org/10.1017/s1743921316011960.
Abstract:
AbstractWe infer the absolute time dependence of kinematic gas temperature along a proposed orbit of molecular clouds in the Central Molecular Zone (CMZ) of the Galactic Center (GC). Ammonia gas temperature maps are one of the results of the “Survey of Water and Ammonia in the Galactic Center” (SWAG, PI: J. Ott); the dynamical model of molecular clouds in the CMZ was taken from Kruijssen et al. (2015). We find that gas temperatures increase as a function of time in both regimes before and after the cloud passes pericenter on its orbit in the GC potential. This is consistent with the recent proposal that pericenter passage triggers gravitational collapse. Other investigated quantities (line width, column density, opacity) show no strong sign of time dependence but are likely dominated by cloud-to-cloud variations.
26
Padoan, Paolo, Tuomas Lunttila, Mika Juvela, Åke Nordlund, David Collins, Alexei Kritsuk, Michael Normal, and Sergey Ustyugov. "Magnetic Fields in Molecular Clouds." Proceedings of the International Astronomical Union 6, S271 (June 2010): 187–96. http://dx.doi.org/10.1017/s1743921311017601.
Abstract:
AbstractSupersonic magneto-hydrodynamic (MHD) turbulence in molecular clouds (MCs) plays an important role in the process of star formation. The effect of the turbulence on the cloud fragmentation process depends on the magnetic field strength. In this work we discuss the idea that the turbulence is super-Alfvénic, at least with respect to the cloud mean magnetic field. We argue that MCs are likely to be born super-Alfvénic. We then support this scenario based on a recent simulation of the large-scale warm interstellar medium turbulence. Using small-scale isothermal MHD turbulence simulation, we also show that MCs may remain super-Alfvénic even with respect to their rms magnetic field strength, amplified by the turbulence. Finally, we briefly discuss the comparison with the observations, suggesting that super-Alfvénic turbulence successfully reproduces the Zeeman measurements of the magnetic field strength in dense MC clouds.
27
Sharma, Ekta, Maheswar Gopinathan, Archana Soam, Chang Won Lee, and T. R. Seshadri. "Core orientations and magnetic fields in isolated molecular clouds." Monthly Notices of the Royal Astronomical Society 517, no. 1 (October 11, 2022): 1138–55. http://dx.doi.org/10.1093/mnras/stac2487.
Abstract:
ABSTRACT Molecular clouds are sites of star formation. Magnetic fields are believed to play an important role in their dynamics and shaping morphology. We aim to study any possible correlation that might exist between the magnetic fields orientation inside the clouds and the magnetic fields at envelope scales and their connection with respect to the observed morphology of the selected clouds. We examine the magnetic field orientation towards the clouds L1512, L1523, L1333, L1521E, L1544, L1517, L1780, and L183, using optical and Planck polarization observations. We also found the correlation between the ambient magnetic field and core orientations derived using Astrodendrogram on the Herschel 250 $\mu$m data. We find that the magnetic fields derived from optical and Planck agree with each other. The derived magnetic fields are aligned along the observed emission of each cloud as seen in Herschel 250 $\mu$m data. We also find that the relative orientation between the cores and the magnetic fields is random. This lack of correlation may arise due to the fact that the core orientation could also be influenced by the different magnetization within individual clouds at higher densities or the feedback effects which may vary from cloud to cloud. The estimated magnetic field strength and the mass-to-flux ratio suggest that all the clouds are in a magnetically critical state except L1333, L1521E, and L183, where the cloud envelope could be strongly supported by the magnetic field lines.
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Braine, J., A. Hughes, E. Rosolowsky, P. Gratier, D. Colombo, S. Meidt, and E. Schinnerer. "Rotation of molecular clouds in M 51." Astronomy & Astrophysics 633 (December 23, 2019): A17. http://dx.doi.org/10.1051/0004-6361/201834613.
Abstract:
The grand-design spiral galaxy M 51 was observed at 40 pc resolution in CO(1–0) by the PAWS project. A large number of molecular clouds were identified and we search for velocity gradients in two high signal-to-noise subsamples, containing 682 and 376 clouds. The velocity gradients are found to be systematically prograde oriented, as was previously found for the rather flocculent spiral M 33. This strongly supports the idea that the velocity gradients reflect cloud rotation, rather than more random dynamical forces, such as turbulence. Not only are the gradients prograde, but their ∂v/∂x and ∂v/∂y coefficients follow galactic shear in sign, although with a lower amplitude. No link is found between the orientation of the gradient and the orientation of the cloud. The values of the cloud angular momenta appear to be an extension of the values noted for galactic clouds despite the orders of magnitude difference in cloud mass. Roughly 30% of the clouds show retrograde velocity gradients. For a strictly rising rotation curve, as in M 51, gravitational contraction would be expected to yield strictly prograde rotators within an axisymmetric potential. In M 51, the fraction of retrograde rotators is found to be higher in the spiral arms than in the disk as a whole. Along the leading edge of the spiral arms, a majority of the clouds are retrograde rotators. While this work should be continued on other nearby galaxies, the M 33 and M 51 studies have shown that clouds rotate and that they rotate mostly prograde, although the amplitudes are not such that rotational energy is a significant support mechanism against gravitation. In this work, we show that retrograde rotation is linked to the presence of a spiral gravitational potential.
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Knez, C., M. Moore, S. Travis, R. Ferrante, J. Chiar, A. Boogert, L. Mundy, et al. "Comparing ice composition in dark molecular clouds." Proceedings of the International Astronomical Union 4, S251 (February 2008): 47–48. http://dx.doi.org/10.1017/s1743921308021157.
Abstract:
AbstractWe present 5–20 μm Spitzer/IRS spectroscopy toward stars behind dark molecular clouds. We present preliminary results from the Serpens dark cloud to show the variation between environments within a cloud. We are surveying 3 clouds with varying levels of star formation activity. Serpens has the highest level of activity from our 3 clouds. We show that location as well extinction can cause variations in ice composition. We also find that some lines of sight contain organic molecules such as methane and methanol, and the first detection of acetylene ice in the interstellar medium. We believe the high extinction lines of sight have been enriched by star formation activity near those lines of sight.
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Henshaw, Jonathan D. "Molecular gas kinematics of the CMZ: Great oaks from little acorns grow." Proceedings of the International Astronomical Union 11, S322 (July 2016): 85–89. http://dx.doi.org/10.1017/s1743921316011972.
Abstract:
AbstractThe central molecular zone (CMZ) hosts some of the most massive and dense molecular clouds and star clusters in the Galaxy, offering an important window into star formation under extreme conditions. Star formation in this extreme environment may be closely linked to the 3-D distribution and orbital dynamics of the gas. Here I discuss how our new, accurate description of the {l,b,v} structure of the CMZ is helping to constrain its 3-D geometry. I also present the discovery of a highly-regular, corrugated velocity field located just upstream from the dust ridge molecular clouds (which include G0.253+0.016 and Sgr B2). The extremes in this velocity field correlate with a series of massive (~ 104 M⊙) cloud condensations. The corrugation wavelength (~23 pc) and cloud separation (~8 pc) closely agree with the predicted Toomre (~17 pc) and Jeans (~6 pc) lengths, respectively. I conclude that gravitational instabilities are driving the formation of molecular clouds within the Galactic Centre gas stream. Furthermore, I suggest that these seeds are the historical analogues of the dust ridge molecular clouds – possible progenitors of some of the most massive and dense molecular clouds in the Galaxy. If our current best understanding for the 3-D geometry of this system is confirmed, these clouds may pinpoint the beginning of an evolutionary sequence that can be followed, in time, from cloud condensation to star formation.
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Rosolowsky, Erik, Annie Hughes, Adam K. Leroy, Jiayi Sun, Miguel Querejeta, Andreas Schruba, Antonio Usero, et al. "Giant molecular cloud catalogues for PHANGS-ALMA: methods and initial results." Monthly Notices of the Royal Astronomical Society 502, no. 1 (January 15, 2021): 1218–45. http://dx.doi.org/10.1093/mnras/stab085.
Abstract:
ABSTRACT We present improved methods for segmenting CO emission from galaxies into individual molecular clouds, providing an update to the cprops algorithms presented by Rosolowsky & Leroy. The new code enables both homogenization of the noise and spatial resolution among data, which allows for rigorous comparative analysis. The code also models the completeness of the data via false source injection and includes an updated segmentation approach to better deal with blended emission. These improved algorithms are implemented in a publicly available Python package, pycprops. We apply these methods to 10 of the nearest galaxies in the PHANGS-ALMA survey, cataloguing CO emission at a common 90 pc resolution and a matched noise level. We measure the properties of 4986 individual clouds identified in these targets. We investigate the scaling relations among cloud properties and the cloud mass distributions in each galaxy. The physical properties of clouds vary among galaxies, both as a function of galactocentric radius and as a function of dynamical environment. Overall, the clouds in our target galaxies are well-described by approximate energy equipartition, although clouds in stellar bars and galaxy centres show elevated line widths and virial parameters. The mass distribution of clouds in spiral arms has a typical mass scale that is 2.5× larger than interarm clouds and spiral arms clouds show slightly lower median virial parameters compared to interarm clouds (1.2 versus 1.4).
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Zhang, Miaomiao. "Distances to Nearby Molecular Clouds Traced by Young Stars." Astrophysical Journal Supplement Series 265, no. 2 (April 1, 2023): 59. http://dx.doi.org/10.3847/1538-4365/acc1e8.
Abstract:
Abstract I present a catalog of distances to 63 molecular clouds located within ∼2.5 kpc of the Sun. The cloud distances are derived based on utilizing the Gaia DR3 parallaxes of the young stellar objects (YSOs). By identifying AllWISE YSO candidates (YSOCs) with infrared excesses and combining them with published YSOC catalogs, I compile an all-sky YSOC sample that is devoid of a significant proportion of contaminants. Using Gaia DR3 astrometric measurements, I associate over 3000 YSOCs with 63 local clouds and obtain the average distance to each cloud by fitting the YSOC parallax distribution within the cloud. I find good agreements with typical scatter of ≲10% between my new cloud distances and previous distance estimates. Unlike cloud distances obtained using stellar extinction, my catalog provides distances to the relatively dense areas of local clouds, which makes them more appropriate references for investigating the physical properties of nearby dense regions.
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Tanvir, Tabassum S., and James E. Dale. "Collision between molecular clouds – I. The effect of the cloud virial ratio in head-on collisions." Monthly Notices of the Royal Astronomical Society 494, no. 1 (March 13, 2020): 246–58. http://dx.doi.org/10.1093/mnras/staa665.
Abstract:
ABSTRACT In a series of papers, we investigate the effect of collisions between turbulent molecular clouds on their structure, evolution, and star formation activity. In this paper, we look into the role of the clouds’ initial virial ratios. Three different scenarios were examined: both clouds initially bound, one cloud bound and one unbound, and both clouds initially unbound. Models in which one or both clouds are bound generate filamentary structures aligned along the collision axis and discernible in position–position and position–velocity space. If neither cloud is bound, no filaments result. Unlike in previous simulations of collisions between smooth clouds, owing to the substructure created in the clouds by turbulence before the collisions, dissipation of kinetic energy by the collision is very inefficient and in none of our simulations is sufficient bulk kinetic energy lost to render the clouds bound. Simulations where both clouds are bound created twice as much stellar mass than the bound–unbound model, and both these scenarios produced much more stellar mass than the simulation in which both clouds are unbound. Each simulation was also compared with a control run in which the clouds do not collide. We find the bound–bound collision increases the overall star formation efficiency by a factor of approximately two relative to the control, but that the bound–unbound collision produces a much smaller increase, and the collision has very little effect on the unbound–unbound cloud collision.
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Inutsuka, Shu-ichiro. "The role of magnetic field in the formation and evolution of filamentary molecular clouds." Proceedings of the International Astronomical Union 14, A30 (August 2018): 100. http://dx.doi.org/10.1017/s1743921319003557.
Abstract:
AbstractRecent observations have emphasized the importance of the formation and evolution of magnetized filamentary molecular clouds in the process of star formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds. In this review we summarize our current understanding of various processes that are required in describing the filamentary molecular clouds. Especially we can explain a robust formation mechanism of filamentary molecular clouds in a shock compressed layer, which is in analogy to the making of “Sushi.” We also discuss the origin of the mass function of cores.
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Zucker, Catherine, Joshua S. Speagle, Edward F. Schlafly, Gregory M. Green, Douglas P. Finkbeiner, Alyssa Goodman, and João Alves. "A compendium of distances to molecular clouds in the Star Formation Handbook." Astronomy & Astrophysics 633 (January 2020): A51. http://dx.doi.org/10.1051/0004-6361/201936145.
Abstract:
Accurate distances to local molecular clouds are critical for understanding the star and planet formation process, yet distance measurements are often obtained inhomogeneously on a cloud-by-cloud basis. We have recently developed a method that combines stellar photometric data with Gaia DR2 parallax measurements in a Bayesian framework to infer the distances of nearby dust clouds to a typical accuracy of ∼5%. After refining the technique to target lower latitudes and incorporating deep optical data from DECam in the southern Galactic plane, we have derived a catalog of distances to molecular clouds in Reipurth (2008, Star Formation Handbook, Vols. I and II) which contains a large fraction of the molecular material in the solar neighborhood. Comparison with distances derived from maser parallax measurements towards the same clouds shows our method produces consistent distances with ≲10% scatter for clouds across our entire distance spectrum (150 pc−2.5 kpc). We hope this catalog of homogeneous distances will serve as a baseline for future work.
36
Pudritz, Ralph E. "Formation of structure in star-forming clouds." Canadian Journal of Physics 68, no. 9 (September 1, 1990): 808–23. http://dx.doi.org/10.1139/p90-118.
Abstract:
Star formation occurs in massive, dense, molecular clouds in the interstellar medium. These clouds have a rich substructure consisting of dense clumps and extended filaments. Since stars only form within these dense clumps, any fundamental theory of star formation must predict their physical properties. This review focusses on the physics of molecular clouds and discusses in this context a particular mechanism for the formation of structure that is well supported by the observations. Strong hydromagnetic waves are likely to be excited in molecular clouds since it is observed that cloud magnetic fields have energy densities close to gravity. These waves support the cloud against global gravitational collapse by providing an effective wave "pressure". This review also shows that waves may control the formation of structure in molecular clouds.
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Jaffe, D. T. "Warm Molecular Clouds." Symposium - International Astronomical Union 150 (1992): 311–15. http://dx.doi.org/10.1017/s0074180900090239.
Abstract:
Warm molecular gas is important in a large range of astronomical contexts. We discuss here determinations of the temperature and mass of warm material in protostellar disks and cores, photon dominated regions, and molecular material shocked by protostellar outflows. We then compare these results to heating and cooling models. The models of dense cores and photon dominated regions are not adequate to explain the large amounts of warm material observed. This conclusion raises the possibility that there may be other heating mechanisms at work in these regions which theorists have not yet included in their models.
38
BALLY, J. "Interstellar Molecular Clouds." Science 232, no. 4747 (April 11, 1986): 185–93. http://dx.doi.org/10.1126/science.232.4747.185.
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Liu, Lijie, Martin Bureau, Leo Blitz, Timothy A. Davis, Kyoko Onishi, Mark Smith, Eve North, and Satoru Iguchi. "WISDOM Project – IX. Giant molecular clouds in the lenticular galaxy NGC 4429: effects of shear and tidal forces on clouds." Monthly Notices of the Royal Astronomical Society 505, no. 3 (May 27, 2021): 4048–85. http://dx.doi.org/10.1093/mnras/stab1537.
Abstract:
ABSTRACT We present high spatial resolution (≈12 pc) Atacama Large Millimeter/submillimeter Array 12CO(J = 3–2) observations of the nearby lenticular galaxy NGC 4429. We identify 217 giant molecular clouds within the 450 pc radius molecular gas disc. The clouds generally have smaller sizes and masses but higher surface densities and observed linewidths than those of Milky Way disc clouds. An unusually steep size–linewidth relation ($\sigma \propto R_{\rm c}^{0.8}$) and large cloud internal velocity gradients (0.05–0.91 km s−1 pc−1) and observed virial parameters (〈αobs,vir〉 ≈ 4.0) are found, which appear due to internal rotation driven by the background galactic gravitational potential. Removing this rotation, an internal virial equilibrium appears to be established between the self-gravitational (Usg) and turbulent kinetic (Eturb) energies of each cloud, i.e. $\langle \alpha _{\rm sg,vir}\equiv \frac{2E_{\rm turb}}{\vert U_{\rm sg}\vert }\rangle \approx 1.3$. However, to properly account for both self and external gravity (shear and tidal forces), we formulate a modified virial theorem and define an effective virial parameter $\alpha _{\rm eff,vir}\equiv \alpha _{\rm sg,vir}+\frac{E_{\rm ext}}{\vert U_{\rm sg}\vert }$ (and associated effective velocity dispersion). The NGC 4429 clouds then appear to be in a critical state in which the self-gravitational energy and the contribution of external gravity to the cloud’s energy budget (Eext) are approximately equal, i.e. $\frac{E_{\rm ext}}{\vert U_{\rm sg}\vert }\approx 1$. As such, 〈αeff,vir〉 ≈ 2.2 and most clouds are not virialized but remain marginally gravitationally bound. We show this is consistent with the clouds having sizes similar to their tidal radii and being generally radially elongated. External gravity is thus as important as self-gravity to regulate the clouds of NGC 4429.
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Li, P. S., C. F. McKee, and R. I. Klein. "Structure and Dynamics of Magnetized Dark Molecular Clouds." Proceedings of the International Astronomical Union 10, H16 (August 2012): 386. http://dx.doi.org/10.1017/s1743921314011557.
Abstract:
Massive infrared dark clouds (IRDCs) are believed to be the precursors to star clusters and massive stars (e.g. Bergin & Tafalla 2007). The supersonic, turbulent nature of molecular clouds in the presence of magnetic fields poses a great challenge in understanding the structure and dynamics of magnetized molecular clouds and the star formation therein. Using the high-order radiation-magneto-hydrodynamic adaptive mesh refinement (AMR) code ORION2 (Li et al. 2012), we perform a large-scale driven-turbulence simulation to reveal the 3D filamentary structure and dynamical state of a highly supersonic (thermal Mach number = 10) and strongly magnetized (plasma β=0.02) massive infrared dark molecular cloud. With the high resolution afforded by AMR, we follow the dynamical evolution of the cloud in order to understand the roles of strong magnetic fields, turbulence, and self-gravity in shaping the cloud and in the formation of dense cores.
41
Koda, Jin, Linda Watson, Françoise Combes, Monica Rubio, Samuel Boissier, Masafumi Yagi, David Thilker, et al. "First Detection of the Molecular Cloud Population in the Extended Ultraviolet Disk of M83." Astrophysical Journal 941, no. 1 (December 1, 2022): 3. http://dx.doi.org/10.3847/1538-4357/ac9dfc.
Abstract:
Abstract We report a CO(J = 3−2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with the Atacama Large Millimeter/submillimeter Array. The observed 1 kpc2 region is at about 1.24 times the optical radius (R 25) of the disk, where CO(J = 2–1) was previously not detected. The detection and nondetection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A—with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3−2), range from 8.2 × 102 to 2.3 × 104 M ⊙ and from 2.7 × 102 to 7.5 × 103 M ⊙, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is common. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The common cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3−2) is an efficient tracer of molecular clouds even in low-metallicity environments.
42
Shull, Peter, John Dyson, and Franz Kahn. "A Model of SNR Evolution for an O-Star in a Cloudy ISM." International Astronomical Union Colloquium 101 (1988): 231–34. http://dx.doi.org/10.1017/s0252921100102416.
Abstract:
AbstractWe present an analytical model of SNR evolution in a cloudy interstellar medium for a single progenitor star of spectral type 05 V. The model begins with the progenitor on the zero-age main sequence, includes the effects of the star’s wind and ionizing photons, and ends with the SNR’s assimilation by the ISM. We assume that the ISM consists of atomic clouds, molecular clouds, and a hot intercloud phase. The type of SNR that results bears a strong resemblance to N63A in the Large Magellanic Cloud.
43
Dessauges-Zavadsky, Miroslava, Johan Richard, Françoise Combes, Daniel Schaerer, Wiphu Rujopakarn, and Lucio Mayer. "Molecular clouds in a Milky Way progenitor at z = 1." Proceedings of the International Astronomical Union 15, S352 (June 2019): 269–73. http://dx.doi.org/10.1017/s1743921319008949.
Abstract:
AbstractThanks to the remarkable ALMA capabilities and the unique configuration of the Cosmic Snake galaxy behind a massive galaxy cluster, we could resolve molecular clouds down to 30 pc linear physical scales in a typical Milky Way progenitor at z = 1.036, through CO(4–3) observations performed at the ∼ 0.2″ angular resolution. We identified 17 individual giant molecular clouds. These high-redshift molecular clouds are clearly different from their local analogues, with 10–100 times higher masses, densities, and internal turbulence. They are offset from the Larson scaling relations. We argue that the molecular cloud physical properties are dependent on the ambient interstellar conditions particular to the host galaxy. We find these high-redshift clouds in virial equilibrium, and derive, for the first time, the CO-to-H2 conversion factor from the kinematics of independent molecular clouds at z = 1. The measured large clouds gas masses demonstrate the existence of parent gas clouds with masses high enough to allow the in-situ formation of similarly massive stellar clumps seen in the Cosmic Snake galaxy in comparable numbers. Our results support the formation of molecular clouds by fragmentation of turbulent galactic gas disks, which then become the stellar clumps observed in distant galaxies.
44
Li, Hua-Bai. "Magnetic Fields in Molecular Clouds—Observation and Interpretation." Galaxies 9, no. 2 (June 8, 2021): 41. http://dx.doi.org/10.3390/galaxies9020041.
Abstract:
The Zeeman effect and dust grain alignment are two major methods for probing magnetic fields (B-fields) in molecular clouds, largely motivated by the study of star formation, as the B-field may regulate gravitational contraction and channel turbulence velocity. This review summarizes our observations of B-fields over the past decade, along with our interpretation. Galactic B-fields anchor molecular clouds down to cloud cores with scales around 0.1 pc and densities of 104–5 H2/cc. Within the cores, turbulence can be slightly super-Alfvénic, while the bulk volumes of parental clouds are sub-Alfvénic. The consequences of these largely ordered cloud B-fields on fragmentation and star formation are observed. The above paradigm is very different from the generally accepted theory during the first decade of the century, when cloud turbulence was assumed to be highly super-Alfvénic. Thus, turbulence anisotropy and turbulence-induced ambipolar diffusion are also revisited.
45
Niwa, Takahiro, Yoichi Itoh, Kengo Tachihara, Yumiko Oasa, Kazuyoshi Sunada, and Koji Sugitani. "Radio observation of molecular clouds around the W5-East triggered star-forming region." Proceedings of the International Astronomical Union 2, S237 (August 2006): 454. http://dx.doi.org/10.1017/s1743921307002426.
Abstract:
It is known that most of stars are formed as clusters (Lada & Lada 2003, ARAA 41, L57) and clusters are formed by triggering. However, the relationships of molecular clouds' conditions and properties of formed stars by triggering is not well studied. To clarify differences between triggered and spontaneous star formation through physical properties of molecular clouds (e.g. mass, density, morphology), we observed the W5-East HII region. The W5-East HII region is located at 2 kpc and has a 10 pc extent of HII region. This region has 3 Bright Rimmed Clouds (BRCs; Sugitani et al. 1991, ApJS 77, S59), which are interface between HII regions and molecular clouds, and known as sites of triggered star formation. The molecular clouds surround the W5-East (Karr et al. 2003, ApJ, 595, 900), thus we expect molecular clouds morphology is affected by the HII region and the cloud evolution is supposed to be dominated by the expanding HII region.
46
Yuan, Lixia, Ji Yang, Fujun Du, Xunchuan Liu, Yang Su, Qing-Zeng Yan, Xuepeng Chen, et al. "On the Spatial Distribution of 13CO Structures within 12CO Molecular Clouds." Astrophysical Journal 944, no. 1 (February 1, 2023): 91. http://dx.doi.org/10.3847/1538-4357/acac26.
Abstract:
Abstract We look into the 2851 12CO molecular clouds harboring 13CO structures to reveal the distribution of the projected angular separations and radial velocity separations between their internal 13CO structures. The projected angular separations are determined using the minimal spanning tree algorithm. We find that ∼50% of the angular separations fall in a narrow range of ∼3′–7′ with a median of ∼5′, and the corresponding radial velocity separations mainly range from ∼0.3 to 2.5 km s−1. The mean and standard deviation of the angular separations of the internal 13CO structures within 12CO clouds appear to be universal, independent of the 12CO cloud angular areas and the counts of their internal 13CO structures. We also reveal a scaling relation between the 12CO cloud angular area and its harbored 13CO structure count. These results suggest there is a preferred angular separation between 13CO structures in these 12CO clouds, considering the distance effects. According to that, we propose an alternative picture for the assembly and destruction of molecular clouds: there is a fundamental separation for the internal structures of molecular clouds, the build-up and destruction of molecular clouds proceeds under this fundamental unit.
47
Yan, Qing-Zeng, Bo Zhang, Ye Xu, Sufen Guo, Jean-Pierre Macquart, Zheng-Hong Tang, and Andrew John Walsh. "Distances to molecular clouds at high galactic latitudes based on Gaia DR2." Astronomy & Astrophysics 624 (March 29, 2019): A6. http://dx.doi.org/10.1051/0004-6361/201834337.
Abstract:
We report the distances of molecular clouds at high Galactic latitudes (|b| > 10°) derived from parallax and G-band extinction (AG) measurements in the second Gaia data release, Gaia DR2. Aided by Bayesian analyses, we determined distances by identifying the breakpoint in the extinction AG toward molecular clouds and using the extinction AG of Gaia stars around molecular clouds to confirm the breakpoint. We used nearby star-forming regions, such as Orion, Taurus, Cepheus, and Perseus, whose distances are well known to examine the reliability of our method. By comparing with previous results, we found that the molecular cloud distances derived from this method are reliable. The systematic error in the distances is approximately 5%. In total, 52 molecular clouds have well-determined distances, most of which are at high Galactic latitudes, and we provide reliable distances for 13 molecular clouds for the first time.
48
Genzel, R., G. J. Stacey, C. H. Townes, A. Poglitsch, and N. Geis. "158μm [CII] Mapping of the Galactic Center Molecular Clouds." Symposium - International Astronomical Union 136 (1989): 151–55. http://dx.doi.org/10.1017/s0074180900186450.
Abstract:
We have made 55″ resolution maps of the 158 μm [CII] emission line in the region of the curved, thermal filaments and the +20 / +50 kms−1 molecular clouds in Sgr A. The [CII] emission is spatially well correlated with the radio continuum in the filaments. The large intensity of the [CII] radiation excludes shocks as the origin of the ionization and we conclude that the curved filaments are most likely photo-ionized HII regions at the surface of dense molecular clouds. Our [CII] maps of the +20 / +50 kms−1 clouds indicate that the +50 kms−1 cloud is close to (<10pc) Sgr A west while the more massive +20 kms−1 cloud is at a greater distance from the center (>30pc).
49
Veeraraghavan, Shoba, and Gary A. Fuller. "Characterisation of spatial structure in molecular clouds." Symposium - International Astronomical Union 147 (1991): 505–7. http://dx.doi.org/10.1017/s0074180900240205.
Abstract:
Two topological tools for studying the global structure of molecular clouds, the genus and the contour-crossing statistic, are discussed. Preliminary results for the Taurus molecular cloud complex are presented.
50
Veeraraghavan, Shoba, and Gary A. Fuller. "Characterisation of spatial structure in molecular clouds." Symposium - International Astronomical Union 147 (1991): 505–7. http://dx.doi.org/10.1017/s0074180900199589.
Abstract:
Two topological tools for studying the global structure of molecular clouds, the genus and the contour-crossing statistic, are discussed. Preliminary results for the Taurus molecular cloud complex are presented.