- ID:
- ivo://CDS.VizieR/J/ApJ/853/171
- Title:
- mm point sources in the extended Sgr B2 cloud
- Short Name:
- J/ApJ/853/171
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report ALMA observations with resolution ~0.5" at 3mm of the extended Sgr B2 cloud in the Central Molecular Zone (CMZ). We detect 271 compact sources, most of which are smaller than 5000au. By ruling out alternative possibilities, we conclude that these sources consist of a mix of hypercompact HII regions and young stellar objects (YSOs). Most of the newly detected sources are YSOs with gas envelopes that, based on their luminosities, must contain objects with stellar masses M*>~8M_{sun}_. Their spatial distribution spread over a ~12x3pc region demonstrates that Sgr B2 is experiencing an extended star formation event, not just an isolated "starburst" within the protocluster regions. Using this new sample, we examine star formation thresholds and surface density relations in Sgr B2. While all of the YSOs reside in regions of high column density (N(H_2_)>~2x10^23^cm^-2^), not all regions of high column density contain YSOs. The observed column density threshold for star formation is substantially higher than that in solar vicinity clouds, implying either that high-mass star formation requires a higher column density or that any star formation threshold in the CMZ must be higher than in nearby clouds. The relation between the surface density of gas and stars is incompatible with extrapolations from local clouds, and instead stellar densities in Sgr B2 follow a linear {Sigma}_*_-{Sigma}_gas_ relation, shallower than that observed in local clouds. Together, these points suggest that a higher volume density threshold is required to explain star formation in CMZ clouds.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/638/293
- Title:
- 1.1mm sources in the Perseus Molecular Cloud
- Short Name:
- J/ApJ/638/293
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have completed a 1.1mm continuum survey of 7.5{deg}^2^ of the Perseus Molecular Cloud using Bolocam at the Caltech Submillimeter Observatory. This represents the largest millimeter or submillimeter continuum map of Perseus to date. Our map covers more than 30,000 31" (FWHM) resolution elements to a 1{sigma} rms of 15mJy/beam. We detect a total of 122 cores above a 5{sigma} point-source mass detection limit of 0.18M_{sun}_, assuming a dust temperature of T_D_=10K, 60 of which are new millimeter or submillimeter detections.
- ID:
- ivo://CDS.VizieR/J/A+A/624/A108
- Title:
- Modeling interstellar sulfur depletion
- Short Name:
- J/A+A/624/A108
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the "missing" sulfur is in the form of organo-sulfur species that are trapped on grains.
- ID:
- ivo://CDS.VizieR/J/A+A/607/A118
- Title:
- Models for molecular transitions
- Short Name:
- J/A+A/607/A118
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a method to interpret molecular observations and molecular line ratios in nearby extragalactic regions. Ab initio grids of time dependent chemical models, varying in gas density, temperature, cosmic ray ionization rate, and radiation field, are used as input to RADEX calculations. Tables of abundances, column densities, theoretical line intensities, and line ratios for some of the most used dense gas tracers are provided. The degree of correlation as well as degeneracy inherent in molecular ratios is discussed. Comparisons of the theoretical intensities with example observations are also provided. We find that, within the parameters space explored, chemical abundances can be constrained by a well defined set of gas density-gas temperature-cosmic ray ionization rate for the species we investigate here. However, line intensities, as well as, more importantly, line ratios, from different chemical models can be very similar leading to a clear degeneracy. We also find that the gas subjected to a galactic cosmic ray ionization rate will not necessarily have reached steady state by 1 million years. The species most affected by time dependency effects are HCN and CS, both high density tracers. We use our ab initio method to fit an example set of data from two galaxies (M82 and, NGC 253). We find that (i) molecular line ratios can be easily matched even with erroneous individual line intensities; (ii) no set of species can be matched by a one-component ISM; (iii) a species may be a good tracer of an energetic process but only under specific density and temperature conditions. We provide tables of chemical abundances and line intensities ratios for some of the most commonly observed extragalactic tracers of dense gas for a grid of models. We show that by taking into consideration the chemistry behind each species and the individual line intensities, many degeneracies that arise by just using molecular line ratios can be avoided. Finally we show that using a species or a ratio as a tracer of an individual energetic process (e.g. cosmic rays, UV) ought to be done with caution.
- ID:
- ivo://CDS.VizieR/J/ApJ/705/144
- Title:
- Molecular and atomic gas in the LMC. II.
- Short Name:
- J/ApJ/705/144
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We compare the CO (J=1-0) and HI emission in the Large Magellanic Cloud in three dimensions, i.e., including a velocity axis in addition to the two spatial axes, with the aim of elucidating the physical connection between giant molecular clouds (GMCs) and their surrounding HI gas. The CO(J=1-0) data set is from the second NANTEN CO survey (Fukui et al. 2008, Cat. J/ApJS/178/56) and the HI data set is from the merged Australia Telescope Compact Array (ATCA) and Parkes Telescope surveys (Kim et al. 2003ApJS..148..473K).
- ID:
- ivo://CDS.VizieR/J/A+A/658/A54
- Title:
- Molecular cloud assoc. to Milky Way spiral arms
- Short Name:
- J/A+A/658/A54
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The morphology of the Milky Way is still a matter of debate. In order to shed light on the uncertainty surrounding the Galactic structure, in this paper, we study the imprint of spiral arms on the molecular gas distribution and properties. To do so, we take full advantage of the SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the ^13^CO(2-1) line at an angular resolution of 28". We analyse the influences of spiral arms by considering the features of the molecular gas emission as a whole across the longitude- velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to those in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ much from the emission between the arms. On average, spiral arms show masses per unit length of ~10^5^-10^6^M_{sun}_/kpc. This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.
- ID:
- ivo://CDS.VizieR/J/ApJ/872/121
- Title:
- Molecular cloud cores in the GC 50km/s cloud
- Short Name:
- J/ApJ/872/121
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Galactic center 50km/s molecular cloud (50MC) is the most remarkable molecular cloud in the Sagittarius A region. This cloud is a candidate for the massive star formation induced by cloud-cloud collision (CCC) with a collision velocity of ~30km/s that is estimated from the velocity dispersion. We observed the whole of the 50MC with a high angular resolution (~2.0"x1.4") in Atacama Large Millimeter/submillimeter Array cycle 1 in the H^13^CO^+^ J=1-0 and C^34^S J=2-1 emission lines. We identified 241 and 129 bound cores with a virial parameter of less than 2, which are thought to be gravitationally bound, in the H^13^CO^+^ and C^34^S maps using the clumpfind algorithm, respectively. In the CCC region, the bound H^13^CO^+^ and C^34^S cores are 119 and 82, whose masses are 68% and 76% of those in the whole 50MC, respectively. The distribution of the core number and column densities in the CCC are biased to larger densities than those in the non-CCC region. The distributions indicate that the CCC compresses the molecular gas and increases the number of the dense bound cores. Additionally, the massive bound cores with masses of >3000M_{sun}_ exist only in the CCC region, although the slope of the core mass function (CMF) in the CCC region is not different from that in the non-CCC region. We conclude that the compression by the CCC efficiently formed massive bound cores even if the slope of the CMF is not changed so much by the CCC.
- ID:
- ivo://CDS.VizieR/J/A+A/548/A52
- Title:
- Molecular cloud formation in M33
- Short Name:
- J/A+A/548/A52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The amount of H_2_ present in spiral galaxies remains uncertain, particularly in the dim outer regions and in low-metallicity environments. We present high-resolution CO(1-0) observations with the Plateau de Bure interferometer of the most distant molecular cloud in the local group galaxy M33. The cloud is a single entity rather than a set of smaller clouds within the broad beam of the original single-dish observations. The interferometer and single-dish fluxes are very similar and the line widths are indistinguishable, despite the difference in beamsize.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A125
- Title:
- Molecular cloud in Corona Australis
- Short Name:
- J/A+A/615/A125
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalogue of prestellar and starless cores within the Corona Australis molecular cloud using photometric data from the Herschel Space Observatory. At a distance of d~130pc, Corona Australis is one of the closest star-forming regions. Herschel has taken multi-wavelength data of Corona Australis with both the Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al. 2010A&A...518L...3G) and the Photodetector Array Camera and Spectrometer (PACS; Poglitsch et al. 2010A&A...518L...2P) photometric cameras in a parallel mode with wavelengths in the range 70um to 500um. A complete sample of starless and prestellar cores and embedded protostars is identified. Other results from the Herschel Gould Belt Survey have shown spatial correlation between the distribution of dense cores and the filamentary structure within the molecular clouds. We go further and show correlations between the properties of these cores and their spatial distribution within the clouds, with a particular focus on the mass distribution of the dense cores with respect to their filamentary proximity. We find that only lower-mass starless cores form away from filaments, while all of the higher-mass prestellar cores form in close proximity to or directly on the filamentary structure. This result supports the paradigm that prestellar cores mostly form on filaments. We analyse the mass distribution across the molecular cloud, finding evidence that the region around the Coronet appears to be at a more dynamically advanced evolutionary stage in comparison to the rest of the clumps within the cloud.
- ID:
- ivo://CDS.VizieR/J/ApJS/182/131
- Title:
- Molecular clouds and clumps in the GRS
- Short Name:
- J/ApJS/182/131
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS) of ^13^CO J=1->0 emission covers Galactic longitudes 18<l<55.7{deg} and Galactic latitudes |b|<=1{deg}. Using the SEQUOIA array on the FCRAO 14m telescope, the GRS fully sampled the ^13^CO Galactic emission (46" angular resolution on a 22" grid) and achieved a spectral resolution of 0.21km/s. Because the GRS uses ^13^CO, an optically thin tracer, rather than ^12^CO, an optically thick tracer, the GRS allows a much better determination of column density and also a cleaner separation of velocity components along a line of sight. With this homogeneous, fully sampled survey of ^13^CO emission, we have identified 829 molecular clouds and 6124 clumps throughout the inner Galaxy using the CLUMPFIND algorithm. Here we present details of the catalog and a preliminary analysis of the properties of the molecular clouds and their clumps. Moreover, we compare clouds inside and outside of the 5kpc ring and find that clouds within the ring typically have warmer temperatures, higher column densities, larger areas, and more clumps compared with clouds located outside the ring. This is expected if these clouds are actively forming stars. This catalog provides a useful tool for the study of molecular clouds and their embedded young stellar objects.
