Chemical Composition and Molecular Abundances of Molecular Clouds

1985 ◽  
pp. 23-43 ◽  
Author(s):  
M. Guelin
Keyword(s):  
Chemical Composition ◽  
Molecular Clouds ◽  
Molecular Abundances

scholarly journals Mapping the 13CO/C18O abundance ratio in the massive star-forming region G29.96−0.02

2018 ◽  
Vol 617 ◽  
pp. A14 ◽  
Author(s):  
S. Paron ◽  
M. B. Areal ◽  
M. E. Ortega
Keyword(s):  
Molecular Clouds ◽  
Abundance Ratio ◽  
High Mass ◽  
Local Thermal Equilibrium ◽  
Mass Star ◽  
Galactocentric Distance ◽  
Star Forming ◽  
Physical Conditions ◽  
The Galaxy ◽  
Molecular Abundances

Aims. Estimating molecular abundances ratios from directly measuring the emission of the molecules toward a variety of interstellar environments is indeed very useful to advance our understanding of the chemical evolution of the Galaxy, and hence of the physical processes related to the chemistry. It is necessary to increase the sample of molecular clouds, located at different distances, in which the behavior of molecular abundance ratios, such as the 13CO/C18O ratio, is studied in detail. Methods. We selected the well-studied high-mass star-forming region G29.96−0.02, located at a distance of about 6.2 kpc, which is an ideal laboratory to perform this type of study. To study the 13CO/C18O abundance ratio (X13∕18) toward this region, we used 12CO J = 3–2 data obtained from the CO High-Resolution Survey, 13CO and C18O J = 3–2 data from the 13CO/C18O (J = 3–2) Heterodyne Inner Milky Way Plane Survey, and 13CO and C18O J = 2–1 data retrieved from the CDS database that were observed with the IRAM 30 m telescope. The distribution of column densities and X13∕18 throughout the extension of the analyzed molecular cloud was studied based on local thermal equilibrium (LTE) and non-LTE methods. Results. Values of X13∕18 between 1.5 and 10.5, with an average of about 5, were found throughout the studied region, showing that in addition to the dependency of X13∕18 and the galactocentric distance, the local physical conditions may strongly affect this abundance ratio. We found that correlating the X13∕18 map with the location of the ionized gas and dark clouds allows us to suggest in which regions the far-UV radiation stalls in dense gaseous components, and in which regions it escapes and selectively photodissociates the C18O isotope. The non-LTE analysis shows that the molecular gas has very different physical conditions, not only spatially throughout the cloud, but also along the line of sight. This type of study may represent a tool for indirectly estimating (from molecular line observations) the degree of photodissociation in molecular clouds, which is indeed useful to study the chemistry in the interstellar medium.

scholarly journals A sensitive λ 3 mm line survey of L483

2019 ◽  
Vol 625 ◽  
pp. A147 ◽  
Author(s):  
M. Agúndez ◽  
N. Marcelino ◽  
J. Cernicharo ◽  
E. Roueff ◽  
M. Tafalla
Keyword(s):  
Chemical Composition ◽  
Chemical Characterization ◽  
Isotopic Ratios ◽  
Dark Cloud ◽  
Carbon Chains ◽  
Prestellar Cores ◽  
Low Mass ◽  
Line Survey ◽  
Molecular Abundances

An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had their molecular composition characterized in detail. Here we present a λ 3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30 m telescope in the 80–116 GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines (FWHM ~ 0.5 km s−1) and low rotational temperatures (≲10 K). Of particular interest among the detected molecules are the cis isomer of HCOOH, the complex organic molecules HCOOCH3, CH3OCH3, and C2H5OH, a wide variety of carbon chains, nitrogen oxides like N2O, and saturated molecules like CH3SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH+, CNCN, NCO, H2NCO+, and NS+) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293−2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C3S, SO2/C2S, and CH3SH/C2S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving 34S and 33S), while there are also several isotopic anomalies like an extreme depletion in 13C for one of the two isotopologs of c-C3H2, a drastic enrichment in 18O for SO and HNCO (SO being also largely enriched in 17O), and different abundances for the two 13C substituted species of C2H and the two 15N substituted species of N2H+. We report the first detection in space of some minor isotopologs like c-C3D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.

Molecular abundances in the high-latitude molecular clouds

1988 ◽  
Vol 326 ◽  
pp. 909 ◽  
Author(s):  
Loris Magnani ◽  
Leo Blitz ◽  
Jan G. A. Wouterloot
Keyword(s):  
High Latitude ◽  
Molecular Clouds ◽  
Molecular Abundances

Chemical Abundances and Isotope Ratios in Molecular Clouds

1996 ◽  
Vol 13 (2) ◽  
pp. 202-203
Author(s):  
M. R. Hunt
Keyword(s):  
Observational Data ◽  
Molecular Clouds ◽  
Isotope Ratios ◽  
Magellanic Clouds ◽  
Chemical Abundances ◽  
Millimetre Wave ◽  
The Galaxy ◽  
Molecular Abundances

AbstractA program to observe millimetre-wave molecular transitions in a number of southern-sky molecular clouds is under way. Molecular clouds in both the Galaxy and the Magellanic Clouds are included in the sample. The aim of the program is to build a body of observational data which can be used to derive molecular abundances in southern-sky molecular clouds.

scholarly journals Reproducing the CO-to-H2 conversion factor in cosmological simulations of Milky-Way-mass galaxies

2020 ◽  
Vol 499 (1) ◽  
pp. 837-850
Author(s):  
Laura C Keating ◽  
Alexander J Richings ◽  
Norman Murray ◽  
Claude-André Faucher-Giguère ◽  
Philip F Hopkins ◽  
...  
Keyword(s):  
Molecular Clouds ◽  
Milky Way ◽  
Conversion Factor ◽  
Star Formation Rate ◽  
Formation Rate ◽  
Giant Molecular Clouds ◽  
Post Processing ◽  
Subgrid Model ◽  
Local Sources ◽  
Molecular Abundances

ABSTRACT We present models of CO(1–0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1–0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H2 abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ∼3 pc in cold gas (T < 300 K) for both CO and H2, is able to reproduce both the observed CO(1–0) luminosity and inferred CO-to-H2 conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds.

First molecular cloud measurement with Irbene RT-32 radio telescope

2020 ◽  
pp. 39-44
Author(s):  
A ABERFELDS ◽  
A VASYUNIN
Keyword(s):  
Radio Emission ◽  
Radio Astronomy ◽  
Molecular Clouds ◽  
Massive Stars ◽  
Young Star ◽  
Point Of View ◽  
Star Forming ◽  
Emission Point ◽  
Chemical Evaluation ◽  
Molecular Abundances

This paper describes the efforts of the Ventspils International Radio Astronomy Center (VSRC) astrochemist and observation groups to study the formation of massive stars from chemical evaluation and radio emission point of view. By observing all four selected sources chemists group observations can provide important feedback to models, mainly an information for molecules with maser emission. Based on detection of masers in young stellar object (YSO) observations provide information that there are parts of molecular clouds where gas density and molecular abundances are higher by a few orders than in typical young star forming clouds.

scholarly journals Molecules in the circumnuclear disk of the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 78-82
Author(s):  
Nanase Harada ◽  
Denise Riquelme ◽  
Serena Viti ◽  
Karl Menten ◽  
Miguel Requena-Torres ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Shock Waves ◽  
Molecular Clouds ◽  
Galactic Center ◽  
Cosmic Ray ◽  
Central Star ◽  
Ionization Rate ◽  
Data Sets ◽  
X Rays ◽  
Circumnuclear Disk

AbstractWithin a few parsecs around the central black hole A*, chemistry in the dense molecular cloud material of the circumnuclear disk (CND) can be affected by many energetic phenomena such as high UV-flux from the massive central star cluster, X-rays from A*, shock waves, and an enhanced cosmic-ray flux. Recently, spectroscopic surveys with the IRAM 30 meter and the APEX 12 meter telescopes of substantial parts of the 80–500 GHz frequency range were made toward selected positions in and near the CND. These data sets contain lines from the molecules HCN, HCO+, HNC, CS, SO, SiO, CN, H2CO, HC3N, N2H+, H3O+ and others. We conduct Large Velocity Gradient analyses to obtain column densities and total hydrogen densities, n, for each species in molecular clouds located in the southwest lobe of the CND. The data for the above mentioned molecules indicate 105 cm−3 ≲ n < 106 cm−3, which shows that the CND is tidally unstable. The derived chemical composition is compared with a chemical model calculated using the UCL_CHEM code that includes gas and grain reactions, and the effects of shock waves. Models are run for varying shock velocities, cosmic-ray ionization rates, and number densities. The resulting chemical composition is fitted best to an extremely high value of cosmic-ray ionization rate ζ ∼ 10−14 s−1, 3 orders of magnitude higher than the value in regular Galactic molecular clouds, if the pre-shock density is n=105 cm−3.

scholarly journals The Submillimeter Wave Astronomy Satellite

1990 ◽  
Vol 123 ◽  
pp. 251-251 ◽  
Author(s):  
G.J. Melnick
Keyword(s):  
Fine Structure ◽  
Chemical Composition ◽  
Ground State ◽  
Molecular Clouds ◽  
Large Scale ◽  
Rotational Transition ◽  
Submillimeter Wave ◽  
Important Species ◽  
Structure Transition ◽  
Large Scale Survey

AbstractThe Submillimeter Wave Astronomy Satellite (SWAS) is a NASA Small-Explorer Class experiment whose objective is to study both the chemical composition and the thermal balance in dense (NH2 > 103 cm−3) molecular clouds and, by observing many clouds throughout our galaxy, relate these conditions to the processes of star formation. To conduct this study SWAS will be capable of carrying out both pointed and scanning observations simultaneously in the lines of four important species: (1) the H2O (110–101) 556.963 GHz ground-state ortho transition, (2) the O2 (3,3–1,2) 487.249 GHz transition, (3) the CI (3P1 – 3P0) 492.162 GHz ground-state fine structure transition, and (4) the 13CO (J = 5–4) 550.926 GHz rotational transition. These atoms and molecules are predicted to be among the most abundant within molecular clouds and, because they possess low-lying transitions with energy differences (ΔE/k) between 15 and 30K (temperatures typical of many molecular clouds), these species are believed to be dominant coolants of the gas as it collapses to form stars and planets. A large-scale survey in these lines is virtually impossible from any platform within the atmosphere due to telluric absorption.

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