Issue |
A&A
Volume 679, November 2023
|
|
---|---|---|
Article Number | A112 | |
Number of page(s) | 36 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202346136 | |
Published online | 29 November 2023 |
Characterizing the line emission from molecular clouds
II. A comparative study of California, Perseus, and Orion A★
1
Observatorio Astronomico Nacional (IGN),
Alfonso XII 3,
28014
Madrid, Spain
e-mail: m.tafalla@oan.es; a.usero@oan.es
2
Department of Astrophysics, University of Vienna,
Türkenschanzstrasse 17,
1180
Vienna, Austria
e-mail: alvaro.hacar@univie.ac.at
Received:
13
February
2023
Accepted:
13
September
2023
Aims. We aim to characterize and compare the molecular-line emission of three clouds whose star-formation rates span one order of magnitude: California, Perseus, and Orion A.
Methods. We used stratified random sampling to select positions representing the different column density regimes of each cloud and observed them with the IRAM 30 m telescope. We covered the 3 mm wavelength band and focused our analysis on CO, HCN, CS, HCO+, HNC, and N2H+.
Results. We find that the line intensities depend most strongly on the H2 column density, with which they are tightly correlated. A secondary effect, especially visible in Orion A, is a dependence of the line intensities on the gas temperature. We explored a method that corrects for temperature variations and show that, when it is applied, the emission from the three clouds behaves very similarly. CO intensities vary weakly with column density, while the intensity of traditional dense-gas tracers such as HCN, CS, and HCO+ varies almost linearly with column density. N2H+ differs from all other species in that it traces only cold dense gas. The intensity of the rare HCN and CS isotopologs reveals additional temperature-dependent abundance variations. Overall, the clouds have similar chemical compositions that, as the depth increases, are sequentially dominated by photodissociation, gas-phase reactions, molecular freeze-out, and stellar feedback in the densest parts of Orion A. Our observations also allowed us to calculate line luminosities for each cloud, and a comparison with literature values shows good agreement. We used our HCN(1–0) data to explore the behavior of the HCN conversion factor, finding that it is dominated by the emission from the outermost cloud layers. It also depends strongly on the gas kinetic temperature. Finally, we show that the HCN/CO ratio provides a gas volume density estimate, and that its correlation with the column density resembles that found in extragalactic observations.
Key words: ISM: abundances / ISM: molecules / ISM: individual objects: California / ISM: individual objects: Persus / ISM: individual objects: Orion A / ISM: structure
Full Tables B.1–B.2 are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/679/A112
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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