Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium
There is a robust polyatomic chemistry in diffuse, partially molecular interstellar gas that is readily accessible in absorption at radio/millimeter/submillimeter wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known...
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| Format: | Article |
| Language: | English |
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IOP Publishing
2023-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/acae83 |
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| author | Harvey Liszt Maryvonne Gerin |
| author_facet | Harvey Liszt Maryvonne Gerin |
| author_sort | Harvey Liszt |
| collection | DOAJ |
| description | There is a robust polyatomic chemistry in diffuse, partially molecular interstellar gas that is readily accessible in absorption at radio/millimeter/submillimeter wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H _2 . Here we consider the use of proxies for hydrogen column densities N (H _2 ) and N (H) = N (H i )+2 N (H _2 ) based on measurements of HCO ^+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing H i , H _2 and CO toward stars and active galactic nuclei. We consider the use of HCO ^+ as a proxy for H _2 and show that the assumption of a relative abundance N (H _2 ) = N (HCO ^+ )/3 × 10 ^−9 gives the same view of the atomic–molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N (HCO ^+ ) ≳ 6 × 10 ^11 cm ^−2 or N (H _2 ) ≳ 2 × 10 ^20 cm ^−2 . Wide variations in the integrated CO J = 1 − 0 brightness W _CO and N (CO)/ N (H _2 ) imply equivalent variations in the CO-H _2 conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N (H)/ E ( B − V ) = 6.2 × 10 ^21 H cm ^−2 (mag) ^−1 overall or 6.8 × 10 ^21 H cm ^−2 (mag) ^−1 for sightlines at E ( B − V ) ≤ 0.08 mag lacking H _2 are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3 × 10 ^21 H cm ^−2 (mag) ^−1 . |
| first_indexed | 2024-03-12T03:38:20Z |
| format | Article |
| id | doaj.art-f991e8d6f6014b91b18d33ee12e4e580 |
| institution | Directory Open Access Journal |
| issn | 1538-4357 |
| language | English |
| last_indexed | 2024-03-12T03:38:20Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj.art-f991e8d6f6014b91b18d33ee12e4e5802023-09-03T13:07:59ZengIOP PublishingThe Astrophysical Journal1538-43572023-01-01943217210.3847/1538-4357/acae83Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar MediumHarvey Liszt0https://orcid.org/0000-0002-6116-1911Maryvonne Gerin1National Radio Astronomy Observatory, 520 Edgemont Road , Charlottesville, VA 22903, USA ; hliszt@nrao.eduLERMA, Observatoire de Paris, PSL Research University , CNRS, Sorbonne Université, France ; maryvonne.gerin@observatoiredeparis.psl.euThere is a robust polyatomic chemistry in diffuse, partially molecular interstellar gas that is readily accessible in absorption at radio/millimeter/submillimeter wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H _2 . Here we consider the use of proxies for hydrogen column densities N (H _2 ) and N (H) = N (H i )+2 N (H _2 ) based on measurements of HCO ^+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing H i , H _2 and CO toward stars and active galactic nuclei. We consider the use of HCO ^+ as a proxy for H _2 and show that the assumption of a relative abundance N (H _2 ) = N (HCO ^+ )/3 × 10 ^−9 gives the same view of the atomic–molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N (HCO ^+ ) ≳ 6 × 10 ^11 cm ^−2 or N (H _2 ) ≳ 2 × 10 ^20 cm ^−2 . Wide variations in the integrated CO J = 1 − 0 brightness W _CO and N (CO)/ N (H _2 ) imply equivalent variations in the CO-H _2 conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N (H)/ E ( B − V ) = 6.2 × 10 ^21 H cm ^−2 (mag) ^−1 overall or 6.8 × 10 ^21 H cm ^−2 (mag) ^−1 for sightlines at E ( B − V ) ≤ 0.08 mag lacking H _2 are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3 × 10 ^21 H cm ^−2 (mag) ^−1 .https://doi.org/10.3847/1538-4357/acae83Diffuse interstellar cloudsInterstellar atomic gasInterstellar molecules |
| spellingShingle | Harvey Liszt Maryvonne Gerin Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium The Astrophysical Journal Diffuse interstellar clouds Interstellar atomic gas Interstellar molecules |
| title | Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium |
| title_full | Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium |
| title_fullStr | Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium |
| title_full_unstemmed | Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium |
| title_short | Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium |
| title_sort | molecular hydrogen and its proxies hco and co in the diffuse interstellar medium |
| topic | Diffuse interstellar clouds Interstellar atomic gas Interstellar molecules |
| url | https://doi.org/10.3847/1538-4357/acae83 |
| work_keys_str_mv | AT harveyliszt molecularhydrogenanditsproxieshcoandcointhediffuseinterstellarmedium AT maryvonnegerin molecularhydrogenanditsproxieshcoandcointhediffuseinterstellarmedium |