An Ultraviolet Study of CO Chemistry in the Magellanic Clouds
How does molecular cloud chemistry change with metallicity? In this work, we study the relation between molecular hydrogen (H _2 ) and carbon monoxide (CO) at 1/2 and 1/5 solar metallicity using ultraviolet absorption spectroscopy obtained as part of the UV Legacy Library of Young Stars as Essential...
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IOP Publishing
2025-01-01
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| Series: | The Astronomical Journal |
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| Online Access: | https://doi.org/10.3847/1538-3881/adf33a |
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| author | Kirill Tchernyshyov Jessica K. Werk Julia Roman-Duval |
| author_facet | Kirill Tchernyshyov Jessica K. Werk Julia Roman-Duval |
| author_sort | Kirill Tchernyshyov |
| collection | DOAJ |
| description | How does molecular cloud chemistry change with metallicity? In this work, we study the relation between molecular hydrogen (H _2 ) and carbon monoxide (CO) at 1/2 and 1/5 solar metallicity using ultraviolet absorption spectroscopy obtained as part of the UV Legacy Library of Young Stars as Essential Standards Hubble Space Telescope program. We determine CO column densities or upper limits for a sample of 50 lines of sight through the Large and Small Magellanic Clouds (LMC and SMC). ^12 CO is detected along eight lines of sight and ^13 CO is detected along two. Combining our new CO column densities with ${N}_{{{\rm{H}}}_{2}}$ measurements from the literature, we find that the evolution of ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ from the Milky Way to the LMC and SMC is a relatively shallow function of metallicity. Taking N _CO > 3 × 10 ^15 cm ^−2 as a threshold value above which CO emission is likely to be detectable at the distance of the Magellanic Clouds, the ${{\rm{log}}}_{10}\,{N}_{{{\rm{H}}}_{2}}$ at which a sightline has a 50% probability of having N _CO above this threshold is 20.8 in the Milky Way, 20.9 in the LMC, and 21.1 in the SMC. This is an 0.3 dex change in threshold ${{\rm{log}}}_{10}{N}_{{{\rm{H}}}_{2}}$ over an 0.7 dex change in metallicity. We compare our measurements with ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ relations from literature chemical models and find that the measured relations agree best with models in which the dynamical timescale is longer than the chemical timescale for H _2 but shorter than the chemical timescale for CO. |
| format | Article |
| id | doaj-art-1ce0f87c75954ea69ddd5a5cfa2ffefe |
| institution | Kabale University |
| issn | 1538-3881 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astronomical Journal |
| spelling | doaj-art-1ce0f87c75954ea69ddd5a5cfa2ffefe2025-08-22T12:35:34ZengIOP PublishingThe Astronomical Journal1538-38812025-01-01170317610.3847/1538-3881/adf33aAn Ultraviolet Study of CO Chemistry in the Magellanic CloudsKirill Tchernyshyov0https://orcid.org/0000-0003-0789-9939Jessica K. Werk1https://orcid.org/0000-0002-0355-0134Julia Roman-Duval2https://orcid.org/0000-0001-6326-7069Department of Astronomy, University of Washington , Seattle, WA, USA ; ktcherny@gmail.comDepartment of Astronomy, University of Washington , Seattle, WA, USA ; ktcherny@gmail.comSpace Telescope Science Institute , Baltimore, MD, USAHow does molecular cloud chemistry change with metallicity? In this work, we study the relation between molecular hydrogen (H _2 ) and carbon monoxide (CO) at 1/2 and 1/5 solar metallicity using ultraviolet absorption spectroscopy obtained as part of the UV Legacy Library of Young Stars as Essential Standards Hubble Space Telescope program. We determine CO column densities or upper limits for a sample of 50 lines of sight through the Large and Small Magellanic Clouds (LMC and SMC). ^12 CO is detected along eight lines of sight and ^13 CO is detected along two. Combining our new CO column densities with ${N}_{{{\rm{H}}}_{2}}$ measurements from the literature, we find that the evolution of ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ from the Milky Way to the LMC and SMC is a relatively shallow function of metallicity. Taking N _CO > 3 × 10 ^15 cm ^−2 as a threshold value above which CO emission is likely to be detectable at the distance of the Magellanic Clouds, the ${{\rm{log}}}_{10}\,{N}_{{{\rm{H}}}_{2}}$ at which a sightline has a 50% probability of having N _CO above this threshold is 20.8 in the Milky Way, 20.9 in the LMC, and 21.1 in the SMC. This is an 0.3 dex change in threshold ${{\rm{log}}}_{10}{N}_{{{\rm{H}}}_{2}}$ over an 0.7 dex change in metallicity. We compare our measurements with ${N}_{{\rm{CO}}}\,({N}_{{{\rm{H}}}_{2}})$ relations from literature chemical models and find that the measured relations agree best with models in which the dynamical timescale is longer than the chemical timescale for H _2 but shorter than the chemical timescale for CO.https://doi.org/10.3847/1538-3881/adf33aInterstellar absorptionInterstellar mediumInterstellar moleculesUltraviolet spectroscopy |
| spellingShingle | Kirill Tchernyshyov Jessica K. Werk Julia Roman-Duval An Ultraviolet Study of CO Chemistry in the Magellanic Clouds The Astronomical Journal Interstellar absorption Interstellar medium Interstellar molecules Ultraviolet spectroscopy |
| title | An Ultraviolet Study of CO Chemistry in the Magellanic Clouds |
| title_full | An Ultraviolet Study of CO Chemistry in the Magellanic Clouds |
| title_fullStr | An Ultraviolet Study of CO Chemistry in the Magellanic Clouds |
| title_full_unstemmed | An Ultraviolet Study of CO Chemistry in the Magellanic Clouds |
| title_short | An Ultraviolet Study of CO Chemistry in the Magellanic Clouds |
| title_sort | ultraviolet study of co chemistry in the magellanic clouds |
| topic | Interstellar absorption Interstellar medium Interstellar molecules Ultraviolet spectroscopy |
| url | https://doi.org/10.3847/1538-3881/adf33a |
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