Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds
The cosmic-ray ionization rate (CRIR, ζ _2 ) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as ${{\rm{H}}}_{3}^{+}$ , OH ^+ , and H _2 O ^+ , are hard to detect and require th...
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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/acbf34 |
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| author | Gan Luo Zhi-Yu Zhang Thomas G. Bisbas Di Li Ping Zhou Ningyu Tang Junzhi Wang Pei Zuo Nannan Yue |
| author_facet | Gan Luo Zhi-Yu Zhang Thomas G. Bisbas Di Li Ping Zhou Ningyu Tang Junzhi Wang Pei Zuo Nannan Yue |
| author_sort | Gan Luo |
| collection | DOAJ |
| description | The cosmic-ray ionization rate (CRIR, ζ _2 ) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as ${{\rm{H}}}_{3}^{+}$ , OH ^+ , and H _2 O ^+ , are hard to detect and require the presence of background massive stars for absorption measurements. In this work, we propose an alternative method to infer the CRIR in diffuse clouds using the abundance ratios of OH/CO and HCO ^+ /CO. We have analyzed the response of chemical abundances of CO, OH, and HCO ^+ on various environmental parameters of the interstellar medium in diffuse clouds and found that their abundances are proportional to ζ _2 . Our analytic expressions give an excellent calculation of the abundance of OH for ζ _2 ≤ 10 ^−15 s ^−1 , which are potentially useful for modeling chemistry in hydrodynamical simulations. The abundances of OH and HCO ^+ were found to monotonically decrease with increasing density, while the CO abundance shows the opposite trend. With high-sensitivity absorption transitions of both CO (1–0) and (2–1) lines from Atacama Large Millimeter/submillimeter Array, we have derived the H _2 number densities ( ${n}_{{{\rm{H}}}_{2}}$ ) toward 4 lines of sight; assuming a kinetic temperature of T _k = 50 K, we find a range of (0.14 ± 0.03–1.2 ± 0.1) × 10 ^2 cm ^−3 . By comparing the observed and modeled HCO ^+ /CO ratios, we find that ζ _2 in our diffuse gas sample is in the range of ${1.0}_{-1.0}^{+14.8}$ × 10 ${}^{-16}\mbox{--}{2.5}_{-2.4}^{+1.4}$ × 10 ^−15 s ^−1 . This is ∼2 times higher than the average value measured at higher extinction, supporting an attenuation of CRs as suggested by theoretical models. |
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| issn | 1538-4357 |
| language | English |
| last_indexed | 2024-03-12T04:35:16Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj.art-e2f1fe42a7304b66b5fed950ce2a2b802023-09-03T09:56:23ZengIOP PublishingThe Astrophysical Journal1538-43572023-01-0194629110.3847/1538-4357/acbf34Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse CloudsGan Luo0https://orcid.org/0000-0002-1583-8514Zhi-Yu Zhang1https://orcid.org/0000-0002-7299-2876Thomas G. Bisbas2https://orcid.org/0000-0003-2733-4580Di Li3https://orcid.org/0000-0003-3010-7661Ping Zhou4https://orcid.org/0000-0002-5683-822XNingyu Tang5https://orcid.org/0000-0002-2169-0472Junzhi Wang6https://orcid.org/0000-0001-6106-1171Pei Zuo7https://orcid.org/0000-0003-3948-9192Nannan Yue8https://orcid.org/0000-0003-0355-6875School of Astronomy and Space Science, Nanjing University , Nanjing 210093, People's Republic of China ; luogan@nju.edu.cn; Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) , Ministry of Education, Nanjing 210093, People's Republic of ChinaSchool of Astronomy and Space Science, Nanjing University , Nanjing 210093, People's Republic of China ; luogan@nju.edu.cn; Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) , Ministry of Education, Nanjing 210093, People's Republic of ChinaResearch Center for Intelligent Computing Platforms , Zhejiang Laboratory, Hangzhou 311100, People's Republic of China ; tbisbas@zhejianglab.comResearch Center for Intelligent Computing Platforms , Zhejiang Laboratory, Hangzhou 311100, People's Republic of China ; tbisbas@zhejianglab.com; CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences , Beijing 100101, People's Republic of China; NAOC-UKZN Computational Astrophysics Centre, University of KwaZulu-Natal , Durban 4000, South AfricaSchool of Astronomy and Space Science, Nanjing University , Nanjing 210093, People's Republic of China ; luogan@nju.edu.cn; Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University) , Ministry of Education, Nanjing 210093, People's Republic of ChinaDepartment of Physics, Anhui Normal University , Wuhu, Anhui 241002, People's Republic of ChinaSchool of Physical Science and Technology, Guangxi University , Nanning 530004, People's Republic of ChinaKavli Institute for Astronomy and Astrophysics, Peking University , Beijing, 5 Yiheyuan Road, Haidian District, Beijing 100871, People's Republic of China; International Centre for Radio Astronomy Research (ICRAR), University of Western Australia , Crawley, WA 6009, AustraliaKavli Institute for Astronomy and Astrophysics, Peking University , Beijing, 5 Yiheyuan Road, Haidian District, Beijing 100871, People's Republic of ChinaThe cosmic-ray ionization rate (CRIR, ζ _2 ) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as ${{\rm{H}}}_{3}^{+}$ , OH ^+ , and H _2 O ^+ , are hard to detect and require the presence of background massive stars for absorption measurements. In this work, we propose an alternative method to infer the CRIR in diffuse clouds using the abundance ratios of OH/CO and HCO ^+ /CO. We have analyzed the response of chemical abundances of CO, OH, and HCO ^+ on various environmental parameters of the interstellar medium in diffuse clouds and found that their abundances are proportional to ζ _2 . Our analytic expressions give an excellent calculation of the abundance of OH for ζ _2 ≤ 10 ^−15 s ^−1 , which are potentially useful for modeling chemistry in hydrodynamical simulations. The abundances of OH and HCO ^+ were found to monotonically decrease with increasing density, while the CO abundance shows the opposite trend. With high-sensitivity absorption transitions of both CO (1–0) and (2–1) lines from Atacama Large Millimeter/submillimeter Array, we have derived the H _2 number densities ( ${n}_{{{\rm{H}}}_{2}}$ ) toward 4 lines of sight; assuming a kinetic temperature of T _k = 50 K, we find a range of (0.14 ± 0.03–1.2 ± 0.1) × 10 ^2 cm ^−3 . By comparing the observed and modeled HCO ^+ /CO ratios, we find that ζ _2 in our diffuse gas sample is in the range of ${1.0}_{-1.0}^{+14.8}$ × 10 ${}^{-16}\mbox{--}{2.5}_{-2.4}^{+1.4}$ × 10 ^−15 s ^−1 . This is ∼2 times higher than the average value measured at higher extinction, supporting an attenuation of CRs as suggested by theoretical models.https://doi.org/10.3847/1538-4357/acbf34Interstellar absorptionInterstellar mediumInterstellar moleculesInterstellar abundancesMolecular cloudsChemical abundances |
| spellingShingle | Gan Luo Zhi-Yu Zhang Thomas G. Bisbas Di Li Ping Zhou Ningyu Tang Junzhi Wang Pei Zuo Nannan Yue Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds The Astrophysical Journal Interstellar absorption Interstellar medium Interstellar molecules Interstellar abundances Molecular clouds Chemical abundances |
| title | Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds |
| title_full | Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds |
| title_fullStr | Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds |
| title_full_unstemmed | Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds |
| title_short | Abundance Ratios of OH/CO and HCO+/CO as Probes of the Cosmic-Ray Ionization Rate in Diffuse Clouds |
| title_sort | abundance ratios of oh co and hco co as probes of the cosmic ray ionization rate in diffuse clouds |
| topic | Interstellar absorption Interstellar medium Interstellar molecules Interstellar abundances Molecular clouds Chemical abundances |
| url | https://doi.org/10.3847/1538-4357/acbf34 |
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