KMS of Xinjiang Astronomical Observatory, CAS
| Physical properties of CO-dark molecular gas traced by C+ | |
| Tang, Ningyu1,2; Li, Di1,3; Heiles, Carl4; Wang, Shen1,2; Pan, Zhichen1,2; Wang, Jun-Jie1 | |
| 2016-09-01 | |
| Source Publication | Astronomy & Astrophysics
 |
| ISSN | 1432-0746 |
| Volume | 593Pages:A42 |
| Abstract | Context. Neither HI nor CO emission can reveal a significant quantity of so-called dark gas in the interstellar medium (ISM). It is considered that CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark gas. Determination of physical properties of DMG is critical for understanding ISM evolution. Previous studies of DMG in the Galactic plane are based on assumptions of excitation temperature and volume density. Independent measurements of temperature and volume density are necessary. Aims. We intend to characterize physical properties of DMG in the Galactic plane based on C+ data from the Herschel open time key program, namely Galactic Observations of Terahertz C + (GOT C +) and HI narrow self-absorption (HINSA) data from international HI 21 cm Galactic plane surveys. Methods. We identified DMG clouds with HINSA features by comparing HI, C+, and CO spectra. We derived the HI excitation temperature and HI column density through spectral analysis of HINSA features. The HI volume density was determined by utilizing the on-the-sky dimension of the cold foreground HI cloud under the assumption of axial symmetry. The column and volume density of H-2 were derived through excitation analysis of C+ emission. The derived parameters were then compared with a chemical evolutionary model. Results. We identified 36 DMG clouds with HINSA features. Based on uncertainty analysis, optical depth of HI tau(HI) of 1 is a reasonable value for most clouds. With the assumption of tau(HI) = 1, these clouds were characterized by excitation temperatures in a range of 20 K to 92 K with a median value of 55 K and volume densities in the range of 6.2 x 10(1) cm(-3) to 1.2 x 10(3) cm(-3) with a median value of 2.3 x 10(2) cm(-3). The fraction of DMG column density in the cloud (f(DMG)) decreases with increasing excitation temperature following an empirical relation f(DMG) = 2.1 x 10(-3)T(ex), (tau(HI) = 1) + 1.0. The relation between f(DMG) and total hydrogen column density N-H is given by f(DMG) = 1.0 - 3.7 x 10(20)/N-H. We divided the clouds into a high extinction group and low extinction group with the dividing threshold being total hydrogen column density N-H of 5.0 x 10(21) cm(-2) (A(V) = 2.7 mag). The values of f(DMG) in the low extinction group (A(V) <= 2.7 mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of similar to 10 Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group (A(V) > 2.7 mag). Compared to clouds in the low extinction group (A(V) <= 2.7 mag), clouds in the high extinction group (A(V) > 2.7 mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group (A(V) > 2.7 mag) are 6.6 x 10(2) times smaller than the canonical value in the Milky Way. Conclusions. The molecular gas seems to be the dominate component in these clouds. The high percentage of DMG in clouds of the high extinction group (A(V) > 2.7 mag) may support the idea that molecular clouds are forming from pre-existing molecular gas, i.e., a cold gas with a high H-2 content but that contains a little or no CO content. |
| Correspondent Email | nytang@nao.cas.cn ; dili@nao.cas.cn |
| Keyword | Ism: Clouds Evolution Ism: Molecules |
| Subtype | Article |
| DOI | 10.1051/0004-6361/201528055 |
| Indexed By | SCI ; EI |
| Language | 英语 |
| WOS Keyword | GALACTIC PLANE SURVEY ; KINEMATIC DISTANCE AMBIGUITY ; NARROW SELF-ABSORPTION ; 158 MU-M ; H-I ; STAR-FORMATION ; INTERSTELLAR CLOUDS ; RADIAL-DISTRIBUTION ; SOLAR NEIGHBORHOOD ; TRANSLUCENT CLOUDS |
| WOS Research Area | Astronomy & Astrophysics |
| WOS Subject | Astronomy & Astrophysics |
| WOS ID | WOS:000385820100036 |
| Project Number | 2012CB821802 ; 2015CB857100 ; 20130421 |
| Funding Organization | Technology under State Key Development Program for Basic Research (973 program) ; National Key Basic Research Program of China (973 Program) ; China Ministry of Science ; Guizhou Scientific Collaboration Program |
| Citation statistics | |
| Document Type | 期刊论文 |
| Identifier | http://ir.xao.ac.cn/handle/45760611-7/1607 |
| Collection | 110米口径全可动射电望远镜（QTT）_110米大口径全可动射电望远镜关键技术研究（973项目）_课题1：大样本射电天体的观测和前沿问题研究 |
| Corresponding Author | Tang, Ningyu; Li, Di |
| Affiliation | 1.National Astronomical Observatories, CAS, 100012 Beijing, PR China 2.University of Chinese Academy of Sciences, 100049 Beijing, PR China 3.Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China 4.Astronomy Department, University of California, Berkeley, CA 94720, USA |
| Recommended Citation GB/T 7714 | Tang, Ningyu,Li, Di,Heiles, Carl,et al. Physical properties of CO-dark molecular gas traced by C+[J]. Astronomy & Astrophysics,2016,593:A42. |
| APA | Tang, Ningyu,Li, Di,Heiles, Carl,Wang, Shen,Pan, Zhichen,&Wang, Jun-Jie.(2016).Physical properties of CO-dark molecular gas traced by C+.Astronomy & Astrophysics,593,A42. |
| MLA | Tang, Ningyu,et al."Physical properties of CO-dark molecular gas traced by C+".Astronomy & Astrophysics 593(2016):A42. |
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