| Citation: |
Jian-jun Zhou, Run-xia Li, Donghui Quan, Jarken Esimbek, Yu-xin He, Da-lei Li, Xin-di Tang, Gang Wu, Wei-guang Ji, Zheng-xue Chang, Xia Zhang. Effect of Optical Depth on Study of Chemical Properties of Massive Star Forming Clumps[J]. Chinese Journal of Chemical Physics , 2020, 33(1): 114-118. DOI: 10.1063/1674-0068/cjcp1911212
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Past observations have shown that CN, HCN, HNC, N2H+ and HCO+ could be used to study the chemical evolution of massive star forming clumps [1-3]. However, these studies did not always yield fully consistent results. Sanhueza et al. [2] suggested that N2H+/HCO+ abundance ratio could serve as a chemical clock for massive star formation, whereas Hoq et al. [1] found that the N2H+/HCO+ abundance ratio shows no discernable trend from quiescent to protostellar, or to HII/PDR stage.
Because these previous studies usually use the molecular data from single point observations, their results may be affected by the distance. The massive stars are believed to be formed in clusters. Newly formed OB stars may affect the chemical properties of nearby star forming clumps. In addition, the chemical properties of star forming regions may have obvious spatial variations [4, 5].
We studied the global chemical properties of massive star forming clumps at different stages using CN(1-0), HCN(1-0), HNC(1-0), HCO+(1-0) and N2H+(1-0) lines, and obtained improved results [6, 7]. Abundances of these four molecules are mainly dominated by H2 column density and thus they cannot be used to trace the evolution of massive star forming clumps [8]. Further studies are necessary to find more appropriate tracers.
Gerner et al. studied the chemical evolution of 59 massive star forming clumps by observing N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) with the IRAM 30m millimeter telescope [9]. H13CO+(1-0) and HN13C(1-0) lines are considered to be optically thin [2], and high sensitivity also improves N2H+(1-0) and HCN(1-0) data. Here we study the data from Gerner et al. [9].
Gerner et al. [9] observed N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) toward 59 massive star forming clumps with the IRAM 30m millimeter telescope. The corresponding beam size is 29 arcsecond, the sensitivity is ∼0.03 K, and the spectral resolution is ∼0.6 km/s. The sample of Gerner et al. [9] includes 19 IRDCs (infrared dark clouds) and 20 HMPOs (high-mass protostellar objects) as well as 11 HMCs (hot molecular cores) and 9 UCHIIs (ultra-compact regions). Following Zhang et al. [7], we classify IRDCs as stage A, and classify HMPO, HMC and UCHII clumps as stage B. We obtained the integrated intensities of N2H+, H13CO+, HCN and HN13C from Table A.1 of Gerner et al. [9], and plotted the histogram of the integral intensity and the ratios of four kinds of molecules in FIG. 1 and FIG. 2. We obtained the column densities of N2H+, H13CO+, HCN and HN13C from Table A.3 of Gerner et al. [9], which were derived with several assumptions such as local thermodynamic equilibrium, optical thinness and the initially chosen typical temperatures (see Section 4.3 of Gerner et al. [9]). We estimated their abundances with the beam averaged H2 column densities which were also listed in their Table A.3, which were derived with several assumptions such as local thermodynamic equilibrium, optical. The H2 column densities were derived from their radio fluxes at 850 μm, or 870 μm, or 1.2 mm. We plotted the histograms of abundances of these four molecules for stages A and B in FIG. 3, and abundance ratios between them in FIG. 4. We plotted the abundances of N2H+, H13CO+, HCN and HN13C versus H2 column densities to study the relationship between molecular abundance and H2 column density. Whether observing optically thin molecular lines with high angular resolution is necessary to study the chemical evolution of massive star forming clumps.
The sample of Gerner et al. [9] includes 19 IRDCs (infrared dark clouds) and 20 HMPOs (high-mass protostellar objects) as well as 11 HMCs (hot molecular cores) and 9 UCHIIs (ultra-compact regions). IRDCs are the objects reaching densities of 104 cm−3 and thus become detectable as cold dense molecular clouds. HMPOs host actively accreting protostars with mass ≥8M, which show internal emission sources at mid-infrared wavelengths. In the HMC stage, the central sources heat the surrounding environments, evaporating molecular-rich ices and raising the molecular complexity in the gas. Finally, the UV-radiation from the embedded protostars ionizes the surrounding gas and ultra-compact HII regions are formed (UCHII stage). Gerner et al. [9] observed N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) toward these 59 sources with the IRAM 30 m millimeter telescope. The corresponding beam size is 29 arcsecond, the sensitivity is ∼ 0.03 K, and the spectral resolution is ∼0.6 km/s.
We obtained the integrated intensities of N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) from Table A.1 of Gerner et al. [9]. Following Zhang et al. [7], we classify IRDCs as stage A, and classify HMPO, HMC and UCHII clumps as stage B. We found that all integrated intensities of N2H+(1-0), H13CO+(1-0), HCN(1-0) and HN13C(1-0) show an increasing trend from stage A to stage B (see FIG. 1). This is similar to the conclusion of Zhang et al. [7]. We noted that the median values of the integrated intensities of N2H+, H13CO+, and HN13C for stage B are nearly twice of the corresponding values of stage A. The median value of the integrated intensity of HCN for stage B is nearly four times of that at stage A. This is probably because that the integrated intensities of Gerner et al. [9] come from single point observations, which could better trace the chemical properties of occurring star formation. Meanwhile, the integrated intensities of Zhang et al. [7] come from mapping observation, which is the globally averaged values of the whole star forming clumps. It should be noted that such differences may also be caused by different excitation temperatures used for estimating column densities of these four molecules [9].
We plotted the histograms of integrated intensity ratios between these four molecules for stages A and B in FIG. 2. The integrated intensity ratios of N2H+/H13CO+, N2H+/HCN, N2H+/H13CO+, N2H+/HN13C, and H13CO+/HN13C show similar trends of variations from stage A to B to those in Zhang et al. [7].
We obtained the column densities of N2H+, H13CO+, HCN and HN13C from Table A.3 of Gerner et al. [9], which were derived with several assumptions such as local thermodynamic equilibrium, optical thinness and the initially chosen typical temperatures (see Section 4.3 of Gerner et al. [9]). We estimated their abundances with the beam averaged H2 column densities which were also listed in their Table A.3. The H2 column densities were derived from their radio fluxes at 850 μm, or 870 μm, or 1.2 mm. We plotted the histograms of abundances of these four molecules for stages A and B in FIG. 3. The abundances of HCN, HN13C, H13CO+ and N2H+ display an increasing trend from stage A to stage B. This is similar to the result from Zhang et al. [7]. Median values of the abundances of HN13C, H13CO+ increase nearly 10 times from stage A to stage B. This suggests that these two molecules are more sensitive to star formation activities than the previous four molecules/ions, and therefore they are suitable for tracing the evolutionary stage of star formation. As we mentioned above, such difference may also be attributed to the different excitation temperatures used for estimating column densities of these four molecules [9].
We plotted the histograms of abundance ratios between these four molecules for stages A and B in FIG. 4. Abundance ratios of HCN/HN13C, H13CO+/HCN, H13CO+/HN13C show an increasing trend from stage A to stage B, while N2H+/H13CO+ ratio shows a decreasing trend from stage A to stage B. This is also similar to the conclusion of Zhang et al. [7]. However, abundance ratios of N2H+/HCN and N2H+/HN13C display decreasing trends which are contrary to the results of Zhang et al. [7]. The reason is that the data of Gerner et al. [9] were obtained by single point observations, which focused on where star formation is taking place, and is strongly associated with star formation activities. These results suggest that abundances of HCN and HN13C in star forming regions increase faster from stage A to stage B than that of N2H+. We further arrange the growth of abundances of these four molecules from stage A to stage B in the order of rapidity from the highest to the lowest to be H13CO+>HCN>HN13C>N2H+.
We plotted the abundances of N2H+, H13CO+, HCN and HN13C versus H2 column densities in FIG. 5. The abundance of HCN shows a clear decreasing trend with increasing H2 column density. This is similar to the result from Li et al. [8]. The abundances of H13CO+ and HN13C display a relatively weak decreasing trend with increasing H2 column density, so optically thinner molecules are barely affected by H2 column density. Abundance of N2H+ shows no obvious trend with the variation of H2 column density. This is consistent with the result of Li et al. [8]. On the other hand, abundance ratios of N2H+/H13CO+, N2H+/HCN, N2H+/HN13C, H13CO+/HCN, H13CO+/HNC and HCN/HN13C as a function of H2 column density show obvious variations (see FIG. 6). All these supports the idea that optically thinner lines could better trace the chemical properties of massive star forming clumps.
We present the chemical properties of massive star forming clumps derived from H13CO+(1-0), HN13C(1-0), N2H+(1-0) and HCN(1-0) data from Gerner et al. [9], compared with our previous results derived from N2H+(1-0), HCO+(1-0), HCN(1-0) and HNC(1-0) data [7, 8]. Our main conclusions are listed in the following. (i) Compared with our previous results, the integrated intensities of H13CO+(1-0), HN13C (1-0), N2H+(1-0) and HCN(1-0) and the ratios between them increase much faster from stage A to stage B. This may be because these molecular lines were observed by single point observations, and therefore they are strongly associated with star forming activities. (ii) For the same reason as the above, the increasing trend of the abundances of these four molecules from stage A to stage B, in the order of the fastest to lowest, is H13CO+, HCN, HN13C and N2H+. This is different from the conclusion from Zhang et al. [7]. (iii) The abundances of H13CO+ and HN13C display a weak decreasing trend with increasing H2 column density. We suggest these two molecules may not be very optically thin. Abundance ratios of N2H+/H13CO+, N2H+/HCN, N2H+/HN13C, H13CO+/HCN, H13CO+/HNC and HCN/HN13C as a function of H2 column density show no obvious variations, either. (iv) Higher angular resolution observations of optically thin molecular lines are necessary to study the chemical evolution of massive star forming clumps.
This work was supported by the Open Program of the Key Laboratory of Xinjiang Uygur Autonomous Region (No.2019D04023) and the National Natural Science foundation of China (No.11973076). It was also partially funded by the National Natural Science foundation of China (No.11433008, No.11603063, No.11703074 and No.11703073), and the CAS "Light of West China" Program (No.2018-XBQNXZ-B-024, No.2016-QNXZ-B-23, and No.2016-QNXZ-B-22).
†Part of the special topic on “The International Workshop on Astrochemistry (2019)”
| [1] |
S. Hoq, J. M. Jackson, J. B. Foster, P. Sanhueza, A. Guzmán, J. S. W. C. Claysmith, J. M. Rathborne, T. Vasyunina, and A. Vasyunin, Astrophys. J. 777, 157 (2013). doi: 10.1088/0004-637X/777/2/157
|
| [2] |
P. Sanhueza, J. M. Jackson, J. B. Foster, G. Garay, A. Silva, and S. C. Finn, Astrophsical J. 756, 60 (2012). doi: 10.1088/0004-637X/756/1/60
|
| [3] |
T. Vasyunina, H. Linz, T. Henning, I. Zinchenko, H. Beuther, and M. Voronkov, Astrophys. J. 527, A88 (2011).
|
| [4] |
P. Schilke, C. M.Walmsley, G. P. des Forets, E. Roueff, D. R. Flower, and S. Guilloteau, Astron. Astrophys. 256, 595 (1992).
|
| [5] |
M. Tafalla, P. C. Myers, P. Caselli, C. M. Walmsley, and A. Crapsi, Astron. Astrophys. 455, 577 (2006). doi: 10.1051/0004-6361:20065311
|
| [6] |
X. H. Han, J. J. Zhou, J. Z. Wang, J. Esimbek, J. S. Zhang, and N. Wang, Astron. Astrophys. 576, A131 (2015).
|
| [7] |
Y. Zhang, J. Zhou, Y. He, D. Li, W. Ji, Y. Yuan, and W. H. Guo, Astrophys. Space Sci. 361, 191 (2016). doi: 10.1007/s10509-016-2773-5
|
| [8] |
M. Li, J. Zhou, J. Esimbek, D. Quan, Y. He, Q. Li, and C. Zhu, Astrophys. J. Suppl. Ser. 243, 13 (2019). doi: 10.3847/1538-4365/ab2465
|
| [9] |
T. Gerner, H. Beuther, D. Semenov, H. Linz, T. Vasyunina, S. Bihr, Y. L. Shirley, and T. Henning, Astron. Astrophys. 563, A97 (2014).
|
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