Chinese Journal of Chemical Physics  2020, Vol. 33 Issue (1): 114-118

#### The article information

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

Chinese Journal of Chemical Physics, 2020, 33(1): 114-118

http://dx.doi.org/10.1063/1674-0068/cjcp1911212

### Article history

Accepted on: December 13, 2019
Effect of Optical Depth on Study of Chemical Properties of Massive Star Forming Clumps
Jian-jun Zhoua,c , Run-xia Lia,b , Donghui Quana,d , Jarken Esimbeka,c , Yu-xin Hea,c , Da-lei Lia,c , Xin-di Tanga,c , Gang Wua,c , Wei-guang Jia,c , Zheng-xue Changa,b , Xia Zhanga,b
Dated: Received on November 23, 2019; Accepted on December 13, 2019
a. Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China;
b. University of the Chinese Academy of Sciences, Beijing 100080, China;
c. Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Urumqi 830011, China;
d. Department of Chemistry, Eastern Kentucky University, Richmond KY 40475, USA
Abstract: Here we present the study on chemical properties of massive star forming clumps using N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(1-0) data from the literature [Astron. Astrophys. 563, A97 (2014)]. We found that abundances of H$^{13}$CO$^+$ and HN$^{13}$C are affected by H$_2$ column densities. As the median values of these two abundances increase by nearly 10 times from stages A to B, H$^{13}$CO$^+$ and HN$^{13}$C are suitable for tracing the evolution of massive star forming clumps. The order of rapidity in growth of abundances of all the four studied molecules from stages A to B, is H$^{13}$CO$^+$, HCN, HN$^{13}$C, and N$_2$H$^+$, from the highest to the lowest. Our results suggest that the observing optically thin molecular lines with high angular resolution are necessary to study the chemical evolution of massive star forming clumps.
Key words: Stars formation    Clumps    Molecules    Radio lines
Ⅰ. INTRODUCTION

Past observations have shown that CN, HCN, HNC, N$_2$H$^+$ 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 N$_2$H$^+$/HCO$^+$ abundance ratio could serve as a chemical clock for massive star formation, whereas Hoq et al. [1] found that the N$_2$H$^+$/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 N$_2$H$^+$(1-0) lines, and obtained improved results [6, 7]. Abundances of these four molecules are mainly dominated by H$_2$ 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 N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(1-0) with the IRAM 30m millimeter telescope [9]. H$^{13}$CO$^+$(1-0) and HN$^{13}$C(1-0) lines are considered to be optically thin [2], and high sensitivity also improves N$_2$H$^+$(1-0) and HCN(1-0) data. Here we study the data from Gerner et al. [9].

Ⅱ. METHOD

Gerner et al. [9] observed N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(1-0) toward 59 massive star forming clumps with the IRAM 30m millimeter telescope. The corresponding beam size is 29 arcsecond, the sensitivity is $\sim$0.03 K, and the spectral resolution is $\sim$0.6 km/s. The sample of Gerner et al. [9] includes 19 IRDCs (infrared dark clouds) and 20 HMPOs (high-mass protostellar objects) as well as 11 HMCs (hot molecular cores) and 9 UCHIIs (ultra-compact regions). Following Zhang et al. [7], we classify IRDCs as stage A, and classify HMPO, HMC and UCHII clumps as stage B. We obtained the integrated intensities of N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C from Table A.1 of Gerner et al. [9], and plotted the histogram of the integral intensity and the ratios of four kinds of molecules in FIG. 1 and FIG. 2. We obtained the column densities of N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C from Table A.3 of Gerner et al. [9], which were derived with several assumptions such as local thermodynamic equilibrium, optical thinness and the initially chosen typical temperatures (see Section 4.3 of Gerner et al. [9]). We estimated their abundances with the beam averaged H$_2$ column densities which were also listed in their Table A.3, which were derived with several assumptions such as local thermodynamic equilibrium, optical. The H$_2$ column densities were derived from their radio fluxes at 850 $\mathtt{μ}$m, or 870 $\mathtt{μ}$m, 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 N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C versus H$_2$ column densities to study the relationship between molecular abundance and H$_2$ column density. Whether observing optically thin molecular lines with high angular resolution is necessary to study the chemical evolution of massive star forming clumps.

 FIG. 1 The histograms of the beam averaged integrated intensities of N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(1-0) for the stages A and B. The name of the evolutionary stage is given on the top right corner of each panel. The vertical solid lines indicate the median values of the integrated intensities for each evolutionary stage
 FIG. 2 The histograms of the beam averaged integrated intensity ratios of N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/HN$^{13}$C, HCO$^+$/HCN, HCO$^+$/HN$^{13}$C and HCN/HN$^{13}$C for the stages A and B. The name of the evolutionary stage is given on the top right corner of each panel. The vertical solid lines indicate the median values of the integrate intensity ratios for each evolutionary stage
 FIG. 3 The histograms of the beam averaged abundances of N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C for the stages A and B (in logarithm). The name of the evolutionary stage is given on the top right corner of each panel. The vertical solid lines indicate the median values of the abundance for each evolutionary stage
 FIG. 4 The histograms of the beam averaged abundance ratios (in logarithm) of N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/HN$^{13}$C, H$^{13}$CO$^+$/HCN, H$^{13}$CO$^+$/HNC and HCN/HN$^{13}$C for the stages A and B. The name of the evolutionary stage is given on the top right corner of each panel. The vertical solid lines indicate the median values of the abundance ratios for each evolutionary stage
Ⅲ. RESULTS AND DISCUSSION

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 10$^4$ cm$^{-3}$ and thus become detectable as cold dense molecular clouds. HMPOs host actively accreting protostars with mass $\geq$8M, which show internal emission sources at mid-infrared wavelengths. In the HMC stage, the central sources heat the surrounding environments, evaporating molecular-rich ices and raising the molecular complexity in the gas. Finally, the UV-radiation from the embedded protostars ionizes the surrounding gas and ultra-compact HII regions are formed (UCHII stage). Gerner et al. [9] observed N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(1-0) toward these 59 sources with the IRAM 30 m millimeter telescope. The corresponding beam size is 29 arcsecond, the sensitivity is $\sim$ 0.03 K, and the spectral resolution is $\sim$0.6 km/s.

A. Integrated intensities and ratios

We obtained the integrated intensities of N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(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 N$_2$H$^+$(1-0), H$^{13}$CO$^+$(1-0), HCN(1-0) and HN$^{13}$C(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 N$_2$H$^+$, H$^{13}$CO$^+$, and HN$^{13}$C 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 N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HN$^{13}$C, and H$^{13}$CO$^+$/HN$^{13}$C show similar trends of variations from stage A to B to those in Zhang et al. [7].

B. Abundances and abundance ratios

We obtained the column densities of N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C 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 H$_2$ column densities which were also listed in their Table A.3. The H$_2$ column densities were derived from their radio fluxes at 850 $\mathtt{μ}$m, or 870 $\mathtt{μ}$m, 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, HN$^{13}$C, H$^{13}$CO$^+$ and N$_2$H$^+$ 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 HN$^{13}$C, H$^{13}$CO$^+$ 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/HN$^{13}$C, H$^{13}$CO$^+$/HCN, H$^{13}$CO$^+$/HN$^{13}$C show an increasing trend from stage A to stage B, while N$_2$H$^+$/H$^{13}$CO$^+$ 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 N$_2$H$^+$/HCN and N$_2$H$^+$/HN$^{13}$C 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 HN$^{13}$C in star forming regions increase faster from stage A to stage B than that of N$_2$H$^+$. 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 H$^{13}$CO$^+$$>$HCN$>$HN$^{13}$C$>$N$_2$H$^+$.

C. The relation between abundances and H$_\mathbf{2}$ column densities

We plotted the abundances of N$_2$H$^+$, H$^{13}$CO$^+$, HCN and HN$^{13}$C versus H$_2$ column densities in FIG. 5. The abundance of HCN shows a clear decreasing trend with increasing H$_2$ column density. This is similar to the result from Li et al. [8]. The abundances of H$^{13}$CO$^+$ and HN$^{13}$C display a relatively weak decreasing trend with increasing H$_2$ column density, so optically thinner molecules are barely affected by H$_2$ column density. Abundance of N$_2$H$^+$ shows no obvious trend with the variation of H$_2$ column density. This is consistent with the result of Li et al. [8]. On the other hand, abundance ratios of N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/HN$^{13}$C, H$^{13}$CO$^+$/HCN, H$^{13}$CO$^+$/HNC and HCN/HN$^{13}$C as a function of H$_2$ 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.

 FIG. 5 The abundances of N$_2$H$^+$, H$^{13}$CO$^+$, HCN, and HN$^{13}$C as a function of H$_2$ column density
 FIG. 6 The beam averaged abundance ratios of N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/HN$^{13}$C, H$^{13}$CO$^+$/HCN, H$^{13}$CO$^+$/HNC and HCN/HN$^{13}$C as a function of H$_2$ column density
Ⅳ. CONCLUSION

We present the chemical properties of massive star forming clumps derived from H$^{13}$CO$^+$(1-0), HN$^{13}$C(1-0), N$_2$H$^+$(1-0) and HCN(1-0) data from Gerner et al. [9], compared with our previous results derived from N$_2$H$^+$(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 H$^{13}$CO$^+$(1-0), HN$^{13}$C (1-0), N$_2$H$^+$(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 H$^{13}$CO$^+$, HCN, HN$^{13}$C and N$_2$H$^+$. This is different from the conclusion from Zhang et al. [7]. (iii) The abundances of H$^{13}$CO$^+$ and HN$^{13}$C display a weak decreasing trend with increasing H$_2$ column density. We suggest these two molecules may not be very optically thin. Abundance ratios of N$_2$H$^+$/H$^{13}$CO$^+$, N$_2$H$^+$/HCN, N$_2$H$^+$/HN$^{13}$C, H$^{13}$CO$^+$/HCN, H$^{13}$CO$^+$/HNC and HCN/HN$^{13}$C as a function of H$_2$ 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.

Ⅴ. ACKNOWLEDGMENTS

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).

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a. 中国科学院新疆天文台，乌鲁木齐 830011;
b. 中国科学院大学，北京 100049;
c. 中国科学院射电重点实验室，乌鲁木齐 830011;
d. 美国东肯塔基大学化学系，里士满 KY 40475