Millimetre spectral line mapping observations towards four massive star-forming H ii regions

Abstract

We present spectral line mapping observations towards four massive star-forming regionsCepheus A, DR21S, S76E and G34.26+0.15 -with the IRAM 30-m telescope at the 2 and 3 mm bands. In total, 396 spectral lines from 51 molecules, one helium recombination line, 10 hydrogen recombination lines and 16 unidentified lines were detected in these four sources. An emission line of nitrosyl cyanide (ONCN, 14 0, 14 -13 0, 13 ) was detected in G34.26+0.15, as the first detection in massive star-forming regions. We found that c-C 3 H 2 and NH 2 D show enhancement in shocked regions, as suggested by the evidence of SiO and/or SO emission. The column density and rotational temperature of CH 3 CN were estimated with the rotational diagram method for all four sources. Isotope abundance ratios of 12 C/ 13 C were derived using HC 3 N and its 13 C isotopologue, which were around 40 in all four massive star-forming regions and slightly lower than the local interstellar value (∼65). The 14 N/ 15 N and 16 O/ 18 O abundance ratios in these sources were also derived using the double isotopic method, which were slightly lower than in the local interstellar medium. Except for Cep A, the 33 S/ 34 S ratios in the other three targets were derived, which were similar to that in the local interstellar medium. The column density ratios of N(DCN)/N(HCN) and N(DCO + )/N(HCO + ) in these sources were more than two orders of magnitude higher than the elemental [D]/[H] ratio, which is 1.5 × 10 −5 . Our results show that the later stage sources, G34.26+0.15 in particular, present more molecular species than earlier stage sources. Evidence of shock activity is seen in all stages studied.

Properties of Dense Molecular Gas along the Major Axis of M82

“…Previous observations of dense-gas tracers have shown that they are mostly optically thick in both Galactic GMCs and external galaxies (Wang et al 2014;Meier et al 2015;Jiménez-Donaire et al 2017;Li et al 2017Li et al , 2020. If we assume that the abundance ratio of 12 C/ 13 C is 40, which is obtained as the average condition of nearby galaxies (Henkel et al 1994(Henkel et al , 2010, the HCN, HCO + , and HNC lines of M82 would be optically thin.…”

“…This ratio measured in two starbursts, NGC 4945 (∼200-400, Henkel et al 2018) and ULIRG Arp 220 (440 +140 −82 , Wang et al 2014), is higher than found in M82 (∼80-250, see Figure 6). In our Milky Way, the 14 N/ 15 N ratio is distributed in a large range (100-600, Li et al 2017), with a possible positive gradient from the center to the outskirts (Chen et al 2021).…”

“…With observations of HCN J = 1 → 0 toward 65 galaxies, Gao & Solomon (2004) found a tight linear correlation between the luminosities of HCN J = 1 → 0 and total infrared emission. However, most dense-gas tracers are normally optically thick both in Galactic giant molecular clouds (GMCs) and in external galaxies (Wang et al 2014;Meier et al 2015;Jiménez-Donaire et al 2017;Li et al 2017Li et al , 2020. Therefore, there is a large uncertainty in estimating the mass of dense gas from a single transition line of a molecule with a high dipole moment, which is similar to the issue of the CO-to-H 2 conversion factors (Narayanan et al 2012;Bolatto et al 2013).…”

Properties of dense molecular gas along the major axis of M 82

“…Under the assumptions of local thermal equilibrium (LTE), uniform gas excitation, constant isotopic abundance ratios, and neglecting radiative transfer by the cosmic microwave background, we can derive optical depths of the main isotopic lines. We adopt the same method as reported in Li et al (2020), in which the line ratio 𝑅 and the optical depth for a specific isotopologue line 𝜏, is related as𝑅 = (1-𝑒 −𝜏 12 )/(1-𝑒 −𝜏 13 ) Due to the large uncertainty of absolute abundance ratio of isotopes in external galaxies, we adopt various 12 C/ 13 C and 14 N/ 15 N abundance ratios reported in the literature: 12 C/ 13 C = 89 (the Solar system, Clayton & Nittler 2004) and 200 (Ultra-Infraredluminous ULIRGs, Romano et al 2017)), and 14 N/ 15 N = 100 (nearby starburst galaxies, Chin et al 1999), 200 (Galactic massive star-forming regions, Li et al 2017), and 290 (the Solar neighbourhood at 𝑅 gc ∼ 7.9 kpc, Adande & Ziurys 2012). Table 5 shows the derived optical depths of HCN, HCO + , and HNC 𝐽 = 1→0.…”

“…With observations of HCN 𝐽 = 1 → 0 toward 65 galaxies, Gao & Solomon (2004) found a tight linear correlation between the luminosities of HCN 𝐽 = 1→ 0 and total infrared emission. However, most dense gas tracers are normally optically thick both in Galactic giant molecular clouds (GMCs) and in external galaxies (Wang et al 2014;Meier et al 2015;Jiménez-Donaire et al 2017;Li et al 2017Li et al , 2020. Therefore, there is a large uncertainty in estimating the dense gas mass from a single transition line of a high dipole moment molecule, which is similar to the issue of the CO-to-H2 conversion factors (Narayanan et al 2012;Bolatto et al 2013).…”