Interstellar HOCN in the Galactic center region

Abstract

Aims. Our aim is to confirm the interstellar detection of cyanic acid, HOCN, in the Galactic center clouds. It has previously been tentatively detected only in Sgr B2(OH). Methods. We used a complete line survey of the hot cores Sgr B2(N) and (M) in the 3 mm range, complemented by additional observations carried out with the IRAM 30 m telescope at selected frequencies in the 2 mm band and towards four additional positions in the Sgr B2 cloud complex in the 2 and 3 mm bands. The spectral survey was analysed in the local thermodynamical equilibrium approximation (LTE) by modeling the emission of all identified molecules simultaneously. This allowed us to distinguish weak features of HOCN from the rich line spectrum observed in Sgr B2(N) and (M). Lines of the more stable (by 1.1 eV) isomer isocyanic acid, HNCO, in these sources, as well as those of HOCN and HNCO towards the other positions, were analysed in the LTE approximation as well. Results. Four transitions of HOCN were detected in a quiescent molecular cloud in the Galactic center at a position offset in (RA, Dec) by (20 , 100 ) from the hot core source Sgr B2(M), confirming its previous tentative interstellar detection. Up to four transitions were detected toward five other positions in the Sgr B2 complex, including the hot cores Sgr B2(M), (S), and (N). A fairly constant abundance ratio of ∼0.3−0.8% for HOCN relative to HNCO was derived for the extended gas components, suggesting a common formation process of these isomers.

The puzzling behavior of HNCO isomers in molecular clouds

“…Four HCNO lines -namely the 4−3 (ν = 91 751.320 MHz, E u = 11 K), 5−4 (ν = 114 688.383 MHz, E u = 16 K), 6−5 (ν = 137 624.934 MHz, E u = 23 K), and 9−8 (ν = 206 430.497 MHz, E u = 50 K) transitions -were searched for in several positions toward SgrB2 (see Table 1), using ∼1 h per position and transition. To compare our intensities with the previous HOCN observations (Brünken et al 2010), we observed the 4 04 −3 03 and 7 07 −6 06 lines in all SgrB2 positions. For that purpose, we used 1 http://www.iram.fr/IRAMFR/GILDAS 40 min per source and transition.…”

“…The chemistry of the CHNO isomers, where the term refers to all four species, seems to be rather complex. While in Marcelino et al (2009), the observed abundances of HCNO and HNCO were reproduced well with a steady-state, gas-phase model, Brünken et al (2009aBrünken et al ( , 2010 proposed models with both gas-phase and grain-surface chemistry to explain the isomeric abundances of HOCN and HNCO in the molecular clouds of the GC. In this paper we report observations of HOCN and HCNO toward different sources in order to shed some light on the possible formation pathways of these species in both the gas phase and on dust surfaces.…”

“…However, HCNO has not been detected toward the direction of the cyanopolyyne peak of TMC-1 or in the Orion Hot Core region. Cyanic acid (HOCN) is detected in several positions in the giant molecular cloud SgrB2 (Brünken et al 2009a(Brünken et al , 2010, after its characterization in the laboratory in the cm-and mmwave bands and a tentative detection in SgrB2(OH) based on previous spectral line surveys (Brünken et al 2009a). The reported HNCO/HOCN abundance ratio in the Galactic center (GC) clouds is between 40-400.…”

“…Since there is not an obvious common precursor for them, their relative abundances in molecular clouds might provide some clues concerning the respective contributions of gas-phase and grain processes to the chemistry. A&A 516, A105 (2010) Two isomeric forms of HNCO have recently been detected in space, HCNO (Marcelino et al 2009) and HOCN (Brünken et al 2010). Fulminic acid (HCNO) is observed in three dark cloud cores and toward the low-mass star-forming region L1527, with an abundance ratio of HNCO/HCNO between 20 and 60.…”

“…In this paper we report observations of HOCN and HCNO toward different sources in order to shed some light on the possible formation pathways of these species in both the gas phase and on dust surfaces. We looked for HCNO in several positions in SgrB2 associated with hot or warm environments, where HOCN had previously been detected (see Brünken et al 2010), while HOCN was searched for in the cold to lukewarm dense cores observed in Marcelino et al (2009). We have also included in our search the well-studied low-mass protostar IRAS 16293-2422, where HNCO has been found to be prominent (Bisschop et al 2008), and a rich organic chemistry has been widely observed as a result of mantle grain desorption and, possibly, warm gas chemistry (see, e.g., Cazaux et al 2003;Kuan et al 2004;Garrod & Herbst 2006;Garrod et al 2008, and references therein).…”