A line confusion limited millimeter survey of Orion KL I. Sulfur carbon chains

Abstract

We perform a sensitive (line confusion limited), single-side band spectral survey towards Orion KL with the IRAM 30 m telescope, covering the following frequency ranges: 80−115.5 GHz, 130−178 GHz, and 197−281 GHz. We detect more than 14 400 spectral features of which 10 040 have been identified up to date and attributed to 43 different molecules, including 148 isotopologues and lines from vibrationally excited states. In this paper, we focus on the study of OCS, HCS + , H 2 CS, CS, CCS, C 3 S, and their isotopologues. In addition, we map the OCS J = 18−17 line and complete complementary observations of several OCS lines at selected positions around Orion IRc2 (the position selected for the survey). We report the first detection of OCS ν 2 = 1 and ν 3 = 1 vibrationally excited states in space and the first detection of C 3 S in warm clouds. Most of CCS, and almost all C 3 S, line emission arises from the hot core indicating an enhancement of their abundances in warm and dense gas. Column densities and isotopic ratios have been calculated using a large velocity gradient (LVG) excitation and radiative transfer code (for the low density gas components) and a local thermal equilibrium (LTE) code (appropriate for the warm and dense hot core component), which takes into account the different cloud components known to exist towards Orion KL, the extended ridge, compact ridge, plateau, and hot core. The vibrational temperature derived from OCS ν 2 = 1 and ν 3 = 1 levels is 210 K, similar to the gas kinetic temperature in the hot core. These OCS high energy levels are probably pumped by absorption of IR dust photons. We derive an upper limit to the OC 3 S, H 2 CCS, HNCS, HOCS + , and NCS column densities. Finally, we discuss the D/H abundance ratio and infer the following isotopic abundances: 12 C/ 13 C = 45 ± 20, 32 S/ 34 S = 20 ± 6, 32 S/ 33 S = 75 ± 29, and 16 O/ 18 O = 250 ± 135.

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

“…16 O/ 18 O With the double isotope method, we derived the 16 O/ 18 O isotopic ratios with H 13 CO + (1-0), HC 18 O + (1-0) and the ratio of 12 C/ 13 C. The local ISM value (557 ± 30) is slightly higher than our results (Wilson 1999), which are about 422, 415, 532 and 288 in Cep A, S76E DR21S and G34, respectively. The ratio of ∼288 found in G34 is consistent with 287 ± 27 found in Orion derived from OCS and its isotopologues (Tercero et al 2010). The fractionation of oxygen can be ignored because of its high first ionization potential (Langer et al 1984).…”

“…The 12 C/ 13 C isotope abundance ratios were derived from the line ratio between HC 3 N and its isotopologues, assuming similar astrochemical processes. The 12 C/ 13 C isotope ratios are about 40 in all four sources (Table 2), which are slightly lower than the ratio of 60-70 found in the local interstellar medium (ISM; Milam et al 2005), but agree well with the result in Orion (∼40; Persson et al 2007;Tercero et al 2010). The rarer isotope 13 C is a secondary element that can be produced from 12 C in the carbonnitrogen-oxygen (CNO) cycle in low-and intermediate-mass stars (Prantzos, Aubert & Audouze 1996), while the main isotope of carbon 12 C, as a primary element, is mainly produced by the triple alpha reaction (Timmes, Woosley & Weaver 1995).…”

Trans-cismolecular photoswitching in interstellar space

“…Following our previous studies of the Bar (Cuadrado et al 2015;Goicoechea et al 2016) we run a model of an isobaric PDR (P th /k = 10 8 K cm −3 ) illuminated by χ = 4 × 10 4 times the mean interstellar radiation field (Draine 1978). For photons in the λ = 2000−3000 Å range, we adopt N ph (λ) = 4 × 10 4 × 732 × λ 0.7 photon cm −2 s −1 Å −1 at the PDR edge (A V = 0) (van Dishoeck & Black 1982). We use a constant dust composition and size distribution that reproduces a standard interstellar extinction curve (Cardelli et al 1989).…”

“…The observed line parameters are consistent with emission from the hot core itself. This is dense, n H of a few 10 7 cm −3 , and hot gas at approximatley 200 K Tercero et al 2010), and also from the more extended warm gas (approximatley 60 K) Appendix F: Detected cis-and trans-HCOOH lines towards the Orion Bar PDR …”

“…We observed three prototypical interstellar sources known to display a very rich chemistry and bright molecular line emission: (i) the Orion Bar photodissociation region (PDR): the edge of the Orion cloud irradiated by ultraviolet (UV) photons from nearby massive stars (e.g., Goicoechea et al 2016); (ii) the Orion hot core: warm gas around massive protostars (e.g., Tercero et al 2010); and (iii) Barnard 1-b (B1-b): a cold dark cloud (e.g., Cernicharo et al 2012). The two latter sources are shielded from strong UV radiation fields.…”