2017A&A...603A.124C


C.D.S. - SIMBAD4 rel 1.7 - 2021.02.24CET22:53:22

2017A&A...603A.124C - Astronomy and Astrophysics, volume 603A, 124-124 (2017/7-1)

Complex organic molecules in strongly UV-irradiated gas.

CUADRADO S., GOICOECHEA J.R., CERNICHARO J., FUENTE A., PETY J. and TERCERO B.

Abstract (from CDS):

We investigate the presence of complex organic molecules (COMs) in strongly UV-irradiated interstellar molecular gas. We have carried out a complete millimetre (mm) line survey using the IRAM 30 m telescope towards the edge of the Orion Bar photodissociation region (PDR), close to the H2 dissociation front, a position irradiated by a very intense far-UV (FUV) radiation field. These observations have been complemented with 8.5'' resolution maps of the H2CO JKa,Kc=51,5-41,4 and C18O J=3-2 emission at 0.9mm. Despite being a harsh environment, we detect more than 250 lines from COMs and related precursors: H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH, CH3CN, CH2NH, HNCO, H213CO, and HC3N (in decreasing order of abundance). For each species, the large number of detected lines allowed us to accurately constrain their rotational temperatures (Trot) and column densities (N). Owing to subthermal excitation and intricate spectroscopy of some COMs (symmetric- and asymmetric-top molecules such as CH3CN and H2CO, respectively), a correct determination of N and Trot requires building rotational population diagrams of their rotational ladders separately. The inferred column densities are in the 1011-1013cm–2 range. We also provide accurate upper limit abundances for chemically related molecules that might have been expected, but are not conclusively detected at the edge of the PDR (HDCO, CH3O, CH3NC, CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN). A non-thermodynamic equilibrium excitation analysis for molecules with known collisional rate coefficients suggests that some COMs arise from different PDR layers but we cannot resolve them spatially. In particular, H2CO and CH3CN survive in the extended gas directly exposed to the strong FUV flux (Tk=150-250K and Td≤60K), whereas CH3OH only arises from denser and cooler gas clumps in the more shielded PDR interior (Tk=40-50K). The non-detection of HDCO towards the PDR edge is consistent with the minor role of pure gas-phase deuteration at very high temperatures. We find a HCO/H2CO/CH3OH~=1/5/3 abundance ratio. These ratios are different from those inferred in hot cores and shocks. Taking into account the elevated gas and dust temperatures at the edge of the Bar (mostly mantle-free grains), we suggest the following scenarios for the formation of COMs: (i) hot gas-phase reactions not included in current models; (ii) warm grain-surface chemistry; or (iii) the PDR dynamics is such that COMs or precursors formed in cold icy grains deeper inside the molecular cloud desorb and advect into the PDR.

Abstract Copyright: © ESO, 2017

Journal keyword(s): astrochemistry - surveys - photon-dominated region (PDR) - ISM: molecules - ISM: abundances - ISM: abundances

Simbad objects: 17

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Number of rows : 17

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2021
#notes
1 [JCC87] IRAS 4A Y*O 03 29 10.49 +31 13 30.8           ~ 622 1
2 Barnard 1 MoC 03 33 16.3 +31 07 51           ~ 270 0
3 [HKM99] B1-b cor 03 33 20.32 +31 07 21.5           ~ 171 0
4 TMC-1 MoC 04 41 45.9 +25 41 27           ~ 1397 0
5 NAME OMC-1 MoC 05 35 14 -05 22.4           ~ 1038 2
6 NAME Orion-KL SFR 05 35 14.16 -05 22 21.5           ~ 2048 1
7 NAME Orion Bright Bar reg 05 35 22.30 -05 24 33.0           ~ 742 0
8 NAME Horsehead Nebula DNe 05 40 59.0 -02 27 30           ~ 457 0
9 NAME ORI MOL CLOUD MoC 05 56 -01.8           ~ 887 1
10 NAME Mon R2 HII 06 07 47.58 -06 22 42.6           ~ 663 2
11 IRC +10216 C* 09 47 57.40632 +13 16 43.5648     10.96     C9,5e 2177 0
12 M 82 IG 09 55 52.430 +69 40 46.93 9.61 9.30 8.41     ~ 5421 6
13 IRAS 16293-2422 cor 16 32 22.56 -24 28 31.8           ~ 1076 1
14 LDN 1689B DNe 16 34 42.1 -24 36 11           ~ 143 0
15 NAME Sgr B2 MoC 17 47 20.4 -28 23 07           ~ 1978 1
16 LDN 1157 DNe 20 39 06.4 +68 02 13           ~ 500 0
17 [DE95] LDN 1157 B1 out 20 39 11 +68 01.3           ~ 191 0

    Equat.    Gal    SGal    Ecl

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2021.02.24-22:53:22

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