2016A&A...585A..76W


C.D.S. - SIMBAD4 rel 1.7 - 2019.11.13CET14:31:07

2016A&A...585A..76W - Astronomy and Astrophysics, volume 585A, 76-76 (2016/1-1)

Far-infrared study of tracers of oxygen chemistry in diffuse clouds.

WIESEMEYER H., GUESTEN R., HEYMINCK S., HUEBERS H.W., MENTEN K.M., NEUFELD D.A., RICHTER H., SIMON R., STUTZKI J., WINKEL B. and WYROWSKI F.

Abstract (from CDS):

The chemistry of the diffuse interstellar medium rests upon three pillars: exothermic ion-neutral reactions (``cold chemistry''), endothermic neutral-neutral reactions with significant activation barriers (``warm chemistry''), and reactions on the surfaces of dust grains. While warm chemistry becomes important in the shocks associated with turbulent dissipation regions, the main path for the formation of interstellar OH and H2 O is that of cold chemistry. The aim of this study is to observationally confirm the association of atomic oxygen with both atomic and molecular gas phases, and to understand the measured abundances of OH and OH+ as a function of the available reservoir of H2. We obtained absorption spectra of the ground states of OH, OH+ and Oiwith high-velocity resolution, with GREAT onboard SOFIA, and with the THz receiver at the APEX. We analyzed them along with ancillary spectra of HF and CH from HIFI. To deconvolve them from the hyperfine structure and to separate the blend that is due to various velocity components on the sightline, we fit model spectra consisting of an appropriate number of Gaussian profiles using a method combining simulated annealing with downhill simplex minimization. Together with HF and/or CH as a surrogate for H2, and Hiλ21 cm data, the molecular hydrogen fraction fNH2=N(H2)/(N(H)+2N(H2)) can be determined. We then investigated abundance ratios as a function of fNH2. The column density of Oiis correlated at a high significance with the amount of available molecular and atomic hydrogen, with an atomic oxygen abundance of 3x10–4 relative to H nuclei. While the velocities of the absorption features of OH and OH+ are loosely correlated and reflect the spiral arm crossings on the sightline, upon closer inspection they display an anticorrespondence. The arm-to-interarm density contrast is found to be higher in OH than in OH+. While both species can coexist, with a higher abundance in OH than in OH+, the latter is found less frequently in absence of OH than the other way around, which is a direct consequence of the rapid destruction of OH+ by dissociative recombination when not enough H2 is available. This conjecture has been substantiated by a comparison between the OH/OH+ ratio with fNH2, showing a clear correlation. The hydrogen abstraction reaction chain OH+(H2,H)H2 O+(H2,H)H3 O+ is confirmed as the pathway for the production of OH and H2 O. Our estimate of the branching ratio of the dissociative recombination of H3 O+ to OH and H2 O is confined within the interval of 84 to 91%, which matches laboratory measurements (74 to 83%). A correlation between the linewidths and column densities of OH+ features is found to be significant with a false-alarm probability below 5%. Such a correlation is predicted by models of interstellar MHD turbulence. For OH the same correlation is found to be insignificant because there are more narrow absorption features. While it is difficult to assess the contributions of warm neutral-neutral chemistry to the observed abundances, it seems fair to conclude that the predictions of cold ion-neutral chemistry match the abundance patterns we observed.

Abstract Copyright:

Journal keyword(s): ISM: abundances - ISM: atoms - ISM: clouds - ISM: lines and bands - ISM: molecules - ISM: structure

CDS comments: G329.031 and G332.81 not identified.

Simbad objects: 26

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Number of rows : 26

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2020
#notes
1 NAME Per Arm PoG 03 30 +45.0           ~ 1195 0
2 M 42 HII 05 35 17.3 -05 23 28           ~ 3658 0
3 NAME Car Arm PoG 10 00 -60.0           ~ 367 1
4 NAME Crux Arm PoG 12 30 -60.0           ~ 81 0
5 NAME Car-Sgr Arm PoG 15 00 -40.0           ~ 412 1
6 OH 327.29 -0.58 cor 15 53 07.65 -54 37 06.1           ~ 37 0
7 NAME Nor Arm PoG 16 00 -50.0           ~ 316 0
8 Caswell OH 330.953-00.182 Mas 16 09 52.43 -51 54 54.0           ~ 10 0
9 Caswell OH 332.824-00.548 Mas 16 20 10.23 -50 53 18.0           ~ 3 0
10 HD 154368 s*b 17 06 28.3687923960 -35 27 03.759955171 6.10 6.63 6.13 7.12   O9.5Iab 260 0
11 MMB G351.581-00.353 cor 17 25 24.9262 -36 12 42.383           ~ 27 0
12 NAME Sgr B2 MoC 17 47 20.4 -28 23 07           ~ 1856 1
13 MMB G010.472+00.027 cor 18 08 38.20 -19 51 50.1           ~ 35 0
14 Caswell OH 010.623-00.384 Mas 18 10 28.65 -19 55 49.5           ~ 4 0
15 W 31c HII 18 10 29.1 -19 56 05           ~ 299 0
16 NGC 6618 OpC 18 20 47 -16 10.3           ~ 1456 0
17 GAL 034.4+00.23 HII 18 53 17.4 +01 24 55           ~ 73 1
18 [ARM2000] G034.257+00.154 Mas 18 53 18.67 +01 14 58.5           ~ 5 0
19 NAME Sgr Arm PoG 19 00 -30.0           ~ 496 1
20 W 49n HII 19 10 13.2 +09 06 12           ~ 403 3
21 W 49 SFR 19 10 20 +09 07.7           ~ 502 1
22 W 51e2 Rad 19 23 43.90 +14 30 34.8           ~ 176 0
23 W 51 SNR 19 23 50 +14 06.0           ~ 1119 1
24 NAME Galactic Bulge reg ~ ~           ~ 3243 0
25 NAME Scutum Spiral Arm PoG ~ ~           ~ 256 0
26 NAME Galactic Bar reg ~ ~           ~ 824 0

    Equat.    Gal    SGal    Ecl

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2019.11.13-14:31:07

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