J/A+A/600/A2 Hydrogen in diffuse molecular clouds (Winkel+, 2017)
Hydrogen in diffuse molecular clouds in the Milky Way.
Atomic column densities and molecular fraction along prominent lines of sight.
Winkel B., Wiesemeyer H., Menten K.M., Sato M., Brunthaler A.,
Wyrowski F., Neufeld D., Gerin M., Indriolo N.
<Astron. Astrophys. 600, A2 (2017)>
=2017A&A...600A...2W 2017A&A...600A...2W (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way; H I data ; Interstellar medium ; Spectroscopy
Keywords: astrochemistry - ISM: abundances - ISM: molecules
Abstract:
Recent sub-millimeter and far-infrared wavelength observations of
absorption in the rotational ground state lines of various simple
molecules against distant Galactic continuum sources have opened the
possibility to study the chemistry of diffuse molecular clouds Milky
Way-wide. In order to calculate abundances the column densities of
molecular and atomic hydrogen, HI, must be known.
We aim at determining the atomic hydrogen column densities for diffuse
clouds located on the sight lines toward a sample of prominent high
mass star forming regions that were intensely studied with the HIFI
instrument onboard Herschel.
Based on Jansky Very Large Array data, we employ the 21-cm HI
absorption-line technique to construct profiles of the HI opacity
versus radial velocity toward our target sources. These profiles are
combined with lower resolution archival data of extended HI emission
to calculate the HI column densities of the individual clouds along
the sight lines. We employ Bayesian inference to estimate the
uncertainties of the derived quantities.
Our study delivers reliable estimates of the atomic hydrogen column
density for a large number of diffuse molecular clouds at various
Galactocentric distances. Together with column densities of molecular
hydrogen derived from its surrogates observed with HIFI, the
measurements can be used to characterize the clouds and, e.g.,
investigate the dependence of their chemistry on the molecular
fraction.
Description:
To study the Hydrogen in diffuse molecular clouds in the Milky Way,
eight sources were observed with the Jansky Very Large Array in
spectral line mode at 21-cm. The targets have bright continuum
emission and were used as background sources for HI absorption
spectroscopy. The brightest positions in the background continuum
images were used to define several sight lines per source and to
extract associated HI absorption spectra.
To obtain spin temperatures and corrected HI column densities,
suitable Off-position spectra were acquired, making use of the three
Galactic plane surveys: Canadian Galactic Plane Survey (CGPS, Taylor
et al., 2003AJ....125.3145T 2003AJ....125.3145T), Southern Galactic Plane Survey (SGPS,
McClure-Griffiths et al., 2005ApJS..158..178M 2005ApJS..158..178M), and VLA Galactic Plane
Survey (VGPS, Stil et al., 2006AJ....132.1158S 2006AJ....132.1158S).
Furthermore, for some of the sight lines 550-GHz HF spectra are
available, observed with the Herschel space telescope. With these, the
molecular hydrogen column density can be estimated, and in combination
with the HI column densities, the molecular fraction is determined.
The calculation of all result spectra was done with Bayesian
Markov-Chain Monte Carlo using the Python PyMC3 framework (Patil,
Huard, and Fonnesbeck, DOI: 10.18637/jss.v035.i04).
In the FITS binary tables, provided here, the results of our Bayes
sampling are contained. For each sight line, we provide seven spectra
per quantity (i.e, the -3, -2, -1, 0, 1, 2, and 3 confidence levels)
plus the full Bayes chains (1000 steps) for the following variables:
Tcsou, Tcbg, tau, Tspin, NHI, NH2, and F{H2/H}, because the
latter are not Normal distributed.
Objects:
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RA (2000) DE Designation(s)
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17 47 20.4 -28 23 07 SGRB2 = SGR B2
18 10 28.6 -19 55 51 G10.62-0.39 = W 31
18 14 39.0 -17 52 03 W33A = W 33A
18 47 34.6 -01 12 43 G31.41+0.31 = GAL 031.41+00.31
18 53 18.5 +01 14 58 G34.26+0.15 = GAL 034.26+00.15
19 10 13.2 +09 06 12 W49N = W 49N
19 23 43.9 +14 30 30 W51E4 = W 51E4
20 39 01.0 +42 22 50 DR21(OH) = DR 21 (OH)
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
sources.dat 67 8 List of observed sources
sightlin.dat 70 31 List of analyzed sight lines
fits/* . 8 Individial fits tables with distributions and
Bayes chain
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See also:
J/A+A/542/A110 : Neutral gas in the Milky Way halo (Ben Bekhti+, 2012)
Byte-by-byte Description of file: sources.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Name Source name
13- 14 I2 h RAh Right ascension (J2000.0) of phase center
16- 17 I2 min RAm Right ascension (J2000.0) of phase center
19- 22 F4.1 s RAs Right ascension (J2000.0) of phase center
24 A1 --- DE- Declination sign (J2000.0) of phase center
25- 26 I2 deg DEd Declination (J2000.0) of phase center
28- 29 I2 arcmin DEm Declination (J2000.0) of phase center
31- 32 I2 arcsec DEs Declination (J2000.0) of phase center
34- 40 F7.4 deg GLON Galactic longitude of phase center
42- 48 F7.4 deg GLAT Galactic latitude of phase center
50 I1 --- SLnum Number of extracted sight lines
52- 67 A16 --- FileName Associated FITS file in subdirectory fits
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Byte-by-byte Description of file: sightlin.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- Name Source name
13- 16 A4 --- ID Sight line ID
18- 19 I2 h RAh Right ascension (J2000.0) of sight line
21- 22 I2 min RAm Right ascension (J2000.0) of sight line
24- 27 F4.1 s RAs Right ascension (J2000.0) of sight line
29 A1 --- DE- Declination sign (J2000.0) of sight line
30- 31 I2 deg DEd Declination (J2000.0) of sight line
33- 34 I2 arcmin DEm Declination (J2000.0) of sight line
36- 37 I2 arcsec DEs Declination (J2000.0) of sight line
39- 45 F7.4 deg GLON Galactic longitude of sight line
47- 53 F7.4 deg GLAT Galactic latitude of sight line
55- 70 A16 --- FileName Associated FITS file in subdirectory fits
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Acknowledgements:
Benjamin Winkel, bwinkel(at)mpifr.de
(End) Patricia Vannier [CDS] 26-Jan-2017