J/A+A/612/A37 CH 149um spectra of 4 molecular clouds (Wiesemeyer+, 2018)
Unveiling the chemistry of interstellar CH.
Spectroscopy of the 2 THz N = 2 ← 1 ground state line.
Wiesemeyer H., Guesten R., Menten K.M., Duran C.A., Csengeri T.,
Jacob A.M., Simon R., Stutzki J., Wyrowski F.
<Astron. Astrophys. 612, A37 (2018)>
=2018A&A...612A..37W 2018A&A...612A..37W (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Spectroscopy
Keywords: ISM: abundances - ISM: clouds - ISM: lines and bands -
ISM: molecules - ISM: structure
Abstract:
The methylidyne radical CH is commonly used as a proxy for molecular
hydrogen in the cold, neutral phase of the interstellar medium. The
optical spectroscopy of CH is limited by interstellar extinction,
whereas far-infrared observations provide an integral view through the
Galaxy. While the HF ground state absorption, another H2 proxy in
diffuse gas, frequently suffers from saturation, CH remains transparent
both in spiral-arm crossings and high-mass star forming regions,
turning this light hydride into a universal surrogate for H2.
However, in slow shocks and in regions dissipating turbulence its
abundance is expected to be enhanced by an endothermic production
path, and the idea of a "canonical" CH abundance needs to be
addressed.
The N=2←1 ground state transition of CH at λ149um has become
accessible to high-resolution spectroscopy thanks to GREAT, the German
Receiver for Astronomy at Terahertz Frequencies aboard the
Stratospheric Observatory for Infrared Astronomy, SOFIA. Its
unsaturated absorption and the absence of emission from the star
forming regions makes it an ideal candidate for the determination of
column densities with a minimum of assumptions. Here we present an
analysis of four sightlines towards distant Galactic star forming
regions, whose hot cores emit a strong far-infrared dust continuum
serving as background signal. Moreover, if combined with the
sub-millimeter line of CH at λ560um, environments forming
massive stars can be analyzed. For this we present a case study on the
"proto-Trapezium" cluster W3 IRS5.
While we confirm the global correlation between the column densities
of HF and those of CH, both in arm and interarm regions, clear
signposts of an over-abundance of CH are observed towards lower
densities. However, a significant correlation between the column
densities of CH and HF remains. A characterization of the hot cores in
the W3 IRS5 proto-cluster and its envelope demonstrates that the
sub-millimeter/far-infrared lines of CH reliably trace not only diffuse
but also dense, molecular gas.
In diffuse gas, at lower densities a quiescent ion-neutral chemistry
alone cannot account for the observed abundance of CH. Unlike the
production of HF, for CH+ and CH, vortices forming in turbulent,
diffuse gas may be the setting for an enhanced production path.
However, CH remains a valuable tracer for molecular gas in
environments reaching from diffuse clouds to sites of high-mass star
formation.
Description:
The observations were performed on several SOFIA flights in the
observatory's cycle 4 (F298 and F301 on 2016 May 18 and 24,
respectively, F346 on 2016 November 8, and F369 on 2017 February 3).
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 123 5 Synopsis of velocity-integrated column densities
of CH, HF, and OH on the sightline to W49 N
list.dat 120 4 List of fits files
fits/* . 4 Individual fits files
--------------------------------------------------------------------------------
See also:
J/A+A/490/213 : 1.4 and 3.4mm interferometry of W3 IRS5 (Rodon+, 2008)
J/A+A/567/L5 : W3(OH) high angular resolution 7mm images (Dzib+, 2014)
J/ApJ/647/418 : Ground-state OH masers in W3(OH) study (Fish+, 2006)
J/ApJ/668/331 : OH masers in W3(OH) (Fish+, 2007)
J/ApJ/393/149 : W49N H2O maser outflow: distance and kinematics (Winn+, 1992)
J/ApJ/429/253 : W49N H2O masers (Gwinn, 1994)
J/A+A/582/A64 : W51/e2 and G34.3+0.2 IRAM spectra (Lykke+, 2015)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 A11 --- Name Spiral arm designation
13 A1 --- n_Name [abc] Note on Name (1)
16- 19 F4.1 km/s Vlow Lower value of velocity interval
22- 25 F4.1 km/s Vup Upper value of velocity interval
27- 31 F5.2 10+13cm-2 N(CH) CH velocity-integrated column density
33- 36 F4.2 10+13cm-2 E_N(CH) Error on N(CH) (upper value) (4)
38- 41 F4.2 10+13cm-2 e_N(CH) Error on N(CH) (lower value) (4)
43- 47 F5.3 10+13cm-2 N(HF) HF velocity-integrated column density
49- 53 F5.3 10+13cm-2 E_N(HF) Error on N(HF) (upper value) (4)
55- 59 F5.3 10+13cm-2 e_N(HF) Error on N(HF) (lower value) (4)
61- 65 F5.2 10+13cm-2 N(OH) OH velocity-integrated column density
67- 70 F4.2 10+13cm-2 E_N(OH) Error on N(OH) (upper value) (4)
72- 75 F4.2 10+13cm-2 e_N(OH) Error on N(OH) (lower value) (4)
77- 80 F4.2 --- fNH2 Molecular hydrogen fraction,
2N(H2)/(N(HI)+2N(H2)) (2)
82- 85 F4.2 --- E_fNH2 Error on fNH2 (upper value) (4)
87- 90 F4.2 --- e_fNH2 Error on fNH2 (lower value) (4)
92- 96 F5.3 --- N(HF)/N(CH) Molecular ratio, N(HF)/N(CH) (3)
98-102 F5.3 --- E_N(HF)/N(CH) Error on N(HF)/N(CH) (upper value) (4)
104-108 F5.3 --- e_N(HF)/N(CH) Error on N(HF)/N(CH) (lower value) (4)
110-113 F4.2 --- N(OH)/N(CH) Molecular ratio, N(OH)/N(CH) (3)
115-118 F4.2 --- E_N(OH)/N(CH) Error on N(OH)/N(CH) (upper value) (4)
120-123 F4.2 --- e_N(OH)/N(CH) Error on N(OH)/N(CH) (lower value) (4)
--------------------------------------------------------------------------------
Note (1): Notes as follows:
a = Analysis and error estimates inaccurate (velocity interval contains
hot-core environment of unknown excitation, HF absorption is partially
saturated)
b = Near- and far-side crossing of Sagittarius spiral arm
c = Far-side crossing
Note (2): are derived from HF (with N(HF)/N(H2)=1.4x10-8) and from
HI λ21cn data (Winkle et al., 2017, further references therein).
Note (3): Bayesian error estimates (accounting for the correlation between the
column densities of the reported ratios).
Note (4): Error estimates are based on the normalized chi2 of the fits shown
in Fig. 2, and on a 5% uncertainty in the continuum levels.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 43 A19 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss)
45- 52 F8.5 GHz bFreq Lower value of frequency interval
54- 60 F7.5 GHz BFreq Upper value of frequency interval
62- 70 E9.4 Hz dFreq Frequency resolution
72- 73 I2 Kibyte size Size of FITS file
75- 98 A24 --- FileName Name of FITS file, in subdirectory fits
100-120 A21 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Helmut Wiesemeyer, hwiese(at)mpifr.de
(End) Patricia Vannier [CDS] 11-Jan-2018