J/A+A/626/A28 M8 and Her 36 C2H and c-C3H2 data (Tiwari+, 2019)
Observational study of hydrocarbons in the bright photodissociation region of
Messier 8.
Tiwari M., Menten K.M., Wyrowski F., Perez-Beaupuits J.P., Lee M.-Y.,
Kim W.-J.
<Astron. Astrophys. 626, A28 (2019)>
=2019A&A...626A..28T 2019A&A...626A..28T (SIMBAD/NED BibCode)
ADC_Keywords: H II regions ; Interstellar medium ; Radio lines ; Spectroscopy
Keywords: astrochemistry - radiative transfer - ISM: abundances -
HII regions - photon-dominated region - submillimeter: ISM
Abstract:
Hydrocarbons are ubiquitous in the interstellar medium, but their
formation is still not well understood, depending on the physical
environment they are found in. Messier 8 (M8) is host to one of the
brightest Hii regions and photodissociation regions (PDRs) in our
galaxy. With the observed C2H and c-C3H2 data toward M8, we aim
at obtaining their densities and abundances and to shed some light on
their formation mechanism.
Using the Atacama Pathfinder Experiment (APEX) 12m, and the Institut
de Radioastronomie Millimetrique (IRAM) 30m telescopes, we performed
a line survey toward Herschel 36 (Her 36), which is the main ionizing
stellar system in M8, and an imaging survey within 1.3x1.3pc around
Her 36 of various transitions of C2H and C3H2. We used both
Local Thermodynamic Equilibrium (LTE) and non-LTE methods to determine
the physical conditions of the emitting gas along with the column
densities and abundances of the observed species, which we compared
with (updated) gas phase photochemical PDR models. In order to examine
the role of polycyclic aromatic hydrocarbons (PAHs) in the formation
of small hydrocarbons and to investigate their association with the
Hii region, the PDR and the molecular cloud, we compared archival
Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE)
8µm and the Spectral and Photometric Imaging Receiver (SPIRE)
250µm continuum images with the C2H emission maps.
We observed a total of three rotational transitions of C2H with
their hyperfine structure components and four rotational transitions
of C3H2 with ortho and para symmetries toward the HII region and
the PDR of M8. Fragmentation of PAHs seems less likely to contribute
to the formation of small hydrocarbons as the 8 m emission does not
follow the distribution of C2H emission, which is more associated
with the molecular cloud toward the north-west of Her 36. From the
quantitative analysis, we obtained abundances of 10-8 and 10-9 for
C2H and c-C3H2 respectively, and volume densities of the
hydrocarbon emitting gas in the range n(H2)∼5x104-5x106cm-3.
The observed column densities of C2H and c-C3H2 are reproduced
reasonably well by our PDR models. This supports the idea that in
high-UV flux PDRs, gas phase chemistry is sufficient to explain
hydrocarbon abundances.
Description:
Data cubes and spectra of the observed C2H, c-C3H2, C and CO
toward M8. They contain both temperature and velocity information of
the hydrocarbons, C and CO emission in a region 120"x120" around the
bright stellar system Herschel 36.
Objects:
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RA (2000) DE Designation(s)
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18 03 40.32 -24 22 42.9 Her 36 = NAME Herschel 36
18 03 37 -24 23.2 M 8 = NAME Lagoon Nebula
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 223 12 List of fits spectra and datacubes
fits/* . 12 Individual fits spectra and datacubes
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Byte-by-byte Description of file: list.dat
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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- 25 I6 --- Nx Number of pixels along X-axis
27- 28 I2 --- Ny ? Number of pixels along Y-axis
for the datacubes
30- 33 I4 --- Nz ? Number of slices for the datacubes
35- 57 A23 "datime" Obs.Date Observation date
59- 74 E16.11 Hz Obs.Freq Observed frequency (RestFreq for the spectra)
76- 87 E12.6 Hz bFreq ? Lower value of relative frequency interval
for the spectra
89- 99 E11.6 Hz BFreq ? Upper value of relative frequency interval
for the spectra
101-106 I6 Hz dFreq ? Frequency resolution for the spectra
108-115 F8.1 m/s bVrad ? Lower value of Vrad interval
for the datacubes
117-124 F8.1 m/s BVrad ? Upper value of Vrad interval
for the datacubes
126-134 F9.4 m/s dVrad ? Vrad resolution for the datacubes
136-140 I5 Kibyte size Size of FITS file
142-165 A24 --- FileName Name of FITS file, in subdirectory fits
167-223 A57 --- Title Title of the FITS file
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Acknowledgements:
Maitraiyee Tiwari, mtiwari(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 05-Apr-2019