J/A+A/677/A152 Horsehead ALMA images (Hernandez-Vera+, 2023)
The extremely sharp transition between molecular and ionized gas in the
Horsehead nebula.
Hernandez-Vera C., Guzman V.V., Goicoechea J.R., Maillard V., Pety J.,
Le Petit F., Gerin M., Bron E., Roueff E., Abergel A., Schirmer T.,
Carpenter J., Gratier P., Gordon K., Misselt K.
<Astron. Astrophys. 677, A152 (2023)>
=2023A&A...677A.152H 2023A&A...677A.152H (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds; Millimetric/submm sources; Carbon monoxide
Keywords: astrochemistry - ISM: clouds - ISM: molecules -
ISM: photon-dominated region (PDR)
Abstract:
Massive stars can determine the evolution of molecular clouds by
eroding and photo-evaporating their surfaces with strong ultraviolet
(UV) radiation fields. Moreover, UV radiation is relevant in setting
the thermal gas pressure in star-forming clouds, whose influence can
extend across various spatial scales, from the rims of molecular
clouds to entire star-forming galaxies. Probing the fundamental
structure of nearby molecular clouds is therefore crucial to
understand how massive stars shape their surrounding medium and how
fast molecular clouds are destroyed, specifically at their
UV-illuminated edges, where models predict an intermediate zone of
neutral atomic gas between the molecular cloud and the surrounding
ionized gas whose size is directly related to the exposed physical
conditions. We present the highest angular resolution (∼0.5",
corresponding to 207 au) and velocity-resolved images of the molecular
gas emission in the Horsehead nebula, using CO J=3-2 and HCO+ J=4-3
observations with the Atacama Large Millimeter/submillimeter Array
(ALMA). We find that CO and HCO+ are present at the edge of the cloud,
very close to the ionization (H+/H) and dissociation fronts (H/H_2),
suggesting a very thin layer of neutral atomic gas (<650au) and a
small amount of CO-dark gas (AV=0.006-0.26mag) for stellar UV
illumination conditions typical of molecular clouds in the Milky Way.
The new ALMA observations reveal a web of molecular gas filaments with
an estimated thermal gas pressure of Pth=(2.3-4.0)*106K/cm3,
and the presence of a steep density gradient at the cloud edge that
can be well explained by stationary isobaric photo-dissociation region
(PDR) models with pressures consistent with our estimations. However,
in the HII region and PDR interface, we find Pth,PDR>Pth,HII,
suggesting the gas is slightly compressed. Therefore, dynamical
effects cannot be completely ruled out and even higher angular
observations will be needed to unveil their role.
Description:
Emission maps and datacubes of Horsehead nebula taken with
multiple-pointing mosaic mode using ALMA 12m array, the Atacama
Compact Array (ACA) and Total Power (single-dish) observations. We
observe a field-of-view (FoV) of 50"x50" of the Horsehead edge
centered on RA=05:40:53, DEC=-02:27:45 (J2000).
Objects:
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RA (2000) DE Designation(s)
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05 40 53 -02 27 45 Horsehead nebula = Barnard 33
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 124 5 List of fits maps and datacubes
fits/* . 5 Individual fits maps and datacubes
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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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- 28 I4 --- Ny Number of pixels along Y-axis
30- 31 I2 --- Nz ? Number of slices for the datacubes
33- 55 A23 "datime" Obs.date Observation date
57- 62 I6 Kibyte size Size of FITS file
64- 87 A24 --- FileName Name of FITS file, in subdirectory fits
89-124 A36 --- Title Title of the FITS file
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Acknowledgements:
Claudio Hernandez-Vera, chernandez(at)astro.puc.cl
(End) Patricia Vannier [CDS] 19-Jul-2023