J/MNRAS/482/2681 Hypercompact H II regions in the Galactic Plane (Yang+, 2019)
A search for hypercompact H II regions in the Galactic Plane.
Yang A.Y., Thompson M.A., Tian W.W., Bihr S., Beuther H., Hindson L.
<Mon. Not. R. Astron. Soc., 482, 2681-2696 (2019)>
=2019MNRAS.482.2681Y 2019MNRAS.482.2681Y (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; H II regions ; Infrared ; Radio continuum ;
Millimetric/submm sources
Keywords: ISM: HII regions - infrared: ISM - radio continuum: ISM -
submillimetre: ISM
Abstract:
We have carried out the largest and most unbiased search for
hypercompact (HC) HII regions. Our method combines four
interferometric radio continuum surveys (THOR, CORNISH, MAGPIS, and
White2005) with far-infrared and sub-mm Galactic Plane surveys to
identify embedded HII regions with positive spectral indices; 120
positive spectrum HII regions have been identified from a total sample
of 534 positive spectral index radio sources. None of these HII
regions, including the known HC HII regions recovered in our search,
fulfills the canonical definition of an HC HII region at 5GHz. We
suggest that the current canonical definition of HC HII regions is
not accurate and should be revised to include a hierarchical structure
of ionized gas that results in an extended morphology at 5GHz.
Correlating our search with known ultracompact (UC) HII region
surveys, we find that roughly half of detected UC HII regions have
positive spectral indices, instead of more commonly assumed flat and
optically thin spectra. This implies a mix of optically thin and thick
emission and has important implications for previous analyses which
have so far assumed optically thin emission for these objects.
Positive spectrum HII regions are statistically more luminous and
possess higher Lyman continuum fluxes than HII regions with flat or
negative indices. Positive spectrum HII regions are thus more likely
to be associated with more luminous and massive stars. No differences
are found in clump mass, linear diameter, or luminosity-to-mass ratio
between positive spectrum and non-positive spectrum HII regions.
Description:
Our starting point was the CORNISH (Coordinated Radio 'N' Infrared
Survey for High-mass star formation), which we used to form a base
5GHz radio source catalogue. CORNISH was a sensitive (∼0.4mJy/beam)
and high-resolution (∼1.5") 5GHz survey of a section of the Galactic
Plane (10<l<65° and |b|<1°), using the JVLA in B and BnA
configuration. CORNISH detected 3062 continuum sources greater than
7σ (Hoare et al. 2012PASP..124..939H 2012PASP..124..939H; Purcell et al.
2013ApJS..205....1P 2013ApJS..205....1P, Cat. J/ApJS/205/1).
The Multi-Array Galactic Plane Imaging Survey (MAGPIS) has the highest
sensitivity and resolution of the 1.4GHz surveys that we have used,
with a resolution of ∼6" and noise level ∼0.3mJy/beam (Helfand et al.
2006AJ....131.2525H 2006AJ....131.2525H, Cat. J/AJ/131/2525). A catalogue has been
published for the survey region 5<l<32° and |b|<0.8°. We used
image cutouts available from the MAPGIS Website to identify 1.4GHz
counterparts to CORNISH sources in the uncatalogued region of the
Galactic Plane. To cover the remainder of the CORNISH survey region,
we have used 1-2GHz catalogues and data from The HI, OH, Recombination
line survey of the Milky Way (THOR, Bihr et al. 2016A&A...588A..97B 2016A&A...588A..97B,
Cat. J/A+A/588/A97; Beuther et al. 2016A&A...595A..32B 2016A&A...595A..32B) and the 1.4GHz
VLA survey White et al. (2005AJ....130..586W 2005AJ....130..586W, Cat. J/AJ/130/586),
hereafter White2005.
Once the 1.4-5GHz radio spectral index of the objects has been
derived, we select those objects with a positive spectral index and
confirm that they are embedded within molecular cloud clumps by
examining cutout images from the ATLASGAL (Schuller et al.
2009A&A...504..415S 2009A&A...504..415S) and Hi-GAL (Molinari et al. 2016A&A...591A.149M 2016A&A...591A.149M,
Cat. J/A+A/591/A149) surveys.
We found a total of 120 HII regions with positive radio spectral
indices, shown in Table A1 and we obtained a sample of 534 objects
with a 1.4 to 5GHz spectral index greater than zero, listed in Table
A2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 88 121 Information of 120 young positive spectrum HII
regions
tablea2.dat 88 534 Information of total 534 positive spectrum radio
objects
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Byte-by-byte Description of file: tablea1.dat tablea2.dat
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Bytes Format Units Label Explanations
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1- 17 A17 --- Name CORNISH source name (GLLL.llll+BB.bbbb)
19 A1 --- f_Name Flag on name (1)
21- 28 F8.5 deg GLON Galactic longitude
30- 37 F8.5 deg GLAT Galactic latitude
39- 46 F8.2 mJy F5GHz Flux density at 5GHz
48- 54 F7.2 mJy e_F5GHz Error on F5GHz
56- 60 F5.2 arcsec ADiam Angular diameter at 5GHz from CORNISH
62- 65 F4.2 arcsec e_ADiam Error on ADiam
67- 73 F7.2 mJy F1.4GHz Flux densities at 1.4GHz from THOR, MAGPIS and
White et al. (2005AJ....130..586W 2005AJ....130..586W,
Cat. J/AJ/130/586)
75- 78 F4.2 mJy e_F1.4GHz ? Error on F1.4GHz (2)
80- 83 F4.2 --- alpha Spectral index
85- 88 F4.2 --- e_alpha ? Error on SpIdx
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Note (1): Symbol * means that those objects are detected at both 5GHz and 1.4GHz
with lower limit of the spectral indices as they are extended at
1.4GHz
Note (2): Flux densities of some sources at 1.4GHz with no errors refer to the
1.4GHz noise level at the source position, indicating that these
sources are only detected at 5GHz and so have lower limits of spectral
indices
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 30-Jun-2022