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A search for hypercompact HII regions in the Galactic Plane

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A search for hypercompact HII regions in the Galactic Plane. / Yang, A. Y.; Thompson, M. A.; Tian, W. W.; Bihr, S.; Beuther, H.; Hindson, L.

In: Monthly Notices of the Royal Astronomical Society, Vol. 482, No. 2, 11.01.2019, p. 2681–2696.

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Yang, A. Y. ; Thompson, M. A. ; Tian, W. W. ; Bihr, S. ; Beuther, H. ; Hindson, L. / A search for hypercompact HII regions in the Galactic Plane. In: Monthly Notices of the Royal Astronomical Society. 2019 ; Vol. 482, No. 2. pp. 2681–2696.

Bibtex

@article{fbbabca5c3684e1e89a45776f9c36f92,
title = "A search for hypercompact HII regions in the Galactic Plane",
abstract = "We have carried out the largest and most unbiased search for hypercompact (HC) H II 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 H II regions with positive spectral indices; 120 positive spectrum H II regions have been identified from a total sample of 534 positive spectral index radio sources. None of these H II regions, including the known HC H II regions recovered in our search, fulfills the canonical definition of an HC H II region at 5 GHz. We suggest that the current canonical definition of HC H II regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) H II region surveys, we find that roughly half of detected UC H II 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 H II regions are statistically more luminous and possess higher Lyman continuum fluxes than H II regions with flat or negative indices. Positive spectrum H II 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 H II regions.",
keywords = "Infrared: ISM, ISM: H II regions, Radio continuum: ISM, Submillimetre: ISM",
author = "Yang, {A. Y.} and Thompson, {M. A.} and Tian, {W. W.} and S. Bihr and H. Beuther and L. Hindson",
note = "22 pages, 8 figures, 6 tables, accepted by MNRAS",
year = "2019",
month = "1",
day = "11",
doi = "10.1093/mnras/sty2811",
language = "English",
volume = "482",
pages = "2681–2696",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "2",

}

RIS

TY - JOUR

T1 - A search for hypercompact HII regions in the Galactic Plane

AU - Yang, A. Y.

AU - Thompson, M. A.

AU - Tian, W. W.

AU - Bihr, S.

AU - Beuther, H.

AU - Hindson, L.

N1 - 22 pages, 8 figures, 6 tables, accepted by MNRAS

PY - 2019/1/11

Y1 - 2019/1/11

N2 - We have carried out the largest and most unbiased search for hypercompact (HC) H II 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 H II regions with positive spectral indices; 120 positive spectrum H II regions have been identified from a total sample of 534 positive spectral index radio sources. None of these H II regions, including the known HC H II regions recovered in our search, fulfills the canonical definition of an HC H II region at 5 GHz. We suggest that the current canonical definition of HC H II regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) H II region surveys, we find that roughly half of detected UC H II 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 H II regions are statistically more luminous and possess higher Lyman continuum fluxes than H II regions with flat or negative indices. Positive spectrum H II 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 H II regions.

AB - We have carried out the largest and most unbiased search for hypercompact (HC) H II 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 H II regions with positive spectral indices; 120 positive spectrum H II regions have been identified from a total sample of 534 positive spectral index radio sources. None of these H II regions, including the known HC H II regions recovered in our search, fulfills the canonical definition of an HC H II region at 5 GHz. We suggest that the current canonical definition of HC H II regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) H II region surveys, we find that roughly half of detected UC H II 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 H II regions are statistically more luminous and possess higher Lyman continuum fluxes than H II regions with flat or negative indices. Positive spectrum H II 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 H II regions.

KW - Infrared: ISM

KW - ISM: H II regions

KW - Radio continuum: ISM

KW - Submillimetre: ISM

UR - http://www.scopus.com/inward/record.url?scp=85058482841&partnerID=8YFLogxK

U2 - 10.1093/mnras/sty2811

DO - 10.1093/mnras/sty2811

M3 - Article

VL - 482

SP - 2681

EP - 2696

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 2

ER -