J/A+A/706/A280 Molecular gas associated with HII regions (Khan+, 2026)
Exploring the interplay between molecular and ionized gas in HII regions.
Khan S., Jacob A.M., Rugel M.R., Urquhart J.S., Neupane S., Wyrowski F.,
Brunthaler A., Pandian J.D., Gong Y., Barlach Christensen I., Menten K.M.
<Astron. Astrophys. 706, A280 (2026)>
=2026A&A...706A.280K 2026A&A...706A.280K (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; H II regions ; Radio continuum ; Spectroscopy
Keywords: stars: formation - stars: massive - ISM: clouds - ISM: molecules
Abstract:
Massive stars strongly impact their natal environments and influence
subsequent star formation through feedback mechanisms including
shocks, outflows and radiation. HII regions are key laboratories for
studying this impact. To understand such feedback, it is crucial to
characterize the physical conditions of the dense molecular gas in
which these regions are embedded.
We aim to constrain the kinetic
temperature and H2 volume density of massive star-forming clumps
associated with HII regions using multiple p-H2CO transitions. The
non-LTE analysis yielded kinetic temperatures (Tkin) ranging from
33.7K to 265K and H2 densities (n(H2)) between 0.8x104 to
1.05x107cm-3, providing a detailed characterization of the
dense molecular gas contained in these clumps.
We observed the JKaKc transitions of p-H2CO (within its J=3-2
and 4-3 states) with the Atacama Pathfinder EXperiment (APEX) 12m
submillimeter telescope, using the nFLASH230 and SEPIA345 receivers
toward a sample of 61 HII regions. We derived spectral line
parameters via multicomponent Gaussian fitting, which was then used to
constrain the physical conditions determined using PyRADEX, a
non-local thermodynamic equilibrium (LTE) radiative transfer code in
combination with Markov chain Monte Carlo analysis.
The non-LTE analysis yielded kinetic temperatures (Tkin) ranging from
33.7K to 265K and H2 densities (n(H2)) between 0.8x104 and
1.05x107cm-3, providing a detailed characterization of the dense
-olecular gas contained in these clumps. In addition to the p-H2CO
emission arising from the targeted clump, a large fraction (57%) of
the sources exhibited multiple p-H2CO components, with the secondary
components being characterized by a higher Tkin and broader line
widths. Investigation of the nature of the secondary component
revealed its association with supersonic nonthermal motions and
turbulent gas. When comparing the physical properties of the molecular
gas and dust components with those of the ionized gas, we found that
parameters directly linked to the central high-mass star, such as
bolometric luminosity (Lbol) and Lyman continuum photon rate (NLyc),
show stronger and more systematic correlations. These findings
emphasize the role of the central star in governing the interplay
between the molecular and ionized gas. In our sample of HII
regions, the pressure of the neutral gas systematically exceeds that
of the ionized gas. This suggests that the surrounding neutral
molecular medium can hinder or slow down the expansion of HII regions
due to its higher pressure. However, given the limited spatial
resolution, a definitive conclusion on the role of molecular gas in
confining HII regions cannot be made until high resolution
observations are obtained.
Description:
We observed the JKaKc transitions of p-H2CO (within its J=3-2 and
4-3 states) with the Atacama Pathfinder EXperiment (APEX) 12m
submillimeter telescope using the nFLASH230 and SEPIA345 receivers
towards a sample of 61 HII regions. Spectral line parameters are
derived via multi-component Gaussian fitting, which was then used to
constrain the physical conditions determined using PyRADEX, a
non-local thermodynamic equilibrium (LTE) radiative transfer code in
combination with Markov Chain Monte Carlo (MCMC) analysis.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tablec1.dat 163 105 Line fitting parameters of the p-H2CO lines
observed with nFLASH230
tablec2.dat 162 105 Line fitting parameters of the p-H2CO lines
observed with SEPIA345
tablec3.dat 105 105 Derived physical properties of molecular clouds
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See also:
J/A+A/565/A75 : ATLASGAL: dust condensations in Galactic plane
(Csengeri+, 2014)
J/A+A/611/A6 : ATLASGAL massive clumps H2CO data (Tang+, 2018)
J/MNRAS/473/1059 : Complete sample of Galactic clump properties
(Urquhart+, 2018)
J/A+A/627/A175 : GLOSTAR. Radio Source Catalogue I (Medina+, 2019)
J/MNRAS/510/3389 : High-mass star formation evolutionary trends
(Urquhart+, 2022)
J/A+A/689/A196 : GLOSTAR GPS. Radio source catalog (Medina+, 2024)
J/A+A/689/A81 : GLOSTAR GPS. X. Galactic HII region catalog (Khan+, 2024)
Byte-by-byte Description of file: tablec1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- GLOSTAR Name of the HII region in the
GLOSTAR RRL catalog (G1)
16- 33 A18 --- ATLASGAL Name of the associated ATLASGAL clump
35 A1 --- component [abc] Secondary component name (G2)
37- 40 F4.2 --- etanFLASH Beam filling factor for nFLASH observation
42- 48 F7.2 km/s Vlsr ? Velocity of the component obtained from
H2CO(303-202)
50- 54 F5.2 K TmbH2CO1 ? Amplitude of the component of
H2CO(303-202)
56- 60 F5.2 K e_TmbH2CO1 ? Error in the amplitude of the component of
H2CO(303-202)
62- 67 F6.2 K.km/s tdvH2CO1 ? Integrated intensity of the component of
H2CO(303-202)
69- 73 F5.2 K.km/s e_tdvH2CO1 ? Error in the Integrated intensity of the
component of H2CO(303-202)
75- 80 F6.2 km/s WH2CO1 ? Linewidth of the component of
H2CO(303-202)
82- 86 F5.2 km/s e_WH2CO1 ? Error in the Linewidth of the component of
H2CO(303-202)
88- 92 F5.2 K TmbH2CO2 ? Amplitude of the component of
H2CO(322-221)
94- 98 F5.2 K e_TmbH2CO2 ? Error in the Amplitude of the component of
H2CO(322-221)
100-105 F6.2 K.km/s tdvH2CO2 ? Integrated intensity of the component of
H2CO(322-221)
107-111 F5.2 K.km/s e_tdvH2CO2 ? Error in the Integrated intensity of the
component of H2CO(322-221)
113-118 F6.2 km/s WH2CO2 ? Linewidth of the component of
H2CO(322-221)
120-125 F6.2 km/s e_WH2CO2 ? Error in the Linewidth of the component of
H2CO(322-221)
127-131 F5.2 K TmbH2CO3 ? Amplitude of the component of
H2CO(321-220)
133-137 F5.2 K e_TmbH2CO3 ? Error in the Amplitude of the component of
H2CO(321-220)
139-144 F6.2 K.km/s tdvH2CO3 ? Integrated intensity of the component of
H2CO(321-220)
146-150 F5.2 K.km/s e_tdvH2CO3 ? Error in the Integrated intensity of the
component of H2CO(321-220)
152-157 F6.2 km/s WH2CO3 ? Linewidth of the component of
H2CO(321-220)
159-163 F5.2 km/s e_WH2CO3 ? Error in the Linewidth of the component of
H2CO(321-220)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tablec2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- GLOSTAR Name of the HII region in the GLOSTAR RRL
catalog (G1)
16- 33 A18 --- ATLASGAL Name of the associated ATLASGAL clump
35 A1 --- component [abc] Secondary component name (G2)
37- 40 F4.2 --- etaSEPIA Beam filling factor for SEPIA observation
42- 48 F7.2 km/s Vlsr ? Velocity of the component obtained from
H2CO(303-202)
50- 54 F5.2 K TmbH2CO4 ? Amplitude of the component of
H2CO(404-303)
56- 60 F5.2 K e_TmbH2CO4 ? Error in the Amplitude of the component of
H2CO(404-303)
62- 67 F6.2 K.km/s tdvH2CO4 ? Integrated intensity of the component of
H2CO(404-303)
69- 73 F5.2 K.km/s e_tdvH2CO4 ? Error in the Integrated intensity of the
component of H2CO(404-303)
75- 80 F6.2 km/s WH2CO4 ? Linewidth of the component of
H2CO(404-303)
82- 86 F5.2 km/s e_WH2CO4 ? Error in the Linewidth of the component of
H2CO(404-303)
88- 92 F5.2 K TmbH2CO5 ? Amplitude of the component of
H2CO(423-322)
94- 98 F5.2 K e_TmbH2CO5 ? Error in the Amplitude of the component of
H2CO(423-322)
100-105 F6.2 K.km/s tdvH2CO5 ? Integrated intensity of the component of
H2CO(423-322)
107-111 F5.2 K.km/s e_tdvH2CO5 ? Error in the Integrated intensity of the
component of H2CO(423-322)
113-118 F6.2 km/s WH2CO5 ? Linewidth of the component of
H2CO(423-322)
120-124 F5.2 km/s e_WH2CO5 ? Error in the Linewidth of the component of
H2CO(423-322)
126-130 F5.2 K TmbH2CO6 ? Amplitude of the component of
H2CO(422-321)
132-136 F5.2 K e_TmbH2CO6 ? Error in the Amplitude of the component of
H2CO(422-321)
138-143 F6.2 K.km/s tdvH2CO6 ? Integrated intensity of the component of
H2CO(422-321)
145-149 F5.2 K.km/s e_tdvH2CO6 ? Error in the Integrated intensity of the
component of H2CO(422-321)
151-156 F6.2 km/s WH2CO6 ? Linewidth of the component of
H2CO(422-321)
158-162 F5.2 km/s e_WH2CO6 ? Error in the Linewidth of the component of
H2CO(422-321)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tablec3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 14 A14 --- GLOSTAR Name of the HII region in the GLOSTAR RRL
catalog (G1)
16- 33 A18 --- ATLASGAL Name of the associated ATLASGAL clump
35 A1 --- component Secondary component name (G2)
37- 43 F7.2 K Tkin ? Non-LTE Kinetic temperature
45- 49 F5.2 K E_Tkin ? Upper error in the non-LTE Kinetic
temperature
51- 55 F5.2 K e_Tkin ? Lower error in the non-LTE Kinetic
temperature
57- 61 F5.2 [cm-3] lognH2 ? Log of H2 density
63- 67 F5.2 [cm-3] E_lognH2 ? Log of upper error in the H2 density
69- 73 F5.2 [cm-3] e_lognH2 ? Log of lower error in the H2 density
75- 80 F6.2 [cm-2] logNH2CO ? Log of para-H2CO column density
82- 86 F5.2 [cm-2] E_logNH2CO ? Log of upper error in the para-H2CO
column density
88- 92 F5.2 [cm-2] e_logNH2CO ? Log of lower error in the para-H2CO
column density
94- 98 F5.2 --- logXH2CO ? Log of fractional abundance of para-H2CO
100-105 F6.2 [cm-2] logNH2 ? Log of H2 column density
--------------------------------------------------------------------------------
Global notes:
Note (G1): GLOSTAR RRL catalog, Khan et al., 2024A&A...689A..81K 2024A&A...689A..81K,
Cat. J/A+A/689/A81.
Note (G2) : a, b, c indicates the secondary velocity components.
--------------------------------------------------------------------------------
Acknowledgements:
Sarwar Khan, skhan(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 29-Dec-2025