J/A+A/708/A201 Formaldehyde observations (Barlach Christensen+, 2026)
Formaldehyde as a densitometer and thermometer in Cygnus-X, the GLOSTAR pilot
region, and M8. Utilizing the H2CO ground-state transition.
Barlach Christensen I., Gieser C., Wyrowski F., Nguyen H., Hoang T.D.,
Veena V.S., Beuther H., Kahle K.A., Gong Y., Menten K.M.
<Astron. Astrophys. 708, A201 (2026)>
=2026A&A...708A.201B 2026A&A...708A.201B (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Radio lines
Keywords: astrochemistry - stars: formation - ISM: abundances - evolution -
ISM: molecules - submillimeter: ISM
Abstract:
Measurements of the physical conditions in molecular clumps are key
to our understanding of star formation. Formaldehyde (H2CO) is a
molecule prevalent in these regions that can be used as diagnostic for
the physical conditions in them.
Here we explore a technique for determining the volume density and gas
kinetic temperature in molecular clumps across various evolutionary
phases and environments. The ground-state transition of H2CO has a
critical density of ncrit∼104cm-3, allowing us to use this
molecule as a densitometer at n≤105cm-3 and lessen the
discrepancy between the measurements between gas densities derived
from molecular tracers and those derived of dust observations.
The clumps have been observed with the IRAM 30-m telescope, marking
the first extensive survey of the H2CO (10,1-00,0) line across a
large sample of sources. These observations were complemented by the
H2CO J=3-2 lines, obtained using the APEX telescope. These clumps
have been surveyed in three regions, the Cygnus-X giant molecular
cloud complex, the GLOSTAR pilot region covering the Galactic plane at
longitudes 28°≤l≤36°, and the molecular cloud associated
with the HII regions in the Lagoon nebula (M8).
We analyzed a total of 127 clumps, including 78 from Cygnus-X, 12 from
the GLOSTAR pilot region, and 37 from M8. We derived the gas kinetic
temperature, volume densities and H2CO column densities using
radiative transfer modeling with pyradex+emcee in 102 clumps. We
reproduce the observed line intensities in the sources with volume
densities n(H2)=5.4x104-3.8x105cm-3, gas kinetic temperatures
Tgas=16-219K, and H2CO column densities
N(H2CO)=6.0x1012-1.6x1015cm-2.
The gas kinetic temperatures obtained from the non-LTE modeling
with pyradex+emcee agree well with the LTE gas kinetic temperature
obtained from the ratio of H2CO (30,3-20,2) and
H2CO (32,1-22,0) lines at densities n(H2)≤105.5cm-3.
However, we find that, at higher densities, LTE temperatures derived
from this ratio are overestimated by up to 0.5dex. The measured volume
densities are consistent with the volume densities obtained from dust
continuum measurements, thereby probing the bulk of the gas.
Furthermore, we find that the volume densities and dust temperatures
increase, with increasing evolutionary phase. The newly available
ground-state transition of H2CO allows to constrain the physical
conditions in various phases of star formation more effectively.
Description:
Observations using the IRAM 30-m and the APEX telescopes.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea.dat 45 137 Overview of source list (tables A1-A4)
tableb.dat 66 126 Integrated intensities of formaldehyde
transitions (tables B1-B4)
tablec.dat 63 102 Modeled physical conditions (table C1)
tabled.dat 41 102 Evolutionary sequences (table D1)
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Byte-by-byte Description of file: tablea.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Source Source ID
13 A1 --- n_Source [*] Note on Source
15- 16 I2 h RAh Right ascension (J2000)
18- 19 I2 min RAm Right ascension (J2000)
21- 24 F4.1 s RAs Right ascension (J2000)
26 A1 --- DE- Declination sign (J2000)
27- 28 I2 deg DEd Declination (J2000)
30- 31 I2 arcmin DEm Declination (J2000)
33- 34 I2 arcsec DEs Declination (J2000)
36- 40 F5.2 kpc d ? Distance (1)
42- 45 F4.2 kpc e_d ? Distance error
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Note (1): Distance of Cygnus-X adopted from Rygl et al. (2012A&A...539A..79R 2012A&A...539A..79R),
distance for M8 clumps adopted from Damiani et al. (2017A&A...604A.135D 2017A&A...604A.135D).
GLOSTAR source distances determined using the Parallax-Based Distance
Calculator, as described in Reid et al. (2019ApJ...885..131R 2019ApJ...885..131R).
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Byte-by-byte Description of file: tableb.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Source Source ID
14- 18 F5.2 K.km/s H2CO(101-000) ? H2CO(101-000) integrated intensity
20- 23 F4.2 K.km/s e_H2CO(101-000) ? H2CO(101-000) integrated intensity error
25- 29 F5.2 K.km/s H2CO(303-202) ? H2CO(303-202) integrated intensity
31- 34 F4.2 K.km/s e_H2CO(303-202) ? H2CO(303-202) integrated intensity error
36- 40 F5.2 K.km/s H2CO(322-221) ? H2CO(322-221) integrated intensity
42- 45 F4.2 K.km/s e_H2CO(322-221) ? H2CO(322-221) integrated intensity error
47- 51 F5.2 K.km/s H2CO(321-220) ? H2CO(321-220) integrated intensity
53- 56 F4.2 K.km/s e_H2CO(321-220) ? H2CO(321-220) integrated intensity error
58- 61 F4.2 km/s Linewidth Average linewidth
63- 66 F4.2 km/s e_Linewidth Average linewidth error
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Byte-by-byte Description of file: tablec.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Source Source ID
14- 17 F4.2 cm-3 n(H2) Maximum likelihood of H2 volume density
from pyradex+emcee
19- 22 F4.2 cm-3 e_n(H2) H2 volume density lower limit of 1sigma
24- 27 F4.2 cm-3 E_n(H2) H2 volume density upper limit of 1sigma
29- 32 F4.2 K Tkin Maximum likelihood of gas kinetic temperature
from pyradex+emcee
34- 37 F4.2 K e_Tkin Gas kinetic temperature lower limit of 1sigma.
39- 42 F4.2 K E_Tkin Gas kinetic temperature upper limit of 1sigma.
44- 48 F5.2 cm-2 N(H2CO) Maximum likelihood of H2CO column density
from pyradex+emcee
50- 53 F4.2 cm-2 e_N(H2CO) H2CO column density lower limit of 1sigma.
55- 58 F4.2 cm-2 E_N(H2CO) H2CO column density upper limit of 1sigma.
60- 63 A4 --- nlines Number of lines utilized in the modeling (1)
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Note (1): If 3(2), the third line is H2CO (32,2-22,1) (218.475GHz).
If 3(3), the third line is H2CO (32,1-22,0) (218.760GHz).
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Byte-by-byte Description of file: tabled.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- Source Source ID
14- 16 A3 --- Continuum [Yes No] Continuum emission detected in
the source
18- 20 A3 --- 70um [Yes No] 70 micron emission detected in
the source
22- 24 A3 --- 24um [Yes No] 24 micron emission detected in
the source
26- 28 A3 --- 3.8um [Yes No] 3.8 micron emission detected in
the source
30- 41 A12 --- EvolStage Evolutionary stage (1)
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Note (1): Determined evolutionary stage based on the four emission criteria.
The criteria for the four evolutionary stages are described in Urquhart et al.
(2022ApJ...940..111U 2022ApJ...940..111U). This method was used for the clumps in Cygnus-X and M8.
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Acknowledgements:
Ivalu Barlach Christensen, ivalubarlach(at)gmail.com
(End) Patricia Vannier [CDS] 27-Feb-2026