J/A+A/643/A36 HI-to-H2 transition in local diffuse ISM dataset (Bellomi+, 2020)
3D chemical structure of diffuse turbulent ISM.
I. Statistics of the HI-to-H2 transition.
Bellomi E., Godard B., Hennebelle P., Valdivia V., Pineau Des Forets G.,
Lesaffre P., Perault M.
<Astron. Astrophys., 643, A36 (2020)>
=2020A&A...643A..36B 2020A&A...643A..36B (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium
Keywords: ISM: structure - ISM: molecules - ISM: kinematics and dynamics -
ISM: clouds - methods: numerical - methods: statistical
Abstract:
The amount of data collected by spectrometers from radio to
ultraviolet (UV) wavelengths opens a new era where the statistical and
chemical information contained in the observations can be used
concomitantly to investigate the thermodynamical state and the
evolution of the interstellar medium (ISM).
In this paper, we study the statistical properties of the HI-to-H2
transition observed in absorption in the local diffuse and multiphase
ISM. Our goal is to identify the physical processes that control the
probability of occurrence of any line of sight and the origins of the
variations of the integrated molecular fraction from one line of sight
to another.
The turbulent diffuse ISM is modeled using the RAMSES code, which
includes detailed treatments of the magnetohydrodynamics, the thermal
evolution of the gas, and the chemistry of H2. The impacts of the UV
radiation field, the mean density, the turbulent forcing, the integral
scale, the magnetic field, and the gravity on the molecular content of
the gas are explored through a parametric study that covers a wide
range of physical conditions. The statistics of the HI-to-H2
transition are interpreted through analytical prescriptions and
compared with the observations using a modified and robust version of
the Kolmogorov-Smirnov test. The analysis of the observed background
sources shows that the lengths of the lines of sight follow a flat
distribution in logarithmic scale from ∼100pc to ∼3kpc. Without taking
into account any variation of the parameters along a line of sight or
from one line of sight to another, the results of one simulation,
convolved with the distribution of distances of the observational
sample, are able to simultaneously explain the position, the width,
the dispersion, and most of the statistical properties of the
HI-to-H2 transition observed in the local ISM. The tightest
agreement is obtained for a neutral diffuse gas modeled over ∼200pc,
with a mean density nH=1-2cm-3, illuminated by the standard
interstellar UV radiation field, and stirred up by a large-scale
compressive turbulent forcing. Within this configuration, the 2D
probability histogram of the column densities of H and H2,
poetically called the kingfisher diagram, is remarkably stable and is
almost unaltered by gravity, the strength of the turbulent forcing,
the resolution of the simulation, or the strength of the magnetic
field Bx, as long as Bx<4µG. The weak effect of the
resolution and our analytical prescription suggest that the column
densities of HI are likely built up in large-scale warm neutral medium
and cold neutral medium (CNM) structures correlated in density over
∼20pc and ∼10pc, respectively, while those of H2 are built up in CNM
structures between ∼3 and ∼10pc. Combining the chemical and
statistical information contained in the observations of HI and H2
sheds new light on the study of the diffuse matter. Applying this new
tool to several atomic and molecular species is a promising
perspective to understanding the effects of turbulence, magnetic
field, thermal instability, and gravity on the formation and evolution
of molecular clouds.
Description:
The observational sample studied in this work is built from the
database of Gudennavar et al. (2012, Cat. J/ApJS/199/8) who compiled
existing data of atomic and molecular lines observed in absorption
toward several thousand sources, including stars and AGNs. Limiting
this catalog to observations or tentative detections of HI, H2, and of
the reddening E(B-V), and removing the data associated to the
Magellanic Cloud or high redshift extragalactic environments (e.g.,
Tumlinson et al., 2002ApJ...566..857T 2002ApJ...566..857T; Cartledge et al.,
2005ApJ...630..355C 2005ApJ...630..355C; Welty & Crowther, 2010, Cat. J/MNRAS/404/1321;
Noterdaeme et al., 2007A&A...469..425N 2007A&A...469..425N), we obtain a sample of 360
sources which form, to date, the most complete set of observations of
the HI-to-H2 transition in the local diffuse ISM.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 76 360 Observational dataset used in this work
refs.dat 66 48 References
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See also:
J/ApJS/199/8 : Interstellar column densities compilation (Gudennavar+, 2012)
Byte-by-byte Description of file: tablea1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 15 A15 --- Name Source ID
17- 23 F7.3 deg GLON Galactic longitude
25- 31 F7.3 deg GLAT Galactic latitude
33- 38 F6.3 kpc Dist ?=- Distance (1)
40- 44 F5.3 mag E(B-V) ?=- Extinction
46- 47 I2 --- r_E(B-V) ? Reference for extinction, in refs.dat file
50 A1 --- l_logN(HI) Limit flag on logN(HI)
51- 55 F5.2 [cm-2] logN(HI) ?=- HI column density
57- 58 I2 --- r_logN(HI) ? Reference for logN(HI), in refs.dat file
61 A1 --- l_logN(H2) Limit flag on logN(H2)
62- 66 F5.2 [cm-2] logN(H2) ?=- H2 column density
68- 69 I2 --- r_logN(H2) ? Reference for logN(H2), in refs.dat file
72- 76 F5.2 [cm-2] logNH Total proton column density (2)
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Note (1): The distance of each source is computed from the parallax measured by
Gaia if the data is given in the DR2 catalog (Gaia Collaboration 2018,
Cat. I/345); otherwise, the distance of the source is taken from
Gudennavar et al. (2012, Cat. J/ApJS/199/8).
Note (2): The total proton column densities NH are computed as N(HI)+N(H2) if
the column densities of HI and H2 are available, or derived from the reddening
E(B-V) as NH=5.8x1021E(B-V)cm-2 assuming a standard Galactic extinction
curve and the average interstellar ratio RV=AV*E(B-V)=3.1
(Fitzpatrick & Massa, 1986ApJ...307..286F 1986ApJ...307..286F; Fitzpatrick, 1999PASP..111...63F 1999PASP..111...63F).
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Byte-by-byte Description of file: refs.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Ref Reference code
4- 22 A19 --- BibCode BibCode
24- 45 A22 --- Aut Author's name
48- 66 A19 --- Com Comments
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 20-Jan-2021