J/A+A/665/A77 HI-H2. exploring the role of magnetic field (Skalidis+, 2022)
HI-H2 transition: Exploring the role of the magnetic field;
A case study towards the Ursa Major cirrus.
Skalidis R., Tassis K., Panopoulou G.V., Pineda J.L., Gong Y.,
Mandarakas N., Blinov D., Kiehlmann S., Kypriotakis J.A.
<Astron. Astrophys. 665, A77 (2022)>
=2022A&A...665A..77S 2022A&A...665A..77S (SIMBAD/NED BibCode)
ADC_Keywords: Polarization ; Interstellar medium ; Magnetic fields
Keywords: polarization - ISM: magnetic fields -
ISM: individual objects: North Celestial Pole Loop
(except planetary nebulae) - ISM: clouds - ISM: molecules
Abstract:
Atomic gas in the diffuse interstellar medium (ISM) is organized in
filamentary structures. These structures usually host cold and dense
molecular clumps. The Galactic magnetic field is considered to play an
important role in the formation of these clumps. Our goal is to
explore the role of the magnetic field in the HI-H2 transition
process. We targeted a filamentary cloud where gas transitions from
atomic to molecular. This cloud is located at the edges of an
expanding structure, known as the North Celestial Pole Loop (NCPL). We
probed the magnetic field properties of the cloud with optical
polarization observations. We performed multi-wavelength spectroscopic
observations of different species in order to probe the gas phase
properties of the cloud. We identified two distinct sub-regions within
the cloud. One of the regions hosts purely atomic gas, while the other
is dominated by molecular gas although most of it is CO-dark. The
estimated plane-of-the-sky magnetic field strength between the two
regions remains constant within uncertainties and lies in the range
20-30 micro Gauss. The total magnetic field strength does not scale
with density which implies that gas is compressed along the field
lines. We also found that turbulence is sub-Alfvenic. The HI velocity
gradients are in general perpendicular to the mean magnetic field
orientation, except for the region close to the CO clump where they
tend to become parallel. The latter is likely related to gas
undergoing gravitational infall. The magnetic field morphology of the
target cloud is parallel to the HI column density structure of the
cloud in the atomic region, while it tends to become perpendicular to
the HI structure in the molecular region.
Description:
The polarization data were obtained with the RoboPol polarimeter
mounted at the 1.3m telescope at the Skinakas Observatory in Crete,
Greece. All observations were carried out in the R band. All
measurements are included; even those with low signal to noise ratio
in the degree of polarization.
We carried out CO (1-0) observations toward the target cloud with
the Purple Mountain Observatory 13.7m telescope (PMO-13.7m) from
June 3 to 15, 2019 (project code: 19C002).
We used the Heinrich Hertz Submillimeter Telescope on Mt. Graham,
Arizona, to measure the J=2-1 transition of CO toward the target
cloud. The data were collected on March 1, 3, and 4, 2019. Following
Bieging et al. (2014ApJS..214....7B 2014ApJS..214....7B), we used the ALMA Band 6 sideband
separating mixers dual polarization receiver to observe the CO (J=2-1)
(230.5GHz) line. The spectrometers were filter banks with 0.25MHz
bandwidth and 256 filters, providing a spectral resolution of 0.33km/s.
Objects:
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RA (2000) DE Designation(s)
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09 35 +70.0 Ursa Major cirrus = NAME UMa Cirrus
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
um-robop.dat 157 272 Polarization data
list.dat 109 2 List of fits datacubes
fits/* . 2 Individual fits datacubes
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See also:
I/284 : The USNO-B1.0 Catalog (Monet+ 2003)
I/352 : Distances to 1.47 billion stars in Gaia EDR3 (Bailer-Jones+, 2021)
Byte-by-byte Description of file: um-robop.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- USNOB USNO-B1.0 id (1)
14- 22 F9.5 deg RAdeg Right Ascension (J2000) (1)
24- 31 F8.5 deg DEdeg Declination (J2000) (1)
33- 41 E9.3 --- p Degree of polarization (2)
44- 52 E9.3 --- e_p Observational uncertainty in
polarization degree (2)
53- 61 F9.5 deg evpa Electric vector polarization angle (3)
63- 71 F9.5 deg e_evpa Observational uncertainty in
polarization angle (3)
73- 82 E10.4 --- q q Stokes parameter (2)
85- 92 E8.3 --- e_q Error in q (3)
94-103 E10.4 --- u u Stokes parameter (2)
106-113 E8.3 --- e_u Error in u (3)
115-126 F12.6 pc dist Star distance (4)
128-138 F11.6 pc b_dist Lower star distance limit (4)
140-151 F12.6 pc B_dist Upper star distance limit (4)
153-157 F5.2 mag Rmag R band magnitude (1)
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Note (1): Stars' ids, coordinates, and magnitudes correspond to those in the
USNO-B1.0 catalog Monet et al. (2003AJ....125..984M 2003AJ....125..984M, Cat. I/284).
Note (2): The polarization degree, and the q, u Stokes parameters are
dimensionless. The degree of polarization has not been debiased.
Note (3): The Electric Vector Polarization Angle (EVPA) is measured in the
celestial coordinate system starting from the North direction and increasing
towards the East and is given in the range [-90,90], as specified by the IAU.
Note (4): Stars' distances have been extracted from the Bailer-Jones catalog
(Bailer-Jones et al., 2021AJ....161..147B 2021AJ....161..147B, Cat. I/352).
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 30 I3 --- Nz Number of slices
32- 39 F8.1 m/s bVRAD Lower value of VRAD interval
41- 47 F7.1 m/s BVRAD Lower value of VRAD interval
49- 55 F7.3 m/s dVRAD VRAD resolution
57- 61 I5 Kibyte size Size of FITS file
63- 79 A17 --- FileName Name of FITS file, in subdirectory fits
81-109 A29 --- Title Title of the FITS file
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Acknowledgements:
Raphael Skalidis, rskalidis(at)physics.uoc.gr
(End) Patricia Vannier [CDS] 04-Jul-2022