H_I-H₂ transition: Exploring the role of the magnetic field

Abstract

Context. 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. Aims. Our goal is to explore the role of the magnetic field in the H i-H 2 transition process. Methods. We targeted a diffuse ISM filamentary cloud toward the Ursa Major cirrus where gas transitions from atomic to molecular. We probed the magnetic field properties of the cloud with optical polarization observations. We performed multiwavelength spectroscopic observations of different species in order to probe the gas phase properties of the cloud. We observed the CO (J=1-0) and (J=2-1) lines in order to probe the molecular content of the cloud. We also obtained observations of the [C ii] 157.6 µm emission line in order to trace the CO-dark H 2 gas and estimate the mean volume density of the cloud. Results. We identified two distinct subregions within the cloud. One of the regions is mostly atomic, 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 13 − 30 µG. The total magnetic field strength does not scale with density. This implies that gas is compressed along the field lines. We also found that turbulence is trans-Alfvénic, with M A ≈ 1. In the molecular region, we detected an asymmetric CO clump whose minor axis is closer, with a 24 • deviation, to the mean magnetic field orientation than the angle of its major axis. The H i 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. This phenomenon is likely related to gas undergoing gravitational infall. The magnetic field morphology of the target cloud is parallel to the H i column density structure of the cloud in the atomic region, while it tends to become perpendicular to the H i structure in the molecular region. On the other hand, the magnetic field morphology seems to form a smaller offset angle with the total column density shape (including both atomic and molecular gas) of this transition cloud. Conclusions. In the target cloud where the H i-H 2 transition takes place, turbulence is trans-Alfvénic, and hence the magnetic field plays an important role in the cloud dynamics. Atomic gas probably accumulates preferentially along the magnetic field lines and creates overdensities where molecular gas can form. The magnetic field morphology is probed better by the total column density shape of the cloud, and not its H i column density shape.

On the Origin of the North Celestial Pole Loop

“…Using PanSTARRS photometry, Schlafly et al (2014) found a much larger distance of about 350 ± 25 pc for the Ursa Major MBM 30 cloud, placing it at about the same distance as Polaris. Using the 3D dust extinction map by Green et al (2018), Tritsis et al (2019 found a distance of about 300 pc toward the western part of the Ursa Major complex, consistent with recent polarimetric data presented in Skalidis et al (2022) In this work, we find that complexes in the NCPL, as well as the diffuse material that surrounds them, have distances varying from about 310 to 450 pc (Section 3).…”

“…In a gaseous disk affected by supernovae at a rate on the order of 1 per 50 yr galaxy wide, interconnected cavities and tunnels are expected from the growth of clusters and chains of connected supernova remnants (Cox & Smith 1974). Cox & Smith (1974) predicted that the resulting tunnel network would have low density, high temperature, and a very low magnetic field strength; the field properties seem consistent with the minimum of emission at 408 MHz inside the cavity (M91) and the enhancement of the magnetic field strength in complexes along the loop (Heiles 1989;Myers et al 1995;Pound & Goodman 1997;Tritsis et al 2019;Skalidis et al 2022).…”

“…If at a distance of 150 pc. But see the larger distance estimate in Section 3.4 See alsoTritsis et al (2019) andSkalidis et al (2022) for complementary methods probing the plane-of-sky component.…”

“…The North Celestial Pole Loop (NCPL; Heiles 1984Heiles , 1989Meyerdierks et al 1991, hereafter M91), first referred to as the Polar ridge by Fejes & Wesselius (1973), has been of interest not only because of its distinctive circular geometry on the sky and relatively high gas density for its position high above the Galactic plane, but also for the formation of molecules in individual clouds embedded within diffuse (dusty) H I gas (e.g., H 2 CO, NH 3 , CO, and C II; see Magnani et al 1985;Heithausen et al 1987;Mebold et al 1987;Magnani et al 1988;Grossmann et al 1990;Skalidis et al 2022). Among these clouds are some of the high-latitude molecular clouds (HLCs) discovered by Blitz et al (1984) and cataloged by Magnani et al (1985, hereafter MBM).…”

“…The Spider, named for its prominent "legs" emanating from a central "body," is located at the "top" of the loop (in Galactic coordinates, as used throughout this paper); it has been examined in complementary studies of thermal dust emission and molecules along with H I (Barriault et al 2010;Planck Collaboration XXIV 2011;Martin et al 2015;Blagrave et al 2017). The Ursa Major complex, including MBM clouds 27-30, is located further east; extensive studies have provided valuable information on the kinematic, magnetic field, and turbulent structure of the diffuse ISM, and on the transition from the neutral atomic to the molecular phase (Myers et al 1995;Pound & Goodman 1997;Miville-Deschênes et al 2003;Tritsis et al 2019;Skalidis et al 2022).…”