skip to main content

Title: Length-scales and dynamics of Carina’s Western wall

We present a variety of analyses of the turbulent dynamics of the boundary of a photodissociation region (PDR) in the Carina Nebula using high resolution ALMA observations. Using principal component analysis, we suggest that the turbulence in this molecular cloud is driven at large scales. Analysis of the centroid velocity structure functions indicate that the turbulence is dominated by shocks rather than local (in k-space) transport of energy. We further find that length-scales in the range 0.02–0.03 pc are important in the dynamics of this cloud and this finding is supported by analysis of the dominant emission structure length-scale. These length-scales are well resolved by the observational data and we conclude that the apparent importance of this range of scales is physical in origin. Given that it is also well within the range strongly influenced by ambipolar diffusion, we conclude that it is not primarily a product of turbulence alone, but is more likely to be a result of the interplay between gravity and turbulence. Finally, through comparison of these results with previous observations of H2 emission from the Western Wall, we demonstrate that observations of a PDR can be used to probe the internal structure of the undisturbed portion of a molecular cloud.

more » « less
Author(s) / Creator(s):
; ;
Publisher / Repository:
Oxford University Press
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Medium: X Size: p. 5427-5438
p. 5427-5438
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Photodissociation regions (PDRs), where the (far-)ultraviolet light from hot young stars interact with the gas in surrounding molecular clouds, provide laboratories for understanding the nature and role of feedback by star formation on the interstellar medium. While the general nature of PDRs is well understood—at least under simplified conditions—the detailed dynamics and chemistry of these regions, including gas clumping, evolution over time, etc., can be very complex. We present interferometric observations of the 21 cm atomic hydrogen line, combined with [Cii] 158μm observations, toward the nearby reflection nebula IC 63. We find a clumpy Histructure in the PDR, and a ring morphology for the Hiemission at the tip of IC 63. We further unveil kinematic substructure, of the order of 1 km s−1, in the PDR layers and several legs that will disperse IC 63 in <0.5 Myr. We find that the dynamics in the PDR explain the observed clumpy Hidistribution and lack of a well-defined Hi/H2transition front. However, it is currently not possible to conclude whether Hiself-absorption and nonequilibrium chemistry also contribute to this clumpy morphology and missing Hi/H2transition front.

    more » « less
  2. Context. The role of large-scale magnetic fields in the evolution of star-forming regions remains elusive. Its investigation requires the observational characterization of well-constrained molecular clouds. The Monoceros OB 1 molecular cloud is a large complex containing several structures that have been shown to be engaged in an active interaction and to have a rich star formation history. However, the magnetic fields in this region have only been studied on small scales. Aims. We study the large-scale magnetic field structure and its interplay with the gas dynamics in the Monoceros OB 1 east molecular cloud. Methods. We combined observations of dust polarized emission from the Planck telescope and CO molecular line emission observations from the Taeduk Radio Astronomy Observatory 14-metre telescope. We calculated the strength of the plane-of-sky magnetic field using a modified Chandrasekhar-Fermi method and estimated the mass-over-flux ratios in different regions of the cloud. We used the comparison of the velocity and intensity gradients of the molecular line observations with the polarimetric observations to trace dynamically active regions. Results. The molecular complex shows an ordered large-scale plane-of-sky magnetic field structure. In the northern part, it is mostly orientated along the filamentary structures, while the southern part shows at least two regions with distinct magnetic field orientations. Our analysis reveals a shock region in the northern part right between two filamentary clouds that, in previous studies, were suggested to be involved in a collision. The magnetic properties of the north-main and north-eastern filaments suggest that these filaments once formed a single one, and that the magnetic field evolved together with the material and did not undergo major changes during the evolution of the cloud. In the southern part, we find that either the magnetic field guides the accretion of interstellar matter towards the cloud or it is dragged by the matter falling towards the main cloud. Conclusions. The large-scale magnetic field in the Monoceros OB 1 east molecular cloud is tightly connected to the global structure of the complex. In the northern part, it seems to serve a dynamically important role by possibly providing support against gravity in the direction perpendicular to the field and to the filament. In the southern part, it is probably the most influential factor governing the morphological structure by guiding possible gas inflow. A study of the whole Monoceros OB 1 molecular complex at large scales is necessary to form a global picture of the formation and evolution of the Monoceros OB 1 east cloud and the role of the magnetic field in this process. 
    more » « less
  3. Abstract We have measured the gas temperature in the IC 63 photodissociation region (PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with SOFIA/EXES. We divide the PDR into three regions for analysis based on the illumination from γ Cas: sunny, ridge, and shady. Constructing rotation diagrams for the different regions, we obtain temperatures of T ex = 562 − 43 + 52 K toward the ridge and T ex = 495 − 25 + 28 K in the shady side. The H 2 emission was not detected on the sunny side of the ridge, likely due to the photodissociation of H 2 in this gas. Our temperature values are lower than the value of T ex = 685 ± 68 K using the S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. using lower spatial resolution ISO-SWS data at a different location of the IC 63 PDR. This difference indicates that the PDR is inhomogeneous and illustrates the need for high-resolution mapping of such regions to fully understand their physics. The detection of a temperature gradient correlated with the extinction into the cloud, points to the ability of using H 2 pure rotational line spectroscopy to map the gas temperature on small scales. We used a PDR model to estimate the FUV radiation and corresponding gas densities in IC 63. Our results shows the capability of SOFIA/EXES to resolve and provide detailed information on the temperature in such regions. 
    more » « less
  4. 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 µ m 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. 
    more » « less
  5. The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the 13 CO (2–1) line at an angular resolution of 28′′. We analyse the influences of the spiral arms by considering the features of the molecular gas emission as a whole across the longitude–velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of ~10 5 –10 6 M ⊙ kpc −1 . This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one. 
    more » « less