Molecular Cloud Formation out of the Atomic Phase

This thesis is dedicated to study observationally the formation of molecular cloud formation out of the atomic phase. In the framework of 'The HI, OH, Recombination Line survey of the Milky Way' (THOR), we used the Very Large Array (VLA) to map the 21 cm HI line, 4 OH transitions, up to 19 H alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way. The goal of this thesis is to characterize and study the atomic and molecular content of molecular clouds. Observations of the giant molecular cloud complex associated with the W43 star formation complex revealed large quantities of optically thick HI emission. We used strong continuum sources to measure the optical depth and used this information to correct the HI column density and hence the HI mass. Without these corrections, the mass estimate is at least a factor of 2.4 too low. In addition, we observed HI column densities up to N(HI) ~ 150 M_sun/pc^2, which is in contradiction to current cloud formation models. Furthermore, we present a catalog of ~4400 continuum sources, extracted from the first half of the THOR survey. Due to the broad bandwidth from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 bright sources, which allows us to distinguish between thermal and non-thermal radiation. Using this information, we can confirm four super nova remnant candidates. Beside the direct scientific studies of these evolved sources, this catalog is the basis for prospective absorption studies of the HI and OH lines. By extracting HI self absorption (HISA) features of the molecular filament GMF38.1-32.4, we examined directly the cold and dense atomic hydrogen and compared it to the molecular counterpart. We studied the kinematics and column density probability distribution functions (PDFs) and found a log-normal shape for them, which indicates turbulent motion. Furthermore, we might observe several evolutionary stages within the filament, which will be helpful to validate theoretical models and simulations

Kinematic and thermal structure at the onset of high-mass star formation

Context. Even though high-mass stars are crucial for understanding a diversity of processes within our galaxy and beyond, their formation and initial conditions are still poorly constrained. Aims: We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps? Methods: To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH3 (1, 1) and (2, 2) lines within six very young high-mass star-forming regions comprised of infrared dark clouds (IRDCs), along with ISO-selected far-infrared emission sources (ISOSS) with the Karl G. Jansky Very Large Array (VLA) and the Effelsberg 100 m Telescope. Results: The molecular line data allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3-5'', corresponding to ~0.05 pc at a typical source distance of 2.5 kpc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. Several infrared emission sources show ammonia temperature peaks up to 30 K, whereas other infrared emission sources show no enhanced kinetic gas temperature in their surrounding. We report an upper limit for the linewidth of ~1.3 km s-1, at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s-1 and gradients of 5 to 10 km s-1 pc-1. These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s-1 pc-1. This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources

Hierarchical fragmentation and collapse signatures in a high-mass starless region

Aims: We study the fragmentation and collapse properties of the dense gas during the onset of high-mass star formation. Methods: We observed the massive (~800 M⊙) starless gas clump IRDC 18310-4 with the Plateau de Bure Interferometer (PdBI) at subarcsecond resolution in the 1.07 mm continuum and N2H+(3-2) line emission. Results: Zooming from a single-dish low-resolution map to previous 3 mm PdBI data, and now the new 1.07 mm continuum observations, the substructures hierarchically fragment on the increasingly smaller spatial scales. While the fragment separations may still be roughly consistent with pure thermal Jeans fragmentation, the derived core masses are almost two orders of magnitude larger than the typical Jeans mass at the given densities and temperatures. However, the data can be reconciled with models using non-homogeneous initial density structures, turbulence, and/or magnetic fields. While most subcores remain (far-)infrared dark even at 70 μm, we identify weak 70 μm emission toward one core with a comparably low luminosity of ~16 L⊙, supporting the notion of the general youth of the region. The spectral line data always exhibit multiple spectral components toward each core with comparably small line widths for the individual components (in the 0.3 to 1.0 km s-1 regime). Based on single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio ≤ 0.25. We propose that the likely origin of these spectral properties may be the global collapse of the original gas clump that results in multiple spectral components along each line of sight. Even within this dynamic picture the individual collapsing gas cores appear to have very low levels of internal turbulence

Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ∼ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the 13CO emission. The peak velocities of the HISA and 13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1 is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020 to 1021 cm−2 . The column density of molecular hydrogen, traced by 13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and 13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motionsbeing the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Y.W., H.B., S.B., and J.D.S. acknowledge support from the European Research Council under the Horizon 2020 Framework Program via the ERC Consolidator Grant CSF-648505, and R.S.K. via the ERC AdvancedGrant 339177 (STARLIGHT). H.B., S.C.O.G., and M.R. acknowledge support from the Deutsche Forschungsgemeinschaft in the Collaborative Research Center (SFB 881) “The Milky Way System” (subproject B1, B2, B8). R.S.K. and S.C.O.G. also acknowledge support from the DFG via Germany’s Excellence Strategy EXC-2181/1 - 390900948 (the Heidelberg STRUCTURES Cluster of Excellence). This work was carried out in part at the Jet Propulsion Laboratory which is operated for NASA by the California Institute of Technology. N.S. acknowledges support by the french ANR and the german DFG through the project “GENESIS” (ANR-16-CE92-0035-01/DFG1591/2-1). F.B. acknowledges funding from the European Union’s Horizon 2020 research and innovation program (grant agreement No 726384). This research made use of Astropy and affiliated packages, a community-developed core Python package for Astronomy (Astropy Collaboration 2018), Python package SciPy3 , APLpy, an open-source plotting package for Python (Robitaille & Bressert 2012), and software TOPCAT (Taylor 2005). The authors thank the anonymous referee for the constructive comments that improve the paper

THOR Extended Observations: Spectral Line (3D)

This dataset consists of extended spectral line observations taken by the Very Large Array (VLA), over l = 15° − 67°, |b| ≤ 1°. There are a variety of data products available. The first includes HI 21 cm observations with a bandwidth of 2 MHz and a channel width of 1.953 kHz. This yields a velocity range of ±200 km/s and a spectral resolution of 0.41 km/s. Additional data products available include 4 OH lines (1612, 1665, 1667 and 1720 MHz) and 19 Hα radio recombination lines. As of Fall 2016, they provide the first half of the survey data (HI, OH, and RRL data, along with continuum data) for public download here.</a

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Continuum sources from the THOR survey between 1 and 2 GHz

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): “THOR: The H i, OH, Recombination line survey of the Milky Way”. We observed a significant portion (~100 deg2) of the Galactic plane in the first quadrant of the Milky Way in the 21 cm H i line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2 GHz. In this paper we present a catalog of the continuum sources in the first half of the survey (l = 14.0−37.9° and l = 47.1−51.2°, | b | ≤ 1.1°) at a spatial resolution of 10−25″, depending on the frequency and sky position with a spatially varying noise level of ~0.3−1 mJy beam-1. The catalog contains ~4400 sources. Around 1200 of these are spatially resolved, and ~1000 are possible artifacts, given their low signal-to-noise ratios. Since the spatial distribution of the unresolved objects is evenly distributed and not confined to the Galactic plane, most of them are extragalactic. Thanks to the broad bandwidth of the observations from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 sources. The spectral index distribution reveals a double-peaked profile with maxima at spectral indices of α ≈ −1 and α ≈ 0, corresponding to steep declining and flat spectra, respectively. This allows us to distinguish between thermal and non-thermal emission, which can be used to determine the nature of each source. We examine the spectral index of ~300 known H ii regions, for which we find thermal emission with spectral indices around α ≈ 0. In contrast, supernova remnants (SNR) show non-thermal emission with α ≈ −0.5 and extragalactic objects generally have a steeper spectral index of α ≈ −1. Using the spectral index information of the THOR survey, we investigate potential SNR candidates. We classify the radiation of four SNR candidates as non-thermal, and for the first time, we provide strong evidence for the SNR origin of these candidates