Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138

We present three epochs of VLBA observations of Galactic HI absorption toward the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes VLBA data from observations in 1999 and 2002 along with a reexamination of 1995 VLBA data. Improved data reduction and imaging techniques have led to an order of magnitude improvement in sensitivity compared to previous work. With these new data we confirm the previously detected milliarcsecond scale spatial variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05. The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In addition, for the first time we see clear evidence for temporal variations in the HI opacity over the seven year time span of our three epochs of data. We also attempted to detect the magnetic field strength in the HI gas using the Zeeman effect. From this analysis we have been able to place a 3 sigma upper limit on the magnetic field strength per pixel of ~45 muG. We have also been able to calculate for the first time the plane of sky covering fraction of the small scale HI gas of ~10%. We also find that the line widths of the milliarcsecond sizescale HI features are comparable to those determined from previous single dish measurements toward 3C~138, suggesting that the opacity variations cannot be due to changes in the HI spin temperature. From these results we favor a density enhancement interpretation for the small scale HI structures, although these enhancements appear to be of short duration and are unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A

VLA Observations of the "Eye of the Tornado"- the High Velocity \HII Region G357.63-0.06

The unusual supernova remnant candidate G357.7-0.1 and the compact source G357.63-0.06 have been observed with the Very Large Array at 1.4 and 8.3 GHz. The H92$\alpha$ line (8.3 GHz) was detected from the compact source with a surprising velocity of about -210 km/s indicating that this source is an \HII region, is most likely located at the Galactic center, and is unrelated to the SNR. The \HI absorption line (1.4 GHz) data toward these sources supports this picture and suggests that G357.7-0.1 lies farther away than the Galactic center.Comment: Latex, 14 pages including 4 figures. Accepted to A

Self-gravitating disc candidates around massive young stars

DHF gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG. JDI gratefully acknowledges support from the DISCSIM project, grant agreement 341137, funded by the European Research Council under ERC-2013-ADG. CJC acknowledges support from STFC grant ST/M001296/1.There have been several recent detections of candidate Keplerian discs around massive young protostars. Given the relatively large disc-to-star mass ratios in these systems, and their young ages, it is worth investigating their propensity to becoming self-gravitating. To this end, we compute self-consistent, semi-analytic models of putative self-gravitating discs for five candidate disc systems. Our aim is not to fit exactly the observations, but to demonstrate that the expected dust continuum emission from marginally unstable self-gravitating discs can be quite weak, due to high optical depth at the mid-plane even at millimetre wavelengths. In the best cases, the models produce ‘observable’ disc masses within a factor of <1.5 of those observed, with mid-plane dust temperatures comparable to measured temperatures from molecular line emission. We find in two cases that a self-gravitating disc model compares well with observations. If these discs are self-gravitating, they satisfy the conditions for disc fragmentation in their outer regions. These systems may hence have as-yet-unresolved low-mass stellar companions, and are thus promising targets for future high angular resolution observations.PostprintPeer reviewe

OH Zeeman Magnetic Field Detections Toward Five Supernova Remnants Using the VLA

We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to measure their magnetic field strengths using the Zeeman effect. We detected all 12 of the bright ($S_{\nu} > 200$ mJy) OH (1720 MHz) masers previously detected by Frail et al. (1996) and Green et al. (1997) and measured significant magnetic fields (i.e. $> 3\sigma$) in ten of them. Assuming that the ``thermal'' Zeeman equation can be used to estimate $\mid\vec{B}\mid$ for OH masers, our estimated fields range from 0.2 to 2 mG. These magnetic field strengths are consistent with the hypothesis that ambient molecular cloud magnetic fields are compressed via the SNR shock to the observed values. Magnetic fields of this magnitude exert a considerable influence on the properties of the cloud with the magnetic pressures ($10^{-7} - 10^{-9}$ erg cm$^{-3}$) exceeding the pressure in the ISM or even the thermal pressure of the hot gas interior to the remnant. This study brings the number of galactic SNRs with OH (1720 MHz) Zeeman detections to ten.Comment: 23 pages, 14 figures, accepted to ApJ, for higher resolution images of Figs 4,11, and 12 see http://www.pa.uky.edu/~brogan/brog_publ.htm

The (Re-)Discovery of G350.1-0.3: A Young, Luminous Supernova Remnant and Its Neutron Star

We present an XMM-Newton observation of the long-overlooked radio source G350.1-0.3. The X-ray spectrum of G350.1-0.3 can be fit by a shocked plasma with two components: a high-temperature (1.5 keV) region with a low ionization time scale and enhanced abundances, plus a cooler (0.36 keV) component in ionization equilibrium and with solar abundances. The X-ray spectrum and the presence of non-thermal, polarized, radio emission together demonstrate that G350.1-0.3 is a young, luminous supernova remnant (SNR), for which archival HI and 12-CO data indicate a distance of 4.5 kpc. The diameter of the source then implies an age of only ~900 years. The SNR's distorted appearance, small size and the presence of 12-CO emission along the SNR's eastern edge all indicate that the source is interacting with a complicated distribution of dense ambient material. An unresolved X-ray source, XMMU J172054.5-372652, is detected a few arcminutes west of the brightest SNR emission. The thermal X-ray spectrum and lack of any multi-wavelength counterpart suggest that this source is a neutron star associated with G350.1-0.3, most likely a "central compact object", as seen coincident with other young SNRs such as Cassiopeia A.Comment: 6 pages, uses emulateapj. One B/W figure, one color figure. Minor text changes and update to Fig 2 following referee's report. ApJ Letters, in pres

Evidence for a Massive Protocluster in S255N

S255N is a luminous far-infrared source that contains many indications of active star formation but lacks a prominent near-infrared stellar cluster. We present mid-infrared through radio observations aimed at exploring the evolutionary state of this region. Our observations include 1.3mm continuum and spectral line data from the Submillimeter Array, VLA 3.6cm continuum and 1.3cm water maser data, and multicolor IRAC images from the Spitzer Space Telescope. The cometary morphology of the previously-known UCHII region G192.584-0.041 is clearly revealed in our sensitive, multi-configuration 3.6cm images. The 1.3mm continuum emission has been resolved into three compact cores, all of which are dominated by dust emission and have radii < 7000AU. The mass estimates for these cores range from 6 to 35 Msun. The centroid of the brightest dust core (SMA1) is offset by 1.1'' (2800 AU) from the peak of the cometary UCHII region and exhibits the strongest HC3N, CN, and DCN line emission in the region. SMA1 also exhibits compact CH3OH, SiO, and H2CO emission and likely contains a young hot core. We find spatial and kinematic evidence that SMA1 may contain further multiplicity, with one of the components coincident with a newly-detected H2O maser. There are no mid-infrared point source counterparts to any of the dust cores, further suggesting an early evolutionary phase for these objects. The dominant mid-infrared emission is a diffuse, broadband component that traces the surface of the cometary UCHII region but is obscured by foreground material on its southern edge. An additional 4.5 micron linear feature emanating to the northeast of SMA1 is aligned with a cluster of methanol masers and likely traces a outflow from a protostar within SMA1. Our observations provide direct evidence that S255N is forming a cluster of intermediate to high-mass stars.Comment: 34 pages, 11 figures, accepted for publication in The Astronomical Journa