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We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains (⁠>0.005−0.1μm), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion.

We present the Green Bank 100m Telescope (GBT) mapping observations of CS 1-0, and the Submillimeter Array (SMA) 157-pointings mosaic of the 0.86 mm dust continuum emission as well as several warm and dense gas tracers, in the central ~20 pc area in Galactic Center. The unprecedentedly large field-of-view and the high angular resolution of our SMA dust image allow the identification of abundant 0.1-0.2 pc scale dense gas clumps. We found that in the Galactic Center, the Class I methanol masers are excellently correlated with the dense gas clumps. However, on the ~0.1 pc scale, these dense gas clumps still have a extremely large linewidth (FWHM~10-20 km/s). Simple calculations suggest that the identified clumps can be possibly the pressurized gas reservoir feeding the formation of 1-10 solar-mass stars. These gas clumps may be the most promising candidates for ALMA to resolve the high-mass star-formation in the Galactic center., Proceedings to appear in "Massive Young Star Clusters Near and Far: From the Milky Way to Reionization", 2013 Guillermo Haro Conference. Eds. Y. D. Mayya, D. Rosa-Gonzalez, & E. Terlevich, INAOE and AMC. 4 pages, 2 figures

We present the Submillimeter Array (SMA) 157-pointing mosaic in 0.86 mm dust continuum emission with 5$".1\times4"$.2 angular resolution, and the NRAO Green Bank 100m Telescope (GBT) observations of the CS/C$^{34}$S/$^{13}$CS 1-0 and SiO 1-0 emission with $\le20"$$\times18"$ angular resolution. The dust continuum image marginally resolves at least several tens of 10-10$^{2}$ $M_{\odot}$ dense clumps in the 5$'$ field including the circumnuclear disk (CND) and the exterior gas streamers. There is very good agreement between the high resolution dust continuum map of the CND and all previous molecular line observations. As the dust emission is the most reliable optically thin tracer of the mass, free from most chemical and excitation effects, we demonstrate the reality of the abundant localized structures within the CND, and their connection to external gas structures. From the spectral line data, the velocity dispersions of the dense clumps and their parent molecular clouds are $\sim$10-20 times higher than their virial velocity dispersions. This supports the idea that the CND and its immediate environment may not be stationary or stable structures. Some of the dense gas clumps are associated with 22 GHz water masers and 36.2 GHz and 44.1 GHz CH$_{3}$OH masers. However, we do not find clumps which are bound by the gravity of the enclosed molecular gas. Hence, the CH$_{3}$OH or H$_{2}$O maser emission may be due to strong (proto)stellar feedback, which may be dispersing some of the gas clumps., Comment: 7 pages, 15 figures; Accepted by ApJ

We are developing the Precision Array for Probing the Epoch of Reionization (PAPER) to detect 21cm emission from the early Universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a 4-antenna array in the low-RFI environment of Western Australia and an 8-antenna array at our prototyping site in Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e-5 steradians at 154 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra ($C_\ell$) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the cosmic variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at $\ell=100$ for a 1.46-MHz band centered at 164.5 MHz. This sensitivity level is approximately three orders of magnitude in temperature above the level of the fluctuations in 21cm emission associated with reionization., 13 pages, 14 figures, submitted to AJ. Revision 2 corrects a scaling error in the x axis of Fig. 12 that lowers the calculated power spectrum temperature

A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realizing this goal would open a new window on the study of General Relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of an event horizon, and fundamental black hole physics (e.g., spin). Steady long-term progress on improving the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made it extremely likely that this goal will be achieved within the next decade. The most compelling evidence for this is the recent observation by 1.3mm VLBI of Schwarzschild radius scale structure in SgrA*, the compact source of radio, submm, NIR and xrays at the center of the Milky Way. SgrA* is thought to mark the position of a ~4 million solar mass black hole, and because of its proximity and estimated mass presents the largest apparent event horizon size of any black hole candidate in the Universe. Over the next decade, existing and planned mm/submm facilities will be combined into a high sensitivity, high angular resolution "Event Horizon Telescope" that will bring us as close to the edge of black hole as we will come for decades. This white paper describes the science case for mm/submm VLBI observations of both SgrA* and M87 (a radio loud AGN of a much more luminous class that SgrA*). We emphasize that while there is development and procurement involved, the technical path forward is clear, and the recent successful observations have removed much of the risk that would normally be associated with such an ambitious project., Comment: 8 pages, Science White Paper submitted to the ASTRO2010 Decadal Review Panels

We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg with rms scatters of 0.4% and 9 deg over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission., Ap.J.Lett. accepted

We report here BIMA array observations of linear polarization in Sagittarius A*, the compact radio source in the Galactic Center. These observations had a resolution of 20 arcsec x 5 arcsec oriented North-South. We do not detect linear or circular polarization at a level of 1.8% (1-sigma) at 112 GHz. This puts a new constraint on models that show a sharp change in linear polarization fraction at 100 GHz., 6 pages, 2 figures, ApJL, in press

We summarize the results of 7 mm VLBA imaging of Sgr A* and discuss some of the difficulties of accurately constraining the size of Sgr A* with VLBI observations. Our imaging results are fully consistent with the hypothesis that the VLBA image of Sgr A* is a resolved elliptical Gaussian caused by the scattering of an intervening thermal plasma. We show that determination of the minor axis size at 7 mm with the VLBA is very unreliable. We also present new polarimetric observations from the VLA and from the BIMA array of Sgr A*. At 4.8 GHz, we find an upper limit to the polarization of 0.1%. At 86 GHz, we report a marginal detection of $1 \pm 1$% linear polarization. We discuss the effects of interstellar propagation on the linear polarization and consider the significance of very low intrinsic linear polarization in Sgr A*., Comment: 10 pages, 3 figures, to appear in "The Central Parsecs of the Galaxy", eds. H. Falcke, A. Cotera, W. Duschl, F. Melia, M. Rieke, ASP Conf. Series