Ralph A. Gaume

The Third US Naval Observatory CCD Astrograph Catalog (UCAC3)

The second US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC2 was released in 2003 July. Positions and proper motions for 48,330,571 sources (mostly stars) are available on 3 CDs, supplemented with Two Micron All Sky Survey photometry for 99.5% of the sources. The catalog covers the sky area from -90° to +40° declination, going up to +52° in some areas; this completely supersedes the UCAC1 released in 2001. Current epoch positions are obtained from observations with the USNO 8 inch (0.2 m) Twin Astrograph equipped with a 4K CCD camera. The precision of the positions are 15–70 mas, depending on magnitude, with estimated systematic errors of 10 mas or below. Proper motions are derived by using over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as yet unpublished remeasures of the AGK2 plates and scans from the NPM and SPM plates. Proper-motion errors are about 1–3 mas yr-1 for stars to 12th magnitude, and about 4–7 mas yr-1 for fainter stars to 16th magnitude. The observational data, astrometric reductions, results, and important information for the users of this catalog are presented.

Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC 443

We present arcsecond resolution observations from the VLA with full Stokes polarimetry of the ground-state satellite line of the hydroxyl molecule (OH) at 1720.53 MHz (2Π3/2, J = 3/2, F = 2 → 1) toward three Galactic supernova remnants: W28, W44, and IC 443. The total number of individual OH (1720 MHz) spots we detect in each of these three remnants is 41, 25, and 6, respectively. The OH (1720 MHz) features appear to lie along the edge of radio continuum emission from the supernova remnants, but they are displaced behind the leading edge of the shock as traced by the synchrotron emission. The brightness temperatures of the OH (1720 MHz) emission features range from 2 × 104 to 108 K, convincingly demonstrating the maser nature of the OH (1720 MHz) features. We argue that the partially resolved angular diameters that we measure for the masers are neither intrinsic sizes nor scattering disks, but result from a blend of several unresolved maser features near the same velocity. Thus, our computed brightness temperatures are lower limits to the true values. The characteristic antisymmetric S profile, indicative of Zeeman splitting in the weak-field case, is identified in the Stokes V spectrum of several of the brighter maser spots. The derived line-of-sight magnetic fields are of order 0.2 mG and are remarkably constant in both direction and magnitude over regions several parsecs apart. These are the first measurements of postshock magnetic fields in supernova remnants and demonstrate the importance of magnetic pressure in these molecular shocks. The velocity dispersion of the maser features is typically less than a few km s-1, and, except in the special case of W28, the mean maser velocity is equal to the systemic velocity of the remnant. We suggest that the maximum amplification of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support for this point comes from the location of the masers in IC 443, and from molecular observations that allow the shock geometry to be determined. All of our observations are consistent with a model in which the OH (1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a nondissociative shock. Finally, we end with a discussion of the importance of supernova remnants with OH (1720 MHz) maser emission as promising candidates to conduct high-energy searches for the sites of cosmic-ray acceleration.

Angular diameter measurements of stars

Angular diameters determined with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm. The uniform disk diameters resulting from fits to the visibility observations have rms residuals of order 1 percent for the 800 nm measurements and less than 3 percent for the 450 nm measurements. The improvement over previous observations with this instrument is due to improved data analysis and the use of a wider range of baseline lengths. An analysis of the calibration systematics for the Mark III Optical Interferometer is included. There is good agreement between these measurements and previously published data. The changes in uniform disk diameter between wavelengths of 450 and 800 nm agree with models of stellar atmospheres.

TANAMI: tracking active galactic nuclei with austral milliarcsecond interferometry - I. First-epoch 8.4 GHz images

We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at this http URL We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright gamma-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT.

Ionized Gas in Sgr B2 Main on Scales of 0.065 Arcsecond (600 Au)

The ionized gas in the Sagittarius B2 Main star-forming region has been imaged with ~65 mas (600 AU) resolution using the Very Large Array (VLA) at 7 mm in the A configuration. These observations have ~4-5 times the resolution of the previous highest resolution images of this source. These VLA observations show that the central four ultracompact H II (UC H II) regions in Sgr B2 Main (F1, F2, F3, and F4) break up into ~20 separate UC H II regions. The individual sources have small resolved sizes (~10-3 pc), high derived emission measures (EM ~ 109 pc cm-6), and high rms electron densities (ne ~ 106 cm-3). We compare the observed radius and derived emission measure of each of these regions with those expected for a UC H II region in pressure equilibrium with its environment, using previously published temperatures and densities for the Sgr B2 molecular core (nH2=2×107 cm-3). We find that the observed emission measures are in fact higher than those predicted in a simple pressure-confinement model, suggesting that the molecular densities on scales that confine the UC H II regions may be higher than 2 × 107 cm-3.

High resolution centimeter radio continuum and ammonia maser observations of the W51 region of star formation

High angular resolution and high sensitivity observations at wavelengths of 1.3 and 3.6 cm have detected three new ultracompact emission regions in the core of the W51 (regions d and e). In total, five ultracompact continuum objects of diameter <300 to 3000 AU are located in this region. These sources may best be explained as photoionized stellar winds. This suggests that there may exist a quasi stable point in early stellar evolution where observable HII regions are formed by stellar winds around massive stars.

Ammonia in the W3(OH) region

Maps in the (J, K)=(1, 1) and (2, 2) inversion lines of NH 3 made with 3″ angular resolution (=0.03 pc at 2.2 kpc) show the presence of optically thick unresolved emission 5″.2 east of the compact H II region W3(OH). The position agrees with previous HCN results. This clump is about 0″.8 west of the center of the group of H 2 O masers; the virial mass is 18 M ○ .. From the virial mass and estimated radius, 0.006 pc, the average density is 4×10 8 cm −3

High spatial resolution 1.3 centimeter imaging of the NGC 2024 star-forming region

The NGC 2024 star-forming region has been imaged in the (J,K)=(1,1) and (2,2) inversion of NH 3 and the radio continuum at a wavelength of 1.3 cm with a spatial resolution of 3″. THe continuum image shows one pointlike and two extended features found in previous, lower resolution studies. The unresolved source (in a 2″.4 beam) has a 1.3 cm flux density of 19 mJy and coincides with the position of IRS 2. Using the 6 cm data of Snell & Bally, this source has a spectral index (S∞ν α ) of α=1.2±0.2. The ratio continuum emission is likely due to an ionized stellar wind. The mass-loss rate is ≃3.6×10 −7 M ⊙ yr −1

VERY LARGE ARRAY PLUS PIE TOWN ASTROMETRY OF 46 RADIO STARS

We have used the Very Large Array (VLA), linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 46 radio stars in the International Celestial Reference Frame (ICRF). Positions were obtained in the ICRF directly through phase referencing of the stars to nearby ICRF quasars whose positions are accurate at the 0.25 mas level. Radio star positions are estimated to be accurate at the 10 mas level, with position errors approaching a few milli-arcseconds for some of the stars observed. Our measured positions were combined with previous measurements taken from as early as 1978 to obtain proper motion estimates for all 46 stars with average uncertainties of ~1.7 mas/yr. We compared our radio star positions and proper motions with the Hipparcos Catalogue data, and find consistency in the reference frames produced by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the reference frame orientation angles at our mean epoch of 2003.78. No significant spin is found between our radio data frame and the Hipparcos Celestial Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr. Thus, our results are consistent with a non-rotating Hipparcos frame with respect to the ICRF.We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 46 radio stars in the International Celestial Reference Frame (ICRF). Positions were obtained in the ICRF directly through phase referencing of the stars to nearby ICRF quasars whose positions are accurate at the 0.25 mas level. Radio star positions are estimated to be accurate at the 10 mas level, with position errors approaching a few milliarcseconds for some of the stars observed. Our measured positions were combined with previous measurements taken from as early as 1978 to obtain proper-motion estimates for all 46 stars with average uncertainties of ≈1.7 mas yr-1. We compared our radio star positions and proper motions with the Hipparcos Catalogue data and found consistency in the reference frames produced by each data set on the 1 σ level, with errors of ~2.7 mas per axis for the reference frame orientation angles at our mean epoch of 2003.78. No significant spin is found between our radio data frame and the Hipparcos Celestial Reference Frame, with the largest rotation rates of +0.55 and -0.41 mas yr-1 around the x- and z-axes, respectively, with 1 σ errors of 0.36 mas yr-1. Thus, our results are consistent with a nonrotating Hipparcos frame with respect to the ICRF.

High-Resolution Continuum Imaging at 1.3 and 0.7 Centimeters of the W3 IRS 5 Region

High-resolution images of the hypercompact HII regions (HCHII) in W3 IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm are presented. Four HCHII regions were detected with sufficient signal-to-noise ratios to allow the determination of relevant parameters such as source position, size and flux density. The sources are slightly extended in our ∼0.2 ′′ beams; the deconvolved radii are less than 240 AU. A comparison of our data with VLA images taken at epoch 1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between 1989.1 and 2002.5, we find a proper motion of 210 mas at a position angle of 12 ◦ for IRS 5f and a proper motion of 190 mas at a position angle of 50 ◦ for IRS 5a. At the assumed distance to W3 IRS 5, 1.83±0.14 kpc, these offsets translate to proper motions of ∼135 km s -1 and ∼122 km s -1 respectively. These sources are either shock ionized gas in an outflow or ionized gas ejected from high mass stars. We find n o change in the positions of IRS 5d1/d2 and IRS 5b; and we show through a comparison with archival NICMOS 2.2 µm images that these two radio sources coincide with the infrared double constituting W3 IRS 5. These sources contain B or perhaps O stars. The flux densities of the four sources have changed compared to the epoch 1989.1 results. In our epoch 2002.5 data, none of the spectral indicies obtained from flux densities at 1.3 a nd 0.7 cm are consistent with optically thin free-free emission; IRS 5d1/d2 shows the largest increase in flux densi ty from 1.3 cm to 0.7 cm. This may be an indication of free-free optical depth within an ionized wind, a photoevaporating disk, or an accretion flow. It is less likely that this increase is caused by dust emission at 0.7 cm. Subject headings: H II regions‐ISM:individual(W3)‐radio continuum:ISM‐stars: formation