6,967 research outputs found
The Position of Sgr A at the Galactic Center
The absolute position of the compact radio source at the dynamical center of
the Galaxy, Sgr A, was known only to an accuracy of in spite of its
accurate location with respect to near-IR stellar sources to within 30
milliarcsecond (mas). To remedy this poor positional accuracy, we have selected
15 high-resolution, high-frequency VLA observations of Sgr A carried out in
the last 13 years and determined the weighted average position with the average
epoch 1992.4 to be at , [1950] =
29\dsec3076, , or
, [2000] = 40\dsec0383,
which agrees with earlier
published values to within the error bars of the earlier measurements.
An accurate absolute position of Sgr A can be useful for its identification
with sources at other wavelengths, particularly, in soft and hard X-rays with
implications for the models of a massive black hole at the Galactic center.Comment: 11 pages, one figure and one table. ApJL (in press
A Data Exchange Standard for Optical (Visible/IR) Interferometry
This paper describes the OI Exchange Format, a standard for exchanging
calibrated data from optical (visible/infrared) stellar interferometers. The
standard is based on the Flexible Image Transport System (FITS), and supports
storage of the optical interferometric observables including squared visibility
and closure phase -- data products not included in radio interferometry
standards such as UV-FITS. The format has already gained the support of most
currently-operating optical interferometer projects, including COAST, NPOI,
IOTA, CHARA, VLTI, PTI, and the Keck Interferometer, and is endorsed by the IAU
Working Group on Optical Interferometry. Software is available for reading,
writing and merging OI Exchange Format files.Comment: 26 pages, 1 figur
ALMA and VLA Observations: Evidence for Ongoing Low-mass Star Formation near Sgr A*
Using the VLA, we recently detected a large number of protoplanetary disk
(proplyd) candidates lying within a couple of light years of the massive black
hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the
young massive stars located near Sgr A*. Similar to Orion proplyds, the strong
UV radiation from the cluster of massive stars at the Galactic center is
expected to photoevaporate and photoionize the circumstellar disks around
young, low mass stars, thus allowing detection of the ionized outflows from the
photoionized layer surrounding cool and dense gaseous disks. To confirm this
picture, ALMA observations detect millimeter emission at 226 GHz from five
proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We
present the derived disk masses for four sources as a function of the assumed
dust temperature. The mass of protoplanetary disks from cool dust emission
ranges between 0.03 -- 0.05 solar mass. These estimates are consistent with the
disk masses found in star forming sites in the Galaxy. These measurements show
the presence of on-going star formation with the implication that gas clouds
can survive near Sgr A* and the relative importance of high vs low-mass star
formation in the strong tidal and radiation fields of the Galactic center.Comment: 13 pages, 3 figures, MNRAS (in press
Rhodium Pyrazolate Complexes as Potential CVD Precursors
Reaction of 3,5-(CF3)(2)PzLi with [Rh(mu-Cl)(eta(2)-C2H4)(2)](2) or [Rh(mu-Cl)(PMe3)(2)](2) in Et2O gave the dinuclear complexes [Rh(eta(2)-C2H4)(2)(mu-3,5-(CF3)(2)-Pz)](2) (1) and [Rh-2(mu-Cl)(mu-3,5-(CF3)(2)-Pz) (PMe3)(4)] (2) respectively (3,5-(CF3)(2)Pz = bis-trifluoromethyl pyrazolate). Reaction of PMe3 with [Rh(COD)(mu-3,5-(CF3)(2)-Pz)](2) in toluene gave [Rh(3,5-(CF3)(2)-Pz)(PMe3)(3)] (3). Reaction of 1 and 3 in toluene (1 : 4) gave moderate yields of the dinuclear complex [Rh(PMe3)(2)(mu-3,5-(CF3)(2)-Pz)](2) (4). Reaction of 3,5-(CF3)(2)PzLi with [Rh(PMe3)(4)]Cl in Et2O gave the ionic complex [Rh(PMe3)(4)][3,5-(CF3)(2)-Pz] (5). Two of the complexes, 1 and 3, were studied for use as CVD precursors. Polycrystalline thin films of rhodium (fcc-Rh) and metastable-amorphous films of rhodium phosphide (Rh2P) were grown from 1 and 3 respectively at 170 and 130 degrees C, 0.3 mmHg in a hot wall reactor using Ar as the carrier gas (5 cc min(-1)). Thin films of amorphous rhodium and rhodium phosphide (Rh2P) were grown from 1 and 3 at 170 and 130 degrees C respectively at 0.3 mmHg in a hot wall reactor using H-2 as the carrier gas (7 cc min(-1)).Welch Foundation F-816Petroleum Research Fund 47014-ACSNSF 0741973Chemistr
Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects near Sgr A*
We present radio images within 30 of Sgr A* based on recent VLA
observations at 34 GHz with 7.8 microJy sensitivity and resolution
milliarcseconds (mas). We report 44 partially resolved compact
sources clustered in two regions in the E arm of ionized gas that orbits Sgr
A*. These sources have size scales ranging between ~50 and 200 mas (400 to 1600
AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the
bow-shock sources previously identified in the near-IR but associated with
massive stars, these 34 GHz sources do not appear to have near-IR counterparts
at 3.8 m. We interpret these sources as a candidate population of
photoevaporative protoplanetary disks (proplyds) that are associated with newly
formed low mass stars with mass loss rates ~10^{-7} - 10^{-6} solar mass per
year and are located at the edge of a molecular cloud outlined by ionized gas.
The disks are externally illuminated by strong Lyman continuum radiation from
the ~100 OB and WR massive stars distributed within 10'' of Sgr A*. The
presence of proplyds implies current in-situ star formation activity near Sgr
A* and opens a window for the first time to study low mass star, planetary and
brown dwarf formations near a supermassive black hole.Comment: 13 pages, 4 figures, ApJL (in press
VLBI Observations of a Complete Sample of Radio Galaxies V. 3C346 and 4C31.04: two Unusual CSS Sources
We present observations at 1.7 and 8.4 GHz of two Compact Steep Spectrum
(CSS) sources from a complete sample of low-intermediate power radio galaxies.
3C346 shows an asymmetric structure with a one-sided ``jet'' and ``hot spot''.
Present observations suggest that the classification of this source as a CSS is
inappropriate, and that it is a common radio galaxy at a small angle to the
line of sight. Its properties are in agreement with the predictions of unified
schemes models. 4C31.04 shows more complex structure with the possibility of a
centrally located flat spectrum core in between two close lobes. We suggest
that this source could be a low redshift Compact Symmetric Object.Comment: 15 pages, LATEX, uuenconde ps figures To be published in the
Astrophysical Journal, October 20th issu
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
Going nuclear: gene family evolution and vertebrate phylogeny reconciled
Gene duplications have been common throughout vertebrate evolution, introducing paralogy and so complicating phylogenctic inference from nuclear genes. Reconciled trees are one method capable of dealing with paralogy, using the relationship between a gene phylogeny and the phylogeny of the organisms containing those genes to identify gene duplication events. This allows us to infer phylogenies from gene families containing both orthologous and paralogous copies. Vertebrate phylogeny is well understood from morphological and palaeontological data, but studies using mitochondrial sequence data have failed to reproduce this classical view. Reconciled tree analysis of a database of 118 vertebrate gene families supports a largely classical vertebrate phylogeny
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