8,426 research outputs found
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
Pixelization and Dynamic Range in Radio Interferometry
This study investigates some of the consequences of representing the sky by a
rectangular grid of pixels on the dynamic range of images derived from radio
interferometric measurements. In particular, the effects of image pixelization
coupled to the CLEAN deconvolution representation of the sky as a set of
discrete delta functions can limit the dynamic range obtained when representing
bright emission not confined to pixels on the grid. Sky curvature effects on
non-coplanar arrays will limit the dynamic range even if strong sources are
centered on a pixel in a "fly's eye" representation when such pixel is not
located at the corresponding facet's tangent point. Uncertainties in the
response function of the individual antennas as well as in the calibration of
actual data due to ionospheric, atmospheric or other effects will limit the
dynamic range even when using grid-less subtraction (i.e. in the visibility
domain) of strong sources located within the field of view of the observation.
A technique to reduce these effects is described and examples from an
implementation in the Obit package are given. Application of this technique
leads to significantly superior results without a significant increase in the
computing time.Comment: 8 pages, 4 figures (one double, one triple), to appear in Astronomy
and Astrophysics (accepted: September 5, 2008
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
Study program on /30-100 GHz/ electronically steerable antenna systems Quarterly report, 20 Apr. - 20 Jul. 1967
Far field patterns, element design, and feed systems for electronically steerable antenna system
Submillimeter satellite radiometer first semiannual engineering progress report
Development of 560 GHz fourth harmonic mixer and 140 GHz third harmonic generator for use in radiomete
Magnetoelectric birefringence revisited
Electromagnetic wave propagation inside isotropic material media
characterized by dielectric coefficients and
is examined. The regime of the eikonal approximation is
considered. The Hadamard method of field disturbances is used and the
dispersion relations are obtained by solving the Fresnel equation. Some
applications of the formalism are presented. Particularly, birefringence
phenomena induced by applied external fields are derived and discussed. It is
shown that magnetoelectric birefringence effect can occur even without the
presence of Kerr and Cotton-Mouton effects, provided the physical system
satisfies certain conditions.Comment: 9 pages, 1 figure, LaTe
Submillimeter satellite radiometer Final engineering report
All solid-state superheterodyne Dicke radiometer for submillimeter wavelength
Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit
By explicitly considering surface roughness at the atomic level, we
quantitatively show that the thermal conductivity of Si nanowires can be lower
than Casimir's classical limit. However, this violation only occurs for deep
surface degradation. For shallow surface roughness, the Casimir formula is
shown to yield a good approximation to the phonon mean free paths and
conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact
treatment of roughness scattering is in stark contrast with a previously
proposed perturbative approach, which is found to overpredict scattering rates
by an order of magnitude. The obtained results suggest that a complete
theoretical understanding of some previously published experimental results is
still lacking.Comment: 11 pages, 4 figure
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
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