414 research outputs found
Millimeter-Wave Aperture Synthesis Imaging of Vega: Evidence for a Ring Arc at 95 AU
We present the first millimeter-wave aperture synthesis map of dust around a
main sequence star. A 3'' resolution image of 1.3 mm continuum emission from
Vega reveals a clump of emission 12'' from the star at PA 45 deg, consistent
with the location of maximum 850 micron emission in a lower resolution
JCMT/SCUBA map. The flux density is 4.0+/-0.9 mJy. Adjacent 1.3 mm peaks with
flux densities 3.4+/-1.0 mJy and 2.8+/-0.9 mJy are located 14'' and 13'' from
the star at PA 67 deg and 18 deg, respectively. An arc-like bridge connects the
two strongest peaks. There is an additional 2.4 +/-0.8 mJy peak to the SW 11''
from the star at PA 215 deg and a marginal detection, 1.4+/-0.5 mJy, at the
stellar position, consistent with photospheric emission. An extrapolation from
the 850 micron flux, assuming F_{1.3mm-0.85mm} proportional to lambda^{-2.8},
agrees well with the total detected flux for Vega at 1.3 mm, and implies a dust
emissivity index, beta, of 0.8. We conclude that we have detected all but a
very small fraction of the dust imaged by SCUBA in our aperture synthesis map
and that these grains are largely confined to segments of a ring of radius 95
AU.Comment: 4 pages, 1 figure, 1 table, accepted for publication in Astrophysical
Journal Letter
High-resolution imaging at the SOAR telescope
Bright single and binary stars were observed at the 4.1-m telescope with a
fast electron-multiplication camera in the regime of partial turbulence
correction by the visible-light adaptive optics system. We compare the angular
resolution achieved by simple averaging of AO-corrected images (long-exposure),
selection and re-centering (shift-and-add or "lucky" imaging) and speckle
interferometry. The effect of partial AO correction, vibrations, and image
post-processing on the attained resolution is shown. Potential usefulness of
these techniques is evaluated for reaching the diffraction limit in
ground-based optical imaging. Measurements of 75 binary stars obtained during
these tests are given and objects of special interest are discussed. We report
tentative resolution of the astrometric companion to Zeta Aqr B. A concept of
advanced high-resolution camera is outlined.Comment: Accepted for publication in PASP. 14 pages, 9 figures, 2 tabl
The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs
We report parallax measurements for 70 ultracool dwarfs (UCDs). Using both
literature values and our sample, we report new polynomial relations between
spectral type and M. Including resolved L/T transition binaries in the
relations, we find no reason to differentiate between a "bright" (unresolved
binary) and "faint" (single source) sample across the L/T boundary. Isolating
early T dwarfs, we find that the brightening of T0-T4 sources is prominent in
M where there is a [1.2 - 1.4] magnitude difference. A similar yet
dampened brightening of [0.3 - 0.5] magnitude happens at M and a plateau
or dimming of [-0.2 - -0.3] magnitude is seen in M. Comparing with
evolutionary models that vary gravity, metallicity, and cloud thickness we find
that a near constant temperature of 1200 100 K along a narrow spectral
subtype of T0-T4 is required to account for the brightening and color magnitude
diagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significant
population of both L and T dwarfs which are red or potentially "ultra-cloudy"
compared to the models, many of which are known to be young indicating a
correlation between enhanced photospheric dust and youth. For the low
surface-gravity or young companion L dwarfs we find that 8 out of 10 are at
least [0.2-1.0] magnitude underluminous in M and/or M compared to
equivalent spectral type objects. We speculate that this is a consequence of
increased dust opacity and conclude that low-surface gravity L dwarfs require a
completely new spectral-type/absolute magnitude polynomial for analysis.Comment: 65 pages, Accepted for publication to Ap
L- and M-band imaging observations of the Galactic Center region
We present near-infrared H-, K-, L- and M-band photometry of the Galactic
Center from images obtained at the ESO VLT in May and August 2002, using the
NAOS/CONICA (H and K) and the ISAAC (L and M) instruments. The large field of
view (70" x 70") of the ISAAC instrument and the large number of sources
identified (L-M data for 541 sources) allows us to investigate colors, infrared
excesses and extended dust emission. Our new L-band magnitude calibration
reveals an offset to the traditionally used calibrations, which we attribute to
the use of the variable star IRS7 as a flux calibrator. Together with new
results on the extinction towards the Galactic Center (Scoville et al. 2003;
Raab 2000), our magnitude calibration results in stellar color properties
expected from standard stars and removes any necessity to modify the K-band
extinction. The large number of sources for which we have obtained L-M colors
allows us to measure the M-band extinction to A_M=(0.056+-0.006)A_V
(approximately =A_L), a considerably higher value than what has so far been
assumed. L-M color data has not been investigated previously, due to lack of
useful M-band data. We find that this color is a useful diagnostic tool for the
preliminary identification of stellar types, since hot and cool stars show a
fairly clear L-M color separation. This is especially important if visual
colors are not available, as in the Galactic Center. For one of the most
prominent dust embedded sources, IRS3, we find extended L- and M-band continuum
emission with a characteristic bow-shock shape. An explanation for this
appearance is that IRS3 consists of a massive, hot, young mass-losing star
surrounded by an optically thick, extended dust shell, which is pushed
northwest by wind from the direction of the IRS16 cluster and SgrA*.Comment: 24 pages, 7 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
The Vela Pulsar in the Near-Infrared
We report on the first detection of the Vela pulsar in the near-infrared with
the VLT/ISAAC in the Js and H bands. The pulsar magnitudes are Js=22.71 +/-
0.10 and H=22.04 +/- 0.16. We compare our results with the available
multiwavelength data and show that the dereddened phase-averaged optical
spectrum of the pulsar can be fitted with a power law F_nu propto
nu^(-alpha_nu) with alpha_nu = 0.12 +/- 0.05, assuming the color excess
E(B-V)=0.055 +/-0.005 based on recent spectral fits of the emission of the Vela
pulsar and its supernova remnant in X-rays. The negative slope of the pulsar
spectrum is different from the positive slope observed over a wide optical
range in the young Crab pulsar spectrum. The near-infrared part of the Vela
spectrum appears to have the same slope as the phase-averaged spectrum in the
high energy X-ray tail, obtained in the 2-10 keV range with the RXTE. Both of
these spectra can be fitted with a single power law suggesting their common
origin. Because the phase-averaged RXTE spectrum in this range is dominated by
the second X-ray peak of the pulsar light curve, coinciding with the second
main peak of its optical pulse profile, we suggest that this optical peak can
be redder than the first one. We also detect two faint extended structures in
the 1.5''-3.1'' vicinity of the pulsar, projected on and aligned with the
south-east jet and the inner arc of the pulsar wind nebula, detected in X-rays
with Chandra. We discuss their possible association with the nebula.Comment: 12 pages, 8 figures, accepted for publication in A&A, the associated
near-infrared images in the fits format are available at
http://www.ioffe.ru/astro/NSG/obs/vela-ir
Color Transformations for the 2MASS Second Incremental Data Release
Transformation equations are presented to convert colors and magnitudes
measured in the AAO, ARNICA, CIT, DENIS, ESO, LCO (Persson standards), MSSSO,
SAAO, and UKIRT photometric systems to the photometric system inherent to the
2MASS Second Incremental Data Release. The transformations have been derived by
comparing 2MASS photometry with published magnitudes and colors for stars
observed in these systems. Transformation equations have also been derived
indirectly for the Bessell & Brett (1988) and Koornneef (1983) homogenized
photometric systems.Comment: To appear in AJ, May 200
The mineralogy, geometry and mass-loss history of IRAS 16342-3814
We present the 2-200 um Infrared Space Observatory (ISO) spectrum and 3.8-20
um ISAAC and TIMMI2 images of the extreme OH/IR star IRAS 16342-3814. Amorphous
silicate absorption features are seen, together with crystalline silicate
absorption features up to almost 45 um. No other OH/IR star is known to have
crystalline silicate features in absorption up to these wavelengths. This
suggests that IRAS 16342-3814 must have, or recently had, an extremely high
mass-loss rate. Preliminary radiative transfer calculations suggest that the
mass-loss rate may be as large as 10^{-3} Msun/yr. The 3.8 um ISAAC image shows
a bipolar reflection nebula with a dark equatorial waist or torus, similar to
that seen in optical Hubble Space Telescope (HST) images. The position angle of
the nebula decreases significantly with increasing wavelength, suggesting that
the dominant source of emission changes from scattering to thermal emission.
Still, even up to 20 um the nebula is oriented approximately along the major
axis of the nebula seen in the HST and ISAAC images, suggesting that the torus
must be very cold, in agreement with the very red ISO spectrum. The 20 um image
shows a roughly spherically symmetric extended halo, approximately 6'' in
diameter, which is probably due to a previous phase of mass-loss on the AGB,
suggesting a transition from a (more) spherically symmetric to a (more) axial
symmetric form of mass-loss at the end of the AGB. We estimate the maximum dust
particle sizes in the torus and in the reflection nebula to be 1.3 and 0.09 um
respectively. The size of the particles in the torus is large compared to
typical ISM values, but in agreement with high mass-loss rate objects like AFGL
4106 and HD161796. We discuss the possible reason for the difference in
particle size between the torus and the reflection nebula.Comment: Accepted for publication by A&
The Vega Debris Disk -- A Surprise from Spitzer
We present high spatial resolution mid- and far-infrared images of the Vega
debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS).
The disk is well resolved and its angular size is much larger than found
previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and
105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are
circular, smooth and without clumpiness at all three wavelengths. The radial
surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86
AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the
disk can be modeled as an axially symmetric and geometrically thin disk, viewed
face-on, with the surface particle number density following an r^-1 power law.
The disk radiometric properties are consistent with a range of models using
grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger
than 180 um and at radii of 86-200 AU from the star, can reproduce the observed
850 um flux, while its emission does not violate the observed MIPS profiles.
This ring could be associated with a population of larger asteroidal bodies
analogous to our own Kuiper Belt. Cascades of collisions starting with
encounters amongthese large bodies in the ring produce the small debris that is
blown outward by radiation pressure to much larger distances where we detect
its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS
results. This rate would require a very massive asteroidal reservoir for the
dust to be produced in a steady state throughout Vega's life. Instead, we
suggest that the disk we imaged is ephemeral and that we are witnessing the
aftermath of a large and relatively recent collisional event, and subsequent
collisional cascade.Comment: 13 pages, 17 figures, accepted for publication in ApJ. (Figures 2,
3a, 3b and 4 have been degraded to lower resolutions.
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