643 research outputs found
HST/ACS Images of the GG Tauri Circumbinary Disk
Hubble Space Telescope Advanced Camera for Surveys images of the young binary
GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002
and are the most detailed of this system to date. The confirm features
previously seen in the disk including: a "gap" apparently caused by shadowing
from circumstellar material; an asymmetrical distribution of light about the
line of sight on the near edge of the disk; enhanced brightness along the near
edge of the disk due to forward scattering; and a compact reflection nebula
near the secondary star. New features are seen in the ACS images: two short
filaments along the disk; localized but strong variations in disk intensity
("gaplets"); and a "spur" or filament extending from the reflection nebulosity
near the secondary. The back side of the disk is detected in the V band for the
first time. The disk appears redder than the combined light from the stars,
which may be explained by a varied distribution of grain sizes. The brightness
asymmetries along the disk suggest that it is asymmetrically illuminated by the
stars due to extinction by nonuniform circumstellar material or the illuminated
surface of the disk is warped by tidal effects (or perhaps both). Localized,
time-dependent brightness variations in the disk are also seen.Comment: 28 pages, 7 figures, accepted for publication in the Astronomical
Journa
Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution
We present an H-band image of the light scattered from circumstellar dust
around the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ 803, HD
197481), obtained with the Keck adaptive optics system. We resolve the disk
both vertically and radially, tracing it over 17-60 AU from the star. Our AU
Mic observations thus offer the possibility to probe at high spatial resolution
(0.04" or 0.4 AU per resolution element) for morphological signatures of the
debris disk on Solar-System scales. Various sub-structures (dust clumps and
gaps) in the AU Mic disk may point to the existence of orbiting planets. No
planets are seen in our H-band image down to a limiting mass of 1 M_Jup at >20
AU, although the existence of smaller planets can not be excluded from the
current data. Modeling of the disk surface brightness distribution at H-band
and R-band, in conjunction with the optical to sub-millimeter spectral energy
distribution, allows us to constrain the disk geometry and the dust grain
properties. We confirm the nearly edge-on orientation of the disk inferred from
previous observations, and deduce an inner clearing radius <=10 AU. We find
evidence for a lack of small grains in the inner (<60 AU) disk, either as a
result of primordial disk evolution, or because of destruction by
Poynting-Robertson and/or corpuscular drag. A change in the power-law index of
the surface brightness profile is observed near 33 AU, similar to a feature
known in the profile of the beta Pic circumstellar debris disk. By comparing
the time scales for inter-particle collisions and Poynting-Robertson drag
between the two systems, we argue that the breaks are linked to one of these
two processes.Comment: 17 pages, 7 figures, 1 table; accepted by Ap
HST and Spitzer Observations of the HD 207129 Debris Ring
A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in
scattered visible light with the ACS coronagraph on the Hubble Space Telescope
and in thermal emission using MIPS on the Spitzer Space Telescope at 70 microns
(resolved) and 160 microns (unresolved). Spitzer IRS (7-35 microns) and MIPS
(55-90 microns) spectrographs measured disk emission at >28 microns. In the HST
image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and
is inclined by 60 degrees from pole-on. At 70 microns it appears partially
resolved and is elongated in the same direction and with nearly the same size
as seen with HST in scattered light. At 0.6 microns the ring shows no
significant brightness asymmetry, implying little or no forward scattering by
its constituent dust. With a mean surface brightness of V=23.7 mag per square
arcsec, it is the faintest disk imaged to date in scattered light.Comment: 28 pages, 8 figure
Hubble and Spitzer Space Telescope Observations of the Debris Disk around the Nearby K Dwarf HD 92945
[ABRIDGED] We present the first resolved images of the debris disk around the
nearby K dwarf HD 92945. Our F606W (V) and F814W (I) HST/ACS coronagraphic
images reveal an inclined, axisymmetric disk consisting of an inner ring
2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface
brightness declines slowly with increasing radius 3".0-5".1 (65-110 AU) from
the star. A precipitous drop in the surface brightness beyond 110 AU suggests
that the outer disk is truncated at that distance. The radial surface-density
profile is peaked at both the inner ring and the outer edge of the disk. The
dust in the outer disk scatters neutrally but isotropically, and it has a low
V-band albedo of 0.1. We also present new Spitzer MIPS photometry and IRS
spectra of HD 92945. These data reveal no infrared excess from the disk
shortward of 30 micron and constrain the width of the 70 micron source to < 180
AU. Assuming the dust comprises compact grains of astronomical silicate with a
surface-density profile described by our scattered-light model of the disk, we
successfully model the 24-350 micron emission with a minimum grain size of
a_min = 4.5 micron and a size distribution proportional to a^-3.7 throughout
the disk, but with a maximum grain size of 900 micron in the inner ring and 50
micron in the outer disk. Our observations indicate a total dust mass of ~0.001
M_earth. However, they provide contradictory evidence of the dust's physical
characteristics: its neutral V-I color and lack of 24 micron emission imply
grains larger than a few microns, but its isotropic scattering and low albedo
suggest a large population of submicron-sized grains. The dynamical causes of
the disk's morphology are unclear, but recent models of dust creation and
transport in the presence of migrating planets indicate an advanced state of
planet formation around HD 92945.Comment: 29 pages, 10 figures; to be published in The Astronomical Journa
Hubble space telescope advanced camera for surveys coronagraphic observations of the dust surrounding HD 100546
We present HST ACS coronagraphic observations of HD 100546, a B9.5 star, 103 pc away from the Sun, taken in the F435W, F606W, and F814W bands. Scattered light is detected up to 14" from the star. The observations are consistent with the presence of an extended flattened nebula with the same inclination as the inner disk. The well-known "spiral arms" are clearly observed and trail the rotating disk material. Weaker arms never before reported are also seen. The interarm space becomes brighter, but the structures become more neutral in color at longer wavelengths, which is not consistent with models that assume that they are due to the effects of a warped disk. Along the major disk axis, the colors of the scattered light relative to the star are Ī(F435W - F606W) ā 0.0-0.2 mag and Ī(F435W - F814W) ā 0.5-1 mag. To explain these colors, we explore the role of asymmetric scattering, reddening, and large minimum sizes on ISM-like grains. We conclude that each of these hypotheses by itself cannot explain the colors. The disk colors are similar to those derived for Kuiper Belt objects, suggesting that the same processes responsible for their colors may be at work here. We argue that we are observing only the geometrically thick, optically thin envelope of the disk, while the optically thick disk responsible for the far-IR emission is undetected. The observed spiral arms are then structures on this envelope. The colors indicate that the extended nebulosity is not a remnant of the infalling envelope but reprocessed disk material
A Resolved Debris Disk Around the Nearby G Star HIP 32480
The Herschel Space Observatory is providing unprecedented sensitivity and angular resolution in the far-infrared. The DUNES Key Project (DUst around NEarby Stars, PI Carlos Eiroa) has finished its survey of 133 FGK stars within 25 pc of the Sun using the PACS photometer at 100 and 160 microns. We report the detection of a resolved debris ring around HIP 32480, a GO star 16.5 parsecs distant. The ring is almost 300 AU in diameter and inclined 30 degrees from edge-on. We present a thermal emission model for the system that fits the Spitzer spectroscopy and Herschel images of the system. We find a minimum grain-size of 4 microns in the main ring and a distinct warm dust population interior to it. Faint detached emission features just outside the ring may trace a separate, more distant ring in the system. The non-detection of the ring in archival HST/ACS coronagraphic images limits the dust grain albedo in the ring to be no more than 10%
Discovery of an 86 AU Radius Debris Ring Around HD 181327
HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have
revealed the presence of a ring-like disk of circumstellar debris seen in 1.1
micron light scattered by the disk grains, surrounded by a di use outer region
of lower surface brightness. The annular disk appears to be inclined by 31.7
+/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The
total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r <
5.0") of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy
percent of the light from the scattering grains appears to be confined in a 36
AU wide annulus centered on the peak of the radial surface brightness (SB)
profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of
thermal emission estimated from IRAS and Spitzer flux densities assuming
blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally
symmetric, exhibits directionally preferential scattering well represented by a
Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an
azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy
arcsec^-2. No photocentric offset is seen in the ring relative to the position
of the central star. A low surface brightness diffuse halo is seen in the
NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted
coronagraphic observations reveal a faint outer nebulosity, asymmetrically
brighter to the North of the star. We discuss models of the disk and properties
of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU
at the 87.6 AU radius of maximum surface density, and a total maximum dust mass
of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~
4 M(moon).Comment: 45 pages, 15 figures, accepted for publication in Ap
The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing
We present the first multicolor view of the scattered light disk of the Herbig Ae star HD 163296, based on coronagraphic observations from the Hubble Space Telescope Advanced Camera for Surveys (HST ACS). Radial profile fits of the surface brightness along the disk's semimajor axis indicate that the disk is not continuously flared, and extends to ~540 AU. The disk's color (V ā I) = 1.1 at a radial distance of 3.5'' is redder than the observed stellar color (V ā I) = 0.15. This red disk color might be indicative of either an evolution in the grain size distribution (i.e., grain growth) and/or composition, both of which would be consistent with the observed nonflared geometry of the outer disk. We also identify a single ansa morphological structure in our F435W ACS data, which is absent from earlier epoch F606W and F814W ACS data, but corresponds to one of the two ansae observed in archival HST Space Telescope Imaging Spectrograph (STIS) coronagraphic data. Following transformation to similar bandpasses, we find that the scattered light disk of HD 163296 is 1 mag arcsec^(ā2) fainter at 3.5'' in the STIS data than in the ACS data. Moreover, variations are seen in (1) the visibility of the ansa(e) structures, (2) the relative surface brightness of the ansa(e) structures, and (3) the (known) intrinsic polarization of the system. These results indicate that the scattered light from the HD 163296 disk is variable. We speculate that the inner disk wall, which Sitko et al. suggests has a variable scale height as diagnosed by near-IR SED variability, induces variable self-shadowing of the outer disk. We further speculate that the observed surface brightness variability of the ansa(e) structures may indicate that the inner disk wall is azimuthally asymmetric
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