509 research outputs found
Evidence for dust accumulation just outside the orbit of Venus
To contribute to the knowledge of dynamics of interplanetary dust we are
searching for structures in the spatial distribution of interplanetary dust
near the orbit of Venus. To this end we study the radial gradient of zodiacal
light brightness, as observed by the zodiacal light photometer on board the
Helios space probes on several orbits from 1975 to 1979. The cleanest data
result from Helios B (= Helios 2) launched in January 1976. With respect to the
general increase of zodiacal light brightness towards the Sun, the data show an
excess brightness of a few percent for positions of the Helios space probe just
outside the orbit of Venus. We consider this as evidence for a dust ring
associated with the orbit of Venus, somewhat similar to that found earlier
along the Earth's orbit.Comment: 7 pages, 8 figures, Astronomy&Astrophysics, accepte
NICMOS Images of the GG Tau Circumbinary Disk
We present deep, near-infrared images of the circumbinary disk surrounding
the pre-main-sequence binary star, GG Tau A, obtained with NICMOS aboard the
Hubble Space Telescope. The spatially resolved proto-planetary disk scatters
roughly 1.5% of the stellar flux, with a near-to-far side flux ratio of ~1.4,
independent of wavelength, and colors that are comparable to the central
source; all of these properties are significantly different from the earlier
ground-based observations. New Monte Carlo scattering simulations of the disk
emphasize that the general properties of the disk, such as disk flux, near side
to far side flux ratio and integrated colors, can be approximately reproduced
using ISM-like dust grains, without the presence of either circumstellar disks
or large dust grains, as had previously been suggested. A single parameter
phase function is fitted to the observed azimuthal variation in disk flux,
providing a lower limit on the median grain size of 0.23 micron. Our analysis,
in comparison to previous simulations, shows that the major limitation to the
study of grain growth in T Tauri disk systems through scattered light lies in
the uncertain ISM dust grain properties. Finally, we use the 9 year baseline of
astrometric measurements of the binary to solve the complete orbit, assuming
that the binary is coplanar with the circumbinary ring. We find that the
estimated 1 sigma range on disk inner edge to semi-major axis ratio, 3.2 <
Rin/a < 6.7, is larger than that estimated by previous SPH simulations of
binary-disk interactions.Comment: 40 pages, 8 postscript figures, accepted for publication in Ap
High spatial resolution mid-infrared observations of the low-mass young star TW Hya
We want to improve knowledge of the structure of the inner few AU of the
circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have
suggested the existence of a large inner hole, possibly caused by interactions
with a growing protoplanet. We used interferometric observations in the N-band
obtained with the MIDI instrument on the Very Large Telescope Interferometer,
together with 10 micron spectra recorded by the infrared satellite Spitzer. The
fact that we were able to determine N-band correlated fluxes and visibilities
for this comparatively faint source shows that MIR interferometry can be
applied to a large number of low-mass young stellar objects.
The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI
(7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength
regime is amorphous. According to the correlated flux measured with MIDI, most
of the crystalline material is in the inner, unresolved part of the disk, about
1 AU in radius. The visibilities exclude the existence of a very large (3-4 AU
radius) inner hole in the circumstellar disk of TW Hya, which was required in
earlier models. We propose instead a geometry of the inner disk where an inner
hole still exists, but at a much reduced radius, with the transition from zero
to full disk height between 0.5 and 0.8 AU, and with an optically thin
distribution of dust inside. Such a model can comply with SED and MIR
visibilities, as well as with visibility and extended emission observed in the
NIR at 2 micron. If a massive planet was the reason for this inner hole, as has
been speculated, its orbit would have to be closer to the star than 0.3 AU.
Alternatively, we may be witnessing the end of the accretion phase and an early
phase of an inward-out dispersal of the circumstellar disk.Comment: 13 pages, 9 figures, accepted by A&
Structure Function Scaling of a 2MASS Extinction Map of Taurus
We compute the structure function scaling of a 2MASS extinction map of the
Taurus molecular cloud complex. The scaling exponents of the structure
functions of the extinction map follow the Boldyrev's velocity structure
function scaling of supersonic turbulence. This confirms our previous result
based on a spectral map of 13CO J=1-0 covering the same region and suggests
that supersonic turbulence is important in the fragmentation of this
star--forming cloud.Comment: submitted to Ap
The structure of disks around intermediate-mass young stars from mid-infrared interferometry. Evidence for a population of group II disks with gaps
The disks around Herbig Ae/Be stars are commonly divided into group I and
group II based on their far-infrared spectral energy distribution, and the
common interpretation for that is flared and flat disks. Recent observations
suggest that many flaring disks have gaps, whereas flat disks are thought to be
gapless. The different groups of objects can be expected to have different
structural signatures in high-angular-resolution data. Over the past 10 years,
the MIDI instrument on the Very Large Telescope Interferometer has collected
observations of several tens of protoplanetary disks. We model the large set of
observations with simple geometric models. A population of radiative-transfer
models is synthesized for interpreting the mid-infrared signatures. Objects
with similar luminosities show very different disk sizes in the mid-infrared.
Restricting to the young objects of intermediate mass, we confirm that most
group I disks are in agreement with being transitional. We find that several
group II objects have mid-infrared sizes and colors overlapping with sources
classified as group I, transition disks. This suggests that these sources have
gaps, which has been demonstrated for a subset of them. This may point to an
intermediate population between gapless and transition disks. Flat disks with
gaps are most likely descendants of flat disks without gaps. Gaps, potentially
related to the formation of massive bodies, may therefore even develop in disks
in a far stage of grain growth and settling. The evolutionary implications of
this new population could be twofold. Either gapped flat disks form a separate
population of evolved disks, or some of them may further evolve into flaring
disks with large gaps. The latter transformation may be governed by the
interaction with a massive planet, carving a large gap and dynamically exciting
the grain population in the disk.Comment: 24 pages, 11 figures, A&A in pres
The First Detection of Spatially Resolved Mid-Infrared Scattered Light from a Protoplanetary Disk
We report spatially resolved 11.8 micron images, obtained at the W. M. Keck
10 m telescope, of the protoplanetary disk around the pre--main-sequence star
HK Tau B. The mid-infrared morphology and astrometry of HK Tau B with respect
to HK Tau A indicate that the flux observed in the mid-infrared from HK Tau B
has been scattered off the upper surface of its nearly edge-on disk. This is
the first example of a protoplanetary disk observed in scattered light at
mid-infrared wavelengths. Monte Carlo simulations of this disk show that the
extent (FWHM =0."5, or 70 AU) of the scattered light nebula in the mid-infrared
is very sensitive to the dust size distribution. The 11.8 micron measurement
can be best modelled by a dust grain population that contains grains on the
order of 1.5-3 micron in size; grain populations with exclusively sub-micron
grain sizes or power law size distributions that extend beyond 5 micron cannot
reproduce the observed morphology. These grains are significantly larger than
those expected in the ISM implying that grain growth has occurred; whether this
growth is a result of dust evolution within the disk itself or had originally
occurred within the dark cloud remains an open question.Comment: 11 pages, 1 postscript figure, accepted for publication in ApJ
The dusty disk around VV Ser
We have carried out observations at millimeter and centimeter wavelengths
towards VV Ser using the Plateau de Bure Interferometer and the Very Large
Array. This allows us to compute the SED from near infrared to centimeter
wavelengths. The modeling of the full SED has provided insight into the dust
properties and a more accurate value of the disk mass.
The mass of dust in the disk around VV Ser is found to be about 4 10^(-5)
Msun, i.e. 400 times larger than previous estimates. Moreoever, the SED can
only be accounted for assuming dust stratification in the vertical direction
across the disk. The existence of small grains (0.25--1 micron) in the disk
surface is required to explain the emission at near- and mid-infrared
wavelengths. The fluxes measured at millimeter wavelengths imply that the dust
grains in the midplane have grown up to very large sizes, at least to some
centimeters.Comment: To appear in Ap
VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre
The dust enshrouded star IRS~3 in the central light year of our galaxy was
partially resolved in a recent VLTI experiment. The presented observation is
the first step in investigating both IRS~3 in particular and the stellar
population of the Galactic Centre in general with the VLTI at highest angular
resolution. We will outline which scientific issues can be addressed by a
complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge
Four Brown Dwarfs in the Taurus Star-Forming Region
We have identified four brown dwarfs in the Taurus star-forming region. They
were first selected from and CCD photometry of 2.29 square degrees
obtained at the Canada-France-Hawaii Telescope. Subsequently, they were
recovered in the 2MASS second incremental data release point source catalog.
Low-resolution optical spectra obtained at the William Herschel telescope allow
us to derive spectral types in the range M7--M9. One of the brown dwarfs has
very strong H emission (EW=-340 \AA). It also displays Br
emission in an infrared spectrum obtained with IRCS on the Subaru telescope,
suggesting that it is accreting matter from a disk. The \ion{K}{1} resonance
doublet and the \ion{Na}{1} subordinate doublet at 818.3 and 819.5 nm in these
Taurus objects are weaker than in field dwarfs of similar spectral type,
consistent with low surface gravities as expected for young brown dwarfs. Two
of the objects are cooler and fainter than GG Tau Bb, the lowest mass known
member of the Taurus association. We estimate masses of only 0.03 M for
them. The spatial distribution of brown dwarfs in Taurus hints to a possible
anticorrelation between the density of stars and the density of brown dwarfs.Comment: ApJ Letters (in press
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