22,796 research outputs found
CARMA interferometric observations of 2MASS J044427+2512: the first spatially resolved observations of thermal emission of a brown dwarf disk
We present CARMA 1.3 mm continuum data of the disk surrounding the young
brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular
resolution of the CARMA observations (0.16 arcsec) allows us to spatially
resolve for the first time the thermal emission from dust around a brown dwarf.
We analyze the interferometric visibilities and constrain the disk outer radius
adopting disk models with power-law radial profiles of the dust surface
density. In the case of a power-law index equal to or lower than 1, we obtain a
disk radius in the range of about 15 - 30 AU, while larger disks are inferred
for steeper radial profiles. By combining this information on the disk spatial
extent with the sub-mm spectral index of this source we find conclusive
evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss
the implications of our results on the models of dust evolution in
proto-planetary disks and brown dwarf formation.Comment: 14 pages, 3 figures, Accepted for publication in ApJ Letter
Large-Scale Asymmetries in the Transitional Disks of SAO 206462 and SR 21
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
in the dust continuum (690 GHz, 0.45 mm) and 12CO J=6-5 spectral line emission,
of the transitional disks surrounding the stars SAO 206462 and SR 21. These
ALMA observations resolve the dust-depleted disk cavities and extended gaseous
disks, revealing large-scale asymmetries in the dust emission of both disks. We
modeled these disks structures with a ring and an azimuthal gaussian, where the
azimuthal gaussian is motivated by the steady-state vortex solution from Lyra &
Lin (2013). Compared to recent observations of HD 142527, Oph IRS 48, and LkHa
330, these are low-contrast (< 2) asymmetries. Nevertheless, a ring alone is
not a good fit, and the addition of a vortex prescription describes these data
much better. The asymmetric component encompasses 15% and 28% of the total disk
emission in SAO 206462 and SR 21 respectively, which corresponds to a lower
limit of 2 MJup of material within the asymmetry for both disks. Although the
contrast in the dust asymmetry is low, we find that the turbulent velocity
inside it must be large (~20% of the sound speed) in order to drive these
azimuthally wide and radially narrow vortex-like structures. We obtain
residuals from the ring and vortex fitting that are still significant, tracing
non-axisymmetric emission in both disks. We compared these submillimeter
observations with recently published H-band scattered light observations. For
SR 21 the scattered light emission is distributed quite differently from the
submillimeter continuum emission, while for SAO 206462 the submillimeter
residuals are suggestive of spiral-like structure similar to the near-IR
emission.Comment: ApJL, in press. 6 pages, 3 figures, 2 table
NICMOS2 hubble space telescope observations of the embedded cluster associated with Mon R2: Constraining the substellar initial mass function
We have analyzed Hubble Space Telescope NICMOS2 F110W-, F160W-, F165M-, and F207M-band images covering the central 1' × 1' region of the cluster associated with Mon R2 in order to constrain the initial mass function (IMF) down to 20M_J. The flux ratio between the F165M and F160W bands was used to measure the strength of the water-band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 and 3300 K. These objects are placed in the H-R diagram together with sources observed by Carpenter et al. to estimate an age of ~1 Myr for the low-mass cluster population. By constructing extinction-limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high- and low-mass objects. For stars with estimated masses between 0.1 and 1.0 M_☉ for a 1 Myr population with A_V ≤ 19 mag, we find that 27% ± 9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to or slightly lower than the fraction found in other star-forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0 M_☉ to the number of objects in the mass interval 0.02-0.08 M_☉ by forming the ratio R^(**) = N(0.08-1 M_☉)/N(0.02-0.08 M_☉) for objects in an extinction-limited sample complete for A_V ≤ 7 mag. The ratio is found to be R^(**) = 2.2 ± 1.3, assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier. The ratio is similar to the ratios observed toward the Orion Nebula Cluster and IC 348, as well as the ratio derived in the 28 deg^2 survey of Taurus by Guieu et al
Recognition of 3-D Objects from Multiple 2-D Views by a Self-Organizing Neural Architecture
The recognition of 3-D objects from sequences of their 2-D views is modeled by a neural architecture, called VIEWNET that uses View Information Encoded With NETworks. VIEWNET illustrates how several types of noise and varialbility in image data can be progressively removed while incornplcte image features are restored and invariant features are discovered using an appropriately designed cascade of processing stages. VIEWNET first processes 2-D views of 3-D objects using the CORT-X 2 filter, which discounts the illuminant, regularizes and completes figural boundaries, and removes noise from the images. Boundary regularization and cornpletion are achieved by the same mechanisms that suppress image noise. A log-polar transform is taken with respect to the centroid of the resulting figure and then re-centered to achieve 2-D scale and rotation invariance. The invariant images are coarse coded to further reduce noise, reduce foreshortening effects, and increase generalization. These compressed codes are input into a supervised learning system based on the fuzzy ARTMAP algorithm. Recognition categories of 2-D views are learned before evidence from sequences of 2-D view categories is accumulated to improve object recognition. Recognition is studied with noisy and clean images using slow and fast learning. VIEWNET is demonstrated on an MIT Lincoln Laboratory database of 2-D views of jet aircraft with and without additive noise. A recognition rate of 90% is achieved with one 2-D view category and of 98.5% correct with three 2-D view categories.National Science Foundation (IRI 90-24877); Office of Naval Research (N00014-91-J-1309, N00014-91-J-4100, N00014-92-J-0499); Air Force Office of Scientific Research (F9620-92-J-0499, 90-0083
Searching for circumplanetary disks around LkCa 15
We present Karl G. Jansky Very Large Array (VLA) observations of the 7 mm
continuum emission from the disk surrounding the young star LkCa 15. The
observations achieve an angular resolution of 70 mas and spatially resolve the
circumstellar emission on a spatial scale of 9 AU. The continuum emission
traces a dusty annulus of 45 AU in radius that is consistent with the dust
morphology observed at shorter wavelengths. The VLA observations also reveal a
compact source at the center of the disk, possibly due to thermal emission from
hot dust or ionized gas located within a few AU from the central star. No
emission is observed between the star and the dusty ring, and, in particular,
at the position of the candidate protoplanet LkCa 15 b. By comparing the
observations with theoretical models for circumplanetary disk emission, we find
that if LkCa~15~b is a massive planet (>5 M_J) accreting at a rate greater than
1.e-6 M_J yr^{-1}, then its circumplanetary disk is less massive than 0.1 M_J,
or smaller than 0.4 Hill radii. Similar constraints are derived for any
possible circumplanetary disk orbiting within 45 AU from the central star. The
mass estimate are uncertain by at least one order of magnitude due to the
uncertainties on the mass opacity. Future ALMA observations of this system
might be able to detect circumplanetary disks down to a mass of 5.e-4 M_J and
as small as 0.2 AU, providing crucial constraints on the presence of giant
planets in the act of forming around this young star.Comment: Accepted for publication on Ap
Multilevel Multiple Imputation in presence of interactions, non-linearities and random slopes Imputazione Multipla Multilivello in presenza di interazioni, non-linearit`a e pendenze casuali
Ozone profile measurements at McMurdo Station Antarctica during the spring of 1987
During the Antarctic spring of 1986, 33 ozone soundings were conducted from McMurdo Station. These data indicated that the springtime decrease in ozone occurred rapidly between the altitudes of 12 and 20 km. During 1987, these measurements were repeated with 50 soundings between 29 August and 9 November. Digital conversions of standard electrochemical cell ozonesondes were again employed. The ozonesonde pumps were individually calibrated for flow rate as the high altitude performance of these pumps have been in question. While these uncertainties are not large in the region of the ozone hole, they are significant at high altitude and apparently resulted in an underestimate of total ozone of about 7 percent (average) as compared to the Total Ozone Mapping Spectrometer (TOMS) in 1986, when the flow rate recommended by the manufacturer was used. At the upper altitudes (approx. 30 km) the flow rate may be overestimated by as much as 15 percent using recommended values (see Harder et al., The UW Digital Ozonesonde: Characteristics and Flow Rate Calibration, poster paper, this workshop). These upper level values are used in the extrapolation, at constant mixing ratio, required to complete the sounding for total ozone. The first sounding was on 29 August, prior to major ozone depletion, when 274 DU total ozone (25 DU extrapolated) was observed. By early October total ozone had decreased to the 150 DU range; it then increased during mid-October owing to motion of the vortex and returned to a value of 148 DU (29 DU extrapolated) on 27 October
Interferometric Evidence for Resolved Warm Dust in the DQ Tau System
We report on near-infrared (IR) interferometric observations of the
double-lined pre-main sequence (PMS) binary system DQ Tau. We model these data
with a visual orbit for DQ Tau supported by the spectroscopic orbit & analysis
of \citet{Mathieu1997}. Further, DQ Tau exhibits significant near-IR excess;
modeling our data requires inclusion of near-IR light from an 'excess' source.
Remarkably the excess source is resolved in our data, similar in scale to the
binary itself ( 0.2 AU at apastron), rather than the larger circumbinary
disk ( 0.4 AU radius). Our observations support the \citet{Mathieu1997}
and \citet{Carr2001} inference of significant warm material near the DQ Tau
binary.Comment: 14 pgs, 3 figures, ApJL in pres
ALMA observations of the debris disk around the young Solar Analog HD 107146
We present ALMA continuum observations at a wavelength of 1.25 mm of the
debris disk surrounding the 100 Myr old solar analog HD 107146. The
continuum emission extends from about 30 to 150 AU from the central star with a
decrease in the surface brightness at intermediate radii. We analyze the ALMA
interferometric visibilities using debris disk models with radial profiles for
the dust surface density parametrized as i) a single power-law, ii) a single
power-law with a gap, and iii) a double power-law. We find that models with a
gap of radial width AU at a distance of AU from the central
star, as well as double power-law models with a dip in the dust surface density
at AU provide significantly better fits to the ALMA data than single
power-law models. We discuss possible scenarios for the origin of the HD 107146
debris disk using models of planetesimal belts in which the formation of
Pluto-sized objects trigger disruptive collisions of large bodies, as well as
models which consider the interaction of a planetary system with a planetesimal
belt and spatial variation of the dust opacity across the disk. If future
observations with higher angular resolution and sensitivity confirm the
fully-depleted gap structure discussed here, a planet with a mass of
approximately a few Earth masses in a nearly circular orbit at AU
from the central star would be a possible explanation for the presence of the
gap.Comment: (38 pages, 7 figures, accepted for publication in ApJ
Growth mechanisms of perturbations in boundary layers over a compliant wall
The temporal modal and nonmodal growth of three-dimensional perturbations in
the boundary-layer flow over an infinite compliant flat wall is considered.
Using a wall-normal velocity/wall-normal vorticity formalism, the dynamic
boundary condition at the compliant wall admits a linear dependence on the
eigenvalue parameter, as compared to a quadratic one in the canonical
formulation of the problem. This greatly simplifies the accurate calculation of
the continuous spectrum by means of a spectral method, thereby yielding a very
effective filtering of the pseudospectra as well as a clear identification of
instability regions. The regime of global instability is found to be matching
the regime of the favorable phase of the forcing by the flow on the compliant
wall so as to enhance the amplitude of the wall. An energy-budget analysis for
the least-decaying hydroelastic (static-divergence, traveling-wave-flutter and
near-stationary transitional) and Tollmien--Schlichting modes in the parameter
space reveals the primary routes of energy flow. Moreover, the flow exhibits a
slower transient growth for the maximum growth rate of a superposition of
streamwise-independent modes due to a complex dependence of the wall-boundary
condition with the Reynolds number. The initial and optimal perturbations are
compared with the boundary-layer flow over a solid wall; differences and
similarities are discussed. Unlike the solid-wall case, viscosity plays a
pivotal role in the transient growth. A slowdown of the maximum growth rate
with the Reynolds number is uncovered and found to originate in the transition
of the fluid-solid interaction from a two-way to a one-way coupling. Finally, a
term-by-term energy budget analysis is performed to identify the key
contributors to the transient growth mechanism
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