107 research outputs found
A Disk-Wind Model for the Near-Infrared Excess Emission in Protostars
Protostellar systems, ranging from low-luminosity T Tauri and Herbig Ae stars
to high-luminosity Herbig Be stars, exhibit a near-infrared (NIR) excess in
their spectra that is dominated by a bump in the monochromatic luminosity with
a peak near 3 microns. The bump can be approximated by a thermal emission
component of temperature 1500 K that is of the order of the sublimation
temperature of interstellar dust grains. In the currently popular "puffed up
rim" scenario, the bump represents stellar radiation that propagates through
the optically thin inner region of the surrounding accretion disk and is
absorbed and reemitted by the dust that resides just beyond the dust
sublimation radius, Rsub. However, this model cannot account for the strongest
bumps measured in these sources, and it predicts a large secondary bounce in
the interferometric visibility curve that is not observed. In this paper we
present an alternative interpretation, which attributes the bump to reemission
of stellar radiation by dust that is uplifted from the disk by a centrifugally
driven wind. Winds of this type are a leading candidate for the origin of the
strong outflows associated with protostars, and there is observational evidence
for disk winds originating on scales ~Rsub. Using a newly constructed Monte
Carlo radiative transfer code, we show that this model can account for the NIR
excess emission even in bright Herbig Ae stars such as AB Auriga and MWC 275,
and that it successfully reproduces the basic features of the visibilities
measured in these protostars. We argue that a robust dusty outflow in these
sources could be self-limiting to a relatively narrow launching region between
Rsub and 2Rsub. Finally, we suggest that our model could also naturally account
for the NIR and scattered-light variability exhibited by a source like MWC 275,
which may be triggered by the uplifting of dust clouds from the disk.Comment: 19 pages, 8 figures. Updated submitted version to refereed and
accepted one (accepted 8/29/2012 for publication in The Astrophysical
Journal
Radiation thermo-chemical models of protoplanetary discs. III. Impact of inner rims on Spectral Energy Distributions
We study the hydrostatic density structure of the inner disc rim around
HerbigAe stars using the thermo-chemical hydrostatic code ProDiMo. We compare
the Spectral Energy Distributions (SEDs) and images from our hydrostatic disc
models to that from prescribed density structure discs. The 2D continuum
radiative transfer in ProDiMo includes isotropic scattering. The dust
temperature is set by the condition of radiative equilibrium. In the
thermal-decoupled case the gas temperature is governed by the balance between
various heating and cooling processes. The gas and dust interact thermally via
photoelectrons, radiatively, and via gas accommodation on grain surfaces. As a
result, the gas is much hotter than in the thermo-coupled case, where the gas
and dust temperatures are equal, reaching a few thousands K in the upper disc
layers and making the inner rim higher. A physically motivated density drop at
the inner radius ("soft-edge") results in rounded inner rims, which appear
ring-like in near-infrared images. The combination of lower gravity pull and
hot gas beyond ~1 AU results in a disc atmosphere that reaches a height over
radius ratio z/r of 0.1 while this ratio is 0.2 only in the thermo-coupled
case. This puffed-up disc atmosphere intercepts larger amount of stellar
radiation, which translates into enhanced continuum emission in the 3- 30
micron wavelength region from hotter grains at ~500 K. We also consider the
effect of disc mass and grain size distribution on the SEDs self-consistently
feeding those quantities back into the gas temperature, chemistry, and
hydrostatic equilibrium computation.Comment: Accepted to MNRA
The Inner Rim of YSO Disks: Effects of dust grain evolution
Dust-grain growth and settling are the first steps towards planet formation.
An understanding of dust physics is therefore integral to a complete theory of
the planet formation process. In this paper, we explore the possibility of
using the dust evaporation front in YSO disks (`the inner rim') as a probe of
the dust physics operating in circumstellar disks. The geometry of the rim
depends sensitively on the composition and spatial distribution of dust. Using
radiative transfer and hydrostatic equilibrium calculations we demonstrate that
dust growth and settling can curve the evaporation front dramatically (from a
cylindrical radius of about 0.5 AU in the disk mid-plane to 1.2 AU in the disk
upper layers for an A0 star). We compute synthetic images and interferometric
visibilities for our representative rim models and show that the current
generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly
constrain the dust properties of circumstellar disks, shedding light on the
relatively poorly understood processes of grain growth, settling and turbulent
mixing.Comment: 26 pages, 9 figures. Accepted for publication in Ap
Mid-infrared size survey of Young Stellar Objects: Description of Keck segment-tilting experiment and basic results
The mid-infrared properties of pre-planetary disks are sensitive to the
temperature and flaring profiles of disks for the regions where planet
formation is expected to occur. In order to constrain theories of planet
formation, we have carried out a mid-infrared (wavelength 10.7 microns) size
survey of young stellar objects using the segmented Keck telescope in a novel
configuration. We introduced a customized pattern of tilts to individual mirror
segments to allow efficient sparse-aperture interferometry, allowing full
aperture synthesis imaging with higher calibration precision than traditional
imaging. In contrast to previous surveys on smaller telescopes and with poorer
calibration precision, we find most objects in our sample are partially
resolved. Here we present the main observational results of our survey of 5
embedded massive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Ori
system, and 5 emission-line objects of uncertain classification. The observed
mid-infrared sizes do not obey the size-luminosity relation found at
near-infrared wavelengths and a companion paper will provide further modelling
analysis of this sample. In addition, we report imaging results for a few of
the most resolved objects, including complex emission around embedded massive
protostars, the photoevaporating circumbinary disk around MWC 361A, and the
subarcsecond binaries T Tau, FU Ori and MWC 1080.Comment: Accepted by Astrophysical Journal. 38 pages. 9 figure
Strong Near-Infrared Emission Interior to the Dust-Sublimation Radius of Young Stellar Objects MWC275 and AB Aur
Using the longest optical-interferometeric baselines currently available, we
have detected strong near-infrared (NIR) emission from inside the
dust-destruction radius of Herbig Ae stars MWC275 and AB Aur. Our
sub-milli-arcsecond resolution observations unambiguously place the emission
between the dust-destruction radius and the magnetospheric co-rotation radius.
We argue that this new component corresponds to hot gas inside the
dust-sublimation radius, confirming recent claims based on spectrally-resolved
interferometry and dust evaporation front modeling.Comment: 12 pages, 4 figures, Accepted for publication in ApJ
Pre-transitional disk nature of the AB Aur disk
The disk around AB Aur was imaged and resolved at 24.6\,m using the
Cooled Mid-Infrared Camera and Spectrometer on the 8.2m Subaru Telescope. The
gaussian full-width at half-maximum of the source size is estimated to be 90
6 AU, indicating that the disk extends further out at 24.6\,m than
at shorter wavelengths. In order to interpret the extended 24.6\,m image,
we consider a disk with a reduced surface density within a boundary radius
, which is motivated by radio observations that suggest a reduced inner
region within about 100 AU from the star. Introducing the surface density
reduction factor for the inner disk, we determine that the best match
with the observed radial intensity profile at 24.6\,m is achieved with
=88 AU and =0.01. We suggest that the extended emission at
24.6\,m is due to the enhanced emission from a wall-like structure at the
boundary radius (the inner edge of the outer disk), which is caused by a jump
in the surface density at . Such reduced inner disk and geometrically
thick outer disk structure can also explain the more point-like nature at
shorter wavelengths. We also note that this disk geometry is qualitatively
similar to a pre-transitional disk, suggesting that the AB Aur disk is in a
pre-transitional disk phase.Comment: 10 pages, 4 figures, accepted for publication in Ap
High Resolution K-band Spectroscopy of MWC 480 and V1331 Cyg
We present high resolution (R=25,000-35,000) K-band spectroscopy of two young
stars, MWC 480 and V1331 Cyg. Earlier spectrally dispersed (R=230)
interferometric observations of MWC 480 indicated the presence of an excess
continuum emission interior to the dust sublimation radius, with a spectral
shape that was interpreted as evidence for hot water emission from the inner
disk of MWC 480. Our spectrum of V1331 Cyg reveals strong emission from CO and
hot water vapor, likely arising in a circumstellar disk. In comparison, our
spectrum of MWC 480 appears mostly featureless. We discuss possible ways in
which strong water emission from MWC 480 might go undetected in our data. If
strong water emission is in fact absent from the inner disk, as our data
suggest, the continuum excess interior to the dust sublimation radius that is
detected in the interferometric data must have another origin. We discuss
possible physical origins for the continuum excess.Comment: 29 pages, 5 figures, to appear in Ap
Characterizing the IYJ Excess Continuum Emission in T Tauri Stars
We present the first characterization of the excess continuum emission of
accreting T Tauri stars between optical and near-infrared wavelengths. With
nearly simultaneous spectra from 0.48 to 2.4 microns acquired with HIRES and
NIRSPEC on Keck and SpeX on the IRTF, we find significant excess continuum
emission throughout this region, including the I, Y, and J bands, which are
usually thought to diagnose primarily photospheric emission. The IYJ excess
correlates with the excess in the V band, attributed to accretion shocks in the
photosphere, and the excess in the K band, attributed to dust in the inner disk
near the dust sublimation radius, but it is too large to be an extension of the
excess from these sources. The spectrum of the excess emission is broad and
featureless, suggestive of blackbody radiation with a temperature between 2200
and 5000 K. The luminosity of the IYJ excess is comparable to the accretion
luminosity inferred from modeling the blue and ultraviolet excess emission and
may require reassessment of disk accretion rates. The source of the IYJ excess
is unclear. In stars of low accretion rate, the size of the emitting region is
consistent with cooler material surrounding small hot accretion spots in the
photosphere. However, for stars with high accretion rates, the projected area
is comparable to or exceeds that of the stellar surface. We suggest that at
least some of the IYJ excess emission arises in the dust-free gas inside the
dust sublimation radius in the disk.Comment: Accepted to ApJ, 31 pages, 21 figure
A Comprehensive Study of Proto-Planetary Disks around Herbig Ae Stars using Long-Baseline Infrared Interferometry.
Planetary systems are born in circumstellar disks around young stellar objects (YSOs) and the disk is thought to play a major role in the evolution of planetary systems. A good understanding of disk structure and its time evolution is therefore essential in comprehending planet formation, planet migration and the diversity of planetary systems. In this thesis, I use high angular resolution observations and state-of-the-art radiative transfer modeling to probe circumstellar disk structure and validate current disk models.
First, I discuss models and observations of the gas-dust transition region in YSOs. The dust component in circumstellar disks gets truncated at a finite radius from the central star, inside of which it is too hot for dust to survive. The truncated disk forms an ``evaporation front'' whose shape depends sensitively on dust properties. The possibility of using the front as a probe of the dust physics operating in circumstellar disks is explored. The Center for High Angular Resolution Astronomy~(CHARA) near-infrared~(near-IR) array is used to resolve out the evaporation front in the Herbig Ae stars MWC275 and AB Aur, and the presence of an additional near-IR opacity source within the ``conventional'' dust destruction radius is reported. Second, I describe comprehensive disk models that simultaneously explain the spectral energy distribution (from UV to milli-meter ) and long-baseline interferometry (from near-IR to mm) of Herbig Ae stars. The models are constrained with a wide range of data drawn from the literature as well as new interferometric observations in the K-band with the CHARA array and in the mid-IR with the novel Keck Segment Tilting Experiment. I show that the mid-IR size of MWC275 relative to AB~Aur is small, suggesting that dust grains in the outer disk of MWC275 are significantly more evolved/settled than the grains in the AB~Aur disk.
I conclude with a discussion on exciting prospects for measuring the gas-disk morphology on scales of fractions of an AU with the CHARA array, introducing a new powerful tool to understand the ``star-disk connection''.Ph.D.Astronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/61678/1/atannirk_1.pd
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