520 research outputs found
The HR 4796A Debris System: Discovery of Extensive Exo-Ring Dust Material
The optically and IR bright, and starlight-scattering, HR 4796A ring-like
debris disk is one of the most (and best) studied exoplanetary debris systems.
The presence of a yet-undetected planet has been inferred (or suggested) from
the narrow width and inner/outer truncation radii of its r = 1.05" (77 au)
debris ring. We present new, highly sensitive, Hubble Space Telescope (HST)
visible-light images of the HR 4796A circumstellar debris system and its
environment over a very wide range of stellocentric angles from 0.32" (23 au)
to ~ 15" (1100 au). These very high contrast images were obtained with the
Space Telescope Imaging Spectrograph (STIS) using 6-roll PSF-template
subtracted coronagraphy suppressing the primary light of HR 4796A and using
three image plane occulters and simultaneously subtracting the background light
from its close angular proximity M2.5V companion. The resulting images
unambiguously reveal the debris ring embedded within a much larger,
morphologically complex, and bi-axially asymmetric exoring scattering
structure. These images at visible wavelengths are sensitive to, and map, the
spatial distribution, brightness, and radial surface density of micron size
particles over 5 dex in surface brightness. These particles in the exo-ring
environment may be unbound from the system and interacting with the local ISM.
Herein we present a new morphological and photometric view of the larger than
prior seen HR 4796A exoplanetary debris system with sensitivity to small
particles at stellocentric distances an order of magnitude greater than has
previously been observed.Comment: 28 pages, 17 figures, accepted for publication in the Astronomical
Journal 21 December 201
The 0.5 micrometer-2.2 micrometer Scattered Light Spectrum of the Disk Around TW Hya
We present a 0.5-2.2micron scattered light spectrum of the circumstellar disk around TW Hya from a combination of spatially resolved HST STIS spectroscopy and NICMOS coronagraphic images of the disk. \Ve investigate the morphology at the disk at distances> 40 AU over this wide range of wavelengths. We measure the surface brightness, azimuthal symmetry, and spectral character of the disk as a function of radius. We find that the scattering efficiency of the dust is largely neutral to blue over the observed wavelengths. We find a good fit to the data over a wide range of distances from the star if we use a model disk with a partial gap of 30% depth at 80 AU and with steep disk truncation exterior to 100 AU. If the gap is caused by a planetary companion in the process of accreting disk gas, it must be less than 20 Solar mass
The 0.5-2.22 micrometer Scattered Light Spectrum of the Disk around TW Hya: Detection of a Partially Filled Disk Gap at 80 AU*
We present a 0.5-2.2 micrometer scattered light spectrum of the circumstellar disk around TW Hya from a combination of spatially resolved Hubble Space Telescope STIS spectroscopy and NICMOS coronagraphic images of the disk. We investigate the morphology of the disk at distances greater than 40 AU over this wide range of wavelengths, and identify the presence of a depression in surface brightness at approximately 80 AU that could be caused by a gap in the disk. Additionally, we quantify the surface brightness, azimuthal symmetry, and spectral character of the disk as a function of radius. Our analysis shows that the scattering efficiency of the dust is largely neutral to blue over the observed wavelengths. We model the disk as a steady -disk with an ad hoc gap structure. The thermal properties of the disk are selfconsistently calculated using a three-dimensional radiative transfer code that uses ray tracing to model the heating of the disk interior and scattered light images. We find a good fit to the data over a wide range of distances from the star if we use a model disk with a partially filled gap of 30% depth at 80 AU and with a self-similar truncation knee at 100 AU. The origin of the gap is unclear, but it could arise from a transition in the nature of the disk's dust composition or the presence of a planetary companion. Based on scalings to previous hydrodynamic simulations of gap-opening criteria for embedded proto-planets, we estimate that a planetary companion forming the gap could have a mass between 6 and 28 solar mass
EXCEDE Technology Development III: First Vacuum Tests
This paper is the third in the series on the technology development for the
EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission
concept, which in 2011 was selected by NASA's Explorer program for technology
development (Category III). EXCEDE is a 0.7m space telescope concept designed
to achieve raw contrasts of 1e6 at an inner working angle of 1.2 l/D and 1e7 at
2 l/D and beyond. This will allow it to directly detect and spatially resolve
low surface brightness circumstellar debris disks as well as image giant
planets as close as in the habitable zones of their host stars. In addition to
doing fundamental science on debris disks, EXCEDE will also serve as a
technological and scientific precursor for any future exo-Earth imaging
mission. EXCEDE uses a Starlight Suppression System (SSS) based on the PIAA
coronagraph, enabling aggressive performance.
We report on our continuing progress of developing the SSS for EXCEDE, and in
particular (a) the reconfiguration of our system into a more flight-like
layout, with an upstream deformable mirror and an inverse PIAA system, as well
as a LOWFS, and (b) testing this system in a vacuum chamber, including IWA,
contrast, and stability performance. The results achieved so far are 2.9e-7
contrast between 1.2-2.0 l/D and 9.7e-8 contrast between 2.0-6.0 l/D in
monochromatic light; as well as 1.4e-6 between 2.0-6.0 l/D in a 10% band, all
with a PIAA coronagraph operating at an inner working angle of 1.2 l/D. This
constitutes better contrast than EXCEDE requirements (in those regions) in
monochromatic light, and progress towards requirements in broadband light. Even
though this technology development is primarily targeted towards EXCEDE, it is
also germane to any exoplanet direct imaging space-based telescopes because of
the many challenges common to different coronagraph architectures and mission
requirements.Comment: 12 pages, 12 figures, to be published in proceedings of SPIE
Astronomical Telescopes + Instrumentation (2014
Space-Based Coronagraphic Imaging Polarimetry of the TW Hydrae Disk: Shedding New Light on Self-Shadowing Effects
We present Hubble Space Telescope Near-Infrared Camera and Multi-Object
Spectrometer coronagraphic imaging polarimetry of the TW Hydrae protoplanetary
disk. These observations simultaneously measure the total and polarized
intensity, allowing direct measurement of the polarization fraction across the
disk. In accord with the self-shadowing hypothesis recently proposed by Debes
et al., we find that the total and polarized intensity of the disk exhibits
strong azimuthal asymmetries at projected distances consistent with the
previously reported bright and dark ring-shaped structures (~45-99 au). The
sinusoidal-like variations possess a maximum brightness at position angles near
~268-300 degrees and are up to ~28% stronger in total intensity. Furthermore,
significant radial and azimuthal variations are also detected in the
polarization fraction of the disk. In particular, we find that regions of lower
polarization fraction are associated with annuli of increased surface
brightness, suggesting that the relative proportion of multiple-to-single
scattering is greater along the ring and gap structures. Moreover, we find
strong (~20%) azimuthal variation in the polarization fraction along the
shadowed region of the disk. Further investigation reveals that the azimuthal
variation is not the result of disk flaring effects, but instead from a
decrease in the relative contribution of multiple-to-single scattering within
the shadowed region. Employing a two-layer scattering surface, we hypothesize
that the diminished contribution in multiple scattering may result from
shadowing by an inclined inner disk, which prevents direct stellar light from
reaching the optically thick underlying surface component.Comment: Accepted for publication in the Astrophysical Journal, 2018 May 3: 17
pages, 9 figures, 2 table
The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters
We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3
pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object
Spectrometer (NICMOS). Combining these data with new ground-based K-band
spectra (R=800) and existing spectral types and photometry and the models of
D'Antona & Mazzitelli, we find that the distributions of ages of comparable
samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of
\~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass
IMFs in these sites of clustered star formation are similar over a wide range
of stellar densities and other environmental conditions. With current data, we
cannot rule out modest variations in the substellar mass functions among these
clusters. We then make the best estimate of the true form of the IMF in the
Trapezium by using the evolutionary models of Baraffe et al. and an empirically
adjusted temperature scale and compare this mass function to recent results for
the Pleiades and the field. All of these data are consistent with an IMF that
is flat or rises slowly from the substellar regime to about 0.6 Msun, and then
rolls over into a power law that continues from about 1 Msun to higher masses
with a slope similar to or somewhat larger than the Salpeter value of 1.35. For
the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun
and probably, after a modest completeness correction, even from 0.01-0.02 Msun.
These data include ~50 likely brown dwarfs. We test the predictions of theories
of the IMF against various properties of the observed IMF.Comment: 34 pages, 13 figures, for color image see
http://cfa-www.harvard.edu/~kluhman/trap/colorimage.jp
Does the Debris Disk around HD 32297 Contain Cometary Grains?
We present an adaptive optics imaging detection of the HD 32297 debris disk
at L' (3.8 \microns) obtained with the LBTI/LMIRcam infrared instrument at the
LBT. The disk is detected at signal-to-noise per resolution element ~ 3-7.5
from ~ 0.3-1.1" (30-120 AU). The disk at L' is bowed, as was seen at shorter
wavelengths. This likely indicates the disk is not perfectly edge-on and
contains highly forward scattering grains. Interior to ~ 50 AU, the surface
brightness at L' rises sharply on both sides of the disk, which was also
previously seen at Ks band. This evidence together points to the disk
containing a second inner component located at 50 AU. Comparing the
color of the outer (50 /AU ) portion of the disk at L' with
archival HST/NICMOS images of the disk at 1-2 \microns allows us to test the
recently proposed cometary grains model of Donaldson et al. 2013. We find that
the model fails to match the disk's surface brightness and spectrum
simultaneously (reduced chi-square = 17.9). When we modify the density
distribution of the model disk, we obtain a better overall fit (reduced
chi-square = 2.9). The best fit to all of the data is a pure water ice model
(reduced chi-square = 1.06), but additional resolved imaging at 3.1 \microns is
necessary to constrain how much (if any) water ice exists in the disk, which
can then help refine the originally proposed cometary grains model.Comment: Accepted to ApJ January 13, 2014. 12 pages (emulateapj style), 9
figures, 1 tabl
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