1,170 research outputs found
A study of participation in the Eastern Massachusetts Chapter of National Association of Social Workers
Thesis (M.S.)--Boston Universit
Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti
We present ~0.4 resolution images of CO(3-2) and associated continuum
emission from the gas-bearing debris disk around the nearby A star 49 Ceti,
observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We
analyze the ALMA visibilities in tandem with the broad-band spectral energy
distribution to measure the radial surface density profiles of dust and gas
emission from the system. The dust surface density decreases with radius
between ~100 and 310 au, with a marginally significant enhancement of surface
density at a radius of ~110 au. The SED requires an inner disk of small grains
in addition to the outer disk of larger grains resolved by ALMA. The gas disk
exhibits a surface density profile that increases with radius, contrary to most
previous spatially resolved observations of circumstellar gas disks. While ~80%
of the CO flux is well described by an axisymmetric power-law disk in Keplerian
rotation about the central star, residuals at ~20% of the peak flux exhibit a
departure from axisymmetry suggestive of spiral arms or a warp in the gas disk.
The radial extent of the gas disk (~220 au) is smaller than that of the dust
disk (~300 au), consistent with recent observations of other gas-bearing debris
disks. While there are so far only three broad debris disks with well
characterized radial dust profiles at millimeter wavelengths, 49 Ceti's disk
shows a markedly different structure from two radially resolved gas-poor debris
disks, implying that the physical processes generating and sculpting the gas
and dust are fundamentally different.Comment: 20 pages, 8 figures, accepted for publication in ApJ March 31, 2017
(submitted Nov 2016
HST-STIS Spatially Resolved Spectroscopy and Coronagraphic Imaging of the TW Hydrae Circumstellar Disk
We present the first spatially resolved spectrum of scattered light from the
TW Hydrae protoplanetary disk. This nearly face-on disk is optically thick,
surrounding a classical T Tauri star in the nearby 10 Myr old TW Hya
association. The spectrum was taken with the HST-STIS CCD, providing resolution
R ~ 360 over the wavelength range 5250 - 10300 A. Spatially resolved
spectroscopy of circumstellar disks is difficult due to the high contrast ratio
between the bright star and faint disk. Our novel observations provide optical
spectra of scattered light from the disk between 40 AU and 155 AU from the
star. The scattered light has the same color as the star (gray scattering) at
all radii, except the innermost region. This likely indicates that the
scattering dust grains are larger than about 1 micron all the way out to large
radii. From the spectroscopic data, we also obtained radial profiles of the
integrated disk brightness at two position angles, over almost the same region
as previously observed in HST-WFPC2 and NICMOS coronagraphic images (35 AU to
173 AU from the star). The profiles have the same shape as the earlier ones,
but show a small azimuthal asymmetry in the disk not previously noted. Our STIS
broad-band coronagraphic images of TW Hya confirm the reality of this
asymmetry, and show that the disk surface brightness interior to 140 AU has a
sinusoidal dependence on azimuthal angle. The maximum brightness occurs at a
position angle of 233.6 +/- 5.7 degrees East of North. This might be caused by
the combination of forward-scattering and an increase in inclination in the
inner region of the disk, suggesting that the TW Hya disk has a warp like that
seen in the Beta Pictoris debris disk.Comment: 18 pages, 9 figures (some in color). First version submitted to ApJ
2004-08-29, posted to astro-ph 2004-10-10. Revised version accepted for
publication in ApJ 2004-12-07, posted to astro-ph 2004-12-0
Optical Coronagraphic Spectroscopy of AU Mic: Evidence of Time Variable Colors?
We present coronagraphic long slit spectra of AU Mic's debris disk taken with
the STIS instrument aboard the Hubble Space Telescope (HST). Our spectra are
the first spatially resolved, scattered light spectra of the system's disk,
which we detect at projected distances between approximately 10 and 45 AU. Our
spectra cover a wavelength range between 5200 and 10200 angstroms. We find that
the color of AU Mic's debris disk is bluest at small (12-35 AU) projected
separations. These results both confirm and quantify the findings qualitatively
noted by Krist et al. (2005), and are different than IR observations that
suggested a uniform blue or gray color as a function of projected separation in
this region of the disk. Unlike previous literature that reported the color of
AU Mic's disk became increasingly more blue as a function of projected
separation beyond approximately 30 AU, we find the disk's optical color between
35-45 AU to be uniformly blue on the southeast side of the disk and
decreasingly blue on the northwest side. We note that this apparent change in
disk color at larger projected separations coincides with several fast, outward
moving "features" that are passing through this region of the southeast side of
the disk. We speculate that these phenomenon might be related, and that the
fast moving features could be changing the localized distribution of sub-micron
sized grains as they pass by, thereby reducing the blue color of the disk in
the process. We encourage follow-up optical spectroscopic observations of the
AU Mic to both confirm this result, and search for further modifications of the
disk color caused by additional fast moving features propagating through the
disk.Comment: Accepted by AJ, 13 pages, 8 figures, 1 tabl
The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
The edge-on disk surrounding the nearby young star Beta Pictoris is the
archetype of the "debris disks", which are composed of dust and gas produced by
collisions and evaporation of planetesimals, analogues of Solar System comets
and asteroids. These disks provide a window on the formation and early
evolution of terrestrial planets. Previous observations of Beta Pic concluded
that the disk gas has roughly solar abundances of elements [1], but this poses
a problem because such gas should be rapidly blown away from the star, contrary
to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we
report the detection of singly and doubly ionized carbon (CII, CIII) and
neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these
abundant volatile species permits a much more complete gas inventory. Carbon is
extremely overabundant relative to every other measured element. This appears
to solve the problem of the stable gas disk, since the carbon overabundance
should keep the gas disk in Keplerian rotation [3]. New questions arise,
however, since the overabundance may indicate the gas is produced from material
more carbon-rich than the expected Solar System analogues.Comment: Accepted for publication in Nature. PDF document, 12 pages.
Supplementary information may be found at
http://www.dtm.ciw.edu/akir/Documents/roberge_supp.pdf *** Version 2 :
Removed extraneous publication information, per instructions from the Nature
editor. No other changes mad
The Exozodiacal Dust Problem for Direct Observations of ExoEarths
Debris dust in the habitable zones of stars - otherwise known as exozodiacal
dust - comes from extrasolar asteroids and comets and is thus an expected part
of a planetary system. Background flux from the Solar System's zodiacal dust
and the exozodiacal dust in the target system is likely to be the largest
source of astrophysical noise in direct observations of terrestrial planets in
the habitable zones of nearby stars. Furthermore, dust structures like clumps,
thought to be produced by dynamical interactions with exoplanets, are a
possible source of confusion. In this paper, we qualitatively assess the
primary impact of exozodical dust on high-contrast direct imaging at optical
wavelengths, such as would be performed with a coronagraph. Then we present the
sensitivity of previous, current, and near-term facilities to thermal emission
from debris dust at all distances from nearby solar-type stars, as well as our
current knowledge of dust levels from recent surveys. Finally, we address the
other method of detecting debris dust, through high-contrast imaging in
scattered light. This method is currently far less sensitive than thermal
emission observations, but provides high spatial resolution for studying dust
structures. This paper represents the first report of NASA's Exoplanet
Exploration Program Analysis Group (ExoPAG).Comment: 21 pages, 5 figures, 2 tables. Accepted for publication in PASP
2012-06-0
FUSE and HST STIS Observations of Hot and Cold Gas in the AB Aurigae System
We present the first observations of a Herbig Ae star with a circumstellar
disk by the Far Ultraviolet Spectroscopic Explorer (FUSE), as well as a
simultaneous observation of the star obtained with the Hubble Space Telescope
Space Telescope Imaging Spectrograph (STIS). The spectra of AB Aurigae show
emission and absorption features arising from gasses that have a wide range in
temperature, from hot OVI emission to cold molecular hydrogen and CO
absorption. Emissions from the highly ionized species OVI and CIII present in
the FUSE spectrum are redshifted, while absorption features arising from
low-ionization species like OI, NI, and SiII are blueshifted and show
characteristic stellar wind line-profiles. We find the total column density of
molecular hydrogen toward AB Aur from the FUSE apectrum, N(H_2) = (6.8 +/- 0.5)
x 10^19 cm^-2. The gas kinetic temperature of the molecular hydrogen derived
from the ratio N(J=1)/N(J=0) is 65 +/- 4 K. The column density of the CO
observed in the STIS spectrum is N(CO) = (7.1 +/- 0.5) x 10^13 cm^-2, giving a
CO/H_2 ratio of (1.04 +/- 0.11) x 10^-6. We also use the STIS spectrum to find
the column density of HI, permitting us to calculate the total column density
of hydrogen atoms, the fractional abundance of H_2, and the gas-to-dust ratio.Comment: 5 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted for
publication in ApJ Letter
Tensor interaction constraints from beta decay recoil spin asymmetry of trapped atoms
We have measured the angular distribution of recoiling daughter nuclei
emitted from the Gamow-Teller decay of spin-polarized Rb. The
asymmetry of this distribution vanishes to lowest order in the Standard Model
(SM) in pure Gamow-Teller decays, producing an observable very sensitive to new
interactions. We measure the non-SM contribution to the asymmetry to be
= 0.015 0.029 (stat) 0.019 (syst), consistent with the SM
prediction. We constrain higher-order SM corrections using the measured
momentum dependence of the asymmetry, and their remaining uncertainty dominates
the systematic error. Future progress in determining the weak magnetism term
theoretically or experimentally would reduce the final errors. We describe the
resulting constraints on fundamental 4-Fermi tensor interactions.Comment: 11 pages, 13 figures; v2 published in Phys. Rev. C, with referee
clarifications and figures improved for black-and-whit
X-raying the AU Microscopii debris disk
AU Mic is a young, nearby X-ray active M-dwarf with an edge-on debris disk.
Debris disk are the successors of the gaseous disks usually surrounding
pre-main sequence stars which form after the first few Myrs of their host
stars' lifetime, when - presumably - also the planet formation takes place.
Since X-ray transmission spectroscopy is sensitive to the chemical composition
of the absorber, features in the stellar spectrum of AU Mic caused by its
debris disk can in principle be detected. The upper limits we derive from our
high resolution Chandra LETGS X-ray spectroscopy are on the same order as those
from UV absorption measurements, consistent with the idea that AU Mic's debris
disk possesses an inner hole with only a very low density of sub-micron sized
grains or gas.Comment: 11 pages, 10 figures, accepted for publication in A&
Interstellar Polarization in the Taurus Dark Clouds, Wavelength Dependent Position Angles and Cloud Structure Near TMC-1
We use polarimetric observations of two stars (HD29647, HD283809) in the
general direction of TMC-1 in the Taurus Dark Cloud to investigate grain
properties and cloud structure in this region. We show the data to be
consistent with a simple two-component model, in which general interstellar
polarization in the Taurus Cloud is produced by a widely distributed cloud
component with relatively uniform magnetic field orientation; the light from
stars close to TMC-1 suffers additional polarization arising in one (or more)
subcloud(s) with larger average grain size and different magnetic field
directions compared with the general trend. Towards HD29647, in particular, we
show that the unusually low degree of visual polarization relative to
extinction is due to the presence of distinct cloud components in the line of
sight with markedly different magnetic field orientations. Stokes parameter
calculations allow us to separate out the polarization characteristics of the
individual components. Results are fit with the Serkowski empirical formula to
determine the degree and wavelength of maximum polarization. Whereas lambda_max
values in the widely distributed material are similar to the average (0.55um)
for the diffuse interstellar medium, the subcloud in line of sight to
HD~283809, the most heavily reddened star in our study, has lambda_max approx.
0.73um, indicating the presence of grains about 30% larger than this average.
Our model also predicts detectable levels of circular polarization toward both
HD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical
Journal, vol 48
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