753 research outputs found
Band-Limited Coronagraphs using a halftone-dot process: II. Advances and laboratory results for arbitrary telescope apertures
The band-limited coronagraph is a nearly ideal concept that theoretically
enables perfect cancellation of all the light of an on-axis source. Over the
past years, several prototypes have been developed and tested in the
laboratory, and more emphasis is now on developing optimal technologies that
can efficiently deliver the expected high-contrast levels of such a concept.
Following the development of an early near-IR demonstrator, we present and
discuss the results of a second-generation prototype using halftone-dot
technology. We report improvement in the accuracy of the control of the local
transmission of the manufactured prototype, which was measured to be less than
1%.
This advanced H-band band-limited device demonstrated excellent contrast
levels in the laboratory, down to 10-6 at farther angular separations than 3
lambda/D over 24% spectral bandwidth. These performances outperform the ones of
our former prototype by more than an order of magnitude and confirm the
maturity of the manufacturing process.
Current and next generation high-contrast instruments can directly benefit
from such capabilities. In this context, we experimentally examine the ability
of the band-limited coronagraph to withstand various complex telescope
apertures.Comment: Accepted in ApJ - under pres
Detection of Weak Circumstellar Gas around the DAZ White Dwarf WD 1124-293: Evidence for the Accretion of Multiple Asteroids
Single metal polluted white dwarfs with no dusty disks are believed to be
actively accreting metals from a circumstellar disk of gas caused by the
destruction of asteroids perturbed by planetary systems. We report, for the
first time, the detection of circumstellar Ca~II gas in absorption around the
DAZ WD~1124-293, which lacks an infrared excess. We constrain the gas to 7
and 32000~AU, and estimate it to be at 54~R,
well within WD~1124-293's tidal disruption radius. This detection is based on
several epochs of spectroscopy around the Ca~II H and K lines
(=3968\AA, 3933\AA) with the MIKE spectrograph on the Magellan/Clay
Telescope at Las Campanas Observatory. We confirm the circumstellar nature of
the gas by observing nearby sightlines and finding no evidence for gas from the
local interstellar medium. Through archival data we have measured the
equivalent width of the two photospheric Ca lines over a period of 11 years. We
see 5-7\% epoch-to-epoch variation in equivalent widths over this time
period, and no evidence for long term trends. The presence of a circumstellar
gas implies a near edge-on inclination to the system, thus we place limits to
short period transiting planetary companions with R R using
the WASP survey. The presence of gas in orbit around WD~1124-293 implies that
most DAZs could harbor planetary systems. Since 25-30\% of white dwarfs show
metal line absorption, the dynamical process for perturbing small bodies must
be robust.Comment: 31 pages with 9 figures; accepted to Ap
Cool Customers in the Stellar Graveyard IV: Spitzer Search for Mid-IR excesses Around Five DAs
Hydrogen atmosphere white dwarfs with metal lines, so-called DAZs, require
external accretion of material to explain the presence of weak metal line
absorption in their photospheres. The source of this material is currently
unknown, but could come from the interstellar medium, unseen companions, or
relic planetesimals from asteroid belt or Kuiper belt analogues. Accurate
mid-infrared photometry of these white dwarfs provide additional information to
solve the mystery of this accretion and to look for evidence of planetary
systems that have survived post main sequence evolution. We present {\em
Spitzer} IRAC photometry accurate to 3% for four DAZs and one DA with
circumstellar absorption lines in the UV. We search for excesses due to unseen
companions or circumstellar dust disks. We use {\em Hubble Space Telescope}
NICMOS imaging of these white dwarfs to gauge the level of background
contamination to our targets as well as rule out common proper motion
companions to WD 1620-391. All of our targets show no excesses due to
companions 20 M, ruling out all but very low mass companions to these
white dwarfs at all separations. No excesses due to circumstellar disks are
observed, and we place limits on what types of disks may still be present.Comment: 18 pages, 8 figures, Accepted to A
A Spitzer Space Telescope Study of the Debris Disks around four SDSS White Dwarfs
We present Spitzer Space Telescope data of four isolated white dwarfs that
were previously known to harbor circumstellar gaseous disks. IRAC photometry
shows a significant infrared excess in all of the systems, SDSS0738+1835,
SDSS0845+2257, SDSS1043+0855 and SDSS1617+1620, indicative of a dusty extension
to those disks. The 4.5-micron excesses seen in SDSS0738, SDSS0845, and
SDSS1617 are 7.5, 5.7 and 4.5 times the white dwarf contribution, respectively.
In contrast, in SDSS1043, the measured flux density at 4.5 microns is only 1.7
times the white dwarf contribution. We compare the measured IR excesses in the
systems to models of geometrically thin, optically thick disks, and find that
we are able to match the measured SEDs to within 3 sigma of the uncertainties,
although disks with unfeasibly hot inner dust temperatures generally provide a
better fit than those below the dust sublimation temperature. Possible
explanations for the dearth of dust around SDSS1043+0855 are briefly discussed.
Including our previous study of SDSS1228+1040, all five white dwarfs with
gaseous debris disks have significant amounts of dust around them. It is
evident that gas and dust can coexist around these relatively warm, relatively
young white dwarfs.Comment: 20 pages, including 4 figures. Accepted to Ap
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
T Cell Migration from Inflamed Skin to Draining Lymph Nodes Requires Intralymphatic Crawling Supported by ICAM-1/LFA-1 Interactions.
T cells are the most abundant cell type found in afferent lymph, but their migration through lymphatic vessels (LVs) remains poorly understood. Performing intravital microscopy in the murine skin, we imaged T cell migration through afferent LVs in vivo. T cells entered into and actively migrated within lymphatic capillaries but were passively transported in contractile collecting vessels. Intralymphatic T cell number and motility were increased during contact-hypersensitivity-induced inflammation and dependent on ICAM-1/LFA-1 interactions. In vitro, blockade of endothelial cell-expressed ICAM-1 reduced T cell adhesion, crawling, and transmigration across lymphatic endothelium and decreased T cell advancement from capillaries into lymphatic collectors in skin explants. In vivo, T cell migration to draining lymph nodes was significantly reduced upon ICAM-1 or LFA-1 blockade. Our findings indicate that T cell migration through LVs occurs in distinct steps and reveal a key role for ICAM-1/LFA-1 interactions in this process
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