377 research outputs found
Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation
We report the discovery of a circumstellar disk around the young A0 star, HR
4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By
fitting a model of the emission from a flat dusty disk to an image at
lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face
on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission
does not decrease with radius as expected for circumstellar disks but increases
outward from the star, peaking near both ends of the elongated structure. We
simulate this appearance by varying the inner radius in our model and find an
inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to
the radial distance of our own Kuiper belt and may suggest a source of dust in
the collision of cometesimals. By contrast with the appearance at 20.8 microns,
excess emission at lambda = 12.5 microns is faint and concentrated at the
stellar position. Similar emission is also detected at 20.8 microns in residual
subtraction of the best-fit model from the image. The intensity and ratio of
flux densities at the two wavelengths could be accounted for by a tenuous dust
component that is confined within a few AU of the star with mean temperature of
a few hundred degrees K, similar to that of zodiacal dust in our own solar
system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that
its disk is in a transitional planet-forming stage, between that of massive
gaseous proto-stellar disks and more tenuous debris disks such as the one
detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in
gzipped tar file. Accepted for publication in Astrophysical Journal Letters.
http://upenn5.hep.upenn.edu/~davidk/hr4796.htm
The Inner Rings of Beta Pictoris
We present Keck images of the dust disk around Beta Pictoris at 17.9 microns
that reveal new structure in its morphology. Within 1" (19 AU) of the star, the
long axis of the dust emission is rotated by more than 10 degrees with respect
to that of the overall disk. This angular offset is more pronounced than the
warp detected at 3.5" by HST, and in the opposite direction. By contrast, the
long axis of the emission contours at ~ 1.5" from the star is aligned with the
HST warp. Emission peaks between 1.5" and 4" from the star hint at the presence
of rings similar to those observed in the outer disk at ~ 25" with HST/STIS. A
deconvolved image strongly suggests that the newly detected features arise from
a system of four non-coplanar rings. Bayesian estimates based on the primary
image lead to ring radii of 14+/-1 AU, 28+/-3 AU, 52+/-2 AU and 82+/-2 AU, with
orbital inclinations that alternate in orientation relative to the overall disk
and decrease in magnitude with increasing radius. We believe these new results
make a strong case for the existence of a nascent planetary system around Beta
Pic.Comment: 5 pages, 2 figures, PDF format. Published in ApJL, December 20,200
Widespread atomic gas emission reveals the rotation of the beta Pictoris disk
We present high resolution Na I D spectroscopy of the beta Pic disk, and the
resonantly scattered sodium emission can be traced from less than 30 AU to at
least 140 AU from the central star. This atomic gas is co-existent with the
dust particles, suggestive of a common origin or source. The disk rotates
towards us in the south-west and away from us in the north-east. The velocity
pattern of the gas finally provides direct evidence that the faint linear
feature seen in images of the star is a circumstellar disk in Keplerian
rotation. From modelling the spatial distribution of the Na I line profiles we
determine the effective dynamical mass to be 1.40 +/- 0.05 M_sun, which is
smaller than the stellar mass, 1.75 M_sun. We ascribe this difference to the
gravity opposing radiation pressure in the Na I lines. We argue that this is
consistent with the fact that Na is nearly completely ionised throughout the
disk (Na I/Na < 10^-4). The total column density of sodium gas is N(Na) = 10^15
cm^-2.Comment: 9 pages, including 6 figs (fig.1 in colour). Accepted by ApJ
Metagenomic analysis of the turkey gut RNA virus community
Viral enteric disease is an ongoing economic burden to poultry producers worldwide, and despite considerable research, no single virus has emerged as a likely causative agent and target for prevention and control efforts. Historically, electron microscopy has been used to identify suspect viruses, with many small, round viruses eluding classification based solely on morphology. National and regional surveys using molecular diagnostics have revealed that suspect viruses continuously circulate in United States poultry, with many viruses appearing concomitantly and in healthy birds. High-throughput nucleic acid pyrosequencing is a powerful diagnostic technology capable of determining the full genomic repertoire present in a complex environmental sample. We utilized the Roche/454 Life Sciences GS-FLX platform to compile an RNA virus metagenome from turkey flocks experiencing enteric disease. This approach yielded numerous sequences homologous to viruses in the BLAST nr protein database, many of which have not been described in turkeys. Our analysis of this turkey gut RNA metagenome focuses in particular on the turkey-origin members of the Picornavirales, the Caliciviridae, and the turkey Picobirnaviruses
Three years of harvest with the vector vortex coronagraph in the thermal infrared
For several years, we have been developing vortex phase masks based on
sub-wavelength gratings, known as Annular Groove Phase Masks. Etched onto
diamond substrates, these AGPMs are currently designed to be used in the
thermal infrared (ranging from 3 to 13 {\mu}m). Our AGPMs were first installed
on VLT/NACO and VLT/VISIR in 2012, followed by LBT/LMIRCam in 2013 and
Keck/NIRC2 in 2015. In this paper, we review the development, commissioning,
on-sky performance, and early scientific results of these new coronagraphic
modes and report on the lessons learned. We conclude with perspectives for
future developments and applications.Comment: To appear in SPIE proceedings vol. 990
Detection of cool dust around the G2V star HD 107146
We report the detection of dust emission at sub-millimeter wavelengths from
HD 107146, a G2V star with an age estimated to lie between 80 and 200 Myr. The
emission is resolved at 450um with a size 300 AU x 210 AU. A fit to the
spectral energy distribution gives a dust temperature of 51 K and dust mass of
0.10 Earth masses. No excess emission above the photosphere was detected at
18um showing that there is very little warm dust and implying the presence of a
large inner hole, at least 31 AU (~ 1'') in radius, around the star. The
properties of this star-disk system are compared with similar observations of
other systems. We also discuss prospects for future observations that may be
able to determine whether the inner hole is maintained by the dynamical effect
of an unseen orbiting companion.Comment: accepted by the Astrophysical Journa
Warm molecular gas and kinematics in the disc around HD 100546
The disc around the Herbig Ae/Be star HD 100546 is one of the most
extensively studied discs in the southern sky. Although there is a wealth of
information about its dust content and composition, not much is known about its
gas and large scale kinematics. We detect and study the molecular gas in the
disc at spatial resolution from 7.7" to 18.9" using the APEX telescope. The
lines 12CO J=7-6, J=6-5, J=3-2, 13CO J=3-2 and [C I] 3P2-3P1 are observed,
diagnostic of disc temperature, size, chemistry, and kinematics. We use
parametric disc models that reproduce the low-J 12CO emission from Herbig~Ae
stars and vary the basic disc parameters - temperature, mass and size. Using
the molecular excitation and radiative transfer code RATRAN we fit the observed
spectral line profiles. Our observations are consistent with more than 0.001
Msun of molecular gas in a disc of approximately 400 AU radius in Keplerian
rotation around a 2.5 Msun star, seen at an inclination of 50 degrees. The
detected 12CO lines are dominated by gas at 30-70~K. The non-detection of the
[C I] line indicates excess ultraviolet emission above that of a B9 type model
stellar atmosphere. Asymmetry in the 12CO line emission suggests that one side
of the outer disc is colder by 10-20~K than the other, possibly due to a shadow
by a warped geometry of the inner disc. Pointing offsets, foreground cloud
absorption and asymmetry in the disc extent are excluded scenarios. Efficient
heating of the outer disc ensures that low- and high-J 12CO lines are dominated
by the outermost disc regions, indicating a 400 AU radius. The 12CO J=6--5 line
arises from a disc layer higher above disc midplane, and warmer by 15-20~K than
the layer emitting the J=3--2 line. The existing models of discs around Herbig
Ae stars, assuming a B9.5 type model stellar atmosphere overproduce the [CI]
3P2--3P1 line intensity from HD 100546 by an order of magnitude.Comment: 9pages, 3figures, Accepted for publication in Astronomy &
Astrophysic
Stellar Encounters with the Beta Pictoris Planetesimal System
We use data from the Hipparcos Catalog and the Barbier-Brossat & Figon (2000)
catalog of stellar radial velocities to test the hypothesis that the Beta Pic
planetesimal disk was disrupted by a close stellar encounter. We trace the
space motions of 21,497 stars and discover 18 that have passed within 5 pc of
Beta Pic in the past 1 Myr. Beta Pic's closest encounter is with the K2III star
HIP 27628 (0.6 pc), but dynamically the most important encounter is with the
F7V star HIP 23693 (0.9 pc). We calculate the velocity and eccentricity changes
induced by the 18 perturbations and conclude that they are dynamically
significant if planetesimals exist in a Beta Pic Oort cloud. We provide a
first-order estimate for the evolutionary state of a Beta Pic Oort cloud and
conclude that the primary role of these stellar perturbations would be to help
build a comet cloud rather than destroy a pre-existing structure. The stellar
sample is 20% complete and motivates future work to identify less common close
interactions that would significantly modify the observed circumstellar disk.
For future radial velocity study we identify six stars in the Hipparcos Catalog
that may have approached Beta Pic to within 0.1 pc and therefore remain as
candidate disk perturbers.Comment: 23 pages, 5 figures, Accepted for publication in Ap
An overview of the mid-infrared spectro-interferometer MATISSE: science, concept, and current status
MATISSE is the second-generation mid-infrared spectrograph and imager for the
Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric
instrument will allow significant advances by opening new avenues in various
fundamental research fields: studying the planet-forming region of disks around
young stellar objects, understanding the surface structures and mass loss
phenomena affecting evolved stars, and probing the environments of black holes
in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the
spectral domain of current optical interferometers by offering the L and M
bands in addition to the N band. This will open a wide wavelength domain,
ranging from 2.8 to 13 um, exploring angular scales as small as 3 mas (L band)
/ 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared
imaging - closure-phase aperture-synthesis imaging - with up to four Unit
Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE
will offer a spectral resolution range from R ~ 30 to R ~ 5000. Here, we
present one of the main science objectives, the study of protoplanetary disks,
that has driven the instrument design and motivated several VLTI upgrades
(GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a
description of the signal on the detectors and an evaluation of the expected
performances. We also discuss the current status of the MATISSE instrument,
which is entering its testing phase, and the foreseen schedule for the next two
years that will lead to the first light at Paranal.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June
2016, 11 pages, 6 Figure
Gas Lines from the 5-Myr old Optically Thin Disk around HD141569A
At the distance of 99-116 pc, HD141569A is one of the nearest HerbigAe stars that is surrounded by a tenuous disk, probably in transition between a massive primordial disk and a debris disk. We observed the fine-structure lines of OI at 63 and 145 micron and the CII line at 157 micron with the PACS instrument onboard the Herschel Space Telescope as part of the open-time large programme GASPS. We complemented the atomic line observations with archival Spitzer spectroscopic and photometric continuum data, a ground-based VLT-VISIR image at 8.6 micron, and 12CO fundamental ro-vibrational and pure rotational J=3-2 observations. We simultaneously modeled the continuum emission and the line fluxes with the Monte Carlo radiative transfer code MCFOST and the thermo-chemical code ProDiMo to derive the disk gas- and dust properties assuming no dust settling. The models suggest that the oxygen lines are emitted from the inner disk around HD141569A, whereas the [CII] line emission is more extended. The CO submillimeter flux is emitted mostly by the outer disk. Simultaneous modeling of the photometric and line data using a realistic disk structure suggests a dust mass derived from grains with a radius smaller than 1 mm of 2.1E-7 MSun and from grains with a radius of up to 1 cm of 4.9E-6 MSun. We constrained the polycyclic aromatic hydrocarbons (PAH) mass to be between 2E-11 and 1..4E-10 MSun assuming circumcircumcoronene (C150H30) as the representative PAH. The associated PAH abundance relative to hydrogen is lower than those found in the interstellar medium (3E-7) by two to three orders of magnitude. The disk around HD141569A is less massive in gas (2.5 to 4.9E-4 MSun or 67 to 164 MEarth) and has a flat opening angle (\u3c10%). [abridged
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