207 research outputs found
A Multi-Wavelength Analysis of Dust and Gas in the SR 24S Transition Disk
We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm
continuum observations of the SR 24S transition disk with an angular resolution
(12 au radius). We perform a multi-wavelength investigation by
combining new data with previous ALMA data at 0.45 mm. The visibilities and
images of the continuum emission at the two wavelengths are well characterized
by a ring-like emission. Visibility modeling finds that the ring-like emission
is narrower at longer wavelengths, in good agreement with models of dust
trapping in pressure bumps, although there are complex residuals that suggest
potentially asymmetric structures. The 0.45 mm emission has a shallower profile
inside the central cavity than the 1.3 mm emission. In addition, we find that
the CO and CO (J=2-1) emission peaks at the center of the
continuum cavity. We do not detect either continuum or gas emission from the
northern companion to this system (SR 24N), which is itself a binary system.
The upper limit for the dust disk mass of SR 24N is , which gives a disk mass ratio in dust between the two
components of . The current ALMA observations may imply that either
planets have already formed in the SR 24N disk or that dust growth to mm-sizes
is inhibited there and that only warm gas, as seen by ro-vibrational CO
emission inside the truncation radii of the binary, is present.Comment: Accepted for publication in Ap
Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud
The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a
survey of EROS-2 lightcurves for stars with large irregular brightness
variations typical of the R Coronae Borealis (RCB) class. However, the visible
spectrum showing emission lines including the Balmer and Paschen series as well
as many Fe II lines is emphatically not that of an RCB star. This star has all
of the characteristics of a typical UX Ori star. It has a spectral type of
approximately A2 and has excited an H II region in its vicinity. However, if it
is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows
irregular drops in brightness of up to 2 mag, and displays the reddening and
"blueing" typical of this class of stars. Its spectrum, showing a combination
of emission and absorption lines, is typical of a UX Ori star that is in a
decline caused by obscuration from the circumstellar dust. SSTISAGE1C
J050756.44-703453.9 has a strong IR excess and significant emission is present
out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires
that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large
extended diffuse envelope to fit both the mid- and far-IR dust emission. This
star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only
the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure
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Association of Systemic Inflammation With Retinal Vascular Caliber in Patients With AIDS.
PurposeTo evaluate relationships among retinal vascular caliber and biomarkers of systemic inflammation in patients with AIDS.MethodsA total of 454 participants with AIDS had retinal vascular caliber (central retinal artery equivalent and central retinal vein equivalent) determined from enrollment retinal photographs by reading center graders masked to clinical and biomarker information. Cryopreserved plasma specimens were assayed for inflammatory biomarkers, including C-reactive protein (CRP), IL-6, interferon-γ inducible protein (IP)-10, kynurenine/tryptophan (KT) ratio, and intestinal fatty acid binding protein (I-FABP).ResultsIn the simple linear regression of retinal vascular caliber on plasma biomarkers, elevated CRP, IL-6, and IP-10 were associated with retinal venular dilation, and elevated KT ratio with retinal arteriolar narrowing. In the multiple linear regression, including baseline characteristics and plasma biomarkers, AMD was associated with dilation of retinal arterioles (mean difference: 9.1 μm; 95% confidence interval [CI] 5.2, 12.9; P < 0.001) and venules (mean difference, 10.9 μm; 95% CI, 5.3, 16.6; P < 0.001), as was black race (P < 0.001). Hyperlipidemia was associated with retinal venular narrowing (mean difference, -7.5 μm; 95% CI, -13.7, -1.2; P = 0.02); cardiovascular disease with arteriolar narrowing (mean difference, -5.2 μm; 95% CI, -10.3, -0.1; P = 0.05); age with arteriolar narrowing (slope, -0.26 μm/year; 95% CI, -0.46, -0.06; P = 0.009); and IL-6 with venular dilation (slope, 5.3 μm/standard deviation log10[plasma IL-6 concentration]; 95% CI, 2.7, 8.0; P < 0.001).ConclusionsThese data suggest that retinal vascular caliber is associated with age, race, AMD, hyperlipidemia, cardiovascular disease, and selected biomarkers of systemic inflammation
H_2 Emission From Disks Around Herbig Ae and T Tauri Stars
We present the initial results of a deep ISO-SWS survey
for the low J pure rotational emission lines of H2
toward a number of Herbig Ae and T Tauri stars.
The objects are selected to be as isolated as possible
from molecular clouds, with a spectral energy distribution
characteristic of a circumstellar disk. For most
of them the presence of a disk has been established
directly by millimeter interferometry. The S (1) line is
detected in most sources with a peak flux of 0.3-1 Jy.
The S(0) line is definitely seen in 2 objects: GG Tau
and HD 163296. The observations suggest the presence
of "warm" gas at T_(kin) ≈ 100 K with a mass of a
few % of the total gas+ dust mass, derived assuming
a gas-to-dust ratio of 100:1. The S(1) peak flux does
not show a strong correlation with spectral type of
the central star or continuum flux at 1.3 millimeter.
Possible origins for the warm gas seen in H_2 are discussed,
and comparisons with model calculations are
made
Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
CONTEXT - Low-mass stars form with disks in which the coagulation of grains
may eventually lead to the formation of planets. It is not known when and where
grain growth occurs, as models that explain the observations are often
degenerate. A way to break this degeneracy is to resolve the sources under
study. AIMS - To find evidence for the existence of grains of millimetre sizes
in disks around in T Tauri stars, implying grain growth. METHODS - The
Australia Telescope Compact Array (ATCA) was used to observe 15 southern T
Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3
millimetre. The five Lupus sources were also observed with the Submillimeter
Array (SMA) at 1.4 millimetre. Our new data are complemented with data from the
literature to determine the slopes of the spectral energy distributions in the
millimetre regime. RESULTS - Ten sources were detected at better than 3sigma
with the ATCA, with sigma ~1-2 mJy, and all sources that were observed with the
SMA were detected at better than 15sigma, with sigma ~4 mJy. Six of the sources
in our sample are resolved to physical radii of ~100 AU. Assuming that the
emission from such large disks is predominantly optically thin, the millimetre
slope can be related directly to the opacity index. For the other sources, the
opacity indices are lower limits. Four out of six resolved sources have opacity
indices <~1, indicating grain growth to millimetre sizes and larger. The masses
of the disks range from < 0.01 to 0.08 MSun, which is comparable to the minimum
mass solar nebula. A tentative correlation is found between the millimetre
slope and the strength and shape of the 10-micron silicate feature, indicating
that grain growth occurs on similar (short) timescales in both the inner and
outer disk.Comment: 13 pages, 7 figures, 5 tables, accepted for publication in A&
Relation between the luminosity of young stellar objects and their circumstellar environment
We present a new model-independent method of comparison of NIR visibility
data of YSOs. The method is based on scaling the measured baseline with the
YSO's distance and luminosity, which removes the dependence of visibility on
these two variables. We use this method to compare all available NIR visibility
data and demonstrate that it distinguishes YSOs of luminosity >1000L_sun
(low-L) from YSOs of <1000L_sun (high-L). This confirms earlier suggestions,
based on fits of image models to the visibility data, for the difference
between the NIR sizes of these two luminosity groups. When plotted against the
``scaled'' baseline, the visibility creates the following data clusters: low-L
Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. The T Tau
cluster is similar to the low-L Herbig Ae/Be cluster, which has ~7 times
smaller ``scaled'' baselines than the high-L Herbig Be cluster. We model the
shape and size of clusters with different image models and find that low-L
Herbig stars are the best explained by the uniform brightness ring and the halo
model, T Tauri stars with the halo model, and high-L Herbig stars with the
accretion disk model. However, the plausibility of each model is not well
established. Therefore, we try to build a descriptive model of the
circumstellar environment consistent with various observed properties of YSOs.
We argue that low-L YSOs have optically thick disks with the optically thin
inner dust sublimation cavity and an optically thin dusty outflow above the
inner disk regions. High-L YSOs have optically thick accretion disks with high
accretion rates enabling gas to dominate the NIR emission over dust. Although
observations would favor such a description of YSOs, the required dust
distribution is not supported by our current understanding of dust dynamics.Comment: 20 pages, 12 figures, Accepted for publication in the Astrophysical
Journa
The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry
To trace the accretion and outflow processes around YSOs, diagnostic spectral
lines such as the BrG 2.166 micron line are widely used, although due to a lack
of spatial resolution, the origin of the line emission is still unclear.
Employing the AU-scale spatial resolution which can be achieved with infrared
long-baseline interferometry, we aim to distinguish between theoretical models
which associate the BrG line emission with mass infall or mass outflow
processes. Using the VLTI/AMBER instrument, we spatially and spectrally
(R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296,
HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the
adjacent continuum. All objects (except MWC297) show an increase of visibility
within the BrG emission line, indicating that the BrG-emitting region in these
objects is more compact than the dust sublimation radius. For HD98922, our
quantitative analysis reveals that the line-emitting region is compact enough
to be consistent with the magnetospheric accretion scenario. For HD163296,
HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the
most likely line-emitting mechanism. We search for general trends and find that
the size of the BrG-emitting region does not seem to depend on the basic
stellar parameters, but correlates with the H-alpha line profile shape. We find
evidence for at least two distinct BrG line-formation mechanisms. Stars with a
P-Cygni H-alpha line profile and a high mass-accretion rate seem to show
particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with
a double-peaked or single-peaked H-alpha-line profile show a significantly more
extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar
wind or a disk wind.Comment: 20 pages; 11 figures; Accepted by A&A; a high quality version of the
paper can be obtained at
http://www.skraus.eu/papers/kraus.HAeBe-BrGsurvey.pd
Detection of an inner gaseous component in a Herbig Be star accretion disk: Near- and mid-infrared spectro-interferometry and radiative transfer modeling of MWC 147
We study the geometry and the physical conditions in the inner (AU-scale)
circumstellar region around the young Herbig Be star MWC 147 using
long-baseline spectro-interferometry in the near-infrared (NIR K-band,
VLTI/AMBER observations and PTI archive data) as well as the mid-infrared (MIR
N-band, VLTI/MIDIobservations). The emission from MWC 147 is clearly resolved
and has a characteristic physical size of approx. 1.3 AU and 9 AU at 2.2 micron
and 11 micron respectively (Gaussian diameter). The spectrally dispersed AMBER
and MIDI interferograms both show a strong increase in the characteristic size
towards longer wavelengths, much steeper than predicted by analytic disk models
assuming power-law radial temperature distributions. We model the
interferometric data and the spectral energy distribution of MWC 147 with 2-D,
frequency-dependent radiation transfer simulations. This analysis shows that
models of spherical envelopes or passive irradiated Keplerian disks (with
vertical or curved puffed-up inner rim) can easily fit the SED, but predict
much lower visibilities than observed; the angular size predicted by such
models is 2 to 4 times larger than the size derived from the interferometric
data, so these models can clearly be ruled out. Models of a Keplerian disk with
optically thick gas emission from an active gaseous disk (inside the dust
sublimation zone), however, yield a good fit of the SED and simultaneously
reproduce the absolute level and the spectral dependence of the NIR and MIR
visibilities. We conclude that the NIR continuum emission from MWC 147 is
dominated by accretion luminosity emerging from an optically thick inner
gaseous disk, while the MIR emission also contains contributions from the
outer, irradiated dust disk.Comment: 44 pages, 15 figures, accepted for publication in The Astrophysical
Journal. The quality of the figures was slightly reduced in order to comply
with the astro-ph file-size restrictions. You can find a high-quality version
of the paper at http://www.mpifr-bonn.mpg.de/staff/skraus/papers/mwc147.pd
A low optical depth region in the inner disk of the HerbigAe star HR5999
Circumstellar disks surrounding young stars are known to be the birthplaces
of planets, and the innermost astronomical unit is of particular interest. We
present new long-baseline spectro-interferometric observations of the HerbigAe
star, HR5999, obtained in the H and K bands with the AMBER instrument at the
VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. We
spatially resolve the circumstellar material and reconstruct images using the
MiRA algorithm. In addition, we interpret the interferometric observations
using models that assume that the near-infrared excess is dominated by the
emission of a circumstellar disk. We compare the images reconstructed from the
VLTI measurements to images obtained using simulated model data. The K-band
image reveals three main elements: a ring-like feature located at ~0.65 AU, a
low surface brightness region inside, and a central spot. At the maximum
angular resolution of our observations (1.3 mas), the ring is resolved while
the central spot is only marginally resolved, preventing us from revealing the
exact morphology of the circumstellar environment. We suggest that the ring
traces silicate condensation, i.e., an opacity change, in a circumstellar disk
around HR 5999. We build a model that includes a ring at the silicate
sublimation radius and an inner disk of low surface brightness responsible for
a large amount of the near-infrared continuum emission. The model successfully
fits the SED, visibilities, and closure phases, and provides evidence of a low
surface brightness region inside the silicate sublimation radius. This study
provides additional evidence that in HerbigAe stars, there is material in a low
surface brightness region, probably a low optical depth region, located inside
the silicate sublimation radius and of unknown nature.Comment: 11 pages, 10 figure
A parameter study of self-consistent disk models around Herbig AeBe stars
We present a parameter study of self-consistent models of protoplanetary
disks around Herbig AeBe stars. We use the code developed by Dullemond and
Dominik, which solves the 2D radiative transfer problem including an iteration
for the vertical hydrostatic structure of the disk. This grid of models will be
used for several studies on disk emission and mineralogy in followup papers. In
this paper we take a first look on the new models, compare them with previous
modeling attempts and focus on the effects of various parameters on the overall
structure of the SED that leads to the classification of Herbig AeBe stars into
two groups, with a flaring (group I) or self-shadowed (group II) SED. We find
that the parameter of overriding importance to the SED is the total mass in
grains smaller than 25um, confirming the earlier results by Dullemond and
Dominik. All other parameters studied have only minor influences, and will
alter the SED type only in borderline cases. We find that there is no natural
dichotomy between group I and II. From a modeling point of view, the transition
is a continuous function of the small dust mass. We also show that moderate
grain growth produces spectra with weak or no 1um feature, both for flaring
(Group I) and non-flaring (Group II) sources. The fact that sources with weak
features have been found mostly in Group I sources is therefore surprising and
must be due to observational biases or evolutionary effects.Comment: 12 pages, 10 figures, Astronomy and Astrophysics, in pres
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