356 research outputs found
The Mid-Infrared Emitting Dust Around AB Aur
Using the Keck I telescope, we have obtained 11.7 micron and 18.7 micron
images of the circumstellar dust emission from AB Aur, a Herbig Ae star. We
find that AB Aur is probably resolved at 18.7 micron with an angular diameter
of 1.2" at a surface brightness of 3.5 Jy/arcsec^2. Most of the dust mass
detected at millimeter wavelengths does not contribute to the 18.7 micron
emission, which is plausibly explained if the system possesses a relatively
cold, massive disk. We find that models with an optically thick, geometrically
thin disk, surrounded by an optically thin spherical envelope fit the data
somewhat better than flared disk models.Comment: ApJ in press, 4 color figure
ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
We present Infrared Space Observatory (ISO) spectra of fourteen isolated
Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar
dust. These spectra show large star-to-star differences, in the emission
features of both carbon-rich and oxygen-rich dust grains. The IR spectra were
combined with photometric data ranging from the UV through the optical into the
sub-mm region. We defined two key groups, based upon the spectral shape of the
infrared region. The derived results can be summarized as follows: (1) the
continuum of the IR to sub-mm region of all stars can be reconstructed by the
sum of a power-law and a cool component, which can be represented by a black
body. Possible locations for these components are an optically thick,
geometrically thin disc (power-law component) and an optically thin flared
region (black body); (2) all stars have a substantial amount of cold dust
around them, independent of the amount of mid-IR excess they show; (3) also the
near-IR excess is unrelated to the mid-IR excess, indicating different
composition/location of the emitting material; (4) remarkably, some sources
lack the silicate bands; (5) apart from amorphous silicates, we find evidence
for crystalline silicates in several stars, some of which are new detections;
(6) PAH bands are present in at least 50% of our sample, and their appearance
is slightly different from PAHs in the ISM; (7) PAH bands are, with one
exception, not present in sources which only show a power-law continuum in the
IR; their presence is unrelated to the presence of the silicate bands; (8) the
dust in HAEBE stars shows strong evidence for coagulation; this dust processing
is unrelated to any of the central star properties (such as age, spectral type
and activity).Comment: 15 pages, accepted by A&
Silicate Emission in the TW Hydrae Association
The TW Hydrae Association is the nearest young stellar association. Among its
members are HD 98800, HR 4796A, and TW Hydrae itself, the nearest known
classical T Tauri star. We have observed these three stars spectroscopically
between 3 and 13 microns. In TW Hya the spectrum shows a silicate emission
feature that is similar to many other young stars with protostellar disks. The
11.2 micron feature indicative of significant amounts of crystalline olivine is
not as strong as in some young stars and solar system comets. In HR 4796A, the
thermal emission in the silicate feature is very weak, suggesting little in the
way of (small silicate) grains near the star. The silicate band of HD 98800
(observed by us but also reported by Sylvester and Skinner (1996)) is
intermediate in strength between TW Hya and HR 4796.Comment: 22 pages, 11 figures, LaTeX2e and AAS LaTeX macros v5.0. Accepted for
publication in A
Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data
We propose a method that uses near-infrared plus uvby photometry to identify
potentially extensive circumstellar dusty environment about F and A stars. The
method has been applied to a sample of ~900 metal rich reddened F stars with
2MASS and uvby data, suggesting the presence of circumstellar dust emitting in
the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most,
if not all, of them are likely pre-mainsequence (PMS). They seem to be
consistent with being a continuation of the class of Herbig Ae/Be PMS stars
into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84
(spectral types later than ~F5), which may provide new clues to the PMS
evolution of stars with 1 to 2 solar mass. We present a list of 21 most
conspicuous candidate stars with circumstellar dust. About half of them are
associated with the extended star-forming region around rho Oph. The brightest
of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the
presence of heated dust emitting in the far infrared. Also in this list, HD
81270 is reported as a very unusual star moving away from the Galactic plane at
a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002
May
Looking for Pure Rotational H_2 Emission from Protoplanetary Disks
We report on a limited search for pure-rotational molecular hydrogen emission
associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1
and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle
Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high
spectral and spatial resolution of our observations lead to more stringent
limits on narrow line emission close to the source than previously achieved.
One star, AB Aur, shows a possible (2sigma) H_2 detection, but further
observations are required to make a confident statement. Our non-detections
suggest that a significant fraction, perhaps all, of previously reported H_2
emission towards these objects could be extended on scales of 5" or more.Comment: 14 pages including 2 figures. Accepted by ApJ Letter
FU Orionis - The MIDI/VLTI Perspective
We present the first mid-infrared interferometric measurements of FU Orionis.
We clearly resolve structures that are best explained with an optically thick
accretion disk. A simple accretion disk model fits the observed SED and
visibilities reasonably well and does not require the presence of any
additional structure such as a dusty envelope. The inclination and also the
position angle of the disk can be constrained from the multibaseline
interferometric observations. Our disk model is in general agreement with most
published near-infrared interferometric measurements. From the shape and
strength of the 8-13 micrometer spectrum the dust composition of the accretion
disk is derived for the first time. We conclude that most dust particles are
amorphous and already much larger than those typically observed in the ISM.
Although the high accretion rate of the system provides both, high temperatures
out to large radii and an effective transport mechanism to distribute
crystalline grains, we do not see any evidence for crystalline silicates
neither in the total spectrum nor in the correlated flux spectra from the inner
disk regions. Possible reasons for this non-detection are mentioned. All
results are discussed in context with other high-spatial resolution
observations of FU Ori and other FU Ori objects. We also address the question
whether FU Ori is in a younger evolutionary stage than a classical TTauri star.Comment: 41 pages (aastex style), 11 figures, 8 tables, accepted by Ap
The composition of circumstellar gas and dust in 51 Oph
We analyze ISO archive data of the nearby bright emission-line star 51 Oph,
previously classified as a proto-planetary system similar to beta Pic. The
infrared spectrum reveals the presence of gas-phase emission bands of hot
(approx 850 K) CO, CO_2, H_2O and NO. In addition to this, partially
crystalline silicate dust is present. The solid-state bands and the energy
distribution are indicative of dust that has formed recently, rather than of
debris dust. The presence of hot molecular gas and the composition of the
circumstellar dust are highly unusual for a young star and are reminiscent of
what is found around evolved (AGB) stars, although we exclude the possibility
of 51 Oph belonging to this group. We suggest several explanations for the
nature of 51 Oph, including a recent episode of mass loss from a Be star, and
the recent destruction of a planet-sized body around a young star.Comment: 4 pages, 3 figures, accepted by A&A (letters
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 Circumstellar Disk of HD 141569 Imaged with NICMOS
Coronagraphic imaging with the Near Infrared Camera and Multi Object
Spectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4'')
radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected
light image at 1.1 micron shows the disk oriented at a position angle of 356
+/- 5 deg and inclined to our line of sight by 51 +/- 3 deg; the intrinsic
scattering function of the dust in the disk makes the side inclined toward us,
the eastern side, brighter. The disk flux density peaks 185 AU (1.''85) from
the star and falls off to both larger and smaller radii. A region of depleted
material, or a gap, in the disk is centered 250 AU from the star. The dynamical
effect of one or more planets may be necessary to explain this morphology.Comment: 4 pages, LaTeX with emulateapj.sty and epsfig.sty, 4 postscript
figures, Accepted to ApJ Letter
Making FORS2 fit for exoplanet observations (again)
For about three years, it was known that precision spectrophotometry with
FORS2 suffered from systematic errors that made quantitative observations of
planetary transits impossible. We identified the Longitudinal Atmospheric
Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged
in a project to exchange the LADC prisms with the uncoated ones from FORS1.
This led to a significant improvement in the depth of FORS2 zero points, a
reduction in the systematic noise, and should make FORS2 again competitive for
transmission spectroscopy of exoplanets.Comment: To appear in the March issue of the ESO Messenge
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