36 research outputs found
Evidence for grain growth in T Tauri disks
In this article we present the results from mid-infrared spectroscopy of a
sample of 14 T Tauri stars with silicate emission. The qualitative analysis of
the spectra reveals a correlation between the strength of the silicate feature
and its shape similar to the one which was found recently for the more massive
Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical
spectra of amorphous olivine with different grain sizes suggests that this
correlation is indicating grain growth in the disks of T Tauri stars. Similar
mechanisms of grain processing appear to be effective in both groups of young
stars.Comment: 4 pages A&A lette
Coagulation of small grains in disks: the influence of residual infall and initial small-grain content
Turbulent coagulation in protoplanetary disks is known to operate on
timescale far shorter than the lifetime of the disk. In the absence of
mechanisms that replenish the small dust grain population, protoplanetary disks
would rapidly lose their continuum opacity-bearing dust. This is inconsistent
with infrared observations of disks around T Tauri stars and Herbig Ae/Be
stars, which are usually optically thick at visual wavelengths and show
signatures of small (a<~ 3um) grains. A plausible replenishing mechanism of
small grains is collisional fragmentation or erosion of large dust aggregates,
which model calculations predict to play an important role in protoplanetary
disks. If optically thick disks are to be seen as proof for ongoing
fragmentation or erosion, then alternative explanations for the existence of
optically thick disks must be studied carefully. In this study we explore two
scenarios. First, we study the effect of residual, low-level infall of matter
onto the disk surface. We find that infall rates as low as 10^{-11} Msun/yr
can, in principle, replenish the small grain population to a level that keeps
the disk marginally optically thick. However, it remains to be seen if the
assumption of such inflow is realistic for star+disk systems at the age of
several Myrs, at which winds and jets are expected to have removed any residual
envelope. In summary, fragmentation or erosion still appear to be the most
promising processes to explain the abundant presence of small grains in old
disks.Comment: 10 pages, 4 figures, A&A in pres
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
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Analysis of the dust evolution in the circumstellar disks of TTauri stars
We present a compositional analysis of 8-13um spectra of 32 young stellar
objects (YSOs). Our sample consists of 5 intermediate-mass stars and 27
low-mass stars. While the spectra and first scientific results have already
been published by Przygodda et al. (2003) and Kessler-Silacci et al. (2004) we
perform a more detailed analysis of the 10um silicate feature. In our analysis
we assume that this emission feature can be represented by a linear
superposition of the wavelength-dependent opacity
describing the optical properties of silicate grains with different chemical
composition, structure, and grain size. The determination of an adequate
fitting equation is another goal of this study. Using a restricted number of
fitting parameters we investigate which silicate species are necessary for the
compositional fitting. Particles with radii of 0.1um- and 1.5um consisting of
amorphous olivine and pyroxene, forsterite, enstatite, and quartz have been
considered. Only compact, homogeneous dust grains have been used in the
presented fitting procedures. In this context we show that acceptable fitting
results can also be achieved if emission properties of porous silicate grains
are considered instead. Although some previous studies give reasons for the
similarity between the dust in circumstellar disks of TTauri stars and Herbig
Ae/Be stars, a quantitative comparison has been missing, so far. Therefore, we
conclude with a discussion of the results of a 10um spectroscopic survey of van
Boekel et al. (2005) who focus on Herbig Ae/Be stars, the higher mass
counterparts of T Tauri stars and draw comparisons to this and other studies.
We find that the results of our study of T Tauri systems partly agree with
previous studies of Herbig Ae/Be stars.Comment: 17 pages, 6 figure
The radial distribution of dust species in young brown dwarf disks
We present a study of the radial distribution of dust species in young brown
dwarf disks. Our work is based on a compositional analysis of the 10 and 20
micron silicate emission features for brown dwarfs in the Taurus-Auriga
star-forming region. A fundamental finding of our work is that brown dwarfs
exhibit stronger signs of dust processing in the cold component of the disk,
compared to the higher mass T Tauri stars in Taurus. For nearly all of our
targets, we find a flat disk structure, which is consistent with the stronger
signs of dust processing observed in these disks. For the case of one brown
dwarf, 2M04230607, we find the forsterite mass fraction to be a factor of ~3
higher in the outer disk compared to the inner disk region. Simple large-scale
radial mixing cannot account for this gradient in the dust chemical
composition, and some local crystalline formation mechanism may be effective in
this disk. The relatively high abundance of crystalline silicates in the outer
cold regions of brown dwarf disks provides an interesting analogy to comets. In
this context, we have discussed the applicability of the various mechanisms
that have been proposed for comets on the formation and the outward transport
of high-temperature material. We also present Chandra X-ray observations for
two Taurus brown dwarfs, 2M04414825 and CFHT-BD-Tau 9. We find 2M04414825,
which has a ~12% crystalline mass fraction, to be more than an order of
magnitude brighter in X-ray than CFHT-BD-Tau 9, which has a ~35% crystalline
mass fraction. Combining with previous X-ray data, we find the inner disk
crystalline mass fractions to be anti-correlated with the X-ray strength.Comment: Accepted in MNRA
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&
Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4
We present accurate mid-infrared observations of the disk around the young,
bona-fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in
the 7.9, 10.4 and 12.3 micron filters, from which we infer the presence of a
prominent, broad silicate emission feature. The shape of the silicate feature
is dominated by emission from 2 micron amorphous olivine grains. Such grains,
being an order of magnitude larger than those in the interstellar medium, are a
first proof of dust processing and grain growth in disks around brown dwarfs.
The object's spectral energy distribution is below the prediction of the
classical flared disk model but higher than that of the two-layer flat disk. A
good match can be achieved by using an intermediate disk model with strongly
reduced but non-zero flaring. Grain growth and dust settling processes provide
a natural explanation for this disk geometry and we argue that such
intermediate flaring might explain the observations of several other brown
dwarf disks as well.Comment: Accepted for publication in Astronomy & Astrophysics Letters, 4.5
pages with 1 figur
Mid-IR observations of circumstellar disks -- Part III: A mixed sample of PMS stars and Vega-type objects
We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis
object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained
with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members
of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands
are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144
N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the
Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm
wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no
indication for circumstellar matter at mid-infrared wavelengths is found
towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the
T Tauri system HD 155555.
The silicate emission features of the remaining eight sources are modelled
with a mixture of silicates of different grain sizes and composition.
Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous
grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS
144 S. Both small grains and crystalline dust is found for the Vega-type HD
123356, with a dominance of small amorphous grains. We show that the infrared
emission of the binary HD 123356 is dominated by its late-type secondary, but
optical spectroscopy is still required to confirm the age of the system and the
spectral class of the companion. For most targets this is their first
mid-infrared spectroscopic observation. We investigate trends between stellar,
disk and silicate properties and confirm correlations of previous studies.
Several objects present an exciting potential for follow-up high-resolution
disk studies.Comment: Accepted for publication in A&
A 10 micron spectroscopic survey of Herbig Ae star disks: grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Ae stars in
the 10 micrometer spectral region. We perform compositional fits of the spectra
based on properties of homogeneous as well as inhomogeneous spherical
particles, and derive the mineralogy and typical grain sizes of the dust
responsible for the 10 m emission. Several trends are reported that can
constrain theoretical models of dust processing in these systems: i) none of
the sources consists of fully pristine dust comparable to that found in the
interstellar medium, ii) all sources with a high fraction of crystalline
silicates are dominated by large grains, iii) the disks around more massive
stars (M >~ 2.5 M_sun, L >~ 60 L_sun) have a higher fraction of crystalline
silicates than those around lower mass stars, iv) in the subset of lower mass
stars (M <~ 2.5 M_sun) there is no correlation between stellar parameters and
the derived crystallinity of the dust. The correlation between the shape and
strength of the 10 micron silicate feature reported by van Boekel et al. (2003)
is reconfirmed with this larger sample. The evidence presented in this paper is
combined with that of other studies to present a likely scenario of dust
processing in Herbig Ae systems. We conclude that the present data favour a
scenario in which the crystalline silicates are produced in the innermost
regions of the disk, close to the star, and transported outward to the regions
where they can be detected by means of 10 micron spectroscopy. Additionally, we
conclude that the final crystallinity of these disks is reached very soon after
active accretion has stopped.Comment: 22 pages, 14 figures, accepted for publication in A&A. Note: this
submission was replaced on 26.04.2005: we used incorrect terminology in
figure 6 and the discussion of this figure. The vertical axis label of figure
6 has been corrected and now reads "Normalized 11.3/9.8 Flux Ratio", in the
discussion of this figure (section 4.2) "continuum subtracted" has been
replaced by "normalized