1,667 research outputs found
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&
ISO spectroscopy of circumstellar dust in the Herbig Ae systems AB Aur and HD 163296
Using both the Short- and Long-wavelength Spectrometers on board the Infrared
Space Observatory (ISO), we have obtained infrared spectra of the Herbig Ae
systems AB Aur and HD 163296. In addition, we obtained ground-based N band
images of HD 163296. Our results can be summarized as follows: (1) The main
dust components in AB Aur are amorphous silicates, iron oxide and PAHs; (2) The
circumstellar dust in HD 163296 consists of amorphous silicates, iron oxide,
water ice and a small fraction of crystalline silicates; (3) The infrared
fluxes of HD 163296 are dominated by solid state features; (4) The colour
temperature of the underlying continuum is much cooler in HD 163296 than in AB
Aur, pointing to the existence of a population of very large (mm sized) dust
grains in HD 163296; (5) The composition and degree of crystallization of
circumstellar dust are poorly correlated with the age of the central star. The
processes of crystallization and grain growth are also not necessarily coupled.
This means that either the evolution of circumstellar dust in protoplanetary
disks happens very rapidly (within a few Myr), or that this evolution is
governed by factors other than stellar mass and age.Comment: 6 pages, 2 figures, accepted for publication in Astronomy &
Astrophysic
Medium-separation binaries do not affect the first steps of planet formation
The first steps of planet formation are marked by the growth and
crystallization of sub-micrometer-sized dust grains accompanied by dust
settling toward the disk midplane. In this paper we explore whether the first
steps of planet formation are affected by the presence of medium-separation
stellar companions. We selected two large samples of disks around single and
binary T Tauri stars in Taurus that are thought to have only a modest age
spread of a few Myr. The companions of our binary sample are at projected
separations between 10 and 450 AU with masses down to about 0.1 solar masses.
We used the strength and shape of the 10 micron silicate emission feature as a
proxy for grain growth and for crystallization respectively. The degree of dust
settling was evaluated from the ratio of fluxes at two different mid-infrared
wavelengths. We find no statistically significant difference between the
distribution of 10 micron silicate emission features from single and binary
systems. In addition, the distribution of disk flaring is indistinguishable
between the single and binary system samples. These results show that the first
steps of planet formation are not affected by the presence of a companion at
tens of AU.Comment: To appear in the Astrophysical Journa
Binarity as a key factor in protoplanetary disk evolution: Spitzer disk census of the eta Chamaeleontis cluster
The formation of planets is directly linked to the evolution of the
circumstellar (CS) disk from which they are born. The dissipation timescales of
CS disks are, therefore, of direct astrophysical importance in evaluating the
time available for planet formation. We employ Spitzer Space Telescope spectra
to complete the CS disk census for the late-type members of the ~8 Myr-old eta
Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess
emission. We find that the presence of a CS disk is anti-correlated with
binarity, with all but one disk associated with single stars. With nine single
stars in total, about 80% retain a CS disk. Of the six known or suspected close
binaries the only CS disk is associated with the primary of RECX 9. No
circumbinary disks have been detected. We also find that stars with disks are
slow rotators with surface values of specific angular momentum j = 2-15 j_sun.
All high specific angular momentum systems with j = 20-30 j_sun are confined to
the primary stars of binaries. This provides novel empirical evidence for
rotational disk locking and again demonstrates the much shorter disk lifetimes
in close binary systems compared to single star systems. We estimate the
characteristic mean disk dissipation timescale to be ~5 Myr and ~9 Myr for the
binary and single star systems, respectively.Comment: Accepted by ApJ
DIGIT survey of far-infrared lines from protoplanetary disks I
[abridged] We present far-infrared spectroscopic observations of PMS stars
taken with Herschel/PACS as part of the DIGIT key project. The sample includes
22 Herbig AeBe and 8 T Tauri sources. Multiple atomic fine structure and
molecular lines are detected at the source position: [OI], [CII], CO, OH, H_2O,
CH^+. The most common feature is the [OI] 63micron line detected in almost all
of the sources followed by OH. In contrast with CO, OH is detected toward both
Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab
model fit to the OH lines indicates column densities of 10^13 < N_OH < 10^16
cm^-2, emitting radii 15 < r < 100 AU and excitation temperatures 100 < T_ex <
400 K. The OH emission thus comes from a warm layer in the disk at intermediate
stellar distances. Warm H_2O emission is detected through multiple lines toward
the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe
systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall,
Herbig AeBe sources have higher OH/H_2O abundance ratios across the disk than
do T Tauri disks, from near- to far-infrared wavelengths. Far-infrared CH^+
emission is detected toward HD 100546 and HD 97048. The slab model suggests
moderate excitation (T_ex ~ 100 K) and compact (r ~ 60 AU) emission in the case
of HD 100546. The [CII] emission is spatially extended in all sources where the
line is detected. This suggests that not all [CII] emission is associated with
the disk and that there is a substantial contribution from diffuse material
around the young stars. The flux ratios of the atomic fine structure lines are
consistent with a disk origin for the oxygen lines for most of the sources.Comment: Accepted for publication in A&
On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars
Based on the SED, Herbig stars have been categorized into two observational
groups, reflecting their overall disk structure: group I members have disks
with a higher degree of flaring than their group II counterparts. We
investigate the 5-35 um Spitzer IRS spectra of a sample of 13 group I sources
and 20 group II sources. We focus on the continuum emission to study the
underlying disk geometry. We have determined the [30/13.5] and [13.5/7]
continuum flux ratios. The 7-um flux excess with respect to the stellar
photosphere is measured, as a marker for the strength of the near-IR emission
produced by the inner disk. We have compared our data to self-consistent
passive-disk model spectra, for which the same quantities were derived. We
confirm the literature result that the difference in continuum emission between
group I and II sources can largely be explained by a different amount of small
dust grains. However, we report a strong correlation between the [30/13.5] and
[13.5/7] flux ratios for Meeus group II sources. Moreover, the [30/13.5] flux
ratio decreases with increasing 7-um excess for all targets in the sample. To
explain these correlations with the models, we need to introduce an artificial
scaling factor for the inner disk height. In roughly 50% of the Herbig Ae/Be
stars in our sample, the inner disk must be inflated by a factor 2 to 3 beyond
what hydrostatic calculations predict. The total disk mass in small dust grains
determines the degree of flaring. We conclude, however, that for any given disk
mass in small dust grains, the shadowing of the outer (tens of AU) disk is
determined by the scale height of the inner disk (1 AU). The inner disk
partially obscures the outer disk, reducing the disk surface temperature. Here,
for the first time, we prove these effects observationally.Comment: 4 pages, 3 figures, accepted by A&
Spectral Energy Distributions of T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with ISO LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passive
circumstellar disks constructed by Chiang & Goldreich (1997). New features
include (1) account for a range of particle sizes, (2) employment of
laboratory-based optical constants of representative grain materials, and (3)
numerical solution of the equations of radiative and hydrostatic equilibrium
within the original 2-layer (disk surface + disk interior) approximation. We
explore how the spectral energy distribution (SED) of a face-on disk depends on
grain size distributions, disk geometries and surface densities, and stellar
photospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,
including spectra from the Long Wavelength Spectrometer (LWS) aboard the
Infrared Space Observatory (ISO), are fitted with our models. Silicate emission
bands from optically thin, superheated disk surface layers appear in nearly all
systems. Water ice emission bands appear in LWS spectra of 2 of the coolest
stars. Infrared excesses in several sources are consistent with vertical
settling of photospheric grains. While this work furnishes further evidence
that passive reprocessing of starlight by flared disks adequately explains the
origin of infrared-to-millimeter wavelength excesses of young stars, we
emphasize how the SED alone does not provide sufficient information to
constrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin
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 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
An emission ring at 20 microns around the HAEBE star AB Aurigae: unveiling the disc structure
Isolated HAEBE stars are believed to represent an intermediate stage of
objects between young stellar objects surrounded by massive, optically thick,
gaseous and dusty disks and Vega like stars surrounded by debris disks. The
star AB Aur is already known for being surrounded by an intermediate-stage dust
disk emitting a fairly large infrared and (sub-)millimetric excess. Until now,
the outer disk structure has only been resolved at millimeter wavelengths and
at optical wavelength coronographic imaging. We have obtained 20 microns images
which show an unexpected ellipse-shaped disk structure in emission at a
distance of about 260 AU from the central star. Large azimuthal asymmetries in
brightness can be noticed and the center of the ellipse does not coincide with
the star. A simple, pure geometrical model based on an emission ring of uniform
surface brightness, but having an intrinsic eccentricity succeeds in fitting
the observations. These observations give for the first time clues on a very
peculiar structure of pre-main-sequence disk geometry, i.e. a non uniform
increase in the disk thickness unlike the common usual sketch of a disk with a
constant flaring angle. They provide also valuable informations on the disk
inclination as well as its dust composition; at such a large distance from the
star, only transient heating of very small particles can explain such a bright
ring of emission at mid-infrared wavelengths. Finally, the increase of
thickness inferred by the model could be caused by disk instabilities; the
intrinsic eccentricity of the structure might be a clue to the presence of a
massive body undetected yet
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