1,727 research outputs found
The Onset of Planet Formation in Brown Dwarf Disks
The onset of planet formation in protoplanetary disks is marked by the growth
and crystallization of sub-micron-sized dust grains accompanied by dust
settling toward the disk mid-plane. Here we present infrared spectra of disks
around brown dwarfs and brown dwarf candidates. We show that all three
processes occur in such cool disks in a way similar or identical to that in
disks around low- and intermediate-mass stars. These results indicate that the
onset of planet formation extends to disks around brown dwarfs, suggesting that
planet formation is a robust process occurring in most young circumstellar
disks.Comment: Published in Science 2005, vol 310, 834; 3 pages in final format, 4
figures + 8 pages Supporting Online Material. For final typeset, see
http://www.sciencemag.org/cgi/content/abstract/310/5749/834?eto
High spatial resolution mid-infrared observations of the low-mass young star TW Hya
We want to improve knowledge of the structure of the inner few AU of the
circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have
suggested the existence of a large inner hole, possibly caused by interactions
with a growing protoplanet. We used interferometric observations in the N-band
obtained with the MIDI instrument on the Very Large Telescope Interferometer,
together with 10 micron spectra recorded by the infrared satellite Spitzer. The
fact that we were able to determine N-band correlated fluxes and visibilities
for this comparatively faint source shows that MIR interferometry can be
applied to a large number of low-mass young stellar objects.
The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI
(7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength
regime is amorphous. According to the correlated flux measured with MIDI, most
of the crystalline material is in the inner, unresolved part of the disk, about
1 AU in radius. The visibilities exclude the existence of a very large (3-4 AU
radius) inner hole in the circumstellar disk of TW Hya, which was required in
earlier models. We propose instead a geometry of the inner disk where an inner
hole still exists, but at a much reduced radius, with the transition from zero
to full disk height between 0.5 and 0.8 AU, and with an optically thin
distribution of dust inside. Such a model can comply with SED and MIR
visibilities, as well as with visibility and extended emission observed in the
NIR at 2 micron. If a massive planet was the reason for this inner hole, as has
been speculated, its orbit would have to be closer to the star than 0.3 AU.
Alternatively, we may be witnessing the end of the accretion phase and an early
phase of an inward-out dispersal of the circumstellar disk.Comment: 13 pages, 9 figures, accepted by A&
Context-dependent detection of fungal parasites in four ant species
The reaction between the allyl radical (C3H5˙) and acetylene (C2H2) in a heated microtubular reactor has been studied at the VUV beamline of the Swiss Light Source. The reaction products are sampled from the reactor and identified by their photoion mass-selected threshold photoelectron spectra (ms-TPES) by means of imaging photoelectron photoion coincidence spectroscopy. Cyclopentadiene is identified as the sole reaction product by comparison of the measured photoelectron spectrum with that of cyclopentadiene. With the help of quantum-chemical computations of the C5H7 potential energy surface, the C2H2 + C3H5˙ association reaction is confirmed to be the rate determining step, after which H-elimination to form C5H6 is prompt in the absence of re-thermalization at low pressures. The formation of cyclopentadiene as the sole product from the allyl + acetylene reaction offers a direct path to the formation of cyclic hydrocarbons under combustion relevant conditions. Subsequent reactions of cyclopentadiene may lead to the formation of the smallest polycyclic aromatic molecule, naphthalene
Molecular Signatures in the Near Infrared Dayside Spectrum of HD 189733b
We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5
microns using the NICMOS instrument on the Hubble Space Telescope. The emergent
spectrum contains significant modulation, which we attribute to the presence of
molecular bands seen in absorption. We find that water (H2O), carbon monoxide
(CO), and carbon dioxide (CO2) are needed to explain the observations, and we
are able to estimate the mixing ratios for these molecules. We also find
temperature decreases with altitude in the ~0.01 < P < ~1 bar region of the
dayside near-infrared photosphere and set an upper limit to the dayside
abundance of methane (CH4) at these pressures.Comment: 13 pages, 3 figures. accepted in Astrophysical Journal Letter
Tracing the development of dust around evolved stars: The case of 47 Tuc
We observed mid-infrared (7.5-22 mum) spectra of AGB stars in the globular
cluster 47 Tuc with the Spitzer telescope and find significant dust features of
various types. Comparison of the characteristics of the dust spectra with the
location of the stars in a logP-K-diagram shows that dust mineralogy and
position on the AGB are related. A 13 mum feature is seen in spectra of low
luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 mum.
The spectra of the most luminous stars are dominated by the amorphous silicate
bending vibration centered at 9.7 mum. For 47 Tuc AGB stars, we conclude that
early on the AGB dust consisting primarily of Mg-, Al- and Fe oxides is formed.
With further AGB evolution amorphous silicates become the dominant species.Comment: 2 figures, accepted for publication in ApJ Letter
Dust in the wind: Crystalline silicates, corundum and periclase in PG 2112+059
We have determined the mineralogical composition of dust in the Broad
Absorption Line (BAL) quasar PG 2112+059 using mid-infrared spectroscopy
obtained with the Spitzer Space Telescope. From spectral fitting of the solid
state features, we find evidence for Mg-rich amorphous silicates with olivine
stoichiometry, as well as the first detection of corundum (Al_2O_3) and
periclase (MgO) in quasars. This mixed composition provides the first direct
evidence for a clumpy density structure of the grain forming region. The
silicates in total encompass 56.5% of the identified dust mass, while corundum
takes up 38 wt.%. Depending on the choice of continuum, a range of mass
fractions is observed for periclase ranging from 2.7% in the most conservative
case to 9% in a less constrained continuum. In addition, we identify a feature
at 11.2 micron as the crystalline silicate forsterite, with only a minor
contribution from polycyclic aromatic hydrocarbons. The 5% crystalline silicate
fraction requires high temperatures such as those found in the immediate quasar
environment in order to counteract rapid destruction from cosmic rays.Comment: 2 figure
Deeply embedded objects and shocked molecular hydrogen: The environment of the FU Orionis stars RNO 1B/1C
We present Spitzer IRAC and IRS observations of the dark cloud L1287. The mid-infrared (MIR) IRAC images show deeply embedded infrared sources in the vicinity of the FU Orionis objects RNO 1B and RNO 1C suggesting their association with a small young stellar cluster. For the first time we resolve the MIR point source associated with IRAS 00338+6312 which is a deeply embedded intermediate-mass protostar driving a known molecular outflow. The IRAC colors of all objects are consistent with young stars ranging from deeply embedded Class 0/I sources to Class II objects, part of which appear to be locally reddened. The two IRS spectra show strong absorption bands by ices and dust particles, confirming that the circumstellar environment around RNO 1B/1C has a high optical depth. Additional hydrogen emission lines from pure rotational transitions are superimposed on the spectra. Given the outflow direction, we attribute these emission lines to shocked gas in the molecular outflow powered by IRAS 00338+6312. The derived shock temperatures are in agreement with high velocity C-type shocks
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
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
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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