209 research outputs found
FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure
Context. Planets are thought to eventually form from the mostly gaseous (~99%
of the mass) disks around young stars. The density structure and chemical
composition of protoplanetary disks are affected by the incident radiation
field at optical, FUV, and X-ray wavelengths, as well as by the dust
properties.
Aims. The effect of FUV and X-rays on the disk structure and the gas chemical
composition are investigated. This work forms the basis of a second paper,
which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+
observed with facilities such as Spitzer and Herschel.
Methods. A grid of 240 models is computed in which the X-ray and FUV
luminosity, minimum grain size, dust size distribution, and surface density
distribution are varied in a systematic way. The hydrostatic structure and the
thermo-chemical structure are calculated using ProDiMo.
Results. The abundance structure of neutral oxygen is stable to changes in
the X-ray and FUV luminosity, and the emission lines will thus be useful
tracers of the disk mass and temperature. The C+ abundance distribution is
sensitive to both X-rays and FUV. The radial column density profile shows two
peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and
other heavy elements have a very strong response to X-rays, and the column
density in the inner disk increases by two orders of magnitude from the lowest
(LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV
confines the Ne+ ionized region to areas closer to the star at low X-ray
luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to
higher temperatures in the inner disk and higher ionization fractions in the
outer disk. The line fluxes and profiles are affected by the effects on these
species, thus providing diagnostic value in the study of FUV and X-ray
irradiated disks around T Tauri stars. (abridged)Comment: 47 pages, accepted by Astronomy and Astrophysics, a high resolution
version of the paper is located at
http://www.astro.rug.nl/~meijerink/disk_paperI_xrays.pd
The Binarity of Eta Carinae and its Similarity to Related Astrophysical Objects
I examine some aspects of the interaction between the massive star Eta
Carinae and its companion, in particular during the eclipse-like event, known
as the spectroscopic event or the shell event. The spectroscopic event is
thought to occur when near periastron passages the stellar companion induces
much higher mass loss rate from the primary star, and/or enters into a much
denser environment around the primary star. I find that enhanced mass loss rate
during periastron passages, if it occurs, might explain the high eccentricity
of the system. However, there is not yet a good model to explain the presumed
enhanced mass loss rate during periastron passages. In the region where the
winds from the two stars collide, a dense slow flow is formed, such that large
dust grains may be formed. Unlike the case during the 19th century Great
Eruption, the companion does not accrete mass during most of its orbital
motion. However, near periastron passages short accretion episodes may occur,
which may lead to pulsed ejection of two jets by the companion. The companion
may ionize a non-negligible region in its surrounding, resembling the situation
in symbiotic systems. I discuss the relation of some of these processes to
other astrophysical objects, by that incorporating Eta Car to a large class of
astrophysical bipolar nebulae.Comment: Updated version. ApJ, in pres
Uncertainties in water chemistry in disks: An application to TW Hya
Context. This paper discusses the sensitivity of water lines to chemical
processes and radiative transfer for the protoplanetary disk around TW Hya. The
study focuses on the Herschel spectral range in the context of new line
detections with the PACS instrument from the Gas in Protoplanetary Systems
project (GASPS). Aims. The paper presents an overview of the chemistry in the
main water reservoirs in the disk around TW Hya. It discusses the limitations
in the interpretation of observed water line fluxes. Methods. ... (abbreviated)
Results. We report new line detections of p-H2O (3_22-2_11) at 89.99 micron and
CO J=18-17 at 144.78 micron for the disk around TW Hya. Disk modeling shows
that the far-IR fine structure lines ([OI], [CII]) and molecular submm lines
are very robust to uncertainties in the chemistry, while the water line fluxes
can change by factors of a few. The water lines are optically thick,
sub-thermally excited and can couple to the background continuum radiation
field. The low-excitation water lines are also sensitive to uncertainties in
the collision rates, e.g. with neutral hydrogen. The gas temperature plays an
important role for the [OI] fine structure line fluxes, the water line fluxes
originating from the inner disk as well as the high excitation CO, CH+ and OH
lines. Conclusions. Due to their sensitivity on chemical input data and
radiative transfer, water lines have to be used cautiously for understanding
details of the disk structure. Water lines covering a wide range of excitation
energies provide access to the various gas phase water reservoirs (inside and
outside the snow line) in protoplanetary disks and thus provide important
information on where gas-phase water is potentially located. Experimental
and/or theoretical collision rates for H2O with atomic hydrogen are needed to
diminish uncertainties from water line radiative transfer.Comment: accepted for publication in A&
Water depletion in the disk atmosphere of Herbig AeBe stars
We present high resolution (R = 100,000) L-band spectroscopy of 11 Herbig
AeBe stars with circumstellar disks. The observations were obtained with the
VLT/CRIRES to detect hot water and hydroxyl radical emission lines previously
detected in disks around T Tauri stars. OH emission lines are detected towards
4 disks. The OH P4.5 (1+,1-) doublet is spectrally resolved as well as the
velocity profile of each component of the doublet. Its characteristic
double-peak profile demonstrates that the gas is in Keplerian rotation and
points to an emitting region extending out to ~ 15-30 AU. The OH, emission
correlates with disk geometry as it is mostly detected towards flaring disks.
None of the Herbig stars analyzed here show evidence of hot water vapor at a
sensitivity similar to that of the OH lines. The non-detection of hot water
vapor emission indicates that the atmosphere of disks around Herbig AeBe stars
are depleted of water molecules. Assuming LTE and optically thin emission we
derive a lower limit to the OH/H2O column density ratio > 1 - 25 in contrast to
T Tauri disks for which the column density ratio is 0.3 -- 0.4.Comment: Accepted for publication in Ap
Dust in Brown Dwarfs IV. Dust formation and driven turbulence on mesoscopic scales
Dust formation in brown dwarf atmospheres is studied by utilising a model for
driven turbulence in the mesoscopic scale regime. We apply a pseudo-spectral
method where waves are created and superimposed within a limited wavenumber
interval. The turbulent kinetic energy distribution follows the Kolmogoroff
spectrum which is assumed to be the most likely value. Such superimposed,
stochastic waves may occur in a convectively active environment. They cause
nucleation fronts and nucleation events and thereby initiate the dust formation
process which continues until all condensible material is consumed. Small
disturbances are found to have a large impact on the dust forming system. An
initially dust-hostile region, which may originally be optically thin, becomes
optically thick in a patchy way showing considerable variations in the dust
properties during the formation process. The dust appears in lanes and curls as
a result of the interaction with waves, i.e. turbulence, which form larger and
larger structures with time. Aiming on a physical understanding of the
variability of brown dwarfs, related to structure formation in substellar
atmospheres, we work out first necessary criteria for small-scale closure
models to be applied in macroscopic simulations of dust forming astrophysical
systems.Comment: A&A accepted, 20 page
Herschel evidence for disk flattening or gas depletion in transitional disks
Transitional disks are protoplanetary disks characterized by reduced near-
and mid-infrared emission with respect to full disks. This characteristic
spectral energy distribution indicates the presence of an optically thin inner
cavity within the dust disk believed to mark the disappearance of the
primordial massive disk. We present new Herschel Space Observatory PACS spectra
of [OI] 63 micron for 21 transitional disks. Our survey complements the larger
Herschel GASPS program "Gas in Protoplanetary Systems" (Dent et al. 2013) by
quadrupling the number of transitional disks observed with PACS at this
wavelength. [OI] 63 micron traces material in the outer regions of the disk,
beyond the inner cavity of most transitional disks. We find that transitional
disks have [OI] 63 micron line luminosities two times fainter than their full
disk counterparts. We self consistently determine various stellar properties
(e.g. bolometric luminosity, FUV excess, etc.) and disk properties (e.g. disk
dust mass, etc.) that could influence the [OI] 63 micron line luminosity and we
find no correlations that can explain the lower [OI] 63 micron line
luminosities in transitional disks. Using a grid of thermo-chemical
protoplanetary disk models, we conclude that either transitional disks are less
flared than full disks or they possess lower gas-to-dust ratios due to a
depletion of gas mass. This result suggests that transitional disks are more
evolved than their full disk counterparts, possibly even at large radii.Comment: Accepted for publication in ApJ; 52 pages, 16 figures, 8 table
[OI] disk emission in the Taurus star forming region
The structure of protoplanetary disks is thought to be linked to the
temperature and chemistry of their dust and gas. Whether the disk is flat or
flaring depends on the amount of radiation that it absorbs at a given radius,
and on the efficiency with which this is converted into thermal energy. The
understanding of these heating and cooling processes is crucial to provide a
reliable disk structure for the interpretation of dust continuum emission and
gas line fluxes. Especially in the upper layers of the disk, where gas and dust
are thermally decoupled, the infrared line emission is strictly related to the
gas heating/cooling processes. We aim to study the thermal properties of the
disk in the oxygen line emission region, and to investigate the relative
importance of X-ray (1-120 Angstrom) and far-UV radiation (FUV, 912-2070
Angstrom) for the heating balance there. We use [OI] 63 micron line fluxes
observed in a sample of protoplanetary disks of the Taurus/Auriga star forming
region and compare it to the model predictions presented in our previous work.
The data were obtained with the PACS instrument on board the Herschel Space
Observatory as part of the Herschel Open Time Key Program GASPS (GAS in
Protoplanetary diskS), published in Howard et al. (2013). Our theoretical grid
of disk models can reproduce the [OI] absolute fluxes and predict a correlation
between [OI] and the sum Lx+Lfuv. The data show no correlation between the [OI]
line flux and the X-ray luminosity, the FUV luminosity or their sum. The data
show that the FUV or X-ray radiation has no notable impact on the region where
the [OI] line is formed. This is in contrast with what is predicted from our
models. Possible explanations are that the disks in Taurus are less flaring
than the hydrostatic models predict, and/or that other disk structure aspects
that were left unchanged in our models are important. ..abridged..Comment: 9 pages, accepted for publication in A&
Optical linear polarization in ultra cool dwarfs: A tool to probe dust in the ultra cool dwarf atmospheres
Aims.Recent studies have detected linear polarization in L dwarfs in the
optical I band. Theoretical models have been developed to explain this
polarization. These models predict higher polarization at shorter wavelengths.
We discuss the polarization in the R and I band of 4 ultra cool dwarfs.
Methods.We report linear polarization measurements of 4 ultra cool dwarfs in
the R and I bands using the Intermediate dispersion Spectrograph and Imaging
System (ISIS) mounted on the 4.2m William Herschel Telescope (WHT). Results.As
predicted by theoretical models, we find a higher degree of polarization in the
R band when compared to polarization in the I band for 3/4 of these ultra cool
dwarfs. This suggests that dust scattering asymmetry is caused by oblateness
>.We also show how these measurements fit the theoretical models. A case for
variability of linear polarization is found, which suggests the presence of
randomly distributed dust clouds. We also discuss one case for the presence of
a cold debris disk.Comment: 7 pages, 2 figure
CO ro-vibrational lines in HD100546: A search for disc asymmetries and the role of fluorescence
We have studied the emission of CO ro-vibrational lines in the disc around
the Herbig Be star HD100546 with the final goal of using these lines as a
diagnostic to understand inner disc structure in the context of planet
formation. High-resolution IR spectra of CO ro-vibrational emission at eight
different position angles were taken with CRIRES at the VLT. From these spectra
flux tables, CO ro-vibrational line profiles, and population diagrams were
produced. We have investigated variations in the line profile shapes and line
strengths as a function of slit position angle. We used the thermochemical disc
modelling code ProDiMo based on the chemistry, radiation field, and temperature
structure of a previously published model for HD100546. Comparing observations
and the model, we investigated the possibility of disc asymmetries, the
excitation mechanism (UV fluorescence), the geometry, and physical conditions
of the inner disc. The observed CO ro-vibrational lines are largely emitted
from the inner rim of the outer disc at 10-13 AU. The line shapes are similar
for all v levels and line fluxes from all vibrational levels vary only within
one order of magnitude. All line profile asymmetries and variations can be
explained with a symmetric disc model to which a slit correction and pointing
offset is applied. Because the angular size of the CO emitting region (10-13
AU) and the slit width are comparable the line profiles are very sensitive to
the placing of the slit. The model reproduces the line shapes and the fluxes of
the v=1-0 lines as well as the spatial extent of the CO ro-vibrational
emission. It does not reproduce the observed band ratios of 0.5-0.2 with higher
vibrational bands. We find that lower gas volume densities at the surface of
the inner rim of the outer disc can make the fluorescence pumping more effcient
and reproduce the observed band ratios.Comment: 20 pages, 21 figure
LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths
We present a new code for solving the molecular and atomic excitation and
radiation transfer problem in a molecular gas and predicting emergent spectra.
This code works in arbitrary three dimensional geometry using unstructured
Delaunay latices for the transport of photons. Various physical models can be
used as input, ranging from analytical descriptions over tabulated models to
SPH simulations. To generate the Delaunay grid we sample the input model
randomly, but weigh the sample probability with the molecular density and other
parameters, and thereby we obtain an average grid point separation that scales
with the local opacity. Our code does photon very efficiently so that the slow
convergence of opaque models becomes traceable. When convergence between the
level populations, the radiation field, and the point separation has been
obtained, the grid is ray-traced to produced images that can readily be
compared to observations. Because of the high dynamic range in scales that can
be resolved using this type of grid, our code is particularly well suited for
modeling of ALMA data. Our code can furthermore deal with overlapping lines of
multiple molecular and atomic species.Comment: 13 pages, 12 figures, Accepted by A&A on 06/08/201
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