212 research outputs found
Measuring Organic Molecular Emission in Disks with Low Resolution Spitzer Spectroscopy
We explore the extent to which Spitzer IRS spectra taken at low spectral
resolution can be used in quantitative studies of organic molecular emission
from disks surrounding low mass young stars. We use Spitzer IRS spectra taken
in both the high and low resolution modules for the same sources to investigate
whether it is possible to define line indices that can measure trends in the
strength of the molecular features in low resolution data. We find that trends
in HCN emission strength seen in the high resolution data can be recovered in
low resolution data. In examining the factors that influence the HCN emission
strength, we find that the low resolution HCN flux is modestly correlated with
stellar accretion rate and X-ray luminosity. Correlations of this kind are
perhaps expected based on recent observational and theoretical studies of inner
disk atmospheres. Our results demonstrate the potential of using the large
number of low resolution disk spectra that reside in the Spitzer archive to
study the factors that influence the strength of molecular emission from disks.
Such studies would complement results for the much smaller number of
circumstellar disks that have been observed at high resolution with IRS
A Spitzer Survey of Mid-Infrared Molecular Emission from Protoplanetary Disks II: Correlations and LTE Models
We present an analysis of Spitzer-IRS observations of H2O, OH, HCN, C2H2, and
CO2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse
sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections
and strengths of most mid-IR molecular emission features are correlated with
each other, suggesting a common origin and similar excitation conditions. We
note that the line detection efficiency is anti-correlated with the 13/30 um
SED spectral slope, which is a measure of the degree of grain settling in the
disk atmosphere. We also note a correlation between detection efficiency and
H-alpha equivalent width, and tentatively with accretion rate, suggesting that
accretional heating contributes to line excitation. If detected, H2O line
fluxes are correlated with the mid-IR continuum flux, and other co-varying
system parameters, such as L_star. However, significant sample variation,
especially in molecular line ratios, remains. LTE models of the H2O emission
show that line strength is primarily related to the best-fit emitting area, and
this accounts for most source-to-source variation in H2O emitted flux. Best-fit
temperatures and column densities cover only a small range of parameter space,
near 10^{18} cm-2 and 450 K for all sources, suggesting a high abundance of H2O
in many planet-forming regions. Other molecules have a range of excitation
temperatures from ~500-1500 K, also consistent with an origin in planet-forming
regions. We find molecular ratios relative to water of ~10^{-3} for all
molecules, with the exception of CO, for which n(CO)/n(H2O)~1. However, LTE
fitting caveats and differences in the way thermo-chemical modeling results are
reported make comparisons with such models difficult, and highlight the need
for additional observations coupled with the use of line-generating radiative
transfer codes
Transitional disks and their origins: an infrared spectroscopic survey of Orion A
Transitional disks are protoplanetary disks around young stars, with inner
holes or gaps which are surrounded by optically thick outer, and often inner,
disks. Here we present observations of 62 new transitional disks in the Orion A
star-forming region. These were identified using the \textit{Spitzer Space
Telescope}'s Infrared Spectrograph and followed up with determinations of
stellar and accretion parameters using the Infrared Telescope Facility's SpeX.
We combine these new observations with our previous results on transitional
disks in Taurus, Chamaeleon I, Ophiuchus and Perseus, and with archival X-ray
observations. This produces a sample of 105 transitional disks of "cluster" age
3 Myr or less, by far the largest hitherto assembled. We use this sample to
search for trends between the radial structure in the disks and many other
system properties, in order to place constraints on the possible origins of
transitional disks. We see a clear progression of host star accretion rate and
the different disk morphologies. We confirm that transitional disks with
complete central clearings have median accretion rates an order of magnitude
smaller than radially continuous disks of the same population. Pre-transitional
disks --- those objects with gaps that separate inner and outer disks --- have
median accretion rates intermediate between the two. Our results from the
search for statistically significant trends, especially related to ,
strongly support that in both cases the gaps are far more likely to be due to
the gravitational influence of Jovian planets or brown dwarfs orbiting within
the gaps, than to any of the photoevaporative, turbulent or grain-growth
processes that can lead to disk dissipation. We also find that the fraction of
Class II YSOs which are transitional disks is large, 0.1-0.2, especially in the
youngest associations.Comment: 96 pages, 25 figures, resubmitted to Ap
Atomic Diagnostics of X-ray Irradiated Protoplanetary Disks
We study atomic line diagnostics of the inner regions of protoplanetary disks
with our model of X-ray irradiated disk atmospheres which was previously used
to predict observable levels of the NeII and NeIII fine-structure transitions
at 12.81 and 15.55mum. We extend the X-ray ionization theory to sulfur and
calculate the fraction of sulfur in S, S+, S2+ and sulfur molecules. For the
D'Alessio generic T Tauri star disk, we find that the SI fine-structure line at
25.55mum is below the detection level of the Spitzer Infrared Spectrometer
(IRS), in large part due to X-ray ionization of atomic S at the top of the
atmosphere and to its incorporation into molecules close to the mid-plane. We
predict that observable fluxes of the SII 6718/6732AA forbidden transitions are
produced in the upper atmosphere at somewhat shallower depths and smaller radii
than the neon fine-structure lines. This and other forbidden line transitions,
such as the OI 6300/6363AA and the CI 9826/9852AA lines, serve as complementary
diagnostics of X-ray irradiated disk atmospheres. We have also analyzed the
potential role of the low-excitation fine-structure lines of CI, CII, and OI,
which should be observable by SOFIA and Herschel.Comment: Accepted by Ap
Recommended from our members
An Experiment with Depressurization Tests as Indicators of Radon Availability in 6 New Jersey Houses
PAH emission from Herbig AeBe stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained
with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of
the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and
seven of the spectra show PAH emission, but no silicate emission at 10 microns.
The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features
decrease with stellar temperature, indicating that the PAHs are less
photo-processed in cooler radiation fields. The apparent low level of photo
processing in HAeBe stars, relative to other PAH emission sources, implies that
the PAHs are newly exposed to the UV-optical radiation fields from their host
stars. HAeBe stars show a variety of PAH emission intensities and ionization
fractions, but a narrow range of PAH spectral classifications based on
positions of major PAH feature centers. This may indicate that, regardless of
their locations relative to the stars, the PAH molecules are altered by the
same physical processes in the proto-planetary disks of intermediate-mass
stars. Analysis of the mid-IR spectral energy distributions indicates that our
sample likely includes both radially flared and more flattened/settled disk
systems, but we do not see the expected correlation of overall PAH emission
with disk geometry. We suggest that the strength of PAH emission from HAeBe
stars may depend not only on the degree of radial flaring, but also on the
abundance of PAHs in illuminated regions of the disks and possibly on the
vertical structure of the inner disk as well.Comment: 52 pages, 12 figure
On the origin of [NeII] 12.81 micron emission from pre-main sequence stars: Disks, jets, and accretion
(Abridged) We have conducted a study of [NeII] line emission based on a
sample of 92 pre-main sequence stars mostly belonging to the infrared Class II,
including 13 accreting transition disk objects and 14 objects driving jets and
outflows. We find several significant correlations between L[NeII] and stellar
parameters, in particular LX and the wind mass loss rate, dM/dt. Most
correlations are, however, strongly dominated by systematic scatter. While
there is a positive correlation between L[NeII] and LX, the stellar mass
accretion rate, dMacc/dt, induces a correlation only if we combine the largely
different subsets of jet sources and stars without jets. Our results suggest
that L[NeII] is bi-modally distributed, with separate distributions for the two
subsamples. The jet sources show systematically higher L[NeII], by 1-2 orders
of magnitude with respect to objects without jets. Jet-driving stars also tend
to show higher mass accretion rates. We therefore hypothesize that the trend
with dMacc/dt reflects a trend with dM/dt that is more physically relevant for
[NeII] emission. L[NeII] measured for objects without known outflows and jets
is found to agree with simplified calculations of [NeII] emission from disk
surface layers if the measured stellar X-rays are responsible for heating and
ionizing of the gas. The large scatter in L[NeII] may be introduced by
variations of disk properties and the irradiation spectrum, as previously
suggested. The systematically enhanced [NeII] flux from jet sources clearly
suggests a role for the jets themselves, as previously demonstrated by a
spatially resolved observation of the outflow system in the T Tau triple.Comment: accepted for Astronomy & Astrophysics, 25 pages, 11 figures -
revisions: affiliation added, NeII flux unit in Table 3 correcte
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