162 research outputs found
CO Rovibrational Emission as a Probe of Inner Disk Structure
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width ~0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust
The narrow, inner CO ring around the magnetic Herbig Ae star, HD 101412
We describe and model emission lines in the first overtone band of CO in the
magnetic Herbig Ae star HD 101412. High-resolution CRIRES spectra reveal
unusually sharp features which suggest the emission is formed in a thin disk
centered at 1 AU with a width 0.32 AU or less. A wider disk will not fit the
observations. Previous observations have reached similar conclusions, but the
crispness of the new material brings the emitting region into sharp focus.Comment: Accepted as Astronomy and Astrophysics Letter; 4 pages, 5 figure
Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
Carbon Monoxide is a commonly used IR/sub-mm tracer of gas in protoplanetary
disks. We present an analysis of ultraviolet CO emission in {HST}-COS spectra
for 12 Classical T Tauri stars. Several ro-vibrational bands of the CO A^1\Pi -
X^1\Sigma^+ (Fourth Positive) electronic transition system are spectrally
resolved from emission of other atoms and H_2. The CO A^1\Pi v'=14 state is
populated by absorption of Ly\alpha photons, created at the accretion column on
the stellar surface. For targets with strong CO emission, we model the Ly\alpha
radiation field as an input for a simple fluorescence model to estimate CO
rotational excitation temperatures and column densities. Typical column
densities range from N_{CO} = 10^{18} - 10^{19} cm^{-2}. Our measured
excitation temperatures are mostly below T_{CO} = 600 K, cooler than typical
M-band CO emission. These temperatures and the emission line widths imply that
the UV emission originates in a different population of CO than that which is
IR-emitting. We also find a significant correlation between CO emission and the
disk accretion rate M_{acc} and age. Our analysis shows that ultraviolet CO
emission can be a useful diagnostic of CTTS disk gas
The Volatile Composition of the Split Ecliptic comet 73P/Schwassmann-Wachmann 3: A Comparison of Fragments C and B
The composition of fragments C and B of the Jupiter-family comet 73P/Schwassmann-Wachmann 3 (SW3) was investigated in early April of 2006 at IR wavelengths using high-dispersion echelle spectroscopy. Both fragments were depleted in ethane, and C was depleted in most forms of volatile carbon. In particular, fragment C shows a severe depletion of CH_(3)OH but a "normal" abundance of HCN (which has a similar volatility). Thermal processing is a possible explanation, but since fragment B is perhaps sublimating fresher material because of the frequent outbursts and fragmentation, the observed depletions might have cosmogonic implications. The chemistry of the volatile ices in SW3, like in the Oort Cloud comet C/1999 S4 (LINEAR), may be associated with sublimation of icy mantles from precometary grains followed by subsequent gas-phase chemistry and recondensation
Molecular Gas in the Inner 1 AU of the TW Hya and GM Aur Transitional Disks
We report the detection of CO rovibrational emission from the transitional disks around the T Tauri stars TW Hya and GM Aur. Transitional disks are characterized by significant mid- to far-infrared (IR) dust emission combined with a relative deficit in the near-IR, indicating the presence of an optically thick outer disk but a reduced surface density of small dust grains in the inner disk. Kinematic fits to the resolved CO emission lines demonstrate that they arise from within the tenuous inner disk. Excitation diagram analyses yield rotational temperatures also consistent with small emission radii as well as densities implying dynamically significant amounts of gas in the inner disk and a gas-to-small dust grain ratio in excess of that in dense clouds. Nevertheless, gas densities are not high enough to maintain current accretion rates for more than a few hundred years without replenishment, and transfer of gas from the outer to inner disk is therefore likely required
High-resolution 5 ÎŒm Spectroscopy of Transitional Disks
We present high-resolution M-band (~5 ÎŒm) spectra of 14 transitional disksâcircumstellar disks with an optically thick outer zone but an inner region significantly depleted of small dust grainsâobtained with NIRSPEC on the Keck II telescope. We detect CO emission from nine disks, and show that for the majority of these systems, the emission originates in the depleted inner disk region. We find that the presence of high 5 ÎŒm veiling, strong CO emission, and high accretion rates are usually correlated, suggesting that at least two classes of transitional disks existâthose nearly completely cleared, and those only partially depleted, within their transition radius. Cleared inner disks are consistent with the presence of a close stellar companion, or with formation by photoevaporation. Of the cleared transitional disks, at least two (HD 98800 B and CoKu Tau/4) are known to be circumbinary with projected binary separations of several AU or less. Partially depleted inner disks most often have CO that extends to small (âŸ1 AU) radii, but compared to "classical" disks the CO excitation temperature is lower and the emission radii are larger than that expected for dust sublimation. These disks are consistent with the presence of a giant planet, and inconsistent with having been formed by photoevaporation. Although the inner regions of such disks are vertically optically thin in dust emission, line-of-sight opacities from the star can be large, and the complex physical and chemical processes therein make it difficult to derive a fiducial CO abundance with respect to molecular hydrogen. Thus, CO M-band lines are best suited to providing lower bounds as to the total inner disk gas mass. Amongst the partially depleted sources, veiling measurements and CO emission models demonstrate a great diversity of inner disk gas content and gas/dust ratios, suggesting a variety of planet-forming environments
Evidence for a Snow Line beyond the Transitional Radius in the TW Hya Protoplanetary Disk
We present an observational reconstruction of the radial water vapor content near the surface of the TW Hya transitional protoplanetary disk, and report the first localization of the snow line during this phase of disk evolution. The observations are comprised of Spitzer-IRS, Herschel-PACS, and Herschel-HIFI archival spectra. The abundance structure is retrieved by fitting a two-dimensional disk model to the available star+disk photometry and all observed H_2O lines, using a simple step-function parameterization of the water vapor content near the disk surface. We find that water vapor is abundant (~10^(â4) per H_2) in a narrow ring, located at the disk transition radius some 4 AU from the central star, but drops rapidly by several orders of magnitude beyond 4.2 AU over a scale length of no more than 0.5 AU. The inner disk (0.5-4 AU) is also dry, with an upper limit on the vertically averaged water abundance of 10^(â6) per H_2. The water vapor peak occurs at a radius significantly more distant than that expected for a passive continuous disk around a 0.6 M_â star, representing a volatile distribution in the TW Hya disk that bears strong similarities to that of the solar system. This is observational evidence for a snow line that moves outward with time in passive disks, with a dry inner disk that results either from gas giant formation or gas dissipation and a significant ice reservoir at large radii. The amount of water present near the snow line is sufficient to potentially catalyze the (further) formation of planetesimals and planets at distances beyond a few AU
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
Multi-Epoch Observations of HD69830: High Resolution Spectroscopy and Limits to Variability
The main-sequence solar-type star HD69830 has an unusually large amount of
dusty debris orbiting close to three planets found via the radial velocity
technique. In order to explore the dynamical interaction between the dust and
planets, we have performed multi-epoch photometry and spectroscopy of the
system over several orbits of the outer dust. We find no evidence for changes
in either the dust amount or its composition, with upper limits of 5-7% (1
per spectral element) on the variability of the {\it dust spectrum}
over 1 year, 3.3% (1 ) on the broad-band disk emission over 4 years,
and 33% (1 ) on the broad-band disk emission over 24 years. Detailed
modeling of the spectrum of the emitting dust indicates that the dust is
located outside of the orbits of the three planets and has a composition
similar to main-belt, C-type asteroids asteroids in our solar system.
Additionally, we find no evidence for a wide variety of gas species associated
with the dust. Our new higher SNR spectra do not confirm our previously claimed
detection of HO ice leading to a firm conclusion that the debris can be
associated with the break-up of one or more C-type asteroids formed in the dry,
inner regions of the protoplanetary disk of the HD69830 system. The modeling of
the spectral energy distribution and high spatial resolution observations in
the mid-infrared are consistent with a 1 AU location for the emitting
material
- âŠ