Planets are often invoked as the cause of inferred gaps or inner clearings in transition disks. These putative planets would interact with the remnant circumstellar disk, accreting gas and generating substantial luminosity. Here I explore the expecte
d appearance of accreting protoplanets at a range of evolutionary states. I compare synthetic spectral energy distributions with the handful of claimed detections of substellar-mass companions in transition disks. While observed fluxes of candidate companions are generally compatible with accreting protoplanets, challenges remain in reconciling the extended structure inferred in observed objects with the compact emission expected from protoplanets or circumplanetary disks. I argue that a large fraction of transition disks should harbor bright protoplanets, and that more may be detected as larger telescopes open up additional parameter space.
We present multiple epochs of near-IR spectroscopy for a sample of 25 young stars, including T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra of the BrGamma transition of h
ydrogen, with a resolution of ~3500. Epochs were taken over a span of >1 year, sampling time-spacings of roughly one day, one month, and one year. The majority of our targets show BrGamma emission, and in some cases these are the first published detections. Time-variability is seen in approximately half of the targets showing BrGamma emission. We compare the observed variability with expectations for rotationally-modulated accretion onto the central stars and time-variable continuum emission or extinction from matter in the inner disk. Our observations are not entirely consistent with models of rotationally-modulated magnetospheric accretion. Further monitoring, over a larger number of epochs, will facilitate more quantitative constraints on variability timescales and amplitudes, and a more conclusive comparison with theoretical models.
We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ~2000. Our observations span the K-band, and include the Br gamma transition of Hydrogen and
the v=2-0 and v=3-1 transitions of carbon monoxide. For several targets we also present data from Keck/NIRSPEC that provide higher spectral resolution, but a seeing-limited spatial resolution, of the same spectral features. We analyze the Br gamma emission in the context of both disk and infall/outflow models, and conclude that the Br gamma emission traces gas at very small stellocentric radii, consistent with the magnetospheric scale. However some Br gamma-emitting gas also seems to be located at radii of >0.1 AU, perhaps tracing the inner regions of magnetically launched outflows. CO emission is detected from several objects, and we generate disk models that reproduce both the KI and NIRSPEC data well. We infer the CO spatial distribution to be coincident with the distribution of continuum emission in most cases. Furthermore the Br gamma emission in these objects is roughly coincident with both the CO and continuum emission. We present potential explanations for the spatial coincidence of continuum, Br gamma, and CO overtone emission, and explore the implications for the low occurrence rate of CO overtone emission in young stars. Finally, we provide additional discussion of V1685 Cyg, which is unusual among our sample in showing large differences in emitting region size and spatial position as a function of wavelength.
We present near-IR spectra of a sample of T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra with a resolution of ~3500. Here we present spectra of the v=2->0 and v=3->1 band
heads of ro-vibrational transitions of carbon monoxide. We observed these spectra over multiple epochs spaced by a few days and approximately one month. Several of our targets show CO emission or absorption features. However we see little evidence of variability in these features across multiple epochs. We compare our results with previous observations, and discuss the physical implications of non-variable CO emission across the sampled timescales.
We present imaging observations at 1.3 mm wavelength of Class I protostars in the Taurus star forming region, obtained with the CARMA interferometer. Of an initial sample of 10 objects, we detected and imaged millimeter wavelength emission from 9. On
e of the 9 is resolved into two sources, and detailed analysis of this binary protostellar system is deferred to a future paper. For the remaining 8 objects, we use the CARMA data to determine the basic morphology of the millimeter emission. Combining the millimeter data with 0.9 micron images of scattered light, Spitzer IRS spectra, and broadband SEDs (all from the literature), we attempt to determine the structure of the circumstellar material. We consider models including both circumstellar disks and envelopes, and constrain the masses (and other structural parameters) of each of these components. We show that the disk masses in our sample span a range from <0.01 to >0.1 Msun. The disk masses for our sample are significantly higher than for samples of more evolved Class II objects. Thus, Class I disk masses probably provide a more accurate estimate of the initial mass budget for star and planet formation. However, the disk masses determined here are lower than required by theories of giant planet formation. The masses also appear too low for gravitational instability, which could lead to high mass accretion rates. Even in these Class I disks, substantial particle growth may have hidden much of the disk mass in hard-to-see larger bodies.
We present Keck Interferometer observations of the three prototypical FU Orionis stars, FU Ori, V1057 Cyg, and V1515 Cyg. With a spatial resolution of a few milli-arcseconds and a spectral resolution of 2000, our near-infrared observations spatially
resolve gas and dust emission extending from stellocentric radii of ~0.05 AU to several AU. We fit these data with accretion disk models where each stellocentric radius of the disk is represented by a supergiant-type stellar emission spectrum at the disk temperature. A disk model is consistent with the data for FU Ori, although we require some local asymmetry in the disk. For V1057 Cyg the disk model does not fit our data well, especially compared to the fit quality achieved for FU Ori. We speculate that a disk wind may be contributing substantially to the observed near-IR emission in this source. The data for V1515 Cyg are noisier than the data obtained for the other two objects, and do not strongly constrain the validity of an accretion disk model.
We present two epochs of observations of TW Hya from the high-dispersion near-IR spectrograph ARIES at the MMT. We detect strong emission from the Brackett gamma transition of hydrogen, indicating an accretion rate substantially larger than previousl
y estimated using hydrogen line emission. The Brackett gamma line-strength varies across our two observed epochs. We also measure circumstellar-to-stellar flux ratios (i.e., veilings) that appear close to zero in both epochs. These findings suggest that TW Hya experiences episodes of enhanced accretion while the inner disk remains largely devoid of dust. We discuss several physical mechanisms that may explain these observations.
The ASTrometric and phase-Referenced Astronomy (ASTRA) project will provide phase referencing and astrometric observations at the Keck Interferometer, leading to enhanced sensitivity and the ability to monitor orbits at an accuracy level of 30-100 mi
croarcseconds. Here we discuss recent scientific results from ASTRA, and describe new scientific programs that will begin in 2010-2011. We begin with results from the self phase referencing (SPR) mode of ASTRA, which uses continuum light to correct atmospheric phase variations and produce a phase-stabilized channel for spectroscopy. We have observed a number of protoplanetary disks using SPR and a grism providing a spectral dispersion of ~2000. In our data we spatially resolve emission from dust as well as gas. Hydrogen line emission is spectrally resolved, allowing differential phase measurements across the emission line that constrain the relative centroids of different velocity components at the 10 microarcsecond level. In the upcoming year, we will begin dual-field phase referencing (DFPR) measurements of the Galactic Center and a number of exoplanet systems. These observations will, in part, serve as precursors to astrometric monitoring of stellar orbits in the Galactic Center and stellar wobbles of exoplanet host stars. We describe the design of several scientific investigations capitalizing on the upcoming phase-referencing and astrometric capabilities of ASTRA.
We present spatially resolved near-IR spectroscopic observations of 15 young stars. Using a grism spectrometer behind the Keck Interferometer, we obtained an angular resolution of a few milli-arcseconds and a spectral resolution of 230, enabling prob
es of both gas and dust in the inner disks surrounding the target stars. We find that the angular size of the near-IR emission typically increases with wavelength, indicating hot, presumably gaseous material within the dust sublimation radius. Our data also clearly indicate Brackett-gamma emission arising from hot hydrogen gas, and suggest the presence of water vapor and carbon monoxide gas in the inner disks of several objects. This gaseous emission is more compact than the dust continuum emission in all cases. We construct simple physical models of the inner disk and fit them to our data to constrain the spatial distribution and temperature of dust and gas emission components.
We present high angular resolution observations with the Keck Interferometer, high dispersion spectroscopic observations with Keck/NIRSPEC, and near-IR photometric observations from PAIRITEL of a sample of 11 solar-type T Tauri stars in 9 systems. We
use these observations to probe the circumstellar material within 1 AU of these young stars, measuring the circumstellar-to-stellar flux ratios and angular size scales of the 2.2 micron emission. Our sample spans a range of stellar luminosities and mass accretion rates, allowing investigation of potential correlations between inner disk properties and stellar or accretion properties. We suggest that the mechanism by which the dusty inner disk is truncated may depend on the accretion rate of the source; in objects with low accretion rates, the stellar magnetospheres may truncate the disks, while sublimation may truncate dusty disks around sources with higher accretion rates. We have also included in our sample objects that are known to be highly variable (based on previous photometric and spectroscopic observations), and for several sources, we obtained multiple epochs of spectroscopic and interferometric data, supplemented by near-IR photometric monitoring, to search for inner disk variability. While time-variable veilings and accretion rates are observed in some sources, no strong evidence for inner disk pulsation is found.
