G. T. van Belle

The Palomar Testbed Interferometer

The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in 1995 July. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. The three fixed 40 cm apertures can be combined pairwise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 2.2 μm and active delay lines with a range of ±38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing.

Imaging the Surface of Altair

Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both.

Radii and Effective Temperatures for G, K, and M Giants and Supergiants

?????We present new interferometric observations for 74 luminous red stars, made in the near-infrared. We show that our 2.2 ?m uniform-disk diameters agree with other near-infrared diameter determinations (lunar occultations and interferometers) for 22 stars measured in common with ours. From our new data, we derive effective temperatures that are compared with our previous work and with comparable observations made by lunar occultations at Kitt Peak. The combined data set yields 91 luminosity class II, II?III, and III stars that have well-determined spectral types spanning the range from about K0 to about M8. There are 83 stars in the sample that define an approximately linear relationship between spectral type and effective temperature for giants, with a dispersion of 192 K at each spectral type. Eight of the stars have temperatures that are roughly 750 K too low for their spectral types. These stars are not known to be at the high-luminosity end of the range of stars observed and are not recognized as binary stars. At present, we have no explanation for their low effective temperatures. We also show that Hipparcos parallaxes, combined with our angular diameters, yield linear radii precise enough to see differences in the average radius between luminosity class II and luminosity class III stars.

The near-infrared size-luminosity relations for Herbig Ae/Be disks

We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependenceonluminosityandeffectivetemperature.Formosttargets,themeasurednear-infraredsizes(rangingfrom0.2to 4AU)supportasimple diskmodelpossessingacentralopticallythin(dust-free) cavity,ringedbyhotdustemitting at theexpected sublimation temperatures (Ts � 1000–1500 K).Furthermore, wefindatightcorrelation of disksizewith source luminosity R / L 1 =2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0–B3 stars) are undersized compared to predictions of the ‘‘optically thin cavity’’ model, likely because of optically thick gas within the inner AU. Subject headingg accretion, accretion disks — circumstellar matter — instrumentation: interferometers — radiative transfer — stars: formation — stars: pre–main-sequence

First results from the chara array. I. An interferometric and spectroscopic study of the fast rotator α leonis (Regulus)

We report on K-band interferometric observations of the bright, rapidly rotating star Regulus (type B7 V) made with the CHARA Array on Mount Wilson, California. Through a combination of interferometric and spectroscopic measurements, we have determined for Regulus the equatorial and polar diameters and temperatures, the rotational velocity and period, the inclination and position angle of the spin axis, and the gravity darkening coefficient. These first results from the CHARA Array provide the first interferometric measurement of gravity darkening in a rapidly rotating star and represent the first detection of gravity darkening in a star that is not a member of an eclipsing binary system.

Observations and modeling of the inner disk region of T tauri stars

We present observations of four T Tauri stars using long baseline infrared interferometry from the Palomar Testbed Interferometer. The target sources, T Tau N, SU Aur, RY Tau, and DR Tau, are all known to be surrounded by dusty circumstellar disks. The observations directly trace the inner regions (<1 AU) of the disk and can be used to constrain the physical properties of this material. For three of the sources observed, the infrared emission is clearly resolved. We first use geometric models to characterize the emission region size, which ranges from 0.04 to 0.3 AU in radius. We then use Monte Carlo radiation transfer models of accretion disks to jointly model the spectral energy distribution and the interferometric observations with disk models including accretion and scattering. With these models, we are able to reproduce the data set with extended emission arising from structures larger than 10 mas contributing less than 6% of the K-band emission, consistent with little or no envelope remaining for these class II sources [d log(λFλ)/d log λ ≈ -2-0 in the infrared]. The radiation transfer models have inner radii for the dust similar to the geometric models; however, for RY Tau, emission from gas within the inner dust radius contributes significantly to the model flux and visibility at infrared wavelengths. The main conclusion of our modeling is that emission from inner gas disks (between the magnetic truncation radius and the dust destruction radius) can be a significant component in the inner disk flux for sources with large inner dust radii.

CONSTRAINTS ON CIRCUMSTELLAR DISK PARAMETERS FROM MULTIWAVELENGTH OBSERVATIONS: T TAURI AND SU AURIGAE

We present circumstellar disk models for two pre-main-sequence objects, T Tau and SU Aur. The models are based on interferometric data from infrared and millimeter wavelengths and infrared photometry from the literature. The physical properties of the disk are examined by calculating parameter probabilities based on a passive, flat-disk model. The model adequately fits the data for SU Aur but not for T Tau. We find that there are significant differences in the physical parameters suggested by the individual data sets. The size of the inner disk radius as implied by the infrared interferometry data (~tenths of AU) is larger than expected for a flat-disk model. This discrepancy is discussed in consideration of more complex disk models that include the presence of a hot, inner region or wall in the disk.

Interferometer Observations of Subparsec-Scale Infrared Emission in the Nucleus of NGC 4151

We report novel, high angular resolution interferometric measurements that imply that the near-infrared nuclear emission in NGC 4151 is unexpectedly compact. We have observed the nucleus of NGC 4151 at 2.2 μm using the two 10 m Keck telescopes as an interferometer and find a marginally resolved source ≤0.1 pc in diameter. Our measurements rule out models in which a majority of the K-band nuclear emission is produced on scales larger than this size. The interpretation of our measurement most consistent with other observations is that the emission mainly originates directly in the central accretion disk. This implies that active galactic nucleus unification models invoking hot, optically thick dust may not be applicable to NGC 4151.

INFRARED INTERFEROMETRIC OBSERVATIONS OF YOUNG STELLAR OBJECTS

We present infrared observations of four young stellar objects made using the Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147, and SU Aur, the 2.2 μm emission is resolved at the PTIs nominal fringe spacing of 4 mas, while the emission region of AB Aur is overresolved on this scale. We fit the observations with simple circumstellar material distributions and compare our data to the predictions of accretion disk models inferred from spectral energy distributions. We find that the infrared emission region is tenths of an AU in size for T Tau and SU Aur and ~1 AU for MWC 147.

FU Orionis Resolved by Infrared Long-Baseline Interferometry at a 2 AU Scale

We present the first infrared interferometric observations of a young stellar object with a spatial projected resolution better than 2 AU. The observations were obtained with the Palomar Testbed Interferometer (PTI). FU Orionis exhibits a visibility of V2=0.72 ± 0.07 for a 103 ± 5 m-projected baseline at λ=2.2 μm. On the spatial scale probed by the PTI, the data are consistent with both a binary system scenario (a maximum magnitude difference of 2.7 ± 0.5 mag and the smallest separation of 0.35 ± 0.05 AU) and a standard luminous accretion disk model ( ~6 × 10−5 M☉ yr-1), where the thermal emission dominates the stellar scattering, and are inconsistent with a single stellar photosphere.