Benjamin F. Lane

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.

Resolved Inner Disks around Herbig Ae/Be Stars

We have observed 14 Herbig Ae/Be (HAEBE) sources with the long-baseline near-IR Palomar Testbed Interferometer. All except two sources are resolved at 2.2 μm, with angular sizes generally 5 mas. We determine the size scales and orientations of the 2.2 μm emission using various models: uniform disks, Gaussians, uniform rings, flat accretion disks with inner holes, and flared disks with puffed-up inner rims. Although it is difficult to distinguish different radial distributions, we are able to place firm constraints on the inclinations of most sources; seven objects display significantly inclined morphologies. The inner disk inclinations derived from our near-IR data are generally compatible with the outer disk geometries inferred from millimeter interferometric observations, implying that HAEBE disks are not significantly warped. Using the derived inner disk sizes and inclinations, we compute the spectral energy distributions (SEDs) for two simple physical disk models and compare these with observed SEDs compiled from the literature and new near-IR photometry. While geometrically flat accretion disk models are consistent with the data for the earliest spectral types in our sample (MWC 297, V1685 Cyg, and MWC 1080), the later type sources are explained better through models incorporating puffed-up inner disk walls. The different inner disk geometries may indicate different accretion mechanisms for early- and late-type HAEBE stars.

Dynamical Masses of the Binary Brown Dwarf GJ 569 Bab

We have obtained new images and high-resolution (R ~ 22,400) near-infrared (1.2400-1.2575 μm) spectra of each component of the brown dwarf binary GJ 569 Bab using the adaptive optics facility of the Keck II telescope and the NIRSPEC spectrometer. These data have allowed us to improve the determination of the astrometric orbit and to measure radial velocities of the components. We have used the astrometric and spectroscopic measurements to derive the dynamical mass of each brown dwarf and the systemic velocity of the pair by means of a χ2 fitting technique. From various considerations, the mass of each component is likely in the range 0.034-0.070 M☉ (GJ 569 Bb) and 0.055-0.087 M☉ (GJ 569 Ba). This implies that the mass ratio q of the binary is greater than 0.4, the most likely value being q = 0.75-0.85. Adopting 0.072 M☉ as the most conservative location of the substellar limit for solar metallicity, our analysis confirms GJ 569 Bb as the first genuine brown dwarf known without any theoretical assumptions. We have compared the dynamical masses of GJ 569 Ba and Bb, and their effective temperatures and luminosities, to the predictions of state-of-the-art theoretical evolutionary isochrones, finding that models exhibit good performance in the regime of high substellar masses if the binary is about a few hundred million years old. However, the surface gravities of GJ 569 Ba (M8.5 V) and Bb (M9 V) derived from our spectral analysis (the observed data have been compared to the latest synthetic spectra) appear to be smaller than the values provided by the evolutionary models.

Near-Infrared Interferometric Measurements of Herbig Ae/Be Stars

We have observed the Herbig Ae/Be sources AB Aur, VV Ser, V1685 Cyg (BD +40°4124), AS 442, and MWC 1080 with the Palomar Testbed Interferometer, obtaining the longest baseline near-IR interferometric observations of this class of objects. All of the sources are resolved at 2.2 μm with angular size scales generally 5 mas, consistent with the only previous near-IR interferometric measurements of Herbig Ae/Be stars, by Millan-Gabet and collaborators. We determine the angular size scales and orientations predicted by uniform-disk, Gaussian, ring, and accretion disk models. Although it is difficult to distinguish different radial distributions, we are able to place firm constraints on the inclinations of these models, and our measurements are the first that show evidence for significantly inclined morphologies. In addition, the derived angular sizes for the early-type Herbig Be stars in our sample, V1685 Cyg and MWC 1080, agree reasonably well with those predicted by the face-on accretion disk models used by Hillenbrand and collaborators to explain observed spectral energy distributions. In contrast, our data for the later-type sources AB Aur, VV Ser, and AS 442 are somewhat inconsistent with these models and may be explained better through the puffed-up inner disk models of Dullemond and collaborators.

The Orbit of the Brown Dwarf Binary Gliese 569B

We present photometric, astrometric, and spectroscopic observations of the nearby (9.8 pc) low-mass binary Gl 569Bab (in turn being a companion to the early-M star Gl 569A) made with the Keck adaptive optics facility. Having observed Gl 569Bab since 1999 August, we are able to see orbital motion and to determine the orbital parameters of the pair. We find the orbital period to be 892 ± 25 days, the semimajor axis to be 0.90 ± 0.02 AU, the eccentricity to be 0.32 ± 0.02, and the inclination of the system to be 34° ± 3° (1 σ). The total mass is found to be 0.123 M☉ (3 σ). In addition, we have obtained low-resolution (R = 1500-1700) near-infrared spectra of each of the components in the J and K bands. We determine the spectral types of the objects to be M8.5 V (Gl 569Ba) and M9 V (Gl 569Bb) with an uncertainty of half a subclass. We also present new J- and K-band photometry that allows us to place the objects in the H-R diagram accurately. Most likely the binary system is comprised of two brown dwarfs with a mass ratio of 0.89 and with an age of approximately 300 Myr.

Spectrally Dispersed K-Band Interferometric Observations of Herbig Ae/Be Sources: Inner Disk Temperature Profiles

We use spectrally dispersed near-IR interferometry data to constrain the temperature profiles of sub-AU-sized regions of 11 Herbig Ae/Be sources. We find that a single-temperature ring does not reproduce the data well. Rather, models incorporating radial temperature gradients are preferred. These gradients may arise in a dusty disk, or may reflect separate gas and dust components with different temperatures and spatial distributions. Comparison of our models with broadband spectral energy distributions suggests the latter explanation. The data support the view that the near-IR emission of Herbig Ae/Be sources arises from hot circumstellar dust and gas in sub-AU-sized disk regions. Intriguingly, our derived temperature gradients appear systematically steeper for disks around higher mass stars. It is not clear, however, whether this reflects trends in relative dust/gas contributions or gradients within individual components.

Differential Astrometry of Subarcsecond Scale Binaries at the Palomar Testbed Interferometer

We have used the Palomar Testbed Interferometer to perform very high precision differential astrometry on the 025 separation binary star HD 171779. In 70 minutes of observation, we achieve a measurement uncertainty of ≈9 μas in one axis, consistent with theoretical expectations. Night-to-night repeatability over four nights is at the level of 16 μas. This method of very narrow angle astrometry may be extremely useful for searching for planets with masses as small as 0.5MJ around a previously neglected class of stars—so-called speckle binaries. It will also provide measurements of stellar parameters such as masses and distances, useful for constraining stellar models at the 10-3 level.

New insights on the AU-scale circumstellar structure of FU Orionis

We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models. We are able to constrain the geometry of the disk, including an inclination of 55 deg and a position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data (at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 AU at the 130 deg position angle and with a magnitude difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object.

Interferometric Measurement of the Angular Sizes of Dwarf Stars in the Spectral Range K3-M4

We have used the Palomar Testbed Interferometer to measure the angular diameter of five dwarf stars of spectral types K3-M4. Using the 110 m baseline and observing in H and K bands allows us to measure angular diameters with an accuracy of 2%-8% for stars with apparent angular diameters approaching 1 milliarcsecond. We provide results for both uniform-disk and limb-darkened models and compare our results with theoretical predictions. At the current level of precision our measurements are consistent with most widely accepted models, but further observations should be able to provide useful empirical constraints.

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.