Ben R. Oppenheimer

Direct Detection of Galactic Halo Dark Matter

The Milky Way galaxy contains a large, spherical component which is believed to harbor a substantial amount of unseen matter. Recent observations indirectly suggest that as much as half of this “dark matter” may be in the form of old, very cool white dwarfs, the remnants of an ancient population of stars as old as the galaxy itself. We conducted a survey to find faint, cool white dwarfs with large space velocities, indicative of their membership in the galaxys spherical halo component. The survey reveals a substantial, directly observed population of old white dwarfs, too faint to be seen in previous surveys. This newly discovered population accounts for at least 2 percent of the halo dark matter. It provides a natural explanation for the indirect observations, and represents a direct detection of galactic halo dark matter.

TESTS OF THE ACCELERATING UNIVERSE WITH NEAR-INFRARED OBSERVATIONS OF A HIGH-REDSHIFT TYPE Ia SUPERNOVA

We have measured the rest-frame B-, V -, and I-band light curves of a high-redshift type Ia supernova (SN Ia), SN 1999Q (z \ 0.46), using the Hubble Space Telescope (HST ) and ground-based near-infrared detectors. A goal of this study is the measurement of the color excess, a sensitive indicator of E B~I , interstellar or intergalactic dust, which could aUect recent cosmological measurements from high-redshift SNe Ia. Our observations disfavor a 30% opacity of SN Ia visual light by dust as an alternative to an accelerating universe. This statement applies to both Galactic-type dust (rejected at the 3.4 p con—dence level) and grayer dust (grain size ( 0.1 km, rejected at the 2.3¨2.6 p con—dence level) as proposed by Aguirre. The rest-frame I-band light curve shows the secondary maximum 1 month after the B maximum typical of nearby SNe Ia of normal luminosity, providing no indication of evolution as a func- tion of redshift out to z B 0.5. An expanded set of similar observations could improve the constraints on any contribution of extragalactic dust to the dimming of high-redshift SNe Ia. Subject headings: cosmology: observationsdistance scalesupernovae: general

Palomar adaptive optics project: status and performance

We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 micrometer), the system has achieved Strehl ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 micrometer). Predictions of the systems performance based on the analysis of real-time wavefront sensor telemetry data and an analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.

SPECTRAL ENERGY DISTRIBUTION AND BOLOMETRIC LUMINOSITY OF THE COOL BROWN DWARF GLIESE 229B

Infrared broadband photometry of the cool brown dwarf Gliese 229B extending in wavelength from 0.8 to 10.5 micron is reported. These results are derived from both new data and reanalyzed, previously published data. Existing spectral data reported have been rereduced and recalibrated. The close proximity of the bright Gliese 229A to the dim Gliese 229B required the use of special techniques for the observations and also for the data analysis. We describe these procedures in detail. The observed luminosity between 0.8 and 10.5 micron is (4.9 +/- 0.6) x 10(exp -6) solar luminosity. The observed spectral energy distribution is in overall agreement with a dust-free model spectrum by Tsuji et al. for T(eff) approx. equal to 900 K. If this model is used to derive the bolometric correction, the best estimate of the bolometric luminosity is 6.4 x 10(exp -6) solar luminosity and 50% of this luminosity ties between 1 and 2.5 microns. Our best estimate of the effective temperature is 900 K. From the observed near-infrared spectrum and the spectral energy distribution, the brightness temperatures (T(sub B) are estimated. The highest, T(sub B) = 1640 K, is seen at the peak of the J band spectrum, while the lowest, T(sub B) is less than or equal to 600 K, is at 3.4 microns, which corresponds to the location of the fundamental methane band.

Observations of Ultracool White Dwarfs

We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object.

Companion detection limits with adaptive optics coronagraphy

We presented a detailed observational study of the capabilities of the Palomar Adaptive Optics System and the PHARO near infrared camera in coronagraphic mode. The camera provides two different focal plane occulting masks consisting of completely opaque circular disks of diameter 0.433 arcsec and 0.965 arcsec, both within the cryogenic dewar. In addition, three different pupil plane apodizing masks (a.k.a. Lyot masks) are provided which downsize the beam. The six different combinations of Lyot mask and focal plane mask provide for different levels of suppression of the point spread function of a bright star centered on the focal plane mask. We obtained images of the bright nearby star Gliese 614 with all six different configurations in the K-band filter. Herein, we provide an analysis of the dynamic range achievable with these configurations. The dynamic range (the ratio of the primary star intensity to the intensity of the faintest point source detectable in the images) is a complicated function of not only the angular separation of the primary star and companion, but also of the azimuthal angle because of the complex point spread function of the primary star, which is also wavelength dependent. However, beyond 2.5 arcseconds from the star, regardless of the wavelength of the observation, the detection limit of a companion is simply the limiting magnitude of the image, as determined by the sensitivity of the PHARO camera. Within that radius, the dynamic range is at least 8 magnitudes at the 5(sigma) level and as high as 12 in a one second exposure. This represents a substantial gain over similar techniques without adaptive optics, which are generally limited to radii beyond two arcsec. We provide a quantitative discussion and recommendation for the optimal configuration along with a detailed comparison with recent theoretical predictions of AO coronagraphic performance.

Detection of a very low mass companion to the astrometric binary Gliese 105A

Optical coronagraph images of the high proper motion astrometric binary Gl 105A reveal a very red companion, Gl 105C, located 3.27 arcsecs from Gl 105A at P.A. 287 deg. At this location, it is not clear whether Gl 105C can fully account for the astrometric perturbation of Gl 105A. Aperture photometry gives I(sub C) = 12.6 and R(sub C) - I(sub C) = 3.7 for Gl 105C, indicating that it is a very low mass M dwarf. Using the observed I(sub C), an empirical M(sub I) versus I-K relation, and an assumed distance of 8.2 pc to Gl 105A, M(sub K) = 9.7 is derived for Gl 105C. An empirical mass-M(sub K) relation for low-mass stars suggest a mass of 0.084 solar mass for Gl 105C, which is just above the minimum mass for stable hydrogen burning. Gl 105C was not detected in previous K-band searches; its detection demonstrates the usefulness of optical coronagraphy for identifying very low mass objects.

Astronomical coronagraphy with high-order adaptive optics systems

Space surveillance systems have recently been developed that exploit high order adaptive optics systems to take diffraction limited images in visible light on 4 meter class telescopes. Most astronomical targets are faint, thus driving astronomical AO systems towards larger subapertures, and thus longer observing wavelengths for diffraction limited imaging at moderate Strehl ratio. There is, however, a particular niche that can be exploited by turning these visible light space surveillance systems to astronomical use at infrared wavelengths. At the longer wavelengths, the Strehl ratio rises dramatically, thus placing more light into the diffracted Airy pattern compared to the atmospheric halo. A Lyot coronagraph can be used to suppress the diffracted light from an on axis star, and observe faint companions and debris disks around nearby, bright stars. These very high contrast objects can only be observed with much higher order adaptive optics systems than are presently available to the astronomical community. We describe simulations of high order adaptive optics coronagraphs, and outline a project to deploy an astronomical coronagraph at the Air Force AEOS facility at the Maui Space Surveillance System.

Studies of Herbig-Haro Objects with the Palomar Adaptive Optics System

Herbig-Haro objects are bright optical emission-line sources associated with tightly collimated jets ejected from pre-main- sequence stars. Only a few hundred are known. In optical images, they appear to be dense knots of material at the outer ends of the jets, and often exhibit streaming wake morphologies suggestive of bow shocks. Their optical spectra show characteristics of high-velocity shocks, with line-widths typically 100 km/s. HH objects often occur in pairs consistent with the bipolar morphology of outflows from YSOs; when radio maps of NH3 are made, high-density central regions consistent with collimating disks are seen. HH objects also often appear in a series along a jet, presumably where the jet undergoes a particularly energetic interaction with the ambient medium. Adaptively-corrected near-infrared studies of HH objects can reveal much about their workings at fine spatial scales. Narrow-band NIR filters sensitive to transitions of molecular hydrogen and other selected species are excellent tracers of shock excitation, and many HH objects have been observed to show complex structure in these lines down to the arc second level. By pushing to higher spatial resolution with adaptive optics, much more detailed information about the nature of the shock fronts may be obtained. In this paper we describe our first observations of HH objects with the AO system on the Palomar 200-inch telescope.

Searches for Galactic Halo Remnants

There has recently been renewed interest in the possibility that very old, cool white dwarfs may make a significant contribution to the Galactic dark halo mass budget. New results, both theoretical and observational, have shown that such objects have blue optical/NIR colors despite their low atmospheric temperatures. These results have prompted a number of new searches for very cool degenerates, particularly using archival Schmidt plates. Follow-up spectroscopy and imaging are being pursued using telescopes with apertures ≤ 4-m. We discuss these results and the likely direction of future searches with emphasis on the role that ‘small’ telescopes have played and will play in this topical area of research.