Arthur D. Code

Empirical effective temperatures and bolometric corrections for early-type stars

An empirical effective temperature for a star can be found by measuring its apparent angular diameter and absolute flux distribution. The angular diameters of 32 bright stars in the spectral range O5f to F8 have recently been measured with the stellar interferometer at Narrabri Observatory, and their absolute flux distributions have been found by combining observations of ultraviolet flux from the Orbiting Astronomical Observatory (OAO-2) with ground-based photometry. In this paper these data have been combined to derive empirical effective temperatures and bolometric corrections for these 32 stars. (AIP)

Polarization from scattering in blobs

We describe a Monte Carlo method for calculating polarization due to scattering in various geometries. We apply the method in this paper to the case of scattering in a spherical cloud illuminated by parallel rays. The polarization is largest in small optical depth blobs, where single scattering dominates. The polarization decreases as optical depth increases until a limit is reached where scattering occurs on the surface, and increasing optical depth has little effect on the results. For a given optical depth, decreasing the albedo leads to an increase of the degree of polarization of the scattered light because single scattering increases relative to multiple scattering. As the optical depth of the blob increases, the scattered flux becomes increasingly backward throwing. In the high albedo blobs, the maximum polarization becomes skewed towards the illuminated hemisphere at high optical depths. We discuss the applications to polarization in supergiants and R CrB stars, and scattering in a clumpy interstellar medium.

The first spectropolarimetric study of the wavelength dependence of interstellar polarization in the ultraviolet

The first UV spectropolarimetry along six lines of sight with significant interstellar polarization is reported. The observations were obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) during the Astro-1 mission. HD 37903, HD 62542 and HD 99264 show a wavelength dependence which follows the Serkowski relation extrapolated into the UV. HD 25443 and Alpha Cam have UV polarization well in excess of the Serkowski extrapolation. HD 197770 clearly shows a polarization bump which closely matches the 2175 A extinction feature. This bump polarization can be fitted by small aligned graphite disks. The differences along various lines of sight might be the result of differences in the environments which affect the size and alignment of the grains.

Ultraviolet Interstellar Polarization of Galactic Starlight.I.Observations by the Wisconsin Ultraviolet Photo Polarimeter Experiment

The Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) ew twice as part of NASAs Astro Spacelab missions in 1990 December and 1995 March. A systematic survey of the interstellar polarization in the ultraviolet was one of the main projects on both ights. The program was carefully crafted to 1) sample the galactic plane as uniformly as practicable, 2) explore sight lines of diverse chemical composition and morphology, and 3) measure the shape of the UV polarization through the full range of known wavelengths of peak polarization in the optical. We present here Astro-2 data for 20 previously unobserved sight lines and combine these with previously published UV data and with optical observations from the University of Wisconsins ground based facilities and elsewhere. We thus have spectropolarimetry from 1500 A to 10,000 A for 35 galactic objects in which the polarization appears to be dominated by the interstellar component. The extrapolation of the empirical Serkowski formula based only on visual data does not provide a reliable representation of the UV polarization. We nd that there are substantial diierences in the amount of UV polarization relative to that in the visual. This may indicate that the small and large aligned grain populations are somewhat independent and this may in turn provide a new diagnostic of varying conditions in the interstellar medium. Finally there are several cases in which the UV polarization is enhanced in the 2000 A to 3000 A range.

Imaging of the gravitational lens system PG 1115+080 with the Hubble Space Telescope

This paper is the first of a series presenting observations of gravitational lenses and lens candidates, taken with the Wide Field/Planetary Camera (WFPC) of the Hubble Space Telescope (HST). We have resolved the gravitational lens system PG 1115 + 080 into four point sources and a red, extended object that is presumably the lens galaxy; we present accurate relative intensities, colors, and positions of the four images, and lower accuracy intensity and position of the lens galaxy, all at the epoch 1991.2. Comparison with earlier data shows no compelling evidence for relative intensity variations between the QSO components having so far been observed. The new data agree with earlier conclusions that the system is rather simple, and can be produced by the single observed galaxy. The absence of asymmetry in the HST images implies that the emitting region of the quasar itself has an angular radius smaller than about 10 milliarcsec (100 pc for H_0=50, q_0=0.5).

The First Ultraviolet and Optical Spectropolarimetry of the B[e] Star HD 50138

We report the first ultraviolet spectropolarimetry of the B[e] star HD 50138, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment aboard the Astro-2 Space Shuttle mission in 1995 March. The data cover the spectral range 1500-3200 A. Spectropolarimetry obtained contemporaneously in the range of 3800-10,000 A in the visual-to-near-IR region are presented as well. The presence of intrinsic polarization is detected from the UV to the near-IR. Strong evidence of a thin gaseous disk around the star is found. An almost flat wavelength dependence of the intrinsic polarization in the optical spectral region indicates that electron scattering, rather than dust scattering, is the dominant polarizing mechanism, although a small contribution due to dust scattering cannot be completely ruled out. A small inclination of the disk away from edge-on with respect to the line of sight is suggested. Comparison with similar polarimetric observations obtained for different objects that have the same circumstellar geometry shows that the envelope of HD 50138 probably has a rather large density contrast between the equatorial and polar regions that does not produce a position angle flip like that observed in other objects, notably HD 45677.

Astronomical radiation measurements. III - An analysis of far-ultraviolet filter observations of stars.

Far UV filter observations of main sequence and giant stars analyzed for attainable magnitude accuracy, comparing observations with model atmospheres

Ultraviolet photometry from the orbiting astronomical observatory. XXI. Absolute energy distribution of stars in the ultraviolet

The absolute energy distribution in the ultraviolet is given for the stars ..cap alpha.. Vir, eta UMa, and ..cap alpha.. Leo. The calibration is based upon absolute heterochromatic photometry betweeen 2920 and 1370 A carried out with an Aerobee sounding rocket. The fundamental radiation standard is the synchrotron radiation from 240 MeV electrons in the University of Wisconsin Physical Ssience Laboratory synchrotron storage ring. On the basis of the sounding rocket calibration the preliminary OAO-2 spectrometer calibration has been revised; the fluxes for the three program stars are tabulated in energy per second per centimeter/sup 2/ per unit wavelength interval. (AIP)

First ultraviolet spectropolarimetry of Be stars from the Wisconsin Ultraviolet Photo-Polarimeter Experiment

The first UV spectropolarimetric observations of Be stars are presented. They were obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro-1 mission. WUPPE data on the Be stars Zeta Tau and Pi Aqr, along with near-simultaneous optical data obtained at the Pine Bluff Observatory (PBO). Combined WUPPE and PBO data give polarization as a function of wavelength across a very broad spectral region, from 1400 to 7600 A. Existing Be star models predicted increasing polarization toward shorter wavelengths in the UV, but this is not supported by the WUPPE observations. Instead, the observations show a constant or slightly declining continuum polarization shortward of the Balmer jump, and broad UV polarization dips around 1700 and 1900 A, which may be a result of Fe-line-attenuation effects on the polarized flux. Supporting evidence for this conclusion comes from the optical data, in which decreases in polarization across Fe II lines in Zeta Tau were discovered.

Evidence for a bipolar nebula around the peculiar B(e) star HD 45677 from ultraviolet spectropolarimetry

We report the first ultraviolet spectropolarimetry of the peculiar B-type emission-line star with infrared excess HD 45677. The observations were obtained during the 1990 December Astro-l space shuttle mission with the Wisconsin Ultraviolet Photo-Polarimeter Experiment, and cover the spectral range 1400-3220 A. We also present the first optical spectropolarimetry, extending from the atmospheric cutoff to about 7600 A. The observed UV/optical linear polarization displays a strong increase toward shorter wavelengths indicative of scattering by circumstellar dust. The position angle of the intrinsic polarization flips by 90 deg in the near-UV as expected from a bipolar reflection nebula.