Search

Ultraviolet astronomy is an important tool for the study of the interplanetary medium and diffuse, angularly extended emissions in planetary/comet atmospheres and their near space environments. We describe a new technique for high etendue observations of emission lines at R > 10 5 with an all-reflective spatial heterodyne spectrometer and polarmetric sampling of these lines with an ultraviolet optimized waveplate-Brewster mirror combination. The resulting system is themo-mechanically stable, has light collecting power substantially greater than HST for extended emissions, despite having a volume of several x 10 -3 m 3 . This makes the SHS polarimeter ideal for spacecraft applications. We describe the SHS and polarmetric optical techniques and provide a discussion of its planned development for studies of interplanetary hydrogen.

This paper describes the science drivers and first generation instrumentation capabilities of the Southern African Large Telescope (SALT), due to begin science operations in late 2004/early 2005. First generation instruments are confined to the visible spectrum, but optimized for UV performance, with capability to ~320 nm. Instrumentation will have access to a circular 8 arcmin diameter science field, with guidance objects outside of this region ( days. Time resolved studies are an important aspect of the overall SALT science drivers. Special efforts are being made to ensure high time resolution capability by employing frame transfer CCDs on two of the first-light instruments, the imaging spectrograph (PFIS/IMPALAS) and imaging camera (SALTICAM). Time resolutions of ~50 ms for spectroscopy, with zero dead-time, are planned. Instrument capabilities, which include polarimetry, Fabry-Perot imaging spectroscopy and high resolution fibre-fed spectroscopy (HRS/CELESTIA), will ensure that the major science goals of SALT's partners are realized.

Linear spectropolarimetry of spectral lines is a neglected field in astronomy, largely because of the lack of instrumentation. Techniques that have been applied, but rarely, include investigation of the dynamics of scattering envelopes through the polarization of electron- or dust-scattered nebular light. Untried techniques include promising new magnetic diagnostics like the Hanle Effect in the far-ultraviolet and magnetic realignment in the visible. The University of Wisconsin Space Astronomy Lab is developing instrumentation for such investigations. In the visible, the Prime Focus Imaging Spectrograph (PFIS) is a first light instrument for the Southern African Large Telescope (SALT), which at an aperture of 11m will be the largest single telescope in the Southern Hemisphere. Scheduled for commissioning in late 2004, PFIS is a versatile high-throughput imaging spectrograph using volume-phase holographic gratings for spectroscopic programs from 320nm to 900nm at resolutions of R=500 to R=6000. A dual-etalon Fabry-Perot subsystem enables imaging spectroscopy at R=500 and R=3000 or 12,500. The polarization subsystem, consisting of a very large calcite polarizing beam-splitter used in conjunction with half- and quarter-wave Pancharatnam superachromatic plates, allow linear or circular polarimetric measurements in any of the spectroscopic modes. In the FUV, the Far-Ultraviolet SpectroPolarimeter (FUSP) is a sounding rocket payload, scheduled for its first flight in 2003, that will obtain the first high-precision spectropolarimetry from 105 - 150 nm, and the first astronomical polarimetry of any kind below 130 nm. The 50 cm primary mirror of the telescope is F/2.5. At the prime focus are the polarimetric optics, a stressed lithium fluoride rotating waveplate, followed by a synthetic diamond Brewster-angle mirror. The spectrometer uses an aberration-corrected spherical holographic grating and a UV-sensitized CCD detector, for a spectral resolution of R=1800.

We present visible-wavelength spectropolarimetric measurements of the emission-line star MWC 349A and its close optical companion MWC 349B, conducted with the HPOL spectropolarimeter on both the 0.9 m telescope at the University of Wisconsin's Pine Bluff Observatory and the 3.5 m WIYN telescope on Kitt Peak. Our measurements allow us to estimate the interstellar polarization contribution and thus constrain the intrinsic polarization of MWC 349A, which we find to be consistent in position angle with the dusty disk seen perpendicular to the bipolar outflow. Our analysis reopens the possibility that MWC 349A may be part of the Cyg OB2 association, suggests that it is not a physical companion to MWC 349B, and supports the classification of MWC 349A as a pre-main sequence B[e] star., Comment: 16 pages, 7 figures, accepted for publication in AJ

We review the characteristics of SALT and discuss the major science drivers, both of which will decide the choice of a fist-light instrument package. Current science drivers call for instrumentation similar to that of the Hobby-Eberly Telescope, on which SLAT is closely modeled, with fiber-fed and direct spectroscopy in the visible/near-IR being the major observational mode delivering the best astrophysical returns. The suite will include multi-object spectroscopic capability over a continuous wavelength range of at least 400 nm to 1300 nm, with possible extended capabilities to the blue and near-IR. Factors which will decide these limits are discussed, which include mirror coatings and fiber transmission. Other specific science drivers, which may influence instrument choice or design, include photometry, imaging spectroscopy and spectropolarimetry. Possibility exist for different or novel approaches to instrument designs. Together with potential enhancements of the telescope design, these will have some impact on instrument choice. The nature of the first light instrument suite is currently under discussion and will probably be decided by late-2000.

The Space Astronomy Laboratory has built and flown a very- low-cost (approximately 50K) star tracker and digital imaging system with embedded compression. The star tracker is suitable for all rocket and spacecraft applications, and provides pitch, yaw, and roll updates at rates up to 10 Hz. The digital imaging subsystem uses a novel NASA-funded scheme of `progressive image transmission' in which the image is sent out over a very-low-bandwidth channel, such as a telemetry downlink, in such a way that it can be reconstructed `on the fly' and updated as more data arrive. Large (768 X 474) useful images can be obtained over a 4- kbit/s downlink in as little as 10 seconds. This device can act as an aspect camera, a deployment monitor, or a science imager in situations where low bandwidth is desired or high bandwidth is not available.

The new territory of ultraviolet imaging polarimetry is being explored at the University of Wisconsin Space Astronomy Lab by a sounding rocket payload, the Wide-Field Imaging Survey Polarimeter ('WISP'). WISP uses an 8-inch F/1.9 all-reflective Schmidt telescope, a large rotatable stressed CaF2 waveplate, and a fixed polarizing mirror coated with an opaque monolayer of ZrO2, illuminated at the Brewster angle. The payload has flown successfully four times, targeting the Pleiades Reflection Nebula, the Large Magellanic Cloud, comet Hale-Bopp, and the diffuse light of our Galaxy. An improved payload in the concept stage is Cosmic Ultraviolet Polarimetric Imaging Device ('CUPID'), which would use a 20- inch Gregorian Paul-Baker telescope, WISP-style polarimetric optics, and multilayer reflective coatings to achieve wide- field imaging polarimetry with more than 15 times the sensitivity of WISP. It is to be used to separate the Galactic and Cosmological components of the ultraviolet diffuse background using their polarimetric properties. High resolution stellar spectropolarimetry in the far ultraviolet is to be pioneered by a sounding rocket payload in the design stage, the Far-Ultraviolet Spectropolarimeter ('FUSP'). This instrument is to have a spectral resolution of 0.07 nm and a spectral coverage from 105 to 145 nm. It uses a 20-inch telescope with polarimetric optics at the prime focus, and a far ultraviolet spectrometer using an aberration-corrected holographic grating. The polarimetric analyzer will be a thin LiF stressed waveplate, followed by a Brewster-angle polarizer of natural diamond. Scientific goals include diagnosis of the geometry and magnetic fields in stellar envelopes via resonance scattering and the Hanle Effect.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on eight years of spectropolarimetric monitoring of the WR140 binary. The broad-band linear polarization decreased systematically after the 1985 periastron passage. By 1991, it settled to a constant value at which it has remained through the 1993 periastron passage. We do not detect, in data taken after 1989, a line-effect in He II λ4686. This suggests either that the continuum and the line emission scatter at the same region, or that any intrinsic polarization has been below our detection limit. We conclude that the presently observed polarization of WR140 is consistent with interstellar foreground polarization.

Two sounding rockets payloads under development at the Space Astronomy Laboratory are intended to explore different forms of ultraviolet astronomical polarimetry as a follow-up to the successful mission of the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE). The unknown territory of diffuse-object ultraviolet polarimetry is to be explored by the Wide- Field Imaging Survey Polarimeter (WISP). WISP is a quite unusual integrated fast telescope/polarimeter optimized for the wavelengths 135 - 260 nm. The first WISP flight is scheduled for Fall 1993, targeting the Pleiades reflection nebula. Just in its early development is another sounding rocket payload, the Far-Ultraviolet Spectropolarimeter. This instrument is to have a resolution of better than 0.1 nm and a spectral coverage from 105 to 145 nm. It will consist of a 0.4 m parabolic primary with polarimetric optics at the prime focus and a far ultraviolet spectrometer. The polarimetric analyzer will be a thin stressed-LiF waveplate, followed by a diamond Brewster-angle polarizer.

In December 1990, the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) on the Astro-1 shuttle Spacelab mission obtained approximately eight hours of pioneering ultraviolet spectropolarimetric observations on more than 20 galactic and extragalactic objects. WUPPE is a 0.5 m Cassegrain telescope coupled to a Monk-Gilleson spectrometer with a magnesium fluoride polarizing beamsplitter preceded by magnesium fluoride halfwave retarders. Two spectra from 135 to 330 nm with orthogonal polarization are detected simultaneously by a pair of intensified photodiode arrays. The first flight measured for the first time the ultraviolet polarization induced by interstellar dust, the UV intrinsic polarization of the rapidly rotating `Be' stars, and the large polarization in a `hidden' active galactic nucleus. A second flight of Astro is currently scheduled for 1994.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A B STR ACT Portions of the Moon were observed by the Wisconsin Ultraviolet Photopolarimeter Experiment (WUPPE) on 1995 March 12, 14 and 17, and represent the first ultraviolet (UV) spectropolarimetric observations of the Moon. The polarimetric observations confirm that a change in the dominant scattering process occurs in the UV, changing from volume scattering in the near-UV to surface scattering in the far-UV. The data are investigated empirically. It is found that Umov’s relationship holds when the polarization is perpendicular to the scattering plane. It is also found that the degree of polarization can be modelled by a phase-angle-dependent polarization modified by a wavelength-dependent depolarization factor. The scattering function for each observation is determined.

We have obtained low-resolution visible spectroscopy and polarimetry of R Coronae Borealis covering 8 years from 1986 to 1993 at Pine Bluff Observatory. The spectra clearly show large CN and C2 band variations correlated with pulsational phase and B-V color, similar to those previously seen for RY Sagittarii. During the declines of 1986, 1989 and 1993, the measured polarization increased in a manner consistent with previous declines. However, measurements of the position angle of the polarization in these declines do not support the result of Stanford et al. that there is a preferred of CN and C2 are associated with declines or suspected dust formation episodes. Many of the lowest band strengths are found in years when no decline takes place. However, large band strengths are not only associated with declines. For instance, in 1991, the band strengths were very large throughout the observing season and significant polarization variations were also seen even though no decline occurred that year. These variations are interpreted as being due to dust formation episodes not along the line of sight. It has been suggested that a very large pulsation observed in 1990 may be a small dust formation episode. The molecular band strengths measured during this pulsation are the largest measured in this study. However, only small variations in polarization were measured during the pulsation cycle. There is also some evidence that the polarization variations are correlated with the pulsations of R CrB in 1990.

We present the first ultraviolet spectropolarimetric observations of the Be star PP Car, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro 1 mission. Usable polarization data were obtained from 1400 to 2330 A, along with a good spectrum from 1400 to 3200 A. These data show a lower polarization shortward of the Balmer jump than had been predicted by standard models, and a broad UV polarization dip around 1900 A is seen. These results are in agreement with those found from the WUPPE observations of two other Be stars, Xi Tau and Pi Aqr, which were published earlier. All these observations are an important probe of the Be circumstellar envelopes and demonstrate the need for the inclusion of metal-line effects in circumstellar disk models of Be star UV polarization.

We present the first ultraviolet spectropolarimetric observations of the Type 2 Seyfert galaxy NGC 1068 obtained by the Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE). In this communication we discuss the continuum polarization in the ultraviolet. Shortward of 2500 A the polarization becomes constant at 16 percent at a position angle of 112 deg. This is in general agreement with the 1983 prediction of Miller and Antonucci (1983) and their thick torus model, in which a hidden Type 1 Seyfert nucleus is seen in light scattered by electrons outside the obscured nuclei.

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.

During the 1990 December Astro-1 Space Shuttle mission, spectropolarimetry was conducted in the wavelength region from 1400 to 3200 A of the Wolf-Rayet stars EZ CMa (WN5) and Theta Mus (WC6 + O9.5I) with the Wisconsin Ultraviolet Photo-Polarimeter Experiment. The UV polarization of EZ CMa displays features which correspond to emission lines. This indicates a large, about 0.8 percent, intrinsic UV-continuum polarization, and provides further evidence that the wind of EZ CMa is highly distorted. The polarization of Theta Mus does not change across emission lines, or the strong interstellar 2200 A feature. The polarization decreases smoothly to shorter wavelengths, at constant position angle. The combined UV-optical polarization spectrum of Theta Mus can be described well with interstellar polarization following a Serkowski law.

A time-dependent spectropolarimetric data set of HD 191765 in the wavelength range 3159-7593 A is presented. At all epochs the present observations display a large and strongly wavelength-dependent continuum polarization and reduced levels of polarization across the emission lines. The data imply a significant intrinsic continuum polarization which requires a general deviation of the electron distribution from spherical symmetry. The global shape is quite stable as a function of time; small fluctuations may arise from localized density/temperature changes. The line polarizations are consistent with an axisymmetric wind geometry and ionization stratification. A qualitative model for polarization in a Wolf-Rayet atmosphere is developed. It is argued that the blueward rise of the continuum polarization in HD 191765 can be explained if the density in the wind is high, resulting in a competition of thermal and electron-scattering continuum opacity in the vertical.

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.

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.

UV spectropolarimetric observations of the early-type supergiant stars P Cygni and Kappa Cassiopeiae obtained with the Wisconsin UV Photo-Polarimeter Experiment (WUPPE) are presented. Contrary to model predictions, the intrinsic polarization of P Cyg remains constant in the Balmer continuum except for a broad dip between 2600 and 3000 A. This decrease in polarization is discussed in terms of possible Fe line blanketing effects. The intrinsic position angle is essentially constant in the UV except for a rotation across the feature between 2600 and 3000 A. The intrinsic polarization of Kappa Cas is small; most of the observed polarization is interstellar in nature. The Serkowski curve extrapolated into the UV is found to fall consistently below the observed polarization. A slight rotation in position angle from optical to UV wavelengths is noted. Very good agreement is found between the mean continuum polarization obtained by WUPPE and the broadband UV polarization measurement obtained by Gehrels (1968).

Recent models of Wolf-Rayet star winds have been tailored to EZ CMa, and make predictions of the envelope structure and location of line-emitting regions. It is discussed how the wind structure of EZ CMa can be probed observationally through electron distribution integrals as measured by spectropolarimetry, and then present, analyze, and interpret a time-dependent spectropolarimetric data set of EZ CMa. The observations further the view of an electron-scattering wind that is axisymmetric, rotating, and expanding, with a variable mass-loss rate being responsible for the quasi-periodic polarimetric variability. It is demonstrated that the emission lines of EZ CMa are partially polarized, indicating that line photons are electron-scattered in the wind. The polarization in N V lambda 4945 and N IV lambda 4058 is observed to be larger than that of He II lambda 4686 and He I lambda 5876, as expected from ionization stratification.

Results of optical linear spectropolarimetric measurements of the hypergiant P Cygni are presented. The data indicate that both the intrinsic polarization and the position angle vary with time. It is found that, prior to removing the interstellar polarization vector, the polarization varies by 0.642 percent, while the position angle takes on values between 27.0 and 39.8 deg. The wavelength dependence of the polarization curve also changes with time, indicating that the intrinsic polarization curve is not always constant with the wavelength. After a model is determined for interstellar polarization, it is found that the amplitude of the intrinsic polarization of P Cyg varies from 0.04 to 0.48 percent, with a change of 0.12 percent on successive nights. It is inferred from this large variation in the polarization on such a short timescale that the scattering material lies close to the star. Several possible geometries to describe the circumstellar material of P Cygni are presented.

Spectropolarimetric observations from 5000 to 8000 A have been obtained for comets P/Austin (1982g) and P/Churyumov-Gerasimenko (1982f). The observations were spaced over phase angles of 50–125° for comet Austin and 10–40° for comet Churyumov-Gerasimenko. The use of spectropolarimetry allowed an evaluation of continuum polarization without molecular line contamination. Especially for comet Churyumov-Gerasimenko, the curve of polarization versus phase angle resembles curves for asteroids, where the polarization is negative (electric vector maximum parallel to the scattering plane) for phase angles less than 20° and the most negative polarization is from −1 to −2%. The negative polarization at backscattering angles may be due to multiple scattering in agglomerated grains, as assumed for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in comet dust, polarization measurements may imply a low albedo, less than 0.08. The polarization of comet Austin remained steady during a large change in the dust production rate. Both comets increased continuum flux by a factor of 2 near perihelion. The continuum of comet Churyumov-Gerasimenko had the shape of the solar spectrum with derivations less than 5%. The equivalent width of spectral features of C 2 , NH 2 , and O varied as r −2 .

There is evidence from visible wavelength polarimetry that the matter in the winds of early-type stars is not distributed uniformly about the stars. In this paper equations for the observable polarization from an axisymmetric distribution of electrons are derived first from the basic point source model of Brown and McLean (1977) and then from extended atmosphere radiative transfer theory. A factor is derived which corrects the Brown and McLean model for the finite size of stars. This finite disk factor is found to reduce the magnitude of the predicted polarization to less than half of the point source prediction. The effects of having the mass outflow concentrated in a plume are considered. The effects of absorption in an unocculted plume are shown to give rise to a wavelength-dependent polarization that is described using a 'polarization color' parameter. A continuum opacity index is defined that allows for a straightforward interpretation of the polarization of stars of differing effective temperatures.

We present preliminary results obtained between March and August of 1987 of a two-year spectropolarimetric study of 10 RV Tauri stars. The observations were made at the University of Wisconsin's Pine Bluff Observatory using the 36 inch cassegrain telescope with a Lyot polarimeter. The spectral resolution of the instrument was 6 A and covered the range between 4350 A and 7500 A. The polarimetry covered the same wavelength range but is binned in four broad bands at 4828 A, 5361 A, 6025 A, and 6877 A.

An investigation of the nature of the thermal diode background in integrating photodiode detectors is presented. The Weckler's (1967) nonlinear model was extended to the general situation of comparable dark and photocurrents, and the model with a self-scanned diode array detector was verified. It was found that a naive linear subtraction of a scan of background from a scan of signal plus background introduces a small multiplicative error in the apparent gain of the detector; the fractional gain error is approximately equal to the fraction of diode saturation due to integrated dark current and does not depend on the amount of illumination. A simple linear correction algorithm is described which allows the use of integration times for which diode background becomes a large fraction of diode saturation allowing detection of lower light levels. A sample application of the correction formula allows operation at a convenient dry-ice pumped-alcohol temperature for exposure times of up to 200 s.

Spectropolarimetric observations of R Coronae Borealis obtained during the brightness minimum of 1986 are presented. When combined with previous polarization observations of brightness minima, the distribution of observations in the Q-U plane suggests that ejections of dust clouds occur in a preferred plane about the star, in contrast to the standard model for the R CrB-type stars, which holds that clouds are ejected from all parts of the stellar surface. The possibility of an ejection mechanism connected with nonradial pulsations is discussed as the most likely explanation of the preferred plane.

A search was conducted for evidence of a coronal region at the base of the winds of Epsilon Ori and Kappa Ori, by means of high signal-to-noise observations at the forbidden lines of Fe X, at 6574 A, and Fe XIV, 5303 A. Both stars have been detected as soft X-ray sources, and show anomalously strong O VI lines in their UV spectra. Large coronal emission measures were expected from the total X-ray flux and Auger-enhanced ionization, but the fact that the iron coronal lines were not detected places new limits on the emission measure if the total temperature is in the range of 700,000-3,000,000 or more than 1,000,000 for Kappa Ori and 2,000,000 for Epsilon Ori. It is suggested that at least some of the X-rays arise, not from the base corona, but from source features farther out in the wind.