Kenneth H. Nordsieck

Prime focus imaging spectrograph for the Southern African Large Telescope: optical design

The University of Wisconsin - Madison, together with Rutgers University and the South African Astronomical Observatory, is designing and building an imaging spectrograph for the Prime Focus Instrument Package of the Southern African Large Telescope (SALT). The Prime Focus Imaging Spectrograph (PFIS) will be a versatile instrument specializing in very high throughput, low and medium resolution (R=500-12,500) imaging spectroscopy, using volume phase holographic (VPH) gratings and a double etalon Fabry-Perot interferometer, and spectropolarimetry from 320 to 900 nm. The optical design includes all transmissive optics for high efficiency and compactness. To maintain throughput in the ultraviolet, only fused silica, CaF2 and NaCl are used. As NaCl is very hygroscopic, the design uses NaCl as the inner element in sealed triplets only. For the highest possible first-order spectral resolution, the collimated beam size is 150 mm - the maximum for practical Fabry-Perot etalons. The F/2.2 camera can be articulated to tune the efficiency of the VPH gratings; a complement of six gratings (5 VPH and 1 standard transmission grating) has been designed to fill the resolution-wavelength space available to the instrument. Linear, circular, and all-stokes spectropolarimetry will be performed through the use of Pancharatnam superachromatic waveplates and a Wollaston beamsplitter.

Prime Focus Imaging Spectrograph for the Southern African Large Telescope: operational modes

The Prime Focus Imaging Spectrograph (PFIS) will be the workhorse first-light instrument on the Southern African Large Telescope (SALT). Scheduled for commissioning in late 2004, PFIS is a versatile high-throughput imaging spectrograph with a complement of 5 volume-phase holographic gratings for spectroscopic programs from 3200Å to 9000Å at resolutions of R=1500 to R=6000. A magazine of 6 longslits and 30 custom laser-milled slitmasks enables single- or multi-object spectroscopy over an 8 arcminute diameter field. With the gratings stowed, a dual-etalon Fabry-Perot subsystem enables imaging spectroscopy at R=500, R=3000, and R=12,500. The polarization subsystem, consisting of a polarizing beam-splitter used in conjunction with half- and quarter-wave plates, allow linear or circular polarimetric measurements in ANY of the spectroscopic modes. Three mosaiced rapid-readout frame-transfer CCDs provide the capability for time-resolved sampling at rates in excess of 10 Hz. Combinations of these subsystems permit novel observing modes for specialized scientific programs. Examples include high-time resolution multi-object spectral polarizmetry of accreting compact objects, and Fabry-Perot polarimetry or imaging spectral polarimetry of nebulae and stellar clusters. The demands of queue-scheduled observing on a fixed-altitude telescope require that the instrument be capable of rapid reconfiguration between modes.

The metallicity extremes of the Sagittarius dSph: SALT spectroscopy of PNe

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) contains a youngest stellar population with [O/H] ≈ -0.2 and age t > 1 Gyr, and an oldest population with [O/H] = -2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12 + log(O/H) = 8.57 ± 0.02 dex. The other PN studied, BoBn 1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn 1 yield the original stellar metallicity, corresponding to 12 + log(O/H) = 6.72 ± 0.16 dex which is 1/110 of the solar value. The actual [O/H] is much higher: third dredge-up enriched the material by a factor of ∼12 in oxygen, ∼240 in nitrogen and ∼70 in neon. Neon as well as nitrogen and oxygen content may have been produced in the intershell of low-mass asymptotic giant branch (AGB) stars. Well defined broad WR lines are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn 1. This puts the fraction of [WR]-type central PNe stars to 67 per cent for dSph galaxies.

MULTI-WAVELENGTH OBSERVATIONS OF SUPERNOVA 2011ei: TIME-DEPENDENT CLASSIFICATION OF TYPE IIb AND Ib SUPERNOVAE AND IMPLICATIONS FOR THEIR PROGENITORS

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within � 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (& 12,000 km s −1 ) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v � 0.13c and a progenitor star mass-loss rate of u M � 1.4 × 10 −5 M⊙ yr −1 (assuming wind velocity vw = 10 3 km s −1 ). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN2011ei are consistent with the explosion of a lower-mass (3 4 M⊙), compact (R∗ . 1 × 10 11 cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN2011ei’s rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star’s evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations. Subject headings: supernovae: general — supernova: individual (SN2011ei)

PySALT: the SALT science pipeline

PySALT is the python/PyRAF-based data reduction and analysis pipeline for the Southern African Large Telescope (SALT), a modern 10m class telescope with a large user community consisting of 13 partner institutions. The two first generation instruments on SALT are SALTICAM, a wide-field imager, and the Robert Stobie Spectrograph (RSS). Along with traditional imaging and spectroscopy modes, these instruments provide a wide range of observing modes, including Fabry-Perot imaging, polarimetric observations, and high-speed observations. Due to the large user community, resources available, and unique observational modes of SALT, the development of reduction and analysis software is key to maximizing the scientific return of the telescope. PySALT is developed in the Python/PyRAF environment and takes advantage of a large library of open-source astronomical software. The goals in the development of PySALT are: (1) Provide science quality reductions for the major operational modes of SALT, (2) Create analysis tools for the unique modes of SALT, and (3) Create a framework for the archiving and distribution of SALT data. The data reduction software currently provides support for the reduction and analysis of regular imaging, high-speed imaging, and long slit spectroscopy with planned support for multi-object spectroscopy, high-speed spectroscopy, Fabry-Perot imaging, and polarimetric data sets. We will describe the development and current status of PySALT and highlight its benefits through early scientific results from SALT.

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.

Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy

We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope of a luminous IR galaxy (LIRG) IRAS 19115-2124. Thes ...

The Hanle Effect as a Diagnostic of Magnetic Fields in Stellar Envelopes. I. Theoretical Results for Integrated Line Profiles

The Hanle effect concerns the modification of polarized resonance-line scattering by magnetic fields; thus, it can be used as a diagnostic of stellar magnetic fields. The Hanle effect has been used to determine the field strength and distribution of magnetic structures present in prominences of the Sun. To investigate its potential use in stellar astronomy, the simplified case of an optically thin axisymmetric ring illuminated by a stellar point source is considered. The results are then used to derive the polarization from polar plumes, equatorial disks, and spherical shells. The integrated line polarization is calculated for axisymmetric rings with a variety of magnetic field orientations, and in every case the polarization is proportional to sin2 i (where i is the viewing inclination), just as in the zero field case. It is also found that the Hanle effect can significantly alter the integrated line polarization. In some cases the position angle of the polarization in the line can be rotated by 90° relative to the zero field case. We consider the Hanle effect as a possible diagnostic of magnetic fields in stellar winds with prominent ultraviolet and visible resonance lines. For these lines the diagnostic has sensitivity in the range of 1-1000 G. The Zeeman effect is not normally applicable for diagnosing magnetic fields in stellar winds in the subkilogauss range; thus, the Hanle effect should provide an especially useful new method of determining magnetic fields in stars other than the Sun. Possibilities for measuring the fields in early-type stars using ultraviolet observations is discussed.

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.

Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings

We report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration. The ghost is caused by light reflected off the detector surface, recollimated by the camera, recombined by, and reflected from, the grating, and reimaged by the camera onto the detector. This recombination can occur in two different ways. We observe this ghost in two spectrographs being developed by the University of Wisconsin-Madison: the Robert Stobie Spectrograph for the Southern African Large Telescope, and the Bench Spectrograph for the WIYN 3.5 m telescope. The typical ratio of the brightness of the ghost relative to the integrated flux of the spectrum is of order 10 4 , implying a recombination efficiency of the VPH gratings of order 10 3 or higher, consistent with the output of rigorous coupled wave analysis. Any spectrograph employing VPH gratings, including grisms, in Littrow configuration will suffer from this ghost, although the general effect is not intrinsic to VPH gratings themselves and has been observed in systems with conventional gratings in non-Littrow configurations. We explain the geometric configurations that can result in the ghost, as well as a more general prescription for predicting its position and brightness on the detector. We make recommendations for mitigating the ghost effects for spectrographs and gratings currently built. We further suggest design modifications for future VPH gratings to eliminate the problem entirely, including tilted fringes and/or prismatic substrates. We discuss the resulting implications for the spectrograph performance metrics.