Stephane Beland

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

THE DATA REDUCTION PIPELINE for the APACHE POINT OBSERVATORY GALACTIC EVOLUTION EXPERIMENT

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III, explores the stellar populations of the Milky Way using the Sloan 2.5-m telescope linked to a high resolution (R~22,500), near-infrared (1.51-1.70 microns) spectrograph with 300 optical fibers. For over 150,000 predominantly red giant branch stars that APOGEE targeted across the Galactic bulge, disks and halo, the collected high S/N (>100 per half-resolution element) spectra provide accurate (~0.1 km/s) radial velocities, stellar atmospheric parameters, and precise (~0.1 dex) chemical abundances for about 15 chemical species. Here we describe the basic APOGEE data reduction software that reduces multiple 3D raw data cubes into calibrated, well-sampled, combined 1D spectra, as implemented for the SDSS-III/APOGEE data releases (DR10, DR11 and DR12). The processing of the near-IR spectral data of APOGEE presents some challenges for reduction, including automated sky subtraction and telluric correction over a 3 degree diameter field and the combination of spectrally dithered spectra. We also discuss areas for future improvement.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE)

National Science Foundation [AST-1109178, AST-1616636]; Gemini Observatory; Spanish Ministry of Economy and Competitiveness [AYA-2011-27754]; NASA [NNX12AE17G]; Hungarian Academy of Sciences; Hungarian NKFI of the Hungarian National Research, Development and Innovation Office [K-119517]; Alfred P. Sloan Foundation; National Science Foundation; U.S. Department of Energy Office of Science

METAL DEPLETION AND WARM H2 IN THE BROWN DWARF 2M1207 ACCRETION DISK

We present new far-ultraviolet observations of the young M8 brown dwarf 2MASS J12073346- 3932539, which is surrounded by an accretion disk. The data were obtained using the Hubble Space Telescope-Cosmic Origins Spectrograph. Moderate resolution spectra (R � 17,000 - 18,000) obtained in the 1150 - 1750 u and 2770 - 2830 u bandpasses reveal H2 emission excited by H I Lyphotons, several ionization states of carbon (C I - C IV), and hot gas emission lines of He II and N V (T � 10 4 - 10 5 K). Emission from some species that would be found in a typical thermal plasma at this temperature (Si II, Si III, Si IV, and Mg II) are not detected. The non-detections indicate that these refractory elements are depleted into grains, and that accretion shocks dominate the production of the hot gas observed on 2MASS J12073346-3932539. We use the observed C IV luminosity to constrain the mass accretion rate in this system. We use the kinematically broadened H2 profile to confirm that the majority of the molecular emission arises in the disk, measure the radius of the inner hole of the disk (Rhole � 3R∗), and constrain the physical conditions of the warm molecular phase of the disk (T(H2) � 2500 - 4000 K). A second, most likely unresolved H2 component is identified. This feature is either near the stellar surface in the region of the accretion shock or in a molecular outflow, although the possibility that this Jovian-like emission arises on the day-side disk of a 6 MJ companion (2M1207b) cannot be conclusively ruled out. In general, we find that this young brown dwarf disk system is a low-mass analog to classical T Tauri stars that are observed to produce H2 emission from a warm layer in their disks, such as the well studied TW Hya and DF Tau systems. Subject headings: stars: brown dwarfs (2MASS J12073346-3932539) — accretion disks — ultraviolet: stars

Performance of the apache point observatory galactic evolution experiment (APOGEE) high-resolution near-infrared multi-object fiber spectrograph

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) uses a dedicated 300-fiber, narrow-band near-infrared (1.51-1.7 μm), high resolution (R~22,500) spectrograph to survey approximately 100,000 giant stars across the Milky Way. This three-year survey, in operation since late-summer 2011 as part of the Sloan Digital Sky Survey III (SDSS III), will revolutionize our understanding of the kinematical and chemical enrichment histories of all Galactic stellar populations. We present the performance of the instrument from its first year in operation. The instrument is housed in a separate building adjacent to the 2.5-m SDSS telescope and fed light via approximately 45-meter fiber runs from the telescope. The instrument design includes numerous innovations including a gang connector that allows simultaneous connection of all fibers with a single plug to a telescope cartridge that positions the fibers on the sky, numerous places in the fiber train in which focal ratio degradation had to be minimized, a large mosaic-VPH (290 mm x 475 mm elliptically-shaped recorded area), an f/1.4 six-element refractive camera featuring silicon and fused silica elements with diameters as large as 393 mm, three near-infrared detectors mounted in a 1 x 3 mosaic with sub-pixel translation capability, and all of these components housed within a custom, LN2-cooled, stainless steel vacuum cryostat with dimensions 1.4-m x 2.3-m x 1.3-m.

Taking Advantage of the Cos Time-tag Capability: Observations of Pulsating Hot DQ White Dwarfs and Discovery of a New One

We present an analysis of the ultraviolet light curves of five Hot DQ white dwarfs recently observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). These light curves were constructed by extracting the time-tag information from the FUV and NUV spectroscopic data. Single-color light curves were thus produced in 60 s time bins. The Fourier analysis of these data successfully recovers the main pulsation modes of the three stars previously known to be variable from ground-based observations. We also report the discovery of pulsations in another object, SDSS J1153+0056, making it only the fifth member of the new class of variable Hot DQ stars and the first pulsating white dwarf to be discovered from space-based observations. The relatively high amplitudes of the modes observed in the FUV (2-4 times that observed in the optical) as well as the high fraction of stars variable in our sample suggest that most, if not all, Hot DQ white dwarfs might be pulsating at some level when observed at high enough sensitivity. Our results also underline the vast potential of the time-tag capability of the HST/COS combination.

HST-COS far-ultraviolet detector: final ground calibration

The flight microchannel plate detectors to be used in the Cosmic Origins Spectrograph, a fourth generation instrument for the Hubble Space Telescope, have been calibrated in the laboratory before being integrated into the spectrograph. This paper presents the results of these calibrations that include measurements of the detector quantum efficiency, spatial resolution, spatial linearity, flat field, electronic livetime and the local count rate limit.

Gain sag in the FUV detector of the Cosmic Origins Spectrograph

The Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) uses a large-format cross delay line (XDL) detector in its Far Ultraviolet (FUV) channel. While obtaining spectra, light falls non-uniformly on the detector due to the optical design and the spectral properties of the object being observed; in particular, bright emission lines from geocoronal Lyman-alpha can fall on the detector in more than 20 locations. As a result, some areas of the detector have received a much greater exposure than others. This non-uniform illumination has led to a time- and position-dependent change in the gain of the microchannel plates, which causes variations in the overall detector performance. We will discuss the effects of this gain sag on the science data, and discuss mitigation strategies which are being implemented in order to maximize the detector lifetime.

Hubble Space Telescope: Cosmic Origins Spectrograph FUV detector initial on-orbit performance

The Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) in May 2009 during Servicing Mission 4 (SM4). This paper discusses the initial on-orbit performance of the HST-COS far ultraviolet (FUV) detector designed and built by the Experimental Astrophysics Group at the Univ. of California, Berkeley. The HST-COS FUV detector is an open face, photon counting, microchannel plate (MCP) based device employing a cross delay line (XDL) readout. The detector consists of two separate, end-to-end segments (2x 85mm x 10mm - 179mm x 10mm total with a gap between segments), each digitized within a 16384x1024 space. The input surface is curved to match the Rowland circle of HST-COS. The CsI photocathode and open face nature result in sensitivity from <900Å to ~1750Å. Spatial resolution is approximately 25-30μm. Comparisons of on-orbit behavior relative to expectations from ground testing are performed. Areas of discussion include background (rate and morphology), sensitivity (system throughput and short wavelength response), and imaging performance (apparent spatial resolution and flat field fixed pattern). A measured increase in the MCP gain relative to ground testing is also discussed.

Algorithms for correcting geometric distortions in delay-line anodes

Time-delay anodes are typically used in conjunction with microchannel plates to provide photon counting and two- dimensional imaging. The anode and associated electronics are used to compute the centroid of the charge cloud from the microchannel plate stack. The computation is done in analog circuitry and reported as a digital value. The analog nature of the time-delay anode makes them susceptible to variations in the correlation between physical space and the reported digital value. These variations, both local and global, must be corrected as part of the data reduction of scientific data. If left uncorrected in spectral data, for example, these variations would result in inaccurate wavelength identifications and distorted spectral line profiles. This work describes successful algorithms for correcting the dominant distortions present in a time-delay anode; geometric (local) and thermal (global) distortions. These algorithms were developed as part of the data reduction pipelines for the Cosmic Origins Spectrograph (COS), a fourth generation instrument for the Hubble Space Telescope, and the Far Ultraviolet Spectroscopic Explorer (FUSE).