David J. Osip

The Host Galaxy of GRB 031203: Implications of Its Low Metallicity, Low Redshift, and Starburst Nature

We present Keck/NIRSPEC near-IR images and Magellan/IMACS optical spectroscopy of the host galaxy of GRB 031203. The host is an actively star-forming galaxy at z ¼ 0:1055 � 0:0001. This is the lowest red- shift GRB to date, aside from GRB 980425. From the hydrogen Balmer lines, we infer an extinction of AV ¼ 3:62 � 0:25 or a total reddening ET(BV ) ¼ 1:17 � 0:1 toward the sight line to the nebular regions. After correcting for reddening, we perform an emission-line analysis and derive an ISM temperature of T ¼ 13400 � 2000 K and electron density of ne ¼ 300 cm � 3 . These imply a metallicity ½O=H �¼� 0:72 � 0:15 dex and a roughly solar abundance pattern for N, Ne, S, and Ar. Integrating H� , we infer a dust-corrected star formation rate (SFR) of more than 11 Myr � 1 . These observations have the following implications: (1) The galaxy has a low K 0 -band luminosity LL � =5, typical of GRB host galaxies. (2) The low redshift indicates GRB 031203 had an isotropic-equivalent � -ray energy release smaller than all previous confirmed GRB events. The burst discovery raises the likelihood of identifying many additional low-z, low-flux events with Swift .( 3) The large SFR, low metallicity, and inferred hard radiation field are suggestive of massive star formation, supporting the collapsar model. (4) Several lines of evidence argue against the identification of GRB 031203 as an X-ray flash event.

Starspots and Spin-orbit Alignment in the WASP-4 Exoplanetary System

We present photometry of 4 transits of the exoplanet WASP-4b, each with a precision of approximately 500 ppm and a time sampling of 40-60 s. We have used the data to refin e the estimates of the system parameters and ephemerides. During two of the transits we observed a short-lived, low-amplitude anomaly that we interpret as the occultation of a starspot by the planet. We also found evidence for a pair of similar anomalies in previously published photometry. The recurrence of these anomalies suggests that the stellar rotation axis is nearly aligned with the orbital axis, or else the star spot wo uld not have remained on the transit chord. By analyzing the timings of the anomalies we find the sky-projec ted stellar obliquity to be � = -1 +14 -12 degrees. This result is consistent with (and more constraining than) a rec ent observation of the Rossiter-McLaughlin effect. It suggests that the planet migration mechanism preserved the initially low obliquity, or else that tidal evolution has realigned the system. Future applications of this method using data from the CoRoT and Kepler missions will allow spin-orbit alignment to be probed for many other exoplanets. Subject headings:planetary systems — stars: individual (WASP-4=USNO-B1.0 0479-0948995)

IMACS: The Inamori-Magellan Areal Camera and Spectrograph on Magellan-Baade

The Inamori-Magellan Areal Camera and Spectrograph (IMACS) is a wide-field, multipurpose imaging spectrograph on the Magellan-Baade telescope at Las Campanas Observatory. IMACS has two channels—f/2 and f/4, each with an 8K × 8K pixel mosaic of CCD detectors, that service the widest range of capabilities of any major spectrograph. These include wide-field imaging at two scales, 0.20 pixel-1 and 0.11 pixel-1, single-object and multislit spectroscopy, integral-field spectroscopy with two 5 × 7 areas sampled at 0.20 pixel-1 (Durham IFU), a multiobject echelle (MOE) capable of N ~ 10 simultaneous full-wavelength R ≈ 20,000 spectra, the Maryland-Magellan Tunable Filter (MMTF), and an image-slicing reformatter for dense-pack multislit work (GISMO). Spectral resolutions of 8 < R < 5000 are available through a combination of prisms, grisms, and gratings, and most modes are instantly available in any given IMACS configuration. IMACS has a spectroscopic efficiency over 50% in f/2 multislit mode (instrument only) and, by the AΩ figure of merit (telescope primary surface area times instrument field of view ), IMACS scores 5.7 m2 deg2, compared with 3.1 for VIMOS on VLT3 and with 2.0 for DEIMOS on Keck2. IMACS is the most versatile, and—for wide-field optical spectroscopy—the most powerful spectrograph on the planet.

The recent expansion of Pluto's atmosphere.

Stellar occultations—the passing of a relatively nearby body in front of a background star—can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Plutos atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of ∼1,215u2009km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Plutos atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Plutos atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

IMACS: the wide-field imaging spectrograph on Magellan-Baade

Bigelow & Dressler1 reported on the design and construction of IMACS - the Inamori-Magellan Areal Camera and Spectrograph. IMACS was installed on the Magellan-Baade 6.5-m telescope at the Carnegie Institutions Las Campanas Observatory in Chile in August, 2003, and was phased into regular operation in the remaining months of that year (Osip et al2). IMACS is now the most-used instrument on the Baade telescope, accounting for 63% of the nights available for astronomy in the 2005 observing year. IMACS has two basic operating modes. A single 6-inch beam refractive collimator feeds either (1) an f/4 all-spherical refractive camera delivering 0.11 arcsec/pixel, or (2) a double-asphere refractive camera with oil-coupled multiplets producing a scale of 0.20 arcsec/pixel. The detector for both foci is an 8K x 8K mosaic camera of 8 SITe 2K x 4K 15 μ CCDs. The collimator and f/4 camera have performed to design specifications and have delivered 0.45 arcsec images across the 15 arcmin square field. The f/2 camera has delivered images of 0.55 to 0.65 arcsec across its 27 arcmin diameter field in excellent seeing (FWHM ~ 0.40 arcsec). The f/4 camera uses 6-inch reflecting gratings to obtain spectroscopy at multiple resolutions ranging from R=1350-9375; the f/2 camera uses three 6-inch grisms to achieve resolutions of R=450, 600, and 900 over its larger field. We routinely cut hundreds of slits in 30-inch diameter, stainless steel, spherical-shell slitmasks with a commercial laser system. Alignment procedures for observing are simple and efficient, typically requiring 5-10 minutes per set-up. IMACS - an unusually versatile instrument - includes an IFU built by Durham University with two 5 x 8 (f/2) or 4 x 7 (f/4) apertures, each sampled by 1000 optical fibers. A Multi-Object Echelle mode, which can obtain 10-15 full wavelength R=20000 spectra, has been fully tested and has now started regular operation. The Maryland-Magellan Tunable Filter (MMTF) has been lab tested and will be commissioned in June 2006. In early 2007, Gladders Image-Slicing Multislit Option (GISMO) will be ready for testing, and a second Mosaic CCD camera - which will simplify operations, increase sensitivity, and allow rapid access to both f/2 and f/4 modes - is under construction. We report on the design challenges posed and met by the variety of operating modes and stringent performance requirements. We describe some issues encountered in the past two years in bringing such a complex, multi-mode instrument to the Magellan Observatory.

THE TRANSIT LIGHT CURVE PROJECT. XI. SUBMILLIMAGNITUDE PHOTOMETRY OF TWO TRANSITS OF THE BLOATED PLANET WASP-4b*

We present photometry of two transits of the giant planet WASP-4b with a photometric precision of 400-800 parts per million and a time sampling of 25-40 s. The two midtransit times are determined to within 6 s. Together with previously published times, the data are consistent with a constant orbital period, giving no compelling evidence for period variations that would be produced by a satellite or additional planets. Analysis of the new photometry, in combination with stellar-evolutionary modeling, gives a planetary mass and radius of 1.237 ± 0.064 M Jup and 1.365 ± 0.021 R Jup, respectively. The planet is 15% larger than expected based on previously published models of solar-composition giant planets. With data of the quality presented here, the detection of transits of a super-Earth of radius 1.75 R ⊕ would have been possible.

A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)

We present infrared images and spectra of comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger program to observe comets inside of 5 AU from the Sun with the Spitzer Space Telescope. The nucleus of comet 2P/Encke was observed at two vastly different phase angles (20° and 63°). Model fits to the spectral energy distributions of the nucleus suggest that comet Enckes infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence. The observed emission from comet Enckes dust coma is best modeled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 μm, and the silicate contribution by mass to the submicron dust coma is constrained to <31%. Comet 67P/Churyumov-Gerasimenko was observed with distinct coma emission in excess of a model nucleus at a heliocentric distance of 5.0 AU. The coma detection suggests that sublimation processes are still active or grains from recent activity remain near the nucleus. Comet C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the spectrum obtained at 3.2 AU, and we derive a silicate-to-carbon dust ratio of 0.6. The ratio is an order of magnitude lower than that derived for comets 9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).

MMTF: The Maryland-Magellan Tunable Filter

This paper describes the Maryland-Magellan Tunable Filter (MMTF) on the Magellan-Baade 6.5 m telescope. MMTF is based on a 150 mm clear aperture Fabry-Perot (FP) etalon that operates in low orders and provides transmission bandpass and central wavelength adjustable from ~5 A to ~15 A and from ~5000 A to over ~9200xa0A, respectively. It is installed in the Inamori Magellan Areal Camera and Spectrograph and delivers an image quality of ~05 over a field of view of 27 in diameter (monochromatic over ~10). This versatile and easy-to-operate instrument has been used over the past three years for a wide variety of projects. This paper first reviews the basic principles of FP tunable filters, and then provides a detailed description of the hardware and software associated with MMTF and the techniques developed to observe with this instrument and reduce the data. The main lessons learned in the course of the commissioning and implementation of MMTF are highlighted next, before concluding with a brief outlook on the future of MMTF and of similar facilities which are soon coming on line.

Megacam: A Wide-Field CCD Imager for the MMT and Magellan

Megacam is a large-format optical camera that can be operated at the f/5 Cassegrain foci of the MMT on Mount Hopkins, Arizona, and the Magellan Clay telescope at Las Campanas Observatory, Chile. Megacams focal plane is composed of 36 closely packed e2v CCD42-90 CCDs, each with 2048 × 4608 pixels, assembled in an 18,432 × 18,432 array. Two additional CCD42-90s are provided for autoguiding and focus control. The CCDs have 13.5 μm square pixels that subtend at the f/5 foci, yielding a 25 × 25 field-of-view. The camera system includes a focal plane shutter, two filter wheels, two liquid nitrogen reservoirs, a central chamber that holds the CCD mosaic array, and two electronics boxes. Megacam is equipped with a variety of broadband and narrowband filters. Software features include automatic calculation of twilight flat exposure times.

Lack of Transit Timing Variations of OGLE-TR-111b: A Re-Analysis with Six New Epochs

We present six new transits of the exoplanet OGLE-TR-111b observed with the Magellan Telescopes in Chile between 2008 April and 2009 March. We combine these new transits with five previously published transit epochs for this planet between 2005 and 2006 to extend the analysis of transit timing variations (TTVs) reported for this system. We derive a new planetary radius value of 1.019 ± 0.026 RJ , which is intermediate to the previously reported radii of 1.067 ± 0.054 RJ and 0.922 ± 0.057 RJ . We also examine the TTV and duration change claims of Diaz et al. Our analysis of all 11 transit epochs does not reveal any points with deviations larger than 2σ, and most points are well within 1σ. Although the transit duration nominally decreases over the four year span of the data, systematic errors in the photometry can account for this result. Therefore, there is no compelling evidence for either a timing or a duration variation in this system. Numerical integrations place an upper limit of about 1 M ⊕ on the mass of a potential second planet in a 2:1 mean-motion resonance with OGLE-TR-111b.