Deriving Iodine-free spectra for high-resolution echelle spectrographs

We describe a new method to derive clean, iodine-free spectra directly from observations acquired using high-resolution echelle spectrographs equipped with iodine cells. The main motivation to obtain iodine-free spectra is to use portions of the spectrum that are superimposed with the dense forest of iodine absorption lines, in order to retrieve lines that can be used to monitor the magnetic activity of the star, helping to validate candidate planets. In short, we provide a straight-forward methodology to clean the spectra by using the forward model used to derive radial velocities, the Line Spread Function information plus the stellar spectrum without iodine to reconstruct and subtract the iodine spectrum from the observations. We show our results using observations of the star $\tau$ Ceti acquired with the PFS, HIRES and UCLES spectrographs, reaching an iodine-free spectrum correction at the $\sim$1% RMS level. We additionally discuss the limitations and further applications of the method.Comment: 15 pages, 7 figures. Accepted for publication in A

The Great Circle Camera: A New Drift Scanning Instrument

We discuss the design, construction, and use of a new class of scanning camera that eliminates a critical limitation of standard CCD drift-scan observations. A standard scan, which involves no correction for the differential drift rates and curved stellar paths across the field-of-view, suffers from severe image degradation even when one observes at moderate declinations. Not only does this effect limit the area of the sky over which drift scanning is viable, but as detector sizes increase, CCD mosaics become standard, and dome/telescope seeing improves, the area of sky for which scanning is acceptable (image degradation \ltsim seeing) will be further reduced unless some action is taken. By modifying the scan path (the path on the sky traced by signal accumulated along a single CCD column) to lie along a great circle on the sky rather than along a path of constant declination, image degradation is minimized. In this paper, we discuss the design and implementation of a stage that rotates and translates the CCD during a drift-scan exposure so that the scan path is along a great circle on the sky. Data obtained during the commissioning run of the Great Circle Camera at the Las Campanas 1-m telescope are presented.Comment: Second attempt at a readable archival file. 7 pages (gzip'ed and uuencoded postscript). A version of the preprint with Figures 2 and 3 can be obtained from D. Zaritsky. Accepted for publication in PAS

Neutron-Capture Nucleosynthesis in the First Stars

Recent studies suggest that metal-poor stars enhanced in carbon but containing low levels of neutron-capture elements may have been among the first to incorporate the nucleosynthesis products of the first generation of stars. We have observed 16 stars with enhanced carbon or nitrogen using the MIKE Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present radial velocities, stellar parameters, and detailed abundance patterns for these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and other heavy elements are detected. In four stars, these heavy elements appear to have originated in some form of r-process nucleosynthesis. In one star, a partial s-process origin is possible. The origin of the heavy elements in the rest of the sample cannot be determined unambiguously. The presence of elements heavier than the iron group offers further evidence that zero-metallicity rapidly-rotating massive stars and pair instability supernovae did not contribute substantial amounts of neutron-capture elements to the regions where the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor stars with low levels of neutron-capture elements were enriched by products of zero-metallicity supernovae only, then the presence of these heavy elements indicates that at least one form of neutron-capture reaction operated in some of the first stars.Comment: Accepted for publication in the Astrophysical Journal (36 pages, 26 figures

The Luminosity Function of Galaxies in the Las Campanas Redshift Survey

We present the $R$-band luminosity function for a sample of 18678 galaxies, with average redshift $z = 0.1$, from the Las Campanas Redshift Survey. The luminosity function may be fit by a Schechter function with $M^* = -20.29 \pm 0.02 + 5 \log h$, $\alpha = -0.70 \pm 0.05$, and $\phi^* = 0.019 \pm 0.001 \ h^3$~Mpc$^{-3}$, for absolute magnitudes$-23.0 \leq M - 5 \log h \leq -17.5$. We compare our luminosity function to that from other redshift surveys; in particular our normalization is consistent with that of the Stromlo-APM survey, and is therefore a factor of two below that implied by the$b_J \approx 20$bright galaxy counts. Our normalization thus indicates that much more evolution is needed to match the faint galaxy count data, compared to minimal evolution models which normalize at$b_J \approx 20$. Also, we show that our faint-end slope$\alpha = -0.7$, though ``shallower'' than typical previous values$\alpha = -1$, results primarily from fitting the detailed shape of the LCRS luminosity function, rather than from any absence of intrinsically faint galaxies from our survey. Finally, using [OII] 3727 equivalent width$W_{\lambda} = 5$~\AA \ as the dividing line, we find significant differences in the luminosity functions of emission and non-emission galaxies, particularly in their$\alpha$values. Emission galaxies have Schechter parameters$M^* = -20.03 \pm 0.03 + 5 \log h$and$\alpha = -0.9 \pm 0.1$, while non-emission galaxies are described by$M^* = -20.22 \pm 0.02 + 5 \log h$and$\alpha = -0.3 \pm 0.1$. (abridged abstract)Comment: 41 pages, including 13 postscript figures, uses AASTEX v4.0 style files. Important clarification of R-band definition, plus correction of luminosity densities and updated references. Main conclusions unchanged. Final version to appear in Ap

The Texture of the Universea a

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71428/1/j.1749-6632.1984.tb23343.x.pd

Background-Limited Imaging in the Near-Infrared with Warm InGaAs Sensors: Applications for Time-Domain Astronomy

We describe test observations made with a customized 640 x 512 pixel Indium Gallium Arsenide (InGaAs) prototype astronomical camera on the 100" DuPont telescope. This is the first test of InGaAs as a cost-effective alternative to HgCdTe for research-grade astronomical observations. The camera exhibits an instrument background of 113 e-/sec/pixel (dark + thermal) at an operating temperature of -40C for the sensor, maintained by a simple thermo-electric cooler. The optical train and mechanical structure float at ambient temperature with no cold stop, in contrast to most IR instruments which must be cooled to mitigate thermal backgrounds. Measurements of the night sky using a reimager with plate scale of 0.4 arc seconds / pixel show that the sky flux in Y is comparable to the dark current. At J the sky brightness exceeds dark current by a factor of four, and hence dominates the noise budget. The sensor read noise of ~43e- falls below sky+dark noise for exposures of t>7 seconds in Y and 3.5 seconds in J. We present test observations of several selected science targets, including high-significance detections of a lensed Type Ia supernova, a type IIb supernova, and a z=6.3 quasar. Deeper images are obtained for two local galaxies monitored for IR transients, and a galaxy cluster at z=0.87. Finally, we observe a partial transit of the hot JupiterHATS34b, demonstrating the photometric stability required over several hours to detect a 1.2% transit depth at high significance. A tiling of available larger-format sensors would produce an IR survey instrument with significant cost savings relative to HgCdTe-based cameras, if one is willing to forego the K band. Such a camera would be sensitive for a week or more to isotropic emission from r-process kilonova ejecta similar to that observed in GW170817, over the full 190 Mpc horizon of Advanced LIGO's design sensitivity for neutron star mergers.Comment: 13 pages, 12 figures, submitted to A

A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars

We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10-year observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coude Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by approx. 0.25 dex for red giants and approx. 0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] < -3.5, 84 stars with [Fe/H] < -3.0, and 210 stars with [Fe/H] < -2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] < -2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.Comment: Accepted for publication in the Astronomical Journal. 60 pages, 59 figures, 18 tables. Machine-readable versions of the long tables can be found in the ancillary data file