arXiv subjects

Kuenley Chiu

Publications and source records attributed to Kuenley Chiu.

24 records · Page 2Linked to original sources

arXiv2007

The Late-T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

(Abridged) We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5+/-0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

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arXiv2006

The Optical and Near-Infrared Properties of 2837 Quasars in the UKIRT Infrared Deep Sky Survey (UKIDSS)

The UKIRT Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of hemispheric imaging projects to extend the work of the Two Micron All Sky Survey (2MASS) by reaching three magnitudes deeper in YJHK imaging, to K=18.2 (5-sigma, Vega) over wide fields. Better complementing existing optical surveys such as the Sloan Digital Sky Survey (SDSS), the resulting public imaging catalogues provide new photometry of rare object samples too faint to be reached previously. The first data release of UKIDSS has already surpassed 2MASS in terms of photons gathered, and using this new dataset we examine the near-infrared properties of 2837 quasars found in the SDSS and newly catalogued by the UKIDSS in ~189 square degrees. The matched quasars include the RA range 22hr to 4hr on the Southern Equatorial Stripe (SDSS Stripe 82), an area of significant future followup possibilities with deeper surveys and pointed observations. The sample covers the redshift and absolute magnitude ranges 0.08<z<5.03 and -29.5<M_i<-22.0, and 98 per cent of SDSS quasars have matching UKIDSS data. We discuss the photometry, astrometry, and various colour properties of the quasars. We also examine the effectiveness of quasar/star separation using the near-infrared passbands. The combination of SDSS ugriz photometry with the YJHK near-infrared photometry from UKIDSS over large areas of sky has enormous potential for advancing our understanding of the quasar population.

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arXiv2006

The Stellar Mass Density at z~6 from Spitzer Imaging of i-drop Galaxies

We measure the ages, stellar masses, and star formation histories of z~6 galaxies, observed within 1Gyr of the Big Bang. We use imaging from HST and Spitzer from the public "Great Observatories Origins Deep Survey", coupled with ground-based near-infrared imaging, to measure the spectral energy distributions from 0.8-5microns, spanning the rest-frame UV and optical. From our sample of ~50 i-drop Lyman-break star-forming galaxies in GOODS-South with z'(AB)<27mag, we focus on ~30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40%) have evidence for substantial Balmer-breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of 200-700Myr, implying formation redshifts of 7 7 could have played a major role in reionizing the universe.

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arXiv2006

SDSS J1534+1615AB: A Novel T Dwarf Binary Found with Keck Laser Guide Star Adaptive Optics and the Potential Role of Binarity in the L/T Transition

We have resolved the newly discovered T dwarf SDSS J1534+1615 into a 0.11'' binary using the Keck sodium laser guide star adaptive optics system. With an integrated-light spectral type of T3.5, this binary provides a new benchmark for studying the distinctive J-band brightening previously noted among early and mid-T dwarfs, using two brown dwarfs with different spectral types but having a common metallicity and age and very similar surface gravities. We estimate spectral types of T1.5+/-0.5 and T5.5+/-0.5 for the two components based on their near-IR colors, consistent with modeling the integrated-light spectrum as the blend of two components. The observed near-IR flux ratios are unique compared to all previously known substellar binaries: the component that is fainter at H and K' is brighter at J. This inversion of the near-IR fluxes is a manifestation of the J-band brightening within this individual binary system. Therefore, SDSS 1534+1615 demonstrates that the brightening can be intrinsic to ultracool photospheres (e.g., arising from cloud disruption and/or rapid increase in cloud sedimentation) and does not necessarily result from physical variations among the observed ensemble of T dwarfs (e.g., a range in masses, ages and/or metallicities). We suggest that the apparently large amplitude of the J-band brightening may be due to a high incidence of unresolved binaries and that the true amplitude of the phenomenon could be more modest. This scenario would imply that truly single objects in these spectral subclasses are relatively rare, in agreement with the small effective temperature range inferred for the L/T transition.

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arXiv2005

The Color Selection of Quasars from Redshifts 5 to 10: Cloning and Discovery

We present simulations of quasar colors, magnitudes, and numbers at redshifts 5 6 epoch taking advantage of the new UKIDSS near-infrared filter set, and we examine the redshift intervals of maximum productivity, discussing color selection and survey depth issues. On the basis of the SDSS sample, we find that the surface density of z>4.7 quasars increases by a factor of 3X by extending 0.7 i-magnitudes deeper than the SDSS spectroscopic survey limit of i=20.2 -- correspondingly we predict a total of ~400 faint quasars in the SDSS main area that have redshift z>4.7 and magnitudes $<20.9.

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arXiv2005

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

We report the discovery of a fifth image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0.2'' from the center of the brightest galaxy in the lensing cluster is detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves and thereby the mass distribution of the lensing cluster.

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