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A Dusty Component to the Gaseous Debris Disk Around the White Dwarf SDSS J1228+1040
We present Infrared Spectrometer And Array Camera (ISAAC) spectroscopyand ISAAC, UKIDSS, and Spitzer Space Telescope broadband photometry ofSDSS J1228+1040—a white dwarf for which evidence of a gaseousmetal-rich circumstellar disk has previously been found from opticalemission lines. The data show a clear excess in the near- andmid-infrared (IR), providing compelling evidence for the presence ofdust in addition to the previously identified gaseous debris disk aroundthe star. The IR excess can be modeled in terms of an optically thickbut geometrically thin disk. We find that the inner disk temperaturesmust be relatively high (~1700 K) in order to fit the spectral energydistribution in the near-IR. These data provide the first evidence forthe coexistence of both gas and dust in a disk around a white dwarf, andshow that their presence is possible even around moderately hot (~22,000K) stars.
| Spectral Irradiance Calibration in the Infrared. XIV. The Absolute Calibration of 2MASS
Element by element, we have combined the optical components in the threecameras of the Two Micron All Sky Survey (2MASS), and incorporateddetector quantum efficiency curves and site-specific atmospherictransmissions, to create three relative spectral response curves (RSRs).We provide the absolute 2MASS attributes associated with ``zeromagnitude'' in the JHKs bands so that these RSRs may be usedfor synthetic photometry. The RSRs tie 2MASS to the``Cohen-Walker-Witteborn'' framework of absolute photometry and stellarspectra for the purpose of using 2MASS data to support the developmentof absolute calibrators for the Infrared Array Camera and pairwisecross-calibrators between all three SIRTF instruments. We examine therobustness of these RSRs to changes in water vapor within a night. Wecompare the observed 2MASS magnitudes of 33 stars (converted from theprecision optical calibrators of Landolt and Carter-Meadows intoabsolute infrared calibrators from 1.2 to 35 μm) with ourpredictions, thereby deriving 2MASS ``zero-point offsets'' from theensemble. These offsets are the final ingredients essential to merge2MASS JHKs data with our other absolutely calibrated bandsand stellar spectra, and to support the creation of faint calibrationstars for SIRTF.
| Spectral Irradiance Calibration in the Infrared. XIII. ``Supertemplates'' and On-Orbit Calibrators for the SIRTF Infrared Array Camera
We describe the technique that will be used to develop a set of on-orbitcalibrators for the Infrared Array Camera (IRAC) and demonstrate thevalidity of the method for stars with spectral types either K0-M0 III orA0-A5 V. For application to SIRTF, the approach is intended to operatewith all available optical, near-infrared (NIR), and mid-infrared (MIR)photometry and to yield complete absolute spectra from UV to MIR. Oneset of stars is picked from Landolt's extensive network of optical(UBVRI) calibrators, the other from the Carter-Meadows set of faint IRstandards. Traceability to the ``Cohen-Walker-Witteborn'' framework ofabsolute photometry and stellar spectra is assured. The method is basedon the use of either ``supertemplates,'' which represent the intrinsicshapes of the spectra of K0-M0 III stars from far-UV (1150 Å) toMIR (35 μm) wavelengths, or Kurucz synthetic spectra for A0-A5 Vstars. Each supertemplate or Kurucz model is reddened according to theindividual star's extinction and is normalized using availablecharacterized optical photometry. This paper tests our capability topredict NIR (JHK) magnitudes using supertemplates or models constrainedby Hipparcos/Tycho or precision ground-based optical data. We provideabsolutely calibrated 0.275-35.00 μm spectra of 33 Landolt andCarter-Meadows optical standard stars to demonstrate the viability ofthis technique, and to offer a set of IR calibrators 100-1000 timesfainter than those we have previously published. As an indication ofwhat we can expect for actual IRAC calibration stars, we have calculatedthe absolute uncertainties associated with predicting the IRACmagnitudes for the faintest cool giant and hot dwarf in this new set ofcalibration stars.
| Faint members of the Chamaeleon I cloud
We present a survey of the central ~ 100 arcmin(2) of the Chamaeleon Istar forming cloud, including objective prism spectroscopy in theH? region and deep imaging in the near-infrared. We estimate theexpected number of very low mass objects within the survey, taking as areference the higher mass members identified in previous studies, andassuming different ages and slopes of the initial mass function of theChamaeleon I population. A new approach is introduced to estimate thecontribution of background objects to the counts of low luminositysources. This method takes advantage of the fact that the contributionof Chamaeleon I members should be negligible at the faintest magnitudescovered by our survey for any reasonable shape of the initial massfunction. K-band source counts indicate the absence of a significantpopulation of very low mass stars, implying that the initial massfunction at very low masses, approximated by a power law, has a form Phi({cal M}) d {cal M} ~ {cal M}(-1) d{cal M} or flatter. This conclusionis in qualitative agreement with the discovery of six new emission lineobjects in the objective prism survey, and with the fact that only 2-3faint objects are detected in the region of the (J-H), (H-K) diagramdiagnostic of near infrared excesses of circumstellar origin. The massesof the new emission line objects, derived from recent pre-main sequenceevolutionary tracks, are found to be near, and possibly below, thehydrogen burning limit, and their ages to be younger than 3 x 10(6)years. One of them is found to be a bona-fide brown dwarf, and itsdetection in a deep ROSAT exposure makes it the first, and so far theonly, brown dwarf known to emit X-rays (Neuhäuser &Comerón 1998, Science, 282, 83). The near-infrared properties ofthe H? emission objects suggest that, unlike at higher masses,strong H? emission near the hydrogen-burning limit is notaccompanied by infrared excess detectable in the K band. Comparing thenumbers of very low mass objects expected from K band counts with thenumber of new H? -emitting members, for which we derive individualmasses and ages, we find that the spectroscopic survey samples theinitial mass function completely, or nearly completely, down to thehydrogen-burning limit. Based on observations collected at the EuropeanSouthern Observatory, Chile, program 58.E-0429.
| Linear and circular imaging polarimetry of the Chamaeleon infrared nebula
We present linear and circular imaging polarimetry observations of theChamaeleon infrared nebula, a bipolar reflection nebula in theChamaeleon I dark cloud, at near-infrared (JHK_n) wavelengths. These areamongst the first imaging circular polarimetry results for astar-forming region. The detection of both high degrees of linearpolarization and a significant degree of circular polarization in theextended nebulosity allows us to comment on the scattering geometry andthe range of particle sizes present. We develop a model incorporating apolarized source which can successfully account for the observed linearand circular polarimetry and for the asymmetries in nebular brightness(the `bright rim' structures) seen in this and other objects (e.g., NGC2261/R Mon). In order to do so, the model requires a non-axisymmetricillumination of the nebula, and we discuss possible origins for thisasymmetry, including disruption of a circumstellar disc by binaryprotostars.
| Fainter Southern JHK Standards Suitable for Infrared Arrays
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995MNRAS.276..734C&db_key=AST
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