John T. Rayner

SpeX: A Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager for the NASA Infrared Telescope Facility

We present the design, construction, and performance of SpeX, a medium-resolution 0.8-5.5 mm cryogenic spectrograph and imager, now in operation at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The design uses prism cross-dispersers and gratings to provide resolving powers up to R ∼ 2000 simultaneously across 0.8-2.4, 1.9-4.2, or 2.4-5.5 mm, with a 15 long slit. A high-throughput low-resolution prism mode is also provided for faint-object and occultation spectroscopy. Single-order 60 long-slit R ∼ 200 modes with resolving powers up to are available for extended objects. The spectrograph employs an R ∼ 2000

Spextool: A Spectral Extraction Package for SpeX, a 0.8-5.5 Micron Cross-Dispersed Spectrograph

ABSTRACT We describe an IDL‐based package for the reduction of spectral data obtained with SpeX, a medium‐resolution, 0.8–5.5 μm cross‐dispersed spectrograph and imager for the NASA Infrared Telescope Facility. The package, called Spextool, carries out all the procedures necessary to produce fully reduced spectra including preparation of calibration frames, processing and extraction of spectra from science frames, wavelength calibration of spectra, and flux calibration of spectra. The package incorporates an “optimal extraction” algorithm for point‐source data and also generates realistic error arrays associated with the extracted spectra. Because it is fairly quick and easy to use, requiring minimal user interaction, Spextool can be run by observers at the telescope to estimate the signal‐to‐noise ratio of their data. We describe the procedures incorporated into Spextool and show examples of extracted spectra.

A method of correcting near-infrared spectra for telluric absorption

We present a method for correcting near-infrared medium-resolution spectra for telluric absorption. The method makes use of a spectrum of an A0 V star, observed near in time and close in air mass to the target object, and a high-resolution model of Vega, to construct a telluric correction spectrum that is free of stellar absorption features. The technique was designed specifically to perform telluric corrections on spectra obtained with SpeX, a 0.8-5.5 mm medium-resolution cross-dispersed spectrograph at the NASA Infrared Telescope Facility, and uses the fact that for medium resolutions there exist spectral regions uncontaminated by atmospheric absorption lines. However, it is also applicable (in a somewhat modified form) to spectra obtained with other near-infrared spectrographs. An IDL-based code that carries out the procedures is available for downloading via the World Wide Web.

THE INFRARED TELESCOPE FACILITY (IRTF) SPECTRAL LIBRARY: COOL STARS

We present a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using the Two Micron All Sky Survey photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry. The library is available in digital form from the IRTF Web site.

Atmospheric Parameters of Field L and T Dwarfs

We present an analysis of the 0.95-14.5 ?m spectral energy distributions of nine field ultracool dwarfs with spectral types ranging from L1 to T4.5. Effective temperatures, gravities, and condensate cloud sedimentation efficiencies are derived by comparing the data to synthetic spectra computed from atmospheric models that self-consistently include the formation of condensate clouds. Overall, the model spectra fit the data well, although the agreement at some wavelengths remains poor due to remaining inadequacies in the models. Derived effective temperatures decrease steadily through the L1-T4.5 spectral types, and we confirm that the effective temperatures of ultracool dwarfs at the L/T transition are nearly constant, decreasing by only ~200 K from spectral types L7.5 to T4.5. The condensate cloud properties vary significantly among the L dwarfs in our sample, ranging from very thick clouds to relatively thin clouds with no particular trend with spectral type. The two objects in our sample with very red -->J ? Ks colors are, however, best fitted with synthetic spectra that have thick clouds, which hints at a possible correlation between the near-infrared colors of L dwarfs and the condensate cloud properties. The fits to the two T dwarfs in our sample (T2 and T4.5) also suggest that the clouds become thinner in this spectral class, in agreement with previous studies. Restricting the fits to narrower wavelength ranges (i.e., individual photometric bands) almost always yields excellent agreement between the data and models. Limitations in our knowledge of the opacities of key absorbers such as FeH, VO, and CH4 at certain wavelengths remain obvious, however. The effective temperatures obtained by fitting the narrower wavelength ranges can show a large scatter compared to the values derived by fitting the full spectral energy distributions; deviations are typically ~200 K and, in the worst cases, up to 700 K.

Spectral Energy Distributions for Disk and Halo M Dwarfs

We have obtained infrared (1-2.5 μm) spectroscopy for 42 halo and disk dwarfs with spectral types M1-M6.5. These data are compared to synthetic spectra generated by the latest model atmospheres of Allard & Hauschildt. Photospheric parameters metallicity, effective temperature, and radius are determined for the sample.We find good agreement between observation and theory except for known problems due to incomplete molecular data for metal hydrides and H2O. The metal-poor M subdwarfs are well matched by the models, as oxide opacity sources are less important in this case. The derived effective temperatures for the sample range from 3600 to 2600 K; at these temperatures grain formation and extinction are not significant in the photosphere. The derived metallicities range from solar to 1/10 solar. The radii and effective temperatures derived agree well with recent models of low-mass stars. The spectra are available in electronic form upon request.

IRCS : Infrared Camera and Spectrograph for the Subaru Telescope

A 1-5 micrometers IR camera and spectrograph (IRCS) is described. The IRCS will be a facility instrument for the 8.2 m Subaru Telescope at Mauna Kea. It consists of two sections, a spectrograph and a camera section. The spectrograph is a cross-dispersed echelle that will provide a resolving power of 20,000 with a slit width of 0.15 arcsec and two-pixel sampling. The camera section serves as a slit viewer and as a camera with two pixel scales, 0.022 arcsec/pixel and 0.060 arcsec/pixel. Grisms providing 400-1400 resolving power will be available. Each section will utilize an ALADDIN II 1024 X 1024 InSb array. The instrument specifications are optimized for 2.2 micrometers using the adaptive optics and the tip-tilt secondary systems of the Subaru Telescope.

A symmetrically pulsed jet of gas from an invisible protostar in Orion

Young stars are thought to accumulate most of their mass through an accretion disk, which channels the gas and dust of a collapsing cloud onto the central protostellar object. The rotational and magnetic forces in the star–disk system often produce high-velocity jets of outflowing gas. These jets can in principle be used to study the accretion and ejection history of the system, which is hidden from direct view by the dust and dense gas of the parent cloud. But the structures of these jets are often too complex to determine which features arise at the source and which are the result of subsequent interactions with the surrounding gas. Here we present infrared observations of a very young jet driven by an invisible protostar in the vicinity of the Horsehead nebula in Orion. These observations reveal a sequence of geyser-like eruptions occurring at quasi-regular intervals and with near-perfect mirror symmetry either side of the source. This symmetry is strong evidence that such features must be associated with the formation of the jet, probably related to recurrent or even chaotic instabilities in the accretion disk.

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 ∼1,215 km. 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.

Physical Parameters of Two Very Cool T Dwarfs

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 μm spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 μm spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log Lbol/L☉ = -5.67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g = 5.00-5.37, M = 33-58 MJup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H] ~ 0.3), and log Lbol/L☉ = -5.31, Teff = 850-950 K, log g = 4.80-5.42, M = 25-66 MJup, and age = 1-10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies that are most likely due to the incomplete CH4 opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed.