Craig W. McMurtry

Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C

Condensate clouds are a salient feature of L dwarf atmospheres, but have been assumed to play little role in shaping the spectra of the coldest T-type brown dwarfs. Here we report evidence of condensate opacity in the near-infrared spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data verify the low-temperature nature of this source, indicating a T8 spectral classification, log10 L bol/L ? = ?5.62 ? 0.03, T eff = 650 ? 25 K, and a mass at or below the deuterium burning limit. The data also reveal enhanced emission at the K band associated with youth (low surface gravity) and supersolar metallicity, reflecting the properties of the Ross 458 system (age = 150-800?Myr, [Fe/H] = +0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C, we confirm a low surface gravity and supersolar metallicity, while the temperature differs depending on the presence (635+25 ?35 K) or absence (760+70 ?45 K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar temperatures (595+25 ?45 K), but distinct surface gravities (log g = 4.0-4.5 cgs versus 5.0-5.5 cgs) and metallicities ([M/H] +0.2 versus ?0.2). In all three cases, cloudy models provide better fits to the spectral data, significantly so for Ross 458C. These results indicate that clouds are an important opacity source in the spectra of young cold T?dwarfs and should be considered when characterizing planetary-mass objects in young clusters and directly imaged exoplanets. The characteristics of Ross 458C suggest that it could itself be regarded as a planet, albeit one whose cosmogony does not conform with current planet formation theories.

Constraints on the universal C IV mass density at z ∼ 6 from early infrared spectra obtained with the Magellan fire spectrograph

We present a new determination of the intergalactic C IV mass density at 4.3 5.8 obtained with the newly commissioned Folded-Port Infrared Echellette (FIRE) spectrograph on the Magellan Baade telescope, coupled with six observations of northern objects taken from the literature. We confirm the presence of a downturn in the C IV abundance at (z) = 5.66 by a factor of 4.1 relative to its value at (z) = 4.96, as measured in the same sight lines. In the FIRE sample, a strong system previously reported in the literature as C IV at z = 5.82 is re-identified as Mg II at z = 2.78, leading to a substantial downward revision in {Omega}{sub Civ} for these prior studies. Additionally, we confirm the presence of at least two systems with low-ionization C II, Si II, and O I absorption but relatively weak signal from C IV. The latter systems may be of interest if the downward trend in {Omega}{sub Civ} at high redshift is driven in part by ionization effects.

FIRE: a near-infrared cross-dispersed echellette spectrometer for the Magellan telescopes

FIRE (the Folded-port InfraRed Echellette) is a prism cross-dispersed infrared spectrometer, designed to deliver singleobject R=6000 spectra over the 0.8-2.5 micron range, simultaneously. It will be installed at one of the auxiliary Nasmyth foci of the Magellan 6.5-meter telescopes. FIRE employs a network of ZnSe and Infrasil prisms, coupled with an R1 reflection grating, to image 21 diffraction orders onto a 2048 × 2048, HAWAII-2RG focal plane array. Optionally, a user-controlled turret may be rotated to replace the reflection grating with a mirror, resulting in a singleorder, longslit spectrum with R ~ 1000. A separate, cold infrared sensor will be used for object acquisition and guiding. Both detectors will be controlled by cryogenically mounted SIDECAR ASICs. The availability of low-noise detectors motivates our choice of spectral resolution, which was expressly optimized for Magellan by balancing the scientific demand for increased R with practical limits on exposure times (taking into account statistics on seeing conditions). This contribution describes that analysis, as well as FIREs optical and opto-mechanical design, and the design and implementation of cryogenic mechanisms. Finally, we will discuss our data-flow model, and outline strategies we are putting in place to facilitate data reduction and analysis.

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC.

The FIRE infrared spectrometer at Magellan: construction and commissioning

We describe the construction and commissioning of FIRE, a new 0.8-2.5μm echelle spectrometer for the Magellan/ Baade 6.5 meter telescope. FIRE delivers continuous spectra over its full bandpass with nominal spectral resolution R = 6000. Additionally it offers a longslit mode dispersed by the prisms alone, covering the full z to K bands at R ~ 350. FIRE was installed at Magellan in March 2010 and is now performing shared-risk science observations. It is delivering sharp image quality and its throughput is sufficient to allow early observations of high redshift quasars and faint brown dwarfs. This paper outlines several of the new or unique design choices we employed in FIREs construction, as well as early returns from its on-sky performance.

Development of sensitive long-wave infrared detector arrays for passively cooled space missions

Abstract. The near-earth object camera (NEOCam) is a proposed infrared space mission designed to discover and characterize most of the potentially hazardous asteroids larger than 140 m in diameter that orbit near the Earth. NASA has funded technology development for NEOCam, including the development of long wavelength infrared detector arrays that will have excellent zodiacal background emission-limited performance at passively cooled focal plane temperatures. Teledyne Imaging Sensors has developed and delivered for test at the University of Rochester the first set of approximately 10 μm cutoff, 1024×1024 pixel HgCdTe detector arrays. Measurements of these arrays show the development to be extremely promising: noise, dark current, quantum efficiency, and well depth goals have been met by this technology at focal plane temperatures of 35 to 40 K, readily attainable with passive cooling. The next set of arrays to be developed will address changes suggested by the first set of deliverables.

2Kx2K InSb for astronomy

Raytheon Vision Systems is under contract to develop 2K × 2K InSb Focal Plane Arrays (FPA) for the ORION and NEWFIRM projects teaming with NOAO, NASA, and USNO. This paper reviews the progress in the ORION, NEWFIRM, and the JWST projects, showing bare mux readout noise at 30 K of 2.4 e- and InSb dark current as low as 0.01 e-/s. Several FPAs have been fabricated to date and the ongoing improvements for the fabrication of FPAs will be discussed. The FPA and packaging designs are complete, resulting in a design that has self-aligning features for ease in FPA replacement at position of the focal plane assembly with alignment accuracy in the focus direction of ± 12 μm. The ORION/NEWFIRM modules are 2-side buttable to easily form 4K × 4K mosaics while the Phoenix modules, developed under the JWST development program, are 3-side buttable for ease in forming 4K × 2NK mosaics where N can be any integer. This paper will include FPA QE, dark current and noise performance, FPA reliability, and module-to-module flatness capabilities.

Burst noise in the HAWAII-1RG multiplexer

Burst noise (also known as popcorn noise and random telegraph signal/noise) is a phenomenon that is understood to be a result of defects in the vicinity of a p-n junction. It is characterized by rapid level shifts in both positive and negative directions and can have varying magnitudes. This noise has been seen in both HAWAII-1RG and HAWAII-2RG multiplexers and is under investigation. We have done extensive burst noise testing on a HAWAII-1RG multiplexer, where we have determined a significant percentage of pixels exhibit the phenomenon. In addition, the prevalence of small magnitude transitions make sensitivity of detection the main limiting factor. Since this is a noise source for the HAWAII-1RG multiplexer, its elimination would make the HAWAII-1RG and the HAWAII-2RG even lower noise multiplexers.

Spitzer space telescope: dark current and total noise prediction for InSb detector arrays in the infrared array camera (IRAC) for the post-cryogen era

During the expected 5+ years of operation, the Spitzer Space Telescope is and will continue to produce outstanding infrared images and spectra, and greatly further scientific understanding of our universe. The Spitzer Space Telescopes instruments are cryogenically cooled to achieve low dark current and low noise. After the cryogens are exhausted, the Spitzer Space Telescope will only be cooled by passively radiating into space. The detector arrays in the IRAC instrument are expected to equilibrate at approximately 30K. The two shortest wavelength channels (3.6 and 4.5 micron) employ InSb detector arrays and are expected to function and perform with only a modest degradation in sensitivity. Thus, an extended mission is possible for Spitzer. We present the predicted dark current, noise, quantum efficiency and image residuals for the 3.6 and 4.5 micron IRAC channels in the post-cryogen era.

James Webb Space Telescope: characterization of flight candidate NIR InSb arrays

The James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope, will draw on recent improvements in infrared array technologies to achieve its goals and mission. In order to best meet the goals of JWST, NASA is funding a competition between two near infrared detector technologies: InSb detector arrays from Raytheon Vision Systems and HgCdTe detector arrays from Rockwell Scientific. The University of Rochester, in collaboration with Raytheon, is testing near infrared InSb detectors in a 2048 x 2048 array format to meet the stringent requirements for JWST. Results from characterization under top level requirements, such as noise, quantum efficiency, well capacity, pixel operability, etc., are discussed. Dark current and its contribution to the total noise are analyzed.