arXiv subjects

A. Mainzer

Publications and source records attributed to A. Mainzer.

34 records · Page 2Linked to original sources

arXiv2012

Characterizing Subpopulations within the Near Earth Objects with NEOWISE: Preliminary Results

We present the preliminary results of an analysis of the sub-populations within the near-Earth asteroids, including the Atens, Apollos, Amors, and those that are considered potentially hazardous using data from the Wide-field Infrared Survey Explorer (WISE). In order to extrapolate the sample of objects detected by WISE to the greater population, we determined the survey biases for asteroids detected by the project's automated moving object processing system (known as NEOWISE) as a function of diameter, visible albedo, and orbital elements. Using this technique, we are able to place constraints on the number of potentially hazardous asteroids (PHAs) larger than 100 m and find that there are $\sim4700\pm1450$ such objects. As expected, the Atens, Apollos, and Amors are revealed by WISE to have somewhat different albedo distributions, with the Atens being brighter than the Amors. The cumulative size distributions of the various near-Earth object (NEO) subgroups vary slightly between 100 m and 1 km. A comparison of the observed orbital elements of the various sub-populations of the NEOs with the current best model is shown.

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arXiv2011

NEOWISE Studies of Asteroids with Sloan Photometry: Preliminary Results

We have combined the NEOWISE and Sloan Digital Sky Survey data to study the albedos of 24,353 asteroids with candidate taxonomic classifications derived using Sloan photometry. We find a wide range of moderate to high albedos for candidate S-type asteroids that are analogous to the S-complex defined by previous spectrophotometrically-based taxonomic systems. The candidate C-type asteroids, while generally very dark, have a tail of higher albedos that overlaps the S types. The albedo distribution for asteroids with a photometrically derived Q classification is extremely similar to those of the S types. Asteroids with similar colors to (4) Vesta have higher albedos than the S types, and most have orbital elements similar to known Vesta family members. Finally, we show that the relative reflectance at 3.4 and 4.6 $μ$m is higher for D-type asteroids and suggest that their red visible and near-infrared spectral slope extends out to these wavelengths. Understanding the relationship between size, albedo, and taxonomic classification is complicated by the fact that the objects with classifications were selected from the visible/near-infrared Sloan Moving Object Catalog, which is biased against fainter asteroids, including those with lower albedos.

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arXiv2011

NEOWISE Studies of Spectrophotometrically Classified Asteroids: Preliminary Results

The NEOWISE dataset offers the opportunity to study the variations in albedo for asteroid classification schemes based on visible and near-infrared observations for a large sample of minor planets. We have determined the albedos for nearly 1900 asteroids classified by the Tholen, Bus and Bus-DeMeo taxonomic classification schemes. We find that the S-complex spans a broad range of bright albedos, partially overlapping the low albedo C-complex at small sizes. As expected, the X-complex covers a wide range of albedos. The multi-wavelength infrared coverage provided by NEOWISE allows determination of the reflectivity at 3.4 and 4.6 $μ$m relative to the visible albedo. The direct computation of the reflectivity at 3.4 and 4.6 $μ$m enables a new means of comparing the various taxonomic classes. Although C, B, D and T asteroids all have similarly low visible albedos, the D and T types can be distinguished from the C and B types by examining their relative reflectance at 3.4 and 4.6 $μ$m. All of the albedo distributions are strongly affected by selection biases against small, low albedo objects, as all objects selected for taxonomic classification were chosen according to their visible light brightness. Due to these strong selection biases, we are unable to determine whether or not there are correlations between size, albedo and space weathering. We argue that the current set of classified asteroids makes any such correlations difficult to verify. A sample of taxonomically classified asteroids drawn without significant albedo bias is needed in order to perform such an analysis.

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arXiv2011

NEOWISE Observations of Near-Earth Objects: Preliminary Results

With the NEOWISE portion of the \emph{Wide-field Infrared Survey Explorer} (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 $μ$m, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniformity in sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981$\pm$19 NEAs larger than 1 km and 20,500$\pm$3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32$\pm$0.14 below 1.5 km. This power law slope produces $\sim13,200\pm$1,900 NEAs with $D>$140 m. Although previous studies predict another break in the cumulative size distribution below $D\sim$50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100-1000 m are lower than previous estimates. The numbers of near-Earth comets will be the subject of future work.

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arXiv2011

A variable mid-infrared synchrotron break associated with the compact jet in GX 339-4

Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Wide-field Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically-thin slope of ~-0.7 and a jet radiative power of >6x10^{35} erg/s (d/8 kpc)^2 are measured. A ~24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ~95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of ~3-22 microns that WISE is sensitive to, and more. Based on four-band simultaneous mid-infrared detections, the break lies at ~5x10^{13} Hz in at least two epochs of observation, consistent with a magnetic field B~1.5x10^4 G assuming a single-zone synchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of ~10 on relatively-short timescales.

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arXiv2011

Thermal Model Calibration for Minor Planets Observed with WISE/NEOWISE: Comparison with IRAS

With thermal infrared observations detected by the NEOWISE project, we have measured diameters for 1742 minor planets that were also observed by the Infrared Astronomical Satellite (IRAS). We have compared the diameters and albedo derived by applying a spherical thermal model to the objects detected by NEOWISE and find that they are in generally good agreement with the IRAS values. We have shown that diameters computed from NEOWISE data are often less systematically biased than those found with IRAS. This demonstrates that the NEOWISE dataset can provide accurate physical parameters for the >157,000 minor planets that were detected by NEOWISE.

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arXiv2011

New Young Star Candidates in the Taurus-Auriga Region as Selected From WISE

The Taurus Molecular Cloud subtends a large solid angle on the sky, in excess of 250 square degrees. The search for legitimate Taurus members to date has been limited by sky coverage as well as the challenge of distinguishing members from field interlopers. The Wide-field Infrared Survey Explorer (WISE) has recently observed the entire sky, and we take advantage of the opportunity to search for young stellar object (YSO) candidate Taurus members from a ~260 square degree region designed to encompass previously-identified Taurus members. We use near- and mid-infrared colors to select objects with apparent infrared excesses and incorporate other catalogs of ancillary data to present: a list of rediscovered Taurus YSOs with infrared excesses (taken to be due to circumstellar disks), a list of rejected YSO candidates (largely galaxies), and a list of 94 surviving candidate new YSO-like Taurus members. There is likely to be contamination lingering in this candidate list, and follow-up spectra are warranted.

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arXiv2011

Thermal Model Calibration for Minor Planets Observed with WISE/NEOWISE

With the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010), we have observed over 157,000 minor planets (Mainzer et al. 2011). Included in these are a number of near-Earth objects, Main Belt Asteroids, and irregular satellites which have well-measured physical properties via radar, occultation and in situ imaging. We have used these objects to validate models of thermal models using the WISE measurements, as well as the color corrections derived in Wright et al. (2010) for the four WISE bandpasses as a function of effective temperature. We have used 50 objects with diameters measured by radar, occultation or in situ imaging to characterize the systematic errors implicit in using the WISE data with a faceted spherical NEATM model to compute diameters and albedos. By using the previously measured diameters and H magnitudes with a spherical NEATM model, we compute the predicted fluxes after applying the color corrections given in Wright et al. (2010) in the WISE bands and compare them to the measured magnitudes. We find minimum systematic flux errors of 5-10%, yielding minimum relative diameter and albedo errors of ~10% and ~20%, respectively. Visible albedos for the objects are computed and compared to the albedos at 3.4 and 4.6 microns, which contain a mix of reflected sunlight and thermal emission for most asteroids. We derive a linear relationship between subsolar temperature and effective temperature, which allows the color corrections given in Wright et al. (2010) to be used for asteroids by computing only subsolar temperature instead of a faceted thermal model. The thermal models derived in this paper are not intended to supplant previous measurements made using radar or spacecraft imaging; rather, we have used them to characterize the errors that should be expected when computing diameters and albedos of WISE asteroids using a spherical NEATM model.

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arXiv2011

FIRE Spectroscopy of Five Late-type T Dwarfs Discovered with the Wide-field Infrared Survey Explorer

We present the discovery of five late-type T dwarfs identified with the Wide-field Infrared Survey Explorer (WISE). Low-resolution near-infrared spectroscopy obtained with the Magellan Folded-port InfraRed Echellette (FIRE) reveal strong water and methane absorption in all five sources, and spectral indices and comparison to spectral templates indicate classifications ranging from T5.5 to T8.5:. The spectrum of the latest-type source, WISE J1812+2721, is an excellent match to that of the T8.5 companion brown dwarf Wolf 940B. WISE-based spectrophotometric distance estimates place these T dwarfs at 12-13 pc from the Sun, assuming they are single. Preliminary fits of the spectral data to the atmosphere models of Saumon & Marley indicate effective temperatures ranging from 600 K to 930 K, both cloudy and cloud-free atmospheres, and a broad range of ages and masses. In particular, two sources show evidence of both low surface gravity and cloudy atmospheres, tentatively supporting a trend noted in other young brown dwarfs and exoplanets. In contrast, the high proper motion T dwarf WISE J2018-7423 exhibits a suppressed K-band peak and blue spectrophotometric J-K colors indicative of an old, massive brown dwarf; however, it lacks the broadened Y-band peak seen in metal-poor counterparts. These results illustrate the broad diversity of low-temperature brown dwarfs that will be uncovered with WISE.

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arXiv2011

Preliminary Results from NEOWISE: An Enhancement to the Wide-field Infrared Survey Explorer for Solar System Science

The \emph{Wide-field Infrared Survey Explorer} has surveyed the entire sky at four infrared wavelengths with greatly improved sensitivity and spatial resolution compared to its predecessors, the \emph{Infrared Astronomical Satellite} and the \emph{Cosmic Background Explorer}. NASA's Planetary Science Division has funded an enhancement to the \WISE\ data processing system called "NEOWISE" that allows detection and archiving of moving objects found in the \WISE\ data. NEOWISE has mined the \WISE\ images for a wide array of small bodies in our Solar System, including Near-Earth Objects (NEOs), Main Belt asteroids, comets, Trojans, and Centaurs. By the end of survey operations in February 2011, NEOWISE identified over 157,000 asteroids, including more than 500 NEOs and $\sim$120 comets. The NEOWISE dataset will enable a panoply of new scientific investigations.

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arXiv2011

ExploreNEOs III: Physical characterization of 65 low-deltaV NEOs

Space missions to NEOs are being planned at all major space agencies, and recently a manned mission to an NEO was announced as a NASA goal. Efforts to find and select suitable targets (plus backup targets) are severely hampered by our lack of knowledge of the physical properties of dynamically favorable NEOs. In particular, current mission scenarios tend to favor primitive low-albedo objects. For the vast majority of NEOs the albedo is unknown. Here we report new constraints on the size and albedo of 65 NEOs with rendezvous deltaV < 7 km/s. Our results are based on thermal-IR flux data obtained in the framework of our ongoing (2009--2011) ExploreNEOs survey (Trilling et al. 2010) using NASA's "Warm Spitzer" space telescope. As of 2010 July 14, we have results for 293 objects in hand (including the 65 low-deltaV NEOs presented here); before the end of 2011 we expect to have measured the size and albedo of ~ 700 NEOs (including probably ~ 160 low-deltaV NEOs). While there are reasons to believe that primitive volatile-rich materials are universally low in albedo, the converse need not be true: the orbital evolution of some dark objects likely has caused them to lose their volatiles by coming too close to the Sun. For all our targets, we give the closest perihelion distance they are likely to have reached (using orbital integrations from Marchi et al. 2009) and corresponding upper limits on the past surface temperature. Low-deltaV objects for which both albedo and thermal history may suggest a primitive composition include (162998) 2001 SK162, (68372) 2001 PM9, and (100085) 1992 UY4.

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arXiv2010

The First Ultra-Cool Brown Dwarf Discovered by the Wide-field Infrared Survey Explorer

We report the discovery of the first new ultra-cool brown dwarf found with the Wide-field Infrared Survey Explorer (WISE). The object's preliminary designation is WISEPC J045853.90+643451.9. Follow-up spectroscopy with the LUCIFER instrument on the Large Binocular Telescope indicates that it is a very late-type T dwarf with a spectral type approximately equal to T9. Fits to an IRTF/SpeX 0.8-2.5 micron spectrum to the model atmospheres of Marley and Saumon indicate an effective temperature of approximately 600 K as well as the presence of vertical mixing in its atmosphere. The new brown dwarf is easily detected by WISE, with a signal-to-noise ratio of ~36 at 4.6 microns. Current estimates place it at a distance of 6 to 10 pc. This object represents the first in what will likely be hundreds of nearby brown dwarfs found by WISE that will be suitable for follow up observations, including those with the James Webb Space Telescope. One of the two primary scientific goals of the WISE mission is to find the coolest, closest stars to our Sun; the discovery of this new brown dwarf proves that WISE is capable of fulfilling this objective.

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arXiv2010

ExploreNEOs I: Description and first results from the Warm Spitzer NEO Survey

We have begun the ExploreNEOs project in which we observe some 700 Near Earth Objects (NEOs) at 3.6 and 4.5 microns with the Spitzer Space Telescope in its Warm Spitzer mode. From these measurements and catalog optical photometry we derive albedos and diameters of the observed targets. The overall goal of our ExploreNEOs program is to study the history of near-Earth space by deriving the physical properties of a large number of NEOs. In this paper we describe both the scientific and technical construction of our ExploreNEOs program. We present our observational, photometric, and thermal modeling techniques. We present results from the first 101 targets observed in this program. We find that the distribution of albedos in this first sample is quite broad, probably indicating a wide range of compositions within the NEO population. Many objects smaller than one kilometer have high albedos (>0.35), but few objects larger than one kilometer have high albedos. This result is consistent with the idea that these larger objects are collisionally older, and therefore possess surfaces that are more space weathered and therefore darker, or are not subject to other surface rejuvenating events as frequently as smaller NEOs.

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arXiv2010

Mid-Infrared Variability from the Spitzer Deep, Wide-Field Survey

We use the multi-epoch, mid-infrared Spitzer Deep, Wide-Field Survey to investigate the variability of 474,179 objects in 8.1 deg^2 of the NDWFS Bootes field. We perform a Difference Image Analysis of the four available epochs between 2004 and 2008, focusing on the deeper 3.6 and 4.5 micron bands. We find that 1.1% of the studied sample meet our standard selection criteria for being classed as a variable source. We require that the 3.6 and 4.5 micron light-curves are strongly correlated (r>0.8) and that their joint variance exceeds that for all sources with the same magnitude by 2 sigma. We then examine the mid-IR colors of the variable sources and match them with X-ray sources from the XBootes survey, radio catalogs, 24 micron-selected AGN candidates, and spectroscopically identified AGNs from the AGN and Galaxy Evolution Survey (AGES). Based on their mid-IR colors, most of the variable sources are AGNs (76%), with smaller contributions from stars (11%), galaxies (6%), and unclassified objects. Most of the stellar, galaxy and unclassified sources are false positives. For our standard selection criteria, 11-12% of the mid-IR counterparts to X-ray sources, 24 micron-selected AGN candidates and spectroscopically identified AGNs show variability. Mid-IR AGN variability can be well described by a single power-law structure function with a power-law index of 0.5 at both 3.6 and 4.5 microns, and an amplitude of 0.1 mag on rest-frame time scales of 2 years. The variability amplitude is higher for shorter rest-frame wavelengths and lower luminosities. (Abridged)

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arXiv2009

The Spitzer Deep, Wide-Field Survey

The Spitzer Deep, Wide-Field Survey (SDWFS) is a four-epoch infrared survey of ten square degrees in the Bootes field of the NOAO Deep Wide-Field Survey using the IRAC instrument on the Spitzer Space Telescope. SDWFS, a Cycle four Spitzer Legacy project, occupies a unique position in the area-depth survey space defined by other Spitzer surveys. The four epochs that make up SDWFS permit -- for the first time -- the selection of infrared-variable and high proper motion objects over a wide field on timescales of years. Because of its large survey volume, SDWFS is sensitive to galaxies out to z~3 with relatively little impact from cosmic variance for all but the richest systems. The SDWFS datasets will thus be especially useful for characterizing galaxy evolution beyond z~1.5. This paper explains the SDWFS observing strategy and data processing, presents the SDWFS mosaics and source catalogs, and discusses some early scientific findings. The publicly-released, full-depth catalogs contain 6.78, 5.23, 1.20, and 0.96 x 10e5 distinct sources detected to the average 5-sigma, 4" diameter, aperture-corrected limits of 19.77, 18.83, 16.50, and 15.82 Vega mag at 3.6, 4.5, 5.8, and 8.0 micron, respectively. The SDWFS number counts and color-color distribution are consistent with other, earlier Spitzer surveys. At the 6 min integration time of the SDWFS IRAC imaging, more than 50% of isolated FIRST radio sources and more than 80% of on-axis XBootes sources are detected out to 8.0 micron. Finally, we present the four highest proper motion IRAC-selected sources identified from the multi-epoch imaging, two of which are likely field brown dwarfs of mid-T spectral class.

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arXiv2008

Spitzer-MIPS Observations of the eta Cha Young Association

We have mapped the eta Chamaeleontis young stellar association in the far-infrared with the Multiband Imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope. All sixteen members within the map region were detected at 24 um, along with five members at 70 um and two at 160 um. Ten stars show far-infrared excess emission indicating the presence of circumstellar disks; six of these have central clearings as evidenced by the onset of excess emission at >5 um. No new infrared excess sources are identified among the 113 2MASS field stars with 24 um photometry but not seen as X-ray sources, indicating that membership lists derived from X-ray surveys are reasonably complete. Circumstellar disks in the eta Cha association span the range from 1e-1 to 1e-4 in their fractional infrared luminosity, with a median Ldust /Lstar of 0.04. The presence of optically thick, optically thin, and intermediate optical depth disks within the same stellar group, in combination with the large fraction of disks with inner holes, indicates that the Cha association represents a crucial stage in circumstellar disk evolution.

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