arXiv subjects

John Stauffer

Publications and source records attributed to John Stauffer.

46 records · Page 3Linked to original sources

arXiv2007

The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer

We provide an overview of the Spitzer Legacy Program ``Formation and Evolution of Planetary Systems" (FEPS) which was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between 2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric observations of solar-type stars. With a sample of ~ 328 stars ranging in age from ~ 3 Myr to ~ 3 Gyr, we trace the evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks through to older collisionally generated debris disks. When completed, our program will help define the time scales over which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a function of time, and establish the diversity of mature planetary architectures. In addition to the observational program, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of circumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic and molecular gas line emission. Together with the observations, these efforts will provide astronomical context for understanding whether our Solar System - and its habitable planet - is a common or a rare circumstance. Additional information about the FEPS project can be found on the team website: feps.as.arizona.edu

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arXiv2006

Star Formation in the Era of the Three Great Observatories

This paper summarizes contributions and suggestions as presented at the Chandra Workshop Star Formation in the Era of Three Great Observatories conducted in July 2005. One of the declared goals of the workshop was to raise recognition within the star formation research community about the sensible future utilization of the space observatories Spitzer, Hubble, and Chandra in their remaining years of operation to tackle imminent questions of our understanding of stellar formation and the early evolution of stars. A white paper was generated to support the continuous and simultaneous usage of observatory time for star formation research. The contents of this paper have been presented and discussed at several other meetings during the course of 2005 and January 2006.

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arXiv2004

Chandra X-ray observations of Young Clusters II. Orion Flanking Fields Data

We present results of Chandra observations of two flanking fields (FF) in Orion, outside the Orion Nebula Cluster (ONC). The observations were taken with the ACIS-I camera with an exposure time of about 48 ks each field. We present a catalog of 417 sources, which includes X-ray luminosity, optical and infrared photometry and X-ray variability information. We have found 91 variable sources, 33 of which have a flare-like light curve, and 11 of which have a pattern of a steady increase or decrease over a 10 hour period. The optical and infrared photometry for the stars identified as X-ray sources are consistent with most of these objects being pre-main sequence stars with ages younger than 10 Myr. We present evidence for an age difference among the X-ray selected samples of NGC 2264, Orion FF, and ONC, with NGC 2264 being the oldest, and ONC being the youngest.

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arXiv2004

Chandra X-ray observations of Young Clusters I. NGC2264 Data

We present results of a Chandra observation of a field in NGC 2264. The observations were taken with the ACIS-I camera with an exposure time of 48.1 ks. We present a catalog of 263 sources, which includes X-ray luminosity, optical and infrared photometry and X-ray variability information. We found 41 variable sources, 14 of which have a flare-like light curve, and 2 of which have a pattern of a steady increase or decrease over a 10 hour period. The optical and infrared photometry for the stars identified as X-ray sources are consistent with most of these objects being pre-main sequence stars with ages younger than 3 Myr.

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arXiv2003

Spectral Irradiance Calibration in the Infrared. XIII. "Supertemplates" and On-Orbit Calibrators for SIRTF's Infrared Array Camera (IRAC)

We describe the technique used to develop on-orbit calibrators for IRAC and validate the method for either K0-M0III or A0-A5V stars. For SIRTF, the approach uses all available optical, NIR, and MIR photometry, and yields absolute spectra from UV to MIR. One set of stars is from Landolt's network of optical standards, the other from Carter-Meadows IR standards. Traceability to the Cohen-Walker- Witteborn framework of absolute photometry and spectra is assured. The method is based on using either "supertemplates" to represent the intrinsic shapes of the spectra of K0-M0IIIs from 1150A to 35 um, or Kurucz synthetic spectra for A0-5V stars. Each supertemplate/model is reddened according to a star's extinction and normalized by characterized optical photometry. This paper tests our ability to predict NIR (JHK) magnitudes from supertemplates or models constrained by Hippa- rcos/Tycho or precision ground-based optical data. We offer absolute calibrated spectra of 33 optical standards to demonstrate the viability of this technique for a set of IR calibrators 100-1000 times fainter than we have previously publ- ished. We calculate the absolute uncertainties associated with predicting IRAC mags for the faintest cool giant and hot dwarf in this new set of calibrators.

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arXiv2002

The Brown Dwarf Deficit in Taurus: Evidence for a Non-Universal IMF

We present the results of a deep, optical/IR wide field imaging survey of selected fields in the nearby (d ~ 140 pc) Taurus star-forming region. We report the discovery of 9 new members with spectral types M5.75-M9.5. We derive an Initial Mass Function encompassing 54% of the known members in Taurus. Comparison with dense regions like the Trapezium Cluster in Orion shows that Taurus has produced x2 less brown dwarfs. We suggest that the lower frequency of brown dwarfs in Taurus may result from the low-density star-forming environment, leading to larger minimum Jeans masses.

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arXiv2000

Ages of late spectral type Vega-like stars

We have estimated the ages of eight late-type Vega-like stars by using standard age dating methods for single late-type stars, e.g., location on the color magnitude diagram, Li 6708 angstrom absorption, CaII H&K emission, X-ray luminosity, and stellar kinematic population. With the exception of the very unusual pre-main sequence star system HD 98800, all the late-type Vega-like stars are the same age as the Hyades cluster (600-800 Myr) or older.

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arXiv1998

Keck Spectra of Pleiades Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Edge in the Pleiades

We have obtained intermediate resolution spectra of eleven candidate brown dwarf members of the Pleiades open cluster using the Keck II telescope and LRIS spectrograph. Our primary goal was to determine the location of the "lithium depletion edge" in the Pleiades and hence to derive a precise age for the cluster. All but one of our 11 program objects have radial velocities appropriate for Pleiades members, have moderately strong H alpha emission, and have spectral types M6 to M8.5, as expected for their (R-I) colors. We have constructed a color-magnitude diagram for the faint end of the Pleiades main sequence, including only stars for which high S/N spectra in the region of the lithium 6708 Angstrom absorption line have been obtained. These data allow us to accurately determine the Pleiades single-star lithium depletion edge at I(c0) = 17.80, R-I(c0) = 2.20, spectral type = M6.5. By reference to theoretical evolutionary models, this converts fairly directly into an age for the Pleiades of 125 Myr. This is significantly older than the age that is normally quoted, but does agree with some other recent estimates.

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arXiv1998

The Problem of Hipparcos Distances to Open Clusters: I. Constraints from Multicolor Main Sequence Fitting

Parallax data from the Hipparcos mission allow the direct distance to open clusters to be compared with the distance inferred from main sequence (MS) fitting. There are surprising differences between the two distance measurements, which could lead to consequences of significant astrophysical importance. We examine the different possibilities, focusing on MS fitting in both metallicity-sensitive B-V and metallicity-insensitive V-I for five well-studied systems (the Hyades, Pleiades, Alpha Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and Alpha Per agree with the MS fitting distances in both colors; there is a possible conflict for Praesepe and Coma Ber. The Hipparcos distance to the Pleiades disagrees with the MS fitting distance in both colors at more than the three sigma level. Changes in the cluster metal abundance, helium abundance, reddening and age-related effects are all shown to be unlikely to explain the puzzling behavior for the Pleiades. We present evidence for spatially dependent systematic errors at the 1 mas level in the parallaxes of Pleiades stars. The implications of this result are discussed.

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arXiv1998

Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368

We report an upper limit of 9 x 10^{12} cm-2 on the column density of water in the translucent cloud along the line of sight toward HD 154368. This result is based upon a search for the C-X band of water near 1240 Åcarried out using the Goddard High Resolution Spectrograph of the Hubble Space Telescope. Our observational limit on the water abundance together with detailed chemical models of translucent clouds and previous measurements of OH along the line of sight constrain the branching ratio in the dissociative recombination of H_3O+ to form water. We find at the $3σ$ level that no more than 30% of dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum also yielded high-resolution observations of the Mg II doublet at 1239.9 Åand 1240.4 Å, allowing the velocity structure of the dominant ionization state of magnesium to be studied along the line of sight. The Mg II spectrum is consistent with GHRS observations at lower spectral resolution that were obtained previously but allow an additional velocity component to be identified.

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