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Hoard D.W.,Max Planck Institute for Astronomy | Hoard D.W.,Eureka Scientific Inc. | Debes J.H.,US Space Telescope Science Institute | Wachter S.,Max Planck Institute for Astronomy | And 2 more authors.
Astrophysical Journal | Year: 2013

We have compiled photometric data from the Wide-field Infrared Survey Explorer All Sky Survey and other archival sources for the more than 2200 objects in the original McCook & Sion Catalog of Spectroscopically Identified White Dwarfs. We applied color-selection criteria to identify 28 targets whose infrared spectral energy distributions depart from the expectation for the white dwarf (WD) photosphere alone. Seven of these are previously known WDs with circumstellar dust disks, five are known central stars of planetary nebulae, and six were excluded for being known binaries or having possible contamination of their infrared photometry. We fit WD models to the spectral energy distributions of the remaining ten targets, and find seven new candidates with infrared excess suggesting the presence of a circumstellar dust disk. We compare the model dust disk properties for these new candidates with a comprehensive compilation of previously published parameters for known WDs with dust disks. It is possible that the current census of WDs with dust disks that produce an excess detectable at K-band and shorter wavelengths is close to complete for the entire sample of known WDs to the detection limits of existing near-IR all-sky surveys. The WD dust disk candidates now being found using longer wavelength infrared data are drawn from a previously underrepresented region of parameter space, in which the dust disks are overall cooler, narrower in radial extent, and/or contain fewer emitting grains. © 2013. The American Astronomical Society. All rights reserved.


Lopez L.A.,University of California at Santa Cruz | Ramirez-Ruiz E.,University of California at Santa Cruz | Huppenkothen D.,University of Amsterdam | Badenes C.,Tel Aviv University | And 2 more authors.
Astrophysical Journal | Year: 2011

Supernova remnants (SNRs) are a complex class of sources, and their heterogeneous nature has hindered the characterization of their general observational properties. To overcome this challenge, in this paper, we use statistical tools to analyze the Chandra X-ray images of Galactic and Large Magellanic Cloud SNRs. We apply two techniques, a power-ratio method (a multipole expansion) and wavelet-transform analysis, to measure the global and local morphological properties of the X-ray line and thermal emission in 24 SNRs. We find that Type Ia SNRs have statistically more spherical and mirror-symmetric thermal X-ray emission than core-collapse (CC) SNRs. The ability to type SNRs based on thermal emission morphology alone enables, for the first time, the typing of SNRs with weak X-ray lines and those with low-resolution spectra. Based on our analyses, we identify one source (SNR G344.7-0.1) as originating from a CC explosion that was of unknown origin previously; we also confirm the tentative Type Ia classifications of G337.2-0.7 and G272.2-3.2. Although the global morphology is indicative of the explosion type, the relative morphology of the X-ray line emission within SNRs is not: all sources in our sample have well-mixed ejecta, irrespective of stellar origin. In particular, we find that 90% of the bright metal-line-emitting substructures are spatially coincident and have similar scales, even if the metals arise from different burning processes. Moreover, the overall X-ray line morphologies within each SNR are the same, with <6% differences. These findings reinforce observationally that hydrodynamical instabilities can efficiently mix ejecta in Type Ia and CC SNRs. The only exception is W49B, which can be attributed to its jet-driven/bipolar explosive origin. Based on comparative analyses across our sample, we describe several observational constraints that can be used to test hydrodynamical models of SNR evolution; notably, the filling factor of X-ray emission decreases with SNR age. © 2011. The American Astronomical Society. All rights reserved.


Peacock M.B.,Michigan State University | Zepf S.E.,Michigan State University | MacCarone T.J.,University of Southampton | Kundu A.,Eureka Scientific Inc.
Astrophysical Journal | Year: 2011

Accurate stellar population synthesis models are vital in understanding the properties and formation histories of galaxies. In order to calibrate and test the reliability of these models, they are often compared with observations of star clusters. However, relatively little work has compared these models in the ugriz filters, despite the recent widespread use of this filter set. In this paper, we compare the integrated colors of globular clusters in the Sloan Digital Sky Survey (SDSS) with those predicted from commonly used simple stellar population (SSP) models. The colors are based on SDSS observations of M31's clusters and provide the largest population of star clusters with accurate photometry available from the survey. As such, it is a unique sample with which to compare SSP models with SDSS observations. From this work, we identify a significant offset between the SSP models and the clusters' g - r colors, with the models predicting colors which are too red by g r∼0.1. This finding is consistent with previous observations of luminous red galaxies in the SDSS, which show a similar discrepancy. The identification of this offset in globular clusters suggests that it is very unlikely to be due to a minority population of young stars. The recently updated SSP model of Maraston & Strömbäck better represents the observed g - r colors. This model is based on the empirical MILES stellar library, rather than theoretical libraries, suggesting an explanation for the g - r discrepancy. © 2011. The American Astronomical Society. All rights reserved..


Waldron W.L.,Eureka Scientific Inc. | Cassinelli J.P.,University of Wisconsin - Madison
Astrophysical Journal Letters | Year: 2010

A controversy has developed regarding the stellar wind mass loss rates in O stars. The current consensus is that these winds may be clumped, which implies that all previously derived mass loss rates using density-squared diagnostics are overestimated by a factor of ≈2. However, arguments based on Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the P V resonance line doublet suggest that these rates should be smaller by another order of magnitude, provided that P V is the dominant phosphorous ion among these stars. Although a large mass loss rate reduction would have a range of undesirable consequences, it does provide a straightforward explanation of the unexpected symmetric and un-shifted X-ray emission-line profiles observed in high-energy resolution spectra. But acceptance of such a large reduction then leads to a contradiction with an important observed X-ray property: the correlation between He-like ion source radii and their equivalent X-ray continuum optical depth unity radii. Here we examine the phosphorous ionization balance since the P V fractional abundance, q (P V), is fundamental to understanding the magnitude of this mass loss reduction. We find that strong emission line radiation in the XUV energy band (defined here as 54 to 124 eV) can significantly reduce q (P V). Furthermore, owing to the unique energy distribution of these XUV lines, there is a negligible impact on the S V fractional abundance (a key component in the FUSE mass loss argument). We conclude that large reductions in O star mass loss rates are not required, and the X-ray optical depth unity relation remains valid. © 2010. The American Astronomical Society. All rights reserved.


Pratley L.,Victoria University of Wellington | Johnston-Hollitt M.,Victoria University of Wellington | Dehghan S.,Victoria University of Wellington | Sun M.,Eureka Scientific Inc.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present an in-depth, multiwavelength study of the radio galaxy PKS J0334-3900, which resides at the centre of Abell 3135. The spectro-polarimetric radio observations are combined with spectroscopic optical and X-ray data to illustrate the use of head-tail radio galaxies in revealing properties of the intracluster medium (ICM). Australia Telescope Compact Array (ATCA) observations at 1.4, 2.5, 4.6 and 8.6 GHz are presented with a detailed analysis of the morphology and spectral indices which give physical parameters to constrain the dynamical history of the galaxy. Using these constraints we produce a simulation of PKS J0334-3900. We find that this particular head-tail morphology can be induced via a combination of orbital motion due to a binary companion and the relative motion through the ICM. New Chandra images of A3135 are presented from which we obtain a cluster electron density of ne,0 ≤ (1.06 ± 0.11) × 10-3 cm-3, a global temperature of 2.4 +0.51-0.38 keV and a lower limit to the radio jet power of PKS J0334-3900 of 1.6 × 1044 erg s-1. A new redshift analysis of the cluster from available spectroscopic data demonstrates A3135 to be comprised of galaxies with 0.058 ≤ z < 0.066 and gives a new mean cluster redshift of 0.06228 ± 0.00015.We also uncovered a background subgroup within the range 0.066 ≤ z < 0.070. Additionally, ATCA Stokes Q and U data of Abell 3135 were used to obtain rotation measure values along the line of sight to PKS J0334-3900. Using our simulation, we are able to infer the distance between the jets along the line of sight to be 154 ± 16 kpc, which when combined with the difference in rotation measure between the jets provides a novel new way to estimate the average magnetic field within a cluster. A lower limit to the cluster magnetic field was calculated to be 0.09 ± 0.03 μG. From these results, we have shown that different techniques can be combined from the observations of head-tail galaxies to infer information on the cluster environment, showing them to be an important class of objects in next generation all sky surveys. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Sun M.,Eureka Scientific Inc.
New Journal of Physics | Year: 2012

Galaxy groups are the least massive systems where the bulk of baryons begin to be accounted for. Not simply the scaled-down versions of rich clusters following self-similar relations, galaxy groups are ideal systems to study baryon physics, which is important for both cluster cosmology and galaxy formation. We review the recent observational results on the hot gas in galaxy groups. The first part of this paper is on the scaling relations, including x-ray luminosity, entropy, gas fraction, baryon fraction and metal abundance. Compared to clusters, groups have a lower fraction of hot gas around the center (e.g. r < r 2500), but may have a comparable gas fraction at large radii (e.g. r 2500 < r < r 500). Better constraints on the group gas and baryon fractions require sample studies with different selection functions and deep observations at r > r 500 regions. The hot gas in groups is also iron-poor at large radii (0.3r 500-0.7 r 500). The iron content of the hot gas within the central regions (r < 0.3r 500) correlates with the group mass, in contrast to the trend of the stellar mass fraction. It remains to be seen where the missing iron in low-mass groups is. In the second part, we discuss several aspects of x-ray cool cores in galaxy groups, including their difference from cluster cool cores, radio AGN heating in groups and the cold gas in group cool cores. Because of the vulnerability of the group cool cores to radio AGN heating and the weak heat conduction in groups, group cool cores are important systems to test the AGN feedback models and the multiphase cool-core models. At the end of the paper, some outstanding questions are listed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Roberts M.S.E.,Eureka Scientific Inc.
AIP Conference Proceedings | Year: 2011

There has recently been a large increase in the number of known eclipsing radio millisecond pulsars in the Galactic field, many of which are associated with Fermi γ-ray sources. All are in tight binaries (P b< 24hr) many of which are classical "black widows" with very low mass companions (M c<<0.1M ⊙) but some are "redbacks" with probably non-degenerate low mass companions (M c∼0.2M ⊙). I review the new discoveries, briefly discuss the distance uncertainties and the implications for high-energy emission. © 2011 American Institute of Physics.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 148.51K | Year: 2010

This award will support an observational program to analyze high-contrast, high spatial resolution images of young stars with circumstellar disks. The principal goals will be to find planets directly or to look for morphological structures in the disks caused by the presence of planets. Much of the work will be to analyze the results of an extensive coronographic survey, but there will also be new observations.

The results of the analysis will explore the processes involved in the birth, early evolution, and architecture of exoplanets located in the outer regions of both protoplanetary and debris disks. Members of this research team also are engaged in a program which involves beginning students in research, especially students from underrepresented groups, at the University of Washington.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: STELLAR ASTRONOMY & ASTROPHYSC | Award Amount: 10.00K | Year: 2010

This award will provide partial support for the conference Cool Stars 16, to be held in Seattle, Washington in August, 2010.

This meeting will bring together students and professionals in the large community of stellar astronomy. This biennial conference, one of the largest regularly-held topical meetings in astrophysics, concentrates on cool stars with temperatures less than that of the Sun. The subject matter includes all aspects of the evolution of low-mass stars, including their formation and ultimate death. The conference will also explores connections between stellar and solar research.


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: Computing Ed for 21st Century | Award Amount: 488.13K | Year: 2014

The Council for Opportunity in Education, in collaboration with TERC, seeks to advance the understanding of social and cultural factors that increase retention of women of color in computing; and implement and evaluate a mentoring and networking intervention for undergraduate women of color based on the projects research findings. Computing is unique because it ranks as one of the STEM fields that are least populated by women of color, and because while representation of women of color is increasing in nearly every other STEM field, it is currently decreasing in computing - even as national job prospects in technology fields increase. The project staff will conduct an extensive study of programs that have successfully served women of color in the computing fields and will conduct formal interviews with 15 professional women of color who have thrived in computing to learn about their educational strategies. Based on those findings, the project staff will develop and assess a small-scale intervention that will be modeled on the practices of mentoring and networking which have been established as effective among women of color who are students of STEM disciplines. By partnering with Broadening Participation in Computing Alliances and local and national organizations dedicated to diversifying computing, project staff will identify both women of color undergraduates to participate in the intervention and professionals who can serve as mentors to the undergraduates in the intervention phase of the project. Assisting the researchers will be a distinguished Advisory Board that provides expertise in broadening the representation of women of color in STEM education. The external evaluator will provide formative and summative assessments of the projects case study data and narratives data using methods of study analysis and narrative inquiry and will lead the formative and summative evaluation of the intervention using a mixed methods approach. The intervention evaluation will focus on three variables: 1) students attitudes toward computer science, 2) their persistence in computer science and 3) their participant attitudes toward, and experiences in, the intervention.

This project extends the PIs previous NSF-funded work on factors that impact the success of women of color in STEM. The project will contribute an improved understanding of the complex challenges that women of color encounter in computing. It will also illuminate individual and programmatic strategies that enable them to participate more fully and in greater numbers. The ultimate broader impact of the project should be a proven, scalable model for reversing the downward trend in the rates at which women of color earn bachelors degrees in computer science.

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