Publications (34)65.36 Total impact
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Article: Erratum: "Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low-metallicity Small Magellanic Cloud (SAGE-SMC). II. Cool Evolved Stars" (2011, AJ, 142, 103)
aj. 05/2012; 143:127. -
Article: The Circumstellar Environment of R Coronae Borealis: White Dwarf Merger or Final Helium Shell Flash?
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ABSTRACT: In 2007, R Coronae Borealis (R CrB) went into an historically deep and long decline. In this state, the dust acts like a natural coronagraph at visible wavelengths, allowing faint nebulosity around the star to be seen. Imaging has been obtained from 0.5 to 500 micron with Gemini/GMOS, HST/WFPC2, Spitzer/MIPS, and Herschel/SPIRE. Several of the structures around R CrB are cometary globules caused by wind from the star streaming past dense blobs. The estimated dust mass of the knots is consistent with their being responsible for the R CrB declines if they form along the line of sight to the star. In addition, there is a large diffuse shell extending up to 4 pc away from the star containing cool 25 K dust that is detected all the way out to 500 micron. The SED of R CrB can be well fit by a 150 AU disk surrounded by a very large diffuse envelope which corresponds to the size of the observed nebulosity. The total masses of the disk and envelope are 10^-4 and 2 M(Sun), respectively, assuming a gas-to-dust ratio of 100. The evidence pointing toward a white-dwarf merger or a final-helium-shell flash origin for R CrB is contradictory. The shell and the cometary knots are consistent with a fossil planetary nebula. Along with the fact that R CrB shows significant Lithium in its atmosphere, this supports the final-helium-shell flash. However, the relatively high inferred mass of R CrB and its high fluorine abundance support a white-dwarf merger.10/2011; -
Conference Proceeding: A SAGE Overview of AGB Stars in the Small Magellanic Cloud
Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants; 09/2011 -
Article: Herschel detects a massive dust reservoir in supernova 1987A.
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ABSTRACT: We report far-infrared and submillimeter observations of supernova 1987A, the star whose explosion was observed on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160,000 light years away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 kelvin at a rate of about 220 times the luminosity of the Sun. The intensity and spectral energy distribution of the emission suggest a dust mass of about 0.4 to 0.7 times the mass of the Sun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts.Science 07/2011; 333(6047):1258-61. · 31.20 Impact Factor -
Article: The Dust Properties of Two Hot R Coronae Borealis Stars and a Wolf-Rayet Central Star of a Planetary Nebula: in Search of a Possible Link
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ABSTRACT: We present new Spitzer/IRS spectra of two hot R Coronae Borealis (RCB) stars, one in the Galaxy,V348 Sgr, and one lying in the LMC, HV 2671. These two objects may constitute a link between the RCB stars and the late Wolf-Rayet ([WCL]) class of central stars of planetary nebula (CSPNe) such as CPD -56 8032 that has little or no hydrogen in their atmospheres. HV 2671 and V348 Sgr are members of a rare subclass that has significantly higher effective temperatures than most RCB stars, but sharing the traits of hydrogen deficiency and dust formation that define the cooler RCB stars. The [WC] CSPNe star, CPD -56 8032, displays evidence for dual-dust chemistry showing both PAHs and crystalline silicates in its mid-IR spectrum. HV 2671 shows strong PAH emission but shows no sign of having crystalline silicates. The spectrum of V348 Sgr is very different from those of CPD -56 8032 and HV 2671. The PAH emission seen strongly in the other two stars is not present. Instead, the spectrum is dominated by a broad emission centered at about 8.2 micron. The mid-IR spectrum of CPD -56 8032 shows emission features that may be associated with C60. The other two stars do not show evidence for C60. HV 2671 has also been detected by Herschel/PACS and SPIRE. V348 Sgr and CPD -56 8032 have been detected by AKARI/FIS. These data were combined with Spitzer, IRAS, 2MASS and other photometry to produce their spectral energy distributions from the visible to the far-IR. Monte Carlo radiative transfer modeling was used to study the circumstellar dust around these stars. HV 2671 and CPD -56 8032 require both a flared inner disk with warm dust and an extended diffuse envelope with cold dust to to fit their SEDs. The SED of V348 Sgr can be fit with a much smaller disk and envelope.06/2011; -
Article: Fundamental stellar parameters in 47 Tucanae (Mcdonald+, 2011)
VizieR Online Data Catalog. 05/2011; 219:30023. -
Article: Fundamental Parameters, Integrated Red Giant Branch Mass Loss, and Dust Production in the Galactic Globular Cluster 47 Tucanae
apjs. 04/2011; 193:23. -
Article: A Panchromatic View of NGC 602: Time-resolved Star Formation with the Hubble and Spitzer Space Telescopes
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ABSTRACT: We present the photometric catalogs for the star-forming cluster NGC 602 in the wing of the Small Magellanic Cloud covering a range of wavelengths from optical (HST/ACS F555W, F814W, and SMARTS/ANDICAM V, I) to infrared (Spitzer/IRAC 3.6, 4.5, 5.8, and 8 μm and MIPS 24 μm). Combining this with Infrared Survey Facility near-infrared photometry (J, H, Ks ), we compare the young main sequence (MS) and pre-main sequence (PMS) populations prominent in the optical with the current young stellar object (YSO) populations revealed by the infrared. We analyze the MS and PMS population with isochrones in color-magnitude diagrams to derive ages and masses. The optical data reveal ~565 PMS candidates, low-mass Stage III YSOs. We characterize ~40 YSOs by fitting their spectral energy distributions to a grid of models (Robitaille et al.) to derive luminosities, masses, and evolutionary phase (Stages I-III). The higher resolution HST images reveal that ~70% of the YSO candidates are either multiples or protoclusters. For YSOs and PMS sources found in common, we find a consistency in the masses derived. We use the YSO mass function to derive a present-day star formation rate of ~0.2-1.0 M ☉ yr–1 kpc–2, similar to the rate derived from the optical star formation history suggesting a constant star formation rate for this region. We demonstrate a progression of star formation from the optical star cluster center to the edge of the star-forming dust cloud. We derive lifetimes of a few 105 years for the YSO Stages I and II.The Astrophysical Journal 03/2011; 730(2):78. · 6.02 Impact Factor -
Article: A Panchromatic View OF NGC 602: Time-Resolved Star Formation with the Hubble and Spitzer Space Telescopes
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ABSTRACT: We present the photometric catalogs for the star-forming cluster NGC 602 in the wing of the Small Magellanic Cloud covering a range of wavelengths from optical HST/ACS (F555W, F814W) and SMARTS/ANDICAM (V, I) to infrared (Spitzer/IRAC 3.6, 4.5, 5.8, and 8 micron and MIPS 24 micron). Combining this with IRSF (InfraRed Survey Facility) near-infrared photometry (J, H, Ks), we compare the young main sequence (MS) and pre-main sequence (PMS) populations prominent in the optical with the current young stellar object (YSO) populations revealed by the infrared (IR). We analyze the MS and PMS population with isochrones in color-magnitude diagrams to derive ages and masses. The optical data reveal ~565 PMS candidates, low mass Stage III YSOs. We characterize ~40 YSOs by fitting their spectral energy distributions (SEDs) to a grid of models (Robitaille et al. 2007) to derive luminosities, masses and evolutionary phase (Stage I-III). The higher resolution HST images reveal that ~70% of the YSO candidates are either multiples or protoclusters. For YSOs and PMS sources found in common, we find a consistency in the masses derived. We use the YSO mass function to derive a present-day star-formation rate of ~0.2-1.0 Msun/yr/kpc^2, similar to the rate derived from the optical star formation history suggesting a constant star formation rate for this region. We demonstrate a progression of star formation from the optical star cluster center to the edge of the star forming dust cloud. We derive lifetimes of a few 10^5 years for the YSO Stages I and II. Comment: 55 pages, 18 Figures; High resolution images available form author Accepted to ApJ12/2010; -
Article: Young Stellar Objects in the Large Magellanic Cloud Star-forming Region N206
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ABSTRACT: We present analysis of the energetic star-forming region Henize 206 (N206) located near the southern edge of the Large Magellanic Cloud (LMC) based on photometric data from the Spitzer Surveying the Agents of Galaxy Evolution (SAGE-LMC; IRAC 3.6, 4.5, 5.8, 8.0 μm and MIPS 24 μm), Infrared Survey Facility near-infrared survey (J, H, K s ), and the Magellanic Clouds Photometric Survey (MCPS UBVI) covering a wavelength range of 0.36-24 μm. Young stellar object (YSO) candidates are identified based upon their location in infrared color-magnitude space and classified by the shapes of their spectral energy distributions in comparison with a pre-computed grid of YSO models. We identify 116 YSO candidates: 102 are well characterized by the YSO models, predominately Stage I, and 14 may be multiple sources or young sources with transition disks. Careful examination of the individual sources and their surrounding environment allows us to identify a factor of ~14.5 more YSO candidates than have already been identified. The total mass of these well-fit YSO candidates is ~520 M ☉. We calculate a current star formation rate of 0.27 × 10–1 M ☉ yr–1 kpc–2. The distribution of YSO candidates appears to follow shells of neutral material in the interstellar medium.The Astrophysical Journal 09/2010; 721:357-368. · 6.02 Impact Factor -
Article: Period Luminosity Relationships and Mass-Loss Rates of Asymptotic Giant Branch Stars
Spitzer Proposal. 06/2010; -
Article: Spitzer Analysis of H II Region Complexes in the Magellanic Clouds: Determining a Suitable Monochromatic Obscured Star Formation Indicator
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ABSTRACT: H II regions are the birth places of stars, and as such they provide the best measure of current star formation rates (SFRs) in galaxies. The close proximity of the Magellanic Clouds allows us to probe the nature of these star forming regions at small spatial scales. To study the H II regions, we compute the bolometric infrared flux, or total infrared (TIR), by integrating the flux from 8 to 500 μm. The TIR provides a measure of the obscured star formation because the UV photons from hot young stars are absorbed by dust and re-emitted across the mid-to-far-infrared (IR) spectrum. We aim to determine the monochromatic IR band that most accurately traces the TIR and produces an accurate obscured SFR over large spatial scales. We present the spatial analysis, via aperture/annulus photometry, of 16 Large Magellanic Cloud (LMC) and 16 Small Magellanic Cloud (SMC) H II region complexes using the Spitzer Space Telescope's IRAC (3.6, 4.5, 8 μm) and MIPS (24, 70, 160 μm) bands. Ultraviolet rocket data (1500 and 1900 Å) and SHASSA Hα data are also included. All data are convolved to the MIPS 160 μm resolution (40 arcsec full width at half-maximum), and apertures have a minimum radius of 35''. The IRAC, MIPS, UV, and Hα spatial analysis are compared with the spatial analysis of the TIR. We find that nearly all of the LMC and SMC H II region spectral energy distributions (SEDs) peak around 70 μm at all radii, from ~10 to ~400 pc from the central ionizing sources. As a result, we find the following: the sizes of H II regions as probed by 70 μm are approximately equal to the sizes as probed by TIR (70 pc in radius); the radial profile of the 70 μm flux, normalized by TIR, is constant at all radii (70 μm ~ 0.45TIR); the 1σ standard deviation of the 70 μm fluxes, normalized by TIR, is a lower fraction of the mean (0.05-0.12 out to ~220 pc) than the normalized 8, 24, and 160 μm normalized fluxes (0.12-0.52); and these results are the same for the LMC and the SMC. From these results, we argue that 70 μm is the most suitable IR band to use as a monochromatic obscured star formation indicator because it most accurately reproduces the TIR of H II regions in the LMC and SMC and over large spatial scales. We also explore the general trends of the 8, 24, 70, and 160 μm bands in the LMC and SMC H II region SEDs, radial surface brightness profiles, sizes, and normalized (by TIR) radial flux profiles. We derive an obscured SFR equation that is modified from the literature to use 70 μm luminosity, SFR(M ☉ yr–1) = 9.7(0.7) × 10–44 L 70(ergs s–1), which is applicable from 10 to 300 pc distance from the center of an H II region. We include an analysis of the spatial variations around H II regions between the obscured star formation indicators given by the IR and the unobscured star formation indicators given by UV and Hα. We compute obscured and unobscured SFRs using equations from the literature and examine the spatial variations of the SFRs around H II regions.The Astrophysical Journal 05/2010; 716(1):453. · 6.02 Impact Factor -
Article: Is Dust Forming on the Red Giant Branch in 47 Tuc?
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ABSTRACT: Using Spitzer IRAC observations from the SAGE-SMC Legacy program and archived Spitzer IRAC data, we investigate dust production in 47 Tuc, a nearby massive Galactic globular cluster. A previous study detected infrared excess, indicative of circumstellar dust, in a large population of stars in 47 Tuc, spanning the entire Red Giant Branch (RGB). We show that those results suffered from effects caused by stellar blending and imaging artifacts and that it is likely that no stars below about 1 mag from the tip of the RGB are producing dust. The only stars that appear to harbor dust are variable stars, which are also the coolest and most luminous stars in the cluster. Comment: Accepted for publication in ApJL02/2010; -
Conference Proceeding: Two Magellanic Cloud Star-Forming Clusters from Optical to Infrared
American Astronomical Society Meeting Abstracts #215; 01/2010 -
Conference Proceeding: Infrared Variable Stars in the Small Magellanic Cloud Using the Spitzer SAGE-SMC Survey
American Astronomical Society Meeting Abstracts #215; 01/2010 -
Conference Proceeding: HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) in the Magellanic Clouds
American Astronomical Society Meeting Abstracts #215; 01/2010 -
Article: Lifting the Dusty Veil with Near- and Mid-Infrared Photometry. II. A Large-Scale Study of the Galactic Infrared Extinction Law
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ABSTRACT: We combine near-infrared (Two Micron All Sky Survey) and mid-infrared (Spitzer-IRAC) photometry to characterize the IR extinction law (1.2-8 μm) over nearly 150° of contiguous Milky Way midplane longitude. The relative extinctions in five passbands across these wavelength and longitude ranges are derived by calculating color excess ratios for G and K giant red clump stars in contiguous midplane regions and deriving the wavelength dependence of extinction in each one. Strong, monotonic variations in the extinction law shape are found as a function of angle from the Galactic center, symmetric on either side of it. These longitudinal variations persist even when dense interstellar regions, known a priori to have a shallower extinction curve, are removed. The increasingly steep extinction curves toward the outer Galaxy indicate a steady decrease in the absolute-to-selective extinction ratio (RV ) and in the mean dust grain size at greater Galactocentric angles. We note an increasing strength of the 8 μm extinction inflection at high Galactocentric angles and, using theoretical dust models, show that this behavior is consistent with the trend in RV . Along several lines of sight where the solution is most feasible, A λ/AKs as a function of Galactic radius (R GC) is estimated and shown to have a Galactic radial dependence. Our analyses suggest that the observed relationship between extinction curve shape and Galactic longitude is due to an intrinsic dependence of the extinction law on Galactocentric radius.The Astrophysical Journal 11/2009; 707(1):510. · 6.02 Impact Factor -
Article: Lifting the Dusty Veil II: A Large-Scale Study of the Galactic Infrared Extinction Law
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ABSTRACT: We combine near-infrared (2MASS) and mid-infrared (Spitzer-IRAC) photometry to characterize the IR extinction law (1.2-8 microns) over nearly 150 degrees of contiguous Milky Way midplane longitude. The relative extinctions in 5 passbands across these wavelength and longitude ranges are derived by calculating color excess ratios for G and K giant red clump stars in contiguous midplane regions and deriving the wavelength dependence of extinction in each one. Strong, monotonic variations in the extinction law shape are found as a function of angle from the Galactic center, symmetric on either side of it. These longitudinal variations persist even when dense interstellar regions, known a priori to have a shallower extinction curve, are removed. The increasingly steep extinction curves towards the outer Galaxy indicate a steady decrease in the absolute-to-selective extinction ratio (R_V) and in the mean dust grain size at greater Galactocentric angles. We note an increasing strength of the 8 micron extinction inflection at high Galactocentric angles and, using theoretical dust models, show that this behavior is consistent with the trend in R_V. Along several lines of sight where the solution is most feasible, A_lambda/A_Ks as a function of Galactic radius is estimated and shown to have a Galactic radial dependence. Our analyses suggest that the observed relationship between extinction curve shape and Galactic longitude is due to an intrinsic dependence of the extinction law on Galactocentric radius. Comment: Accepted to ApJ10/2009; -
Article: A spatially resolved study of photoelectric heating and [C II] cooling in the LMC. Comparison with dust emission as seen by SAGE
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ABSTRACT: Context. Photoelectric heating is a dominant heating mechanism for many phases of the interstellar medium. We study this mechanism throughout the Large Magellanic Cloud (LMC). Aims. We aim to quantify the importance of the [C II] cooling line and the photoelectric heating process of various environments in the LMC and to investigate which parameters control the extent of photoelectric heating. Methods. We use the BICE [C II] map and the Spitzer/SAGE infrared maps. We examine the spatial variations in the efficiency of photoelectric heating: photoelectric heating rate over power absorbed by grains, i.e. the observed [C II] line strength over the integrated infrared emission. We correlate the photoelectric heating efficiency and the emission from various dust constituents and study the variations as a function of H emission, dust temperatures, and the total infrared luminosity. The observed variations are interpreted in a theoretical framework. From this we estimate radiation field, gas temperature, and electron density. Results. We find systematic variations in photoelectric efficiency. The highest efficiencies are found in the diffuse medium, while the lowest coincide with bright star-forming regions (~1.4 times lower). The [C II] line emission constitutes 1.32% of the far infrared luminosity across the whole of the LMC. We find correlations between the [C II] emission and ratios of the mid infrared and far infrared bands, which comprise various dust constituents. The correlations are interpreted in light of the spatial variations of the dust abundance and by the local environmental conditions that affect the dust emission properties. As a function of the total infrared surface brightness, S_(TIR), the [C II] surface brightness can be described as: S_([C II]) = 1.25 S^(0.69)_(TIR)[10^(-3) erg s^(-1) cm^(-2) sr^(-1)], for S_(TIR) ≳ 3.2 x 10^(-4) erg s^(-1) cm^(-2) sr^(-1). We provide a simple model of the photoelectric efficiency as a function of the total infrared luminosity. We find a power-law relation between radiation field and electron density, consistent with other studies. The [C II] emission is well-correlated with the 8 µm emission, suggesting that the polycyclic aromatic hydrocarbons play a dominant role in the photoelectric heating process.åp. 02/2009; 494:647-661. -
Article: Variable Evolved Stars and Young Stellar Objects Discovered in the Large Magellanic Cloud Using the SAGE Survey
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ABSTRACT: We present initial results and source lists of variable sources in the Large Magellanic Cloud (LMC) for which we detect thermal infrared variability from the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, which had two epochs of photometry separated by 3 months. The SAGE survey mapped a 7° × 7° region of the LMC using the Infrared Array Camera (IRAC) and the MIPS instruments on board Spitzer. Variable sources are identified using a combination of the IRAC 3.6, 4.5, 5.8, 8.0 μ bands and the MIPS 24 μ bands. An error-weighted flux difference between the two epochs is used to assess the variability. Of the ~3 million sources detected at both epochs, we find ~2000 variable sources for which we provide electronic catalogs. Most of the variable sources can be classified as asymptotic giant branch (AGB) stars. A large fraction (>66%) of the extreme AGB stars are variable and only smaller fractions of carbon-rich (6.1%) and oxygen-rich (2.0%) stars are detected as variable sources. We also detect a population of variable young stellar object candidates.The Astronomical Journal 01/2009; 137(2):3139. · 4.03 Impact Factor
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Institutions
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2006–2011
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University of Wisconsin - Madison
- Department of Astronomy
Madison, MS, USA
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