Robert D. Gehrz

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (169)447.58 Total impact

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    ABSTRACT: We present pre-perihelion infrared 8 to 31 micron spectrophotometric and imaging observations of comet C/2012 K1 (Pan-STARRS), a dynamically new Oort Cloud comet, conducted with NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) facility (+FORCAST) in 2014 June. As a "new" comet (first inner solar system passage), the coma grain population may be extremely pristine, unencumbered by a rime and insufficiently irradiated by the Sun to carbonize its surface organics. The comet exhibited a weak 10 micron silicate feature ~1.18 +/- 0.03 above the underlying best-fit 215.32 +/- 0.95 K continuum blackbody. Thermal modeling of the observed spectral energy distribution indicates that the coma grains are fractally solid with a porosity factor D = 3 and the peak in the grain size distribution, a_peak = 0.6 micron, large. The sub-micron coma grains are dominated by amorphous carbon, with a silicate-to-carbon ratio of 0.80 (+0.25) (- 0.20). The silicate crystalline mass fraction is 0.20 (+0.30) (-0.10), similar to with other dynamically new comets exhibiting weak 10 micron silicate features. The bolometric dust albedo of the coma dust is 0.14 +/- 0.01 at a phase angle of 34.76 degrees, and the average dust production rate, corrected to zero phase, at the epoch of our observations was Afrho ~ 5340~cm.
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    ABSTRACT: In this program we propose to use a total of 0.8 hr to obtain 3.6 and 4.5 micron photometry of SNR 1987A at four additional epochs beyond day 10000 after the explosion. The emission in these two IRAC bands may arise from a hot dust component residing in the equatorial ring (ER) with a distinctly different spectral shape and temperature from the dominant 180 K silicate dust component in the ER. The dust in the ER is collisionally-heated by the SN blast wave that also gives rise to the soft X-ray emission from the ER. The intensity in the mid-IR emission (24 micron) was generally well correlated with that of the X-ray emission. However, the continued monitoring of the 3.6 and 4.5 micron emission now seems to show that at these wavelengths the IR emission has begun to fade, and is no longer tracking the brightness of the soft X-ray emission. These differences could stem from a variety of causes, including the sputtering of the dust or changes in the morphology of the ER. Ongoing X-ray observations of the remnant are taking place. Supplementing these with IR observations is essential for determining the nature and the evolution of this hot dust component. Finally, the observations may still reveal the appearance of a new emission component from the SN ejecta which is currently interacting with the reverse shock. These observations will complete the record of Spitzer's observations of SN 1987A, spanning more than 12 years from launch to end of mission.
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    ABSTRACT: Nearby resolved dwarf galaxies provide excellent opportunities for studying the dust-producing late stages of stellar evolution over a wide range of metallicity (-2.7 < [Fe/H] < -1.0). Here, we describe DUSTiNGS (DUST in Nearby Galaxies with Spitzer): a 3.6 and 4.5 micron post-cryogen Spitzer Space Telescope imaging survey of 50 dwarf galaxies within 1.5 Mpc that is designed to identify dust-producing Asymptotic Giant Branch (AGB) stars and massive stars. The survey includes 37 dwarf spheroidal, 8 dwarf irregular, and 5 transition-type galaxies. This near-complete sample allows for the building of statistics on these rare phases of stellar evolution over the full metallicity range. The photometry is >75% complete at the tip of the Red Giant Branch for all targeted galaxies, with the exception of the crowded inner regions of IC 10, NGC 185, and NGC 147. This photometric depth ensures that the majority of the dust-producing stars, including the thermally-pulsing AGB stars, are detected in each galaxy. The images map each galaxy to at least twice the half-light radius to ensure that the entire evolved star population is included and to facilitate the statistical subtraction of background and foreground contamination, which is severe at these wavelengths. In this overview, we describe the survey, the data products, and preliminary results. We show evidence for the presence of dust-producing AGB stars in 8 of the targeted galaxies, with metallicities as low as [Fe/H] = -1.9, suggesting that dust production occurs even at low metallicity.
    The Astrophysical Journal Supplement Series 11/2014; 216(1). DOI:10.1088/0067-0049/216/1/10 · 14.14 Impact Factor
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    ABSTRACT: Asymptotic Giant Branch (AGB) stars may be the dominant stellar dust source in galaxies, potentially driving galactic chemical evolution. Specifically, the dustiest ``extreme'' AGB stars, which comprise <5% of the AGB population, provide more than 70% of the AGB-produced dust in the Magellanic Clouds (Riebel et al. 2012; Boyer et al. 2012). Despite their importance, these stars have only been studied in detail in the Magellanic Clouds, which cover a limited range in metallicity. Here, we present the first results of the DUST In Nearby Galaxies with Spitzer (DUSTINGS) post-cryogen Spitzer program. The DUSTINGS program was designed to identify and characterize these extreme AGB stars in a complete infrared census of resolvable Local Group (<2 Mpc) dwarf galaxies, which span a wide range of galactic environments. We find hundreds of extreme AGB star candidates and estimate their dust-injection rates.
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    ABSTRACT: In this program we propose to use a total of 0.4 hr to obtain 3.6 and 4.5 micron photometry of SNR 1987A at two additional epochs beyond day 9800 after the explosion. The emission in these two IRAC bands arises from a hot dust component residing in the equatorial ring (ER) with a distinctly different spectral shape and temperature from the dominant 180 K silicate dust component in the ER. The dust in the ER is collisionally-heated by the SN blast wave that also gives rise to the soft X-ray emission from the ER. The intensity in the IR emission is generally well correlated with that of the X-ray emission. However, the most recent X-ray observations have showed a difference in the evolution of their respective light curves. These differences could stem from a variety of causes, including the sputtering of the dust or changes in the morphology of the ER. Ongoing X-ray observations of the remnant are taking place. Supplementing these with IR observations is essential for determining the nature and the evolution of this hot dust component. Finally, the observations may reveal the appearance of a new emission component from the SN ejecta which is currently interacting with the reverse shock.
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    ABSTRACT: In this program we propose to use a total of 0.4 hr to obtain 3.6 and 4.5 micron photometry of SNR 1987A at two additional epochs beyond day 9500 after the explosion. The emission in these two IRAC bands arises from a hot dust component residing in the equatorial ring (ER) with a distinctly different spectral shape and temperature from the dominant 180 K silicate dust component in the ER. The dust in the ER is collisionally-heated by the SN blast wave that also gives rise to the soft X-ray emission from the ER. The intensity in the IR emission is generally well correlated with that of the X-ray emission. However, the most recent X-ray observations have showed a difference in the evolution of their respective light curves. These differences could stem from a variety of causes, including the sputtering of the dust or changes in the morphology of the ER. Ongoing X-ray observations of the remnant are taking place. Supplementing these with IR observations is essential for determining the nature and the evolution of this hot dust component. Finally, the observations may reveal the appearance of a new emission component from the SN ejecta which is currently interacting with the reverse shock.
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    ABSTRACT: We present Spitzer Space Telescope mid-infrared IRS spectra, supplemented by ground-based optical observations, of the classical novae V1974 Cyg, V382 Vel, and V1494 Aql more than 11, 8, and 4 years after outburst respectively. The spectra are dominated by forbidden emission from neon and oxygen, though in some cases, there are weak signatures of magnesium, sulfur, and argon. We investigate the geometry and distribution of the late time ejecta by examination of the emission line profiles. Using nebular analysis in the low density regime, we estimate lower limits on the abundances in these novae. In V1974 Cyg and V382 Vel, our observations confirm the abundance estimates presented by other authors and support the claims that these eruptions occurred on ONe white dwarfs. We report the first detection of neon emission in V1494 Aql and show that the system most likely contains a CO white dwarf.
    The Astrophysical Journal 06/2012; 755(1). DOI:10.1088/0004-637X/755/1/37 · 6.28 Impact Factor
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    ABSTRACT: Recent infrared (IR) observations of freshly-formed dust in supernova remnants (SNRs) have yielded significantly lower dust masses than predicted by theoretical models and measured from high redshift observations. The Crab Nebula's pulsar wind is thought to be sweeping up freshly-formed supernova (SN) dust along with the ejected gas. The evidence for this dust was found in the form of an IR excess in the integrated spectrum of the Crab and in extinction against the synchrotron nebula that revealed the presence of dust in the filament cores. We present the first spatially resolved emission spectra of dust in the Crab Nebula acquired with the Infrared Spectrograph aboard the Spitzer Space Telescope. The IR spectra are dominated by synchrotron emission and show forbidden line emission from from S, Si, Ne, Ar, O, Fe, and Ni. We derived a synchrotron spectral map from the 3.6 and 4.5 microns images, and subtracted this contribution from our data to produce a map of the residual continuum emission from dust. The dust emission appears to be concentrated along the ejecta filaments and is well described by an amorphous carbon or silicate grain compositions. We find a dust temperature of 55+/- 4 K for silicates and 60 +/- 7 K for carbon grains. The total estimated dust mass is 0.0012-0.012 solar masses, well below the theoretical dust yield predicted for a core-collapse supernova. Our grain heating model implies that the dust grain radii are relatively small, unlike what is expected for dust grains formed in a Type IIP SN.
    The Astrophysical Journal 05/2012; 753(1). DOI:10.1088/0004-637X/753/1/72 · 6.28 Impact Factor
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    ABSTRACT: Presolar grains in meteorites and interplanetary dust particles (IDPs) carry non-solar isotopic signatures pointing to origins in supernovae, giant stars, and possibly other stellar sources. There have been suggestions that some of these grains condensed in the ejecta of classical nova outbursts, but the evidence is ambiguous. We report neon and helium compositions in particles captured on stratospheric collectors flown to sample materials from comets 26P/Grigg-Skjellerup and 55P/Tempel-Tuttle that point to condensation of their gas carriers in the ejecta of a neon (ONe) nova. The absence of detectable 3He in these particles indicates space exposure to solar wind (SW) irradiation of a few decades at most, consistent with origins in cometary dust streams. Measured 4He/20Ne, 20Ne/22Ne, 21Ne/22Ne and 20Ne/21Ne isotope ratios, and a low upper limit on 3He/4He, are in accord with calculations of nucleosynthesis in neon nova outbursts. Of these, the uniquely low 4He/20Ne and high 20Ne/22Ne ratios are the most diagnostic, reflecting the large predicted 20Ne abundances in the ejecta of such novae. The correspondence of measured Ne and He compositions in cometary matter with theoretical predictions is evidence for the presence of presolar grains from novae in the early solar system.
    The Astrophysical Journal 04/2012; 742(2). DOI:10.1088/0004-637X/742/2/86 · 6.28 Impact Factor
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    ABSTRACT: Binary mass transfer via Roche-lobe overflow (RLOF) is a key channel for producing stripped-envelope Wolf-Rayet (WR) stars and may be critical to account for SN Ib/c progenitors. RY Scuti is an extremely rare example of a massive binary star caught in this brief but important phase. Its toroidal nebula indicates equatorial mass loss during RLOF, while the mass-gaining star is apparently embedded in an opaque accretion disk. RY Scuti's toroidal nebula has two components: an inner ionised double-ring system, and an outer dust torus that is twice the size of the ionised rings. We present two epochs of Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of the ionised gas rings. Proper motions show that the inner ionised rings and the outer dust torus came from two separate ejection events roughly 130 and 250 yr ago. This suggests that RLOF in massive contact binaries can be accompanied by eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate that the repeating outbursts may arise in the mass gainer from instabilities associated with a high accretion rate. If discrete mass-loss episodes in other RLOF binaries are accompanied by luminous outbursts, they might contribute to the population of extragalactic optical transients. When RLOF ends for RY Scuti, the overluminous mass gainer, currently surrounded by an accretion disk, will probably become a B[e] supergiant and may outshine the hotter mass-donor star that should die as a Type Ib/c supernova.
    Monthly Notices of the Royal Astronomical Society 05/2011; 418. DOI:10.1111/j.1365-2966.2011.19614.x · 5.23 Impact Factor
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    ABSTRACT: The cryogenic Spitzer campaign to monitor the evolution of SNR 1987A has succeeding in revealing a steady change in the brightness of emission from silicate dust in equatorial ring (ER) of the SN progenitor. The change in brightness is well-correlated with the X-ray emission. However, the Spitzer IRAC and IRS data unexpectedly revealed at second hotter dust component. The spectra did not provide a distinct spectroscopic signature of this dust, but the time series of observations indicated that it may be evolving at a slightly different rate from the dominant silicate component. In this program we will use a total of 0.4 hr to obtain 3.6 and 4.5 micron photometry of SNR 1987A at two additional epochs. The initial warm observation has revealed a definite flattening of the light curve at these wavelengths. Presently, we lack sufficient information to identify the physical processes responsible for this trend. The requested observations will reveal the long-term behavior of the light curve which may even show the onset of a decline. The continued evolution of this hot dust component will provide important and unique information on the nature and fate of this dust component, and on the origin and morphology of the ER.
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    ABSTRACT: Highly evolved stars (massive stars and intermediate-mass Asymptotic Giant Branch -or AGB- stars) can become heavily enshrouded in dust. This material is ejected into the interstellar medium (ISM), driving galactic chemical evolution. The dwarf galaxies of the Local Group (LG) are ideal laboratories for studying dusty evolved stars since their stellar populations are resolvable and they span a wide range in metallicity, luminosity, star formation history, ISM content, and interaction history. The majority of known resolvable LG dwarf galaxies (d < 2 Mpc) remains unobserved with IRAC or were observed with inadequate areal coverage and/or depth for detecting the dusty stellar population. We propose to complete a uniform census of LG dwarf galaxies with IRAC that is designed to detect and characterize the circumstellar dust around evolved stars, especially those obscured in optical and near-infrared surveys. The dust-producing phase is brief, so it is rare in low-luminosity dwarf galaxies. We must therefore observe a complete sample of dwarf galaxies to build equivalent statistics at each metallicity. Our immediate science goals are to (1) analyze dust content and mass loss in AGB stars, (2) generate intermediate-age star formation histories using AGB stars, (3) analyze the infrared properties of massive evolved stars, and (4) use this survey as a pathfinder for JWST science. These observations and the resulting database will leave a valuable and lasting Spitzer legacy.
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    ABSTRACT: We request DDT observations of the recurrent nova T Pyx, whose 2011 eruption was long overdue. Spitzer/IRAC observations at 3.6 and 4.5 microns will complement the 1-2.5 micron data we are getting from the ground, and the >50 micron data we are getting from Herschel (DDT approved). For the first time we will get infra-red data on an erupting nova from 1-100 microns, throwing new and unique insight into the evolution of a recurrent nova.
  • R. D. Gehrz · E. E. Becklin
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    ABSTRACT: We examine the ejecta evolution of the classical nova V1065 Centauri, constructing a detailed picture of the system based on spectrophotometric observations obtained from 9 to approximately 900 days post-outburst with extensive coverage from optical to mid-infrared wavelengths. We estimate a reddening toward the system of E(B-V) = 0.5 ± 0.1, based upon the B – V color and analysis of the Balmer decrement, and derive a distance estimate of 8.7+2.8 –2.1 kpc. The optical spectral evolution is classified as P o fe N ne A o according to the CTIO Nova Classification system of Williams et al. Photoionization modeling yields absolute abundance values by number, relative to solar of He/H = 1.6 ± 0.3, N/H = 144 ± 34, O/H = 58 ± 18, and Ne/H = 316 ± 58 for the ejecta. We derive an ejected gas mass of Mg = (1.6 ± 0.2) × 10–4 M ☉. The infrared excess at late epochs in the evolution of the nova arises from dust condensed in the ejecta composed primarily of silicate grains. We estimate a total dust mass, Md , of order (0.2-3.7) × 10–7 M ☉, inferred from modeling the spectral energy distribution observed with the Spitzer IRS and Gemini-South GNIRS spectrometers. Based on the speed class, neon abundance, and the predominance of silicate dust, we classify V1065 Cen as an ONe-type classical nova.
    The Astronomical Journal 10/2010; 140(5):1347. DOI:10.1088/0004-6256/140/5/1347 · 4.05 Impact Factor
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    ABSTRACT: We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a 5 year period spanning the epochs between days 6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant (SNR) and its radiative output is dominated by the interaction of the SN blast wave with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by emission from ~180 K silicate dust, collisionally-heated by the hot X-ray emitting gas with a temperature and density of ~5x10^6 K and 3x10^4 cm-3, respectively. The mass of the radiating dust is ~1.2x10^(-6) Msun on day 7554, and scales linearly with IR flux. The infrared to soft-X-ray flux ratio is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of of this ratio suggests that very little grain processing or gas cooling have occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased with a time dependence of t^(0.87), t being the time since the first encounter between the blast wave and the ER. These observations are consistent with the transitioning of the blast wave from free expansion to a Sedov phase as it propagates into the main body of the ER.
    The Astrophysical Journal 09/2010; 722(1):425. DOI:10.1088/0004-637X/722/1/425 · 6.28 Impact Factor
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    ABSTRACT: Low-resolution, mid-infrared Spitzer/IRS spectral maps are presented for three nearby, low-metallicity dwarf galaxies (NGC 55, NGC 3109 and IC 5152) for the purpose of examining the spatial distribution and variation of polycyclic aromatic hydrocarbon (PAH) emission. The sample straddles a metallicity of 12+log(O/H)~8.0, a transition point below which PAH intensity empirically drops and the character of the interstellar medium changes. We derive quantitative radiances of PAH features and atomic lines on both global and spatially-resolved scales. The Spitzer spectra, combined with extensive ancillary data from the UV through the mid-infrared, allow us to examine changes in the physical environments and in PAH feature radiances down to a physical scale of 50 pc. We discuss correlations between various PAH emission feature and atomic line radiances. The (6.2 micron)/(11.3 micron), (7.7 micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron), and (8.6 micron)/(6.2 micron) PAH radiance ratios are found to be independent of position across all three galaxies, although the ratios do vary from galaxy to galaxy. As seen in other galaxies, we find no variation in the grain size distribution as a function of local radiation field strength. Absolute PAH feature intensities as measured by a ratio of PAH/(24 micron) radiances are seen to vary both positionally within a given galaxy, and from one galaxy to another when integrated over the full observed extent of each system. We examine direct comparisons of CC mode PAH ratios (7.7 micron)/(6.2 micron) and (8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7 micron)/(11.3 micron). We find little variation in either mode, and no difference in trends between modes. While the local conditions change markedly over the observed regions of these galaxies, the properties of PAH emission show a remarkable degree of uniformity. Comment: Astrophysical Journal, in press
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    ABSTRACT: The cryogenic Spitzer campaign to monitor the evolution of SNR 1987A has succeeding in revealing a steady change in the brightness of emission from silicate dust in equatorial ring of the SN progenitor. The change in brightness is well correlated with the X-ray emission. However, the Spitzer IRAC and IRS data unexpectedly revealed a second, hotter dust component. The data did not provide a distinct spectroscopic signature of this dust, but did indicate that it may be evolving at a slightly different rate than the dominant silicate component. In this program we will use a total of 0.4 hr to obtain 3.6 and 4.5 micron photometry of SNR 1987A at two additional epochs. This will allow us to determine the continued evolution of this hot dust component, which provides unique constraints on the location, composition, grain size distribution, and possible destruction of this dust.
  • R. D. Gehrz · E. E. Becklin
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    ABSTRACT: The joint U.S. and German Stratospheric Observatory for Infrared Astronomy (SOFIA) will be a premier facility for studying the physics and chemistry of the stellar evolution process for many decades. SOFIA's first-generation instrument complement includes broadband imagers, moderate resolution spectrographs capable of resolving broad features due to dust and large molecules, and high resolution spectrometers suitable for kinematic studies of molecular and atomic gas lines at km/s resolution. SOFIA spectroscopic science applications will be discussed, with special emphasis on investigations related to infrared spectroscopy of astrophysical gas, grains, and ices. Examples will be given of imaging and spectroscopic studies of protostars, obscured sources in molecular cloud cores, circumstellar disks around young stellar objects, remnants of nova and supernova explosions, and winds of evolved stellar systems.
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    ABSTRACT: We report the first results of imaging the Carina Nebula with Spitzer/IRAC, providing a catalog of point sources and YSOs based on SED fits. We discuss several aspects of the extended emission, including dust pillars that result when a clumpy molecular cloud is shredded by massive star feedback. There are few "extended green objects" (EGOs) normally taken as signposts of outflow activity, and none of the HH jets detected optically are seen as EGOs. A population of "extended red objects" tends to be found around OB stars, some with clear bow-shocks. These are dusty shocks where stellar winds collide with flows off nearby clouds. Finally, the relative distributions of O stars and subclusters of YSOs as compared to dust pillars shows that while some YSOs are located within pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by massive star feedback. As pillars are destroyed, they leave newly formed stars in their wake, which are then subsumed into the young OB association. Altogether, the current generation of YSOs shows no strong deviation from a normal IMF. The number of YSOs suggests a roughly constant star-formation rate over the past 3Myr, implying that star formation in pillars constitutes an important mechanism to construct unbound OB associations. Accelerated pillars may give birth to O-type stars that, after several Myr, could appear to have formed in isolation. Comment: 25 pages, 15 figures, MNRAS accepted
    Monthly Notices of the Royal Astronomical Society 04/2010; DOI:10.1111/j.1365-2966.2010.16792.x · 5.23 Impact Factor

Publication Stats

2k Citations
447.58 Total Impact Points

Institutions

  • 1990–2012
    • University of Minnesota Duluth
      • Department of Physics
      Duluth, Minnesota, United States
  • 2003–2004
    • University of Minnesota Twin Cities
      • School of Physics and Astronomy
      Minneapolis, Minnesota, United States
  • 1997
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1976–1987
    • University of Wyoming
      Ларами, Wyoming, United States
  • 1981
    • University of Texas at Austin
      Austin, Texas, United States