Robert D. Gehrz

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (150)403.62 Total impact

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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.
    01/2014;
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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.
    Spitzer Proposal. 10/2013;
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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.
    Spitzer Proposal. 12/2012;
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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). · 6.73 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; · 6.73 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). · 6.73 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. · 5.52 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.
    Spitzer Proposal. 05/2011;
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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.
    Spitzer Proposal. 05/2011;
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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.
    Spitzer Proposal. 04/2011;
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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. · 4.97 Impact Factor
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    ABSTRACT: We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a five year period spanning the epochs between days ~6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant 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 ~5 × 106 K and ~3 × 104 cm–3, respectively. The mass of the radiating dust is ~1.2 × 10–6 M ☉ on day 7554 and scales linearly with IR flux. Comparison of the IR data with the soft X-ray flux derived from Chandra observations shows that the IR-to-X-ray flux ratio, IRX, is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of IRX is most consistent with the scenario that very little grain processing or gas cooling has occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased by a factor of ~5 with a time dependence of t'0.87± 0.20, 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, as also suggested by X-ray observations. The constant spectral shape of the IR emission provides strong constraints on the density and temperature of the shocked gas in which the interaction takes place. Silicate grains, with radii of ~0.2 μm and temperature of T ~ 180 K, best fit the spectral and temporal evolution of the ~8-30 μm data. The IR spectra also show the presence of a secondary population of very small, hot (T 350 K), featureless dust. If these grains spatially coexist with the silicates, then they must have shorter lifetimes. The data show slightly different rates of increase of their respective fluxes, lending some support to this hypothesis. However, the origin of this emission component and the exact nature of its relation to the silicate emission is still a major unsolved puzzle.
    The Astrophysical Journal 09/2010; 722(1):425. · 6.73 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
    09/2010;
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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.
    Spitzer Proposal. 06/2010;
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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; · 5.52 Impact Factor
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    ABSTRACT: We propose to exploit the wide-field capabilities of the NEWFIRM camera to carry out a near-infrared emission-line imaging survey of the Triangulum Galaxy - M33. At a distance of 840 kpc, M33 is the nearest star-forming spiral galaxy of low inclination - where the entire disk can be resolved from the ground at ~4 parsec scales. Its rich assortment of star-forming environments span a significant range of metallicities and intensities, thus enabling a diagnostic study of the environmental factors that mediate star-forming activity (and vice versa). We intend to image the entire galaxy in the light of ionized iron (1.64um), molecular hydrogen (2.12um), recombining ionized hydrogen (2.16um), and associated continua (using the broadband J, H, and K_S bands). The resulting images of spectral-line and continuum emission will reveal important indices of the multiphase star-forming environments and their variations across the galaxy. Comparisons with complementary ultraviolet, optical, mid-infrared, radio, and X-ray surveys of M33 will lead to further insights on the structuring and powering of stellar-nebular "ecosystems" in star-forming galaxies - including the more powerful starbursting systems at high redshift.
    NOAO Proposal. 08/2009;
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    The Astronomical Journal 05/2009; 137(6):5155. · 4.97 Impact Factor
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    ABSTRACT: This white paper is submitted to the Astronomy and Astrophysics 2010 Decadal Survey (Astro2010)1 Committee on the State of the Profession to emphasize the potential of the Stratospheric Observatory for Infrared Astronomy (SOFIA) to contribute to the training of instrumentalists and observers, and to related technology developments. This potential goes beyond the primary mission of SOFIA, which is to carry out unique, high priority astronomical research. SOFIA is a Boeing 747SP aircraft with a 2.5 meter telescope. It will enable astronomical observations anywhere, any time, and at most wavelengths between 0.3 microns and 1.6 mm not accessible from ground-based observatories. These attributes, accruing from the mobility and flight altitude of SOFIA, guarantee a wealth of scientific return. Its instrument teams (nine in the first generation) and guest investigators will do suborbital astronomy in a shirt-sleeve environment. The project will invest $10M per year in science instrument development over a lifetime of 20 years. This, frequent flight opportunities, and operation that enables rapid changes of science instruments and hands-on in-flight access to the instruments, assure a unique and extensive potential - both for training young instrumentalists and for encouraging and deploying nascent technologies. Novel instruments covering optical, infrared, and submillimeter bands can be developed for and tested on SOFIA by their developers (including apprentices) for their own observations and for those of guest observers, to validate technologies and maximize observational effectiveness.
    04/2009;
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    ABSTRACT: We present the third and final part of a census of Asymptotic Giant Branch (AGB) stars in Local Group dwarf irregular galaxies. Papers I and II presented the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 are presented, along with a more thorough treatment of background galaxy contamination than was presented in papers I and II. We find that at least a small population of completely optically obscured AGB stars exists in each galaxy, regardless of the galaxy's metallicity, but that higher-metallicity galaxies tend to harbor more stars with slight IR excesses. The optical incompleteness increases for the redder AGB stars, in line with the expectation that some AGB stars are not detected in the optical due to large amounts of extinction associated with in situ dust production. Overall, there is an underrepresentation of 30% - 40% in the optical AGB within the 1 sigma errors for all of the galaxies in our sample. This undetected population is large enough to affect star formation histories derived from optical color-magnitude diagrams. As measured from the [3.6] - [4.5] color excesses, we find average stellar mass-loss rates ranging from 3.1E-7 - 6.6E-6 solar masses per year, and integrated galaxy mass-loss rates ranging from 4.4E-5 - 1.4E-3 solar masses per year. The integrated mass-loss rate is sufficient to sustain the current star formation rate in only LGS 3 and DDO 210, requiring either significant non-dusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.
    The Astrophysical Journal 03/2009; 697. · 6.73 Impact Factor
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    Eric E. Becklin, Robert D. Gehrz
    Spie Newsroom. 01/2009;

Publication Stats

2k Citations
403.62 Total Impact Points

Institutions

  • 2000–2012
    • University of Minnesota Duluth
      • Department of Physics
      Duluth, Minnesota, United States
  • 2003–2010
    • University of Minnesota Twin Cities
      • School of Physics and Astronomy
      Minneapolis, MN, United States
  • 1981
    • University of Texas at Austin
      Austin, Texas, United States
  • 1976–1978
    • University of Wyoming
      Laramie, Wyoming, United States