A. Tanner

Mississippi State University, Mississippi, United States

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Publications (85)204.37 Total impact

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    ABSTRACT: We used the CHARA Array to directly measure the angular diameter of HD 69830, home to three Neptune mass planets and an asteroid belt. Our measurement of 0.674+/-0.014 milli-arcseconds for the limb-darkened angular diameter of this star leads to a physical radius of R$_*$ = 0.9058$\pm$0.0190 R\sun and luminosity of L* = 0.622+/-0.014 Lsun when combined with a fit to the spectral energy distribution of the star. Placing these observed values on an Hertzsprung-Russel (HR) diagram along with stellar evolution isochrones produces an age of 10.6+/-4 Gyr and mass of 0.863$\pm$0.043 M\sun. We use archival optical echelle spectra of HD 69830 along with an iterative spectral fitting technique to measure the iron abundance ([Fe/H]=-0.04+/-0.03), effective temperature (5385+/-44 K) and surface gravity (log g = 4.49+/-0.06). We use these new values for the temperature and luminosity to calculate a more precise age of 7.5+/-Gyr. Applying the values of stellar luminosity and radius to recent models on the optimistic location of the habitable zone produces a range of 0.61-1.44 AU; partially outside the orbit of the furthest known planet (d) around HD 69830. Finally, we estimate the snow line at a distance of 1.95+/-0.19 AU, which is outside the orbit of all three planets and its asteroid belt.
    12/2014;
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    ABSTRACT: We outline an approach yielding proper motions with higher precision than exists in present catalogs for a sample of stars in the Kepler field. To increase proper motion precision we combine first moment centroids of Kepler pixel data from a single Season with existing catalog positions and proper motions. We use this astrometry to produce improved reduced proper motion diagrams, analogous to a Hertzsprung-Russell diagram, for stars identified as Kepler Objects of Interest. The more precise the relative proper motions, the better the discrimination between stellar luminosity classes. With UCAC4 and PPMXL epoch 2000 positions (and proper motions from those catalogs as quasi-bayesian priors) astrometry for a single test Channel (21) and Season (0) spanning two years yields proper motions with an average per-coordinate proper motion error of 1.0 millisecond of arc per year, over a factor of three better than existing catalogs. We apply a mapping between a reduced proper motion diagram and an HR diagram, both constructed using HST parallaxes and proper motions, to estimate Kepler Object of Interest K-band absolute magnitudes. The techniques discussed apply to any future small-field astrometry as well as the rest of the Kepler field.
    The Astronomical Journal 08/2014; 148(6). · 4.97 Impact Factor
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    ABSTRACT: We have built and commissioned gas absorption cells for precision spectroscopic radial velocity measurements in the near-infrared in the H and K bands. We describe the construction and installation of three such cells filled with 13CH4, 12CH3D, and 14NH3 for the CSHELL spectrograph at the NASA Infrared Telescope Facility (IRTF). We have obtained their high-resolution laboratory Fourier Transform spectra, which can have other practical uses. We summarize the practical details involved in the construction of the three cells, and the thermal and mechanical control. In all cases, the construction of the cells is very affordable. We are carrying out a pilot survey with the 13CH4 methane gas cell on the CSHELL spectrograph at the IRTF to detect exoplanets around low mass and young stars. We discuss the current status of our survey, with the aim of photon-noise limited radial velocity precision. For adequately bright targets, we are able to probe a noise floor of 7 m/s with the gas cell with CSHELL at cassegrain focus. Our results demonstrate the feasibility of using a gas cell on the next generation of near-infrared spectrographs such as iSHELL on IRTF, iGRINS, and an upgraded NIRSPEC at Keck.
    Proc SPIE 09/2013;
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    ABSTRACT: We have built and commissioned a prototype agitated non-circular core fiber scrambler for precision spectroscopic radial velocity measurements in the near-infrared H band. We have collected the first on-sky performance and modal noise tests of these novel fibers in the near-infrared at H and K bands using the CSHELL spectrograph at the NASA InfraRed Telescope Facility (IRTF). We discuss the design behind our novel reverse injection of a red laser for co-alignment of star-light with the fiber tip via a corner cube and visible camera. We summarize the practical details involved in the construction of the fiber scrambler, and the mechanical agitation of the fiber at the telescope. We present radial velocity measurements of a bright standard star taken with and without the fiber scrambler to quantify the relative improvement in the obtainable blaze function stability, the line spread function stability, and the resulting radial velocity precision. We assess the feasibility of applying this illumination stabilization technique to the next generation of near-infrared spectrographs such as iSHELL on IRTF and an upgraded NIRSPEC at Keck. Our results may also be applied in the visible for smaller core diameter fibers where fiber modal noise is a significant factor, such as behind an adaptive optics system or on a small < 1 meter class telescope such as is being pursued by the MINERVA and LCOGT collaborations.
    Proc SPIE 09/2013;
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    ABSTRACT: We present the results of an infrared spectroscopic survey of 23 late-M dwarfs with the NIRSPEC echelle spectrometer on the Keck II telescope. Using telluric lines for wavelength calibration, we are able to achieve measurement precisions of down to 45 m s{sup -1} for our late-M dwarfs over a one- to four-year long baseline. Our sample contains two stars with radial velocity (RV) variations of >1000 m s{sup -1}. While we require more measurements to determine whether these RV variations are due to unseen planetary or stellar companions or are the result of starspots known to plague the surface of M dwarfs, we can place upper limits of <40 M{sub J} sin i on the masses of any companions around those two M dwarfs with RV variations of <160 m s{sup -1} at orbital periods of 10-100 days. We have also measured the rotational velocities for all the stars in our late-M dwarf sample and offer our multi-order, high-resolution spectra over 2.0-2.4 {mu}m to the atmospheric modeling community to better understand the atmospheres of late-M dwarfs.
    The Astrophysical Journal Supplement Series 11/2012; 203(1). · 16.24 Impact Factor
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    ABSTRACT: In December 2010, NASA created a Science Definition Team (SDT) for WFIRST, the Wide Field Infra-Red Survey Telescope, recommended by the Astro 2010 Decadal Survey as the highest priority for a large space mission. The SDT was chartered to work with the WFIRST Project Office at GSFC and the Program Office at JPL to produce a Design Reference Mission (DRM) for WFIRST. Part of the original charge was to produce an interim design reference mission by mid-2011. That document was delivered to NASA and widely circulated within the astronomical community. In late 2011 the Astrophysics Division augmented its original charge, asking for two design reference missions. The first of these, DRM1, was to be a finalized version of the interim DRM, reducing overall mission costs where possible. The second of these, DRM2, was to identify and eliminate capabilities that overlapped with those of NASA's James Webb Space Telescope (henceforth JWST), ESA's Euclid mission, and the NSF's ground-based Large Synoptic Survey Telescope (henceforth LSST), and again to reduce overall mission cost, while staying faithful to NWNH. This report presents both DRM1 and DRM2.
    08/2012;
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    ABSTRACT: We report on our analysis of HST/NICMOS snapshot high resolution images of 255 stars in 201 systems within ~10 parsecs of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas/pixel, NICMOS can easily resolve binaries with sub-arcsecond separations in the 19".5x19".5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0{-0.0}^{+3.5}% for L companions to M dwarfs in the separation range of 5 to 70 AU, and 2.3_{-0.7}^{+5.0}% for L and T companions to M dwarfs in the separation range of 10 to 70 AU. We also discuss trends in the color-magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called "brown dwarf desert" extends to binary systems with low mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.
    The Astronomical Journal 06/2012; 144(2). · 4.97 Impact Factor
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    ABSTRACT: We have built a single gas absorption cell for precision spectroscopic radial velocity measurements in the near-infrared. We are currently carrying out a survey with the CSHELL spectrograph at the NASA InfraRed Telescope Facility to detect exoplanets around red, low mass, and young stars. We discuss the current status of our survey, with the aim of 20 m/s long-term photon-noise limited radial velocity precision at 2.3 microns. We present the design of a near-infrared fiber scrambler with first light in May 2012 with CSHELL at IRTF. The fiber scrambler makes use of non-circular core fibers to stabilize the illumination of the slit and echelle grating against changes in seeing, focus, guiding and other sources of systematic radial velocity noise, complementing the wavelength calibration of a gas cell.
    05/2012;
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    ABSTRACT: We present a high-precision infrared radial velocity study of late-type stars using spectra obtained with NIRSPEC at the W. M. Keck Observatory. Radial velocity precisions of 50 m/s are achieved for old field mid-M dwarfs using telluric features for precise wavelength calibration. Using this technique, 20 young stars in the {\beta} Pic (age ~12 Myr) and TW Hya (age ~8 Myr) Associations were monitored over several years to search for low mass companions; we also included the chromospherically active field star GJ 873 (EV Lac) in this survey. Based on comparisons with previous optical observations of these young active stars, radial velocity measurements at infrared wavelengths mitigate the radial velocity noise caused by star spots by a factor of ~3. Nevertheless, star spot noise is still the dominant source of measurement error for young stars at 2.3 {\mu}m, and limits the precision to ~77 m/s for the slowest rotating stars (v sin i < 6 km/s), increasing to ~168 m/s for rapidly rotating stars (v sin i > 12 km/s). The observations reveal both GJ 3305 and TWA 23 to be single-lined spectroscopic binaries; in the case of GJ 3305, the motion is likely caused by its 0.09" companion, identified after this survey began. The large amplitude, short-timescale variations of TWA 13A are indicative of a hot Jupiter-like companion, but the available data are insufficient to confirm this. We label it as a candidate radial velocity variable. For the remainder of the sample, these observations exclude the presence of any 'hot' (P < 3 days) companions more massive than 8 MJup, and any 'warm' (P < 30 days) companions more massive than 17 MJup, on average. Assuming an edge-on orbit for the edge-on disk system AU Mic, these observations exclude the presence of any hot Jupiters more massive than 1.8 MJup or warm Jupiters more massive than 3.9 MJup.
    The Astrophysical Journal 02/2012; 749(1). · 6.73 Impact Factor
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    ABSTRACT: RECONS (www.recons.org, REsearch Consortium On Nearby Stars) has been using astrometric techniques since 1999 to search for massive planets orbiting more than 130 nearby red and white dwarfs. Because of their proximity, nearby stars are natural locations to search for other solar systems --- the stars provide increased fluxes, larger astrometric perturbations, and higher probabilities for eventual resolution of planets than similar stars at larger distances. Unlike radial velocity searches, our astrometric effort is most sensitive to Jovian planets in Jovian orbits, i.e. those that span decades. We have discovered stellar companions with masses of a few hundred Jupiters, brown dwarf companions with masses of a few tens of Jupiters, and are now pushing into the realm of planets with masses of a few Jupiters around the nearest red dwarfs. Several previously unknown companions have been imaged via Gemini-AO observations, but we have also detected perturbations caused by enigmatic companions that elude direct detection. As we sweep through the mass regimes of stars to exoplanets for companions, we are now able to assess the various populations --- stars are common as companions, whereas brown dwarfs and massive planets are rare. We outline what we have discovered so far and place our exoplanet search results in context with an overview of the census of more than 60 stars with exoplanets known within 25 pc. This effort is supported by the NSF through grant AST-0908402 and via observations made possible by the SMARTS Consortium.
    01/2012;
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    ABSTRACT: Since 1999, RECONS (www.recons.org, REsearch Consortium On Nearby Stars) has been using astrometric techniques to search for massive planets orbiting ∼100 nearby stars. Unlike radial velocity searches, our astrometric effort is most sensitive to Jovian planets in Jovian orbits, i.e. those that span decades. To date, we have detected companion stars and brown dwarfs, as well as one enigmatic unseen companion that eludes direct detection, and are currently pushing into the realm of Jovian planets. As a result, we are now able to assess the population of companions orbiting the nearest stars.
    10/2011;
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    ABSTRACT: The main-sequence solar-type star HD69830 has an unusually large amount of dusty debris orbiting close to three planets found via the radial velocity technique. In order to explore the dynamical interaction between the dust and planets, we have performed multi-epoch photometry and spectroscopy of the system over several orbits of the outer dust. We find no evidence for changes in either the dust amount or its composition, with upper limits of 5-7% (1 $\sigma$ per spectral element) on the variability of the {\it dust spectrum} over 1 year, 3.3% (1 $\sigma$) on the broad-band disk emission over 4 years, and 33% (1 $\sigma$) on the broad-band disk emission over 24 years. Detailed modeling of the spectrum of the emitting dust indicates that the dust is located outside of the orbits of the three planets and has a composition similar to main-belt, C-type asteroids asteroids in our solar system. Additionally, we find no evidence for a wide variety of gas species associated with the dust. Our new higher SNR spectra do not confirm our previously claimed detection of H$_2$O ice leading to a firm conclusion that the debris can be associated with the break-up of one or more C-type asteroids formed in the dry, inner regions of the protoplanetary disk of the HD69830 system. The modeling of the spectral energy distribution and high spatial resolution observations in the mid-infrared are consistent with a $\sim$ 1 AU location for the emitting material.
    The Astrophysical Journal 08/2011; 743(1). · 6.73 Impact Factor
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    ABSTRACT: RECONS (Research Consortium On Nearby Stars) began using the CTIO 0.9m telescope in 1999 to carry out an extensive survey of the nearby stars under the auspices of the NOAO Surveys Program. This effort expanded during the SMARTS era from its original astrometric focus to discover new nearby stars via trigonometric parallaxes to include a reconnaissance of the solar neighborhood using photometric and spectroscopic techniques as well. In this talk, we will highlight the truly long-term science that has only been possible because of the SMARTS Consortium. Up to a decade of astrometric (and photometric) observations of more than 100 of the nearest red and white dwarfs have been made, in an effort to reveal unseen stellar, brown dwarf, and massive planetary companions. What we are finding is that red dwarfs have many stellar companions, but few brown dwarfs or massive planets. We will also highlight statistics that show that red dwarfs really are in charge of the solar neighborhood, and present results on our new photometric effort to identify hundreds more red dwarfs within 25 pc. Finally, when making careful observations of more than 500 nearby stars, a few pop out as extraordinary targets. Among the most intriguing are a possible neutron star only 30 pc away, a few highly unusual white dwarfs, and a bizarre red dwarf binary in which the components are twins in every measureable way ... except that one is four times brighter than another. This work has been supported by the National Science Foundation (AST 05-07711 and 09-08402), NASA's Space Interferometry Mission, Georgia State University, and Northern Arizona University.
    05/2011;
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    ABSTRACT: We provide precise J2000/epoch 2000 coordinates and cross-identifications to sources in the 2MASS Point Source Catalog for nearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs of nearby stars. The only Gliese objects where we were not successful are two Gliese sources that are actually QSOs; two proposed companions to brighter stars, which we believe do not exist; four stars included in one of the catalogs but identified there as only optical companions; one probable plate flaw; and two stars that simply remain unrecovered. For the 4251 recovered stars, 2693 have coordinates based on Hipparcos positions, 1549 have coordinates based on 2MASS data, and 9 have positions from other astrometric sources. All positions have been calculated at epoch 2000 using proper motions from the literature, which are also given here. (2 data files).
    VizieR Online Data Catalog. 03/2011;
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    ABSTRACT: We propose to examine ~400 nearby southern M dwarfs for companions at wide separations (1 arcsec - 3 arcmin) using the 0.9m at CTIO as part of a larger systematic study of ~1200 nearby M dwarfs. The results, when combined with a complementary northern search at Lowell Observatory and a SuperCOSMOS blinking search for companions to 10 arcmin, will allow statistical analyses of the nearby M dwarf population, refinement of the solar neighborhood membership roster, and improvement of the mass function for these objects at the end of the main sequence. This robust sample includes a carefully selected list of red dwarfs that have trigonometric parallaxes placing them nearer than 25 pc, which eliminates biases inherent to photometrically-selected samples. This will be the largest, most comprehensive, study ever done of the multiplicity of the most common stars in the Galaxy. The proposed observations will comprise a significant portion of the PI's PhD thesis.
    NOAO Proposal. 02/2011;
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    ABSTRACT: RECONS (Research Consortium on Nearby Stars, www.recons.org) is expanding its careful reconnaissance of the solar neighborhood from the original 10 parsec core sample to 25 parsecs. The resulting RECONS Database will expand the number of stellar systems for which we have detailed observations (astrometry, photometry, spectroscopy) and derived information (metallicity, multiplicity, exoplanets) from about 250 systems to 4000-6000 systems. Here we outline the new-and-improved 25 parsec sample, significantly updated from the NStars effort of a decade ago. To date, only 2000 systems have trigonometric parallaxes placing them within 25 parsecs, with more than 10% contributed by the RECONS effort at CTIO. In fact, an accurate estimate of the expected population is elusive, as new systems are being found as close as 4 parsecs, making extrapolations to 25 parsecs highly uncertain. We look forward to upcoming contributions made to the census by RECONS, SkyMapper, Pan-STARRS, LSST, and Gaia, and discuss how each of these efforts will play a role. During the next decade, census work is important because of surging interest in the nearest stars --- our stellar neighbors will offer the best answers to such fundamental questions as "What types of stars really populate the Galaxy?", "How many planets orbit nearby stars?", and "Is there life on any of those planets?" This work is currently supported by the NSF under AST 09-08402.
    01/2011;
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    ABSTRACT: With the development of extreme high contrast ground-based adaptive optics instruments and space missions aimed at detecting and characterizing Jupiter- and terrestrial-mass planets, it is critical that each target star be thoroughly vetted to determine whether it is a viable target, given both the instrumental design and scientific goals of the program. With this in mind, we have conducted a high-contrast imaging survey of mature AFGKM stars with the PALAO/PHARO instrument on the Palomar 200 inch telescope. The survey reached sensitivities sufficient to detect brown dwarf companions at separations of >50 AU . The results of this survey will be utilized both by future direct imaging projects such as GPI, SPHERE, and P1640 and indirect detection missions such as SIM Lite. Out of 84 targets, all but one have no close-in (0.45–1″) companions and 64 (76%) have no stars at all within the 25″ field of view. The sensitivity contrasts in the passband ranged from 4.5 to 10 for this set of observations. These stars were selected as the best nearby targets for habitable planet searches because of their long-lived habitable zones ( >1 billion years). We report two stars, GJ 454 and GJ 1020, with previously unpublished proper motion companions. In both cases, the companions are stellar in nature and are most likely M dwarfs based on their absolute magnitudes and colors. Based on our mass sensitivities and level of completeness, we can place an upper limit of ∼17% on the presence of brown dwarf companions with masses >40 at separations of >1″ . We also discuss the importance of including statistics on those stars with no detected companions in their field of view for the sake of future companion searches and an overall understanding of the population of low-mass objects around nearby stars.
    Publications of the Astronomical Society of the Pacific 10/2010; 122(896):1195-1206. · 3.69 Impact Factor
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    ABSTRACT: A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in scattered visible light with the ACS coronagraph on the Hubble Space Telescope (HST) and in thermal emission using MIPS on the Spitzer Space Telescope at λ = 70 μm (resolved) and 160 μm (unresolved). Spitzer IRS (λ = 7-35 μm) and MIPS (λ = 55-90 μm) spectrographs measured disk emission at λ> 28 μm. In the HST image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and is inclined by 60° from pole-on. At 70 μm, it appears partially resolved and is elongated in the same direction and with nearly the same size as seen with HST in scattered light. At 0.6 μm, the ring shows no significant brightness asymmetry, implying little or no forward scattering by its constituent dust. With a mean surface brightness of V = 23.7 mag arcsec^(–2), it is the faintest disk imaged to date in scattered light. We model the ring's infrared spectral energy distribution (SED) using a dust population fixed at the location where HST detects the scattered light. The observed SED is well fit by this model, with no requirement for additional unseen debris zones. The firm constraint on the dust radial distance breaks the usual grain size-distance degeneracy that exists in modeling of spatially unresolved disks, and allows us to infer a minimum grain size of ~2.8 μm and a dust size distribution power-law spectral index of –3.9. An albedo of ~5% is inferred from the integrated brightness of the ring in scattered light. The low-albedo and isotropic scattering properties are inconsistent with Mie theory for astronomical silicates with the inferred grain size and show the need for further modeling using more complex grain shapes or compositions. Brightness limits are also presented for six other main-sequence stars with strong Spitzer excess around which HST detects no circumstellar nebulosity (HD 10472, HD 21997, HD 38206, HD 82943, HD 113556, and HD 138965).
    The Astronomical Journal 09/2010; · 4.97 Impact Factor
  • 08/2010;
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    ABSTRACT: We provide precise J2000, epoch 2000 coordinates and cross-identifications to sources in the 2MASS point source catalog for nearly all stars in the Gliese, Gliese and Jahreiss, and Woolley catalogs of nearby stars. The only Gliese objects where we were not successful are two Gliese sources that are actually QSOs, two proposed companions to brighter stars which we believe do not exist, four stars included in one of the catalogs but identified there as only optical companions, one probable plate flaw, and two stars which simply remain un-recovered. For the 4251 recovered stars, 2693 have coordinates based on Hipparcos positions, 1549 have coordinates based on 2MASS data, and 9 have positions from other astrometric sources. All positions have been calculated at epoch 2000 using proper motions from the literature, which are also given here. Comment: accepted to PASP, Full version of Table 1 available electronically
    06/2010;

Publication Stats

1k Citations
204.37 Total Impact Points

Institutions

  • 2012–2014
    • Mississippi State University
      • Department of Physics and Astronomy
      Mississippi, United States
  • 2009–2012
    • Georgia State University
      • Department of Physics and Astronomy
      Atlanta, Georgia, United States
  • 2006–2011
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States
  • 2003
    • W. M. Keck Observatory
      Hilo, Hawaii, United States
  • 1970–2003
    • University of California, Los Angeles
      • • Institute of Geophysics and Planetary Physics
      • • Department of Physics and Astronomy
      Los Angeles, CA, United States