K. C. Roth

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (80)248.88 Total impact

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    ABSTRACT: The Gemini Remote Access to CFHT ESPaDONS Spectrograph has achieved first light of its experimental phase in May 2014. It successfully collected light from the Gemini North telescope and sent it through two 270 m optical fibers to the the ESPaDOnS spectrograph at CFHT to deliver high-resolution spectroscopy across the optical region. The fibers gave an average focal ratio degradation of 14% on sky, and a maximum transmittance of 85% at 800nm. GRACES achieved delivering spectra with a resolution power of R = 40,000 and R = 66,000 between 400 and 1,000 nm. It has a ~8% throughput and is sensitive to target fainter than 21st mag in 1 hour. The average acquisition time of a target is around 10 min. This project is a great example of a productive collaboration between two observatories on Maunakea that was successful due to the reciprocal involvement of the Gemini, CFHT, and NRC Herzberg teams, and all the staff involved closely or indirectly.
    09/2014;
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    ABSTRACT: We present the discovery and subsequent spectroscopy with Gemini-North of the optical afterglow of the Swift gamma-ray burst (GRB) 140515A. The spectrum exhibits a well-detected continuum at wavelengths longer than 8915 Angs with a steep decrement to zero flux blueward of 8910 Angs due to Ly-alpha absorption at redshift z~6.33. Some transmission through the Lyman-alpha forest is present at 5.2<z<5.733, but none is detected at higher redshift, consistent with previous measurements from quasars and GRB 130606A. We model the red damping wing of Lyman-alpha in three ways that provide equally good fits to the data: (a) a single host galaxy absorber at z=6.327 with log(N_HI)=18.62+/-0.08; (b) pure intergalactic medium (IGM) absorption from z=6.0 to z=6.328 with a constant neutral hydrogen fraction of x_HI=0.056+0.011-0.027; and (c) a hybrid model with a host absorber located within an ionized bubble of radius 10 comoving Mpc in an IGM with x_HI=0.12+/-0.05 (x_HI<0.21 at the 2-sigma level). Regardless of the model, the sharpness of the dropoff in transmission is inconsistent with a substantial neutral fraction in the IGM at this redshift. No narrow absorption lines from the host galaxy are detected, indicating a host metallicity of [Z/H]<~ -0.8. Even if we assume that all of the hydrogen absorption is due to the host galaxy, the column is unusually low for a GRB sightline, similar to two out of the other three highest-redshift bursts with measured log(N_HI). This is possible evidence that the escape fraction of ionizing photons from normal star-forming galaxies increases at z>~6.
    05/2014;
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    ABSTRACT: In the past decade, several rapidly-evolving transients have been discovered whose timescales and luminosities are not easily explained by traditional supernovae (SN) models. The sample size of these objects has remained small due, at least in part, to the challenge of detecting short timescale transients with traditional survey cadences. Here we present the results from a search within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly-evolving and luminous transients. We identify 10 new transients with a time above half-maximum of less than 12 days and -16.5 > M > -20 mag. This increases the number of known events in this region of SN phase space by roughly a factor of three. The median redshift of the PS1-MDS sample is z=0.275 and they all exploded in star forming galaxies. In general, the transients possess faster rise than decline timescale and blue colors at maximum light (g - r < -0.2). Best fit blackbodies reveal photospheric temperatures/radii that expand/cool with time and explosion spectra taken near maximum light are dominated by a blue continuum, consistent with a hot, optically thick, ejecta. We find it difficult to reconcile the short timescale, high peak luminosity (L > 10^43 erg/s), and lack of UV line blanketing observed in many of these transients with an explosion powered mainly by the radioactive decay of Ni-56. Rather, we find that many are consistent with either (1) cooling envelope emission from the explosion of a star with a low-mass extended envelope which ejected very little (<0.03 M_sun) radioactive material, or (2) a shock breakout within a dense, optically thick, wind surrounding the progenitor star. After calculating the detection efficiency for objects with rapid timescales in the PS1-MDS we find a volumetric rate of 3000 - 5500 events/yr/Gpc^3 (1-6% of the core-collapse SN rate at z=0.2).
    05/2014;
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    ABSTRACT: The Gemini Multi-Object Spectrographs (GMOS) at both Gemini North and South have provided crucial access to longslit, MOS and IFU moderate resolution optical spectroscopy for the Gemini international partnership for over a decade. The interim installment of e2v deep depletion CCDs at GMOS-N in November 2011, providing enhanced red sensitivity, was the first major upgrade for either GMOS since the implementation of the Nod&Shuffle mode in 2002. We present plans to replace the original EEV detectors in GMOS-S with new Hamamatsu CCDs, extending wavelength coverage out beyond 1.03 microns. GMOS-N upgrade to Hamamatsu CCDs will follow the successful deployment on GMOS-S. With the extension of GMOS sensitivity further to long wavelengths it becomes even more attractive to extend the number of observing modes to include adaptive optics imaging and spectroscopy. As has already been demonstrated with GEMS/GMOS-S imaging, adaptive optics in the 0.8-1 micron wavelength regime on Gemini can effectively transform IQ70 conditions to IQ20 and more than double the spatial resolution over the natural seeing. We present plans to move forward with plans to enable GMOS + adaptive optics as a regular user mode at both sites.
    01/2014;
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    ABSTRACT: We present the Pan-STARRS1 discovery of the long-lived and blue transient PS1-11af, which was also detected by GALEX with coordinated observations in the near-ultraviolet (NUV) band. PS1-11af is associated with the nucleus of an early-type galaxy at redshift z=0.4046 that exhibits no evidence for star formation or AGN activity. Four epochs of spectroscopy reveal a pair of transient broad absorption features in the UV on otherwise featureless spectra. Despite the superficial similarity of these features to P-Cygni absorptions of supernovae (SNe), we conclude that PS1-11af is not consistent with the properties of known types of SNe. Blackbody fits to the spectral energy distribution are inconsistent with the cooling, expanding ejecta of a SN, and the velocities of the absorption features are too high to represent material in homologous expansion near a SN photosphere. However, the constant blue colors and slow evolution of the luminosity are similar to previous optically-selected tidal disruption events (TDEs). The shape of the optical light curve is consistent with models for TDEs, but the minimum accreted mass necessary to power the observed luminosity is only ~0.002M_sun, which points to a partial disruption model. A full disruption model predicts higher bolometric luminosities, which would require most of the radiation to be emitted in a separate component at high energies where we lack observations. In addition, the observed temperature is lower than that predicted by pure accretion disk models for TDEs and requires reprocessing to a constant, lower temperature. Three deep non-detections in the radio with the VLA over the first two years after the event set strict limits on the production of any relativistic outflow comparable to Swift J1644+57, even if off-axis.
    The Astrophysical Journal 09/2013; 780(1). · 6.73 Impact Factor
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    ABSTRACT: We present high signal-to-noise ratio Gemini and MMT spectroscopy of the optical afterglow of the gamma-ray burst (GRB) 130606A at redshift z=5.913, discovered by Swift. This is the first high-redshift GRB afterglow to have spectra of comparable quality to those of z~6 quasars. The data exhibit a smooth continuum at near-infrared wavelengths that is sharply cut off blueward of 8410 Angs due to absorption from Ly-alpha at redshift z~5.91, with some flux transmitted through the Ly-alpha forest between 7000-7800 Angs. We use column densities inferred from metal absorption lines to constrain the metallicity of the host galaxy between a lower limit of [Si/H]>-1.7 and an upper limit of [S/H]<-0.5 set by the non-detection of S II absorption. We demonstrate consistency between the dramatic evolution in the transmission fraction of Ly-alpha seen in this spectrum over the redshift range z=4.9 to 5.85 with that previously measured from observations of high-redshift quasars. There is an extended redshift interval of Delta-z=0.12 in the Ly-alpha forest at z=5.77 with no detected transmission, leading to a 3-sigma upper limit on the mean Ly-alpha transmission fraction of <0.2% (or tau_eff(Ly-alpha) > 6.4). This is comparable to the lowest-redshift Gunn-Peterson troughs found in quasar spectra. We set a 2-sigma upper limit of 0.11 on the neutral fraction of the IGM at the redshift of the GRB from the lack of a Ly-alpha red damping wing, assuming a model with a constant neutral density. Some Ly-beta and Ly-gamma transmission is detected in this redshift window, indicating that it is not completely opaque, and hence that the IGM is nonetheless mostly ionized at these redshifts. GRB 130606A thus for the first time realizes the promise of GRBs as probes of the first galaxies and cosmic reionization.
    The Astrophysical Journal 06/2013; 774(1). · 6.73 Impact Factor
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    ABSTRACT: We present observations and analysis of PS1-10bzj, a superluminous supernova (SLSN) discovered in the Pan-STARRS Medium Deep Survey at a redshift z = 0.650. Spectroscopically, PS1-10bzj is similar to the hydrogen-poor SLSNe 2005ap and SCP 06F6, though with a steeper rise and lower peak luminosity (M_bol = -21.4 mag) than previous events. We construct a bolometric light curve, and show that while PS1-10bzj's energetics were less extreme than previous events, its luminosity still cannot be explained by radioactive nickel decay alone. We explore both a magnetar spin-down and circumstellar interaction scenario and find that either can fit the data. PS1-10bzj is located in the Extended Chandra Deep Field South and the host galaxy is imaged in a number of surveys, including with the Hubble Space Telescope. The host is a compact dwarf galaxy (M_B ~ -18 mag, diameter < 800 pc), with a low stellar mass (M_* ~ 2.4 * 10^7 M_sun), young stellar population (\tau_* ~ 5 Myr), and a star formation rate of ~ 2-3 M_sun/yr. The specific star formation rate is the highest seen in a SLSN host so far (~ 100 Gyr^{-1}). We detect the [O III]\lambda 4363 line, and find a low metallicity: 12+(O/H) = 7.8 +/- 0.2 (~ 0.1 Z_sun). Together, this indicates that at least some of the progenitors of SLSNe come from young, low-metallicity populations.
    The Astrophysical Journal 03/2013; 771(2). · 6.73 Impact Factor
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    ABSTRACT: We present the Pan-STARRS1 discovery of PS1-10afx, a unique hydrogen-deficient superluminous supernova (SLSN) at z=1.388. The light curve peaked at z_P1=21.7 mag, making PS1-10afx comparable to the most luminous known SNe, with M_u = -22.3 mag. Our extensive optical and NIR observations indicate that the bolometric light curve of PS1-10afx rose on the unusually fast timescale of ~12 d to the extraordinary peak luminosity of 4.1e44 erg/s (M_bol = -22.8 mag) and subsequently faded rapidly. Equally important, the SED is unusually red for a SLSN, with a color temperature of 6800 K near maximum light, in contrast to previous H-poor SLSNe, which are bright in the UV. The spectra more closely resemble those of a normal SN Ic than any known SLSN, with a photospheric velocity of 11,000 km/s and evidence for line blanketing in the rest-frame UV. Despite the fast rise, these parameters imply a very large emitting radius (>5e15 cm). We demonstrate that no existing theoretical model can satisfactorily explain this combination of properties: (i) a nickel-powered light curve cannot match the combination of high peak luminosity with the fast timescale; (ii) models powered by the spindown energy of a rapidly-rotating magnetar predict significantly hotter and faster ejecta; and (iii) models invoking shock breakout through a dense circumstellar medium cannot explain the observed spectra or color evolution. The host galaxy is well detected in pre-explosion imaging with a luminosity near L*, a star formation rate of 15 M_sun/yr, and is fairly massive (2e10 M_sun), with a stellar population age of 1e8 yr, also in contrast to the dwarf hosts of known H-poor SLSNe. PS1-10afx is distinct from known examples of SLSNe in its spectra, colors, light-curve shape, and host galaxy properties, suggesting that it resulted from a different channel than other hydrogen-poor SLSNe.
    The Astrophysical Journal 01/2013; 767(2). · 6.73 Impact Factor
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    ABSTRACT: The GMOS-N instrument was upgraded with new CCDs in October 2011, improving the instrument sensitivity at both red and blue wavelengths. The deep depletion devices are manufactured by e2v (42-90 with multi-layer 3 coating) and extend the useful wavelength range of GMOS-N to 0.98 microns (compared to 0.94 microns previously). These detectors also exhibit much lower fringing than the original EEV detectors that had been in use since GMOS-N was commissioned in 2002. All other characteristics of the new detectors (readout speed, pixel size and format, detector controller, noise, gain) are similar to the original CCDs. Operating the new detectors in all amps mode (2 per CCD) has effectively improved the readout speed by a factor of 2. The new devices were selected to provide a quick and relatively simply upgrade route while technical issues with the Hamamatsu devices, originally planned for the upgrade, were investigated and resolved. We discuss the rationale for this interim upgrade, the upgrade process and attending issues. The new detectors have been used for science since November 2011. We present commissioning results illustrating the resulting gain in sensitivity over the original detector package. Gemini is still committed to installing Hamamatsu devices, which will further extend the useful wavelength range of GMOS to 1.03 microns, in both North and South GMOS instruments. We discuss the status of the Hamamatsu project and the current planned schedule for these future upgrades.
    Proc SPIE 09/2012;
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    ABSTRACT: The Gemini Observatories primarily operate a multi-instrument queue, with observers selecting observations that are best suited to weather and seeing conditions. The Target of Opportunity (ToO) observing mode is intended to allow observation of targets that cannot be specified in advance but which have a well defined external trigger such as distant supernovae or Gamma Ray bursts. In addition, the instrument and configuration best suited to observe the ToO may depend on properties of the event, such as brightness and redshift which again are impossible to know in advance. Queue observing naturally lends itself to Target of Opportunity (ToO) support since the time required to switch between programs and instruments is very short, and the staff observer is trained to operate all the available instruments and modes. Gemini Observatory has supported pre-approved ToO programs since beginning queue operations, and has implemented a rapid (less than 15 minutes response time) ToO mode since 2005. ToOs comprise a significant fraction of the queue (20-25% of the highest ranking band) nowadays. We discuss the ToO procedures, the statistics of rapid ToOs observing at Gemini North Observatory, the science related to GRBs and supernovae that this important mode has enabled.
    Proceedings of the International Astronomical Union 09/2012; 7(S279):345-346.
  • Katherine Roth, R. Schiavon, K. Chiboucas, G. Gimeno
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    ABSTRACT: The GMOS-N instrument was upgraded with new e2v deep-depletion devices in November 2011. The new detectors extend and improve the sensitivity at both ends of the optical spectrum by factors of up to 1.45 in the blue and > 2 longward of 850 nm. These CCDs, being deep-depletion devices, are significantly thicker than the original CCDs and produce spectral images affected much less by fringing. This provides a significantly improved sky subtraction at long wavelengths, which is dominated by a dense population of atmospheric emission lines. We present commissioning results including detector characteristics, spectroscopic throughputs, cosmic ray rates, nod & shuffle dark features, and spectral PSFs.
    01/2012;
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    ABSTRACT: We present the discovery of two ultra-luminous supernovae (SNe) at z ~ 0.9 with the Pan-STARRS1 Medium-Deep Survey. These SNe, PS1-10ky and PS1-10awh, are amongst the most luminous SNe ever discovered, comparable to the unusual transients SN 2005ap and SCP 06F6. Like SN 2005ap and SCP 06F6, they show characteristic high luminosities (M_bol ~ -22.5 mag), blue spectra with a few broad absorption lines, and no evidence for H or He. We have constructed a full multi-color light curve sensitive to the peak of the spectral energy distribution in the rest-frame ultraviolet, and we have obtained time-series spectroscopy for these SNe. Given the similarities between the SNe, we combine their light curves to estimate a total radiated energy over the course of explosion of (0.9-1.4) x 10^51 erg. We find photospheric velocities of 12,000-19,000 km/s with no evidence for deceleration measured across ~3 rest-frame weeks around light-curve peak, consistent with the expansion of an optically-thick massive shell of material. We show that, consistent with findings for other ultra-luminous SNe in this class, radioactive decay is not sufficient to power PS1-10ky, and we discuss two plausible origins for these events: the initial spin-down of a newborn magnetar in a core-collapse SN, or SN shock breakout from the dense circumstellar wind surrounding a Wolf-Rayet star.
    The Astrophysical Journal 07/2011; 743(2). · 6.73 Impact Factor
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    ABSTRACT: We present spectroscopic observations of GRB091127 (z=0.490) at the peak of the putative associated supernova, SN2009nz. Subtracting a late-time spectrum of the host galaxy, we isolate the contribution of SN2009nz and uncover broad features typical of nearby GRB-SNe. This establishes unambiguously that GRB091127 was accompanied by a broad-lined Type Ic SN, and links a cosmological long burst with a standard energy release (E_gamma,iso ~ 1.1e52 erg) to a massive star progenitor. The spectrum of SN2009nz closely resembles that of SN2006aj, with SN2003dh also providing an acceptable match, but has significantly narrower features than SNe 1998bw and 2010bh, indicative of a lower expansion velocity. The photospheric velocity inferred from the SiII 6355 absorption feature, v_ph ~ 17,000 km/s, is indeed closer to that of SNe 2006aj and 2003dh than to the other GRB-SNe. Combining the measured velocity with the light curve peak brightness and width, we estimate the following explosion parameters: M_Ni ~ 0.35 M_sun, E_K ~ 2.3e51 erg, and M_ej ~ 1.4 M_sun, similar to those of SN2006aj. These properties indicate that SN2009nz follows a trend of lower M_Ni for GRB-SNe with lower E_K and M_ej. Equally important, since GRB091127 is a typical cosmological burst, the similarity of SN2009nz to SN2006aj either casts doubt on the claim that XRF060218/SN2006aj was powered by a neutron star, or indicates that the nature of the central engine is encoded in the SN properties but not in the prompt emission. Future spectra of GRB-SNe at z > 0.3, including proper subtraction of the host galaxy contribution, will shed light on the full dispersion of SN properties for standard long GRBs, on the relation between SNe associated with sub-energetic and standard GRBs, and on a potential dispersion in the associated SN types.
    06/2011;
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    ABSTRACT: We present plans for the commissioning of the new GMOS-N red-sensitive science detectors, currently being integrated into a new focal plane assembly at the NRC HIA. These Hamamatsu CCDs provide significantly higher quantum efficiency than the existing detectors at red optical wavelengths (longward of ~ 700 nm), with > 80% QE at 900 nm falling to ~10% QE at 1.05 mum. This upgrade not only improves current operations with GMOS-N, but also opens new spectral ranges and potential observing modes (eg. use with Altair, the Gemini-N AO module). Care has been taken to ensure that Nod & Shuffle will still be supported, since accurate sky subtraction is increasingly important at longer wavelengths due to the increased density of sky lines. The commissioning plan aims to demonstrate the improvement in current modes while minimizing the period of GMOS-N downtime for science use. The science commissioning is currently scheduled for mid-November 2010.
    Proc SPIE 07/2010;
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    ABSTRACT: Target of opportunity observations (ToO) are an integral part of multi-instrument queue operations at Gemini Observatory. ToOs comprise a significant fraction of the queue (20-25% of the highest ranking band) and with the advent of large survey telescopes (eg. Pan-STARRS, LSST) dedicated to searching for transient events this fraction may reasonably be expected to increase significantly in the coming years. While some important aspects of ToO execution at Gemini Observatory are managed automatically (eg. trigger alerts, data distribution), other areas such as duplications checking, scheduling and relative priority determination still require manual intervention. In order to increase efficiency and improve our commitment to ToOs and queue observing in general, these aspects need to be formalized and incorporated into improved phase 2 checking, automated queue scheduling and on-the-fly nightly plan generation software. We discuss the different flavors of ToOs supported at Gemini Observatory and how each kind is scheduled with respect to existing queue observations. We present ideas for formalizing these practices into a system of dynamical prioritization which automatically self adjusts as new ToO observations are triggered, high priority targets become endangered, and timing windows near expiration.
    Proc SPIE 07/2010;
  • Dolores M. Coulson, Katherine C. Roth
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    ABSTRACT: The Gemini Multi-Object Spectrograph (GMOS-N), with a field of view of 5.5 × 5.5 arc minutes, was used to obtain r' band images of the Keck II laser beam. The data samples the Rayleigh scattered laser beam at low elevations and the sodium spot at the highest elevation. The Rayleigh scattered part of the beam is large at low elevations, filling the GMOS-N field of view, with high surface brightness. At higher elevations (85 deg - 89.5 deg) it gets smaller and fainter. We also present data taken on the laser spot which we see at an elevation of 89.625, corresponding to a height in the atmosphere of ~100km. In addition, GMOS-N spectra and GMOS-N on-instrument wavefront sensor (OIWFS) data have been collected that allow us to characterize the effect that lasers from other telescopes might have on GMOS-N data. The OIWFS works at wavelengths which include the sodium D band.
    Proc SPIE 07/2010;
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    ABSTRACT: We present the spectroscopic discovery of a broad-lined Type Ic supernova (SN 2010bh) associated with the nearby long-duration gamma-ray burst (GRB) 100316D. At z = 0.0593, this is the third-nearest GRB-SN. Nightly optical spectra obtained with the Magellan telescopes during the first week after explosion reveal the gradual emergence of very broad spectral features superposed on a blue continuum. The supernova features are typical of broad-lined SNe Ic and are generally consistent with previous supernovae associated with low-redshift GRBs. However, the inferred velocities of SN 2010bh at 21 days after explosion are a factor of ~2 times larger than those of the prototypical SN 1998bw at similar epochs, with v ~ 26,000 km/s, indicating a larger explosion energy or a different ejecta structure. A near-infrared spectrum taken 13.8 days after explosion shows no strong evidence for He I at 1.083 microns, implying that the progenitor was largely stripped of its helium envelope. The host galaxy is of low luminosity (M_R ~ -18.5 mag) and low metallicity (Z < 0.4 Z_solar), similar to the hosts of other low-redshift GRB-SNe. Comment: 6 pages, 4 figures, 1 table, submitted to ApJ Letters
    04/2010;
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    ABSTRACT: The Lunar CRater Observation and Sensing Satellite (LCROSS) Centaur rocket impacted a permanently shadowed crater near the south pole of the Moon at 11:31 UTC 2009 October 09. Gemini, one of several telescopes in a coordinated network observing the impact, conducted observations using NIFS to obtain 3D K-band imaging spectroscopy to detect water ice in the ejected plume of material. The spectral slope of the NIFS data can constrain the grain size and height distribution as the plume evolves, measuring the total mass and the water ice concentration in the plume. These observations provided an engineering challenge for Gemini, including the need to track non-sidereal with constantly changing track rates and guide on small bright moon craters, in order to keep the impact site within the NIFS field-of-view. High quality images taken by GMOS-N, NIRI and the acquisition camera during engineering periods at specific lunar libration and illumination were also used by the LCROSS ground based observing team to supplement slit positioning and offset plans for other ground based observatories. LCROSS mission support and engineering has resulted in improved telescope functionality for non-sidereal targets, including the ability to upload and import target ephemerides directly into the TCS, starting in semester 2010B. In this poster we present the engineering results and observing improvements which will facilitate enhanced user capabilities of the Gemini telescopes arising from the intensive LCROSS support challenge. Gemini Observatory is operated by AURA, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (United States), the STFC (United Kingdom), the NRC (Canada), CONICYT (Chile), the ARC (Australia), Ministério da Ciência e Tecnologia (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). In part this research was supported by NASA through contracts to SWRI and NSF grant AST-0706980 to the U. Minnesota.
    01/2010;
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    ABSTRACT: We report on measurements of interstellar O VI, H2, P II, Si II, Ar I, and Fe II absorption along the line of sight to Sk -67°05, a B0 Ia star in a diffuse H II region in the western edge of the Large Magellanic Cloud (LMC). We find log N(O ) = 14.40 ± 0.04 in the Milky Way component and, using the C IV column density from previous IUE observations, N(C )/N(O ) = 1.00 ± 0.16, a value similar to other halo measurements made with the Far Ultraviolet Spectroscopic Explorer. In the LMC component, log N(O ) = 13.89 ± 0.05 and N(C )/N(O ) < 0.4 (3 σ), since only an upper limit on N(C IV) is available. Along this sight line, the LMC is rich in molecular hydrogen [log N(H2) = 19.50 ± 0.08]; in the Milky Way, log N(H2) = 14.95 ± 0.08. A two-component fit for the excitation temperature of the molecular gas in the LMC gives T01 = 59 ± 5 K for J = 0, 1 and Tex = 800 ± 330 K for J = 3, 4, 5. For the Milky Way, T01 = 99 K; no excitation temperature could be determined for the higher rotational states. The Milky Way and LMC gas-phase [Fe/P] abundances are ~0.6 and ~0.7 dex lower, respectively, than solar system abundances. These values are similar to [Fe/Zn] measurements for the Milky Way and LMC toward SN 1987A.
    The Astrophysical Journal 12/2008; 538(1):L39. · 6.73 Impact Factor
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    ABSTRACT: The metal-deficient starburst galaxy I Zw 18 has been observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) in a search for H2 molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H2 at the radial velocity of the galaxy. The upper limit for the diffuse H2 column density is found to be very low, N(H2) 1015 cm-2 (10 σ), unlike our Galaxy where H2 is generally present for even low H I column densities. Although the H I column density here is as high as N(H I) ≈ 2 × 1021 cm-2, we observe 2N(H2)/N(H I) 10-6. We cannot exclude the possibility that some H2 could be in very dense, small, and discrete clumps that cannot be detected with the present observation. However, the remarkable absence of diffuse H2 in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H atoms), the high ultraviolet flux, and the low density of the H I cloud surrounding the star-forming regions. Thus, having eliminated diffuse H2 as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by H I and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.
    The Astrophysical Journal 12/2008; 538(1):L77. · 6.73 Impact Factor

Publication Stats

1k Citations
248.88 Total Impact Points

Institutions

  • 2013
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2000–2013
    • Gemini Observatory
      Hilo, Hawaii, United States
    • The Royal Observatory, Edinburgh
      Edinburgh, Scotland, United Kingdom
  • 1993–2013
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 2011
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2000–2008
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 2006
    • California Institute of Technology
      • Division of Physics, Mathematics, and Astronomy
      Pasadena, California, United States
  • 2003
    • Carnegie Institution for Science
      Washington, West Virginia, United States
  • 1999
    • Honolulu University
      Honolulu, Hawaii, United States
  • 1989–1998
    • Northwestern University
      Evanston, Illinois, United States