Publications

2015
Margutti R, Guidorzi C, Lazzati D, Milisavljevic D, Kamble A, Laskar T, Parrent J, Gehrels NC, Soderberg AM. Dust in the Wind: the Role of Recent Mass Loss in Long Gamma-Ray Bursts. The Astrophysical Journal [Internet]. 2015;805. Publisher's VersionAbstract
We study the late-time (t\gt 0.5 days) X-ray afterglows of nearby (z\lt0.5) long gamma-ray bursts (GRBs) with Swift and identify a populationof explosions with slowly decaying, super-soft (photon index {{{Γ}}x}\gt 3) X-ray emission that is inconsistent with forwardshock synchrotron radiation associated with the afterglow. Theseexplosions also show larger-than-average intrinsic absorption(N{{H}x,i}\gt 6× {{10}21}c{{m}-2}) and prompt γ-ray emission with extremely longduration ({{T}90}\gt 1000 s). The chance association of thesethree rare properties (i.e., large N{{H}x,i}, super-soft{{{Γ }}x}, and extreme duration) in the same class ofexplosions is statistically unlikely. We associate these properties withthe turbulent mass-loss history of the progenitor star that enriched andshaped the circumburst medium. We identify a natural connection betweenN{{H}x,i}, {{{Γ }}x}, and {{T}90}in these sources by suggesting that the late-time super-soft X-raysoriginate from radiation reprocessed by material lost to the environmentby the stellar progenitor before exploding (either in the form of a dustecho or as reprocessed radiation from a long-lived GRB remnant), andthat the interaction of the explosion's shock/jet with the complexmedium is the source of the extremely long prompt emission. However,current observations do not allow us to exclude the possibility thatsuper-soft X-ray emitters originate from peculiar stellar progenitorswith large radii that only form in very dusty environments.
Laskar T, Berger E, Margutti R, Perley D, Zauderer BA, Sari R'em, Fong W-fai. Energy Injection in Gamma-Ray Burst Afterglows. The Astrophysical Journal [Internet]. 2015;814. Publisher's VersionAbstract
We present multi-wavelength observations and modeling of gamma-raybursts (GRBs) that exhibit a simultaneous re-brightening in their X-rayand optical light curves, and are also detected at radio wavelengths. Weshow that the re-brightening episodes can be modeled by injection ofenergy into the blastwave and that in all cases the energy injectionrate falls within the theoretical bounds expected for a distribution ofenergy with ejecta Lorentz factor. Our measured values of thecircumburst density, jet opening angle, and beaming-corrected kineticenergy are consistent with the distribution of these parameters forlong-duration GRBs at both z˜ 1 and z≳ 6, suggesting that thejet launching mechanism and environment of these events are similar tothat of GRBs that do not have bumps in their light curves. However,events exhibiting re-brightening episodes have lower radiativeefficiencies than average, suggesting that a majority of the kineticenergy of the outflow is carried by slow-moving ejecta, which is furthersupported by steep measured distributions of the ejecta energy as afunction of Lorentz factor. We do not find evidence for reverse shocksover the energy injection period, implying that the onset of energyinjection is a gentle process. We further show that GRBs exhibitingsimultaneous X-ray and optical re-brightenings are likely the tail of adistribution of events with varying rates of energy injection, formingthe most extreme events in their class. Future X-ray observations of GRBafterglows with Swift and its successors will thus likely discoverseveral more such events, while radio follow-up and multi-wavelengthmodeling of similar events will unveil the role of energy injection inGRB afterglows.
2014
Berger E, Zauderer BA, Chary R-R, Laskar T, Chornock R, Tanvir NR, Stanway ER, Levan AJ, Levesque EM, Davies JE. ALMA Observations of the Host Galaxy of GRB 090423 at z = 8.23: Deep Limits on Obscured Star Formation 630 Million Years after the Big Bang. The Astrophysical Journal [Internet]. 2014;796. Publisher's VersionAbstract
We present rest-frame far-infrared (FIR) and optical observations of thehost galaxy of GRB 090423 at z = 8.23 from the Atacama Large MillimeterArray (ALMA) and the Spitzer Space Telescope, respectively. The hostremains undetected to 3σ limits of F ν(222 GHz)<~ 33 μJy and F ν(3.6 μm) <~ 81 nJy. The FIRlimit is about 20 times fainter than the luminosity of the local ULIRGArp 220 and comparable to the local starburst M 82. Comparing this withmodel spectral energy distributions, we place a limit on the infrared(IR) luminosity of L IR(8-1000 μm) <~ 3 ×1010 L , corresponding to a limit on theobscured star formation rate of SFRIR ≲ 5 M yr-1. For comparison, the limit on theunobscured star formation rate from Hubble Space Telescope rest-frameultraviolet (UV) observations is SFRUV <~ 1 M yr-1. We also place a limit on the hostgalaxy stellar mass of M * <~ 5 × 107 M (for a stellar population age of 100 Myr and constantstar formation rate). Finally, we compare our millimeter observations tothose of field galaxies at z >~ 4 (Lyman break galaxies, Lyαemitters, and submillimeter galaxies) and find that our limit on the FIRluminosity is the most constraining to date, although the field galaxieshave much larger rest-frame UV/optical luminosities than the host of GRB090423 by virtue of their selection techniques. We conclude that GRBhost galaxies at z >~ 4, especially those with measured interstellarmedium metallicities from afterglow spectroscopy, are an attractivesample for future ALMA studies of high redshift obscured star formation.
Laskar T, Berger E, Tanvir N, Zauderer BA, Margutti R, Levan A, Perley D, Fong W-fai, Wiersema K, Menten K, et al. GRB 120521C at z ~ 6 and the Properties of High-redshift γ-Ray Bursts. The Astrophysical Journal [Internet]. 2014;781. Publisher's VersionAbstract
We present optical, near-infrared, and radio observations of theafterglow of GRB 120521C. By modeling the multi-wavelength data set, wederive a photometric redshift of z ≈ 6.0, which we confirm with a lowsignal-to-noise ratio spectrum of the afterglow. We find that a modelwith a constant-density environment provides a good fit to the afterglowdata, with an inferred density of n <~ 0.05 cm-3.The radio observations reveal the presence of a jet break at tjet ≈ 7 d, corresponding to a jet opening angle ofθjet ≈ 3°. The beaming-corrected γ-rayand kinetic energies are E γ ≈ EK ≈ 3× 1050 erg. We quantify the uncertainties in ourresults using a detailed Markov Chain Monte Carlo analysis, which allowsus to uncover degeneracies between the physical parameters of theexplosion. To compare GRB 120521C to other high-redshift bursts in auniform manner we re-fit all available afterglow data for the two otherbursts at z >~ 6 with radio detections (GRBs 050904 and 090423). Wefind a jet break at t jet ≈ 15 d for GRB 090423, incontrast to previous work. Based on these three events, we find thatγ-ray bursts (GRBs) at z >~ 6 appear to explode inconstant-density environments, and exhibit a wide range of energies anddensities that span the range inferred for lower redshift bursts. On theother hand, we find a hint for narrower jets in the z >~ 6 bursts,potentially indicating a larger true event rate at these redshifts.Overall, our results indicate that long GRBs share a common progenitorpopulation at least to z ~ 8.
Lunnan R, Chornock R, Berger E, Laskar T, Fong W, Rest A, Sanders NE, Challis PM, Drout MR, Foley RJ, et al. Hydrogen-poor Superluminous Supernovae and Long-duration Gamma-Ray Bursts Have Similar Host Galaxies. The Astrophysical Journal [Internet]. 2014;787. Publisher's VersionAbstract
We present optical spectroscopy and optical/near-IR photometry of 31host galaxies of hydrogen-poor superluminous supernovae (SLSNe),including 15 events from the Pan-STARRS1 Medium Deep Survey. Our samplespans the redshift range 0.1 <~ z <~ 1.6, and is the firstcomprehensive host galaxy study of this specific subclass of cosmicexplosions. Combining the multi-band photometry and emission-linemeasurements, we determine the luminosities, stellar masses, starformation rates, and metallicities. We find that, as a whole, the hostsof SLSNe are a low-luminosity (langMB rang ≈ -17.3 mag),low stellar mass (langM *rang ≈ 2 × 108 M) population, with a high median specific star formationrate (langsSFRrang ≈ 2 Gyr-1). The median metallicity ofour spectroscopic sample is low, 12 + log (O/H) ≈ 8.35 ≈ 0.45 Z, although at least one host galaxy has solarmetallicity. The host galaxies of H-poor SLSNe are statisticallydistinct from the hosts of GOODS core-collapse SNe (which cover asimilar redshift range), but resemble the host galaxies of long-durationgamma-ray bursts (LGRBs) in terms of stellar mass, SFR, sSFR, andmetallicity. This result indicates that the environmental causes leadingto massive stars forming either SLSNe or LGRBs are similar, and inparticular that SLSNe are more effectively formed in low metallicityenvironments. We speculate that the key ingredient is large core angularmomentum, leading to a rapidly spinning magnetar in SLSNe and anaccreting black hole in LGRBs.
Fong W, Berger E, Metzger BD, Margutti R, Chornock R, Migliori G, Foley RJ, Zauderer BA, Lunnan R, Laskar T, et al. Short GRB 130603B: Discovery of a Jet Break in the Optical and Radio Afterglows, and a Mysterious Late-time X-Ray Excess. The Astrophysical Journal [Internet]. 2014;780. Publisher's VersionAbstract
We present radio, optical/NIR, and X-ray observations of the afterglowof the short-duration Swift and Konus-Wind GRB 130603B, and uncover abreak in the radio and optical bands at ≈0.5 day after the burst,best explained as a jet break with an inferred jet opening angle of≈4°-8°. GRB 130603B is only the third short GRB with a radioafterglow detection to date, and represents the first time that a jetbreak has been evident in the radio band. We model the temporalevolution of the spectral energy distribution to determine the burstexplosion properties and find an isotropic-equivalent kinetic energy of≈(0.6-1.7) × 1051 erg and a circumburst density of≈5 × 10-3-30 cm-3. From theinferred opening angle of GRB 130603B, we calculate beaming-correctedenergies of E γ ≈ (0.5-2) × 1049erg and E K ≈ (0.1-1.6) × 1049 erg. Alongwith previous measurements and lower limits we find a median openingangle of ≈10°. Using the all-sky observed rate of 10Gpc-3 yr-1, this implies a true shortGRB rate of ≈20 yr-1 within 200 Mpc, the AdvancedLIGO/VIRGO sensitivity range for neutron star binary mergers. Finally,we uncover evidence for significant excess emission in the X-rayafterglow of GRB 130603B at >~ 1 day and conclude that the additionalenergy component could be due to fall-back accretion or spin-down energyfrom a magnetar formed following the merger.
2013
Berger E, Zauderer BA, Levan A, Margutti R, Laskar T, Fong W, Mangano V, Fox DB, Tunnicliffe RL, Chornock R, et al. The Afterglow and ULIRG Host Galaxy of the Dark Short GRB 120804A. The Astrophysical Journal [Internet]. 2013;765. Publisher's VersionAbstract
We present the optical discovery and subarcsecond optical and X-raylocalization of the afterglow of the short GRB 120804A, as well asoptical, near-IR, and radio detections of its host galaxy. X-rayobservations with Swift/XRT, Chandra, and XMM-Newton extending toδt ≈ 19 days reveal a single power-law decline. The opticalafterglow is faint, and comparison to the X-ray flux indicates that GRB120804A is "dark," with a rest-frame extinction of A host V ≈ 2.5 mag (at z = 1.3). The intrinsic neutral hydrogencolumn density inferred from the X-ray spectrum, N H, int(z =1.3) ≈ 2 × 1022 cm-2, iscommensurate with the large extinction. The host galaxy exhibits redoptical/near-IR colors. Equally important, JVLA observations at≈0.9-11 days reveal a constant flux density of F ν(5.8GHz) = 35 ± 4 μJy and an optically thin spectrum,unprecedented for GRB afterglows, but suggestive instead of emissionfrom the host galaxy. The optical/near-IR and radio fluxes are well fitwith the scaled spectral energy distribution of the local ultraluminousinfrared galaxy (ULIRG) Arp 220 at z ≈ 1.3, with a resulting starformation rate of x ≈ 300 M yr-1.The inferred extinction and small projected offset (2.2 ± 1.2kpc) are also consistent with the ULIRG scenario, as is the presence ofa companion galaxy at the same redshift and with a separation of about11 kpc. The limits on radio afterglow emission, in conjunction with theobserved X-ray and optical emission, require a circumburst density of n~ 10-3 cm-3, an isotropic-equivalentenergy scale of E γ, iso ≈ E K, iso ≈7 × 1051 erg, and a jet opening angle of θj >~ 11°. The expected fraction of luminous infraredgalaxies in the short GRB host sample is ~0.01 and ~0.25 (for purestellar mass and star formation weighting, respectively). Thus, theobserved fraction of two events in about 25 hosts (GRBs 120804A and100206A) appears to support our previous conclusion that short GRBstrack both stellar mass and star formation activity.
Fong W, Berger E, Chornock R, Margutti R, Levan AJ, Tanvir NR, Tunnicliffe RL, Czekala I, Fox DB, Perley DA, et al. Demographics of the Galaxies Hosting Short-duration Gamma-Ray Bursts. The Astrophysical Journal [Internet]. 2013;769. Publisher's VersionAbstract
We present observations of the afterglows and host galaxies of threeshort-duration gamma-ray bursts (GRBs): 100625A, 101219A, and 110112A.We find that GRB 100625A occurred in a z = 0.452 early-type galaxy witha stellar mass of ≈4.6 × 109 M and a stellar population age of ≈0.7 Gyr, and GRB 101219A originatedin a star-forming galaxy at z = 0.718 with a stellar mass of ≈1.4× 109 M , a star formation rate of≈16 M yr-1, and a stellarpopulation age of ≈50 Myr. We also report the discovery of theoptical afterglow of GRB 110112A, which lacks a coincident host galaxyto i >~ 26 mag, and we cannot conclusively identify any field galaxyas a possible host. From afterglow modeling, the bursts have inferredcircumburst densities of ≈10-4-1cm-3 and isotropic-equivalent gamma-ray and kineticenergies of ≈1050-1051 erg. These three eventshighlight the diversity of galactic environments that host short GRBs.To quantify this diversity, we use the sample of 36 Swift short GRBswith robust associations to an environment (~1/2 of 68 short burstsdetected by Swift to 2012 May) and classify bursts originating from four
Chornock R, Berger E, Fox DB, Lunnan R, Drout MR, Fong W-fai, Laskar T, Roth KC. GRB 130606A as a Probe of the Intergalactic Medium and the Interstellar Medium in a Star-forming Galaxy in the First Gyr after the Big Bang. The Astrophysical Journal [Internet]. 2013;774. Publisher's VersionAbstract
We present high signal-to-noise ratio Gemini and MMT spectroscopy of theoptical afterglow of the gamma-ray burst (GRB) 130606A at redshift z =5.913, discovered by Swift. This is the first high-redshift GRBafterglow to have spectra of comparable quality to those of z ≈ 6quasars. The data exhibit a smooth continuum at near-infraredwavelengths that is sharply cut off blueward of 8410 Å due toabsorption from Lyα at redshift z ≈ 5.91, with some fluxtransmitted through the Lyα forest between 7000 and 7800 Å.We use column densities inferred from metal absorption lines toconstrain the metallicity of the host galaxy between a lower limit of[Si/H] >~ -1.7 and an upper limit of [S/H] <~ -0.5 setby the non-detection of S II absorption. We demonstrate consistencybetween the dramatic evolution in the transmission fraction of Lyαseen in this spectrum over the redshift range z = 4.9-5.85 with thatpreviously measured from observations of high-redshift quasars. There isan extended redshift interval of Δz = 0.12 in the Lyα forestat z = 5.77 with no detected transmission, leading to a 3σ upper
Zauderer BA, Berger E, Margutti R, Levan AJ, Olivares E. F, Perley DA, Fong W, Horesh A, Updike AC, Greiner J, et al. Illuminating the Darkest Gamma-Ray Bursts with Radio Observations. The Astrophysical Journal [Internet]. 2013;767. Publisher's VersionAbstract
We present X-ray, optical, near-infrared (IR), and radio observations ofgamma-ray bursts (GRBs) 110709B and 111215A, as well as optical andnear-IR observations of their host galaxies. The combination of X-raydetections and deep optical/near-IR limits establish both bursts as"dark." Sub-arcsecond positions enabled by radio detections lead torobust host galaxy associations, with optical detections that indicate z<~ 4 (110709B) and z ≈ 1.8-2.9 (111215A). We therefore concludethat both bursts are dark due to substantial rest-frame extinction.Using the radio and X-ray data for each burst we find that GRB 110709Brequires A_V^host≳ 5.3 mag and GRB 111215A requiresA_V^host≳ 8.5 mag (assuming z = 2). These are among the largestextinction values inferred for dark bursts to date. The two bursts alsoexhibit large neutral hydrogen column densities of N H, int>~ 1022 cm-2 (z = 2) as inferred fromtheir X-ray spectra, in agreement with the trend for dark GRBs.Moreover, the inferred values are in agreement with the GalacticAV -N H relation, unlike the bulk of the GRBpopulation. Finally, we find that for both bursts the afterglow emissionis best explained by a collimated outflow with a total beaming-correctedenergy of E γ + EK ≈ (7-9) ×1051 erg (z = 2) expanding into a wind medium with a highdensity, \dot{M}≈ (6{--}20)× 10^{-5} M yr-1 (n ≈ 100-350 cm-3 at≈1017 cm). While the energy release is typical of longGRBs, the inferred density may be indicative of larger mass-loss ratesfor GRB progenitors in dusty (and hence metal rich) environments. Thisstudy establishes the critical role of radio observations indemonstrating the origin and properties of dark GRBs. Observations withthe JVLA and ALMA will provide a sample with sub-arcsecond positions androbust host associations that will help to shed light on obscured starformation and the role of metallicity in GRB progenitors.
Laskar T, Berger E, Zauderer BA, Margutti R, Soderberg AM, Chakraborti S, Lunnan R, Chornock R, Chandra P, Ray A. A Reverse Shock in GRB 130427A. The Astrophysical Journal [Internet]. 2013;776. Publisher's VersionAbstract
We present extensive radio and millimeter observations of the unusuallybright GRB 130427A at z = 0.340, spanning 0.67-12 days after the burst.We combine these data with detailed multi-band UV, optical, NIR, andSwift X-ray observations and find that the broadband afterglow emissionis composed of distinct reverse shock and forward shock contributions.The reverse shock emission dominates in the radio/millimeter and at<~ 0.1 days in the UV/optical/NIR, while the forward shock emissiondominates in the X-rays and at >~ 0.1 days in the UV/optical/NIR. Wefurther find that the optical and X-ray data require a wind circumburstenvironment, pointing to a massive star progenitor. Using the combinedforward and reverse shock emission, we find that the parameters of theburst include an isotropic kinetic energy of E K, iso ≈ 2× 1053 erg, a mass loss rate of \dot{M}\approx 3\times10^{-8} M yr-1 (for a wind velocityof 1000 km s-1), and a Lorentz factor at thedeceleration time of Γ(200 s) ≈ 130. Due to the low density andlarge isotropic energy, the absence of a jet break to ≈15 days placesonly a weak constraint on the opening angle, θj >~2.°5, and therefore a total energy of E γ +EK >~ 1.2 × 1051 erg, similar to othergamma-ray bursts (GRBs). The reverse shock emission is detectable inthis burst due to the low circumburst density, which leads to a slowcooling shock. We speculate that this property is required for thedetectability of reverse shocks in radio and millimeter bands. Followingon GRB 130427A as a benchmark event, observations of future GRBs withthe exquisite sensitivity of the Very Large Array and ALMA, coupled withdetailed modeling of the reverse and forward shock contributions, willtest this hypothesis.
2011
Laskar T, Berger E, Chary R-R. Exploring the Galaxy Mass-metallicity Relation at z ~ 3-5. The Astrophysical Journal [Internet]. 2011;739. Publisher's VersionAbstract
Long-duration gamma-ray bursts (GRBs) provide a premier tool forstudying high-redshift star-forming galaxies thanks to their extremebrightness and association with massive stars. Here we use GRBs to studythe galaxy stellar mass-metallicity (M *-Z) relation at z ~3-5, where conventional direct metallicity measurements are extremelychallenging. We use the interstellar medium metallicities of long GRBhosts derived from afterglow absorption spectroscopy, in conjunctionwith host galaxy stellar masses determined from deep Spitzer 3.6 μmobservations of 20 GRB hosts. We detect about 1/4 of the hosts with MAB(I) ≈ -21.5 to -22.5 mag and place a limit of MAB(I) >~ -19 mag on the remaining hosts from a stackinganalysis. Using these observations, we present the first rest-frameoptical luminosity distribution of long GRB hosts at z >~ 3 and findthat it is similar to the distribution of long GRB hosts at z ~ 1. Incomparison to Lyman-break galaxies at the same redshift, GRB hosts aregenerally fainter, but the sample is too small to rule out an overallsimilar luminosity function. On the other hand, the GRB hosts appear tobe more luminous than the population of Lyα emitters at z ~ 3-4.Using a conservative range of mass-to-light ratios for simple stellarpopulations (with ages of 70 Myr to ~2 Gyr), we infer the host stellarmasses and present mass-metallicity measurements at z ~ 3-5 (langzrang≈ 3.5). We find that the detected GRB hosts, with M * ≈2 × 1010 M sun, display a wide range ofmetallicities, but that the mean metallicity at this mass scale, Z ≈0.3 Z sun, is lower than measurements at z <~ 3. Combinedwith stacking of the non-detected hosts with M * <~ 3× 109 M sun and Z <~ 0.1 Zsun, we find tentative evidence for the existence of an M*-Z relation at z ~ 3.5 and continued evolution of thisrelation to systematically lower metallicities from z ~ 2.
2010
Kruse EA, Berger E, Knapp GR, Laskar T, Gunn JE, Loomis CP, Lupton RH, Schlegel DJ. Chromospheric Variability in Sloan Digital Sky Survey M Dwarfs. II. Short-timescale Hα Variability. The Astrophysical Journal [Internet]. 2010;722 :1352-1359. Publisher's VersionAbstract
We present the first comprehensive study of short-timescalechromospheric Hα variability in M dwarfs using the individual 15minute spectroscopic exposures for 52, 392 objects from the SloanDigital Sky Survey. Our sample contains about103-104 objects per spectral type bin in the rangeM0-M9, with a typical number of three exposures per object (ranging upto a maximum of 30 exposures). Using this extensive data set, we find
Laskar T, Fabian AC, Blundell KM, Erlund MC. Inverse-Compton X-rays from giant radio galaxies at z ~ 1. Monthly Notices of the Royal Astronomical Society [Internet]. 2010;401 :1500-1504. Publisher's VersionAbstract
We report XMM-Newton observations of three FR II radio galaxies atredshifts between 0.85 and 1.34, which show extended diffuse X-rayemission within the radio lobes, likely due to inverse-Comptonup-scattering of the cosmic microwave background. Under this assumption,through spectrum fitting together with archival Very Large Array radioobservations, we derive an independent estimate of the magnetic field inthe radio lobes of 3C 469.1 and compare it with the equipartition value.We find concordance between these two estimates as long as the turnoverin the energy distribution of the particles occurs at a Lorentz factorin excess of ~250. We determine the total energy in relativisticparticles in the radio-emitting lobes of all three sources to rangebetween 3 × 1059 and 8 × 1059erg. Thenuclei of these X-ray sources are heavily-absorbed powerful activegalactic nuclei.
2009
Laskar T, Soderblom DR, Valenti JA, Stauffer JR. The Metallicity of the HD 98800 System. The Astrophysical Journal [Internet]. 2009;698 :660-665. Publisher's VersionAbstract
Pre-main-sequence (PMS) binaries and multiples enable critical tests ofstellar models if masses, metallicities, and luminosities of thecomponent stars are known. We have analyzed high-resolution, highsignal-to-noise echelle spectra of the quadruple-star system HD 98800and using spectrum synthesis computed fits to the composite spectrum fora full range of plausible stellar parameters for the components. Weconsistently find that subsolar metallicity yields fits with lowerχ2 values, with an overall best fit of [M/H] = -0.20± 0.10. This metallicity appears to be consistent with PMSevolutionary tracks for the measured masses and luminosities of thecomponents of HD 98800 but additional constraints on the system andmodeling are needed.
Soderblom DR, Laskar T, Valenti JA, Stauffer JR, Rebull LM. The Metallicity of the Pleiades. The Astronomical Journal [Internet]. 2009;138 :1292-1295. Publisher's VersionAbstract
We have measured the abundances of Fe, Si, Ni, Ti, and Na in 20 Pleiadesstars with T eff values near solar and with low vsin i usinghigh-resolution, high signal-to-noise echelle spectra. We have validatedour procedures by also analyzing 10 field stars of a range oftemperatures and metallicities that were observed by Valenti andFischer. Our result for the Pleiades is [Fe/H] = +0.03 ± 0.02± 0.05 (statistical and systematic). The average of publishedmeasurements for the Pleiades is +0.042 ± 0.021.
2008
Gupta S, Saini TD, Laskar T. Direction dependence and non-Gaussianity in the high-redshift supernova data. Monthly Notices of the Royal Astronomical Society [Internet]. 2008;388 :242-246. Publisher's VersionAbstract
The most detailed constraints on the accelerating expansion of theuniverse and the nature of dark energy are derived from thehigh-redshift supernova data, assuming that the luminosity distanceversus redshift relation is isotropic and the errors in the measurementsare Gaussian. There is a possibility that there is a systematicdirection dependence in the data, either due to uncorrected, knownphysical processes or because there are tiny departures from thecosmological principle, making the universe slightly anisotropic. Toinvestigate this possibility, we introduce a statistic based on extremevalue theory and apply it to the gold data sets from Riess et al. Ouranalysis indicate a systematic, directional dependence in the supernovadata in both sets, which using the bootstrap distribution comes to about80 per cent level of confidence for Riess et al. and 90 per cent forRiess et al. Equally importantly, we show that while the 2007 data fitΛ cold dark matter (ΛCDM) model better than the 2004 data,the level of non-Gaussianity, quantified by departures of our statisticfrom the Gaussian predictions has become worse. In fact, we find thatRiess et al. data lie totally outside the distribution obtained byassuming the noise to be Gaussian.
Nath BB, Laskar T, Shull JM. Dust Sputtering by Reverse Shocks in Supernova Remnants. The Astrophysical Journal [Internet]. 2008;682. Publisher's VersionAbstract
We consider the sputtering of dust grains, believed to be formed incooling supernovae ejecta, under the influence of reverse shocks. In theregime of self-similar evolution of reverse shocks, we can follow theevolution of ejecta density and temperature analytically as a functionof time in different parts of the ejecta, and calculate the sputteringrate of graphite and silicate grains embedded in the ejecta as theyencounter the reverse shock. Through analytic (one-dimensional)

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