Browsing by Subject "supernovae"

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CfAIR2: Near-Infrared Light Curves of 94 Type Ia Supernovae
(2015-09) Friedman, Andrew S.; Wood-Vasey, W. M.; Marion, G. H.; Challis, Peter; Mandel, Kaisey S.; Bloom, Joshua S.; Modjaz, Maryam; Narayan, Gautham; Hicken, Malcom; Foley, Ryan J.; Klein, Christopher R.; Starr, Dan L.; Morgan, Adam; Rest, Armin; Blake, Cullen H.; Miller, Adam A.; Falco, Emilio E.; Wyatt, William F.; Mink, Jessica; Skrutskie, Mmichael F.; Kirshner, Rrobert P.; Marion, G. H.
CfAIR2 is a large, homogeneously reduced set of near-infrared (NIR) light curves (LCs) for Type Ia supernovae (SNe Ia) obtained with the 1.3 m Peters Automated InfraRed Imaging TELescope. This data set includes 4637 measurements of 94 SNe Ia and 4 additional SNe Iax observed from 2005 to 2011 at the Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona. CfAIR2 includes JHKs photometric measurements for 88 normal and 6 spectroscopically peculiar SN Ia in the nearby universe, with a median redshift of z similar to 0.021 for the normal SN Ia. CfAIR2 data span the range from -13 days to +127 days from B-band maximum. More than half of the LCs begin before the time of maximum, and the coverage typically contains similar to 13-18 epochs of observation, depending on the filter. We present extensive tests that verify the fidelity of the CfAIR2 data pipeline, including comparison to the excellent data of the Carnegie Supernova Project. CfAIR2 contributes to a firm local anchor for SN cosmology studies in the NIR. Because SN Ia are more nearly standard candles in the NIR and are less vulnerable to the vexing problems of extinction by dust, CfAIR2 will help the SN cosmology community develop more precise and accurate extragalactic distance probes to improve our knowledge of cosmological parameters, including dark energy and its potential time variation.
Confined Population III Enrichment And The Prospects For Prompt Second-Generation Star Formation
(2012-12) Ritter, Jeremy S.; Safranek-Shrader, Chalence; Gnat, Orly; Milosavljevic, Milos; Bromm, Volker; Ritter, Jeremy S.; Safranek-Shrader, Chalence; Milosavljevic, Milos; Bromm, Volker
It is widely recognized that nucleosynthetic output of the first Population III supernovae was a catalyst defining the character of subsequent stellar generations. Most of the work on the earliest enrichment was carried out assuming that the first stars were extremely massive and that the associated supernovae were unusually energetic, enough to completely unbind the baryons in the host cosmic minihalo and disperse the synthesized metals into the intergalactic medium. Recent work, however, suggests that the first stars may in fact have been somewhat less massive, with a characteristic mass scale of a few tens of solar masses. We present a cosmological simulation following the transport of the metals synthesized in a Population III supernova assuming that it had an energy of 1051 erg, compatible with standard Type II supernovae. A young supernova remnant is inserted in the first star's relic H II region in the free expansion phase and is followed for 40 Myr employing adaptive mesh refinement and Lagrangian tracer particle techniques. The supernova remnant remains partially trapped within the minihalo, and the thin snowplow shell develops pronounced instability and fingering. Roughly half of the ejecta turn around and fall back toward the center of the halo, with 1% of the ejecta reaching the center in similar to 30 kyr and 10% in similar to 10 Myr. The average metallicity of the combined returning ejecta and the pristine filaments feeding into the halo center from the cosmic web is similar to 0.001-0.01 Z(circle dot), but the two remain unmixed until accreting onto the central hydrostatic core that is unresolved at the end of the simulation. We conclude that if Population III stars had less extreme masses, they promptly enriched the host minihalos with metals and triggered Population II star formation.
Do Hydrogen-Deficient Carbon Stars Have Winds?
(2009-06) Geballe, T. R.; Rao, N. Kameswara; Clayton, Geoffrey C.; Rao, N. Kameswara
We present high resolution spectra of the five known hydrogen-deficient carbon (HdC) stars in the vicinity of the 10830 angstrom line of neutral helium. In R Coronae Borealis (RCB) stars the He I line is known to be strong and broad, often with a P Cygni profile, and must be formed in the powerful winds of those stars. RCB stars have similar chemical abundances as HdC stars and also share greatly enhanced O-18 abundances with them, indicating a common origin for these two classes of stars, which has been suggested to be white dwarf mergers. A narrow He I absorption line may be present in the hotter HdC stars, but no line is seen in the cooler stars, and no evidence for a wind is found in any of them. The presence of wind lines in the RCB stars is strongly correlated with dust formation episodes so the absence of wind lines in the HdC stars, which do not make dust, is as expected.
The Effect Of Turbulent Intermittency On The Deflagration To Detonation Transition In Supernova Ia Explosions
(2008-07) Pan, Lubin; Wheeler, J. Craig; Scalo, John; Pan, Lubin; Wheeler, J. Craig; Scalo, John
We examine the effects of turbulent intermittency on the deflagration to detonation transition (DDT) in Type Ia supernovae. The Zel'dovich mechanism for DDT requires the formation of a nearly isothermal region of mixed ash and fuel that is larger than a critical size. We primarily consider the hypothesis by Khokhlov et al. and Niemeyer and Woosley that the nearly isothermal, mixed region is produced when the flame makes the transition to the distributed regime. We use two models for the distribution of the turbulent velocity fluctuations to estimate the probability as a function of the density in the exploding white dwarf that a given region of critical size is in the distributed regime due to strong local turbulent stretching of the flame structure. We also estimate lower limits on the number of such regions as a function of density. We find that the distributed regime, and hence perhaps DDT, occurs in a local region of critical size at a density at least a factor of 2-3 larger than predicted for mean conditions that neglect intermittency. This factor makes the transition density much larger than the empirical value from observations in most situations. We also consider the intermittency effect on the more stringent conditions for DDT by Lisewski et al. and Woosley. We find that a turbulent velocity of 10(8) cm s(-1) in a region of size 10(6) cm, as required by Lisewski et al., is rare. We expect that intermittency has a weaker effect on the Woosley model with a stronger DDT criterion. The predicted transition density from this criterion remains below 10(7) g cm(-3) after accounting for intermittency using our intermittency models.
From Shock Breakout To Peak And Beyond: Extensive Panchromatic Observations Of The Type Ib Supernova 2008D Associated With Swift X-Ray Transient 080109
(2009-09) Modjaz, M.; Li, W.; Butler, N.; Chornock, Ryan; Perley, D.; Blondin, S.; Bloom, Joshua S.; Filippenko, Alexei V.; Kirshner, Robert P.; Kocevski, D.; Poznanski, D.; Hicken, M.; Foley, Ryan J.; Stringfellow, Guy S.; Berlind, Perry; Navascues, D. B. Y.; Blake, C. H.; Bouy, H.; Brown, Warren R.; Challis, Peter; Chen, H.; de Vries, W. H.; Dufour, Patrick; Falco, E.; Friedman, A.; Ganeshalingam, Mohan; Garnavich, Peter; Holden, B.; Illingworth, G.; Lee, N.; Liebert, James; Marion, G. H.; Olivier, S. S.; Prochaska, J. X.; Silverman, Jeffrey M.; Smith, N.; Starr, D.; Steele, Thea N.; Stockton, A.; Williams, G. G.; Wood-Vasey, W. M.; Marion, G.H.
We present extensive early photometric (ultraviolet through near-infrared) and spectroscopic (optical and near-infrared) data on supernova (SN) 2008D as well as X-ray data analysis on the associated Swift X-ray transient (XRT) 080109. Our data span a time range of 5 hr before the detection of the X-ray transient to 150 days after its detection, and a detailed analysis allowed us to derive constraints on the nature of the SN and its progenitor; throughout we draw comparisons with results presented in the literature and find several key aspects that differ. We show that the X-ray spectrum of XRT 080109 can be fit equally well by an absorbed power law or a superposition of about equal parts of both power law and blackbody. Our data first established that SN 2008D is a spectroscopically normal SN Ib (i.e., showing conspicuous He lines) and showed that SN 2008D had a relatively long rise time of 18 days and a modest optical peak luminosity. The early-time light curves of the SN are dominated by a cooling stellar envelope (for Delta t approximate to 0.1-4 days, most pronounced in the blue bands) followed by (56)Ni decay. We construct a reliable measurement of the bolometric output for this stripped-envelope SN, and, combined with estimates of E(K) and M(ej) from the literature, estimate the stellar radius R(star) of its probable Wolf-Rayet progenitor. According to the model of Waxman et al. and Chevalier & Fransson, we derive R(star)(W07) = 1.2 +/- 0.7 R(circle dot) and R(star)(CF08) = 12 +/- 7 R(circle dot), respectively; the latter being more in line with typical WN stars. Spectra obtained at three and four months after maximum light show double-peaked oxygen lines that we associate with departures from spherical symmetry, as has been suggested for the inner ejecta of a number of SN Ib cores.
High-Resolution Spectroscopy Of Extremely Metal-Poor Stars In The Least Evolved Galaxies: Ursa Major II And Coma Berenices
(2010-01) Frebel, Anna; Simon, Joshua D.; Geha, Marla; Willman, Beth; Frebel, Anna
We present spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and Coma Berenices obtained with the Keck/High Resolution Echelle Spectrometer (HIRES). These observations include the first high-resolution spectroscopic observations of extremely metal-poor ([Fe/H] < -3.0) stars not belonging to the MW halo field star population. We obtain abundance measurements and upper limits for 26 elements between carbon and europium. The entire sample of stars spans a range of -3.2 < [Fe/H] < -2.3, and we confirm that each galaxy contains a large intrinsic spread of Fe abundances. A comparison with MW halo stars of similar metallicities reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the light, alpha, and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (-2.5 less than or similar to [Fe/H] less than or similar to -1.0) in more luminous dwarf spheroidal galaxies, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H] greater than or similar to -3.0. Not only are our results broadly consistent with a galaxy formation model that predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H] > -2.5) of the MW halo, but they also suggest that the faintest known dwarfs may be the primary contributors to the metal-poor end of the MW halo metallicity distribution.
Improved Log(gf) Values for Lines of Ti I and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937 (Accurate Transition Probabilities for Ti I)
(2013-04) Lawler, James E.; Guzman, A.; Wood, M. P.; Sneden, Christopher; Cowan, John J.; Sneden, Christopher
New atomic transition probability measurements for 948 lines of Ti I are reported. Branching fractions from Fourier transform spectra and from spectra recorded using a 3 m echelle spectrometer are combined with published radiative lifetimes from laser-induced fluorescence measurements to determine these transition probabilities. Generally good agreement is found in comparisons to the NIST Atomic Spectra Database. The new Ti I data are applied to re-determine the Ti abundance in the photospheres of the Sun and metal-poor star HD 84937 using many lines covering a range of wavelength and excitation potential to explore possible non-local thermal equilibrium effects. The variation of relative Ti/Fe abundance with metallicity in metal-poor stars observed in earlier studies is supported in this study.
M31N-2007-06B: A Nova in the M31 Globular Cluster Bol 111
(2007-12) Shafter, A. W.; Quimby, R. M.; Quimby, R. M.
We report spectroscopic observations of the nova M31N-2007-06b, which was found to be spatially coincident with the M31 globular cluster Bol 111. This nova is the first out of more than 700 discovered in M31 over the past century to be associated with one of the galaxy's globular clusters. A total of three spectra of the nova were obtained 3, 6, and 36 days after discovery. The data reveal broad (FWHM similar to 3000 km s(-1)) Balmer, N II, and N III emission lines and show that the nova belongs to the He/N spectroscopic class. The He/N class of novae are relatively rare, making up roughly 15% of the novae with measured spectra in M31 and roughly 20% - 25% of the Galactic novae for which spectroscopic data are available. The implications of a nova, particularly an He/N nova, occurring in a globular cluster are discussed.
Multi-Color Optical and Near-Infrared Light Curves of 64 Stripped-Envelope Core-Collapse Supernovae
(2014-08) Bianco, F. B.; Modjaz, M.; Hicken, M.; Friedman, A.; Kirshner, R. P.; Bloom, J. S.; Challis, P.; Marion, G. H.; Wood-Vasey, W. M.; Rest, A.; Marion, G. H.
We present a densely sampled, homogeneous set of light curves of 64 low-redshift (z less than or similar to 0.05) stripped-envelope supernovae (SNe of Type IIb, Ib, Ic, and Ic-BL). These data were obtained between 2001 and 2009 at the Fred L. Whipple Observatory (FLWO) on Mount Hopkins in Arizona, with the optical FLWO 1.2 m and the near-infrared (NIR) Peters Automated Infrared 1.3 m telescopes. Our data set consists of 4543 optical photometric measurements on 61 SNe, including a combination of UBVRI, UBVr'i', and u'BVr'i', and 1919 JHK(s) NIR measurements on 25 SNe. This sample constitutes the most extensive multi-color data set of stripped-envelope SNe to date. Our photometry is based on template-subtracted images to eliminate any potential host-galaxy light contamination. This work presents these photometric data, compares them with data in the literature, and estimates basic statistical quantities: date of maximum, color, and photometric properties. We identify promising color trends that may permit the identification of stripped-envelope SN subtypes from their photometry alone. Many of these SNe were observed spectroscopically by the Harvard-Smithsonian Center for Astrophysics (CfA) SN group, and the spectra are presented in a companion paper. A thorough exploration that combines the CfA photometry and spectroscopy of stripped-envelope core-collapse SNe will be presented in a follow-up paper.
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
(2013-10) Hinshaw, G.; Larson, D.; Komatsu, Eiichiro; Spergel, D. N.; Bennett, C. L.; Dunkley, J.; Nolta, M. R.; Halpern, M.; Hill, R. S.; Odegard, N.; Page, L.; Smith, K. M.; Weiland, J. L.; Gold, B.; Jarosik, N.; Kogut, A.; Limon, M.; Meyer, S. S.; Tucker, G. S.; Wollack, E.; Wright, E. L.; Komatsu, Eiichiro
We present cosmological parameter constraints based on the final nine-year Wilkinson Microwave Anisotropy Probe (WMAP) data, in conjunction with a number of additional cosmological data sets. The WMAP data alone, and in combination, continue to be remarkably well fit by a six-parameter Delta CDM model. When WMAP data are combined with measurements of the high-l cosmic microwave background anisotropy, the baryon acoustic oscillation scale, and the Hubble constant, the matter and energy densities, Omega(b)h(2), Omega(c)h(2), and Omega(Lambda), are each determined to a precision of similar to 1.5%. The amplitude of the primordial spectrum is measured to within 3%, and there is now evidence for a tilt in the primordial spectrum at the 5 sigma level, confirming the first detection of tilt based on the five-year WMAP data. At the end of the WMAP mission, the nine-year data decrease the allowable volume of the six-dimensional Delta CDM parameter space by a factor of 68,000 relative to pre-WMAP measurements. We investigate a number of data combinations and show that their Delta CDM parameter fits are consistent. New limits on deviations from the six-parameter model are presented, for example: the fractional contribution of tensor modes is limited to r < 0.13 (95% CL); the spatial curvature parameter is limited to Omega(k) = -0.0027(-0.0038)(+0.0039); the summed mass of neutrinos is limited to Sigma m(nu) < 0.44 eV (95% CL); and the number of relativistic species is found to lie within N-eff = 3.84 +/- 0.40, when the full data are analyzed. The joint constraint on N-eff and the primordial helium abundance, Y-He, agrees with the prediction of standard big bang nucleosynthesis. We compare recent Planck measurements of the Sunyaev-Zel'dovich effect with our seven-year measurements, and show their mutual agreement. Our analysis of the polarization pattern around temperature extrema is updated. This confirms a fundamental prediction of the standard cosmological model and provides a striking illustration of acoustic oscillations and adiabatic initial conditions in the early universe.
Oxygen Isotopic Ratios In Cool R Coronae Borealis Stars
(2010-05) Garcia-Hernandez, D. Anibal; Lambert, David L.; Rao, N. Kameswara; Hinkle, Ken. H.; Eriksson, Kiell; Lambert, David L.
We investigate the relationship between R Coronae Borealis (RCB) stars and hydrogen-deficient carbon (HdC) stars by measuring precise (16)O/(18)O ratios for five cool RCB stars. The (16)O/(18)O ratios are derived by spectrum synthesis from high-resolution (R similar to 50,000) K-band spectra. Lower limits to the (16)O/(17)O and (14)N/(15)N ratios as well as Na and S abundances (when possible) are also given. RCB stars in our sample generally display less (18)O than HdC stars-the derived (16)O/(18)O ratios range from 3 to 20. The only exception is the RCB star WX CrA, which seems to be an HdC-like star with (16)O/(18)O = 0.3. Our result of a higher (16)O/(18)O ratio for the RCB stars must be accounted for by a theory of the formation and evolution of HdC and RCB stars. We speculate that a late dredge-up of products of He burning, principally (12)C and (16)O, may convert an (18)O-rich HdC star into an (18)O-poor RCB star as the H-deficient star begins its final evolution from a cool supergiant to the top of the white dwarf cooling track.
Single-Degenerate Type Ia Supernovae Are Preferentially Overluminous
(2015-06) Fisher, Robert; Jumper, Kevin; Jumper, Kevin
Recent observational and theoretical progress has favored merging and helium-accreting sub-Chandrasekhar mass white dwarfs (WDs) in the double-degenerate and the double-detonation channels, respectively, as the most promising progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of rapidly accreting Chandrasekhar mass WDs in the single-degenerate channel remains more mysterious then ever. In this paper, we clarify the nature of ignition in Chandrasekhar-mass single-degenerate SNe Ia by analytically deriving the existence of a characteristic length scale which establishes a transition from central ignitions to buoyancy-driven ignitions. Using this criterion, combined with data from three-dimensional simulations of convection and ignition, we demonstrate that the overwhelming majority of ignition events within Chandrasekhar-mass WDs in the single-degenerate channel are buoyancy-driven, and consequently lack a vigorous deflagration phase. We thus infer that single-degenerate SNe Ia are generally expected to lead to overluminous 1991T-like SNe Ia events. We establish that the rates predicted from both the population of supersoft X-ray sources (SSSs) and binary population synthesis models of the single-degenerate channel are broadly consistent with the observed rates of overluminous SNe Ia, and suggest that the population of SSSs are the dominant stellar progenitors of SNe 1991T-like events. We further demonstrate that the single-degenerate channel contribution to the normal and failed 2002cx-like rates is not likely to exceed 1% of the total SNe Ia rate. We conclude with a range of observational tests of overluminous SNe Ia which will either support or strongly constrain the single-degenerate scenario.
SN 2006oz: Rise Of A Super-Luminous Supernova Observed By The SDSS-II SN Survey
(2012-05) Leloudas, G.; Chatzopoulos, E.; Dilday, B.; Gorosabel, J.; Vinko, J.; Gallazzi, A.; Wheeler, J. C.; Bassett, B.; Fischer, J. A.; Frieman, J. A.; Fynbo, J. P. U.; Goobar, A.; Jelinek, M.; Malesani, D.; Nichol, R. C.; Nordin, J.; Ostman, L.; Sako, M.; Schneider, D. P.; Smith, M.; Sollerman, J.; Stritzinger, M. D.; Thone, C. C.; Postigo, A. D.; Chatzopoulos, E.; Vinko, J.; Wheeler, J. C.
Context. A new class of super-luminous transients has recently been identified. These objects reach absolute luminosities of M-u < -21, lack hydrogen in their spectra, and are exclusively discovered by non-targeted surveys because they are associated with very faint galaxies. Aims. We aim to contribute to a better understanding of these objects by studying SN 2006oz, a newly-recognized member of this class. Methods. We present multi-color light curves of SN 2006oz from the SDSS-II SN Survey that cover its rise time, as well as an optical spectrum that shows that the explosion occurred at z similar to 0.376. We fitted black-body functions to estimate the temperature and radius evolution of the photosphere and used the parametrized code SYNOW to model the spectrum. We constructed a bolometric light curve and compared it with explosion models. In addition, we conducted a deep search for the host galaxy with the 10 m GTC telescope. Results. The very early light curves show a dip in the g-and r-bands and a possible initial cooling phase in the u-band before rising to maximum light. The bolometric light curve shows a precursor plateau with a duration of 6-10 days in the rest-frame. A lower limit of M-u < -21.5 can be placed on the absolute peak luminosity of the SN, while the rise time is constrained to be at least 29 days. During our observations, the emitting sphere doubled its radius to similar to 2 x 10(15) cm, while the temperature remained hot at similar to 15 000 K. As for other similar SNe, the spectrum is best modeled with elements including O II and Mg II, while we tentatively suggest that Fe III might be present. The host galaxy is detected in gri with 25.74 +/- 0.19, 24.43 +/- 0.06, and 24.14 +/- 0.12, respectively. It is a faint dwarf galaxy with M-g = -16.9. Conclusions. We suggest that the precursor plateau might be related to a recombination wave in a circumstellar medium (CSM) and discuss whether this is a common property of all similar explosions. The subsequent rise can be equally well described by input from a magnetar or by ejecta-CSM interaction, but the models are not well constrained owing to the lack of post-maximum observations, and CSM interaction has difficulties accounting for the precursor plateau self-consistently. Radioactive decay is less likely to be the mechanism that powers the luminosity. The host is a moderately young and star-forming, but not a starburst, galaxy.
Supernova Asymmetries
(2007-10) Wheeler, J. C.; Maund, J. R.; Akiyama, S.; Wheeler, J. C.; Maund, J. R.
All core collapse supernovae are strongly aspherical. The >Bochum event> with velocity components displaced symmetrically about the principal H alpha line, strongly suggests that SN 1987A was a bi-polar rather than a uni-polar explosion. While there is a general tendency to display a single prominant axis in images and spectropolarimetry, there is also growing evidence for frequent departures from axisymmetry. There are various mechanisms that might contribute to large scale departures from spherical symmetry: jet-induced processes, the spherical shock accretion instability (SASI) and associated phenomena, and non-axisymmetric instabilities (NAXI). The MRI gives inevitable production of large toroidal magnetic fields. In sum: no Omega without B. The role of magnetic fields, non-axisymmetric instabilities, and of the de-leptonization phase are discussed.
A Theoretical Investigation of Supernovae Progenitors
(2016) Nance, Sarafina; Wheeler, J. Craig
The Transitional Stripped-Envelope Sn 2008Ax: Spectral Evolution And Evidence For Large Asphericity
(2011-09) Chornock, Ryan; Filippenko, Alexei V.; Li, W.; Marion, G. H.; Foley, Ryan J.; Modjaz, M.; Rafelski, M.; Becker, G. D.; de Vries, W. H.; Garnavich, Peter; Jorgenson, R. A.; Lynch, D. K.; Malec, A. L.; Moran, E. C.; Murphy, M. T.; Rudy, R. J.; Russell, R. W.; Silverman, Jeffrey M.; Steele, Thea N.; Stockton, A.; Wolfe, A. M.; Woodward, C. E.; Marion, G. H.; Stockton, A.
Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock breakout, presenting the rare opportunity to study a core-collapse SN beginning with the initial envelope-cooling phase immediately following shock breakout. We present an extensive sequence of optical and near-infrared spectra, as well as three epochs of optical spectropolarimetry. Our initial spectra, taken two days after shock breakout, are dominated by hydrogen Balmer lines at high velocity. However, by maximum light, He I lines dominated the optical and near-infrared spectra, which closely resembled those of normal Type Ib supernovae (SNe Ib) such as SN 1999ex. This spectroscopic transition defines Type IIb SNe, but the strong similarity of SN 2008ax to normal SNe Ib beginning near maximum light, including an absorption feature near 6270 angstrom due to H alpha at high velocities, suggests that many objects classified as SNe Ib in the literature may have ejected similar amounts of hydrogen as SN 2008ax, roughly a few x 0.01 M-circle dot. Only the unusually early discovery of SN 2008ax allowed us to observe the spectroscopic signatures of the hydrogen-rich outer ejecta. Early-time spectropolarimetry (six and nine days after shock breakout) revealed strong line polarization modulations of 3.4% across H alpha, indicating the presence of large asphericities in the outer ejecta and possibly that the spectrum of SN 2008ax could be dependent on the viewing angle. After removal of interstellar polarization, the continuum shares a common polarization angle with the hydrogen, helium, and oxygen lines, while the calcium and iron absorptions are oriented at different angles. This is clear evidence of deviations from axisymmetry even in the outer ejecta. Intrinsic continuum polarization of 0.64% only nine days after shock breakout shows that the outer layers of the ejecta were quite aspherical. A single epoch of late-time spectropolarimetry as well as the shapes of the nebular line profiles demonstrate that asphericities extended from the outermost layers all the way down to the center of this core-collapse SN. SN 2008ax may in some ways be an extragalactic analog of the explosion giving rise to Cassiopeia A, which has recently been determined to be a remnant of an SN IIb.
Type Iax Supernovae: A New Class Of Stellar Explosion
(2013-04) Foley, Ryan J.; Challis, Peter J.; Chornock, Ryan; Ganeshalingam, Mohan; Li, W.; Marion, G. H.; Morrell, Nidia I.; Pignata, G.; Stritzinger, Maximillian D.; Silverman, Jeffrey M.; Wang, X.; Anderson, J. P.; Filippenko, Alexei V.; Freedman, W. L.; Hamuy, Mario; Jha, Saurabh W.; Kirshner, Robert P.; McCully, C.; Persson, S. E.; Phillips, Mark M.; Reichart, D. E.; Soderberg, Alicia M.; Silverman, Jeffrey M.
We describe observed properties of the Type Iax class of supernovae (SNe Iax), consisting of SNe observationally similar to its prototypical member, SN 2002cx. The class currently has 25 members, and we present optical photometry and/ or optical spectroscopy for most of them. SNe Iax are spectroscopically similar to SNe Ia, but have lower maximum-light velocities (2000 less than or similar to broken vertical bar v broken vertical bar less than or similar to 8000 km s(-1)), typically lower peak magnitudes (-14.2 >= M-V,M-peak greater than or similar to -18.9 mag), and most have hot photospheres. Relative to SNe Ia, SNe Iax have low luminosities for their light-curve shape. There is a correlation between luminosity and light-curve shape, similar to that of SNe Ia, but offset from that of SNe Ia and with larger scatter. Despite a host-galaxy morphology distribution that is highly skewed to late-type galaxies without any SNe Iax discovered in elliptical galaxies, there are several indications that the progenitor stars are white dwarfs (WDs): evidence of C/ O burning in their maximum-light spectra, low (typically similar to 0.5 M-circle dot) ejecta masses, strong Fe lines in their late-time spectra, a lack of X-ray detections, and deep limits on massive stars and star formation at the SN sites. However, two SNe Iax show strong He lines in their spectra. The progenitor system and explosion model that best fits all of the data is a binary system of a C/ O WD that accretes matter from a He star and has a deflagration. At least some of the time, this explosion will not disrupt the WD. The small number of SNe in this class prohibit a detailed analysis of the homogeneity and heterogeneity of the entire class. We estimate that in a given volume there are 31(-13)(+17) SNe Iax for every 100 SNe Ia, and for every 1M(circle dot) of iron generated by SNe Ia at z = 0, SNe Iax generate similar to 0.036M(circle dot). Being the largest class of peculiar SNe, thousands of SNe Iax will be discovered by LSST. Future detailed observations of SNe Iax should further our understanding of both their progenitor systems and explosions as well as those of SNe Ia.
Type II-P Supernovae From The SDSS-II Supernova Survey And The Standardized Candle Method
(2010-01) D'Andrea, Chris B.; Sako, Masao; Dilday, Benjamin; Frieman, Joshua A.; Holtzman, Jon; Kessler, Richard; Konishi, Kohki; Schneider, Donald P.; Sollerman, Jesper; Wheeler, J. Craig; Yasuda, Naoki; Cinabro, David; Jha, Saurabh; Nichol, Robert C.; Lampeitl, Hubert; Smith, Mathew; Atlee, David W.; Bassett, Bruce; Castander, Francisco J.; Goobar, Ariel; Miquel, Ramon; Nordin, Jakob; Ostman, Linda; Prieto, Jose L.; Quimby, Robert; Riess, Adam G.; Stritzinger, Maximillian; Wheeler, J. Craig
We apply the Standardized Candle Method (SCM) for Type II Plateau supernovae (SNe II-P), which relates the velocity of the ejecta of a SN to its luminosity during the plateau, to 15 SNe II-P discovered over the three season run of the Sloan Digital Sky Survey-II Supernova Survey. The redshifts of these SNe-0.027 < z < 0.144-cover a range hitherto sparsely sampled in the literature; in particular, our SNe II-P sample contains nearly as many SNe in the Hubble flow (z > 0.01) as all of the current literature on the SCM combined. We find that the SDSS SNe have a very small intrinsic I-band dispersion (0.22 mag), which can be attributed to selection effects. When the SCM is applied to the combined SDSS-plus-literature set of SNe II-P, the dispersion increases to 0.29 mag, larger than the scatter for either set of SNe separately. We show that the standardization cannot be further improved by eliminating SNe with positive plateau decline rates, as proposed in Poznanski et al. We thoroughly examine all potential systematic effects and conclude that for the SCM to be useful for cosmology, the methods currently used to determine the Fe II velocity at day 50 must be improved, and spectral templates able to encompass the intrinsic variations of Type II-P SNe will be needed.
Ultraviolet Spectroscopy Of Type IIb Supernovae: Diversity And The Impact Of Circumstellar Material
(2015-04) Ben-Ami, Sagi; Hachinger, Stephan; Gal-Yam, Avishay; Mazzali, Paolo A.; Filippenko, Alexei V.; Horesh, Assaf; Matheson, Thomas; Modjaz, Maryam; Sauer, Daniel N.; Silverman, Jeffrey M.; Smith, Nathan; Yaron, Ofer; Silverman, Jeffrey M.
We present new Hubble Space Telescope (HST) multi-epoch ultraviolet (UV) spectra of the bright Type IIb SN 2013df, and undertake a comprehensive analysis of the set of four SNe IIb for which HST UV spectra are available (SN 1993J, SN 2001ig, SN 2011dh, and SN 2013df). We find strong diversity in both continuum levels and line features among these objects. We use radiative-transfer models that fit the optical part of the spectrum well, and find that in three of these four events we see a UV continuum flux excess, apparently unaffected by line absorption. We hypothesize that this emission originates above the photosphere, and is related to interaction with circumstellar material (CSM) located in close proximity to the SN progenitor. In contrast, the spectra of SN 2001ig are well fit by single-temperature models, display weak continuum and strong reverse-fluorescence features, and are similar to spectra of radioactive 56Ni-dominated SNe Ia. A comparison of the early shock-cooling components in the observed light curves with the UV continuum levels which we assume trace the strength of CSM interaction suggests that events with slower cooling have stronger CSM emission. The radio emission from events having a prominent UV excess is perhaps consistent with slower blast-wave velocities, as expected if the explosion shock was slowed down by the CSM that is also responsible for the strong UV, but this connection is currently speculative as it is based on only a few events.