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Rollin Thomas

Rollin Thomas
Big Data Architect
Data and Analytics Services
Phone: (510) 486-4697
Fax: (510) 486-6459
Mailstop: 59-4010A
Berkeley, CA 94720 US

Biographical Sketch

Rollin Thomas is a Big Data Architect in the Data and Analytics Services group.  Prior to joining NERSC in 2015, he was a Staff Scientist in the Computational Research Division.  He has worked on numerical simulations of supernova atmospheres, observation and analysis of supernova spectroscopy data, and data management for supernova cosmology experiments.  Rollin has served as a member of the Nearby Supernova Factory, is a builder on the Dark Energy Survey, and is a full member of the Large Synoptic Survey Telescope Dark Energy Science Collaboration.  He holds a B.S. in physics from Purdue University and a Ph.D. in astrophysics from the University of Oklahoma.

Journal Articles

Friesen, B., Baron, E., Parrent, J. T., Thomas, R., C., Branch, D., Nugent, P., Hauschildt, P. H., Foley, R. J., Wright, D. E., Pan, Y.-C., Filippenko, A. V., Clubb, K. I., Silverman, J. M., Maeda, K. Shivvers, I., Kelly, P. L., Cohen, D. P., Rest, A., Kasen, D.,"Optical and ultraviolet spectroscopic analysis of SN 2011fe at late times",Monthly Notices of the Royal Astronomical Society,February 27, 2017,467:2392-2411,doi: 10.1093/mnras/stx241

We present optical spectra of the nearby Type Ia supernova SN 2011fe at 100, 205, 311, 349 and 578 d post-maximum light, as well as an ultraviolet (UV) spectrum obtained with the Hubble Space Telescope at 360 d post-maximum light. We compare these observations with synthetic spectra produced with the radiative transfer code phoenix. The day +100 spectrum can be well fitted with models that neglect collisional and radiative data for forbidden lines. Curiously, including these data and recomputing the fit yields a quite similar spectrum, but with different combinations of lines forming some of the stronger features. At day +205 and later epochs, forbidden lines dominate much of the optical spectrum formation; however, our results indicate that recombination, not collisional excitation, is the most influential physical process driving spectrum formation at these late times. Consequently, our synthetic optical and UV spectra at all epochs presented here are formed almost exclusively through recombination-driven fluorescence. Furthermore, our models suggest that the UV spectrum even as late as day +360 is optically thick and consists of permitted lines from several iron-peak species. These results indicate that the transition to the ‘nebular’ phase in Type Ia supernovae is complex and highly wavelength dependent.

Smith, M., Sullivan, M., D'Andrea, C. B., Castander, F. J., Casas, R., Prajs, S., Papadopoulos, A., Nichol, R. C., Karpenka, N. V., Bernard, S. R., Brown, P., Cartier, R., Cooke, J., Curtin, C., Davis, T. M., Finley, D. A., Foley, R. J., Gal-Yam, A., Goldstein, D. A., González-Gaitán, S., Gupta, R. R., Howell, D. A., Inserra, C., Kessler, R., Lidman, C., Marriner, J., Nugent, P., Pritchard, T. A., Sako, M., Smartt, S., Smith, R. C., Spinka, H., Thomas, R. C., Wolf, R. C., Zenteno, A., Abbott, T. M. C., Benoit-Lévy, A., Bertin, E., Brooks, D., Buckley-Geer, E., Carnero Rosell, A., Carrasco Kind, M., Carretero, J., Crocce, M., Cunha, C. E., da Costa, L. N., Desai, S., Diehl, H. T., Doel, P., Estrada, J., Evrard, A. E., Flaugher, B., Fosalba, P., Frieman, J., Gerdes, D. W., Gruen, D., Gruendl, R. A., James, D. J., Kuehn, K., Kuropatkin, N., Lahav, O., Li, T. S., Marshall, J. L., Martini, P., Miller, C. J., Miquel, R., Nord, B., Ogando, R., Plazas, A. A., Reil, K., Romer, A. K., Roodman, A., Rykoff, E. S., Sanchez, E., Scarpine, V., Schubnell, M., Sevilla-Noarbe, I., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., Tarle, G., Walker, A. R., Wester, W.,"DES14X3taz: A Type I Superluminous Supernova Showing a Luminous, Rapidly Cooling Initial Pre-peak Bump",The Astrophysical Journal Letters,2016,doi: 10.3847/2041-8205/818/1/L8

We present DES14X3taz, a new hydrogen-poor superluminous supernova (SLSN-I) discovered by the Dark Energy Survey (DES) supernova program, with additional photometric data provided by the Survey Using DECam for Superluminous Supernovae. Spectra obtained using Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy on the Gran Telescopio CANARIAS show DES14X3taz is an SLSN-I at z = 0.608. Multi-color photometry reveals a double-peaked light curve: a blue and relatively bright initial peak that fades rapidly prior to the slower rise of the main light curve. Our multi-color photometry allows us, for the first time, to show that the initial peak cools from 22,000 to 8000 K over 15 rest-frame days, and is faster and brighter than any published core-collapse supernova, reaching 30% of the bolometric luminosity of the main peak. No physical 56Ni-powered model can fit this initial peak. We show that a shock-cooling model followed by a magnetar driving the second phase of the light curve can adequately explain the entire light curve of DES14X3taz. Models involving the shock-cooling of extended circumstellar material at a distance of ≃400 {\text{}}{R}⊙ are preferred over the cooling of shock-heated surface layers of a stellar envelope. We compare DES14X3taz to the few double-peaked SLSN-I events in the literature. Although the rise times and characteristics of these initial peaks differ, there exists the tantalizing possibility that they can be explained by one physical interpretation.

Baron, E., Hoeflich, P., Friesen, B., Sullivan, M., Hsiao, E., Ellis, R. S., Gal-Yam, A., Howell, D. A., Nugent, P. E., Dominguez, I., Krisciunas, K., Phillips, M. M., Suntzeff, N., Wang, L., and Thomas, R. C.,"Spectral models for early time SN 2011fe observations",Monthly Notices of the Royal Astronomical Society,2015,454:2549,doi: 10.1093/mnras/stv1951

We use observed UV through near-IR spectra to examine whether SN 2011fe can be understood in the framework of Branch-normal Type Ia supernovae (SNe Ia) and to examine its individual peculiarities. As a benchmark, we use a delayed-detonation model with a progenitor metallicity of Z/20. We study the sensitivity of features to variations in progenitor metallicity, the outer density profile, and the distribution of radioactive nickel. The effect of metallicity variations in the progenitor have a relatively small effect on the synthetic spectra. We also find that the abundance stratification of SN 2011fe resembles closely that of a delayed-detonation model with a transition density that has been fit to other Branch-normal SNe Ia. At early times, the model photosphere is formed in material with velocities that are too high, indicating that the photosphere recedes too slowly or that SN 2011fe has a lower specific energy in the outer ≈0.1 M than does the model. We discuss several explanations for the discrepancies. Finally, we examine variations in both the spectral energy distribution and in the colours due to variations in the progenitor metallicity, which suggests that colours are only weak indicators for the progenitor metallicity, in the particular explosion model that we have studied. We do find that the flux in the U band is significantly higher at maximum light in the solar metallicity model than in the lower metallicity model and the lower metallicity model much better matches the observed spectrum.

Fakhouri, H. K., Boone, K., Aldering, G., Antilogus, P., Aragon, C., Bailey, S., Baltay, C., Barbary, K., Baugh, D., Bongard, S., Buton, C., Chen, J., Childress, M., Chotard, N., Copin, Y., Fagrelius, P., Feindt, U., Fleury, M., Fouchez, D., Gangler, E., Hayden, B., Kim, A. G., Kowalski, M., Leget, P.-F., Lombardo, S., Nordin, J., Pain, R., Pecontal, E., Pereira, R., Perlmutter, S., Rabinowitz, D., Ren, J., Rigault, M., Rubin, D., Runge, K., Saunders, C., Scalzo, R., Smadja, G., Sofiatti, C., Strovink, M., Suzuki, N., Tao, C., Thomas, R. C., and Weaver, B. A.,"Improving Cosmological Distance Measurements Using Twin Type Ia Supernovae",The Astrophysical Journal,2015,815:58,doi: 10.1088/0004-637X/815/1/58

We introduce a method for identifying “twin” Type Ia supernovae (SNe Ia) and using them to improve distance measurements. This novel approach to SN Ia standardization is made possible by spectrophotometric time series observations from the Nearby Supernova Factory (SNfactory). We begin with a well-measured set of SNe, find pairs whose spectra match well across the entire optical window, and then test whether this leads to a smaller dispersion in their absolute brightnesses. This analysis is completed in a blinded fashion, ensuring that decisions made in implementing the method do not inadvertently bias the result. We find that pairs of SNe with more closely matched spectra indeed have reduced brightness dispersion. We are able to standardize this initial set of SNfactory SNe to 0.083 ± 0.012 mag, implying a dispersion of 0.072 ± 0.010 mag in the absence of peculiar velocities. We estimate that with larger numbers of comparison SNe, e.g., using the final SNfactory spectrophotometric data set as a reference, this method will be capable of standardizing high-redshift SNe to within 0.06–0.07 mag. These results imply that at least 3/4 of the variance in Hubble residuals in current SN cosmology analyses is due to previously unaccounted-for astrophysical differences among the SNe.

Kessler, R., Marriner, J., Childress, M., Covarrubias, R., D'Andrea, C. B., Finley, D. A., Fischer, J., Foley, R. J., Goldstein, D., Gupta, R. R., Kuehn, K., Marcha, M., Nichol, R. C., Papadopoulos, A., Sako, M., Scolnic, D., Smith, M., Sullivan, M., Wester, W., Yuan, F., Abbott, T., Abdalla, F. B., Allam, S., Benoit-Levy, A., Bernstein, G. M., Bertin, E., Brooks, D., Carnero Rosell, A., Carrasco Kind, M., Castander, F. J., Crocce, M., da Costa, L. N., Desai, S., Diehl, H. T., Eifler, T. F., Fausti Neto, A., Flaugher, B., Frieman, J., Gerdes, D. W., Gruen, D., Gruendl, R. A., Honscheid, K., James, D. J., Kuropatkin, N., Li, T. S., Maia, M. A. G., Marshall, J. L., Martini, P., Miller, C. J., Miquel, R., Nord, B., Ogando, R., Plazas, A. A., Reil, K., Romer, A. K., Roodman, A., Sanchez, E., Sevilla-Noarbe, I., Smith, R. C., Soares-Santos, M., Sobreira, F., Tarle, G., Thaler, J., Thomas, R. C., Tucker, D., and Walker, A. R.,"The Difference Imaging Pipeline for the Transient Search in the Dark Energy Survey",The Astronomical Journal,2015,150:172,doi: 10.1088/0004-6256/150/6/172

We describe the operation and performance of the difference imaging pipeline (DiffImg) used to detect transients in deep images from the Dark Energy Survey Supernova program (DES-SN) in its first observing season from 2013 August through 2014 February. DES-SN is a search for transients in which ten 3 deg2 fields are repeatedly observed in the g, r, i, z passbands with a cadence of about 1 week. The observing strategy has been optimized to measure high-quality light curves and redshifts for thousands of Type Ia supernovae (SNe Ia) with the goal of measuring dark energy parameters. The essential DiffImg functions are to align each search image to a deep reference image, do a pixel-by-pixel subtraction, and then examine the subtracted image for significant positive detections of point-source objects. The vast majority of detections are subtraction artifacts, but after selection requirements and image filtering with an automated scanning program, there are ∼130 detections per deg2 per observation in each band, of which only ∼25% are artifacts. Of the ∼7500 transients discovered by DES-SN in its first observing season, each requiring a detection on at least two separate nights, Monte Carlo (MC) simulations predict that 27% are expected to be SNe Ia or core-collapse SNe. Another ∼30% of the transients are artifacts in which a small number of observations satisfy the selection criteria for a single-epoch detection. Spectroscopic analysis shows that most of the remaining transients are AGNs and variable stars. Fake SNe Ia are overlaid onto the images to rigorously evaluate detection efficiencies and to understand the DiffImg performance. The DiffImg efficiency measured with fake SNe agrees well with expectations from a MC simulation that uses analytical calculations of the fluxes and their uncertainties. In our 8 “shallow” fields with single-epoch 50% completeness depth ∼23.5, the SN Ia efficiency falls to 1/2 at redshift z ≈ 0.7; in our 2 “deep” fields with mag-depth ∼24.5, the efficiency falls to 1/2 at z ≈ 1.1. A remaining performance issue is that the measured fluxes have additional scatter (beyond Poisson fluctuations) that increases with the host galaxy surface brightness at the transient location. This bright-galaxy issue has minimal impact on the SNe Ia program, but it may lower the efficiency for finding fainter transients on bright galaxies.

Yuan, F., Lidman, C., Davis, T. M., Childress, M., Abdalla, F. B., Banerji, M., Buckley-Geer, E., Carnero Rosell, A., Carollo, D., Castander, F. J., D'Andrea, C. B., Diehl, H. T., Cunha, C. E., Foley, R. J., Frieman, J., Glazebrook, K., Gschwend, J., Hinton, S., Jouvel, S., Kessler, R., Kim, A. G., King, A. L., Kuehn, K., Kuhlmann, S., Lewis, G. F., Lin, H., Martini, P., McMahon, R. G., Mould, J., Nichol, R. C., Norris, R. P., O'Neill, C. R., Ostrovski, F., Papadopoulos, A., Parkinson, D., Reed, S., Romer, A. K., Rooney, P. J., Rozo, E., Rykoff, E. S., Sako, M., Scalzo, R., Schmidt, B. P., Scolnic, D., Seymour, N., Sharp, R., Sobreira, F., Sullivan, M., Thomas, R. C., Tucker, D., Uddin, S. A., Wechsler, R. H., Wester, W., Wilcox, H., Zhang, B., Abbott, T., Allam, S., Bauer, A. H., Benoit-Levy, A., Bertin, E., Brooks, D., Burke, D. L., Carrasco Kind, M., Covarrubias, R., Crocce, M., da Costa, L. N., DePoy, D. L., Desai, S., Doel, P., Eifler, T. F., Evrard, A. E., Fausti Neto, A., Flaugher, B., Fosalba, P., Gaztanaga, E., Gerdes, D., Gruen, D., Gruendl, R. A., Honscheid, K., James, D., Kuropatkin, N., Lahav, O., Li, T. S., Maia, M. A. G., Makler, M., Marshall, J., Miller, C. J., Miquel, R., Ogando, R., Plazas, A. A., Roodman, A., Sanchez, E., Scarpine, V., Schubnell, M., Sevilla-Noarbe, I., Smith, R. C., Soares-Santos, M., Suchyta, E., Swanson, M. E. C., Tarle, G., Thaler, J., and Walker, A. R.,"OzDES multifibre spectroscopy for the Dark Energy Survey: first-year operation and results",Monthly Notices of the Royal Astronomical Society,2015,452:3047,doi: 10.1093/mnras/stv1507

The Australian Dark Energy Survey (OzDES) is a five-year, 100-night, spectroscopic survey on the Anglo-Australian Telescope, whose primary aim is to measure redshifts of approximately 2500 Type Ia supernovae host galaxies over the redshift range 0.1 < z < 1.2, and derive reverberation-mapped black hole masses for approximately 500 active galactic nuclei and quasars over 0.3 < z < 4.5. This treasure trove of data forms a major part of the spectroscopic follow-up for the Dark Energy Survey for which we are also targeting cluster galaxies, radio galaxies, strong lenses, and unidentified transients, as well as measuring luminous red galaxies and emission line galaxies to help calibrate photometric redshifts. Here, we present an overview of the OzDES programme and our first-year results. Between 2012 December and 2013 December, we observed over 10 000 objects and measured more than 6 000 redshifts. Our strategy of retargeting faint objects across many observing runs has allowed us to measure redshifts for galaxies as faint as mr = 25 mag. We outline our target selection and observing strategy, quantify the redshift success rate for different types of targets, and discuss the implications for our main science goals. Finally, we highlight a few interesting objects as examples of the fortuitous yet not totally unexpected discoveries that can come from such a large spectroscopic survey.

Goldstein, D. A., D'Andrea, C. B., Fischer, J. A., Foley, R. J., Gupta, R. R., Kessler, R., Kim, A. G., Nichol, R. C., Nugent, P. E., Papadopoulos, A., Sako, M., Smith, M., Sullivan, M., Thomas, R. C., Wester, W., Wolf, R. C., Abdalla, F. B., Banerji, M., Benoit-Levy, A., Bertin, E., Brooks, D., Carnero Rosell, A., Castander, F. J., da Costa, L. N., Covarrubias, R., DePoy, D. L., Desai, S., Diehl, H. T., Doel, P., Eifler, T. F., Fausti Neto, A., Finley, D. A., Flaugher, B., Fosalba, P., Frieman, J., Gerdes, D., Gruen, D., Gruendl, R. A., James, D., Kuehn, K., Kuropatkin, N., Lahav, O., Li, T. S., Maia, M. A. G., Makler, M., March, M., Marshall, J. L., Martini, P., Merritt, K. W., Miquel, R., Nord, B., Ogando, R., Plazas, A. A., Romer, A. K., Roodman, A., Sanchez, E., Scarpine, V., Schubnell, M., Sevilla-Noarbe, I., Smith, R. C., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., Tarle, G., Thaler, J., and Walker, A. R.,"Automated Transient Identification in the Dark Energy Survey",The Astronomical Journal,2015,150:82,doi: 10.1088/0004-6256/150/3/82

We describe an algorithm for identifying point-source transients and moving objects on reference-subtracted optical images containing artifacts of processing and instrumentation. The algorithm makes use of the supervised machine learning technique known as Random Forest. We present results from its use in the Dark Energy Survey Supernova program (DES-SN), where it was trained using a sample of 898,963 signal and background events generated by the transient detection pipeline. After reprocessing the data collected during the first DES-SN observing season (2013 September through 2014 February) using the algorithm, the number of transient candidates eligible for human scanning decreased by a factor of 13.4, while only 1.0% of the artificial Type Ia supernovae (SNe) injected into search images to monitor survey efficiency were lost, most of which were very faint events. Here we characterize the algorithm’s performance in detail, and we discuss how it can inform pipeline design decisions for future time-domain imaging surveys, such as the Large Synoptic Survey Telescope and the Zwicky Transient Facility. An implementation of the algorithm and the training data used in this paper are available at at http://portal.nersc.gov/project/dessn/autoscan.

Kim, A. G., Padmanabhan, N., Aldering, G., Allen, S. W., Baltay, C., Cahn, R. N., D'Andrea, C. B., Dalal, N., Dawson, K. S., Denney, K. D., Eisenstein, D. J., Finley, D. A., Freedman, W. L., Ho, S., Holz, D. E., Kasen, D., Kent, S. M., Kessler, R., Kuhlmann, S., Linder, E. V., Martini, P., Nugent, P. E., Perlmutter, S., Peterson, B. M., Riess, A. G., Rubin, D., Sako, M., Suntzeff, N. V., Suzuki, N., Thomas, R. C., Wood-Vasey, W. M., and Woosley, S. E.,"Distance probes of dark energy",Astroparticle Physics,2015,63:2,doi: 10.1016/j.astropartphys.2014.05.007

This document presents the results from the Distances subgroup of the Cosmic Frontier Community Planning Study (Snowmass 2013). We summarize the current state of the field as well as future prospects and challenges. In addition to the established probes using Type Ia supernovae and baryon acoustic oscillations, we also consider prospective methods based on clusters, active galactic nuclei, gravitational wave sirens and strong lensing time delays.

Sasdelli, M., Hillebrandt, W., Aldering, G., Antilogus, P., Aragon, C., Bailey, S., Baltay, C., Benitez-Herrera, S., Bongard, S., Buton, C., Canto, A., Cellier-Holzem, F., Chen, J., Childress, M., Chotard, N., Copin, Y., Fakhouri, H. K., Feindt, U., Fink, M., Fleury, M., Fouchez, D., Gangler, E., Guy, J., Ishida, E. E. O., Kim, A. G., Kowalski, M., Kromer, M., Lombardo, S., Mazzali, P. A., Nordin, J., Pain, R., Pecontal, E., Pereira, R., Perlmutter, S., Rabinowitz, D., Rigault, M., Runge, K., Saunders, C., Scalzo, R., Smadja, G., Suzuki, N., Tao, C., Taubenberger, S., Thomas, R. C., Tilquin, A., and Weaver, B. A.,"A metric space for Type Ia supernova spectra",Monthly Notices of the Royal Astronomical Society,2015,447:1247,doi: 10.1093/mnras/stu2416

We develop a new framework for use in exploring Type Ia supernovae (SNe Ia) spectra. Combining principal component analysis (PCA) and partial least square (PLS) analysis we are able to establish correlations between the principal components (PCs) and spectroscopic/photometric SNe Ia features. The technique was applied to ˜120 SN and ˜800 spectra from the Nearby Supernova Factory. The ability of PCA to group together SNe Ia with similar spectral features, already explored in previous studies, is greatly enhanced by two important modifications: (1) the initial data matrix is built using derivatives of spectra over the wavelength, which increases the weight of weak lines and discards extinction, and (2) we extract time evolution information through the use of entire spectral sequences concatenated in each line of the input data matrix. These allow us to define a stable PC parameter space which can be used to characterize synthetic SN Ia spectra by means of real SN features. Using PLS, we demonstrate that the information from important previously known spectral indicators (namely the pseudo-equivalent width of Si II 5972 Å/Si II 6355 Å and the line velocity of S II 5640 Å/Si II 6355 Å) at a given epoch is contained within the PC space and can be determined through a linear combination of the most important PCs. We also show that the PC space encompasses photometric features like B/V magnitudes, B - V colours and SALT2 parameters c and x1. The observed colours and magnitudes, which are heavily affected by extinction, cannot be reconstructed using this technique alone. All the above-mentioned applications allowed us to construct a metric space for comparing synthetic SN Ia spectra with observations.

Saunders, C., Aldering, G., Antilogus, P., Aragon, C., Bailey, S., Baltay, C., Bongard, S., Buton, C., Canto, A., Cellier-Holzem, F., Childress, M., Chotard, N., Copin, Y., Fakhouri, H. K., Feindt, U., Gangler, E., Guy, J., Kerschhaggl, M., Kim, A. G., Kowalski, M., Nordin, J., Nugent, P., Paech, K., Pain, R., Pecontal, E., Pereira, R., Perlmutter, S., Rabinowitz, D., Rigault, M., Rubin, D., Runge, K., Scalzo, R., Smadja, G., Tao, C., Thomas, R. C., Weaver, B. A., and Wu, C.,"Type Ia Supernova Distance Modulus Bias and Dispersion from K-correction Errors: A Direct Measurement Using Light Curve Fits to Observed Spectral Time Series",The Astrophysical Journal,2015,800:57,doi: 10.1088/0004-637X/800/1/57

We estimate systematic errors due to K-corrections in standard photometric analyses of high-redshift Type Ia supernovae. Errors due to K-correction occur when the spectral template model underlying the light curve fitter poorly represents the actual supernova spectral energy distribution, meaning that the distance modulus cannot be recovered accurately. In order to quantify this effect, synthetic photometry is performed on artificially redshifted spectrophotometric data from 119 low-redshift supernovae from the Nearby Supernova Factory, and the resulting light curves are fit with a conventional light curve fitter. We measure the variation in the standardized magnitude that would be fit for a given supernova if located at a range of redshifts and observed with various filter sets corresponding to current and future supernova surveys. We find significant variation in the measurements of the same supernovae placed at different redshifts regardless of filters used, which causes dispersion greater than ~0.05 mag for measurements of photometry using the Sloan-like filters and a bias that corresponds to a 0.03 shift in w when applied to an outside data set. To test the result of a shift in supernova population or environment at higher redshifts, we repeat our calculations with the addition of a reweighting of the supernovae as a function of redshift and find that this strongly affects the results and would have repercussions for cosmology. We discuss possible methods to reduce the contribution of the K-correction bias and uncertainty.

Friesen, B., Baron, E., Wisniewski, J. P., Parrent, J. T., Thomas, R. C., Miller, Timothy R., and Marion, G. H.,"Near-infrared Line Identification in Type Ia Supernovae during the Transitional Phase",The Astrophysical Journal,2014,792:120,doi: 10.1088/0004-637X/792/2/120

We present near-infrared synthetic spectra of a delayed-detonation hydrodynamical model and compare them to observed spectra of four normal Type Ia supernovae ranging from day +56.5 to day +85. This is the epoch during which supernovae are believed to be undergoing the transition from the photospheric phase, where spectra are characterized by line scattering above an optically thick photosphere, to the nebular phase, where spectra consist of optically thin emission from forbidden lines. We find that most spectral features in the near-infrared can be accounted for by permitted lines of Fe II and Co II. In addition, we find that [Ni II] fits the emission feature near 1.98 μm, suggesting that a substantial mass of 58Ni exists near the center of the ejecta in these objects, arising from nuclear burning at high density.

"Type Ia supernova bolometric light curves and ejected mass estimates from the Nearby Supernova Factory",Monthly Notices of the Royal Astronomical Society,2014,440:1498,doi: 10.1093/mnras/stu350

We present a sample of normal Type Ia supernovae (SNe Ia) from the Nearby Supernova Factory data set with spectrophotometry at sufficiently late phases to estimate the ejected mass using the bolometric light curve. We measure 56Ni masses from the peak bolometric luminosity, then compare the luminosity in the 56Co-decay tail to the expected rate of radioactive energy release from ejecta of a given mass. We infer the ejected mass in a Bayesian context using a semi-analytic model of the ejecta, incorporating constraints from contemporary numerical models as priors on the density structure and distribution of 56Ni throughout the ejecta. We find a strong correlation between ejected mass and light-curve decline rate, and consequently 56Ni mass, with ejected masses in our data ranging from 0.9 to 1.4 M. Most fast-declining (SALT2 x1 < -1) normal SNe Ia have significantly sub-Chandrasekhar ejected masses in our fiducial analysis.

McCully, C., Jha, S. W., Foley, R. J., Chornock, R., Holtzman, J. A., Balam, D. D., Branch, D., Filippenko, A. V., Frieman, J., Fynbo, J., Galbany, L., Ganeshalingam, M., Garnavich, P. M., Graham, M. L., Hsiao, E. Y., Leloudas, G., Leonard, D. C., Li, W., Riess, A. G., Sako, M., Schneider, D. P., Silverman, J. M., Sollerman, J., Steele, T. N., Thomas, R. C., Wheeler, J. C., and Zheng, C.,"Hubble Space Telescope and Ground-based Observations of the Type Iax Supernovae SN 2005hk and SN 2008A",The Astrophysical Journal,2014,786:134,doi: 10.1088/0004-637X/786/2/134

We present Hubble Space Telescope (HST) and ground-based optical and near-infrared observations of SN 2005hk and SN 2008A, typical members of the Type Iax class of supernovae (SNe). Here we focus on late-time observations, where these objects deviate most dramatically from all other SN types. Instead of the dominant nebular emission lines that are observed in other SNe at late phases, spectra of SNe 2005hk and 2008A show lines of Fe II, Ca II, and Fe I more than a year past maximum light, along with narrow [Fe II] and [Ca II] emission. We use spectral features to constrain the temperature and density of the ejecta, and find high densities at late times, with ne >~ 109 cm-3. Such high densities should yield enhanced cooling of the ejecta, making these objects good candidates to observe the expected "infrared catastrophe," a generic feature of SN Ia models. However, our HST photometry of SN 2008A does not match the predictions of an infrared catastrophe. Moreover, our HST observations rule out a "complete deflagration" that fully disrupts the white dwarf for these peculiar SNe, showing no evidence for unburned material at late times. Deflagration explosion models that leave behind a bound remnant can match some of the observed properties of SNe Iax, but no published model is consistent with all of our observations of SNe 2005hk and 2008A.

Friesen, B., Baron, E., Branch, D., Chen, B., Parrent, J., Thomas, R. C.,"Supernova Resonance-scattering Line Profiles in the Absence of a Photosphere",The Astrophysical Journal Supplements Series,2012,203:1,doi: 10.1088/0067-0049/203/1/12

In supernova (SN) spectroscopy relatively little attention has been given to the properties of optically thick spectral lines in epochs following the photosphere's recession. Most treatments and analyses of post-photospheric optical spectra of SNe assume that forbidden-line emission comprises most if not all spectral features. However, evidence exists that suggests that some spectra exhibit line profiles formed via optically thick resonance-scattering even months or years after the SN explosion. To explore this possibility, we present a geometrical approach to SN spectrum formation based on the "Elementary Supernova" model, wherein we investigate the characteristics of resonance-scattering in optically thick lines while replacing the photosphere with a transparent central core emitting non-blackbody continuum radiation, akin to the optical continuum provided by decaying 56Co formed during the explosion. We develop the mathematical framework necessary for solving the radiative transfer equation under these conditions and calculate spectra for both isolated and blended lines. Our comparisons with analogous results from the Elementary Supernova code SYNOW reveal several marked differences in line formation. Most notably, resonance lines in these conditions form P Cygni-like profiles, but the emission peaks and absorption troughs shift redward and blueward, respectively, from the line's rest wavelength by a significant amount, despite the spherically symmetric distribution of the line optical depth in the ejecta. These properties and others that we find in this work could lead to misidentification of lines or misattribution of properties of line-forming material at post-photospheric times in SN optical spectra.

Parrent, J. T., Howell, D. A., Friesen, B., Thomas, R. C., Fesen, R. A., Milisavljevic, D., Bianco, F. B., Dilday, B., Nugent, P., Baron, E., Arcavi, I., Ben-Ami, S., Bersier, D., Bildsten, L., Bloom, J., Cao, Y., Cenko, S. B., Filippenko, A. V., Gal-Yam, A., Kasliwal, M. M., Konidaris, N., Kulkarni, S. R., Law, N. M., Levitan, D., Maguire, K., Mazzali, P. A., Ofek, E. O., Pan, Y., Polishook, D., Poznanski, D., Quimby, R. M., Silverman, J. M., Sternberg, A., Sullivan, M., Walker, E. S., Xu, Dong, Buton, C., Pereira, R.,"Analysis of the Early-time Optical Spectra of SN 2011fe in M101",The Astrophysical Journal Letters,2012,752, doi: 10.1088/2041-8205/752/2/L26

The nearby Type Ia supernova (SN Ia) SN 2011fe in M101 (cz = 241 km s–1) provides a unique opportunity to study the early evolution of a "normal" SN Ia, its compositional structure, and its elusive progenitor system. We present 18 high signal-to-noise spectra of SN 2011fe during its first month beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This gives a clear picture of how various line-forming species are distributed within the outer layers of the ejecta, including that of unburned material (C+O). We follow the evolution of C II absorption features until they diminish near maximum light, showing overlapping regions of burned and unburned material between ejection velocities of 10,000 and 16,000 km s–1. This supports the notion that incomplete burning, in addition to progenitor scenarios, is a relevant source of spectroscopic diversity among SNe Ia. The observed evolution of the highly Doppler-shifted O I λ7774 absorption features detected within 5 days post-explosion indicates the presence of O I with expansion velocities from 11,500 to 21,000 km s–1. The fact that some O I is present above C II suggests that SN 2011fe may have had an appreciable amount of unburned oxygen within the outer layers of the ejecta.

Thomas, R. C.; Aldering, G.; Antilogus, P.; Aragon, C.; Bailey, S.; Baltay, C.; Bongard, S.; Buton, C.; Canto, A.; Childress, M.; Chotard, N.; Copin, Y.; Fakhouri, H. K.; Gangler, E.; Hsiao, E. Y.; Kerschhaggl, M.; Kowalski, M.; Loken, S.; Nugent, P.; Paech, K.; Pain, R.; Pecontal, E.; Pereira, R.; Perlmutter, S.; Rabinowitz, D.; Rigault, M.; Rubin, D.; Runge, K.; Scalzo, R.; Smadja, G.; Tao, C.; Weaver, B. A.; Wu, C.; (The Nearby Supernova Factory); Brown, P. J.; Milne, P. A.,"Type Ia Supernova Carbon Footprints",Astrophysical Journal,December 2011,743:27,

We present convincing evidence of unburned carbon at photospheric velocities in new observations of five Type Ia supernovae (SNe Ia) obtained by the Nearby Supernova Factory. These SNe are identified by examining 346 spectra from 124 SNe obtained before +2.5 days relative to maximum. Detections are based on the presence of relatively strong C II λ6580 absorption "notches" in multiple spectra of each SN, aided by automated fitting with the SYNAPPS code. Four of the five SNe in question are otherwise spectroscopically unremarkable, with ions and ejection velocities typical of SNe Ia, but spectra of the fifth exhibit high-velocity (v > 20, 000 km s–1) Si II and Ca II features. On the other hand, the light curve properties are preferentially grouped, strongly suggesting a connection between carbon-positivity and broadband light curve/color behavior: three of the five have relatively narrow light curves but also blue colors and a fourth may be a dust-reddened member of this family. Accounting for signal to noise and phase, we estimate that 22+10 – 6% of SNe Ia exhibit spectroscopic C II signatures as late as –5 days with respect to maximum. We place these new objects in the context of previously recognized carbon-positive SNe Ia and consider reasonable scenarios seeking to explain a physical connection between light curve properties and the presence of photospheric carbon. We also examine the detailed evolution of the detected carbon signatures and the surrounding wavelength regions to shed light on the distribution of carbon in the ejecta. Our ability to reconstruct the C II λ6580 feature in detail under the assumption of purely spherical symmetry casts doubt on a "carbon blobs" hypothesis, but does not rule out all asymmetric models. A low volume filling factor for carbon, combined with line-of-sight effects, seems unlikely to explain the scarcity of detected carbon in SNe Ia by itself. http://dx.doi.org/10.1088/0004-637X/743/1/27

Nugent, Peter E.; Sullivan, Mark; Cenko, S. Bradley; Thomas, Rollin C.; Kasen, Daniel; Howell, D. Andrew; Bersier, David; Bloom, Joshua S.; Kulkarni, S. R.; Kandrashoff, Michael T.; Filippenko, Alexei V.; Silverman, Jeffrey M.; Marcy, Geoffrey W.; Howard, Andrew W.; Isaacson, Howard T.; Maguire, Kate; Suzuki, Nao; Tarlton, James E.; Pan, Yen-Chen; Bildsten, Lars; Fulton, Benjamin J.; Parrent, Jerod T.; Sand, David; Podsiadlowski, Philipp; Bianco, Federica B.; Dilday, Benjamin; Graham, Melissa L.; Lyman, Joe; James, Phil; Kasliwal, Mansi M.; Law, Nicholas M.; Quimby, Robert M.; Hook, Isobel M.; Walker, Emma S.; Mazzali, Paolo; Pian, Elena; Ofek, Eran O.; Gal-Yam, Avishay; Poznanski, Dovi,"Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star",Nature,December 2011,480:344-347,

Type Ia supernovae have been used empirically as `standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor. http://dx.doi.org/10.1038/nature10644

Krisciunas, Kevin; Li, Weidong; Matheson, Thomas; Howell, D. Andrew; Stritzinger, Maximilian; Aldering, Greg; Berlind, Perry L.; Calkins, M.; Challis, Peter; Chornock, Ryan; Conley, Alexander; Filippenko, Alexei V.; Ganeshalingam, Mohan; Germany, Lisa; González, Sergio; Gooding, Samuel D.; Hsiao, Eric; Kasen, Daniel; Kirshner, Robert P.; Howie Marion, G. H.; Muena, Cesar; Nugent, Peter E.; Phelps, M.; Phillips, Mark M.; Qiu, Yulei; Quimby, Robert; Rines, K.; Silverman, Jeffrey M.; Suntzeff, Nicholas B.; Thomas, Rollin C.; Wang, Lifan,"The Most Slowly Declining Type Ia Supernova 2001ay",Astrophysical Journal,September 2011,142:74,

We present optical and near-infrared photometry, as well as ground-based optical spectra and Hubble Space Telescope ultraviolet spectra, of the Type Ia supernova (SN) 2001ay. At maximum light the Si II and Mg II lines indicated expansion velocities of 14,000 km s–1, while Si III and S II showed velocities of 9000 km s–1. There is also evidence for some unburned carbon at 12,000 km s–1. SN 2001ay exhibited a decline-rate parameter of Δm 15(B) = 0.68 ± 0.05 mag; this and the B-band photometry at t +25 day past maximum make it the most slowly declining Type Ia SN yet discovered. Three of the four super-Chandrasekhar-mass candidates have decline rates almost as slow as this. After correction for Galactic and host-galaxy extinction, SN 2001ay had MB = –19.19 and MV = –19.17 mag at maximum light; thus, it was not overluminous in optical bands. In near-infrared bands it was overluminous only at the 2σ level at most. For a rise time of 18 days (explosion to bolometric maximum) the implied 56Ni yield was (0.58 ± 0.15)/α M , with α = L max/E Ni probably in the range 1.0-1.2. The 56Ni yield is comparable to that of many Type Ia SNe. The "normal" 56Ni yield and the typical peak optical brightness suggest that the very broad optical light curve is explained by the trapping of γ rays in the inner regions. http://dx.doi.org/10.1088/0004-6256/142/3/74

Quimby, R. M.; Kulkarni, S. R.; Kasliwal, M. M.; Gal-Yam, A.; Arcavi, I.; Sullivan, M.; Nugent, P.; Thomas, R.; Howell, D. A.; Nakar, E.; Bildsten, L.; Theissen, C.; Law, N. M.; Dekany, R.; Rahmer, G.; Hale, D.; Smith, R.; Ofek, E. O.; Zolkower, J.; Velur, V.; Walters, R.; Henning, J.; Bui, K.; McKenna, D.; Poznanski, D.; Cenko, S. B.; Levitan, D.,"Hydrogen-poor superluminous stellar explosions",Nature,June 2011,474:487-489,

Supernovae are stellar explosions driven by gravitational or thermonuclear energy that is observed as electromagnetic radiation emitted over weeks or more. In all known supernovae, this radiation comes from internal energy deposited in the outflowing ejecta by one or more of the following processes: radioactive decay of freshly synthesized elements (typically 56Ni), the explosion shock in the envelope of a supergiant star, and interaction between the debris and slowly moving, hydrogen-rich circumstellar material. Here we report observations of a class of luminous supernovae whose properties cannot be explained by any of these processes. The class includes four new supernovae that we have discovered and two previously unexplained events (SN 2005ap and SCP 06F6) that we can now identify as members of the same class. These supernovae are all about ten times brighter than most type Ia supernova, do not show any trace of hydrogen, emit significant ultraviolet flux for extended periods of time and have late-time decay rates that are inconsistent with radioactivity. Our data require that the observed radiation be emitted by hydrogen-free material distributed over a large radius (~1015 centimetres) and expanding at high speeds (>104 kilometres per second). These long-lived, ultraviolet-luminous events can be observed out to redshifts z>4.

Childress, M.; Aldering, G.; Aragon, C.; Antilogus, P.; Bailey, S.; Baltay, C.; Bongard, S.; Buton, C.; Canto, A.; Chotard, N.; Copin, Y.; Fakhouri, H. K.; Gangler, E.; Kerschhaggl, M.; Kowalski, M.; Hsiao, E. Y.; Loken, S.; Nugent, P.; Paech, K.; Pain, R.; Pecontal, E.; Pereira, R.; Perlmutter, S.; Rabinowitz, D.; Runge, K.; Scalzo, R.; Thomas, R. C.; Smadja, G.; Tao, C.; Weaver, B. A.; Wu, C.,"Keck Observations of the Young Metal-poor Host Galaxy of the Super-Chandrasekhar-mass Type Ia Supernova SN 2007if",Astrophysical Journal,May 2011,733:3,

We present Keck LRIS spectroscopy and g-band photometry of the metal-poor, low-luminosity host galaxy of the super-Chandrasekhar-mass Type Ia supernova SN 2007if. Deep imaging of the host reveals its apparent magnitude to be mg = 23.15 ± 0.06, which at the spectroscopically measured redshift of z helio = 0.07450 ± 0.00015 corresponds to an absolute magnitude of Mg = -14.45 ± 0.06. Galaxy g - r color constrains the mass-to-light ratio, giving a host stellar mass estimate of log(M */M sun) = 7.32 ± 0.17. Balmer absorption in the stellar continuum, along with the strength of the 4000 Å break, constrains the age of the dominant starburst in the galaxy to be t burst = 123+165-77 Myr, corresponding to a main-sequence turnoff mass of M/M sun = 4.6+2.6-1.4. Using the R 23 method of calculating metallicity from the fluxes of strong emission lines, we determine the host oxygen abundance to be 12 + log(O/H)KK04 = 8.01 ± 0.09, significantly lower than any previously reported spectroscopically measured Type Ia supernova host galaxy metallicity. Our data show that SN 2007if is very likely to have originated from a young, metal-poor progenitor. http://dx.doi.org/10.1088/0004-637X/733/1/3

Sullivan, M.; Kasliwal, M. M.; Nugent, P. E.; Howell, D. A.; Thomas, R. C.; Ofek, E. O.; Arcavi, I.; Blake, S.; Cooke, J.; Gal-Yam, A.; Hook, I. M.; Mazzali, P.; Podsiadlowski, P.; Quimby, R.; Bildsten, L.; Bloom, J. S.; Cenko, S. B.; Kulkarni, S. R.; Law, N.; Poznanski, D.,"The Subluminous and Peculiar Type Ia Supernova PTF 09dav",Astrophysical Journal,May 2011,732:118,

PTF 09dav is a peculiar subluminous Type Ia supernova (SN) discovered by the Palomar Transient Factory (PTF). Spectroscopically, it appears superficially similar to the class of subluminous SN1991bg-like SNe, but it has several unusual features which make it stand out from this population. Its peak luminosity is fainter than any previously discovered SN1991bg-like SN Ia (MB ~ -15.5), but without the unusually red optical colors expected if the faint luminosity were due to extinction. The photospheric optical spectra have very unusual strong lines of Sc II and Mg I, with possible Sr II, together with stronger than average Ti II and low velocities of ~6000 km s-1. The host galaxy of PTF09dav is ambiguous. The SN lies either on the extreme outskirts (~41 kpc) of a spiral galaxy or in an very faint (MR >= -12.8) dwarf galaxy, unlike other 1991bg-like SNe which are invariably associated with massive, old stellar populations. PTF 09dav is also an outlier on the light-curve-width-luminosity and color-luminosity relations derived for other subluminous SNe Ia. The inferred 56Ni mass is small (0.019 ± 0.003 M sun), as is the estimated ejecta mass of 0.36 M sun. Taken together, these properties make PTF 09dav a remarkable event. We discuss various physical models that could explain PTF 09dav. Helium shell detonation or deflagration on the surface of a CO white dwarf can explain some of the features of PTF 09dav, including the presence of Sc and the low photospheric velocities, but the observed Si and Mg are not predicted to be very abundant in these models. We conclude that no single model is currently capable of explaining all of the observed signatures of PTF 09dav. http://dx.doi.org/10.1088/0004-637X/732/2/118

Chotard, N.; Gangler, E.; Aldering, G.; Antilogus, P.; Aragon, C.; Bailey, S.; Baltay, C.; Bongard, S.; Buton, C.; Canto, A.; Childress, M.; Copin, Y.; Fakhouri, H. K.; Hsiao, E. Y.; Kerschhaggl, M.; Kowalski, M.; Loken, S.; Nugent, P.; Paech, K.; Pain, R.; Pecontal, E.; Pereira, R.; Perlmutter, S.; Rabinowitz, D.; Runge, K.; Scalzo, R.; Smadja, G.; Tao, C.; Thomas, R. C.; Weaver, B. A.; Wu, C.; Nearby Supernova Factory,"The reddening law of type Ia supernovae: separating intrinsic variability from dust using equivalent widths",Astronomy & Astrophysics,May 2011,529:L4,

We employ 76 type Ia supernovae (SNe Ia) with optical spectrophotometry within 2.5 days of B-band maximum light obtained by the Nearby Supernova Factory to derive the impact of Si and Ca features on the supernovae intrinsic luminosity and determine a dust reddening law. We use the equivalent width of Si ii λ4131 in place of the light curve stretch to account for first-order intrinsic luminosity variability. The resulting empirical spectral reddening law exhibits strong features that are associated with Ca ii and Si ii λ6355. After applying a correction based on the Ca ii H&K equivalent width we find a reddening law consistent with a Cardelli extinction law. Using the same input data, we compare this result to synthetic rest-frame UBVRI-like photometry to mimic literature observations. After corrections for signatures correlated with Si ii λ4131 and Ca ii H&K equivalent widths and introducing an empirical correlation between colors, we determine the dust component in each band. We find a value of the total-to-selective extinction ratio, RV = 2.8 ± 0.3. This agrees with the Milky Way value, in contrast to the low RVvalues found in most previous analyses. This result suggests that the long-standing controversy in interpreting SN Ia colors and their compatibility with a classical extinction law, which is critical to their use as cosmological probes, can be explained by the treatment of the dispersion in colors, and by the variability of features apparent in SN Ia spectra. http://dx.doi.org/1 0.1051/0004-6361/201116723

Posters

Annette Greiner, Evan Racah, Shane Canon, Jialin Liu, Yunjie Liu, Debbie Bard, Lisa Gerhardt, Rollin Thomas, Shreyas Cholia, Jeff Porter, Wahid Bhimji, Quincey Koziol, Prabhat,"Data-Intensive Supercomputing for Science",Berkeley Institute for Data Science (BIDS) Data Science Faire,May 3, 2016,

Review of current DAS activities for a non-NERSC audience.