Adam A. Miller

Supernova 2007bi as a pair-instability explosion

Stars with initial masses such that 10 ≤ Minitial ≤ 100, where is the solar mass, fuse progressively heavier elements in their centres, until the core is inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion—an iron-core-collapse supernova. By contrast, extremely massive stars with Minitial ≥ 140 (if such exist) develop oxygen cores with masses, Mcore, that exceed 50, where high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron–positron pairs occurs before oxygen ignition and leads to a violent contraction which triggers a nuclear explosion that unbinds the star in a pair-instability supernova. Transitional objects with 100 < Minitial < 140 may end up as iron-core-collapse supernovae following violent mass ejections, perhaps as a result of brief episodes of pair instability, and may already have been identified. Here we report observations of supernova SN 2007bi, a luminous, slowly evolving object located within a dwarf galaxy. We estimate the exploding core mass to be Mcore ≈ 100, in which case theory unambiguously predicts a pair-instability supernova. We show that >3 of radioactive 56Ni was synthesized during the explosion and that our observations are well fitted by models of pair-instability supernovae. This indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic stellar mass limit, which perhaps result from processes similar to those that created the first stars in the Universe.

Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe

Weidong Li1, Joshua S. Bloom1, Philipp Podsiadlowski2, Adam A. Miller1, S. Bradley Cenko1, Saurabh W. Jha3, Mark Sullivan2, D. Andrew Howell4,5, Peter E. Nugent6,1, Nathaniel R. Butler7, Eran O. Ofek8,9, Mansi M. Kasliwal10, Joseph W. Richards1,11, Alan Stockton12, Hsin-Yi Shih12, Lars Bildsten5,13, Michael M. Shara14, Joanne Bibby14, Alexei V. Filippenko1, Mohan Ganeshalingam1, Jeffrey M. Silverman1, S. R. Kulkarni8, Nicholas M. Law15, Dovi Poznanski16, Robert M. Quimby8, Curtis McCully3, Brandon Patel3, & Kate Maguire2Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models: double-degenerate (involving two white dwarfs in a close binary system) and single-degenerate models. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

The exceptionally luminous type II-linear supernova 2008es

We report on our early photometric and spectroscopic observations of the extremely luminous Type II supernova (SN) 2008es. SN 2008es, with an observed peak optical magnitude of mV = 17.8 and at a redshift z = 0.213, has a peak absolute magnitude of MV = ?22.3, making it the second most luminous SN ever observed. The photometric evolution of SN 2008es exhibits a fast decline rate (~ 0.042 mag d?1), similar to the extremely luminous Type II-Linear (II-L) SN 2005ap. We show that SN 2008es spectroscopically resembles the luminous Type II-L SN 1979C. Although the spectra of SN 2008es lack the narrow and intermediate-width line emission typically associated with the interaction of an SN with the circumstellar medium of its progenitor star, we argue that the extreme luminosity of SN 2008es is powered via strong interaction with a dense, optically thick circumstellar medium. The integrated bolometric luminosity of SN 2008es yields a total radiated energy at ultraviolet and optical wavelengths of 1051 erg. Finally, we examine the apparently anomalous rate at which the Texas Supernova Search has discovered rare kinds of SNe, including the five most luminous SNe observed to date, and find that their results are consistent with those of other modern SN searches.

Discovery of Precursor Luminous Blue Variable Outbursts in Two Recent Optical Transients: The Fitfully Variable Missing Links UGC 2773-OT and SN 2009ip

We present progenitor-star detections, light curves, and optical spectra of supernova (SN) 2009ip and the 2009 optical transient in UGC 2773 (U2773-OT), which were not genuine SNe. Precursor variability in the decade before outburst indicates that both of the progenitor stars were luminous blue variables (LBVs). Their pre-outburst light curves resemble the S Doradus phases that preceded giant eruptions of the prototypical LBVs η Carinae and SN 1954J (V12 in NGC 2403), with intermediate progenitor luminosities. Hubble Space Telescope detections a decade before discovery indicate that the SN 2009ip and U2773-OT progenitors were supergiants with likely initial masses of 50-80 M ☉ and 20 M ☉, respectively. Both outbursts had spectra befitting known LBVs, although in different physical states. SN 2009ip exhibited a hot LBV spectrum with characteristic speeds of 550 km s–1, plus evidence for faster material up to 5000 km s–1, resembling the slow Homunculus and fast blast wave of η Carinae. In contrast, U2773-OT shows a forest of narrow absorption and emission lines comparable to that of S Dor in its cool state, plus [Ca II] emission and an infrared excess indicative of dust, similar to SN 2008S and the 2008 optical transient in NGC 300 (N300-OT). The [Ca II] emission is probably tied to a dusty pre-outburst environment, and is not a distinguishing property of the outburst mechanism. The LBV nature of SN 2009ip and U2773-OT may provide a critical link between historical LBV eruptions, while U2773-OT may provide a link between LBVs and the unusual dust-obscured transients SN 2008S and N300-OT. Future searches will uncover more examples of precursor LBV variability of this kind, providing key clues that may help unravel the instability driving LBV eruptions in massive stars.

IMPROVED STANDARDIZATION OF TYPE II-P SUPERNOVAE: APPLICATION TO AN EXPANDED SAMPLE

In the epoch of precise and accurate cosmology, cross-confirmation using a variety of cosmographic methods is paramount to circumvent systematic uncertainties. Owing to progenitor histories and explosion physics differing from those of Type Ia supernovae (SNeIa), Type II-plateau supernovae (SNeII-P) are unlikely to be affected by evolution in the same way. Based on a new analysis of 17 SNeII-P, and on an improved methodology, we find that SNeII-P are good standardizable candles, almost comparable to SNeIa. We derive a tight Hubble diagram with a dispersion of 10% in distance, using the simple correlation between luminosity and photospheric velocity introduced by Hamuy and Pinto. We show that the descendent method of Nugent etal. can be further simplified and that the correction for dust extinction has low statistical impact. We find that our SN sample favors, on average, a very steep dust law with total to selective extinction RV < 2. Such an extinction law has been recently inferred for many SNeIa. Our results indicate that a distance measurement can be obtained with a single spectrum of a SNII-P during the plateau phase combined with sparse photometric measurements.

Illuminating gravitational waves: A concordant picture of photons from a neutron star merger

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Observations of a binary neutron star merger at multiple wavelengths can be explained by an off-axis relativistic cocoon model. Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.

A Massive Progenitor of the Luminous Type IIn Supernova 2010jl

The bright, nearby, recently discovered supernova (SN) 2010jl is a luminous Type IIn SN. Here, we report archival Hubble Space Telescope (HST) observations of its host galaxy UGC 5189A taken roughly 10 yr prior to explosion, as well as early-time optical spectra of the SN. The HST images reveal a luminous, blue point source at the position of the SN, with an absolute magnitude of –12.0 in the F300W filter. If it is not just a chance alignment, the source at the SN position could be (1) a massive young (<6 Myr) star cluster in which the SN resided, (2) a quiescent, luminous blue star with an apparent temperature around 14,000 K, (3) a star caught during a bright outburst akin to those of luminous blue variables, or (4) a combination of option (1) and option (2) or (3). Although we cannot confidently choose between these possibilities with the present data, any of them imply that the progenitor of SN 2010jl had an initial mass above 30 M_☉. This reinforces mounting evidence that many SNe IIn result from very massive stars, that massive stars can produce visible SNe without collapsing quietly to black holes, and that massive stars can sometimes retain their H envelopes until shortly before explosion. Standard stellar evolution models fail to account for these observed properties.

Type Ia Supernovae Strongly Interacting with Their Circumstellar Medium

Owing to their utility for measurements of cosmic acceleration, Type Ia supernovae (SNe Ia) are perhaps the best-studied class of SNe, yet the progenitor systems of these explosions largely remain a mystery. A rare subclass of SNe Ia shows evidence of strong interaction with their circumstellar medium (CSM), and in particular, a hydrogen-rich CSM; we refer to them as SNe Ia-CSM. In the first systematic search for such systems, we have identified 16 SNe Ia-CSM, and here we present new spectra of 13 of them. Six SNe Ia-CSM have been well studied previously, three were previously known but are analyzed in depth for the first time here, and seven are new discoveries from the Palomar Transient Factory. The spectra of all SNe Ia-CSM are dominated by Hα emission (with widths of ~2000 km s^(–1)) and exhibit large Hα/Hβ intensity ratios (perhaps due to collisional excitation of hydrogen via the SN ejecta overtaking slower-moving CSM shells); moreover, they have an almost complete lack of He I emission. They also show possible evidence of dust formation through a decrease in the red wing of Hα 75-100 days past maximum brightness, and nearly all SNe Ia-CSM exhibit strong Na I D absorption from the host galaxy. The absolute magnitudes (uncorrected for host-galaxy extinction) of SNe Ia-CSM are found to be –21.3 mag ≤ M_R ≤ –19 mag, and they also seem to show ultraviolet emission at early times and strong infrared emission at late times (but no detected radio or X-ray emission). Finally, the host galaxies of SNe Ia-CSM are all late-type spirals similar to the Milky Way, or dwarf irregulars like the Large Magellanic Cloud, which implies that these objects come from a relatively young stellar population. This work represents the most detailed analysis of the SN Ia-CSM class to date.

Evidence for an FU orionis-like outburst from a classical T tauri star

We present pre- and post-outburst observations of the new FU Orionis-like young stellar object PTF 10qpf (also known as LkHα 188-G4 and HBC 722). Prior to this outburst, LkHα 188-G4 was classified as a classical T Tauri star (CTTS) on the basis of its optical emission-line spectrum superposed on a K8-type photosphere and its photometric variability. The mid-infrared spectral index of LkHα 188-G4 indicates a Class II-type object. LkHα 188-G4 exhibited a steady rise by ~1 mag over ~11 months starting in August 2009, before a subsequent more abrupt rise of >3 mag on a timescale of ~2 months. Observations taken during the eruption exhibit the defining characteristics of FU Orionis variables: (1) an increase in brightness by ≳ 4 mag, (2) a bright optical/near-infrared reflection nebula appeared, (3) optical spectra are consistent with a G supergiant and dominated by absorption lines, the only exception being Hα which is characterized by a P Cygni profile, (4) near-infrared spectra resemble those of late K-M giants/supergiants with enhanced absorption seen in the molecular bands of CO and H_(2)O, and (5) outflow signatures in H and He are seen in the form of blueshifted absorption profiles. LkHα 188-G4 is the first member of the FU Orionis-like class with a well-sampled optical to mid-infrared spectral energy distribution in the pre-outburst phase. The association of the PTF 10qpf outburst with the previously identified CTTS LkHα 188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the disk. The early identification of PTF 10qpf as an FU Orionis-like variable will enable detailed photometric and spectroscopic observations during its post-outburst evolution for comparison with other known outbursting objects.

THE MASSIVE PROGENITOR OF THE TYPE II-LINEAR SUPERNOVA 2009kr*

We present early-time photometric and spectroscopic observations of supernova (SN) 2009kr in NGC 1832. We find that its properties to date support its classification as Type II-linear (SN II-L), a relatively rare subclass of core-collapse supernovae (SNe). We have also identified a candidate for the SN progenitor star through comparison of pre-explosion, archival images taken with WFPC2 on board the Hubble Space Telescope with SN images obtained using adaptive optics plus NIRC2 on the 10 m Keck-II telescope. Although the host galaxys substantial distance (~26 Mpc) results in large uncertainties in the relative astrometry, we find that if this candidate is indeed the progenitor, it is a highly luminous (M^0_V = –7.8 mag) yellow supergiant with initial mass ~18-24 M_⊙. This would be the first time that an SN II-L progenitor has been directly identified. Its mass may be a bridge between the upper initial mass limit for the more common Type II-plateau SNe and the inferred initial mass estimate for one Type II-narrow SN.