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Iair Arcavi
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Hydrogen-rich core collapse supernovae, known as Type II supernovae, are the most common type of stellar explosion realized in nature. They are defined by the presence of prominent hydrogen lines in their spectra. Type II supernovae are observed only in star-forming galaxies, and several events have been directly linked to massive star progenitors. Five main subclasses are identified: Type IIP (displaying a plateau in their light curve), Type IIL (displaying a light curve decline), Type IIn (displaying narrow emission lines), Type IIb (displaying increasingly strong He features with time) and 87A-likes (displaying long-rising light curves similar to that of SN 1987A). Type IIP supernovae have been robustly established as the explosions of red supergiants, while the progenitors of Type IILs remain elusive. Type IIns are likely linked to luminous blue variables, Type IIb progenitors may be interacting binary systems and the prototype of the 87A-like class was observed to be the explosion of a blue supergiant. The diversity in progenitor mass, metallicity, binarity and rotation is likely responsible for the diversity in observed explosion types, but the connection between progenitor parameters and supernova properties is not yet entirely understood theoretically nor fully mapped observationally. New observational methods for constraining this connection are currently being implemented, including the analyses of large samples of events, making use of very early data (obtained hours to days from explosion) and statistical studies of host-galaxy properties.
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We present a study of optical and near-infrared (NIR) spectra along with the light curves of SN 2013ai. These data range from discovery until 380 days after explosion. SN 2013ai is a fast declining type II supernova (SN II) with an unusually long rise time; $18.9pm2.7$d in $V$ band and a bright $V$ band peak absolute magnitude of $-18.7pm0.06$ mag. The spectra are dominated by hydrogen features in the optical and NIR. The spectral features of SN 2013ai are unique in their expansion velocities, which when compared to large samples of SNe II are more than 1,000 kms faster at 50 days past explosion. In addition, the long rise time of the light curve more closely resembles SNe IIb rather than SNe II. If SN 2013ai is coeval with a nearby compact cluster we infer a progenitor ZAMS mass of $sim$17 M$_odot$. After performing light curve modeling we find that SN 2013ai could be the result of the explosion of a star with little hydrogen mass, a large amount of synthesized $^{56}$Ni, 0.3-0.4 M$_odot$, and an explosion energy of $2.5-3.0times10^{51}$ ergs. The density structure and expansion velocities of SN 2013ai are similar to that of the prototypical SN IIb, SN 1993J. However, SN 2013ai shows no strong helium features in the optical, likely due to the presence of a dense core that prevents the majority of $gamma$-rays from escaping to excite helium. Our analysis suggests that SN 2013ai could be a link between SNe II and stripped envelope SNe.
We present our study of OGLE-2014-SN-073, one of the brightest Type II SN ever discovered, with an unusually broad lightcurve combined with high ejecta velocities. From our hydrodynamical modelling we infer a remarkable ejecta mass of $60^{+42}_{-16}$~M$_odot$, and a relatively high explosion energy of $12.4^{+13.0}_{-5.9} times10^{51}$~erg. We show that this object belongs, with a very small number of other hydrogen-rich SNe, to an energy regime that is not explained by standard core-collapse (CC) neutrino-driven explosions. We compare the quantities inferred by the hydrodynamical modelling with the expectations of various exploding scenarios, trying to explain the high energy and luminosity released. We find some qualitative similarities with pair-instabilities SNe, although a prompt injection of energy by a magnetar seems also a viable alternative to explain such extreme event.
We present SN 2020jfo, a Type IIP supernova in the nearby galaxy M61. Optical light curves from the Zwicky Transient Facility, complemented with data from Swift and near-IR photometry are presented. The 350-day duration bolometric light curve exhibits a relatively short (~ 65 days) plateau. This implies a moderate ejecta mass (~ 5 Msun). A series of spectroscopy is presented, including spectropolarimetric observations. The nebular spectra are dominated by Halpha but also reveal emission lines from oxygen and calcium. Comparisons to synthetic nebular spectra indicate an initial progenitor mass of about 12 Msun. Stable nickel is present in the nebular spectrum, with a super-solar Ni/Fe ratio. Several years of pre-discovery data are examined, but no signs of pre-cursor activity is found. Pre-explosion Hubble Space Telescope imaging reveals a probable progenitor star, detected only in the reddest band and is fainter than expected for stars in the 10 - 15 Msun range, in tension with the analysis of the LC and the nebular spectral modeling. We present two additional core-collapse SNe monitored by the ZTF, which also have nebular Halpha-dominated spectra. This illustrates how the absence or presence of interaction with circumstellar material affect both the LCs and in particular the nebular spectra. Type II SN 2020amv has a LC powered by CSM interaction, in particular after about 40 days when the LC is bumpy and slowly evolving. The late-time spectra show strong Halpha emission with a structure suggesting emission from a thin, dense shell. The evolution of the complex three-horn line profile is reminiscent of that observed for SN 1998S. SN 2020jfv has a poorly constrained early-time LC, but shows a transition from a hydrogen-poor Type IIb to a Type IIn, where the nebular spectrum after the light-curve rebrightening is dominated by Halpha, although with an intermediate line width.
Hydrogen-rich supernovae, known as Type II (SNe II), are the most common class of explosions observed following the collapse of the core of massive stars. We use analytical estimates and population synthesis simulations to assess the fraction of SNe II progenitors that are expected to have exchanged mass with a companion prior to explosion. We estimate that 1/3 to 1/2 of SN II progenitors have a history of mass exchange with a binary companion before exploding. The dominant binary channels leading to SN II progenitors involve the merger of binary stars. Mergers are expected to produce a diversity of SN II progenitor characteristics, depending on the evolutionary timing and properties of the merger. Alternatively, SN II progenitors from interacting binaries may have accreted mass from their companion, and subsequently been ejected from the binary system after their companion exploded. We show that the overall fraction of SN II progenitors that are predicted to have experienced binary interaction is robust against the main physical uncertainties in our models. However, the relative importance of different binary evolutionary channels is affected by changing physical assumptions. We further discuss ways in which binarity might contribute to the observed diversity of SNe II by considering potential observational signatures arising from each binary channel. For supernovae which have a substantial H-rich envelope at explosion (i.e., excluding Type IIb SNe), a surviving non-compact companion would typically indicate that the supernova progenitor star was in a wide, non-interacting binary. We argue that a significant fraction of even Type II-P SNe are expected to have gained mass from a companion prior to explosion.
We investigate correlated gravitational wave and neutrino signals from rotating core-collapse supernovae with simulations. Using an improved mode identification procedure based on mode function matching, we show that a linear quadrupolar mode of the core produces a dual imprint on gravitational waves and neutrinos in the early post-bounce phase of the supernova. The angular harmonics of the neutrino emission are consistent with the mode energy around the neutrinospheres, which points to a mechanism for the imprint on neutrinos. Thus, neutrinos carry information about the mode amplitude in the outer region of the core, whereas gravitational waves probe deeper in. We also find that the best-fit mode function has a frequency bounded above by $sim 420$ Hz, and yet the modes frequency in our simulations is $sim 15%$ higher, due to the use of Newtonian hydrodynamics and a widely used pseudo-Newtonian gravity approximation. This overestimation is particularly important for the analysis of gravitational wave detectability and asteroseismology, pointing to limitations of pseudo-Newtonian approaches for these purposes, possibly even resulting in excitation of incorrect modes. In addition, mode frequency matching (as opposed to mode function matching) could be resulting in mode misidentification in recent work. Lastly, we evaluate the prospects of a multimessenger detection of the mode using current technology. The detection of the imprint on neutrinos is most challenging, with a maximum detection distance of $sim!1$ kpc using the IceCube Neutrino Observatory. The maximum distance for detecting the complementary gravitational wave imprint is $sim!5$ kpc using Advanced LIGO at design sensitivity.
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