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 Study of the transient sky: Observations and modelling

Collaborations: Stephane Blondin, Luc Dessart, Joseph Anderson, Alejandro Clocchiatti, Francisco Forster, Mario Hamuy, Felipe Olivares, Giuliano Pignata, Jose Luis Prieto.

Collaborations with ongoing surveys: Numerous surveys of the transient sky are presently active. Blondin and Dessart are part of the ESO large program ePESSTO (operated at the NTT La Silla telescope), which aims at monitoring spectroscopically a large number of relatively bright SNe, either standard SNe nearby or super-luminous SNe further away. Blondin is part of the working groups on "Progenitors and explosion physics of SNe Ia" and "Faint or fast-evolving thermonuclear transients". Dessart is part of the group on luminous Type II SNe (led by Joe Anderson), Type II SNe from low-luminosity (low-metallicity) hosts (led by C. Gutierrez), and early-time Type II SNe with signatures of interaction (led by G. Terreran). These boundaries are loose and, for example, Dessart was part of the study of OGLE-2014-073 led by Terreran et al. on a super-luminous Type II SN or of the study of the electromagnetic counterpart to gravitational wave event GW170817.

Analysis and modelling of the sample of Type II SNe: obtained by the Carnegie Supernova Project and the Calan-Tololo survey. This work started in 2013 as a collaboration with Hamuy and Anderson, and as part of the Ph.D. thesis of Claudia Gutierrez and Thomas de Jaeger. The spectral analysis is in part completed (Gutierrez+17ab). The main goal now is to understand the origin of the diversity of Type II SNe, including the diversity in bolometric light curves, color evolution, and spectral properties (for example, the diversity seen in the Halpha line profile morphology). This diversity likely originates from a diversity in progenitor envelopes (in particular the H-envelope mass and extent), in progenitor cores (in relation to compactness, He core mass, and the potential presence of rotation), and perhaps in environments (for example in the case of explosion occuring in progenitors with a dense circumstellar envelope).

Study of Type II SNe at low metallicity: In Dessart+13,14, we showed that the metal line strength seen in simulations of Type II SN spectra correlates with the progenitor metallicity. In the case of weak mixing during and after explosion, all lines of metals beyond Na show a correlation. With a stronger mixing of core products, this correlation may hold only for Fe. Still, it offers a spectroscopic diagnostic for environment metallicity independent of more standard (but uncertain) nebular analyses. The correlation is corroborated by previous observations, although these lacked a significant number of events at low metallicity (Anderson+16). We have since obtained ESO-VLT and ePESSTO time to observe SNe in low-metallicity hosts (i.e. at metallicities below that of the Small Magellanic Cloud; SMC). In Gutierrez et al (in prep.), we show that the correlation holds still and supports the use of Type II SNe as metallicity indicators. This project also offers an original investigation of Type II SNe at low (sub SMC) metallicity, which has been still largely unexplored until now because of the nature of targeted surveys.

Study of shock breakout and early-time radiation from core-collapse SNe: Gal Yam+14 and Yaron+17 have recently observed that standard core-collapse SNe of Type IIb and II from supergiant progenitors exhibit signatures of interaction with circumstellar material in the direct vicinity of the star. Theoretically, work by Shiode and Quataert (2014) and Fuller (2017) suggest that progenitor envelope excitation by waves emitted in the stellar core may cause enhanced mass loss in the ultimate stages of massive star evolution. An important question is whether this phenomenon is ubiquitous. This is at present not known observationaly nor theoretically. In Dessart+17, using radiation hydrodynamics and non-LTE radiative transfer, we investigated the properties of the CSM required to explain the observations of Gal-Yam+ and Yaron+. We found that these may be compatible with an over-loading of the atmosphere of the red-supergiant surface, in the sense that wave excitation would be sufficient to lift material off the surface but insufficient to drive that material to infinity. Such massive stars would therefore be exploding in a cocoon of material. With further observations from ePESSTO (led by Terreran), we will investigate the intriguing observations of Gal-Yam+ and Yaron+.

Study of super-luminous SNe: Since the discovery of super-luminous Type Ic SNe by Quimby et al. (2011), this class of object has received special attention, with dedicated follow-up programs (Pan-STARRS, PTF, ePESSTO) to characterize them both photometrically and spectroscopically (e.g. Inserra+13; Nicholl+13,14; Yan+16,17; De Cia+17). The general consensus is that these events stem from massive carbon-rich Wolf-Rayet star explosions under the influence of a magnetar. Because they eject about 10Msun of oxygen, they must represent the explosion of the most massive stars we presently know (jerkstrand+17). Super-luminous Type II SNe that show no sign of interaction have now also been observed and are thought to be powered through the same process (e.g., Dessart18). Using our wide collaboration of observers, we will continue to study and model these events because they provide an opportunity to investigate the properties of compact objects at birth and provide some clues on the extreme properties that pre-supernova cores may have. The main difference with standard core collapse SNe is likely fast rotation, although it is at present unclear how supergiant stars may preserve a fast spinning core until their death. Another topic is to investigate how such super-luminous SNe could be used as probes of the distant Universe, for example for metallicity determinations.

Study of Hydrogen-rich interacting SNe: reveal the complexity of pre-SN mass loss in massive stars. At present, the mechanisms identified to explain these events are somewhat primitive. For example, there is no good theory yet available to explain the 1843 outburst of Eta Carina, an event that probably has counterparts in a number of hydrogen-rich super-luminous interacting SNe like SN2006gy. Interpreting these Type IIn SNe requires detailed modelling. In Dessart+15,16,17, we showed how a combination of radiation hydrodynamics and non-LTE radiative transfer could offer important clues on these events. We can identify 98S-like events, which correspond to stars exploding in a cocoon of material of limited mass and extent. Events like 94W probably arise from nuclear fashes in low mass red-supergiant stars (Chugai 2016, Dessart+16). Events like 2010jl suggest that some stars lose mass at an incredible rate for decades before exploding (e.g. Owocki+04). Still, numerous SNe IIn do not look like these three prototypes, for reasons that are unclear. It may be asymmetry, but it may also be that some configurations have not been covered in previous studies. Hence, the first project is to broaden further the range of ejecta/CSM configurations to capture the full diversity of events. We will confront our results to the growing sample of SNe IIn, for example gathered by ASSASN (collaboration with J.L. Prieto).

The explosion mechanism and progenitors of Type Ia SNe: are still subject to debate to this day, but the combination of high-quality observations from Chilean colleagues and state-of-the-art modeling by UMI researchers Blondin and Dessart can effectively address this outstanding astrophysical problem. In Blondin et al. (2017, 2018), we showed that nebular-phase spectra of SNe Ia (ASSASN project with J.L. Prieto) bore signatures of the mass of the exploding progenitor White Dwarf star, which was found to be significantly below the Chandrasekhar limit for low-luminosity events. In addition, the analysis and modelling of near-infrared (NIR) observations of SNe Ia obtained by the Carnegie Supernova Project II will allow to characterize their higher accuracy as distance indicators in this wavelength range, and provide a strong theoretical basis for future NIR transient surveys with WFIRST, Euclid, and JWST. Last, the analysis of spectro-polarimetric observations of SNe Ia (collaboration with A. Clocchiatti) will constrain the level of explosion asymmetries and their impact on the predicted spectra and light curves.

The SuperNova IDentification (SNID) code of Blondin & Tonry (2007) is the primary spectroscopic classification tool used by the ePESSTO and ASSASN transient surveys. It is however in need of an upgrade, both of its spectroscopic template library (through the addition of more recent observations) and of the core algorithm (relatively simple cross-correlation in Fourier space). We will take advantage of the presence of mathematicians/statisticians within the "Millennium Institute of Astrophysics" (MAS) to explore new ideas for the statistical framework used by SNID for classification and model/data comparisons, such as Gaussian processes, Bayesian inference, deep learning etc.

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