superluminous supernovae

myphot I lead a group funded by the European Research Council working on finding and understanding the most luminous supernovae in the Universe. These are 50-100 times brighter than normal supernovae and are very bright in the ultraviolet region of the spectrum. These attributes mean that they will be detectable in the high redshift Universe  with future facilities such as the James Web Space Telescope and Large Synoptic Survey Telescope. They also have the, as yet unexplained, property of being exclusively located in small dwarf galaxies that are actively forming stars. We think that this means that they are produced by low metallicity massive stars. This is another reason to think that they will dominate the searches for bright transient sources in the high redshift Universe when the metal content of galaxies was lower then we see now in the local Universe, and when the starformation rate of the Universe was at its highest.

My teams work on this ERC project is based around finding these superluminous supernovae with the Pan-STARRS and PESSTO projects.  Here are some highlights of the work. The scientific work has been led by the postdocs on the grant Cosimo Inserra and Anders Jerkstrand, PhD students Matt Nicholl and Janet Chen. The project has been enabled by the excellent work of Ken Smith, Dave Young and Darryl Wright in building the computing projects Pan-STARRS Survey for Transients, and PESSTO.

PESSTO is now the world's leading public classification engine - we classify the most supernovae of any project. And Pan-STARRS is  the world's leading public supernova discovery survey. The compiled statistics by the International Astronomical Union's server are here.

Through PESSTO and the public repository WISeREP, we make all our spectra that we have taken as part of this ERC project public. Anders Jerkstrand's page makes all models public.

This ERC project is close to producing 100 refereed papers through Pan-STARRS and PESSTO in particular. And the papers specifically on superluminous supernovae are linked below.

Anders is now a Marie Curie fellow at MPA in Munich, Janet has recently won a Humboldt fellowship, and Matt won the RAS Penston Prize for best thesis in UK astronomy for 2015. He is now a postdoc at Havard's Centre for Astrophysics. 

magnetars as engines

myphot To produce the high luminosity seen in the explosions, theoretical work (most notably Lars Bildsten & Dan Kasen and  Stan  Woosley in 2010) suggested that magnetized, rapidly rotating neutron stars could power the lightcurves as they spin down. We showed in two papers in 2013 that our lightcurve data could be well reproduced with these models. Details are in Inserra et al. (2013),  and Nicholl et al. (2013). The image above is taken from the Nature letter of Nicholl et al. (2013).  The semi-analytic code for fitting the magnetar model described in Inserra et al. (2013),   is available on Anders Jerkstrand's page here.

It is still possible that interaction with dense circumstellar shells produces some of these explosions, however the data are quite well reproduced with central engines. We have also shown that we don't think any of them can be pair-instability explosions - in a series of papers Nicholl et al. (2013),    and in  McCrum,et al. MNRAS, (20 14) plus in the model papers of (Jerkstrand et al, 2017, ApJ, in press) and in (Jerkstrand, Smartt & Heger 2016)

explosions of the most massive stars

myphot In an extensive analysis of all the superluminous supernovae data to date, we showed that the ejecta masses are likely to be the highest of any know supernova type. This plot from Nicholl et al. (2015) - "On the diversity of superluminous supernovae: ejected mass as the dominant factor", shows that the ejected masses range from around 4 to 30 solar masses. As the ejecta are seemingly free from hydrogen and helium, these large masses of carbon-oxygen-magnesium dominated material suggest that the progenitor stars  likely have initial masses in the region of 15 to 60 solar masses.  We collected VLT and PESSTO spectra of these in the nebular phase, and Anders Jerkstrand applied his radiative transfer models to estimate the  masses and density of the ejecta in our most recent paper (Jerkstrand et al, 2017, ApJ, in press). For the most slowly evolving explosions, we find that more than 10 solar masses of oxygen is required to produce the strong lines and the ejecta must be highly clumped (the filling factor requried is f < 0.01). 

locations in dwarf galaxies

papers resulting from ERC project

My papers specifically on the subject of superluminous supernovae, arising from this project are on this ADS Private library here   (28 to date, November 2016).

These have quantified the physics of explosions, ejecta masses and compositions, the relative rates and how to find them in surveys, and their host galaxies.