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Joseph Lyman




I am a post-doctoral fellow in the Astrophysics group at Warwick, starting in January 2014 having obtained my PhD at Liverpool John Moores University.

My research is concerned primarily with observing and characterising astrophysical transients. I have particular interest in constraining the progenitors of core-collapse supernovae through observations of their explosions and the environments they inhabit within their hosts galaxies. Alongside this I study unusual or peculiar subclasses of supernovae, the progenitors of which are poorly understood. I have also been involved in the ongoing attempts to discover and characterise the electromagnetic counterparts to gravitational wave signals.

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Supernovae are seen to explode in or around the bright discs and bulges of their host galaxies. This is expected since these bright regions constain the vast majority of the stars in a galaxy. Calcium-rich supernovae, a peculiar subclass of supernovae, however, have a strong tendancy to explode in the remote out-reaches of their host galaxies where there are very few stars. Quite why they prefer these solitary locations has been a puzzle. Suggestions have been made that they could be formed in very faint (and thus difficult to detect) systems at these remote locations.

Using observations taken with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT), I have investigated the explosion sites of these unusual explosions to search for signs of any potential birth places, such as faint dwarf galaxies, that may have been undetected by other observations.

SN2005E and it's explosion site


In the first panel, SN2005E, a member of the Ca-rich supernova class, is seen in the bottom right. Returning to observe this location with HST long after the supernova has faded (second panel), we find no evidence for any birth site (last panel - a zoom in of the square on the middle panel).

Ca-rich supernovae are truly lonely

Thanks to the extreme depth of the observations taken with HST and VLT, the fact that we see no sources at the explosion sites of any of the Ca-rich supernovae we have looked at allows us to rule out their formation in faint underlying systems (such as globular clusters or dwarf galaxies).


Runaway couples?

These supernovae really are exploding where they have no business to be. Since there are no obvious birth sites for the supernovae, and the fact that the number of stars in these remote locations is so small, we can consider if these explosions are the result of high-velocity pairs of stars that have been flung from their galaxies at hundreds of kilometres per second.

These systems could comprise of a neutron star and a white dwarf. The neutron star is formed when a very massive star collapses under its own gravity and makes a supernova of its own. When it is formed it undergoes a 'kick', accelerating it to large velocities. Since most stars are in binary systems, a companion star can be dragged along for the ride. The supernovae would then be a result of the companion being eventually ripped apart by the neutron star, after the pair have travelled a significant distance to these remote out-reaches of their host galaxy. To the right is an artist's impression of such a system having being ejected from its galaxy.

Further info

Lonely Supernovae May Have Been Kicked Out Of Their Galaxies

Hubble observes calcium-rich supernovae

The results are contained in two papers: The progenitors of calcium-rich transients are not formed in situ and Hubble Space Telescope observations of the host galaxies and environments of calcium-rich supernovae.

Core-collapse supernovae (CCSNe) are thought to arise from the deaths of stars at least 8 times more massive than the Sun. Beyond this cutoff, there is debate as to exactly how different kinds of massive stars die and specifically how their varied deaths produce the varied types of supernova we see.

I have used literature data of CCSN to produce a method of creating the bolometric light curve of a CCSN (which have traditionally been observationally expensive to create) from relatively little data. This method was then applied to a large sample of CCSN in order to determine the properties of their explosions, and thus inform on their progenitor stars.

Bolometric corrections for CCSNe

Bolometric corrections for CCSNe B-Ic

With just an optical colour, one can estimate the bolometric light curve of a CCSN over a wide range of epochs with a typical rms of the scatter about the relations $< 0.1$ mag. The case for $B-I$ is shown above.

Bolometric lightcurves of CCSNe

Bolometric light curves of CCSNe

Using the bolometric correction method, a large catalogue of bolometric light curves has been created for stripped-envelope CCSNe. These are types of CCSN that show little or no hydrogen in their specta - it is thought that the massive hydrogen envelopes of the progenitor stars has been largely lost before exploding, resulting in an absence of hydrogen.

There are two (main) mechanisms that can strip the hydrogen envelope from a progenitor star. In this first case the progenitor star is simply that massive and luminous that it sheds these outer layers itself, through very strong stellar winds, over the course of its life.This requires very massive stars, typically at least 20 to 30 times as massive as the Sun. In the second case the star can be more modestly massive and it is the presence of a binary companion that strips this envelope through its gravitational influence. This second mechanism can work for lower mass progenitors (8-20 times the mass of the Sun). Thus if we can determine the masses of the proenitor stars we can distinguish between these mechanisms.

Modelling the bolometric light curves of CCSNe

By employing a simple model to the catalogue of bolometric light curves, one can extract estimates for the explosion parameters of the supernovae. One such parameter is the mass of material that is ejected during the supernova, $M_\textrm{ej}$. This is intrinsically linked to the mass of the star when it was born and we can thus use it, by comparing to results from models of stellar evolution, to constrain the mass range of stripped-envelope CCSN progenitors.


Above is a plot of the distributions of $M_\textrm{ej}$ for different types of stripped-envelope CCSNe (IIb, Ib, Ic, Ic-BL). The ranges for $M_\textrm{ej}$ from stellar evolution modelling are indicated by gray bars. As can be seen, the distributions are best decribed by binary stars that are between 8 and 20 times the mass of the Sun when formed ($M_\textrm{init}$). There is very little contribution from more massive stars, either single or binary.

This indicates that the progenitors of stripped-envelope CCSNe are less massive than previously thought, with the binary interaction mechanism being mainly responsible for stripping the hydrogen envelopes of the progenitor stars. A lack of large values for $M_\textrm{ej}$, which would be expected from more massive stars, has dramatic consequences for the fates of these more massive stars. It may indeed be the case that these stars do not produce a luminous supernova, instead directly collapsing to a black hole.

Further info

The results are presented in the following papers:
Bolometric corrections for optical light curves of core-collapse supernovae and Bolometric light curves and explosion parameters of 38 stripped-envelope core-collapse supernovae

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