Prof Paolo Mazzali
Dr Phil James
Dr David Bersier
Ms Stacey Habergham (PhD student)
Mr Joe Lyman (PhD student)
Elena Pian (Pisa, Italy)
Joe Anderson (University of Chile, Chile)
Members of the PTF SNe follow-up (too many to mention them all):
Mark Sullivan (Oxford, UK)
Avishay Gal-Yam (Weizmann Institute, Israel)
Supernovae are exploding stars. They happen when specific types of stars reach a point in their lives when they cannot support their own weight. This triggers a catastrophic series of events that liberates an enormous amount of energy.
There are two types of SNe: i) the explosion of white dwarf star. This happens when a white dwarf has swallowed mass from a companion star and can't take it any more. The whole star explodes. This is a type Ia supernova.
ii) those coming from massive stars (mass above 8 times the mass of the Sun) that reach the end of their evolution. The core collapses to a neutron star or a black hole and the rest of the star is ejected and shines brightly. This is a core-collapse supernova.
The brightness of a SN as a function of time is what we call a light curve. Different types of SNe have different light curves and this is one way to distinguish them. Another way is to take a spectrum. This has led to a detailed classification of supernovae. Type Ia SNe come from the explosion of a white dwarf. All other types are core-collapse SNe. Among these we distinguish SN II that have strong lines of hydrogen in their spectra, SNe Ib that have no hydrogen but do have helium in their spectra, and then type Ic SNe that have neither hydrogen nor helium.
One of the major issues in the field is what type of star becomes what type of SN. In other words, we want to know what are the progenitors of each type of SN. This is the main theme of the research done at the ARI.
Correlation with Hα light
One of the most fundamental questions in supernova research concerns the nature of the progenitor stars that result in supernovae of the different types listed above, and the physical properties that determine the variety of spectroscopic and light-curve signatures seen in different supernova. Searches for progenitor stars in pre-explosion imaging has resulted in the clear identification of SNIIP (the most common core-collapse type) with red supergiant stars, but this approach has so far failed to detect progenitor stars for any other SN type. An alternative is to use the information encoded in the local stellar population in which supernovae are observed to occur, to put statistical constraints on the likely progenitor stars for different supernova types. We have particularly been using the spatial correlation of supernovae with Hα light, which is a good tracer of the youngest, highest-mass stars that dominate the ionising light output of normal galaxies.
This approach was first employed by James & Anderson (2006) using a small sample of SNe (12 Ia, 30 II and 8 Ibc) that had occurred within the nearby, star-forming galaxies of the HαGS survey. The strength of correlation with Hα emission was quantified through an analysis of the pixels within continuum-subtracted Hα images, and in particular the pixel that contained the location of the SN. For each SN, the analysis returns a number between zero (for SNe that occur in regions with no detectable Hα) and one (which requires the SN to occur in the pixel with the most intense Hα emission). The distributions of values for the 50 SNe of the initial study are shown below, colour-coded by SN type:
Even for this small sample, clear differences are apparent. Unsurprisingly, given that they are frequently found in elliptical and lenticular galaxies, many SNIa are found in regions with no star formation. More interesting is the significant fraction of SNII with values of this association parameter that are either zero or very low. This may indicate that many or all of the progenitor stars are at the bottom of the mass range for core-collapse, approx. 8 solar masses, or that many of their progenitors are high-velocity stars which move far from the HII regions where they form, prior to exploding. The type Ic SNe are very different; these almost perfectly trace the Hα emission and hence the very high-mass stars thought to be responsible for this emission. The SNIb are intermediate between the SNII and the SNIc, and thus we have evidence for a progenitor mass sequence, II < Ib < Ic.
Supernovae in starburst environments
We have been using core-collapse supernovae as a tracer of the star-formation process in the central regions of disturbed and merging galaxies, which have been shown to host some of the most intense bursts of star formation seen the local Universe. As expected, these regions are prodigious sources of core-collapse supernovae, with, for example, seven having been observed in the merger system Arp 299 (below) in the last 22 years.
More surprising are the types of supernovae we are finding in these starbursts. In Arp299, of the 6 SNe with clear classifications, none is a IIP, the most common type observed in 'normal' galaxies. Instead, the core-collapse supernovae in starbursts are dominated by 'stripped envelope' types, that have lost a large fraction of their outer envelopes before exploding. These unusual ratios of supernova types are found across the full sample of disturbed galaxies we have investigated (figure below taken from Habergham et al. 2010; see this paper for further details):
The exact reason for this envelope loss is still being investigated, but it is plausibly linked to extremely high mass in the progenitor stars. If this is the explanation for the unusual pattern of supernovae, these regions must be forming stars with an Initial Mass Function that is biased to high mass stars, to a remarkable extent.
Data reduction pipeline
We are collecting photometric data of PTF SNe with the Liverpool Telescope (LT). As this represents many thousands of images, it is unthinkable to process each image individually. To solve this problem PhD student Joe Lyman has developed a data reduction pipeline that extracts the photometry for any SNe observed with the LT.
A SN usually occurs in a galaxy hence the light coming from the SN is "contaminated" by its host galaxy. To remedy this problem, we subtract the galaxy via a software procedure that leaves the SN light untouched. This is done by using an image of the host galaxy obtained about one year after the SN maximum. PhD student Joe Lyman has developed a data reduction pipeline that takes care of all this and extracts the photometry for any SNe. This pipeline works with the data we are routinely collecting with the Liverpool Telescope (LT) but it has been tested and works for data obtained on other telescopes as well.
One of our goals in observing many SNe is to build an atlas of light curves for core-collapse SNe. We want to model the light curves with an analytical procedure that allows us to extract the fundamental parameters of the explosion: mass of nickel synthesized, ejected mass, explosion energy. These parameters are crucial to building accurate computer models.
The method relies on the bolometric light curve, i.e. the total energy output of the SN across the electromagnetic spectrum. We do not have access to the whole spectrum but most of the energy comes out in the optical which we have. Furthermore, we are quantifying the part that we may be missing. The method relies not only on light curves but also on a measurement of the expansion velocity near maximum light. This can be obtained easily as part of the spectroscopic follow-up of PTF SNe. Many candidate SNe are observed near maximum light in order to determine their spectroscopic types (Ia, II, Ib, etc.); we use these spectra to measure expansion velocities. Bolometric light curve and expansion velocity allows us to then obtain the explosion parameters.