Massive Stars & supernova progenitors

Core-collapse supernovae are explosions of the most massive stars in the universe and are responsible for producing every element in the periodic table heavier than iron. However it seems that some of the more unusual supernovae may have been boosted by the most powerful magnets in the universe.

In recent years, the discovery of a number of unusually luminous stellar explosions in small, distant galaxies is challenging our previous ideas about these events. It is no exaggeration to say that the field has been revolutionized by the new generation of surveys, such as Pan-STARRS1. The early discoveries of these explosions suggest that these super-luminous (100x more luminous than a typical exploding star)
supernovae are quite diverse. They are remarkable bright, one hundred times brighter than the explosion of a massive star (at least 7-8 times our Sun). They explode in small and faint galaxies with an abundance of metal elements lesser than what we observe in the Milky Way, and until now none of these super luminous objects have been found closer than 1.5 billion light years.

Although these Supernovae are explosions in galaxies far, far away, the astronomers are confident that they are not consequences of a planet being destroyed by a laser being fired from a small moon (“That’s no moon, it’s a space-station”). Indeed, some scientifically valid ideas have been proposed. To explain the high luminosity of such events. The likely suspects are: 1) having an extra source of energy boosting the total explosion energy; or 2) converting all the energy into light (generally only 10% of energy is transformed into light). Thanks to Pan-STARRS1 we had the opportunity to study five of these rare supernovae at distances close enough to allow us to have the best sample of data so far. Supernovae brighten over several days as the explosion lights up the gas around the dying star. Following this they gradually fade with time. We were able to collect data until roughly 300 days after the peak luminosity (a phase called “tail” of the luminosity pattern) and monitor the fading of the light coming from the explosion. Revealing for the first time an unexpected flattening of the luminosity at this stage. This new evidence lead us to look for an extra source of energy boosting the supernova’s output during this phase.

We looked for physical and feasible possibilities and we found that a magnetar could be a really good candidate for the additional power source. What is a magnetar? When a star explodes, it can leave behind a rotating and magnetic stellar remnant, called neutron star (abbreviated NS). If the NS spins rapidly and its magnetic field is exceptionally strong magnetic field it is called a magnetar. This can have a magnetic field one hundred thousand billion times stronger than that of the Earth and ten billion times stronger than a typical fridge magnet!. This magnetar then rapidly spins down, deposits its rotational energy into a supernova explosion and thus significantly enhances the luminosity. Thanks to a semi-analytical model, we plugged in magnetar in a normal SN event being able to reproduce the entire luminosity behaviour until the late phase, thus we could say that we caught a magnetar by the tail!

We reached a remarkable result, but this is only a starting point because, as scientists, we have to explore all possible scenarios and repeat the experiment (in our case this means using the model on as many SNe as possible) to make it trustworthy. Our journey to the mysteries of these luminous explosions has just begun!

Stars, such as our Sun, may appear to be ever-present sources of heat and light, but they are highly dynamic objects which will eventually reach the end of their evolutionary paths and die. Although the death of the Sun will be comparatively quiet, more massive stars (with masses greater than 8 times that of the Sun) can be ripped apart when their iron core collapses to an extremely dense stellar remnant known as a neutron star and a shock wave driven by the production of a huge number of particles called neutrinos blasts away the outer layers of material in a supernova explosion. This ejected material does not go to waste however and the elements created, such as carbon and oxygen, provide the Universe with the building blocks it needs for the formation of rocky planets like Earth and all living things upon it.

The process outlined above will lead to only one of the different types of known supernovae, called core-collapse supernovae. These core-collapse type events can then be further divided into sub-classes (type Ib/c, type II) depending on the mass of the progenitor star which explodes. One of the most luminous type II explosions was recorded at 100 billion times the brightness of the Sun and could easily be identified despite being 4.7 billion lightyears away. A second type, a thermonuclear supernova (type Ia), is thought to occur when a dead remnant of a star like the Sun known as a white dwarf star which happens to be in a close binary system accretes matter from its companion, pushing it past its upper mass limit (the Chandrasekhar limit) of 1.4 solar masses. In the last two years incredibly bright supernovae (100 times brighter than an average SNe) have been discovered which may require a new physical explanation for their luminosity and observations from the Pan-STARRS 1 survey (PS1) will help further our understanding of these ultra-luminous events and better define the very interesting physics which leads to them.

Pre-supernova (top) and post-supernova (bottom) PS1 images showing the discoveries of a supernova with a clear host (left) and a hostless supernova (an orphan, right).

The basic discovery and classification of supernovae is a two-part process. Difference imaging, where nightly images are subtracted from a previously obtained reference image to highlight any objects with a variable flux, is used to detect these very characteristic transients. This is essentially an interstellar spot-the-difference and some examples of PS1 images containing supernovae can be seen in the images to the left. After the initial discovery of a transient that is thought to be a supernovae, follow up spectroscopy must be done in order to confirm the nature of the variable object (it might be simply a variable star or an active galactic nucleus). If it is a supernovae, the spectra can be used to determine elements involved in the explosion and thus deduce its type. By compiling large datasets of confirmed events, the rates of different species of supernovae can be estimated giving us invaluable information on stellar populations within various forms of galaxies and the intergalactic medium.

Up until now, both professional and amateur searches for supernovae have concentrated their efforts on large galaxies with high star formation rates to ensure a large number of successful discoveries. The PS1 survey is not a dedicated search for transients but hosts a multitude of varied science projects meaning that the Hawai’i based, 1.8m telescope, with its 7 square degree field of view focused onto the world’s largest CCD (1.4 gigapixels), surveys massive, unbiased portions of the sky every night. This results in supernovae discoveries in unexpected places as demonstrated in the right hand set of images above. This type of transient is known as an Orphan due to its apparent lack of association with any other object. The object may very well have a host galaxy, but one that is simply too faint to be imaged with the PS1 survey suggesting that it is either at a high redshift or has a low proportion of elements heavier than Helium. If the galaxy has a high redshift (and thus is at a great distance from us) the high luminosity of supernovae events allow us to probe deeper into the universe than the photometric limit of a telescope may at first suggest. If the event occurs within a faint host galaxy, the type of supernovae can give us evidence as to the population in these mysterious stellar neighbourhoods. A possible progenitor channel for the ultra-luminous supernovae mentioned earlier may be pair instability in stars which are quite deficient in the heavy chemical elements (like carbon and all heavier elements). Previous searches in relatively nearby, metal rich galaxies have therefore missed these exiting and currently obscure phenomena.

The lightcurve of a hostless SNIIn from PS1 photometry. The different coloured points represent brightness measurements in different filters (g, r, i and z).

Throughout the 3 year period of my PhD at Queen’s University Belfast, under the supervision of Professor Stephen Smartt and Dr. Rubina Kotak, I am hoping to classify as many of the orphans from the PS1 survey as possible, offering statistical analysis and comparison of their rates with current estimates. The PS1 telescope has an r-band photometric limit deeper than magnitude 21 (over one million times fainter than what the human eye can see on a dark night) which is easily capable of detecting these events and producing thorough and defined lightcurves, such as the one shown on the right. Of the almost 250 supernovae with faint or no visible hosts already compiled, at least 3 are thought to fall into the ’ultra-luminous’ category and with hundreds of new transients being detected by PS1 each week, more are sure to follow. With a little luck I will be able to obtain decisive experimental data and perform intense follow-up observations of some of these objects in an attempt to further our understanding of this latest addition to the study of supernovae. Another group, the Palomar Transient Factory, is also looking for these fascinating events (more information on their project can be found here). Possible collaborative work with this team, and some constructive rivalry, will ensure that the science goals presented here are carried out to optimal completeness.