My research deals with different kinds of cosmic explosions and transient phenomena including thermonuclear and core-collapse supernovae, super-luminous supernovae, gamma-ray bursts, tidal disruption events and gravitational waves. Furthermore, I study the galaxies and the locations where these bright events occur.
Thermonuclear Supernovae
These explosions are better known as Type Ia supernovae. The fact that they can be used to measure distances across the Universe, by standardizing their luminosity, led to the discovery that the Universe is dominated by a mysterious Dark Energy that forces it to accelerate. This discovery was awarded by the 2011 Nobel Prize in Physics. It is widely believed that SNe Ia result from the thermonuclear explosions of white dwarfs, although the details remain unkwown. Understanding their nature and sharpening their properties as distance indicators remain one of the biggest goals of astrophysics and cosmology.
The way that I first studied SNe Ia was through observations at nebular phases. These are stages significantly after the explosion, when the outer layers have become transparent and it has become possible to peer deeper into the ejecta. In 2009, together with my collaborators, we presented a study of SN 2003hv extending to more than 2 years after maximum light. We were thus able to constrain important physical explosion parameters, showing that an infrared catastrophe did not occur and that positrons, created during radioactive decays, did not escape the ejecta. Following our publication, I had the pleasure of participating in a collaboration, led by Keiichi Maeda, that resulted to a series of papers linking nebular spectral lines of SNe Ia to the explosion asymmetry. The viewing angle from which we look at an asymmetric explosion can lead to second order variations in the SN Ia colors and magnitudes.
I am also interested in SNe Ia that show strong signs of interaction with pre-existing circumstellar material (the so-called SNe Ia-CSM), which was expelled by the progenitor system prior to the explosion. In a study I led, I proposed that a sub-class of over-luminous SNe Ia, which show a higher degree of association with SNe Ia-CSM, have likely single degenerate progenitors.
Core-collapse Supernovae
These explosions result from the gravitational collapse of massive stars (more than 8 times heavier than our Sun). Depending on how much of their outer envelope they maintain prior to the explosion, they have different spectroscopic signatures and they can be divided in several sub-classes. The most usual are Type II supernovae that show hydrogen in their spectra. For many of them, we confidently know that they result from the explosions of Red Super-Giants. But if the progenitor star, for one reason or another, was stripped off its hydrogen envelope by the time of collapse, the explosion will appear as Type Ib/c (also collectively known as stripped-envelope supernovae). SNe Ib are the ones that show helium in their spectra, while SNe Ic do not. Furthemore, some particularly energetic SNe Ic are linked to GRBs.
Within the field of core-collapse supernovae, my research has mostly focused on stripped supernovae. The way that I have studied those was predominantly through their environments, or the environments of their prospective progenitors. These are widely believed to be Wolf-Rayet stars but a confirmation is pending. In a study we conducted, it was shown that the locations of WR stars within their host galaxies are indeed compatible with those of SNe Ib/c, and even long GRBs, while the opposite would have been quite an upset. Furthermore, this extends to the sub-types of WR stars: WN star locations are more closely related to those of SNe Ib, while WC star locations to those of SNe Ic, as also predicted by theory. Another open question regarding SNe Ib/c is how does exactly the stripping take place: is it done through metal-driven stellar winds (in which case SNe Ib should result by more metal-poor progenitors than SNe Ic) or through binary interactions? By obtaining spectra at the exact locations of SNe Ib/c, I studied their properties, such as the local metallicity and the minimum ages of the local stellar populations. Within our sample, SNe Ib do show a small preference for more metal-poor environments, compared to SNe Ic, but this difference is not statistically significant. Within some assumptions, the locations of a sub-sample of these events appear older than what is expected by the evolution of single stars more massive than 25-30 solar masses. This might indicate that they come from lower mass binaries.
I am also interested in the rich and diverse zoo of interacting transients. Interaction with a circumstellar medium can easily hide the true nature of the underlying supernova. An exciting object that we have been following recently with my collaborators is SN 2015bh. For this object it is not even clear whether the star has exploded or it is still alive! SN 2015bh took place in a galaxy that we have studied extensively as in the recent past it was home to more exciting supernovae.
Super-luminous Supernovae
Astronomers have rutinely discovered extragalactic supernovae during the last century. Surpizingly, however, the brightest and most spectacular of these explosions had escaped us until 2005! The reason is that these events tend to occur in faint, dwarf galaxies that were not monitored during the traditional supernova searches (that target bright nearby galaxies). Some of these supernovae, such as the famous SN 2006gy, show narrow lines of hydrogen in their spectra that originate in the collision of the ejecta with a surrounding circumstellar medium. Other events, however, do not show traces of hydrogen. What powers their luminosity remains a complete mystery.
I got initially interested in super-luminous supernovae when I realized that a SN that we observed in 2006, during the course of the SDSS Supernova Survey, was one of these guys! What is special about SN 2006oz is that it demonstrates a dip in the very early light curves (or a plateau in its bolometric light curve), which was inaccessible to previous observations of similar, hydrogen-poor, events. In our study, we suggested that this early plateau phase might be due to a recombination wave in a oxygen-rich circumstellar medium. Since then, more events have been found to have similar behavior and it has been suggested that double-peaked light curves are common in H-poor SLSNe. In any case, this is an important clue in understanding their nature.
Motivated by the fact that the first SLSNe were found in unusual environments, I initiated the SUperluminous Supernova Host galaxIES (SUSHIES) survey. In our first paper, we studied the spectroscopic properties of a large sample of SLSNe and concluded that H-poor SLSNe have a tendency to occur in extreme emission line galaxies. This means that they show a preference for young, intensively star-forming and metal poor environments and that their progenitors might be very young (where young in astronomy means just a few million years old!) This effort continues with my collaborator Steve Schulze in charge of paper "SUSHIES II". In addition, in order to study the geometry, and thereby the nature, of SLSNe, I embarked on a project to obtain polarimetry of these objects. To this end, I presented the first polarimetric observations of a H-poor SLSN, obtained with the Very Large Telescope. For this object we did not detect a significant polarisation (or polarisation evolution), meaning that this object did not show significant deviations from spherical symmetry during the phases that we observed it. However, this was only the start and me and my collaborators are working on collecting more data. Furthermore, I actively study SLSNe as part of both the iPTF and the PESSTO surveys.
Gamma-Ray Bursts
Working at DARK, it was difficult not to get interested in GRBs! These are very energetic phenomena, defined by their gamma-ray emission, and are exclusively detected from space. From Earth, we are able to observe their optical afterglows that, among other properties, allow us to establish their distance. Due to their immense luminosities, GRBs can be detected out to the highest redshifts. They are traditionally clasified as long or short, depending on their duration. Long GRBs have been linked to energetic Type Ic supernovae and the collapse of massive stars.
My involvement in GRB research was mainly through my participation in Target of Opportunity observations of these events. This basically means that when the Swift satellite detected a GRB, it sent an sms to my phone! I was then supposed to contact a telescope, tell them to observe, look at the data and, perhaps, issue a telegram informing fellow astronomers of the results. This happened during the few weeks per year that I was on alert and it could sometimes become annoying, since GRBs tend to happen in the worst moments (in the middle of the night, during important sport events, etc). This is one of the reasons that I am not doing this actively any more! But other times it could become very exciting, especially when the event we were observing turned out to be exceptional. Such an example was when I was the first to observe and locate Sw 1644+57, that is believed to be a rare tidal disruption event, after it occured. I am involved in several GRB-related publications as a co-author and I am still collaborating with my colleagues on these topics.
Tidal Disruption Events
Black holes are among the most fascinating objects in the Universe: their gravitational field is so strong that even light itself cannot escape! Stellar-mass black holes are the remnants of stars that have exploded as supernovae and they have masses up to a few tens that of the Sun. There exist, however, black holes way more massive than this: supermassive black holes weighing as much as many million Suns are found in the centre of (almost) every galaxy. If a star passes too close to such a megamoth, the tidal forces it will experience will be so strong that they can tear it apart! This is what we call a tidal disruption event. This phenomenon was predicted a few decades ago but it is only recently that we have started finding events that potentially fit this description.
The tidal disruption event that I have worked on was pretty special! ASASSN-15lh was initially proposed to be a superluminous supernova, and in fact the most luminous supernova ever observed. Our study, however, suggests that this event is much more likely to be the tidal disruption of a low-mass star by a particularly massive black hole. This conclusion is supported by: the location of ASASSN-15lh near the nucleus of a red passive galaxy, the peculiar light curve and temperature evolution, and the composition and ionisation condition of the gas illuminated by the radiation. As mentioned, however, the mass of the black hole is particularly high, exceeding 100 million times the mass of the Sun. Normally, a black hole of this mass would swallow a star in one go and the disruption would happen inside the event horizon, where we would not be able to see it. We have therefore concluded that this monstrous black hole must be spinning rapidly, as this is the only way for the disrupt to happen where the radiation will be able to escape. A spinning black hole can also more easily explain the huge luminosity of ASASSN-15lh.
Gravitational Waves
Here I should really be more specific because what I am after, as an optical astronomer, are mostly the electromagnetic counterparts of gravitational waves. The gravitational waves themselves are not electromagnetic radiation (i.e. light) but ripples of spacetime itself, generated by accelerating masses. The existence of gravitational waves was predicted by Einstein in 1916 as part of his general theory of relativity. The first direct detection of gravitational waves took place in 2015 when LIGO detected GW150914, the merger of two black holes of 36 and 29 solar masses. The signal was so undoubtedly beautiful and close to the theoretical prediction that it immediately erased any doubts about its nature, and rightfully led to a Nobel prize.
But a black hole merger such as GW150914 is not expected to produce any electromagnetic counterpart (because by definition black holes do not let any radiation escape). For this, we had to wait two more years (and 4-5 black hole mergers more) to witness the next most violent merger in the Universe: the one between two neutron stars! Such an event was long predicted to produce (i) a short GRB and (ii) a faint, red, afterglow dubbed a kilonova. This was exactly what was observed for GW170817, allowing us to see, for the first time, light from a source that also emitted gravitational waves! I participated in the study by Smartt et al. (2018), contributing with calculations of the kilonova bolometric light curve and spectroscopic comparisons ruling out any similarity of this event with previous transients. I am now part of the large European collaboration ENGRAVE for the study of gravitational wave counterparts with the ESO facilities.
