Research



Ths Sun is seen to move from East to West every day. The appearance and position of the Moon change from night to night. The planets wander across the sky. Comets or even new stars sometimes appear. In short, some of the most familiar astronomical phenomena involve variability, either in brightness, shape, or position. Although it is nothing new, the study of phenomena that involve a change over time are nowadays called time-domain astronomy. Most of my research focuses on variable objects, near or far.



The restless Universe      

For a long time, catching variable or transient objects was based on luck. We now have the technological means to observe the sky in a systematic way. In the past three decades or so, astronomers have become more and more ambitious in the area of sky they observe and how often a patch of sky is observed. I am part of the All-Sky Automated Survey for SuperNovae, ASAS-SN that observes the whole sky every 2-3 nights. This survey is using small 14cm telescopes equipped with CCDs. There are two units, one in each hemisphere, each having four telescopes. The telescopes have a 4.5 x 4.5 degree field of view which allows us to observe about 10,000 thousand square degrees per night. In a typical night, about two thousands images will be obtained. Obviously, such a large amount of data can only be handled by using efficient software.

The main focus of the project is to find supernovae (SNe) but anything that varies, for any physical reason, will be detected. We find plenty of classical variable stars like RR Lyrae, Cepheids or Long Period Variables. We also find variable Active Galactic Nuclei, including the "changing look" NGC 2617, plenty of flares from M dwarf stars, lots of cataclysmic variables (CVs). We have also found tidal disruption events, stars being ripped apart when they get too close to a supermassive black hole.


Stars going boom: Gamma-ray bursts and supernovae      

Gamma-ray bursts are just that: brief flashes of gamma-rays. Gamma rays are like ordinary light but it carries a lot more "punch". We now know that GRBs mark the birth of a black hole. GRBs involve extreme physical conditions and are pushing almost every limit in terms of matter density, magnetic field, speed of matter.
We now think that GRBs lasting longer than about two seconds are caused by the collapse of a massive star (somewhere between 20 and 40 times the mass of the Sun) and is accompanied by a supernova (see below for supernovae). In almost every case where we can find evidence of a SN accompanying a GRB, we have found such evidence.
As for short GRBs, those lasting less than about two seconds, they are thought to arise from the merger of two neutron stars or a neutron star and a black hole.

The supernova (SN) phenomenon is a spectacular display, a kind of cosmic fireworks that marks the end of the evolution of two type of stars.


Stars going Up and Down: Cepheids and variable stars      

Stars are very much like a simple pendulum. A pendulum is stable when it is perfectly vertical. When it is moved away from its equilibrium position, it tries to get back to it, "overshoots (goes too far), then moves back, etc. If there is no friction, the pendulum can oscillate forever. Stars do the same. They can shine at a constant luminosity but they can also be a little away from their equilibrium, in which case they will pulsate. They will appear periodically bright and faint. Go to the web site of the American Association of Variable Star Observers (AAVSO); it has a lot of background information on variable stars.

One particular type of variable stars I am interested in are Cepheids. They vary in size and luminosity extremely regularly. As time-keepers, they are much more accurate than most wrist-watches one can buy but this is not their main use. It turns out that the pulsation period of a Cepheid — the time it takes for a whole cycle of getting-faint-and-getting-bright-again, typically between a few days and a few weeks — is related to its average luminosity. This means that if we measure the period of a Cepheid, we can measure its luminosity, hence its distance. This Period_Luminosity relation (PL) is what makes Cepheids very useful to measure distance in the local Universe. Most of my work on Cepheids is connected to their use as distance indicators.

One particular issue I have been working on is the metallicity dependence of the PL relation. To build a large sample of Cepheids all at the same distance, we have surveyed the nearby galaxy M33. Complete results are not yet out but you can already check out the M33 variability survey page