Thematics
Stars exist with a broad range of physical parameters. We know best the closest star to the Earth, the Sun, but many more exist. The GAPHE undertakes researches to better understand the most massive objects of the stellar population. With their high masses, temperatures, and luminosities, those stars multiply the superlatives. This is not just a record-holder game. Indeed, such stars have immense impacts on their environment. Their ionizing radiation is able to heat up the circumstellar medium, giving birth to bright nebulae. In addition, their intense radiation field is able to lift the external layers or the stars and accelerate them to high speeds: a typical O star ejects in its surrounding space 5 1014 tons of matter every second at a velocity of 7 million km/h! Such energetic stellar winds as well as intermittent eruptions and the final explosions as supernovae truly shape their neighbourhood, injecting also large amounts of chemical elements such as those we see around us on Earth. The GAPHE studies massive stars from their birth to their last moments, following their evolution and interactions at all stages.
Twinkle, twinkle, large star...
Stars are rarely perfectly constant: their spectrum as well as their luminosity usually vary, on several timescales and with various amplitudes! In massive stars, this variability may occur because of several processes. First, the stellar surface itself may change with time because of pulsations or localized spots. Second, the winds themselves are unstable and may possess large-scale structures (linked to the surface spots or to the action of strong magnetic fields). Third, the stars may loose material which then forms a disk surrounding them (the so-called Be stars) and such disks evolve with time, with building or dissipating events. The GAPHE aims at identifying such variabilities, characterizing them in detail, and ascertaining their nature. We have done so for OB stars, but also for their evolved descendants, the LBV and Wolf-Rayet stars.
Multiplying the output
While the Sun is a single star, most massive stars live in couples (called binaries) or in even larger families. The GAPHE has specialized in the study of such multiple objects as they provide invaluable information. Indeed, it is not easy to measure the physical parameters of a star as one can obviously not put celestial objects on a scale or near a rule! Everything has to be done from afar and, fortunately, binaries can help. In a binary, both stars are orbiting each other and this orbital motion directly depends on the system parameters. This motion can be detected through changes in velocity (using the Doppler effect), in position (astrometry/interferometry), and/or in brightness (e.g. when there are eclipses). The GAPHE performs many studies of massive binaries, either in large studies of clusters or in focused analyses of specific objects.
As the two massive stars may be quite close to each other, additional phenomena appear. For example, tidal forces can distort the stars’ shapes but also modify their orbit, leading to the so-called apsidal motion. The GAPHE determines the amount of apsidal motion and then analyses it to get information on the internal structure of the stars which cannot be obtained otherwise. Another example is colliding winds. Indeed, if a binary is composed of two massive stars, they will both emit strong and fast stellar winds and these winds will inevitably collide in the space between the two stars. Such collisions display signatures throughout the whole electromagnetic spectrum and the GAPHE monitored these collisions, pinpointed their signatures, and derived from them the characteristics of the winds.
Once the stellar properties are known, the GAPHE uses them to enlighten the evolutionary paths of massive stars. As many such stars live in multiple systems, interactions between the stars may occur: transfer of material from one star to its companion, merger of two objects... The details of these processes are still uncertain, so the GAPHE poses strong constraints on them using large sets of observational material. For example, it allows to find that fast rotation is not always a result of such interactions or that a companion might be a stripped star (i.e. victim of stellar cannibalism) and not a black hole...
