RESEARCH

 

RESEARCH INTERESTS

My research revolves around the intrinsic variability of stars.

 

MASSIVE STARS

Every atom composing all forms of life on Earth originate from the deaths of hot, luminous, massive stars. Indeed, with initial masses at least about eight times greater than that of the Sun, massive stars are the ones that finish their lives as supernovae, which are key processes responsible for the enrichment of the interstellar medium with chemical elements that serve to form new stars, new planetary systems, and life on habitable planets. This leads us to wonder so much about the nature of these massive stars.

O-type stars and their descendants Wolf-Rayet stars break the record of being both the hottest and most luminous ones. Part of my research interests consists in exploring how the brightness of these stars may vary over time, and if so, what kind of intrinsic phenomena cause the brightness variations. Particularly, the long-term photometric monitoring of key O-type stars such as ξ Per and ζ Pup with the MOST microsatellite and the BRITE nanosatellites led us to the surprising discovery that these stars may exhibit bright spots at their surface, and that these bright surface inhomogeneities turn out to drive the formation of large-scale spiral-like corotating interaction regions in the stellar wind. Moreover, some kind of stochastic photospheric light variability is concurrently detected, exhibiting strong correlation with the clumpy behaviour of the wind in the case of ζ Pup. The artist's impression of ζ Pup above depicts these findings. The exact origins of the bright surface spots and the stochastically-triggered surface features are currently amongst the key topics that we are exploring. One possibility is that they could be closely related to the presence of sub-surface convection zones in the enormous radiative envelopes of massive stars, as predicted by some theories. On the one hand, a sub-surface convection zone due to the opacity peak induced by the partial ionization of iron was found to be capable of sustaining the generation of small-scale magnetic fields that could occasionally breach through the surface and create magnetic bright surface spots. On the other hand, the stochastically-triggered surface features might be the manifestations of core-convection internal gravity waves, gravity waves generated within a sub-surface convection zone, and/or convective turbulence in the latter. Lastly, the cherry on the cake: are these two kinds of surface features universal amongst O-type stars? More observations are underway!

RED DWARFS

While hot, luminous, massive stars populate the upper-left corner of the Hertzsprung-Russell diagram, cool low-mass red (KM-type) dwarfs literally lie in the corresponding antipode. Red dwarfs (especially the smallest and coolest ones - M dwarfs) are the most adequate targets for the search for transiting Earth-like habitable exoplanets. Indeed, in addition to the prevalence of M dwarfs in galaxies (e.g. it has been estimated that ∼75% of all the stars in the Milky Way are M dwarfs), observational results from the nominal Kepler mission estimated that every late-K and early-M dwarf should host about one planet 0.5−4 times the radius of the Earth, and about one quarter of such systems should have a rocky planet falling in the habitable zone of the host star. Furthermore, the smaller the host star the deeper the transit signals of their exoplanets. Additionally, thanks to the dimness and low temperatures of M dwarfs, their habitable zones are much closer than those of their hotter and brighter counterparts, augmenting the geometric probability of detection of transit signals.

In this lower-right part of the HR diagram, my research consists in characterizing the activity of red dwarfs, especially by mapping their surface active regions that induce rotational modulation of their brightness variations as observed by the Kepler mission. Then, our knowledge of their surface spot coverage and occurrence rate can give us hints on e.g. the possibility of having active latitudes at a given epoch (like in the case of the Sun, yielding the butterfly diagram over the long term), or even the possibility of having dominant active longitudes (the flip-flop phenomenon, as also seen in the Sun), which may constrain the properties of their small-scale and large-scale magnetic fields, both essential in determining the habitability of their planets.

 
 

NEWS

"BRITE space mission reveals the origins of fundamental structures in the wind of the supergiant star Zeta Puppis"

 

©2018 Tahina Ramiaramanantsoa