ν Indi is a bright star (visual magnitude mv = 5.3) visible with the naked-eye from the southern hemisphere. By using ground data (ESO telescopes), space data (Gaia and Tess missions) and by combining very diverse spectroscopic, astrometric, kinematic or asteroseismological information, an international team including two researchers from the Department of Astrophysics / AIM Laboratory of CEA-Saclay was able to determine the epoch, between 11.6 and 13.2 billion years ago, of a collision between our galaxy and a small dwarf galaxy, Gaia-Enceladus. This work is published in the journal Nature Astronomy, January 2020.
This result is based on a detailed study of the properties of the star, using informations from ground and space observatories. Extremely fine spectroscopic data (obtained at ESO with the 3.6m and 2.2m telescopes) allowed to measure the chemical composition of the star, whose elements are by their presence and abundance tracers of the life of the star and its environment. They provide relevant information on its history leading here in two hypotheses: ν Indi is born in the Milky Way or belongs to the galaxy Gaia-Enceladus. This ambiguity has been resolved by analyzing the star's motion, possible thanks to the data collected by the Gaia telescope. Gaia is an ESA space mission launched in 2013 and dedicated to the very precise measurement of the position, distance and motion of stars in the Galaxy.
Left: Composition of the images taken by the TESS satellite of a part of the sky showing the ? Indi position (blue circle), the Milky Way map (bottom left) and the South Ecliptic Pole (top). Data collected by the TESS satellite (credit: J. T. Mackereth).
Right: Spectral power density as a function of the frequency of the star ? Indi (data obtained by TESS over a 27-day period). The main oscillation modes are indicated by number/color. ? -Indy is a star that is on the so-called subgiant phase, a phase where the hydrogen in the star's core is depleted and during which nuclear reactions are initiated at the boundary of the same core. During this phase, which is very fast in the cycle of the star's evolution, the frequencies of the dipole oscillation modes (marked 1 in the figure) are very sensitive to the age of the star. The measurement of these modes thus makes it possible to determine with precision the age of the star, a method used in the work presented here.
Finally, the precise determination of the age of the star completed this survey. This was made possible by the study of the oscillation modes of ν Indi (asteroseismology) using data collected by NASA's TESS mission (Transiting Exoplanet Survey Satellite). TESS, launched in 2018, scans almost the entire sky in search of planets orbiting stars. At the same time, these observations make it possible to study the properties of the star around which potential planets orbit. Analysis of the TESS data has thus made it possible to constrain the age of the star and determine that it was born within and at the beginning of the life of the Milky Way, 11.5 billion years ago, but that the collision with the Gaia-Enceladus galaxy had significantly affected its trajectory in the Milky Way. Determining the age of the star makes it possible to define an upper limit on the date of the Milky Way/Gaia-Enceladus collision, which researchers estimate to be between 11.6 and 13.2 billion years ago.
This work, published in the journal Nature Astronomy, shows the potential of the combination of multiple studies, both for the understanding of stars and for the understanding of the history of the Galaxy.
- « Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi "
William J. Chaplin et al., New Astronomy, 13 January 2020
See also : le press release from the University of Birmingham
Rédaction : Lisa Bugnet, Rafael Garcia, C. Gouifffès