Multiplicity as a tracer of...





1. Statistics

At the beginning of my PhD, our knowledge of the multiplicity properties of ultracool dwarfs was limited to casual observations of random multiple systems. The goal of my PhD was to perform a systematic study of these properties in different environments. The surveys I conducted or participated in using HST and adaptive optics contributed to show that the binary frequency is monotonically decreasing from stars to substellar (brown dwarfs) objects, that the distribution of separations is similar to that of more massive stars but shifted towards smaller separations, and that the distribution of mass ratios peaks towards unity. These observations brought new constrains to the models of formation, which so far fail to reproduce these results.


2. Dynamical masses

One of the ultimate goals of stellar and brown dwarf theories is an accurate determination of their mass based on spectroscopic characteristics and luminosity. While the mass-luminosity relationship has been accurately calibrated for low mass stars, the degeneracy in the mass-luminosity relation for ultra-cool dwarfs makes it difficult to pin down their properties. Luminosities and effective temperatures are indeed function of both age and mass so that an older, slightly more massive object can exhibit the same effective temperature as a younger, less massive one. Model-independent dynamical masses are required to calibrate the mass-luminosity relation.

Moreover, the knowledge of the mass function is a crucial step toward the understanding of their formation, and requires an accurate determination of their masses in different environments. But up to now it relies primarily on un-calibrated theoretical mass-luminosity relationships. The determination of dynamical masses thus provides not only the best tests for the theory but also a cornerstone in the understanding of their formation. In the course of my research, I have measured the orbital parameters and dynamical masses of brown dwarfs, providing the first calibration and benchmark measurements for the models of brown dwarf evolution.


... and planetary formation

The formation of planet must be strongly affected by the presence of a stellar companion. Studying the properties of multiplicity of planet host stars and comparing the characteristics of planets in multiple and single star systems holds important clues on the formation mechanisms at work.


In the context of a project led by my collaborators J. Lillo-Box and D. Barrado y Navascués, I am participating to a study of multiplicity among a sample of planet host stars discovered by the Kepler satellite.

When observed at high spatial resolution, a number of these stars have one or sometimes several visual companions. Follow-up observations are on-going to confirm the nature of these companions and whether they are physically bound to the planet host star or just a chance alignment in the sky.

But independently of their nature, the presence of these companions has important consequences on the physical properties of the planets orbiting these stars.

... stellar formation

The multiplicity properties of a population are directly related to the formation mechanisms. The study of multiplicity can be split in two parts: a statistical study of their properties (frequency, distribution of orbital periods and mass ratio), and the measurement of dynamical masses.

© Last Update: 13-02-2017 by H. Bouy

Photograph Credit: Yuri Beletsky