Jonathan Gagné – PhD

Postdoctoral researcher at Carnegie Institution of Washington
Former Phd student, Université de Montréal

See current webpage

Jonathan’s PhD project was focused on the search for brown dwarfs and young low-mass stars (spectral types greater than M5) belonging to moving groups in the solar neighbourhood. Moving groups are relatively young (10 to 150 million years) ensembles of stars. They formed together from the same molecular cloud, and their age of several million years is sufficient to get rid of interstellar dust remaining from early stages of formation, as well as for star members to slowly start dispersing. However, it is still apparent that they belong to a given moving group because they still have very similar galactic velocities (hence the name “moving group”).

The project has many benefits:

  1. It will allow to understand the statistics of their population through their initial mass function (IMF; an histogram of the population as a function of mass). Many questions are still open concerning this IMF; for example, it is not known how it depends upon the environment.
  2. It will help to understand the atmospheres of gas giant exoplanets, particularly difficult to study because of their host stars, which are very close and many times brighter, hence blinding our instruments. In fact, the atmospheres of young and very low-mass brown dwarfs display a dazzling resemblance to those of the few gas giants whose atmospheres we have been able to study.
  3. Identifying potential subtle differences between the atmospheres of planetary-mass brown dwarfs and those of exoplanets will help answer the question of where to draw the line between what are called low-mass, isolated objects “brown dwarfs” and “planets.” This issue is still blurry, and leads to heated debates among researchers.
  4. It will allow to build a compelling target list for the search of exoplanets using direct imaging. In this case, the contrast between low-mass stars and young planets will be lower than that of typical systems.

In order to identify a large quantity of new candidate members in those moving groups, researchers at iREx have developed a statistical tool named BANYAN, which is based on Bayesian inference. Jonathan’s main contribution is to continue developing this tool (see BANYAN II) and to adapt and use it to look for candidate members with even lower masses, covering the regime of low-mass stars, brown dwarfs, and even planetary masses.

This project and the BANYAN II tool have already generated a number of scientific results:

  • The identification of new candidate members of moving groups from the set of already known young brown dwarfs (Gagné et al. 2014a).
  • The discovery of the 2 coldest (and hence lowest-mass) brown dwarfs in the TW Hydrae association (Gagné et al. 2014b) – the coldest one having an estimated mass of 12 Jupiter masses.
  • The discovery of the candidate free-floating planet CFBDSIR2149 (Delorme et al., 2012), with an estimated mass of 4 to 7 Jupiter masses.
  • The discovery of an 11 to 12 Jupiter mass companion to a binary low-mass star, candidate member of a moving group (Delorme et al., 2013; Gagné et al., in preparation).
  • The discovery of dozens of new young brown dwarfs, including a few with estimated masses as low as 10 times that of Jupiter (Gagné et al., in preparation).
  • The discovery of another 13 Jupiter mass companion to a low-mass star (Artigau et al., in preparation).
  • The discovery of several hundred new brown dwarf candidate members of moving groups (Gagné et al., in preparation).
  • The discovery of the coldest brown dwarf candidate member of the Argus moving group (Gagné et al., in preparation).

Another aspect of Jonathan’s dissertation is the use of near-infrared radial velocity measurements to detect exoplanets around low-mass stars. He uses the CSHELL instrument at IRTF to aim at tight-orbit gaseous exoplanets with masses between those of Neptune and Jupiter. He is involved in this project led by Peter Plavchan at the Infrared Processing and Data Analysis Center (IPAC; Caltech) as part of a 6-month IPAC fellowship made in 2014.


René DoyonDavid Lafrenière


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