Brown Dwarfs

Brown dwarfs occupy a peculiar position as bridges between stars and planets, and understanding their physics has important implications in other fields: from star and planet formation and evolution, to dense-matter physics and galaxy evolution.

Brown dwarfs are among the latest additions to our collection of sky’s objects. Even though their existence was first proposed back in the 1960s, the first brown dwarfs were not discovered until the 1990s. This delay is not due to brown dwarfs being rare; at least 15% of “stellar” objects in the Solar Neighborhood are brown dwarfs. The delay is due to brown dwarfs’ extreme faintness: unlike stars, brown dwarfs are not massive enough to sustain core hydrogen burning. As a result, a brown dwarf’s luminosity is governed by the left-over heat from their formation. As time passes, brown dwarfs cool and dim, and their spectral energy distributions increasingly shift to near-infrared wavelengths. Their discovery required a combination of better search techniques, spectroscopic followup, and observational sensitivity at infrared wavelengths.

image credit: Joergens, Viki

Astronomy is currently at a similar turning point: the next generation of space telescopes, such as the James Webb Space Telescope (JWST), and ground-based extremely large telescopes, such as the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT), will achieve unprecedented sensitivity in the infrared, where brown dwarfs’ spectra peak. This will enable the detection and study of brown dwarfs that are colder, older, and further away, including the oldest brown dwarfs in the Galaxy: those found in globular star clusters.

Image: TMT in front of globular cluster M 22

Members of a globular cluster share to a first degree the same age, chemical composition and distance from the Sun, properties generally determined from observations of stars on the red giant branch, main sequence turn-off or white dwarfs. For the first time, we will have large samples of brown dwarfs for which these fundamental properties are known to high accuracy, allowing us to break many of the observational degeneracies that arise from their cooling nature. For instance, while brown dwarfs in the vicinity of the Sun possess a range of masses and ages, and hence broad diversity in temperatures, luminosities and colors; those in globular clusters lie along a single age and metallicity-dependent cooling sequence tracing mass. This will enable robust determination of these fundamental parameters and analysis of broader properties such as the substellar mass function, in a manner not currently possible in the field (due to degeneracies) or nearby open clusters (due to small number statistics).

image credit: Adam Burgasser

In my paper, I propose a new method of measuring the age of globular clusters, completely independent from the ones used until now, namely the turn-off method and the white dwarf cooling sequence method. The idea behind the age estimation is simple. As it burns hydrogen, a low mass star is stable, and changes in its luminosity are very small on the order of billions of years. Brown dwarfs, on the other hand, have no source of energy, and they cool down as they get older, becoming fainter and fainter. For this reason, we expect the transition region between the the low-mass star and the brown dwarf regimes to be characterized, in a globular cluster, by a dearth of objects, or a gap, as function of luminosity. By identifying the brightest brown dwarfs, or the end of the gap, it is possible to determine the age of a globular cluster. If the brown dwarfs are properly modelled, this method of age estimation can be very accurate, and does not rely on the knowledge of the cluster's distance from Earth.

Image: mass gap at the end of the main sequence in the globular cluster M4. The yellow dots indicate the position at 12 Gyr of mass-gap objects with mass in even steps of 0.0003 solar masses, while black dots are the same at 13 Gyr. I used MESA for modelling the interior of the brown dwarfs and Phoenix models for the atmosphere.

Because brown dwarfs in globular clusters have been cooling since almost the beginning of time, the gap is particularly enhanced in these systems. It also means that brown dwarfs in globular clusters are exceedingly faint, currently below the detection limit of the best telescopes. Deep HST observations in globular clusters already reach the end of the main sequence, and while some candidate brown dwarfs have been detected in M4, they remain too faint for follow-up spectroscopic confirmation and characterization.

The ability to study brown dwarfs in globular clusters over the next decade will usher in a new way of exploring the astrophysical properties of these unique objects, while in turn providing new opportunities for studying the star formation history of clusters and the Milky Way Galaxy at large.

Image: JWST in front of a simulated JWST image of globular cluster 47 Tucanae.