Star Clusters
I have many research project that involve observations of star clusters. Below, I describe our JWST program to observe brown dwarfs in the globular cluster 47 Tucanae and our program to look for massive white dwarfs in open clusters.
Brown Dwarfs in Globular clusters
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 work, I proposed 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.
To observe brown dwarf in a globular cluster for the first time, I designed and led a successful JWST proposal during Cycle 1 to observe the globular cluster 47 Tucanae and detect and study the brown dwarf cooling sequence, as well as hunt for ancient planetary systems around white dwarfs. With extremely well-characterized properties (distance, age, metallicity), the location and detailed structure of the gap between low-mass stars and brown dwarfs and of the brown dwarf sequence can be directly compared to the predictions of state-of-the-art models and help us break the degeneracies in age, mass, and composition that affect current models of brown dwarf evolution, constraining the physics of brown dwarfs atmospheres and interiors.
Image: JWST in front of a simulated JWST image of globular cluster 47 Tucanae.
Open Clusters and the IFMR
The maximum mass of a star that is capable of forming a white dwarf is an important astrophysical quantity: it affects the rate of core-collapse supernovae, the rate of formation of neutron stars and the chemical enrichment and star formation rate of galaxies. The lower the maximum mass is, the higher the number of supernova explosions, as well as the number of neutron stars formed. Additionally, the amount of heavy elements pumped back into the interstellar medium increases along with the dust production and likely the star formation rate in a galaxy. This quantity is, however, still poorly constrained. A related open question is the shape of the relation between the initial mass of main-sequence progenitors and the final mass of white dwarfs (initial-final mass relation or IMFR). Such relation is most poorly understood at the high-mass end and has important implications for the physics of convection and stellar winds in evolved stars.
One of the goals of my research program is to find massive white dwarfs in open clusters to constrain these quantities, as white dwarfs that are cluster members allow us to estimate the main sequence mass of their progenitor stars. Gaia’s precise astrometry allows to identify stars that are cluster members with high confidence. Additionally, my collaborators and I developed a “reconstructing” technique to identify former cluster members that escaped in the past, allowing to study the evolution of open clusters in the Galaxy. Using these techniques, we found the most massive white dwarfs with the most massive progenitors known in clusters, constraining the high end of the IFMR.
Image: IFMR for cluster white dwarfs whose progenitor mass is greater than 2 solar masses and whose cluster membership has been confirmed with Gaia. The new white dwarfs discovered within our program are shown in red. The lines are fitted to the data assuming only one slope (dot-and-dashes) or allowing a break (solid); the quality of the fits is similar with the current data.