Faherty’s scientific research is focused on the lowest mass components of star formation. That category includes both the smallest stars as well as brown dwarfs — objects that do not have sustained stable Hydrogen burning. As one goes from the highest mass brown dwarfs at 75 Jupiter masses to the lowest, the line between substellar mass objects and planets becomes blurred. Consequently both the coldest class of brown dwarfs and the youngest become analogous to giant exoplanets. Faherty’s research over the years has concentrated on these planet-like objects thus much of her research crosses into the exoplanet regime.

Another large aspect of Faherty’s research has been on mapping the local solar neighborhood. Through the Backyard Worlds: Planet 9 citizen science project that she co-founded with scientists Marc Kuchner, Aaron Meisner, and Adam Schneider, Faherty and colleagues have made tremendous progress in understanding the number and distribution of the coldest compact sources that form through the star formation process. Faherty has also been making 3D maps of the local neighborhood using planetarium enabled software and data from the European Space Agency’s Gaia telescope. Investigating the velocity distributions of stars within a few hundred light years including co-moving stars, brown dwarfs, and their giant planet companions has led to important advances in our understanding of the evolution of stars near the Sun.

Scientific Interests

The Brown Dwarf Kinematics Project (BDKP) 1

An important aspect of Faherty’s research includes measuring and analyzing the positions and velocities of low mass stars and brown dwarfs. In 2006, she initiated the Brown Dwarf Kinematics Project (BDKP) in order to use astrometric measurements to investigate fundamental physical properties of the brown dwarf population. A velocity analysis of the BDKP sample has led to a correlation between near-infrared color, kinematics, and age. Analysis of luminosity trends and model comparisons for normal, low-surface gravity and subdwarfs in the BDKP sample led to observational distinctions between the sub-populations. Parallaxes combined with the wealth of photometric and spectral data led to a library of spectral energy distributions and an establishment of bolometric luminosity trends for the brown dwarf population.

Recently, the Backyard Worlds: Planet 9 collaboration that Faherty co-founded has become a primary source for new nearby brown dwarf discoveries. Analysis of the most complete census of low mass constituents of the solar neighborhood by many on the Backyard Worlds team has provided important insights on the cut-off mass of the star formation process. Faherty continues to work toward new discoveries in the solar neighborhood as well as toward using distances and space velocities to discern age, atmosphere, metallicity, and binary information on objects.

Low Gravity Brown Dwarfs 2

Brown dwarfs suffer from an age mass degeneracy. For compact objects with temperatures less than ~3000K, without knowing the age of a given source it is extremely difficult to differentiate between low mass stars, brown dwarfs, and giant exoplanets. A young hot planet, a juvenile age warm brown dwarf, and an old very low temperature star can all share the same spectral features. The effective temperature of a given object is the primary impactor on the chemistry that sculpts the spectral energy distribution. Therefore the same molecular features that dominate low mass stars, will dominate in hot planets. In order to use photometric, spectral, or luminosity information to differentiate between stars, brown dwarfs, and planets, we must identify features that are gravity hence age sensitive. Gravity properties are discernible in low temperature objects but they must also be disentangled from metallicity, binary, and atmospheric effects.

Faherty has worked on a sample of brown dwarfs that were isolated for having spectral characteristics defined as low surface gravity. Using parallaxes and photometry, Faherty has focused on determining their absolute magnitudes, bolometric luminosities, and positions on color magnitude diagrams. Using velocities, Faherty has focused on determining memberships in known nearby moving groups or young stellar associations. With ages determined, the coldest young brown dwarfs joined a small sample of objects that blur the definition between small brown dwarf and giant planet.

CoMoving Brown Dwarfs 3

Brown dwarfs that are found co-moving with higher mass stars are goldmine discoveries. Ages are extremely difficult to discern for individual brown dwarfs because there are a limited number of observational characteristics that lend clues to how old the object is. Main sequence stars (e.g. solar analogs) have far more diagnostics available for determining their age. Moreover, with high resolution spectroscopy or more novel techniques on lower resolution data, metallicity and chemical abundance information can be extracted for a main sequence star. If a brown dwarf is found at a similar distance and with a common proper motion to a higher mass star, then co-evality can be assumed and all of the diagnostics for the primary can be used to evaluate the secondary.

Faherty has been using precise astrometric measurements of the brown dwarf population — both those in the literature and ones she has collaborated on obtaining — to ascertain a robust sample of benchmark objects co-moving with well studied main sequence stars. The distribution of mass ratios, separations, and binding energies for these systems likely yield clues to both giant planet and brown dwarf formation. Faherty has also been using the spectral retrieval method to evaluate compositional information for the brown dwarf companions to compare and contrast with the higher mass main sequence star values. This is a promising method for determining formation signatures as well as grounding atmospheric anomalies in brown dwarf data.

Kinematic Structures Near the Sun 4

The parallaxes and proper motions of stars allow us to investigate co-evolving structures near the Sun. In all likelihood most stars are born in clusters that are dense when they are born and then dissipate over time. Using spatial and velocity maps of the few hundred light year volume around the Sun we have the best glimpse at how stars are born and evolve. The European Space Agency’s Gaia DR2 observatory currently scanning the sky at the Earth-Sun’s 2nd Lagrange point has been revolutionizing our understanding of the structure of the Galaxy.

Faherty has been using the Gaia catalog to investigate kinematic structures from their highest mass components to the lowest. In collaboration with astronomer Jonathan Gagne, Faherty has been using the bayesian inference tool called BANYAN to determine individual members of known groups as well as to define entirely new groups near the Sun. Using NASA’s TESS survey, Faherty and team are also using photometric variability data to investigate the color - period relation for known young moving groups, associations, clusters, and star forming regions near the Sun.