Collaborative Research: Determining Properties of Brown Dwarf Atmospheres With High-Precision Spectro-Polarimetry

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Objects that form in the Galaxy with masses less than about 8% that of the Sun cannot sustain the nuclear fusion necessary to become a true star. These substellar objects, called brown dwarfs (BDs), are similar in many ways to the gas giant planets in our own Solar System, but there are significant differences. A research collaboration between California Institute of Technology and the University of California-Berkeley will observe both brown dwarfs and giant planets outside the solar system (extra-solar giant planets: EGPs) to better understand these similarities and differences. The researchers will carry out an extensive astronomical survey of BDs and EGPs using telescopes. Their new observations will provide essential information to help understand the formation and physical properties of EGPs. The work will involve graduate and undergraduate students at both universities, and the investigators will develop a new exhibit on BDs and exoplanets at Palomar Observatory's Greenway Visitor Center, drawing on activities and results from their research. The near-infrared (NIR) emission spectra of brown dwarfs and extra-solar giant planets are shaped by condensate clouds and hazes, but a detailed description of these clouds' spatial distributions and microphysics is lacking. As a result, atmospheric retrieval codes yield degenerate abundance results when applied to current typically low-resolution, low signal-to-noise EGP spectroscopy. This program will provide a spectro-polarimetric NIR sequence for BDs and EGPs in order to inform a new generation of three-dimensional modeling tools, incorporating global circulation models and cloud microphysics. The outcome will be a large-scale survey of spectro-polarimetric cloud signatures in BDs, complemented with a smaller spectro-polarimetric survey of companion BDs and EGPs, in the NIR (J and H bands). Compared with photometry, polarimetry is more sensitive to the presence and surface distribution of clouds and hazes. The proposed survey will ultimately help differentiate between formation pathways such as star-like cloud collapse, planet-like disk instability, and core accretion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.