# # Program information file # PROGRAM_ID 2024A075 PROGRAM_TITLE Test and calibrating ultracool atmospheric models with new benchmark brown dwarfs from Gaia and Pan-STARRS PROGRAM_INV1 Zenghua Zhang PROGRAM_INV2 Richard Smart PROGRAM_INV3 Maria Cruz Galvez Ortiz PROGRAM_INV4 Hugh Jones PROGRAM_INV5 Sayan Baig PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG Brown dwarfs [BD] are the last major population discovered on the HR diagram. They are crucial to constrain the mass function, and to provide reference in the characterization of exoplanets. However, BDs have mass-age degeneracy; their dusty atmosphere are also diversified at different metallicities; and have rapidly changing atmospheres at the L/T transition. Some spectral features of L dwarfs can not be reproduced by atmospheric models. Predictions from different models are not consistent. A sample of benchmark BDs covering wide ranges of age, Teff, [Fe/H] are needed for the test and calibration of theoretical models. We discovered 49 star + L dwarf wide binaries from Gaia and Pan-STARRS [Dec > -30degrees], which are ideal benchmark BDs. Two primaries are white dwarfs, four are members of moving groups, ten F-G dwarf primaries are bright enough to have precise [Fe/H] measurement from Gaia DR4. Eight primaries have LAMOST optical spectra. We also obtained optical spectra for three primaries with SOAR/Goodman. Our team also submitted a proposal to the GTC to obtained optical spectra for these L dwarfs. Three L dwarf candidates have spectra from SDSS, X-sooter, SpeX, which confirmed their spectral types. We propose to observe these L dwarf secondaries and 16 late-M dwarf primaries with SpeX, for spectral classification, model fitting, and radial velocity measurement. With such a well constrained benchmark BD sample, we can test and calibrate the latest ultra-cool atmospheric models, which is crucial for the characterisations of large sample of new distant BDs that will be discovered with Euclid an LSST, and more and more direct imaged gaseous exoplanets. PROGRAM_ABSTRACT_END