Calibrating Ultracool Dwarfs: Optical Template Spectra, Bolometric Corrections, and chi Values

• Published in: Publications of the Astronomical Society of the Pacific 2014-07, Vol.126 (941), p.642-659
• Main Authors: Schmidt, Sarah J, West, Andrew A, Bochanski, John J, Hawley, Suzanne L, Kielty, Collin
• Format: Article
• Language: English
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/677403

We present optical template spectra, bolometric corrections, and chi values for ultracool dwarfs. The templates are based on spectra from the Sloan Digital Sky Survey (SDSS) and the Astrophysical Research Consortium 3.5 m telescope. The spectral features and overall shape of the L dwarf templates are consistent with previous spectroscopic standards and the templates have a radial velocity precision of ~10-20 km s super(-1). We calculate bolometric fluxes (accurate to 10-20%) for 101 late-M and L dwarfs from SDSS, 2MASS, and WISE photometry, SDSS spectra, and BT-Settl model spectra. We find that the 3-band and J-band bolometric corrections for late-M and L dwarfs have a strong correlation with z-J and J-KS colors, respectively. The new chi values, which can be used to convert Ha equivalent widths to activity strength, are based on spectrophotometrically calibrated SDSS spectra and the new bolometric fluxes. While the measured chi values have typical uncertainties of ~20%, ultracool dwarf models show the continuum surrounding Ha can vary by up to an order of magnitude with changing surface gravity. Our semiempirical chi values are one to two orders of magnitude larger than previous chi values for mid-to-late L dwarfs, indicating that the upper limits for Ha activity strength on the coolest L dwarfs have been underestimated.