Calibrating Ultracool Dwarfs

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

Authors:

Schmidt et al

Abstract:

We present optical template spectra, bolometric corrections, and χ 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−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 z- 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 χ values, which can be used to convert Hα equivalent widths to activity strength, are based on spectrophotometrically calibrated SDSS spectra and the new bolometric fluxes. While the measured χ values have typical uncertainties of ∼20\%, ultracool dwarf models show the continuum surrounding Hα can vary by up to an order of magnitude with changing surface gravity. Our semi-empirical χ values are one to two orders of magnitude larger than previous χ values for mid- to late-L dwarfs, indicating that the upper limits for Hα activity strength on the coolest L dwarfs have been underestimated.