Understanding Fundamental Properties and Atmospheric Features of Subdwarfs via a Case Study of SDSS J125637.13-022452.4

We present the distance-calibrated spectral energy distribution (SED) of the sdL3.5 subdwarf SDSS J125637.13-022452.4 (J1256-0224) using its Gaia parallax and its resultant bolometric luminosity and semi-empirical fundamental parameters, as well as updated UVW velocities. The SED of J1256-0224 is compared to field-age and low-gravity dwarfs of the same effective temperature (Teff) and bolometric luminosity. In the former comparison, we find that the SED of J1256-0224 is brighter than the field source in the optical, but dims in comparison beyond the J-band, where it becomes fainter than the field from the H through W2 bands. Compared to the young source, its fainter at all wavelengths. We conclude that J1256-0224 is depleted of condensates compared to both objects. A near infrared band-by-band analysis of the spectral features of J1256-0224 is done and is compared to the equivalent Teff sample. From this analysis, we find a peculiar behavior of the J-band K I doublets whereby the 1.17 micron doublet is stronger than the field or young source as expected, while the 1.25 micron doublet shows indications of low gravity. In examining a sample of 4 other subdwarfs with comparable data we confirm this trend across different subtypes indicating that the 1.25 micron doublet is a poor indicator of gravity for low metallicity objects. In the K-band analysis of J1256-0224 we detect the 2.29 micron CO line of J1256-0224, previously unseen in the low-resolution SpeX data. We also present fundamental parameters using Gaia parallaxes for 9 additional subdwarfs with spectral types M7 - L7 for comparison. The 10 subdwarfs are placed in a temperature sequence and we find a poor linear correlation with spectral type. We present polynomial relations for absolute magnitude in JHKW1W2, effective temperature, and bolometric luminosity versus spectral type for subdwarfs.