iSHELL K-band Survey of Class I and Flat Spectrum Sources: Magnetic Field Measurements in the Protostellar Phase

We perform the first magnetic field strength survey of Class I and Flat Spectrum (FS) sources using K -band observations with iSHELL. We obtained new observations of 42 Class I and FS sources and additionally included 10 sources from the archive. We detect photospheric lines in 44 of the sources, in addition to Br γ , H 2 , and CO emission in several objects. We model the photospheric absorption lines of 32 Class I and FS sources and measure their magnetic field strengths, K -band temperatures, gravities, projected rotational velocities, and infrared veiling values. We put the physical properties of Class I and FS sources in context by comparing them to the values derived for a sample of Class II sources. We find that (a) the average magnetic field strength of Class I and FS sources 〈 B 〉 = 2.0 ± 0.15 kG is consistent with the average magnetic field strength of Class II sources 〈 B 〉 = 1.8 ± 0.15 kG, and (b) the average gravity of Class I and FS objects log g = 3.43 ± 0.07 is lower than the average gravity of Class II sources log g = 3.75 ± 0.04 , although there is significant overlap between both gravity distributions. Furthermore, using stellar evolutionary models, we deduce that Class I and FS sources have a median age of ∼0.6 Myr, and are, as a group, younger than the Class II stars with a median age of ∼3 Myr. Our results confirm that Class I and FS sources host strong magnetic fields on their photospheres. Thus, it is likely that these sources accrete disk material through a magnetosphere similar to the more evolved T Tauri stars.