iSHELL $K$-band Survey of Class I and Flat Spectrum Sources: Magnetic field measurements in the protostellar phase

TL;DR

This study measures magnetic fields in early-stage protostars (Class I and Flat Spectrum sources) using $K$-band spectroscopy, revealing strong magnetic fields similar to more evolved stars, and providing insights into their physical properties and evolution.

Contribution

First magnetic field strength survey of Class I and FS protostars using $K$-band observations, comparing their properties to Class II sources and inferring their evolutionary status.

Findings

Average magnetic field strength ~2.0 kG in Class I/FS sources

Class I/FS sources are younger (~0.6 Myr) than Class II (~3 Myr)

Class I/FS sources host strong magnetic fields, suggesting magnetospheric accretion

Abstract

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$\gamma$, 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 $\langle B \rangle=2.0\pm0.15$ kG is consistent with the average magnetic field strength of Class II sources $\langle B \rangle=1.8\pm0.15$ kG, and b) the average gravity of Class I and FS objects $\log{g}=3.43\pm0.07$ is lower than the average gravity of Class II sources $\log{g}=3.75\pm0.04$, 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 $\sim$0.6 Myr, and are, as a group, younger than the Class II stars with a median age of $\sim$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.