Measuring the magnetic field of young stars using iSHELL observations: BP Tau and V347 Aur

TL;DR

This study measures magnetic fields of young stars BP Tau and V347 Aur using near-infrared spectroscopy, confirming previous results for BP Tau and providing first measurements for V347 Aur, highlighting starspots' impact on stellar parameters.

Contribution

Introduces a new near-infrared spectroscopic method to measure magnetic fields in young stars, including first measurements for V347 Aur, and analyzes starspots' effects on stellar properties.

Findings

BP Tau has a magnetic field strength of approximately 2.5 kG.

V347 Aur's magnetic field strength is measured at about 1.36 kG.

Starspots significantly affect temperature and age estimates of young stars.

Abstract

While it has been suggested that there is a connection between the magnetic properties and the internal structure of young stars, there have not been enough magnetic measurements to firmly establish such a correlation at the earliest ages. Here, we contribute to this endeavor by presenting stellar parameters and magnetic field strength measurements of BP Tau and V347 Aur, both stars observed with the near-infrared spectrograph iSHELL. We first test the accuracy of our method by fitting synthetic stellar spectra to a sample of nine main and post-main-sequence stars. We report uncertainties of $\sigma_{\rm Teff}$ = 91 K in temperature and $\sigma_{\rm log(g)}$ = 0.14 in gravity. We then apply the modeling technique to BP Tau and measure a surface magnetic field strength of $\langle \rm B \rangle $ = 2.5$^{+0.15}_{-0.16}$ kG, confirming literature results. For this star, however, we obtain a much lower temperature value than previous optical studies ($\Delta \rm T \sim 400$ K) and interpret this significant temperature difference as due to the relatively higher impact of starspots at near-infrared wavelengths than at optical wavelengths. We further apply this technique to the class I protostellar source V347 Aur and measure for the first time its magnetic field strength $\langle \rm B \rangle = $ 1.36$^{+0.06}_{-0.05}$ kG and its surface gravity log(g) = 3.25$^{+0.14}_{-0.14}$. Lastly, we combine our measurements with pre-main-sequence stellar evolutionary models and illustrate the effects produced by starspots on the retrieved masses and ages of young stars.