Dynamical Masses and Stellar Evolutionary Model Predictions of M-Stars

Kavli Affiliate: Gregory J. Herczeg

| First 5 Authors: Jamila Pegues, Ian Czekala, Sean M. Andrews, Karin I. Öberg, Gregory J. Herczeg

| Summary:

In this era of Gaia and ALMA, dynamical stellar mass measurements provide
benchmarks that are independent of observations of stellar characteristics and
their uncertainties. These benchmarks can then be used to validate and improve
stellar evolutionary models, which can lead to both imprecise and inaccurate
mass predictions for pre-main-sequence, low-mass stars. We present the
dynamical stellar masses derived from disks around three M-stars (FP Tau,
J0432+1827, and J1100-7619) using ALMA observations of $^{12}$CO (J=2–1) and
$^{13}$CO (J=2–1) emission. These are the first dynamical stellar mass
measurements for J0432+1827 and J1100-7619 and the most precise measurement for
FP Tau. Fiducial stellar evolutionary model tracks, which do not include any
treatment of magnetic activity, agree with the dynamical measurement of
J0432+1827 but underpredict the mass by $sim$60% for FP Tau and $sim$80%
for J1100-7619. Possible explanations for the underpredictions include
inaccurate assumptions of stellar effective temperature, undetected binarity
for J1100-7619, and that fiducial stellar evolutionary models are not complex
enough to represent these stars. In the former case, the stellar effective
temperatures would need to be increased by $sim$40K to $sim$340K to reconcile
the fiducial model predictions with the dynamically-measured masses. In the
latter case, we show that the dynamical masses can be reproduced using results
from stellar evolutionary models with starspots, which incorporate fractional
starspot coverage to represent the manifestation of magnetic activity. Folding
in low-mass M-stars from the literature and assuming that the stellar effective
temperatures are imprecise but accurate, we find tentative evidence of a
relationship between fractional starspot coverage and observed effective
temperature for these young, cool stars.

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