Dynamical Masses and Stellar Evolutionary Model Predictions of M Stars

Abstract

In this era of Gaia and ALMA, dynamical stellar mass measurements, derived from spatially and spectrally resolved observations of the Keplerian rotation of circumstellar disks, 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, the latter of which can lead to both imprecise and inaccurate mass predictions for pre-main-sequence, low-mass (≤0.5 M ⊙) 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 12CO (J = 2–1) and 13CO (J = 2–1) emission. These are the first dynamical stellar mass measurements for J0432+1827 and J1100–7619 (0.192 ± 0.005 M ⊙ and 0.461 ± 0.057 M ⊙, respectively) and the most precise measurement for FP Tau (0.395 ± 0.012 M ⊙). Fiducial stellar evolutionary model tracks, which do not include any treatment of magnetic activity, agree with the dynamical stellar mass measurement of J0432+1827 but underpredict the mass by ∼60% for FP Tau and by ∼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 amounts ranging from ∼40 to ∼340 K to reconcile the fiducial stellar evolutionary 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.

Demographics of young stars and their protoplanetary disks: lessons learned on disk evolution and its connection to planet formation

“…Similarly, eclipsing binaries provide a further tests of the models (Stassun et al 2014;Rizzuto et al 2020) but can only be applied to a limited number of systems. In recent years, the availability of spectrally resolved observations of CO emission from disks with the Atacama Large Millimeter/submillimeter Array (ALMA) has also enabled the use of dynamical stellar mass estimates to test models (e.g., Czekala et al 2016;Yen et al 2018;Sheehan et al 2019;Simon et al 2017Simon et al , 2019Premnath et al 2020;Pegues et al 2021). The results from these works are still diverse, with some studies showing that a better agreement with dynamical mass estimates is reached when using magnetic models in the range M ∼ 0.4 -1 M (Simon et al 2019) or M ∼ 0.6 -1.3 M (Braun et al 2021).…”

Uncovering the radio emission of low-mass systems

“…However, there are some important discrepancies between observations and models for low-mass objects (e.g. : Pegues et al 2021) which have been attributed (among other things) to the scarcity of well-known dynamical masses of such objects (Hillenbrand et al 2004, Mathieu et al 2007, Pegues et al 2021. In this sense, we will also present here preliminary results of our VLA and EVN campaings targeted towards M dwarfs systems and UCDs, respectively.…”

Gas Disk Sizes from CO Line Observations: A Test of Angular Momentum Evolution

“…The 12 CO J = 2 − 1 emission from the FP Tau disk was recently observed in an ALMA Chemistry program with ∼ 0. 2 resolution (Pegues et al 2021). We also included the measurements for CX Tau, which was reported as the first example with very high gas-to-dust disk size ratios (R CO /R mm ∼ 4, Facchini et al 2019).…”