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Register a new userThe theoretical instability strip of M dwarf stars
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Cristina Rodriguez-Lopez
Publication date
2013
fields
Physics
and research's language is
English
Authors
C. Rodriguez-Lopez
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The overstability of the fundamental radial mode in M dwarf models was theoretically predicted by Rodriguez-Lopez et al. (2012). The periods were found to be in the ranges ~25-40 min and ~4-8 h, depending on stellar age and excitation mechanism. We have extended our initial M dwarf model grid in mass, metallicity, and mixing length parameter. We have also considered models with boundary conditions from PHOENIX NextGen atmospheres to test their influence on the pulsation spectra. We find instability of non-radial modes with radial orders up to k=3, degree l=0-3, including p and g modes, with the period range extending from 20 min up to 11 h. Furthermore, we find theoretical evidence of the potential of M dwarfs as solar-like oscillators.
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By using a non-local and time-dependent convection theory, we have calculated radial and low-degree non-radial oscillations for stellar evolutionary models with $M=1.4$--3.0,$mathrm{M}_odot$. The results of our study predict theoretical instability strips for $delta$ Scuti and $gamma$ Doradus stars, which overlap with each other. The strip of $gamma$ Doradus is slightly redder in colour than that of $delta$ Scuti. We have paid great attention to the excitation and stabilization mechanisms for these two types of oscillations, and we conclude that radiative $kappa$ mechanism plays a major role in the excitation of warm $delta$ Scuti and $gamma$ Doradus stars, while the coupling between convection and oscillations is responsible for excitation and stabilization in cool stars. Generally speaking, turbulent pressure is an excitation of oscillations, especially in cool $delta$ Scuti and $gamma$ Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermal convection, on the other hand, is a damping mechanism against oscillations that actually plays the major role in giving rise to the red edge of the instability strip. Our study shows that oscillations of $delta$ Scuti and $gamma$ Doradus stars are both due to the combination of $kappa$ mechanism and the coupling between convection and oscillations, and they belong to the same class of variables at the low-luminosity part of the Cepheid instability strip. Within the $delta$ Scuti--$gamma$ Doradus instability strip, most of the pulsating variables are very likely hybrids that are excited in both p and g modes.
The analysis of 14 periodograms of EZ Lyn for the data spaced over 565 d in 2012--2014 (2-3.5 yr after 2010 outburst) yielded the existence of the stable signals around 100 c/d and three signals around 310 c/d, 338 c/d and 368 c/d (the corresponding periods are 864 s, 279 s, 256 s and 235 s). We interpret them as independent non-radial pulsations of the white dwarf in EZ Lyn, but a possibility that a linear combination of frequency at 100 c/d and harmonic of orbital period could produce the frequency at 368 c/d also cannot be excluded. The signal at 100 c/d was detected during the first stay in the instability strip as a transient one. The period at 338 c/d, is a known non-radial pulsation EZ Lyn entered the instability strip after the 2010 outburst. We detected the signals around 310 c/d and 368 c/d for the first time. We applied the two-dimensional least absolute shrinkage and selection operator (Lasso) analysis for the first time to explore the behavior of these signals on the scale of hours for nightly runs of observations having duration of 6-12 hr. The Lasso analysis revealed the simultaneous existence of all three frequencies (310 c/d, 338 c/d and 368 c/d) for majority of nights of observations, but with variable amplitudes and variable drifts of frequencies by 2-6 percents on a time scale of ~5-7 hr. The largest drift we detected corresponded to 17.5 s in period in ~5 hours.
M dwarf stars are the most common stars in the Galaxy, dominating the population of the Galaxy by numbers at faint magnitudes. Precise and accurate stellar parameters for M dwarfs are of crucial importance for many studies. However, the atmospheric parameters of M dwarf stars are difficult to be determined. In this paper, we present a catalog of the spectroscopic stellar parameters ($T_{eff}$ and [M/H]) of $sim$ 300,000 M dwarf stars observed by both LAMOST and Gaia using Stellar Label Machine (SLAM). We train a SLAM model using LAMOST spectra with APOGEE Data Release 16 (DR16) labels with $2800 lt T_{eff} lt 4500$K and $-2 lt [M/H] lt 0.5$ dex. The SLAM $T_{eff}$ is in agreement to within $sim 50$K compared to the previous study determined by APOGEE observation, and SLAM [M/H] agree within 0.12 dex compared to the APOGEE observation. We also set up a SLAM model trained by BT-Settl atmospheric model, with random uncertainties (in cross-validation) to 60K and agree within $sim 90$K compared to previous study.
M dwarf stars are currently the main targets in searches for potentially habitable planets. However, their winds have been suggested to be harmful to planetary atmospheres. Here, in order to better understand the winds of M dwarfs and also infer their physical properties, we perform a one-dimensional magnetohydrodynamic parametric study of winds of M dwarfs that are heated by dissipation of Alfven waves. These waves are triggered by sub-surface convective motions and propagate along magnetic field lines. Here, we vary the magnetic field strength and density at the wind base (chromosphere), while keeping the same relative wave amplitude ($0.1 B_0$) and dissipation lenghtscale. We find that our winds very quickly reach isothermal temperatures with mass-loss rates proportional to base density square. We compare our results with Parker wind models and find that, in the high-beta regime, both models agree. However, in the low-beta regime, the Parker wind underestimates the terminal velocity by around one order of magnitude and mass-loss rate by several orders of magnitude. We also find that M dwarfs could have chromospheres extending to 18% to 180% of the stellar radius. We apply our model to the planet-hosting star GJ 436 and find, from X-ray observational constraints, $dot{M}<7.6times 10^{-15},M_{odot}~text{yr}^{-1}$. This is in agreement with values derived from the Lyman-alpha transit of GJ 436b, indicating that spectroscopic planetary transits could be used as a way to study stellar wind properties.
M-dwarf stars below a certain mass are convective from their cores to their photospheres. These fully convective objects are extremely numerous, very magnetically active, and the likely hosts of many exoplanets. Here we study, for the first time, dynamo action in simulations of stratified, rotating fully convective M-dwarf stars. Importantly, we use new techniques to capture the correct full ball geometry down to the center of the star. We find surprising dynamo states in these systems, with the global-scale mean fields confined strongly to a single hemisphere, in contrast to prior stellar dynamo solutions. These hemispheric-dynamo stars are likely to have profoundly different interactions with their surroundings, with important implications for exoplanet habitability and stellar spindown.
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