The outer atmospheres of cool M dwarfs
Final Report Abstract
In this project a set of one-dimensional parametrized chromosphere models for a stellar sample of M-type stars in the effective temperature range 3500 ± 50 K was calculated and compared to actual CARMENES observations. The synthetic spectra of single models have turned out to be able to represent inactive stars in the lines of Na I D , Hα, and a Ca II IRT line simultaneously, while a linear combination of at least two models is needed to simultaneously model the chromospheric lines of active stars. We could further show that this two-component fit approach can be interpreted as a filling factor, in the sense that the enhanced activity originates only in parts of the stellar surface similar to the Sun. While the chosen temperature structures are ad hoc, the shape of the temperature structure of the models representing the inactive stars is actually comparable to the VAL C model for the Sun, i.e., a steep temperature gradient in the lower chromosphere is followed by a plateau-like structure in the upper chromosphere. Our best-fit inactive models resemble this general structure of the VAL C model, but the temperatures and column mass densities differ, of course. As far as active stars are concerned, the temperature structure rather indicates a steeper temperature gradient in the upper chromosphere than a plateau-like structure. We then used these models to consider the predicted He I 10830 Å line and specifically considered inactive models shifted in column mass density to obtain denser chromospheres with corresponding higher activity levels. Going from low to high activity levels, our models predict that the He I 10830 Å line first goes into absorption and then reaches a maximum depth, after which fill-in sets in. Our models reproduce the point of maximum He I 10830 Å line at around absorption at ∼ 300 mÅ. The Hα line shows the same qualitative behavior, but reaches the turning point at far lower activity levels. In contrast, the Na I D2 and the bluest Ca II IRT lines only show fill-in with increasing activity levels. Furthermore, the He I D3 line never shows absorption in our models, but directly goes into emission with increasing activity levels. By investigating the He I IR line as a function of the EUV radiation field arising from the models themselves, we find that the most inactive models are highly sensitive to changesin EUV radiation, but with further increases of the EUV radiation, the line absorption tends to saturate. Artificially suppressing EUV emission and thus the PR mechanism altogether, collisional excitation was shown to gain in importance for the formation of the He I 10830 Å line as activity levels increase beyond the point of maximum He I 10830 Å line absorption. The in-vestigation of chromosphere models showed that the PR mechanism and the collisional excitation both contribute to the He I 10830 Å line formation, with the PR mechanism dominating the low-activity regime. This is in line with former results on the He I 10830 Å line formation in the Sun. The whole stellar CARMENES sample shows the He I IR line in absorption. Our current study demonstrates that one-dimensional PHOENIX model atmospheres with a parameterized temperature structure does reproduce the observed behavior of the He I 10830 Å line for stars in the spectral range M2/3 V stars. Its strong response to EUV irradiation in the low-activity regime makes the He I 10830 Å line a promising proxy of this otherwise inaccessible wavelength range. The comparably weak response of the line to changes in the EUV radiation field that start already at moderate activity levels is favorable for planetary transmission spectroscopy of active stars, where temporally constant reference spectra are crucial for success.
Publications
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(2019): "The CARMENES search for exoplanets around M dwarfs. The He I triplet at 10830 Å across the M dwarf sequence", Astronomy and Astrophysics, Volume 632, id. A24
Fuhrmeister et al.
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(2020): "The CARMENES search for exoplanets around M dwarfs", Astronomy and Astrophysics, Volume 623, id. A136
Hintz, D. et al.
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(2020): "The CARMENES search for exoplanets around M dwarfs", Astronomy and Astrophysics, Volume 638, id. A115
Hintz, D. et al.
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(2020): "The CARMENES search for exoplanets around M dwarfs. Variability of the He I line at 10 830", Astronomy and Astrophysics, Volume 640, id. A52
Fuhrmeister et al.
