Observation and modelling of main-sequence star chromospheres – XV. New constraints on the dynamo mechanisms for dM1 stars

With the help of measures of rotation, radius and metallicity for a selected sample of dM1 stars (with Teff= 3460 ± 60 K), we aim to set new constraints on the dynamo mechanisms.We recover 913 high-resolution spectra for 97 different M1 dwarfs from the European Southern Observatory and Observatoire de Haute Provence data bases. We present 660 new measurements of the Ca ii resonance lines and 913 new measurements of the Hα line for dM1 stars. We also compile other measurements available in the literature. In total, we obtain 2216 measures of the Ca ii lines for 113 different dM1 stars. This represents the largest compilation of chromospheric line measurements at a single spectral type. We cross-correlate these magnetic activity indicators with various stellar parameters to set new constraints on the dynamo mechanisms and on the properties of the outer atmosphere. We find a correlation of the Ca ii line mean equivalent width with the absolute magnitude and the metallicity. We correct the Ca ii line measures from the metallicity effect and find that the surface flux in the Ca ii lines grows roughly as the power of 3.6 of the stellar radius. This corrected flux is a direct measure of magnetic activity at the chromospheric level. We find that the total magnetic activity level grows roughly as the power of 5.6 of the stellar radius. This trend is confirmed by the correlation between the Hα line and absolute magnitude and the Hα line luminosity and stellar radius: the Hα luminosity grows roughly as the volume of the star for low activity dM1 stars and as the power of roughly 5/2 of the stellar radius for dM1e stars. The advantage of the Hα line is that its formation in not dependent on metallicity. In contrast to the Ca ii line, we find no correlation between LX and the absolute magnitude. We find that LX roughly correlates with the Ca ii luminosity although the correlation is not very good. This correlation shows that LX grows as the power of 3/2 of the Ca ii luminosity, i.e. the coronal emission grows faster than the chromospheric emission. We find a correlation between the corrected Ca ii line equivalent width and P/sin i, i.e. the Ca ii surface flux grows as the power of −1.5 of the rotation period. We also find a correlation between FX, the X-ray surface flux, and P/sin i: FX∝ (P/sin i)−3.7. In other words, the coronal emission is much more dependent on the rotation period than the chromospheric emission. We find that the level of magnetic activity in dM1 stars is more dependent on the stellar radius than on rotation at the chromospheric level. We discuss the implications of these results on the models of stellar dynamos

Observation and modelling of main‐sequence star chromospheres – XVIII. Observations of the Ca ii resonance lines and Hα line for dM4 stars and dK5 stars

We use 665 high‐resolution spectra for 60 different dM4 stars and 1088 high‐resolution spectra for 97 different dK5 stars from the European Southern Observatory (ESO) and Observatoire de Haute Provence (OHP) data bases. We present 179 new measurements of the Ca ii resonance lines and 615 new measurements of the Hα line for dM4 stars. We also present 701 new measurements of the Ca ii resonance lines and 1088 new measurements of the Hα line for dK5 stars. We also compiled other measurements available in the literature.We cross‐correlate the activity diagnostics, namely the Ca ii resonance line characteristics, the Hα line characteristics and LX. We set new constraints on some empirical relationships between these parameters that are important for constraining the chromospheric modelling of these stars. We study the Ca ii line mean equivalent width (EW) as a function of absolute magnitude (Mv) for three spectral types: dK5, dM1 and dM4. We show that the magnetic activity level is rather constant with Mv for both dM4 and dK5 stars in contrast to dM1 stars for which the magnetic activity level increases with diminishing Mv. In other words, only dM1 stars show this particular dependence of the level of magnetic activity with Mv. From the correlations of the magnetic activity indices with P/sin i, we find that the Ca ii EW and LX grow increasingly faster as the spectral type increases from dK5, dM1 to dM4. The exponents in the Ca ii–P/sin i correlations are −0.80, −1.53 and −3.72 for dK5 stars, dM1 stars and dM4 stars, respectively. We also find that the X‐ray luminosity grows faster than the chromospheric Ca ii emission when the rotation rate increases. Moreover, we found that the exponent on P/sin i for both the Ca ii and LX correlations is about twice smaller for dK5 stars than for dM1 stars. Therefore, the level of magnetic activity in dK5 and dM4 stars is more dependent on rotation than on the stellar radius, the opposite result to that found for dM1 stars

Observation and modelling of main-sequence star chromospheres – XIV. Rotation of dM1 stars

We have measured v sin i for a selected sample of dM1-type stars. We give 114 measurements of v sin i for 88 different stars, and six upper detection limits. These are the first measurements of v sin i for most of the stars studied here. This represents the largest sample of v sin i measurements for M dwarfs at a given spectral type. For these measurements, we used four different spectrographs: HARPS (ESO), SOPHIE (OHP), ÉLODIE (OHP) and UVES (ESO). Two of these spectrographs (HARPS and SOPHIE) are particularly stable in wavelength since they were designed for exoplanet searches.We measured v sin i down to an accuracy of 0.3 km s−1 for the highest resolution spectrographs and a detection limit of about 1 km s−1. We show that this unprecedented accuracy for M dwarfs in our data set is possible because all the targets have the same spectral type. This is an advantage and it facilitates the determination of the narrowest line profiles for v sin i∼ 0. Although it is possible to derive the zero-point profiles using several spectral types at a time. These values were combined with other measurements taken from the literature. The total sample represents detected rotation for 100 stars (10 dM1e and 90 dM1 stars). We confirm our finding of Paper VII that the distribution of the projected rotation period is bimodal for dM1 stars with a much larger sample, i.e. there are two groups of stars: the fast rotators with P/sin i∼ 4.5 d and the slow rotators with P/sin i∼ 14.4 d. There is a gap between these two groups. We find that the distribution of stars as a function of P/sin i has two very abrupt cuts, below 10 d and above 18 d. There are very few stars observed out of this range 10–18 d. We also observe that the distribution increases slightly from 18 to 10 d. We find that the M1 subdwarfs (very low metallicity dwarfs) rotate with an average period of P/sin i∼ 7.2 d, which is about twice faster as the main group of normal M1 dwarfs. We also find a correlation for P/sin i to decrease with stellar radius among dM1e stars. Such a trend is also observed in dM1 stars. We also derive metallicity and radius for all our target stars using the same method as in Paper VII. We notably found that 11 of our target stars are subdwarfs with metallicities below −0.5 dex

Observation and modelling of main-sequence star chromospheres – X. Radiative budgets on Gl 867A and AU Mic (dM1e), and a two-component model chromosphere for Gl 205 (dM1)

We report on high-resolution observations of two dM1 stars: Gl 867A, an active dM1e star, and Gl 205, a less active dM1 star. The wavelength coverage is from 3890 to 6820 Å with a resolving power of about 45 000. The difference spectrum of these two stars allows us to make a survey of spectral lines sensitive to magnetic activity. We chose these two stars because, to within measurement errors, they have very close properties: Gl 867A has R= 0.726 R⊙, [M/H]= 0.080 dex and Teff= 3416 K, and Gl 205 has R= 0.758 R⊙, [M/H]= 0.101 dex and Teff= 3493 K.We find that besides traditional chromospheric lines, many photospheric lines are ‘filled-in’ in the active star spectrum. These differences are, most of the time, weak in absolute fluxes but can be large in terms of differences in the spectral-line equivalent widths. We calculate the differences in surface fluxes between these two stars for many spectral lines. We derive the radiative budgets for two dM1e stars: Gl 867A and AU Mic. We show that the sum of the numerous spectral lines represents a significant fraction of the radiative cooling of the outer atmosphere. We also re-investigate the cooling from the continuum from the visible to the extreme ultraviolet; we find that earlier predictions of the calculations of Houdebine et al. (Paper V) are in good agreement with observations. We emphasize that if this radiative cooling is chromospheric in character, then in chromospheric model calculations, we should include the radiative losses in Ca i, Cr i, V i, Ti i and Fe i. From simple constraints, we derive model chromospheres for quiescent and active regions on Gl 205. We show that the quiescent regions have a strong absorption Hα profile. The plage regions show a filled-in intermediate activity Hα profile. We also present possible spectral line profiles of quiescent and active regions on Gl 867A

Observation and modelling of main-sequence star chromospheres – XIX. FIES and FEROS observations of dM1 stars

We present 187 high-resolution spectra for 62 different M1 dwarfs from observations obtained with the FIbre-fed Echelle Spectrograph (FIES) on the Nordic Optical Telescope (NOT) and from observations with the Fibre-fed Extended Range Echelle Spectrograph (FEROS) from the European Southern Observatory (ESO) data base. We also compiled other measurements available in the literature.We observed two stars, Gl 745A and Gl 745B, with no Ca ii line core emission and Hα line equivalent widths (EWs) of only 0.171 and 0.188 Å, respectively. We also observed another very low activity M1 dwarf, Gl 63, with an Hα line EW of only 0.199 Å. These are the lowest activity M dwarfs ever observed and are of particular interest for the non-local thermodynamic equilibrium radiative transfer modelling of M1 dwarfs. Thanks to the high signal-to-noise ratio of most of our spectra, we were able to measure the Ca ii H&K full width at half-maximum (FWHM) for most of our stars. We find good correlations between the FWHM values and the mean Ca ii line EW for dM1 stars. Then the FWHM seems to saturate for dM1e stars. Our previous models of M1 dwarfs can reproduce the FWHM for dM1e stars and the most active dM1 stars, but fail to reproduce the observations of lower activity M1 dwarfs. We believe this is due to an effect of metallicity. We also investigate the dependence of the Hα line FWHM as a function of its EW. We find that the models globally agree with the observations including subwarfs, but tend to produce too narrow profiles for dM1e stars. We re-investigate the correlation between the Ca ii line mean EW and the absolute magnitude. With our new data that notably include several M1 subdwarfs, we find a slightly different and better correlation with a slope of −0.779 instead of −0.909. We also re-investigate the variations of the Hα line EW as a function of radius and find that the EW increases continuously with increasing radius. This confirms our previous finding that the level of magnetic activity in M1 dwarfs increases with the radius. For the first time, we investigate the Wilson–Bappu correlation for a given spectral type. We find a rather linear correlation for stars of absolute magnitude greater than 9.6, but below this value the FWHM seems to saturate. In fact, we show that these Wilson–Bappu type correlations are activity–FWHM correlations and are due to the diminishing column mass of the transition region with decreasing activity level

Observation and modelling of main-sequence star chromospheres – XVI. Rotation of dK5 stars

Using two different spectrographs, High Accuracy Radial velocity Planet Search (HARPS) (European Southern Observatory) and SOPHIE (Observatoire de Haute Provence), we have measured v sin i for a sample of dK5 stars. These are the first measurements of v sin i for most of the stars studied here.We measured v sin i to an accuracy of 0.3 km s−1 and a detection limit of about 0.5–1 km s−1. All our targets have similar (R − I)c colour. This is an advantage and facilitates the determination of the narrowest line profiles for v sin i ∼ 0. In our total sample, we detected rotation for 22 stars (three dK5e and 19 dK5 stars), and we did not detect rotation in a further 22 stars. This result shows that there are many dK5 slow rotators, and many more than for dM1 stars. We also report on a newly discovered dK5e star, McC 522, which is also the fastest rotator in our sample. We determine radii and effective temperatures for all our target stars. The effective temperatures were derived using the (R − I)c colour and empirical far-red colour–effective temperature correlations, and the radii were derived from the standard formulae relating Mbol, bolometric correction and Teff. We find that the distribution of P/sin i (the projected rotation period) is rather homogeneous, i.e. the distribution of the 22 detected stars as a function of P/sin i is approximately flat and does not show any maximum, unlike for dM1 stars, a close spectral type. We find that the distribution in v sin i is bimodal, as in dM1 stars, with fast dK5e rotators and slower dK5 rotators

A Helicity-Based Method to Infer the CME Magnetic Field Magnitude in Sun and Geospace: Generalization and Extension to Sun-Like and M-Dwarf Stars and Implications for Exoplanet Habitability

Patsourakos et al. (Astrophys. J. 817, 14, 2016) and Patsourakos and Georgoulis (Astron. Astrophys. 595, A121, 2016) introduced a method to infer the axial magnetic field in flux-rope coronal mass ejections (CMEs) in the solar corona and farther away in the interplanetary medium. The method, based on the conservation principle of magnetic helicity, uses the relative magnetic helicity of the solar source region as input estimates, along with the radius and length of the corresponding CME flux rope. The method was initially applied to cylindrical force-free flux ropes, with encouraging results. We hereby extend our framework along two distinct lines. First, we generalize our formalism to several possible flux-rope configurations (linear and nonlinear force-free, non-force-free, spheromak, and torus) to investigate the dependence of the resulting CME axial magnetic field on input parameters and the employed flux-rope configuration. Second, we generalize our framework to both Sun-like and active M-dwarf stars hosting superflares. In a qualitative sense, we find that Earth may not experience severe atmosphere-eroding magnetospheric compression even for eruptive solar superflares with energies ~ 10^4 times higher than those of the largest Geostationary Operational Environmental Satellite (GOES) X-class flares currently observed. In addition, the two recently discovered exoplanets with the highest Earth-similarity index, Kepler 438b and Proxima b, seem to lie in the prohibitive zone of atmospheric erosion due to interplanetary CMEs (ICMEs), except when they possess planetary magnetic fields that are much higher than that of Earth.Comment: http://adsabs.harvard.edu/abs/2017SoPh..292...89

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-

Biosignatures from Earth-Like Planets Around M Dwarfs

Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible, near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs, AD Leo and GJ 643, and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earth's was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.Comment: 34 pages, 10 figures, Astrobiology, in pres

Progress in semi-empirical modelling of main-sequence stellar chromospheres

We present the results of a long term research programme on the outer atmospheres of main-sequence dwarfs. Combining NLTE-radiation transfer calculations with high resolution spectroscopic observations have led to significant progress in understanding chromospheric physical properties and spectral signatures. We emphasize that in order to unravel the extremely complex physics of the outer atmosphere and its energy source, magnetic field and acoustic wave dissipation, one must isolate the influence of all stellar parameters.</jats:p