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 – 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 – 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

2MASS 1315-2649: A High Proper Motion L Dwarf with Strong H-alpha Emission

In Hall (2002) I reported that 2MASSI J1315309-264951 is an L dwarf with strong H-alpha emission. Two spectroscopic epochs appeared to show that the H-alpha was variable, decreasing from 121 Angstroms to 25 Angstroms EW, which I interpreted as a flare during the first observation. Gizis (2002) independently discovered this object, and his intermediate spectroscopic epoch shows H-alpha with 97 Angstroms EW. A new fourth epoch of spectroscopy again shows a very large H-alpha EW (124 Angstroms), confirming this object to be a persistent, strong H-alpha emitter. Whether the H-alpha is steady (like 2MASS 1237+6526) or from continuous strong flaring (like PC0025+0447) remains unclear. Imaging confirms that 2MASS 1315-2649 has a high proper motion (0.71"/year), corresponding to a transverse velocity of ~76 km/s at its distance of ~23 pc. Thus 2MASS 1315-2649 is consistent with being >~2 Gyr old and therefore relatively massive. If that is so, the correlation of H-alph activity with mass found by Gizis et al. (2000) would seem to support the continuous strong flaring scenario, though it does not rule out a brown dwarf binary accretion scenario.Comment: 2 pages, ApJL accepte

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

Principal component analysis-based inversion of effective temperatures for late-type stars

We show how the range of application of the principal component analysis-based inversion method of Paletou et al. (2015) can be extended to late-type stars data. Besides being an extension of its original application domain, for FGK stars, we also used synthetic spectra for our learning database. We discuss our results on effective temperatures against previous evaluations made available from Vizier and Simbad services at CDS.Comment: Accepted for publication in A&

\'Etale homotopy equivalence of rational points on algebraic varieties

It is possible to talk about the \'etale homotopy equivalence of rational points on algebraic varieties by using a relative version of the \'etale homotopy type. We show that over $p$-adic fields rational points are homotopy equivalent in this sense if and only if they are \'etale-Brauer equivalent. We also show that over the real field rational points on projective varieties are \'etale homotopy equivalent if and only if they are in the same connected component. We also study this equivalence relation over number fields and prove that in this case it is finer than the other two equivalence relations for certain generalised Ch\^atelet surfaces.Comment: New title, rewritten introduction, 48 pages. To appear in Algebra & Number Theor

Multiwavelength observations of a giant flare on CN Leonis I. The chromosphere as seen in the optical spectra

Flares on dM stars contain plasmas at very different temperatures and thus affect a wide wavelength range in the electromagnetic spectrum. While the coronal properties of flares are studied best in X-rays, the chromosphere of the star is observed best in the optical and ultraviolet ranges. Therefore, multiwavelength observations are essential to study flare properties throughout the atmosphere of a star. We analysed simultaneous observations with UVES/VLT and XMM-Newton of the active M5.5 dwarf CN Leo (Gl 406) exhibiting a major flare. The optical data cover the wavelength range from 3000 to 10000 Angstrom. From our optical data, we find an enormous wealth of chromospheric emission lines occurring throughout the spectrum. We identify a total of 1143 emission lines, out of which 154 are located in the red arm, increasing the number of observed emission lines in this red wavelength range by about a factor of 10. Here we present an emission line list and a spectral atlas. We also find line asymmetries for H I, He I, and Ca II lines. For the last, this is the first observation of asymmetries due to a stellar flare. During the flare onset, there is additional flux found in the blue wing, while in the decay phase, additional flux is found in the red wing. We interpret both features as caused by mass motions. In addition to the lines, the flare manifests itself in the enhancement of the continuum throughout the whole spectrum, inverting the normal slope for the net flare spectrum.Comment: 15 pages, accepted by A&