Transgenic Animals

Mullan

Bercu

et al. 2017

ApJ

We study the rotation-activity correlations (RACs) in a sample stars from spectral type dK4 to dM4. We study RACs using chromospheric data and coronal data. We study the Ca ii line surface fluxes-P/ sin i RACs. We fit the RACs with linear homoscedastic and heteroscedastic regression models. We find that these RACs differ substantially from one spectral sub-type to another. For dM3 and dM4 stars, we find that the RACs cannot be described by a simple model, but instead that there may exist two distinct RAC behaviors for the low activity and the high activity stellar sub-samples respectively. Although these results are preliminary and will need confirmation, the data suggest that these distinct RACs may be associated with different dynamo regimes.We also study R'HK as a function of the Rossby number R0. We find that: (i) For dK4 stars, we confirm R'HK as a function of R0 agrees well with previous results for F-G-K stars. (ii) In dK6, dM2, dM3 and dM4 stars, we find that, at a given R0, the values of R'HK lie a factor of 3, 10, 20 and 90 respectively below the F-G-K RAC. Our results suggest a significant decrease in the efficiency of the dynamo mechanism(s) as regards chromospheric heating before and at dM3, i.e. before and at the TTCC.We also show that the ratio of coronal heating to chromospheric heating LX /LHK increases by a factor of 100 between dK4 and dM4 stars.

Section: Dynamos: Unsaturated and Saturatedsupporting

confidence: 58%

The Rotation–activity Correlations in K and M Dwarfs. II. New Constraints on the Dynamo Mechanisms in Late-K and M Dwarfs before and at the Transition to Complete Convection^∗

Mullan

Bercu

et al. 2017

ApJ

“…Finally, we combine our present measurements with the measurements of rotation found for dK5 stars by Houdebine (2011b, hereafter Paper XVI) and for dM4 stars by Houdebine (2012) in order to constrain the rotation–activity relationships at these spectral types. Rotation has been studied mostly for main‐sequence fast rotators (e.g.…”

Section: Introductionmentioning

confidence: 93%

“…Here, we also have the advantage that we know the rotational broadening for 19 slow rotator dK5 stars (Paper XVI) and 10 slow rotator dM4 stars (Houdebine 2012, hereafter Paper XVII). Our rotation periods P /sin i were inferred from the spectral line rotational broadening v sin i (see references above).…”

Section: Magnetic Activity and Rotationmentioning

confidence: 99%

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★

Monthly Notices of the Royal Astronomical Society

2012

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.

“…In previous studies (e.g. Houdebine & Stempels 1997Paper VI, Houdebine 2008 Paper VII, Papers XV, XVIII) we have chosen to isolate samples of stars according to their effective temperatures, i.e. based on photospheric conditions.…”

Section: Selection Of Our Sample Starsmentioning

confidence: 99%

Dynamics of Rotation in M Dwarfs: Indications for a Change in the Dynamo Regime in Stars at the Onset of Complete Convection

Mullan

2015

ApJ

We have measured v sin i with high precision for a sample of dM3 stars (86 targets). We detected rotation in 82 stars (73 dM3 stars, and 9 dM3e stars). We compare our measurements of v sin i for all the stars in our dM0, dM2, dM3 and dM4 samples to those from other authors. We find a good agreement down to v sin i values of less than 1 km s −1 . The mean of the differences between measurements is only 0.42 km s −1 .We find that the distribution of P/sini for our dM3 stars is different from the distribution of P/sini among our samples of dM2 and dM4 stars. The mean rotation rate for the dM3 stars (excluding dM3e and sdM3 stars) is significantly slower (25.8 days) than for dM2 (14.4 days) and dM4 stars (11.4 days). Analogous behavior also emerges among the faster rotators (dMe stars): we find that a longer rotation period also occurs at spectral sub-type dM3e. Our data suggest that, as regards the rotational properties of lower main sequence stars, spectral sub-type dM3 stands out as exhibiting unusual slow rotation compared to that of adjoining sub-types. Our data lead us to suggest that the unusual rotational properties of M3 dwarfs may represent a signature of the transition to complete convection (TTCC).