Abstract
We study the rotation–activity correlations (RACs) in a sample of stars from spectral type dK4 to dM4. We study RACs using chromospheric data and coronal data. We study the Ca
ii
line ...surface fluxes-
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
as a function of the Rossby number
R
0
. We find (i) for dK4 stars,
as a function of
R
0
agrees well with previous results for F-G-K stars and (ii) in dK6, dM2, dM3, and dM4 stars, at a given
R
0
, the values of
lie at 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 transition to complete convection. We also show that the ratio of coronal heating to chromospheric heating
L
X
/
L
HK
increases by a factor of 100 between dK4 and dM4 stars.
The reliable determination of rotation-activity correlations (RACs) depends on precise measurements of the following stellar parameters: T sub(eff), parallax, radius, metallicity, and rotational ...speed v sin i. In this paper, our goal is to focus on the determination of these parameters for a sample of K and M dwarfs. In a future paper (Paper II), we will combine our rotational data with activity data in order to construct RACs. Here, we report on a determination of effective temperatures based on the (R-I)C color from the calibrations of Mann et al. and Kenyon & Hartmann for four samples of late-K, dM2, dM3, and dM4 stars. We also determine stellar parameters (T sub(eff), log(g), and M/H) using the principal component analysis-based inversion technique for a sample of 105 late-K dwarfs. We compile all effective temperatures from the literature for this sample. We determine empirical radius-M/H correlations in our stellar samples. This allows us to propose new effective temperatures, stellar radii, and metallicities for a large sample of 612 late-K and M dwarfs. Our mean radii agree well with those of Boyajian et al. We analyze HARPS and SOPHIE spectra of 105 late-K dwarfs, and we have detected v sin i in 92 stars. In combination with our previous v sin i measurements in M and K dwarfs, we now derive P/sin i measures for a sample of 418 K and M dwarfs. We investigate the distributions of P/sin i, and we show that they are different from one spectral subtype to another at a 99.9% confidence level.
Abstract
Empirical correlations between stellar parameters such as rotation or radius and magnetic activity diagnostics require estimates of the effective temperatures and the stellar radii. The aim ...of this study is to propose simple methods that can be applied to large samples of stars in order to derive estimates of the stellar parameters. Good empirical correlations between red/infrared colors (e.g., (
R
–
I
)
C
) and effective temperatures have been well established for a long time. The more recent (
R
–
I
)
C
color–
T
eff
correlation using the data of Mann et al. (hereafter M15) and Boyajian et al. (hereafter B12) shows that this color can be applied as a temperature estimate for large samples of stars. We find that the mean scatter in
T
eff
relative to the (
R
–
I
)
C
–
T
eff
relationship of B12 and M15 data is only ±3
σ
= 44.6 K for K dwarfs and ±3
σ
= 39.4 K for M dwarfs. These figures are small and show that the (
R
–
I
)
C
color can be used as a first-guess effective temperature estimator for K and M dwarfs. We derive effective temperatures for about 1910 K and M dwarfs using the calibration of (
R
–
I
)
C
color–
T
eff
from B12 and M15 data. We also compiled
T
eff
and metallicity measurements available in the literature using the VizieR database. We determine
T
eff
for 441 stars with previously unknown effective temperatures. We also identified 21 new spectroscopic binaries and one triple system from our high-resolution spectra.
PolarBase is an evolving database that contains all stellar data collected with the ESPaDOnS and NARVAL high-resolution spectropolarimeters, in their reduced form, as soon as they become public. As ...of early 2014, observations of 2000 stellar objects throughout the Hertzsprung-Russell diagram are available. Intensity spectra are available for all targets, and the majority of the observations also include simultaneous spectra in circular or linear polarization, with the majority of the polarimetric measurements being performed only in circularly polarized light (Stokes V). Observations are associated with a cross-correlation pseudoline profile in all available Stokes parameters, greatly increasing the detectability of weak polarized signatures. Stokes V signatures are detected for more than 300 stars of all masses and evolutionary stages, and linear polarization is detected in 35 targets. The detection rate in Stokes V is found to be anticorrelated with the stellar effective temperature. This unique set of Zeeman detections offers the first opportunity to run homogeneous magnetometry studies throughout the H-R diagram. The Web interface of PolarBase is available at http://polarbase.irap.omp.eu.
Context. We present an automated procedure that simultaneously derives the effective temperature Teff, surface gravity log g, metallicity Fe/H, and equatorial projected rotational velocity vsini for ...“normal” A and Am stars. The procedure is based on the principal component analysis (PCA) inversion method, which we published in a recent paper . Aims. A sample of 322 high-resolution spectra of F0-B9 stars, retrieved from the Polarbase, SOPHIE, and ELODIE databases, were used to test this technique with real data. We selected the spectral region from 4400−5000 Å as it contains many metallic lines and the Balmer Hβ line. Methods. Using three data sets at resolving powers of R = 42 000, 65 000 and 76 000, about ~6.6 × 106 synthetic spectra were calculated to build a large learning database. The online power iteration algorithm was applied to these learning data sets to estimate the principal components (PC). The projection of spectra onto the few PCs offered an efficient comparison metric in a low-dimensional space. The spectra of the well-known A0- and A1-type stars, Vega and Sirius A, were used as control spectra in the three databases. Spectra of other well-known A-type stars were also employed to characterize the accuracy of the inversion technique. Results. We inverted all of the observational spectra and derived the atmospheric parameters. After removal of a few outliers, the PCA-inversion method appeared to be very efficient in determining Teff, Fe/H, and vsini for A/Am stars. The derived parameters agree very well with previous determinations. Using a statistical approach, deviations of around 150 K, 0.35 dex, 0.15 dex, and 2 km s-1 were found for Teff, log g, Fe/H, and vsini with respect to literature values for A-type stars. Conclusions. The PCA inversion proves to be a very fast, practical, and reliable tool for estimating stellar parameters of FGK and A stars and for deriving effective temperatures of M stars.
We evaluate an adaptive Gaussian quadrature integration scheme suitable for the numerical evaluation of generalized redistribution in frequency functions. The latter are indispensable ingredients for ...“full non-LTE” radiation transfer computations, assuming potential deviations of the velocity distribution of massive particles from the usual Maxwell–Boltzmann distribution. A first validation is made with computations of the usual Voigt profile.
Large-scale magnetic topologies of mid M dwarfs Morin, J.; Donati, J.-F.; Petit, P. ...
Monthly Notices of the Royal Astronomical Society,
10/2008, Letnik:
390, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present in this paper, the first results of a spectropolarimetric analysis of a small sample (∼20) of active stars ranging from spectral type M0 to M8, which are either fully convective or possess ...a very small radiative core. This study aims at providing new constraints on dynamo processes in fully convective stars. This paper focuses on five stars of spectral type ∼M4, i.e. with masses close to the full convection threshold (≃0.35 M⊙), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarized profiles. We find that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by ∼1 yr; we find the magnetic topologies to be globally stable on this time-scale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field.
The general context of this study is the inversion of stellar fundamental parameters from high-resolution Echelle spectra. We aim at developing a fast and reliable tool for the post-processing of ...spectra produced by ESPaDOnS and Narval spectropolarimeters. Our inversion tool relies on principal component analysis. It allows reducing dimensionality and defining a specific metric for the search of nearest neighbours between an observed spectrum and a set of observed spectra taken from the Elodie stellar library. Effective temperature, surface gravity, total metallicity, and projected rotational velocity are derived. Various tests presented in this study that were based solely on information coming from a spectral band centred on the Mg i b-triplet and had spectra from FGK stars are very promising.
From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD ...190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars. We reconstruct the large-scale magnetic geometry of the targets as a low-order (ℓ < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component.
Aims. We aim at a highly sensitive search for weak magnetic fields in main sequence stars of intermediate mass, by scanning classes of stars with no previously reported magnetic members. After ...detecting a weak magnetic field on the normal, rapidly rotating A-type star Vega, we concentrate here on the bright star Sirius A, taken as a prototypical, chemically peculiar, moderately rotating Am star. Methods. We employed the NARVAL and ESPaDOnS high-resolution spectropolarimeters to collect 442 circularly polarized spectra, complemented by 60 linearly polarized spectra. Using a list of about 1100 photospheric spectral lines, we computed a cross correlation line profile from every spectrum, leading to a signal-to-noise ratio of up to 30 000 in the polarized profile. Results. We report the repeated detection of circularly polarized, highly asymmetric signatures in the line profiles, interpreted as Zeeman signatures of a large-scale photospheric magnetic field, with a line-of-sight component equal to 0.2 ± 0.1 G. Conclusions. This is the first polarimetric detection of a surface magnetic field on an Am star. Using rough estimates of the physical properties of the upper layers of Sirius A, we suggest that a dynamo operating in the shallow convective envelope cannot account for the field strength reported here. Together with the magnetic field of Vega, this result confirms that a new class of magnetic objects exists among non Ap/Bp intermediate-mass stars, and it may indicate that a significant fraction of tepid stars are magnetic.
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