ABSTRACT
Flares and coronal mass ejections (CMEs) are very powerful events in which energetic radiation and particles are ejected within a short time. These events thus can strongly affect planets ...that orbit these stars. This is particularly relevant for planets of M-stars, because these stars stay active for a long time during their evolution and yet potentially habitable planets orbit at short distance. Unfortunately, not much is known about the relation between flares and CMEs in M-stars as only very few CMEs have so far been observed in M-stars. In order to learn more about flares and CMEs on M-stars, we monitored the active M-star EV Lac spectroscopically at high resolution. We find 27 flares with energies between 1.6 × 1031 and 1.4 × 1032 erg in $\rm H\alpha$ during 127 h of spectroscopic monitoring and 49 flares with energies between 6.3 × 1031 and 1.1 × 1033 erg during the 457 h of Transiting Exoplanet Survey Satellite (TESS) observation. Statistical analysis shows that the ratio of the continuum flux in the TESS band to the energy emitted in $\rm H\alpha$ is 10.408 ± 0.026. Analysis of the spectra shows an increase in the flux of the He ii 4686 Å line during the impulsive phase of some flares. In three large flares, we detect a continuum source with a temperature between 6900 and 23 000 K. In none of the flares we find a clear CME event indicating that these must be very rare in active M-stars. However, in one relatively weak event, we found an asymmetry in the Balmer lines of ${\sim}220\, \rm km\, s^{-1}$, which we interpret as a signature of an erupting filament.
Context.
Flares and coronal mass ejections (CMEs) are important for the evolution of the atmospheres of planets and their potential habitability, particularly for planets orbiting M stars at a ...distance <0.4 AU. Detections of CMEs on these stars have been sparse, and previous studies have therefore modelled their occurrence frequency by scaling up solar relations. However, because the topology and strength of the magnetic fields on M stars is different from that of the Sun, it is not obvious that this approach works well.
Aims.
We used a large number of high-resolution spectra to study flares, CMEs, and their dynamics of the active M dwarf star AD Leo. The results can then be used as reference for other M dwarfs.
Methods.
We obtained more than 2000 high-resolution spectra (
R
~ 35 000) of the highly active M dwarf AD Leo, which is viewed nearly pole on. Using these data, we studied the behaviour of the spectral lines H
α
, H
β
, and He
I
5876 in detail and investigated asymmetric features that might be Doppler signatures of CMEs.
Results.
We detected numerous flares. The largest flare emitted 8.32 × 10
31
erg in H
β
and 2.12 × 10
32
erg in H
α
. Although the spectral lines in this and other events showed a significant blue asymmetry, the velocities associated with it are far below the escape velocity.
Conclusions.
Although AD Leo shows a high level of flare activity, the number of CMEs is relatively low. It is thus not appropriate to use the same flare-to-CME relation for M dwarfs as for the Sun.
We present a search for stellar activity (flares and mass ejections) in a sample of 28 stars in the young open cluster Blanco-1. We use optical spectra obtained with European Southern Observatory's ...Visible Multi-Object Spectrograph installed on the Very Large Telescope. From the total observing time of ∼5 h, we find four Hα flares but no distinct indication of coronal mass ejections (CMEs) on the investigated dK–dM stars. Two flares show ‘dips’ in their light curves right before their impulsive phases which are similar to previous discoveries in photometric light curves of active dMe stars. We estimate an upper limit of <4 CMEs per day per star and discuss this result with respect to a empirical estimation of the CME rate of main-sequence stars. We find that we should have detected at least one CME per star with a mass of ≤ 3 × 1017 g depending on the star's X-ray luminosity, but the estimated Hα fluxes associated with these masses are below the detection limit of our observations. We conclude that the parameter which mainly influences the detection of stellar CMEs using the method of Doppler-shifted emission caused by moving plasma is not the spectral resolution/velocity but the flux/mass of the CME.
ABSTRACT
Detections of stellar coronal mass ejections (CMEs) are still rare. Observations of strong Balmer line asymmetries during flare events have been interpreted as being caused by CMEs. Here, we ...aim to estimate the maximum possible Balmer line fluxes expected from CMEs to infer their detectability in spectroscopic observations. Moreover, we use these results together with a model of intrinsic CME rates to infer the potentially observable CME rates for stars of different spectral types under various observing conditions, as well as the minimum required observing time to detect stellar CMEs in Balmer lines. We find that generally CME detection is favoured for mid- to late-type M dwarfs, as they require the lowest signal-to-noise ratio for CME detection, and the fraction of observable-to-intrinsic CMEs is largest. They may require, however, longer observing times than stars of earlier spectral types at the same activity level, as their predicted intrinsic CME rates are lower. CME detections are generally favoured for stars close to the saturation regime, because they are expected to have the highest intrinsic rates; the predicted minimum observing time to detect CMEs on just moderately active stars is already >100 h. By comparison with spectroscopic data sets including detections as well as non-detections of CMEs, we find that our modelled maximum observable CME rates are generally consistent with these observations on adopting parameters within the ranges determined by observations of solar and stellar prominences.
We present a companion of the ≤2 Myr young classical T Tauri star GQ Lup in the Lupus star forming region at $140 \pm 50$ pc from imaging, astrometry, and spectroscopy. With direct K-band imaging ...using VLT/NACO, we detected an object 6 mag fainter than GQ Lup located $0.7^{\prime \prime}$ west of it. Compared to images obtained 2 to 5 years earlier with Subaru/CIAO and HST/PC, this object shares the proper motion of GQ Lup by 5 and $7\sigma$, respectively, hence it is a co-moving companion. Its $K-L'$ color is consistent with a spectral type early to mid L. Our NACO K-band spectrum yields spectral type M9-L4 with H2O and CO absorption, consistent with the new GAIA-Dusty template spectrum for $\log~g \simeq 2$ to 3 and $T_{\rm eff} \simeq 2000$ K with ~2 Rjup radius at ∼140 pc, hence few Jupiter masses. Using the theoretical models from Wuchterl & Tscharnuter (2003), Burrows et al. (1997), and Baraffe et al. (2002), the mass lies between 1 and 42 Jupiter masses.
ABSTRACT
Coronal mass ejections (CMEs) may have major importance for planetary and stellar evolution. Stellar CME parameters, such as mass and velocity, have yet not been determined statistically. So ...far only a handful of stellar CMEs has been detected mainly on dMe stars using spectroscopic observations. We therefore aim for a statistical determination of CMEs of solar-like stars by using spectroscopic data from the ESO phase 3 and Polarbase archives. To identify stellar CMEs, we use the Doppler signal in optical spectral lines being a signature of erupting filaments that are closely correlated to CMEs. We investigate more than 3700 h of on-source time of in total 425 dF-dK stars. We find no signatures of CMEs and only few flares. To explain this low level of activity, we derive upper limits for the non-detections of CMEs and compare those with empirically modelled CME rates. To explain the low number of detected flares, we adapt a flare power law derived from EUV data to the H α regime, yielding more realistic results for H α observations. In addition, we examine the detectability of flares from the stars by extracting Sun-as-a-star H α light curves. The extrapolated maximum numbers of observable CMEs are below the observationally determined upper limits, which indicates that the on-source times were mostly too short to detect stellar CMEs in H α. We conclude that these non-detections are related to observational biases in conjunction with a low level of activity of the investigated dF-dK stars.
Most extrasolar planet discoveries using radial velocity measurements have been for solar-like G-stars. In order to understand better the role stellar mass for the formation of planets we must learn ...more about the frequency of planetary companions around both high- and low-mass stars. Radial velocity searches for planets around high mass main-sequence stars are difficult due to the paucity of lines and often rapid rotation of these early-type stars. On the other hand, evolved stars that have moved off the main sequence offer us the possibility of searching for planets around higher mass stars by means of precise radial velocity measurements. Here we present radial velocity measurements for the star HD 13189 using measurements taken at the Thüringer Landessternwarte Tautenburg, the Harlan J. Smith Telescope at McDonald Observatory, and the Hobby-Eberly Telescope. We classify the spectral type of this star as K2 with luminosity class II. The radial velocity measurements show long-period variations with a period of 472 days and an amplitude of 173 m s-1. The Ca II S-index is consistent with an inactive star and this shows no variations with the radial velocity period. We also investigated possible changes in the line shapes by measuring spectral line bisectors. These show no variations with the radial velocity period. We interpret the 472-day period as being caused by a sub-stellar companion. Based on the estimated absolute magnitude and a comparison to evolutionary tracks we estimate the mass of the progenitor star between 2 and 7 $M_\odot$ which results in a projected mass of the companion of m sin i = 8–20 MJ. HD 13189 may be the most massive star known to possess an extrasolar planet. This suggests that the formation of giant planets can also occur around early-type stars. HD 13189 also shows significant short term radial velocity variability on time scales of days that is most likely due to stellar oscillations. This behavior is typical for K giant stars.
Aims. We investigate the nature of the long-period radial velocity variations in α Tau first reported over 20 yr ago. Methods. We analyzed precise stellar radial velocity measurements for α Tau ...spanning over 30 yr. An examination of the Hα and Ca II λ8662 spectral lines, and Hipparcos photometry was also done to help discern the nature of the long-period radial velocity variations. Results. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, Hα equivalent width measurements and Hipparcos photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 ± 0.90 d, eccentricity, e = 0.10 ± 0.05, and a radial velocity amplitude, K = 142.1 ± 7.2 m s-1. Evolutionary tracks yield a stellar mass of 1.13 ± 0.11 M⊙, which corresponds to a minimum companion mass of 6.47 ± 0.53 MJup with an orbital semi-major axis of a = 1.46 ± 0.27 AU. After removing the orbital motion of the companion, an additional period of ≈520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of Hα and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The ~520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by ~10 yr. This might be due to an activity cycle. Conclusions. The data presented here provide further evidence of a planetary companion to α Tau, as well as activity-related radial velocity variations.
Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to show a flat featureless transmission spectrum in the infrared and a tentative Rayleigh scattering slope in the optical. We ...conducted an optical transmission spectroscopy project to assess the impacts of stellar activity and to determine whether or not GJ 3470b hosts a hydrogen-rich gas envelop. Methods. We observed three transits with the low-resolution Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias, and one transit with the high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) at the 8.2 m Very Large Telescope. Results. From the high-resolution data, we find that the difference of the CaiiH+K lines in- and out-of-transit is only 0.67 + or - 0.22%, and determine a magnetic filling factor of about 10-15%. From the low-resolution data, we present the first optical transmission spectrum in the 435-755 nm band, which shows a slope consistent with Rayleigh scattering. Conclusions. After exploring the potential impacts of stellar activity in our observations, we confirm that Rayleigh scattering in an extended hydrogen-helium atmosphere is currently the best explanation. Further high-precision observations that simultaneously cover optical and infrared bands are required to answer whether or not clouds and hazes exist at high-altitude.
Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the ...magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list of M dwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism.
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