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Register a new userThe quest for stellar coronal mass ejections in late-type stars: I. Investigating Balmer-line asymmetries of single stars in Virtual Observatory data
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Flares and CMEs can have deleterious effects on their surroundings: they can erode atmospheres of orbiting planets over time and also have high importance in stellar evolution. Most of the CME detections in the literature are single events found serendipitously sparse for statistical investigation. We aimed to gather a large amount of spectral data of M-dwarfs to drastically increase the number of known events to make statistical analysis possible in order to study the properties of potential stellar CMEs. Using archive data we investigated asymmetric features of Balmer-lines, that could indicate the Doppler-signature of ejected material. Of more than 5500 spectra we found 478 with line asymmetries--including nine larger events--on 25 objects, with 1.2-19.6 events/day on objects with line asymmetries. Most events are connected with enhanced Balmer-line peaks, suggesting these are connected to flares similar to solar events. Detected speeds mostly do not reach surface escape velocity: the typical observed maximum velocities are in the order of 100-300km/s , while the typical masses of the ejecta were in the order of $10^{15}-10^{18}$g. Statistical analysis suggests that events are more frequent on cooler stars with stronger chromospheric activity. Detected maximum velocities are lower than those observed on the Sun, while event rates were somewhat lower than we could expect from the solar case. These findings may support the idea that most of the CMEs could be suppressed by strong magnetic field. Alternatively, it is possible that we can observe only an early low-coronal phase before CMEs are accelerated at higher altitudes. Our findings could indicate that later-type, active dwarfs could be a safer environment for exoplanetary systems CME-wise than previously thought, and atmosphere loss due to radiation effects would play a stronger role in exoplanetary atmosphere evolution than CMEs.
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The stellar magnetic field completely dominates the environment around late-type stars. It is responsible for driving the coronal high-energy radiation (e.g. EUV/X-rays), the development of stellar winds, and the generation transient events such as flares and coronal mass ejections (CMEs). While progress has been made for the first two processes, our understanding of the eruptive behavior in late-type stars is still very limited. One example of this is the fact that despite the frequent and highly energetic flaring observed in active stars, direct evidence for stellar CMEs is almost non-existent. Here we discuss realistic 3D simulations of stellar CMEs, analyzing their resulting properties in contrast with solar eruptions, and use them to provide a common framework to interpret the available stellar observations. Additionally, we present results from the first 3D CME simulations in M-dwarf stars, with emphasis on possible observable signatures imprinted in the stellar corona.
Type II radio bursts are observed in the Sun in association with many coronal mass ejections (CMEs. In view of this association, there has been an expectation that, by scaling from solar flares to the flares which are observed on M dwarfs, radio emission analogous to solar Type II bursts should be detectable in association with M dwarf flares. However, several surveys have revealed that this expectation does not seem to be fulfilled. Here we hypothesize that the presence of larger global field strengths in low-mass stars, suggested by recent magneto-convective modeling, gives rise to such large Alfven speeds in the corona that it becomes difficult to satisfy the conditions for the generation of Type II radio bursts. As a result, CMEs propagating in the corona/wind of a flare stars are expected to be radio-quiet as regards Type II bursts. In view of this, we suggest that, in the context of Type II bursts, scaling from solar to stellar flares is of limited effectiveness.
Coronal mass ejections (CMEs) are explosive events that occur basically daily on the Sun. It is thought that these events play a crucial role in the angular momentum and mass loss of late-type stars, and also shape the environment in which planets form and live. Stellar CMEs can be detected in optical spectra in the Balmer lines, especially in Halpha, as blue-shifted extra emission/absorption. To increase the detection probability one can monitor young open clusters, in which the stars are due to their youth still rapid rotators, and thus magnetically active and likely to exhibit a large number of CMEs. Using ESO facilities and the Nordic Optical Telescope we have obtained time series of multi-object spectroscopic observations of late-type stars in six open clusters with ages ranging from 15 Myrs to 300 Myrs. Additionally, we have studied archival data of numerous active stars. These observations will allow us to obtain information on the occurrence rate of CMEs in late-type stars with different ages and spectral types. Here we report on the preliminary outcome of our studies.
Hydrogen Balmer lines are commonly used as spectroscopic effective temperature diagnostics of late-type stars. However, the absolute accuracy of classical methods that are based on one-dimensional (1D) hydrostatic model atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To investigate this, we carry out 3D non-LTE calculations for the Balmer lines, performed, for the first time, over an extensive grid of 3D hydrodynamic STAGGER model atmospheres. For H$alpha$, H$beta$, and H$gamma$, we find significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer wings tend to be stronger in 3D models, particularly for H$gamma$, while the inner wings can be weaker in 3D models, particularly for H$alpha$. For H$alpha$, we also find significant 3D LTE versus 3D non-LTE differences (non-LTE effects): in warmer stars ($T_{text{eff}}approx6500$K) the inner wings tend to be weaker in non-LTE models, while at lower effective temperatures ($T_{text{eff}}approx4500$K) the inner wings can be stronger in non-LTE models; the non-LTE effects are more severe at lower metallicities. We test our 3D non-LTE models against observations of well-studied benchmark stars. For the Sun, we infer concordant effective temperatures from H$alpha$, H$beta$, and H$gamma$; however the value is too low by around 50K which could signal residual modelling shortcomings. For other benchmark stars, our 3D non-LTE models generally reproduce the effective temperatures to within $1sigma$ uncertainties. For H$alpha$, the absolute 3D effects and non-LTE effects can separately reach around 100K, in terms of inferred effective temperatures. For metal-poor turn-off stars, 1D LTE models of H$alpha$ can underestimate effective temperatures by around 150K. Our 3D non-LTE model spectra are publicly available, and can be used for more reliable spectroscopic effective temperature determinations.
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 which are closely correlated to CMEs. We investigate more than 3700 hours 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{alpha} regime, yielding more realistic results for H{alpha} observations. In addition we examine the detectability of flares from the stars by extracting Sun-as-a-star H{alpha} 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{alpha}. 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.
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