Active dMe Stars Research Articles

Stellar flare oscillations: evidence for oscillatory reconnection and evolution of MHD modes

Here, we report on the detection of a range of quasi-periodic pulsations (20–120 s; QPPs) observed during flaring activity of several magnetically active dMe stars, namely AF Psc, CR Dra, GJ 3685A, Gl 65, SDSS J084425.9+513830, and SDSS J144738.47+035312.1 in the GALEX NUV filter. Based on a solar analogy, this work suggests that many of these flares may be triggered by external drivers creating a periodic reconnection in the flare current sheet or an impulsive energy release giving rise to an avalanche of periodic bursts that occur at time intervals that correspond to the detected periods, thus generating QPPs in their rising and peak phases. Some of these flares also show fast QPPs in their decay phase, indicating the presence of fast sausage mode oscillations either driven externally by periodic reconnection or intrinsically in the post-flare loop system during the flare energy release.

A search for flares and mass ejections on young late-type stars in the open cluster Blanco-1★

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.

Observation and modelling of main sequence star chromospheres

Aims. We report on high resolution observations of chromospheric lines for M and K dwarfs. Methods. We observed up to 13 spectral lines per star: the Hα ,H β ,H � ,P a � ,P a 8, CaII H, CaII IR triplet, CaI 6572A, NaI D1/D2 and HeI D3 lines. We observed two dMe stars, one dM(e) star, 5 dM stars and 3 dK stars. Results. We observed a self-reversal in the emission core of the CaII H line for the brightest stars only, indicating a rather optically thick region of formation. We present original spectra of the NaI doublet and the CaII IR triplet for active dMe stars and less active dM stars. Core emission is detected in the Sodium lines and the CaII IR triplet lines for the most active M dwarf AU Mic. We investigate the difference spectra between active dMe stars and dM stars and show that these provide interesting new constraints for the NLTE-radiation transfer modelling of the chromospheres. In our sample, emission Hα profiles have a rather homogeneous FWHM of about 1.5 A. This, according to our previous modelling, can be interpreted as the signature of a rather constant temperature break in the chromosphere. We found that one of our targets (MCC 332) is a binary with a faint but active Hα emission component. – –

Spectral Indications of Density Variability in the Corona of AD Leonis

Direct comparison of high-resolution X-ray spectra of the active dMe star AD Leo, observed three times with Chandra, shows variability of key density-sensitive lines, possibly due to flaring activity. In particular, a significant long-duration enhancement of the coronal density is indicated by the Ne IX and Si XIII He-like triplets, and possibly also by density-sensitive Fe XXI line ratios.

The unusual X-ray and EUV flare activity of Wolf 630AB

We study the X-ray and extreme ultraviolet (EUV) behaviour of the flare star Wolf 630AB (V1054 Oph, Gl 644AB, HD 152751) using the ROSAT all-sky survey data from 1990 August. The simultaneous X-ray and EUV observations revealed a complex and unusual series of interplay. Indeed, the star was in a state of almost continuous flare activity for the two days of the X-ray observations. We saw a total of five X-ray flares (plus a subflare) and at least as many flares were also seen in the EUV data. We suggest that the series of flares observed on Wolf 630AB originated in a single (probably complex) active region and illustrate, in a unique way, the evolution of flare activity in the stellar corona of a very active dMe star.

The Neupert Effect in Active Stellar Coronae: Chromospheric Evaporation and Coronal Heating in the dMe Flare Star Binary UV Ceti

Evidence for coronal heating by chromospheric evaporation in flares of active dMe stars is presented through observations of the Neupert effect in high-frequency microwaves and soft X-rays. The Neupert effect, as originally found in solar flares, manifests itself in a close similarity between the soft X-ray light curve and the time integral of the simultaneous microwave light curve. It is interpreted as the signature of the accumulation of hot plasma due to heating by accelerated electrons in the chromosphere.We used the ROSAT and ASCA soft X-ray observatories and the Very Large Array (VLA) radio telescope (at 6 cm and 3.6 cm wavelengths) to monitor simultaneously the nearby dMe flare star binary Gliese 65 A+ B = UV Ceti during 9 hours on each of two consecutive days. We find several weakly polarized radio events that start contemporaneously (within a few minutes) with X-ray flares and then peak and decay as the X-ray flares develop gradually. A striking similarity to the temporal evolution of solar gradual events is found. We argue that the Neupert effect is best observed in relatively hard bands of the soft X-ray emission, but that its presence can be inferred from the much softer bands commonly used for stellar observations by use of the solar analogy. Together with spectral hardness observations of soft X-rays, the data suggest the operation of chromospheric evaporation on UV Cet. The observations thus indicate a causal relation between the nonthermal and thermal energies of the underlying electron populations. We find that stellar flares are, relative to solar flares, X-ray-weak. The ratio between the total energy radiated into the radio and the soft X-ray bands closely matches the corresponding ratio between the quiescent luminosities of active stars, perhaps implying similar mechanisms and similar efficiencies for the quiescent emission and for larger, single flares. Estimating the total kinetic energy in the electrons from the radio flux, we find that only a part is observed in soft X-rays, a discrepancy well known from solar flares.

A Search for Microflaring Activity on dMe Flare Stars. I. Observations of the dM8e Star CN Leonis

Microflares are frequent, short-duration, energetically weak disturbances occurring in the nonradiatively heated regions of the Sun and other magnetically active stars. They are thought to be the low-energy extension of flares commonly seen on active dMe stars and may be a major source of heating the chromosphere and corona of cool stars in general. In this paper we describe rapid time sequence UV photometry of the dMe star CN Leo taken with the High Speed Photometer (HSP) aboard the Hubble Space Telescope (HST). The filter was centered at 240 nm, near wavelengths at which flares are expected to have maximum intensity and the stellar background is small. During 2 hr of on-source observing, a total of 32 flarelike events were detected, with integrated counts ranging from 12 to more than 14,000. In most cases the events had integrated energy ranging between 10<SUP>27</SUP> and 10<SUP>28</SUP> ergs and can be classified as microflares. A considerable fine structure was seen in these events, with substantial variations sometimes occurring on timescales of less than 1 s. The occurrence rates for the smaller events showed a power-law distribution, with a slope comparable to that seen for larger events observed from the ground. Extrapolating the occurrence rate relation to nanoflare energies indicates a predicted count rate that is significantly smaller than that observed, suggesting that the nanoflares have a different energy distribution than the larger events.

New EUV Selected Flare Stars

The ROSAT Wide Field Camera all-sky EUV survey of 1990-91 identified over 200 active late-type stars (Pounds et al. 1993). Most of these were not known to be active prior to the survey and follow-up optical spectroscopy has been successful in selecting the most active dMe stars. These stars are characterized by their Balmer line emission which is direct evidence for an active chromosphere. We present photometric monitoring data for 4 new dMe stars, including the discovery of flaring activity on all 4 stars.The observations were made at the South African Astronomical Observatory at Sutherland near Capetown in October 1994, using the 0.5m and 1.9m telescopes for photometry and spectrometry, respectively. The 0.5m telescope is equipped with an automated, pulse-counting photometer. Flare monitoring of all the stars was done in the U-band with either 5 or 10 s integration times. All the stars monitored were between magnitude V = 11 and 13. A total of 15 flares were detected, including at least one on each star (Table 1). Comparison stars were measured in standard UBVRI photometric bands for each star.

A Major Optical Flare on the Recently Discovered X-ray Active dMe Star G102-21

During UBV photoelectric measurements of G102-21 - a dM3 star recently discovered as a remarkably active X-ray source by Micela et al. (1995) - we have observed the flare shown in Fig. 1 (left panel). The observed flux increase at flare maximum and the energy output (cf. Table 1) make this event one of the largest amplitude and most energetic flares ever detected on UV Cet-type stars.The ratios EU/EB (=8.9) and EU/EV (=3.9) are more than 7 and 2 times larger, respectively, than the mean values found for typical dMe flares by Lacy et al. (1976). Moreover, assuming the empirical relation between the absolute magnitude MV and the mean energy dissipation during a flare (Gurzadyan 1980), the energy released in the U band exceeds the predicted one by a factor of ∼ 3 · 103. Clearly, we observed a rather peculiar and rare event that does not appear to be a typical flare on red-dwarf stars.

Coronal activity from AB Dor and RST 137B

Two 12-h synthesis observations of the active star AB Dor (HD 36705) made with the Molongo Observatory Synthesis Telescope (MOST) as part of a 1990 international campaign of observations are presented. A weak variable source was detected near the stellar position during both sessions. The derived brightness temperatures are consistent with a gyrosynchrotron emission model. The complete sample of MOST observations of AB Dor made since 1985 reveals a quiescent level of radio emission with fare-like increases, and a discrepancy of ∼ 5 arcsec between the 843-MHz source and the optical position. An analysis of the variations in position of the radio source with flux density suggests that this discrepancy may be the result of confusion with the nearby, active dMe star RST 137B

VARIABILITY OF CHROMOSPHERIC LINES IN LATE-TYPE DWARF STARS

ABSTRACT The results of a high-resolution spectroscopic synoptic program of observations of key chromospheric diagnostics in a sample of dwarf stars are reported. Low-level variability in the strength of the chromospheric H-alpha absorption line is found. The nature of the variability implies that the operative chromospheric heating mechanism is partially characterized by a transient component. This fact, combined with the detectability of the low-level variability, suggests that magnetic field-related, nonradiative atmospheric heating is widespread in the dM stars. High-amplitude variability in the H-alpha emission strength of the active dMe star BY Dra was also observed. Synoptic observations of the He I 5876 A absorption line in three solar-type stars revealed low-level variability in the strength of this feature.

Atmospheric activity in red dwarf stars

Radiation losses from quiescent and flaring regions of red dwarf atmospheres are compared and found to be equally important. The flare emission on all wavelengths approaches 1% of the stellar bolometric luminosity for active dMe stars, but is 100 times less for some dM stars. The discriminating parameter is flare frequency, which may vary with stellar mass, age, rotation rate or other quantities. Radiation losses from chromospheric emission lines are larger than losses from transition region lines. H I is the most important element in late dMe stars, while Ca II dominates the chromospheric radiation loss from early dMe and dKe stars. The corona is less significant in dKe stars, but dominates over chromospheric radiation losses in dMe stars. A sharp drop in coronal importance is seen near dM5e, where stars become fully convective. All forms of radiation losses have a common cause, probably to be found in the convection zone. The magnetic field may be the instrument that brings mechanical energy into the outer atmosphere, where it is radiated into space.