D. Montes, J. López-Santiago, I. Crespo-Chacón, M.J. Fernández-Figueroa,

Universidad Complutense de Madrid, Dpt. de Astrofísica, Facultad de Ciencias Físicas, E-28040 Madrid, SpainE-mail: dmg@astrax.fis.ucm.es, WWW: http://www.ucm.es/info/Astrof/users/dmg/dmg.html

AbstractThe analysis of high resolution optical spectroscopic observations of different kinds of late-type stars indicates that chromospheric flare phenomena in these stars take place at very different scales than in the solar case. Evidences of chromospheric microflaring activity have been found in the broad wings detected in the excess Hα emission profiles of very active binary systems and weak-lined T Tauri stars (WTTS). High and moderate-energy, but low frequency flares have been detected in young, single and rapid-rotator K dwarfs such as LQ Hya and PW And. Strong (several orders of magnitude larger than in the Sun) and long-duration (several days) flares have been found in chromospherically active binaries like V711 Tau and 2RE J0743+224. Moreover, when we carefully analyse high-temporal (15 s) resolution spectroscopic observations of typical flare stars (UV Cet type) we find very frequent low amplitude flare-like events in addition to the typical flares. These large ranges of energy and frequency in the flares detected in late-type stars point out the strong influence of stellar properties as temperature, rotation rate, age, and binarity on the magnetic reconnection process that originates flares.

Flares are believed to result from the release of magnetic energy stored in the corona through reconnection. Many types of cool stars produce flares, sometimes at levels several orders of magnitude more energetic than their solar counterparts. In the dMe stars (or UV Cet type stars) optical flares are a common phenomenon (see Crespo-Chacón et al. 2004, CS13, ESA SP). In more luminous stars, flares are usually only detected through UV or X-ray observations, although optical flares have been detected in young early K dwarfs like LQ Hya (Montes et al. 1999, MNRAS, 303, 45) and PW And (López-Santiago et al. 2003, A&A 411, 489) and other K dwarfs members of young moving groups (Montes et al. 2004 , CS13, ESA SP). Strong and long-duration flares have been found in chromospherically active binaries (RS CVn and BY Dra types) like UX Ari and II Peg (Montes et al. 1996, A&A, 310, L29; 1997, A&A, 125,263) and V711 Tau (García-Alvarez et al. 2003, A&A, 397, 285). In this contribution we summarize the behaviour of the detected optical flares in these different kinds of cool stars.

Flare stars

PW AndThis K2 dwarf is a young single star member of the Local Association (see López-Santiago et al. 2003, A&A 411, 489). We have observed this star during nine different observing runs from 1999 to 2002. The spectra at different epoch always show strong emission in Hα, Ca II H&K and Ca II IRT lines, and excess emission in the other Balmer lines in the subtracted spectra. During three of these observing runs we have detected variations from one day to the other in the optical chromospheric lines typical of flare like events (increase of the Balmeremission lines broad emission components, He I D3 goes in emission, etc.). The more energetic flare was detected during the HET-HRS 2001/12 observing run (see Fig. 8).

High and moderate-energy, but low frequency flaresYoung, single and rapid-rotators K dwarfs

Strong and long duration flaresChromospherically active binaries (RS CVn and BY Dra types)

Frequent low amplitude flares + very strong flaresFlare stars (UV Cet type)

HeI D3Hα

LQ Hya

Flare 1

Flare 2

Flare 3

Flare 4

Flare 5

Flare 6

Flare 7

Flare 8

Flare 9

Flare 10

Flare 11

LQ Hya is a very active young, rapidly rotating, single K2 dwarf. We have detected a strong flare on 1993 December 22 (Montes et al. 1999, MNRAS, 303, 45). In addition to the typically flare-enhanced emission lines (Hα and Hβ), we observe He I D3 going into emission, plus excess emission (after subtraction of the quiescent spectrum) in other He I and several strong neutral metal lines (Mg I b).

Fig. 6: HeI D3 and Hα line profiles during the strong flare observed in LQ Hya (Montes et al. 1999). Note that the contribution of the broad component is higher at the beginning and maximum of the flare.

Fig. 8: HeI D3 and Hα line profiles during one of the flares (HET-HRS 2001/12 observing run ) detected PW And (López-Santiago et al. 2003). EW(Hα) increase a factor 2.1. Note the broad Hα emission component during the flare and the HeI D3 line in emission.

Fig. 7: The change of EW of several optical chromospheric lines during the flare

2RE J0743+224 2RE J0743+224 (BD +23 1799) is a chromospherically active binary selected by X-ray and EUV emission (Jeffries et al. 1995; Montes & Ramsey 1998, A&A 340, L5). A dramatic increase in the chromosphericemissions is detected during our observations (12-21 January 1998). We interpret this behavior as an unusual long-duration (>8 days) flare based on a) the temporal evolution of the event, b) the broad component observed in the Hα line profile, c) the detection of He I D3 line in emission and d) a filled-in of the He I λ6678 Ǻ line.We detect a Li I λ6708 Ǻ line enhancement which is clearly related with the temporal evolution of the flare. The maximum Li Ienhancement occurs just after the maximum chromospheric emission observed in the flare. We suggest that this Li I is produced by spallation reactions in the flare.

Fig. 9: Hα line profile during the long duration flare observed in 2RE J0743+224 (Montes & Ramsey 1998). Note that the contribution of the broad component is higher at the beginning and maximum of the flare.

Fig. 10:Temporal evolution of the Hα and Li I EW during the flare.

Fig. 11: HeI D3 and Hα line profiles during another strong flare (HET-HRS 2001/12 observing run ) detected in 2RE J0743+224

UX AriHα

V711 Tau

AD Leo

During the MUSICOS (MUlti-SIteCOntinuous Spectroscopy)1998 campaign (21 November to 13 December 1998) V711 Tau was observed almost continuously for more than8 orbits of 2.8d. Two large optical flares were observed (García-Alvarez, Montes et al. 2003, A&A, 397, 285).

Fig. 12: Hα and HeID3 line profiles during a strong flare detected in the CAB UX Ari (Montes et al. 1996, A&A, 310, L29)

Fig. 13: Hα and HeID3 line profiles during two strong flares detected in the CAB V711 Tau(García-Alvarez, Montes, et al. 2003)

Flare 2Flare 1

Fig. 14: Observed blue and red spectra of AD Leo in its quiescent state and in the maximum phase of a flare. The observed spectrum of the inactive reference star Gl 687B (M3.5V) is plotted for comparison.

Fig. 15: Evolution of the equivalent width (EW) of the Hβline of AD Leo during the 4 first nights of observations.

AD Leo has been analyzed using high temporal resolution spectroscopic observations Crespo-Chacón et al. (2004a, b) and Montes et al. (2004), Although we did not detect strong flares, we found very interesting short and weak variations in the emission lines with properties very similar to flares that are produced with high temporal frequency.The EW(Hβ) changes in a factor of ~1.3 in the Flare 3 up to a factor of 1.7 in the Flare 2. The duration of the observed flares ranges from ~22 to 43 minutes (see Fig. 15).The same flares have been also detected using the rest of the emission lines from Hδ to H11, including the Ca II H&K lines. In addition to the flares, small changes in the emission lines at shorter time scales are also observed.

In additional observations of flare (UV Cet type) stars with spectral types from K5V to M5V we have found (Crespo-Chacón et al. 2004, CS13, ESA SP) different kinds of flare-like events.The strongest flares have been marked with the symbol while the symbol has been used to indicate other changes that could be due to flares of lower intensity (hereafter type A); different magnetic reconnection processes that, decreasing in efficiency, occurs sequentially within the same flare (hereafter type B); or other kind of variations during the quiescent state at shorter temporal scales (hereafter type C).

Fig. 16: Evolution of the EW ratio (relative to the quiescent state) of the different chromospheric lines during the strongest AD Leo flare of the night 3 (Flare 6).

Fig. 17: CR Dra has flares and variations type A, B and C. The gradual decay phase of a flare together with changes type B can be seen in the night 4 and, in the night 5, several noticeable flares take place within a flare of greater intensity (total duration > 48 min, ∆EW ~0.53Å).

Fig. 18: YZ CMi has flares and changes type B, showing a very strong and long duration flare (duration > 145 min, ∆EW >7.4Å) in the night 5.

CR Dra

YZ CMi

MicroflaresIn our analysis of the Hα line, using the spectral subtraction technique, in chromospherically active binary systems (CABS) and in weak-lined T Tauri stars (WTTS) and young single stars, we have found that in some stars the subtracted Hα emission line profile has very broad wings, and is not well matched using a single-Gaussian fit. These profiles have therefore been fitted using two Gaussian components: a narrow component having a FWHM of 45-90 km/s and a broad component with a FWHM ranging from 133 to 470 km/s. This broad component could be interpreted as arising from microflaring that occurs in the chromosphere by similarity with the broad components also found by other authors in the chromospheric Mg II h & k lines and in several transition region lines of active stars. The microflares are frequent, short-duration, energetically weak disturbances, i.e. they are the low-energy extension of flares, and therefore have large-scale motions associated that could explain the broad wings observed in these lines.

Fig. 1: Hα line profile of the CAB EZ Peg (Montes et al. 1998, A&A 330, 155). Observed and synthetic profiles in the left panel and subtracted profile in the right panel. A broad Hα emission component (microflaring) is observed. A two Gaussian components fit is necessary.

A correlation between the contribution of the broad components and the degree of stellar activity seems to be present (see Fig. 3).In some cases the line is asymmetric and the fit is better matched when the broad component is blue-shifted or red-shifted with respect to the narrow component. These asymmetries are also observed during the impulsive and gradual decay phases of solar and stellar flares, and favour the interpretation of the broad component as due to upward and downward motions produced by microflaring in the chromosphere.

Fig. 4: Hα line profile of the CAB V711 Tau (Montes et al. 1997, A&AS 125, 263). Note that a changing broad Hα emission component (microflaring) is observed at different epochs and orbital phases.

Fig. 2: As Fig. 1 for the CAB HU Vir (Montes et al. 2000, A&AS 146, 203). A broad Hα emission component (microflaring) is observed. at different orbital phases. Note that at some orbital phases the broad component is blue-shifted or red-shifted with respect to the narrow component .

Fig. 5: As Fig. 1 for three WTTS (Poncet, Montes et al. 1998). A broad Hα emission component (microflaring) is observed in these three active young stars.

Fig. 3: EW of the broad component vs. EW of the total emission.

Hα