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, Spain
E-mail: dmg@astrax.fis.ucm.es, WWW: http://www.ucm.es/info/Astrof/users/dmg/dmg.html
Abstract
Flare stars
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.
The 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.
Microflares
High and moderate-energy, but low frequency flares
In 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 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
chromosphere by similarity with the broad components also component (microflaring) is observed. A two Gaussian components fit is necessary.
found by other authors in the chromospheric Mg II h & k lines
and in several transition region lines of active stars. The Fig. 2: As Fig. 1 for the
microflares are frequent, short-duration, energetically weak CAB HU Vir (Montes et al.
disturbances, i.e. they are the low-energy extension of flares, and 2000, A&AS 146, 203). A
broad
Hα
emission
therefore have large-scale motions associated that could explain component (microflaring)
is observed. at different
the broad wings observed in these lines.
orbital phases. Note that at
some orbital phases the
broad component is blueshifted or red-shifted with
respect to the narrow
component .
Hα
Young, single and rapid-rotators K dwarfs
LQ Hya
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. 7: The change of EW of several optical chromospheric lines during the flare
Fig. 3: EW of the broad
component vs. EW of the total
emission.
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. 5:
As Fig. 1 for
three
WTTS
(Poncet, Montes et
al. 1998). A broad
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 redshifted 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.
Hα
emission
component
(microflaring) is
observed in these
three
active
young stars.
Fig. 8: HeI D3
and Hα line profiles during one of the flares (HET-HRS 2001/12 observing run ) detected PW And
PW And
This 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 Balmer
emission lines broad emission components,
He I D3 goes in emission, etc.). The more
energetic flare was detected during the HETHRS 2001/12 observing run (see Fig. 8).
(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.
Strong and long duration flares
2RE J0743+224
Hα
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 chromospheric
emissions
is
detected
during
our
observations (12-21 January 1998). We
interpret this behavior as an unusual longduration (>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 I
enhancement 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.
UX Ari
Hα
during the flare.
Fig. 12: Hα
and HeI
D3 line profiles during a
strong flare detected in the
CAB UX Ari (Montes et al.
I EW
V711 Tau
During the MUSICOS
(MUlti-SIte
COntinuous
Spectroscopy)
1998
campaign (21
November
to
13
December 1998) V711
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.
Flare stars (UV Cet type)
AD Leo has been analyzed using high temporal
resolution spectroscopic observations CrespoChacón 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.
HeI D3
1996, A&A, 310, L29)
Fig. 10:Temporal evolution of the Hα and Li
Frequent low amplitude flares + very strong flares
AD Leo
Chromospherically active binaries (RS CVn and BY Dra types)
Tau was observed
almost
continuously
for more than
8 orbits of 2.8d. Two
large optical flares
were observed (García-
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).
CR Dra
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.
Flare 2
Flare 6
Flare 7
Flare 1
Flare 9
Flare 4
Flare 3
Flare 8
Flare 5
Fig. 15:
Evolution of the
equivalent width (EW) of the Hβ
line of AD Leo during the 4 first
nights of observations.
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).
YZ CMi
Flare 2
Fig. 13: Hα and HeI
D3 line profiles during
Fig. 11: HeI D3 and Hα line
profiles during another strong flare (HET-HRS 2001/12
observing run ) detected in 2RE J0743+224
two strong flares detected
in the CAB V711 Tau
(García-Alvarez, Montes, et
al. 2003)
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:
Flare 11
Fig. 16:
Alvarez, Montes et al.
2003, A&A, 397, 285).
Flare 1
Flare 10
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.