Astron. Astrophys. 340, L5–L9 (1998) ASTRONOMY
AND

ASTROPHYSICS

Letter to the Editor

Li I enhancement during a long-duration stellar flare

David Montes1,2,? and Lawrence W. Ramsey2,?

1 Departamento de Astrofı́sica, Facultad de Fı́sicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
(e-mail: dmg@astrax.fis.ucm.es)

2 The Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Laboratory, University Park, PA 16802, USA

Received 28 September 1998 / Accepted 14 October 1998

Abstract. We report the possible detection of a Lii λ6708Å
line enhancement during an unusual long-duration optical flare
in the recently discovered, X-ray/EUV selected, chromospher-
ically active binary 2RE J0743+224. The Lii equivalent width
(EW) variations follow the temporal evolution of the flare and
large changes are observed in the intensity of the line. The max-
imum Li i enhancement (40% in EW) occurs just after the max-
imum chromospheric emission observed in the flare. A signif-
icant increase of the6Li/7Li isotopic ratio is also detected. No
significant simultaneous variations are detected in other photo-
spheric lines. Neither line blends nor starspots seem to be the
primary cause of the observed Lii line variation. From all this
we suggest that this Lii enhancement is produced by spallation
reactions during the flare.

Key words: stars: activity – stars: chromospheres – stars: bi-
naries: spectroscopic – stars: flare – stars: abundances – stars:
individual (2RE J0743+224)

1. Introduction

The resonance doublet of Lii at λ6708Å is an important di-
agnostic of age in late-type stars since it is easy destroyed by
thermonuclear reactions in the stellar interiors. However, Lii
observations of several types of chromospherically active stars,
such as pre-main sequence stars, late-type dwarfs in open clus-
ters, as well as post-main sequence objects, show a significant
range of abundances. It is well-known that a large number of
chromospherically active binaries (CAB hereafter) show Lii
abundances higher than the normal values characteristic of stars
of the same mass and evolutionary stage (Pallavicini et al. 1992,
Liu et al. 1993; Ferńandez-Figueroa et al. 1993; Randich et al.
1994; Barrado et al. 1997, 1998; Montes et al. 1998). This line
is very temperature sensitive because of its low ionization po-
tential of 5.37 eV. Thus the equivalent width (EW) should be
enhanced in dark spots but reduced in the bright facular re-
gions as demonstrated by solar observations (Giampapa 1984).

Send offprint requests to: David Montes
? Guest observer at McDonald Observatory

A number of researchers (Fekel 1996; Barrado 1996; Hussain
et al. 1997; Ferńandez & Miranda 1998; Neuhäuser et al. 1998)
have investigated starspots as the possible cause of the observed
Li i abundances spreads. However, Li can also be produced by
low energy spallation reactions in stellar flares (Fowler et al.
1955; Canal 1974; Canal et al. 1975). Some evidence of Li pro-
duction by spallation have been found in the Sun (Livshits 1997;
Livingston et al. 1997), but no detection has been reported in
other stars. Spallation has only been discussed as a possibil-
ity to explain the high Li abundances observed in some stars
(Pallavicini et al. 1992; Mathioudakis et al. 1995; Favata et al.
1996).

In this letter we describe the Lii λ6708Å line enhancement
detected during the observations of an unusual long-duration op-
tical flare in the recently discovered, X-ray/EUV selected CAB
2RE J0743+224, and discuss the possible causes.

2. Observations and analysis

High resolution (0.16̊A) observations of this star were obtained
during a 10 night run 1998 January 12-21 using the 2.1m tele-
scope at McDonald Observatory and the Sandiford Cassegrain
Echelle Spectrograph (McCarthy et al. 1993). The spectra in the
Li i line region are plotted in Fig. 1. These observations are an-
alyzed in detail in a separate paper (Montes & Ramsey 1998b)
where stellar properties, orbital parameters, and chromospheric
behavior are discussed. We found that this star, previously clas-
sified as single-lined spectroscopic binary (Jeffries et al. 1995),
is a double-lined spectroscopic binary (SB2) with a K1 III pri-
mary and an orbital period of 10 days.

A dramatic increase in the chromospheric emissions (Hα
and Caii IRT lines) is detected during the observations (see
the temporal evolution of the Hα EW in the upper left panel of
Fig. 2). The increase of the emission start the 3th night (1998
January 15) reach a maximum on the 5th night and at the end
of the observations (1998 January 22) the chromospheric lines
had not yet recovered the quiescent value. The total duration
of the event was evidently larger than 8 days. We interpret this
behaviour as an unusual long-duration flare based on a) the tem-
poral evolution of the event, b) the broad component observed

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L6 D. Montes & L.W. Ramsey: Lii enhancement during a long-duration stellar flare

Fig. 1.Spectra of 2RE J0743+224 in the Lii λ6708Å line region from
1998 January 13 to 22. A K1 III reference star (HR 5340) is also plotted
for comparison, with the more intense photospheric lines identified.
The wavelength position of the lines of both components are marked.

in the Hα line profile, c) the detection of the Hei D3 line in
emission and d) a filled-in of the Hei λ6678Å line. A detailed
description of the flare is given in Montes & Ramsey (1998a,
b).

Here we analyze the spectra in the Lii λ6708Å line region.
The Li i absorption feature is clearly observed (Fig. 1) and ap-
pears centered at the wavelength corresponding to the primary
component with no evidence for a contribution from the sec-
ondary. The mean EW is 130 m̊A which is a little bit above the
normal value for this kind of star, typically< 100 mÅ, (Randich
et al. 1994; Barrado et al. 1997, 1998). In addition, a small ab-
sorption feature corresponding to the Lii λ6104Å is marginally
detected in the spectrum taken the 4th night. A careful analysis
of the Li i λ6708Å line indicates that the line profile, EW, and
intensity, I, are changing during the observations. The measured
EW and I are given in Table 1 and the EW is plotted in Fig 2,
where we can see that the increase of Lii line follows the tempo-
ral evolution of the flare. The maximum Lii enhancement (50%
in EW) occurs just after the maximum chromospheric emission
observed in the flare (5th night). However, any real variations of
the Li i EW (in excess of the uncertainties) are clearly indicated
on nights 1 and 7 only.

Fig. 2.Measured EW for Hα, Li i 6708Å and other photospheric lines
during the flare. The dotted line is the mean EW for each line.

In order to test if these variations are real we have also
measured the EW of other photospheric lines. We have selected
several isolated lines in the same spectral order as the Lii line;
Fei λ6710.3Å and Fei λ6703.6Å which are close and similar
in strength to the Lii line. In addition we study the more intense
lines Fei λ6663.4Å and Cai λ6717.7Å. Other intense lines
included in different spectral orders were also measured. All are
neutral lines and, as the Lii line, the EW should be enhanced in
a different way depending of their excitational potential when
the the temperature decrease. These lines are listed in Table 2
together with their excitational potential (χ) and the mean EW
(EW), the standard deviation (σ), and the peak to peak variation
(EWmax - EWmin). The measured EW for each night is plotted
in Fig 2, the errors bars have been calculated taking into account
the error in placing of the continuum and theσ obtained from
repeated measures in each line. As can be seen the variations
in these lines are very small and, contrary to the Lii line no
correlation with the temporal evolution of the flare is evident.
The peak to peak variation in the Lii line is a factor 3 larger than
in the other lines and theσ of similar strength lines is 3 times
smaller. Furthermore, no clear systematic behavior is observed
with different excitation potentials.

3. Discussion and conclusions

One source of variability of the Lii λ6708Å line could due to
blending with TiO bands at 6707.29, 6707.92, and 6708.16Å

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D. Montes & L.W. Ramsey: Lii enhancement during a long-duration stellar flare L7

Table 1.Measured Lii λ6708Å line parameters

HJD EW I ∆ Ca - Li λ0
6Li/7Li

(2450000+) (̊A) (Å) (Å)

826.8794 0.102 0.146 9.892 6707.789 0.
827.8537 0.122 0.149 9.899 6707.782 0.
828.8925 0.133 0.161 9.898 6707.783 0.
829.8929 0.126 0.165 9.886 6707.795 0.
830.9239 0.136 0.164 9.889 6707.792 0.
831.9308 0.138 0.177 9.880 6707.801 0.
832.9098 0.153 0.178 9.861 6707.820 0.06
835.8891 0.130 0.144 9.853 6707.828 0.11

and CN bands at 6707.64̊A as these molecular bands also be-
come stronger at the lower temperatures of the starspots. How-
ever, the calculations of the Lii abundance in sunspots that take
into account these bands (Engvold et al. 1970; Ritzenhoff et
al. 1997) concluded that the molecular blend is of lower im-
portance. Thus it seems reasonable assume that this effect in
the stellar spectra is also negligible. At this spectral resolution
the Li i line is blended with the nearby Fei λ6707.41Å line,
which is clearly seen in the spectra of the inactive and Li free
star HR 5340 (K1 III). The mean EW measured in these spectra
is 20 mÅ, which is much smaller than the observed Lii EW.
Furthermore, as the other photospheric lines we measured did
not show significant variations, we conclude that this line does
not produce the variation in the measured Lii EW. Due to the
SB2 nature of this binary in some orbital phases the lines of
the primary component could be blended with the lines from
the secondary (see Fig. 1). This is clearly seen in the spectrum
from the 5th night (which is very close the conjunction). The
EW of the strongest lines measured during this night are notice-
ably larger than in the rest of the nights due to the contribution
(≈ 15%) to the EW from the secondary. We have corrected the
EW for this effect by subtracting the EW of the lines in the
secondary measured at other orbital phases where the lines are
not blended. These corrected values are what we have plotted
in Fig 2. However, for weak lines with EW similar to the Lii
line the contribution of the secondary seems to be negligible
and this effect is not observed. In conclusion the observed Lii
line variations do not seem to be due to any kind of line blends.

The Li i line variations could be caused by possible cold
spots and faculae on the stellar surface (see review by Fekel
1996). Since this line is very temperature sensitive, the EW
should be enhanced in dark spots but reduced in the bright fac-
ular regions as shown by solar observations (Grevesse 1968;
Traub & Roesler 1971; Giampapa 1984). While Giampapa
(1984) suggests this can substantially alter the EW in stellar
spectra, other authors find this is not the case. No detection of
Li i EW variations in six active dwarfs have been reported by
Boesgaard (1991). The calculations of Soderblom et al. (1993)
indicate that the effect is only significant when the fraction of
the surface covered by spots is very high (see also Stuik et
al. 1997). Pallavicini et al. (1993) show by means of spectral
synthesis simulations that the effects may be less pronounced

Table 2.Photospheric line parameters

Line Id.(M) λ χ EW σ EWmax−min

(Å) (eV) (Å) (Å)

Li i (1) 6707.8 0.000 0.130 0.014 0.051

Cai (18) 6462.566 2.523 0.344 0.008 0.024
Cai (18) 6471.660 2.526 0.192 0.006 0.017
Cai (1) 6572.781 0.000 0.173 0.007 0.023
Cai (32) 6717.685 2.709 0.225 0.009 0.024

Fei (268) 6546.245 2.758 0.174 0.006 0.021
Fei (111) 6663.446 2.424 0.180 0.005 0.016
Fei (268) 6703.573 2.758 0.101 0.005 0.017
Fei (34) 6710.310 1.485 0.094 0.003 0.009

Ni i (43) 6643.641 1.676 0.172 0.005 0.019

than suggested by Giampapa (1984) and found no evidence that
changes in the EW is correlated with the photometric variabil-
ity due to starspots in four active stars. The simulations done
by Barrado (1996) also indicated smaller changes in the EW
and even, in certain cases, the presence of faculae can cancel
these changes, leaving the EW unaltered. By application of the
Doppler imaging technique, Hussain et al. (1997) show that the
Li i behaves in much the same way that conventional Doppler
imaging Cai and Fei lines do. Until now significant variations
in the Li i EW have been found only in some stars with very high
Li i abundances such as pre-main sequence stars (Patterer et al.
1993; Ferńandez & Miranda 1998; Neuhäuser et al. 1998) and
other young and very active stars (Robinson et al. 1986; Jeffries
et al. 1994; Soderblom et al. 1996). Large Lii EW variation has
been observed at larger V band amplitude in V410 Tau. Here a
peak to peak variability the Lii EW of 0.12Å has been found
by Ferńandez & Miranda (1998) when the amplitude in V was
0.6 mag, while little or no variation have been reported by pre-
vious work on this star at lower V amplitudes (Basri et al. 1991;
Patterer et al. 1993; Martı́n 1993; Welty & Ramsey 1995). This
result is confirmed on the young star Par 1724 by Neuhäuser et
al. (1998). However, in CAB little or no variations have been
previously reported (Pallavicini et al. 1993). Recent observa-
tions (Berdyugina et al. 1998) of the extremely CAB II Peg,
which exhibits high V band variations and spot filling factors,
show very small Lii EW variations (10 m̊A), poorly correlated
with quasi-simultaneous photometric observations. The Lii EW
variations that we observe are clearly larger than those reported
in other CAB with similar activity levels and Lii abundance.
Indeed, in other stars that exhibit large Lii EW variations other
photospheric lines exhibit similar EW variations (Fernández &
Miranda 1998), contrary to the behavior we report here. Taking
into account all these facts, the starspots that we infer on the
surface of 2RE J0743+224 from the analysis of the TiO 7055Å
band (Montes & Ramsey 1998b), do not seem to be the primary
cause of the observed Lii line variation.

The possibility of detecting Lii abundance inhomogeneities
resulting from spallation reactions in the solar photosphere have
been discussed by Hultqvist (1974, 1977). Evidence for such

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L8 D. Montes & L.W. Ramsey: Lii enhancement during a long-duration stellar flare

Li formation have been found through the deexcitation line
Li (478 keV) that results fromα-α reactions. This line has been
detected byγ-ray spectral observations of solar flares with OSO-
7 (Chupp et al. 1973), SMM (Murphy et al. 1990) and Yohkoh
(Yoshimori et al. 1994; Kotov et al. 1996). Recent calculation of
Li production in solar flares by Livshits (1997) agree withγ line
observations and suggest that enhancement of Li, especially in
the intensity of the Lii λ6708Å line, should be observed in the
Sun and other active stars. Evidence for a Li enhancement at one
umbral position, during a solar flare is reported by Livingston
et al. (1997). In other stars (including the UV Ceti flares stars)
no evidences of production of Li by nuclear reactions have been
previously reported. The possibility of Li production have been
discussed only in terms of the energy required (Ryter et al. 1970;
Karpen & Worden 1979), as a possibility to explain the high Li
abundances observed in CAB (Pallavicini et al. 1992) and stars
with high flare activity (Mathioudakis et al. 1995), to explain
the widespread presence of Li in very cool dwarfs (Favata et al.
1996), or the6Li detection in Population II stars (Deliyanis &
Malaney 1995).

Another signature of Li production from spallation reactions
is that the6Li/7Li isotopic ratio should increase. The predicted
6Li/7Li ratio for the Li produced by spallation is≈ 0.4 (Au-
douze 1970) or≈ 0.5 (Walker et al. 1985). The ratio measured
in the Sun is between 0.01 and 0.04 (Traub & Roesler 1971;
Müller et al. 1975). In Population I stars it is≤ 0.04 (Andersen
et al. 1984; Maurice et al. 1984; Rebolo et al. 1986; Pallavicini
et al. 1987), and in Population II stars it is≈ 0.05 in the two
halo stars in which6Li has been positively detected, but still
lower upper limits are found for a larger number of other stars.
(Smith et al. 1993; Hobbs & Thorburn 1994, 1997). In order to
estimate the6Li/7Li in our high resolution spectra we adopt a
method used by Herbig (1964) based on the shift of the center
of gravity (cog) of the Lii blend toward longer wavelengths as
the 6Li fraction increases. If each Lii component is weighted
by its gf-value, pure7Li would produce a cog wavelength of
6707.8117̊A while pure6Li would be 6707.9713̊A. For weak
lines, intermediate mixtures would yield a wavelength,λ0, be-
tween the two isotopes that would be weighted by the6Li/7Li
ratio as6Li/7Li = (λ0 - 6707.8117) / (6707.9713 -λ0). To deter-
mine the value ofλ0 we have measured the difference between
the Li i and Cai features (∆ = Ca i - Li i) where we adopt
a wavelength of 6717.681̊A for the Cai line. We give these
values and the corresponding6Li/7Li ratio obtained in Table 1.
The largest difference in the inferred Lii) wavelengths amounts
to only 0.046Å, or 30% of one resolution element, and the ef-
fects of the blended Fei, CN, and TiO lines on these inferred,
apparent wavelengths of the Lii line are uncertain. In order
to estimate the possible errors we have also measured this dif-
ference,∆, for other photospheric lines included in the same
spectral order than the Lii feature. The other line∆’s do not
show any trend during the observations and theσ with respect to
the mean value is≈ 0.008. The Lii line ∆’s shows a tendency
to decrease toward the end of the flare, attaining a maximum
difference 0.05. This significant change in∆ and thus in the

6Li/7Li ratio is consistent with increasing6Li during the flare
as is predicted for the production of Lii by spallation reactions.

The Li i EW variations that we observe are clearly correlated
with the temporal evolution of the flare, and large changes are
observed in the core of the Lii line, as predict the models of Li
production in flares (Livshits 1997). Thus taking into account
that the other possible causes of variability have been mini-
mized above we suggest that this Lii is produced by spallation
reactions in the flare. The observed6Li/7Li ratio also support
this hypothesis. Neither the EW variation or the6Li/7Li varia-
tion are entirely convincing by themselves but together they are
strongly suggestive. This is the first time that such Lii enhance-
ment associate with a stellar flare is reported, and probably the
long-duration of this flare is a key factor for this detection.

Acknowledgements.This work was supported by the Universidad
Complutense de Madrid and the Spanish Dirección General de Investi-
gacíon Cient́ıfica y Técnica (DGICYT) under grant PB94-0263, and by
National Science Foundation (NSF) grant AST 92-18008. We thank the
staff of McDonald observatory for their allocation of observing time
and their assistance with our observations.

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	Introduction
	Observations and analysis
	Discussion and conclusions