4 The Li I  $\mathsf{\lambda}$6707.8 line

The resonance doublet of Li  I at $\lambda$6707.8 Å is an important diagnostic of age in late-type stars since it is destroyed easily by thermonuclear reactions in the stellar interior. This line is included in our echelle spectra in all the observing runs. At this spectral resolution and with the rotational velocity ($v\sin{i}$ > 8 km s^-1) of the observed stars the Li  I line is blended with the nearby Fe  I $\lambda$6707.41 Å line. We have corrected the total measured equivalent width, EW(Li  I+Fe  I), by subtracting the EW of Fe  I calculated from the empirical relationship with (B-V) given by Soderblom et al. (1990). The obtained values are given in Table 3 and plotted in Fig. 2 versus their spectral type.

In order to obtain an estimate of the ages of our stars we compare their EW(Li  I) with those of stars in well-known young open clusters of different ages. In the EW(Li  I) versus spectral type diagram (Fig. 2) we have overplotted the upper envelope of the Li  I EW of IC 2602 (10-35 Myr), the Pleiades (78-125 Myr), and the Hyades (600 Myr), open clusters which cover the range of ages of the MGs studied here. For the Pleiades we adopt the upper envelope determined by Neuhäuser et al. (1997) with data from Soderblom et al. (1993b) and García López et al. (1994) and the lower envelope given by Soderblom et al. (1993b). In the case of IC 2602 we have not adopted the upper envelope given by Neuhäuser et al. (1997) with data from Randich et al. (1997) and Stauffer et al. (1997) because they have used EW(Li  I) not corrected for the EW(Fe  I) and we have determined a new upper envelope with corrected EW(Li  I) and using, in addition, new data provided by Randich et al. (2001). Finally for the Hyades (600 Myr) we have used the upper envelope adopted by Soderblom et al. (1993b).

Representative spectra in the Li  I line region of the star sample are plotted in Fig. 3. As can be seen in this figure a prominent Li  I absorption line is observed in the stars classified as possible members of the LA except GJ 211. The other LA stars have EW(Li  I) between the lower and upper envelope of the Pleiades (see Fig. 2), except EK Dra and HD 77407 which seem to be younger (EW(Li  I) between the upper envelopes of the Pleiades and IC 2602). The possible members of the UMa have a lower Li  I absorption line corresponding to the greater age of the UMa group (EW(Li  I) between the upper envelope of the Hyades and the lower envelope of the Pleiades). The YD stars LQ Hya have a EW(Li  I) similar to the upper envelope of the Pleiades. The EW(Li  I) of GJ 577 is well above the upper envelope of the Hyades, and no Li  I line is detected in GJ 3706.

 

 

Table 3: Radial velocities and Li  I EW.

Name	Obs.	MJD	$V_{\rm hel} \pm \sigma_{V}$	$\overline{V_{\rm hel}}$ $\pm$ $\sigma_V$	EW(LiI+FeI)	EW(LiI)	$\overline{EW{\rm (LiI)}}$
			(km s-1)	(km s-1)	(mÅ)	(mÅ)	(mÅ)
LA
PW And	2.2 m 99	51384.1732	-11.76 $\pm$ 0.59	-10.99 $\pm$ 0.11	290	269	271
	2.2 m 99	51385.0401	-11.49 $\pm$ 0.23		283	262
	2.2 m 99	51386.1011	-12.50 $\pm$ 0.37		292	271
	2.2 m 99	51387.0396	-10.51 $\pm$ 0.42		290	269
	2.2 m 99	51388.1258	-11.95 $\pm$ 0.26		277	256
	2.2 m 99	51389.0818	-11.11 $\pm$ 0.47		296	275
	NOT 99	51508.8690	-9.41 $\pm$ 0.25		305	284
	NOT 99	51509.9022	-10.28 $\pm$ 0.28		300	279
V368 Cep	2.2 m 99	51384.0420	-16.41 $\pm$ 0.39	-16.67 $\pm$ 0.11	229	213	207
	2.2 m 99	51385.0085	-17.02 $\pm$ 0.19		222	206
	2.2 m 99	51386.1725	-16.58 $\pm$ 0.24		231	215
	2.2 m 99	51387.0935	-16.83 $\pm$ 0.32		225	209
	2.2 m 99	51388.0347	-16.74 $\pm$ 0.32		209	193
	2.2 m 99	51389.0296	-16.50 $\pm$ 0.32		226	210
	NOT 99	51509.8749	-15.72 $\pm$ 0.38		220	204
V383 Lac	2.2 m 99	51384.0141	-19.55 $\pm$ 0.49	-20.19 $\pm$ 0.12	266	249	257
	2.2 m 99	51384.1565	-19.51 $\pm$ 0.49		265	248
	2.2 m 99	51384.9920	-20.59 $\pm$ 0.22		273	256
	2.2 m 99	51386.0848	-19.98 $\pm$ 0.33		277	260
	2.2 m 99	51387.0233	-20.61 $\pm$ 0.40		273	256
	2.2 m 99	51388.0023	-20.02 $\pm$ 0.22		277	260
	2.2 m 99	51388.9987	-20.19 $\pm$ 0.36		284	267
EP Eri	NOT 99	51508.9395	17.54 $\pm$ 0.11	17.54 $\pm$ 0.11	231	211	208
	NOT 99	51509.9946			225	206
DX Leo	NOT 99	51509.2471	8.33 $\pm$ 0.11	8.13 $\pm$ 0.08	200	184	186
	NOT 99	51510.2689	8.00 $\pm$ 0.13		196	180
	INT 00	51562.1797			188	172
	INT 00	51564.2121	8.16 $\pm$ 0.24		196	180
	INT 00	51566.1720	7.63 $\pm$ 0.24		183	167
GJ 211	INT 00	51566.0980	0.26 $\pm$ 0.17	0.26 $\pm$ 0.17	21	2	2
HD 77407	INT 00	51564.1974	4.72 $\pm$ 0.23	4.43 $\pm$ 0.17	168	160	162
	INT 00	51566.1601	4.08 $\pm$ 0.25		173	165
EK Dra	INT 00	51563.3055	-19.37 $\pm$ 0.48	-20.64 $\pm$ 0.33	198	189	195
	INT 00	51566.3100	-21.80 $\pm$ 0.46		201	202
UMa
V834 Tau	NOT 99	51509.0815	0.59 $\pm$ 0.17	0.27 $\pm$ 0.11	91	65	60
	NOT 99	51510.1062	0.07 $\pm$ 0.16		86	60
	INT 00	51566.0563	-0.13 $\pm$ 0.35		80	54
$\pi ^{1}$ UMa	NOT 99	51510.2862	-14.87 $\pm$ 0.15	-14.45 $\pm$ 0.13	117	107	106
	INT 00	51564.1889	-12.98 $\pm$ 0.33		116	106
	INT 00	51566.1501	-13.82 $\pm$ 0.34		115	105
GJ 503.2	INT 00	51566.2634	-9.26 $\pm$ 0.29	-9.26 $\pm$ 0.29	101	92	92
Others
LQ Hya	NOT 99	51509.2635	6.80 $\pm$ 0.41	8.26 $\pm$ 0.19	259	237	243
	NOT 99	51510.3008	7.97 $\pm$ 0.29		277	255
	INT 00	51564.1761	10.30 $\pm$ 0.50		255	233
	INT 00	51566.1127	8.82 $\pm$ 0.37		268	246
GJ 577	2.2 m 99	51384.8466	-6.45 $\pm$ 0.15	-6.48 $\pm$ 0.10	160	148	145
	2.2 m 99	51386.8451	-6.52 $\pm$ 0.27		165	153
	2.2 m 99	51388.8314	-6.49 $\pm$ 0.14		145	133
GJ 3706	INT 00	51564.2814	-2.60 $\pm$ 0.21	-2.60 $\pm$ 0.21	18	1	1