6 Individual results

In this section we describe the individual results about stellar parameters, kinematic, lithium, chromospheric activity, and membership of the different SKG for each star of the sample.

6.1 PW And (HD 1405)

This solar neighborhood Pleiades-age K2 dwarf (Ambruster et al. 1998) is a fast rotator with a photometric period $P_{\rm phot}$ = 1.745 days (Hooten & Hall 1990) and $v\sin{i}$ = 23.4 km s^-1 (Fekel 1997). Strassmeier et al. (1988) listed this star as a chromospherically active binary candidate, however, Griffin (1992) found no evidence of variability in their radial velocities, indicating it is a single star. The mean radial velocity determined by us ( $v_{\rm r} = -10.99$ km s^-1) is very similar to the values given by Fehrenbach & Burnage (1982) ( $v_{\rm r} = -11.5$ km s^-1) and Griffin (1992) ( $v_{\rm r} = -10.4$ km s^-1) supporting the single nature of this star. We have found, however, small amplitude radial velocity variations which follow the rotational period (1.75 days) of the star. These variations can be associated with photospheric spots that disturb the line profile of this rapidly rotating star. Bidelman (1985) and Christian et al. (2001) reported moderate Ca  II H & K emission and the H$\alpha$ line in emission in this star. Chromospheric and transition region UV emission fluxes have been reported by Ambruster et al. (1998) and Wood et al. (2000). In addition, it has been detected by the ROSAT-satellite as the 2RE J001820+305 source (Pye et al. 1995; Kreysing et al. 1995; Thomas et al. 1998; Rutledge et al. 2000), and by the EUVE-satellite as the EUVE J0018+309 source (Malina et al. 1994; Christian et al. 2001). In the eight spectra of this star that we have analysed, we have found intense emission in the Ca  II H & K and H$\epsilon$ lines, strong H$\alpha$ emission above the continuum (see Fig. 5), excess chromospheric emissions in the other Balmer lines, and emission reversal in the Ca  II IRT lines (Fig. 6). We have detected variations in the excess emission of the different chromospheric lines, specially in the Balmer lines. High Li  I abundance indicative of a Pleiades-age star is reported by Ambruster et al. (1998). The EW(Li  I) = 271 mÅ$\$ we have determined confirms it is a young star. The space motions and all the other spectroscopic properties we have analysed prove its LA membership.

6.2 V368 Cep (HD 220140, HIP 115147)

This star is the optical counterpart of the X-ray source H 2311+77 (Pravdo et al. 1985) and has been identified as member of the LA and classified as a post T Tauri star (Chugainov et al. 1991a, 1993; Ambruster et al. 1998; Kahanpää et al. #Kahanpä&). The spectral type of this star in the literature ranges from G5 to K2V, but the recent photometric observations of Kahanpää et al. (#Kahanpä&) support a K1V spectral type. It is a rapidly rotating and spotted star with a photometric period $P_{\rm phot}$ = 2.74 d (Kahanpää et al. #Kahanpä&). and $v\sin{i}$ = 16.1 km s^-1 (Fekel 1997). From our six spectra taken in July 1999 and another one taken in November 1999 we have determined a constant radial velocity with a mean value of -16.67 km s^-1, which is within the range given in the literature (15-17 km s^-1, Chugainov et al. 1991a), supporting its classification as a constant-velocity star. Evidence of magnetic activity including strong Ca  II H & K emission, IUE-, EUVE- and ROSAT-satellite detections have been reported for this star (Bianchi et al. 1991; Malina et al. 1994; Pye et al. 1995). In our spectra we observe intense emission in the Ca  II H & K and H$\epsilon$ lines, strong and variable excess chromospheric emissions in the Balmer lines, specially in H$\alpha$ (Fig. 5), and the Ca  II IRT lines in emission superimposed on the corresponding absorption (Fig. 6). Chugainov et al. (1991a) report a Li  I $\lambda$6707.8 Å$\$ line stronger than the Ca  I $\lambda$ 6717 Å$\$ line with a EW(Li  I) = 288 mÅ. In our spectra the Li  I absorption feature is also stronger than the Ca  I line but the mean EW we have obtained after the correction of the Fe  I line is EW(Li  I) = 207 mÅ which is similar to EW(Li  I) observed in Pleiades stars of this spectral type (Fig. 2). However, this EW(Li  I) is not high enough to consider the star as a post T Tauri star (see the EW(Li  I) vs. spectral type diagram by Martín 1997). Both kinematic and spectroscopic criteria indicate V368 Cep is a bona fide member of the LA.

6.3 V383 Lac (BD+48 3686)

Recent spectroscopic and photometric studies of V383 Lac (Mulliss & Bopp 1994; Jeffries 1995; Henry et al. 1995; Fekel 1997; Osten & Saar 1998) concluded it is a single active K1V star with an age less than of the Pleiades and with a rapid rotation. These authors report a photometric period $P_{\rm phot}$ = 2.42 days and $v\sin{i}$ ranging from 14 to 20 km s^-1. The mean radial velocity determined from our seven spectra ($v_{\rm r}$ = -20.19 km s^-1) shows no evidence of variability and it is in agreement with the range of values (from -19.4 to -22.1 km s^-1) given in the literature, supporting the conclusion that it is a single star. Mulliss & Bopp (1994) found the H$\alpha$ and Ca  II IRT lines filled by emission. It has been detected as an extreme ultraviolet source (Pye et al. 1995; Lampton et al. 1997). In our spectra, which cover more than one stellar rotation, we have found notable emission in the Ca  II H & K and H$\epsilon$ lines, excess chromospheric emission in the Balmer lines and emission reversal in the Ca  II IRT lines. During one of the nights a noticeable increase in the excess emission is detected, showing the H$\alpha$ line large emission wings. This variation could be due to a small-scale flare or to the transit of an active region. We have determined a EW(Li  I) = 257 mÅ, similar to the values of 250 and 277 mÅ given by Mulliss & Bopp (1994) and Jeffries (1995) respectively. This high EW(Li  I) (close to the upper envelope of the Pleiades) indicates it is a young star. The space motions and all the other spectroscopic characteristic we have analysed in this star confirm it is a member of the LA.

6.4 EP Eri (HD 17925, GJ 117, HIP 13402)

This is a very nearby (8 pc), very young (high Li  I abundance) active K2-type dwarf with a rotation period of 6.85 days (Cutispoto 1992; Henry et al. 1995). The presence of an unresolved companion in this star has been suggested by Henry et al. (1995) based on the variable widths of the photospheric absorption lines reported in the literature ($v\sin{i}$ range from 3 to 8; see Fekel 1997). Wood et al. (2000) also suggest that an unresolved secondary can be contributing to the emission Mg  II h and k lines. However, no evidence or velocity variability is reported in the literature (Halbwachs et al. 2000) and the radial velocity we have determined (17.5 km s^-1) is in good agreement with that of Beavers & Eitter (1986) 18.8 km s^-1, and Henry et al. (1995) 18.1 km s^-1. Cutispoto et al. (2001) also indicate that the binary hypothesis does not seem to be consistent with the Hipparcos photometric data. Strong Ca  II H & K emission and a filled-in H$\alpha$line have been found by Pasquini et al. (1988) and Henry et al. (1995). Chromospheric and transition region UV emission fluxes have been reported by Ambruster et al. (1998) and Wood et al. (2000). It is also an X-ray and EUV source (Favata et al. 1995; Jeffries 1995; Lampton et al. 1997). In our spectra we have found Ca  II K emission and excess chromospheric emission in the H$\alpha$ and the Ca  II IRT lines. EP Eri is a young star as indicated by the strong lithium line detected by Cayrel de Strobel & Cayrel (1989). We have measured a EW(Li  I) = 208 mÅ, similar to the 197 mÅ$\$ given by Favata et al. (1995) and the 205 mÅ$\$ obtained by Jeffries (1995). Recently, additional evidence of youth has been reported. An age of 80 Myr has been estimated by Lachaume et al. (1999) and a IR excess (ISO 60 $\mu$m) has been detected in this star and attributed by Habing et al. (2001) to a circumstellar disk (Vega-like). Ambruster et al. (1998) identified it as member of the LA, and the kinematic data analysed by Cayrel de Strobel & Cayrel (1989) show that the birth-place of this star is associated with the Scorpio-Centaurus complex. The position in the (U, V) ad (U, W) planes we have determined as well as the spectroscopic criteria are in agreement with its LA membership.

6.5 DX Leo (HD 82443, GJ 354.1, HIP 46843)

This is a nearby, young and active K0 dwarf with space motion very similar to the LA (Soderblom & Clements 1987; Ambruster et al. 1998; Gaidos et al. 2000). Optical flux modulation with a period of 5.4 days attributed to cool photospheric spots have been found by Henry et al. (1995), Messina et al. (1999b), and Gaidos et al. (2000). A projected rotational velocity $v\sin{i}$ = 6.2 is given by Fekel (1997). DX Leo is a single star as indicates the constant radial velocity we have determined in our spectra, $v_{\rm r}$ = 8.13 km s^-1, and the values of $v_{\rm r}$ = 8.2, 8.9, and 8.25 km s^-1 given by Duquennoy et al. (1991), Griffin (1994), and Henry et al. (1995) respectively. Strong chromospheric and transition region line emissions have been reported by Soderblom & Clements (1987), Basri et al. (1989), Ambruster et al. 1998; Strassmeier et al. (2000), and Wood et al. (2000). It is also a X-ray and EUV source (Pye et al. 1995; Hünsch et al. 1999). In our spectra, taken in two different epochs, we have found noticeable emission in the Ca  II H & K and Ca  II IRT lines, and excess chromospheric emission in the H$\alpha$ line. We have obtained a EW(Li  I) = 198 mÅ$\$ which is within the range observed in the Pleiades and similar to the value of EW(Li  I) = 187 mÅ$\$ given by Strassmeier et al. (2000). The space motion we have determined and all the spectroscopic criteria we have analysed indicate DX Leo is a young star member of the LA.

6.6 GJ 211 (HD 37394, HR 1925)

This is a nearby star classified as a possible member of the LA (Jeffries & Jewell 1993; Gaidos et al. 2000). It is a K1V slowly rotating star ( $P_{\rm phot}$ = 10.86 d, and $v\sin{i}$ = 4.0 km s^-1, Gaidos et al. 2000) but with evidence of chromospheric activity (emission in the Ca  II H & K and Mg  II h & k lines, Soderblom & Clements 1987) and coronal activity (EUV and X-ray emission Hünsch et al. 1999). It has even been classified by some authors (Gershberg et al. 1999) as a flare star. Unlike EP Eri, Habing et al. (2001) have not detected evidence of circumstellar disk in the IR (ISO 60 $\mu$m) flux of this star. Constant radial velocity has been reported for this star with values ranging from -0.2 to 2 km s^-1, which are in agreement with the $v_{\rm r} = 0.26$ km s^-1 we have determined. In our spectrum we observe small excess chromospheric emission in the Balmer lines and Ca  II IRT lines and a very small EW(Li  I) (2.0 mÅ) in agreement with the EW(Li  I) = 1.3$\,\pm\,$3.2 mÅ$\$ reported by Gaidos et al. (2000). The space motions calculated by us are consistent with the LA but the low level of chromospheric emission and very small EW(Li  I) indicate it is not a young star and probably it is not a member of the LA.

Figure 6: Spectra in the Ca  II IRT (8498, 8542 Å) line region for our star sample. Observed and synthetic spectra in the left panel and subtracted spectra in the right panel.

6.7 HD 77407 (BD+38 1993, HIP 44458)

This G0 star was included in the study of the Hyades and Sirius MGs by Eggen (1986), but not identified as a member of any of these MGs. It is an X-ray/EUV source detected by ROSAT and EUVE (Lampton et al. 1997) and also detected as a stellar radio source by Helfand et al. (1999). However, it is a very little-studied star, and no previous determinations of radial velocity, rotation, chromospheric activity and lithium have been reported in the literature. Using our two spectra of this star we have determined a mean radial velocity of 4.43 km s^-1, which together with the astrometric data results in a Galactic space motion (U, V, W) similar to the LA. Eggen's kinematic criteria also confirm their membership of the LA. Our spectra show that it is a slow rotating star ($v\sin{i}$ $\approx$ 7 km s^-1) with a notable chromospheric excess emission in the $H\beta$, $H\alpha$, and Ca  II IRT lines. The EW(Li  I) = 162 mÅ$\$ we have determined lies above the upper EW(Li  I) envelope of the Pleiades (as also reported by Wichmann & Schmitt 2001), indicating that it is a very young star and therefore a probable member of the LA.

6.8 EK Dra (HD 129333, GJ 559.1A, HIP 71631)

This nearby G1.5V spotted and very active star (Strassmeier & Rice 1998; Strassmeier et al. 2000) was previously identified as a member of the LA (Chugainov 1991; Chugainov et al. 1991b; Soderblom & Clements 1987) and considered as a young solar analog (Dorren & Guinan 1994). It is a rapidly-rotating star with $v\sin{i}$ 17.3 km s^-1 and with a photometric period ranging in the literature from 2.6 to 2.8 days (Chugainov et al. 1991b; Dorren & Guinan 1994; Strassmeier & Rice 1998; Messina et al. 2001; DePasquale et al. 2001). EK Dra can be treated essentially as a single star, although Duquennoy & Mayor (1991) have suggested that it may be a member of a long-period ($\approx$12.5 yr) binary system with radial velocity variations between -21 and -32 km s^-1 and $v_{\rm0}$ = -23.1 km s^-1. Dorren & Guinan (1994) found a mean radial velocity of -24 km s^-1, Fehrenbach et al. (1997) give -18 km s^-1 and we have obtained in our spectra a mean value of -20.6 km s^-1. High levels of magnetic activity have been detected in this star: strong Ca  II H & K emission (Soderblom 1985); variable UV chromospheric emission lines (Dorren & Guinan 1994; Saar & Bookbinder 1998); and X-ray and EUV emission (Güdel et al. 1997; Audard et al. 2000). In our spectra we observe a small emission reversal in the Ca  II IRT lines and a notable excess emission in the $H\alpha$ and $H\beta$ lines. Strong Li  I absorption has been reported previously in the literature for this star (see Wichmann & Schmitt 2001). In our spectra we have determined a EW(Li  I) of 198 mÅ, which is between the upper envelopes of the Pleiades and IC 2602, indicating that this star is significantly younger than the Pleiades open cluster. The space motion, high level of magnetic activity, and strong Li  I absorption observed in this star confirm that it is a member of the LA.

6.9 V834 Tau (HD 29697, GJ 174, HIP 21818)

This very active K4V star was considered for some time (Strassmeier et al. 1993) as candidate to chromospherically active binary. However, the constant radial velocities measured by several authors (Fouts & Sandage 1986; Henry et al. 1995; Halbwachs et al. 2000) and our radial velocity determination ( $v_{\rm r} = 0.27$ km s^-1) indicate that it is a single star. Light variations were first discovered in this star by Chugainov (1981), later, Henry et al. (1995) determined a photometric period of 3.936 days and Fekel (1997) measured a rotational velocity, $v\sin{i}$ = 9.5 km s^-1. Exceptional strong Ca  II H & K emission lines (Young et al. 1989), modest H$\alpha$ emission above the continuum (Rutten et al. 1989; Henry et al. 1995) and Radio, X-ray and EUV emission (Güdel 1992; Pye et al. 1995; Hünsch et al. 1999) have been detected in this star. In our spectra taken at two different epochs we have found strong emission in the Ca  II K line, the H$\alpha$ line in emission above the continuum with a central self-absorption, a filled-in absorption H$\beta$ line, and the Ca  II IRT lines in emission. Chugainov (1991) and Eggen (1996) listed V834 Tau as a young disk star and Chugainov (1991) suggested it as a possible member of the LA. However, the Galactic velocity components we have determined indicate it is a possible member of the UMa. The lithium line has been detected in this star; Henry et al. (1995) give a EW(Li  I) = 79 mÅ, and we have determined in our spectra a EW(Li  I) = 60 mÅ. This notable EW(Li  I) (see Fig. 2) indicates it is a young star and confirms its membership of the UMa.

6.10 $\pi ^{1}$ UMa (HD 72905, GJ 311, HIP 42438)

This nearby G1.5V active star, considered as proxy of the young Sun (Bochanski et al. 2001), has been classified as a possible member of the UMa moving group by Soderblom & Clements (1987); Soderblom & Mayor (1993a); Gaidos (1998); and Gaidos et al. (2000). It has a $v\sin{i}$ = 9.5 km s^-1 (Fekel 1997) and a short rotation period ($P \approx$ 4.8 days) as measured by periodic light variations due to starspots by Gaidos et al. (2000) and Bochanski et al. (2001). The mean radial velocity we have determined ($v_{\rm r}$ = -14.45 km s^-1) is similar to the value reported by Duquennoy et al. (1991) ($v_{\rm r}$ = -12.66 km s^-1). $\pi ^{1}$ UMa has high levels of chromospheric and coronal activity (Soderblom & Mayor 1993a,b; Dorren & Guinan 1994). A superflare (Schaefer et al. 2000) was detected in this star in the X-ray band by the EXOSAT satellite (Landini et al. 1986). In our observations we have found small excess chromospheric emissions in the H$\alpha$, and the Ca  II IRT lines. We have determined a EW(Li  I) = 106 mÅ, very close to the value of 96 mÅ given by Soderblom et al. (1993a). This EW(Li  I) is intermediate between the Hyades upper envelope and Pleiades lower envelope, corresponding to the age of the UMa. Our new calculation of the galactic velocity components and the spectroscopic criteria above-described are in agreement with the membership of this star to the UMa.

6.11 GJ 503.2 (HD 115043, BD+57 1425, HIP 64532)

This nearby G2V star is classified as a member of the UMa group by Eggen (1992) and Soderblom & Mayor (1993a). It is a slow rotating star with $v\sin{i}$ = 7.5 km s^-1 (Soderblom & Mayor 1993b). Duquennoy et al. (1991) reported a constant radial velocity ($v_{\rm r}$ = -8.86 km s^-1) for this star, which is very similar to the value determined in our spectrum ($v_{\rm r}$ = -9.26 km s^-1) confirming it is a single star. Evidence of magnetic activity has been found in the X-ray by the ROSAT (Hünsch et al. 1999) and in the ultraviolet by the IUE (Soderblom & Clement 1987) and the HST (Lamzin 2000). Moderate Ca  II H & K chromospheric emission is reported by Soderblom (1985). In our spectra we have found a very small fill-in in the H$\alpha$ and Ca  II IRT lines. The Galactic velocity components we have determined and the kinematic criteria are compatible with the star being a member of the UMa. In addition, the EW(Li  I) of 92 mÅ$\$ determined in our spectrum, which is similar to the value of 77 mÅ$\$ given by Soderblom et al. (1993a), indicates an age intermediate between the Hyades and Pleiades corresponding to the UMa. All this supports that GJ 503.2 is a bona fide member of the UMa.

6.12 LQ Hya (HD 82558, GJ 355, HIP 46816)

This star is a young single K2 dwarf classified as a BY Dra variable (Fekel et al. 1986). It is a rapidly rotating star with v sini = 26.5 km s^-1 (Donati 1999) and with a photometric rotational period of 1.600881 days (Strassmeier et al. 1997). It is a very active star, as indicated by emission in several chromospheric and transition region lines, even with occasional flares (see Montes et al. 1999 and references therein). EUV and X-ray emission and X-ray flares have been detected in this star (see Covino et al. 2001 and references therein). In our four spectra taken at two different epochs we observed strong emission in the Ca  II K line, the H$\alpha$ line in emission above the continuum with an intensity similar to that observed in the quiescent spectra by Montes et al. (1999), a filled-in absorption H$\beta$ line, and the Ca  II IRT lines in emission. We have determined a mean radial velocity, $v_{\rm r}$ = 8.26 km s^-1, very close to the mean value of 7.3 km s^-1 reported by Fekel et al. (1986), 7.5 km s^-1 given by Vilhu et al. (1991), and 9.0 km s^-1 by Donati et al. (1997), confirming it is a constant radial velocity star. Eggen (1984b) suggested that LQ Hya may be a member of the HS, Fekel et al. (1986), using their new radial velocity, found it should be considered only as a YD star, however, Chugainov (1991) and Ambruster et al. (1998) listed this star as a member of the LA. The U, V and W velocity components we have calculated using the astrometric data from Tycho-2 Catalogue and the radial velocity determined by us indicate that this star is a YD star but not a member of the LA. It is, however, a young star as pointed out by the strong lithium absorption line (EW(Li)  I = 234  mÅ) reported by Fekel et al. (1986). In our spectra we have obtained a similar mean EW(Li)  I of 243 mÅ, which is close to the upper envelope of the Pleiades cluster.

6.13 GJ 577 (IU Dra, HD 134319, HIP 73869)

Messina & Guinan (1998) and Messina et al. (1999a) considered this G5V star as a proxy for the young Sun and classified it as a probable member of the HS according with its U, V, W components and parallax. These authors found high levels of photospheric magnetic activity in this star and reported a photometric rotation period of 4.448 days. The mean radial velocity we have determined with our three spectra ($v_{\rm r}$ = -6.48 km s^-1) is very close to the constant radial velocity ($v_{\rm r}$ = -6.38 km s^-1) given by Duquennoy et al. (1991), confirming it is likely a single star. Soderblom (1985) found moderate chromospheric emission in this star. Moderate Ca  II H & K emission is observed in our spectra, however, the H$\epsilon$ line is not in emission. In addition, a notable filling-in is detected in the H$\alpha$ and Ca  II IRT lines. The behaviour of the lithium ($\lambda$6707.8 line) in this star has not been previously reported in the literature. We have determined in our spectra a EW(Li  I) of 145 mÅ$\$which is well above the upper envelope of the Hyades and close to the lower envelope of the Pleiades (see Fig. 2). Even though this star could be considered as member of the HS based on its position in the (U, V) plane and the kinematic criteria, the EW(Li  I) indicates it is too young to be a member of the HS.

6.14 GJ 3706 (HD 105631, BD+41 2276, HIP 59280)

This K0V star was classified as a member of the IC 2391 supercluster by Eggen (1991). The radial velocity we have determined for this star ($v_{\rm r}$ = -2.6 km s^-1) is in agreement with the previous value ($v_{\rm r}$ = -3.1 km s^-1) reported by Duflot et al. (1995). This star is listed by Hünsch et al. (1999) as source of X-ray detected by the ROSAT-satellite and Strassmeier et al. (2000) found slight Ca  II H&K emission. In our spectrum we have found a very low level of chromospheric activity (no filled-in is detected in H$\alpha$ and a very slight filled-in is observed in the Ca  II IRT lines). The Li  I absorption line is practically not detected in our spectrum (EW(Li  I) = 1.4 mÅ) indicating it is not a young star and probably not a member of the IC 2391 SC, in spite of the fact that the kinematic criteria point out that it is a member of this SC.