HIGH-RESOLUTION SPECTROSCOPY OF VEGA-LIKE STARS .2. AGE INDICATORS, ACTIVITY AND CIRCUMSTELLAR GAS

We have completed a high-resolution optical study of 14 stars classifi ed as Vega-like, having an IR excess attributable to dust emission. Su rface lithium abundances were measured for the four G-and K-type stars of the sample, to test the suggestion that these Vega-like stars are intermediate in evolutionary state between pre-main-sequence objects a nd established main-sequence stars. Abundances ranged from a very high value in the G5e star HD 143006, implying a very low age of only 1 My r, to below the limit of measurement for the K2V star HD 23362, which we conclude to be already well established on the main sequence. The e mission-line characteristics of all the stars in our sample were studi ed to compare with those seen in the classical pre-main-sequence Herbi g Ae/Be (HAeBe) stars and T Tauri stars. Activity levels ranged from s tars showing Little or no activity, such as HD 23362, to those exhibit ing extensive activity, such as the A9/F0Ve star HD 144432, which show ed distinctive P Cyg profiles in its spectrum, and HD 143006, which is young enough to be included in the T Tauri class of stars. The A2/3e star HD 35187 shows evidence of variability in its H alpha and HeI lam bda 5876 lines, with four other A-type stars in our sample also showin g evidence of He I lambda 5876 activity in the form of emission or abs orption. We interpret the excess absorption and/or emission in the lam bda 5876 line as providing direct evidence for ongoing accretion activ ity on these systems. We find that the emission characteristics of the H alpha, Na I D, He I and Ca II K lines are not significantly differe nt from those of HAeBe stars and T Tauri stars. Some of our sample hav e also been previously classified as pre-main-sequence or candidate pr e-main-sequence stars, which would seem to suggest that there is no di stinct boundary between Vega-like stars and the HAeBe and T Tauri star s. The surface gravities of the A-and F-type stars in our sample imply that they have already reached the main sequence, consistent with the short time-scales to reach the main sequence predicted for stars of t heir mass and the fact that they are not located close to star formati on regions. On the other hand, the ages derived for the three emission -line G-and K-type stars in our sample imply that they have probably n ot yet reached the main sequence. It is likely that these emission-lin e Vega-like stars represent the intermediate stage between classical p re-main-sequence stars with 'dusty' IR excesses and stars that are wel l established on the main sequence. We also studied our sample for evi dence of optical circumstellar gas absorption features. Of the 14 star s, seven show evidence for narrow absorption lines in their spectra. M ost of these appear to be of interstellar origin. One of these stars, HD 144432, has a narrow absorption component in the absorption trough of its Na I D P Cyg profile, at a heliocentric velocity of -91 km s(-1 ), which excludes an interstellar origin and therefore more likely ori ginates in its circumstellar environment. We also detect narrow absorp tion lines in the spectrum of HD 158643 (51 Oph) arising from excited- state Fe II lines, which can only come from the circumstellar environm ent of the star.