Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.290..165d&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 290, Issue 1, pp. 165-185.
Astronomy and Astrophysics
Astronomy
49
Techniques: Spectroscopic, Circumstellar Matter, Stars: Activity, Stars: Evolution
Scientific paper
We have completed a high-resolution optical study of 14 stars classified as Vega-like, having an IR excess attributable to dust emission. Surface 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 and established main-sequence stars. Abundances ranged from a very high value in the G5e star HD 143006, implying a very low age of only 1Myr, 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 emission-line characteristics of all the stars in our sample were studied to compare with those seen in the classical pre-main-sequence Herbig Ae/Be (HAeBe) stars and T Tauri stars. Activity levels ranged from stars showing little or no activity, such as HD 23362, to those exhibiting extensive activity, such as the A9/F0Ve star HD 144432, which showed 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 Hα and Hei lambda5876 lines, with four other A-type stars in our sample also showing evidence of Hei lambda5876 activity in the form of emission or absorption. We interpret the excess absorption and/or emission in the lambda5876 line as providing direct evidence for ongoing accretion activity on these systems. We find that the emission characteristics of the Hα, Nai D, Hei and Caii K lines are not significantly different from those of HAeBe stars and T Tauri stars. Some of our sample have also been previously classified as pre-main-sequence or candidate pre-main-sequence stars, which would seem to suggest that there is no distinct boundary between Vega-like stars and the HAeBe and T Tauri stars. 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 their mass and the fact that they are not located close to star formation 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 not yet reached the main sequence. It is likely that these emission-line Vega-like stars represent the intermediate stage between classical pre-main-sequence stars with `dusty' IR excesses and stars that are well established on the main sequence. We also studied our sample for evidence of optical circumstellar gas absorption features. Of the 14 stars, seven show evidence for narrow absorption lines in their spectra. Most 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 Nai D P Cyg profile, at a heliocentric velocity of -91kms^-1, which excludes an interstellar origin and therefore more likely originates in its circumstellar environment. We also detect narrow absorption lines in the spectrum of HD 158643 (51 Oph) arising from excited-state Feii lines, which can only come from the circumstellar environment of the star.
Barlow Michael J.
Dunkin Sarah K.
Ryan Sean G.
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