Spectroscopy of southern active stars. II - HD 32918, HD 82558, BD - 22 deg 3467, AB Doradus (HD 36705) and RST 137 B

Details Spectroscopy of southern active stars. II - HD 32918, HD 82558, BD - 22 deg 3467, AB Doradus (HD 36705) and RST 137 B Spectroscopy of southern active stars. II - HD 32918, HD 82558, BD - 22 deg 3467, AB Doradus (HD 36705) and RST 137 B

Jan 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991a%26a...241..167v&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 241, no. 1, Jan. 1991, p. 167-175.

Astronomy and Astrophysics

Astrophysics

51

Cool Stars, H Alpha Line, Metallicity, Stellar Activity, Stellar Spectrophotometry, Dwarf Stars, Emission Spectra, Lithium, Pre-Main Sequence Stars, Southern Sky, Stellar Atmospheres, Stellar Activity, Chromospheres Of Stars, Late-Type Stars, Mass Loss, Stellar Rotation

Scientific paper

We have observed the southern active cool stars HD 32918 (K1 IIIp), HD 82558 (K2 V), BD-22°3467 (G8 III), AB Doradus (HD 36705, K1 IV) and Rst 137 B (M 3-5), each of which lies close to the chromospheric- coronal saturation limit. The fractional Hα-luminosities LHα/Lbol of the stars were close to 10-4. No significant radial velocity variations were detected in any of the stars.
With the variations of the Fe I 6546-line profiles, as a function of the 9.55 d rotation period, the existence of a cool spot-region of HD 32918 was traced. The spectra were analysed by a surface imaging technique by Piskunov et al. (1990). The resulting image showed a large group of cool spots at the equatorial belt at one side of the star, and a smaller and less cool one extending to higher latitudes at the opposite longitude. Old photometry suggests that the equatorial spots might have been permanent during 7 yr at the same (active) longitude. The Hα-emission of HD 32918 was variable and asymmetric, comprising possibly of chromospheric and wind (or expanding chromosphere) components. The He I 5876 (D 3) absorption was remarkably strong in HD 32918, pointing to the existence of an extended and low density chromosphere.
BD-22°3467, the cool companion to the hot central star of the planetary nebula Abell 35, experiences a photometric wave with a 0.77 d period (Jasniewicz & Acker, 1988). Our observations, performed around the same time with this photometry, resolved a blue-shifted Hα-emission at the minimum light. This behaviour was opposite to that found in HD 32918. The υ sin i value (90 km s-1) of BD-22°3467 is compatible with the interpretation that the 0.77 d photometric cycle is the star's rotation period.
The Hα-emission of HD 82558 was stronger than in the previous observations by Fekel et al. (1986) and variable, but with no obvious rotational modulation. The Hα-emission strengths of the young pre-main-sequence pair AB Dor - Rst 137 B are compatible with those of the Pleiades stars (WHα vs. R - I, Stauffer & Hartmann 1986), giving additional support to the small age of the system. Rst 137 B has the He I 5876 triplet in strong emission (EW = 750±80 mA), and its photospheric lines are broad (υ sin i around 50 km s-1). These facts suggest that Rst 137 B is an extremely active M star. In AB Dor we detected two Hα-transients (the first in emission and the second one in absorption).
In the fastest rotators of our sample, AB Dor (0.51 d) and HD 82558 (1.66 d), we detected no He I D3 absorption nor emission. The physics of the Hα and He D3 transitions seem to be related. In low density and active chromospheres (with large optical thickness and filling factor) the lines appear in absorption (photo-ionization dominated plasma), while in very high density chromospheres (Rst 137 B) they are in emission (collisionally dominated plasma). Intermediate cases show no absorption nor emission (filled-in level, AB Dor as a typical example).
The lithium-abundances, as determined by us, suggest that AB Dor (logɛ(Li) = 3.1) and HD82558 (logɛ(Li) = 2.6) are premain-sequence stars, while HD 32918 (logɛ(Li) = 1.6-2.0) is most probably an evolved object.

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