Short-term variability of the CA II K line in the pre-main sequence Herbig AE start HD 250550

Details Short-term variability of the CA II K line in the pre-main sequence Herbig AE start HD 250550 Short-term variability of the CA II K line in the pre-main sequence Herbig AE start HD 250550

Apr 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991a%26a...244..166c&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 244, no. 1, April 1991, p. 166-180.

Astronomy and Astrophysics

Astrophysics

23

Astronomical Spectroscopy, Line Spectra, Pre-Main Sequence Stars, Stellar Mass Ejection, Variable Stars, Calcium, Potassium, Red Shift, Stellar Models, Stars: Pre-Main Sequence, Stars: Variable, Stars: Activity Of, Stars: Chromospheres Of, Stars: Mass Loss

Scientific paper

We present observations of the short-term variability in the Ca II K line of the pre-main sequence Herbig Ae star HD 250550. In February 1986 and in January 1988, we obtained quasi-continuous series of Ca II K spectra for this star, combining observations at the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii, at the Isaak Newton Telescope at La Palma in the Canary Islands (only in 1986), and at the Observatoire de Haute-Provence in France (only in 1988).
We find that the Ca II K line of HD 250550 is subject to strong variations with a time scale of a few hours. The line is structured into a narrow component which does not seem to vary, and several broad components (one redshifted emission component, one absorption component centered at rest wavelength and one blue shifted absorption component) which appear as highly variable.
The short-term variability can be decomposed into a smooth variation, affecting principally the red edge of the blueshifted absorption component, plus a sudden broadening, appearing first in the central component, then latter in the blueshifted absorption component. The data strongly suggest that the smooth variations are periodic, with a period of 41+9-5h.
We propose to interpret the periodic part of the variations in terms of the magnetic stream model, already envisaged for two other Herbig Ae stars, AB Aur, and HD 163296. In this model, fast and slow streams alternating on the line of sight as the star rotates modulate the line with the star's rotation period. The stream structure is presumably controlled by a surface magnetic field. The additional sudden broadening of the absorption components can be interpreted as a perturbation originating from the star's surface and affecting successively different regions of the wind as it is advected by the flow. The data presented here do not allow to determine the physical nature of this perturbation, but they provide several useful constraints on the location and size of the regions of formation of the various parts of the line.

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