Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...314..821p&link_type=abstract
Astronomy and Astrophysics, v.314, p.821-834
Astronomy and Astrophysics
Astrophysics
42
Stars: Pre-Main Sequence, Circumstellar Matter, Stars: Imaging, Stars: Individual: Su Aurigae
Scientific paper
Series of high-resolution echelle spectra of the T Tauri star SU Aurigae were collected during 3 observing periods. Simultaneous photoelectric UBV photometry was made during one period. We have analysed selected spectral orders, covering the Balmer lines from Hα to Hδ, He I 5876A, the Na I D lines and Ca II 8662A in addition to a number of photospheric absorption lines. We confirm and extend the results found in two similar studies by Giampapa et al. (1993ApJS...89..321G) and Johns & Basri (1995ApJ...449..341J & 1995AJ....109.2800J). Features which have been persistent over the years are the simultaneous occurrence of mass infall and outflow in the line-of-sight to the star. The outflow is seen as blueshifted absorption components in Hα and Hβ, but not in the higher Balmer lines. The inflow is most evident through the redshifted absorption components in the differential profiles (SU Aur minus standard) of Hβ and the higher Balmer lines, while there is also a redshifted absorption-like component superimposed on the red wing of the Hα emission. This indicates an outflow of gas in the outer part of the circumstellar envelope/wind of the star and a simultaneous inflow/accretion in the innermost region of the envelope. No free-fall acceleration of the inflowing gas was found from measured accretion component velocities from Hα to Hδ. We found variations of the inflow velocity with an amplitude of about 50km/s on a time-scale of 3 days, similar to the period found earlier in the intensity variations of Hα and Hβ. There is a correlation between the variability of the He I 5876A absorption line intensity and the variability in intensity of the redshifted Balmer absorption lines. This indicates that the He I line formation is dependent on the accretion. We identify the periodic variations with modulation of the projected inflow velocity by the stellar rotation and find that a model with magnetically channeled accretion in a dipole field inclined to the axis of stellar rotation describes the observations in a natural way. SU Aur is seen almost edge on. The time-scale of variability in the Hα blueshifted absorptions at radial velocities of -100 to -200km/s is less than one day. This implies a stellar wind acceleration region within 3 stellar radii and we find that the wind decelerates outside this region. The process of outflow is not steady but rather impulsive, which may result in the formation of expanding shells. The traces of such decelerated shells at large distance from the star are observed as a more stable blueshifted absorption at about -50km/s in the profiles of the Hα and Na I D-lines. During one period we observed a very strong event of mass ejection where the material was seen to decelerate over 3 days. We have also made surface (Doppler) imaging of the stellar surface using photospheric Fe I line profiles, which are variable in intensity and indicate the presence of stellar surface temperature inhomogeneities. The simultaneous UBV photometry provides constraints on such modeling, although due to the lack of data we do not include the photometry in the temperature mapping procedure. However, the derived temperature map is consistent with the lack of rotationally modulated variability in the B-V photometry.
Gahm Goesta F.
Gullbring Erik
Hackman Thomas
Ilyin I. I.
Loden K.
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