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Astron. Astrophys. 321, 497-512 (1997)

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3. Character of the variations

By comparing the variations in the equivalent width of the strongest line, H [FORMULA] (lower plots in Fig. 2) with the I-band CCD-photometry (upper), one can see three different kinds of variability in DG Tau: the stars showed either rapid line variations on time scales of order our time resolution of 10 minutes (see the third panel in Fig. 2 for JD 2248513), smooth increases or decreases on time scales of hours (the fourth panel for JD 2248514), or the stars were essentially constant (the last panel for JD 2248515). DI Cep, however, showed smooth variations only. The amplitudes of the variations of the equivalent widths of H [FORMULA] within a few hours were on average [FORMULA] times smaller than the night-to-night variations(Table 2), but some nights show variations on time scales of hours or less with an amplitude comparable to that of the night-to-night variations. The Paschen, Ca II, and He I lines show the same night-to-night variations as H [FORMULA], but are much more variable on shorter timescales, with typical amplitudes of 50-100%. The amplitudes of the continuum variations on time scales of an hour or less are at least 10 times smaller than the amplitudes of the night-to-night variations.

[FIGURE] Fig. 2. Simultaneous measurements of the equivalent width of H [FORMULA] (lower) and relative photometry in Johnson I (upper) in DG Tau. The equivalent width and the relative intensity are plotted against UT time in hours. Each bin corresponds to one observing night (JD 2 448 511 - 2 448 515).


Table 2. Variations within a night and from night to night1

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998