Volume 559, November 2013
|Number of page(s)||5|
|Published online||01 November 2013|
Using data from four different instruments over about a decade to detect subtle effects might be problematic in terms of stability and cross-instrumental effects. All are echelle spectrographs; HARPS, FEROS, and HEROS are attached by fiber-link, while UVES is mounted on a gravity invariant Nasmyth platform. The three ESO instruments are monitored and partly thermally controlled for stability. All spectra were used in the heliocentric reference frame. The pixel sampling of the spectra is from 0.1 Å (HEROS) over 0.03 Å (FEROS) down to about 0.01 Å (UVES and HARPS). This is much smaller than the width of the observed variations, which are several tens of km s-1 even for the most narrow ones, the high-velocity “dents”. The observed line depths differ by almost 2% of the continuum, at a typical S/N of the average spectra of about 1000. The observed effect is not systematically different between FEROS−FEROS and OTHER−FEROS comparisons.
Since the variations are very oversampled and have a consistent appearance and evolution through all data sets, regardless of the instrument, a cross-instrumental/stability effect can be excluded.
To allow an assessment of the circumstellar disk strength independent of the Hα equivalent width, the seasonal Hα profiles are shown in Fig. B.1 in a similar format to the one given by Vinicius et al. (2006) in their Fig. 12. In a shell star, the net effect of a
weakly developed disk on the overall Hα equivalent width can vanish if, as happened in Achernar in 2001, the contributions from line emission and shell absorption just cancel one another out. Note as well the unusual Balmer-line CQE seen in the FEROS 2001 and somewhat more strongly in UVES 2004 data.
In the proof-of-concept model, a gravity-darkening value of β = 0.25 was used, even though lower values of β are typically derived observationally. We note that β = 0.2 was adopted by Domiciano de Souza et al. (2012), though not derived. The model we use can compute spectra for different values of β, but the current formalism for computing the corresponding stellar parameters, in particular Teff and L, has been derived with a fixed β = 0.25. For this reason, modeling with a fixed Teff is only possible with β = 0.25, a shortcoming that will be overcome in a more detailed dedicated work. To demonstrate that the effect does not vanish for a different β, we show models computed with constant Tpole instead, for β = 0.20 (Fig. C.1). Compared to the models shown in Fig. 4, the “dents” are unaltered in Ca ii3933 and the Si ii lines, and slightly less pronounced in He i lines, but still clearly present in He i 3927, 4009, 4121, 4144. The most significant difference is that the Balmer line wings are no longer reproduced (and Balmer lines are generally stronger), which, however, is an effect dominated by keeping Tpole fixed rather than Teff, and otherwise unrelated to the value of β.
Hα difference profiles with respect to FEROS 2000 for all observing seasons. Telluric lines were not corrected for.
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Same as Fig. 4, with models computed for a fixed Tpole = 16 900 K and β = 0.20.
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© ESO, 2013
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