## 2. Veiling derivation: the state of the artThe veiling of a T Tauri star has been defined as the ratio of
its excess flux to either the local photospheric continuum of the
comparison template, named Two main classes of algorithms have been proposed to extract the
veiling from CTTS spectra. The first class was introduced by HHKHS and
is based on a point to point comparison between the photospheric
absorption HHKHS proposed to extract the veiling where the index Let us summarize the conclusions of CH99 with respect to the bias in veiling calculations from Eq. (1). There are two classes of bias coming from: 1) bad noise ratio estimates and, 2) spectral mismatches of any kind, real or due to systematic errors, between the object and the comparison template. Recently, Guenther et al. (1999) have shown that the presence of strong magnetic fields in T Tauri stars may lead to wrongly determined spectral types and may induce veiling overestimates. Let be the calculated veiling and be the difference between the true and the calculated veiling. Let us assume for the moment that there is no spectral mismatches. The veiling bias is obviously equal to zero if the input noise ratio in Eq. (1) represents the real noise ratio. On the other hand, the relative veiling bias is bounded by a positive upper limit and a negative lower limit: where means in Eq. (1) and (resp. ) is the variance with wavelength - square of the spectral constrast - in noise units of the template (resp. object) spectrum in the selected spectral bandpass, i.e.: The 's are the parameters scaling the bias. Eqs. (2) and (3) clearly show that when the object and the template noises tend to zero, the upper and the lower limits of the veiling bias also tend to zero and the solution of Eq. (1) becomes more and more independent of any input parameter . This simple result allows us to consider the problem of spectral mismatches. Let us now assume relatively not noisy spectra
( and
). Any variation of the calculated
veiling as a function of the input
value in Eq. (1) must be attributed to spectral mismatches between the
object and the template spectra. In this case, as we do not know the
true object underlying photospheric spectrum, it is no longer possible
to strictly bound the object veiling with minimum and maximum values.
But, it is still possible to compute the "extreme" veiling values
and
. CH99 has shown that if the
difference is equal to zero, then
Eq. (1) is exact and the bias is dominated by the apparent veiling of
the template, if any [see Eq. (12) of CH99]. Otherwise, the difference
can be interpreted as the goodness
of the fit to the veiling equation. In practice, there is © European Southern Observatory (ESO) 1999 Online publication: September 13, 1999 |