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Astron. Astrophys. 346, 82-86 (1999)

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3. The accretion disc contamination

We first binned the V616 Mon and template star spectra onto a logarithmic wavelength scale using a [FORMULA] interpolation scheme to minimize data smoothing (Stover et al. 1980). We then optimally subtracted the template star from the V616 Mon spectra using the standard procedure (see MRW) as follows. Taking the accretion disc contribution as a flat continuum, we fit the V616 Mon spectra with this plus a variable fraction of the template star. The fraction of light from the template star, (f), is adjusted to minimise the residual scatter between the spectra. The scatter is measured by carrying out the subtraction and then computing the [FORMULA] between the residual spectrum and a smoothed version of the residual spectrum. This is done to remove any large-scale structure. The [FORMULA] for different fractions of the template star is computed and fitted with a parabola, and the fraction at the minimum value of [FORMULA] is taken to be the best fit. The above analysis was performed only on the 2.292 µ 12CO(2,0) bandhead in the summed spectrum of V616 Mon. Other bandheads were not used because of poor atmospheric subtraction and bad pixels. Fig. 1 shows the summed spectra of V616 Mon along with the template K3V star. The residual light spectrum (i.e. the accretion disc light) is also shown.

In addition to using HD42606 (K3V) we also used template stars of other spectral types [HD42606 (K0V), HD42606 (K5V) and HD42606 (K7V)] in order to determine the sensitivity of f to the assumed spectral type. We find that for the summed spectra of V616 Mon, the minimum value for [FORMULA] (1.01, a good fit) is obtained using the K3V secondary star, which contributes 75[FORMULA]17 percent of the flux in the IR (the disc fraction is 25 per cent). We also fitted the region used in the V616 Mon spectrum for the optimal subtraction with a constant and obtained a much worse fit with a [FORMULA] of 2.7 thereby showing that the absorption feature is real as the 99.9 per cent level.

The large uncertainties we obtain for the accretion disc light suggest that our result is consistent with the secondary star contributing all the light to the observed flux. In order to determine a lower limit to the fraction of light from the secondary, we simulated a spectrum in which all the light arises from the secondary star. This was done using the K3V template star and adding noise to the data such that the signal-to-noise was the same as the V616 Mon spectrum. We then performed the optimal subtraction and determined the 2-[FORMULA] per cent lower limit to the fraction of light arising from the secondary to be 73 per cent. This value is comparable with the fraction of light arising from the secondary star deduced from our spectrum of V616 Mon, suggesting V616 Mon most probably contains a companion star which contributes all the light in the IR.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999