The near-infrared spectrum of U Sagittae at primary eclipse

Abstract

We have observed the Algol type binary U Sagittae before and during primary eclipse and near the first quadrature in the spectral region 8300–8760 Å with the CARELEC spectrometer at the coudé focus of the 193 cm telescope of the Haute Provence Observatory. The purpose was to study the near IR spectrum of the secondary (a G4 giant) free from the contribution of the hot primary (a B8 main sequence star) and to compare it with those of some standard stars included between F5 and K3 as well as with the composite spectrum at phase 0.236. The comparison of the spectrum of the secondary with that of three standard stars surprisingly indicates that it is very similar to that of the F5 standard instead, as expected, to that of the G5 standard star. The disagreement between the spectral classification in the optical range and in the near-infrared range is explained as due to the fact that the irradiation from the primary is more effective in the IR, where the continuous absorption coefficient of the secondary is maximum, than in the optical. In fact the heating in the IR probably affects just the upper layers, producing a stronger observable effect than in the optical, where the heating affects deeper layers.