CONTINUOUS ABSORPTION BY HE-2+ AND HE-2+ IN COOL WHITE-DWARFS

Photodissociation cross sections and absorption coefficients of the He2+ and H2+ molecular ions for transitions from the X2SIGMA+ ground state to the vibrational continuum of the A2SIGMA+ excited state have been determined for photon wavelengths lambda between 50 nm and 20 mum and for temperatures T between 4200 and 50,400 K and between 3150 and 25,200 K, respectively. The cross sections are obtained through a fully quantum-mechanical method, and it is shown that inclusion of the high-lying rotational states is important for determination of the correct long-wavelength cross section amplitude and the peak wavelength. Free-free absorption coefficients are also obtained using the semiclassical method of Bates. The detection possibility of He-2(+) and H-2(+) in cool white dwarfs is discussed and shown to be insignificant. An effectively temperature independent Cutoff in the He-2(+) photodissociation cross section in the EUV at approximately 60 nm appears to be the only possible observable feature. It is suggested that He-2(+) may be responsible for an observed UV flux deficiency in some cool DB stars as well as in the DZ stars WD 0552 - 041 and WD 1334 + 039. We further suggest that the star WD 0752 - 676 currently assumed to be helium-rich may actually have a hydrogen-dominated atmosphere.