CHROMOSPHERIC MODELS FOR ALTAIR (A7 IV-V)

The star, Altair (A7 IV-V), is clearly shown to have Ly alpha emission of chromospheric origin, while no evidence is found for the Mg II emission reported in previous investigations. We present non-LTE semiempirical models incorporating partial redistribution of the chromosphere of Altair that reproduce the observed Ly alpha emission and the Mg II resonance absorption at 2800 Angstrom. We unambiguously establish that chromospheres exist at spectral types as early as A7 on the main sequence, and we also demonstrate that it is very unlikely that the observed emission originates in a corotating expanding wind. This result represents a new challenge for chromospheric heating theories. It may indicate that both differential rotation and convective layers, at least near the equator, exist in this fast rotating (v sin i = 220 km s(-1)) star.