Feasibility of UV astronomy by balloon-borne observations. II. Stellar gradients in the near ultraviolet.

Summary. The analysis of 48 spectrophotometric stellar distributions obtained during a balloon-flight is performed by means of two absolute gradients defined shortward of the Baimer discontinuity. It is found that for normal stars of luminosity classes V, IV or III the measured stellar fluxes are lower than the predicted fluxes for wavelengths shorter than 3000 A and for stars of spectral types later than B 5. This effect does not seem to occur for stars of luminosity class II orI. Rapid rotators (v sin i> 150 km/s) show a variation of the ultraviolet gradient (a diminution of color tempera- ture) which is at least two times greater than that predicted by theory for spectral types in the range B 1 V - B 5 V. This disagreement between theory and observation disappears for later spectral types. There is the evidence that the anomalous "reddening by rotation" found for earlier spectral types is of the same amplitude in the near UV as in the blue-visible spectral region. Key words: ultraviolet astronomy - stellar gradients - stellar rotation