Temperatures of Delta CEP and Nonvariable Supergiants

Abstract

This study is a comparison of the energy distributions of nonvariable supergiants and δ Cep with model atmospheres. The data include lUE spectra from 1700 to 3200 A, and broadband B, V, R_C, I_C, J, H, and K. The model atmosphere comparisons cover a range of temperatures, gravities, and microturbulence. Both the nonvariables and δ Cep are a good fit to the Kraft (B-V)₀-temperature relation. The δ Cep observations match energy distributions using reasonable values of gravity and microturbulence. For the hotter phases, a larger gravity is required than for the cooler phases, but this value results in a good fit to energy distribution, including the excess ultraviolet flux found previously. Similarly, the energy distribution for the piston phase can also be reproduced, but a large value of microturbulence (larger than ≃6 km s^-1) would be very difficult to accommodate. The most interesting result concerns the nonradiative flux found by Morossi et al. [A&A, 277, 173 (1993)] at the shortest wavelengths. it is found in nonvariable supergiants, however it is nonexistent or much reduced in the S Cep observations. This suggests an alteration to the atmospheric structure caused by pulsation. This is in agreement with and explains the recent ROSAT observation of the Cepheid ζ Gem by Sasselov & Sabbey [RMxA&A, 29, 215 (1994)]. It was not detected, meaning that it's x-ray flux is at least 20 times less than comparable nonvariable supergiants. Pulsation apparently inhibits upper atmosphere heating, so that a hot corona is not formed.