The Structure of the Atmosphere of the K-Type Component of Zeta Aurigae.

Abstract

view Abstract Citations (12) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Structure of the Atmosphere of the K-Type Component of Zeta Aurigae. Wilson, O. C. Abstract This investigation is based upon spectrograms of 10.4 A/mm dispersion obtained during the 1939- 1940 eclipse. * Curves of growth were constructed for four atmospheric levels observed during ingress and for three during egress. Relative gf-values for Fe I lines were taken from the laboratory data of King and King. For other elements the relative gf's were calculated from the measures of lines in the solar-flash spectrum made by H. H. Lane. Doppler widths, L~XD, of the theoretical curves of growth which best fit the observations, range from 0.08 A for a height above the limb of 0.8 X 106 km to 0.16 A for a height of 20.6 X 1O°km, with cor- responding turbulent velocities of 6.5 and 13.0 km/sec, respectively. It appears, therefore, that the turbu- lence increases with height in theatmosphere. Excitation temperatures determined by comparison of the populations of the a~F and a3F states of Fe i with that of the ground state, a5D, also increase with height. Mean values range from 37800 at h = 0.8 X 106 km to 5660° at Ii = 13.5)< 10~ km. Density gradients in the atmosphere are readily evaluated by means of the curves of growth. For all atomic and ionic states included in this investigation the gradients are steepest at the lowest levels and tend to become less steep with increasing height. Moreover, except for Ca i in the lowest level, all the density gradients are nearly the same. If the densities are expressed as a function of height by means of the formula n = noe~, the observations yield a mean value of a = 2.3 X 1012 cm'. The observed gradient is compared with those calculated on the assumption of hydrostatic equilibrium. The latter hypothesis leads to gradients of the order of twenty times larger than observed for H to one thousand times larger for Fe and elements of similar atomic weight. McCrea's theory of turbulent support is also investigated. If the turbulent velocities obtained from the curves of growth are used in McCrea's equa- tion, the resulting gradient is still about ten times too large. It cannot be decided at the present time whether this discrepancy is real or due to an accumulation of errors in the data. The ionization in the atmosphere of the K-type component is investigated, chiefly at heights of 7 X 10~ km and 14 X 1O~ km, for which the data appear most reliable. Thermodynamic equilibrium is first assumed; the combination of the Boltzmann and Saha equations for H, together with the standard ionization equation for Ca, then permit an evaluation of the temperature, T. The latter is found to be nearly the same as that of the K-type star itself, and the electron density at height 7 X 10~ km is 1O~ per cm3, on the assumption that hydrogen supplies effectively all the electrons. It is next assumed that the ionization of Ca is governed by the dilute radiation of the B-type star and Pannekoek's equation is applied. It is shown that the observed ionization of Ca requires a higher electron density than appears to be available for H, and hence it is unlikely that the B-type star is responsible for the ionization of Ca and atoms of similar I.P. This conclusion leads to an investigation of the opacity of the atmosphere of the K-type star as a function of wave length. Menzel's opacity formula is shown to provide ample opacity at X «= 2000 A to screen most of the atmosphere from the ionizing radiation of the companion, while still insuring transparency in the regions ordinarily observed. On the basis of the same equation it is suggested that the eclipses may begin appreciably earlier in the ultraviolet than in the ordinary photo- graphic region. A tentative explanation of the rise of excitation temperature with height is based upon the metastability of the atomic states concerned and the variation of atmospheric opacity with height and wave length. INTRODUCTIO Publication: The Astrophysical Journal Pub Date: March 1948 DOI: 10.1086/145001 Bibcode: 1948ApJ...107..126W full text sources ADS | Related Materials (1) Reprint: 1948CMWCI.742....1W