W Virginis is a 17-day variable star which is considered to be the prototype of population II Cepheids. An analysis of the physical conditions in W Virginis during its cyclic variations has been made from the following data: High dispersion (10 Å/mm.) Coude plates were measured for radial velocities (by R. F. Sanford) and lines intensities which yielded curves of growth. Also used were a light curve in one color (Gordon and Kron) and colors (Whitford and Code).
The observations indicate an expansion of about 36 x 106 km. and then a subsequent contraction. The first indication of a new expansion wave is the appearance of hydrogen emission lines, formed deep in the atmosphere. Later the outward-moving region of gas produces absorption lines like that of an F-type star. These gain in strength until maximum expansion. This is also a time of minimum electron pressure and nearly minimum temperature. During the contraction the electron pressure, temperature, and opacity rapidly increase. Also just after maximum expansion the appearance of a new set of hydrogen emission lines from deep in the atmosphere indicates the start of a new outward-moving wave. There is a time of several days during which absorption lines are seen from the two masses of gas: the one falling downward and the other moving upward. As the spectral features of the downward-moving region fade, those of the upward-moving region increase toward maximum strength. Data derived from the two simultaneous sets of absorption lines indicate very different conditions in the two regions.
It was found that relative radii derived from light and color curves could not be compared with displacements derived from the radial velocity curve, because, perhaps, the regions predominantly forming the continuous and line spectra have different motions. The extremely red colors and the large apparent temperature gradient, both particularly at maximum expansion, may be due to the presence of an extended atmosphere.
