Binary Star Rotation from Line Profiles

Abstract

Rotation measures for binaries and single stars need to be put on a firm basis. From an astrophysical point of view, asynchronous rotation in Algol type binaries is mainly a consequence of mass transfer, and thus can provide evolutionary information. Also, rotation is a neglected area of stellar astronomy, especially on the observational side, and it is appropriate to develop our knowledge simply because unexpected benefits nearly always accompany rapid observational advances into unexplored territory. This contribution includes results on RY Per, RW Mon, DM Per, RY Gem, TU Mon and Algol (Table 1). F₁ is the ratio of spin angular speed to mean orbital angular speed for the primary star while F₂ is the corresponding ratio for the secondary star. F₁= 1 signifies synchronous rotation. Table 1 lists the values of F1 obtained from three methods. F₁(dc) refers to the value obtained by the differential correction method while F₁(simplex) refers to the value obtained from the simplex method. F₁(lc) is the photometric F1 value. The published spectroscopic F₁ values in Table 1 are averages found by Van Hamme and Wilson (1990, Ai 100, 1981) from the listed references. The last column in Table 1 gives the equatorial velocity corresponding to the F₁(dc) value. The fitted spectral line profiles are shown in Figures 1, 2, and 3. A more complete list of binaries observed to date by G.J.Peters is given in Table 2. In this table the term “noisy lines” refers to those for which the spectral distributions have large scatter. Any blending is usually with lines of the same star because Algol-type secondary stars are much less luminous than the primaries. The term “clean lines” refers to those which are reasonably free of scatter and blending. The resolution of the CCD as it was used is 0.15 Angstroms per pixel.

Traditional methods for determining stellar rotation rates from photospheric spectral line profiles are subjective, do not provide standard error estimates, and neglect most or all other broadening mechanisms. Essentially impersonal results of greatly improved reliability are now potentially attainable, due to advances in the areas of observations, modeling, and parameter adjustment. Improvements in observations are coming along naturally because of developments in detectors, such as CCD’s and reticons. It is important for modeling and parameter adjustments to keep pace with the improved observations. At the University of Florida, we have developed a model which produces computed line profiles (for theory, see Mihalas, D. 1978. Stellar Atmospheres. San Francisco, W.H. Freeman and Company) which are Doppler broadened by superposition of local intrinsic profiles over the surfaces of model stars (WD model). Effects included are thermal Doppler broadening, turbulence, damping, rotation, gravity darkening, limb darkening, reflection, tidal distortion, eclipses, and instrumental broadening. Phase smearing will be added soon. Among the parameters are the usual binary star parameters (including the F₁ and F₂ rotation parameters and the surface temperature (T_eff)), turbulent velocity (V_turb), effective damping constant (Γ), and the number of absorbers in the line of sight (Nf). Our simplex and differential corrections programs for parameter fitting are now being used on CCD line profiles of additional Algol primaries recently observed by G.J.Peters.

Part of this research was supported by grant NGT 40015 from the Florida Space Grant Consortium.