A photoelectric study of U Sagittae.
Abstract
U Sagittae is an eclipsing binary system consisting of a B9 component and a larger, much fainter G~ component. The pflmary minimum is a deep, total eclipse of the B9 component; the secondary minimum is a shallow, annular eclipse of the G5 component. Tidal and rotational forces distort the G~ star into an approximately triaxial ellipsoid whose axes are in the ratio 1.00: o.88: 0.83. The purpose of the present investigation was to secure photometric data accurate enough to determine (I) the coefficient of limb darkening of the B9 component in two color regions, and (2) the amount of distortion of the G~ component. The photoelectric observations of U Sagittae at primary minimum, with a blue-sensitive photocell, have been shown to be precise enough to derive an accurate value of the limb darkening coefficient of the B9 component. A least-squares method was extended to include the determination of the mass ratio of the two components. The mass ratio was determinate with a precision half that of the spectroscopic method, and was indirectly determined through the effect on the light- curve of the distorted shape of the fainter, eclipsing G~ component. The effect of this distortion is considerable, but it is shown that a combination of fictitious elements would provide a solution that would satisfy all but the most precise observations. The spectroscopic and photometric mass ratios are in agreement within their probable errors. Observations with a red-sensitive photocell were not precise enough in themselves to derive either an accurate darkening coefficient or an accurate mass ratio. A method of analysis was developed by which it was possible, by a comparison of the observed light-curves in the blue and the red, to determine very accurately the difference in limb darkening between the two color regions. This method, applicable to similar observations of other stars, should yield differential limb darkening ?oefficients precise enough to afford valuable checks on astrophysical theories of stellar atmospheres. The time of primary minimum was determined very precisely in both colors and there is no evidence that this time is a function of the wave length of observation. The time of secondary n~inimum was not precisely enough determined to distinguish between the two hypotheses, (i) a rotating line of apsides or (2) a third component of low mass, necessary to explain the variation of period of U Sagittae. New methods of rectification, based partly on theory and partly on the observations of the light-curve between minima, were developed and applied. The resulting corrections in the solution of primary minimum are small but not negligible. It is shown that the conventional rectification procedures lead to an anomalous result when applied to the depth of the secondary minimum in the blue. Flower Observatory, Upper Darby, Pa.
- Publication:
-
The Astronomical Journal
- Pub Date:
- February 1947
- DOI:
- 10.1086/105963
- Bibcode:
- 1947AJ.....52..125I