INVESTIGATION O φ Cas(F 0Ia) SUPERGIANT ATMOSPHERE

Abstract

Background: The stars of Cassiopeia, the celestial Queen, are so prominent that we tend to overlook the many others of considerable interest. Such is the case with fifth magnitude (4.98) Phi Cassiopeiae, whose Greek letter ranks way down the alphabet, number 21 out of 24. Situated between Ruchbah (Delta Cas) and the two Marfaks (Theta and Mu), the star is the subject of some contention. Right on the edge of a prominent open cluster called NGC 457 (popularly called the "ET" or "Owl" Cluster), Phi has long been thought to be the brightest member (and even called NGC 457 no. 136). Or not. As seen in a great many apparent double stars, just because two things line up does not mean that they are related. (As another example, look at Aldebaran and the Hyades.) And the consideration is important, as Phi Cas is of a rare breed, a class F (F0) bright supergiant that has even been tossed into an ethereal bin of stars called "yellow hypergiants." Materials and Method: The relative motions between the star and cluster are not sufficiently the same or different to allow a decision. Parallax would seem to settle the case. The cluster is at a distance of 7900 light years, while the star comes in at 2300 light years, making it a foreground object. Results: The statistical error of measurement, however, is perilously high, and there is a significant chance of the star lying as far as 4500 light years and perhaps even as far as the cluster. Using the actual parallax distance and an allowance of 1.7 magnitudes of absorption by interstellar dust gives an absolute magnitude for the star (what the magnitude would be at 32.6 light years) of -6.0, not bright enough to make the ranks of bright supergiants (which is around -8.5). Only if we put the star at the cluster's distance does it reach the required brightness. Conclusion: The atmosphere of supergiant φ Cas(F 0Ia) is investigated by the method of atmospheric model. Effective temperature and surface of gravity are determined by comparing the observed and theoretical values of photometrical indices [c1], Q, and equivalent widths of Balmer lines: Teff = 7350±200 K, logg=0,4±0.2.The microturbulence parameter is evaluated as ξt=7.5 km/s on the basis of studies of FeII lines. The chemical composition of the star is determined. In the atmosphere of φ Cas(F 0Ia) the C turned out to be a deficit, N, and Na in excess, other investigated elements practically display solar content.