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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
HD 161817 is one of the four prototype field horizontal- branch A-type stars. It was classified as a FHB star by Oke, Greenstein and Gunn (1964) from photoelectric scans. Extensive observations have been made of this star since this time. Four-color measures show that the star falls in the same regions of the four-color diagrams as do stars on the BHB of globular clusters. Spectroscopic analyses indicate that the star has the low metal abundance characteristic of Pop II stars. Recently I have found that when HD 161817 is listed in a database (such as SIMBAD in Strasbourg) it is qualified as a variable star. I have, in my own photo- metric database, over twenty years of four-color measures of this star. The residuals in the four-color indices show no trend with time, but scatter around the mean value for each color with mean probable errors of \pm0.008 in (b-y), \pm0.012 in c_1 and \pm0.009 in m_1. In the Russian Variable Star Catalog the assignment of variability comes from Geneva photometric observations. Some time ago I received a copy of the original observations which were made from ESO, at La Silla in Chile. From Chile HD 161817 has a zenith distance of about 70 degrees, so even at the best of times the star is low in the observer's sky. The observations which contributed to the "variability" of the star were made far off the meridian, so the airmass was high. In my observations I usually keep the airmass lower than 1.2; here the airmass was near a value of 2. I believe that the observations that disagreed with the usual values for HD 161817 were incorrectly reduced to outside the atmosphere and thus they should be disregarded when considering the stability of the star in four-color photo- metry. The poster paper displays the measures in my database and show that, within the rms errors of the observations, there are no signs of photometric variability.
Program listing for Wednesday