By-Products of a MACHO Search in the Bright Star Catalogue (Abstract)

Volume 23 number 2 (1995)

Download this article (pdf)

Dorrit Hoffleit

Astronomy Department, Yale University, New Haven, CT

Abstract

(Abstract only) Upon the request of NASA scientist Bradley Schaefer, a search has been made forMACHO (MAssive Compact Halo Object) events among the approximately 9100 stars in TheBright Star Catalogue (Hoffleit 1982). No conclusive discoveries could be made, as none ofthe stars for which only a single maximum was reported in a literature search had enoughobservations to yield a light curve around the maximum. The shape of the curve is characteriestic and therfore crucial for the identification of a MACHO transit. With insufficient data for isolated single observations at maximum, there are not more than ten suspectedvariables that might have had observed MACHO events, but all with insufficient data to besure. Seven of these are components of double stars, with separations ranging from 0.5" to 30.4". Depending on the position angle between the component and the relative proper motions of the double and the lensing object, there might be two MACHO events, but separated by large time intervals. (With a favorable position angle and a separation of the components by30", and the relative proper motion of the double and the MACHO object as fast as the 10"annual prooper motion of Barnard's star, the two MACHO events would occur three years apart; for a separation of 1.0", the interval would be about a month, but all intervals are presumable longer.) In view of the fact that Mt. Stromlo observers, after monitoring some eight million stars approximately 500 times each, found only four MACHOs, it is not surprising that no definitive cases could be found among about 2,000 suspected variables inThe Bright Star Catalogue.In the course of the search, numerous double stars were noted to have one componentvariable but with insufficient observations to ascertain the characteristics of the variation, andin some cases even to identify which component is the suspected variable. I have compiled a catalogue of 33 such instances where the separations of the components exceed 5". In a few cases it seemed that the only maximum for a bright star occurred in Al Sufi's catalogue in the10th century. After examining numerous early catalogues from Ptolemy in the 2nd century,then Tycho Brahe about 1570, and later, none of the Al Sufi maxima qualified as MACHO events, being confirmed by both Ptolemy and Tycho, a span of some 1400 years.J. E. Gore, around 1900, wrote extensively on Al Sufi magnitude determinations and gavetables suggesting that some 38 bright stars showed secular variations, important considerationsfor stellar evolution. All but eight of these could definitely be ruled out on the basis ofprobable errors in the determinations and likely systematic differences between observers. The eight remaining do show over a magnitude difference between the early and modern magnitude estimates. If secular, the changes are not linear with time. They could even suggest the possibility of periods on the order of millennia. As usual, the need for extensive variable star observations continues to proliferate.ReferenceHoffleit, D. 1982, The Bright Star Catalogue, Yale University Observatory, New Haven, CT.