A photometric pilot study on Sonneberg archival patrol plates^*

How many “constant” stars are in fact long-term variables?

¹ Sonneberg Observatory, Sternwartestr. 32, 96515 Sonneberg, Germany e-mail: pk@4pisysteme.de
² Instituto de Astronomia, Universidad Catolica del Norte, Avenida Angamos 0610, Antofagasta, Chile

Accepted: 7 June 2004

Abstract

The light curves of 216 arbitrarly chosen field stars and of 23 known variables in the Aur/Tau/Ori region were derived () from scanned, blue-sensitive archival patrol plates, covering a total of 34 years (1961-1995). We achieved a photometric accuracy of 0.07 ... 0.12 mag in spite of rather unfavourable locations of most stars near the plate borders. 17 field stars turned out to be variables, most of them with time scales of 1000-8000 days in the form of slow waves with amplitudes between 0.1 and 0.3 mag, i.e. below the threshold of traditional variable searches on photographic plates. About 50% of these new long-term variables exhibit drifts indicating periodic or erratic variability at much longer time scales than covered here. For the 23 known variables we achieved improvements in their periods and amplitudes and detected long-term variations (drifts, waves) in about 50% of them. The above fraction of low-amplitude long-term variables among field stars implies that a total of about 45 000 new variables should be detectable in the Sonneberg patrol plate archive. They will represent a new, hitherto not investigated population of variable stars with a possibly significant impact on our understanding of the stellar interior and evolution.