Research Note

The pulsating λ Bootis star 15 Andromedae: results from a three-site photometry campaign

¹ Astronomical Observatory of Odessa National University, Marazlievskaya 1v, 65014 Odessa, Ukraine e-mail: tnd@te.net.ua
² Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria e-mail: handler@astro.univie.ac.at
³ Instituto de Astrofisica de Andalucia, C.S.I.C., Apdo. 3004, 18080 Granada, Spain

Received: 27 September 2007
Accepted: 29 November 2007

Abstract

Context.The λ Bootis stars are a chemically peculiar subgroup of main sequence A/F stars. Several of these fall into the instability strip of the δ Scuti stars and the whole spectroscopic group shows a high incidence of pulsational variability.

Aims.We investigated the pulsational behaviour of the λ Bootis star 15 And (HD 221756) to determine as many intrinsic mode frequencies as possible for asteroseismic applications.

Methods.The star was included as a secondary target for three telescopes during a campaign devoted to the β Cep stars 12 and 16 Lac, due to its proximity to these targets. 302.8 h of time-resolved photometry spanning 161.2 d were acquired and analyzed.

Results.Our frequency analysis revealed (2) c/d as the main pulsation frequency, with an amplitude of only 1.9(1) mmag. The residual periodogram suggests the presence of additional frequencies in an interval from 20 to 25 c/d, with very low amplitudes. We succeeded in detecting a second frequency (9) with an amplitude of 0.6(1) mmag. Low-frequency light variations of the comparison star HD 220575 were revealed as well.

Conclusions.The pulsational behaviour of 15 And as a typical λ Boo-type star is compared with that of δ Scuti stars with normal abundances and with metallic-line δ Scuti stars. We discuss the value of λ Boo stars in understanding accretion and mixing in pulsating stars. The difficulties in the selection of reliable comparison stars for such high-precision research are pointed out.