EDP Sciences
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Volume 480, Number 1, March II 2008
Page(s) 187 - 191
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078750

A&A 480, 187-191 (2008)
DOI: 10.1051/0004-6361:20078750

Research Note

The pulsating $\lambda$ Bootis star 15 Andromedae: results from a three-site photometry campaign

T. N. Dorokhova1, G. Handler2, N. I. Dorokhov1, E. Rodriguez3, and S. N. Udovichenko1

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)

Context.The $\lambda$ Bootis stars are a chemically peculiar subgroup of main sequence A/F stars. Several of these fall into the instability strip of the $\delta$ Scuti stars and the whole spectroscopic group shows a high incidence of pulsational variability.
Aims.We investigated the pulsational behaviour of the $\lambda$ 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 $\beta$ 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 f1 = 24.7887(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 f2 = 22.2580(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 $\lambda$ Boo-type star is compared with that of $\delta$ Scuti stars with normal abundances and with metallic-line $\delta$ Scuti stars. We discuss the value of $\lambda$ 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.

Key words: stars: chemically peculiar -- stars: individual: HD 221756 -- stars: individual: HD 220575 -- stars: oscillations -- stars: variables: $\delta$ Scuti

© ESO 2008