V473 Lyr, a modulated, period-doubled Cepheid, and U TrA, a double-mode Cepheid observed by MOST

Molnár, L. and Derekas, A. and Szabó, R. and Matthews, J. M. and Cameron, C. and Moffat, A. F. J. and Richardson, N. D. and Csák, B. and Dózsa, Á. and Reed, P. and Szabados, L. and Heathcote, B. and Bohlsen, T. and Cacella, P. and Luckas, P. and Sódor, Á. and Skarka, M. and Szabó, Gy. M. and Plachy, E. and Kovács, J. and Evans, N. R. and Kolenberg, K. and Collins, K. A. and Pepper, J. and Stassun, K. G. and Rodriguez, J. E. and Siverd, R. J. and Henden, A. and Mankiewicz, L. and .Zarnecki, A. F. and Cwiek, A. and Sokolowski, M. and Pál, A. and Guenther, D. B. and Kuschnig, R. and Rowe, J. and Rucinski, S. M. and Sasselov, D. and Weiss, W. W. (2017) V473 Lyr, a modulated, period-doubled Cepheid, and U TrA, a double-mode Cepheid observed by MOST. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 466 (4). pp. 4009-4020. ISSN 0035-8711

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Abstract

Space-based photometric measurements first revealed low-amplitude irregularities in the pulsations of Cepheid stars, but their origins and how commonly they occur remain uncertain. To investigate this phenomenon, we present MOST space telescope photometry of two Cepheids. V473 Lyrae is a second-overtone, strongly modulated Cepheid, while U Trianguli Australis is a Cepheid pulsating simultaneously in the fundamental mode and first overtone. The nearly continuous, high-precision photometry reveals alternations in the amplitudes of cycles in V473 Lyr, the first case of period doubling detected in a classical Cepheid. In U TrA, we tentatively identify one peak as the fX or 0.61-type mode often seen in conjunction with the first radial overtone in Cepheids, but given the short length of the data, we cannot rule out that it is a combination peak instead. Ground-based photometry and spectroscopy were obtained to follow two modulation cycles in V473 Lyr and to better specify its physical parameters. The simultaneous data yield the phase lag parameter (the phase difference between maxima in luminosity and radial velocity) of a second-overtone Cepheid for the first time. We find no evidence for a period change in U TrA or an energy exchange between the fundamental mode and the first overtone during the last 50 yr, contrary to earlier indications. Period doubling in V473 Lyr provides a strong argument that mode interactions do occur in some Cepheids and we may hypothesize that it could be behind the amplitude modulation, as recently proposed for Blazhko RR Lyrae stars.

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