The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data

Silvotti, R. and Schuh, S. and Kim, S-L. and Lutz, R. and Reed, M. and Paparó, Margit and Molnár, László (2018) The sdB pulsating star V391 Peg and its putative giant planet revisited after 13 years of time-series photometric data. ASTRONOMY & ASTROPHYSICS, 611. ISSN 0004-6361

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Abstract

V391 Peg (alias HS 2201+2610) is a subdwarf B (sdB) pulsating star that shows both p- and g-modes. By studying the arrival times of the p-mode maxima and minima through the O-C method, in a previous article the presence of a planet was inferred with an orbital period of 3.2 years and a minimum mass of 3.2 M_Jup. Here we present an updated O-C analysis using a larger data set of 1066 h of photometric time series ( 2.5× larger in terms of the number of data points), which covers the period between 1999 and 2012 (compared with 1999-2006 of the previous analysis). Up to the end of 2008, the new O-C diagram of the main pulsation frequency (f₁) is compatible with (and improves) the previous two-component solution representing the long-term variation of the pulsation period (parabolic component) and the giant planet (sine wave component). Since 2009, the O-C trend of f₁ changes, and the time derivative of the pulsation period (p^.) passes from positive to negative; the reason of this change of regime is not clear and could be related to nonlinear interactions between different pulsation modes. With the new data, the O-C diagram of the secondary pulsation frequency (f₂) continues to show two components (parabola and sine wave), like in the previous analysis. Various solutions are proposed to fit the O-C diagrams of f₁ and f₂, but in all of them, the sinusoidal components of f₁ and f₂ differ or at least agree less well than before. The nice agreement found previously was a coincidence due to various small effects that are carefully analyzed. Now, with a larger dataset, the presence of a planet is more uncertain and would require confirmation with an independent method. The new data allow us to improve the measurement of p^. for f₁ and f₂: using only the data up to the end of 2008, we obtain p^.₁ = (1.34 ± 0.04) × 10^-12 and p^.₂ = (1.62 ± 0.22) × 10^-12. The long-term variation of the two main pulsation periods (and the change of sign of p^.₁) is visible also in direct measurements made over several years. The absence of peaks near f₁ in the Fourier transform and the secondary peak close to f₂ confirm a previous identification as l = 0 and l = 1, respectively, and suggest a stellar rotation period of about 40 days. The new data allow constraining the main g-mode pulsation periods of the star. The complete set of data shown in Fig. 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A85Based on observations obtained at the following observatories: WHT 4.2m, TNG 3.6m, Calar Alto 2.2m, NOT 2.5m, Loiano 1.5m, LOAO 1.0m, MDM 1.3m, Moletai 1.6m, MONET-North 1.2m, Piszkéstető 1.0m, Mercator 1.2m, Wise 1.0m, Lulin 1.0m, Baker 0.6m.

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