V391 Peg: Identification of the two main pulsation modes from ULTRACAM u′g′r′ amplitudes

Abstract

V391 Peg (HS 2201+2610) is an extreme horizontal branch subdwarf B (sdB) star, it is an hybrid pulsator showing p-and g-mode oscillations, and hosts a 3.2/sin i MJup planet at an orbital distance of about 1.7 AU. In order to improve the characterization of the star, we measured the pulsation amplitudes in the u g r Sloan photometric bands using ULTRACAM at the William Herschel 4.2 m telescope and we compared them with theoretical values. The preliminary results presented in this article conclusively show that the two main pulsation periods at 349.5 and 354.1 s are a radial and a dipole mode respectively. This is the first time that the degree index of multiple modes has been uniquely identified for an sdB star as faint as V391 Peg (B = 14.4), proving that multicolor photometry is definitely an efficient technique to constrain mode identification, provided that the data have a high enough quality.

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

“…The low amplitude of the secondary peak suggests a low inclination. This interpretation is consistent with the previous identification of f 2 as an l = 1 mode by Silvotti et al (2010). A rotation period of ∼40 days would be compatible with the distribution of rotation periods as recently measured by the Kepler spacecraft in a sample of 18 sdB g-mode pulsators (see Zong 2017 and references therein).…”

“…For this reason, after several tests we decided to split our analysis of the amplitude spectrum into three steps with data sets of different length and different frequency resolution. It is useful to recall here that the two main pulsation modes of V391 Peg were identified as l = 0 and l = 1 from high-precision multi-color photometry obtained with ULTRACAM at the WHT (Silvotti et al 2010). We show below that this identification is well supported by the current analysis.…”

“…After prewhitening, this excess of power is not completely removed near 180 µHz, suggesting that further low-amplitude frequencies are present in that region. modes seem the most natural explanation, but the l = 0 identification (Silvotti et al 2010), which is confirmed by the new data, does not help as we cannot invoke resonant mode coupling between the components of a multiplet nor resonance between modes linked by linear combinations that we do not see. The irregular behavior of f 1 agrees to a certain extent with recent Kepler results, which showed that sdB pulsation frequencies are in general less stable than previously believed.…”

“…The data obtained in October 2007 at the Piszkéstető, Loiano, and Lulin Observatories were collected without any filter in order to maximize the signal-to-noise ratio (S/N) of that run. The differences introduced by the different filters in terms of amplitudes or phases of the pulsation modes were considered and found to be negligible 1 The WHT data were simultaneously obtained with ULTRACAM in three photometric bands (u, g, and r) but only the g-band data are used in this article, while multi-band data were previously used to identify the main pulsation modes of V391 Peg (Silvotti et al 2010). …”

“…mass of 3.2 M Jup was found, with an orbital period of 3.2 years, corresponding to an orbital distance of about 1.7 AU. The presence of the planet was inferred by measuring the arrival times of the maxima and minima of the stellar light, given that V391 Peg is a pulsating subdwarf B (sdB) star with at least four p-mode pulsation periods between 344 and 354 s (Silvotti et al 2002(Silvotti et al , 2010, and a few longer-period g-modes (Lutz et al 2009). A recent review on hot subdwarfs of spectral type O and B is given by Heber (2016).…”