We present the results of a multisite photometric campaign on the pulsating B-type hot subdwarf star Balloon 090100001 (Bal09). The star is one of the two known hybrid hot subdwarfs with both long- and short-period oscillations, theoretically attributed to g and p modes. The campaign involved eight telescopes with three obtaining UBVR data, four B-band data and one Strömgren uvby photometry. The campaign covered 48 nights, providing a temporal resolution of 0.36 μHz with a detection threshold of about 0.2 mmag in B-filter data. Bal09 has the richest pulsation spectrum of any known pulsating subdwarf B star, and our analysis detected 114 frequencies including 97 independent and 17 combination ones. Most of the 24 g-mode frequencies are between 0.1 and 0.4 mHz. Of the remaining 73, presumably p modes, 72 group into four distinct regions near 2.8, 3.8, 4.7 and 5.5 mHz. The density of frequencies requires that some modes must have degrees ℓ larger than 2. The modes in the 2.8 mHz region have the largest amplitudes. The strongest mode (f 1) is most likely radial, while the remaining ones in this region form two nearly symmetric multiplets: a triplet and quintuplet, attributed to rotationally split ℓ = 1 and 2 modes, respectively. We find clear increases of splitting in both multiplets between the 2004 and 2005 observing campaigns, amounting to ∼15 per cent on average. The observed splittings imply that the rotational rate in Bal09 depends on stellar latitude and is the fastest on the equator. We also speculate on the possible reasons for the changes of splitting. The only plausible explanation we find is torsional oscillation. This hypothesis, however, needs to be verified in the future by detailed modelling. In this context, it is very important to monitor the splittings on a longer time-scale as their behaviour may help to explain this interesting phenomenon. The amplitudes of almost all terms detected in both 2004 and 2005 were found to vary. This is evident even during one season; for example, amplitudes of modes f8 and fC were found to change by a factor of 2-3 within about 50 d during 2005. We use a small grid of models to constrain the main mode (f1), which most likely represents the radial fundamental pulsation. The groups of p-mode frequencies appear to lie in the vicinity of the consecutive radial overtones, up to the third one. Despite the large number of g-mode frequencies observed, we failed to identify them, most likely because of the disruption of asymptotic behaviour by mode trapping. The observed frequencies were not, however, fully exploited in terms of seismic analysis which should be done in the future with a larger grid of reliable evolutionary models of hot subdwarfs.


Physics, Astronomy, and Materials Science

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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2009 The authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


Stars: individual: Balloon 090100001, Stars: oscillations, Subdwarf

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Monthly Notices of the Royal Astronomical Society