A search for solar - like oscillations in K giants in the globular cluster M 4 ⋆

Context. To expand the range in the colour-magnitude diagram where asteroseismology can be applied, we organized a photometry campaign to find evidence for solar-like oscillations in giant stars in the globular cluster M4. Aims. The aim was to detect the comb-like p-mode structure characteristic for solar-like oscillations in the amplitude spectra. The two dozen main target stars are in the region of the bump stars and have luminosities in the range 50–140 L⊙. Methods. We collected 6160 CCD frames and light curves for about 14 000 stars were extracted. The frames consist of exposures in the Johnson B, V and R bands and were obtained at three different telescopes. Three different software packages were applied to obtain the lowest possible photometric noise level. The resulting light curves have been analysed for signatures of oscillations using a variety of methods. Results. We obtain high quality light curves for the K giants, but no clear oscillation signal is detected. This is a surprise as the noise levels achieved in the amplitude spectra should permit oscillations to be seen at the levels predicted by extrapolating from stars at lower luminosities. In particular, when we search for the signature of oscillations in a large number of stars we might expect to see common features in the power spectra, but even here we fall short of having clear evidence of oscillations. Conclusions. High precision differential photometry is possible even in very crowded regions like the core of M4. Solar-like oscillations are probably present in K giants, but the amplitudes are lower than classical scaling laws predict. The reasons may be that the lifetime of the modes are short or the driving mechanism is relatively inefficient in giant stars.