Rotational velocities of blue straggler stars in the globular cluster M 55

By using high-resolution spectra acquired with FLAMES-GIRAFFE at the ESO/VLT, we measured the radial and rotational velocities of 115 stars in the Galactic globular cluster M 55. After field decontamination based on the radial velocity values, the final sample of member stars is composed of 32 blue straggler stars (BSSs) and 76 reference stars populating the red giant and horizontal branches of the cluster. In agreement with previous findings, the totality of red giant branch stars has negligible rotation (<10 km s^-1) and horizontal branch stars have rotational velocities of 40 km s^-1 at most. In contrast, the BSS rotational velocity distribution shows a long tail extending up to ~200 km s^-1, with 15 BSSs (out of 32), spinning faster than 40 km s^-1. By defining the threshold for fast rotating BSSs at 40 km s^-1, this sets the percentage of these stars at 47 ±14%. Such a large value has never been found before in any globular clusters. It is roughly comparable to that measured in other loose systems (ω Centauri, M 4, and NGC 3201) and significantly larger than that observed in high-density clusters (as 47 Tucanae, NGC 6397, NGC 6752, and M 30). This evidence supports a scenario whereby recent BSS formation is occurring in low-density environments. We also find that the BSS rotational velocity tends to decrease for decreasing luminosity, as found for another loose cluster of the sample, namely, NGC 3201.