Fast Rotating Blue Straggler Stars in the globular cluster NGC 1851

TL;DR

The study measures projected rotational velocities for blue straggler stars in the globular cluster NGC 1851 using high-resolution FLAMES-GIRAFFE spectra to test environmental effects on BSS formation. It identifies 4 fast-rotating BSSs out of 15 (about 27%), with reference RGB/HB stars rotating slowly, and finds a monotonic decrease in the fast-rotator fraction with cluster concentration and central density. The results support environment-driven BSS formation, potentially linked to binary evolution and dynamical age, though the small sample size limits definitive conclusions. These findings add NGC 1851 to the growing evidence that cluster environment shapes BSS rotation and formation pathways, motivating larger multi-cluster surveys.

Abstract

In this work we study the rotational velocities of a sample of blue straggler stars (BSSs) and reference stars belonging to the Galactic globular cluster NGC 1851, using high-resolution spectra acquired with FLAMES-GIRAFFE at the ESO/VLT. After field decontamination based on radial velocities and proper motions, the final sample of member stars is composed of 15 BSSs and 45 reference stars populating the red giant and horizontal branches of the cluster. In agreement with previous findings, the totality of reference stars has negligible rotation (lower than 15 km/s). In contrast, we find high values of rotational velocity (up to $\sim$ 150 km/s) for a sub-sample of BSSs. By defining the threshold for fast rotating BSSs at 40 km/s, we found 4 fast-rotating BSSs out of 15, corresponding to a percentage of 27 $\pm$ 14 %. This results delineates a monotonically decreasing trend (instead of a step function) between the percentage of fast spinning BSSs and the central concentration and density of the host cluster, supporting a scenario where recent BSS formation preferentially occurs in low-density environments from the evolution of binary systems.

Figures (8)

• Figure 1: CMD of NGC 1851 (black dots) with the surveyed BSSs, RGB and HB stars marked, respectively, as blue, red, and green circles. A set of BASTI evolutionary tracks (Pietrinferni+21) with masses ranging from 0.9 to 1.5 $M_\odot$ are overplotted as cyan lines. A BASTI isochrone of 11 Gyr is overplotted as orange line, and a BASTI HB model is overplotted as green line.
• Figure 2: Radial velocity of the observed stars as a function of their distance from the cluster center. Member BSSs, RGB and HB stars are marked as blue, red and green circles, respectively. The gray circles correspond to Galactic field interlopers. The black dashed line marks the average radial velocity of the member star sample.
• Figure 3: Comparison between the observed spectrum (black line) and the best-fit synthetic spectrum (red line) of four BSSs with different rotational velocities (see labels).
• Figure 4: Rotational velocity distribution of the 15 BSSs analyzed in this paper. Left arrows indicate the upper limit of 15 km s$^{-1}$ adopted for very slow rotating BSSs.
• Figure 5: Optical CMD of NGC 1851 with the position of fast and slowly rotating BSSs analyzed in this work highlighted with red and blue circles, respectively.