NGTS clusters survey. I. Rotation in the young benchmark open cluster Blanco 1

TL;DR

This study measures rotation periods for 127 stars in the 115-million-year-old Blanco 1 cluster, revealing patterns in stellar rotation and the influence of companions, with implications for stellar angular momentum evolution.

Contribution

It provides the first extensive rotation period dataset for Blanco 1, employing multiple methods, and compares these results with the Pleiades to understand stellar angular momentum evolution.

Findings

Rotation periods range from ~2 to 10 days for mid-F to mid-K stars.

Stars in multiple systems tend to rotate faster than single stars.

Rotation period distributions are similar between Blanco 1 and Pleiades for single stars.

Abstract

We determine rotation periods for 127 stars in the ~115 Myr old Blanco 1 open cluster using ~200 days of photometric monitoring with the Next Generation Transit Survey (NGTS). These stars span F5-M3 spectral types (1.2 $\gtrsim M \gtrsim$ 0.3 M$_{\odot}$) and increase the number of known rotation periods in Blanco 1 by a factor of four. We determine rotation periods using three methods: Gaussian process (GP) regression, generalised autocorrelation (G-ACF) and Lomb-Scargle (LS) periodograms, and find that GPs and G-ACF are more applicable to evolving spot modulation patterns. Between mid-F and mid-K spectral types, single stars follow a well-defined rotation sequence from ~2 to 10 days, whereas stars in photometric multiple systems typically rotate faster. This may suggest that the presence of a moderate-to-high mass ratio companion inhibits angular momentum loss mechanisms during the early pre-main sequence, and this signature has not been erased at ~100 Myr. The majority of mid-F to mid-K stars display evolving modulation patterns, whereas most M stars show stable modulation signals. This morphological change coincides with the shift from a well-defined rotation sequence (mid-F to mid-K stars) to a broad rotation period distribution (late-K and M stars). Finally, we compare our rotation results for Blanco 1 to the similarly-aged Pleiades: the single star populations in both clusters possess consistent rotation period distributions, which suggests that the angular momentum evolution of stars follows a well-defined pathway that is, at least for mid-F to mid-K stars, strongly imprinted by ~100 Myr.