Measuring Rotation Periods in Crowded Star Clusters with TESS: A Proof of Concept with NGC 3532

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Abstract The Transiting Exoplanet Survey Satellite (TESS) has observed nearly the entire sky, producing Full-Frame Images (FFIs) every 30 minutes (Cycles 1–2), 10 minutes (Cycles 3–4), and now 200 s (Cycle 5+), over 27 day sectors. Light curves extracted from FFIs can be used to measure stellar rotation periods ( P rot ) in nearby open clusters, and are well suited for studying low-mass stars (≲1.2 M ⊙ ) younger than ≲1 Gyr, whose P rot are generally still ≤15 days. A challenge to exploiting TESS data fully is its 21″ pixel size, which can cause strong signals from one source to contaminate those of nearby stars, particularly in the crowded environments found, for example, in the more distant and/or richest clusters. We conducted a test with the young (≈350 Myr old), moderately distant (470 pc), and rich open cluster NGC 3532 ( N ⋆ > 3000), which has an extensive P rot catalog from ground-based photometry, to examine the reliability of P rot obtained from TESS data. We recovered 69% of the literature periods from at least one of the three TESS cycles in which NGC 3532 was observed before any quality analysis. We then used all available TESS data for low-mass members of NGC 3532 and, applying a set of quality cuts that combined information from TESS and Gaia, measured P rot for 885 cluster stars, adding 706 new P rot to the existing catalog. We conclude that, when considered with appropriate caution, TESS data for stars in crowded fields can yield reliable P rot measurements.

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