Discovery of 79 $\delta$ Scuti Stars in NGC 3532 Suggests a Decrease of Pulsator Occurrence with Age

Abstract

Many A-F type stars do not display $\delta$ Scuti pulsations, despite being located within the instability strip. Open clusters provide a unique opportunity to study $\delta$ Scuti pulsations among coeval populations with uniform chemical composition. Here we use data from the TESS Mission to discover 79 $\delta$ Scuti pulsators in the 300 Myr old open cluster NGC 3532, the largest number found within a single open cluster to-date. We report a $50\pm5\%$ pulsator fraction in NGC 3532, considerably lower than in younger stellar populations, such the Pleiades (110 Myr), NGC 2516 (100 Myr), and the Cep-Her Complex ($\leq\,$80 Myr), and similar to the pulsator fraction found among field star samples. We introduce the concept of pulsator occurrence, which corrects for incompleteness, and find it to be $63\pm6\%$. For the stars that do pulsate, we find that the hotter stars occupy a distinct branch in the color-magnitude diagram (CMD) due to faster rotation ($>\,$150 km/s) than their non-pulsating counterparts. These results suggest that pulsator occurrence decreases with age and that rapid rotation is important in maintaining $\delta$ Scuti pulsations over time. We also investigate the Period-Luminosity (P-L) relation and the $\nu_{\rm max}$--$T_{\rm eff}$ relation of $\delta$ Scuti stars in NGC 3532. We find much scatter in the P-L relation of the dominant mode and two distinct branches in the $\nu_{\rm max}$--$T_{\rm eff}$ relation, similar to the Cep-Her Complex.