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

TL;DR

This study discovers 79 $\delta$ Scuti stars in the 300 Myr old open cluster NGC 3532 using TESS data, revealing a decrease in pulsator occurrence with age and highlighting the role of rapid rotation in pulsation maintenance.

Contribution

It provides the largest sample of $\delta$ Scuti stars in a single open cluster and introduces the concept of pulsator occurrence, linking pulsation to age and rotation.

Findings

Pulsator fraction in NGC 3532 is 50 extpm5",

Pulsator occurrence is 63 extpm6",

Pulsator occurrence decreases with age and rapid rotation influences pulsation

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.