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

TL;DR

The study uses TESS observations of the ~300 Myr open cluster NGC 3532 to identify 79 δ Scuti pulsators among 247 A/F-type members, introducing pulsator occurrence as a completeness-corrected metric that exceeds the simple observed pulsator fraction. It finds a center-instability-strip pulsator fraction of 50±5% and a pulsator occurrence of 63±6%, indicating a decline in pulsation with age and highlighting rapid rotation as a key factor for sustaining pulsations over time. The work reveals two branches in the ν_max–T_eff relation and substantial scatter in the P–L relation, with many stars likely pulsating in overtone modes; Δν is measured for one star at ~6.8 d^{-1}. These results imply that age and rotation jointly regulate δ Scuti pulsations in clusters and provide a framework for comparing pulsation statistics across stellar populations, with implications for stellar evolution and asteroseismology.

Abstract

Many A-F type stars do not display $δ$ Scuti pulsations, despite being located within the instability strip. Open clusters provide a unique opportunity to study $δ$ Scuti pulsations among coeval populations with uniform chemical composition. Here we use data from the TESS Mission to discover 79 $δ$ 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 $δ$ Scuti pulsations over time. We also investigate the Period-Luminosity (P-L) relation and the $ν_{\rm max}$--$T_{\rm eff}$ relation of $δ$ Scuti stars in NGC 3532. We find much scatter in the P-L relation of the dominant mode and two distinct branches in the $ν_{\rm max}$--$T_{\rm eff}$ relation, similar to the Cep-Her Complex.

Figures (14)

• Figure 1: Example amplitude spectra of three $\delta$ Scuti stars in NGC 3532, from aperture photometry (black) and PSF photometry (red). The stars are ordered by Gaia vbroad, and the TIC IDs and Gaia $G_{\rm BP}-G_{\rm RP}$ are provided.
• Figure 2: Stacked amplitude spectra of all 79 $\delta$ Scuti stars in NGC 3532. Darker colors denote larger amplitudes. Stars are ordered by Gaia $G_{\rm BP}-G_{\rm RP}$, with bluest stars on top. The dashed red line marks 5 ${\rm d^{-1}}$, which is commonly used as the boundary between $\delta$ Scuti pulsations and other sources of low frequency variability 2002MNRAS.333..251H2010arXiv1007.3176H2019MNRAS.485.2380M. Each amplitude spectrum is labeled with its corresponding star and color (without dereddening).
• Figure 3: Scatter plot showing the log skewness of peak amplitudes in aperture and PSF photometry from TGLC. We apply two lower boundaries in log skewness, 0.75 for both aperture and PSF photometry. Points are color-coded by the signal-to-noise ratio (SNR) of the highest peak in the periodogram. The gray dashed line is the 1:1 line.
• Figure 4: Left: Dereddened Gaia Color-Magnitude Diagram (CMD) of NGC 3532. Color-mapped points are stars with a Gaia vbroad measurement, and gray points have no vbroad measurement. Star markers with red outlines are $\delta$ Scuti pulsators, while circular points are non-pulsators. Gray pluses are stars we did not search. Furthermore we plot MIST isochrones with age 300 Myr and [Fe/H] = -0.07 dex. The dotted isochrone is a non-rotating model, and the solid isochrone is a rotating model at 40% critical rotation speed. An empirically derived red instability strip (IS) edge from 2024ApJ...972..137G is shown as the dashed magenta line. Right: As Left except in $\log L$ vs. $T_{\rm eff}$ space. Luminosities and $T_{\rm eff}$ values come from the TIC catalog 2019AJ....158..138S. The magenta lines now show the empirically derived IS bounds from 2019MNRAS.485.2380M.
• Figure 5: Vertical distance above the 40% critical isochrone (solid orange curve in the left panel of Figure \ref{['fig:cmd']}) vs. $G_{\rm BP}-G_{\rm RP}$. Color-mapped points are stars with a vbroad measurement. Star points are $\delta$ Scuti stars, and circular points and gray pluses are non-pulsators. Stars with red edges are $\delta$ Scuti stars that suffer from blending (described in § \ref{['subsec:blend']}). This scatter plot shows that the $\delta$ Scuti stars fall farther from the isochrone and rotate more rapidly than the non-pulsators on the lower branch. This is most apparent at $G_{\rm BP}-G_{\rm RP}\lesssim 0.25$. To show this more explicitly, we found the mean vbroad of stars on the upper branch, designated as the region between the two gray dotted lines, where $\langle\texttt{vbroad}\rangle\approx184$ km/s. This is about twice the rotation seen on the lower branch, where $\langle\texttt{vbroad}\rangle\approx95$ km/s. Roughly two-thirds of the $\delta$ Scuti stars in NGC 3532 fall on the upper branch.