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TESS Detects Pulsating Stars in the Pleiades Cluster

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an infrared image of the Pleiades star cluster
This unusual view of the familiar Pleiades star cluster was created with data from the Wide-field Infrared Survey Explorer (WISE) spacecraft. [NASA/JPL-Caltech/UCLA]

NASA’s newest planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has been tracking down exoplanets since 2018. Its high-cadence observations make it adept at untangling the workings of bright, nearby stars, and a recent research article has used TESS data to study stars in one of the most familiar clusters: the Pleiades.

The Persistent Mystery of Pulsating Stars

diagram showing the location of stars on the instability strip with respect to other stars

Approximate temperatures and luminosities of stars on the instability strip compared to main-sequence and other categories of stars. [Adapted from ESO; CC BY 4.0]

Astronomers have known for centuries that many stars in the night sky vary in brightness. Over time, we’ve refined our understanding of variable stars, and researchers now recognize that most variable stars lie within a small range of temperatures and luminosities referred to as the instability strip. Many — but not all — of the stars on the instability strip pulsate, their luminosities changing periodically as their radii grow and shrink over time.

A persistent question in the study of variable stars is why some stars on the instability strip are variable and others are not — how is it possible for two stars with nearly identical temperatures and luminosities to behave so differently? As a step toward answering this question, a collaboration led by Timothy Bedding (University of Sydney) used precise spacecraft data to search for pulsating stars in the nearby Pleiades cluster.

plot of the pulsation spectra of 35 stars

Pulsation spectra of the 35 pulsating stars observed by TESS. The stars are ordered according to their color as measured by the Gaia spacecraft, with bluer stars near the top and redder stars near the bottom. Click to enlarge. [Bedding et al. 2023]

Gaia and TESS

First, the team used precise stellar positions and distances cataloged by the Gaia spacecraft to determine which stars belonged to the Pleiades cluster, which is about 450 light-years away. Combining these likely cluster members with a handful of nearby stars that appear to have recently escaped the cluster, Bedding and collaborators collected a sample of 89 stars. Ten stars in this sample are already known to vary in brightness.

TESS observed most of these stars over a period of several weeks in 2021. Using the finely time-resolved TESS data (and some Kepler data for a star in the sample that moved off the edge of the TESS detector), the team detected a particular kind of behavior called δ Scuti pulsations in 36 stars, 30 of which were not previously known to vary. The pulsation spectra of these stars demonstrate that stars with similar ages, temperatures, and chemical compositions can pulsate in dramatically different ways.

Questions Continue

Two plots show the colors and magnitudes of the pulsating stars relative to the non-pulsating stars.

Top: Colors and magnitudes of 89 Pleiades stars with spectral classes A and F. The filled purple circles indicate pulsating stars. Bottom: A histogram showing the colors of pulsating (blue) and not pulsating (orange) stars in the sample. Click to enlarge. [Bedding et al. 2023]

By plotting the observed brightness of these stars against their colors, Bedding and coauthors determined where the stars fell with respect to the instability strip. The pulsating stars spanned a band 0.45 magnitude wide, and 72% of Pleiades stars falling within this band showed pulsations. For stars located in the center of the instability strip, 84% are pulsators, which the team notes is an unusually high percentage.

Given the remarkably high percentage of pulsators in the cluster, Bedding and collaborators noted that the question isn’t so much why some stars pulsate and others do not, but rather why similar stars have such different pulsations. Though the question remains open, one thing is clear: Gaia and TESS data are powerful tools for tracking down variable stars.

Citation

“TESS Observations of the Pleiades Cluster: A Nursery for δ Scuti Stars,” Timothy R. Bedding et al 2023 ApJL 946 L10. doi:10.3847/2041-8213/acc17a