Asteroseismology of the young open cluster NGC 2516

Abstract

Asteroseismic modelling of isolated stars presents significant challenges due to the difficulty in accurately determining stellar parameters, particularly the stellar age. These challenges can be overcome by observing stars in open clusters whose coeval members share an initial chemical composition. The light curves from the all-sky survey by the Transiting Exoplanet Survey Satellite (TESS) allow us to investigate and analyse stellar variations in clusters with an unprecedented level of detail for the first time. We aim to detect gravity-mode oscillations in the early-type main-sequence members of the young open cluster NGC\,2516 to deduce their internal rotation rates. We selected the member stars with no more than mild contamination as our sample. We analysed the full-frame image light curves, which provide nearly continuous observations in the first and third years of TESS monitoring. We also collected high-resolution spectra using the Fiber-fed Extended Range Optical Spectrograph for the g-mode pulsators, with the aim assessing the Gaia effective temperatures and gravities and preparing for future seismic modelling. By fitting the theoretical isochrones to the colour-magnitude diagram of a cluster, we determined an age of and inferred that the extinction at 550\,nm ($A_0$) is We identified stars with surface-brightness modulations: with gravity (g-)mode pulsations (gamma \,Doradus or slowly pulsating B-type stars) and with pressure (p-)mode pulsations (delta \,Sct stars). When sorted by colour index, the amplitude spectra of the delta \,Sct stars show a distinct ordering and reveal a discernible frequency-temperature relationship. The near-core rotation rates, measured from period spacing patterns in two slowly pulsating B-type (SPB) stars and nine gamma \,Dor stars, reach up to $3\ d^ $. This is at the high end of the values found from Kepler\/ data of field stars of similar variability type. The gamma \,Dor stars of NGC\,2516 have internal rotation rates as high as 50<!PCT!> of their critical value, whereas the SPB stars exhibit rotation rates close to their critical rate. Although the B-type stars are rotating rapidly, we did not find long-term brightness and colour variations in the mid-infrared, which suggests that there are no disc or shell formation events in our sample. We also discussed the results of our spectroscopic observations for the g-mode pulsators.

Age uncertainties of red giants due to cumulative rotational mixing of progenitors calibrated by asteroseismology

“…Our work shows the importance of taking into account transport processes in the stellar interior during the entire main sequence when age-dating red giants for high-precision Galactic archaeological studies. Asteroseismology of young and medium-aged open clusters with a variety of metallicities and both main sequence and red giant pulsators among their members offers the best way to calibrate internal mixing and angular momentum transport processes across the entire nuclear evolution phases of intermediate-mass stars (see Fritzewski et al 2024;Li et al 2024, for initial attempts of such modelling). It is mostly consistent with the FLAME pipeline.…”

Probability distributions of initial rotation velocities and core-boundary mixing efficiencies of γ Doradus stars

“…UBC 1 includes one γ Dor member with a near-core rotation frequency measurement, with a value of 0.544 ± 0.009 d −1 , which is in agreement with our results for the distribution of rotation rates near the ZAMS. On the other hand, measurements of the near-core rotation frequency of γ Dor stars in the even younger open cluster NGC 2516 (102 ± 15 Myr, also solar metallicity) reveal eight of the 11 g-mode pulsators to have near-core rotation frequencies around 3 d −1 , while the other three have values between 1 d −1 and about 2.2 d −1 (Li et al 2024). At this estimated cluster age, these stars should be even closer to the ZAMS than the γ Dor member of UBC 1.…”