Extended Main-sequence Turnoffs in the Double Cluster h and χ Persei: The Complex Role of Stellar Rotation

Abstract

Using Gaia Data Release 2 photometry, we report the detection of extended main-sequence turnoff (eMSTO) regions in the color-magnitude diagrams (CMDs) of the ∼ 14 Myr-old double clusters h and χ Persei (NGC 869 and NGC 884). We find that stars with masses below ∼1.3 M in both h and χ Persei populate narrow main sequences (MSs), while more massive stars define the eMSTO, closely mimicking observations of young Galactic and Magellanic Cloud clusters (with ages older than ∼30 Myr). Previous studies based on clusters older than ∼30 Myr find that rapidly rotating MS stars are redder than slow rotators of similar luminosity, suggesting that stellar rotation may be the main driver of the eMSTO. By combining photometry and projected rotational velocities from the literature of stars in h and χ Persei, we find no obvious relation between the rotational velocities and colors of non-emission-line eMSTO stars, in contrast with what is observed in older clusters. Similarly to what is observed in Magellanic Cloud clusters, most of the extremely rapidly rotating stars, identified by their strong Hα emission lines, are located in the red part of the eMSTOs. This indicates that stellar rotation plays a role in the color and magnitude distribution of MSTO stars. By comparing the observations with simulated CMDs, we find that a simple population composed of coeval stars that span a wide range of rotation rates is unable to reproduce the color spread of the clusters' MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.

Probing the interior physics of stars through asteroseismology

“…Masses of fully mixed convective cores may also be derived from a model-dependent isochrone fitting of the observed extended main-sequence turn-offs (eMSTOs). This was done for numerous young open clusters observed by the Hubble and Gaia space telescopes (Goudfrooij et al, 2018;Li, Sun et al, 2019). Interpretation of the shape and diversity of observed eMSTOs was done mainly by including rotational mixing (Bastian et al, 2018;Gossage et al, 2018) or magnetism (Georgy et al, 2019) in the stellar evolution models.…”

Internal mixing of rotating stars inferred from dipole gravity modes

“…The g modes allow for proper inference of the core masses despite considerable uncertainty for α cbl in the 6-D fitting. The convective core mass expressed as a percentage of total stellar mass ranges from ∼30% near the zero-age-main-sequence (ZAMS) and stays above ∼6% near the terminal-age-main-sequence (TAMS), confirming the need for higher-thanstandard core masses in eclipsing binaries 19 and in young open clusters 37,38 . Through the calculation of Spearman's rank correlation coefficients (Supplementary Table 7) we find a strong correlation between r cc /R and m cc /M as well as between the core masses and radii and the main-sequence evolutionary stage X c /X ini , as expected from theory.…”

The effect of the centrifugal acceleration on period spacings of gravito-inertial modes in intermediate-mass stars

“…In each of the calculated MESA models, we considered D mix to be constant throughout the radiative envelope, adopting values typical for g-mode pulsators (Van Reeth et al 2016;Moravveji et al 2016). -convective core overshooting: this process is of major importance for stellar evolution in the considered mass range, yet it is least known among the various ingredients to be chosen as input physics (Claret & Torres 2019;Johnston et al 2019;Li et al 2019a;Tkachenko et al 2020). Different formalisms exist to describe this overshoot region.…”