Ensemble seismology of Red Clump stars

Ensemble seismology of Red Clump stars Anthony Noll1,2,3, Sarbani Basu4 and Saskia Hekker3,5 1: Institute of Space Sciences (CSIC) ; 2: Institut d’Estudis Espacials de Catalunya 3: Heidelberg Institute for Theoretical Studies (HITS) ; 4: Yale University 5: Heidelberg University Context ●Red Clump stars are core-helium burning stars which went through the Helium flash ●The properties of their cores are ill-constrained, notably: ○Core boundary mixing (overshoot, semi-convection…) ○Rate of the carbon-𝛼 reaction ●Red Clump stars exhibit mixed modes, whose period spacing (𝝙𝚷) is a good indicator of the properties of the core1,2 and has been measured in thousands of stars observed by Kepler3 ●Aim: Fit the observed3 𝝙𝚷 distributions using Monte-Carlo methods and stellar models, assuming different core boundary schemes and reaction rates Grid of tracks computed with MESA, varying masses and metallicities Random sampling, using observational priors (APOKASC 3)4 Computation of the period spacing, linear interpolation, adding of uncertainties Simulated distribution, compared with the observational one from Vrard et al. 2016 Results: Core Boundary mixing Results: Carbon-𝛼 reaction rate Computation of the period spacing: use of the asymptotic formula5, integrated over the region outside the partially mixed (overmixing or semi-convective) region, to mimic the period spacing of modes mainly trapped outside the partially mixed region. If such trapping is not assumed, cannot fit the observed distribution! Brunt-Väisälä frequency G-mode cavity Methodology Decreasing the Carbon-𝛼 rate slightly increases the quality of fit Contact information: [email protected] Observations Models Observations Models 1. Montalbán, Miglio et al., 2013, ApJ, 766, 118 2. Noll, Basu, and Hekker, 2024, A&A, 683, A189 3. Vrard, Mosser, and Samadi, 2016, A&A, 588, A87 4. Pinsonneault, Zinn et al., 2025, ApJS, 276, 69 5. Shibahashi, 1979, PASJ, 31, 87 Models computed using the default carbon-𝛼 nuclear reaction rate (NACRE2, Xu et al. 2013) Models computed using the Maximal Overshoot mixing scheme Best case Best fit found when modes are assumed to be trapped outside a classical semi-convective region, with a 15% lower carbon-𝛼 rate. 6. Constantino, Campbell et al., 2015, MNRAS, 452, 123 7. Castellani, Giannone, and Renzini, 1971, Ap&SS, 10, 355 Maximal Overshoot Semi-convection Overmixing, 𝛼ov= 0.2 Overmixing, 𝛼ov= 0.5 ●Core extended6 such that min(∇rad) = ∇ad ●Equivalent to semi-convection, but without core breathing pulses ●Best fit to the observations! ●Region around the core where ∇rad = ∇ad 7 ●Exhibits core breathing pulses ●Core breathing pulses worsens the fit ●Core extended over 𝑑ov= 𝛼ov𝐻p, with 𝛼ov= 0.2 ●∇ = ∇rad in the overshoot region ●Underestimates the number of stars with low period spacings ●Increasing 𝛼ov worsens the fit Nominal rate × 0.75 Nominal rate × 0.85 Nominal rate × 1.00 Nominal rate × 1.25