Massive Merger Products on the Main Sequence: Do Their Pulsations Betray Them?
Abstract
When two massive main-sequence stars merge, the resulting merger product can continue its life as a seemingly normal main-sequence star. Given that ~10% of massive stars in binary systems are expected to merge, such merger products are relatively common in stellar populations and can lead to errors in inferred stellar properties. Peculiar surface properties - such as chemical enrichment, slow rotation, and strong large-scale magnetic fields - can hint at a star’s merger origin, yet are often insufficient for unambiguous identification on their own. Since such peculiarities are not limited to the stellar surface, we predict how peculiarities in the interior of merger products manifest via the properties of their non-radial stellar pulsations. To do so, we used the outcome of a 3D magnetohydrodynamic simulation of a merger between a 9 and an 8 solar-mass main-sequence star. With the resulting merger product structure mapped to 1D, we evolved it through the main-sequence and computed its gravity and pressure modes. By comparing these predicted asteroseismic signals with those of “genuine single” stars with similar surface properties, we find that the differences in pressure-mode frequencies and mean gravity-mode period spacings exceed typical observational uncertainties. Moreover, the merger product’s gravity-mode period spacing patterns contain features which could technically be exploited in practice. We discuss other aspects related to the potential of asteroseismology to identify main-sequence merger products and the danger of ignoring the potential merger origin of stars. We further assess how well three commonly used 1D merger prescriptions replicate the structural and seismic properties of the 3D merger product and highlight the need for improved merger product models for future (forward) modelling efforts.
Full text
Massive Merger Products on the Main Sequence Do Their Pulsations Betray Them? Jan Henneco 09.07.2025 TASC9 KASC16 ISTA Austria Fabian Schneider, Saskia Hekker, Max Heller, Conny Aerts
J. Henneco – Massive Merger Products on the MS 1 Hiding in plain sight
J. Henneco – Massive Merger Products on the MS This is a star Hiding in plain sight 1
J. Henneco – Massive Merger Products on the MS This is a star This is also a star Hiding in plain sight 1
J. Henneco – Massive Merger Products on the MS This is a star This is also a star Hiding in plain sight 1
J. Henneco – Massive Merger Products on the MS This is also a star Hiding in plain sight This is ‘normal’ main-sequence (MS) star 1
J. Henneco – Massive Merger Products on the MS Hiding in plain sight This is ‘normal’ main-sequence (MS) star This is a MS+MS merger product 1
J. Henneco – Massive Merger Products on the MS Hiding in plain sight This is a MS+MS merger product ✸Could/can you tell from their surfaces? 1
J. Henneco – Massive Merger Products on the MS Hiding in plain sight This is a MS+MS merger product ✸Could/can you tell from their surfaces? à maybe … 1
J. Henneco – Massive Merger Products on the MS ✸3D magnetohydrodynamic simulation of a 9+8 M⊙ MS+MS merger from Schneider+2019 Looking for answers with help from 3D 2
J. Henneco – Massive Merger Products on the MS ✸3D magnetohydrodynamic simulation of a 9+8 M⊙ MS+MS merger from Schneider+2019 ✸Most realistic merger product model currently available Looking for answers with help from 3D 2
J. Henneco – Massive Merger Products on the MS ✸3D magnetohydrodynamic simulation of a 9+8 M⊙ MS+MS merger from Schneider+2019 ✸Most realistic merger product model currently available ✸3D à 1D à Looking for answers with help from 3D 2 r12778 Paxton+2019 a.r.t Jermyn+2023
J. Henneco – Massive Merger Products on the MS ✸3D magnetohydrodynamic simulation of a 9+8 M⊙ MS+MS merger from Schneider+2019 ✸Most realistic merger product model currently available ✸3D à 1D à ✸Find equivalent ‘genuine’ single MS model Looking for answers with help from 3D 2 r12778 Paxton+2019 a.r.t Jermyn+2023
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star Structural differences 3 TAMS r12778
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star ✸Mass discrepancy: 𝚫M = 0.5 M⊙ Structural differences 3 TAMS r12778 17.4 M ⊙ 16.9 M ⊙
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star ✸Mass discrepancy: 𝚫M = 0.5 M⊙ ✸‘Age’ discrepancy: at most 2.3% in Xc Structural differences 3 TAMS r12778 17.4 M ⊙ 16.9 M ⊙
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star ✸Mass discrepancy: 𝚫M = 0.5 M⊙ ✸‘Age’ discrepancy: at most 2.3% in Xc ✸Peculiar chemical profile and gradients Structural differences 3 He enrichment 8 M ⊙ 9 M ⊙
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star ✸Mass discrepancy: 𝚫M = 0.5 M⊙ ✸‘Age’ discrepancy: at most 2.3% in Xc ✸Peculiar chemical profile and gradients Structural differences 3 He enrichment 8 M ⊙ 9 M ⊙
J. Henneco – Massive Merger Products on the MS ✸Merger product vs. genuine single star ✸Mass discrepancy: 𝚫M = 0.5 M⊙ ✸‘Age’ discrepancy: at most 2.3% in Xc ✸Peculiar chemical profile and gradients Structural differences 3 He enrichment 8 M ⊙ 9 M ⊙ convection
J. Henneco – Massive Merger Products on the MS Asteroseismic differences – gravity modes ✸Gravity modes àperiod spacing patterns 4 single v7.0 Townsend & Teitler 2013 Townsend+2018 Goldstein & Townsend 2020
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! Asteroseismic differences – gravity modes 4 single merger v7.0 Townsend & Teitler 2013 Townsend+2018 Goldstein & Townsend 2020
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! ✸Additional component in PSP variability Asteroseismic differences – gravity modes 4 single merger v7.0
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! ✸Additional component in PSP variability à second BV peak Asteroseismic differences – gravity modes 4 single merger v7.0
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! ✸Additional component in PSP variability à second BV peak Asteroseismic differences – gravity modes 4 single merger v7.0
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! ✸Additional component in PSP variability à second BV peak Asteroseismic differences – gravity modes 4 single merger v7.0
J. Henneco – Massive Merger Products on the MS ✸Gravity modes àperiod spacing patterns ✸Relatively small difference in asymptotic period spacing àBUT: larger than currently best uncertainties! ✸Additional component in PSP variability à second BV peak Asteroseismic differences – gravity modes 4 single merger v7.0
J. Henneco – Massive Merger Products on the MS Asteroseismic differences – pressure modes 5 freq.single − freq.merger [1/day] freq.single − freq.merger [μHz]
J. Henneco – Massive Merger Products on the MS ✸Pressure-mode frequency shifts due to He enrichment in envelope Asteroseismic differences – pressure modes 5 freq.single − freq.merger [1/day] freq.single − freq.merger [μHz]
J. Henneco – Massive Merger Products on the MS ✸Pressure-mode frequency shifts due to He enrichment in envelope ✸Shifts decrease with MS age Asteroseismic differences – pressure modes 5 freq.single − freq.merger [1/day] freq.single − freq.merger [μHz]
J. Henneco – Massive Merger Products on the MS ✸Technically measurable shifts in mean period spacing à Is this enough? Can we use asteroseismology to identify them? 7 × merger product genuine single stars
J. Henneco – Massive Merger Products on the MS ✸Technically measurable shifts in mean period spacing à Is this enough? à Not with our current uncertainties on luminosity and effective temperature… Can we use asteroseismology to identify them? 7 × merger product genuine single stars
J. Henneco – Massive Merger Products on the MS ✸Technically measurable shifts in mean period spacing à Is this enough? à Not with our current uncertainties on luminosity and effective temperature… ✸Characterise and model PSP variability à link with internal structure! Can we use asteroseismology to identify them? 7 × merger product genuine single stars
J. Henneco – Massive Merger Products on the MS Takeaways 8 Henneco+2025 j[email protected]
J. Henneco – Massive Merger Products on the MS Takeaways 8 Henneco+2025 j[email protected] ✸Despite its similar appearance to its genuine single counterpart, the main-sequence merger product ✸Has a considerably different structure ✸pulsates measurably different from the genuine single MS star
J. Henneco – Massive Merger Products on the MS Takeaways 8 Henneco+2025 j[email protected] ✸Despite its similar appearance to its genuine single counterpart, the main-sequence merger product ✸Has a considerably different structure ✸pulsates measurably different from the genuine single MS star ✸Shifts in mean period spacings and p-mode frequencies ALONE are not enough to identify the MS merger product ✸together with other diagnostics? ✸PSP variability is promising!
J. Henneco – Massive Merger Products on the MS Takeaways 8 Henneco+2025 j[email protected] ✸Despite its similar appearance to its genuine single counterpart, the main-sequence merger product ✸Has a considerably different structure ✸pulsates measurably different from the genuine single MS star ✸Shifts in mean period spacings and p-mode frequencies ALONE are not enough to identify the MS merger product ✸together with other diagnostics? ✸PSP variability is promising! ✸This is just the beginning! ✸Relatively “boring” merger product à there’s a whole zoo of them!
J. Henneco – Massive Merger Products on the MS Takeaways 9 Henneco+2025 j[email protected] ✸How do we populate this zoo? ✸3D merger simulations are expensive… ✸1D merger prescriptions? (entropy sorting, fast accretion, PyMMAMS)
J. Henneco – Massive Merger Products on the MS Takeaways 9 Henneco+2025 j[email protected] ✸How do we populate this zoo? ✸3D merger simulations are expensive… ✸1D merger prescriptions? (entropy sorting, fast accretion, PyMMAMS) àWe’re not there yet! 1D merger prescriptions fail to reproduce stellar structure and seismic properties (see also Heller et al. in prep.)
J. Henneco – Massive Merger Products on the MS Takeaways 9 Henneco+2025 j[email protected] ✸How do we populate this zoo? ✸3D merger simulations are expensive… ✸1D merger prescriptions? (entropy sorting, fast accretion, PyMMAMS) àWe’re not there yet! 1D merger prescriptions fail to reproduce stellar structure and seismic properties (see also Heller et al. in prep.) ✸Lots of things to include (properly) ✸Rotation ✸Magnetic fields ✸Mode excitation/damping mechanisms ✸…