arXiv subjects

A. Rantala

Publications and source records attributed to A. Rantala.

2 recordsLinked to original sources

arXiv2026

Ballistic mixing: stellar mergers drive internal mixing in extremely massive stars, with implications for proto-globular cluster pollution

Extremely massive stars, with masses of $10^3$-$10^4$ M$_\odot$, have been proposed as potential sources of the chemical enrichment associated with multiple populations in globular clusters. Early on the main sequence, these stars reach central temperatures of 65 to 75 MK, enabling nucleosynthesis through the CNO, NeNa, and MgAl cycles, producing abundance patterns similar to those observed in the so-called cluster's second population. Stellar mergers are among the leading formation channels for extremely massive stars. In this work, we follow the formation of a $\sim6700$ M$_\odot$ star via 253 stellar mergers, drawn from a direct $N$-body star cluster simulation with properties similar to the densest star clusters observed at redshift $z\sim10$. We evolve each star as a 1D stellar model and construct the merger products from mixing and shock heating prescriptions calibrated to 3D simulations of parabolic collisions. We find that entropy-based mixing efficiently transports nuclear-processed material from the convective core to the envelope, while merger-driven heating promotes chemical homogenization. As a result, the stellar surface becomes enriched with H-burning products, producing abundance patterns consistent with those observed in globular cluster multiple populations and supporting the role of merger-formed extremely massive stars as potential sources of early chemical enrichment in proto-globular cluster-like conditions.

astro-ph.GA↗

arXiv2022

Accuracy and precision of triaxial orbit models I: SMBH mass, stellar mass and dark-matter halo

We investigate the accuracy and precision of triaxial dynamical orbit models by fitting two dimensional mock observations of a realistic N-body merger simulation resembling a massive early-type galaxy with a supermassive black hole (SMBH). We show that we can reproduce the triaxial N-body merger remnant's correct black hole mass, stellar mass-to-light ratio and total enclosed mass (inside the half-light radius) for several different tested orientations with an unprecedented accuracy of 5-10%. Our dynamical models use the entire non-parametric line-of-sight velocity distribution (LOSVD) rather than parametric LOSVDs or velocity moments as constraints. Our results strongly suggest that state-of-the-art integral-field projected kinematic data contain only minor degeneracies with respect to the mass and anisotropy recovery. Moroever, this also demonstrates the strength of the Schwarzschild method in general. We achieve the proven high recovery accuracy and precision with our newly developed modeling machinery by combining several advancements: (i) our new semi-parametric deprojection code probes degeneracies and allows to constrain the viewing angles of a triaxial galaxy; (ii) our new orbit modeling code SMART uses a 5-dim orbital starting space to representatively sample in particular near-Keplerian orbits in galaxy centers; (iii) we use a generalised information criterion AICp to optimise the smoothing and to compare different mass models to avoid biases that occur in $χ^2$-based models with varying model flexibilities.

astro-ph.GA↗