Red giant - jet collisions in galactic nuclei I: 3D hydrodynamical model of a few stellar orbits

TL;DR

This study uses 3D hydrodynamical simulations to model how red giants near the Galactic center lose mass due to interactions with nuclear jets, revealing potential observable effects on stellar populations.

Contribution

It provides the first detailed numerical modeling of red giant ablation by galactic jets, quantifying mass loss rates and thermal effects during multiple jet passages.

Findings

Mass loss scales with the square root of time.

Total ablated mass during an AGN phase is about 10^{-4} solar masses.

Stellar surface temperature can increase significantly, affecting spectral type.

Abstract

Several models have been proposed to explain missing red giants (RGs) near the Galactic centre. Recently, a scenario has been suggested that predicts, among other processes, a long-term ablation of the surface layers of RGs during their repetitive passages through the Galactic jet (Zaja\v{c}ek et al., 2020). In this study, we perform detailed three-dimensional numerical modelling of this phenomenon. We calculate the ablation rate of the surface layers of a RG orbiting the supermassive black hole (SMBH) as it passes through the nuclear jet. In particular, we model the jet-star interaction for approximately 10 passages for the closer orbital distance of $10^{-3}\,\text{pc}$ and 2 passages for $10^{-2}\,\text{pc}$. We find that the mass loss due to ablation by the jet behaves with time as $\Delta M_{\star}\propto \sqrt{t}$ and the total ablated mass during a single active galactic nucleus (AGN) phase ($10^5$ years) is $\sim 10^{-4}\,M_{\odot}$. We arrive at similar rates of the stellar ablation for the relatively smaller jet luminosity $10^{42}\,\text{erg}\,\text{s}^{-1}$ as in the previous analytical calculations. For larger jet luminosities of $10^{44}$ and $10^{48}\,\text{erg}\,\text{s}^{-1}$, the ablation rates inferred from $\sim 10$ interactions as well as extrapolated power-law fits are significantly lower than analytical values. Overall, the mass ablation rate per interaction and the extrapolated cumulative mass loss during the jet activity are comparable to the stellar-wind mass loss. For the smallest orbital distance of $10^{-3}\,\text{pc}$, we also track the thermal behaviour of the stellar surface layer, whose temperature appears to grow rapidly during the first 10 passages from $\sim 3600\,{\rm K}$ (spectral type M) to $\sim 8500\,{\rm K}$ (spectral type A). RG-jet interactions can thus lead to observable changes in the nuclear stellar population during the jet existence.