Short-Lived Radioisotopic enrichment from AGB interlopers in low-mass star-forming regions

TL;DR

The paper investigates how short-lived radioisotopes ($^{26}$Al and $^{60}$Fe) can be delivered to protoplanetary disks in low-mass star-forming regions via passing AGB interlopers, offering an alternative to WR winds and supernovae that better preserve disks. Using AMUSE-based N-body simulations coupled with a time-dependent AGB wind enrichment model, the authors perform a broad parameter-space exploration (region size, interloper velocity, encounter timing, and cluster population) and quantify disk enrichment through an outflow-transport scheme. They find that enrichment is widespread and highly sensitive to interloper velocity and the timing of interaction, with walkaway speeds ($ ext{a few}$ km s$^{-1}$) and late-stage AGB winds capable of delivering Solar-system-like levels of $^{26}$Al (and substantial $^{60}$Fe) to multiple disks, sometimes even under near-miss geometries. The results imply that AGB interlopers could significantly contribute to the SLR inventory of planetary systems without the destructive consequences of massive-star feedback, though the overall probability of such interloper events remains uncertain and warrants future observational and statistical study.

Abstract

The decay of Short-Lived Radioisotopes (SLRs) can be a significant source of heating early in protoplanetary systems, though how a protoplanetary disk becomes enriched with these SLRs far above the galactic background level remains an open question. Observational evidence suggests that this enrichment occurs during the period from when the disk forms to when it progresses into a protoplanetary system, and is homogenous throughout the resultant planetary system. Whilst SLRs such as 26Al and 60Fe can be injected into disks through interaction with Wolf-Rayet winds and supernovae, these outflows can disrupt disks, and in the case of low-mass star-forming regions high-mass stars may not form at all. Recent research has determined the existence of AGB ``interlopers'', Asymptotic Giant Branch stars that pass close to or through star-forming regions that could be an alternative source of SLR enrichment to WR winds and supernovae. In this paper we study the effect of AGB interlopers on star-forming regions from a dynamical perspective, determining the enrichment amount of 26Al and 60Fe in disks within small clusters via numerous N-body simulations via a parameter space exploration. We find that enrichment is widespread from AGB stars, with efficient enrichment dependent on the time at which the interloper intersects the star-forming region. Velocity is a factor, though interlopers travelling at 30 km/s are capable of enriching many disks in a star-forming region assuming they encounter a disk when the interloper is more evolved.

Figures (9)

• Figure 1: SLR yield rates and total yields for stars over their AGB phase, derived from the work in karakasStellarYieldsMetalRich2016. Higher-mass stars have significantly shorter AGB phases, but have much higher emission rates and larger total yields.
• Figure 2: A comparison of $^{26}\text{Al}$ and $^{60}\text{Fe}$ enrichment for set A simulations, split by $r_c$. Opaque, larger markers represent disks with a host star between 0.5M_⊙ and 1.5M_⊙. Disks with Solar System levels with a Solar-like host star are possible, though infrequent. Dashed lines represent estimated Solar System formation levels of enrichment based on observational data. Enrichment amount does not vary considerably as $r_c$ is varied, though the probability of the disks undergoing high levels of enrichment decreases significantly. There is a clear inverse relationship between $v_i$ and enrichment of either isotope.
• Figure 3: Enrichment fractions for each simulation in the B set. There is a strong dependency on interloper velocity and yield, with a significant reduction in enrichment as the initial interloper velocity exceeds $10\,km.s^{-1}$. Whilst there is also an inverse relationship with enrichment and interloper AGB phase offset time, this is largely due to a lack of viable protoplanetary disks.
• Figure 4: Interloper distance from the nearest star over time for simulations where the interloper enters the AGB phase immediately after the start of the simulation. Most interlopers do not become trapped in the star-forming region.
• Figure 5: Comparison of enrichment in simulations where interloper $x_i$ and $y_i$ offset are changed. Results are separated by initial interloper velocity. The $10\,km.s^{-1}$ case consistently produces some highly enriched disks, as long as the interloper is given some time to evolve before it interacts with the star-forming region.