Stellar disk destruction by dynamical interactions in the Orion Trapezium star cluster

We compare the observed size distribution of circum stellar disks in the Orion Trapezium cluster with the results of $N$-body simulations in which we incorporated an heuristic prescription for the evolution of these disks. In our simulations, the sizes of stellar disks are affected by close encounters with other stars (with disks). We find that the observed distribution of disk sizes in the Orion Trapezium cluster is excellently reproduced by truncation due to dynamical encounters alone. The observed distribution appears to be a sensitive measure of the past dynamical history of the cluster, and therewith on the conditions of the cluster at birth. The best comparison between the observed disk size distribution and the simulated distribution is realized with a cluster of $N = 2500\pm500$ stars with a half-mass radius of about 0.5\,pc in virial equilibrium (with a virial ratio of $Q = 0.5$, or somewhat colder $Q \simeq 0.3$), and with a density structure according to a fractal dimension of $F \simeq 1.6$. Simulations with these parameters reproduce the observed distribution of circum stellar disks in about 0.2--0.5\,Myr.Comment: submitted to MNRA

The Formation of Solar System Analogs in Young Star Clusters

The Solar system was once rich in the short-lived radionuclide (SLR) $^{26}$Al\, but deprived in $^{60}$Fe. Several models have been proposed to explain these anomalous abundances in SLRs, but none has been set within a self-consistent framework of the evolution of the Solar system and its birth environment. The anomalous abundance in $^{26}$Al may have originated from the accreted material in the wind of a massive \apgt 20\,$M_\odot$ Wolf-Rayet star, but the star could also have been a member of the parental star-cluster instead of an interloper or an older generation that enriched the proto-solar nebula. The protoplanetary disk at that time was already truncated around the Kuiper-cliff (at $45$ au) by encounters with another cluster members before it was enriched by the wind of the nearby Wolf-Rayet star. The supernova explosion of a nearby star, possibly but not necessarily the exploding Wolf-Rayet star, heated the disk to \apgt 1500K, melting small dust grains and causing the encapsulation and preservation of $^{26}$Al into vitreous droplets. This supernova, and possibly several others, caused a further abrasion of the disk and led to its observed tilt of $5.6\pm1.2^\circ$ with respect to the Sun's equatorial plane. The abundance of $^{60}$Fe originates from a supernova shell, but its preservation results from a subsequent supernova. At least two supernovae are needed (one to deliver $^{60}$Fe\, and one to preserve it in the disk) to explain the observed characteristics of the Solar system. The most probable birth cluster then has $N = 2500\pm300$ stars and a radius of $r_{\rm vir} = 0.75\pm0.25$ pc. We conclude that Solar systems equivalent systems form in the Milky Way Galaxy at a rate of about 30 per Myr, in which case approximately 36,000 Solar system analogues roam the Milky Way.Comment: Submitted to A&

The origin of the two populations of blue stragglers in M30

We analyze the position of the two populations of blue stragglers in the globular cluster M30 in the Hertzsprung-Russell diagram. Both populations of blue stragglers are brighter than the cluster's turn-off, but one population (the blue blue-stragglers) align along the zero-age main-sequence whereas the (red) population is elevated in brightness (or colour) by $\sim 0.75$ mag. Based on stellar evolution and merger simulations we argue that the red population, which composes about 40\% of the blue stragglers in M 30, is formed at a constant rate of $\sim 2.8$ blue stragglers per Gyr over the last $\sim 10$ Gyr. The blue population is formed in a burst that started $\sim 3.2$ Gyr ago at a peak rate of $30$ blue stragglers per Gyr$^{-1}$ with an e-folding time scale of $0.93$ Gyr. We speculate that the burst resulted from the core collapse of the cluster at an age of about 9.8 Gyr, whereas the constantly formed population is the result of mass transfer and mergers through binary evolution. In that case about half the binaries in the cluster effectively result in a blue straggler.Comment: Accepted for publication as Letter in A&