The role of stellar collisions for the formation of massive stars

Abstract

We use direct N-body simulations of gas embedded star clusters to study the importance of stellar collisions for the formation and mass accretion history of high-mass stars. Our clusters start in virial equilibrium as a mix of gas and proto-stars. Proto-stars then accrete matter using different mass accretion rates and the amount of gas is reduced in the same way as the mass of stars increases. During the simulations we check for stellar collisions and we investigate the role of these collisions for the build-up of high-mass stars and the formation of runaway stars. We find that a significant number of collisions only occur in clusters with initial half-mass radii r_h < 0.1 pc. After emerging from their parental gas clouds, such clusters end up too compact compared to observed young, massive open clusters. In addition, collisions lead mainly to the formation of a single runaway star instead of the formation of many high mass stars with a broad mass spectrum. We therefore conclude that massive stars form mainly by gas accretion, with stellar collisions only playing a minor role if any at all. Collisions of stars in the pre-main sequence phase might however contribute to the formation of the most massive stars in the densest star clusters and possibly to the formation of intermediate-mass black holes with masses up to a few 100 Msun.Comment: 10 pages, 8 figures, MNRAS in pres

Formation of massive seed black holes via collisions and accretion

“…4 and 5), but that the average collision rate is more consistent with previous studies. For example, Baumgardt & Klessen (2011, Fig. 4) measure a collisional fraction of 0.1-1%, and this result is confirmed by Reinoso et al (2017) for gas free systems.…”

“…In model 5 the fraction reduces to about 10% as due to the uniform accretion the central object in that cluster is less pronounced, and due to the time dependent accretion, the protostellar radii shrink again with decreasing gas reservoir. The latter represents an enhancement by roughly a factor of 10 compared to the corresponding gas-free models or models which assumed smaller protostellar radii (Baumgardt & Klessen 2011;Reinoso et al 2017). Comparing the radially independent accretion models (top row of Fig 3) and the radially dependent models (bottom row), we observe a higher fraction of less massive collision products for the radially independent accretion models.…”

Pre-Mainsequence Stellar Evolution in N-Body Models

“…It is not straightforward to make a direct comparison with Baumgardt & Klessen (2011) because they let their stars form by accretion on to 0.1 M ࣻ cores, at constant accretion rates to populate the IMF, whereas we start with an IMF and evolve our protostars as if they were coeval. However, examining the 6 Railton et al time-integrated cross-section of our analytic fits compared to theirs (see Figure 4) shows that their models systematically underestimate the radii of the preMS stars below 6 M ࣻ compared to ours.…”

“…The background gas that would exist in a protostellar cloud has been neglected, as have the effects of accretion of this gas (see the treatment by Baumgardt & Klessen 2011). Nor have we modelled the effects of stellar accretion discs which would enhance the likelihood of collisions and also change their nature.…”

“…The physics used by Bernasconi & Maeder (1996) is not very different from that used by Tout et al (1999) and indeed the evolutionary tracks in the H-R diagram are very similar once accretion has ceased and the stars contract down Hayashi tracks. The major difference between the models we present here and those of Baumgardt & Klessen (2011) is that all their stars begin as protostellar cores of 0.1 M and accrete at a constant rate until they arrive at a suitable mass to populate an IMF up to 15 M . Their stars then undergo a preMS phase in which the logarithm of their radii shrinks linearly with time until they reach the ZAMS, whereupon they begin main-sequence evolution.…”