Red giant stellar collisions in the Galactic Centre

Abstract

We show that collisions with stellar--mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low--mass giants and AGB stars in the range 1-3 M$_{\odot}$. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculations, restricted three--body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main--sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the $10.5<K<12$ band. However, depleting the brighter ($K<10.5$) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the $10.5<K<12$ giants in a region much larger than is observed. We conclude that collisions with stellar--mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar--mass black holes in the Galactic Centre.Comment: 21 pages, 21 figures, accepted by MNRA

The nuclear cluster of the Milky Way: our primary testbed for the interaction of a dense star cluster with a massive black hole

“…Mostly, collisions between stars or between stars and stellar remnants have been considered (e.g. Dale et al 2009), but recently it has been suggested that collisions between giants and dense clumps of gas in a star forming disk around Sgr A* could have stripped the giants and thus rendered them too dim to be observable with current technology (Amaro-Seoane & Chen 2014). Scenario (1b) has been shown to be a plausible result of the combination of star formation and dynamical evolution of the NSC.…”

The Nuclear Star Cluster of the Milky Way

“…Further inside, the number density of the old -and therefore probably dynamically relaxed -stars is too flat to be consistent with the cusp scenario (Buchholz et al 2009). This observation could be attributed to a number of reasons, like a very long relaxation time (Merritt 2010) or the effect of stellar collisions on giant stars (Dale et al 2009). The presence (or not) of a stellar cusp at the GC must be considered ongoing research.…”

Short- and long-term evolution of a stellar disc around a massive black hole: the role of the cusp, stellar evolution and binaries

“…Similar results from MonteCarlo calculations were found by Freitag et al (2006) However, observations of red-giants in the Galactic center suggest a core-like structure in the inner regions of the GC (γ in the range 0 − 0.5; Do et al 2009;Bartko et al 2010;Genzel et al 2010. Nevertheless, it is not yet clear whether the distribution of red-giants reflect the overall distribution of stars in the GC, and various models had been suggested to explain both a "real" core distribution (Merritt 2010) or an apparent one (i.e., only reflecting the distribution of red-giants; Dale et al 2009;Amaro-Seoane & Chen 2014;Aharon & Perets 2015).…”