On the origins of enigmatic stellar populations in Local Group galactic nuclei

Abstract

We consider the origins of enigmatic stellar populations in four Local Group galactic nuclei, specifically the Milky Way, M31, M32 and M33. These are centrally concentrated blue stars, found in three out of the four nuclear star clusters (NSCs) considered here. Their origins are unknown, but could include blue straggler (BS) stars, extended horizontal branch stars and young recently formed stars. Here, we calculate order-of-magnitude estimates for various collision rates, as a function of the host NSC environment and distance from the cluster centre. These rates are sufficiently high that BSs, formed via collisions between main sequence (MS) stars, could contribute non-negligibly (∼ 1-10% in mass) to every surface brightness profile, with the exception of the Milky Way. Stellar evolution models show that the envelopes of red giant branch (RGB) stars must be nearly completely stripped to significantly affect their photometric appearance, which requires multiple collisions. Hence, the collision rates for individual RGB stars are only sufficiently high in the inner 0.1 pc of M31 and M32 for RGB destruction to occur. Collisions between white dwarfs and MS stars, which should ablate the stars, could offer a steady and significant supply of gas in every NSC in our sample. The gas could either fragment to form new stars, or accrete onto old MS stars already present. Thus, collisional processes could contribute significantly to the observed blue excesses in M31 and M33; future studies should be aimed at better constraining theoretical predictions to compliment existing and future observational data.

Millisecond Pulsars and the Gamma-Ray Excess in Andromeda

“…Finally, we note that some of the neutron stars delivered by the GCs may mass-segregate to some extent and also be successfully exchanged in few-body interactions in binaries that later could lead to the formation of MSPs, which could enhance our predicted rate (Leigh et al 2016). This would give a maximum contribution roughly comparable to the in situ formation scenario, which can account only for ∼ 1/4 of the excess (Eckner et al 2018), being the mass in GCs of the order of the mass of the nuclear star cluster.…”

An overabundance of black hole X-ray binaries in the Galactic Centre from tidal captures

“…D09 find that stripping is only effective in reducing the brightness of giants in the RGB phase (and has little effect on AGB and horizontal branch stars). Furthermore, only close pericenters (rp 15R for a solar type giant) will remove enough material to significantly alter the evolution of the giant (see also Leigh et al 2016). They conclude that 2×10 4 BHs inside of 0.1 pc are required to explain the observed dearth of intermediate luminosity giants.…”

“…However these observations probe only low mass stars ( 2M ), and thus do not constrain the rate of NS/BH formation within the star-forming disks if the top-heavy disk IMF is truncated below a few solar masses. Other nearby galactic nuclei such as M31 possess disks of A stars, but no O and B stars (Leigh et al 2016); in these NSCs at least, the last major episode of star formation occurred 100 Myr ago.…”

“…We also consider models in which stars form at a constant rate with the present day observed profile. This model is unrealistic for the GC, but may be useful for other galactic nuclei with different star formation histories (Leigh et al 2016). These models are summarized in Table 2.…”

“…The large mean stellar densities in these clusters, typically ∼ 10 2 − 10 6 M pc −3 (e.g. Georgiev & Böker 2014), result in correspondingly high rates of collisional interactions (Leigh et al 2016). Stellar-mass compact objects, particularly black holes (BHs) and neutron stars (NSs), play an important role in these environments; for example, they form sources of LIGO and LISA-band gravitational waves (e.g.…”