TY - JOUR
T1 - On the origins of enigmatic stellar populations in Local Group galactic nuclei
AU - Leigh, Nathan W.C.
AU - Antonini, Fabio
AU - Stone, Nicholas C.
AU - Shara, Michael M.
AU - Merritt, David
N1 - Publisher Copyright:
© 2017 The Authors.
PY - 2016/12/1
Y1 - 2016/12/1
N2 - 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 per cent in mass) to every surface brightness profile, with the exception of the MilkyWay. 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 on to 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.
AB - 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 per cent in mass) to every surface brightness profile, with the exception of the MilkyWay. 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 on to 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.
KW - Blue stragglers
KW - Galaxies: nuclei
KW - Galaxy: centre
KW - Stars: black holes
KW - Stars: kinematics and dynamics
UR - http://www.scopus.com/inward/record.url?scp=85021080114&partnerID=8YFLogxK
U2 - 10.1093/MNRAS/STW2018
DO - 10.1093/MNRAS/STW2018
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AN - SCOPUS:85021080114
SN - 0035-8711
VL - 463
SP - 1605
EP - 1623
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 2
ER -