The boring history of Gaia BH3 from isolated binary evolution
arxiv(2024)
摘要
Gaia BH3 is the first observed dormant black hole (BH) with a mass of
≈30 M_⊙ and represents the first confirmation that such massive
BHs are associated with metal-poor stars. Here, we explore the isolated binary
formation channel for Gaia BH3 focusing on the old and metal-poor stellar
population of the Milky Way halo. We use our open-source population synthesis
code SEVN to evolve 3.2 × 10^8 binaries exploring 16 sets of parameters.
We find that systems like Gaia BH3 form preferentially from binaries initially
composed of a massive star (40-60 M_⊙) and a low mass companion (<1
M_⊙) in a wide (P>10^3 days) and eccentric orbit (e>0.6). Such
progenitors do not undergo any Roche-lobe overflow episode during their entire
evolution, so that the final orbital properties of the BH-star system are
essentially determined at the core collapse of the primary star. Low natal
kicks (≈ 10 km/s) significantly favour the formation of Gaia BH3-like
systems, but high velocity kicks up to ≈ 220 km/s are also allowed. We
estimate the formation efficiency for Gaia BH3-like systems in old (t>10 Gyr)
and metal-poor (Z<0.01) populations to be 4 × 10^-8 M_⊙^-1,
representing ≈ 3% of the whole simulated BH-star population. We expect
up to ∼ 3000 BH-star systems in the Galactic halo formed through isolated
evolution, of which ∼ 100 are compatible with Gaia BH3 alike. Gaia
BH3-like systems represent a common product of isolated binary evolution at low
metallicity (Z<0.01), but given the steep density profile of the Galactic
halo we do not expect more than one in the halo at the observed distance of
Gaia BH3. Considering the estimated formation efficiency for both the isolated
and dynamical formation channel, we conclude that they are almost equally
likely to explain the origin of Gaia BH3.
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