A Triple Origin for Twin Blue Stragglers in Close Binaries. (arXiv:1901.11088v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zwart_S/0/1/0/all/0/1">Simon Portegies Zwart</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leigh_N/0/1/0/all/0/1">Nathan W. C. Leigh</a>
We propose a formation mechanism for twin blue stragglers (BSs) in compact
binaries that involves mass transfer from an evolved outer tertiary companion
on to the inner binary via a circumbinary disk. We apply this scenario to the
observed double BS system Binary 7782 in the old open cluster NGC 188, and show
that its observed properties are naturally reproduced within the context of the
proposed model. We predict the following properties for twin BSs: (1) For the
outer tertiary orbit, the initial orbital period should lie between 220 days
$lesssim$ P$_{rm out}$ $lesssim$ 1100 days, assuming initial masses for the
inner binary components of $m_{rm 1} = 1.1$ M$_{odot}$ and $m_{rm 2} =$ 0.9
M$_{odot}$ and an outer tertiary mass of $m_{rm 3} = 1.4$ M$_{odot}$. After
Roche-lobe overflow, the outer star turns into a white dwarf (WD) of mass 0.43
to 0.54,MSun. There is a correlation between the mass of this WD and the
outer orbital period: more massive WDs will be on wider orbits. (3) The
rotational axes of both BSs will be aligned with each other and the orbital
plane of the outer tertiary WD. (4) The BSs will have roughly equal masses,
independent of their initial masses (since the lower mass star accretes the
most). The dominant accretor should, therefore, be more enriched by the
accreted material. Hence, one of the BSs will appear to be more enriched by
either He, C and O or by s-process elements, if the donor started Roche lobe
overflow on, respectively, the red giant or asymptotic giant branch. (5)
Relative to old clusters, twin BSs in close binaries formed from the proposed
mechanism should be more frequent in the Galactic field and open clusters with
ages $lesssim$ 4-6 Gyr, since then the donor will have a radiative envelope.
(6) The orbit of the binary BS will have a small semi-major axis (typically
$aplt 0.3$,au) and be close to circular ($e aplt 0.2$).
We propose a formation mechanism for twin blue stragglers (BSs) in compact
binaries that involves mass transfer from an evolved outer tertiary companion
on to the inner binary via a circumbinary disk. We apply this scenario to the
observed double BS system Binary 7782 in the old open cluster NGC 188, and show
that its observed properties are naturally reproduced within the context of the
proposed model. We predict the following properties for twin BSs: (1) For the
outer tertiary orbit, the initial orbital period should lie between 220 days
$lesssim$ P$_{rm out}$ $lesssim$ 1100 days, assuming initial masses for the
inner binary components of $m_{rm 1} = 1.1$ M$_{odot}$ and $m_{rm 2} =$ 0.9
M$_{odot}$ and an outer tertiary mass of $m_{rm 3} = 1.4$ M$_{odot}$. After
Roche-lobe overflow, the outer star turns into a white dwarf (WD) of mass 0.43
to 0.54,MSun. There is a correlation between the mass of this WD and the
outer orbital period: more massive WDs will be on wider orbits. (3) The
rotational axes of both BSs will be aligned with each other and the orbital
plane of the outer tertiary WD. (4) The BSs will have roughly equal masses,
independent of their initial masses (since the lower mass star accretes the
most). The dominant accretor should, therefore, be more enriched by the
accreted material. Hence, one of the BSs will appear to be more enriched by
either He, C and O or by s-process elements, if the donor started Roche lobe
overflow on, respectively, the red giant or asymptotic giant branch. (5)
Relative to old clusters, twin BSs in close binaries formed from the proposed
mechanism should be more frequent in the Galactic field and open clusters with
ages $lesssim$ 4-6 Gyr, since then the donor will have a radiative envelope.
(6) The orbit of the binary BS will have a small semi-major axis (typically
$aplt 0.3$,au) and be close to circular ($e aplt 0.2$).
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