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Not far away from the Sun, at a distance of around 170pc, four
gravitationally bound subdwarfs were discovered.
The common identifier of this star system is G89-14. Initially, there were
known just two close spectroscopic components in this system, A and B,
orbiting around each other with a period of about 190 days (Carney et al.,
1994). Subsequently, at a separation of 34 arc seconds from this
spectroscopic pair, Allen et al. (2000) found its companion D with common
proper motion (Fig.1, right, and Fig.2). Lately, using speckle interferometry
on the 6-m BTA telescope, the staff of the Group of High Angular Resolution
Methods in Astronomy discovered another companion of the spectroscopic
binary, designated C (Fig.1, left). Thus, it was shown at first that
G89-14 is a system of four components. The detected C component is 50 times
fainter than the spectoscopic pair and it is located at the angular distance
of less than one arc second from it, which made this component invisible for
other researchers.
The uniqueness of this star is that it is the most metal-poor quadruple
system known to date. The atmospheric metal abundance in these four
subdwarfs ([Fe/H] = -1.9) is around 100 times lower than that of the Sun.
This indicates that the system was formed in the environment with low metal
content at the moment when our Galaxy was born, i.e., more than 10 Gyr ago.
We estimated the component masses (MA~0.67, MB~0.24 MC~0.33, MD~0.22
solar masses) and orbital periods of the outer subsystems of G89-14.
The ratio of the orbital periods of the outer subsystems of the quadruple
star, 0.52:3000:650000, is indicative of its high degree of hierarchy and
its internal dynamical stability. The constructed orbit of the system's
motion in the Galaxy as a whole (Fig.3) suggests that G98-14 belongs to
the halo population, i.e. the described quadruple system may greatly
recede from the Galactic plane during its movement around the Galactic centre.
The existence of objects like G89-14 proves the capability of stellar
systems of high multiplicity to survive as a result of dynamical evolution
during the time comparable to the age of our Galaxy.
D.A.Rastegaev, Yu.Yu.Balega, A.F.Maximov, E.V.Malogolovets, V.V.Diachenko
Published in: Astronomy Letters, 2009, vol.35, No. 7, p.466-471
(http://arxiv.org/abs/0906.1818):
Rastegaev D.A. The Most Metal-Poor Quadruple System of Subdwarfs G89-14.
Contact - D.A.Rastegaev
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Fig.1.
On the right: image of G89-14 (POSS archive). On the left: the speckle
interferometric subsystem (image reconstructed from speckle interferograms
by S.L.Komarinskij).
Fig.2.
Location of the G89-14 central part and its outer companion in 1955 (green),
in 1989 (red), and in 1998 (blue). The data are taken from the POSS I and
POSS II archives.
Fig.3.
The constructed centre of mass orbit of the quadruple star G89-14 in our
Galaxy.
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