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The Most Metal-Poor Quadruple System of Subdwarfs G89-14

Russian version

    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
Rastegaev D.A. The Most Metal-Poor Quadruple System of Subdwarfs G89-14.

Contact - D.A.Rastegaev

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.