K. Fuhrmann

ON THE AGE OF GLIESE 504

Direct imaging observations of the solar-type star Gl 504 have recently uncovered a faint companion that, on the supposition that the host star has an age of Myr, was announced to be a MJ Jovian exoplanet. Here we present the observational evidence that Gl 504 A is an evolved turn-off star of about solar age and by inference its faint companion a low-mass brown dwarf. As with our previous work on Gl 504 A several years ago, we suggest the accretion of a substellar object to account for the otherwise unexplained high rotation of Gl 504 A. We also propose that with the distant Gl 504 B we may now well be facing the driving agent for the former merger.

New visual companions of solar-type stars within 25 pc

We report the discovery of faint common-proper-motion companions to the nearby southern solar-type stars HD43162, HD67199, HD114837, HD114853, HD129502, HD165185, HD197214, and HD212330 from near-infrared imaging and astrometry. We also confirm the previously identified tertiary components around HD165401 and HD188088. Since the majority of these stars were already known as binaries, they ascend now to higher-level systems. A particularly interesting case is the G6.5V BY Dratype variable HD43162, which harbors two common-proper-motion companions at distances of 410AU and 2740AU. Our limited study shows that the inventory of common-proper-motion companions around nearby bright stars is still not completely known.

ON THE AGE OF GLIESE 86

Gliese 86 is a nearby planet hosting a visual binary consisting of a K-type primary, Gl 86 A, and a white dwarf secondary, Gl 86 B. In this work, we present a model atmosphere analysis of the M {sub A} = 0.83 M {sub ☉} primary, whose chemistry unambiguously identifies it as a 10 Gyr old disk star. For the secondary, this gives rise to a progenitor mass M {sub prog} = 1.11 ± 0.05 M {sub ☉}, and—by inference with its local sibling o {sup 2} Eri—a white dwarf mass M {sub B} = 0.49 ± 0.02 M {sub ☉}. The discrepancy with the 2-3 Gyr chromospheric age of Gl 86 A, as first noted by Rocha-Pinto, Castilho and Maciel, is thus most plausibly the result of a former accretion of mass and angular momentum from the distant degenerate. In consequence, this also implies that with respect to shorter-period systems, like Sirius or Procyon, one must expect significant wind accretion when they evolve through the planetary nebula phase, such that the system ages cannot be reliably determined from their currently bright primaries.

Archeology of an Ancient Star

We report on the bright and late F-type star HR 3138, which, with respect to its chemistry in the [Mg/H]-[Fe/Mg] abundance plane, we identify as an old Population II member. Evolutionary tracks are, however, in conflict with this finding and instead imply an age of only {tau} = 5.6{sup -1.8}{sub +{sub 2.2}} Gyr (2{sigma}) for HR 3138. We discuss this controversy in light of existing high-precision radial velocity surveys that mostly exclude the case of a blue straggler primary and a white dwarf secondary. While it is realized that a stellar merger can principally solve the issue and there is indeed observational evidence for mass accretion on HR 3138 from the absence of lithium in its photosphere, we also consider the interesting circumstance that HR 3138 lies in the direction to the 350 pc distant, young open cluster NGC 2516. We point to the possibility that the progenitor cloud of this cluster may likewise account for former mass accretion and we argue in particular for a dynamical friction with this cloud as a plausible cause for the strikingly common Galactic rotational velocity of the field star and open cluster.

BRIGHT TIMES FOR AN ANCIENT STAR

Field stars of Population II are among the oldest sources in the Galaxy. Most of their solar-type dwarfs are non-single and, given their extreme age, a significant fraction is accompanied by stellar remnants. Here we report the discovery of the bright F7V star 49 Lib as a massive and very metal-rich Population II field blue straggler, along with evidence for a white dwarf as its dark and unseen companion. 49 Lib is known as a relatively fast-rotating, single-lined spectroscopic binary in a 3 year orbit and with an apparent age of about τ 2.3 Gyr. Its chemistry and kinematics, however, both consistently imply that 49 Lib must be an ancient Population II star at τ 12 Gyr. With reference to the inclination from the astrometric orbit, leading to a M⊙ low-mass white dwarf, and in view of the M⊙ massive, evolved F-type blue straggler star, we demonstrate that 49 Lib must have been the subject of a mostly conservative mass transfer with a near-equal-mass M 1.06 + 1.00 M⊙ G-type binary at birth. For its future evolution, we point to the possibility as a progenitor system toward a type Ia supernova. Most importantly, however, we note that the remarkable metal enrichment of 49 Lib at [Mg/H] = +0.23 and [Fe/H] = −0.11 has principally very relevant implications for the early epoch when the Milky Way came into being.

A MODEL ATMOSPHERE ANALYSIS OF ALPHA AURIGAE A

This work presents the first quantitative composite model atmosphere analysis of Capella, the brightest near-equal-mass spectroscopic binary and principal star of the constellation Auriga. Its high-resolution spectrum leads to a slightly metal-rich object at [Fe/H] = +0.05 ± 0.08 dex. In line with its young age and its kinematics, this consistently associates Capella with the Hyades moving group. The measured projected rotational velocities, vsin i Aa = 3.5 ± 0.8 km s–1 and vsin i Ab = 35.4 ± 3.2 km s–1, both agree with rotational and orbital coplanarity and synchronous orbital rotation for the Aa component. At an orbital period P = 104 d the primarys bound rotation together with the almost zero orbital eccentricity are both key characteristics of this binary and clearly imply that the Aa component must have passed the tip of the giant branch. Whether in that phase Capella also became a mass transfer system remains inconclusive at present, though the high rotational velocity of the less evolved Hertzsprung gap secondary and the very diverse lithium abundances of both its components render this a plausible case.