Metal-poor Red Horizontal Branch Stars Research Articles

A study of two high-velocity red horizontal branch stars

Context. High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of Population II unbound and bound stars enables us better estimate the mass of the halo. Aims. We carried out a detailed spectroscopic and kinematic study of two red horizontal branch stars, CD-41 15048 and HD 214362. Methods. The atmospheric parameters, chemical abundances, and kinematical properties were determined using high-resolution optical spectroscopy and employing the local thermodynamic equilibrium model atmospheres of Kurucz and the spectral analysis code moog. Results. We found that CD-41 15048 and HD 214362 are metal-poor red horizontal branch stars. Their abundance patterns are similar to other metal-poor halo stars already investigated. Our kinematic analysis of the 3D space motions shows that HD 214362 has a highly eccentric (e = 0:95) orbit and passes at 2 kpc from the Galactic center. CD-41 15048, for an adopted distance of 1.3 kpc, has an extreme retrograde motion and travels with very high velocity relative to the Galactocentric reference frame (VGRF = 583 km s 1 ). Conclusions. CD-41 15048 is a bound or an unbound star, depending on the adopted Galactic potential. We also show that the red horizontal branch star BD+09 3223 is another example of a hypervelocity star. Whether it is bound or unbound to the Galaxy depends on the assumed mass and adopted Galactic potential. Possible origins of these two high-velocity stars are briefly discussed. CD-41 15048 and BD+09 3223 are further examples of evolved stars to join the restricted group of hypervelocity stars. Finally, our results seem to contradict the idea that a passage of a star very close to the Galactic center is the only possible origin of hypervelocity stars.

The visitor from an ancient galaxy: A planetary companion around an old, metal-poor red horizontal branch star

AbstractWe report the detection of a planetary companion around HIP 13044, a metal-poor red horizontal branch star belonging to a stellar halo stream that results from the disruption of an ancient Milky Way satellite galaxy. The detection is based on radial velocity observations with FEROS at the 2.2-m MPG/ESO telescope. The periodic radial velocity variation ofP= 16.2 days can be distinguished from the periods of the stellar activity indicators. We computed a minimum planetary mass of 1.25 Mjupand an orbital semimajor axis of 0.116 AU for the planet. This discovery is unique in three aspects: First, it is the first planet detection around a star with a metallicity much lower than few percent of the solar value; second, the planet host star resides in a stellar evolutionary stage that is still unexplored in the exoplanet surveys; third, the planetary system HIP 13044 most likely has an extragalactic origin in a disrupted former satellite of the Milky Way.

The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope (HST) single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters that exhibit red horizontal branches and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post–main-sequence phases, but we lack the detection of main-sequence turnoffs that would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ≈50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, only 3% of solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.