CD-62\degree1346: an extreme halo or hypervelocity CH~star?

TL;DR

This paper presents a detailed spectroscopic and kinematic analysis of the star CD-62°1346, revealing it as a high-velocity, evolved giant with peculiar chemical abundances, potentially classifying it as a hypervelocity star and providing insights into the Galactic halo.

Contribution

It is the first detailed study identifying CD-62°1346 as a hypervelocity red giant star, expanding the known types of such stars beyond main-sequence objects.

Findings

CD-62°1346 is a metal-poor, evolved giant with high carbon and s-element abundances.

The star has a highly eccentric, retrograde orbit with a velocity of 570 km/s.

It is potentially unbound from the Galaxy, depending on the Galactic potential model.

Abstract

High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of unbound and bound Population II stars permits us to better estimate the mass of the halo.} {We carried out a detailed spectroscopic and kinematic study and have significantly refined the distance and the evolutionary state of the star. Its 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 {\sc moog}. We found that CD-62$\degr$1346 is a metal-poor ([Fe/H]=$-$1.7) evolved giant star with $T_{\rm eff}=5300$ K and $\log g=1.7$. The star exhibits high carbon and s-element abundances typical of CH stars. It is also a lead star. Our kinematic analysis of its 3D space motions shows that this star has a highly eccentric ($e=0.91$) retrograde orbit with an apogalactic distance of $\sim 100$ kpc, exceeding by a factor of three the distance to the Magellanic Clouds. The star travels with very high velocity relative to the Galactocentric reference frame ($V_{\rm GRF}=570$ km\,s$^{-1}$). CD-62$\degr$1346 is an evolved giant star and not a subgiant star, as was considered earlier. Whether it is bound or unbound to the Galaxy depends on the assumed mass and on the adopted Galactic potential. We also show that the star HD 5223 is another example of a high-velocity CH star that exceeds the Galactic escape velocity. Possible origins of these two high-velocity stars are briefly discussed. CD-62$\degr$1346 and HD 5223 are the first red giant stars to join the restricted group of hypervelocity stars.