Is UU Herculis a post-AGB star?

TL;DR

This study investigates the evolutionary status of UU Herculis by analyzing high-resolution spectra over five years, revealing it is likely an old, post-AGB star with specific chemical signatures indicating limited dredge-up episodes.

Contribution

The paper provides detailed spectroscopic evidence supporting UU Herculis as a post-AGB star, clarifying its chemical composition and radial velocity behavior.

Findings

UUHer has a high radial velocity (~130 km/sec), indicating an old galactic population.

Chemical analysis shows low iron abundance and signs of a first dredge-up.

No evidence of third dredge-up or Ne-Na cycle dredging-up was found.

Abstract

In order to understand the evolutionary status of the anomalous supergiant UUHer, the prototype of the class of variable supergiants located at high galactic latitudes, we obtained several high-resolution spectra of this star, with the 6m telescope, over 5 years. This material was used for a search of possible temporal variations of the radial velocity at the different depths in the photosphere and for studying the chemical composition. The average radial velocity Vr approximately 130 km/sec suggests that UUHer belongs to the old population of the Galaxy. No systematic dependence of the velocity on depth of the line formation layer or on ionization and excitation potential is observed. The radial velocity of the H_alpha absorption differs strongly from the average photospheric velocity. The iron abundance in the photosphere of UUHer is significantly lower than that of the Sun: [Fe/H]=-1.32. The enhancement of nitrogen relatively to iron content [N/Fe]=+0.40 in combination with the carbon underabundance [C/Fe]=-0.30 suggests that only a first dredge-up episode occurred. The Na content is normal relatively to iron, therefore there is no evidence for dredging-up of Ne-Na cycle products. The heavy s-process metals Y, Ba are depleted relative to H and Fe, which again implies that the third dredge-up did not occur.