GK Car and GZ Nor: two low-luminous, depleted RV Tauri stars

Abstract

ABSTRACT We performed a photometric and spectroscopic analysis of two RV Tauri stars: GK Car and GZ Nor. Both objects are surrounded by hot circumstellar dust. Their pulsation periods, derived from ASAS (All Sky Automated Survey) photometric time-series, have been used to derive their luminosities and distances via the period–luminosity–distance (PLC) relation. In addition, for both objects, Gaia distances are available. The Gaia distances and luminosities are consistent with the values obtained from the PLC relationship. GK Car is at distance of 4.5 ± 1.3 kpc and has a luminosity of 1520 ± 840 L⊙, while GZ Nor is at distance of 8.4 ± 2.3 kpc and has a luminosity of 1240 ± 690 L⊙. Our abundance analysis reveals that both stars show depletion of refractory elements with [Fe/H] = −1.3 and [Zn/Ti] = +1.2 for GK Car and [Fe/H] = −2.0 and [Zn/Ti] = +0.8 for GZ Nor. In the WISE(Wide-Field Infrared Survey Explorer) colour–colour diagram, GK Car is located in the RV Tauri box as originally defined by Lloyd Evans and updated by Gezer et al., while GZ Nor is not. Despite this, we argue that both objects are surrounded by a gravitationally bound disc. As depletion is observed in binaries, we postulate that both stars are binaries as well. RV Tauri stars are generally acknowledged to be post-asymptotic giant branch (post-AGB) stars. Recent studies show that they might be either indeed post-AGB or post-red giant branch (post-RGB) objects depending on their luminosity. For both objects, the derived luminosities are relatively low for post-AGB objects, however, the uncertainties are quite large. We conclude that they could be either post-RGB or post-AGB objects.