A detailed study of the barium central star of the planetary nebula Abell 70

TL;DR

This study investigates the barium star at the center of planetary nebula Abell 70, revealing its rapid rotation, chemical enrichment, and potential binary orbital characteristics through long-term photometry and high-resolution spectroscopy.

Contribution

It provides detailed physical and chemical characterization of the barium star and insights into its binary nature, which were previously unconfirmed.

Findings

The star is a chromospherically-active, late G-type sub-giant with +1 dex barium enhancement.

Photometric variability shows no change in rotation period over ten years.

Radial velocity data suggest a long-period, possibly eccentric binary system.

Abstract

We present a detailed study of the barium star at the heart of the planetary nebula Abell 70. Time-series photometry obtained over a period of more than ten years demonstrates that the barium-contaminated companion is a rapid rotator with temporal variability due to spots. The amplitude and phasing of the photometric variability changes abruptly, however there is no evidence for a change in the rotation period (P = 2.06~d) over the course of the observations. The co-addition of 17 high-resolution spectra obtained with VLT-UVES allow us to measure the physical and chemical properties of the companion, confirming it to be a chromospherically-active, late G-type sub-giant with more than +1~dex of barium enhancement. We find no evidence of radial velocity variability in the spectra, obtained over the course of approximately 130~d with a single additional point some 8 years later, with the radial velocities of all epochs approximately $-$10 \kms{} from the previously measured systemic velocity of the nebula. This is perhaps indicative that the binary has a relatively long period (P $\gtrsim$ 2~yr) and high eccentricity ($e\gtrsim$ 0.3), and that all the observations were taken around radial velocity minimum. However, unless the binary orbital plane is not aligned with the waist of the nebula or the systemic velocity of the binary is not equal to the literature value for the nebula, this would imply an unfeasibly large mass for the nebular progenitor.