Precision Orbit of $\delta$ Delphini and Prospects for Astrometric Detection of Exoplanets

TL;DR

This study combines multiple observational techniques to precisely determine the orbit, masses, and distance of the binary star system δ Delphini, revealing potential for future exoplanet detection around binary stars using astrometry.

Contribution

The paper provides the first full 3D orbital analysis of δ Delphini, including new spectroscopic and interferometric data, and assesses its potential for exoplanet detection with high-precision astrometry.

Findings

Determined stellar masses and distance with high precision.

Identified a puzzling age discrepancy between the binary components.

Estimated exoplanet detection limits around binary stars using differential astrometry.

Abstract

Combining visual and spectroscopic orbits of binary stars leads to a determination of the full 3D orbit, individual masses, and distance to the system. We present a full analysis of the evolved binary system $\delta$ Delphini using astrometric data from the MIRC and PAVO instruments on the CHARA long-baseline interferometer, 97 new spectra from the Fairborn Observatory, and 87 unpublished spectra from Lick Observatory. We determine the full set of orbital elements for $\delta$ Del, along with masses of $1.78 \pm 0.07$ $M_{\odot}$ and $1.62 \pm 0.07$ $M_{\odot}$ for each component, and a distance of $63.61 \pm 0.89$ pc. These results are important in two contexts: for testing stellar evolution models and defining the detection capabilities for future planet searches. We find that the evolutionary state of this system is puzzling, as our measured flux ratios, radii, and masses imply a $\sim$ 200 Myr age difference between the components using standard stellar evolution models. Possible explanations for this age discrepancy include mass transfer scenarios with a now ejected tertiary companion. For individual measurements taken over a span of 2 years we achieve $<10$ $\mu$-arcsecond precision on differential position with 10-minute observations. The high precision of our astrometric orbit suggests that exoplanet detection capabilities are within reach of MIRC at CHARA. We compute exoplanet detection limits around $\delta$ Del, and conclude that if this precision is extended to wider systems we should be able to detect most exoplanets $>2$ M$_{J}$ on orbits $>0.75$ AU around individual components of hot binary stars via differential astrometry.