HIP 3678: a hierarchical triple stellar system in the centre of the planetary nebula NGC 246

TL;DR

This paper reports the discovery of a new low-mass stellar companion to the white dwarf HIP 3678 A within the planetary nebula NGC 246, confirming a hierarchical triple stellar system through astrometric and imaging data.

Contribution

The study identifies and characterizes a new low-mass stellar companion to HIP 3678 A, expanding knowledge of the system's hierarchical structure and demonstrating the use of high-contrast imaging and astrometry.

Findings

Detected a new companion at 1 arcsec separation

Ruled out additional companions between 130 and 5500 au

Estimated companion masses as 0.85 M_sun and 0.1 M_sun

Abstract

We report the detection of a new low-mass stellar companion to the white dwarf HIP 3678 A, the central star of the planetary nebula NGC 246. The newly found companion is located about 1 arcsec (at projected separation of about 500 au) north-east of HIP 3678 A, and shares a common proper motion with the white dwarf and its known comoving companion HIP 3678 B. The hypothesis that the newly detected companion is a non-moving background object can be rejected on a significance level of more than 8 $\sigma$, by combining astrometric measurements from the literature with follow-up astrometry, obtained with Wild Field Planetary Camera 2/\textit{Hubble Space Telescope} and NACO/Very Large Telescope. From our deep NACO imaging data, we can rule out additional stellar companions of the white dwarf with projected separations between 130 up to 5500 au. In the deepest high-contrast NACO observation, we achieve a detection limit in the \textit{Ks} band of about 20 mag, which allows the detection of brown dwarf companions with masses down to 36$\,\rmn{M_{jup}}$ at an assumed age of the system of 260 Myr. To approximate the masses of the companions HIP 3678 B and C, we use the evolutionary Baraffe et al. models and obtain about 0.85\,M$_{\sun}$ for HIP 3678 B and about 0.1 M$_{\sun}$ for HIP 3678 C. According to the derived absolute photometry, HIP 3678 B should be a early to mid-K dwarf (K2--K5), while HIP 3678 C should be a mid-M dwarf with a spectral type in the range between M5 and M6.