A new white dwarf companion to the $\Delta \mu$ star GJ 3346

Abstract

We present the discovery of a white dwarf companion at 3.6" from GJ3346, a nearby ($\pi \sim$42 mas) K star observed with SPHERE@VLT as part of an open time survey for faint companions to objects with significant proper motion discrepancies ($\Delta \mu$) between Gaia DR1 and Tycho-2. Syrius-like systems like GJ3346AB, which include a main sequence star and a white dwarf, can be difficult to detect because of the intrinsic faintness of the latter. They have, however, been found to be common contaminants for direct imaging searches. White dwarfs have in fact similar brightness to sub-stellar companions in the infrared, while being much brighter in the visible bands like those used by Gaia. Combining our observations with Gaia DR2 and with several additional archival data sets, we were able to fully constrain the physical properties of GJ3346B, such as its effective temperature (11$\times$10${}^3\pm$500 K) as well as the cooling age of the system (648$\pm$58 Myrs). This allowed us to better understand the system history and to partially explains the discrepancies previously noted in the age indicators for this objects. Although further investigation is still needed, it seems that GJ3346, which was previously classified as young, is in fact most likely to be older than 4 Gyrs. Finally, given that the mass (0.58$\pm$0.01$M_{\odot }$)} and separation (85 au) of GJ3346B are compatible with the observed $\Delta \mu$, this discovery represents a further confirmation of the potential of this kind of dynamical signatures as selection methods for direct imaging surveys targeting faint, sub-stellar companions.

10 pages, 7 figures

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