Investigating three Sirius-like systems with SPHERE
Authors: Gratton R. , D’Orazi V. , Pacheco T. A. , Zurlo A. , Desidera S. , Meléndez J. , Mesa D. , Claudi R. , Janson M. , Langlois M. , Rickman E. , Samland M. , Moulin T. , Soenke C. , Cascone E. , Ramos J. , Rigal F. , Avenhaus H. , Beuzit J. L. , Biller B. , Boccaletti A. , Bonavita M. , Bonnefoy M. , Brandner W. , Chauvin G. , Cudel M. , Daemgen S. , Delorme P. , Desgrange C. , Engler N. , Feldt M. , Fontanive C. , Galicher R. , Garufi A. , Gasparri D. , Ginski C. , Girard J. , Hagelberg J. , Hunziker S. , Kasper M. , Keppler M. , Lagrange A.-M. , Lannier J. , Lazzoni C. , Le Coroller H. , Ligi R. , Lombart M. , Maire A.-L. , Mayer M. R. , Mazevet S. , Ménard F. , Mouillet D. , Perrot C. , Peretti S. , Petrus S. , Potier A. , Rouan D. , Schmid H. M. , Schmidt T. O. B. , Sissa E. , Stolker T. , Salter G. , Vigan A. , Wildi F. 
Published: December 2020

Context. Sirius-like systems are relatively wide binaries with a separation from a few to hundreds of au; they are composed of a white dwarf (WD) and a companion of a spectral type earlier than M0. Here we consider main sequence (MS) companions, where the WD progenitor evolves in isolation, but its wind during the former asymptotic giant branch (AGB) phase pollutes the companion surface and transfers some angular momentum. They are rich laboratories to constrain stellar models and binary evolution.

Aims. Within the SpHere INfrared survey for Exoplanet survey that uses the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large Telescope, our goal is to acquire high contrast multi-epoch observations of three Sirius-like systems, HD 2133, HD 114174, and CD-56 7708 and to combine this data with archive high resolution spectra of the primaries, TESS archive, and literature data.

Methods. These WDs are easy targets for SPHERE and were used as spectrophotometric standards. We performed very accurate abundance analyses for the MS stars using methods considered for solar analogs. Whenever possible, WD parameters and orbits were obtained using Monte Carlo Markov chain methods.

Results. We found brighter J and K magnitudes for HD 114174B than obtained previously and extended the photometry down to 0.95 μm. Our new data indicate a higher temperature and then shorter cooling age (5.57 ± 0.02 Gyr) and larger mass (0.75 ± 0.03 M⊙) for this WD than previously assumed. Together with the oldest age for the MS star connected to the use of the Gaia DR2 distance, this solved the discrepancy previously found with the age of the MS star. The two other WDs are less massive, indicating progenitors of ∼1.3 M⊙ and 1.5 − 1.8 M⊙ for HD 2133B and CD-56 7708B, respectively. In spite of the rather long periods, we were able to derive useful constraints on the orbit for HD 114174 and CD-56 7708. They are both seen close to edge-on, which is in agreement with the inclination of the MS stars that are obtained coupling the rotational periods, stellar radii, and the projected rotational velocity from spectroscopy. The composition of the MS stars agrees fairly well with expectations from pollution by the AGB progenitors of the WDs: HD 2133A has a small enrichment of n-capture elements, which is as expected for pollution by an AGB star with an initial mass < 1.5 M⊙; CD-56 7708A is a previously unrecognized mild Ba-star, which is also expected due to pollution by an AGB star with an initial mass in the range of 1.5 − 3.0 M⊙; and HD 114174 has a very moderate excess of n-capture elements, which is in agreement with the expectation for a massive AGB star to have an initial mass > 3.0 M⊙.

Conclusions. On the other hand, none of these stars show the excesses of C that are expected to go along with those of n-capture elements. This might be related to the fact that these stars are at the edges of the mass range where we expect nucleosynthesis related to thermal pulses. More work, both theoretical and observational, is required to better understand this issue.