Journal
ASTRONOMY & ASTROPHYSICS
Volume 554, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201321179
Keywords
stars: individual: HD 8049; binaries: visual; techniques: high angular resolution; techniques: spectroscopic; brown dwarfs; white dwarfs
Categories
Funding
- PRIN INAF
- U.S. National Science Foundation [1009203]
- French National Research Agency (ANR) through the GUEPARD project [ANR10-BLANC0504-01]
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1009203] Funding Source: National Science Foundation
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Context. As is the case for all techniques involved in the research for exoplanets, direct imaging has to take into account the probability of so-called astrophysical false positives, which are phenomena that mimic the signature of objects we are seeking. Aims. In this work we present the case of a false positive found during a direct-imaging survey conducted with VLT/NACO. A promising exoplanet candidate was detected around the K2-type star HD 8049 in July 2010. Its contrast of Delta H = 7.05 at 1.57 arcsec allowed us to guess the presence of a 35 M-Jup companion at 50 projected AU, for the nominal system age and heliocentric distance. Methods. To check whether it was gravitationally bound to the host star, as opposed to an unrelated background object, we re-observed the system one year later and concluded a high probability of a bound system. We also used radial velocity measurements of the host star, spanning a time range of similar to 30 yr, to constrain the companion's mass and orbital properties, as well as to probe the host star's spectral age indicators and general spectral energy distribution. We also obtained for the companion U-band imaging with EFOSC and near-infrared spectroscopy for the companion. Results. Combining all these information we conclude that the companion of HD 8049 is a white dwarf (WD) with temperature T-eff = 18 800 +/- 2100 K and mass M-WD = 0.56 +/- 0.08 M-circle dot. The significant radial velocity trend coupled with the imaging data indicate that the most probable orbit has a semi-major axis of about 50 AU. The discrepancy between the age indicators suggested against a bona-fide young star. The moderately large level of chromospheric activity and fast rotation, mimicking the properties of a young star, might be induced by the exchange of mass with the progenitor of the WD. This example demonstrates some of the challenges in determining accurate age estimates and identifications of faint companions.
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