First light of the VLT planet finder SPHERE II. The physical properties and the architecture of the young systems PZ Telescopii and HD 1160 revisited

Abstract

Context. The young systemsPZ Tel and HD 1160, hosting known low-mass companions, were observed during the commissioning of the new planet finder of the Very Large Telescope (VLT) SPHERE with several imaging and spectroscopic modes. Aims. We aim to refine the physical properties and architecture of both systems. Methods. We use SPHERE commissioning data and dedicated Rapid Eye Mount (REM) observations, as well as literature and unpublished data from VLT/SINFONI, VLT/NaCo, Gemini/NICI, and Keck/NIRC2. Results. We derive new photometry and confirm the short-term (P = 0.94 d) photometric variability of the star PZ Tel A with values of 0.14 and 0.06 mag at optical and near-infrared wavelengths, respectively. We note from the comparison to literature data spanning 38 yr that the star also exhibits a long-term variability trend with a brightening of ~0.25 mag. The 0.63−3.8 μm spectral energy distribution of PZ Tel B (separation ~25 AU) allows us to revise its physical characteristics: spectral type M7 ± 1, T_(eff) = 2700 ± 100 K, log(g) < 4.5 dex, luminosity log(L/L_⊙) = −2.51 ± 0.10 dex, and mass 38−72 M_J from "hot-start" evolutionary models combining the ranges of the temperature and luminosity estimates. The 1−3.8 μm SED of HD 1160 B (~85 au) suggests a massive brown dwarf or a low-mass star with spectral type M6.0^(+1.0)_(-0.5), T_(eff) = 3000 ± 100 K, subsolar metallicity [M/H] = −0.5−0.0 dex, luminosity log(L/L_⊙) = −2.81 ± 0.10 dex, and mass 39−166 M_J. The physical properties derived for HD 1160 C (~560 au) from K_sL′-band photometry are consistent with the discovery study. The orbital study of PZ Tel B confirms its deceleration and the high eccentricity of its orbit (e > 0.66). For eccentricities below 0.9, the inclination, longitude of the ascending node, and time of periastron passage are well constrained. In particular, both star and companion inclinations are compatible with a system seen edge-on. Based on "hot-start" evolutionary models, we reject other brown dwarf candidates outside 0.25" for both systems, and giant planet companions outside 0.5" that are more massive than 3 MJ for the PZ Tel system. We also show that K1−K2 color can be used along with YJH low-resolution spectra to identify young L-type companions, provided high photometric accuracy (≤0.05 mag) is achieved. Conclusions. SPHERE opens new horizons in the study of young brown dwarfs and giant exoplanets using direct imaging thanks to high-contrast imaging capabilities at optical (0.5−0.9 μm) and near-infrared (0.95−2.3 μm) wavelengths, as well as high signal-to-noise spectroscopy in the near-infrared domain (0.95−2.3 μm) from low resolutions (R ~ 30−50) to medium resolutions (R ~ 350).

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