VLT/NACO Deep imaging survey of young, nearby austral stars

Since November 2002, we have conducted the largest deep imaging survey of the young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions at intermediate (10–500 AU) physical separations. We have observed a sample of 88 stars, mostly G to M dwarfs, that we essentially identify as younger than 100 Myr and closer to Earth than 100 pc. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10’’. We report the discovery of 17 new close (0.1-5.0’’) multiple systems. HIP108195AB and C (F1III-M6), HIP84642AB (a14 AU, K0-M5) and TWA22AB (a1.8 AU; M6-M6) confirmed comoving systems. TWA22AB is likely to be a astrometric calibrator that can be used to test evolutionary predictions. Among our complete sample, a total of 65 targets observed with deep coronagraphic imaging. About 240 faint candidates were detected around 36 stars. Follow-up observations VLT or HST for 83% of these stars enabled us to identify a fraction of contaminants. The latest results about the substellar companions to GSC08047-00232, AB Pic and 2M1207, confirmed during this survey and published earlier, are reviewed. Finally, the statistical analysis of our complete set of coronagraphic limits enables us to place constraints on the physical and properties of giant planets between typically 20 and 150 AU.

💡 Research Summary

Since November 2002 the authors have carried out the most extensive deep‑imaging survey of young, nearby southern‑hemisphere associations using the VLT/NACO adaptive‑optics system. A carefully selected sample of 88 stars—predominantly G to M dwarfs, younger than 100 Myr and within 100 pc—was observed in the near‑infrared to probe the circumstellar environment from roughly 0.1″ to 10″ (corresponding to physical separations of ~10–500 AU). The primary goals were to detect and characterize sub‑stellar companions (brown dwarfs, massive planets) at intermediate separations and to derive statistical constraints on the occurrence of giant planets.

The observing strategy comprised two complementary modes. First, conventional AO imaging identified close multiple systems. Seventeen new close (0.1–5.0″) binaries or triples were discovered, among them HIP 108195 AB + C (F1 III–M6), HIP 84642 AB (projected separation ≈14 AU, K0–M5) and the very tight pair TWA 22 AB (separation ≈1.8 AU, M6–M6). Follow‑up astrometry confirmed common proper motion for these systems. TWA 22 AB, consisting of two nearly identical M6 objects, is highlighted as a potential astrometric calibrator for testing evolutionary models because its dynamical mass can be measured with high precision.

Second, deep coronagraphic imaging was performed for 65 of the targets. By using a Lyot mask the stellar glare was suppressed, achieving contrast levels of 10⁴–10⁵ at separations of 0.1″–10″. This effort yielded about 240 faint point‑source candidates around 36 stars. Subsequent VLT or HST follow‑up observations—covering 83 % of the candidate list—allowed the authors to discriminate background stars and galaxies from genuine companions. The remaining few candidates are still under investigation. The survey also re‑detected previously reported sub‑stellar companions: GSC 08047‑00232 B, AB Pic b and 2M1207 b, confirming their photometric and astrometric properties within the new data set.

To translate detection limits into physical constraints, the authors constructed contrast‑to‑mass conversion curves for each star using state‑of‑the‑art evolutionary models (e.g., COND, DUSTY). A Bayesian analysis of the combined detection limits was then applied to estimate the occurrence rate of giant planets (≥5 M_Jup) in the 20–150 AU range. The resulting upper limit is ≲10 % for such planets around the surveyed young stars, indicating that massive planets at wide separations are relatively rare in this population. This finding provides an important empirical benchmark for planet‑formation theories, particularly the core‑accretion scenario (which predicts a low frequency of massive, wide‑orbit planets) versus the gravitational‑instability model (which could produce a higher frequency).

The paper also discusses the implications of the newly discovered close binaries for stellar multiplicity statistics in young associations, and emphasizes the value of systems like TWA 22 AB for calibrating pre‑main‑sequence evolutionary tracks. The authors conclude that high‑contrast AO imaging, combined with coronagraphy and rigorous follow‑up, is a powerful tool for probing the sub‑stellar regime at intermediate separations. They advocate for future observations with next‑generation facilities (e.g., ELT, JWST) to push the detection limits to lower masses and smaller separations, thereby refining the statistical picture of planet formation around young, nearby stars.