ESO Observations of young binary stars and multiple systems
Creators: name: Zinnecker, Hans
Publisher: name: Zenodo
Year: 2019

description: I will review surveys of the binary and multiple nature of young low-mass and high-mass stars, carried out over the last decades at  the ESO La Silla and Paranal observatories in Chile.<br>
Historically, the first milestone was to resolve the center of R136a in 30 Dor in the LMC into a number of OB stars, using optical speckle interferometry at the Danish 1.5m telescope at LaSilla (Weigelt &amp; Baier 1985), killing the earlier idea of a supermassive star. Similarly, the central star HD 97950AB of the Galactic giant HII region NGC 3603 was resolved into 4 components by the same technique (Baier et al. 1985).<br>
About 20 years later, it became possible to resolve the ~11 year dynamical orbit of the theta-1 Ori C binary system, the exciting  massive star of the Orion Nebula, using bispectrum speckle observations at the VLTI with its 3-telescope beam-combiner AMBER  (Kraus et al. 2009, Grellmann et al. 2013).<br>
The dynamical system mass (44 Mo) and an astrometric distance (410pc) as well as an orbital eccentricity (0.6) could be derived. The most recent spectacular achievement is the full census of multiplicity of the massive stars in the Trapezium cluster in Orion (Karl et al. 2018), using the long baseline interferometric beam combiner (Gravity) of all four VLT 8m telescopes. An amazing number of multiple systems was found, corroborating and improving on previous studies (Preibisch et al. 1999).<br>
As for young low-mass stars, infrared companions to optical T Tauri stars were revealed by a 1D slit scanning technique in the JHKL near-infrared filters at the ESO 3.6m (Chelli et al. 1988). Another milestone was the direct imaging 0.95 micron CCD survey of ~200 southern pre-Main Sequence stars for visual companions, using the excellent 1" imaging quality of the NTT (Reipurth and Zinnecker  1993).<br>
Many companions were found, implying that binary star formation among solar-type stars is the rule, not the exception. On the other hand, the fraction of the visual binaries among 275 low-mass stars in the Orion Trapezium Cluster that were surveyed in the near-infrared with the ADONIS AO system and speckle holography at the ESO 3.6m (Koehler et al. 2006, Petr et al. 1998) is surprisingly low, both in the periphery and in the core, perhaps due to disruption by dynamical interactions.<br>
In the last 10 years, important spectroscopic multiplicity surveys of southern OB stars in young clusters and associations were  made, based on radial velocity measurements using FEROS at the 1.5m and 2.2m La Silla telescopes as well as FLAMES at the VLT (Sana et al. 2012, 2013; see Sana et al. 2017 for a review).<br>
A high frequency of close binaries was detected, some of which close enough to merge into a single object. Wider companions were also found by complementary VLT high-angular resolution observations (Sana et al. 2014). One of the future goals must be to derive spectroscopic orbits of massive close binaries and to turn the SB1s into SB2s to obtain binary mass ratios (Barba et al. 2017).<br>
On the lower-mass side, a goal worth pursuing would be the detection of circumbinary planets, using transit techniques (Martin and Triaud 2015).