An investigation of binary stars in the Pleiades with high contrast and spatial resolution

Abstract

Context. It is widely recognized that binarity is a very common feature during star formation; however, different regions and clusters display significantly different binary fractions. This appears to be related to the initial density of the star forming region. The Pleiades cluster has been shown to have a binary fraction similar to that of field stars in the solar neighborhood. Aims. We have taken advantage of a passage of the Moon over the Pleiades cluster, to investigate at high angular resolution two small but similar and consistent samples of cluster members and field stars. Methods. We have employed the technique of lunar occultations (LO). Although LO in the Pleiades have been reported several times in the past, we have for the first time employed the superior performance of the VLT, achieving about 1 milliarcsecond (mas) resolution and detecting sources as faint as K ≈ 12 mag. Results. We have recorded 17 LO light curves of stars recognized as cluster members, and 17 of field stars. The average magnitude was K ≈ 8.5 mag and the average limiting resolution ≈0. 002, for both samples. We detect 5 binaries among the cluster members, and 2 among the sample of field stars. Three systems are new detections, while two were previously only suspected to be binaries. The projected separations are in the range 0. 017 to 0. 642 and the brightness ratios in the range 1 to 90. One more star is suspected to have a companion. Conclusions. Although the fraction of binary detections appears higher among the members than among the non-members, the two samples are too small to draw firm conclusions. Our observations show that, in spite of extensive investigations carried out previously in the Pleiades cluster, the binarity census is not yet complete. We have detected significant orbital motion in four binary systems, which stand up as candidates for dynamical mass estimations.

Wide binaries are rare in open clusters

“…Binary stars in clusters can be identified by their overluminosity (Stauffer 1984;Rubenstein & Bailyn 1997;Khalaj & Baumgardt 2013;Sheikhi et al 2016), lunar occultation (Richichi et al 2012), radial velocity techniques (Neill Reid & Mahoney 2000) or by direct imaging (Bouvier et al 1997;Patience et al 2002;Garcia et al 2015;Hillenbrand et al 2018). However these studies are typically limited to relatively close separations with only the direct imaging surveys going out to separations of a few hundred AU.…”

Dynamical masses for the Pleiades binary system HII-2147

“…There are no observations of HII-2147 in the literature that resolve the pair, although several attempts have been made at both optical and near-infrared (NIR) wavelengths (Bouvier et al 1997;Metchev & Hillenbrand 2009;Mason et al 2009), including a lunar occultation observation by Richichi et al (2012).…”

Robo-AO Discovery and Basic Characterization of Wide Multiple Star Systems in the Pleiades, Praesepe, and NGC 2264 Clusters

“…In the Pleiades cluster, 34 binary systems (including 2 pairs that were observed twice and identified as binaries of each other, making 32 unique pairs) were found among 212 targets. HII 1306, a 0.60 separation and ∆m opt =0.58 m system was also identified as a binary by Richichi et al (2012), who reported separation 0.65 and ∆m K =0.33 m . HII 134, a 1.84 separation and ∆m opt =0.27 m system was also identified as a binary by Bouvier et al (1997), who reported separation 1.83 and ∆m K =0.13 m .…”

“…Notably, the majority of their newly resolved systems had been identified previously as photometric binary candidates (Raboud & Mermilliod 1998a). Richichi et al (2012) identified several Pleiades binaries from lunar occulation observations. In Praesepe, Bettis (1975), Jaschek (1976), and Pinfield et al (2003) identified photometric binary candidates in this cluster as well.…”

“…Notably, the majority of their newly resolved systems had been identified previously as photometric binary candidates (Raboud & Mermilliod;1998). Richichi et al (2012) identified several Pleiades binaries from lunar occulation observations.…”