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

Abstract

We identify and roughly characterize 66 candidate binary star systems in the Pleiades, Praesepe, and NGC 2264 star clusters, based on robotic adaptive optics imaging data obtained using Robo-AO at the Palomar 60″ telescope. Only ∼10% of our imaged pairs were previously known. We detect companions at red optical wavelengths, with physical separations ranging from a few tens to a few thousands of au. A three-sigma contrast curve generated for each final image provides upper limits to the brightness ratios for any undetected putative companions. The observations are sensitive to companions with a maximum contrast of ∼6 m at larger separations. At smaller separations, the mean (best) raw contrast at 2″ is 3 8 (6 m ), at 1″ is 3 0 (4 5), and at 0 5 is 1 9 (3 m ). Pointspread function subtraction can recover nearly the full contrast in the closer separations. For detected candidate binary pairs, we report separations, position angles, and relative magnitudes. Theoretical isochrones appropriate to the Pleiades and Praesepe clusters are then used to determine the corresponding binary mass ratios, which range from 0.2 to 0.9 in = q m m 2 1 . For our sample of roughly solar-mass (FGK type) stars in NGC 2264 and sub-solarmass (K and early M-type) primaries in the Pleiades and Praesepe, the overall binary frequency is measured at ∼15.5%±2%. However, this value should be considered a lower limit to the true binary fraction within the specified separation and mass ratio ranges in these clusters, given that complex and uncertain corrections for sensitivity and completeness have not been applied.

Wide binaries are rare in open clusters

“…To estimate our completeness we use the work of Hillenbrand et al (2018) as a comparison survey. This work searched for companions to Pleiades and Praesepe members using adaptive optics.…”

“…While most of their companions were at projected separations of less than an arcsecond and undetected by us, we recovered two of the four companions to stars that were in both our member selections and Hillenbrand et al ( 2018)'s sample which had separations of 1.0-1.5 arcseconds, two of the three with 1.5-2.0 arcsecond separations and all three of the companions wider than that. Some of the Hillenbrand et al (2018) targets did not appear in our sample as they fell outside our sky search area for each cluster, had no astrometric solution in Gaia or were not selected as members by our algorithm. For companions from Hillenbrand et al (2018) that are wider than two arcseconds we do not recover four Pleiades systems.…”

“…Some of the Hillenbrand et al (2018) targets did not appear in our sample as they fell outside our sky search area for each cluster, had no astrometric solution in Gaia or were not selected as members by our algorithm. For companions from Hillenbrand et al (2018) that are wider than two arcseconds we do not recover four Pleiades systems. Both components in all of these systems are excluded as Pleiades members due to their parallaxes with three (s5035799, HII 659 and s5197248) having proper motions that lie outside our cluster distribution.…”

“…We found that three of the objects (s5035799, HII 659 and s5197248) had proper motions that lay outside the proper motion distribution of Pleiades members found by Deacon & Hambly (2004) while obviously DH 800 was selected as a Pleiades member. This suggests that the absence of three wide Pleiades binaries from Hillenbrand et al (2018) is likely due to those binaries being field interlopers or objects in the kinematic outskirts of the cluster which we exclude from our Pleiades member sample rather than any bias we have against selecting wide binary components as cluster members. The additional proper motion information of Gaia only serves to further refine the input member list that was available to Hillenbrand et al (2018), it does not call the existence of any of their binaries into question.…”

“…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.…”