Multiplicity and clustering in Taurus star-forming region

Abstract

Aims. This work analyses the spatial distribution of stars in Taurus with a specific focus on multiple stars and wide pairs in order to derive new constraints on star formation and early dynamical evolution scenarios. Methods. We collected the multiplicity data of stars in Taurus to build an up-to-date stellar/multiplicity catalog. We first present a general study of nearest-neighbor statistics on spatial random distribution, comparing its analytical distribution and moments to those obtained from Monte Carlo samplings. We introduce the one-point correlation Ψ function to complement the pair correlation function and define the spatial regimes departing from randomness in Taurus. We then perform a set of statistical studies to characterize the binary regime that prevails in Taurus. Results. The Ψ function in Taurus has a scale-free trend with a similar exponent as the correlation function at small scale. It extends almost 3 decades up to ~60 kAU showing a potential extended wide binary regime. This was hidden in the correlation function due to the clustering pattern blending. Distinguishing two stellar populations, single stars versus multiple systems (separation ≤1 kAU), within Class II/III stars observed at high angular resolution, we highlight a major spatial neighborhood difference between the two populations using nearest-neighbor statistics. The multiple systems are three times more likely to have a distant companion within 10 kAU when compared to single stars. We show that this is due to the presence of most probable physical ultra-wide pairs (UWPs, defined as such from their mutual nearest neighbor property), that are themselves generally composed of multiple systems containing up to five stars altogether. More generally, our work highlights; 1) a new large population of candidate UWPs in Taurus within the range 1–60 kAU in Taurus and 2) the major local structural role they play up to 60 kAU. There are three different types of UWPs; either composed of two tight and comparatively massive stars (MM), by one single and one multiple (SM), or by two distant low-mass singles (SS) stars. These UWPs are biased towards high multiplicity and higher-stellar-mass components at shorter separations. The multiplicity fraction per ultra-wide pair with separation less than 10 kAU may be as high as 83.5 ± 19.6%. Conclusions. We suggest that these young pre-main sequence UWPs may be pristine imprints of their spatial configuration at birth resulting from a cascade fragmentation scenario of the natal molecular core. They could be the older counterparts, at least for those separated by less than 10 kAU, to the ≤0.5 Myr prestellar cores/Class 0 multiple objects observed at radio/millimeter wavelengths.