[Paper Review] New observational frontiers in the multiplicity of young stars

Gaspard Duchêne, Eduardo Delgado-Donate+3 more|arXiv (Cornell University)|Feb 28, 2006

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper reviews observational advances in the multiplicity of young low-mass stars, focusing on T Tauri stars and deeply embedded protostars, and presents numerical simulations of core fragmentation. It finds that young stellar systems exhibit high multiplicity rates independent of environment, challenging current simulations that predict stronger environmental dependence and frequent disruption of low-binding-energy systems.

ABSTRACT

It has now been known for over a decade that low-mass stars located in star-forming regions are very frequently members of binary and multiple systems, even more so than main sequence stars in the solar neighborhood. This high multiplicity rate has been interpreted as the consequence of the fragmentation of small molecular cores into a few seed objects that accrete to their final mass from the remaining material and dynamically evolve into stable multiple systems, possibly producing a few ejecta in the process. Analyzing the statistical properties of young multiple systems in a variety of environments therefore represents a powerful approach to place stringent constraints on star formation theories. In this contribution, we first review a number of recent results related to the multiplicity of T Tauri stars. We then present a series of studies focusing on the multiplicity and properties of optically-undetected, heavily embedded protostars. These objects are much younger than the previously studied pre-main sequence stars, and they therefore offer a closer look at the primordial population of multiple systems. In addition to these observational avenues, we present new results of a series of numerical simulations that attempt to reproduce the fragmentation of small molecular cores into multiple systems, and compare these results to the observations.

Motivation & Objective

• To update the understanding of multiplicity rates in young low-mass stars, particularly in T Tauri populations and nearby stellar associations.
• To investigate the multiplicity of optically undetected, deeply embedded protostars to probe the primordial phase of multiple system formation.
• To compare observational multiplicity statistics with numerical simulations of prestellar core fragmentation and dynamical evolution.
• To identify discrepancies between simulations and observations—such as uniform multiplicity across environments and survival of low-binding-energy systems—highlighting open questions in star formation theory.

Proposed method

• Conducting statistical surveys of visual and spectroscopic binaries in young stellar populations, including T Tauri stars in Taurus-Auriga, Ophiuchus, and Orion, and nearby co-moving associations.
• Utilizing high-angular-resolution infrared and radio interferometric observations to detect and characterize tightly bound, deeply embedded protostars.
• Performing numerical simulations of prestellar core collapse and fragmentation using smoothed particle hydrodynamics (SPH) and grid-based codes with sink particles.
• Comparing simulated multiplicity properties (e.g., frequency, binding energy, hierarchical structure) with observed distributions in young systems.
• Incorporating physical effects such as feedback from outflows and photoionization in newer simulations to improve realism.
• Analyzing the evolutionary sequence of Class 0–I–II–III protostars to assess whether multiplicity is disrupted during early dynamical evolution.

Experimental results

Research questions

• RQ1 Why do embedded protostars show a uniform multiplicity rate across different environments, contrary to predictions from numerical simulations?
• RQ2 What causes the observed survival of low-binding-energy multiple systems, which are typically disrupted in numerical simulations of core collapse?
• RQ3 Why are aggregates of more than 4–5 stars on scales of a few thousand AU absent in observations, despite being common in simulations?
• RQ4 To what extent do dynamical interactions and feedback processes in star-forming regions alter the initial multiplicity function of young stellar systems?
• RQ5 How do the multiplicity properties of young stars in low-density clouds compare to those in dense clusters, and what does this imply for the initial conditions of star formation?

Key findings

• The multiplicity rate of young low-mass stars is high and consistent across environments, with embedded protostars showing no significant environmental dependence.
• A significant fraction of young multiple systems are found to be low-binding-energy systems, which are rarely stable in current numerical simulations.
• Observations reveal a lack of systems with more than 4–5 stars on scales of a few thousand AU, contradicting predictions from simulations of core fragmentation.
• The existence of mixed-evolutionary-class systems, such as infrared companions in T Tauri binaries, suggests rapid dynamical evolution and possible phase transitions during early stellar evolution.
• Radio interferometry and high-resolution infrared imaging are critical for detecting the youngest, most embedded protostars and resolving tight multiple systems.
• Numerical simulations of core collapse and fragmentation predict strong environmental dependence in multiplicity, but observed data do not support this trend, indicating a gap in theoretical modeling.