Journal Article

On the binary properties and the spatial and kinematical distribution of young stars

Pavel Kroupa

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 298, issue 1, pages 231-242
Published in print July 1998 | ISSN: 0035-8711
Published online July 1998 | e-ISSN: 1365-2966 | DOI:
On the binary properties and the spatial and kinematical distribution of young stars

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The effects that star cluster concentration and binarity have on observable parameters, which characterize the dynamical state of a population of stars after their birth aggregate dissolves, are investigated. To this end, the correlations between ejection velocity, binary proportion, mean system mass, binary orbital period and mass ratio are quantified for simulated aggregates. These consist of a few hundred low-mass binary and single stars, and have half-mass radii in the range 2.5 to 0.08 pc. The primordial binary-star population is assumed to have a period distribution similar to that observed in Taurus-Auriga for pre-main-sequence binaries. The findings presented here are useful for interpreting correlations between relative locations and proper motions, binary properties and masses of young stellar systems within and surrounding star-forming regions, and of stellar systems escaping from Galactic clusters.

For the low-concentration, binary-rich aggregates, the proportion of binaries decreases monotonically as a function of increasing ejection velocity after aggregate dissolution, as expected. However, this is not the case for initially highly concentrated binary-rich aggregates. The reason for this difference is the interplay between the disruption of binary systems and the initial depth of the potential well from which the stellar systems escape.

After aggregate dissolution, a slowly expanding remnant population remains. It can have a high binary proportion (80 per cent) with a high mean system mass, or a low binary proportion (≲20 per cent) with a low mean system mass, if it was born in a low- or high-concentration aggregate respectively. It follows that adjacent regions on the sky near some star-forming clouds can have young populations with different binary proportions and different mass functions, even if the binary proportion at birth and the initial mass function (IMF) were the same.

Binary systems that are ejected from the aggregate tend to be massive, and their mass ratio tends to be biased towards higher values. The mean system mass is approximately independent of ejection velocity between 2 and 30 km s−1. Dynamical ejection from binary-rich aggregates adds, within 10 Myr, relatively massive systems to regions as far as 300 pc from active star-forming centres. Long-period systems cannot survive accelerations to high velocities. The present experiments show that a long-period (>104 d) binary system with a large velocity (>30 km s−1) cannot be ejected from an aggregate. If such young systems exist, then they will have been born in high-velocity clouds.

Keywords: binaries: general; stars: formation; stars: kinematics; stars: low-mass, brown dwarfs; stars: pre-main-sequence; stars: statistics.

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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