Mass segregation in young stellar clusters

2011

A&A

Context. Field stars are not always single stars, but can often be found in bound double systems. Since binary frequencies in the birth places of stars, young embedded clusters, are sometimes even higher than on average the question arises of how binary stars form in young dense star clusters and how their properties evolve to those observed in the field population. Aims. We assess, the influence of stellar dynamical interactions on the primordial binary population in young dense cluster environments. Methods. We perform numerical N-body simulations of the Orion nebula cluster like star cluster models including primordial binary populations using the simulation code nbody6 ++ . Results. We find two remarkable results that have yet not been reported: The first is that the evolution of the binary frequency in young dense star clusters is independent predictably of its initial value. The time evolution of the normalized number of binary systems has a fundamental shape. The second main result is that the mass of the primary star is of vital importance to the evolution of the binary. The more massive a primary star, the lower the probability that the binary is destroyed by gravitational interactions. This results in a higher binary frequency for stars more massive than 2 M compared to the binary frequency of lower mass stars. The observed increase in the binary frequency with primary mass is therefore most likely not due to differences in the formation process but can be entirely explained as a dynamical effect. Conclusions. Our results allow us to draw conclusions about the past and the future number of binary systems in young dense star clusters and demonstrate that the present field stellar population has been influenced significantly by its natal environments.

Section: Methods and Cluster Setupmentioning

confidence: 52%

Evolution of the binary population in young dense star clusters

Kaczmarek

Olczak

2011

A&A

“…Bonnell & Davies (1998), Olczak et al (2011), Huff & Stahler (2006). It is currently unclear whether clusters are primordially mass-segregated or not.…”

Section: Discussionmentioning

confidence: 99%

Strong effect of the cluster environment on the size of protoplanetary discs?

Vincke

Breslau

2015

A&A

Context. Most stars are born in clusters, thus the protoplanetary discs surrounding the newly formed stars might be influenced by this environment. Isolated star-disc encounters have previously been studied, and it was shown that very close encounters are necessary to completely destroy discs. However, relatively distant encounters are still able to change the disc size considerably. Aims. We quantify the importance of disc-size reduction that is due to stellar encounters in an entire stellar population. Methods. We modelled young, massive clusters of different densities using the code Nbody6 to determine the statistics of stellar encounter parameters. In a second step, we used these parameters to investigate the effect of the environments on the disc size. For this purpose, we performed a numerical experiment with an artificial initial disc size of 10 5 AU. Results. We quantify to which degree the disc size is more sensitive to the cluster environment than to the disc mass or frequency. We show that in all investigated clusters a large portion of discs is significantly reduced in size. After 5 Myr, the fraction of discs smaller than 1000 AU in ONC-like clusters with an average number density of ρ cluster ∼ 60 pc −3 , the fraction of discs smaller than 1000 AU is 65%, while discs smaller than 100 AU make up 15%. These fractions increase to 84% and 39% for discs in denser clusters like IC 348 (ρ cluster ∼ 500 pc −3 ). Even in clusters with a density four times lower than in the ONC (ρ cluster ∼ 15 pc −3 ), about 43% of all discs are reduced to sizes below 1000 AU and roughly 9% to sizes below 100 AU. Conclusions. For any disc in the ONC that initially was larger than 1000 AU, the probability to be truncated to smaller disc sizes as a result of stellar encounters is quite high. Thus, among other effects, encounters are important in shaping discs and potentially forming planetary systems in stellar clusters.

“…This distribution develops within 1 Myr, the estimated age of the ONC, to the stellar distribution observed for the ONC today (Olczak et al 2006). Following the approach by Bonnell & Davies (1998), initially the four most massive stars have been placed in the inner cluster region with r seg = 0.6 · r hm , where r hm is the half-mass radius of the cluster. We model clusters of different density by keeping the size of the cluster constant (r cluster = 2.5 pc) and varying the initial number of stars N stars .…”

Section: Cluster Modelsmentioning

confidence: 91%

Does the mass distribution in discs influence encounter-induced losses in young star clusters?

Steinhausen

2014

A&A

Context. One mechanism for the external destruction of protoplanetary discs in young dense clusters is tidal disruption during the flyby of another cluster member. The degree of mass loss in such an encounter depends, among other parameters, on the distribution of the material within the disc. Previous work showed that this is especially so in encounters that truncate large parts of the outer disc. The expectation is that the number of completely destroyed discs in a cluster also depends on the mass distribution within the discs. Aims. Here we test this hypothesis by determining the influence of encounters on the disc fraction and average disc mass in clusters of various stellar densities for different mass distributions in the discs.Methods. This is done by performing 6 simulations of a variety of cluster environments, where we track the encounter dynamics and determine the mass loss due to these encounters for different disc-mass distributions. Results. We find that although the disc-mass distribution has a significant impact on the disc losses for specific star-disc encounters, the overall disc frequency generally remains unaffected. The reason is that in single encounters the dependence on the mass distribution is strongest if both stars have very different masses. Such encounters are rather infrequent in sparse clusters. In dense clusters these encounters are more common; however, here the disc frequency is largely determined by encounters between low-mass stars such that the overall disc frequency does not change significantly. Conclusions. For tidal disruption the disc destruction in clusters is fairly independent of the actual distribution of the material in the disc. The all determining factor remains the cluster density.