Mass segregation

About: Mass segregation is a research topic. Over the lifetime, 1024 publications have been published within this topic receiving 57729 citations.

How fast is mass segregation happening in hierarchically formed embedded star clusters

Abstract: We investigate the evolution of mass segregation in initially sub-structured young embedded star clusters with two different background potentials mimicking the gas. Our clusters are initially in virial or sub-virial global states and have different initial distributions for the most massive stars: randomly placed, initially mass segregated or even inverse segregation. By means of N-body simulation we follow their evolution for 5 Myr. We measure the mass segregation using the minimum spanning tree method Lambda_MSR and an equivalent restricted method. Despite this variety of different initial conditions, we find that our stellar distributions almost always settle very fast into a mass segregated and more spherical configuration, suggesting that once we see a spherical or nearly spherical embedded star cluster, we can be sure it is mass segregated no matter what the real initial conditions were. We, furthermore, report under which circumstances this process can be more rapid or delayed, respectively.

HST Observations of the Central-Cusp Globular Cluster NGC 6752. The Effect of Binary Stars on the Luminosity Function in the Core

Abstract: We consider the effect of binary stars on the main-sequence luminosity functions observed in the core of globular clusters, with specific reference to NGC 6752. We find that mass segregation results in an increased binary fraction at fainter magnitudes along the main-sequence. If this effect is not taken into account when analyzing luminosity functions, erroneous conclusions can be drawn regarding the distribution of single stars, and the dynamical state of the cluster. In the core of NGC 6752, our HST data reveal a flat luminosity function, in agreement with previous results. However, when we correct for the increasing binary fraction at faint magnitudes, the LF begins to fall immediately below the turn-off. This effect appears to be confined to the inner core radius of the cluster.

The Open Cluster NGC 7789: I. Radial Velocities for Giant Stars

Abstract: A total of 597 radial-velocity observations for 112 stars in the ~1.6 Gyr old open cluster NGC 7789 have been obtained since 1979 with the radial velocity spectrometer at the Dominion Astrophysical Observatory. The mean cluster radial velocity is -54.9 +/- 0.12 km/s and the dispersion is 0.86 km/s, from 50 constant-velocity stars selected as members from this radial-velocity study and the proper motion study of McNamara and Solomon (1981). Twenty-five stars (32%) among 78 members are possible radial-velocity variable stars, but no orbits are determined because of the sparse sampling. Seventeen stars are radial-velocity non-members, while membership estimates of six stars are uncertain. There is a hint that the observed velocity dispersion falls off at large radius. This may due to the inclusion of long-period binaries preferentially in the central area of the cluster. The known radial-velocity variables also seem to be more concentrated toward the center than members with constant velocity. Although this is significant at only the 85% level, when combined with similar result of Raboud and Mermilliod (1994) for three other clusters, the data strongly support the conclusion that mass segregation is being detected.

A census of the near-by Pisces-Eridanus stellar stream -- Commonalities with and disparities from the Pleiades

Abstract: Within a 400~pc sphere around the Sun, we search for members of the Pisces-Eridanus (Psc-Eri) stellar stream in the Gaia Data Release 2 (DR2) data set. We compare basic astrophysical characteristics of the stream with those of the Pleiades. We used a modified convergent-point method to identify stars with 2D - velocities consistent with the space velocity of the Psc-Eri stream and the Pleiades, respectively. We found 1387 members of the Psc-Eri stream in a G magnitude range from 5.1 mag to 19.3 mag at distances between 80 and 380 pc from the Sun. The stream has a nearly cylindrical shape with length and thickness of about 700 pc and 100 pc, respectively. The total stellar mass contained in the stream is about 770 M_Sun, and the members are gravitationally unbound. For the stream we found an age of about 135 Myr. In many astrophysical properties Psc-Eri is comparable to the open cluster M45 (the Pleiades): in its age, its luminosity function (LF), its Present-day mass Function (PDMF) as well as in its total mass. Nonetheless, the two stellar ensembles are completely unlike in their physical appearance. We cautiously give two possible explanations for this disagreement: (i) the star-formation efficiency in their parental molecular clouds was higher for the Pleiades than for Psc-Eri or/and (ii) the Pleiades had a higher primordial mass segregation immediately after the expulsion of the molecular gas of the parental cloud.

Spatial mixing of binary stars in multiple-population globular clusters

Abstract: We present the results of a study aimed at investigating the effects of dynamical evolution on the spatial distribution and mixing of primordial binary stars in multiple-population globular clusters. Multiple stellar population formation models predict that second-generation (SG) stars form segregated in the inner regions of a more extended first-generation (FG) cluster. Our study, based on the results of a survey of N-body simulations, shows that the spatial mixing process for binary stars is more complex than that of single stars since additional processes such as binary ionization, recoil and ejection following binary-single and binary-binary interactions play a key role in determining the spatial distribution of the population of surviving binaries. The efficiency and relative importance of these additional effects depends on the binary binding energy and determines the timescale of the spatial mixing of FG and SG binaries. Our simulations illustrate the role of ionization, recoil and ejection combined with the effects of mass segregation driven by two-body relaxation and show that the complex interplay of all these processes results in a significant extension of the time needed for the complete spatial mixing of FG and SG binaries compared to that of single stars. Clusters in which FG and SG single stars have already reached complete spatial mixing might be characterized by a significant radial gradient in the ratio of the FG-to-SG binary fraction. The implications of the delayed mixing of FG and SG binaries for the differences between the kinematics of the two populations are discussed.