Mass segregation

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

Capture-formed Binaries via Encounters with Massive Protostars

Abstract: Most massive stars are found in the center of dense clusters and have a companion fraction much higher than their lower mass siblings; the massive stars of the Trapezium core in Orion have ~1.5 companions each. This high multiplicity could be a consequence of formation via a capture scenario, or it could be due to fragmentation of the cores that form the massive stars. During stellar formation circumstellar disks appear to be nearly ubiquitous. Their large radii compared to stellar sizes increase the interaction radius significantly, suggesting that disk interactions with neighboring stars could assist in capturing binary companions. This mechanism has been studied for stars of approximately solar mass and found to be inefficient. In this paper we present simulations of interactions between a 22 M☉ star-disk system and less massive impactors in order to study the disk-assisted capture formation of binaries in a regime suited to massive stars. The formation of binaries by capture is found to be much more efficient for massive capturers. We discuss the effects of a mass-dependent velocity dispersion and mass segregation on the capture rates and consider the long-term survival of the resulting binaries in a dense cluster.

Unveiling the core of the globular cluster m15 in the ultraviolet

Abstract: We have obtained deep FUV and NUV images of the inner region of the dense globular cluster M15 with the HST ACS. The FUV - NUV color-magnitude diagram shows a well-defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main-sequence turnoff is clearly visible at FUV 23.5 mag and FUV - NUV 3 mag, and the main-sequence stars form a prominent track that extends at least 2 mag below the main-sequence turnoff. As such, this is the deepest FUV - NUV color-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colors and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variable candidates, one known and one probable SX Phoenicis star, and the well-known low-mass X-ray binary AC 211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyrae stars, and SX Phoenicis exhibit massive variability amplitudes in this wave band (several magnitudes).

Deep HST-WFPC2 photometry of NGC 288. II. The Main Sequence Luminosity Function

Abstract: The Main Sequence Luminosity Function (LF) of the Galactic globular cluster NGC 288 has been obtained using deep WFPC2 photometry. We have employed a new method to correct for completeness and fully account for bin-to-bin migration due to blending and/or observational scatter. The effect of the presence of binary systems in the final LF is quantified and is found to be negligible. There is a strong indication of the mass segregation of unevolved single stars and clear signs of a depletion of low mass stars in NGC 288 with respect to other clusters. The results are in good agreement with the prediction of theoretical models of the dynamical evolution of NGC 288 that take into account the extreme orbital properties of this cluster.

The Main-Sequence Luminosity Function of Palomar 5 from the Hubble Space Telescope

Abstract: A low mass, large core radius, low central concentration, and strong tidal tails suggest that the globular cluster Palomar 5 has lost a large fraction of its initial mass over time. If the dynamical evolution of Pal 5 has been dominated by the effects of mass loss, then theoretical arguments suggest that the luminosity function should be deficient in low-mass stars. Using deep WFPC2 F555W and F814W photometry, we determine the main-sequence luminosity functions both near the cluster center and in a field near the half-light radius. A comparison of these luminosity functions yields no compelling evidence of mass segregation within the cluster, in accord with expectations for low-concentration clusters. On the other hand, a comparison of the global mass function of Pal 5 with that of ω Cen and M55 indicates an increasing deficiency of stars with progressively lower masses. A fit of the observed luminosity function to theoretical models indicates a mass function for Pal 5 of dN/dm ∝ m-0.5, which is notably more deficient in low-mass stars than other globular clusters that have been studied with the Hubble Space Telescope. The flatness of the mass function is consistent with models of the dynamical evolution of globular clusters that have lost ~90% of their original stellar mass. We suggest that, like NGC 6712, Pal 5 has lost a large percentage of its original stellar content as a result of tidal shocking.