Mass segregation

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

A Density Dependence for Protostellar Luminosity in Class I Sources: Collaborative Accretion

Abstract: Class I protostars in three high-mass star-forming regions are found to have correlations among the local projected density of other Class I protostars, the summed flux from these other protostars, and the protostellar luminosity in the WISE 22 μm band. Brighter Class I sources form in higher-density and higher-flux regions, while low luminosity sources form anywhere. These correlations depend slightly on the number of neighbors considered (from 2 to 20) and could include a size-of-sample effect from the initial mass function (i.e., larger numbers include rarer and more massive stars). Luminosities seem to vary by neighborhood with nearby protostars having values proportional to each other and higher density regions having higher values. If Class I luminosity is partially related to the accretion rate, then this luminosity correlation is consistent with the competitive accretion model, although it is more collaborative than competitive. The correlation is also consistent with primordial mass segregation and could explain why the stellar initial mass function resembles the dense core mass function even when cores form multiple stars.

Mass segregation and sequential star formation in NGC 2264 revealed by Herschel

Abstract: Context: The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors are still scarce, so the question concerning the origin and evolution of mass segregation is still open. Aims: Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation. Methods: We separated the Herschel column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions. Results: In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08 M⊙ to 53 M⊙. Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ6 three times that of the whole clumps population and ΛMSR ≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion. Conclusions: Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.

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

Abstract: Aims. Within a sphere of 400 pc radius around the Sun, we aim to search for members of the Pisces-Eridanus (Psc-Eri) stellar stream in the Gaia Data Release 2 data set. We compare basic astrophysical characteristics of the stream with those of the Pleiades.Methods. 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.Results. In a G magnitude range from 5.1 mag to 19.3 mag, we found 1387 members of the Psc-Eri stream at distances between 80 and 380 pc from the Sun. The stream has a nearly cylindrical shape with a length of at least 700 pc and a thickness of 100 pc. The accumulated stellar mass of the 1387 members amounts to about 770 M ⊙ , and the stream is 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, its present-day mass function, as well as in its total mass. Nonetheless, the two stellar ensembles are completely different 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, and/or (ii) the Pleiades had a higher primordial mass segregation immediately after the expulsion of the molecular gas of the parental cloud.

Different Dynamical Ages for the Two Young and Coeval LMC Star Clusters NGC 1805 and NGC 1818 Imprinted on Their Binary Populations

Abstract: The two Large Magellanic Cloud star clusters NGC 1805 and NGC 1818 are approximately the same chronological age (~30 Myr), but show different radial trends in binary frequency. The F-type stars (1.3 - 2.2 MSun) in NGC 1818 have a binary frequency that decreases towards the core, while the binary frequency for stars of similar mass in NGC 1805 is flat with radius, or perhaps bimodal (with a peak in the core). We show here, through detailed N-body modeling, that both clusters could have formed with the same primordial binary frequency and with binary orbital elements and masses drawn from the same distributions (defined from observations of open clusters and the field of our Galaxy). The observed radial trends in binary frequency for both clusters are best matched with models that have initial substructure. Furthermore, both clusters may be evolving along a very similar dynamical sequence, with the key difference that NGC 1805 is dynamically older than NGC 1818. The F-type binaries in NGC 1818 still show evidence of an initial period of rapid dynamical disruptions (which occur preferentially in the core), while NGC 1805 has already begun to recover a higher core binary frequency, owing to mass segregation (which will eventually produce a distribution in binary frequency that rises only towards the core, as is observed in old Milky Way star clusters). This recovery rate increases for higher-mass binaries, and therefore even at one age in one cluster, we predict a similar dynamical sequence in the radial distribution of the binary frequency as a function of binary primary mass.

Dynamics of Dense Stellar Systems Including the Effects of Stellar Collisions

Abstract: Dynamical evolution of dense stellar systems is followed by integrating Fokker-Planck equation including successive mergers between stars. We have assumed that all the tidal captures lead to mergers. The initial cluster is assumed to be a single component Plummer model with mass of individual stars being 0.7 M⊙. The highest mass that is allowed to form through successive mergers in our model is 32 × 0.7M⊙ =22.4 M⊙. The stellar evolution is simulated by removing stars from the cluster assuming that the stellar material escapes as the star finishes the evolution. We have estimated the number of stars that evolve off per unit time using “evolution function” that depends on the mean age and lifetime. The mean ages are estimated from the rates of formation of stars (through merger). While the formation of massive stars leads to the acceleration of core collapse through mass segregation, the indirect heating effect due to stellar evolution makes the core collapse slow. The net effect depends on the initial conditions. The number of high mass stars depends sensitively on the cluster parameters. Core collapse is usually found to be terminated by the indirect heating effect of evolution of moderate mass stars (mainly 2.8 or 5.6 M⊙ in our models). The maximum number stars in highest mass bin (i.e., M=22.4 M⊙) varies from unity to an order of 101 depending on exact initial conditions for the models appropriate for the Galactic center. These numbers are too small to explain the observed HeI emission line stars which are interpreted as envelope stripped high mass (20~ 40 M⊙) stars. However, if the ejected material from the final stage of stellar evolution can form stars efficiently instead of escaping from the potential well of the stellar system, more high mass stars can be found. The presence of massive black hole in the center would also boost the merger rates.