Mass segregation

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

The global mass function of M 15

Abstract: Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to deter- mine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M 15, located ∼7 � from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ∼0.3 M� , as obtained by Paresce & De Marchi (2000) for a stellar field at 4. 6 from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20 �� and at 4. 6), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. The model that simulta- neously best fits the LF at various locations, the surface brightness profile and the velocity dispersion profile suggest that the GMF should resemble a segmented power-law with the following indices: x � 0.8 for stars more massive than 0.8 M� , x � 0.9 for 0.3−0.8 Mand x �− 2.2 at smaller masses (Salpeter's IMF would have x = 1.35). The best fitting model also suggests that the cluster mass is ∼5.4 × 10 5 Mand that the mass-to-light ratio is on average M/LV � 2.1, with M/LV � 3. 7i n the core. A large amount of mass (∼44%) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M 15 without invoking the presence of an intermediate-mass black hole.

The Origin of the Initial Mass Function

Abstract: We review recent advances in our understanding of the origin of the initial mass function (IMF). We emphasize the use of numerical simulations to investigate how each physical process involved in star formation affects the resulting IMF. We stress that it is insufficient to just reproduce the IMF, but that any successful model needs to account for the many observed properties of star-forming regions, including clustering, mass segregation, and binarity. Fragmentation involving the interplay of gravity, turbulence, and thermal effects is probably responsible for setting the characteristic stellar mass. Low-mass stars and brown dwarfs can form through the fragmentation of dense filaments and disks, possibly followed by early ejection from these dense environments, which truncates their growth in mass. Higher-mass stars and the Salpeter-like slope of the IMF are most likely formed through continued accretion in a clustered environment. The effects of feedback and magnetic fields on the origin of the IMF are still largely unclear. Finally, we discuss a number of outstanding problems that need to be addressed in order to develop a complete theory for the origin of the IMF.

The young star cluster NGC 2362 : low-mass population and initial mass function from a Chandra X-ray observation

Abstract: Context. We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium. Aims. Our main purposes are to select cluster members down to low masses and to study their properties as a population (spatial properties, initial mass function, and coronal properties). Methods. We compare existing deep optical photometry and H α data with new X-ray data. We use combined optical and X-ray criteria to select cluster members. Results. We detect 387 X-ray sources down to $\log L_{\rm X} = 29.0$ (erg/s), and identify most of them (308) with star-like objects. The majority (88%) of optically identified X-ray sources are found to be very good candidate low-mass pre-main-sequence stars, with minimal field-object contamination. This increases the known cluster census by a substantial amount at low masses, with respect to previous optical/IR studies. The fraction of stars with active accretion is found to be in the range 5–9%. We find a significantly wider spatial distribution for low-mass stars than for massive stars (mass segregation). We find only a small spread around the low-mass cluster sequence in the HR diagram, indicating that star formation lasted only about 1–2 Myr. We have derived the cluster initial mass function, which appears to flatten (on the low-mass side) at higher masses with respect to other very young clusters. The quiescent X-ray emission of low-mass cluster stars is found to be rather strictly correlated with the stellar bolometric luminosity: the small spread in this correlation puts an upper bound on the amplitude of X-ray variability on time scales longer than one day (e.g., activity cycles) in such young coronal sources. We find significant X-ray spectral differences between low-mass stars brighter and fainter than $\log L_{\rm X} \sim 30.3$ (erg/s), respectively, with X-ray brighter stars showing hotter components ($kT \sim 2$ keV), absent in fainter stars.

Mass Segregation and Equipartition of Energy in Two Globular Clusters with Central Density Cusps.

Abstract: We begin by presenting the analysis of a set of deep B- and V-band images of the central density cusp of the globular cluster M30 (NGC 7099), taken with the Faint Object Camera aboard the Hubble Space Telescope. These images are the first to resolve lower-mass main-sequence stars in the cluster's central 10''. From the positions of individual stars, we measure an improved position for the cluster center; this new position is 2.6'' from the previously known position. We find no evidence of a ``flat'', constant-surface-density core; however, the data do not rule out the presence of a core of radius up to 1.9'' (95% confidence level). We measure a logarithmic cusp slope (d \log \sigma / d \log r$) of -0.76 +- 0.07 (1-sigma) for stars with masses between 0.69 and 0.76 \Msun, and -0.82 +- 0.11 for stars with masses between 0.57 and 0.69 \Msun. We also compare the overall mass function (MF) of the cluster cusp with the MF of a field at r = 4.6' (near the cluster half-mass radius). The observed degree of mass segregation is well matched by the predictions of an isotropic, multimass King model. We then use the Jeans equation to compare the structure of M30 with that of M15, another cusped cluster, using data from this and a previous paper. We find that M30 is very close to achieving equipartition of energy between stellar species, at least over the observed range in mass and radius, while M15 is not. This difference may be a result of the longer relaxation time in the observed field in M15. The data also suggest that the degree of mass segregation within the two cluster cusps is smaller than one would expect from the measurements at larger radius. If so, this phenomenon might be the result of gravothermal oscillations, of centrally-concentrated populations of binaries, or of a ~10^3 \Msun black hole in one or more clusters.

The Gaia-ESO Survey: Structural and dynamical properties of the young cluster Chamaeleon I

Abstract: The young (~2 Myr) cluster Chamaeleon I is one of the closest laboratories to study the early stages of star cluster dynamics in a low-density environment. We studied its structural and kinematical properties combining parameters from the high-resolution spectroscopic survey Gaia-ESO with data from the literature. Our main result is the evidence of a large discrepancy between the velocity dispersion (sigma = 1.14 \pm 0.35 km s^{-1}) of the stellar population and the dispersion of the pre-stellar cores (~0.3 km s^{-1}) derived from submillimeter observations. The origin of this discrepancy, which has been observed in other young star clusters is not clear. It may be due to either the effect of the magnetic field on the protostars and the filaments, or to the dynamical evolution of stars driven by two-body interactions. Furthermore, the analysis of the kinematic properties of the stellar population put in evidence a significant velocity shift (~1 km s^{-1}) between the two sub-clusters located around the North and South main clouds. This result further supports a scenario, where clusters form from the evolution of multiple substructures rather than from a monolithic collapse. Using three independent spectroscopic indicators (the gravity indicator $\gamma$, the equivalent width of the Li line, and the H_alpha 10\% width), we performed a new membership selection. We found six new cluster members located in the outer region of the cluster. Starting from the positions and masses of the cluster members, we derived the level of substructure Q, the surface density \Sigma and the level of mass segregation $\Lambda_{MSR}$ of the cluster. The comparison between these structural properties and the results of N-body simulations suggests that the cluster formed in a low density environment, in virial equilibrium or supervirial, and highly substructured.