Mass segregation

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

The impact of realistic models of mass segregation on the event rate of extreme-mass ratio inspirals and cusp re-growth

Abstract: One of the most interesting sources of gravitational waves (GWs) for LISA is the inspiral of compact objects on to a massive black hole (MBH), commonly referred to as an "extreme-mass ratio inspiral" (EMRI). The small object, typically a stellar black hole (bh), emits significant amounts of GW along each orbit in the detector bandwidth. The slowly, adiabatic inspiral of these sources will allow us to map space-time around MBHs in detail, as well as to test our current conception of gravitation in the strong regime. The event rate of this kind of source has been addressed many times in the literature and the numbers reported fluctuate by orders of magnitude. On the other hand, recent observations of the Galactic center revealed a dearth of giant stars inside the inner parsec relative to the numbers theoretically expected for a fully relaxed stellar cusp. The possibility of unrelaxed nuclei (or, equivalently, with no or only a very shallow cusp) adds substantial uncertainty to the estimates. Having this timely question in mind, we run a significant number of direct-summation $N-$body simulations with up to half a million particles to calibrate a much faster orbit-averaged Fokker-Planck code. We then investigate the regime of strong mass segregation (SMS) for models with two different stellar mass components. We show that, under quite generic initial conditions, the time required for the growth of a relaxed, mass segregated stellar cusp is shorter than a Hubble time for MBHs with $M_\bullet \lesssim 5 \times 10^6 M_\odot$ (i.e. nuclei in the range of LISA). SMS has a significant impact boosting the EMRI rates by a factor of $\sim 10$ for our fiducial models of Milky Way type galactic nuclei.

Exploring the Mass Segregation Effect of X-Ray Sources in Globular Clusters. III. Signs of Binary Disruption in M28

Abstract: Using archival {\it Chandra} observations with a total effective exposure of 323 ks, we derive an updated catalog of point sources in the bulge globular cluster M28. The catalog contains 502 X-ray sources within an area of $\sim475\, \rm arcmin^{2}$, and more than $90\%$ of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in M28, with the projected distance and width of the distribution dip for bright ($L_{X} \gtrsim 4.5\times 10^{30} {\rm\ erg\ \,s^{-1}}$) X-ray sources are larger than the faint ($L_{X} \lesssim 4.5\times 10^{30} {\rm\ erg\ \,s^{-1}}$) sources. The "generalized King model" fitting give a slightly larger average mass for the bright sources ($1.30\pm0.15\,M_{\odot}$) than the faint sources ($1.09\pm0.14\,M_{\odot}$), which support a universal mass segregation delay between heavy objects in GCs. Compared with 47 Tuc and Terzan 5, we show that the dynamical age of M28 is comparable to Terzan 5 and much smaller than 47 Tuc, but it is evolving more fast (i.e., with smaller two-body relaxation timescale) than 47 Tuc. These features may suggest an acceleration effect of cluster dynamical evolution by tidal shock in M28. Besides, we find an abnormal deficiency of X-ray sources in the central region ($R \lesssim 1.5 \rm~arcmin$) of M28 than its outskirts, which indicate that M28 may have suffered an early phase of primordial binary disruption within its central region, and mass segregation effect will erase such a phenomenon as cluster evolve to older dynamical age.

Mass Functions and Mass Segregation in Young Starburst Clusters

Abstract: The Milky Way starburst clusters Arches in the Galactic Center region and NGC 3603 in the Carina spiral arm are studied with the aim to gain deeper insight into the stellar mass distribution in starburst clusters. The dense stellar population in both clusters is resolved in unprecedented detail with high angular resolution, near-infrared instruments. In the case of the Arches cluster, diffraction-limited, adaptive optics observations are analysed, and the achievements and limitations of ground-based vs. space-based diffraction-limited imaging are revealed by comparison with HST/NICMOS data. In the case of NGC 3603, seeing-limited JHKL photometry is used to derive colour-excess fractions, and is complemented by space-based Halpha data, both serving as tracers for circumstellar disks. Disk survival in starburst clusters is discussed. The present-day mass function (MF) of both clusters is deduced from colour-magnitude diagrams. Radial variations in the MFs reveal a heavily mass-segregated core in both starburst clusters. Dynamical timescales are estimated and interpreted with respect to primordial and dynamical segregation. The implications for massive star and cluster formation scenarios are discussed. Evidence for a low-mass cut-off is observed in the Arches MF, but not in NGC 3603, indicating a reduced formation efficiency for M < 10 Msun stars in the Galactic Center. This environmental difference has strong implications for the formation of stellar populations in galactic nuclei and starburst galaxies.

Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-Phase Metallicity Gradients : The Effects of Ram Pressure Stripping

Abstract: Recent observations of galaxies in a cluster at z=0.35 show that their integrated gas-phase metallicities increase with decreasing cluster-centric distance. To test if ram pressure stripping (RPS) is the underlying cause, we use a semi-analytic model to quantify the "observational bias" that RPS introduces into the aperture-based metallicity measurements. We take integral field spectroscopy of local galaxies, remove gas from their outer galactic disks via RPS, and then conduct mock slit observations of cluster galaxies at z=0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of -0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a mean metallicity enhancement of +0.02 dex at a fixed stellar mass. This gas removal and subsequent quenching of star formation preferentially removes low mass cluster galaxies from the observed star-forming population. As only the more massive star-forming galaxies survive to reach the cluster core, RPS produces a cluster-scale stellar mass gradient of -0.05 log(M_*/M_sun)/Mpc. This mass segregation drives the predicted cluster-scale metallicity gradient of -0.03 dex/Mpc. However, the effects of RPS alone can not explain the higher metallicities measured in cluster galaxies at z=0.35. We hypothesize that additional mechanisms including steep internal metallicity gradients and self-enrichment due to gas strangulation are needed to reproduce our observations at z=0.35.

The Initial Mass Function and the Surface Density Profile of NGC 6231

Abstract: We have performed new wide-field photometry of the young open cluster NGC 6231 to study the shape of the initial mass function (IMF) and mass segregation. We also investigated the reddening law toward NGC 6231 from optical to mid-infrared color excess ratios, and found that the total-to-selective extinction ratio is Rv = 3.2, which is very close to the normal value. But many early-type stars in the cluster center show large color excess ratios. We derived the surface density profiles of four member groups, and found that they reach the surface density of field stars at about 10', regardless of stellar mass. The IMF of NGC 6231 is derived for the mass range 0.8 -- 45 Msun. The slope of the IMF of NGC 6231 (Gamma = -1.1 +/- 0.1) is slightly shallower than the canonical value, but the difference is marginal. In addition, the mass function varies systematically, and is a strong function of radius - it is is very shallow at the center, and very steep at the outer ring suggesting the cluster is mass segregated. We confirm the mass segregation for the massive stars (m >~ 8 Msun) by a minimum spanning tree analysis. Using a Monte Carlo method, we estimate the total mass of NGC 6231 to be about 2.6 (+/- 0.6) x 10^3 Msun. We constrain the age of NGC 6231 by comparison with evolutionary isochrones. The age of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr. However the age of the high mass stars depends on the adopted models and is 3.5 +/- 0.5 Myr from the non- or moderately-rotating models of Brott et al. as well as the non-rotating models of Ekstr\"om et al. But the age is 4.0 -- 7.0 Myr if the rotating models of Ekstr\"om et al. are adopted. This latter age is in excellent agreement with the time scale of ejection of the high mass runaway star HD 153919 from NGC 6231, albeit the younger age cannot be entirely excluded.