Mass segregation

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

On strong mass segregation around a massive black hole: Implications for lower-frequency gravitational-wave astrophysics

Abstract: We present, for the first time, a clear $N$-body realization of the {\it strong mass segregation} solution for the stellar distribution around a massive black hole. We compare our $N$-body results with those obtained by solving the orbit-averaged Fokker-Planck (FP) equation in energy space. The $N$-body segregation is slightly stronger than in the FP solution, but both confirm the {\it robustness} of the regime of strong segregation when the number fraction of heavy stars is a (realistically) small fraction of the total population. In view of recent observations revealing a dearth of giant stars in the sub-parsec region of the Milky Way, we show that the time scales associated with cusp re-growth are not longer than $(0.1-0.25) \times T_{rlx}(r_h)$. These time scales are shorter than a Hubble time for black holes masses $\mbul \lesssim 4 \times 10^6 M_\odot$ and we conclude that quasi-steady, mass segregated, stellar cusps may be common around MBHs in this mass range. Since EMRI rates scale as $\mbul^{-\alpha}$, with $\alpha \in [1\4,1]$, a good fraction of these events should originate from strongly segregated stellar cusps.

Old open clusters in the inner Galaxy: FSR 1744, FSR 89 and FSR 31

Abstract: Context. We examine the dynamical survival of intermediate-age/old open clusters in the inner Galaxy. Aims. We aim to establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR 1744, FSR 89 and FSR 31 to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. The current status of the issue dealing with the small number of detected open clusters in the inner Galaxy is discussed. Methods. Properties of the objects are investigated with 2MASS colour–magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic diagrams of structural parameters are used to separate dynamical from highbackground effects affecting such centrally projected open clusters. Results. FSR 1744, FSR 89 and FSR 31 are Gyr-class open clusters located at Galactocentric distances 4.0–5.6 kpc. Compared to nearby open clusters, they have small core and limiting radii. Conclusions. With respect to the small number of open clusters observed in the inner Galaxy, the emerging scenario in the nearinfrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are mass loss by stellar evolution, mass segregation and evaporation. Externally they are tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR 1744, FSR 89 and FSR 31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.

The bound fraction of young star clusters

Abstract: Context. The residual gas within newly formed star clusters is expelled through stellar feedback on timescales ≲ 1 Myr. The subsequent expansion of the cluster results in an unbinding of a fraction of stars, before the remaining cluster members can re-virialize and form a surviving cluster. Aims. We investigate the bound fraction after gas expulsion as a function of initial cluster mass in stars M_(ecl) and gauge the influence of primordial mass segregation, stellar evolution and the tidal field at solar distance. We also assess the impact of the star-formation efficiency e_(SFE) and gas expulsion velocity v_g. Methods. We perform N-body simulations using Sverre Aarseth’s NBODY7 code, starting with compact clusters in their embedded phase and approximate the gas expulsion by means of an exponentially depleting external gravitational field. We follow the process of re-virialization through detailed monitoring of different Lagrange radii over several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and M_(ecl) usually ranging from 5 × 10^3 M_⊙ to 5 × 10^4 M_⊙. Results. The strong impact of the relation between the gas expulsion timescale and the crossing time means that clusters with the same initial core density can have very different bound fractions. The adopted e_(SFE) = 0.33 in the cluster volume results in a distinct sensitivity to v_g over a wide mass range, while a variation of e_(SFE) can make the cluster robust to the rapidly decreasing external potential. We confirm that primordial mass segregation leads to a smaller bound fraction, its influence possibly decreasing with mass. Stellar evolution has a higher impact on lower mass clusters, but heating through dynamical friction could expand the cluster to a similar extent. The examined clusters expand well within their tidal radii and would survive gas expulsion even in a strong tidal field.

The effect of environment on star forming galaxies at redshift 1 - First insight from PACS

Abstract: We use deep 70, 100 and 160 um observations taken with PACS, the Photodetector Array Camera and Spectrometer on board of Herschel, as part of the PACS Evolutionary Probe (PEP) guaranteed time, to study the relation between star formation rate and environment at redshift ~ 1 in the GOODS-S and GOODS-N fields. We use the SDSS spectroscopic catalog to build the local analog and study the evolution of the star formation activity dependence on the environment. At z ~ 1 we observe a reversal of the relation between star formation rate and local density, confirming the results based on Spitzer 24 um data. However, due to the high accuracy provided by PACS in measuring the star formation rate also for AGN hosts, we identify in this class of objects the cause for the reversal of the density-SFR relation. Indeed, AGN hosts favor high stellar masses, dense regions and high star formation rates. Without the AGN contribution the relation flattens consistently with respect to the local analog in the same range of star formation rates. As in the local universe, the specific star formation rate anti-correlates with the density. This is due to mass segregation both at high and low redshift. The contribution of AGN hosts does not affect this anti-correlation, since AGN hosts exhibit the same specific star formation rate as star forming galaxies at the same mass. The same global trends and AGN contribution is observed once the relations are studied per morphological type. We study the specific star formation rate vs stellar mass relation in three density regimes. Our data provides an indication that at M/M_{\odot} > 10^{11} the mean specific star formation rate tends to be higher at higher density, while the opposite trend is observed in the local SDSS star forming sample.

Pulsar-Black Hole Binaries in the Galactic Center

Abstract: Binaries consisting of a pulsar and a black hole (BH) are a holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date. Clues as to where such systems might exist are therefore highly desirable. We show that if the central parsec around Sgr A* harbors a cluster of ~25,000 stellar BHs (as predicted by mass segregation arguments) and if it is also rich in recycled pulsar binaries (by analogy with globular clusters), then 3-body exchange interactions should produce PSR-BHs in the Galactic center. Simple estimates of the formation rate and survival time of these binaries suggest that a few PSR-BHs should be present in the central parsec today. The proposed formation mechanism makes unique predictions for the PSR-BH properties: 1) the binary would reside within ~1 pc of Sgr A*; 2) the pulsar would be recycled, with a period of ~1 to a few tens of milliseconds, and a low magnetic field B ~3 AU. The potential discovery of a PSR-BH binary therefore provides a strong motivation for deep, high-frequency radio searches for recycled pulsars toward the Galactic center.