Deficit Of Stars Research Articles

TIDAL DISRUPTION RATES IN NON-SPHERICAL GALACTIC NUCLEI FORMED BY GALAXY MERGERS

ABSTRACT We explore the rates of tidal disruption events (TDEs) of stars by supermassive black holes (SBHs) in galactic nuclei formed in mergers followed by a formation and coalescence of a binary SBH. Such systems initially have a deficit of stars on low-angular-momentum orbits caused by the slingshot process during the binary SBH stage, which tends to reduce the flux of stars into the SBH compared to the steady-state value. On the other hand, a newly formed galactic nucleus has a non-spherical shape, which enhances the mixing of stars in angular momentum and, thus, the TDE rate. In galaxies with relatively low SBH masses ( ), relaxation times are short enough to wash out the anisotropy in initial conditions, and for more massive SBHs the enhancement of flux due to non-sphericity turns out to be more important than its suppression due to initial anisotropy. Therefore, the present-day TDE rates generally exceed conventional steady-state estimates based on a spherical isotropic approximation. We thus conjecture that the lower observationally inferred TDE rates compared to theoretical predictions cannot be attributed to the depletion of low-angular-momentum stars by SBH binaries.

SUPPRESSION OF STELLAR TIDAL DISRUPTION RATES BY ANISOTROPIC INITIAL CONDITIONS

We compute the rates of capture of stars by supermassive black holes, using time-dependent Fokker--Planck equation with initial conditions that have a deficit of stars on low-angular-momentum orbits. One class of initial conditions has a gap in phase space created by a binary black hole, and the other has a globally tangentially-anisotropic velocity distribution. We find that for galactic nuclei that are younger than ~0.1 relaxation times, the flux of stars into the black hole is suppressed with respect to the steady-state value. This effect may substantially reduce the number of observable tidal disruption flares in galaxies with black hole masses M>10^7 Msun.

Red clump giant stars as tracers of Galactic structure

AbstractBy isolating the red clump giant population in the color-magnitude diagrams and inverting their star counts, we can obtain directly the density distribution of the old stellar population along the line of sight. We have applied this method to several near infrared surveys and obtained information on the disc, bulge and long bar. The disc is well fitted by an exponential distribution in both the galactocentric distance and height, flared and warped in the outer parts, and with a deficit of stars in the inner in-plane regions. The long bar occupies these in-plane regions within R < 3.9 kpc, with approximate dimensions of 7.8 kpc × 1.2 kpc × 0.2 kpc and a position angle of 40-45 deg. The bulge is a triaxial structure, possibly boxy, thicker and shorter than the long bar and with position angle of 10-30 deg.

THE ACS SURVEY OF GALACTIC GLOBULAR CLUSTERS. VI. NGC 6366: A HEAVILY STRIPPED GALACTIC GLOBULAR CLUSTER

We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366. The luminosity function derived from those data extends to M F606W ~ 9, or masses of ~0.3 M ☉. Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation. Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with a particular deficit of stars with masses between 0.5 and 0.7 M ☉. We briefly discuss this interesting anomaly within the context of tidal stripping.

Two Micron Narrowband Imaging of the Sagittarius D HiiRegion

We present 2 μm narrowband images of the core H II region in the Galactic star-forming region Sagittarius D. The emission-line images are centered on 2.17 μm (Brγ) and 2.06 μm (He I). The H II region appears at the edge of a well-defined dark cloud, and the morphology suggests a blister geometry as pointed out in earlier radio continuum work. There is a deficit of stars in general in front of the associated dark cloud indicating the H II region is located in between the Galactic center and the Sun. The lesser spatial extent of the He I line emission relative to Brγ places the effective temperature of the ionizing radiation field below 40,000 K. The He I 2.06 μm to Brγ ratio and Brγ/far-infrared dust emission put Teff at about 36,500-40,000 K as derived from ionization models.