Ultracompact Objects Research Articles

The Dynamical Mass of the Young Cluster W3 in NGC 7252: Heavy-Weight Globular Cluster or Ultra-Compact Dwarf Galaxy?

AbstractWe have measured the dynamical mass of the highly luminous star cluster W3 in the young merger remnant galaxy NGC 7252. The value is Mdyn = (8 ± 2) × 107M⊙, and represents the highest dynamically-confirmed mass for an extra-galactic star cluster so far. The dynamical mass is in excellent agreement with the luminous mass (Maraston et al. 2001). This results from the use of stellar population models that include correctly the brightest AGB stellar phase, dominant in young stellar populations. To classify W3, we employ the fundamental plane of stellar systems (Bender, Burstein & Faber 1992), for the first time in these kinds of studies. We find that W3 lies far from typical Milky Way globular clusters, but it is also far from the heavyweights ωCen in the Milky Way and G1 in M31, because it is too extended for its mass, and from dwarf elliptical galaxies because it is much more compact for its mass. Instead W3 lies close to the ultra-compact Fornax objects (Drinkwater et al. 2003) and to the compact elliptical M32, possibly shedding light on the still mysterious nature of these objects. A previously deserted region of the fundamental plane starts to be populated.

CAUSAL CONFIGURATIONS OF HOMOGENEOUS ENERGY DENSITY IN GENERAL RELATIVITY

If the causality condition [the speed of sound always remains less than that of light in vacuum, i.e. v≤c=1] is imposed on the spheres of homogeneous energy density, the "ratio of the specific heats", γ≤2.59457, constraints the compaction parameter, u[≡(M/a), mass to size ratio in geometrized units] of the dynamically stable configurations ≤0.34056 [corresponding to a surface redshift (za)≤0.771]. Apparently, the maximum value of u obtained in this manner belongs to an absolute upper bound, and gives: (i) The maximum value for static neutron star masses as 5.4 M⊙, if we substitute the density at the surface of the configuration equal to the average nuclear density, E=2×1014 g cm -3 [e.g. Nature, 259, 377 (1976)]. (ii) However, if the density of the static configuration is constrained to the value 1.072×1014 g cm -3, by imposing the empirical result that the minimum rotation period of the fastest rotating pulsar known to date, PSR 1937+21, is 1.558 ms, the maximum mass value for static neutron stars exceed upto 7.4 M⊙. These masses have important implications for the massive compact objects, such as Cyg X-1, Cyg XR-1, LMC-X3, etc., which may not, necessarily, represent black holes. (iii) The minimum rotation periods for a static 1.442 M⊙ neutron star to be 0.3041 ms. (iv) A suitable stable model of ultra-compact objects [u>(1/3)] which has important astrophysical significance.

Dwarf Galaxies in Clusters

Dwarf and Low Surface Brightness (LSB) galaxies are the most numerous type of galaxies in the local Universe. However ubiquitous they are, their properties remain widely unexplored. Dwarf galaxies range from Ultra Compact Objects (UCO), Dwarf Ellipticals (dE), Dwarf Spheroidals (dSph) to Dwarf Irregulars (dIrr) and are generally of low surface brightness. In this paper we focus on the relation between these objects and galaxy clusters. The discussion is centered on three main issues, the faint end of the cluster luminosity function, segregation of dwarfs in clusters and the dwarf galaxy – cluster dynamical state connection. A further analysis of the population and clustering properties of dwarf galaxies in clusters require better statistics. While concluding, we present, as well, future prospects on how to measure distances to dwarfs using the surface brightness fluctuation method so as to increase the number of confirmed dwarf member galaxies in more distant clusters.

Ultra compact objects in the Fornax cluster of galaxies: Globular clusters or dwarf galaxies?

The relation between the Ultra Compact Objects (hereafter UCOs) recently discovered in the Fornax cluster (Drinkwater et al. 2000a; Hilker et al. 1999) and the brightest globular clusters associated with the central galaxy NGC 1399 has been investigated. The question was adressed whether the UCOs constitute a distinct population of objects not linked to globular clusters or whether there is a smooth transition between both populations. Therefore, a spectroscopic survey on compact objects in the central region of the Fornax cluster was carried out with the 2.5 m du Pont telescope (LCO). UCOs and the bright NGC 1399 globular clusters with similar brightness were inspected. 12 GCs from the bright end of the globular cluster luminosity function have been identified as Fornax members. Eight are new members, four were known as members from before. Their magnitude distribution supports a smooth transition between the faint UCOs and the bright globular clusters. There is no evidence for a magnitude gap between both populations. However, the brightest UCO clearly stands out; it is too bright and too large to be accounted for by globular clusters. For the only UCO included in our survey, a relatively high metallicity of [Fe/H] ≃ -0.5 dex is measured.

ULTRACOMPACT STARS WITH MULTIPLE NECKS

We discuss ultracompact stellar objects which have multiple necks in their optical geometry. There are in fact physically reasonable equations of state for which the number of necks can be arbitrarily large. The proofs of these statements rely on a recent regularized formulation of the field equations for static spherically symmetric models due to Nilsson and Uggla. We discuss in particular the equation of state p = ρ - ρ s which plays a central role in this context.

Stable Ultracompact Objects

Ultra-Compact Objects (ucos) have been found tobe interesting astrophysical entities. The existence ofsuch a possible stable object has been discussed byconsidering the stability of Tolman's type VII solution which is one of the few physicallyrelevant exact solutions of Einstein's field equationsfor a static and spherically symmetric massdistribution. A distinctive feature of this solutionamong the others is that the surface density alsodrops to zero (i.e., the continuity of density, and therespective derivative of the metric parameter(λ′) is also assured at the surface of theconfiguration). The stability analysis of this solution,according to the variational method [20], as well as thebinding-energy criteria of fluid spheres shows that theconfiguration remains stable at least up to a central redshift, z0 ≤ 5.09, and surfaceredshift, za ≤ 0.78 (i.e. up to a u≡ mass/size) value ≤0.3428), thereby confirmingthe existence of an ultracompact object (uco) thatcorresponds to a mass to size ratio (u ≡ M/a) ≥ 1/3.

Centrifugal force and ellipticity behaviour of a slowly rotating ultra-compact object

Using the optical reference geometry approach, we have derived a general expression for the ellipticity of a slowly rotating fluid configuration using a Newtonian force balance equation in the conformally projected absolute 3-space, in the realm of general relativity. Further, with the help of the Hartle - Thorne (H - T) metric for a slowly rotating compact object, we have evaluated the centrifugal force acting on a fluid element and also evaluated the ellipticity and found that the centrifugal reversal occurs at around , and the ellipticity maximum at around . The result has been compared with that of Chandrasekhar and Miller which was obtained in the full 4-spacetime formalism.

From Ultracompact to Extended H II Regions

view Abstract Citations (208) References (73) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS From Ultracompact to Extended H II Regions Garcia-Segura, Guillermo ; Franco, Jose Abstract The dynamical evolution of H II regions and wind-driven bubbles in dense clouds is studied. In particular, we address two different issues: (1) the conditions under which ultracompact H II (UCHII) regions can reach pressure equilibrium with their surrounding medium (and thereby stall their expansion) and (2) the appearance of a powerful dynamic instability in expanding H II regions. At pressure equilibrium, the ionized regions become static, and as long as the ionization sources and the ambient gas densities remain about constant, the resulting UCHII regions are stable and long-lived. The equilibrium sizes and densities, Rs,eq ∼3 X 10-2F⅓48T⅔H II, 4P-⅔7 pc and ni,eq ∼4 × 104P7T-1H II, 4 cm-3 (where Fβ8 is the photoionizing flux in units of 1048 s-11, P7 is the pressure in units of 10-7 dyne cm-2, and TH II,4 is the ion temperature in units of 104 K), are similar to those actually observed in UCHII regions. Similarly, ultra- compact wind-driven bubbles can reach pressure equilibrium, and the resulting final sizes are similar to those of UCHII'S. The same is true for a combined ultracompact structure consisting of an interior wind- driven cavity and an external H II region. For nonmoving stars in a constant-density medium, the lifetimes for all types of ultracompact objects only depend on the stellar lifetimes. For cases with a density gradient, depending on the core size and slope of the density distribution, some regions never reach the static equilibrium condition. A powerful dynamic instability appears when cooling is included in the neutral gas swept up by an H II region or a combined wind-H II region structure. This instability was first studied by Giuliani and is associated with the thin-shell instability described by Vishniac. The internal ionization front exacerbates the growth of the thin-shell instability, creating a rapid shell fragmentation, and our numerical simulations confirm the linear analysis of Giuliani. The fragments tend to merge as the evolution proceeds, creating dense and more massive clumps, and are slowly eroded by ionization fronts. Thus the resulting structures have a variety of shapes, sizes, densities, and lifetimes. Intriguing features such as "elephant trunks" and cometary-like globules can easily be explained as a result of this instability. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177769 Bibcode: 1996ApJ...469..171G Keywords: ISM: H II REGIONS; HYDRODYNAMICS; ISM: BUBBLES; ISM: CLOUDS full text sources ADS | data products SIMBAD (4)

Infrared polarimetry and imaging of ultracompact partially ionized optical sources in the orion nebula

Hubble Space Telescope images of the Orion nebula taken with the Wide-Field Camera have revealed subarcsecond structure in several dozen objects which are apparently ionized externally from nearby stars. We have obtained near-IR images and IR polarimetry of the Orion region to search for correlations with the WFC objects. We find that all of the ultracompact WFC objects are associated with IR features of some sort, and that some are associated with strongly polarized IR emission. The object with strongest polarization also shows small IR “lobes”. In addition, we find some previously unreported sources, showing polarized IR emission, outside the field of the HST images, which we believe may be the same sorts of object. We note that the object with strongest polarization has a double-lobed appearance in the K band image.

High-Resolution Centimeter Radio Continuum and Ammonia Maser Observations of the W51 Region of Star Formation

view Abstract Citations (72) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution Centimeter Radio Continuum and Ammonia Maser Observations of the W51 Region of Star Formation Gaume, R. A. ; Johnston, K. J. ; Wilson, T. L. Abstract High angular resolution and high-sensitivity observations at wavelengths of 1.3 and 3.6 cm have detected three new ultracompact emission regions in the core of the W51 (regions d and e). In total, five ultracompact continuum objects of diameter less than 300 to 3000 AU are located in W51 d and e. These sources may best be explained as photoionized stellar winds. This suggests that there may exist a quasi-stable point in early stellar evolution where observable H II regions are formed by stellar winds around massive stars. These observations demonstrate that the taxonomy of H II region classification may be heavily affected by sensitivity, and a lack of complete spatial sampling by interferometric mapping. For example, the W51 d region, previously classified as cometary, is shown by our higher resolution and sensitivity observations also to have the properties normally associated with shell-like regions. These new observations suggest that the bow shock model, which is suggested to explain the morphology of cometary H II regions, but cannot explain shell sources, does not fully explain the W51 d H II region. The J, K = (9, 8) NH3 maser associated with W51 d has been shown to have a minimum brightness temperature of 2.7 × 106 K. This maser is most probably saturated. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173342 Bibcode: 1993ApJ...417..645G Keywords: ISM: H II REGIONS; ISM: INDIVIDUAL ALPHANUMERIC: W51; ISM: MOLECULES; ISM: STRUCTURE; MASERS; RADIO CONTINUUM: INTERSTELLAR full text sources ADS | data products SIMBAD (12)

The radial force on a charged particle in superimposed magnetic fields on Schwarzschild spacetime

Following the approach of optical reference geometry we derive the expression for the total force in the radial direction acting on a charged particle in magnetic fields superimposed on the static Schwarzschild background and show the possible existence of bound orbits for particles in the field of ultra compact objects at distancesr⩽3m wherein the Lorentz force counterbalances both the gravitational and centrifugal forces.

Accretion-driven nonthermal phenomena

Arguments are presented which suggest that accretion disks around black holes and neutron stars may self-generate ordered magnetic fields, from currents associated with orbital motion of the disk material. This leads one to expect electromagnetic extraction of the rotational energy of the disk, and it is proposed that such a magnetic field is the ultimate source of various nonthermal phenomena in accreting ultracompact objects.

Ultracompact (R

The authors investigate the possible existence of ultracompact objects, i.e., configurations with radii less than three times the mass in geometrical units. Within the framework of core envelope models for a comparatively low value of the fiducial (p/ rho c2) at the interface, stable ultracompact objects with causal cores are in fact found to be possible. On the other hand available equations of state valid at higher densities when used throughout the star lead invariably to configurations with radii greater than 3 M. It is also shown that within the framework of core-envelope models stability requirements do not bring down the mass limit.