arXiv subjects

Andreas Just

Publications and source records attributed to Andreas Just.

46 records · Page 3Linked to original sources

arXiv2011

Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample

We employ measurements of the [alpha/Fe] ratio derived from low-resolution (R~2000) spectra of 17,277 G-type dwarfs from the SEGUE survey to separate them into likely thin- and thick-disk subsamples. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, of opposite signs, -20 to -30 km/s/dex for the thin-disk and +40 to +50 km/s/dex for the thick-disk population. The rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample, and exhibits a small trend for the thick-disk subsample. The rotational velocity decreases with distance from the plane for both disk components, with similar slopes (-9.0 {\pm} 1.0 km/s/kpc). Thick-disk stars exhibit a strong trend of orbital eccentricity with metallicity (about -0.2/dex), while the eccentricity does not change with metallicity for the thin-disk subsample. The eccentricity is almost independent of Galactocentric radius for the thin-disk population, while a marginal gradient of the eccentricity with radius exists for the thick-disk population. Both subsamples possess similar positive gradients of eccentricity with distance from the Galactic plane. The shapes of the eccentricity distributions for the thin- and thick-disk populations are independent of distance from the plane, and include no significant numbers of stars with eccentricity above 0.6. Among several contemporary models of disk evolution we consider, radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable.

astro-ph.GA↗

arXiv2011

Efficient Merger of Binary Supermassive Black Holes in Merging Galaxies

In spherical galaxies, binary supermassive black holes (SMBHs) have difficulty reaching sub-parsec separations due to depletion of stars on orbits that intersect the massive binary - the final-parsec problem. Galaxies that form via major mergers are substantially nonspherical, and it has been argued that the centrophilic orbits in triaxial galaxies might provide stars to the massive binary at a high enough rate to avoid stalling. Here we test that idea by carrying out fully self-consistent merger simulations of galaxies containing central SMBHs. We find hardening rates of the massive binaries that are indeed much higher than in spherical models, and essentially independent of the number of particles used in the simulations. Binary eccentricities remain high throughout the simulations. Our results constitute a fully stellar-dynamical solution to the final-parsec problem and imply a potentially high rate of events for low-frequency gravitational wave detectors like LISA.

astro-ph.CO↗

arXiv2010

Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region

We have used the self-consistent vertical disc models of the solar neighbourhood presented in Just & Jahreiss (2010), which are based on different star formation histories (SFR) and fit the local kinematics of main sequence stars equally well, to predict star counts towards the North Galactic Pole (NGP). We combined these four different models with the local main sequence in the filter system of the SDSS and predicted the star counts in the NGP field with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better than 20 percent and the star number densities in the solar neighbourhood are consistent with the observed values. The chi^2 analysis shows that model A is clearly preferred with systematic deviations of a few percent only. The SFR of model A is characterised by a maximum at an age of 10Gyr and a decline by a factor of four to the present day value of 1.4Msun/pc^2/Gyr. The thick disc can be modelled very well by an old isothermal simple stellar population. The density profile can be approximated by a sech^(alpha_t) function. We found a power law index alpha_t=1.16 and a scale height of 800pc corresponding to a vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the solar neighbourhood are thick disc stars.

astro-ph.GA↗

arXiv2009

On the dissolution of star clusters in the Galactic centre. I. Circular orbits

We present N-body simulations of dissolving star clusters close to galactic centres. For this purpose, we developed a new N-body program called nbody6gc based on Aarseth's series of N-body codes. We describe the algorithm in detail. We report about the density wave phenomenon in the tidal arms which has been recently explained by Kuepper et al. (2008). Standing waves develop in the tidal arms. The wave knots or clumps develop at the position, where the emerging tidal arm hits the potential wall of the effective potential and is reflected. The escaping stars move through the wave knots further into the tidal arms. We show the consistency of the positions of the wave knots with the theory in Just et al. (2009). We also demonstrate a simple method to study the properties of tidal arms. By solving many eigenvalue problems along the tidal arms, we construct numerically a 1D coordinate system whose direction is always along a principal axis of the local tensor of inertia. Along this coordinate system, physical quantities can be evaluated. The half-mass or dissolution times of our models are almost independent of the particle number which indicates that two-body relaxation is not the dominant mechanism leading to the dissolution. This may be a typical situation for many young star clusters. We propose a classification scheme which sheds light on the dissolution mechanism.

astro-ph.GA↗

arXiv2007

Escape from the vicinity of fractal basin boundaries of a star cluster

The dissolution process of star clusters is rather intricate for theory. We investigate it in the context of chaotic dynamics. We use the simple Plummer model for the gravitational field of a star cluster and treat the tidal field of the Galaxy within the tidal approximation. That is, a linear approximation of tidal forces from the Galaxy based on epicyclic theory in a rotating reference frame. The Poincaré surfaces of section reveal the effect of a Coriolis asymmetry. The system is non-hyperbolic which has important consequences for the dynamics. We calculated the basins of escape with respect to the Lagrangian points $L_1$ and $L_2$. The longest escape times have been measured for initial conditions in the vicinity of the fractal basin boundaries. Furthermore, we computed the chaotic saddle for the system and its stable and unstable manifolds. The chaotic saddle is a fractal structure in phase space which has the form of a Cantor set and introduces chaos into the system.

astro-ph↗

arXiv2007

N-body Models of Rotating Globular Clusters

We have studied the dynamical evolution of rotating globular clusters with direct $N$-body models. Our initial models are rotating King models; we obtained results for both equal-mass systems and systems composed out of two mass components. Previous investigations using a Fokker-Planck solver have revealed that rotation has a noticeable influence on stellar systems like globular clusters, which evolve by two-body relaxation. In particular, it accelerates their dynamical evolution through the gravogyro instability. We have validated the occurence of the gravogyro instability with direct $N$-body models. In the case of systems composed out of two mass components, mass segregation takes place, which competes with the rotation in the acceleration of the core collapse. The "accelerating" effect of rotation has not been detected in our isolated two-mass $N$-body models. Last, but not least, we have looked at rotating $N$-body models in a tidal field within the tidal approximation. It turns out that rotation increases the escape rate significantly. A difference between retrograde and prograde rotating star clusters occurs with respect to the orbit of the star cluster around the Galaxy, which is due to the presence of a ``third integral'' and chaotic scattering, respectively.

astro-ph↗

arXiv2004

Dynamical friction in flattened systems: A numerical test of Binney's approach

We carry out a set of self-consistent N-body calculations to investigate how important the velocity anisotropy in non-spherical dark-matter haloes is for dynamical friction. For this purpose we allow satellite galaxies to orbit within flattened and live dark-matter haloes (DMHs) with aspect-ratio q_h=0.6 and compare the resulting orbit evolution with a semi-analytic code. This code solves the equation of motion of satellites orbiting within the same DMH and implements, according to the dynamical friction model, either Chandrasekhar's formula, which does not incorporate the velocity anisotropy, or Binney's description of dynamical friction in anisotropic systems. In the numerical and the two semi-analytic models the satellites are given different initial orbital inclinations and orbital eccentricities. We find that Binney's approach successfully describes the overall satellite decay and orbital inclination decrease for the whole set of orbits, with an averaged discrepancy of less than 4 per cent in orbital radius during the first 3 orbits. If Chandrasekhar's expression is used instead, the discrepancy increases to 20 per cent and no inclination decrease can be observed. Binney's treatment therefore appears to provide a significantly improved treatment of dynamical friction in anisotropic systems.

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arXiv2003

Stability and evolution of super-massive stars (SMS)

Highly condensed gaseous objects with masses larger than 5x10^4 M_sun are called super-massive stars. In the quasistationary contraction phase, the hydrostatic equilibrium is determined by radiation pressure and gravitation. The global structure is that of an n=3 polytrope at the stability limit. Small relativistic corrections for example can initiate a free fall collapse due to the 'post Newtonian' instability. Since the outcome of the final collapse -A super-massive black hole or hypernova- depends sensitively on the structure and the size of the object, when the instability sets in, it is important to investigate in more detail the contraction phase of the SMS. If the gaseous object is embedded in a dense stellar system, the central star cluster, the interaction and coupling of both components due to dynamical friction changes the energy balance and evolution of the SMS dramatically. Dynamical friction between stars and gas, which can be estimated semi-analytically (see Just et al. 1986), has three different effects on the two-component system. We discuss in which evolutionary stages and parameter range these interaction processes are relevant and how they can influence the stability and evolution of the SMS.

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arXiv2003

Super-massive stars: Radiative transfer

The concept of central super-massive stars (${\cal M} \ge 5 \times 10^4 M_{\odot}$, where ${\cal M}$ is the mass of the super-massive star) embedded in dense stellar systems was suggested as a possible explanation for high- energy emissions phenomena occurring in active galactic nuclei and quasars (Vilkoviski 1976, Hara 1978), such as X-ray emissions (Bahcall and Ostriker, 1975). SMSs and super-massive black holes are two possibilities to explain the nature of super-massive central objects, and super-massive stars may be an intermediate step towards the formation of super-massive black holes (Rees 1984). Therefore it is important to study such a dense gas-star system in detail. We address here the implementation of radiative transfer in a model which was presented in former work (Amaro-Seoane and Spurzem 2001, Amaro-Seoane et al. 2002). In this sense, we extend here and improve the work done by Langbein et al. (1990) by describing the radiative transfer in super-massive stars using previous work on this subject (Castor 1972).

astro-ph↗

arXiv2001

Super-massive stars: Dense star-gas systems

We use a gaseous model and a semi-analytical approach to study the evolution of a super-massive central gaseous object (a super-massive star, (SMS from now on) in an AGN and its evolution by interactions with the surrounding stellar system. Our future work in this field is outlined, which aims at a more detailed study of energy flows in the interstellar medium, stellar evolution and the relation between QSOs and galaxy formation.

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