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At least 343 records · Page 19Linked to original sources
arXiv2008

Chaotic behavior in the accretion disk

The eccentric luminosity variation of quasars is still a mystery. Analytic results of this behavior ranged from multi-periodic behavior to a purely random process. Recently, we have used nonlinear time-series analysis to analyze the light curve of 3C 273 and found its eccentric behavior may be chaos [L. Liu, Chin. J. Astron. Astrophys. \textbf{6}, 663 (2006)]. This result induces us to look for some nonlinear mechanism to explain the eccentric luminosity variation. In this paper, we propose a simple non-linear accretion disk model and find it shows a kind of chaotic behavior under some circumstances. Then we compute the outburst energy $\triangle F$, defined as the difference of the maximum luminosity and the minimum luminosity, and the mean luminosity $ $. We find that $\triangle F\sim < F >^α$ in the chaotic domain, where $α\approx 1$. In this domain, we also find that $ \sim M^{0.5}$, where $M$ is the mass of central black hole. These results are confirmed by or compatible with some results from the observational data analysis [A. J. Pica and A. G. Smith, Astrophys. J. \textbf{272}, 11 (1983); M. Wold, M. S. Brotherton and Z. Shang, Mon. Not. R. Astron. Soc. \textbf{375}, 989 (2007)].

astro-ph↗
arXiv2007

Macroscopic and Microscopic Paradigms for the Torsion Field: from the Test-Particle Motion to a Lorentz Gauge Theory

Torsion represents the most natural extension of General Relativity and it attracted interest over the years in view of its link with fundamental properties of particle motion. The bulk of the approaches concerning the torsion dynamics focus their attention on their geometrical nature and they are naturally led to formulate a non-propagating theory. Here we review two different paradigms to describe the role of the torsion field, as far as a propagating feature of the resulting dynamics is concerned. However, these two proposals deal with different pictures, i.e., a macroscopic approach, based on the construction of suitable potentials for the torsion field, and a microscopic approach, which relies on the identification of torsion with the gauge field associated with the local Lorentz symmetry. We analyze in some detail both points of view and their implications on the coupling between torsion and matter will be investigated. In particular, in the macroscopic case, we analyze the test-particle motion to fix the physical trajectory, while, in the microscopic approach, a natural coupling between torsion and the spin momentum of matter fields arises.

gr-qc↗
arXiv2007

Search for a Dark Matter annihilation signal from the Sagittarius dwarf galaxy with H.E.S.S

Dwarf Spheroidal galaxies are amongst the best targets to search for a Dark Matter annihilation signal. The annihilation of WIMPs in the center of Sagittarius dwarf spheroidal (Sgr dSph) galaxy would produce high energy gamma-rays in the final state. Observations carried out with the H.E.S.S. array of Imaging Atmospheric Cherenkov telescopes are presented. A careful modelling of the Dark Matter halo profile of Sgr dwarf was performed using latest measurements on its structural parameters. Constraints on the velocity-weighted cross section of Dark Matter particles are derived in the framework of Supersymmetric and Kaluza-Klein models.

astro-ph↗
arXiv2009

The Age Distributions of Clusters and Field Stars in the Small Magellanic Cloud -- Implications for Star Formation Histories

Differences between the inferred star formation histories (SFHs) of star clusters and field stars seem to suggest distinct star formation processes for the two. The Small Magellanic Cloud (SMC) is an example of a galaxy where such a discrepancy is observed. We model the observed age distributions of the SMC clusters and field stars using a new population synthesis code, SPACE, that includes stellar evolution, infant mortality and cluster dissolution. We find that the two observed age distributions can be explained by a single SFH, thus eliminating the need to assume two separate mechanisms for star formation.

astro-ph↗
arXiv2007

A deep XMM-Newton survey of M 31

The deep homogeneous survey of the large Local-Group spiral galaxy M 31 is a milestone project for X-ray astronomy, as it allows a detailed X-ray inventory of an archetypal low-star-formation-rate galaxy like our own. We present first results of the deep XMM-Newton survey, which covers the entire D 25 ellipse. Information from different X-ray energy bands are combined in an X-ray colour image of M 31. In the first 15 observations we found about 1000 sources, the full survey will yield about 2000 X-ray sources. Sources will be classified using hardness ratios, extent, high quality spectra and time variability. In addition the sources will be correlated with catalogues in optical, infra-red and radio wavelengths. Our goal is to study M 31 X-ray binaries and globular cluster sources, supersoft sources, supernova remnants and the hot interstellar medium and separate them from foreground stars and background objects.

astro-ph↗
arXiv2007

An S-brane solution with acceleration and small enough variation of G

An S-brane solution with two non-composite electric branes and a set of l scalar fields is considered. The intersection rule for branes corresponds to the Lie algebra A_2. The solution contains five factor spaces with the fifth one interpreted as ``our'' 3-dimensional space. It is shown that there exists a time interval where accelerating expansion of ``our'' 3-dimensional space is compatible with small enough value of effective gravitational ``constant'' variation.

gr-qc↗
arXiv2007

A small cosmological constant from the modified Brans-Dicke theory - an interplay between different energy scales

In this paper we discuss a model in which the energy density, corresponding to the effective cosmological constant, after the $SU(2)\times U(1)$ symmetry breaking appears to be of the desired order of $10^{-48}÷10^{-47} GeV^{4}$. The model contain two different energy scales, one of which is associated with the Higgs's vacuum expectation value. Another scale is of the order of $10^{21}GeV$ and defines the vacuum expectation value of the Brans-Dicke scalar field, non-minimally coupled to gravity, and sets the value of the Planck mass. Other (dimensionless) parameters are assumed not to contain hierarchical differences. The model is devoid of any fine-tuning and gives a small value of the effective cosmological constant even if the real "bare" cosmological constant is quite large.

gr-qc↗
arXiv2008

Stellar Collisions in Young Clusters: Formation of (Very) Massive Stars?

In young star clusters, the density can be high enough and the velocity dispersion low enough for stars to collide and merge with a significant probability. This has been suggested as a possible way to build up the high-mass portion of the stellar mass function and as a mechanism leading to the formation of one or two very massive stars (M > 150 Msun) through a collisional runaway. I quickly review the standard theory of stellar collisions, covering both the stellar dynamics of dense clusters and the hydrodynamics of encounters between stars. The conditions for collisions to take place at a significant rate are relatively well understood for idealised spherical cluster models without initial mass segregation, devoid of gas and composed of main-sequence (MS) stars. In this simplified situation, 2-body relaxation drives core collapse through mass segregation and a collisional phase ensues if the core collapse time is shorter than the MS lifetime of the most massive stars initially present. The outcome of this phase is still highly uncertain. A more realistic situation is that of a cluster still containing large amounts of interstellar gas from which stars are accreting. As stellar masses increase, the central regions of the cluster contracts. This little-explored mechanism can potentially lead to very high stellar densities but it is likely that, except for very rich systems, the contraction is halted by few-body interactions before collisions set in. A complete picture, combining both scenarios, will need to address many uncertainties, including the role of cluster sub-structure, the dynamical effect of interstellar gas, non-MS stars and the structure and evolution of merged stars.

astro-ph↗
arXiv2007

Radial Dependence of Extinction in Parent Galaxies of Supernovae

The problem of extinction is the most important issue to be dealt with in the process of obtaining true absolute magnitudes of core-collapse (including stripped-envelope) supernovae (SNe). The plane-parallel model, widely used in the past, was shown not to describe extinction adequately. We try to apply an alternative model which introduces radial dependance of extinction in parent galaxies of supernovae. For calculating extinction in our Galaxy we use two different methods and compare the results obtained. Our analysis is primarily focused on a chosen sample of stripped-envelope SNe (Ib/c) for which we find intrinsic peak absolute magnitude $\mathrm{M}_{\mathrm{B}}^{0}=-17.80\pm 0.43$.

astro-ph↗
arXiv2007

Fundamental properties of low-mass stars

Numerous investigations on the fundamental properties of low-mass stars using eclipsing binaries indicate a strong discrepancy between theory and observations that is still awaiting explanation. Current models seem to predict radii for stars less massive than the Sun that are some 10% smaller than observed, while their effective temperatures are some 5% larger. Here we discuss recent new observational data that are relevant to this issue and review the progress made in understanding the origin of the important differences with theoretical calculations. Notably, we provide evidence that stellar activity may be responsible for the mismatch between observations and theory through two different channels: inhibition of convection or effects of a significant starspot coverage. The activity hypothesis is put to a test with observational diagnostics and some of the consequences of the large starspot coverage are evaluated. We conclude that stellar activity likely plays a key role in defining the properties of active low-mass stars and that this should be properly taken into account when investigating young, active stars in clusters or star-forming regions.

astro-ph↗
arXiv2007

The importance of the remnant's mass for VLTP born again times. Implications for V4334 Sgr and V605 Aql

We present numerical simulations of the very late thermal pulse (VLTP) scenario for a wide range of remnant masses. We show that, by taking into account the different possible remnant masses, the fast outburst evolution of V4334 Sgr (a.k.a. Sakurai's Object) and V605 Aql can be reproduced within standard 1D stellar evolution models. A dichotomy in the born again timescales is found, with lower mass remnants evolving in a few years and higher mass remnants (M $\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}$ 0.6 M$_{\odot}$) failing to expand due to the H-flash and, as a consequence, evolving in timescales typical of He-shell flash driven born agains ($\sim 100$ yr).

astro-ph↗
arXiv2008

Kinematic and Chemical constraints on the formation of M31's inner and outer halo

The halo of M31 shows a wealth of substructures that are consistent with satellite accretion. Here we report on kinematic and abundance results from Keck/DEIMOS spectroscopy in the calcium triplet region of over 3500 red giant star candidates along the minor axis and in off-axis spheroid fields of M31. Our data reach out to large radial distances of 160 kpc. The derived velocity distributions show a kinematically cold substructure at 17 kpc that has been reported before. We devise an improved method to measure accurate metallicities from the calcium triplet in low signal-to-noise spectra using a coaddition of the individual lines. The resulting distribution leads us to note an even stronger gradient in the abundance distribution along M31's minor axis than previously detected. The mean metallicity in the outer halo reaches below -2 dex, with individual values as low as -2.6 dex. In the inner spheroid, at 17-19 kpc, we find a sharp decline of ~0.5 dex in metallicity, which roughly coincides with the edge of an extended disk, previously detected from star count maps. A large fraction of red giants in the most distant fields are likely members of M33's overlapping halo. A comparison of our velocities with those predicted by new N-body simulations argues that the event responsible for the giant Stream is most likely not responsible for the full population of the inner halo; we show further that the abundance distribution of the Stream is different from that of the inner halo, from which it becomes evident that the merger event that formed the outer halo cannot have contributed any significant material to the inner spheroid. All this evidence of severe structure changes in the halo suggests a high degree of infall and stochastic abundance accretion governing the build-up of M31's halo. (Abridged)

astro-ph↗
arXiv2007

The RMS Survey: A Galaxy-wide Sample of Massive Young Stellar Objects

Here we describe the Red MSX Source (RMS) survey which is the largest, systematic, galaxy-wide search for massive young stellar objects (MYSOs) yet undertaken. Mid-IR bright point sources from the MSX satellite survey have been followed-up with ground-based radio, millimetre, and infrared observations to identify the contaminating sources and characterise the MYSOs and UCHII regions. With the initial classification now complete the distribution of sources in the galaxy will be discussed, as well as some programmes being developed to exploit our sample.

astro-ph↗
arXiv2007

A Kerr interior

An exact solution of the Einstein field equations is proposed which represents a differentially rotating fluid. As this solution matches the exterior Kerr solution and reduces to the Schwarzschild interior solution by setting the rotational parameter to zero, it could serve as Kerr interior.

astro-ph↗
arXiv2007

Kerr interior surfaces

A recently found interior for the Kerr metric is re-investigated by means of geometrical methods. A surface with nonholonomicity is matched to the surface of the exterior solution.

astro-ph↗
arXiv2007

EeV neutrinos associated with UHECR sources

Electromagnetic energy losses of charged pions and muons suppress the expected high energy, >1E18 eV, neutrino emission from sources of ultrahigh energy, >1E19 eV, cosmic-rays. We show here that >1E19 eV photons produced in such sources by neutral pion decay may escape the sources, thanks to the Klein-Nishina suppression of the pair production cross section, and produce muon pairs in interactions with the cosmic microwave background. The flux of muon decay neutrinos, which are expected to be associated in time and direction with the electromagnetic emission from the sources, may reach a few percent of the Waxman-Bahcall bound. Their detection may allow one to directly identify the sources of >1E19 eV cosmic-rays, and will provide the most stringent constraints on quantum-gravity-induced Lorentz violation.

hep-ph↗
arXiv2007

Observational approaches to understanding dark energy

Illuminating the nature of dark energy is one of the most important challenges in cosmology today. In this review I discuss several promising observational approaches to understanding dark energy, in the context of the recommendations by the U.S. Dark Energy Task Force and the ESA-ESO Working Group on Fundamental Cosmology.

astro-ph↗