In the last 30 days

• Open/close Robertson-Walker cosmological models with perfect fluid in general relativity

  Rishi Kumar Tiwari 2011 Res. Astron. Astrophys. 11 767 Tag this article

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  Einstein's field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for a Robertson-Walker universe by assuming the cosmological term to be proportional to R ^−m ( R is a scale factor and m is a constant). A variety of solutions is presented. The physical significance of the cosmological models has also been discussed.

• Open/close Observational constraints on the early dark energy model

  Lei Feng and Yu-Peng Yang 2011 Res. Astron. Astrophys. 11 751 Tag this article

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  Dark energy can be studied by its influence on the expansion of the Universe. We investigate current constraints on early dark energy (EDE) achievable by the combined observational data from type Ia supernovae (557), baryon acoustic oscillations, the current cosmic microwave background and the observed Hubble parameter. We find that combining these data sets provides powerful constraints on early dark energy and the best fit values of the parameters in 68% and 95% confidence-level regions are: Ω _m0 = 0.2897 ^{+0.0149+0.0207} _{−0.0138−0.0194}, Ω _e = 0.0129 ^{+0.0272+0.0381} _{−0.0129−0.0129}, w ₀ = −1.0415 ^{+0.0891+0.1182} _{−0.109−0.1604}, and h = 0.6988 ^{+0.0059+0.0083} _{−0.0058−0.0081}.

• Open/close Photometric study and preliminary elements of the low-mass ratio W UMa system ASAS 021209+2708.3

  Francesco Acerbi et al 2011 Res. Astron. Astrophys. 11 843 Tag this article

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  We present CCD B and V light curves, obtained in the year 2006, and a photometric solution of the low-mass ratio contact binary ASAS 021209+2708.3. With our data we were able to determine six new times of minimum light and refine the orbital period of the system to 0.3181963 days. The light curves are analyzed using the 2003 version of the Wilson-Devinney program and the analysis was performed with and without adding a spot on the surface of one star because the light curves appear to exhibit a typical O'Connell effect, with Maximum I brighter than Maximum II. The results show that ASAS 021209+2708.3 may be classified as an A-subtype W Ursae Majoris system with a small mass ratio q = 0.1889, a large over-contact degree of f = 0.587, a very small difference between the component temperatures of Δ T = 53 K and an orbital inclination of i = 81°. It is known that deep ( f > 50%), low-mass ratio ( q < 0.25) overcontact binary stars are a very important resource for understanding the phenomena of Blue Straggler/FK Com-type stars. The formations of Blue Straggler stars and FK Com-type stars are unsolved problems in stellar astrophysics. One of the possible explanations for their formation is from the coalescence of W UMa-type overcontact binary systems. The absolute dimensions of ASAS 021209+2708.3 are estimated and its dynamical evolution is inferred.

• Open/close ¹²CO, ¹³CO and C¹⁸O observations along the major axes of nearby bright infrared galaxies

  Qing-Hua Tan et al 2011 Res. Astron. Astrophys. 11 787 Tag this article

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  We present simultaneous observations of ¹²CO, ¹³CO and C ¹⁸O J = 1-0 emission in 11 nearby ( cz < 1000km s ⁻¹) bright infrared galaxies. Both ¹²CO and ¹³CO are detected in the centers of all the galaxies, except for ¹³CO in NGC 3031. We have also detected C ¹⁸O, CS J = 2−1 and HCO ⁺ J = 1−0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of ¹²CO and its isotopes from the PMO 14m telescope. We have conducted half-beam-spaced mapping of M82 over an area of 4' × 2.5' and major axis mapping of NGC 3627, NGC 3628, NGC 4631 and M51. The radial distributions of ¹²CO and ¹³ CO in NGC 3627, NGC 3628 and M51 can be well fitted by an exponential profile. The ¹²CO/ ¹³CO intensity ratio, , decreases monotonically with the galactocentric radius in all mapped sources. The average in the center and disk of the galaxies are 9.9±3.0 and 5.6±1.9, respectively, much lower than the peculiar (~24) found in the center of M82. The intensity ratios of ¹³CO/C ¹⁸O, ¹³CO/HCO ⁺ and ¹³CO/CS (either our or literature data) show little variation with galactocentric radius, in sharp contrast with the greatly varied . This supports the notion that the observed gradient in could be the result of the variations of the physical conditions across the disks. The H ₂ column density derived from C ¹⁸O shows that the Galactic standard conversion factor ( X-factor) overestimates the amount of the molecular gas in M82 by a factor of ~2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks and the gradient in can be used to trace the variations of the X-factor.

• Open/close Mars geodesy, rotation and gravity

  Pascal Rosenblatt and Veronique Dehant 2010 Res. Astron. Astrophys. 10 713 Tag this article

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  This review provides explanations of how geodesy, rotation and gravity can be addressed using radioscience data of an orbiter around a planet or of the lander on its surface. The planet Mars is the center of the discussion. The information one can get from orbitography and radioscience in general concerns the global static gravitational field, the time variation of the gravitational field induced by mass exchange between the atmosphere and the ice caps, the time variation of the gravitational field induced by the tides, the secular changes in the spacecraft's orbit induced by the little moons of Mars named Phobos and Deimos, the gravity induced by particular targets, the Martian ephemerides, and Mars' rotation and orientation. The paper addresses as well the determination of the geophysical parameters of Mars and, in particular, the state of Mars' core and its size, which is important for understanding the planet's evolution. Indeed, the state and dimension of the core determined from the moment of inertia and nutation depend in turn on the percentage of light elements in the core as well as on the core temperature, which is related to heat transport in the mantle. For example, the radius of the core has implications for possible mantle convection scenarios and, in particular, for the presence of a perovskite phase transition at the bottom of the mantle. This is also important for our understanding of the large volcanic province Tharsis on the surface of Mars.

• Open/close Combining optical and X-ray observations of galaxy clusters to constrain cosmological parameters

  Heng Yu and Zong-Hong Zhu 2011 Res. Astron. Astrophys. 11 776 Tag this article

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  Galaxy clusters present unique advantages for cosmological study. Here we collect a new sample of 10 lensing galaxy clusters with X-ray observations to constrain cosmological parameters. The redshifts of the lensing clusters lie between 0.1 and 0.6, and the redshift range of their arcs is from 0.4 to 4.9. These clusters are selected carefully from strong gravitational lensing systems which have both X-ray satellite observations and optical giant luminous arcs with known redshifts. Giant arcs usually appear in the central region of clusters, where mass can be traced with luminosity quite well. Based on gravitational lensing theory and a cluster mass distribution model, we can derive a ratio using two angular diameter distances. One is the distance between lensing sources and the other is that between the deflector and the source. Since angular diameter distance relies heavily on cosmological geometry, we can use these ratios to constrain cosmological models. Moreover, X-ray gas fractions of galaxy clusters can also be a cosmological probe. Because there are a dozen parameters to be fitted, we introduce a new analytic algorithm, Powell's UOBYQA (Unconstrained Optimization By Quadratic Approximation), to accelerate our calculation. Our result demonstrates that this algorithm is an effective fitting method for such a continuous multi-parameter constraint. We find an interesting fact that these two approaches are separately sensitive to Ω _Λ and Ω _M. By combining them, we can get reasonable fitting values of basic cosmological parameters: Ω _M = 0.26 ^+0.04 _−0.04, and Ω _Λ = 0.82 ^+0.14 _−0.16.

• Open/close Computation of the structure of a magnetized strange quark star

  Gholam Hossein Bordbar and Ali Reza Peivand 2011 Res. Astron. Astrophys. 11 851 Tag this article

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  We have calculated some properties of spin polarized strange quark matter (SQM) in a strong magnetic field at zero temperature using the MIT bag model. We showed that the equation of state of spin polarized SQM is stiffer than that for un-polarized cases. We have also computed the structural properties of a spin polarized strange quark star (SQS) and found that the presence of a magnetic field leads to a more stable SQS when compared to the structural properties of an unpolarized SQS.

• Open/close New applications of the H-reversal trajectory using solar sails

  Xiang-Yuan Zeng et al 2011 Res. Astron. Astrophys. 11 863 Tag this article

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  Advanced solar sailing has been an increasingly attractive propulsion system for highly non-Keplerian orbits. Three new applications of the orbital angular momentum reversal (H-reversal) trajectories using solar sails are presented: space observation, heliocentric orbit transfer and collision orbits with asteroids. A theoretical proof for the existence of double H-reversal trajectories (referred to as ‘H2RTs’) is given, and the characteristics of the H2RTs are introduced before a discussion of the mission applications. A new family of H2RTs was obtained using a 3D dynamic model of the two-body frame. In a time-optimal control model, the minimum period H2RTs both inside and outside the ecliptic plane were examined using an ideal solar sail. Due to the quasi-heliostationary property at its two symmetrical aphelia, the H2RTs were deemed suitable for space observation. For the second application, the heliocentric transfer orbit was able to function as the time-optimal H-reversal trajectory, since its perihelion velocity is a circular or elliptic velocity. Such a transfer orbit can place the sailcraft into a clockwise orbit in the ecliptic plane, with a high inclination or displacement above or below the Sun. The third application of the H-reversal trajectory was simulated impacting an asteroid passing near Earth in a head-on collision. The collision point can be designed through selecting different perihelia or different launch windows. Sample orbits of each application were presented through numerical simulation. The results can serve as a reference for theoretical research and engineering design.

• Open/close Dark halos acting as chaos controllers in asymmetric triaxial galaxy models

  Nicolaos D. Caranicolas and Euaggelos E. Zotos 2011 Res. Astron. Astrophys. 11 811 Tag this article

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  We study the regular or chaotic character of orbits in a 3D dynamical model, describing a triaxial galaxy surrounded by a spherical dark halo component. Our numerical experiments suggest that the percentage of chaotic orbits decreases exponentially as the mass of the dark halo increases. A linear increase of the percentage of the chaotic orbits was observed as the scale length of the halo component increases. In order to distinguish between regular and chaotic motion, we chose to use the total angular momentum L _tot of the 3D orbits as a new indicator. Comparison with other, previously used, dynamical indicators, such as the Lyapunov Characteristic Exponent or the P( f) spectral method, shows that the L _tot indicator gives very fast and reliable results for characterizing the nature of orbits in galactic dynamical models.

• Open/close Possible distance indicators in gamma-ray pulsars

  Wei Wang 2011 Res. Astron. Astrophys. 11 824 Tag this article

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  Distance measurement of gamma-ray pulsars is a current challenge in pulsar studies. The Large Area Telescope (LAT) aboard the Fermi gamma-ray observatory discovered more than 70 gamma-ray pulsars including 24 new gamma-selected pulsars with almost no distance information. We study the relation between gamma-ray emission efficiency (η = L _γ/ Ė) and pulsar parameters for young radio-selected gamma-ray pulsars with known distance information in the first gamma-ray pulsar catalog reported by Fermi/LAT We have introduced three generation-order parameters to describe the gamma-ray emission properties of pulsars, and find a strong correlation of η − ζ ₃, a generation-order parameter which reflects γ-ray photon generation in the pair cascade processes induced by magnetic field absorption in a pulsar's magnetosphere. A good correlation of η − B _LC, the magnetic field at the light cylinder radius, is also found. These correlations are the distance indicators in gamma-ray pulsars used to evaluate distances for gamma-selected pulsars. Distances of 25 gamma-selected pulsars are estimated, which could be tested by other distance measurement methods. The physical origin of the correlations may also be interesting for pulsar studies.