Formation of Extrasolar Giant Planets: Core Accretion or Disk Instability?

Earth, Moon, and Planets -     

The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418

We present a study of the origin of infrared (IR) emission in the optically normal, infrared luminous galaxy NGC 4418. By decomposing the stellar absorption features and continua in the range of 3600-8000 A from the Sloan Digital Sky Survey into a set of simple stellar populations, we derive the stellar properties for the nuclear region of NGC 4418. We compare the observed infrared luminosity with the one derived from the starburst model, and find that star-forming activity contributes only 7% to the total IR emission, that as the IR emission region is spatially very compact, the most possible source for the greater part of the IR emission is a deeply embedded AGN, though an AGN component is found to be unnecessary for fitting the optical spectrum.     

The orientations of molecular clouds in the outer Galaxy: evidence for the scale of the turbulence driver?

Supernova (SN) explosions inject a considerable amount of energy into the interstellar medium (ISM) in regions with high-to-moderate star formation rates. In order to assess whether the driving of turbulence by supernovae is also important in the outer Galactic disc, where the star formation rates are lower, we study the spatial distribution of molecular cloud (MC) inclinations with respect to the Galactic plane. The latter contains important information on the nature of the mechanism of energy injection into the ISM. We analyse the spatial correlations between the position angles (PAs) of a selected sample of MCs (the largest clouds in the catalogue of the outer Galaxy published by Heyer et al). Our results show that when the PAs of the clouds are all mapped to values into the  [0°, 90°]  interval, there is a significant degree of spatial correlation between the PAs on spatial scales in the range of 100–800 pc. These scales are of the order of the sizes of individual SN shells in low-density environments such as those prevailing in the outer Galaxy and where the metallicity of the ambient gas is of the order of the solar value or smaller. These findings suggest that individual SN explosions, occurring in the outer regions of the Galaxy and in likewise spiral galaxies, albeit at lower rates, continue to play an important role in shaping the structure and dynamics of the ISM in those regions. The SN explosions we postulate here are likely associated with the existence of young stellar clusters in the far outer regions of the Galaxy and the ultraviolet emission and low levels of star formation observed with the Galaxy Evolution Explorer (GALEX) satellite in the outer regions of local galaxies.     

Determination of the Thickness of Non-Edge-on Disk Galaxies

We propose a method to determine the thickness of non-edge-on disk galaxies from their observed structure of spiral arms, based on the solution of the truly three-dimensional Poisson‘s equation for a logarithmic disturbance of density and under the condition where the self-consistency of the density wave theory is no longer valid. From their measured number of arms, pitch angle and location of the innermost point of the spiral arms, we derive and present the thicknesses of 34 spiral galaxies.     

The direct detection of non-baryonic dark matter in the Galaxy?

It has been argued in a number of recent papers that dark matter is in the form of Jupiter-mass primordial black holes that betray their presence by microlensing quasars. This lensing accounts for a number of characteristic properties of quasar light curves, in both single quasars and gravitationally lensed multiple systems, that are not explained on the basis of intrinsic variation. One prediction of this idea is that Jupiter-mass bodies will be detected by the MACHO experiment as short events of about 2 d duration, although the expected frequency of detection is still very hard to estimate. However, the recent report by the MACHO group of the detection of a Jupiter-mass body in the direction of the Galactic bulge is consistent with this prediction, and is possibly the first direct detection of non-baryonic matter in the Galaxy.     

Double globular clusters in the Galaxy?

Four pairs of visual double star clusters with the lowest mass-to-luminosity ratio have been selected among globular clusters of our Galaxy. By taking into accounts the effects of dynamical friction and compressive gravitational shocks, we conclude that the probability for the pairs to be gravitationally bound is very low.     

Possible Streams of the Globular Clusters in the Galaxy

1 INTRODUCTION The globular cluster (GC hereafter), as the oldest star group in the universe, has been a target that astro- physics has paid close attention to all the time. The near-field (Galaxy) cosmology makes contacts with the far-field cosmology by …     

Kinematics of the Open Cluster System in the Galaxy

Absolute proper motions and radial velocities of 202 open clusters in the solar neighborhood, which can be used as tracers of the Galactic disk, are used to investigate the kinematics of the Galaxy in the solar vicinity, including the mean heliocentric velocity components (u_1,u_2,u_3) of the open cluster system, the characteristic velocity dispersions (σ_1,σ_2,σ_3), Oort constants (A, B) and the large-scale radial motion parameters (C,D) of the Galaxy. The results derived from the observational data of proper motions and radial velocities of a subgroup of 117 thin disk young open clusters by means of a maximum likelihood algorithm are: (u_1,u2,u3) = (-16.1±1.0, -7.9±1.4,-10.4±1.5) kms~(-1), (σ_1,σ_2,σ_3) = (17.0 ±0.7,12.2±0.9,8.0±1.3) kms~(-1),(A,B) = (14.8±1.0,-13.0±2.7) kms~(-1) kpc~(-1) ,and(C,D) = (1.5±0.7,-1.2±1.5) kms~(-1) kpc~(-1) . A discussion on the results and comparisons with what was obtained by other authors is given.     

Are extrasolar oceans common throughout the Galaxy?

Light and cold extrasolar planets such as OGLE 2005‐BLG‐390Lb, a 5.5 Earth masses planet detected via microlensing, could be frequent in the Galaxy according to some preliminary results from microlensing experiments. These planets can be frozen rocky‐ or ocean‐planet, situated beyond the snow line and, therefore, beyond the habitable zone of their system. They can nonetheless host a layer of liquid water, heated by radiogenic energy, underneath an ice shell surface for billions of years, before freezing completely. These results suggest that oceans under ice, like those suspected to be present on icy moons in the Solar system, could be a common feature of cold low‐mass extrasolar planets. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistics of the Instability Strip of β Cephei Stars

We present a study of the β Cephei instability strip based on a sample of 49 stars of this type.After deriving their effective temperatures and luminosities from their observed(B-V),(U-B) colors and parallaxes we find their positions in the HR diagram to be mostly confined to the main sequence,and their masses to lie between 7 M and 30 M.Their distribution on the HR diagram matches well with our previous theoretical instability strip which has an upper bound in the luminosity and rather tight boundaries in the effective temperature.     

Origin of clouds of the lα forest

A model is proposed for the formation of clouds of the L_a forest. Earlier calculations have shown that the UV emission from hot stars must play an important role in the reionization of the pregalactic medium (PGM). The formation of galaxies therefore occurred simultaneously with PGM ionization, and the reionization process was nonlinear. With allowance for this fact, the Gunn-Peterson effect, and the fact that galaxies exist at z ≈ 5, which follows from observations, we can conclude that galaxies began to form earlier, perhaps at z ≥ 10. The observed presence of heavy elements in L_a -forest clouds is evidence that these clouds were formed later than galaxies — from interstellar clouds ejected by galactic wind. The next generation of galaxies might have resulted from an increase in the masses of L_a -forest clouds due to their merging. Translated from Astrofizika, Vol. 43, No. 1, pp. 5-12, January–March, 2000.     

On the Rd Edge of the δ Scuti Instability Strip

The δ Scuti star ctalogue is used to derive the observational locations of such stars on the HR diagram.The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory.The observational instability strip almost overlaps with the theoretical one,but the observed blue and red envelopes are hotter overlaps with the theoretical one,but the observed blue and red envelopes are hotter than the theoretical edges.The distribution of δ Scuti stars in the pulsation strip is not uniform.     

On the Red Edge of the δ Scuti Instability Strip

The S Scuti star catalogue is used to derive the observational locations of such stars on the HR diagram. The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory. The observational instability strip almost overlaps with the theoretical one, but the observed blue and red envelopes are hotter than the theoretical edges. The distribution of S Scuti stars in the pulsation strip is not uniform.     

On the βSPEC Values of Galaxy Clusters During the Merger

Time-dependent, X-ray emission properties of merging subclusters of galaxies are studied by N-body + Hydrodynamic simulations. We consider rather idealized cases, in which two virialized subclusters with the same or different masses collide with each other, eventually merging into one bigger cluster. We find that the various observable quantities, such as the cluster temperature, T_g, the line-of-sight velocity dispersion of galaxies, σ₁, and the specific energy ratio, β_spec ≡ μ m_p σ /k_B T_g (where μ is the mean molecular weight in amu, m_p is the proton mass and k_B is the Boltzmann constant) are distinct in merging clusters from those in isolated clusters. When merging, especially, in addition to having large velocity dispersions, σ_l ≳ 2000 km s^-1, clusters will have large deviations of β_spec from unity; that is, β_spec ≳ 2.0 (or β_spec ≲ 0.5) when seen from the direction parallel (perpendicular) to the collision axis. Temperature profiles are the best indicator of merging process. The calculated temperature maps of merging clusters are compared with those of the observed clusters, such as the Coma cluster, A754, and A401/399 supercluster. These simulated features are in good agreement with what ASCA revealed. Further, we present rather unique features in the temperature profiles that can be caused by a merger (or an encounter) with a subgroup of galaxies. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Monte Carlo Study of the Evolution of the Scale Height of Normal Pulsars in the Galaxy

Based on the undisturbed, finite thickness disk gravitational potential, we carried out 3-D Monte Carlo simulations of normal pulsars. We find that their scale height evolves in a similar way for different velocity dispersions (σv): it first increases linearly with time, reaches a peak, then gradually decreases, and finally approaches a stable asymptotic value. The initial velocity dispersion has a very large influence on the scale height. The time evolution of the scale height is studied. When the magnetic decay age is used as the time variable, the observed scale height has a similar trend as the simulated results in the linear stage, from which we derive velocity dispersions in the range 70- 178km s-1, which are near the statistical result of 90 - 270km s-1 for 92 pulsars with known transverse velocities. If the characteristic age is used as the time variable, then the observed and theoretical curves roughly agree for t > 108 yr only if σv < 25km s-1.     

Spectrophotometry of Jupiter in the Wavelength Range 320–1100 nm: Long-Term Observations of Variations over the Disk

Based on long-term spectrophotometric observations of Jupiter in the wavelength range 320–1100 nm, we investigate the variations of aerosol extinction (at 320–600 nm) and methane–ammonia absorption (at 600–1100 nm) over Jupiter's disk. We give estimates of the optical parameters for the upper cloud layer of the planet, the overlying stratospheric haze, and a Rayleigh atmosphere.     

Constructing a Stäckel Model of the Galaxy: Solving the Problem of Whether the Vertical Density Distribution Is Realistic

Astronomy Letters - Techniques for solving the problem of constructing a Stäckel model by generalizing the potential from the equatorial plane to the entire space are considered. The initial...     

Modelling the Global Solar Corona: III. Origin of the Hemispheric Pattern of Filaments

We consider the physical origin of the hemispheric pattern of filament chirality on the Sun. Our 3D simulations of the coronal field evolution over a period of six months, based on photospheric magnetic measurements, were previously shown to be highly successful at reproducing observed filament chiralities. In this paper we identify and describe the physical mechanisms responsible for this success. The key mechanisms are found to be (1) differential rotation of north – south polarity inversion lines, (2) the shape of bipolar active regions, and (3) evolution of skew over a period of many days. As on the real Sun, the hemispheric pattern in our simulations holds in a statistical sense. Exceptions arise naturally for filaments in certain locations relative to bipolar active regions or from interactions among a number of active regions.