New magnetic field measurements of β Cephei stars and slowly pulsating B stars

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B‐type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B‐type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three β Cephei stars and two stars suspected to be β Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3σ has been diagnosed in two normal B‐type stars, the nitrogen‐rich early B‐type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of β Cephei stars have detected magnetic fields: Out of 13 β Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected β Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non‐magnetic pulsating stars in the H‐R diagram. We find that their domains in the H‐R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i ‐values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Non-minimal coupling of the phantom field and cosmic acceleration

Motivated by the recent interest in phantom fields as candidates for the dark energy component, we investigate the consequences of the phantom field when is minimally coupled to gravity. In particular, the necessary (but insufficient) conditions for the acceleration and superacceleration of the universe are obtained when the non-minimal coupling term is taken into account. Furthermore, the necessary condition for the cosmic acceleration is derived when the phantom field is non-minimally coupled to gravity and baryonic matter is included.     

The Chemical Reactivity of Lunar Dust: From Toxicity to Astrobiology

The chemical reactivity of lunar dust is an important topic of inquiry, of fundamental scientific value and of practical relevance to human exploration of the Moon. Lunar specimens brought back to Earth by the Apollo astronauts provide a key resource for ground-based studies which help to define the initial avenues of inquiry. Even among the limited samples obtained from equatorial exploration sites, however, chemical reactivity analyses indicates that lunar dust is heterogeneous, a finding that parallels heterogeneity revealed by remote sensing studies. The region-to-region variability of lunar dust argues that a full understanding of its chemical reactivity will require in situ analysis, on a region-to-region basis. The data from such investigations will help to shape our understanding of the potential for lunar dust toxicity, and will provide insight into the types of reactions that may occur with when lunar dust interacts with organic molecules on the surface of the Moon.     

Upper estimates for the element number of non-redundant antenna configurations on square and hexagonal grids

Estimates for the maximum number of elements of non-redundant configurations on integer square and hexagonal grids of given sizes are derived (a “non-redundant” configuration implies that the vector differences between its elements are all distinct). When projecting a large 2-D interferometer or a telescope, such an estimate can be used as a guide for evaluating the maximum possible number of antennas in a non-redundant configuration that can be arranged within a given area. The suggested estimates are empirical and based on the available data. They are obtained by reducing the problem to the linear case and by generalizing the method applied therein. Examples of applying the method are presented.     

Accretion magnetar in the close binary system 4U 2206+54

The magneto-rotational evolution of a neutron star in the massive binary system 4U 2206+54 is discussed in light of the recent discovery of its 5555 s rotational period and its average rate of spin-down. We show that this behavior of the neutron star means that its magnetic field exceeds the quantum mechanical critical limit and it is an accretion magnetar. The system’s evolution is explained by wind driven mass transfer without formation of an accretion disk. The constant character of the x-ray source indicates a steady rate of accretion and raises anew the question of the stability of the boundary of the magnetosphere of a star undergoing spherical accretion. A solution to this problem is also a key to determining the mechanism for the slowing down of the star’s rotation.     

Instability of LBV stars against radial oscillations

Hydrodynamic calculations of nonlinear radial oscillations of LBV stars with effective temperatures 1.5 × 10⁴ K ⩽ T _eff ⩽ 3 × 10⁴ K and luminosities 1.2 × 10⁶ L _⊙ ⩽ L ⩽ 1.9 × 10⁶ L _⊙ have been performed. Models for the evolutionary sequences of Population I stars (X = 0.7, Z = 0.02) with initial masses 70M _⊙ ⩽ M _ZAMS ⩽ 90M _⊙ at the initial helium burning stage have been used as the initial conditions. The radial oscillations develop on a dynamical time scale and are nonlinear traveling waves propagating from the core boundary to the stellar surface. The amplitude of the velocity variations for the outer layers is several hundred km s⁻¹, while the bolometric magnitude variations are within ΔM _bol ⩽ 0_· ^m 2. The onset of oscillations is not related to the κ-mechanism and is attributable to the instability of a self-gravitating envelope gas whose adiabatic index is close to its critical value of Γ₁ = 4/3 due to the dominant contribution of radiation in the internal energy and pressure. The interval of magnitude variation periods (6 days ≤ II ≤ 31 days) encompasses all currently available estimates of the microvariability periods for LBV stars, suggesting that this type of nonstationarity is pulsational in origin.     

On the Analysis of the Complex Forbush Decreases of January 2005

In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005 using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized both by significant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable series of Forbush effects (FEs) from 17 January 2005 to 20 January 2005, the Forbush decrease (FD) on 21 January 2005 and the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing on the aforementioned FE cases, with special attention drawn on the 21 January 2005, FD event, which demonstrated several exceptional features testifying its uniqueness. Data from the ACE spacecraft, together with GOES X-ray recordings and LASCO CME coronagraph images were used in conjunction to the ground based recordings of the Worldwide Neutron Monitor Network, the interplanetary data of OMNI database and the geomagnetic activity manifestations denoted by K _p and D _st indices. More than that, cosmic ray characteristics as density, anisotropy and density gradients were also calculated. The results illustrate the state of the interplanetary space that cosmic rays crossed and their corresponding modulation with respect to the multiple extreme solar events of this period. In addition, the western location of the 21 January 2005 solar source indicates a new cosmic ray feature, which connects the position of the solar source to the cosmic ray anisotropy variations. In the future, this feature could serve as an indicator of the solar source and can prove to be a valuable asset, especially when satellite data are unavailable.     

Detection of a star escaping from the Orion Trapezium

During studies of spectra of the star Θ¹ Ori C with high photometric and spectral resolution we have discovered a system of spectral lines indicating a large negative radial velocity (RV). We designate this star as Θ¹ Ori C3.     

Three-dimensional reddening map for stars from 2MASS photometry: The method and the first results

The first results of the construction of a three-dimensional reddening map for stars within 1600 pc of the Sun are presented. Analysis of the distribution of 70 million stars from the 2MASS catalog with the most accurate photometry on the (J-Ks)-Ks diagram supplemented with Monte Carlo simulations has shown that one of the maxima of this distribution corresponds to F-type dwarfs and subgiants with a mean absolute magnitude M _Ks = 2_⊙ ^m 5. The shift of this maximum toward large J-Ks with increasing Ks reflects the reddening of these stars with increasing heliocentric distance. The distribution of the sample of stars over Ks, l, and b cells with a statistically significant number of stars in each cell corresponds to their distribution over three-dimensional spatial cells. As a result, the reddening E(J-Ks) has been determined with an accuracy of 0_· ^m 03 for spatial cells with a side of 100 pc. All of the known large absorbing clouds within 1600 pc of the Sun have manifested themselves in the results obtained. The distances to the near and far edges of the clouds have been determined with a relative accuracy of 15%. The cases where unknown clouds are hidden behind known ones on the same line of sight have been found. The distance dependence of reddening is considered for various Galactic latitudes and longitudes. The absorbing matter of the Gould Belt is shown to manifest itself at latitudes up to 40° and within 600 pc of the Sun. The size and influence of the Gould Belt may have been underestimated thus far. The absorbing matter at latitudes up to 60° and within 1600 pc of the Sun has been found to be distributed predominantly in the first and second quadrants in the southern hemisphere and in the third and fourth quadrants in the northern hemisphere. The warping of the absorbing layer in the near Galaxy apparently manifests itself in this way. A nonrandom orientation of the clouds relative to the Sun is possible. The mass of the baryonic dark matter in solar neighborhoods can then be considerably larger than is generally believed.     

A real-time software backend for the GMRT

The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components, is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection. Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate sized array like the GMRT.