We examine statistical methods for calibrating trigonometric parallaxes to retrieve the absolute magnitudes of stars, using Monte Carlo simulations. Here we consider the case of the zero-point of the period-luminosity relation for Cepheid variables. The method originally proposed by Ratnatunga & Casertano was revisited by introducing a realistic density distribution of sample stars belonging to the catalogue through prior calculations of the photometric distance for each star. It is found that our method gives an unbiased estimate, regardless of any dispersions in their absolute magnitude. We further investigate the reliability of results which depend on the accuracy of parallax. Our finding is that the accuracy ($\sim$1 mas) of Hipparcos parallaxes is not enough to obtain a reliable result due to a large variation among different ensembles of stars. More precise determination of parallaxes to an accuracy of 200 $\mu$as at least, which will be easily realized by the ongoing astrometric space satellites, will give a precise zero-point together with a dispersion in absolute magnitude.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The majority of flare activity arises in active regions which contain sunspots, while CME activity can also originate from decaying active regions and even so-called quiet solar regions which contain a filament. Two classes of CME, namely flare-related CME events and CMEs associated with filament eruption are well reflected in the evolution of active regions, flare related CMEs mainly occur in young active regions containing sunspots and as the magnetic flux of active region is getting dispersed, the filament-eruption related CMEs will become dominant. This is confirmed by statistical analyses.

All the CMEs are, nevertheless, caused by loss of equilibrium of the magnetic structure. With observational examples we show that the association of CME, flare and filament eruption depends on the characteristics of the source regions: (i) the strength of the magnetic field, the amount of possible free energy storage, (ii) the small- and large-scale magnetic topology of the source region as well as its evolution (new flux emergence, photospheric motions, canceling flux), and (iii) the mass loading of the configuration (effect of gravity). These examples are discussed in the framework of theoretical models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Important results of magnetic CP star research in the last decade from photometric and spectroscopic observations are discussed. Hipparcos parallaxes confirm CP stars are Main Sequence stars. Photometric monitoring of the rapidly rotating stars provides evidences for the rotational braking on the Main Sequence. High signal-to-noise spectra with high resolution and time-resolution give strong support for chemical separation processes operating in stellar atmospheres (abundance stratification). There are also observational evidences for the departure of the temperature structure of cool CP star atmospheres from that for normal stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We collected X-ray and optical images for a X-ray flux-limited, complete sample of 93 active galactic nuclei (AGNs) at z<0.1, selected from the ROSAT Bright Survey. By modeling the host galaxy and the central AGN, we decompose the optical emission into nuclear, bulge and disk components. We cross-correlated our sample with the NVSS and FIRST radio catalogs, and, due to their limited sky coverage, 53 matches were found. We find that the nuclear optical luminosity, thought to be associated with the accretion disk surrounding the active black hole, correlates with the X-ray luminosity, the radio luminosity, and the black hole mass.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The orbital motion of comets is difficult to characterize accurately due to the rocket-like outgassing of material from the cometary nucleus. The resulting nongravitational accelerations often appear to be fundamentally stochastic in nature and thus pose severe modeling challenges in orbit determination, especially when the comet has been observed for many revolutions. Even so, new techniques have arisen in recent years that give new insight, not only into the motion of the comets, but also into their physical characteristics and spin states. These approaches include modeling of spin axis precession over many decades and the consideration of the seasonal variation in the thrust from discrete jets acting on a rotating nucleus. Such advances have been enabled, in part, by the increasing efforts and capabilities of comet observers worldwide as more and more comets with longer and longer observing arcs become available for study. In this review we specifically consider the application of the Rotating Jet Model to several space mission targets, indicating how this model can often be used to infer the orientation of a comet's spin axis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We are using VIMOS to conduct a wide-field spectroscopic survey covering fields of $0.5 \times 0.5\,\mathrm{deg^2}$ around the X-ray luminous clusters of galaxies Abell 1689 ($z=0.185$) and Abell 1835 ($z=0.25$). Here we describe the observations and first results on the redshift distribution of subsamples of cluster galaxies to $R\simeq22$ for which we at present have obtained secure redshifts. These subsamples constitute $\sim 40\%$ of the total spectroscopic sample and contain 525 and 630 cluster members in Abell 1689 and Abell 1835, respectively, placing them amongst the largest redshift samples available for any cluster of galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present $VR$ magnification ratios of the double quasar Q0957+561AB, which are inferred from frames taken with the 2.56m Nordic Optical Telescope. From two different photometric techniques ($pho2comC$ and $psfphot$) and a reasonable range for the time delay in the system (415–430 days), we find optical continuum ratios depending on the wavelength. These chromatic ratios are consistent with either differential extinction in a dust system or gravitational microlensing in the deflector. Although the dusty scenario is only viable for a compact dust cloud in the line of sight to the A component, the possible values for the differential extinction and the ratio of total to selective extinction in the $V$ band are surprisingly reasonable. To decide on the true origin of the anomalous ratios (extinction, microlensing or a mixed scenario), we are carrying out new monitoring campaigns and planning detailed observations from the best ground–based and space telescopes.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Solar magnetic field polarity reversal (SMFPR) happens every 10-11 years. However untill now there is no clear understanding in what way this process is carried out. Results represented here are a by-product of the coronal hole study based on taken from Web-archive XUV corona images (Yohkoh and EIT/SOHO), white-light corona pictures(SOHO/LASCO) and Kitt Peak coronal hole (CH) 1083 nm maps. Observations used cover the minimum, maximum and a part of declining phase of 23-d cycle (1996-2004).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Compact groups of galaxies recently have been discovered in association with several strong gravitational lens systems. These groups provide additional convergence to the lensing potential and thus affect the value of $H_0$ derived from the systems. Lens system time delays are now being measured with uncertainties of only a few percent or better. Additionally, vast improvements are being made in incorporating observational constraints such as Einstein ring structures and stellar velocity dispersions into the lens models. These advances are reducing the uncertainties on $H_0$ to levels at which the the effects of associated galaxy groups may contribute significantly to the overall error budget. We describe a dedicated multiwavelength program, using Keck, HST, and Chandra, to find such groups and measure their properties. We present, as a case study, results obtained from observations of the CLASS lens system B1608+656 and discuss the implications for the value of $H_0$ derived from this system.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We analyze the formation process of delta configuration in some well-known super active regions based on the photospheric vector magnetogram observations. It is found that the magnetic field in the initial developing stage of some delta active regions shows the potential-like configuration in the solar atmosphere, the magnetic shear develops mainly near the magnetic neutral line with the magnetic islands of opposite polarities, and the large-scale photospheric twisted field forms late gradually. Some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, proposed to be generated in the sub-atmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and the relationship with magnetic shear in some delta active regions completely.

The photospheric current helicity density is a quantity reflected the local twisted magnetic field and relates to the remain of transfered magnetic helicity in the photosphere, even if the mean current helicity density brings the general chiral property in a layer of solar active regions. As the emergence of new magnetic flux in active regions, the changes of photospheric current helicity density with the injection of magnetic helicity into the corona from the sub-atmosphere can be detected. Because the injective rate of magnetic helicity and photospheric current helicity density contain the different means in the solar atmosphere, the injected magnetic helicity probably is not proportional to its remain (current helicity density) in the photosphere. A evidence is that the rotation of sunspots does not synchronize with the twist of photospheric transverse magnetic field in some active regions (such as, delta active regions) completely, as one believes that the rotation of sunspots reflects the magnetic one and connects with the injection of magnetic helicity. They represent different aspects of magnetic chirality. The synthetical analysis of the observational magnetic helicity parameters actually provides a relative complete picture of magnetic helicity and its transfer in the solar atmosphere.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

This paper presents the results of a comprehensive analysis of the dynamics of the sun's global magnetic structure, solar wind parameters, the interplanetary magnetic field and geomagnetic activity during fast global changes of magnetic fields on the Sun.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observations of chemically peculiar (CP) stars have been conducted for decades and have revealed a variety of spectrum anomalies, most prominent among them are line enhancements of heavy elements. The earlier observations were limited to the optical region and the use of less sensitive detectors, yet are responsible for much of our current characterization of the CP star phenomenon. More recent observations embrace a wider expanse of the electromagnetic spectrum and employ more sensitive detectors that continue to unveil new levels of spectrum peculiarity. The traditional criteria used to distinguish normal from peculiar stars have become blurred, thus in some sense replacing the concept of peculiarity with one of continuity. This presentation will address the observations of the traditional nonmagnetic CP star groups over the past decade, paying particular attention to new avenues of research that have a bearing upon the interpretation of the atmospheres of CP stars and the origins of this phenomenon.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss a new program to measure the parallaxes of a number of L and T dwarfs, objects that bridge the gap between M dwarfs and planets. This pilot project tests the feasibility of using large telescopes with infrared detectors to determine parallaxes at the level of milli-arcseconds (mas). First results show that we are able to achieve the required centroiding precision and simulations indicate that when the final observations come in we should be able to achieve our goal of parallaxes with accuracies of 2 mas. The main problems will be focal plane astrometric distortions and stability.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The trans-Neptunian region is inhabited by multiple dynamical populations, each of which have a complicated structure. For the most part, these structures cannot have been sculpted by the giant planets, once on their current orbital configuration. Thus, they represent important clues to the conditions that existed in the distant past. We argue in this paper, that most of what we see is the result of the outward migration of Neptune. By combining results from various authors, we can reproduce most of the observed properties of the trans-Neptunian region. Several aspects are not yet totally clear, and some may not be totally correct. But, for the first time, we have a view — if not not a detailed model — of how the system formed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We announce the construction of an Atlas of STIS-HST Spectra of Seyfert Galaxies, covering the UV and/or optical spectral range. Nuclear spectra of these galaxies have been extracted in windows of width 0.2″ for an optimized sampling (as this is the width of the slit in most cases), from a long-slit spectrum usually covering 52″. Extranuclear spectra have also been extracted, usually using larger windows to improve signal-to-noise ratio. Spectra obtained with different gratings have been combined in order to provide the largest possible wavelength coverage for each galaxy.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the analysis and results of a new spectroscopic and photometric survey of the central 1.5 ${\rm Mpc}^2$ region of the galaxy cluster A3921 (z=0.094). We detect the presence of two dominant clumps of galaxies with a mass ratio of $\sim$4-5: a main cluster centered on the BCG (A3921-A), and a NW sub-cluster (A3921-B) hosting the second brightest cluster object. By comparing optical results to the X-ray analysis of XMM observations (Belsole et al. 2004), we find that A3921-B is tangentially traversing the main cluster along a SW/NE direction. The analysis of stellar populations of more than one hundred cluster members reveals that the merger event has probably affected the kinematics and spectral properties of the active galaxies in A3921.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The CAIRNS (Cluster And Infall Region Nearby Survey) project is a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. I describe the survey and use the kinematics of galaxies in the infall regions to estimate the cluster mass profiles. At small radii, these mass profiles are consistent with independent mass estimates from X-ray observations and Jeans analysis. I demonstrate the dependence of mass-to-light ratios on environment by combining these mass profiles with Two-Micron All-Sky Survey (2MASS) photometry. Near-infrared light is more extended than mass in these clusters, suggesting that dense cluster cores are less efficient at forming galaxies and/or more efficient at disrupting them. At large radii, galaxy populations in cluster infall regions closely resemble those in the field. The mass-to-light ratio at these radii should therefore be a good probe of the global mass-to-light ratio. The mass-to-light ratio in the infall region yields a surprisingly low estimate of $\Omega_m \sim 0.1$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gaia is an all-sky, high precision astrometric and photometric satellite of the European Space Agency (ESA) due for launch in 2010. Its primary mission is to study the composition, formation and evolution of our Galaxy. Over the course of its five-year mission, Gaia will measure parallaxes and proper motions of every object in the sky brighter than visual magnitude 20, amounting to a billion stars, galaxies, quasars and solar system objects. It will achieve an astrometric accuracy of 10 $\mu$as at $V=15$ – corresponding to a distance accuracy of 1% at 1 kpc – and 200 $\mu$as at $V=20$. With Gaia, tens of millions of stars will have their distances measured to a few percent or better. This is an improvement over Hipparcos by several orders of magnitude in the number of objects, accuracy and limiting magnitude. Gaia will also be equipped with a radial velocity spectrograph, thus providing six-dimensional phase space information for sources brighter than $V$$\sim$ 17. To characterize the objects (which are detected in real time, thus dispensing with the need for an input catalogue), each object is observed in 15 medium and broad photometric bands with an onboard CCD camera. With these capabilities, Gaia will make significant advances in a wide range of astrophysical topics. In addition to producing a detailed kinematical map of stellar populations across our Galaxy, Gaia will also study stellar structure and evolution, discover and characterise thousands of exoplanetary systems (extending down to about ten Earth masses for the nearest systems) and make accurate tests of General Relativity on large scales, to mention just some areas. I give an overview of the mission, its operating principles and its expected scientific contributions. For the latter I provide a quick look in five areas on increasing scale size in the universe: the solar system, exosolar planets, stellar clusters and associations, Galactic structure and extragalactic astronomy.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report on the detection of secondary spectra in five spectroscopic binary systems: HD 434, HD 861, HD 108642, HD 178449, and HD 216608. High signal-to-noise high resolution spectroscopic observations were carried out at the Bulgarian NAO Rozhen as part of an extended project concerned mainly with Am stars in binary systems. Our knowledge about early type binaries has serious gaps. This is true especially when it is only based on older photographic techniques. We concluded that photographic data involving longer orbital periods (where Doppler shifts due to the orbital motion are comparable or even less than the rotational broadening of the spectral lines) and early type stars (that have only a few and usually broad lines) should be revisited or at least used with caution. We demonstrate that for the five systems how CCD observations made with 2-m class telescopes can discover the binary nature or secondary spectra of many currently unresolved SB1 systems. Important astrophysical information such as the atmospheric parameters and the mass ratios are used to unravel previous misinterpretations of the data leading often to spurious orbits.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gravitational microlensing is a powerful tool to detect compact matter on mass scales ranging from roughly $10^{-6} M_\odot$ to $10^6 M_\odot$ . Of particular importance is the fact that lensing is sensitive to both luminous and dark matter. There are two practically important regimes of microlensing: cosmological and Galactic. The former deals with the effect of stellar mass objects at cosmological distances on even more distant quasars, the latter treats stellar mass objects in the Milky Way or its Halo on stars in the Magellanic Clouds, the Galactic Bulge or M31. Microlensing has been detected in about ten quasars, roughly a dozen microlensing events towards stars in the Magellanic Clouds are on record, more than 1000 events have been measured towards stars in the Bulge. The large majority of microlensing events towards quasars and towards stars in the Magellanic Clouds or the Bulge can be explained by ordinary stellar mass objects. A small fraction of the Galactic microlensing events ($< 5 - 10$ %) could potentially be due to dark matter objects, including stellar mass Black Holes. Current and planned experiments will clarify the question whether any dark matter objects are necessary at all to explain microlensing events.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html