The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga

We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the Hipparcos Catalogue andthe ACT Reference Catalogue. The ROSAT selected X-ray sources are found to bea mixed population. A few of them are associated with the Tau-Aur or Orion StarForming Regions (SFR). Some, with distances similar to that of Tau-Aur but withdiscrepant proper motions, are probable or sure Pleiades super-cluster members orother late type young active stars with unresolved nature, more likely to originatein rapidly moving cloudlets, or else having originated from different sites other thanTau-Aur and moved to the present locations. A good many of the non-TTS X-raysources are considered as Hyades cluster members. Some TTS candidates could beforeground pre-main sequence stars or actually young dwarfs not yet depleted oftheir Lithium. Under the hypothesis that the sources we studied are representativeof the ROSAT selected TTS candidates discovered in the outskirts of     

Newly Identified Silicate Carbon Stars from IRAS Low-Resolution Spectra

The discovery of silicate carbon star poses a challenge to the theory of stellar evolution in the late stage, hence it is important to look for more silicate carbon stars. To this end we have carried out cross-identifications between the new IRAS Low-Resolution Spectrum (LRS) database and the new carbon star catalog, CGCS3. We have found nine new silicate carbon stars with silicate features around 10μm and/or 18μm. These newly identified stars are located in the Regions IIIa and VII in the IRAS two-color diagram, which means they indeed have typical far infrared colors of silicate carbon stars. The infrared properties of each of these sources are discussed.     

What can we learn from arcs in clusters of galaxies?

Photon-counting techniques have been applied to a study of the movement of the image of an unresolved star at the 50 cm telescope of the Royal Academy of Sciences, at the Roque de los Muchachos Observatory. The temporal autocorrelation function of such movement has been computed, the results are discussed in terms of atmospheric turbulence and in relation to high-resolution imaging.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

What can we learn from accretion disc eclipse mapping experiments?

The accretion disc eclipse mapping method is an astrotomographic inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs in cataclysmic variables. This paper presents examples of eclipse mapping results that have been key to improve our understanding of accretion physics. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

What Have We Learned from Helioseismology, What Have We Really Learned, and What Do We Aspire to Learn?

Helioseismology has been widely acclaimed as having been a great success: it appears to have answered nearly all the questions that we originally asked, some with unexpectedly high precision. We have learned how the sound speed and matter density vary throughout almost all of the solar interior – something which not so very long ago was generally considered to be impossible – we have learned how the Sun rotates, and we have a beautiful picture, on a coffee cup, of the thermal stratification of a sunspot, and also an indication of the material flow around it. We have tried, with some success at times, to apply our findings to issues of broader relevance: the test of the General Theory of Relativity via planetary orbit precession (now almost forgotten because the issue has convincingly been closed, albeit no doubt temporarily) the solar neutrino problem, the manner of the transport of energy from the centre to the surface of the Sun, the mechanisms of angular-momentum redistribution, and the workings of the solar dynamo. The first two were of general interest to the broad scientific community beyond astronomy, and were, quite rightly, principally responsible for our acclaimed success; the others are still in a state of flux.     

Information from the Kinematics of F and G Stars in the Solar Neighborhood

We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp     

What can we Expect from a New Radio Telescope in Upper Egypt?

The selection criteria for the project of a new radio telescope in Egyptare described. The telescope will have a diameter of 32 m and will belocated in Abu Simbel (Upper Egypt). One major task of the telescope isthe participation in VLBI network operation.     

Red Clump Stars from LAMOST Ⅱ: the outer disc of the Milky Way

We present stellar density maps of the Galactic outer disc with red clump stars from LAMOST data.These samples are separated into younger(mean age~2.7 Gyr)and older(mean age~4.6 Gyr)populations so that they can trace the variation of structures with ages in the range of Galactocentric radius R from 9 to 13.5 kpc.We show that scale heights for both of the two populations increase with R and display radial gradients of 48±6 and 40±4 pc kpc~(-1)for the older and younger populations,respectively.It is evident that flaring occurs in the thin disc populations with a wide range of ages.Moreover,the intensity of flaring does not seem to be significantly related to the age of the thin disc populations.On the other hand,scale lengths of the radial surface density profiles are 4.7±0.5 kpc for the younger population and 3.4±0.2 kpc for the older population,meaning that the younger disc population is more radially extended than the older one.Although the fraction of the younger population mildly increases from 28% at R~9 to about 35% at R~13 kpc,the older population is prominent with a fraction of no less than 65%in the outer disc.     

What is Moss?

TRACE observations of active regions show a peculiar extreme ultraviolet (EUV) emission over certain plage areas. Termed `moss' for its spongy, low-lying, appearance, observations and modeling imply that the phenomenon is caused by thermal conduction from 3–5 MKcoronal loops overlying the plage: moss is the upper transition region emission of hot coronal loops. The spongy appearance is due to the presence of chromospheric jets or `spicules' interspersed with the EUV emission elements. High cadence TRACE observations show that the moss EUV elements interact with the chromospheric jets on 10 s time scales. The location of EUV emission in the moss does not correlate well to the locations of underlying magnetic elements in the chromosphere and photosphere, implying a complex magnetic topology for coronal loop footpoint regions. We summarize here the key observations leading to these conclusions and discuss new implications for understanding the structuring of the outer solar atmosphere. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005286503963     

From Solar System to Exoplanets: What can we learn from Planetary Spectroscopy?

The purpose of this paper is to address the question: Using our knowledge of infrared planetary spectroscopy, what can we learn about the atmospheres of exoplanets? In a first part, a simplified classification of exoplanets, assuming thermochemical equilibrium, is presented, based on their masses and their equilibrium temperatures, in order to propose some possible estimations about their atmospheric composition. In the second part, infrared spectra of planets are discussed, in order to see what...     

What can We Learn about Atmospheric Meteor Ablation and Light Production from Laser Ablation?

Laboratory based laser ablation techniques can be used to study the size of the luminous region, predict spectral features, estimate the luminous efficiency factor, and assess the role of chemically differentiated thermal ablation. A pulsed Nd:YAG laser was used to ablate regions from ordinary and carbonaceous chondrite meteorites. CCD cameras and a digital spectroscope were used to measure the size and spectrum from the cloud of vaporised material. Scanning electron microscope (SEM) based energy dispersive x-ray spectroscopy (EDS) provided elemental abundance values in ablated and unablated regions. These results indicated some degree of differential ablation, with the most significant effect being significant loss of carbon from carbonaceous chondrites. This work suggests that a carbon matrix may play the role of the glue in the two component dustball model.     

What are solar faculae?

Results are presented of observations of the facula area near the solar disc center. Observations were performed at the German Vacuum Tower Telescope of the Observatorio del Teide (Tenerife) with the simultaneous use of two instruments, i.e., TESOS in the Ba IIλ 455.4 nm line to measure intensity variations in the photosphere and, at the same time, TIP in the Fe I (λλ 1564.3–1565.8 nm) line to measure Stokes parameters. Using the Fourier filtering technique, we separated the convective and wave components of the intensity field. Stokes parameters Fe I λ 1564.8 nm and λ 1565.2 nm were inverted by the SIR inversion code to estimate the magnetic field strength. We found that the contrast of intergranular lines of the facula in the continuum is almost independent of the magnetic field strengh (in the range from 30 to 160 mT). This result casts doubt on the assertion that solar faculae are a cluster of magnetic flux tubes. Most likely, due to the decrease of transparency of the matter in a strong (approximately 1 kilogauss) magnetic field, we can see the hot walls of granules.     

Young Age for the Veritas Asteroid Family Confirmed?

We present a comprehensive analysis of the dynamics of Veritas family members located in a chaotic strip centeredat 3.174 AU. A total of 600 chaotic members of the family and their clones were integrated for 100 Myr, and the variation of the distance with respect to the barycenter of the family have been computed forall of them. A simple classification of the prevailing behaviors hasbeen introduced to help identify typical dynamical patterns and states that could affect an estimate of the upper bound to the age of the family. We pointed out the importance of the temporary captures in thequasi-stable states, which occur often enough to affect the statistical analysis of the exits. The results are compatible with the young age (<100 Myr) for the family of Veritas, but we cannot say precisely how young it really is. This revised version was published online in July 2006 with corrections to the Cover Date.     

H_2O Megamasers: Accretion Disks, Jet Interaction, Outflows or Massive Star Formation?

The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H₂O emission. Single-dish monitoring and high-resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1–3 pc were mapped and masses of nuclear engines of order 10⁶–10⁸ M were determined. So far, 50 extragalactic H₂O maser sources have been detected, but few have been studied in detail.     

What makes a planet habitable?

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.     

What can we infer about the underlying physics from burst distributions observed in an RMHD simulation?

We determine that the sizes of bursts in mean-square current density in a reduced magnetohydrodynamic (RMHD) simulation follow a power-law probability density function (PDF). The PDFs for burst durations and waiting time between bursts are clearly not exponential and could also be power-law. This suffices to distinguish their behaviour from the original Bak et al. sandpile model which had exponential waiting time PDFs. However, it is not sufficient to distinguish between turbulence, some other SOC-like models, and other red noise sources.     

Accelerated expansion from?a?modified-quadratic gravity

We investigate the late-time dynamics of a four-dimensional universe based on modified scalar field gravity in which the standard Einstein-Hilbert action R is replaced by f(φ)R+f(R) where f(φ)=φ ² and f(R)=AR ²+BR _μν R ^μν,(A,B)∈ℝ. We discussed two independent cases: in the first model, the scalar field potential is quartic and for this special form it was shown that the universe is dominated by dark energy with equation of state parameter w≈−0.2 and is accelerated in time with a scale factor evolving like a(t)∝t ^5/3 and B+3A≈0.036. When, B+3A→∞ which corresponds for the purely quadratic theory, the scale factor evolves like a(t)∝t ^1/2 whereas when B+3A→0 which corresponds for the purely scalar tensor theory we found when a(t)∝t ^1.98. In the second model, we choose an exponential potential and we conjecture that the scalar curvature and the Hubble parameter vary respectively like R=hH[(f)\dot]/f,h ? \mathbbRR=\eta H\dot{\phi}/\phi,\eta\in\mathbb{R} and H=g[(f)\dot]^c,(g,c) ? \mathbbRH=\gamma\dot{\phi}^{\chi},(\gamma,\chi)\in\mathbb{R}. It was shown that for some special values of  χ, the universe is free from the initial singularity, accelerated in time, dominated by dark or phantom energy whereas the model is independent of the quadratic gravity corrections. Additional consequences are discussed.     

Young Stellar Populations at High z: Where are They?

Our view on the deep universe is strongly biassed towards the starlight that directly escapes from high-redshift galaxies, since we know very little on the fraction of luminosity absorbed by dust. Attempts to correct for this effect directly from the slope of the UV spectra seem to suggest that a significant fraction of the UV flux is extinguished. New constraints are now set on the fraction of dust-enshrouded young stellar populations by the detection of the Cosmic Infrared Background, and faint galaxy counts at IR and submm wavelengths. We briefly review the observations and use a semi-analytic model of galaxy formation and evolution to predict number counts consistent with the background. This revised version was published online in July 2006 with corrections to the Cover Date.     

What Can We Learn from the Geoeffectiveness of the Magnetic Cloud on 2012 July 15-17?

An interplanetary shock and a magnetic cloud(MC)reached the Earth on 2012 July 14 and 15 one after another.The shock sheath and the MC triggered an intense geom...