The chemical composition towards the galactic anti-centre .2. Differential abundance analyses and distance determinations

In a previous paper we have published observational data for 6 early B-type stars having, galactocentric distances of between 10 and 18 kpc. Using LTE line-blanketed model at mosphere techniques we derived their atmospheric parameters, finding that all our targets had similar effective temperatures and surface gravities. In the following study we additionally include two stars which have been presented previously (Rolleston et al. 1993) and found also to have compatible atmospheric parameters to the original programme stars. The homogeneity of this sample allows quantitative line-by-line differential abundance analyses to be carried out which should reliably detect variations in the chemical compositions of the stellar photospheres. We present differential abundances for eight stars, in either young open clusters or the field, with respect to an arbitrarily chosen standard which shows a normal abundance pattern. Our method of calculating distances from the derived atmospheric parameters means that the relative distance scale should be accurate.

The chemical composition towards the galactic anti-centre .1. Observations and model atmosphere analyses

High resolution spectra of six early B-type main-sequence stars having galactocentric distances of between 10 and 18 kpc are presented. We List the equivalent widths for the metal lines and illustrate their hydrogen and helium line profiles. The stars are analysed using LTE line-blanketed model atmosphere techniques to derive atmospheric parameters and surface chemical compositions. All six stars have similar effective temperatures and surface gravities, allowing a reliable comparison of their metal abundances and distances. Significant variations in the photospheric abundances are evident and are discuss the need for a more detailed line-by-line differential analysis to exactly quantify the differences. This will be presented in a companion paper (Smartt et al. 1996).

A transiting companion to the eclipsing binary KIC002856960

We present an early result from an automated search of Kepler eclipsing binary systems for circumbinary companions. An intriguing tertiary signal has been discovered in the short period eclipsing binary KIC002856960. This third body leads to transit-like features in the light curve occurring every 204.2 days, while the two other components of the system display eclipses on a 6.2 hour period. The variations due to the tertiary body last for a duration of \sim1.26 days, or 4.9 binary orbital periods. During each crossing of the binary orbit with the tertiary body, multiple individual transits are observed as the close binary stars repeatedly move in and out of alignment with the tertiary object. We are at this stage unable to distinguish between a planetary companion to a close eclipsing binary, or a hierarchical triply eclipsing system of three stars. Both possibilities are explored, and the light curves presented.

WASP-52b, WASP-58b, WASP-59b, and WASP-60b: Four new transiting close-in giant planets

We present the discovery of four new transiting hot jupiters, detected mainly from SuperWASP-North and SOPHIE observations. These new planets, WASP-52b, WASP-58b, WASP-59b, and WASP-60b, have orbital periods ranging from 1.7 to 7.9 days, masses between 0.46 and 0.94 M_Jup, and radii between 0.73 and 1.49 R_Jup. Their G1 to K5 dwarf host stars have V magnitudes in the range 11.7-13.0. The depths of the transits are between 0.6 and 2.7%, depending on the target. With their large radii, WASP-52b and 58b are new cases of low-density, inflated planets, whereas WASP-59b is likely to have a large, dense core. WASP-60 shows shallow transits. In the case of WASP-52 we also detected the Rossiter-McLaughlin anomaly via time-resolved spectroscopy of a transit. We measured the sky-projected obliquity lambda = 24 (+17/-9) degrees, indicating that WASP-52b orbits in the same direction as its host star is rotating and that this prograde orbit is slightly misaligned with the stellar equator. These four new planetary systems increase our statistics on hot jupiters, and provide new targets for follow-up studies.

Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star

Star formation often occurs within or nearby stellar clusters. Irradiation by nearby massive stars can photoevaporate protoplanetary disks around young stars (so-called proplyds) which raises questions regarding the ability of planet formation to take place in these environments. We investigate the two-dimensional physical and chemical structure of a protoplanetary disk surrounding a low-mass (T Tauri) star which is irradiated by a nearby massive O-type star to determine the survivability and observability of molecules in proplyds. Compared with an isolated star-disk system, the gas temperature ranges from a factor of a few (in the disk midplane) to around two orders of magnitude (in the disk surface) higher in the irradiated disk. Although the UV flux in the outer disk, in particular, is several orders of magnitude higher, the surface density of the disk is sufficient for effective shielding of the disk midplane so that the disk remains predominantly molecular in nature. We also find that non-volatile molecules, such as HCN and H2O, are able to freeze out onto dust grains in the disk midplane so that the formation of icy planetesimals, e.g., comets, may also be possible in proplyds. We have calculated the molecular line emission from the disk assuming LTE and determined that multiple transitions of atomic carbon, CO (and isotopologues, 13CO and C18O), HCO+, CN, and HCN may be observable with ALMA, allowing characterization of the gas column density, temperature, and optical depth in proplyds at the distance of Orion (˜400 pc).

The geometry and ionization structure of the wind in the eclipsing nova-like variables RW Tri and UX UMa

The UV spectra of nova-like variables are dominated by emission from the accretion disk, modified by scattering in a wind emanating from the disk. Here, we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-like variables which have been observed with the Hubble Space Telescope in the far-ultraviolet, in an attempt to constrain the geometry and the ionization structure of their winds. Using our Monte Carlo radiative transfer code, we computed spectra for simply parameterized axisymmetric biconical outflow models and were able to find plausible models for both systems. These reproduce the primary UV resonance lines-N v, Si iv, and C iv-in the observed spectra in and out of eclipse. The distribution of these ions in the wind models is similar in both cases as is the extent of the primary scattering regions in which these lines are formed. The inferred mass-loss rates are 6%-8% of the mass accretion rates for the systems. We discuss the implication of our point models for our understanding of accretion disk winds in cataclysmic variables. © 2010. The American Astronomical Society. All rights reserved.

Analytical approximations to numerical solutions of theoretical emission measure distributions

Emission line fluxes from cool stars are widely used to establish an apparent emission measure distribution, EmdApp(Te), between temperatures characteristic of the low transition region and the low corona. The true emission measure distribution, EmdTrue(Te), is determined by the energy balance and geometry adopted and, with a numerical model, can be used to predict EmdApp(Te), to guide further modelling. The scaling laws that exist between coronal parameters arise from the dimensions of the terms in the energy balance equation. Here, analytical approximations to numerical solutions for EmdTrue(Te) are presented, which show how the constants in the coronal scaling laws are determined. The apparent emission measure distributions show a minimum value at some T0 and a maximum at the mean coronal temperature Tc (although in some stars, emission from active regions can contribute). It is shown that, for the energy balance and geometry adopted, the analytical values of the emission measure and electron pressure at T0 and Tc depend on only three parameters: the stellar surface gravity and the values of T0 and Tc. The results are tested against full numerical solutions for e Eri (K2 V) and are applied to Procyon (a CMi, F5 IV/V). The analytical approximations can be used to restrict the required range of full numerical solutions, to check the assumed geometry and to show where the adopted energy balance may not be appropriate. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Off-center explosions of Chandrasekhar-mass white dwarfs:An explanation of super-bright type la supernovae?

Context. The recent discovery of a very bright type la supernova, SNLS-03D3bb (=SN 2003fg), in the Supernova Legacy Survey (SNLS) has raised the question of whether super-Chandrasekhar-mass white-dwarf stars are needed to explain such bright explosions. Progenitors of this sort could form by mergers of pairs of rather massive white dwarfs. Binary systems of two white dwarfs in close orbit, where their total mass significantly exceeds the Chandrasekhar mass, have not yet been found. Therefore SNLS-03D3bb could establish the first clear case of a double-degenerate progenitor of a (peculiar) type la supernovae. Moreover, if this interpretation is correct, it casts some doubt on the universality of the calibration relations used to make SNe la distance indicators for cosmology. Aims. We aim to evaluate the case for a super-Chandrasekhar-mass progenitor for SNLS-03D3bb in light of previous theoretical work on super-Chandrasekhar-mass explosions. Furthermore, we propose an alternative scenario involving only a Chandrasekhar-mass progenitor. Methods. We present a theoretically motivated critical discussion of the expected observational fingerprints of super-Chandrasekharmass explosions. As an alternative, we describe a simple class of aspherical Chandrasekhar-mass models in which the products of nuclear burning are displaced from the center. We then perform simple radiative transfer calculations to predict synthetic lightcurves for one such off-center explosion model. Results. In important respects, the expected observational consequences of super-Chandrasekhar-mass explosions are not consistent with the observations of SNLS-03D3bb. We demonstrate that the lopsided explosion of a Chandrasekhar-mass white dwarf could provide a better explanation. © ESO 2007.

Multidimensional modelling of X-ray spectra for AGN accretion disc outflows - II

Highly ionized fast accretion disc winds have been suggested as an explanation for a variety of observed absorption and emission features in the X-ray spectra of active galactic nuclei. Simple estimates have suggested that these flows may be massive enough to carry away a significant fraction of the accretion energy and could be involved in creating the link between supermassive black holes and their host galaxies. However, testing these hypotheses, and quantifying the outflow signatures, requires high-quality theoretical spectra for comparison with observations. Here, we describe extensions of our Monte Carlo radiative transfer code that allow us to generate realistic theoretical spectra for a much wider variety of disc wind models than that was possible in our previous work. In particular, we have expanded the range of atomic physics simulated by the code so that L- and M-shell ions can now be included. We have also substantially improved our treatment of both ionization and radiative heating such that we are now able to compute spectra for outflows containing far more diverse plasma conditions. We present example calculations that illustrate the variety of spectral features predicted by parametrized outflow models and demonstrate their applicability to the interpretation of data by comparison with observations of the bright quasar PG1211+143. We find that the major features in the observed 2-10 keV spectrum of this object can be well reproduced by our spectra, confirming that it likely hosts a massive outflow. © 2010 The Authors. Journal compilation © 2010 RAS.

Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass 0.9M

Type Ia supernovae are thought to result from thermonuclear explosions of carbong'oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of 0.9M. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83M to 0.9M. © 2010 Macmillan Publishers Limited. All rights reserved.

Galactic Bulge PNe:Carbon molecules in oxygen-rich environments

Galactic bulge planetary nebulae show evidence of mixed chemistry with emission from both silicate dust and PAHs. This mixed chemistry is unlikely to be related to carbon dredge up, as third dredge-up is not expected to occur in the low mass bulge stars. We show that the phenomenon is widespread, and is seen in 30 nebulae out of our sample of 40. A strong correlation is found between strength of the PAH bands and morphology, in particular, the presence of a dense torus. A chemical model is presented which shows that hydrocarbon chains can form within oxygen-rich gas through gas-phase chemical reactions. We conclude that the mixed chemistry phenomenon occurring in the galactic bulge planetary nebulae is best explained through hydrocarbon chemistry in an UV-irradiated, dense torus. © 2012 International Astronomical Union.

Self-organized electromagnetic field structures in laser-produced counter-streaming plasmas

Self-organization(1,2) occurs in plasmas when energy progressively transfers from smaller to larger scales in an inverse cascade(3). Global structures that emerge from turbulent plasmas can be found in the laboratory(4) and in astrophysical settings; for example, the cosmic magnetic field(5,6,) collisionless shocks in supernova remnants(7) and the internal structures of newly formed stars known as Herbig-Haro objects(8). Here we show that large, stable electromagnetic field structures can also arise within counter-streaming supersonic plasmas in the laboratory. These surprising structures, formed by a yet unexplained mechanism, are predominantly oriented transverse to the primary flow direction, extend for much larger distances than the intrinsic plasma spatial scales and persist for much longer than the plasma kinetic timescales. Our results challenge existing models of counter-streaming plasmas and can be used to better understand large-scale and long-time plasma self-organization.

A J-band detection of the sub-stellar mass donor in SDSS J1433+1011

We present time-resolved J-band spectroscopy of the short-period cataclysmic variable SDSS J143317.78+101123.3. We detect absorption lines from the sub-stellar donor star in this system, which contributes 38 +/- 5 per cent to the J-band light. From the relative strengths of the absorption lines in the J band, we estimate the spectral type of the donor star to be L2 +/- 1. These data are the first spectroscopic detection of a donor with a confirmed sub-stellar mass in a cataclysmic variable, and the spectral type is consistent with that expected from semi-empirical evolutionary models.

Using skew mapping, we have been able to derive an estimate for the radial velocity of the donor of K-d = 520 +/- 60 km/s. This value is consistent with, though much less precise than, predictions from mass determinations found via photometric fitting of the eclipse light curves.

The AstraLux large M-dwarf multiplicity survey

We present the results of an extensive high-resolution imaging survey of M-dwarf multiplicity using the Lucky Imaging technique. The survey made use of the AstraLux Norte camera at the Calar Alto 2.2m telescope and the AstraLux Sur camera at the ESO New Technology Telescope in order to cover nearly the full sky. In total, 761 stars were observed (701M-type and 60 late K-type), among which 182 new and 37 previously known companions were detected in 205 systems. Most of the targets have been observed during two or more epochs, and could be confirmed as physical companions through common proper motion, often with orbital motion being confirmed in addition. After accounting for various bias effects, we find a total M-dwarf multiplicity fraction of 27% ± 3% within the AstraLux detection range of 008-6? (semimajor axes of ~3-227 AU at a median distance of 30pc). We examine various statistical multiplicity properties within the sample, such as the trend of multiplicity fraction with stellar mass and the semimajor axis distribution. The results indicate that M-dwarfs are largely consistent with constituting an intermediate step in a continuous distribution from higher-mass stars down to brown dwarfs. Along with other observational results in the literature, this provides further indications that stars and brown dwarfs may share a common formation mechanism, rather than being distinct populations. © 2012. The American Astronomical Society. All rights reserved.

Photodetachment as destruction mechanism for CN- and C3N- anions in circumstellar envelopes

Absolute photodetachment cross sections of two anions of astrophysical importance CN- and C3N- were measured to be (1.18 +- (0.03)_stat (0.17)_sys) * 10^-17 cm^2 and (1.43 +- (0.14)_stat (0.37)_sys) * 10^-17 cm^2 respectively at the ultraviolet wavelength of 266 nm (4.66 eV). These relatively large values of the cross sections imply that photodetachment can play a major role in the destruction mechanisms of these anions particularly in photon-dominated regions. We have therefore carried out model calculations using the newly measured cross sections to investigate the abundance of these molecular anions in the cirumstellar envelope of the carbon-rich star IRC+10216. The model predicts the relative importance of the various mechanisms of formation and destruction of these species in different regions of the envelope. UV photodetachment was found to be the major destruction mechanism for both CN- and C3N- anions in those regions of the envelope, where they occur in peak abundance. It was also found that photodetachment plays a crucial role in the degradation of these anions throughout the circumstellar envelope.