Atmospheric parameters of the B-supergiant HD 198478 from the UV spectra

We determined atmospheric parameters of the Galactic early B-supergiant HD 198478 (55 Cyg) from the UV silicon lines taken from the high-resolution 1150–1980 Å IUE spectra. TLUSTY numerical code was used to model the stellar atmosphere and to determine the temperature and surface gravity assuming a non-LTE plane parallel hydrostatic stellar atmosphere with microturbulence. The synthesized spectra were broadened by the IUE instrumental profile, rotational and macroturbulent velocity with ROTIN numerical code. The silicon 1264 Å, 1309 Å, 1312 Å, 1417 Å and 1294–1303 Å multiplet lines of different stages of ionization (Si II and Si III) and Balmer Hδ 4101 Å line were modeled, leading to the temperature, surface gravity, rotational and macroturbulent velocity values. Our results have shown that the line broadening cannot be explained by rotational velocity only, but additional macroturbulent velocity component should be taken into account. HD 198478 shows a significant degeneracy in velocity, which means that the individual contributions of the macroturbulence and rotation in the total velocity broadening cannot be distinguished. Adequate fit of TLUSTY models to the observed non-resonant silicon lines suggests that the non-LTE plane-parallel hydrostatic stellar model without wind contribution can be used to explain such lines. We have obtained similar results using the HST STIS spectra in the same procedure, showing that the IUE spectra, despite their lacking quality compared to the STIS spectra, are reliable enough in determination of the B supergiants’ photospheric parameters.     

Summary of the First NUVA Conference Space Astronomy: the UV Window to the Universe

In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El?Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions.     

Solving the mysteries of the diffuse interstellar medium with high-resolution UV spectroscopy

Understanding the complex structure, dynamics, and ionization of gas in the nearby interstellar medium is required before one can realistically model interstellar gas in other galaxies. High-resolution ultraviolet spectra provide the essential data for such studies because the resonance lines of most important atoms, ions, and molecules are located in the ultraviolet, and high spectral resolution is needed to resolve line profiles and determine the velocity structure along a line of sight. I list ten important physical questions concerning interstellar gas that require a more sensitive spectrometer than STIS and the desired spectral resolution to answer these questions.     

Using Active Contours for Semi-Automated Tracking of UV and EUV Solar Flare Ribbons

Solar-flare UV and EUV images show elongated bright “ribbons” that move over time. If these ribbons are assumed to locate the footpoints of magnetic-field lines reconnecting in the corona, then it is clear that studying their evolution can provide important insight into the reconnection process. An image-processing method based on active contours (commonly referred to as “snakes”) is proposed as a method for tracking UV and EUV flare ribbons and is tested on images from the Transition Region and Coronal Explorer (TRACE). This paper introduces the basic concepts of such an approach with a brief overview of the history and theory behind active contours. It then details the specifics of the snake algorithm developed for this work and shows the results of running the algorithm on test images. The results from the application of the developed algorithm are reported for six different TRACE flares (five in UV and one in EUV). The discussion of these results uses the output from an expert tracking the same ribbons by eye as a benchmark, and against these the snake algorithm is shown to compare favourably in certain conditions, but less so in others. The applicability of the automated snake algorithm to the general problem of ribbon tracking is discussed and suggestions for ways to improve the snake algorithm are proposed.     

AGB star intershell abundances inferred from UV spectra of extremely hot post-AGB stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. Almost all of the chemical trace elements in these hot stars can only be identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope played a crucial role for this research.     

Magnetic activity optimal tracers: from radio to X-ray; the relevance of UV astronomy

Observations and analysis of magnetic activity phenomena in the atmospheres of cool stars—e.g., active regions, flares, stellar cycles—give insight into the fundamental processes in the heating of chromospheres, transition regions (TRs), and coronae. Diagnostics of magnetic activity can be found throughout the whole electromagnetic spectrum; from radio wavelengths, where gyrosynchrotron radiation arises from the quiescent and flaring corona, to optical, where important signatures are the Balmer lines and the Ca ii IRT and H&K lines, eventually to UV and X-rays, the latter mainly due to coronal thermal plasma. The UV and EUV ranges contains a plethora of emission lines that are powerful diagnostics for the warm (10?000 K) chromospheres, hot (100?000–800?000 K) TRs and very hot (1–10 MK) coronae. Also very weak coronal winds from cool stars have been identified and characterized thanks to high resolution UV spectra. Here I review the main results from UV observations of cool stars atmospheres and outline what can be expected from future UV imaging and spectroscopy measurements.     

The Algol-type eclipsing binaries RW CrB and VZ Leo: new RI photometric study and search for pulsations

In this study we present the new R and I light curve solutions for the eclipsing binary systems RW CrB and VZ Leo, which for VZ Leo is the first one in the literature. Our new observations have been analyzed using the Wilson-Devinney code from which new geometric and photometric elements are derived. The geometry of both stars is that of a semi-detached binary system where the secondary component fills its Roche lobe while the primary component is well inside. In the case of RW CrB, asymmetry in the light curve was explained by a spot model. The orbital period changes of both systems were studied and the results indicated a period decrease which can be explained by angular momentum loss. We also investigated the possibility of pulsations of the primary components since these systems are mentioned as candidates of δ Sct type pulsation. However, a time-series analysis of the residual curves in the filter I does not indicate any evidence of periodic light variation for both systems. Finally, we compared the results obtained for both binary stars to those of similar systems.     

Spectral behavior of AM Her and QQ Vul in high and intermediate states in the UV

We present low resolution UV spectra of two polar systems, AM Her and QQ Vul from the observations taken by the IUE (International Ultraviolet Explorer) of the period between 1978–1996 and 1983–1996 for both systems respectively, to accomplish a large scale study of what happens to the ultraviolet flux of C IV 1550 Å spectral line during different orbital phases. Two spectra for both systems showing the variations in line fluxes and line widths at different orbital phases in high and intermediate states are presented. We concentrated on calculating the line fluxes and line widths of C IV 1550 Å emission line originating in the accretion stream. Our results show that there is spectral variability for the aformentioned physical parameters at different times, similar to that known for the light curve (Heise and Verbunt, Astron. Astrophys. 189:112, 1988; Gansicke et al., Astron. Astrophys. 303:127, 1995; Kafka and Honeycutt, Astron. J. 125:2188K, 2003). We attribute it to the variations of both density and temperature as a result of changing the mass transfer rate (Hutchings et al., Astron. J. 123:2841H, 2002; King and Lasota, Astron. Astrophys. 140L:16K, 1984) which is responsible for this spectral variability. Also we found that the line fluxes of AM Her are greater than the line fluxes of QQ Vul, while the line widths of both systems are approximately the same.     

Continuous monitoring of comet Holmes from before the 2007 Outburst

The outburst and subsequent brightness evolution of comet 17P/Holmes has been observed using the MMT Observatory’s All-Sky Camera (Pickering 2006) on Mt. Hopkins near Tucson, Arizona, USA. The comet was picked up at the limiting visual magnitude of 5.5 on October 24.38 and tracked by the camera continuously until sunrise four hours later. During this time the comet brightened to visual magnitude 3.5. Comet Holmes was next observed just after sunset on October 25.23 at visual magnitude 2.5, where it remained approximately constant over the next three days. The comet then began to dim slowly and was followed into the early months of 2008 with periods of dense time coverage.     

Remote Sensing of the Heliosphere with the Murchison Widefield Array

The Murchison Widefield Array (MWA) is one of the new technology low frequency radio interferometers currently under construction at an extremely radio-quiet location in Western Australia. The MWA design brings to bear the recent availability of powerful high-speed computational and digital signal processing capabilities on the problem of low frequency high-fidelity imaging with a rapid cadence and high spectral resolution. Solar and heliosphere science are among the key science objectives of the MWA and have guided the array design from its very conception. We present here a brief overview of the design and capabilities of the MWA with emphasis on its suitability for solar physics and remote-sensing of the heliosphere. We discuss the solar imaging and interplanetary scintillation (IPS) science capabilities of the MWA and also describe a new software framework. This software, referred to as Haystack InterPlanetary Software System (HIPSS), aims to provide a common data repository, interface, and analysis tools for IPS data from all observatories across the world.     

Oscillatory Response of the 3D Solar Atmosphere to the Leakage of Photospheric Motion

The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations. The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods, and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating through the transition region into the solar corona. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

UV view of High Mass X-Ray Binary systems (what we know and perspectives for WSO-UV)

High Mass X-Ray Binary systems (HMXRB) are composed by a compact object orbiting around a OB massive star. The mass transfer which takes place from the massive star onto its compact companion is the responsible of the emission of high energy photons (X-rays) observed in these systems (to which they owe the name of their class). The presence of a compact object must exert a clear influence in the outer envelopes of the massive star and, thus, must modify the formation and evolution of its stellar wind. Tidal interactions, mass capture and high energy radiation heating and pressure are among the possible ways in which the compact companion and the X-ray emission will influence the stellar wind of the massive counterpart. The status of the observations of such systems from previous UV missions will be reviewed, and perspectives for the incoming WSO-UV space mission will be reported.     

High-Frequency Evolving Emission Lines for the 25 August 1999 Solar Flare

We analyze a special kind of temporal fine structure in microwave radio emission for the 25 August 1999 solar flare observed by the PMO spectrometer over the range of 4.5 – 7.5 GHz. This flare displays continuum emission after a group of reverse-slope type III bursts around 6 GHz. High-resolution dynamic spectra reveal three evolving emission lines (EELs) following the type III group. They are characterized by isolated, narrow, and continuous emission strips, which display frequency fluctuations with time. Their frequency-drift rates are between −2 and 3 GHz s⁻¹. Distinct from the EELs at lower frequencies, three EELs have a very short duration of a few seconds. They show an average bandwidth of Δf≈330 MHz and a relative bandwidth of Δf/f≈0.057. This is the first time that this kind of fine structure has been observed around 6 GHz.     

Prediction of the Maximum Amplitude and Timing of Sunspot Cycle 24

Precursor techniques, in particular those using geomagnetic indices, often are used in the prediction of the maximum amplitude for a sunspot cycle. Here, the year 2008 is taken as being the sunspot minimum year for cycle 24. Based on the average aa index value for the year of the sunspot minimum and the preceding four years, we estimate the expected annual maximum amplitude for cycle 24 to be about 92.8±19.6 (1-sigma accuracy), indicating a somewhat weaker cycle 24 as compared to cycles 21 – 23. Presuming a smoothed monthly mean sunspot number minimum in August 2008, a smoothed monthly mean sunspot number maximum is expected about October 2012±4 months (1-sigma accuracy).     

Smooth models of overshooting at the base of the solar convective zone

A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N ² is calculated. In local convective theories the N ² profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break.     

Automated Detection of Oscillating Regions in the Solar Atmosphere

Recently observed oscillations in the solar atmosphere have been interpreted and modeled as magnetohydrodynamic wave modes. This has allowed for the estimation of parameters that are otherwise hard to derive, such as the coronal magnetic-field strength. This work crucially relies on the initial detection of the oscillations, which is commonly done manually. The volume of Solar Dynamics Observatory (SDO) data will make manual detection inefficient for detecting all of the oscillating regions. An algorithm is presented that automates the detection of areas of the solar atmosphere that support spatially extended oscillations. The algorithm identifies areas in the solar atmosphere whose oscillation content is described by a single, dominant oscillation within a user-defined frequency range. The method is based on Bayesian spectral analysis of time series and image filtering. A Bayesian approach sidesteps the need for an a-priori noise estimate to calculate rejection criteria for the observed signal, and it also provides estimates of oscillation frequency, amplitude, and noise, and the error in all of these quantities, in a self-consistent way. The algorithm also introduces the notion of quality measures to those regions for which a positive detection is claimed, allowing for simple post-detection discrimination by the user. The algorithm is demonstrated on two Transition Region and Coronal Explorer (TRACE) datasets, and comments regarding its suitability for oscillation detection in SDO are made.     

Grand Minima of Solar Activity and the Mean-Field Dynamo

We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of the dynamo models but requires some specific assumption, such as random fluctuations in dynamo governing parameters. In general, we conclude that a relatively simple and straightforward model is able to reproduce the Grand Minima phenomenology remarkably well, in principle providing us with a possibility of studying the physical nature of Grand Minima.     

Numerical Heliospheric Simulations as Assisting Tool for Interpretation of Observations by STEREO Heliospheric Imagers

The interpretation of multi-spacecraft heliospheric observations and three-dimensional reconstruction of structured and evolving solar wind is challenging. This is especially true for the interpretation of white-light structures observed by the Heliospheric Imagers (HI) onboard STEREO spacecraft since their appearance depends on three-dimensional geometric factors. Numerical simulations can provide global context and suggest what may and may not be observed. We use the heliospheric code ENLIL to simulate various scenarios of well-defined corotating solar wind streams and ejected transient density structures, and we generate from the solutions synthetic white-light images at various locations. We illustrate that corotating interaction regions (CIRs) show up differently in HI-2A and HI-2B and that they may appear as transient structures in HI-2A but not in HI-2B. This asymmetry is caused by differing Thomson scattering responses. Further, we illustrate that a given interplanetary coronal mass ejection (ICME) may exhibit drastically different white-light brightness depending on the observing position and that some ICMEs can eventually reach Earth without being detected by the imagers. Finally, we demonstrate application of the modeling system through simulation of the 24?–?25 January 2007, 31 December 2007 and 26 April 2008 CMEs.     

Charged particles’ areas of three-dimensional motions in a system of two magnetic stars

We study the parametric evolution of the regions where three-dimensional motions of a charged particle are allowed in the combined electromagnetic field produced by two rotating magnetic stars. We discuss the changes in the topology of the zero-velocity surfaces, as well as in the trapping regions of the particle motion for various values of the dipoles’ magnetic moments.