We review some recent developments in theoretical studies on the connection between the progenitor systems of Type Ia supernovae (SNe Ia) and the explosion mechanisms. (1) DD-subCh: In the merging of double C+O white dwarfs (DD scenario), if the carbon detonation is induced near the white dwarf (WD) surface in the early dynamical phase, it could result in the (effectively) sub-Chandrasekhar mass explosion. (2) DD-Ch: If no surface C-detonation is ignited, the WD could grow until the Chandrasekhar mass is reached, but the outcome depends on whether the quiescent carbon shell burning is ignited and burns C+O into O+Ne+Mg. (3) SD-subCh: In the single degenerate (SD) scenario, if the He shell-flashes grow strong to induce a He detonation, it leads to the sub-Chandra explosion. (4) SD-Ch: If the He-shell flashes are not strong enough, they still produce interesting amounts of Si and S near the surface of the C+O WD before the explosion. In the Chandra mass explosion, the central density is high enough to produce electron capture elements, e.g., stable 58Ni. Observations of the emission lines of Ni in the nebular spectra provides useful diagnostics of the sub-Chandra vs. Chandra issue. The recent observations of relatively low velocity carbon near the surface of SNe Ia provide also an interesting constraint on the explosion models.

We introduce a new computer program that is able to determine the flux ratio between the components of SB2 stars from time series of composite spectra. The spectra of the components are decomposed using the KOREL program and compared to synthetic spectra computed on a grid of fundamental stellar parameters. In the result, we obtain the optimized stellar parameters together with the flux ratio. The program is tested and applied to the oscillating eclipsing binary KIC 10661783 observed by the Kepler satellite mission.

The rapidly-accumulating archives of ground-based and spacecraft data worldwide that are being linked together through the International Virtual Observatory Alliance (IVOA) provide the binary star community with unparalleled opportunities for research. The main databases that are available to the astronomical community through the IVOA are discussed. Data from long-lasting spacecraft missions such as IUE are especially valuable for studying long-term variability. Some examples of current research on close binary stars that is being carried through with UV spectra from the IUE archive are presented. Included are the search for O-subdwarf companions to bright Be stars and some results from an ongoing investigation of the Double Periodic Variable phenomenon in Algol binaries.

Our sample of round/elliptical double-shell PNe around central stars (CSs) with H-rich surface chemistry covers all evolutionary phases across the HRD. By means of high-resolution and high-S/N spectra we determine bulk matter velocities of the inner wind-driven rims and the maximum (= post-shock) gas velocities of the surrounding thermally expanding shells. Studying the details of the internal kinematics allows a look at processes of PN formation and at PN expansion history helping, for instance, to determine expansion distances.

We present preliminary results from low resolution spectroscopic observations of planetary nebulae located in the southern Milky Way disk.

Binaries are not always neatly aligned. Previous observations of the DI Herculis system showed that the spin axes of both stars are highly inclined with respect to one another and the orbital axis. Here, we report on our ongoing survey to measure relative orientations of spin-axes in a number of eclipsing binary systems.

These observations will hopefully lead to new insights into star and planet formation, as different formation scenarios predict different degrees of alignment and different dependencies on the system parameters. Measurements of spin-orbit angles in close binary systems will also create a basis for comparison for similar measurements involving close-in planets.

The ultraviolet (UV) domain, in particular shortwards of Lyα, provides unique information to unravel the physical parameters of Central Stars of Planetary Nebulae (CSPNe) and the paths for this elusive final stage of stellar evolution, thanks to a wealth of diagnostic transitions from ionic species not observable at other wavelengths. Intermediate mass stars are the major providers of important elements like C and N. Understanding how they shed most of their initial mass is critical for understanding the chemical enrichment of the ISM. Mass-loss diagnostic lines abound at UV wavelengths, and when the CSPN reaches the hottest T eff before turning on the WD-cooling sequence, and the wind fades, the last wind lines to disappear are found in the far-UV, as well as diagnostic lines for elements such as Ne. This domain also offers a host of H2 transitions, tracing the circum-stellar material expelled in previous phases. UV images and spectra of PNe add critical constraints to their ionization structure and to some abundances. Finally, the recent GALEX sky surveys in two UV bands afforded the first unbiased census of hot white dwarfs (WD) and post-AGB objects in the Milky Way, significantly expanding known catalogs and providing statistical constraints to the initial-final mass relation.

The orbital period is one of the most accessible observables of a cataclysmic variable. It has been a concern for many years that the orbital period distribution of the known systems does not match that predicted by evolutionary theory. The sample of objects discovered by the Sloan Digital Sky Survey has changed this: it shows the long-expected predominance of short-period objects termed the ‘period spike’. The minimum period remains in conflict with theory, suggesting that the angular momentum loss mechanisms are stronger than predicted.

Some Type Ia supernovae (SNe Ia) are suggested to have progenitor white dwarfs (WDs) with mass of up to 2.4–2.8 M⊙, highly exceeding the Chandrasekhar mass limit. We present a new single degenerate (SD) model for SNe Ia progenitors, in which the WD mass can increase by accretion up to 2.3 (2.7) M⊙ from the initial value of 1.1 (1.2) M⊙. The results are consistent with high luminosity SNe Ia such as SN 2003fg, SN 2006gz, SN 2007if, and SN 2009dc. There are three characteristic mass ranges of exploding WDs. In an extreme massive case, differentially rotating WDs explode as a SNe Ia soon after the WD mass exceeds 2.4 M⊙ because of a secular instability at T/|W|~0.14. For a mid mass range of MWD=1.5–2.4 M⊙, which is supported by differential rotation, it takes some spinning-down time until carbon is ignited to induce an SN Ia explosion. For a lower mass range of MWD=1.38–1.5 M⊙, they can be supported by rigid rotation until the angular momentum is lost. We also suggest the ultra super-Chandrasekhar mass SNe Ia are born in young and low metallicity environments.

RS Ophiuchi (RS Oph) is a symbiotic binary consisting of a hot white dwarf accreting from the slow, dense stellar wind of a cool, red giant companion. The system belongs to, and is one of the best studied examples of, an even smaller subclass of binaries known as recurrent novae in which the white dwarf undergoes repeated thermonuclear outbursts. We present 3D smoothed particle hydrodynamics (SPH) models of mass transfer from the red giant to the white dwarf, followed by a nova outburst. We show that the outflow in the former is strongly concentrated towards the binary orbital plane. The nova ejecta is thus constrained in the equatorial directions, resulting in a bipolar outflow. The white dwarf in RS Oph is thought to be close to the Chandrasekhar mass, making the system a likely Type Ia supernova candidate. We discuss the role that such a highly structured circumstellar medium will play in the evolution of the supernova remnant.

The current direct observations of brown dwarfs and exoplanets have been obtained using instruments not specifically designed for overcoming the large contrast ratio between the host star and any wide-separation faint companions. However, we are about to witness the birth of several new dedicated observing platforms specifically geared towards high contrast imaging of these objects. The Gemini Planet Imager, VLT-SPHERE, Subaru HiCIAO, and Project 1640 at the Palomar 5m telescope will return images of numerous exoplanets and brown dwarfs over hundreds of observing nights in the next five years. Along with diffraction-limited coronagraphs and high-order adaptive optics, these instruments also will return spectral and polarimetric information on any discovered targets, giving clues to their atmospheric compositions and characteristics. Such spectral characterization will be key to forming a detailed theory of comparative exoplanetary science which will be widely applicable to both exoplanets and brown dwarfs. Further, the prevalence of aperture masking interferometry in the field of high contrast imaging is also allowing observers to sense massive, young planets at solar system scales (~3–30 AU)— separations out of reach to conventional direct imaging techniques. Such observations can provide snapshots at the earliest phases of planet formation—information essential for constraining formation mechanisms as well as evolutionary models of planetary mass companions. As a demonstration of the power of this technique, I briefly review recent aperture masking observations of the HR 8799 system. Moreover, all of the aforementioned techniques are already extremely adept at detecting low-mass stellar companions to their target stars, and I present some recent highlights.

Most popular computer codes for calculating model stellar and planetary atmospheres are briefly reviewed. A particular emphasis is devoted to our universal computer program Tlusty (model stellar atmospheres and accretion disks), CoolTlusty (a variant of Tlusty for computing model atmospheres of substellar-mass objects such as giant planets and brown dwarfs), and Synspec (an associated spectrum synthesis code). We show the highlights of actual applications of these codes which include extensive grids of fully line-blanketed non-LTE model atmospheres of O and B stars, and grids of model atmospheres of extrasolar giant planets and L and T dwarfs.

We have obtained spectra of 16 PNe in the disk of M31 and determined the abundances of He, N, O, Ne, S and Ar. Here we present the median abundances and compare them with previous M31 PN disk measurements and with PNe in the Milky Way. We also derive the radial oxygen gradient in M31, which is shallower than that in the Milky Way, even accounting for M31's larger disk scale length.

Combining recent mass determinations of Galactic CO white dwarfs and their progenitors with the latest evolutionary models for Asymptotic Giant Branch (AGB) stars, I review the initial-final mass relation (IFMR) of low- and intermediate-mass stars. In particular, I analyze the impact on the IFMR produced by a few critical processes characterizing the AGB phase, namely: the second and third dredge-up events, hot-bottom burning, and mass loss. Their dependence on metallicity and related theoretical uncertainties are briefly discussed.

We demonstrate that moderate exoplanet radial velocity searches are often subject to the effect of the correlated (red) radial velocity noise. When disregarded, this effect may induce strong distortions in the results of the time series analysis and, ultimately, can even lead to false planet detections. We construct a maximum-likelihood algorithm, which is able to manage this issue rather efficiently.

CCD photometric observations of the Algol-type eclipsing binary AT Peg have been obtained. The light curves are analyzed with modern techniques and new geometric and photometric elements are derived. A new orbital period analysis of the system, based on the most reliable timings of minima found in the literature, is presented and apparent period modulations are discussed with respect to the Light-Time effect (LITE) and secular changes in the system. The results of these analyses are compared and interpreted in order to obtain a coherent view of the system's behaviour.

The brightest planetary nebulae achieve similar maximum luminosities, have similar ratios of chemcial abundances, and apparently share similar kinematics in all galaxies. These similarities, however, are not necessarily expected theoretically and appear to hide important evolutionary differences. As predicted theoretically, metallicity appears to affect nebular kinematics, if subtly, and there is a clear variation with evolutionary stage. To the extent that it can be investigated, the internal kinematics for galactic and extragalactic planetary nebulae are similar. The extragalactic planetary nebulae for which kinematic data exist, though, probably pertain to a small range of progenitor masses, so there may still be much left to learn, particularly concerning the kinematics of planetary nebulae that descend from the more massive progenitors.

Type Ia supernovae (SNe Ia) are very successfully used as standard candles on cosmological distance scales, but so far the nature of the progenitor(s) is unclear. A possible scenario for SNe Ia are merging carbon/oxygen white dwarfs with a combined mass exceeding the Chandrasekhar mass. We determine the theoretical rates and delay time distribution of these mergers for two different common envelope prescriptions and metallicities. The shape of the delay time distributions is rather insensitive to the assumptions. The normalization is a factor ~3–13 too low compared to observations.

Dwarf galaxies are crucial to understand the formation and evolution of galaxies, since they constitute the most abundant galaxy population. Abundance ratios and their variations due to star formation and inflow/outflow of gas are key constraints to chemical evolution models. The determination of these abundances in the dwarf galaxies of the Local Universe is thus of extreme importance. NGC 185 is one of the four brightest dwarf companions of M31, but unlike the other three it has an important content of gas and dust. Interestingly enough, in an optical survey of bright nearby galaxies NGC 185 was classified as a Seyfert galaxy based on its integrated emission-line ratios in the nuclear regions. However, although its emission lines formally place it in the category of Seyfert it is probable that this galaxy does not contain a genuine active nucleus. In this contribution, we resume, firstly, our results of an empirical study of the galaxy, on which we characterise its emission-line population and obtain planetary nebulae abundance ratios (Gonçalves et al. 2012). And, secondly, we discuss our attempt to identify the possible ionization mechanisms for NGC 185 enlighting the controversial classification of this galaxy dwarf spheroidal (dSph) as well as Seyfert, via stellar population synthesis and chemical evolution modelling (Martins et al. 2011).

Based on time-dependent radiation-hydrodynamics simulations of the evolution of Planetary Nebulae (PNe), we have carried out a systematic parameter study to address the non-trivial question of how the diffuse X-ray emission of PNe with closed central cavities is expected to depend on the evolutionary state of the nebula, the mass of the central star, and the metallicity of stellar wind and circumstellar matter. We have also investigated how the model predictions depend on the treatment of thermal conduction at the interface between the central ‘hot bubble’ and the ‘cool’ inner nebula, and compare the results with recent X-ray observations. Our study includes models whose properties resemble the extreme case of PNe with Wolf-Rayet type central stars. Indeed, such models are found to produce the highest X-ray luminosities.