We calculate harmonic frequencies of the three most abundant carbonic acid conformers. For this, different model chemistries are investigated with respect to their benefits and shortcomings. Based on these results we use perturbation theory to calculate anharmonic corrections at the ωB97XD/aug-cc-pVXZ, X = D, T, Q, level of theory and compare them with recent experimental data and theoretical predictions. A discrete variable representation method is used to predict the large anharmonic contributions to the frequencies of the stretching vibrations in the hydrogen bonds in the carbonic acid dimer. Moreover, we re-investigate the energetics of the formation of the carbonic acid dimer from its constituents water and carbon dioxide using a high-level extrapolation method. We find that the ωB97XD functional performs well in estimating the fundamental frequencies of the carbonic acid conformers. Concerning the reaction energetics, the accuracy of ωB97XD is even comparable to the high-level extrapolation method. We discuss possibilities to detect carbonic acid in various natural environments such as Earth's and Martian atmospheres

Context. Optical nova lightcurves often have structures, such as rapid declines and recoveries, due to nebular or dusty phases of the ejecta. Nova Cygni 2006 (V2362 Cyg) underwent an unusual brightening after an early rapid decline. The shape of the lightcurve can be compared to that of V1493 Aql, but in that case the whole event was not as bright and only lasted a couple of weeks. V2362 Cyg had a moderately fast decline of t 2 = 9.0 ± 0.5 days before rebrightening, which lasted 250 days after maximum. Aims. We present an analysis of our own spectroscopic investigations in combination with American Association of Variable Star Observers (AAVSO) photometric data covering the whole rebrightening phase until the return to the final decline. Methods. We used medium resolution spectroscopy obtained in ten observing nights in the course of 79 nights to investigate the change of the velocity structure of the ejecta. The publicly available AAVSO photometry was used to analyze the overall properties and the energy of the brightening. Results. Although the behavior of the main outburst (velocity, outburst magnitude, and decline timescales) resembles a "normal" classical nova, the shell clearly underwent a second fast mass ejecting phase, causing the unusual properties. The integrated flux during this event contributes ≈40% to the total radiation energy of the outburst. The evolution of the H α profile during the bump event is obtained by subtracting the emission of the detached shells of the main eruption by a simple optically-thin model. A distance of D ≈ 7.5 +3.0 −2.5 kpc and an interstellar extinction E(B − V) = 0. m 6 ± 0. m 1 was also derived.

Abstract.A standard planetary nebula stays more than 10 000 years in the state of a photoionized nebula. As long as the timescales of the most important ionizing processes are much smaller, the ionization state can be characterized by a static photoionization model and simulated with codes like CLOUDY (Ferland et al. 1998). When the star exhibits a late helium flash, however, its ionizing flux stops within a very short period. The star then re-appears from its opaque shell after a few years (or centuries) as a cold giant star without any hard ionizing photons. Describing the physics of such behavior requires a fully time-dependent radiative transfer model. Pollacco (1999), Kerber et al. (1999) and Lechner & Kimeswenger (2004) used data of the old nebulae around V605 Aql and V4334 Sgr to derive a model of the pre-outburst state of the CSPN in a static model. Their argument was the long recombination time scale for such thin media. With regard to these models Schönberner (2008) critically raised the question whether a significant change in the ionization state (and thus the spectrum) has to be expected after a time of up to 80 years, and whether static models are applicable at all.Keywords. radiation mechanisms: general, ISM: general, planetary nebulae: individual (V604 Aql, V4334 Sgr)We use the transport equation formalism presented in the appendix of Binette et al. (2003) to describe the evolution of the nebula after the rapid change of the central star. As extensions to their model we also take He and the CNO elements into account, both as source of free electrons and as absorbing species. Also, we use a realistic stellar photosphere model for the input radiation field, and, as we investigate the evolution for a large change of the input radiation field, we do not use their monochromatic approach of a single mean frequency. The system is solved using a classical splitting scheme and the pressure and temperature are calculated using an explicit solver. The physical parameters were taken from Osterbrock & Ferland (2006) and Gnat & Sternberg (2007). The initial conditions were calculated by CLOUDY. For the evolution of the "born-again" star in the Hertzsprung-Russell-Diagramm the tracks as used in van Hoof et al. (2007) were applied.The nebulae of the "born-again" central stars recombine extremely slowly. The hydrogen and helium timescales dominate the whole process. Pollacco (1999) raised the question about the oxygen recombination time scale. As single species it would be only about one decade. The coupling of the oxygen processes with that of the helium, however, slows down the recombination, as long as He II exists, to a rate even below that of hydrogen. 412at https://www.cambridge.org/core/terms. https://doi

Abstract. NGC 2438 is a classical multiple shell or halo planetary nebula (PN). Its central star and the main nebula are well studied. Also it was target of various hydrodynamic simulations (Corradi et al. 2000). This initiated a discussion whether the haloes are mainly containing recombined gas (Schönberner et al. 2002), or if they are still ionized (Armsdorfer et al. 2003). An analysis of narrow-band images and long slit spectra at multiple slit positions was done to obtain a deeper look on morphological details and the properties of the outer shell and halo. For this work there was data available from ESO (direct imaging and long slit spectroscopy) and from SAAO (spectroscopic observations using a small slit, scanning over the whole nebula). Using temperature measurements from emission lines resulted in an electron temperature which clearly indicates a fully ionized stage. Additionally measurements of the electron density suggest a variation of the filling factor. Keywords. planetary nebulae: individual (NGC 2438)The data reduction was done using all the available data from ESO and from SAAO. The ESO-MIDAS astronomy data reduction package was used to process our obtained data. To confirm the quality of the results, a comparisons between different nights and wavelengths was done. The variations are typically very small (just up to 10%) and the overall quality of the flux calibration is very high.The temperature in a nebula can be determined from measurements of ratios of intensities of particular pairs of emission lines. The essential line for electron temperature determination in PNe used here is the [OIII] line at 4363Å.A calculation of the gradient of the logarithmic intensity of the different [OIII] emission lines (5007Å ,4958Å and 4363Å) was done. At the western part of the nebula the blue [OIII] line at 4363Å is also clearly visible in the halo (see Fig. 1). That allows for the first time the derivation of an electron temperature in the halo. The shape and gradient of the [OIII] lines are nearly parallel overall. This implies a rather homogeneous electron temperature. The ratio of emission-line strengths is calculated by using the formulas described in Osterbrock & Ferland (2006), where the coefficients result from numerical calculations of collision strengths and transition probabilities.Temperature measurements based on [OIII] lines give an electron temperature of about 10000 to 13000 K in the main nebula. In the inner region of the halo the temperature raises up to about 15000 to 17000 K and then slightly lowers again to a value of about 14000 K at the outer edge of the halo. This temperature measurements clearly indicate a fully ionized stage.To verify this result, some CLOUDY (Ferland et al. 1998)