Mattias Eriksson

Modeling the wind structure of AG Peg by fitting of C IV and N V resonance doublets

The latest outburst of AG Peg has lasted for 150 years, which makes it the slowest nova eruption ever recorded. During the time of IUE observations (1978−1995) line profiles and intensity ratios of the N V and C IV doublet components changed remarkably, and we discuss plausible reasons. One of them is radiative pumping of Fe II which is investigated by studying the fluorescence lines from pumped levels. Three Fe II channels are pumped by C IV and one by N V. The pumping rates of those Fe II channels as derived by the modeling agree well with the strengths of the Fe II fluorescence lines seen in the spectra. We model the C IV and N V resonance doublets in IUE spectra recorded between 1978 and 1995 in order to derive optical depths, expansion velocities, and the emissivities of the red giant wind, the white dwarf wind and their collision region. The derived expansion velocities are ∼60 km s −1 for the red giant wind and ∼700 km s −1 for the white dwarf wind. We also suggest a fast outflow from the system at ∼150 km s −1 . The expansion velocity is slightly higher for N V than for C IV. Emission from the collision region strongly affects the profile of the N V and C IV resonance doublets indicating its existence.

The nature of ultraviolet spectra of AG Pegasi and other symbiotic stars: locations, origins, and excitation mechanisms of emission lines

A detailed study of ultraviolet spectra of the symbiotic star AG Peg has been undertaken to derive the atomic excitation mechanisms and origin of formation for the lines common in symbiotic systems. More than 600 emission lines are observed in spectra from IUE, HST and FUSE of which 585 are identified. Population mechanisms and origin of formation are given for a majority of those lines. Based on the understanding of the AG Peg spectra IUE data of 19 additional symbiotic stars are investigated and differences and similarities of their spectra are discussed. Fe II fluorescence lines pumped by strong emission lines between 1000 and 2000 angstrom are observed in 13 of these systems. Some of the symbiotic systems belonging to the subclass symbiotic novae have more than 100 Fe II fluorescence lines in the ultraviolet wavelength region. Forbidden lines are detected for 13 of the stars, mostly from highly-ionized spectra such as Ar V, Ne V and Mg V. Further, [Mg VI] and [Mg VII] lines are observed in a symbiotic star (AG Dra) for the first time. Five of the symbiotic stars have broad white-dwarf wind profiles (FWHM > 400 km s(-1)) for a few lines in their spectra. The stars with no such broad lines can be divided into two similarly sized groups, one where all lines have FWHM less than 70 km s(-1) and the other where one, a few or all of the broad (FWHM > 400 km s(-1)) lines of AG Peg have an enhanced broad wing (110-140 km s(-1)). (Less)

Bowen excitation of NIII lines in symbiotic stars

We present a semi-empirical equation for prediction of the strengths of those N III lines that are generated by the Bowen mechanism and observed in spectra of symbiotic stars. The equation assumes that the Bowen mechanism is the only source populating the 3d state in N III, and comparisons with observations of the 3s-3p and 3p-3d transitions serve as a test of this assumption. In an ongoing study of symbiotic stars the equation has been applied to two symbiotic novae, RR Tel and AG Peg, by comparing the predicted N III line strengths to observed line intensities. It is clear that besides the Bowen mechanism there is another process, most likely radiative recombination, that contributes to the N III 3d population in AG Peg and is the main population process of this state in RR Tel. It is also clear that a second, not previously considered, N III channel, 2p P-2(1/2)-3d D-2(3/2) at 374.198 angstrom, is pumped by O III in both RR Tel and AG Peg. (Less)

Modeling of C IV pumped fluorescence of Fe II in symbiotic stars

Aims. We describe how the C IV lambda 1548.18 line pumps the 1548.20 and 1548.41 angstrom channels of Fe II in symbiotic stars through the process known as photo-ionization by accidental resonance (PAR). We describe where and why Fe II fluorescence arises in symbiotic stars and whether the Fe II lambda 1548.41 channel can only be activated when there is a white-dwarf wind present in the system. Further, we aim to show how an analysis of the PAR-pumped lines helps to understand the phyisical conditions that they manifest. Methods. We calculate intensities of the C IV-pumped Fe II fluorescence lines in symbiotic stars, corresponding to the y(4)H(11/)2 and w(2)D(3/2) levels, based on a simple geometrical model of the emitting regions. We apply the model to seven symbiotic stars, known to have Fe II fluorescence lines pumped by C IV in their spectra. We compare the predicted intensities to the observed intensities of the selected symbiotic stars. Results. We find that we can reproduce the observed fluorescence intensities of the seven symbiotic stars in our sample, using parameters that are consistent with their known properties. We show that PAR-produced lines can be used as a diagnostic tool to derive important physical parameters of a stellar system. We suggest that the detection of Fe II lines corresponding to the w2D3/2 level in certain symbiotic stars provide evidence of hot shells expanding with velocities of at least hundreds km s(-1) in those systems.