Element Abundance Determination in Hot Evolved 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.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical, Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009, Kodaikanal, India (Springer

Proper-motion age dating of the progeny of Nova Scorpii AD 1437.

'Cataclysmic variables' are binary star systems in which one star of the pair is a white dwarf, and which often generate bright and energetic stellar outbursts. Classical novae are one type of outburst: when the white dwarf accretes enough matter from its companion, the resulting hydrogen-rich atmospheric envelope can host a runaway thermonuclear reaction that generates a rapid brightening. Achieving peak luminosities of up to one million times that of the Sun, all classical novae are recurrent, on timescales of months to millennia. During the century before and after an eruption, the 'novalike' binary systems that give rise to classical novae exhibit high rates of mass transfer to their white dwarfs. Another type of outburst is the dwarf nova: these occur in binaries that have stellar masses and periods indistinguishable from those of novalikes but much lower mass-transfer rates, when accretion-disk instabilities drop matter onto the white dwarfs. The co-existence at the same orbital period of novalike binaries and dwarf novae-which are identical but for their widely varying accretion rates-has been a longstanding puzzle. Here we report the recovery of the binary star underlying the classical nova eruption of 11 March AD 1437 (refs 12, 13), and independently confirm its age by proper-motion dating. We show that, almost 500 years after a classical-nova event, the system exhibited dwarf-nova eruptions. The three other oldest recovered classical novae display nova shells, but lack firm post-eruption ages, and are also dwarf novae at present. We conclude that many old novae become dwarf novae for part of the millennia between successive nova eruptions

TU Bootis: An ambiguous W Ursae Majoris system

The W UMa system TU Boo has a period of 0.32438 days and a spectral type G3. We present here the first photometric analysis, based on photoelectric B and V light curves. The q-search method was used to find the preliminary range of the mass-ratio in order to search for the final solution. The unspotted solution was found by using the unperturbed part of the light curve and applying the DC program of the WD code. Spot models were computed, a one-spot model to explain the O’Connell effect (Max II fainter than Max I) by introducing a cool spot on the larger component, and a two-spot model to explain both the O’Connell effect and the small excess of light just after Max I by assuming, in addition, a bright region on the smaller component near the neck region of the common envelope. TU Boo is an A-type W UMa system with complete eclipses, the secondary minimum being an occultation. However, the derived physical parameters of the system and its light curve anomalies are more common for slightly evolved W-type systems

V 700 Cygni and AW Virginis: Two W-type W UMa systems with spotted components

A photometric analysis of the W UMa systems V 700 Cyg and AW Vir, based on photoelectric B and V light curves, is presented. Since no spectroscopic mass-ratio is available, the q-search method was applied to find the preliminary range of the mass-ratio in order to search for the final solution. First, an unspotted solution was carried out by using the unperturbed parts of the light curves and applying the DC program of the WD code. The final spotted solution was made by adopting spot models for the two systems. In the case of V 700 Cyg, two cool spots were placed on the primary (more massive, larger and cooler) component to explain the O’Connell effect (MaxII fainter than Maxi) and a small deficit of light just before MaxI. In the case of AW Vir, one cool spot was placed on the primary component to explain the relatively large O’Connell effect. Both objects are W-type systems with partial eclipses; V 700 Cyg is fairly evolved while AW Vir is rather unevolved

A multiwavelength study of the classical nova V4169 Sagittarii

We present multiwavelength observations of V4169 Sgr (Nova Sgr 92#2) from 1000A to 2cm for the first year following visual maximum. V4169 Sgr was an `Fe II' type nova, and evolved from P^0^_fe_ to N_0_ in the Tololo nova classification scheme. The ultraviolet spectrum was very similar to that of nova PW Vul, and the near infra-red spectrum was dominated by Lybeta pumped Oi emission in the early decline. There was significant apparent deceleration of absorption lines before optical maximum. High resolution optical spectroscopy reveal unusual details of the evolution of blue-shifted P Cygni absorptions; we show that these are consistent with the advance of an ionisation front through the absorbing material. The evolution of absorption systems and emission line profiles are consistent with accelerating wind models of nova ejection. We find an extinction of E(B-V)=0.35+/-0.05, and a distance of ~6.5kpc. The nova shows significant enrichment of helium and nitrogen relative to solar, but not of carbon or oxygen; however our abundances are probably underestimated