J. Köppen

On the Cosmic Origins of Carbon and Nitrogen

The behaviour of C/O and N/O ratios as a function of metallicity in HII regions in galaxies is characterised, and used to derive rough values of chemical yields from analytic models. These ‘analytic yields’ are used to select the best available numerical yields from published stellar nucleosynthesis calculations. This gives a reasonably coherent picture of the important production sites of carbon and nitrogen, with carbon coming from massive stars and nitrogen from intermediate mass stars. The effects of gas inflow and stellar nucleosynthesis time-delay on element ratios C/O and N/O are expected to be small, but may be detectable for N/O. The small dispersion in values of N/O observed in low-metallicity blue compact dwarf galaxies may indicate that these systems have fairly long-lived quiescent star formation. Brief consideration of the CNO cycles suggests that the most recent values for the 17O(p,α)14N cross-section are certainly better than older values.

A possible origin of the mass–metallicity relation of galaxies

Observations show that galaxies follow a mass-metallicity relation over a wide range of masses. One currently favoured explanation is that less massive galaxies are less able to retain the gas and stellar ejecta and thus may lose the freshly produced metals in the form of galactic outflows. Galaxies with a low current star formation rate have been found to contain star clusters up to a lower mass limit. Since stars are predominately born in clusters, and less massive clusters have been found to be less likely to contain very massive stars, this implies that in environments or at times of low star formation, the stellar initial mass function does not extend to as high masses as during high star formation epochs. It is found that the oxygen yield is reduced by a factor of 30 when the star formation rate is decreased by 3 to 4 orders of magnitude. With this concept, chemical evolution models for galaxies of a range of masses are computed and shown to provide an excellent fit to the mass-metallicity relation derived recently by Tremonti et al. Furthermore, the models match the relation between galaxy mass and effective yield. Thus, the scenario of a variable integrated stellar initial mass function, which is based on the concept of formation of stars in clusters, may offer an attractive alternative or partial explanation of the mass-metallicity relation in galaxies.

Effects of episodic gas infall on the chemical abundances in galaxies

The chemical evolution of galaxies that undergo an episode of massive and rapid accretion of metal-poor gas is in- vestigated with models using both simplified and detailed nucleosynthesis recipes. The rapid decrease of the oxygen abundance during infall is followed by a slower evolution which leads back to the closed-box relation, thus forming a loop in the N/O-O/H diagram. For large excursions from the closed-box relation, the mass of the infalling material needs to be substantially larger than the gas remaining in the galaxy, and the accretion rate should be larger than the star formation rate. We apply this concept to the encounter of high velocity clouds with galaxies of various masses, finding that the observed properties of these clouds are indeed able to cause substantial effects not only in low mass galaxies, but also in the partial volumes in large massive galaxies that would be affected by the collision. Numerical models with detailed nucleosynthesis prescriptions are constructed. We assume star formation timescales and scaled yields that depend on the galactic mass, and which are adjusted to reproduce the average relations of gas fraction, oxygen abundance, and effective oxygen yield observed in irregular and spiral galaxies. The resulting excursions in the N/O-O/H diagram due to a single accretion event involving a high velocity cloud are found to be appreciable, which could thus provide a contribution to the large scatter in the N/O ratio found among irregular galaxies. Nonetheless, the N/O-O/H diagram remains an important indicator for stellar nucleosynthesis.

An Interacting Binary System Powers Precessing Outflows of an Evolved Star

Not Single After All Planetary nebulae form toward the end of the lives of sunlike stars. They appear after the star has shed its outer layers, and radiation from what is left of it ionizes the surrounding medium. Using the Very Large Telescope in Chile, Boffin et al. (p. 773) obtained spectra of the star at the center of Fleming 1, a point-symmetric planetary nebula with rotating bipolar jets. It has long been assumed that jets like these arose from an interacting binary system. Indeed, the data reveal that the central star in Fleming 1 has a companion in a very close orbit. Spectra of a planetary nebula’s central star reveal a companion star responsible for launching the system’s rotating jets. Stars are generally spherical, yet their gaseous envelopes often appear nonspherical when ejected near the end of their lives. This quirk is most notable during the planetary nebula phase, when these envelopes become ionized. Interactions among stars in a binary system are suspected to cause the asymmetry. In particular, a precessing accretion disk around a companion is believed to launch point-symmetric jets, as seen in the prototype Fleming 1. Our finding of a post–common-envelope binary nucleus in Fleming 1 confirms that this scenario is highly favorable. Similar binary interactions are therefore likely to explain these kinds of outflows in a large variety of systems.

Gas stripping in galaxy clusters: A new SPH simulation approach

Aims. The influence of a time-varying ram pressure on spiral galaxies in clusters is explored with a new simulation method based on the N-body SPH/tree code GADGET. Methods. We have adapted the code to describe the interaction of two different gas phases, the diffuse hot intracluster medium (ICM) and the denser and colder interstellar medium (ISM). Both the ICM and ISM components are introduced as SPH particles. As a galaxy arrives on a highly radial orbit from outskirts to cluster center, it crosses the ICM density peak and experiences a time-varying wind. Results. Depending on the duration and intensity of the ISM-ICM interaction, early and late type galaxies in galaxy clusters with either a large or small ICM distribution are found to show different stripping efficiencies, amounts of reaccretion of the extra-planar ISM, and final masses. We compare the numerical results with analytical approximations of different complexity and indicate the limits of the Gunn & Gott simple stripping formula. Conclusions. Our investigations emphasize the role of the galactic orbital history to the stripping amount. We discuss the contribution of ram pressure stripping to the origin of the ICM and its metallicity. We propose gas accumulations like tails, filaments, or ripples to be responsible for stripping in regions with low overall ICM occurrence.

A barium central star binary in the Type I diamond ring planetary nebula Abell 70

Abell 70 (PN G038.1−25.4, hereafter A 70) is a planetary nebula known for its diamond ring appearance due to a superposition with a background galaxy. The previously unstudied central star is found to be a binary consisting of a G8IV–V secondary at optical wavelengths and a hot white dwarf at ultraviolet wavelengths. The secondary shows Ba II and Sr II features enhanced for its spectral type that, combined with the chromospheric Hα emission and possible 20– 30 km s −1 radial velocity amplitude, firmly classifies the binary as a Barium star. The proposed origin of Barium stars is intimately linked to planetary nebulae (PNe) whereby wind accretion pollutes the companion with dredged-up material rich in carbon and s-process elements when the primary is experiencing thermal pulses on the asymptotic giant branch (AGB). A 70 provides further evidence for this scenario together with the other very few examples of Barium central stars. The nebula is found to have Type I chemical abundances with helium and nitrogen enrichment, which when combined with future abundance studies of the central star, will establish A 70 as a unique laboratory for studying s-process AGB nucleosynthesis. We also discuss guidelines to discover more binary central stars with cool secondaries in large orbits that are needed to balance our knowledge of binarity in PNe against the currently better studied post-common-envelope binary central stars.

Chemical compositions and plasma parameters of planetary nebulae with Wolf-Rayet and wels type central stars

Aims. Chemical compositions and other properties of planetary nebulae around central stars of spectral types [WC], [WO], and wels are compared with those of “normal” central stars, in order to clarify the evolutionary status of each type and their interrelation. Methods. We use plasma diagnostics to derive from optical spectra the plasma parameters and chemical compositions of 48 planetary nebulae. We also reanalyze the published spectra of a sample of 167 non-WR PN. The results as well as the observational data are compared in detail with those from other studies of the objects in common. Results. We confirm that [WC], [WO] and wels nebulae are very similar to those “normal” PN: the relation between [N II] and [O III] electron temperatures, abundances of He, N, O, Ne, S and Ar, and the number of ionizing photons show no significant differences. However, some differences are observed in their infrared (IRAS) properties. wels nebulae appear bluer than [WR] PN. The central star’s spectral type is clearly correlated with electron density, temperature and excitation class of the nebula, [WC] nebulae tend to be smaller than the other types. All this corroborates the view of an evolutionary sequence from cool [WC 11] central stars inside dense, low excitation nebulae towards hot [WO 1] stars with low density, high excitation nebulae. The wels PN, however, appear to be a separate class of objects, not linked to WRPN by evolution: nebular excitation, electron temperature and density, and the number of ionizing photons all cover the whole range found in the other types. Their lower mean N/O ratio and slightlylower He/H suggest progenitor stars less massive than for the other PN types. Furthermore, the differences between results of different works are dominated by the differences in observational data rather than differences in the analysis methods.

An inverse method to interpret colour-magnitude diagrams

An inverse method is developed to determine the star formation history, the age-metallicity relation, and the IMF slope from a colour-magnitude diagram. The method is applied to the Hipparcos HR diagram. We found that the thin disk of our Galaxy shows a peak of stellar formation 1.6 Gyr ago. The stars close to the Sun have a solar metallicity and a mean IMF index equal to 3.2. However, the model and the evolutionary tracks do not correctly reproduce the horizontal giant branch.

PFP 1: A Large Planetary Nebula Caught in the First Stages of ISM Interaction

This paper presents (Hα + (Nii)) imaging and spectroscopy of a previously unknown, highly evolved planetary nebula of low excitation which is in the first stages of an interaction with the interstellar medium (ISM). It was discovered serendipitously from AAO/UKST Hα Survey images as part of a project to exploit the survey data and has evaded detection by previous surveys due to its very low surface brightness. It is a remarkable hollow-sphere planetary nebula, some 19 � across, making it one of the largest examples of its type. We estimate a radius of 1.5 pc and a distance of 550 pc as derived from a new Hα surface brightness- radius relation. PFP 1 has near-perfect circular symmetry, broken only at the north-western edge which is coupled with significantly increased (Hα + (Nii)) intensity, both of which provide evidence for an interaction with the ISM. We find a near solar composition for this object with possibly enhanced He and N abundances. A blue central star candidate has been identified from the SuperCosmos Sky Survey data.

Ram pressure stripping of tilted galaxies

Context. Ram pressure stripping of galaxies in clusters can yield gas deficient disks. Previous numerical simulations based on various approaches suggested that, except for near edge-on disk orientations, the amount of stripping depends very little on the inclination angle. Aims. Following our previous numerical and analytical study of face-on stripping, we extend the set of parameters with the disk tilt angle and explore in detail the effects of the ram pressure on the interstellar content (ISM) of tilted galaxies that orbit in various environments of clusters, with compact or extended distributions of the intra-cluster medium (ICM). We further study how results of numerical simulations could be estimated analytically. To isolate the effect of inclination, galaxies on strictly radial orbits are considered. Methods. A grid of numerical simulations with varying parameters is produced using the tree/SPH code GADGET with a modified method for calculating the ISM-ICM interaction. These SPH calculations extend the set of existing results obtained from different codes using various numerical techniques. Results. The simulations confirm the general trend of less stripping at orientations close to edge-on. The dependence on the disk tilt angle is more pronounced for compact ICM distributions, however it almost vanishes for strong ram pressure pulses. Although various hydrodynamical effects are present in the ISM-ICM interaction, the main quantitative stripping results appear to be roughly consistent with a simple scenario of momentum transfer from the encountered ICM. This behavior can also be found in previous simulations. To reproduce the numerical results we propose a fitting formula depending on the disk tilt angle and on the column density of the encountered ICM. Such a dependence is superior to that on the peak ram pressure used in previous simple estimates.