Lithium in a metal-poor external galaxy: ω Centauri

Abstractω Centauri is a massive stellar system which is currently going through the Galactic Halo. Its compact aspect and spheroidal shape have for a long time led to it being classified as a Globular Cluster. However the fact that its stars cover a wide metallicity range (−0.6 < [Fe/H] < −2.1), points to this object as an external galaxy, satellite of the Milky Way. Lithium among warm metal-poor stars shows a roughly constant abundance, the “Spite Plateau”. This has been interpreted as evidence for a primordial origin of the lithium nucleus, at the time of nucleosynthesis. After the physical conditions under which nucleosynthesis occurred, have been constrained by the observations of the fluctuations of the Cosmic Microwave Background, we are facing a “cosmological lithium problem”, namely the primordial lithium was a factor of three to four higher than what observed in the Spite plateau. Several avenues may be taken to solve this conundrum, either relying on fundamental physics or on stellar physics, however the realm of possibilities may be considerably narrowed by observing stellar populations in different galaxies, which have experienced different evolutionary histories. Some of the proposed “solutions” may be clearly ruled out, depending on the observation of lithium in the metal-poor populations of external galaxies. ω Centauri is the only external galaxy amenable to such an investigation in the era of 8m telescopes. We have pushed to its limits FLAMES at the ESO 8.2m telescope to obtain high resolution spectra of the Li i doublet in 91 Turn-Off and Sub-Giant stars at V ~ 18 in ω Centauri. We present our preliminary results on this data which suggest that the Li content in ω Centauri warm stars is comparable to that observed in Galactic Halo field stars of similar metallicities and temperatures. This may effectively rule out a whole class of models which invoke a severe Li depletion through processing of material in an early generation of massive stars.

A New Technique for Characterising Planetary Nebulae in External Galaxies

Planetary Nebulae are proving to be powerful tertiary distance indicators as well as sensitive probes of a galaxy’s velocity field and mass distribution. We present a new technique for determining the positions, luminosities and radial velocities of the PNe in an external galaxy which may enable a one-step measurement of all these parameters.

Wolf-Rayet stars in the Magellanic Clouds

The most striking feature of the Wolf-Rayet stars in the Magellanic Clouds is their subtype distribution. The range of WC subtypes found in the Large and Small Cloud agrees with evolutionary model predictions at the corresponding metallicities. It follows that the WR subtype distribution in any external galaxy is a clear indicator of the metallicity in that galaxy.The fluxes in the emission lines of WC stars appear to be consistent within the LMC. This can be used (with some precautions) in the Milky Way and other galaxies to determine reddening, distance and the numbers of WR stars in compact regions.

The Detailed Magnetic Field Structure in the SW Quadrant of M31

A three-dimensional, arc-like structure in the magnetic field was found coming out of the plane of M31. This structure may be the first Parker-Jeans instability observed in an external galaxy.

Planetary Nebulae as Standard Candles for Extragalactic Distances

Although distances to galactic planetary nebulae are typically uncertain by factors near 2, there appears to be an upper limit to the integrated [O III] λ5007 absolute flux for planetary nebulae. This flux, 1.34 × 10−8 ergs/cm2/sec for a maximally bright planetary nebula at a distance of 1 kpc, can be used as a distance indicator when a sufficient number of planetaries have been identified in an external galaxy. Or one may compute the upper limit to the distance for galactic planetary nebulae based on this calibration.

Cosmological Information from X-Ray Observations

The fact that x-ray astronomy could yield cosmological information was apparent in the earliest rocket flights carried out in 1962 and 1963 (Giacconi et al, 1962, Bowyer et al, 1963). These flights established that diffuse radiation was present around the sky; indeed one of the first theoretical papers commenting on the x-ray results was by Fred Hoyle (1963) showing that the observed background flux was below that predicted for the hot, steady-state universe. A few years later, the first discrete source was credibly identified with an external galaxy (Byram, et al. 1966).