The discovery of the unusual supernova SN1998bw, and its possible association with the ␥-ray burst GRB 980425 1-3 , provide new insights into the explosion mechanism of very massive stars and the origin of some classes of ␥-ray bursts. Optical spectra indicate that SN1998bw is a type Ic supernova 3,4 , but its peak luminosity is unusually high compared with typical type Ic supernovae 3 . Here we report our findings that the optical spectra

Synthetic spectra generated with the parameterized supernova synthetic-spectrum code SYNOW are compared to photospheric-phase spectra of Type Ib supernovae (SNe Ib). Although the synthetic spectra are based on many simplifying approximations, including spherical symmetry, they account well for the observed spectra. Our sample of SNe Ib obeys a tight relation between the velocity at the photosphere, as determined from the Fe II features, and the time relative to that of maximum light. From this we infer that the masses and the kinetic energies of the events in this sample were similar. After maximum light the minimum velocity at which the He I features form usually is higher than the velocity at the photosphere, but the minimum velocity of the ejected helium is at least as low as 7000 kms. Previously unpublished spectra of SN 2000H reveal the presence of hydrogen absorption features, and we conclude that hydrogen lines also were present in SNe 1999di and 1954A. Hydrogen appears to be present in SNe Ib in general, although in most events it becomes too weak to identify soon after maximum light. The hydrogen-line optical depths that we use to fit the spectra of SNe 2000H, 1999di, and 1954A are not high, so only a mild reduction in the hydrogen optical depths would be required to make these events look like typical SNe Ib. Similarly, the He I line optical depths are not very high, so a moderate reduction would make SNe Ib look like SNe Ic.Comment: 21 pages and 24 figures, submitted to Ap

The FOCAP multi‐object fibre optic system on the AAT has been used to obtain spectra for a sample of more than a hundred faint stars in the globular cluster 47 Tuc (NGC 104), from the base of the giant branch to about a magnitude below the main‐sequence turn‐off. The spectra cover the ultraviolet and blue bands of CN and the G band of CH. Quantitative abundances have been derived by comparing the observations with synthetic spectra. The main‐sequence stars can be divided into two approximately equal groups, one having N enhanced by a factor of ∼ 7, and C depleted by 40 per cent, relative to the other, similar to what is inferred for the red giants. The mean heavy‐element abundance, as measured by Ca or Fe, differs between the two groups by no more than 0.01 ± 0.05 dex.  All the available abundance data indicate that the atmospheres of 47 Tuc stars contain different amounts of material which has been processed through the CNO cycle, but not through more advanced stages of nucleosynthesis. In the red giants, this could be a result of the convective dredge‐up of processed material from deep within the stars themselves, as occurs in some more metal‐poor clusters. However, stars on the upper main sequence of globular clusters do not have convective envelopes, while any deep mixing there would have consequences incompatible with other observational data. Thus it seems that the main‐sequence C and N variations must be either primordial or caused by a pollution mechanism. The similarity of the bimodal abundance patterns among the dwarfs and giants then suggests that the dredge up of CNO‐cycled material is not the dominant process in the red giants either, at least not in 47 Tuc.  The CNO‐processed material seems most likely to have come from intermediate‐mass stars on the asymptotic giant branch; various mechanisms by which such material might appear in the main‐sequence stars are discussed. Explanations involving a truly primordial origin, or having successive generations of stars, seem to require rather special ‘tuning’. A somewhat speculative alternative self‐enrichment process is proposed, in which stellar winds from intermediate‐mass stars were captured by existing low‐mass stars, early in the life of the cluster. Whatever the mechanism, the abundance data presumably contain important clues about the formation and early evolution of globular clusters.

We present new spectroscopic and photometric data of the Type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early‐time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core‐collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal Type Ib/c supernova explosions which occur within a He‐rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf–Rayet progenitors still embedded in the He‐rich material lost by the star in recent mass‐loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf–Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a Type Ibn classification for the historical SN 1885A (S‐Andromedae), previously considered as an anomalous Type Ia event with some resemblance to SN 1991bg.

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