3.6. Abundances, Wolf-Rayet Stars and Supernovae
Elaborate simulations have been performed by Friedli et al. (1994) and Friedli and Benz (1995) in order to study the secular evolution of barred galaxies. These are fully self-consistent 3D numerical simulations with stars and gas, including star formation. In these simulations newly formed bars (< 500 Myr) show intense star formation along the bar, whereas star formation in the central region dominates in older bars.
Due to the radial mixing caused by the bar the chemical abundance gradient over the disk is kept low. This is in accordance with observations where a correlation has been found between the strength of the bar and the shallowness of the abundance gradient. However, the simulations also show that the intense star formation in the centre, and in some stages along the bar, steepens the abundance gradient in the inner parts. The authors predict that somewhat older bars should display an abundance distribution with two distinct slopes, steep in the inner regions and shallow in the outer.
The abundance gradient in NGC 1365 was studied by Alloin et al. (1981) who found a shallow overall [O/H] gradient of -0.016 dex kpc-1, which is significantly lower than has been estimated for the Milky Way. A more extensive and more recent study was made by Roy and Walsh (1997), using a fiber optics spectrograph on the Anglo-Australian Telescope, who list line intensities for 55 H II regions. As an overall trend they found the same decline as Alloin et al. However, they found a rather obvious break at a distance R = 185" from the centre, which as they point out would correspond to the 4:-1 resonance (Fig. 13), with the gradient having a moderate slope inside this resonance while being flat at larger radii. This may be in qualitative agreement with the prediction of Friedli and Benz, but the significance of the coincidence with the 4:-1 resonance does not seem clear.
In the most luminous of the nuclear hot spots, number L4 (Fig. 16), A.C. Phillips and Conti (1992) discovered a broad emission feature of C III 5696 which they consider derives from Wolf-Rayet stars of the WC9 subtype. This seems to be the first detection of this subtype outside our Galaxy and of a WC9 star in a giant H II region anywhere. L4 is the most oxygen rich of the 55 H II regions in NGC 1365 observed by Roy and Walsh (1997) which, according to Phillips and Conti, supports the notion that the progenitors of the WC9 stars are of extremely high mass and metal rich. Phillips and Conti estimate the number of WC9 stars in L4 to at least 300, perhaps up to 1 400.
In NGC 1365 two supernovae have appeared over the last four decades. SN 1957C in the NW main spiral arm is of unknown type. It coincides rather closely with an X-ray source (see Section 4.2). SN 1983V appeared in a very compact H II region at the front edge of the bar. This supernova is of Type Ic and has been extensively discussed by Clocchiatti et al. (1997).