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Astron. Astrophys. 346, 769-777 (1999)

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1. Introduction

Galactic bars are the sites of highly diversified star formation activities. Phillips (1993, 1996), García-Barreto et al. (1996) and Martin & Friedli (1997, hereafter MF97) have all showed that along certain bars, mostly found in late-type spirals, star formation (SF) can be quite intense. For instance, the bar of the SBcd galaxy NGC 4731 has a total star formation rate (SFR) of about 1.5 [FORMULA] yr-1 (MF97). However, in other cases, SF in the bar can be very weak or completely absent. This is the case for most of the bars in early-type barred spirals (e.g. NGC 1300, NGC 1512, NGC 3351). The origin of these differences is not yet completely understood. Numerical simulations (Friedli & Benz 1995, MF97, Martinet & Friedli 1997) and observations (Martin & Roy 1995) suggest that the existence of massive star formation in certain bars is a relatively brief event ([FORMULA]0.5-1.0 Gyr) in the evolution of a barred system and that it mostly takes place during the formation of the bar itself. The amplitude and duration of the event is, however, controlled by a complicated combination of different physical processes. For example, the time-dependent bar evolution, initial gas content, amplitude of gas flows, and mechanical energy injected in the interstellar medium (ISM) by supernovae ejecta are all factors that can influence the level of SF activity in bars (MF97). Hence, it is essential to acquire more data on the properties of the H ii regions formed in such environments to better constrain the relative importance of these factors, and consequently significantly improve SF recipes used in numerical simulations.

The morphology, SFRs and other properties of SF along the bars of a sample of eleven spiral galaxies were studied in the first paper in this series (MF97). Including the large diversity in SF activity, MF97 found that the distribution of H ii regions can be highly asymmetrical in the bar. For some SBc spirals, large regions can be present outside the bar major axis. This morphology suggests that the SF process along the bar of late-type spirals is a chaotic process, not strictly confined along the major stellar/gas orbits defined by the barred potential. In such a case, one could expect similar properties for these H ii regions when compared to the disc star forming regions. The situation could be different for bars in earlier types of galaxies for which the star forming regions are generally located next to dust lanes (MF97). For one galaxy of their sample (NGC 7479), MF97 also estimated the amount of gas flowing in the bar and falling into the galaxy center. They found that possibly as much as 75% of the gas in the bar is not transformed into stars. Numerical simulations suggest that this number is very dependent on the presence of bar-induced shocks in the star forming ISM. It is then critical to determine whether there is some signature of these shocks in the H ii regions along the bars.

In this paper, spectrophotometric data are used to derive the physical properties of a sub-sample of H ii regions found along the bars of the galaxies studied by MF97. As a comparison sample, a few regions located in the discs of these galaxies have also been analysed. Using different diagnostic line ratio diagrams as defined by Baldwin et al. (1981) and Veilleux & Osterbrock (1987), the excitation of regions located in bar environments is compared to that of "normal" disc regions (Sect. 3.1). A similar analysis is performed for the electronic density using the appropriate sulfur line ratio (Sect. 3.2). The distributions of the visual extinction for both populations are also studied in Sect. 3.3. Using the H[FORMULA] equivalent width indicator (e.g. Leitherer et al. 1999), we also infer the approximate age of H ii regions (Sect. 3.4). Since large-scale mixing of the chemical composition by bar-driven gas radial flows occurs in galaxy discs (Martin & Roy 1994; Friedli et al. 1994), the O/H distribution in bar environments is also investigated (Sect. 3.5).

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999