Bimodal abundance pattern in M51: evidence for corotation resonance effects

ABSTRACT A chemical evolution model for the bimodal-like abundance distribution in the externalgalaxy M51 recently derived on the basis of HST data for more than a half million red su-pergiants is developed. It is shown that, like in our Galaxy, formation of fine structure of theradial abundance pattern – a rather steep gradient in the internal part of the disc and a plateauin the middle part – is due to the influence of the spiral arms, t he bend in the slope of thedistribution being arose near the corotation resonance. Our model strongly suggests that M51is surrounded by overabundant gas infalling onto its disc.Key words: Galaxies: spiral - galaxies: evolution - galaxies: abundances. 1 INTRODUCTIONUntil recently, it was widely believed that the radial abundance dis-tribution (being measured in logarithmic scale) along the galacticdisc is described by a simple linear function in the Galaxy witha constant gradient. This representation is very persistent by sev-eral researches albeit some irregularities in the radial abundancedistribution along the galactic disc were pointed out by severalresearchers of H II regions and planetary nebulae. NeverthelessTwarog, Ashman & Anthony – Twarog (1997) were the firsts whotried to break this oversimplified point of view, but only aft er An-drievsky et al. (2002, see other papers of this series) the idea that thegradient in the galactic disc is not constant has gained substantialfoundation. Over a large number of elements the authors showedsufficiently definitely that the abundance pattern in the Gal axy isa bimodal, i.e., there is a rather steep gradient in the inner part ofthe disc and a plateau (or a shoulder) in the region including theSun, the bending (or fracture) in the slope of the distribution beinginside the solar circle.The results of Andrievsly et al. are quite reliable since theyused spectroscopic data of Cepheids which, in addition, have suf-ficiently precise distances and due to their brightness are s een in awide spatial region. In our Galaxy, the above abundance pattern isalso supported by the young planetary nebulae (Maciel & Quireza1999; Maciel, Costa & Uchida 2003), OB stars (Daflon & Cunha2004), etc.