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Date and Time of the Query: 2019-04-24 T02:47:50 PDT
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For refcode 1988A&A...206..219R:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1988A&A...206..219R Supernova rates and bursts of star formation O.-G, Richter and M. Rosa Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA The Space Telescope European Coordinating Facility, European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-8046 Garching bei Munchen, Federal Republic of Germany Received December 29, 1987; accepted March 31, 1988 Summary. A new analysis of supernova (SN) rates in spiral and irregular galaxies is presented. Most notably, the concept of a single average SN rate for all "normal" galaxies has to be abandoned regardless of whether one normalizes with galaxy luminosity or mass or treats galaxies as virtually identical test objects. It is shown that normal galaxies belong to two distinct classes as far as the formation of SN II progenitors is concerned: most galaxies are rather inactive while a small percentage of galaxies in a "burst" phase is producing these massive stars vigorously at a rate roughly 70 times that of the "inert" galaxies. This fact is most straightforwardly explained by star formation bursts. The recurrence timescale of such bursts is several 100 times longer than their duration, viz, less than 10^7^ years. Furthermore, the initial mass function during such bursts must be notably different from that determined in the solar neighborhood. It has to be strongly biased toward stars with masses above about 8 M_sun_. For a power law IMF either a change in slope of order 1 and/or a lower mass limit of about 2M_sun_ are necessary to avoid complete exhaustion of the gaseous matter on timescales of order 10^7^ to 10^8^ years. The above "star burst" phenomenon has to be distinguished from the commonly mentioned star burst activities near the nuclei of IRAS galaxies and enigmatic objects like, e.g., M 82. We find that the formation of massive stars follows a fundamentally different pattern than that of low and intermediate mass stars. It occurs episodically and apparently over entire disks of spirals, in particular Sc galaxies. Its relative intensity, however, is apparently only detected through enhanced SN II rates. The salient feature of this activity is the incompatibility of mass consumption rates and available gaseous matter in a constant star formation rate scenario, calling for short duration activity with modified mass functions, i.e., "bimodal" (or biased) star formation in the mass, space and time domains. Key words: supernovae - spiral galaxies - star formation - star bursts
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