VLBI observations of supernova remnants in Messier 82

We have used the European VLBI Network (EVN) at 18 cm to study five of the more compact radio sources in the starburst galaxy M82. The angular resolut ion of the observations is 15 mas, corresponding to 0.2 pc at the distance of M82, The observations reveal shells ranging in diameter from 40 to 90 ma s (0.6 to 1.4 pc), although the strongest source (41.95 + 575) is only marg inally resolved by these measurements (similar to 20 x 10 mas(2)). We have found clear evidence for expansion in one of the shell sources (43. 31 + 592) by re-analysing, in wide-field mode, EVN data taken in 1986. Betw een 1986 and 1997 this source has increased its diameter by 13.6 + 2 mas, c orresponding to an average expansion velocity of 9850 +/- 1500 kms(-1). If we assume that the remnant is in free expansion, this is consistent with a supernova event in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact source (41.95 + 575) which was kno wn to be present in the 1960s. 41.95 + 575 shows no clear evidence for expa nsion (<4000 km s(-1)), consistent with a greater age; this is further evid ence of its anomalous status. Comparison of the EVN images with earlier MER LIN data is also consistent with expansion in at least two more of the sour ces. We discuss the flux density variability of the compact sources in M82 and conclude that, with the exception of 41.95 + 575 and two transient sour ces, there is little evidence for significant changes in flux density of mo st of the remnants since the early 1980s.