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Session 49 - Starburst Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
IUE low resolution spectra collected from new and archival data for the starburst knots (knot 6,10,14,16,18 amp; 21) along the major ridge line and at knot 19 near a supernova remanent within NGC 4449, show evidence of spectacular star formation and the presence of young hot stars with ages less than 12 million years (Myrs). Using a new population synthesis code and a corresponding spectral library of O amp; B stars constructed from high dispersion data, the ages for these starburst knots can be deduced accurately to within 1 to 2 million years by matching specific age sensitive spectral features. A single burst model of star formation best fits knots 6, 19 amp; 21. Knots 6 amp; 19 located along the northern periphery of galaxy exhibit a starburst age of less than 6 MYRS with a stellar content mostly populated by O-type supergiants as seen from slight mass loss (P-Cygni) profiles from Si IV ( 1393,1402 Åand C IV (1548,1550 Ålines. Knot 21 shows a single burst of star formation that ended 12 Myrs ago.
Knots 10,14,16 amp; 18 along the ridge line represent several generations of stars with at least two recent bursts of star formation occurring within the last 12 Myrs. In addition to an older baseline age, each of these knots exhibit strong P-Cygni profiles at N V (1238,1242 ÅSi IV (1393,1402 Åamp; C IV (1548,1550 Åwhich are characteristic of massive O stars aged 5 Myrs or younger. Population synthesis analysis to narrow slit extractions from IUE ELBL data for these knots provide additional evidence of two separate bursts of star formation (one 5 Myrs, the other older). An unusually, low extinction of E(B-V)=0.03 is measured for all knots using a 30 Doradus extinction law by fitting the continuum suggesting a very flat UV extinction law for NGC 4449. These results imply at least two separate episodes of star formation along the major ridge with an ongoing process of star formation possibly in the north and central regions of NGC 4449 where major reservoirs of H I gas resides.
Program listing for Tuesday