The Star Formation Histories and Efficiencies of Two Giant H II Regions in M33

Abstract

view Abstract Citations (22) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Star Formation Histories and Efficiencies of Two Giant H II Regions in M33 Wilson, Christine D. ; Matthews, Brenda C. Abstract We have used UBV photometry to re-identify the OB associations which power the two most luminous H II regions in M33, NGC 604 and NGC 595. Color-magnitude diagrams of the two OB associations reveal a significant difference (2-3 Myr) in the ages of the most recent star formation episode in these two regions. In addition, the presence of evolved, low-mass red supergiants in NGC 595 shows that this region has undergone a prior episode of star formation ∼10-15 Myr ago. These data, combined with the presence of molecular clouds in the heart of NGC 604, suggest that molecular clouds may survive at least one intense episode of massive star formation. The different star formation histories of the two regions provide a good explanation for their different gas structure: in NGC 595 the interstellar medium is primarily atomic, since the massive stars have had enough time to photodissociate most of the molecular gas, while the younger NGC 604 is more typical of star-forming regions and is dominated by molecular gas. The number of Wolf- Rayet and other evolved stars is consistent with the estimated turnoff ages and the number of stars still on the main sequence. The star formation efficiencies (mass of stars per mass of gas) of these two H II regions are up to a factor of 3 larger than the average efficiency in the inner disk of M33 or in Galactic molecular clouds but are still only 2%-5%. The very large Hα luminosities of these regions appear to be a product of the increased star formation efficiency, the large gas reservoir which allows a much larger number of massive stars to be formed, and the tendency for a young coeval stellar population to produce more ionizing photons than a steady state population of similar mass. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176561 arXiv: arXiv:astro-ph/9506104 Bibcode: 1995ApJ...455..125W Keywords: GALAXIES: INDIVIDUAL MESSIER NUMBER: M33; GALAXIES: STAR CLUSTERS; GALAXIES: STELLAR CONTENT; ISM: H II REGIONS; ISM: INDIVIDUAL NGC NUMBER: NGC 604; ISM: INDIVIDUAL NGC NUMBER: NGC 595; STARS: FORMATION; Astrophysics E-Print: 26 pages including 2 figures, uuencoded compressed postscript file (Figure 1 not available electronically). Accepted to ApJ full text sources arXiv | ADS | data products SIMBAD (78) NED (3)