Photoionization Models of NGC 2363 and Their Implications for the Ionizing Star Cluster

Details Photoionization Models of NGC 2363 and Their Implications for the Ionizing Star Cluster Photoionization Models of NGC 2363 and Their Implications for the Ionizing Star Cluster

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...527..110l&link_type=abstract

The Astrophysical Journal, Volume 527, Issue 1, pp. 110-124.

Astronomy and Astrophysics

Astronomy

64

Galaxies: Starburst, Galaxies: Star Clusters, Ism: H Ii Regions, Ism: Abundances, Stars: Wolf-Rayet

Scientific paper

Using the photoionization code CLOUDY, we compute photoionization models for the giant extragalactic H II region NGC 2363 and compare them with optical observational data. We mainly focus on F(Hβ), Ne, EW(Hβ), and the ratios of I(λ5007), I(λ4363), I(λ3727), I(λ6300), I(λ6720), and I(λ4686) to I(Hβ). We discuss the variations of the emission spectra obtained with different input parameters. With low-metallicity models (Z=0.10 Zsolar) we were not able to reproduce the observed features of the spectrum. We review the implications of the λ4686 feature on the stellar population of NGC 2363, showing that it might indicate the presence of Wolf-Rayet (W-R) stars, a fact that would conflict with the metallicity of the region. We suggest several possible solutions to this contradiction, such as inadequate stellar evolutionary tracks, a nonstandard star formation process, and a revised metallicity. Focusing on the last possibility, we further show that the disagreement can be satisfactorily overcome by allowing for spatial temperature fluctuations in the nebula. The presence of temperature fluctuations allows a self-consistent scenario, which naturally accounts for the origin of the fluctuations themselves as a result of injection of mechanical energy by W-R winds and supernova explosions. Accordingly, we show that the metallicity of NGC 2363 has most probably been underestimated and that a value of Z~=0.25 Zsolar is in better agreement with the observational data than the usually adopted value Z~=0.10 Zsolar. We further find that a star formation episode extended over a time interval of ~1.6 Myr gives a better fit than a strictly instantaneous burst. We also derive values for the slope and the high-mass end of the initial mass function, the age of the stellar cluster, and the total gaseous mass of the H II region.

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