The Stellar Initial Mass Function in the Giant HII Region NGC 595

Details The Stellar Initial Mass Function in the Giant HII Region NGC 595 The Stellar Initial Mass Function in the Giant HII Region NGC 595

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....111.1128m&link_type=abstract

Astronomical Journal v.111, p.1128

Astronomy and Astrophysics

Astronomy

28

Galaxies: M33, Hii Regions, Galaxies: Stellar Content

Scientific paper

As part of a larger effort to study the resolved and composite properties of the giant H II regions in Messier 33, we have analyzed multiband HST/WFPC-2 images of NGC 595 in terms of the ionizing cluster's resolved stellar population. Photometric reductions of the PC images yield 100 stars on the UV image, 272 stars on the U image, 345 stars on the B image, and 561 stars on the V image. A total of 267 stars are common to the U, B, and V images while 86 stars are detected on all 4 images. Although some clustering is evident, the degree of central concentration is less than that seen in 30 Doradus. The resulting U-B vs B-V diagram of the resolved stars is used to determine the reddening of each star. The average reddening derived from this diagram is E_B-V_ = 0.36+/-0.28 mag. The dereddened color-M_V_ diagram is best fit by a model cluster having an age of 4.5 +/- 1.0 Myr, and hence initial masses no greater than 51 M_sun_. A total of 13 supergiant stars and 10 candidate WR stars [Drissen et al. (AJ, 105,1400, 1993), and references therein] are identified with M_V_ = -5 to -8 mag. The remainder are main-sequence O-type (98) and early B-type (>145) stars with M_V_ = 1 to -6 mag. The ratio of WR to O stars is WR/O = 0.11+/- 0.01, roughly the same as found in the core of 30 Doradus. The resulting luminosity function has a slope of α = - 0.71. The derived IMF has a slope of {GAMMA} = -1.32+/-0.02 before subtracting a background component, and {GAMMA}= - 1.00+/-0.05 after subtracting a background based on photometry of the surrounding WF images. Integration of the derived IMF down to a lower mass limit of 4 M_sun_ yields a total mass of 7350 M_sun_, while integration down to 0.1 M_sun_ yields a total mass of 18000 M_sun_. The total estimated ionizing luminosity is 5.0 x 10^50^ photons s^-1^, roughly half that which is inferred from the Hα luminosity in this region. This shortfall of ionizing photons can be reconciled by allowing for a spread in the stellar ages, and/or by increasing the modeled EUV luminosity of the stars at the inferred cluster age.

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