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Massive Star Populations in Wolf-Rayet Galaxies

Massive Star Populations in Wolf-Rayet Galaxies. Adam Ginsburg Paper by I.F. Fernandes, R. De Carvalho, T.Contini, and R.R. Gal. Background. Wolf-Rayet stars are high mass, strong stellar wind, hot stars. A phase of evolution of very massive stars Strong winds cause significant mass loss

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Massive Star Populations in Wolf-Rayet Galaxies

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  1. Massive Star Populations in Wolf-Rayet Galaxies Adam Ginsburg Paper by I.F. Fernandes, R. De Carvalho, T.Contini, and R.R. Gal

  2. Background • Wolf-Rayet stars are high mass, strong stellar wind, hot stars. • A phase of evolution of very massive stars • Strong winds cause significant mass loss • Divided into 3 classes: • WN: Nitrogen dominant • WC: Carbon dominant, no Nitrogen • WO: C/O ratio <1 (rare) • WR Galaxies show a broad emission in 4686 HeII, and about ¼ also show strong NIII 4640 emission.

  3. Purpose • Wanted to test predictions of Schaerer and Vaaca model of WR/O star populations in starburst galaxies • Predicted that the number of WR stars relative to O stars would increase with increased metallicity • Also set constraints on starburst period

  4. Observations • 14 WR galaxies observed using Palomar 200 inch telescope and ESONTT • Long-slit spectra (180”) were taken with wavelength coverage 3600-6700 Å for Palomar, 4000-6600 Å for ESO (120”) • Spectral resolution was 5.6-5.9 Å • Images processed using IRAF

  5. Determining Population • Absolute population of WR subtype can be determined from Milky Way WR star average luminosity estimates • Total luminosity of subtype (in one of 3 spectral lines identified) divided by average luminosity gives count • To determine O-type population, assumed that all ionizing photons come from O or WR stars • Presence of shocks eliminated by observing low excitation lines and determining ratio to Hß must be stellar, not AGN or supernova • Total ionizing photons measured from Hß emission

  6. Number Estimates • Absolute number of WR stars was determined using blue bump (4686 Å) and red bump (5808 Å) luminosities • WN stars are indicated by the presence of NIII 4640 Å lines and absence of NV 4604 lines • Flux of the blue hump was measured to count the number of WNL (late) stars • Nebular contributions and WR emission lines were removed, then HeII 4686 Å flux was measured

  7. Spectra

  8. Spectra

  9. Red Bump • WC stars are split into Early and Late type • CIII 5696 Å lines indicate WCL • CIV 5808 Å lines in the absence of CIII 5696 Å indicate WCE • WCN stars cannot be responsible for CIV 5808 emission because the HeII 4686/CIV 5808 ratio in spectra does not agree with WCN spectra

  10. Spectra (red bump)

  11. Determining Population Cont’d • Number of O stars is determined using O7V because the observed level of ionization is low, so they represent the average (Osterbrock & Cohen 1982) • A correction is used to determine the total count of O stars • Ratio of O7V to O stars is a parameter dependent on the time elapsed since the beginning of the burst • Calculated using SV98 model

  12. Populations of WR and O stars

  13. Number ratio vs. Metallicity • The solid lines are SV98 models with different IMFs • The dotted lines are Starburst99 models with different elapsed times • Extended burst more likely for higher metallicity

  14. High Metallicity • Solid line is SV98, dotted line is SGIT00 • Dash-dot is Local Group ratios consistent with constant star formation • Shows best agreement • Low NWC/NWN ratio may be because of mistaken assumption on CIV emission

  15. Extended Burst model • Low NWC/NWN ratio is consistent with extended burst model • Hß equivalent widths support ages >5.3 Myr • At late stages of evolutionary model, WN stars present, WC stars go to zero • Presence of late type (red giant) stars also indicates extended burst likely • Instantaneous burst models fail for low NWR/NO

  16. Age • Ages ranged from 2.6-6.1 Myr • WC stars are present in older galaxies (not instantaneous) • Low Hß equivalent widths in high metallicity galaxies show older

  17. Conclusions • In low-metallicity galaxies, the WR-O ratios fit well with the SV98 evolutionary synthesis model • In high-metallicity galaxies, the ratio was below predictions. An extended burst model is supported by TiO bands in the spectra (indicates older stars present) • Better fit by 2-4Myr extended starburst with Salpeter IMF slope

  18. Future Work • A spectral study in the infrared would provide information about older stellar populations • Spectroscopy in the UV range would provide data on younger stellar populations • These, combined with this paper, would constrain upper and lower limits for the time since the start of the burst

  19. More on the SV98 model • In low metallicity regions, WR stars are formed by binary mass transfer, creating primarily WN stars

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