1 / 20

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

lexiss
Download Presentation

Massive Star Populations in Wolf-Rayet Galaxies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related