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Field O Stars: A Mode of Sparse Star Formation

Field O Stars: A Mode of Sparse Star Formation. Joel Lamb Sally Oey University of Michigan. Background. Dwarf galaxies exhibit inefficient star formation Fundamental properties of global star formation Stellar initial mass function (IMF) N(m) dm  m -2.35 dm (Salpeter, 1955)

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Field O Stars: A Mode of Sparse Star Formation

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  1. Field O Stars: A Mode of Sparse Star Formation Joel Lamb Sally Oey University of Michigan

  2. Background • Dwarf galaxies exhibit inefficient star formation • Fundamental properties of global star formation • Stellar initial mass function (IMF) • N(m) dm  m-2.35 dm (Salpeter, 1955) • Different slope? • e.g. Field Star IMF steeper in SMC (Massey 2002) • Cluster mass function & clustering law • n(M) dM  M-2 dM • n(N*) dN* N*-2 dN* • Low mass, N* cutoff? • Field O stars probe an extreme regime

  3. Target: Field O Stars • Probe the limits of sparse, massive star formation • Origin of Field O Stars • “Tip of the iceberg” • Formed in isolation? • Runaways • IMF of any companions • Separate mode of star formation?

  4. SMC • Spatially complete sample of O stars • 376 total O stars, 91 field stars • SMC O star cluster distribution • n(N*) dN* N*-2.3 dN* (Oey, King, & Parker 2004) • Slope extends to N*=1 • Field O stars • SMC close neighbor • HST ACS

  5. Observations • Targets selected using ground-based imaging • Magellanic Clouds Photometric Survey (MCPS) (Zaritsky et. al. 2002) • New HST ACS SNAP observations • 8 SMC field O stars observed • F555W (V) and F814W (I) filters - probe to F0 V and G0 V stars 1 pc

  6. Search for Companions • Map stellar distribution around the 8 field O stars • F814W image • Probes to 1 M • Stellar density enhancements • Poisson Statistics

  7. “Tip-of-an-Iceberg” • Probability = pn · e-p / n! < 0.01%

  8. Sparse Star Formation • 3 of 8 field O stars show evidence of companions • Galactic study: 12% of field O stars have evidence of clustering (de Wit, et al. 2004) Extremely low mass star-forming regions with O stars

  9. IMF of Companions • Reddening: MCPS (Zaritsky et. al. 2002) • Constrain masses of companion stars • Geneva stellar evolutionary tracks • SMC metallicity (Charbonnel et. al. 1993) • IMF of “tip-of-the-iceberg” clusters • Small samples • Composite IMF

  10. “Tip-of-the-Iceberg” IMF • Flat Composite IMF • Missing low mass stars • Consistent with Salpeter? • Different regime of star formation Salpeter slope = -1.35 Fitted slope = -0.67±0.71 Potentially non-standard IMF

  11. Work In Progress • Analysis of Results • Fit within empirical framework? • Global clustering law • Initial Mass Function • Mecl - mmax relation • mmax,2/mmax Weidner & Kroupa (2006) Oey & Clarke (2005)

  12. Future Work • Remaining stars truly alone? • Completing the spectroscopic survey of SMC field O Stars • Field massive star IMF • Runaway Fraction • Binary Fraction

  13. Conclusions • 3 of 8 observations confirmed as sparse star-forming regions with O stars • Important role in dwarf galaxies? • “Tip-of-the-iceberg” flat IMF • Missing low-mass companions • Different mode of star formation?

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