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 = p n · 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 Potentially non-standard IMF Fitted slope = -0.67±0.71 Salpeter slope = -1.35

11. Work In Progress Analysis of Results –Fit within empirical framework? Global clustering law Initial Mass Function –M ecl - m max relation –m max,2 /m max 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?