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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)
• 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
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?
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
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
Search for Companions
• Map stellar distribution around the 8 field O stars– F814W image
– Probes to 1 M
– Stellar density enhancements
• Poisson Statistics
“Tip-of-an-Iceberg”
• Probability = pn · e-p / n! < 0.01%
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
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
“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
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)
Future Work
• Remaining stars truly alone?
• Completing the spectroscopic survey of SMC field O Stars– Field massive star IMF– Runaway Fraction– Binary Fraction
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?