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?