Cloud Structure and the Origins of the Stellar Initial

Mass Function in ρ-Ophiuchus

Dylan R. NelsonDepartment of Astronomy, University of California, Berkeley

Jonathan J. SwiftInstitute for Astronomy, University of Hawaii, Manoa

Jonathan P. WilliamsInstitute for Astronomy, University of Hawaii, Manoa

- 7 August, 2007 -

Introduction• Stars: fundamental objects in

astronomy– Stellar IMF determines the

evolution of the chemical composition of the universe

• How do we probe the initial conditions of star formation?– Deep, multi-wavelength coverage of

regions of active SF

Motivation• Isolated grav. collapse

ok, but active SF sites more complicated:– environment &

interactions• Nearby dense

molecular clouds:– active low-mass SF– objects in a wide range

of evolutionary stagesM16: Eagle Nebula

“Pillars of Creation”

• Cold & dense cores stars

• DCMF tail has a Salpeter slope!

• Two distributions have the same shape?– “Direct progenitors”

& 1-to-1 mapping

Motte et al., 1998.

Motivation

Alves et al., 2007.

~30%

Motivation• Cold & dense cores

stars• DCMF tail has a

Salpeter slope!• Two distributions

have the same shape?– “Direct progenitors”

& 1-to-1 mapping

• Giant molecular cloud (GMC)– Nearby (D ~ 130pc), Cold (Td ~ 15K)

• Large number of young stars• Large amount of mass in cold molecular gas• Goals:

– Examine relation between clump/stellar populations– Comment on the origins of the stellar IMF– Examine previous claims of similarity between the

DCMF and IMF

ρ-Ophiuchus

AV Dust Extinction

1.2mm Dust Emission

Spitzer IR (IRAC/MIPS)

Molecular Line Tracers (13CO, N2H+)

Stellar Surveys (NIR/MIR)

• Dust emission: high resolution, good tracer• Algorithms and techniques:

– Clumpfind: Contour following & shape independence• Wavelet decomposition & background subtraction

– Gaussclumps: Iterative Gaussian fitting & subtraction– Bayesian: Source likelihood & MCMC photometry

Finding Clumps

• What do we know?– size, mass, location, velocity dispersion

Core Mass Function

“Starless” Cores

Stars Masquerading as Cores

Column Density vs Clump Size

AV ~ 1

AV ~ 2

Substructure

• Virial mass approach:– Balance grav. potential energy U with internal

kinetic support K

Gravitational Stability

• Fit 13CO line for velocity dispersion of clumps:

☺

Gravitational Stability

• Fit seven hyperfine components of the N2H+ (Andre et al., preprint) line for velocity dispersion of clumps:

Gravitational Stability

Conclusions• 1-to-1 mapping of cores stars questionable• Clearly some relationship exists between clump and star

distributions– But, “direct progenitors” idea in doubt– Multiple star systems?

• Big unknowns:– turbulence & fractal nature of the ISM– magnetic/other internal support– outflows, feedback, self-regulation– how do we, really, define a clump?

• What we need: next generation, multi-wavelength, observations w/ higher sensitivity and resolution