# UR #16: Star Cluster Evolution

The undergrad research series is where we feature the research that you’re doing. If you’ve missed the previous installments, you can find them under the “Undergraduate Research” category here.

Did you finish a senior thesis this summer? Or maybe you’re just getting started on an astro research project this semester? If you, too, have been working on a project that you want to share, we want to hear from you! Think you’re up to the challenge of describing your research carefully and clearly to a broad audience, in only one paragraph? Then send us a summary of it!

You can share what you’re doing by clicking on the “Your Research” tab above (or by clicking here) and using the form provided to submit a brief (fewer than 200 words) write-up of your work. The target audience is one familiar with astrophysics but not necessarily your specific subfield, so write clearly and try to avoid jargon. Feel free to also include either a visual regarding your research or else a photo of yourself.

We look forward to hearing from you!

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Bhawna Motwani
Indian Institute of Technology Roorkee, Uttarakhand, India

Bhawna is a final year Integrated Masters student of Physics at IIT Roorkee. This work is a part of her summer research in 2013 with Prof. Pavel Kroupa and Dr. Sambaran Banerjee at the Argelander Institut für Astronomie, Bonn, Germany.

Dynamical Evolution of Young Star Clusters

The much-debated classical scenario of star-cluster formation, first delineated by Hills (1980), suggests that the collapse of a proto-stellar core within a parent molecular gas cloud gives rise to an infant gas-embedded cluster. Subsequently, the residual gas is driven out of the cluster due to kinetic energy from stellar winds and radiation thereby diluting the gravitational cluster potential. However, pertaining to a star-formation efficiency $\epsilon$ <50% (Kroupa 2008) and slow gas-expulsion, the cluster remains fractionally bound and ultimately regains dynamical equilibrium. With the help of NBODY6 (Aarseth 1999) algorithm, we perform N-body simulations to examine the time-evolution of confinement radii ($R_f$) for various mass-fractions f of such emerging clusters. From this, we infer the cluster re-virialization times ($\tau_{vir}$) and bound-mass fractions for a range of initial cluster-mass and half-mass radii. We relate the above properties to stellar evolution and initial mass segregation in the simulation and find that primordially segregated systems virialize faster and possess a lower bound-mass fraction on account of mass loss from heavy stars and 2-body+3-body interactions whereas, stellar evolution does not exhibit significant effect. This research is the first instance where a realistic IMF (Kroupa 2001) has been utilized to perform an extended parameter scan for an N-body cluster model.

The figure depicts typical Lagrange radii $R_{f}$ evolution profile for a computed N-body model with initial mass = 3e4 M_sun and half-mass radius = 0.5 pc. From bottom to top, the curves represent mass fractions from 5% to 99% in steps of 5%. The dark-red lines represent $R_{10}$, $R_{50}$ and $R_{80}$ respectively. The delay time (time after which gas-expulsion starts) is 0.6 Myr.