News

I won the Ann G. Wylie Dissertation Fellowship!

I use a state-of-art radiation-magnetohydrodynamics code RAMSES-RT to simulate star formation in molecular clouds. In particular, I am looking to understand the role the first dense star clusters play in the reionization of the universe and in the production of supermassive black hole seeds.

Resolved simulations of star cluster formation from molecular clouds

Relevant publication: C.-C. He, M. Ricotti, & S. Geen, 2019 MNRAS, 489, 1880-1898

The origin of the stellar initial mass function (IMF) is a long-standing question. Theories exist that could explain the characteristic shapes of the present-day IMF. Simulations considering critical physical processes are necessary to understand the whole mass spectrum of star clusters. Using RAMSES-RT, I conduct a series of simulations of the collapsing of turbulent molecular clouds spanning a wide range of masses and densities, taking into account hydrodynamics, gravity, magnetic fields, and radiative feedback. Stars are created in the simulation as sink particles. The IMFs of the star clusters forming from these simulations have not only characteristic power-law slopes very close to Salpeter, but also the same normalization to a sampled Kroupa IMF without any rescaling. The only assumption is that each sink particle converts ~40% of its mass to a single star and the rest to several smaller stars, a mechanism inferred from the mapping between the observed core mass function (CMF) and stellar IMF which in turn gives implications on the CMF-to-IMF conversion.

Massive stars form later

The shape of the mass function stays virtually the same, while its scale increases over time. In the star formation episode, the masses of forming stars are drawn from a probability function similar to Salpeter. In the beginning, because the number of stars is small, there are few high-mass stars. The apparent behavior is that low- and intermediate-mass stars form first, followed by the most massive stars.

Star formation is feedback regulated — the sound-crossing time

The efficiency of conversion of gas into stars is typically much lower than 100% because energetic processes from massive stars can disperse the cloud before all the gas collapses into protostars, a process called ‘feedback.' Recent work favors ionizing radiation as the primary driver of molecular cloud dispersal for the bulk of star clusters. Based on a set of simulations of star formation from molecular clouds extending a wide range of masses and densities, we find a strong correlation between the star formation timescale regulated by photoionization feedback and the sound-crossing time, $t_{\rm sc}$, defined as the crossing time of the cloud radius with the sound speed in HII region (~ 10 km/s). As an HII region expands due to the pressure gradient in the edge, it wipes out the gas and flattens the density contrast. This process happens in the timescale of several $t_{\rm sc}$.

Massive dense GMCs efficiently form bound clusters

A star formation efficiency of $\sim 15%$ is found as the separation between bound and unbound clusters. GMCs 100 times denser than local ones efficiently form stars that remain gravitationally bounded as the gas is dispersed. They are believed to be progenitors of globular clusters.

Escape of LyC Photons from GMCs

Relevant publication: C.-C. He, M. Ricotti, & S. Geen, 2020 MNRAS 492, 4858

The escape of Lyman continuum (LyC) photons from galaxies into IGM is arguably the most uncertain parameter in models of the epoch of reionization. An underlying but less studied parameter is the escape of LyC from GMCs where stars are born. We post-processed a set of 14 simulations of GMCs of various masses and densities where individual massive stars are well resolved. We calculate the escape of LyC from individual stars, taking into account in the line of sight neutral hydrogen column density and dust extinction, and this is averaged over all stars and over their lifetimes to get a LyC escape fraction, $f_{\rm esc}$, from a GMC. We find that $f_{\rm esc}$ decreases with increasing mass and with decreasing initial density of a GMC. GMCs with densities typical of local star formation regions have negligible $f_{\rm esc}$ (below $0.1$).

This relation is explained by a simple model where two timescales are compared: the star formation timescale, $t_{\rm SF}$, and the lifetime of the dominating UV source, $t_{\rm UV}$. We define the star formation timescale as the length of the episode in which gases are converting into stars. We find that $t_{\rm SF}$ is about several GMC crossing time by a wave at $\sim 10$ km/s, the sound speed in HII regions. This is also the timescale of the GMC getting destructed by the propagation of the ionizing front, reaching to a stage where a significant fraction of the instantaneous LyC emission can escape. The other timescale, $t_{\rm UV}$, is about $3$ Myr for massive clusters and is slightly higher for less massive ones. Longer $t_{\rm UV}$ and shorter $t_{\rm SF}$ implies higher $f_{\rm esc}$. We find a monotonic relation between $f_{\rm esc}$ and the ratio of these two timescales: $f_{\rm esc}$ goes from $0$ to $0.7$ as $t_{\rm UV} / t_{\rm SF}$ goes from $0$ to $5$ where it reaches a plateau.

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First-authored refereed publications

• C.-C. He, M. Ricotti, & S. Geen, 2020, “Simulating Star Clusters Across Cosmic Time - II. Fraction of Ionizing Photons Escaping from Molecular Clouds”, MNRAS, 492, 4858.
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• C.-C. He, M. Ricotti, & S. Geen, 2019, “Simulating Star Clusters Across Cosmic Time - I. Initial Mass Function, Star Formation Rates, and Efficiencies”, MNRAS, 489, 1880-1898
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• C.-C. He & L. Keek, 2016, “Anisotropy of X-Ray Bursts from Neutron Stars with Concave Accretion Disks”, ApJ, 819, 47.
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Courses I teach as a teaching assistant:

• ASTR101, Spring 2021
• ASTR100, Fall 2017

Here I write blogs

• My Julia Learning Notes

Email me at che1234@umd.edu