arXiv subjects

Corey S. Howard

Publications and source records attributed to Corey S. Howard.

5 recordsLinked to original sources

arXiv2019

On the Origin of Multiple Populations During Massive Star Cluster Formation

We investigate the possibility that multiple populations in globular clusters arise as a natural by-product of massive star-cluster formation. We use 3D radiative hydrodynamics simulations for the formation of young massive clusters to track their chemical self-enrichment during their first 5 Myr. These clusters form embedded within filamentary Giant Molecular Clouds by a combination of gas accretion and rapid merging of protoclusters. Chemical enrichment is a dynamic process happening as the young cluster assembles, so that the original (1P) and enriched (2P) subpopulations of stars form almost simultaneously. Here we test two simple and opposite extremes for the injection of enriched material into the intracluster gas: we assume either continuous injection in a way that tracks the star formation rate; or sudden injection by a single instantaneous event. Using helium abundance as a proxy for the enrichment, we find that realistic multiple population features can be reproduced by injecting a total helium mass amounting to a few percent of the cluster's total mass. The differences in individual growth histories can lead to widely differing 1P/2P outcomes. These models suggest that dual or multiple populations can emerge rapidly in massive star clusters undergoing the typical mode of star cluster formation.

astro-ph.GA↗

arXiv2018

A universal route for the formation of massive star clusters in giant molecular clouds

Young massive star clusters (YMCs, with M $\geq$10$^4$ M$_{\odot}$) are proposed modern-day analogues of the globular clusters (GCs) that were products of extreme star formation in the early universe. The exact conditions and mechanisms under which YMCs form remain unknown -- a fact further complicated by the extreme radiation fields produced by their numerous massive young stars. Here we show that GC-sized clusters are naturally produced in radiation-hydrodynamic simulations of isolated 10$^7$ M$_{\odot}$ Giant Molecular Clouds (GMCs) with properties typical of the local universe, even under the influence of radiative feedback. In all cases, these massive clusters grow to GC-level masses within 5 Myr via a roughly equal combination of filamentary gas accretion and mergers with several less massive clusters. Lowering the heavy-element abundance of the GMC by a factor of 10 reduces the opacity of the gas to radiation and better represents the high-redshift formation conditions of GCs. This results in higher gas accretion leading to a mass increase of the largest cluster by a factor of ~4. When combined with simulations of less massive GMCs (10$^{4-6}$ M$_{\odot}$), a clear relation emerges between the maximum YMC mass and the mass of the host GMC. Our results demonstrate that YMCs, and potentially GCs, are a simple extension of local cluster formation to more massive clouds and do not require suggested exotic formation scenarios.

astro-ph.GA↗

arXiv2017

Simulating the UV Escape Fractions from Molecular Cloud Populations in Star-forming Dwarf and Spiral Galaxies

The escape of ultraviolet photons from the densest regions of the interstellar medium (ISM) --- Giant Molecular Clouds (GMCs) --- is a poorly constrained parameter which is vital to understanding the ionization of the ISM and the intergalactic medium. We characterize the escape fraction, f$_{\text{esc,GMC}}$, from a suite of individual GMC simulations with masses in the range 10$^{4-6}$ M$_{\odot}$ using the adaptive-mesh refinement code FLASH. We find significantly different f$_{\text{esc,GMC}}$ depending on the GMC mass which can reach $>$90% in the evolution of 5$\times$10$^4$ and 10$^{5}$ M$_{\odot}$ clouds or remain low at $\sim$5% for most of the lifetime of more massive GMCs. All clouds show fluctuations over short, sub-Myr timescales produced by flickering HII regions. We combine our results to calculate the total escape fraction (f$_{\text{esc,tot}}$) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution (dN/dM$\propto$M$^α$, where $α$ is either -1.5 or -2.5) over fixed time intervals. We find typical f$_{\text{esc,tot}}$ values of 8% for both the dwarf and spiral models. The fluctuations of f$_{\text{esc,tot}}$, however, are much larger for the dwarf models with values as high as 90%. The photons escaping from the 5$\times$10$^4$ and 10$^{5}$ M$_{\odot}$ GMCs are the dominant contributors to f$_{\text{esc,tot}}$ in all cases. We also show that the accompanying star formation rates (SFRs) of our model ($\sim$2$\times$10$^{-2}$ and 0.73 M$_{\odot}$yr$^{-1}$) are consistent with observations of SFRs in dwarf starburst and spiral galaxies, respectively.

astro-ph.GA↗

arXiv2017

Ultra Violet Escape Fractions from Giant Molecular Clouds During Early Cluster Formation

The UV photon escape fraction from molecular clouds is a key parameter for understanding the ionization of the Interstellar Medium (ISM), and extragalactic processes, such as cosmic reionization. We present the ionizing photon flux and the corresponding photon escape fraction (f$_{esc}$) arising as a consequence of star cluster formation in a turbulent, 10$^6$ M$_{\odot}$ GMC, simulated using the code FLASH. We make use of sink particles to represent young, star-forming clusters coupled with a radiative transfer scheme to calculate the emergent UV flux. We find that the ionizing photon flux across the cloud boundary is highly variable in time and space due to the turbulent nature of the intervening gas. The escaping photon fraction remains at $\sim$5% for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr with a maximum f$_{esc}$ of 30% and 37%, respectively. These peaks are due to the formation of large HII regions, that expand into regions of lower density and some of which reach the cloud surface. However, these phases are short lived and f$_{esc}$ drops sharply as the HII regions are quenched by the central cluster passing through high-density material due to the turbulent nature of the cloud. We find an average f$_{esc}$ of 15% with factor of two variations over 1 Myr timescales. Our results suggest that assuming a single value for f$_{esc}$ from a molecular cloud is in general a poor approximation, and that the dynamical evolution of the system leads to large temporal variation.

astro-ph.GA↗

arXiv2013

Cluster formation in molecular clouds: I. stellar populations, star formation rates, and ionizing radiation

We present a model for the radiative output of star clusters in the process of star formation suitable for use in hydrodynamical simulations of radiative feedback. Gas in a clump, defined as a region whose density exceeds 10^4 cm^-3, is converted to stars via the random sampling of the Chabrier IMF. A star formation efficiency controls the rate of star formation. We have completed a suite of simulations which follow the evolution of accretion-fed clumps with initial masses ranging from 0 to 10^5 M_sol and accretion rates ranging from 10^-5 to 10^-1 M_sol yr^-1. The stellar content is tracked over time which allows the aggregate luminosity, ionizing photon rate, number of stars, and star formation rate (SFR) to be determined. For a fiducial clump of 10^4 M_sol, the luminosity is ~4x10^6 L_sol with a SFR of roughly 3x10^-3 M_sol yr^-1. We identify two regimes in our model. The accretion-dominated regime obtains the majority of its gas through accretion and is characterized by an increasing SFR while the reservoir-dominated regime has the majority of its mass present in the initial clump with a decreasing SFR. We show that our model can reproduce the expected number of O stars, which dominate the radiative output of the cluster. We find a nearly linear relationship between SFR and mass as seen in observations. We conclude that our model is an accurate and straightforward way to represent the output of clusters in hydrodynamical simulations with radiative feedback.

astro-ph.GA↗