The First Stars

Pop III stars are the key to the character of primeval galaxies, the first heavy elements, the onset of cosmological reionization, and the seeds of supermassive black holes. Unfortunately, in spite of their increasing sophistication, numerical models of Pop III star formation cannot yet predict the masses of the first stars. Because they lie at the edge of the observable universe, individual Pop III stars will also remain beyond the reach of telescopes for the foreseeable future, and so their properties remain unknown. However, it will soon be possible to constrain their masses by the direct detection of their supernovae and by reconciling their nucleosynthetic yields to the chemical abundances measured in ancient metal-poor stars in the Galactic halo, some of which may be bear the ashes of the first stars. Here, I review current problems on the simulation frontier in Pop III star formation and discuss the best prospects for constraining their properties observationally in the near term.


The Simulation Frontier
Unlike with star formation in the Galaxy today, there is little disagreement over the initial conditions of star formation in the primordial universe. The original numerical simulations suggested that Pop III stars formed in small pregalactic structures known as cosmological minihalos at z ∼ 20 -30, or ∼ 200 Myr after the Big Bang (Bromm, Coppi & Larson, 1999,2001Abel, Bryan & Norman, 2000, 2002Nakamura & Umemura, 2001). These models predicted that the stars formed in isolation, one per halo, and that they were 100 -500 M ⊙ . Pop III stars profoundly transformed the halos that gave birth to them, expelling their baryons in supersonic ionized flows and later exploding as supernovae (e.g. Whalen  In particular, the disk calculations indicate that they are unstable to fragmentation, raising the possibility that Pop III stars may have only been tens of solar masses, not hundreds, and that they may have formed in small swarms of up to a dozen at the centers of primeval halos. Computer models of ionizing UV breakout in the final stages of Pop III protostellar disks have also found that the I-front of the nascent star exits the disk in bipolar outflows that terminate accretion onto the star and mostly evaporate the disk by the time the star reaches ∼ 40 M ⊙ (Hosokawa et al., 2011). This result reinforces the sentiments of some in the community that while the Pop III IMF was top-heavy, primordial stars may only have been 10 -40 M ⊙ .

High-Mass or Low-Mass Pop III Stars?
In spite of their increasing sophistication, these simulations should be taken to be very preliminary for several reasons. First, Pop III accretion disks form in smoothed-particle hydrodynamics (SPH) models but not in adaptive mesh refinement (AMR) simulations, although the AMR models have not evolved the collapse of the halo to the times achieved by SPH calculations. This raises the question of whether one technique better captures the transport of angular momentum out of the center of the cloud than the other, and whether accretion is ultimately spherical or through a disk. Second, the stability of the disk itself remains an open question because although the simulations can now fully resolve the disk they do not yet incorporate all of its relevant physics. In particular, they lack high-order radiation transport, which regulates the thermal state of the disk and its tendency to fragment. Furthermore, the role of primordial magnetic fields in the formation and evolution of the disk is not well understood (Turk et al., 2012;Widrow et al., 2012). The use of sink particles to represent disk fragments in the original SPH simulations of Pop III protostellar disk formation called into question the longevity of the fragments. Once they are created in the simulations they are never destroyed, unlike real fragments that could be torn apart by gravitational torques and viscous forces (Norman, 2010). More recent moving mesh simulations performed with the Arepo code that do not rely on sink particles find that the fragments persist but only evolve the disk for 10 -20 yr (Greif et al., 2012). Perhaps most importantly, no simulation has followed the disks for more than a few centuries, far short of the time required to assemble a massive star. Thus, it remains unclear if the fragments in the disk remain distinct objects or merge with the largest one at the center, building it up into a very massive star over time through protracted, clumpy accretion.

Accretion Cutoff and the Final Masses of Pop III Stars
This latter point directly impacts estimates of final masses for Pop III stars inferred from numerical simulations that attempt to model how ionizing UV from the star reverses infall and evaporates the accretion disk. At the heart of such models is a simple recipe for the evolution of the protostar that provides a prescription for its radius and luminosity as a function of time and acquired mass. The Hosokawa et al. (2011) 2D calculations take the growth of the protostar to be relatively steady, in which case it contracts and settles onto the main sequence at ∼ 30 M ⊙ . At this point the star becomes extremely luminous in ionizing UV radiation that halts accretion onto the star in a few hundred kyr at a final mass of ∼ 40 M ⊙ . If accretion instead turns out to be clumpy, the protostar could remain puffy and cool and reach much larger masses before burning off the disk. A finer point is that all current accretion cutoff simulations evolve both radiation and hydrodynamics on the Courant time, a practice which is known to lead to serious inaccuracies in I-front propagation in density gradients (Whalen & Norman, 2006). Such coarse time steps may result in premature Ifront breakout and accretion cutoff, and hence underestimates of the final mass of the star. Three dimensional simulations with more accurate radiationmatter coupling schemes, both steady and clumpy accretion scenarios, more realistic prescriptions for protostellar evolution based on nucleosynthesis codes such as KEPLER (Weaver et al., 1978;Woosley et al., 2002) and a variety of halo environments may better constrain the Pop III IMF. However, in judging the power of such simulations to model the masses of the first stars, it should be remembered that no simulations realistically bridge the gap in time between the formation and fragmentation of a protostellar disk and its photoevaporation up to a Myr later. We note in passing that fragments can also stop accreting if they are ejected from the disk by 3-body gravitational effects (Greif et al., 2011;Johnson & Khochfar, 2011). These fragments could become very low-mass Pop III stars (∼ 1 M ⊙ ); if so, some of them may live today.

Constraining the Pop III IMF with Stellar Archaeology
Unfortunately, because they lie at the edge of the observable universe, individual Pop III stars will remain beyond the reach of direct detection for decades to come, even with their enormous luminosities (Schaerer 2002) and the advent of the next Although these results suggest that low-mass Pop III stars shouldered the bulk of the chemical enrichment of the early IGM, 40 -60 M ⊙ hypernova explosions, whose energies are intermediate to those of CC and PI SNe, may also have contributed metals at high redshifts (Iwamoto et al. 2005).
To date, the telltale odd-even nucleosynthetic signature of PI SNe has not been found in the fossil abundance record, leading some to assert that Pop III stars could not have been very massive. However, the odd-even effect may have been masked by observational bias in previous surveys (Karlsson et al., 2008). Reconciling Pop III SN yields to the elemental patterns in metal-poor stars is still in its infancy for several reasons. First, only small numbers of extremely metal-poor stars have been discovered to date, and larger sample sizes would better constrain early SN yields. Second, measurements of some elements in low-metallicity stars are challenging and in the past have been subject to systematic error. Finally, there are many intervening hydrodynamical processes between the expulsion of the first metals and their uptake into new stars that are not yet understood.

Finding the First Cosmic Explosions
Detection of Pop III SNe would unambiguously probe the masses of primordial stars for the first time. Since these explosions are 100,000 times brighter than either their progenitors or the primitive galaxies that host them, they could be found by JWST or the Wide-Field Infrared Survey Telescope (WFIRST). However, unlike the Type Ia SNe used to constrain cosmic acceleration, light from primeval supernovae must traverse the vast cosmic web of neutral hydrogen that filled the universe prior to the epoch of reion-  Figure 3. These simulations include radiation hydrodynamical calculations of the SN light curve and spectra in the local frame, cosmological redshifting, and Lyman absorption by intergalactic hydrogen. JWST detection limits at 2 -4 µm are AB magnitude 31 -32, so it is clear that JWST will be able to detect the first cosmic explosions in the universe if they are PI SNe (and even perform spectrometry on them). Even given JWST's very narrow fields of view at high redshifts, recent calculations indicate that at least a few PI SNe should be present in any given JWST survey (Hummel et al. 2012). Also, because WFIRST detection limits will be AB magnitude 26 The extreme NIR luminosities of primordial PI SNe could contribute to a NIR background excess, as has been suggested for Pop III stars themselves, i.e. Kashlinsky (2005). New calculations reveal that enough synchrotron emission from CC SN remnants is redshifted into the 21 cm band above z ∼ 10 to be directly detected by the Square Kilometer Array (SKA) (Meiksen & Whalen 2012). Somewhat more energetic hypernovae could be detected by existing facilities such as the Extended Very-Large Array (eVLA) and eMERLIN. PI SN remnants generally expand into ambient media that are too diffuse to generate a detectable synchrotron signal. Pop III SN event rates make it unlikely that they will be found in absorption at 21 cm at z > 10.
The detection of the first cosmic explosions will be one of the most spectacular discoveries in extragalactic astronomy in the coming decade, opening our first observational window on the era of first light and the end of the cosmic Dark Ages at z ∼ 30. They will unveil the nature of primordial stars and constrain scenarios for early cosmological reionization, the process whereby the universe was gradually transformed from a cold, dark, featureless void into the vast, hot, ionized expanse of galaxies we observe today. At somewhat lower redshifts (z ∼ 10 -15), detections of Pop III supernovae will probe the era of primitive galaxy formation, marking the positions of nascent galaxies on the sky that might otherwise have eluded detection by JWST. Finally, finding the first supernovae could also reveal the masses of the seeds of the supermassive black holes lurking at the centers of massive galaxies today.