Keywords

Population III stars forming in minihalos tend to be relatively inefficient, with each minihalo hosting one or a small number of stars which are more massive than local stars, but still challenging to observe directly at high redshift. Here we explore a possible mechanism for the generation of larger clusters of such stars: a nearby ionizing source that ionizes a late forming halo, delaying its collapse until the halo is sufficiently large enough that the core can self-shield and suffer runaway collapse. We use simulations with a simple but accurate model for the radiative ionizing flux and confirm the basic predictions of previous work: higher ionizing fluxes can delay the collapse to lower redshifts and higher masses, up to an order of magnitude above the atomic cooling limit. In a limited number of runs we also examine the fragmentation of the cores at even higher resolution, using both simple estimates and sink particles to show that the number of fragments is generally small, at most a handful, and that the mass accretion rate on the fragments is of order 10⁻³ M_⊙ yr⁻¹. This rate is sufficiently high enough that the descent on the main sequence (and hence the suppression of accretion) is delayed until the stellar masses are of order 100–1000 M_⊙, but not high enough to produce direct collapse black holes of mass ∼10⁵ M_⊙. The resulting clusters are larger than those produced in minihalos, but are still likely to fall short of being easily detectable in James Webb Space Telescope blind fields.

We present synthetic light curves (LCs) of fallback-powered supernovae based on a neutrino-driven explosion of a 40 ${M}_{\odot }$ zero-metallicity star with significant fallback accretion onto a black hole that was previously simulated by Chan et al. until shock breakout. Here, we investigate the LC properties of the explosion after shock breakout for various fallback models. Without extra power from fallback accretion, the LC is that of an SN IIP with a plateau magnitude of around −14 mag and a plateau duration of 40 days. With extra power for the LC from fallback accretion, however, we find that the transient can be significantly more luminous. The LC shape can be SN 1987A-like or Type IIP-like, depending on the efficiency of the fallback engine. If the accretion disk forms soon after the collapse and more than 1% of the accretion energy is released as the central engine, fallback accretion-powered supernovae become as luminous as superluminous supernovae. We suggest that Type II superluminous supernovae with broad hydrogen features could be related to such hydrogen-rich supernovae powered by fallback accretion. In the future, such superluminous supernovae powered by fallback accretion might be found among the supernovae from the first stars in addition to pair-instability supernovae and pulsational pair-instability supernovae.

High-redshift quasars emit copious X-ray photons that heat the intergalactic medium to temperatures up to ∼10⁶ K. At such high temperatures the primordial gas will not form stars until it is assembled into dark matter halos with masses of up to ∼10¹¹ M_⊙, at which point the hot gas collapses and cools under the influence of gravity. Once this occurs, there is a massive reservoir of primordial gas from which stars can form, potentially setting the stage for the brightest Population (Pop) III starbursts in the early universe. Supporting this scenario, recent observations of quasars at z ∼ 6 have revealed a lack of accompanying Lyα emitting galaxies, consistent with suppression of primordial star formation in halos with masses below ∼10¹⁰ M_⊙. Here we model the chemical and thermal evolution of the primordial gas as it collapses into such a massive halo irradiated by a nearby quasar in the run-up to a massive Pop III starburst. We find that, within ∼100 kpc of the highest-redshift quasars discovered to date, the Lyman–Werner flux produced in the quasar host galaxy may be high enough to stimulate the formation of a direct collapse black hole (DCBH). A survey with single pointings of the NIRCam instrument at individually known high-z quasars may be a promising strategy for finding Pop III stars and DCBHs with the James Webb Space Telescope.

The elemental-abundance signatures of the very first stars are imprinted on the atmospheres of CEMP-no stars, as various evidence suggests they are bona fide second-generation stars. It has recently been recognized that the CEMP-no stars can be subdivided into at least two groups, based on their distinct morphology in the A(C)–[Fe/H] space, indicating the likely existence of multiple pathways for their formation. In this work, we compare the halo CEMP-no group morphology with that of stars found in satellite dwarf galaxies of the Milky Way—a very similar A(C)–[Fe/H] pattern is found, providing clear evidence that halo CEMP-no stars were indeed accreted from their host mini-halos, similar in nature to those that formed in presently observed ultra-faint dwarfs (UFDs) and dwarf spheroidal (dSph) galaxies. We also infer that the previously noted "anomalous" CEMP-no halo stars (with high A(C) and low [Ba/Fe] ratios) that otherwise would be associated with Group I may have the same origin as the Group III CEMP-no halo stars, by analogy with the location of several Group III CEMP-no stars in the UFDs and dSphs and their distinct separation from that of the CEMP-s stars in the A(Ba)–A(C) space. Interestingly, CEMP-no stars associated with UFDs include both Group II and Group III stars, while the more massive dSphs appear to have only Group II stars. We conclude that understanding the origin of the CEMP-no halo stars requires knowledge of the masses of their parent mini-halos, which is related to the amount of carbon dilution prior to star formation, in addition to the nature of their nucleosynthetic origin.

The formation process of Population III (PopIII) stars in the mass-accretion phase is investigated by numerical experiments. The barotropic relation of primordial gas and artificial stiffening of the equation of state in very dense regions (>10¹⁵ cm⁻³) enables us to follow the fragmentation of PopIII circumstellar disks and the merging processes of the fragments. The disk becomes gravitationally unstable to fragmentation, followed by a rapid merger process typically within 100 yr, which roughly corresponds to one orbital time of the circumstellar disk. We also find that the fragmentation of the gas disk around a multiple system, a circumbinary disk, is rare; however, it is frequent in the disk around an individual protostar. We also perform a simulation with standard sink particles, where the number and total mass of sink particles are in rough agreement with those of the stiff equation of state runs. Based on the results of these numerical results, we model the evolution of the number of fragments with a simple phenomenological equation. We find that the average number of fragments is roughly proportional to t^0.3, where t is the elapsed time since the formation of the first protostar. Next, we compare this trend with a number of published numerical studies by scaling the elapsed time according to the scale-free nature of the system. As a result, we find most of the results in the literature agree well with the relation. The present results, combined with previous studies in the literature, imply that the PopIII stars tend to be born not as single stars, but in multiple systems.

Within standard ΛCDM cosmology, Population III star formation in minihalos of mass M_halo ≳ 5 × 10⁵M_⊙ provides the first stellar sources of Lyα photons. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has measured a strong absorption signal of the redshifted 21 cm radiation from neutral hydrogen at z ≈ 17, requiring efficient formation of massive stars before then. In this Letter, we investigate whether star formation in minihalos plays a significant role in establishing the early Lyα background required to produce the EDGES absorption feature. We find that Population III stars are important in providing the necessary Lyα-flux at high redshifts, and derive a best-fitting average Population III stellar mass of ∼750 M_⊙ per minihalo, corresponding to a star formation efficiency of 0.1%. Furthermore, it is important to include baryon-dark matter streaming velocities in the calculation, to limit the efficiency of Population III star formation in minihalos. Without this effect, the cosmic dawn coupling between 21 cm spin temperature and that of the gas would occur at redshifts higher than what is implied by EDGES.

We present observational evidence that an aspherical supernova explosion could have occurred in the first stars in the early universe. Our results are based on the first determination of a Zn abundance in a Hubble Space Telescope/Cosmic Origins Spectrograph high-resolution UV spectrum of a hyper-metal-poor (HMP) star, HE 1327−2326, with ${\rm{[Fe/H]}}(\mathrm{NLTE})=-5.2$. We determine [Zn/Fe] = 0.80 ± 0.25 from a UV Zn i line at 2138 Å, detected at 3.4σ. Yields of a 25 M_⊙ aspherical supernova model with artificially modified densities exploding with E = 5 × 10⁵¹ erg best match the entire abundance pattern of HE 1327−2326. Such high-entropy hypernova explosions are expected to produce bipolar outflows, which could facilitate the external enrichment of small neighboring galaxies. This has already been predicted by theoretical studies of the earliest star-forming minihalos. Such a scenario would have significant implications for the chemical enrichment across the early universe, as HMP carbon-enhanced metal-poor (CEMP) stars such as HE 1327−2326 might have formed in such externally enriched environments.

The near-infrared background between 0.5 and 2 μm contains a wealth of information related to radiative processes in the universe. Infrared background anisotropies encode the redshift-weighted total emission over cosmic history, including any spatially diffuse and extended contributions. The anisotropy power spectrum is dominated by undetected galaxies at small angular scales and a diffuse background of Galactic emission at large angular scales. In addition to these known sources, the infrared background also arises from intrahalo light (IHL) at z < 3 associated with tidally stripped stars during galaxy mergers. Moreover, it contains information on the very first galaxies from the epoch of reionization (EoR). The EoR signal has a spectral energy distribution (SED) that goes to zero near optical wavelengths due to Lyman absorption, while other signals have spectra that vary smoothly with frequency. Due to differences in SEDs and spatial clustering, these components may be separated in a multi-wavelength-fluctuation experiment. To study the extent to which EoR fluctuations can be separated in the presence of IHL, and extragalactic and Galactic foregrounds, we develop a maximum likelihood technique that incorporates a full covariance matrix among all the frequencies at different angular scales. We apply this technique to simulated deep imaging data over a 2 × 45 deg² sky area from 0.75 to 5 μm in 9 bands and find that such a "frequency tomography" can successfully reconstruct both the amplitude and spectral shape for representative EoR, IHL, and the foreground signals.

We present a novel scenario for the formation of carbon-enhanced metal-poor (CEMP) stars. Carbon enhancement at low stellar metallicities is usually considered a consequence of faint or other exotic supernovae. An analytical estimate of cooling times in low-metallicity gas demonstrates a natural bias, which favors the formation of CEMP stars as a consequence of inhomogeneous metal mixing: carbon-rich gas has a shorter cooling time and can form stars prior to a potential nearby pocket of carbon-normal gas, in which star formation is then suppressed due to energetic photons from the carbon-enhanced protostars. We demonstrate that this scenario provides a natural formation mechanism for CEMP stars from carbon-normal supernovae, if inhomogeneous metal mixing provides carbonicity differences of at least one order of magnitude separated by >10 pc. In our fiducial (optimistic) model, 8% (83%) of observed CEMP-no stars ([Ba/Fe] < 0) can be explained by this formation channel. This new scenario may change our understanding of the first supernovae and thereby our concept of the first stars. Future 3D simulations are required to assess the likelihood of this mechanism to occur in typical high-redshift galaxies.

It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period P = 34.757 ± 0.010 days in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002–5104378. The secondary star 2MASS J18082002–5104378 B has a mass ${M}_{2}={0.14}_{-0.01}^{+0.06}\,{M}_{\odot }$, placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002–5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002–5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Pop III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Pop III primaries with masses $10\,{M}_{\odot }\lesssim {M}_{* }\lesssim 100\,{M}_{\odot }$. If true, this inference suggests that solar-mass Pop III stars formed via disk fragmentation could survive to the present day.