Primordial star formation: relative impact of H2 three-body rates and initial conditions

Population III stars are the first stars in the Universe to form at z=20-30 out of a pure hydrogen and helium gas in minihalos of 10^5-10^6 M$_\odot$ . Cooling and fragmentation is thus regulated via molecular hydrogen. At densities above 10^8 cm$^{-3}$, the three-body H2 formation rates are particularly important for making the gas fully molecular. These rates were considered to be uncertain by at least a few orders of magnitude. We explore the impact of new accurate three-body H2 formation rates derived by Forrey (2013) for three different minihalos, and compare to the results obtained with three-body rates employed in previous studies. The calculations are performed with the cosmological hydrodynamics code ENZO (release 2.2) coupled with the chemistry package KROME (including a network for primordial chemistry), which was previously shown to be accurate in high resolution simulations. While the new rates can shift the point where the gas becomes fully molecular, leading to a different thermal evolution, there is no trivial trend in how this occurs. While one might naively expect the results to be inbetween the calculations based on Palla et al. (1983) and Abel et al. (2002), the behavior can be close to the former or the latter depending on the dark matter halo that is explored. We conclude that employing the correct three-body rates is about as equally important as the use of appropriate initial conditions, and that the resulting thermal evolution needs to be calculated for every halo individually.Comment: 10 pages, 9 figures, A&A, 561, A13 (2014

The formation of the primitive star SDSS J102915+172927: effect of the dust mass and the grain-size distribution

Understanding the formation of the extremely metal poor star SDSS-J102915+172927 is of fundamental importance to improve our knowledge on the transition between the first and second generation of stars in the Universe. In this paper, we perform three-dimensional cosmological hydrodynamical simulations of dust-enriched halos during the early stages of the collapse process including a detailed treatment of the dust physics. We employ the astrochemistry package \krome coupled with the hydrodynamical code \textsc{enzo} assuming grain size distributions produced by the explosion of core-collapse supernovae of 20 and 35 M$_\odot$ primordial stars which are suitable to reproduce the chemical pattern of the SDSS-J102915+172927 star. We find that the dust mass yield produced from Population III supernovae explosions is the most important factor which drives the thermal evolution and the dynamical properties of the halos. Hence, for the specific distributions relevant in this context, the composition, the dust optical properties, and the size-range have only minor effects on the results due to similar cooling functions. We also show that the critical dust mass to enable fragmentation provided by semi-analytical models should be revised, as we obtain values one order of magnitude larger. This determines the transition from disk fragmentation to a more filamentary fragmentation mode, and suggests that likely more than one single supernova event or efficient dust growth should be invoked to get such a high dust content.Comment: Accepted on Ap

Formation of carbon-enhanced metal-poor stars in the presence of far ultraviolet radiation

Recent discoveries of carbon-enhanced metal-poor stars like SMSS J031300.36-670839.3 provide increasing observational insights into the formation conditions of the first second-generation stars in the Universe, reflecting the chemical conditions after the first supernova explosion. Here, we present the first cosmological simulations with a detailed chemical network including primordial species as well as C, C$^+$, O, O$^+$, Si, Si$^+$, and Si$^{2+}$ following the formation of carbon-enhanced metal poor stars. The presence of background UV flux delays the collapse from $z=21$ to $z=15$ and cool the gas down to the CMB temperature for a metallicity of Z/Z$_\odot$=10$^{-3}$. This can potentially lead to the formation of lower mass stars. Overall, we find that the metals have a stronger effect on the collapse than the radiation, yielding a comparable thermal structure for large variations in the radiative background. We further find that radiative backgrounds are not able to delay the collapse for Z/Z$_\odot$=10$^{-2}$ or a carbon abundance as in SMSS J031300.36-670839.3.Comment: submitted to ApJ

A high resolution solar atlas for fluorescence calculations

The characteristics required of a solar atlas to be used for studying the fluorescence process in comets are examined. Several sources of low resolution data were combined to provide an absolutely calibrated spectrum from 2250 A to 7000A. Three different sources of high resolution data were also used to cover this same spectral range. The low resolution data were then used to put each high resolution spectrum on an absolute scale. The three high resolution spectra were then combined in their overlap regions to produce a single, absolutely calibrated high resolution spectrum over the entire spectral range

Dark-matter halo mergers as a fertile environment for low-mass Population III star formation

While Population III stars are typically thought to be massive, pathways towards lower-mass Pop III stars may exist when the cooling of the gas is particularly enhanced. A possible route is enhanced HD cooling during the merging of dark-matter halos. The mergers can lead to a high ionization degree catalysing the formation of HD molecules and may cool the gas down to the cosmic microwave background (CMB) temperature. In this paper, we investigate the merging of mini-halos with masses of a few 10$^5$ M$_\odot$ and explore the feasibility of this scenario. We have performed three-dimensional cosmological hydrodynamics calculations with the ENZO code, solving the thermal and chemical evolution of the gas by employing the astrochemistry package KROME. Our results show that the HD abundance is increased by two orders of magnitude compared to the no-merging case and the halo cools down to $\sim$60 K triggering fragmentation. Based on Jeans estimates the expected stellar masses are about 10 M$_\odot$. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.Comment: Submitted to MNRA

The formation of massive primordial stars in the presence of moderate UV backgrounds

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.Comment: Submitted to APJ, comments are welcome. High resolution copy is available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd

How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes

Observations of high redshift quasars at $z>6$ indicate that they harbor supermassive black holes (SMBHs) of a billion solar masses. The direct collapse scenario has emerged as the most plausible way to assemble SMBHs. The nurseries for the direct collapse black holes are massive primordial halos illuminated with an intense UV flux emitted by population II (Pop II) stars. In this study, we compute the critical value of such a flux ($J_{21}^{\rm crit}$) for realistic spectra of Pop II stars through three-dimensional cosmological simulations. We derive the dependence of $J_{21}^{\rm crit}$ on the radiation spectra, on variations from halo to halo, and on the impact of X-ray ionization. Our findings show that the value of $J_{21}^{\rm crit}$ is a few times $\rm 10^4$ and only weakly depends on the adopted radiation spectra in the range between $T_{\rm rad}=2 \times 10^4-10^5$ K. For three simulated halos of a few times $\rm 10^{7}$~M$_{\odot}$, $J_{21}^{\rm crit}$ varies from $\rm 2 \times 10^4 - 5 \times 10^4$. The impact of X-ray ionization is almost negligible and within the expected scatter of $J_{21}^{\rm crit}$ for background fluxes of $J_{\rm X,21} \leq 0.1$. The computed estimates of $J_{21}^{\rm crit}$ have profound implications for the quasar abundance at $z=10$ as it lowers the number density of black holes forming through an isothermal direct collapse by a few orders of magnitude below the observed black holes density. However, the sites with moderate amounts of $\rm H_2$ cooling may still form massive objects sufficient to be compatible with observations.Comment: Accepted for publication in MNRAS, comments are welcom