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Latif M.A.,University of Gottingen | Schleicher D.R.G.,University of Gottingen | Bovino S.,University of Gottingen | Grassi T.,Center for Star and Planet Formation | And 2 more authors.
Astrophysical Journal | Year: 2014

Radiative feedback produced by stellar populations played a vital role in early structure formation. In particular, 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 photodetachment of H-. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times 10 7 M ⊙. 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 J 21 assuming a blackbody radiation spectrum with a characteristic temperature of T rad = 104 K. We further make use of sink particles to follow the evolution for 10,000 yr 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 102-104 solar mass protostars are formed when halos are irradiated by J 21 = 10-500 at z > 10 and suggest a strong relation between the strength of the UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about 0.01-0.1 M ⊙ yr-1 are observed by the end of our simulations. The resulting massive stars are 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. © 2014. The American Astronomical Society. All rights reserved.. Source


Latif M.A.,University of Gottingen | Bovino S.,University of Gottingen | Van Borm C.,University of Gottingen | Van Borm C.,University of Groningen | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

The ability of metal-free gas to cool by molecular hydrogen in primordial haloes is strongly associated with the strength of ultraviolet (UV) flux produced by the stellar populations in the first galaxies. Depending on the stellar spectrum, these UV photons can either dissociate H2 molecules directly or indirectly by photodetachment of H- as the latter provides the main pathway for H2 formation in the early universe. In this study, we aim to determine the critical strength of the UV flux above which the formation of molecular hydrogen remains suppressed for a sample of five distinct haloes at z > 10 by employing a higher order chemical solver and a Jeans resolution of 32 cells. We presume that such flux is emitted by Pop II stars implying atmospheric temperatures of 104 K. We performed three-dimensional cosmological simulations and varied the strength of the UV flux below the Lyman limit in units of J21. Our findings show that the value of J crit 21 varies from halo to halo and is sensitive to the local thermal conditions of the gas. For the simulated haloes, it varies from 400 to 700 with the exception of one halo where J crit 21 ≥ 1500. This has important implications for the formation of direct collapse black holes and their estimated population at z > 6. It reduces the number density of direct collapse black holes by almost three orders of magnitude compared to the previous estimates. © 2014 The Authors.Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Bovino S.,University of Gottingen | Latif M.A.,University of Gottingen | Grassi T.,Center for Star and Planet Formation | Grassi T.,Copenhagen University | Schleicher D.R.G.,University of Gottingen
Monthly Notices of the Royal Astronomical Society | Year: 2014

While Population III (Pop 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 haloes. 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 temperature. In this paper, we investigate the merging of mini-haloes with masses of a few 105Mȯ 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 ~60K triggering fragmentation. Based on Jeans estimates, the expected stellar masses are about 10Mȯ. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Bovino S.,University of Gottingen | Grassi T.,Center for Star and Planet Formation | Grassi T.,Copenhagen University | Schleicher D.R.G.,University of Gottingen | Latif M.A.,University of Gottingen
Astrophysical Journal Letters | Year: 2014

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 Si2 + 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 cosmic microwave background temperature for a metallicity of Z/Z⊙ = 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⊙ = 10-2 or a carbon abundance as in SMSS J031300.36-670839.3. © 2014. The American Astronomical Society. All rights reserved. Source


Vos J.,Catholic University of Leuven | Vos J.,Copenhagen University | Clausen J.V.,Copenhagen University | Jorgensen U.G.,Copenhagen University | And 5 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Recent studies have shown that stellar chromospheric activity, and its effect on convective energy transport in the envelope, is most likely the cause of significant radius and temperature discrepancies between theoretical evolution models and observations. Accurate mass, radius, and abundance determinations from solar-type binaries exhibiting various levels of activity are needed for a better insight into the structure and evolution of these stars. Aims. We aim to determine absolute dimensions and abundances for the solar-type detached eclipsing binary EF Aqr, and to perform a detailed comparison with results from recent stellar evolutionary models. Methods.uvby light curves and uvbyβ standard photometry were obtained with the Strömgren Automatic Telescope. The broadening function formalism was applied on spectra observed with HERMES at the Mercator telescope in La Palma, to obtain radial velocity curves. State-of-the-art methods were applied for the photometric and spectroscopic analyses. Results. Masses and radii with a precision of 0.6% and 1.0% respectively have been established for both components of EF Aqr. The active 0.956 M ⊙ secondary shows star spots and strong Ca II H and K emission lines. The 1.224 M ⊙ primary shows signs of activity as well, but at a lower level. An [Fe/H] abundance of 0.00 ± 0.10 is derived with similar abundances for Si, Ca, Sc, Ti, V, Cr, Co, and Ni. Solar calibrated evolutionary models such as Yonsei-Yale, Victoria-Regina and BaSTI isochrones and evolutionary tracks are unable to reproduce EF Aqr, especially for the secondary, which is 9% larger and 400 K cooler than predicted. Models adopting significantly lower mixing length parameters l/H p remove these discrepancies, as seen in other solar type binaries. For the observed metallicity, Granada models with a mixing length of l/H p = 1.30 (primary) and 1.05 (secondary) reproduce both components at a common age of 1.5 ± 0.6 Gyr. Conclusions. Observations of EF Aqr suggests that magnetic activity, and its effect on envelope convection, is likely to be the cause of discrepancies in both radius and temperature, which can be removed by adjusting the mixing length parameter of the models downwards. © 2012 ESO. Source

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