Time filter

Source Type

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.

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..

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.

Chalapathi Rao N.V.,Banaras Hindu University | Paton C.,Center for Star and Planet Formation | Lehmann B.,Clausthal University of Technology
Geological Journal | Year: 2012

The Mesoproterozoic Narayanpet Kimberlite Field (NKF) is located ~200km north of the well-known Wajrakarur Kimberlite Field (WKF) in the Eastern Dharwar Craton, southern India. Whereas a majority of the WKF occurrences are diamondiferous and contain mantle xenoliths and xenocrysts, their paucity is a characteristic feature of the NKF. This has been attributed alternately to the derivation of NKF magmas from a shallower depth, or to variability in thickness of the Sub-Continental Lithospheric Mantle (SCLM) beneath the Eastern Dharwar Craton. Recently, exploration by De Beers resulted in the discovery of a number of new kimberlite occurrences from the NKF, with some of their geochemical features and radiogenic isotope systematics subsequently becoming available. In this paper, we present detailed petrography, groundmass mineral composition and new bulk-rock geochemistry data for a number of NKF rocks and attempt to further constrain their origin. We also investigate the influence of redox conditions on diamond prospectivity by estimating oxygen fugacity (fO 2) from Fe-Nb oxybarometry on NKF groundmass perovskites. We identify for the first time in the NKF the presence of volcaniclastic (fragmental textured) facies kimberlite belonging to the diatreme portion of the intrusion. Rarity of olivine macrocrysts and the presence of diopside are hallmarks of the NKF kimberlites, in contrast to archetypal kimberlites of southern Africa. Mineral components of the groundmass display features that are characteristic to both archetypal kimberlites and to orangeites, and it is not straight forward to apply conventional mineral-genetic schemes in the nomenclature of the NKF pipes. Low fO 2 of the NKF magma (ΔNNO (nickel-nickel oxide)=-1.9 to -3.2), indistinguishable from that of diamondiferous kimberlites world-wide, indicates that redox conditions were favourable for diamond prospectivity, and that magmatic emplacement could, instead, have played a major role in their low diamond potential. © 2011 John Wiley & Sons, Ltd..

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.

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.

Bergemann M.,Max Planck Institute for Astrophysics | Lind K.,Max Planck Institute for Astrophysics | Collet R.,Max Planck Institute for Astrophysics | Collet R.,Center for Star and Planet Formation | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of Fe for a number of well-studied late-type stars in different evolutionary stages. A new model of the Fe atom was constructed from the most up-to-date theoretical and experimental atomic data available so far. Non-LTE (NLTE) line formation calculations for Fe were performed using 1D hydrostatic marcs and mafags-os model atmospheres, as well as the spatial and temporal average stratifications from full 3D hydrodynamical simulations of stellar convection computed using the stagger code. It is shown that the Fei/Feii ionization balance can be well established with the 1D and mean 3D models under NLTE including calibrated inelastic collisions with Hi calculated from Drawin's formulae. Strong low-excitation Fei lines are very sensitive to the atmospheric structure; classical 1D models fail to provide consistent excitation balance, particularly so for cool metal-poor stars. A better agreement between Fei lines spanning a range of excitation potentials is obtained with the mean 3D models. Mean NLTE metallicities determined for the standard stars using the 1D and mean 3D models are fully consistent. Moreover, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g. the infrared flux method and parallaxes, if one of the stellar parameters is constrained independently. © 2012 The Authors Monthly Notices of the Royal Astronomical Society. © 2012 RAS.

Magic Z.,Copenhagen University | Magic Z.,Center for Star and Planet Formation | Weiss A.,Max Planck Institute for Astrophysics
Astronomy and Astrophysics | Year: 2016

Context. Solar p-mode oscillations exhibit a systematic offset towards higher frequencies due to shortcomings in the 1D stellar structure models, in particular, the lack of turbulent pressure in the superadiabatic layers just below the optical surface, arising from the convective velocity field. Aims. We study the influence of the turbulent expansion, chemical composition, and magnetic fields on the stratification in the upper layers of the solar models in comparison with solar observations. Furthermore, we test alternative ( 3D) averages for improved results on the oscillation frequencies. Methods. We appended temporally and spatially averaged ( 3D) stratifications to 1D models to compute adiabatic oscillation frequencies that we then tested against solar observations. We also developed depth-dependent corrections for the solar 1D model, for which we expanded the geometrical depth to match the pressure stratification of the solar ( 3D) model, and we reduced the density that is caused by the turbulent pressure. Results. We obtain the same results with our ( 3D) models as have been reported previously. Our depth-dependent corrected 1D models match the observations to almost a similar extent as the ( 3D) model. We find that correcting for the expansion of the geometrical depth and the reducing of the density are both equally necessary. Interestingly, the influence of the adiabatic exponent Γ1 is less pronounced than anticipated. The turbulent elevation directly from the ( 3D) model does not match the observations properly. Considering different reference depth scales for the ( 3D) averaging leads to very similar frequencies. Solar models with high metal abundances in their initial chemical composition match the low-frequency part much better. We find a linear relation between the p-mode frequency shift and the vertical magnetic field strength with δvnl = 26.21Bz [μHz/kG], which is able to render the solar activity cycles correctly. © ESO, 2016.

Magic Z.,Copenhagen University | Magic Z.,Center for Star and Planet Formation
Astronomy and Astrophysics | Year: 2016

Context. Stellar structure calculations are able to predict precisely the properties of stars during their evolution. However, convection is still modelled by the mixing length theory; therefore, the upper boundary conditions near the optical surface do not agree with asteroseismic observations. Aims. We want to improve how the outer boundary conditions are determined in stellar structure calculations. Methods. We study realistic 3D stellar atmosphere models to find alternative properties. Results. We find that the asymptotic entropy run of the superadiabatic convective surface layers exhibit a distinct universal stratification when normalized by the entropy minimum and jump. Conclusions. The normalized entropy can be represented by a 5th order polynomial very accurately, and a 3rd order polynomial also yields accurate coefficients. This generic entropy stratification or the solar stratification, when scaled by the entropy jump and minimum, can be used to improve the modelling of superadiabatic surface layers in stellar structure calculations. Furthermore, this finding indicates that surface convection operates in the same way for all cool stars, but requires further scrutiny in order to improve our understanding of stellar atmospheres. © ESO, 2016.

Bollard J.,Center for Star and Planet Formation | Connelly J.N.,Center for Star and Planet Formation | Bizzarro M.,Center for Star and Planet Formation
Meteoritics and Planetary Science | Year: 2015

The CB chondrites are metal-rich meteorites with characteristics that sharply distinguish them from other chondrite groups. Their unusual chemical and petrologic features and a young formation age of bulk chondrules dated from the CBa chondrite Gujba are interpreted to reflect a single-stage impact origin. Here, we report high-precision internal isochrons for four individual chondrules of the Gujba chondrite to probe the formation history of CB chondrites and evaluate the concordancy of relevant short-lived radionuclide chronometers. All four chondrules define a brief formation interval with a weighted mean age of 4562.49±0.21Myr, consistent with its origin from the vapor-melt impact plume generated by colliding planetesimals. Formation in a debris disk mostly devoid of nebular gas and dust sets an upper limit for the solar protoplanetary disk lifetime at 4.8±0.3Myr. Finally, given the well-behaved Pb-Pb systematics of all four chondrules, a precise formation age and the concordancy of the Mn-Cr, Hf-W, and I-Xe short-lived radionuclide relative chronometers, we propose that Gujba may serve as a suitable time anchor for these systems. © The Meteoritical Society, 2015.

Loading Center for Star and Planet Formation collaborators
Loading Center for Star and Planet Formation collaborators