Banerjee S.,Argelander Institute For Astronomie |
Kroupa P.,Helmholtz Institute For Strahlen Und Kernphysik
Astrophysical Journal | Year: 2014
Understanding how distinct, near-spherical gas-free clusters of very young, massive stars shape out of vast, complex clouds of molecular hydrogen is one of the biggest challenges in astrophysics. A popular thought dictates that a single gas cloud fragments into many newborn stars which, in turn, energize and rapidly expel the residual gas to form a gas-free cluster. This study demonstrates that the above classical paradigm remarkably reproduces the well-observed central, young cluster (HD 97950) of the Galactic NGC 3603 star-forming region, in particular, its shape, internal motion, and mass distribution of stars naturally and consistently follow from a single model calculation. Remarkably, the same parameters (star formation efficiency, gas expulsion timescale, and delay) reproduce HD 97950, as were found to reproduce the Orion Nebula Cluster, Pleiades, and R136. The present results therefore provide intriguing evidence of formation of star clusters through single-starburst events followed by significant residual gas expulsion. © 2014. The American Astronomical Society. All rights reserved..
Braithwaite J.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2012
Strong magnetic fields play a crucial role in the removal of angular momentum from collapsing clouds and protostellar discs, and are necessary for the formation of disc winds as well as jets from the inner disc and, indeed, strong large-scale poloidal magnetic fields are observed in protostellar discs at all radii down to ∼10R ⊙. Nevertheless, by the time the star is visible virtually, all of the original magnetic flux has vanished. We explore mechanisms for removing this flux during the formation of the protostar once it is magnetically disconnected from the parent cloud, looking at both radiative and convective protostars. This includes a numerical investigation of buoyant magnetic field removal from convective stars. It is found that if the star goes through a fully convective phase, all remaining flux can easily be removed from the protostar, essentially on an Alfvén time-scale. If, on the other hand, the protostar has no fully convective phase, then some flux can be retained, the quantity depending on the net magnetic helicity, which is probably quite small. Only some fraction of this flux is visible at the stellar surface. We also look at how the same mechanisms could prevent flux from accreting on to the star at all, meaning that mass would only accrete as fast as it is able to slip past the flux. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.
Cantalupo S.,Kavli Institute for Cosmology |
Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011
We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. radamesh uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a 'cell-by-cell' basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a 'classical' Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, radamesh is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (Hi, Hii, Hei, Heii, Heiii, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT code comparison project and a new set of tests aimed at substantiating the new characteristics of radamesh. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during reionization produces a large increase of the predicted gas temperature within the whole Hii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used 'on-the-spot' approximation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.
Papadopoulos P.P.,Argelander Institute For Astronomie |
Pelupessy F.I.,Leiden University
Astrophysical Journal | Year: 2010
We utilize detailed time-varying models of the coupled evolution of stars and the H I, H2, and CO-bright H2 gas phases in galaxy-sized numerical simulations to explore the evolution of gas-rich and/or metal-poor systems, which are expected to be numerous in the early universe. The inclusion of the CO-bright H2 gas phase and the realistic rendering of star formation as an H2-regulated process (and the new feedback processes that this entails) allow the most realistic tracking of strongly evolving galaxies and much better comparison with observations. We find that while galaxies eventually settle into states conforming to the Schmidt-Kennicutt (S-K) relations, significant and systematic deviations of their star formation rates (SFRs) from the latter occur, and are especially pronounced and prolonged for metal-poor systems. The largest such deviations occur for gas-rich galaxies during not only the early evolutionary stages but also during brief periods at later stages. Given that gas-rich and/or metal-poor states of present-epoch galaxies are expected in the early universe while a much larger number of mergers frequently reset non-isolated systems to gas-rich states, even brief periods of sustained deviations of their SFRs from those expected from the S-K relations may come to characterize significant periods of their stellar mass built-up. This indicates potentially serious limitations of S-K-type relations as reliable sub-grid elements of star formation physics in simulations of structure formation in the early universe. We anticipate that galaxies with marked deviations from the S-K relations will be found at high redshifts as unbiased inventories of total gas mass become possible with ALMA and the EVLA. © 2010 The American Astronomical Society. All rights reserved.
Papadopoulos P.P.,Argelander Institute For Astronomie
Astrophysical Journal | Year: 2010
The high-density star formation typical of the merger/starburst events that power the large IR luminosities of ultraluminous infrared galaxies (ULIRGs) (LIR(8-1000 μm) ≳1012 L⊙) throughout the universe results in extraordinarily high cosmic-ray (CR) energy densities of UCR ∼ few × (103-104) UCR,Gal permeating their interstellar medium, a direct consequence of the large supernova remnant number densities in such systems. Unlike far-UV photons emanating from numerous star-forming (SF) sites, these large CR energy densities in ULIRGs will volumetrically heat and raise the ionization fraction of dense (n > 104 cm -3) UV-shielded gas cores throughout their compact SF volumes. Such conditions can turn most of the large molecular gas masses found in such systems and their high redshift counterparts (∼109-1010 Ṁ) into giant CR-dominated regions (CRDRs) rather than ensembles of photon-dominated regions (PDRs) which dominate in less IR-luminous systems where star formation and molecular gas distributions are much more extended. The molecular gas in CRDRs will have a minimum temperature of Tkin ∼ (80-160) K, and very high ionization fractions of x(e) > 10-6 throughout its UV-shielded dense core, which in turn will fundamentally alter the initial conditions for star formation in such systems. Observational tests of CRDRs can be provided by high-J CO and 13CO lines or multi-J transitions of any heavy rotor molecules (e.g., HCN) and their isotopologs. Chemical signatures of very high ionization fractions in dense UV-shielded gas such as low [DCO +]/[HCO+] and high [HCO+]/[CO] abundance ratios would be good probes of CRDRs in extreme starbursts. These tests, along with direct measurements of the high CO line brightness temperatures expected over the areas of compact dense gas disks found in ULIRGs, will soon be feasible as sub-arcsecond interferometric imaging capabilities and sensitivity at millimeter/submillimeter wavelengths improve in the era of ALMA. © 2010. The American Astronomical Society. All rights reserved.