Massive star formation via high accretion rates and early disk-driven outflows

We present an investigation of massive star formation that results from the gravitational collapse of massive, magnetized molecular cloud cores. We investigate this by means of highly resolved, numerical simulations of initial magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing three different cases - an isothermal collapse, a collapse with radiative cooling, and a magnetized collapse - we show that massive stars assemble quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the mass accretion during the collapsing phase is much more efficient than predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that during protostellar assembly the mass accretion reaches 20 - 100 c^3/G. Furthermore, we determined the self-consistent structure of bipolar outflows that are produced in our three dimensional magnetized collapse simulations. These outflows produce cavities out of which radiation pressure can be released, thereby reducing the limitations on the final mass of massive stars formed by gravitational collapse. Moreover, we argue that the extraction of angular momentum by disk-threaded magnetic fields and/or by the appearance of bars with spiral arms significantly enhance the mass accretion rate, thereby helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ, see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf for high resolution figure

Recent advances on IMF research

Here I discuss recent work on brown dwarfs, massive stars and the IMF in general. The stellar IMF can be well described by an invariant two-part power law in present-day star-formation events within the Local Group of galaxies. It is nearly identical in shape to the pre-stellar core mass function. The majority of brown dwarfs follow a separate IMF. Evidence from globular clusters and ultra-compact dwarf galaxies has emerged that IMFs may have been top heavy depending on the star-formation rate density. The IGIMF then ranges from bottom heavy at low galaxy-wide star formation rates to being top-heavy in galaxy-scale star bursts.Comment: 6 pages, LaTeX, to appear in The Labyrinth of Star Formation, 18-22 June 2012, Crete, (eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson, Springer, in press; replaced version: very minor corrections plus the addition of reference Smith & Lucey (2013) on the bottom-heavy IMF in elliptical galaxie

Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores

We use magnetic collapse models to place some constraints on the formation and angular momentum evolution of circumstellar disks which are embedded in magnetized cloud cores. Previous models have shown that the early evolution of a magnetized cloud core is governed by ambipolar diffusion and magnetic braking, and that the core takes the form of a nonequilibrium flattened envelope which ultimately collapses dynamically to form a protostar. In this paper, we focus on the inner centrifugally-supported disk, which is formed only after a central protostar exists, and grows by dynamical accretion from the flattened envelope. We estimate a centrifugal radius for the collapse of mass shells within a rotating, magnetized cloud core. The centrifugal radius of the inner disk is related to its mass through the two important parameters characterizing the background medium: the background rotation rate \Omb and the background magnetic field strength \Bref. We also revisit the issue of how rapidly mass is deposited onto the disk (the mass accretion rate) and use several recent models to comment upon the likely outcome in magnetized cores. Our model predicts that a significant centrifugal disk (much larger than a stellar radius) will be present in the very early (Class 0) stage of protostellar evolution. Additionally, we derive an upper limit for the disk radius as it evolves due to internal torques, under the assumption that the star-disk system conserves its mass and angular momentum even while most of the mass is transferred to a central star.Comment: 23 pages, 1 figure, aastex, to appear in the Astrophysical Journal (10 Dec 1998

DJ-1: A promising therapeutic candidate for ischemia-reperfusion injury.

DJ-1 is a multifaceted protein with pleiotropic functions that has been implicated in multiple diseases, ranging from neurodegeneration to cancer and ischemia-reperfusion injury. Ischemia is a complex pathological state arising when tissues and organs do not receive adequate levels of oxygen and nutrients. When the blood flow is restored, significant damage occurs over and above that of ischemia alone and is termed ischemia-reperfusion injury. Despite great efforts in the scientific community to ameliorate this pathology, its complex nature has rendered it challenging to obtain satisfactory treatments that translate to the clinic. In this review, we will describe the recent findings on the participation of the protein DJ-1 in the pathophysiology of ischemia-reperfusion injury, firstly introducing the features and functions of DJ-1 and, successively highlighting the therapeutic potential of the protein

Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres

Self-similar solutions provide good descriptions for the gravitational collapse of spherical clouds or stars when the gas obeys a polytropic equation of state, $p=K\rho^\gamma$ (with $\gamma\le 4/3$). We study the behaviors of nonradial perturbations in the similarity solutions of Larson, Penston and Yahil, which describe the evolution of the collapsing cloud prior to core formation. Our global stability analysis reveals the existence of unstable bar-modes ($l=2$) when $\gamma\le 1.09$. In particular, for the collapse of isothermal spheres, which applies to the early stages of star formation, the $l=2$ density perturbation relative to the background, $\delta\rho({\bf r},t)/\rho(r,t)$, increases as $(t_0-t)^{-0.352}\propto \rho_c(t)^{0.176}$, where $t_0$ denotes the epoch of core formation, and $\rho_c(t)$ is the cloud central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal shape (prolate bar or oblate disk, depending on initial conditions) as the collapse proceeds. In the context of Type II supernovae, core collapse is described by the $\gamma\simeq 1.3$ equation of state, and our analysis indicates that there is no growing mode (with density perturbation) in the collapsing core before the proto-neutron star forms, although nonradial perturbations can grow during the subsequent accretion of the outer core and envelope onto the neutron star. We also carry out a global stability analysis for the self-similar expansion-wave solution found by Shu, which describes the post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a protostar. We show that this solution is unstable to perturbations of all $l$'s, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To be published in ApJ (V.540, Sept.10, 2000 issue

Jet-Induced Emission-Line Nebulosity and Star Formation in the High-Redshift Radio Galaxy 4C41.17

The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work has shown that there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds and we construct a jet-cloud interaction model to explain the major features revealed by the data. The interaction of a high-powered jet with a dense cloud in the halo of 4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s shocks and induces star formation. The CIII to CIV line ratio and the CIV luminosity emanating from the shock, imply that the pre-shock density in the line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star formation could proceed on a timescale of order a few x 10^6 yrs, well within the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s) emission lines are attributed to the disturbance of the gas cloud by a partial bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular CIV) arise in precursor emission in relatively low metallicity gas. The implied baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line and UV continuum images in some of the highest redshift radio galaxies and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high redshift radio galaxies.Comment: 18 pages, 5 figures; uses astrobib.sty and aaspp4.sty. Better versions of figures available via anonymous from ftp://mso.anu.edu.au:pub/pub/geoff/4C41.1

Molecular Evolution in Collapsing Prestellar Cores

We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogues to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical and adsorption time scales. When the central density n_H of a prestellar core with the Larson-Penston flow rises to 3 10^6 cm^{-3}, the CCS and CO column densities are calculated to show central holes of radius 7000 AU and 4000 AU, respectively, while the column density of N2H+ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical time scale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse time scale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is n_H=3 10^6 cm^{-3}, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27, depending on the collapse time scale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.Comment: 21 pages, 17 figure

HST/NICMOS observations of a proto-brown dwarf candidate

We present deep HST/NICMOS observations peering through the outflow cavity of the protostellar candidate IRAS 04381+2540 in the Taurus Molecular Cloud-1. A young stellar object as central source, a jet and a very faint and close (0.6") companion are identified. The primary and the companion have similar colours, consistent with strong reddening. We argue that the companion is neither a shock-excited knot nor a background star. The colour/magnitude information predicts a substellar upper mass limit for the companion, but the final confirmation will require spectroscopic information. Because of its geometry, young age and its rare low-mass companion, this system is likely to provide a unique insight into the formation of brown dwarfs.Comment: Astronomy & Astrophysics Letters, in press; 4 pages, 2 figure

Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows

Collapse of the rotating magnetized molecular cloud core is studied with the axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the equation of state of the interstellar gas, the molecular cloud cores experience several different phases as collapse proce eds. In the isothermal run-away collapse ($n \lesssim 10^{10}{\rm H_2 cm}^{-3}$), a pseudo-disk is formed and it continues to contract till the opaque core is fo rmed at the center. In this disk, a number of MHD fast and slow shock pairs appear running parallelly to the disk. After the equation of state becomes hard, an adiabatic core is formed, which is separated from the isothermal contracting pseudo-disk by the accretion shock front facing radially outwards. By the effect of the magnetic tension, the angular momentum is transferred from the disk mid-plane to the surface. The gas with excess angular momentum near the surface is finally ejected, which explains the molecular bipolar outflow. Two types of outflows are observed. When the poloidal magnetic field is strong (magnetic energy is comparable to the thermal one), a U-shaped outflow is formed in which fast moving gas is confined to the wall whose shape looks like a capit al letter U. The other is the turbulent outflow in which magnetic field lines and velocity fi elds are randomly oriented. In this case, turbulent gas moves out almost perpendicularly from the disk. The continuous mass accretion leads to the quasistatic contraction of the first core. A second collapse due to dissociation of H$_2$ in the first core follows. Finally another quasistatic core is again formed by atomic hydrogen (the second core). It is found that another outflow is ejected around the second atomic core, which seems to correspond to the optical jets or the fast neutral winds.Comment: submitted to Ap

Binary frequency of very young brown dwarfs at separations smaller than 3 AU

Searches for companions of brown dwarfs by direct imaging mainly probe orbital separations > 3-10 AU. On the other hand, previous radial velocity surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6 AU. It has been speculated that the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. This work extends high-precision radial velocity surveys of brown dwarfs for the first time out to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in ChaI was determined: 18% for the whole sample and 10% for the subsample of ten brown dwarfs and VLMS (M < 0.1 Msun). Two spectroscopic binaries were confirmed, the brown dwarf candidate ChaHa8 (previously discovered by Joergens & Mueller) and the low-mass star CHXR74. Since their orbital separations appear to be 1 AU or greater, the binary frequency at < 1 AU might be less than 10%. Now for the first time companion searches of (young) brown dwarfs cover the whole orbital separation range, and the following observational constraints for models of brown dwarf formation can be derived: (i) the frequency of brown dwarf and very low-mass stellar binaries at 3 AU; i.e. direct imaging surveys do not miss a significant fraction of brown dwarf binaries; (ii) the overall binary frequency of brown dwarfs and very low-mass stars is 10-30 %; (iii) the decline in the separation distribution of brown dwarfs towards smaller separations seems to occur between 1 and 3 AU; (iv) the observed continuous decrease in the binary frequency from the stellar to the substellar regime is confirmed at < 3 AU providing further evidence of a continuous formation mechanism from low-mass stars to brown dwarfs.Comment: 17 pages, 14 figures, Accepted by A&A, minor language editin