We present a self-consistent, bimodal stationary solution for spherically symmetric flows driven by young massive stellar clusters with a central supermassive black hole. We demonstrate that the hydrodynamic regime of the flow depends on the location of the cluster in the 3D (star cluster mechanical luminosity - BH mass - star cluster radius) parameter space. We show that a threshold mechanical luminosity (L_crit) separates clusters which evolve in the BH dominated regime frome those whose internal structure is strongly affected by the radiative cooling. In the first case(below the threshold energy) gravity of the BH separates the flow into two distinct zones: the inner accretion zone and the outer zone where the star cluster wind is formed. In the second case (above the critical luminosity), catastrophic cooling sets in inside the cluster. In this case the injected plasma becomes thermally unstable that inhibits a complete stationary solution. We compared the calculated accretion rates and the BH luminosities with those predicted by the classic Bondi accretion theory and found that Bondi's theory is in good agreement with our results in the case of low mass clusters. However, it substantially underestimates the accretion rates and BH luminosities if the star cluster mechanical luminosity, L_sc, approaches the threshold value (L_sc > 0.1 L_crit).
We use fractional integrals to generalize the description of hydrodynamic accretion in fractal media. The fractional continuous medium model allows the generalization of the equations of balance of mass density and momentum density. These make it possible to consider the general case of spherical hydrodynamic accretion onto a gravitating mass embedded in a fractal medium. The general nature of the solution is similar to the "Bondi solution", but the accretion rate may vary substantially and the dependence on central mass may change significantly depending on dimensionality of the fractal mediu…
We present a grid of spherically symmetric model atmospheres for young pre-MS stars. This grid spans the parameter range $2000\K \leq \Teff \leq 6800\K$ and $2.0 \leq \logg \leq 3.5$ for $M=0.1\Msun$, appropriate for low mass stars and brown dwarfs. A major improvement is the replacement of TiO and \water line lists with the newer line list calculated by the NASA-AMES group, for TiO (about 175 million lines of 5 isotopes) and for \water (about 350 million lines in 2 isotopes). We provide the model structures, spectra and broad-band colors in standard filters in electronic form.
Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS spectra of roughly solar-mass accretors with accretion rates that range from 2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A leads to a higher contrast between the accretion and photospheric emission relative to higher-mass counterparts. The mass accretion rates meas…
Aims : We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods : New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results : …
In this paper we have solved the hydrodynamic equations governing the spherically symmetric isothermal accretion (wind) onto (away from) compact objects using various pseudo-Schwarzschild potentials.These solutions are essentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a flow could be obtained (with the assumption of a one-temperature fluid) if {\it only one} flow parameter (temperature of the flow $T$) is given. Also we have investigated the transonic behaviour of such a flow and showed that for a given $T$, transitions from…
We describe general-relativistically a spherically symmetric stationary fluid accretion onto a black hole. Relativistic effects enhance mass accretion, in comparison to the Bondi model predictions, in the case when backreaction is neglected. That enhancement depends on the adiabatic index and the asymptotic gas temperature and it can magnify accretion by one order in the ultrarelativistic regime.
We introduce a formulation of Eulerian general relativistic hydrodynamics which is applicable for (perfect) fluid data prescribed on either spacelike or null hypersurfaces. Simple explicit expressions for the characteristic speeds and fields are derived in the general case. A complete implementation of the formalism is developed in the case of spherical symmetry. The algorithm is tested in a number of different situations, predisposing for a range of possible applications. We consider the Riemann problem for a polytropic gas, with initial data given on a retarded/advanced time slice of Minkows…
Young stars orbiting in the gravitational potential well of forming star clusters pass through the cluster's dense molecular gas and can experience Bondi-Hoyle accretion from reservoirs outside their individual protostellar cloud cores. Accretion can occur for several million years after the stars form, but before the cluster disperses. This accretion is predominantly onto the disk and not the star. N-body simulations of stars orbiting in three young model clusters containing 30, 300, and 3000 stars are presented. The simulations include the gravitational potential of the molecular gas which s…
I study the formation of Comptonization spectra in spherically symmetric, fast moving media in a flat spacetime. I analyze the mathematical character of the moments of the transfer equation in the system-frame and describe a numerical method that provides fast solutions of the time-independent radiative transfer problem that are accurate in both the diffusion and free-streaming regimes. I show that even if the flows are mildly relativistic (V~0.1, where V is the electron bulk velocity in units of the speed of light), terms that are second-order in V alter the emerging spectrum both quantitativ…
We report on the detection of four rotating massive disks in two regions of high-mass star formation. The disks are perpendicular to known bipolar outflows and turn out to be unstable but long lived. We infer that accretion onto the embedded (proto)stars must proceed through the disks with rates of ~10E-2 Msun/yr.
We investigate the effects of dynamical Comptonization on the emergent radiation spectrum produced by near critical accretion onto a neutron star. The flow dynamics and the transfer of radiation are self--consistently solved in the case of a spherically symmetric, ``cold'', pure scattering flow, including general relativity. A sequence of models, each characterized by the value of the total observed luminosity, was obtained assuming that the spectrum at the star surface is black body in shape. It is found that when the luminosity approaches the Eddington limit dynamical effects become importan…
We compute mass outflow rate $R_{\dot m}$ from relativistic matter accreting quasi-spherically onto Schwarzschild black holes. Taking the pair-plasma pressure mediated shock surface as the {\it effective} boundary layer (of the black hole) from where bulk of the outflow is assumed to be generated, computation of this rate is done using combinations of exact transonic inflow and outflow solutions. We find that $R_{\dot m}$ depends on the initial parameters of the flow, the polytropic index of matter, the degree of compression of matter near the shock surface and on the location of the shock sur…
The equations governing general relativistic, spherically symmetric, hydrodynamic accretion of polytropic fluid onto black holes are solved in Schwarzschild metric to investigate some of the transonic properties of the flow. Only stationary solutions are discussed. For such accretion, it has been shown that real physical sonic points may form {\it even for flow with $γ< {4/3}$ or $γ> {5/3}$}. Behaviour of some flow variables in the close vicinity of the event horizon are studied as a function of specific energy and polytropic index of the flow.
The detection and characterization of accretion processes in the disks surrounding young stars may be directly relevant to studies of planet formation. Especially the study of systems with very low accretion rates (<< 10^{-10} M_sun yr^{-1}) is important, since at those rates radial mixing becomes inefficient and disk material will have to be dissipated into larger bodies at its present location. In these proceedings, we compare the different methods of tracing accretion onto Herbig Ae/Be stars and conclude that high-resolution infrared spectroscopy is currently the only reliable method that o…
A radiation-driven disk wind model is proposed that offers great promise of explaining the extreme mass loss signatures of massive young stellar objects (the BN-type objects and more luminous Herbig Be stars). It is argued that the dense low-velocity winds associated with young late-O/early-B stars would be the consequence of continuing optically-thick accretion onto them. The launch of outflow from a Keplerian disk allows wind speeds of about 200 km/s that are substantially less than the escape speed from the stellar surface. The star itself is not required to be a rapid rotator. Disk irradia…
We present an analysis of the temperature, density, and velocity of the molecular gas in the star-forming core around W51 e2. A previous paper (Ho and Young 1996) describes the kinematic evidence which implies that the core around e2 is contracting onto a young massive star. The current paper presents a technique for modelling the three-dimensional structure of the core by simulating spectral line images of the source and comparing those images to observed data. The primary conclusions of this work are that the molecular gas in e2 is radially contracting at about 5 km/s and that the temperatur…
General results from a 3-5 micron spectroscopic survey of nearby low-mass young stellar objects are presented. L and M-band spectra have been obtained of \~50 low mass embedded young stars using the ISAAC spectrometer mounted on UT1-Antu at Paranal Observatory. For the first time, a consistent census of the CO, H2O ices and the minor ice species CH3OH and OCN- and warm CO gas present around young stars is obtained, using large number statistics and resolving powers of up to R=10000. The molecular structure of circumstellar CO ices, the depletion of gaseous CO onto grains in protoplanetary disk…
Bondi type hydrodynamic accretion of the surrounding matter onto Kerr black hole with an arbitrary rotational parameter is considered. The effects of viscosity, thermal conductivity and interaction with radiation field are neglected. The black hole is supposed to be at rest with respect to matter at infinity. The flow is adiabatic and has no angular momentum. The fact that usually in astrophysics substance far from the black hole has nonrelativistic temperature introduces small parameter to the problem and allows to search for the solution as a perturbation to the accretion of a cold, that is …
We report on the first phase of our study of slightly rotating accretion flows onto black holes. We consider inviscid accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We study accretion flows by means of numerical 2D, axisymmetric, hydrodynamical simulations. Our main result is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geo…
The study of the chemical evolution of gas and dust from pre-stellar dense cores to circumstellar disks around young stars forms an essential part of understanding star- and planet formation. Throughout the collapse- and protostellar phases, simple and complex molecules are formed, many of which deplete onto cold grains and are eventually incorporated into the icy planetesimals of new solar systems. Tracing this chemical evolution provides a wealth of information, not only about the chemical processing in primitive solar nebulae, but also about physical processes which occur in the immediate s…
Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of such emission is uncertain. Although many or perhaps most recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor ~50 increase in the X-ray flux from a young star that is presently undergoing a spectacular optical/I…
The stationary, spherically symmetric accretion of dark energy onto a Schwarzschild black hole is considered in terms of relativistic hydrodynamics. The approximation of an ideal fluid is used to model the dark energy. General expressions are derived for the accretion rate of an ideal fluid with an arbitrary equation of state p=p(ρ) onto a black hole. The black hole mass was found to decrease for the accretion of phantom energy. The accretion process is studied in detail for two dark energy models that admit an analytical solution: a model with a linear equation of state, p=α(ρ-ρ_0), and a Cha…
The nature of the energy source powering anomalous X-ray pulsars is uncertain. Proposed scenarios involve either an ultramagnetized neutron star, or accretion onto a neutron star. We consider the accretion model proposed recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback material following a supernova. We compute the optical, infrared, and submillimeter emission expected from such a disk, including both viscous dissipation and reradiation of X-ray flux impinging on the disk from the pulsar. We find that it is possible with current instruments to put serious constra…
The morphology and stellar content of the Cygnus OB2 association has been determined using 2MASS infrared observations in the J, H, and K bands. The analysis reveals a spherically symmetric association of 2 deg in diameter with a half light radius of 13', corresponding to Rh = 6.4 pc at an assumed distance of 1.7 kpc. The interstellar extinction for member stars ranges from Av = 5m to 20m, which led to a considerable underestimation of the association size and richness in former optical studies. From the infrared colour-magnitude diagram, the number of OB member stars is estimated to 2600 +/- …
Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whethe…
Near-infrared surveys of high-mass star-forming regions start to shed light onto their stellar content. A particular class of objects found in these regions, the so-called massive Young Stellar Objects (YSOs) are surrounded by dense circumstellar material. Several near- and mid-infrared diagnostic tools are used to infer the physical characteristics and geometry of this circumstellar matter. Near-infrared hydrogen emission lines provide evidence for a disk-wind. The profiles of the first overtone of the CO band-heads, originating in the inner 10 AU from the central star, are well fitted assumi…
We report here the first evidence for planetesimal infall onto the very young Herbig Be star LkH$_α$234. These results are based on observations acquired over 31 days using spectroscopy of the sodium D lines, the He I 5876Å, and hydrogen H$_α$ lines. We find Redshifted Absorption Components (RAC) with velocities up to 200 km/s and very mild Blueshifted Absorption Components (BEC) up to 100 km/s in the Na I lines. No correlation is observed between the appearance of the Na I RAC & BEC and the H$_α$ and He I line variability, which suggests that these (Na I RAC & BEC) are formed in a process unr…
Magnetic fields are believed to have a vital role in regulating and shaping the flow of material onto and away from protostars during their initial mass accretion phase. It is becoming increasingly accepted that bipolar outflows are generated and collimated as material is driven along magnetic field lines and centrifugally accelerated off a rotating accretion disk. However, the precise role of the magnetic field is poorly understood and evidence for its shape and structure has not been forthcoming. Here we report imaging circular polarimetry in the near-infrared and Monte Carlo modelling showi…
Observations of the dust and gas toward two young stellar clusters, IRS1 and IRS2, in the NGC2264 star forming region are presented. Continuum emission is used to locate the dusty envelopes around the clusters and individual protostars within and line emission from the J=3-2 transitions of HCO+ and H13CO+ is used to diagnose the gas flows around them. The molecular abundance, velocity centroid and dispersion are approximately constant across the IRS1 clump. With these constraints, the self-absorbed HCO+ lines are modeled as a large scale collapse, with speed v_in=0.3 km/s and mass infall rate …
Using full general relativistic calculations, we investigate the possibility of generation of mass outflow from spherical accretion onto non-rotating black holes. Introducing a relativistic hadronic-pressure-supported steady, standing, spherically-symmetric shock surface around a Schwarzschild black hole as the effective physical barrier that may be responsible for the generation of spherical wind, we calculate the mass outflow rate $R_{\dot m}$ in terms of three accretion parameters and one outflow parameter by simultaneously solving the set of general relativistic hydrodynamic equations desc…
A number of `modified' Newtonian potentials of various forms are available in the literature which accurately approximate some general relativistic effects important for studying accretion discs around a Schwarzschild black hole. Such potentials may be called `pseudo-Schwarzschild' potentials because they nicely mimic the space-time around a non-rotating/slowly rotating compact object. In this paper, we examine the validity of the application of some of these potentials to study the spherically symmetric, transonic, hydrodynamic accretion onto a Schwarzschild black hole. By comparing the value…
The standard picture of accretion is a steady flow of matter from the disc onto the young star - a concept which assumes the star-disc system to be completely isolated. However, in a dense cluster environment star-disc systems do interact gravitationally. The aim here is to estimate the encounter-induced accretion rate in an ONC-like environment. Combining simulations of the cluster dynamics with simulations of the effect of encounters on star-disc systems we determine the likelihood and degree of encounter-triggered accretion processes. We show that accretion bursts triggered by encounters of…
A star forms when a cloud of dust and gas collapses. It is generally believed that this collapse first produces a flattened rotating disk, through which matter is fed onto the embryonic star at the center of the disk. When the temperature and density at the center of the star pass a critical threshold, thermonuclear fusion begins. The remaining disk, which can still contain up to 0.3 times the mass of the star, is then sculpted and eventually dissipated by the radiation and wind from the newborn star. Unfortunately this picture of the structure and evolution of the disk remains speculative bec…
We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is ma…
An optical spectrum of the Elson et al. (1998) candidate luminous white dwarf in the young LMC cluster NGC1818 shows conclusively that it is not a degenerate star. A model atmosphere fit gives T=31,500K and log g=4.4, typical of a garden-variety main sequence B star. However, if it is a true LMC member then the star is under-luminous by almost three magnitudes. Its position in the cluster colour-magnitude diagram also rules out the possibility that this is an ordinary B star. The luminosity is, however, consistent with a ~0.5 solar mass post-AGB or post-EHB object, although if it has evolved v…
I present new results from a continuing program to identify and characterize the low-mass stellar and substellar populations in the young cluster IC 348 (1-10~Myr). Optical spectroscopy has revealed young objects with spectral types as late as M8.25. The intrinsic J-H and H-K colors of these sources are dwarf-like, whereas the R-I and I-J colors appear intermediate between the colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are reproduced well with averages of standard dwarf and giant spectra, suggesting that such averages should be used in the classification of young…
The first spherical accretion model was developed 55 years ago, but the theory is yet far from being complete. The real accretion flow was found to be time-dependent and turbulent. This paper presents the minimal MHD spherical accretion model that separately deals with turbulence. Treatment of turbulence is based on simulations of several regimes of collisional MHD. The effects of freezing-in amplification, dissipation, dynamo action, isotropization, and constant magnetic helicity are self-consistently included. The assumptions of equipartition and magnetic field isotropy are released. Correct…
We report on the identification of 54 embedded clusters around 217 massive protostellar candidates of which 34 clusters are new detections. The embedded clusters are identified as stellar surface density enhancements in the 2 $μ$m All Sky Survey (2MASS) data. Because the clusters are all associated with massive stars in their earliest evolutionary stage, the clusters should also be in an early stage of evolution. Thus the properties of these clusters should reflect properties associated with their formation rather than their evolution. For each cluster, we estimate the mass, the morphological …
Solution for a stationary spherically symmetric accretion of the relativistic perfect fluid with an equation of state $p(ρ)$ onto the Schwarzschild black hole is presented. This solution is a generalization of Michel solution and applicable to the problem of dark energy accretion. It is shown that accretion of phantom energy is accompanied with the gradual decrease of the black hole mass. Masses of all black holes tend to zero in the phantom energy universe approaching to the Big Rip.
Shortly after the addition of a 1/R term to the Einstein-Hilbert action was proposed as a solution to the cosmic-acceleration puzzle, Chiba showed that such a theory violates Solar System tests of gravity. A flurry of recent papers have called Chiba's result into question. They argue that the spherically-symmetric vacuum spacetime in this theory is the Schwarzschild-de Sitter solution, making this theory consistent with Solar System tests. We point out that although the Schwarzschild-de Sitter solution exists in this theory, it is not the unique spherically-symmetric vacuum solution, and it is…
