arXiv subjects

Peter Bodenheimer

Publications and source records attributed to Peter Bodenheimer.

49 records · Page 3Linked to original sources

arXiv2008

Stellar Structure of Dark Stars: a first phase of Stellar Evolution due to Dark Matter Annihilation

Dark Stars are the very first phase of stellar evolution in the history of the universe: the first stars to form (typically at redshifts $z \sim 10-50$) are powered by heating from dark matter (DM) annihilation instead of fusion (if the DM is made of particles which are their own antiparticles). We find equilibrium polytropic configurations for these stars; we start from the time DM heating becomes important ($M \sim 1-10 M_\odot$) and build up the star via accretion up to 1000 M$_\odot$. The dark stars, with an assumed particle mass of 100 GeV, are found to have luminosities of a few times $10^6$ L$_\odot$, surface temperatures of 4000--10,000 K, radii $\sim 10^{14}$ cm, lifetimes of at least $ 0.5$ Myr, and are predicted to show lines of atomic and molecular hydrogen. Dark stars look quite different from standard metal-free stars without DM heating: they are far more massive (e.g. $\sim 800 M_\odot$ for 100 GeV WIMPs), cooler, and larger, and can be distinguished in future observations, possibly even by JWST or TMT.

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arXiv2008

Dark Stars: Dark Matter in the First Stars leads to a New Phase of Stellar Evolution

The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the the universe. This talk presents the story of these Dark Stars. We make predictions that the first stars are very massive ($\sim 800 M_\odot$), cool (6000 K), bright ($\sim 10^6 L_\odot$), long-lived ($\sim 10^6$ years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial DM fuel runs out and fusion sets in, DM annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again.

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arXiv2006

Silicon and Nickel Enrichment in Planet-Host Stars: Observations and Implications for the Core-Accretion Theory of Planet Formation

We present evidence that stars with planets exhibit statistically significant silicon and nickel enrichment over the general metal-rich population. We also present simulations which predict silicon enhancement of planet hosts within the context of the core-accretion hypothesis for giant planet formation. Because silicon and oxygen are both alpha elements, [Si/Fe] traces [O/Fe], so the silicon enhancement in planet hosts predicts that these stars are oxygen-rich as well. We present new numerical simulations of planet formation by core accretion that establish the timescale on which a Jovian planet reaches rapid gas accretion, t_rga, as a function of solid surface density sigma_solid: (t_rga / 1 Myr) = (sigma_solid / 25.0 g cm^{-2})^{-1.44}. This relation enables us to construct Monte Carlo simulations that predict the fraction of star-disk systems that form planets as a function of [Fe/H], [Si/Fe], disk mass, outer disk radius and disk lifetime. Our simulations reproduce both the known planet-metallicity correlation and the planet-silicon correlation reported in this paper. The simulations predict that 16% of Solar-type stars form Jupiter-mass planets, in agreement with 12% predicted from extrapolation of the observed planet frequency-semimajor axis distribution. Although a simple interpretation of core accretion predicts that the planet-silicon correlation should be much stronger than the planet-nickel correlation, we observe the same degree of silicon and nickel enhancement in planet hosts. If this result persists once more planets have been discovered, it might indicate a complexity in the chemistry of planet formation beyond the simple accumulation of solids in the core accretion theory.

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arXiv2005

Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex

We study the formation of a giant gas planet by the core--accretion gas--capture process, with numerical simulations, under the assumption that the planetary core forms in the center of an anti-cyclonic vortex. The presence of the vortex concentrates particles of centimeter to meter size from the surrounding disk, and speeds up the core formation process. Assuming that a planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement results in considerably shorter formation times than are found in standard core--accretion gas--capture simulations. Also, formation of a gas giant is possible in a disk with mass comparable to that of the minimum mass solar nebula.

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arXiv2004

Formation of giant planets in disks with different metallicities

We present the first results from simulations of processes leading to planet formation in protoplanetary disks with different metallicities. For a given metallicity, we construct a two-dimensional grid of disk models with different initial masses and radii ($M_0$, $R_0$). For each disk, we follow the evolution of gas and solids from an early evolutionary stage, when all solids are in the form of small dust grains, to the stage when most solids have condensed into planetesimals. Then, based on the core accretion - gas capture scenario, we estimate the planet-bearing capability of the environment defined by the final planetesimal swarm and the still evolving gaseous component of the disk. We define the probability of planet-formation, $P_p$, as the normalized fractional area in the ($M_0$, $\log R_0$) plane populated by disks that have formed planets inside 5 AU. With such a definition, and under the assumption that the population of planets discovered at $R$ $<$ 5 AU is not significantly contaminated by planets that have migrated from $R$ $>$ 5 AU, our results agree fairly well with the observed dependence between the probability that a star harbors a planet and the star's metal content. The agreement holds for the disk viscosity parameter $α$ ranging from $10^{-3}$ to $10^{-2}$, and it becomes much poorer when the redistribution of solids relative to the gas is not allowed for during the evolution of model disks.

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arXiv2004

Core-Accretion Model Predicts Few Jovian-Mass Planets Orbiting Red Dwarfs

The favored theoretical explanation for giant planet formation -- in both our solar system and others -- is the core accretion model (although it still has some serious difficulties). In this scenario, planetesimals accumulate to build up planetary cores, which then accrete nebular gas. With current opacity estimates for protoplanetary envelopes, this model predicts the formation of Jupiter-mass planets in 2--3 Myr at 5 AU around solar-mass stars, provided that the surface density of solids is enhanced over that of the minimum-mass solar nebula (by a factor of a few). Working within the core-accretion paradigm, this paper presents theoretical calculations which show that the formation of Jupiter-mass planets orbiting M dwarf stars is seriously inhibited at all radial locations (in sharp contrast to solar-type stars). Planet detection programs sensitive to companions of M dwarfs will test this prediction in the near future.

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arXiv2003

On the Surface Heating of Synchronously-Spinning Short-Period Jovian Planets

We consider the atmospheric flow on short-period extra-solar planets through two-dimensional numerical simulations of hydrodynamics with radiation transfer. One side is always exposed to the irradiation from the host star. The other is always in shadow. The temperature of the day side is determined by the equilibrium which the planetary atmosphere establishes with stellar radiation. Part of the thermal energy deposited on the day side is advected to the night side by a current. The radiation transfer, the night-side temperature distribution and by this the spectroscopic signature of the planet are sensitive functions of the atmospheric opacity. If the atmosphere contains grains with an abundanceand size distribution comparable to that of the interstellar medium, shallow heating occurs on the day side and the night side cools well below the day side. The temperature difference decreases as the abundance of grains is reduced. A simple analytic model of the dissipation of the circulation flow and associated kinetic heating is considered. This heating effect occurs mostly near the photosphere, not deep enough to significantly affect the size of planets. We show that the surface irradiation suppresses convection near the photospheric region on the day side. In some cases convection zones appear near the surface on the night side. This structural modification may influence the response and dissipation of tidal disturbances and alter the circularization and synchronization time scales.

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arXiv2003

The Effect of Tidal Inflation Instability on the Mass and Dynamical Evolution of Extrasolar Planets with Ultra-Short Periods

We investigate the possibility of substantial inflation of short-period Jupiter-mass planets, as a result of their internal tidal dissipation associated with the synchronization and circularization of their orbits. We employ the simplest prescription based on an equilibrium model with a constant lag angle for all components of the tide. We show that for young Jupiter-mass planets, with a period less than 3 days, an initial radius about 2 Jupiter radii, and an orbital eccentricity greater than 0.2, the energy dissipated during the circularization of their orbits is sufficiently intense and protracted to inflate their sizes up to their Roche radii.

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arXiv2002

Turbulence in Accretion Disks. Vorticity Generation and Angular Momentum Transport via the Global Baroclinic Instability

In this paper we present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We show by analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that a baroclinic flow is unstable and produces turbulence. These findings are tested for numerical effects by performing a simulation with a barotropic initial condition which shows that imposed turbulence rapidly decays. The turbulence in baroclinic disks transports angular momentum outward and creates a radially inward bound accretion of matter. Potential energy is released and excess kinetic energy is dissipated. Finally the reheating of the gas supports the radial entropy gradient, forming a self consistent process. We measure accretion rates in our 2D and 3D simulations of dotM= - 1E-9 - 1E-7 Msun/yr and viscosity parameters of alpha = 1E-4 - 1E-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the R-phi plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

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arXiv2002

Models of the formation of the planets in the 47 UMa system

Formation of planets in the 47 UMa system is followed in an evolving protoplanetary disk composed of gas and solids. The evolution of the disk is calculated from an early stage, when all solids, assumed to be high-temperature silicates, are in the dust form, to the stage when most solids are locked in planetesimals. The simulation of planetary evolution starts with a solid embryo of ~1 Earth mass, and proceeds according to the core accretion -- gas capture model. Orbital parameters are kept constant, and it is assumed that the environment of each planet is not perturbed by the second planet. It is found that conditions suitable for both planets to form within several Myr are easily created, and maintained throughout the formation time, in disks with $α\approx 0.01$. In such disks, a planet of 2.6 Jupiter masses (the minimum for the inner planet of the 47 UMa system) may be formed at 2.1 AU from the star in \~3 Myr, while a planet of 0.89 Jupiter masses (the minimum for the outer planet) may be formed at 3.95 AU from the star in about the same time. The formation of planets is possible as a result of a significant enhancement of the surface density of solids between 1.0 and 4.0 AU, which results from the evolution of a disk with an initially uniform gas-to-dust ratio of 167 and an initial radius of 40 AU.

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arXiv2000

Turbulent Molecular Cloud Cores: Rotational Properties

The rotational properties of centrally condensed, turbulent molecular cloud cores with velocity fields that are characterized by Gaussian random fields are investigated. It is shown that the observed line width-size relationship can be reproduced if the velocity power spectrum is a power-law with P(k)=k**n and n=-3 to -4. The line-of-sight velocity maps of these cores show velocity gradients that can be interpreted as rotation. For n=-4, both, the deduced values of the angular velocity Omega=1.6 km/s/pc * (R/0.1 pc)**0.5 and the scaling relations between Omega and the core radius R are in very good agreement with the observations. As a result of the dominance of long wavelength modes, the cores also have a net specific angular momentum with an average value of j=7*(10**20)*(R/0.1 pc)**(1.5) cm**2/s with a large spread. Their internal dimensionless rotational parameter is beta=0.03, independent of the scale radius R. In general, the line-of-sight velocity gradient of an individual turbulent core does not provide a good estimate of its internal specific angular momentum. We find however that the distribution of the specific angular momenta of a large sample of cores which are described by the same power spectrum can be determined very accurately from the distribution of their line-of-sight velocity gradients Omega using the simple formula j=p*Omega*R*R where p depends on the density distribution of the core and has to be determined from a Monte-Carlo study. Our results show that for centrally condensed cores the intrinsic angular momentum is overestimated by a factor of 2-3 if p=0.4 is used.

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arXiv1998

Double Core Evolution X. Through the Envelope Ejection Phase

The evolution of binary systems consisting of an asymptotic giant branch star of mass equal to 3 M_sun or 5 M_sun, and a main sequence star of mass equal to 0.4 M_sun or 0.6 M_sun with orbital periods > 200 days has been followed from the onset through the late stages of the common envelope phase. Using a nested grid technique, the three-dimensional hydrodynamical simulations of an asymptotic giant branch star with radii approximately 1 A.U. indicate that a significant fraction of the envelope gas is unbound (about 31% and 23% for binaries of 3 M_sun and 0.4 M_sun, and 5 M_sun and 0.6 M_sun respectively) by the ends of the simulations, and that the efficiency of the mass ejection process is about 40%. While the original volume of the giant is virtually evacuated in the late stages, most of the envelope gas remains marginally bound on the grid. At the ends of our simulations, when the orbital decay timescale exceeds about 5 years, the giant core and companion orbit one another with a period of about 1 day (2.4 days for a binary involving a more evolved giant), although this is an upper limit to the final orbital period. For a binary of 5 M_sun and 0.4 M_sun, the common envelope may not be completely ejected.

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arXiv1995

Fragmentation in a centrally condensed protostar

Hydrodynamical calculations in three space dimensions of the collapse of an isothermal, centrally condensed, rotating 1 M\sol protostellar cloud are presented. A numerical algorithm involving nested subgrids is used to resolve the region where fragmentation occurs in the central part of the protostar. A previous calculation by Boss, which produced a hierarchical multiple system, is evolved further, at comparable numerical resolution, and the end result is a binary, with more than half of the mass of the original cloud, whose orbital separation increases with time as a result of accretion of high-angular momentum material and as a result of merging with fragments that have formed farther out. Repeating the calculation with significantly higher resolution, we find that a sequence of binaries can be induced by fragmentation of circumbinary disks. The stability of the resulting multiple system is investigated Using $n$-body calculations, which indicate that such a system would transform on a short time scale into a more stable hierarchical structure. The outermost and most massive binary which forms in the high-resolution run has properties similar to that of the binary found in the low-resolution calculation. Thus the basic outcome is shown to be independent of the numerical spatial resolution. The high-resolution run, in addition, leads to the formation of a system of smaller fragments, which might be important for the understanding of the origin of close binaries with low-mass components and of low mass single stars.

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