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Fred C. Adams

Publications and source records attributed to Fred C. Adams.

198 records · Page 11Linked to original sources
arXiv1999

The Stellar Initial Mass Function: Constraints from Young Clusters and Theoretical Perspectives

We summarize recent observational and theoretical progress aimed at understanding the origin of the stellar initial mass function (IMF) with specific focus on galactic star-forming regions. We synthesize data from various efforts to determine the IMF in very young, partially-embedded stellar clusters and find: i) no significant variations in the low-mass IMF have been observed between different star-forming regions; and ii) the mass distributions of young stars just emerging from molecular clouds are consistent with having been drawn from the IMF derived from field stars in the solar neighborhood. These results apply only to gross characterizations of the IMF (e.g. the ratio of high to low mass stars); present observations do not rule out more subtle regional differences. Further studies are required in order to assess whether or not there is evidence for a universal turnover near the hydrogen--burning limit. We also provide a general framework for discussing theories of the IMF, and summarize recent work on several physical mechanisms which could play a role in determining the form of the stellar initial mass function.

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arXiv1999

Possible Effects of a Cosmological Constant on Black Hole Evolution

We explore possible effects of vacuum energy on the evolution of black holes. If the universe contains a cosmological constant, and if black holes can absorb energy from the vacuum, then black hole evaporation could be greatly suppressed. For the magnitude of the cosmological constant suggested by current observations, black holes larger than $\sim 4 \times 10^{24}$ g would accrete energy rather than evaporate. In this scenario, all stellar and supermassive black holes would grow with time until they reach a maximum mass scale of $\sim 6 \times 10^{55}$ g, comparable to the mass contained within the present day cosmological horizon.

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arXiv1998

The Gravitational Demise of Cold Degenerate Stars

We consider the long term fate and evolution of cold degenerate stars under the action of gravity alone. Although such stars cannot emit radiation through the Hawking mechanism, the wave function of the star will contain a small admixture of black hole states. These black hole states will emit radiation and hence the star can lose its mass energy in the long term. We discuss the allowed range of possible degenerate stellar evolution within this framework.

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arXiv1998

Dynamics of Circumstellar Disks

We present a series of 2-dimensional hydrodynamic simulations of massive disks around protostars. We simulate the same physical problem using both a `Piecewise Parabolic Method' (PPM) code and a `Smoothed Particle Hydrodynamic' (SPH) code, and analyze their differences. The disks studied here range in mass from $0.05 M_*$ to $1.0 M_*$ and in initial minimum Toomre $Q$ value from 1.1 to 3.0. For this problem, the strengths of the codes overlap only in a limited fashion, but similarities exist in their predictions, including spiral arm pattern speeds and morphological features. Our results represent limiting cases (i.e. systems evolved isothermally) rather than true physical systems. Disks become active from the inner regions outward. From the earliest times, their evolution is a strongly dynamic process rather than a smooth progression toward eventual nonlinear behavior. We calculate approximate growth rates for the spiral patterns; the one-armed ($m=1$) spiral arm is not the fastest growing pattern of most disks. In our SPH simulations, disks with initial minimum $Q=1.5$ or lower break up into proto-binary or proto-planetary clumps. However, these simulations cannot follow the physics important for the flow and must be terminated before the system has completely evolved. At their termination, PPM simulations with similar initial conditions show uneven mass distributions within spiral arms, suggesting that clumping behavior might result if they were carried further. Concern that the point-like nature of SPH exaggerates clumping, that our representation of the gravitational potential in PPM is too coarse, and that our physics assumptions are too simple, suggest caution in interpretation of the clumping in both the disk and torus simulations.

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arXiv1997

Constraints on the Intergalactic Transport of Cosmic Rays

Motivated by recent experimental proposals to search for extragalactic cosmic rays (including anti-matter from distant galaxies), we study particle propagation through the intergalactic medium (IGM). We first use estimates of the magnetic field strength between galaxies to constrain the mean free path for diffusion of particles through the IGM. We then develop a simple analytic model to describe the diffusion of cosmic rays. Given the current age of galaxies, our results indicate that, in reasonable models, a completely negligible number of particles can enter our Galaxy from distances greater than $\sim 100$ Mpc for relatively low energies ($E$ $< 10^6$ GeV/n). We also find that particle destruction in galaxies along the diffusion path produces an exponential suppression of the possible flux of extragalactic cosmic rays. Finally, we use gamma ray constraints to argue that the distance to any hypothetical domains of anti-matter must be roughly comparable to the horizon scale.

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arXiv1997

Possible Stellar Metallicity Enhancements from the Accretion of Planets

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central star. We study the ramifications of planet accretion for the final main sequence metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr, stars with masses near 1 solar mass are predicted to have virtually no metallicity enhancement. On the other hand, early F and late A type stars with masses of 1.5--2.0 solar masses can experience significant metallicity enhancements due to their considerably smaller convection zones during the first 10 Myr of pre-main-sequence evolution. We show that the metallicities of an aggregate of unevolved F stars are consistent with an average star accreting about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr dispersal time.

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arXiv1996

Linear Graviational Instability of Filamentary and Sheet-like Molecular Clouds with Magnetic Fields

We study the linear evolution of small perturbations in self-gravitating fluid systems with magnetic fields. We consider wave-like perturbations to nonuniform filamentary and sheet-like hydrostatic equilibria in the presence of a uniform parallel magnetic field. Motivated by observations of molecular clouds that suggest substantial nonthermal (turbulent) pressure, we adopt equations of state that are softer than isothermal. We numerically determine the dispersion relation and the form of the perturbations in the regime of instability. The form of the dispersion relation is the same for all equations of state considered, for all magnetic field strengths, and for both geometries examined. We demonstrate the existence of a fastest growing mode for the system and study how its characteristics depend on the amount of turbulence and the strength of the magnetic field. Generally, turbulence tends to increase the rate and the length scale of fragmentation. While tending to slow the fragmentation, the magnetic field has little effect on the fragmentation length scale until reaching some threshold, above which the length scale decreases significantly. Finally, we discuss the implications of these results for star formation in molecular clouds.

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arXiv1996

Implications of White Dwarf Galactic Halos

Motivated by recent measurements which suggest that roughly half the mass of the galactic halo may be in the form of white dwarfs, we study the implications of such a halo. We first use current limits on the infrared background light and the galactic metallicity to constrain the allowed initial mass function (IMF) of the stellar population that produced the white dwarfs. The IMF must be sharply peaked about a characteristic mass scale $M_C \approx 2.3 M_\odot$. Since only a fraction of the initial mass of a star is incorporated into the remnant white dwarf, we argue that the mass fraction of white dwarfs in the halo is likely to be 25\% or less, and that 50\% is an extreme upper limit. We use the IMF results to place corresponding constraints on the primordial initial conditions for star formation. The initial conditions must be much more homogeneous and skewed toward higher temperatures ($T_{\rm gas} \sim$ 200 K) than the conditions which lead to the present day IMF. Next we determine the luminosity function of white dwarfs. By comparing this result with the observed luminosity function, we find that the age of the halo population must be greater than $\sim 16$ Gyr. Finally, we calculate the radiative signature of a white dwarf halo. This infrared background is very faint, but is potentially detectable with future observations.

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arXiv1996

A Theory of the IMF for Star Formation in Molecular Clouds

We present models for the initial mass function (IMF) for stars forming within molecular clouds. These models use the idea that stars determine their own masses through the action of powerful stellar outflows. This concept allows us to calculate a semi-empirical mass formula (SEMF), which provides the transformation between initial conditions in molecular clouds and the final masses of forming stars. For a particular SEMF, a given distribution of initial conditions predicts a corresponding IMF. We consider several different descriptions for the distribution of initial conditions in star forming molecular clouds. We first consider the limiting case in which only one physical variable -- the effective sound speed -- determines the initial conditions. In this limit, we use observed scaling laws to determine the distribution of sound speed and the SEMF to convert this distribution into an IMF. We next consider the opposite limit in which many different independent physical variables play a role in determining stellar masses. In this limit, the central limit theorem shows that the IMF approaches a log-normal form. Realistic star forming regions contain an intermediate number of relevant variables; we thus consider intermediate cases between the two limits. Our results show that this picture of star formation and the IMF naturally produces stellar mass distributions that are roughly consistent with observations. This paper thus provides a calculational framework to construct theoretical models of the IMF.

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arXiv1995

Infall Collapse Solutions in the Inner Limit: Radiation Pressure and its Effects on Star Formation

In this paper, we study infall collapse solutions for star formation in the small radius limit where the particle orbits become nearly pressure-free. We generalize previous solutions to simultaneously include the effects of both radiation pressure and angular momentum. The effects of radiation pressure can be modeled using a modified potential; for representative cases of such potentials, we obtain analytical solutions for the density and velocity fields. In general, radiation pressure limits the maximum mass of a forming star by reversing the infall when the star becomes sufficiently large. Our results imply that this maximum mass scale is given by the condition that the turnaround radius $R_R$ (the radius at which the radiation pressure force exceeds the gravitational force) exceeds the centrifugal radius $R_C$ (the angular momentum barrier). The maximum mass scale for a star forming within a rotating collapse flow with radiation pressure depends on the initial conditions, but is generally much larger than for the case of spherical infall considered previously. In particular, stars with masses $M_\ast$ $\sim 100$ $M_\odot$ can form for a fairly wide range of initial conditions.

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arXiv1995

Wave Motions In Molecular Clouds: Results in Two Dimensions

We study the linear evolution of small perturbations in self-gravitating fluid systems in two spatial dimensions; we consider both cylindrical and cartesian (i.e., slab) geometries. The treatment is general, but the application is to molecular clouds. We consider a class of equations of state which heuristically take into account the presence of turbulence; in particular, we consider equations of state which are {\it softer} than isothermal. We take the unperturbed cloud configuration to be in hydrostatic equilibrium. We find a class of wave solutions which propagate along a pressure supported cylinder (or slab) and have finite (trapped) spatial distributions in the direction perpendicular to the direction of propagation. Our results indicate that the dispersion relations for these two dimensional waves have similar forms for the two geometries considered here. Both cases possess a regime of instability and a fastest growing mode. We also find the (perpendicular) form of the perturbations for a wide range of wavelengths. Finally, we discuss the implications of our results for star formation and molecular clouds. The mass scales set by instabilities in both molecular cloud filaments and sheets are generally much larger than the masses of stars. However, these instabilities can determine the length scales for the initial conditions for protostellar collapse.

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arXiv1994

Formal Results Regarding Metric-Space Techniques for the Study of Astrophysical Maps

We extend a newly developed formal system for the description of astrophysical maps. In this formalism, we consider the difference between maps to be the distance between elements of a pseudometric space (the space of all such maps). This ansatz allows us to measure quantitatively the difference between any two maps and to order the space of all maps. For each physical characteristic of interest, this technique assigns an ``output'' function to each map; the difference between the maps is then determined from the difference between their corresponding output functions. In this present study, we show that the results of this procedure are invariant under a class of transformations of the maps and the domains of the maps. In addition, we study the propagation of errors (observational uncertainties) through this formalism. We show that the uncertainties in the output functions can be controlled provided that the signal to noise ratios in the original astrophysical maps are sufficiently high. The results of this paper thus increase the effectiveness of this formal system for the description, classification, and analysis of astrophysical maps.

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arXiv1994

A Quantitative Analysis of IRAS Maps of Molecular Clouds

We present an analysis of IRAS maps of five molecular clouds: Orion, Ophiuchus, Perseus, Taurus, and Lupus. For the classification and description of these astrophysical maps, we use a newly developed technique which considers all maps of a given type to be elements of a pseudometric space. For each physical characteristic of interest, this formal system assigns a distance function (a pseudometric) to the space of all maps; this procedure allows us to measure quantitatively the difference between any two maps and to order the space of all maps. We thus obtain a quantitative classification scheme for molecular clouds. In this present study we use the IRAS continuum maps at 100$μ$m and 60$μ$m to produce column density (or optical depth) maps for the five molecular cloud regions given above. For this sample of clouds, we compute the ``output'' functions which measure the distribution of density, the distribution of topological components, the self-gravity, and the filamentary nature of the clouds. The results of this work provide a quantitative description of the structure in these molecular cloud regions. We then order the clouds according to the overall environmental ``complexity'' of these star forming regions. Finally, we compare our results with the observed populations of young stellar objects in these clouds and discuss the possible environmental effects on the star formation process. Our results are consistent with the recently stated conjecture that more massive stars tend to form in more ``complex'' environments.

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arXiv1994

The Scalar Field Potential in Inflationary Models: Reconstruction and Further Constraints

In this paper, we present quantitative constraints on the scalar field potential for a general class of inflationary models. (1) We first consider the reconstruction of the inflationary potential for given primordial density fluctuation spectra. Our work differs from previous work on reconstruction in that we find a semi-analytic solution for the potential for the case of density fluctuations with power-law spectra. In addition, for the case of more general spectra, we show how constraints on the density fluctuation spectra imply corresponding constraints on the potential. We present a series of figures which show how the shape of the potential depends on the shape of the perturbation spectrum and on the relative contribution of tensor modes. (2) We show that the average ratio $\rave$ of the amplitude of tensor perturbations (gravity wave perturbations) to scalar density perturbations is bounded from above: $\rave \le$ 1.6. We also show that the ratio $\rave$ is proportional to the change $Δϕ$ in the field: $\rave \approx 0.42 Δϕ/\mp$. Thus, if tensor perturbations are important for the formation of structure, then the width $Δϕ$ must be comparable to the Planck mass. (3) We constrain the change $ΔV$ of the potential and the change $Δϕ$ of the inflation field during the portion of inflation when cosmological structure is produced. We find both upper and lower bounds for $Δϕ$ and for $ΔV$. In addition, these constraints are then used to derive a bound on the scale $Λ$, which is the scale of the height of the potential

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arXiv1993

General Analytic Results for Nonlinear Waves and Solitons in Molecular Clouds

We study nonlinear wave phenomena in self-gravitating fluid systems, with a particular emphasis on applications to molecular clouds. This paper presents analytical results for one spatial dimension. We show that a large class of physical systems can be described by theories with a ``charge density'' $q(ρ)$; this quantity replaces the density on the right hand side of the Poisson equation for the gravitational potential. We use this formulation to prove general results about nonlinear wave motions in self-gravitating systems. We show that in order for stationary waves to exist, the total charge (the integral of the charge density over the wave profile) must vanish. This ``no-charge'' property for solitary waves is related to the capability of a system to be stable to gravitational perturbations for arbitrarily long wavelengths. We find necessary and sufficient conditions on the charge density for the existence of solitary waves and stationary waves. We then apply these results to study nonlinear waves in Jeans-type theories, Yukawa theories, and a pseudo-two-dimensional treatment. We study the allowed types of wave behavior for all of these models. Finally, we discuss the implications of this work for molecular cloud structure.

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arXiv1993

General Solutions for Tunneling of Scalar Fields with Quartic Potentials

For the theory of a single scalar field $φ$ with a quartic potential $V(φ)$, we find semi-analytic expressions for the Euclidean action in both four and three dimensions. The action in four dimensions determines the quantum tunneling rate at zero temperature from a false vacuum state to the true vacuum state; similarly, the action in three dimensions determines the thermal tunneling rate for a finite temperature theory. We show that for all quartic potentials, the action can be obtained from a one parameter family of instanton solutions corresponding to a one parameter family of differential equations. We find the solutions numerically and use polynomial fitting formulae to obtain expressions for the Euclidean action. These results allow one to calculate tunneling rates for the entire possible range of quartic potentials, from the thin-wall (nearly degenerate) limit to the opposite limit of vanishing barrier height. We also present a similar calculation for potentials containing $φ^4 \ln φ^2$ terms, which arise in the one-loop approximation to the effective potential in electroweak theory.

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arXiv1992

Baryon Number Diffusion and Instabilities in the Quark/Hadron Phase Transition

Hadron bubbles that nucleate with radius $R_{nuc}$ in a quark sea (if the phase transition is first order) are shown to be unstable to the growth of nonspherical structure when the bubble radii exceed a critical size of $20 - 10^3$ $R_{nuc}$. This instability is driven by a very thin layer of slowly diffusing excess baryon number that forms on the surface of the bubble wall and limits its growth. This instability occurs on a shorter length scale than those studied previously and these effects can thus be important for both cosmology and heavy ion collisions.

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