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A large fraction of stars form within young embedded clusters, and these environments produce a substantial ultraviolet (UV) background radiation field, which can provide feedback on the star formation process. To assess the possible effects of young stellar clusters on the formation of their constituent stars and planets, this paper constructs the expected radiation fields produced by these clusters. We include both the observed distribution of cluster sizes N in the solar neighborhood and an extended distribution that includes clusters with larger N. The paper presents distributions of the FUV and EUV luminosities for clusters with given stellar membership N, distributions of FUV and EUV luminosity convolved over the expected distribution of cluster sizes N, and the corresponding distributions of FUV and EUV fluxes. These flux distributions are calculated both with and without the effects of extinction. Finally, we consider the effects of variations in the stellar initial mass function on these radiation fields. Taken together, these results specify the distributions of radiation environments that forming solar systems are expected to experience.

Stochastic acceleration of electrons interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s millimeter and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated in situ. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air Cerenkov telescopes, notably HESS, have detected TeV emission from the Galactic center, with characteristics hinting at a p-p-induced pion decay process for the g-ray emission. Given (1) the size of this acceleration region measured in the radio band and (2) the wind-injected ISM mapped with Chandra using the diffuse X-rays, it is feasible to test the idea that protons accelerated within ~20 Schwarzschild radii of the black hole produce the TeV emission farther out. We show a fraction of TeV protons scattering about once within ~3 pc of Sagittarius A* and the proton power (~1037 ergs s-1) produced in concert with the 7 mm radio emission matches the TeV luminosity well. This model explains why the TeV source does not vary on a timescale of a year or less. The particle cascade generated by the p-p scatterings also produces bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey.

This paper studies the dynamical evolution of young groups/clusters, with N = 100-1000 members, from their embedded stage out to ages of ~10 Myr. We use N-body simulations to explore how their evolution depends on the system size N and the initial conditions. Motivated by recent observations suggesting that stellar groups begin their evolution with subvirial speeds, this study compares subvirial starting states with virial starting states. Multiple realizations of equivalent cases (100 simulations per initial condition) are used to build up a robust statistical description of these systems, e.g., the probability distribution of closest approaches, the mass profiles, and the probability distribution for the radial location of cluster members. These results provide a framework from which to assess the effects of groups/clusters on the processes of star and planet formation and to study cluster evolution. The distributions of radial positions are used in conjunction with the probability distributions of the expected far-ultraviolet (FUV) luminosities (calculated here as a function of cluster size N) to determine the radiation exposure of circumstellar disks. The distributions of closest approaches are used in conjunction with scattering cross sections (calculated here as a function of stellar mass using ~105 Monte Carlo scattering experiments) to determine the probability of disruption for newly formed solar systems. We use the nearby cluster NGC 1333 as a test case in this investigation. The main conclusion of this study is that clusters in this size range have only a modest effect on forming planetary systems. The interaction rates are low, so that the typical solar system experiences a single encounter with closest approach distance b ~ 1000 AU. The radiation exposure is also low, with median FUV flux G0 ~ 900 (1.4 ergs s-1 cm-2), so that photoevaporation of circumstellar disks is only important beyond 30 AU. Given the low interaction rates and modest radiation levels, we suggest that solar system disruption is a rare event in these clusters.

The EGRET supernova remnants (SNRs) are all expanding into nearby dense molecular clouds, powering a shock at the interface where protons and electrons accelerate to relativistic energies. A viable mechanism for the emission of g-rays in these sources is the decay of neutral pions created in collisions between the relativistic hadrons and protons in the ambient medium. But neutral pion decay alone cannot reproduce the whole high-energy spectrum, particularly below 100 MeV. A pion decay scenario thus requires a lepton population to fill in the lower part of the spectrum via bremsstrahlung emission. This population, however, is constrained by the SNR radio spectrum. Taking our cue from the behavior of Sgr A East, an energetic EGRET SNR at the Galactic center, we here examine the role played in these sources by secondary leptons--electrons and positrons produced in proton-proton scattering events and the ensuing particle cascades. We show that, while secondary leptons cannot account for the g-rays below 100 MeV, they can account for the hard radio spectra observed from the EGRET SNRs. Thus, it appears that both primary and secondary leptons may be important contributors to the overall broadband emission from these sources, but if so, they must radiate most of their energy in different parts of the SNR-cloud environment. We show that shock acceleration in dense cores being overtaken by the expanding SNR shell can naturally lead to such a scenario.

Recent analysis of data sets from two extensive air shower cosmic-ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays toward the Galactic center region that "turns on" around 1018 eV. We demonstrate that the anisotropy could be due to neutrons created in the Galactic center region through charge exchange in proton-proton collisions, where the incident, high-energy protons obey a ~E-2 power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 (ascribed to p-p-induced neutral pion decay at GeV energies), together with a very reasonable spectral index of 2.2, predicts a neutron flux at ~1018 eV fully consistent with the extremely high energy cosmic-ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air Cerenkov telescope at ~TeV energies is almost equally compatible with the ~1018 eV cosmic-ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We find a plausible resolution of this discrepancy in an effective two-source model. Finally, we argue that the shock acceleration is probably occurring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic center. In support of this contention we note that (1) the extended shell of this object could provide both of the sources suggested by the gamma-ray data and (2) the unusually strong magnetic field at this remnant, together with a perpendicular shock geometry, allow for acceleration of protons up to the extreme energies required to explain the cosmic-ray anisotropy. If the connection between the anisotropy and Sagittarius A East could be firmly established, it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near 1019 eV.

Motivated by recent observations that show that starless molecular cloud cores exhibit subsonic inward velocities, we revisit the collapse problem for polytropic gaseous spheres. In particular, we provide a generalized treatment of protostellar collapse in the spherical limit and find semianalytic (self-similar) solutions, corresponding numerical solutions, and purely analytic calculations of the mass infall rates (the three approaches are in good agreement). This study focuses on collapse solutions that exhibit nonzero inward velocities at large radii, as observed in molecular cloud cores, and extends previous work in four ways: (1) the initial conditions allow nonzero initial inward velocity, (2) the starting states can exceed the density of hydrostatic equilibrium so that the collapse itself can provide the observed inward motions, (3) we consider different equations of state, especially those that are softer than isothermal, and (4) we consider dynamic equations of state that are different from the effective equation of state that produces the initial density distribution. This work determines the infall rates over a wide range of parameter space, as characterized by four variables: the initial inward velocity v[?], the overdensity L of the initial state, the index G of the static equation of state, and the index g of the dynamic equation of state. For the range of parameter space applicable to observed cores, the resulting infall rate is about a factor of 2 larger than that found in previous theoretical studies (those with hydrostatic initial conditions and v[?] = 0).

Sgr A East appears to be a single, mixed-morphology 10,000 yr old supernova remnant (SNR) at the Galactic center. It also appears to belong to a class of remnants that have been observed and detected at 1720 MHz, the transition frequency of OH maser emission. However, if the EGRET source 3EG J1746-2852, coincident with the Galactic center, is itself associated with this object, it would endow it with a g-ray luminosity almost 2 orders of magnitude greater than that of the other EGRET-detected SNRs. We here reconsider the viability of a pion-production mechanism as the source of the broadband emission observed from Sgr A East, and show that what connects these objects--and ultimately also accounts for their different g-ray emissivity--is the very important interaction between the expanding SNR shell and the surrounding molecular cloud environment. The singularly high g-ray luminosity of Sgr A East, as well as its unusually steep radio spectral index, can thereby be attributed to the high-density (nH = 103 cm-3), strongly magnetized (B ~ 0.18 mG) environment in which it is located.

Previous treatments of ambipolar diffusion in star-forming molecular clouds do not consider the effects of fluctuations in the fluid fields about their mean values. This paper generalizes the ambipolar diffusion problem in molecular cloud layers to include such fluctuations. Because magnetic diffusion is a nonlinear process, fluctuations can lead to an enhancement of the ambipolar diffusion rate. In addition, the stochastic nature of the process makes the ambipolar diffusion time take on a distribution of different values. In this paper, we focus on the case of long-wavelength fluctuations and find that the rate of ambipolar diffusion increases by a significant factor L ~ 1-10. The corresponding decrease in the magnetic diffusion time helps make ambipolar diffusion more consistent with observations.

The relativistic jets in active galactic nuclei (AGNs) are probably driven by the action of supermassive, spinning black holes. There is very little direct evidence for this, however, since the nuclei of active galaxies are difficult to study. This is now changing with new, high-resolution multiwavelength observations of nearby sources such as Sgr A* at the Galactic center and the nucleus of NGC 6251 (hereafter called NGC 6251*). In this paper, we explore the possibility that the radiative properties of the most compact region in NGC 6251* may be understood in the same sense as Sgr A*, though with some telling differences that may hint at the nature of jet formation. We show that observations of this object with ASCA, ROSAT, HST, and VLBI together may be suggesting a picture in which Bondi-Hoyle accretion from an ambient ionized medium feeds a standard disk accreting at ~ 4.0 x 1022 g s-1. Somewhere near the event horizon, this plasma is heated to more than 1011 K, where it radiates via thermal synchrotron (producing a radio component) and self-Comptonization (accounting for a nonthermal X-ray flux). This temperature is much greater than its virial value and the hot cloud expands at roughly the sound speed (~0.1c), after which it begins to accelerate on a parsec scale to relativistic velocities. In earlier work, the emission from the extended jet has been modeled successfully using nonthermal synchrotron self-Compton processes, with a self-absorbed inner core. In the picture we are developing here, the initial ejection of matter is associated with a self-absorbed thermal radio component that dominates the core emission on the smallest scales. The nonthermal particle distributions responsible for the emission in the extended jet are then presumably energized, e.g., via shock acceleration, within the expanding, hot gas. The power associated with this plasma represents an accretion efficiency of about 0.54, requiring dissipation in a prograde disk around a rapidly spinning black hole (with spin parameter a ~ 1).

Maps of the Galactic electron-positron annihilation radiation show evidence for three distinct and significant features: (1) a central bulge source, (2) emission in the Galactic plane, and (3) an enhancement of emission at positive latitudes above the Galactic center. In this paper, we explore the possibility that Sgr A East, a very prominent radio structure surrounding the Galactic nucleus, may be a significant contributor to the central bulge feature. The motivation for doing so stems from a recently proposed link between this radio object and the EGRET ?-ray source 2EG J1746-2852. If this association is correct, then Sgr A East is also expected to be a source of copious positron production. The results presented here show that indeed Sgr A East must have produced a numerically significant population of positrons, but also that most of them have not yet had sufficient time to thermalize and annihilate. As such, Sgr A East by itself does not appear to be the dominant currentsource of annihilation radiation, but it will be when the positrons have cooled sufficiently and they have become thermalized. This raises the interesting possibility that the bulge component may be a result of the relics of earlier explosive events like the one that produced Sgr A East.

EGRET (20 MeV-30 GeV), on board the Compton Gamma Ray Observatory, has observed high-energy emission from about ~40-50 active galactic nuclei. Theoretical models of this emission based on the upscattering of thermal disk photons by cooling, relativistic electrons can successfully account for the EGRET observations, but they predict a considerably greater X-ray flux than that actually observed in a majority of these sources. This inconsistency may be an indication that the particles are energized during the Compton scattering process, since the X-ray emission is produced by the lowest energy electrons, whose density may be relatively small because of the acceleration. Such a situation may arise as a result of resistive field generation in electromagnetic acceleration schemes, which we here explore. A key feature of this model is the assumed existence of a current associated with the azimuthal component Bph of the underlying magnetic field by a slight imbalance in the energy distributions of outwardly moving, relativistic electrons and protons produced at the disk surface via shock acceleration. The generation of an electric field (via magnetic field line reconnection) is thus required to maintain the current in the presence of a resistivity induced by the radiative drag on the relativistic electrons. We show that the resulting spectrum can exhibit a significant deficit of X-rays compared with gamma rays. In addition, because of the unidirectional flow of the current associated with Bph, this model would predict that the electrons are energized relative to the protons in the outflow only on one side of the disk. They should be decelerated on the reverse side. As such, we would anticipate that any given blazar should have a ~50% probability of being gamma-ray-bright, which appears to be consistent with the observed ratio.

Continuum low-frequency radio observations of the Galactic center reveal the presence of two prominent radio sources, Sgr A East and its surrounding halo, containing nonthermal particle distributions with power-law indexes ~2.5-3.3 and ~2.4, respectively. The central 1-2 pc region is also a source of intense (stellar) UV and (dust-reprocessed) far-IR radiation that bathes these extended synchrotron-emitting structures. A recent detection of g-rays (2EG J1746-2852) from within ~1deg of the Galactic center by EGRET onboard the Compton Gamma Ray Observatory (CGRO) shows that the emission from this environment extends to very high energies. We suggest that inverse Compton scattering between the power-law electrons inferred from the radio properties of Sgr A East and its halo and the UV and IR photons from the nucleus may account for the possibly diffuse g-ray source as well. We show that both particle distributions may be contributing to the g-ray emission, although their relevant strength depends on the actual physical properties (such as the magnetic field intensity) in each source. If this picture is correct, the high-energy source at the Galactic center is extended over several arcminutes, which can be tested with the next generation of g-ray and hard X-ray missions.