J. I. Castor

Bio: J. I. Castor is an academic researcher from Lawrence Livermore National Laboratory. The author has contributed to research in topics: Stars & O-type star. The author has an hindex of 22, co-authored 37 publications receiving 4729 citations. Previous affiliations of J. I. Castor include University of Colorado Boulder & National Institute of Standards and Technology.

Interstellar bubbles. II - Structure and evolution

Abstract: The detailed structure of the interaction of a strong stellar wind with the interstellar medium is presented. First, an adiabatic similarity solution is given which is applicable at early times. Second, a similarity solution is derived which includes the effects of thermal conduction between the hot (about 1 million K) interior and the cold shell of swept-up interstellar matter. This solution is then modified to include the effects of radiative energy losses. The evolution of an interstellar bubble is calculated, including the radiative losses. The quantitative results for the outer-shell radius and velocity and the column density of highly ionized species such as O VI are within a factor 2 of the approximate results of Castor, McCray, and Weaver (1975). The effect of stellar motion on the structure of a bubble, the hydrodynamic stability of the outer shell, and the observable properties of the hot region and the outer shell are discussed.

X-ray-heated coronae and winds from accretion disks: Time-dependent two-dimensional hydrodynamics with adaptive mesh refinement

Abstract: We perform first-time time-dependent, 2-D, axisymmetric hydrodynamic simulations using local adaptive mesh refinement of thermally driven rotating winds from X-ray-irradiated accretion disks. The disk is assumed to flare in height with radius allowing direct exposure from the central X-ray source. The heating and cooling are treated strictly in the optically thin approximation. We adopt two spectra characteristic of active galactic nuclei (AGNs) which have Compton temperatures of {ital T}{sub IC}{approx_equal}1.3{times}10{sup 7} K and 10{sup 8} K. We have computed a number of models which cover a large range in luminosity (0.002{le}{ital L}/{ital L}{sub Eddington}{le}1) and radius ({approx_lt}20 Compton radii). Our models extend and improve on the analytic predictions of Begelman, McKee, & Shields (BMS) for Compton-heated winds by including non-Compton processes such as photoionization heating and line cooling, typical of X-ray-heated winds. These non-Compton processes can be dominant at low temperatures ({approx_lt}10{sup 7} K), thus being important in the wind regions of AGNs. Our results agree well with a number of predictions given by BMS, even when non-Compton processes dominate, suggesting that their analytic approximations of the hydrodynamics of disk winds are applicable to the more general area of X-ray-heated winds. In the regime in which Compton processes dominate (i.e.,more » {ital T}{sub IC}=10{sup 8} K spectrum), we have used our results to improve the analytic predictions of BMS, providing a new expression for the mass-loss rate and a modified view of the wind solution topology. We find that beginning from a basically static state, the time-dependent flow which develops eventually settles into a {ital steady} wind, without any evidence of hydrodynamic instabilities. The solution topology consists of a corona with an exponentially truncated wind at small radii, and a vigorous wind at large radii which can be impeded by gravity for small luminosities.« less

Mass-loss predictions for O and B stars as a function of metallicity

Abstract: We have calculated a grid of massive star wind models and mass-loss rates for a wide range of metal abundances between 1=100 Z=Z 10. The calculation of this grid completes the Vink et al. (2000) mass-loss recipe with an additional parameter Z. We have found that the exponent of the power law dependence of mass loss vs. metallicity is constant in the range between 1/30 Z=Z 3. The mass-loss rate scales as _ M / Z 0:85 v1 p with p = 1:23 for stars with Te > 25 000 K, and p = 1:60 for the B supergiants with Te 25 000 K, and _ M / Z 0:64 for B supergiants with Te < 25 000 K. Although it is derived that the exponent of the mass loss vs. metallicity dependence is constant over a large range in Z, one should be aware of the presence of bi-stability jumps at specic temperatures. Here the character of the line driving changes drastically due to recombinations of dominant metal species resulting in jumps in the mass loss. We have investigated the physical origins of these jumps and have derived formulae that combine mass loss recipes for both sides of such jumps. As observations of dierent galaxies show that the ratio Fe/O varies with metallicity, we make a distinction between the metal abundance Z derived on the basis of iron or oxygen lines. Our mass-loss predictions are successful in explaining the observed mass-loss rates for Galactic and Small Magellanic Cloud O- type stars, as well as in predicting the observed Galactic bi-stability jump. Hence, we believe that our predictions are reliable and suggest that our mass-loss recipe be used in future evolutionary calculations of massive stars at dierent metal abundance. A computer routine to calculate mass loss is publicly available.

A computer program for fast non-LTE analysis of interstellar line spectra

Abstract: The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes. Note: Accepted by AA 18 A4 pages, 11 figures;

Galactic Winds

Abstract: Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges

Mesa isochrones and stellar tracks (mist). i. solar-scaled models

Abstract: This is the first of a series of papers presenting the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a new comprehensive set of stellar evolutionary tracks and isochrones computed using MESA, a state-of-the-art open-source 1D stellar evolution package. In this work, we present models with solar-scaled abundance ratios covering a wide range of ages ($5 \leq \rm \log(Age)\;[yr] \leq 10.3$), masses ($0.1 \leq M/M_{\odot} \leq 300$), and metallicities ($-2.0 \leq \rm [Z/H] \leq 0.5$). The models are self-consistently and continuously evolved from the pre-main sequence to the end of hydrogen burning, the white dwarf cooling sequence, or the end of carbon burning, depending on the initial mass. We also provide a grid of models evolved from the pre-main sequence to the end of core helium burning for $-4.0 \leq \rm [Z/H] < -2.0$. We showcase extensive comparisons with observational constraints as well as with some of the most widely used existing models in the literature. The evolutionary tracks and isochrones can be downloaded from the project website at this http URL

Physics of the Interstellar and Intergalactic Medium

Abstract: This is a comprehensive and richly illustrated textbook on the astrophysics of the interstellar and intergalactic medium--the gas and dust, as well as the electromagnetic radiation, cosmic rays, and magnetic and gravitational fields, present between the stars in a galaxy and also between galaxies themselves. Topics include radiative processes across the electromagnetic spectrum; radiative transfer; ionization; heating and cooling; astrochemistry; interstellar dust; fluid dynamics, including ionization fronts and shock waves; cosmic rays; distribution and evolution of the interstellar medium; and star formation. While it is assumed that the reader has a background in undergraduate-level physics, including some prior exposure to atomic and molecular physics, statistical mechanics, and electromagnetism, the first six chapters of the book include a review of the basic physics that is used in later chapters. This graduate-level textbook includes references for further reading, and serves as an invaluable resource for working astrophysicists. * Essential textbook on the physics of the interstellar and intergalactic medium * Based on a course taught by the author for more than twenty years at Princeton University * Covers radiative processes, fluid dynamics, cosmic rays, astrochemistry, interstellar dust, and more * Discusses the physical state and distribution of the ionized, atomic, and molecular phases of the interstellar medium * Reviews diagnostics using emission and absorption lines * Features color illustrations and detailed reference materials in appendices * Instructor's manual with problems and solutions (available only to teachers)