arXiv subjects

C. E. Jones

Publications and source records attributed to C. E. Jones.

At least 37 records · Page 2Linked to original sources

arXiv2014

Line-Driven Winds Revisited in the Context of Be Stars: $Ω$-slow Solutions with High $k$ Values

The standard, or fast, solutions of m-CAK line-driven wind theory cannot account for slowly outflowing disks like the ones that surround Be stars. It has been previously shown that there exists another family of solutions --- the $Ω$-slow solutions --- that is characterized by much slower terminal velocities and higher mass-loss rates. We have solved the one-dimensional m-CAK hydrodynamical equation of rotating radiation-driven winds for this latter solution, starting from standard values of the line force parameters ($α$, $k$, and $δ$), and then systematically varying the values of $α$ and $k$. Terminal velocities and mass-loss rates that are in good agreement with those found in Be stars are obtained from the solutions with lower $α$ and higher $k$ values. Furthermore, the equatorial densities of such solutions are comparable to those that are typically assumed in ad hoc models. For very high values of $k$, we find that the wind solutions exhibit a new kind of behavior.

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arXiv2014

Using Photometry to Probe the Circumstellar Environment of delta Scorpii

We acquired Johnson BV photometry of the binary Be disk system delta~Scorpii during its 2009, 2010, 2011, and 2012 observing seasons and used it to probe the innermost regions of the disk. We found that several disk building events have occurred during this time, resulting in an overall brightening in the V-band and reddening of the system. In addition to these long-term trends, we found cyclical variability in each observing season on timescales between 60 and 100 days. We were able to reproduce the changes in the magnitude and colour of delta Sco using our theoretical models and found that variable mass-loss rates in the range 2.5-7.0x10^{-9} M_{sun}/yr over ~35 days can reproduce the observed increase in brightness.

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arXiv2012

The Effect of Density on the Thermal Structure of Gravitationally-Darkened Be Star Disks

The effects of gravitational darkening on the thermal structure of Be star disks of differing densities are systematically examined. Gravitational darkening is the decrease of the effective temperature near the equator and the corresponding increase near the poles of a star caused by rapid rotation. We also include the rotational distortion of the star using the Roche Model. Increasing the disk density increases the optical depths in the equatorial plane, resulting in the formation of an inner cool region near the equatorial plane of the disk. High rotation rates result in disks that have temperatures similar to those of a denser disk, namely cooler overall. However the effect of increasing rotation produces additional heating in the upper disk due to the hotter stellar pole. Cool regions in the equatorial plane normally associated with high density are seen in low density models at high rotation rates. Gravitational darkening increases the amount of very cool and very hot material in the disk and decreases the amount of disk material at moderate temperatures. We also present models which study the effect of gravitational darkening on hydrostatically-converged disks, in which the temperature structure is consistent with vertical hydrostatic equilibrium. Because the equatorial regions become cooler, hydrostatically converged models that include gravity darkening have smaller vertical scale heights, and $H/R$ is smaller by as much as 56% near $v_{\rm crit}$. Finally we explore differences in disk temperatures when alternate formulations of gravitational darkening, which lower the temperature difference between the pole and the equator, are used.

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arXiv2011

The Thermal Structure of Gravitationally-Darkened Classical Be Star Disks

The effect of gravitational darkening on models of the thermal structure of Be star disks is systematically studied for a wide range of Be star spectral types and rotation rates. Gravitational darkening causes a reduction of the stellar effective temperature towards the equator and a redirection of energy towards the poles. It is an important physical effect in these star-disk systems because the photoionizing radiation from the central B star is the main energy source for the disk. We have added gravitational darkening to the bedisk code to produce circumstellar disk models that include both the variation in the effective temperature with latitude and the non-spherical shape of the star in the calculation of the stellar photoionizing radiation field. The effect of gravitational darkening on global measures of disk temperature is generally significant for rotation rates above 80% of critical rotation. For example, a B0V model rotating at 95% of critical has a density-averaged disk temperature \approx 2500 K cooler than its nonrotating counterpart. However, detailed differences in the temperature structure of disks surrounding rotating and non-rotating stars reveal a complex pattern of heating and cooling. Spherical gravitational darkening, an approximation that ignores the changing shape of the star, gives good results for disk temperatures for rotation rates less than \approx 80% of critical. However for the highest rotation rates, the distortion of the stellar surface caused by rotation becomes important.

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arXiv2011

The Variability of Halpha Equivalent Widths in Be Stars

Focusing on B-emission stars, we investigated a set of H$α$ equivalent widths calculated from observed spectra acquired over a period of about 4 years from 2003 to 2007. During this time, changes in equivalent width for our program stars were monitored. We have found a simple statistical method to quantify these changes in our observations. This statistical test, commonly called the F ratio, involves calculating the ratio of the external and internal error. We show that the application of this technique can be used to place bounds on the degree of variability of Be stars. This observational tool provides a quantitative way to find Be stars at particular stages of variability requiring relatively little observational data.

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arXiv2009

Be Star Disk Models in Consistent Vertical Hydrostatic Equilibrium

A popular model for the circumstellar disks of Be stars is that of a geometrically thin disk with a density in the equatorial plane that drops as a power law of distance from the star. It is usually assumed that the vertical structure of such a disk (in the direction parallel to the stellar rotation axis) is governed by the hydrostatic equilibrium set by the vertical component of the star's gravitational acceleration. Previous radiative equilibrium models for such disks have usually been computed assuming a fixed density structure. This introduces an inconsistency as the gas density is not allowed to respond to temperature changes and the resultant disk model is not in vertical, hydrostatic equilibrium. In this work, we modify the {\sc bedisk} code of \citet{sig07} so that it enforces a hydrostatic equilibrium consistent with the temperature solution. We compare the disk densities, temperatures, H$α$ line profiles, and near-IR excesses predicted by such models with those computed from models with a fixed density structure. We find that the fixed models can differ substantially from the consistent hydrostatic models when the disk density is high enough that the circumstellar disk develops a cool ($T\lesssim10,000 $K) equatorial region close to the parent star. Based on these new hydrostatic disks, we also predict an approximate relation between the (global) density-averaged disk temperature and the $T_{\rm eff}$ of the central star, covering the full range of central Be star spectral types.

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arXiv2008

Constraining the Physical Parameters of the Circumstellar Disk of chi Ophiuchi

We present a numerical model describing a circularly symmetric gaseous disk around the Be star chi Ophiuchi. The model is constrained by long-baseline interferometric observations that are sensitive to the H-alpha Balmer line emission from the disk. For the first time our interferometric observations spatially resolve the inner region of the circumstellar disk around chi Ophiuchi and we use these results to place a constraint on the physical extent of the H-alpha-emitting region. We demonstrate how this in turn results in very specific constraints on the parameters that describe the variation of the gas density as a function of radial distance from the central star.

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arXiv2008

A Parameter Study of Classical Be Star Disk Models Constrained by Optical Interferometry

We have computed theoretical models of circumstellar disks for the classical Be stars $κ$ Dra, $β$ Psc, and $\upsilon$ Cyg. Models were constructed using a non-LTE radiative transfer code developed by \citet{sig07} which incorporates a number of improvements over previous treatments of the disk thermal structure, including a realistic chemical composition. Our models are constrained by direct comparison with long baseline optical interferometric observations of the H$α$ emitting regions and by contemporaneous H$α$ line profiles. Detailed comparisons of our predictions with H$α$ interferometry and spectroscopy place very tight constraints on the density distributions for these circumstellar disks.

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arXiv2008

Infrared Excess in the Be Star Delta Scorpii

We present infrared photometric observations of the Be binary system delta Scorpii obtained in 2006. The J,H and K magnitudes are the same within the errors compared to observations taken 10 months earlier. We derive the infrared excess from the observation and compare this to the color excess predicted by a radiative equilibrium model of the primary star and its circumstellar disk. We use a non-LTE computational code to model the gaseous envelope concentrated in the star's equatorial plane and calculate the expected spectral energy distribution and Halpha emission profile of the star with its circumstellar disk. Using the observed infrared excess of delta Sco, as well as Halpha spectroscopy bracketing the IR observations in time, we place constraints on the radial density distribution in the circumstellar disk. Because the disk exhibits variability in its density distribution, this work will be helpful in understanding its dynamics.

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arXiv2007

The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia

We have computed radiative equilibrium models for the gas in the circumstellar envelope surrounding the hot, classical Be star $γ$Cassiopeia. This calculation is performed using a code that incorporates a number of improvements over previous treatments of the disk's thermal structure by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their relevant ions. Thus, for the first time, the thermal structure of a Be disk is computed for a gas with a solar chemical composition as opposed to assuming a pure hydrogen envelope. We compare the predicted average disk temperature, the total energy loss in H$α$, and the near-IR excess with observations and find that all can be accounted for by a disk that is in vertical hydrostatic equilibrium with a density in the equatorial plane of $ρ(R)\approx 3$ to $5\cdot 10^{-11} (R/R_*)^{-2.5} \rm g cm^{-3}$. We also discuss the changes in the disk's thermal structure that result from the additional heating and cooling processes available to a gas with a solar chemical composition over those available to a pure hydrogen plasma.

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arXiv2006

Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI

Interferometric observations of two well-known Be stars, gamma Cas and phi Per, were collected and analyzed to determine the spatial characteristics of their circumstellar regions. The observations were obtained using the Navy Prototype Optical Interferometer equipped with custom-made narrowband filters. The filters isolate the H-alpha emission line from the nearby continuum radiation, which results in an increased contrast between the interferometric signature due to the H-alpha-emitting circumstellar region and the central star. Because the narrowband filters do not significantly attenuate the continuum radiation at wavelengths 50 nm or more away from the line, the interferometric signal in the H-alpha channel is calibrated with respect to the continuum channels. The observations used in this study represent the highest spatial resolution measurements of the H-alpha-emitting regions of Be stars obtained to date. These observations allow us to demonstrate for the first time that the intensity distribution in the circumstellar region of a Be star cannot be represented by uniform disk or ring-like structures, whereas a Gaussian intensity distribution appears to be fully consistent with our observations.

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arXiv2005

The Cryogenic Target for the G$^0$ Experiment at Jefferson Lab

A cryogenic horizontal single loop target has been designed, built, tested and operated for the G$^0$ experiment in Hall C at Jefferson Lab. The target cell is 20 cm long, the loop volume is 6.5 l and the target operates with the cryogenic pump fully immersed in the fluid. The target has been designed to operate at 30 Hz rotational pump speed with either liquid hydrogen or liquid deuterium. The high power heat exchanger is able to remove 1000 W of heat from the liquid hydrogen, while the nominal electron beam with current of 40 $μ$A and energy of 3 GeV deposits about 320 W of heat into the liquid. The increase in the systematic uncertainty due to the liquid hydrogen target is negligible on the scale of a parity violation experiment. The global normalized yield reduction for 40 $μ$A beam is about 1.5 % and the target density fluctuations contribute less than 238 ppm (parts per million) to the total asymmetry width, typically about 1200 ppm, in a Q$^2$ bin.

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arXiv2004

A minimum hypothesis explanation for an IMF with a lognormal body and power law tail

We present a minimum hypothesis model for an IMF that resembles a lognormal distribution at low masses but has a distinct power-law tail. Even if the central limit theorem ensures a lognormal distribution of condensation masses at birth, a power-law tail in the distribution arises due to accretion from the ambient cloud, coupled with a non-uniform (exponential) distribution of accretion times.

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arXiv2003

On the Power-Law Tail in the Mass Function of Protostellar Condensations and Stars

We explore the idea that the power-law tail in the mass function of protostellar condensations and stars arises from the accretion of ambient cloud material on to a condensation, coupled with a nonuniform (exponential) distribution of accretion lifetimes. This model allows for the generation of power-law distributions in all star-forming regions, even if condensations start with a lognormal mass distribution, as may be expected from the central limit theorem, and supported by some recent numerical simulations of turbulent molecular clouds. For a condensation mass $m$ with growth rate $dm/dt \propto m$, an analytic three-parameter probability density function is derived; it resembles a lognormal at low mass and has a pure power-law high-mass tail. An approximate power-law tail is also expected for other growth laws, and we calculate the distribution for the plausible case $dm/dt \propto m^{2/3}$. Furthermore, any single time snapshot of the masses of condensations that are still accreting (and are of varying ages) also yields a distribution with a power-law tail similar to that of the IMF.

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arXiv2003

The peculiar circumstellar environment of NGC2024-IRS2

We re-examine the nature of NGC2024-IRS2 in light of the recent discovery of the late O-type star, IRS2b, located 5 arcsec from IRS2. Using L-band spectroscopy, we set a lower limit of Av = 27.0 mag on the visual extinction towards IRS2. Arguments based on the nature of the circumstellar material, favor an Av of 31.5 mag. IRS2 is associated with the UCHII region G206.543-16.347 and the infrared source IRAS 05393-0156. We show that much of the mid-infrared emission towards IRS2, as well as the far infrared emission peaking at ~ 100 micron, do not originate in the direct surroundings of IRS2, but instead from an extended molecular cloud. Using new K-, L- and L'-band spectroscopy and a comprehensive set of infrared and radio continuum measurements from the literature, we apply diagnostics based on the radio slope, the strength of the infrared hydrogen recombination lines, and the presence of CO band-heads to constrain the nature and spatial distribution of the circumstellar material of IRS2. Using simple gaseous and/or dust models of prescribed geometry, we find strong indications that the infrared flux originating in the circumstellar material of IRS2 is dominated by emission from a dense gaseous disk with a radius of about 0.6 AU. At radio wavelengths the flux density distribution is best described by a stellar wind recombining at a radius of about 100 AU. Although NGC2024/IRS2 shares many similarities with BN-like objects, we do not find evidence for the presence of a dust shell surrounding this object. Therefore, IRS2 is likely more evolved.

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arXiv2001

The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales

We decipher intrinsic three-dimensional shape distributions of molecular clouds, cloud cores, Bok globules, and condensations using recently compiled catalogues of observed axis ratios for these objects mapped in carbon monoxide, ammonia, through optical selection, or in continuum dust emission. We apply statistical techniques to compare assumed intrinsic axis ratio distributions with observed projected axis ratio distributions. Intrinsically triaxial shapes produce projected distributions which agree with observations. Molecular clouds mapped in $^{12}$CO are intrinsically triaxial but more nearly prolate than oblate, while the smaller cloud cores, Bok globules, and condensations are also intrinsically triaxial but more nearly oblate than prolate.

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arXiv2001

Intrinsic Shapes of Molecular Cloud Cores

We conduct an analysis of the shapes of molecular cloud cores using recently compiled catalogs of observed axis ratios of individual cores mapped in ammonia or through optical selection. We apply both analytical and statistical techniques to deproject the observed axis ratios in order to determine the true distribution of cloud core shapes. We find that neither pure oblate nor pure prolate cores can account for the observed distribution of core shapes. Intrinsically triaxial cores produce distributions which agree with observations. The best-fit triaxial distribution contains cores which are more nearly oblate than prolate.

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arXiv2000

Evidence for a three-nucleon-force effect in proton-deuteron elastic scattering

Developments in spin-polarized internal targets for storage rings have permitted measurements of 197 MeV polarized protons scattering from vector polarized deuterons. This work presents measurements of the polarization observables A_y, iT_11, and C_y,y in proton-deuteron elastic scattering. When compared to calculations with and without three-nucleon forces, the measurements indicate that three-nucleon forces make a significant contribution to the observables. This work indicates that three-body forces derived from static nuclear properties appear to be crucial to the description of dynamical properties.

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