J. J. MacFarlane

Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C

We report on four Chandra grating observations of the oblique magnetic rotator � 1 Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star’s 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray–emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hotplasma. We investigate these diagnostics as a function of viewing angle andanalyzetheminconjunctionwithnewMHDsimulationsofthemagneticallychanneledwindshockmechanism on � 1 Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray–emitting plasma with rotation phase. Subject headingg stars: early-type — stars: individual (HD 37022) — stars: magnetic fields — stars: mass loss — stars: rotation — stars: winds, outflows — X-rays: stars Online material: color figure

Hot Dense Capsule-Implosion Cores Produced by Z -Pinch Dynamic Hohlraum Radiation

Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.

ROSAT PSPC Observations of 27 Near-Main-Sequence B Stars

In this paper, we report on ROSAT Position Sensitive Proportional Counter (PSPC) observations of 27 near-main-sequence B stars made with unprecedented sensitivity. Contrary to the results of previous surveys, it is found that 75% of the sample stars are X-ray sources, albeit most at modest levels. The X-ray luminosities of the program stars range from 5.6 × 1027 up to 2.2 × 1032 ergs s-1. We find that LX/LBol decreases abruptly beyond about B0 and stabilizes at LX/LBol ≈ 10-8.5 by about B2, with seven nondetections at B2 and later. For the B0 and B1 stars, our modeling suggests that wind attenuation of the X-ray photons is significant, so that the emitted X-ray luminosity, corrected for this attenuation, actually exceeds 10-7LBol in some cases. Presumably, this situation is even more severe for O stars; thus, the well-known LX/LBol ≈ 10-7 law simply may be an artifact of the neglect of wind attenuation. The ROSAT PSPC observations of most of the B stars are very soft, with the notable exception of τ Sco (B0 V). The wind emission measure filling factors that we find for the very early B stars are rather large (roughly 0.1-1). This could be brought into line with theoretical calculations of the line-force instability, wind-shock mechanism if the mass-loss rates of these stars are a few times higher than theory currently predicts. However, the X-rays from stars later than B2 require filling factors greater than unity and thus cannot be produced by any radiation-driven wind-shock mechanism because there is simply not enough wind material to produce the observed X-rays. It is possible that mid- to late-B stars represent some kind of transition to, or hybrid of, wind and coronal X-ray mechanisms.

High Temperature Dynamic Hohlraums on the Pulsed Power Driver Z

In the concept of the dynamic hohlraum an imploding Z pinch is optically thick to its own radiation. Radiation may be trapped inside the pinch to give a radiation temperature inside the pinch greater than that outside the pinch. The radiation is typically produced by colliding an outer Z-pinch liner onto an inner liner. The collision generates a strongly radiating shock, and the radiation is trapped by the outer liner. As the implosion continues after the collision, the radiation temperature may continue to increase due to ongoing PdV (pressure times change in volume) work done by the implosion. In principal, the radiation temperature may increase to the point at which the outer liner burns through, becomes optically thin, and no longer traps the radiation. One application of the dynamic hohlraum is to drive an ICF (inertial confinement fusion) pellet with the trapped radiation field. Members of the dynamic hohlraum team at Sandia National Labs have used the pulsed power driver Z (20 MA, 100 ns) to create...

Experimental investigation of opacity models for stellar interior, inertial fusion, and high energy density plasmas

Theoretical opacities are required for calculating energy transport in plasmas. In particular, understanding stellar interiors, inertial fusion, and Z pinches depends on the opacities of mid-atomic-number elements over a wide range of temperatures. The 150–300 eV temperature range is particularly interesting. The opacity models are complex and experimental validation is crucial. For example, solar models presently disagree with helioseismology and one possible explanation is inadequate theoretical opacities. Testing these opacities requires well-characterized plasmas at temperatures high enough to produce the ion charge states that exist in the sun. Typical opacity experiments heat a sample using x rays and measure the spectrally resolved transmission with a backlight. The difficulty grows as the temperature increases because the heating x-ray source must supply more energy and the backlight must be bright enough to overwhelm the plasma self-emission. These problems can be overcome with the new generation...

High-Resolution Chandra Spectroscopy of τ Scorpii: A Narrow-Line X-Ray Spectrum from a Hot Star

Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, τ Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of τ Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the hot plasma on this star being within a few stellar radii of the photosphere, from at least one He-like complex there is evidence that the X-ray-emitting plasma is located more than a stellar radius above the photosphere. The Chandra spectrum of τ Sco is harder and more variable than those of other hot stars, with the exception of the young magnetized O star θ1 Ori C. We discuss these new results in the context of wind, coronal, and hybrid wind-magnetic models of hot-star X-ray emission.

NEW CHALLENGES FOR WIND SHOCK MODELS: THE CHANDRA SPECTRUM OF THE HOT STARORIONIS

The Chandra spectrum ofOri A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with Nvii Lyand lines from ions in the range Fe xvii-Fexxi. In contrast to the broad lines seen inPup andOri (850 � 40 and 1000 � 240 km s � 1 half-width at half-maximum (HWHM), respec- tively), these lines are broadened to only 430 � 60 km s � 1 HWHM. This is much lower than the measured wind terminal velocity of 2000 km s � 1 . The forbidden, intercombination, and resonance ( fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emis- sion lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line profile generation in hot stars, but none of them offers a fully satisfac- tory explanation for the observed line profiles.

Hot-spot mix in ignition-scale implosions on the NIF

Ignition of an inertial confinement fusion (ICF) target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility (NIF) ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. The Rev. 5 ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive [D. S. Clark et al., Phys. Plasmas 17, 052703 (2010)]. Richtmyer–Meshkov and Rayleigh–Taylor hydrodynamic instabilities seeded by high-mode (50<l<200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase [B. A. Hammel et al., Phys. Plasma...

EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.

Predicted extreme-ultraviolet line emission for nearby main-sequence B stars

The source of the X-ray emission from O and B stars is currently the subject of debate. Shocks propagating through the winds of O and B stars have previously been proposed to explain their observed X-ray luminosities. Strong shocks with velocity jumps of several hundred km/s can heat portions of the wind to 10 6 -10 7 K, producing major sources of X-ray and extreme-ultraviolet (EUV) emission lines. Alternatively, a corona at the base of the wind might also be responsible for the X-ray emission. Here, the characteristics of EUV lines produced in high-temperature X-ray emitting regions of early B stars are investigated