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At least 1,045 records · Page 58
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DOE OSTI2013

THE SPATIOKINEMATICAL STRUCTURE OF H{sub 2}O AND OH MASERS IN THE ''WATER FOUNTAIN'' SOURCE IRAS 18460-0151

Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have observed 22.2 GHz H{sub 2}O and 1612 MHz OH masers in the water fountain source IRAS 18460-0151. The H{sub 2}O maser spectrum has a very wide line-of-sight velocity range ( Almost-Equal-To 310 km s{sup -1}) and consists of three groups of emission features at the blueshifted (-68 km s{sup -1}{approx}< V{sub LSR} {approx}< -17 km s{sup -1}) and redshifted (V{sub LSR} {approx_equal} 240 km s{sup -1}) edges as well as around the systemic velocity (112 km s{sup -1} {approx}< V{sub LSR} {approx}< 133 km s{sup -1}). The first two H{sub 2}O spectral components exhibit a highly collimated high-velocity bipolar jet on the sky, with an angular separation of Almost-Equal-To 120 milliarcsec (mas) (240 AU in linear length) and a three-dimensional flow velocity of Almost-Equal-To 160 km s{sup -1}. The flow dynamical age is estimated to be only Almost-Equal-To 6 yr (at the time of the observation epochs of 2006-2007). Interestingly, the systemic velocity component clearly exhibits a spherically expanding outflow with a radius of Almost-Equal-To 36 AU and a flow velocity of Almost-Equal-To 9 km s{sup -1}. On the other hand, the OH maser spectrum shows double peaks with a velocity separation of Almost-Equal-To 25 km s{sup -1} (V{sub LSR} = 111-116 and 138-141 km s{sup -1}), as typically seen in circumstellar envelopes of OH/IR stars. The angular offset between the velocity-integrated brightness peaks of the two high-velocity H{sub 2}O components is Almost-Equal-To 25 mas (50 AU). The offset direction and the alignment of the redshifted maser spots are roughly perpendicular to the axis of the H{sub 2}O maser flow. High-accuracy astrometry for the H{sub 2}O and OH masers demonstrates that the collimated fast jet and the slowly expanding outflow originate from a single or multiple sources which are located within 15 mas (30 AU). On the other hand, the estimated systemic velocity of the collimated jet (V{sub sys} Almost-Equal-To 87-113 km s{sup -1}) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460-0151.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

AN XMM-NEWTON SURVEY OF THE SOFT X-RAY BACKGROUND. III. THE GALACTIC HALO X-RAY EMISSION

We present measurements of the Galactic halo's X-ray emission for 110 XMM-Newton sight lines selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on {approx}4/5 of our sight lines. The temperature is fairly uniform (median = 2.22 Multiplication-Sign 10{sup 6} K, interquartile range = 0.63 Multiplication-Sign 10{sup 6} K), while the emission measure and intrinsic 0.5-2.0 keV surface brightness vary by over an order of magnitude ({approx}(0.4-7) Multiplication-Sign 10{sup -3} cm{sup -6} pc and {approx}(0.5-7) Multiplication-Sign 10{sup -12} erg cm{sup -2} s{sup -1} deg{sup -2}, respectively, with median detections of 1.9 Multiplication-Sign 10{sup -3} cm{sup -6} pc and 1.5 Multiplication-Sign 10{sup -12} erg cm{sup -2} s{sup -1} deg{sup -2}, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CORE-COLLAPSE SUPERNOVA EQUATIONS OF STATE BASED ON NEUTRON STAR OBSERVATIONS

Many of the currently available equations of state for core-collapse supernova simulations give large neutron star radii and do not provide large enough neutron star masses, both of which are inconsistent with some recent neutron star observations. In addition, one of the critical uncertainties in the nucleon-nucleon interaction, the nuclear symmetry energy, is not fully explored by the currently available equations of state. In this article, we construct two new equations of state which match recent neutron star observations and provide more flexibility in studying the dependence on nuclear matter properties. The equations of state are also provided in tabular form, covering a wide range in density, temperature, and asymmetry, suitable for astrophysical simulations. These new equations of state are implemented into our spherically symmetric core-collapse supernova model, which is based on general relativistic radiation hydrodynamics with three-flavor Boltzmann neutrino transport. The results are compared with commonly used equations of state in supernova simulations of 11.2 and 40 M{sub Sun} progenitors. We consider only equations of state which are fitted to nuclear binding energies and other experimental and observational constraints. We find that central densities at bounce are weakly correlated with L and that there is a moderate influence of the symmetry energy on the evolution of the electron fraction. The new models also obey the previously observed correlation between the time to black hole formation and the maximum mass of an s = 4 neutron star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CONSTRAINTS ON OH MEGAMASER EXCITATION FROM A SURVEY OF OH SATELLITE LINES

We report the results of a full-Stokes survey of all four 18 cm OH lines in 77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey of OHMs that included observations of the OH satellite lines; only four of the 77 OHMs have existing satellite line observations in the literature. Satellite line emission is detected in five sources, three of which are redetections of previously published sources. The two sources with new detections of satellite line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz absorption and 1612 MHz emission have the same structure at some velocities within the source, along with additional broader 1612 MHz emission. This is the first observed example of conjugate satellite lines in an OHM. In the remaining sources, no satellite line emission is observed. The detections and upper limits are generally consistent with models of OHM emission in which all of the 18 cm OH lines have the same excitation temperature. There is no evidence for a significant population of strong satellite line emitters among OHMs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EFFECT OF MEASUREMENT ERRORS ON PREDICTED COSMOLOGICAL CONSTRAINTS FROM SHEAR PEAK STATISTICS WITH LARGE SYNOPTIC SURVEY TELESCOPE

We study the effect of galaxy shape measurement errors on predicted cosmological constraints from the statistics of shear peak counts with the Large Synoptic Survey Telescope (LSST). We use the LSST Image Simulator in combination with cosmological N-body simulations to model realistic shear maps for different cosmological models. We include both galaxy shape noise and, for the first time, measurement errors on galaxy shapes. We find that the measurement errors considered have relatively little impact on the constraining power of shear peak counts for LSST.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

WATER-TRAPPED WORLDS

Although tidally locked habitable planets orbiting nearby M-dwarf stars are among the best astronomical targets to search for extrasolar life, they may also be deficient in volatiles and water. Climate models for this class of planets show atmospheric transport of water from the dayside to the nightside, where it is precipitated as snow and trapped as ice. Since ice only slowly flows back to the dayside upon accumulation, the resulting hydrological cycle can trap a large amount of water in the form of nightside ice. Using ice sheet dynamical and thermodynamical constraints, I illustrate how planets with less than about a quarter the Earth's oceans could trap most of their surface water on the nightside. This would leave their dayside, where habitable conditions are met, potentially dry. The amount and distribution of residual liquid water on the dayside depend on a variety of geophysical factors, including the efficiency of rock weathering at regulating atmospheric CO{sub 2} as dayside ocean basins dry up. Water-trapped worlds with dry daysides may offer similar advantages as land planets for habitability, by contrast with worlds where more abundant water freely flows around the globe.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

ARE THE KEPLER NEAR-RESONANCE PLANET PAIRS DUE TO TIDAL DISSIPATION?

The multiple-planet systems discovered by the Kepler mission show an excess of planet pairs with period ratios just wide of exact commensurability for first-order resonances like 2:1 and 3:2. In principle, these planet pairs could have both resonance angles associated with the resonance librating if the orbital eccentricities are sufficiently small, because the width of first-order resonances diverges in the limit of vanishingly small eccentricity. We consider a widely held scenario in which pairs of planets were captured into first-order resonances by migration due to planet-disk interactions, and subsequently became detached from the resonances, due to tidal dissipation in the planets. In the context of this scenario, we find a constraint on the ratio of the planet's tidal dissipation function and Love number that implies that some of the Kepler planets are likely solid. However, tides are not strong enough to move many of the planet pairs to the observed separations, suggesting that additional dissipative processes are at play.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RESOLVING THE MOTH AT MILLIMETER WAVELENGTHS

HD 61005, also known as ''The Moth'', is one of only a handful of debris disks that exhibit swept-back ''wings'' thought to be caused by interaction with the ambient interstellar medium (ISM). We present 1.3 mm Submillimeter Array observations of the debris disk around HD 61005 at a spatial resolution of 1.''9 that resolve the emission from large grains for the first time. The disk exhibits a double-peaked morphology at millimeter wavelengths, consistent with an optically thin ring viewed close to edge-on. To investigate the disk structure and the properties of the dust grains we simultaneously model the spatially resolved 1.3 mm visibilities and the unresolved spectral energy distribution (SED). The temperatures indicated by the SED are consistent with expected temperatures for grains close to the blowout size located at radii commensurate with the millimeter and scattered light data. We also perform a visibility-domain analysis of the spatial distribution of millimeter-wavelength flux, incorporating constraints on the disk geometry from scattered light imaging, and find suggestive evidence of wavelength-dependent structure. The millimeter-wavelength emission apparently originates predominantly from the thin ring component rather than tracing the ''wings'' observed in scattered light. The implied segregation of large dust grains in the ring is consistent with an ISM-driven origin for the scattered light wings.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

IS THERE A METALLICITY CEILING TO FORM CARBON STARS?-A NOVEL TECHNIQUE REVEALS A SCARCITY OF C STARS IN THE INNER M31 DISK

We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of asymptotic giant branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C stars, contrary to the findings of previous C star searches in other regions of M31. We find only one candidate C star (plus up to six additional, less certain C star candidates), resulting in an extremely low ratio of C to M stars (C/M= (3.3{sup +20}{sub -0.1}) Multiplication-Sign 10{sup -4}) that is one to two orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints to evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A MASS-DEPENDENT YIELD ORIGIN OF NEUTRON-CAPTURE ELEMENT ABUNDANCE DISTRIBUTIONS IN ULTRA-FAINT DWARFS

One way to constrain the nature of the high-redshift progenitors of the Milky Way (MW) is to look at the low-metallicity stellar populations of the different Galactic components today. For example, high-resolution spectroscopy of very metal poor (VMP) stars demonstrates remarkable agreement between the distribution of [Ti/Fe] in the stellar populations of the MW halo and ultra-faint dwarf (UFD) galaxies. In contrast, for the neutron-capture (nc) abundance ratio distributions [(Sr, Ba)/Fe], the peak of the small UFD sample (6 stars) exhibits a significant under-abundance relative to the VMP stars in the larger MW halo sample ({approx}300 stars). We present a simple scenario that can simultaneously explain these similarities and differences by assuming: (1) that the MW VMP stars were predominately enriched by a prior generation of stars which possessed a higher total mass than the prior generation of stars that enriched the UFD VMP stars; and (2) a much stronger mass-dependent yield (MDY) for nc-elements than for the (known) MDY for Ti. Simple statistical tests demonstrate that conditions (1) and (2) are consistent with the observed abundance distributions, albeit without strong constraints on model parameters. A comparison of the broad constraints for these nc-MDY with those derived in the literature seems to rule out Ba production from low-mass supernovae (SNe) and affirms models that primarily generate yields from high-mass SNe. Our scenario can be confirmed by a relatively modest (factor of {approx}3-4) increase in the number of high-resolution spectra of VMP stars in UFDs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

AN EXPLANATION FOR THE DIFFERENT X-RAY TO OPTICAL COLUMN DENSITIES IN THE ENVIRONMENTS OF GAMMA RAY BURSTS: A PROGENITOR EMBEDDED IN A DENSE MEDIUM

We study the {approx}> 10 ratios in the X-ray to optical column densities inferred from afterglow spectra of gamma ray bursts (GRBs) due to gas surrounding their progenitors. We present time-evolving photoionization calculations for these afterglows and explore different conditions of their environment. We find that homogenous models of the environment (constant density) predict X-ray columns similar to those found in the optical spectra, with the bulk of the opacity being produced by neutral material at large distances from the burst. This result is independent of gas density or metallicity. Only models assuming a progenitor immersed in a dense ({approx}10{sup 2-4} cm{sup -3}) cloud of gas (with radius {approx}10 pc), with a strong, declining gradient of density for the surrounding interstellar medium (ISM) are able to account for the large X-ray to optical column density ratios. However, to avoid an unphysical correlation between the size of this cloud and the size of the ionization front produced by the GRB, the models also require that the circumburst medium is already ionized prior to the burst. The inferred cloud masses are {approx}< 10{sup 6} M{sub Sun }, even if low metallicities in the medium are assumed (Z {approx} 0.1 Z{sub Sun }). These cloud properties are consistent with those found in giant molecular clouds and our results support a scenario in which the progenitors reside within intense star formation regions of galaxies. Finally, we show that modeling over large samples of GRB afterglows may offer strong constraints on the range of properties in these clouds, and the host galaxy ISM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COLLISIONLESS DAMPING AT ELECTRON SCALES IN SOLAR WIND TURBULENCE

The dissipation of turbulence in the weakly collisional solar wind plasma is governed by unknown kinetic mechanisms. Two candidates have been suggested to play an important role in the dissipation, collisionless damping via wave-particle interactions and dissipation in small-scale current sheets. High resolution spacecraft measurements of the turbulent magnetic energy spectrum provide important constraints on the dissipation mechanism. The limitations of popular fluid and hybrid numerical schemes for simulation of the dissipation of solar wind turbulence are discussed, and instead a three-dimensional kinetic approach is recommended. We present a three-dimensional nonlinear gyrokinetic simulation of solar wind turbulence at electron scales that quantitatively reproduces the exponential form of the turbulent magnetic energy spectrum measured in the solar wind. A weakened cascade model that accounts for nonlocal interactions and collisionless Landau damping also quantitatively agrees with the observed exponential form. These results establish that a turbulent cascade of kinetic Alfven waves that is terminated by collisionless Landau damping is sufficient to explain the observed magnetic energy spectrum in the dissipation range of solar wind turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A STATISTICAL STUDY OF THE SPECTRAL HARDENING OF CONTINUUM EMISSION IN SOLAR FLARES

The observed hard X-ray and {gamma}-ray continuum in solar flares is interpreted as Bremsstrahlung emission of accelerated non-thermal electrons. It has been noted for a long time that in many flares the energy spectra show hardening at energies around or above 300 keV. In this paper, we first conduct a survey of spectral hardening events that were previously studied in the literature. We then perform a systematic examination of 185 flares from the Solar Maximum Mission. We identify 23 electron-dominated events whose energy spectra show clear double power laws. A statistical study of these events shows that the spectral index below the break ({gamma}{sub 1}) anti-correlates with the break energy ({epsilon}{sub b}). Furthermore, {gamma}{sub 1} also anti-correlates with Fr, the fraction of photons above the break compared to the total photons. A hardening spectrum, as well as the correlations between ({gamma}{sub 1}, {epsilon}{sub b}) and ({gamma}{sub 1}, Fr), provide stringent constraints on the underlying electron acceleration mechanism. Our results support a recent proposal that electrons are being accelerated diffusively at a flare termination shock with a width of the order of an ion inertial length scale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE HALO OCCUPATION DISTRIBUTION OF X-RAY-BRIGHT ACTIVE GALACTIC NUCLEI: A COMPARISON WITH LUMINOUS QUASARS

We perform halo occupation distribution (HOD) modeling of the projected two-point correlation function (2PCF) of high-redshift (z {approx} 1.2) X-ray-bright active galactic nuclei (AGNs) in the XMM-COSMOS field measured by Allevato et al. The HOD parameterization is based on low-luminosity AGNs in cosmological simulations. At the median redshift of z {approx} 1.2, we derive a median mass of 1.02{sub -0.23}{sup +0.21} Multiplication-Sign 10{sup 13} h{sup -1} M{sub sun} for halos hosting central AGNs and an upper limit of {approx}10% on the AGN satellite fraction. Our modeling results indicate (at the 2.5{sigma} level) that X-ray AGNs reside in more massive halos compared to more bolometrically luminous, optically selected quasars at similar redshift. The modeling also yields constraints on the duty cycle of the X-ray AGN, and we find that at z {approx} 1.2 the average duration of the X-ray AGN phase is two orders of magnitude longer than that of the quasar phase. Our inferred mean occupation function of X-ray AGNs is similar to recent empirical measurements with a group catalog and suggests that AGN halo occupancy increases with increasing halo mass. We project the XMM-COSMOS 2PCF measurements to forecast the required survey parameters needed in future AGN clustering studies to enable higher precision HOD constraints and determinations of key physical parameters like the satellite fraction and duty cycle. We find that N {sup 2}/A {approx} 5 Multiplication-Sign 10{sup 6} deg{sup -2} (with N the number of AGNs in a survey area of A deg{sup 2}) is sufficient to constrain the HOD parameters at the 10% level, which is easily achievable by upcoming and proposed X-ray surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

BULK COMPOSITION OF GJ 1214b AND OTHER SUB-NEPTUNE EXOPLANETS

GJ 1214b stands out among the detected low-mass exoplanets, because it is, so far, the only one amenable to transmission spectroscopy. Up to date there is no consensus about the composition of its envelope although most studies suggest a high molecular weight atmosphere. In particular, it is unclear if hydrogen and helium are present or if the atmosphere is water dominated. Here, we present results on the composition of the envelope obtained by using an internal structure and evolutionary model to fit the mass and radius data. By examining all possible mixtures of water and H/He, with the corresponding opacities, we find that the bulk amount of H/He of GJ 1214b is at most 7% by mass. In general, we find the radius of warm sub-Neptunes to be most sensitive to the amount of H/He. We note that all (Kepler-11b,c,d,f, Kepler-18b, Kepler-20b, 55Cnc-e, Kepler-36c, and Kepler-68b) but two (Kepler-11e and Kepler-30b) of the discovered low-mass planets so far have less than 10% H/He. In fact, Kepler-11e and Kepler-30b have 10%-18% and 5%-15% bulk H/He. Conversely, little can be determined about the H{sub 2}O or rocky content of sub-Neptune planets. We find that although a 100% water composition fits the data for GJ 1214b, based on formation constraints the presence of heavier refractory material on this planet is expected, and hence, so is a component lighter than water required. The same is true for Kepler-11f. A robust determination by transmission spectroscopy of the composition of the upper atmosphere of GJ 1214b will help determine the extent of compositional segregation between the atmosphere and the envelope.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

INVESTIGATION OF THE PUZZLING ABUNDANCE PATTERN IN THE STARS OF THE FORNAX DWARF SPHEROIDAL GALAXY

Many works have found unusual characteristics of elemental abundances in nearby dwarf galaxies. This implies that there is a key factor of galactic evolution that is different from that of the Milky Way (MW). The chemical abundances of the stars in the Fornax dwarf spheroidal galaxy (Fornax dSph) provide excellent information for setting constraints on the models of galactic chemical evolution. In this work, adopting the five-component approach, we fit the abundances of the Fornax dSph stars, including {alpha} elements, iron group elements, and neutron-capture elements. For most sample stars, the relative contributions from the various processes to the elemental abundances are not usually in the MW proportions. We find that the contributions from massive stars to the primary {alpha} elements and iron group elements increase monotonically with increasing [Fe/H]. This means that the effect of the galactic wind is not strong enough to halt star formation and the contributions from the massive stars to {alpha} elements did not halt for [Fe/H] {approx}< -0.5. The average contribution ratios of various processes between the dSph stars and the MW stars monotonically decrease with increasing progenitor mass. This is important evidence of a bottom-heavy initial mass function (IMF) for the Fornax dSph, compared to the MW. Considering a bottom-heavy IMF for the dSph, the observed relations of [{alpha}/Fe] versus [Fe/H], [iron group/Fe] versus [Fe/H], and [neutron-capture/Fe] versus [Fe/H] for the dSph stars can be explained.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DISCOVERY OF THE TRANSITION OF A MINI-BROAD ABSORPTION LINE INTO A BROAD ABSORPTION LINE IN THE SDSS QUASAR J115122.14+020426.3

We present the detection of a rare case of dramatic strengthening in the UV absorption profiles in the spectrum of the quasar J115122.14+020426.3 between observations {approx}2.86 yr apart in the quasar rest frame. A spectrum obtained in 2001 by the Sloan Digital Sky Survey shows a C IV ''mini-broad'' absorption line (FWHM = 1220 km s{sup -1}) with a maximum blueshift velocity of {approx}9520 km s{sup -1}, while a later spectrum from the Very Large Telescope shows a significantly broader and stronger absorption line, with a maximum blueshift velocity of {approx}12, 240 km s{sup -1} that qualifies as a broad absorption line. A similar variability pattern is observed in two additional systems at lower blueshifted velocities and in the Ly{alpha} and N V transitions as well. One of the absorption systems appears to be resolved and shows evidence for partial covering of the quasar continuum source (C{sub f} {approx} 0.65), indicating a transverse absorber size of, at least, {approx}6 Multiplication-Sign 10{sup 16} cm. In contrast, a cluster of narrower C IV lines appears to originate in gas that fully covers the continuum and broad emission line sources. There is no evidence for changes in the centroid velocity of the absorption troughs. This case suggests that at least some of the absorbers that produce ''mini-broad'' and broad absorption lines in quasar spectra do not belong to intrinsically separate classes. Here, the ''mini-broad'' absorption line is most likely interpreted as an intermediate phase before the appearance of a broad absorption line due to their similar velocities. While the current observations do not provide enough constraints to discern among the possible causes for this variability, future monitoring of multiple transitions at high resolution will help achieve this goal.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE XMM-NEWTON/EPIC X-RAY LIGHT CURVE ANALYSIS OF WR 6

We obtained four pointings of over 100 ks each of the well-studied Wolf-Rayet star WR 6 with the XMM-Newton satellite. With a first paper emphasizing the results of spectral analysis, this follow-up highlights the X-ray variability clearly detected in all four pointings. However, phased light curves fail to confirm obvious cyclic behavior on the well-established 3.766 day period widely found at longer wavelengths. The data are of such quality that we were able to conduct a search for event clustering in the arrival times of X-ray photons. However, we fail to detect any such clustering. One possibility is that X-rays are generated in a stationary shock structure. In this context we favor a corotating interaction region (CIR) and present a phenomenological model for X-rays from a CIR structure. We show that a CIR has the potential to account simultaneously for the X-ray variability and constraints provided by the spectral analysis. Ultimately, the viability of the CIR model will require both intermittent long-term X-ray monitoring of WR 6 and better physical models of CIR X-ray production at large radii in stellar winds.

79 ASTRONOMY AND ASTROPHYSICS↗