Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds

We propose collisionless damping of fast MHD waves as an important mechanism for the heating and acceleration of winds from rotating stars. Stellar rotation causes magnetic field lines anchored at the surface to form a spiral pattern and magneto-rotational winds can be driven. If the structure is a magnetically dominated, fast MHD waves generated at the surface can propagate almost radially outward and cross the field lines. The propagating waves undergo collisionless damping owing to interactions with particles surfing on magnetic mirrors that are formed by the waves themselves. The damping is especially effective where the angle between the wave propagation and the field lines becomes moderately large ($\sim 20$ to $80^{\circ}$). The angle tends naturally to increase into this range because the field in magneto-rotational winds develops an increasingly large azimuthal component. The dissipation of the wave energy produces heating and acceleration of the outflow. We show using specified wind structures that this damping process can be important in both solar-type stars and massive stars that have moderately large rotation rates. This mechanism can play a role in coronae of young solar-type stars which are rapidly rotating and show X-ray luminosities much larger than the sun. The mechanism could also be important for producing the extended X-ray emitting regions inferred to exist in massive stars of spectral type middle B and later.Comment: 12 pages, including 7 figures, accepted for publication in Ap

E pluribus unum : impact entrepreneurship as a solution to grand challenges

Insufficiency of research and theory on the relationship between entrepreneurship and grand challenges means that we know little about who engages and what repertoires of actions they take to tackle socioenvironmental challenges that transcend firms, markets, and nations, and what sorts of solutions they create. Drawing on the five articles featured in this symposium-and focusing especially on their protagonists or actors, the actions these actors take, and their achievements-we begin to conceptualize an impact entrepreneurship perspective. Following the tenet of e pluribus unum ("out of many, one") and adhering to the doctrine that diverse, decentralized human effort can improve the world, our impact entrepreneurship perspective refers to the development of solutions to grand challenges, in a financially, socially, and environmentally sustainable fashion. All in all, then, this symposium provides a starting point to discuss, conceptualize, study, interpret, and enrich our understanding of impact entrepreneurship and collective action to address grand challenges

An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars

Chandra high energy resolution observations have now been obtained from numerous non-peculiar O and early B stars. The observed X-ray emission line properties differ from pre-launch predictions, and the interpretations are still problematic. We present a straightforward analysis of a broad collection of OB stellar line profile data to search for morphological trends. X-ray line emission parameters and the spatial distributions of derived quantities are examined with respect to luminosity class. The X-ray source locations and their corresponding temperatures are extracted by using the He-like f/i line ratios and the H-like to He-like line ratios respectively. Our luminosity class study reveals line widths increasing with luminosity. Although the majority of the OB emission lines are found to be symmetric, with little central line displacement, there is evidence for small, but finite, blue-ward line-shifts that also increase with luminosity. The spatial X-ray temperature distributions indicate that the highest temperatures occur near the star and steadily decrease outward. This trend is most pronounced in the OB supergiants. For the lower density wind stars, both high and low X-ray source temperatures exist near the star. However, we find no evidence of any high temperature X-ray emission in the outer wind regions for any OB star. Since the temperature distributions are counter to basic shock model predictions, we call this the "near-star high-ion problem" for OB stars. By invoking the traditional OB stellar mass loss rates, we find a good correlation between the fir-inferred radii and their associated X-ray continuum optical depth unity radii. We conclude by presenting some possible explanations to the X-ray source problems that have been revealed by this study.Comment: Published in 2007, ApJ, 668, 456. An Erratum scheduled for publication in 2008, ApJ, 680, is included as an Appendix. The Erratum corrects some tabulated data in 5 tables and 2 figure

Mode Bifurcation and Fold Points of Complex Dispersion Curves for the Metamaterial Goubau Line

In this paper the complex dispersion curves of the four lowest-order transverse magnetic modes of a dielectric Goubau line ($\epsilon>0, \mu>0$) are compared with those of a dispersive metamaterial Goubau line. The vastly different dispersion curve structure for the metamaterial Goubau line is characterized by unusual features such as mode bifurcation, complex fold points, both proper and improper complex modes, and merging of complex and real modes

High Resolution X-ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cygnus OB2 Association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We have obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in the Association. Although our analysis will focus on the X-ray properties of Cyg OB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies of early-type stars, we find that the X-ray properties of Cyg OB2 No. 8a are very similar to those of other OB stars that have been observed using high-resolution X-ray spectroscopy. From analyses of the He-like ion "fir" emission lines, we derive radial distances of the He-like line emission sources and find these fir-inferred radii are consistent with their corresponding X-ray continuum optical depth unity radii. Contrary to other O-star results, the emission lines of Cyg OB2 No. 8a show a large range in line centroid shifts (roughly -800 to +250 km/s). We discuss the implications of our results in light of the fact that Cyg OB2 No. 8a is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.Comment: 36 pages (including 4 tables and 12 figures). LaTeX. Submitted to Ap

Integral Relaxation Time of Single-Domain Ferromagnetic Particles

The integral relaxation time \tau_{int} of thermoactivating noninteracting single-domain ferromagnetic particles is calculated analytically in the geometry with a magnetic field H applied parallel to the easy axis. It is shown that the drastic deviation of \tau_{int}^{-1} from the lowest eigenvalue of the Fokker-Planck equation \Lambda_1 at low temperatures, starting from some critical value of H, is the consequence of the depletion of the upper potential well. In these conditions the integral relaxation time consists of two competing contributions corresponding to the overbarrier and intrawell relaxation processes.Comment: 8 pages, 3 figure

A Coordinated X-ray and Optical Campaign of the Nearby Massive Binary δ\delta Orionis Aa: II. X-ray Variability

We present time-resolved and phase-resolved variability studies of an extensive X-ray high-resolution spectral dataset of the $\delta$ Orionis Aa binary system. The four observations, obtained with Chandra ACIS HETGS, have a total exposure time of ~479 ks and provide nearly complete binary phase coverage. Variability of the total X-ray flux in the range 5-25 $\AA$ is confirmed, with maximum amplitude of about +/-15% within a single ~125 ks observation. Periods of 4.76d and 2.04d are found in the total X-ray flux, as well as an apparent overall increase in flux level throughout the 9-day observational campaign. Using 40 ks contiguous spectra derived from the original observations, we investigate variability of emission line parameters and ratios. Several emission lines are shown to be variable, including S XV, Si XIII, and Ne IX. For the first time, variations of the X-ray emission line widths as a function of the binary phase are found in a binary system, with the smallest widths at phase=0.0 when the secondary $\delta$ Orionis Aa2 is at inferior conjunction. Using 3D hydrodynamic modeling of the interacting winds, we relate the emission line width variability to the presence of a wind cavity created by a wind-wind collision, which is effectively void of embedded wind shocks and is carved out of the X-ray-producing primary wind, thus producing phase-locked X-ray variability.Comment: 36 pages, 14 Tables, 19 Figures, accepted by ApJ, one of 4 related papers to be published togethe