Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds

We present 2D MHD simulations of the radiatively driven outflow from a rotating hot star with a dipole magnetic field aligned with the star's rotation axis. We focus primarily on a model with moderately rapid rotation (half the critical value), and also a large magnetic confinement parameter, $\eta_{\ast} \equiv B_{\ast}^2 R_{\ast}^{2} / \dot{M} V_{\infty} = 600$. The magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk extending from near the Kepler corotation radius outwards. Even with fine-tuning at lower magnetic confinement, none of the MHD models produce a stable Keplerian disk. Instead, material below the Kepler radius falls back on to the stellar surface, while the strong centrifugal force on material beyond the corotation escape radius stretches the magnetic loops outwards, leading to episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperatures of nearly $10^{8}$K, high enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection} represents a novel mechanism for driving magnetic reconnection, and seems a very promising basis for modeling X-ray flares recently observed in rotating magnetic Bp stars like $\sigma$ Ori E.Comment: 5 pages, 3 figures, accepted by ApJ

Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation

Building upon our previous MHD simulation study of magnetic channeling in radiatively driven stellar winds, we examine here the additional dynamical effects of stellar {\em rotation} in the (still) 2-D axisymmetric case of an aligned dipole surface field. In addition to the magnetic confinement parameter $\eta_{\ast}$ introduced in Paper I, we characterize the stellar rotation in terms of a parameter $W \equiv V_{\rm{rot}}/V_{\rm{orb}}$ (the ratio of the equatorial surface rotation speed to orbital speed), examining specifically models with moderately strong rotation $W =$ 0.25 and 0.5, and comparing these to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar and Kepler corotation radius R_{\rm{K}} \approx W^{-2/3} \Rstar, we find rotation effects are weak for models with $R_{\rm{A}} < R_{\rm{K}}$, but can be substantial and even dominant for models with R_{\rm{A}} \gtwig R_{\rm{K}}. In particular, by extending our simulations to magnetic confinement parameters (up to $\eta_{\ast} = 1000$) that are well above those ($\eta_{\ast} = 10$) considered in Paper I, we are able to study cases with $R_{\rm{A}} \gg R_{\rm{K}}$; we find that these do indeed show clear formation of the {\em rigid-body} disk predicted in previous analytic models, with however a rather complex, dynamic behavior characterized by both episodes of downward infall and outward breakout that limit the buildup of disk mass. Overall, the results provide an intriguing glimpse into the complex interplay between rotation and magnetic confinement, and form the basis for a full MHD description of the rigid-body disks expected in strongly magnetic Bp stars like $\sigma$ Ori E.Comment: 14 pp, visit this http://shayol.bartol.udel.edu/massivewiki-media/publications/rotation.pdf for full figure version of the paper. MNRAS, in pres

A Perpetual Harvest Greenhouse System: Integrating Barn, Biofilter, and Greenhouse

A prototype was built to evaluate the performance of an integrated barn-biofilter-greenhouse system. The greenhouse floor in the integrated system consisted of a bed of gravel to store maximum solar energy. A vertical airflow biofilter (3.34 x 3.34 m) was constructed inside a solar energy greenhouse (floor area of 15 x 6.7 m); exhaust air from a barn was passed through the biofilter for odour treatment before being released into the greenhouse. A booster fan was used to provide a steady airflow rate of 1.4 m3/s to the biofilter. Data were collected from October 19 to December 6, 2007. The maximum temperature drop along the 15.5 m long, and insulated (R-20) duct carrying the exhaust air from the hog barn to the biofilter was 7°C. The lowest temperature recorded on top of the biofilter surface was 1.3°C when the biofilter booster fan was not working, while the lowest floor temperature was -3°C. On the coldest day in December, when the biofilter booster fan was not in service, the daily average temperature inside the greenhouse was 4.3°C, whereas the outdoor daily average temperature was -25°C. In order to keep the minimum greenhouse temperature at 10°C, the maximum required volumetric flow rate of barn exhaust air at 15°C was 1.60m3/s. Maximum hydrogen sulfide (H2S) removal efficiency was 55%. The weekly average concentration of carbon dioxide (CO2) inside the greenhouse varied from 841 to 1536 ppm. The system has shown promise at creating an environment suitable for plant growth inside the greenhouse using a waste gas stream from a hog barn to provide both auxiliary heat and enhanced CO2 levels

Magnetic stars from a FEROS cool Ap star survey

New magnetic Ap stars with split Zeeman components are presented. These stars were discovered from observations with the Fibre-fed Extended Range Optical Spectrograph (FEROS) spectrograph at the European Southern Observatory (ESO) 2.2-m telescope. 15 new magnetic stars are analysed here. Several stars with very strong magnetic fields were found, including HD 70702 with a 15-kG magnetic field strength, and HD 168767 with a 16.5-kG magnetic field strength measured using split Zeeman components of spectral lines and by comparison with synthetic calculations. The physical parameters of the stars were estimated from photometric and spectroscopic data. Together with previously published results for stars with strong magnetic fields, the relationship between magnetic field strength and rotation period is discussed

The Rigidly Rotating Magnetosphere of Sigma Ori E

We attempt to characterize the observed variability of the magnetic helium-strong star sigma Ori E in terms of a recently developed rigidly rotating magnetosphere model. This model predicts the accumulation of circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic equilibrium at the intersection between magnetic and rotational equators. We find that the model can reproduce well the periodic modulations observed in the star's light curve, H alpha emission-line profile, and longitudinal field strength, confirming that it furnishes an essentially correct, quantitative description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap

Wind Circulation in Selected Rotating Magnetic Early-B Stars

The rotating magnetic B stars have oblique dipolar magnetic fields and often anomalous helium and metallic compositions. These stars develop co-rotating torus-shaped clouds by channelling winds from their magnetic poles to an anchored planar disk over the magnetic equator. The line absorptions from the cloud can be studied as the complex rotates and periodically occults the star. We describe an analysis of the clouds of four stars (HD184927, beta Cep, sigma Ori E, and HR6684). From line synthesis models, we find that the metallic compositions are spatially uniform over the stars' surfaces. Next, using the Hubeny CIRCUS code, we demonstate that periodic UV continuum fluxes can be explained by the absorption of low-excitation lines. The analysis also quantifies the cloud temperatures, densities, and turbulences, which appear to increase inward toward the stars. The temperatures range from about 12,000K for the weak Fe lines up to temperatures of 33,000K for N V absorptions, which is in excess of temperatures expected from radiative equilibrium. The spectroscopic hallmark of this stellar class is the presence of strong C IV and N V resonance line absorptions at occultation phases and of redshifted emissions at magnetic pole-on phases. The emissions have characteristics which seem most compatible with the generation of high-energy shocks at the wind-cloud interface, as predicted by Babel.Comment: 19 pages, Latex plus 6 figures A&A single-spaced, accepted by Astronomy & Astrophysics. Files available by ftp at nobel.stsci.edu/pub/aapaper

An Atlas of K-line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)

We present a short atlas illustrating the unusual Ca {\sc ii} K-line profiles in upper main sequence stars with anomalous abundances. Slopes of the profiles for 10 cool, magnetic chemically peculiar (CP) stars change abruptly at the very core, forming a deep "nib." The nibs show the same or nearly the same radial velocity as the other atomic lines. The near wings are generally more shallow than in normal stars. In three magnetic CP stars, the K-lines are too weak to show this shape, though the nibs themselves are arguably present. The Ca {\sc ii} H-lines also show deep nibs, but the profiles are complicated by the nearby, strong H$\epsilon$ absorption. The K-line structure is nearly unchanged with phase in $\beta$ CrB and $\alpha$ Cir. Calculations, including NLTE, show that other possibilities in addition to chemical stratification may yield nib-like cores.Comment: 6 pages, 2 figures, and 8 figures; accepted for publication in ApJ

The most rapidly rotating He-strong emission line star: HR7355

Using archival spectroscopic and photometric data, we searched for massive stars with Balmer-emission consistent with magnetically confined circumstellar material. HR 7355 is a formerly unknown He-strong star showing Balmer emission. At V=6.02 mag, it is one of the brightest objects simultaneously showing anomalous helium absorption and hydrogen emission. Among similar objects, only sigma Ori E has so far been subjected to any systematic analysis of the circumstellar material responsible for the emission. We argue that the double-wave photometric period of 0.52d corresponds to the rotation period. In tandem with the high projected equatorial velocity, v sin i=320 km/s, this short period suggests that HR 7355 is the most rapidly rotating He-strong star known to date; a class that was hitherto expected to host stars with slow to moderate rotation only.Comment: 4 pages with 2 figures. Accepted for publication as Research Note by Astronomy and Astrophysic