268 research outputs found

    The Rigidly Rotating Magnetosphere of Sigma Ori E

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    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

    Magnetic Rotator Winds and Keplerian Disks of Hot Stars

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    We consider rotating magnetic stars with winds and disks. We establish a theorem that relates the angular velocity of a disk region with no meridional motion to the angular velocity of the star. Also, we show that for a given value of the magnetic field strength, if the rotation rate is too high or the flow velocity into the shock boundary is too low, a Keplerian disk region will not be formed. We develop a model for the formation of disks in magnetic rotators through the processes of fill-up and diffusion into Keplerian orbits. At the end of the fill-up stage the density of the disk increases significantly and the magnetic force in the disk becomes negligible. We derive analytical expressions for the inner and outer radii of Keplerian disks in terms of the stellar rotation rate. A disk can form if the meridional component Bm of the field at the stellar surface is larger than a minimum value. The radial extent of the Keplerian region becomes larger for larger values of Bm and is largest when Bm equals an optimal value. The strengths of the minimum fields required for Keplerian disk formation in B-type stars varies from order 1G to 10G. In O-type stars they must be of order 500G. Also, we suggest that the stellar magnetic fields may be affected by rotationally driven meridional circulation leading to some of the the observed variations of disks with time.Comment: 44 pages, 1 figure, accepted by Ap

    New Perspectives on the X-ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra

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    We present new X-ray observations of the nearby Herbig Ae star HD 104237 (= DX Cha) with XMM-Newton, whose objective is to clarify the origin of the emission. Several X-ray emission lines are clearly visible in the CCD spectra, including the high-temperature Fe K-alpha complex. The emission can be accurately modeled as a multi-temperature thermal plasma with cool (kT < 1 keV) and hot (kT > 3 keV) components. The presence of a hot component is compelling evidence that the X-rays originate in magnetically confined plasma, either in the Herbig star itself or in the corona of an as yet unseen late-type companion. The X-ray temperatures and luminosity (log Lx = 30.5 ergs/s) are within the range expected for a T Tauri companion, but high resolution Chandra and HST images constrain the separation of a putative companion to less than 1 arcsec. We place these new results into broader context by comparing the X-ray and bolometric luminosities of a sample of nearby Herbig stars with those of T Tauri stars and classical main-sequence Be stars. We also test the predictions of a model that attributes the X-ray emission of Herbig stars to magnetic activity that is sustained by a shear-powered dynamo.Comment: To appear in ApJ (part 1); 43 pages, 8 figures, 5 table

    Chandra Observations of Variable Embedded X-ray sources in Orion. Paper I: Resolving Orion Trapezium

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    We used the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-ray Observatory to perform two observations, separated by three weeks, of the Orion Trapezium region. The zeroth order images on the Advanced CCD Imaging Spectrometer (ACIS) provide spatial resolution of 0.5" and moderate energy resolution. Within a 160"x140" region around the Orion Trapezium we resolve 111 X-ray sources with luminosities between 7x10^{28} ergs/s and 2x10^{32} ergs/s. We do not detect any diffuse emission. Many sources appear much more heavily absorbed, with N_H in the range of 10^22 to 10^23 cm^-2. The main objective of this paper is to study the Orion Trapezium and its close vicinity. Most spectra of the very early type members can be fit with a two-temperature thermal spectrum with a soft component of kT ~ 0.8 keV and a hard component of kT ~ 2 to 3 keV. We discuss these results in the context of stellar wind models. We detect eight additional, mostly variable X-ray sources in the close vicinity of the Trapezium. Five of these X-rays sources are identified with proplyds and we argue that the X-ray emission originates from class I, II and III protostars at the cores of the proplyds.Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical Journa

    The view from above: the relevance of shared aerial drone videos for destination marketing

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    The use of drones to produce videos has generated a large amount of visually appealing footage of various destinations. They attract much attention, but there are issues that affect their production, and their relevance to destination marketing. This research examines YouTube meta-data and spatial overlay analysis of shared aerial drone videos from the United Kingdom (UK). The results suggest that shared aerial drone videos have some unique user-generated content (UGC) characteristics and their spatial distribution tend to favor more populated areas. Theoretical and practical implications for destination marketing are further discussed

    Photoemission Spectroscopy and Atomic Force Microscopy Investigation of Vapor Phase Co-Deposited Silver/Poly(3-hexylthiophene) Composites

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    Nanocomposite matrices of silver/poly(3-hexylthiophene) (P3HT) were prepared in ultrahigh vacuum through vapor-phase co-deposition. Change in microstructure, chemical nature and electronic properties with increasing filler (Ag) content were investigated using in-situ XPS and UPS, and ambient AFM. At least two chemical binding states occur between Ag nanoparticles and sulfur in P3HT at the immediate contact layer but no evidence of interaction between Ag and carbon (in P3HT) was found. AFM images reveal a change in Ag nanoparticles size with concentration which modifies the microstructure and the average roughness of the surface. Under co-deposition, P3HT largely retains its conjugated structures, which is evidenced by the similar XPS and UPS spectra to those of P3HT films deposited on other substrates. We demonstrate here that the magnitude of the barrier height for hole injection and the position of the highest occupied band edge (HOB) with respect to the Fermi level of Ag can be controlled and changed by adjusting the metal (Ag) content in the composite. Furthermore, UPS reveals distinct features related to the C 2p (Sigma states) in the 5-12 eV regions, indicating the presence of ordered P3HT which is different from solution processed films.Comment: Scudier and Wei provided equal contributio

    Single-cell clonal tracking of persistent T-cells in allogeneic hematopoietic stem cell transplantation

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    The critical balance between intended and adverse effects in allogeneic hematopoietic stem cell transplantation (alloHSCT) depends on the fate of individual donor T-cells. To this end, we tracked αβT-cell clonotypes during stem cell mobilization treatment with granulocyte-colony stimulating factor (G-CSF) in healthy donors and for six months during immune reconstitution after transfer to transplant recipients. More than 250 αβT-cell clonotypes were tracked from donor to recipient. These clonotypes consisted almost exclusively of CD8+ effector memory T cells (CD8TEM), which exhibited a different transcriptional signature with enhanced effector and cytotoxic functions compared to other CD8TEM. Importantly, these distinct and persisting clonotypes could already be delineated in the donor. We confirmed these phenotypes on the protein level and their potential for selection from the graft. Thus, we identified a transcriptional signature associated with persistence and expansion of donor T-cell clonotypes after alloHSCT that may be exploited for personalized graft manipulation strategies in future studies
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