Bare quark stars or naked neutron stars? The case of RX J1856.5-3754

In a cool neutron star (T less than or similar to 10(6) K) endowed with a rather highmagnetic field (B greater than or similar to 10(13) G), a phase transition may occur in the outermost layers. As a consequence, the neutron star becomes "bare,'' i.e., no gaseous atmosphere sits on the top of the crust. The surface of a cooling, bare neutron star does not necessarily emit a blackbody spectrum because the emissivity is strongly suppressed at energies below the electron plasma frequency, omega(p). Since omega(p) approximate to 1 keV under the conditions typical of the dense electron gas in the condensate, the emission from a T similar to 100 eV bare neutron star will be substantially depressed with respect to that of a perfect Planckian radiator atmost energies. Here we present a detailed analysis of the emission properties of a bare neutron star. In particular, we derive the surface emissivity for an Fe composition in a range of magnetic fields and temperatures representative of cooling isolated neutron stars, like RX J1856.5 - 3754. We find that the emitted spectrum is strongly dependent on the electron conductivity in the solid surface layers. In the cold electron gas approximation ( no electron-lattice interactions), the spectrum turns out to be a featureless depressed blackbody in the 0.1 - 2 keV band with a steeper low-energy distribution. When damping effects due to collisions between electrons and the ion lattice ( mainly due to electron-phonon interactions) are accounted for, the spectrum is more depressed at low energies and spectral features may be present, depending on the magnetic field strength. Details of the emitted spectrum are found, however, to be strongly dependent on the assumed treatment of the transition from the external vacuum to the metallic surface. The implications of our results for RX J1856.5 - 3754 and other isolated neutron stars are discussed

Test evaluation of fuel cell catalysts Quarterly report, 15 Feb. - 15 May 1967

Catalytic activity of iron compounds for fuel cell catalyst

Ion plasma waves induced by frustrated Debye shielding

The oscillation of electrons, in a sufficiently intense pump wave, frustrates Debye shielding in the direction of the oscillation. One finds that such oscillating electrons cannot shield charge fluctuations over distances smaller than the distance they sample in a plasma period. One consequence is that the frequency of ion waves can be increased from the ion acoustic frequency to the ion plasma frequency in the presence of large enough oscillations. This may explain a number of observations in laser experiments. More generally, any phenomenon involving Debye shielding will be altered by an intense pump wave. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69595/2/PHPAEN-9-1-267-1.pd

Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot

High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in CTTS. In particular, the accretion shock region, where the accreting material is heated to temperatures of a few MK as it continues its inward bulk motion, can be probed by X-ray spectroscopy. To attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS, and constrain the accretion stream geometry. We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods, by measuring the position of a selected sample of emission lines, and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface. Our results indicate that complex magnetic field geometries, such as that of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the CIV resonance doublet at 1550 Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at 0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version after language editor corrections; 16 pages, 8 figures, 6 table

Collective memory and historical narratives: The African American civil rights movement

This study explores how undergraduates, as historical thinkers, learn to interact with history and construct their understanding of the past, and examines the role that primary and secondary sources play in narrative construction and revision. Using the African American civil rights movement as a content focus, participants used images to create initial narratives that reflected their understanding of the movement. Half the participants then read an essay on the movement written by a prominent historian, and the other half examined 18 primary sources that reflected the historian’s interpretation of the movement. Participants then each created a second narrative, again selecting images to depict their understanding of the movement. The results of the study suggest that even as students work with primary sources, they need an effective narrative framework based on recent scholarship to forge powerful counter-narratives that transcend outdated interpretations and historical myths. In terms of teaching and learning about the lengthy struggle for racial justice in the United States, simply encouraging teachers and students to ‘do history’ and conduct their own online research is unlikely to change persistent narrative structures that continue to enable and excuse systemic racism