Ultraviolet corona detection sensor study

The feasibility of detecting electrical corona discharge phenomena in a space simulation chamber via emission of ultraviolet light was evaluated. A corona simulator, with a hemispherically capped point to plane electrode geometry, was used to generate corona glows over a wide range of pressure, voltage, current, electrode gap length and electrode point radius. Several ultraviolet detectors, including a copper cathode gas discharge tube and a UV enhanced silicon photodiode detector, were evaluated in the course of the spectral intensity measurements. The performance of both silicon target vidicons and silicon intensified target vidicons was evaluated analytically using the data generated by the spectroradiometer scans and the performance data supplied by the manufacturers

Surface flux transport modeling for solar cycles 15--21: effects of cycle-dependent tilt angles of sunspot groups

We model the surface magnetic field and open flux of the Sun from 1913 to 1986 using a surface flux transport model, which includes the observed cycle-to-cycle variation of sunspot group tilts. The model reproduces the empirically derived time evolution of the solar open magnetic flux, and the reversal times of the polar fields. We find that both the polar field and the axial dipole moment resulting from this model around cycle minimum correlate with the strength of the following cycle.Comment: Accepted for publication by Ap

Fe XIII coronal line emission in cool M dwarfs

We report on a search for the Fe xiii forbidden coronal line at 3388.1 \AA in a sample of 15 M-type dwarf stars covering the whole spectral class as well as different levels of activity. A clear detection was achieved for LHS 2076 during a major flare and for CN Leo, where the line had been discovered before. For some other stars the situation is not quite clear. For CN Leo we investigated the timing behaviour of the Fe xiii line and report a high level of variability on a timescale of hours which we ascribe to microflare heating.Comment: 13 pages, 10 figure

High purity silica reflective heat shield development

Measurements were made of reflectance in the vacuum ultraviolet down to 0.15 micron. Scattering coefficients (S) and absorption coefficients (K) were also measured. These coefficients express the optical properties and are used directly in a thermodynamic analysis for sizing a heat shield. The effect of the thin silica melt layer formed during entry was also studied from the standpoint of trapped radiant energy

The physics of the Applegate mechanism: Eclipsing time variations from magnetic activity

Since its proposal in 1992, the Applegate mechanism has been discussed as a potential intrinsical mechanism to explain transit timing variations in various kinds of close binary systems. Most analytical arguments presented so far focused on the energetic feasibility of the mechanism, while applying rather crude one- or two-zone prescriptions to describe the exchange of angular momentum within the star. In this paper, we present the most detailed approach to date to describe the physics giving rise to the modulation period from kinetic and magnetic fluctuations. Assuming moderate levels of stellar parameter fluctuations, we find that the resulting binary period variations are one or two orders of magnitude lower than the observed values in RS-CVn like systems, supporting the conclusion of existing theoretical work that the Applegate mechanism may not suffice to produce the observed variations in these systems. The most promising Applegate candidates are low-mass post-common-envelope binaries (PCEBs) with binary separations $\lesssim 1~\mathrm{R}_\odot$ and secondary masses in the range of $0.30~\mathrm{M}_\odot$ and $0.36~\mathrm{M}_\odot$.Comment: 10 pages, 8 figures. Accepted for publication in A&

Spot evolution in the eclipsing binary CoRoT 105895502

Stellar activity is ubiquitous in late-type stars. The special geometry of eclipsing binary systems is particularly advantageous to study the stellar surfaces and activity. We present a detailed study of the 145 d CoRoT light curve of the short-period eclipsing binary CoRoT 105895502. By means of light-curve modeling with Nightfall, we determine the orbital period, effective temperature, Roche-lobe filling factors, mass ratio, and orbital inclination of CoRoT 105895502 and analyze the temporal behavior of starspots in the system. Our analysis shows one comparably short-lived (about 40 d) starspot, remaining quasi-stationary in the binary frame, and one starspot showing prograde motion at a rate of 2.3 deg per day, whose lifetime exceeds the duration of the observation. In the CoRoT band, starspots account for as much as 0.6 % of the quadrature flux of CoRoT 105895502, however we cannot attribute the spots to individual binary components with certainty. Our findings can be explained by differential rotation, asynchronous stellar rotation, or systematic spot evolution.Comment: Accepted in A&

Was 49b: An Overmassive AGN in a Merging Dwarf Galaxy?

We present a combined morphological and X-ray analysis of Was 49, an isolated, dual AGN system notable for the presence of a dominant AGN Was 49b in the disk of the primary galaxy Was 49a, at a projected radial distance of 8 kpc from the nucleus. Using X-ray data from Chandra, NuSTAR, and Swift, we find that this AGN has a bolometric luminosity of L_bol ~ 2 x 10^45 erg/s, with a black hole mass of M_BH=1.3^{+2.9}_{-0.9} x 10^8 M_Sol. Despite its large mass, our analysis of optical data from the Discovery Channel Telescope shows that the supermassive black hole is hosted by a stellar counterpart with a mass of only 5.6^{+4.9}_{-2.6} x 10^9 M_Sol, making the SMBH potentially larger than expected from SMBH-galaxy scaling relations, and the stellar counterpart exhibits a morphology that is consistent with dwarf elliptical galaxies. Our analysis of the system in the r and K bands indicates that Was 49 is a minor merger, with a mass ratio of Was 49a to Was 49b between 1:7 and 1:15. This is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers, and that minor mergers predominantly enhance AGN activity in the primary galaxy.Comment: 11 pages, 6 figures, accepted for publication in Ap

Zero cycles on the moduli space of curves

While the Chow groups of 0-dimensional cycles on the moduli spaces of Deligne-Mumford stable pointed curves can be very complicated, the span of the 0-dimensional tautological cycles is always of rank 1. The question of whether a given moduli point [C,p_1,...,p_n] determines a tautological 0-cycle is subtle. Our main results address the question for curves on rational and K3 surfaces. If C is a nonsingular curve on a nonsingular rational surface of positive degree with respect to the anticanonical class, we prove [C,p_1,...,p_n] is tautological if the number of markings does not exceed the virtual dimension in Gromov-Witten theory of the moduli space of stable maps. If C is a nonsingular curve on a K3 surface, we prove [C,p_1,...,p_n] is tautological if the number of markings does not exceed the genus of C and every marking is a Beauville-Voisin point. The latter result provides a connection between the rank 1 tautological 0-cycles on the moduli of curves and the rank 1 tautological 0-cycles on K3 surfaces. Several further results related to tautological 0-cycles on the moduli spaces of curves are proven. Many open questions concerning the moduli points of curves on other surfaces (Abelian, Enriques, general type) are discussed