Scott Lewis Cully

A coronal mass ejection model for the 1992 July 15 flare on AU Microscopii observed by the extreme ultraviolet explorer

The dM1e flare star AU Microscopii (AU Mic) was observed by the EUVE Deep Survey Instrument on 1992 July 14-18. A large flare was detected in the Deep Survey Lexan/Boron (DS Lex/B)(65-190 A) band and the SW (70-190 A) and MW (140-380 A) spectrometers. The flare consisted of a sharp impulsive peak lasting approximately 2 hours followed by a decaying tail lasting about a day. We present a simple, single temperature, dynamic model for the flare decay which is consistent with the DS Lex/B light curve and reproduces the strongest, high-temperature spectral lines in the released EUVE spectra. In this model, we assume the long decay time is due to an ejected, magnetically confined, low beta plasmoid expanding self similarly in the ambient medium in a manner reminiscent of solar coronal mass ejections. We demonstrate that the long tail of the DS Lex/B light curve can be explained by rapid expansion, causing the plasma to become tenuous sufficiently quickly that it avoids catastrophic radiative cooling. From this model, we estimate the mass of the plasmoid to be approximately = 10(exp 20) g and the total energy of the event to be approximately = 10(exp 36) ergs. These values are approximately 10(exp 4) times as large as those seen during the largest solar coronal mass ejection (CME) events. We argue that the results of our model are consistent with other measurements of stellar flare parameters. We also estimate a mass-loss rate of a few times 10(exp -13) Solar mass/yr and discuss the role of mass loss from dMe stars in the mass balance of the interstellar medium. We estimate the rotational braking timescale from these events to be less than 500 million years and suggest that CMEs may be an important source of angular momentum loss from late-type stars.

Extreme Ultraviolet Explorer deep survey observations of a large flare on AU Microscopii

We have made the first extended observation of a stellar flare in the EUV with 100 s time resolution. The flare was detected on AU Mic by the Extreme Ultraviolet Explorer satellite at 12:38 UT on 1992 July 15 during a 4 d observation from 1992 July 14 to 18. This was a large flare detected in the Lexan/boron (65-190 A) band with an observed peak count rate of 7.0 +/- 0.5 counts/s, corresponding to a peak luminosity of 10 exp 30 erg/s in the Lexan/boron bandpass. This is significantly above the measured quiescent level of 0.4 +/- 0.2 counts/s. The flare consisted of a sharp peak lasting about 2 hr, followed by a decaying tail that lasted more than a day. The total EUV energy of the event is estimated to be 3 x 10 exp 34 ergs. A second, smaller flare was also observed and is described. We conclude that the large emission measures on order of 6 x 10 exp 53/cu cm are due to large volumes with characteristic length scales of order the stellar radius. We compare these EUV observations with stellar flare observations in other bandpasses and estimate the likelihood of seeing similar flares in future observations.

Highly absorptive coating for the vacuum ultraviolet range

We have developed new, highly absorptive coatings for the vacuum UV wavelength range. These coatings display two distinct granularity scales: large structures of a 10-100-microm scale form efficient light traps, upon which are superimposed structures of a submicrometer scale. We present results for the total hemispherical reflectivity at normal incidence for 121.6 nm and at a grazing angle incidence for 17.1, 30.4, 58.4, and 121.6 nm. These measurements were made for the new coatings as well as for various coatings in common use. Absorption of the new coatings is in some cases higher than for the best-known coatings and, in contrast to the latter, they are mechanically robust.

Highly curved microchannel plates

Several spherically curved microchannel plate (MCP) stack configurations were studied as part of an ongoing astrophysical detector development program, and as part of the development of the ALEXIS satellite payload. MCP pairs with surface radii of curvature as small as 7 cm, and diameters up to 46 mm have been evaluated. The experiments show that the gain (greater than 1.5 x 10 exp 7) and background characteristics (about 0.5 events/sq cm per sec) of highly curved MCP stacks are in general equivalent to the performance achieved with flat MCP stacks of similar configuration. However, gain variations across the curved MCPs due to variations in the channel length to diameter ratio are observed. The overall pulse height distribution of a highly curved surface MCP stack (greater than 50 percent FWHM) is thus broader than its flat counterpart (less than 30 percent). Preconditioning of curved MCP stacks gives comparable results to flat MCP stacks, but it also decreases the overall gain variations. Flat fields of curved MCP stacks have the same general characteristics as flat MCP stacks.

Soft x-ray to FUV measurements of the grazing incidence BRDF for a selection of low-reflectance surfaces

On future astronomical instruments for the soft x-ray to FUV, stray light may be a significant cause of background events. Currently, we are engaged in an ongoing program to identify materials that are suitable for use as low- reflectance surfaces in space based instruments. As a result, we have measured the scattering performance in this spectral region, of wide a selection of low-reflectivity materials, produced with a range of processes. We present preliminary measurements of the absolute bidirectional reflectance distribution function (BRDF) for a selection of seven of these materials. Measurements were obtained at a five spectral lines, including strong geocoronal lines, over the wavelength range 44 to 1216 angstrom at near grazing incidence. We find that in most cases for constant incident and scatter angles, the total variation of BRDF with wavelength over this range is only a factor of order ten. We also find that although we have identified materials which in many instances have lower reflectances than bead blasted aluminum, it is still a good choice for most applications given its low cost and convenience.