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Дата изменения: Fri May 5 20:20:19 2000
Дата индексирования: Tue Oct 2 04:04:30 2012
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Поисковые слова: virgo cluster

J.G. Doyle, Research Astronomer

The major research effort in 1999 was again directed towards Solar Physics and in particular towards SOHO activities, although some effort was directed towards the cool star area.

Solar Physics

This work can be broadly divided into four areas, each of which is briefly discussed below. In addition to Gerry Doyle, those involved include Luca Teriaca, Ilía Roussev, Elena Pérez, and Dipankar Banerjee.

Coronal Holes

SUMER/SOHO and CDS/SOHO data were obtained for spectral lines from two ions formed at high temperatures, SiVIII and SiIX. Lines from these ions provide both an estimate of the local electron density and micro-turbulent plasma velocity. From the combined dataset we find a radial dependence of the electron density, varying in the range 1-2$R_{\odot}$ as r^-8, from 2-4$R_{\odot}$ as r^-4 and then as r^-2. By $8R_{\odot}$, the electron density has fallen to $\sim4 \times 10^{3}$cm^-3, from 1.5 x 10⁸ cm^-3 at $1.0R_{\odot}$. Combining the SiVIII half-width at 1/e of the peak intensity with the UVCS/SOHO OVI half-width, we find a small increase of the half-width from 1.0-1.2$R_{\odot}$, then a plateau until 1.5$R_{\odot}$, thereafter a sharp increase until 2$R_{\odot}$, and finally a more gradual increase reaching 550km s^-1 at $3.5R_{\odot}$ . Our data suggest that the magnetohydrodynamic (MHD) waves responsible for the excess line broadening tend to become non-linear as they reach 1.2$R_{\odot}$.

Chromospheric Oscillations

We examine spectral time-series of two lower-chromospheric lines observed with SUMER/SOHO (NI and CII). Intensity power spectra of CII are affected at higher frequencies by streams of emitting structures. Using contrast-enhanced time slices we show that (1) there exists a grain-like pattern which is found in both network and inter-network regions; (2) streams of supersonically moving structures probably outline a wave interference pattern; and (3) the sizes of structures observed in NI are smaller than when observed in CII. At various points our findings disagree with earlier results.

Explosive Events

Two examples of explosive events observed with SUMER/SOHO in transition region spectral lines are reported here; one detected in CIV 1548Å, in a region within the northern polar coronal hole, and the other in OVI 1032Å, in an active region. The event measured in CIV lasted approximately 3 minutes, with velocities reaching around 150km s^-1 in the blue wing and 100km s^-1 in the red wing. The active region events were more energetic and in total lasted $\sim$6m. More precisely, we have at least two consecutive events occurring in a short time interval ($\sim$12m) separated by $\sim$3 arc sec. The explosive events seen in OVI showed a very complex structure of subsonic and supersonic velocity flows, both red-shifted and blue-shifted. The apparent maximum velocity reached in the blue wing was approximately 250km s^-1 and 215km s^-1 in the red wing.

There appears to be evidence of super-granular cells, with the increase in electron density occurring along the network boundaries. At some locations, periodicities of between 8 and 16 minutes are visible in the electron density variations.

In order to learn more about these explosive events, simulations were made assuming semi-circular magnetic flux tubes in a hydrodynamic code. The temporal evolution of the thermodynamic state of the loop was converted into CIV 1548Å line profiles. Departures from ionization equilibrium were assessed for the first time under conditions such as those encountered in explosive events. Work is well advanced on developing a 2-D MHD code.

Doppler Line Shifts

The ultraviolet spectral lines formed at transition region temperatures in the solar atmosphere, show a prevailing line-shift. The velocities increase from a red-shift of $\sim$0km s^-1 at $\sim$20000K, to 10km s^-1 at 1.9 x 10⁵K for the quiet Sun, and to $\sim$15km s^-1 at 1.0 x 10⁵K for the active region. At higher temperature an opposite behaviour is observed. In the quiet Sun a blue-shift of $\sim$2km s^-1 is observed at the NeVIII formation temperature ( 6 x 10⁵K), while in the active region a blue-shifted value around $\sim$8km s^-1 is observed for the same spectral line. With these data we explore the idea that the occurrence of nano-flares in the magnetic loop around the OVI formation temperature could explain the observed red-shift of mid-to-low transition region lines as well as the blue-shift observed in low coronal lines. Performing an integration over the entire period of simulations, a red-shift of $\sim$6km s^-1 is found in CIV, while a blue-shift of $\sim$2km s^-1 and $\sim$10km s^-1 were derived for OVI and NeVIII, respectively, in reasonable agreement with observations.

Cool Stars

These results are divided into three principal areas, as discussed below, and include work by Ferhat Ozeren and Darko Jevremovic.

Algol-Type Binaries

We present radio interferometric observations of the Algol-type binary system V505 Sagitarii made with the ATNF Compact Array at 6cm and 3.6cm over one orbital cycle (1.18 days). The radio flux level shows a clear modulation with evidence of eclipses of the emission region at both conjunctions of the binary, which may indicate the existence of an intra-binary region of activity. This has important consequences for the details of coronal formation and field interaction in active close binary stars.

CVn-Type Binaries

We investigate the extent to which the Wilson-Bappu relationship holds for chromospherically active binaries using the MgII h and k lines of 41 RS CVn stars observed with the International Ultraviolet Explorer satellite (IUE). The resulting fits are different from the relationships obtained for single, less active stars. Within a particular luminosity class the relationship is good, but it tends to break down when we incorporate objects ranging in luminosity from Class I to V. From model calculations there is very little dependence of the MgII line-width on effective temperature. The line-width does however depend on the column mass at the transition region boundary showing increased line-width at lower column mass. There is also a dependence on the column mass adopted for the temperature minimum, although the major and dominant parameter is the surface gravity, g, scaling as g^-1/4. Within a given luminosity class more active objects will show larger line-widths, reflecting a higher column mass deeper in the atmosphere, e.g. at the temperature minimum level.

Carbon and Oxygen-Rich Systems

We have developed a new method to determine the physical properties and the local circumstances of dust shells surrounding Carbon and Oxygen-rich stars for a given pulsation phase. The observed mid-infrared dust emission feature(s), in conjunction with both near-infrared and far-infrared photometry, are modelled from radiative transport calculations through the dust shell using a grid of detailed synthetic model input spectra.