Flare X-ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor obtained with the XMM-Newton satellite are analyzed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyze flare and quiescent spectra, with emphasis on the Fe XVII 15 - 17 angstrom wavelength region. The Fe XVII 16.78 angstrom/ 15.01 angstrom line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 angstrom line photons from the line of sight. The escape probability technique indicates an optical depth of approximate to 0.4 for the 15.01 angstrom line. During the flare, the electron density is 4.4(-1.6)(+2.7) x 10(10) cm(-3), and the fractional Fe abundance is 0.5 +/- 0.1 of the solar photospheric value Using these parameters, a path length of approximate to 8000 km is derived. There is no evidence of opacity in the quiescent X-ray spectrum of the star.

RHESSI and SOHO CDS Observations of Explosive Chromospheric Evaporation

Simultaneous observations of explosive chromospheric evaporation are presented using data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory. For the first time, cospatial imaging and spectroscopy have been used to observe explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images and spectra were used to determine the flux of nonthermal electrons accelerated during the impulsive phase of an M2.2 flare. When we assumed a thick-target model, the injected electron spectrum was found to have a spectral index of similar to 7.3, a low-energy cutoff of similar to 20 keV, and a resulting flux of >= 4 x10(10) ergs cm(-2) s(-1). The dynamic response of the atmosphere was determined using CDS spectra; we found a mean upflow velocity of 230 +/- 38 km s(-1) in Fe (XIX) (592.23 angstrom) and associated downflows of 36 +/- 16 and 43 +/- 22 km s(-1) at chromospheric and transition region temperatures, respectively, relative to an averaged quiet- Sun spectra. The errors represent a 1 j dispersion. The properties of the accelerated electron spectrum and the corresponding evaporative velocities were found to be consistent with the predictions of theory.

Eclipse observations of high-frequency oscillations in active region coronal loops

One of the mechanisms proposed for heating the corona above solar active regions is the damping of magnetohydrodynamic (MHD) waves. Continuing on previous work, we provide observational evidence for the existence of high-frequency MHD waves in coronal loops observed during the August 1999 total solar eclipse. A wavelet analysis is used to identify twenty 4 x 4 arcsec(2) areas showing intensity oscillations. All detections lie in the frequency range 0.15 - 0.25 Hz (7 - 4 s), last for at least 3 periods at a confidence level of more than 99% and arise just outside known coronal loops. This leads us to suggest that they occur in low emission-measure or different temperature loops associated with the active region.

A comparison of theoretical line intensity ratios for Ni XII with extreme ultraviolet observations from the JET tokamak

Recent R-matrix calculations of electron impact excitation rates in Ni XII are used to derive the emission line ratios R-1 = I(154.17 Angstrom)/I(152.15 Angstrom), R-2 = I(152.95 Angstrom)/I(152.15 Angstrom) and R-3 = 1(160.55 Angstrom)/I(152.15 Angstrom). This is the first time (to our knowledge) that theoretical emission line ratios have been calculated for this ion. The ratios are found to be insensitive to changes in the adopted electron density (N-e) when N-e greater than or equal to 5 x 10(11) cm(-3), typical of laboratory plasmas. However, they do vary with electron temperature (T-e), with for example R-1 and R-3 changing by factors of 1.3 and 1.8, respectively, between T-e = 10(5) and 10(6) K. A comparison of the theoretical line ratios with measurements from the Joint European Tents (JET) tokamak reveals very good agreement between theory and observation for R-1, with an average discrepancy of only 7%. Agreement between the calculated and experimental ratios for R-2 and R-3 is less satisfactory, with average differences of 30 and 33%, respectively. These probably arise from errors in the JET instrument calibration curve. However, the discrepancies are smaller than the uncertainties in the R-2 and R-3 measurements. Our results, in particular for R-1, provide experimental support for the accuracy of the Ni XIII line ratio calculations, and hence for the atomic data adopted in their derivation.

A comparison of theoretical Si VIII emission line ratios with observations from SERTS

Recent R-matrix calculations of electron impact excitation rates in N-like Si VIII are used to derive theoretical emission line intensity ratios involving 2s(2)2p(3)-2s2p(4) transitions in the 216 -320 Angstrom wavelength range. A comparison of these with an extensive dataset of solar active region, quiet- Sun, sub-flare and off-limb observations, obtained during rocket flights of the Solar EUV Research Telescope and Spectrograph (SERTS), indicates that the ratio R-1 = I(216.94 Angstrom)/I(319.84 Angstrom) may provide a usable electron density diagnostic for coronal plasmas. The ratio involves two lines of comparable intensity, and varies by a factor of about 5 over the useful density range of 10(8)-10(11) cm(-3). However R-2 = I(276.85 Angstrom)/I(319.84 Angstrom) and R-3 = I(277.05 Angstrom)/I(319.84 Angstrom) show very poor agreement between theory and observation, due to the severe blending of the 276.85 and 277.05 Angstrom lines with Si VII and Mg VII transitions, respectively, making the ratios unsuitable as density diagnostics. The 314.35 Angstrom feature of Si VIII also appears to be blended, with the other species contributing around 20% to the total line flux.

A comparison of theoretical MgVI emission line strengths with active-region observations from serts

R-matrix calculations of electron impact excitation rates in N-like Mg vi are used to derive theoretical electron-density-sensitive emission line ratios involving 2s(2)2p(3)-2s2p(4) transitions in the 269-403 Angstrom wavelength range. A comparison of these with observations of a solar active region, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals good agreement between theory and observation for the 2s(2)2p(3) S-4-2s2p(4) P-4 transitions at 399.28, 400.67, and 403.30 Angstrom, and the 2s(2)2p(3) P-2-2s2p(4) D-2 lines at 387.77 and 387.97 Angstrom. However, intensities for the other lines attributed to Mg vi in this spectrum by various authors do not match the present theoretical predictions. We argue that these discrepancies are not due to errors in the adopted atomic data, as previously suggested, but rather to observational uncertainties or mis-identifications. Some of the features previously identified as Mg vi lines in the SERTS spectrum, such as 291.36 and 293.15 Angstrom, are judged to be noise, while others (including 349.16 Angstrom) appear to be blended.

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2-41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST/STIS spectra of a high- velocity interstellar cloud ((LSR)-L-upsilon = + 80 kms(-1)) towards DI1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20-45 and a spectral resolution of about 6.5 km s(-1) (FWHM), The high-velocity cloud absorption is observed in the lines of C II, O I, Si II, Si III, Si IV and S III. Limits can be placed on the amount of S II and Fe II absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C II and O I, suggests that this high-velocity gas is warm (T-k similar to 10(3)-10(4) K) and predominantly ionized, This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si IV, This sightline also intercepts two other high-velocity clouds that produce weak absorption features at (LSR)-L-upsilon = + 113 and + 130kms(-1) in the STIS spectra.

Observations of H alpha intensity oscillations in a flare ribbon

High-cadence Halpha blue wing observations of a C9.6 solar flare obtained at Big Bear Solar Observatory using the Rapid Dual Imager are presented. Wavelet and time-distance methods were used to study oscillatory power along the ribbon, finding periods of 40 - 80 s during the impulsive phase of the flare. A parametric study found statistically significant intensity oscillations with amplitudes of 3% of the peak flare amplitude, periods of 69 s (14.5 mHz) and oscillation decay times of 500 s. These measured properties are consistent with the existence of flare-induced acoustic waves within the overlying loops.

Multiwavelength observations of the M15 intermediate-velocity cloud

We present Westerbork Synthesis Radio Telescope HI images, Lovell telescope multibeam H I wide-field mapping, William Herschel Telescope long-slit echelle Ca II observations, Wisconsin Halpha Mapper (WHAM) facility images, and IRAS ISSA 60- and 100-mum co-added images towards the intermediate- velocity cloud (IVC) at + 70 km s(-1), located in the general direction of the M15 globular cluster. When combined with previously published Arecibo data, the H I gas in the IVC is found to be clumpy, with a peak H I column density of similar to1.5 x 10(20) cm(-2), inferred volume density (assuming spherical symmetry) of similar to24 cm(-3)/D (kpc) and a maximum brightness temperature at a resolution of 81 x 14 arcsec(2) of 14 K. The major axis of this part of the IVC lies approximately parallel to the Galactic plane, as does the low- velocity H I gas and IRAS emission. The H I gas in the cloud is warm, with a minimum value of the full width at half-maximum velocity width of 5 km s(-1) corresponding to a kinetic temperature, in the absence of turbulence, of similar to540 K. From the H I data, there are indications of two-component velocity structure. Similarly, the Ca II spectra, of resolution 7 km s(-1), also show tentative evidence of velocity structure, perhaps indicative of cloudlets. Assuming that there are no unresolved narrow-velocity components, the mean values of log(10)[N(Ca II K) cm(2)] similar to 12.0 and Ca II/H I similar to2 5 x 10(-8) are typical of observations of high Galactic latitude clouds. This compares with a value of Ca II/H I>10(-6) for IVC absorption towards HD 203664, a halo star of distance 3 kpc, some 3.degrees1 from the main M15 IVC condensation. The main IVC condensation is detected by WHAM in Halpha with central local-standard-of-rest velocities of similar to60-70 km s(-1), and intensities uncorrected for Galactic extinction of up to 1.3 R, indicating that the gas is partially ionized. The FWHM values of the Halpha IVC component, at a resolution of 1degrees, exceed 30 km s(-1). This is some 10 km s(-1) larger than the corresponding H I value at a similar resolution, and indicates that the two components may not be mixed. However, the spatial and velocity coincidence of the Halpha and H I peaks in emission towards the main IVC component is qualitatively good. If the Halpha emission is caused solely by photoionization, the Lyman continuum flux towards the main IVC condensation is similar to2.7 x 10(6) photon cm(-2) s(-1). There is not a corresponding IVC Halpha detection towards the halo star HD 203664 at velocities exceeding similar to60 km s(- 1). Finally, both the 60- and 100-mum IRAS images show spatial coincidence, over a 0.675 x 0 625 deg(2) field, with both low- and intermediate-velocity H I gas (previously observed with the Arecibo telescope), indicating that the IVC may contain dust. Both the Halpha and tentative IRAS detections discriminate this IVC from high-velocity clouds, although the H I properties do not. When combined with the H I and optical results, these data point to a Galactic origin for at least parts of this IVC.

CO and optical observations of the Magellanic Bridge

We present (CO)-C-12(1-0) molecular line and BV CCD observations towards 0311-7651: a region in the Magellanic Bridge where cold atomic Hydrogen has previously been detected by Kobulnicky & Dickey: Additionally, BV images of a comparison held 1 degrees to the South were taken. No CO was detected to a limit of similar to 0.06 Kelvin, and the colour-magnitude diagrams show no evidence for a stellar association in either field.

Ca II K observations of QSOs in the line-of-sight to the Magellanic Bridge

We describe medium-resolution spectroscopic observations taken with the ESO Multi-Mode Instrument (EMMI) in the CaII K line (lambda air = 3933.661 angstrom) towards 7 QSOs located in the line-of-sight to the Magellanic Bridge. At a spectral resolution R =lambda/Delta lambda = 6000, five of the sightlines have a signal-to-noise ( S/N) ratio of similar to 20 or higher. Definite Ca absorption due to Bridge material is detected towards 3 objects, with probable detection towards two other sightlines. Gas-phase CaII K Bridge and Milky Way abundances or lower limits for the all sightlines are estimated by the use of Parkes 21-cm H. emission line data. These data only have a spatial resolution of 14 arcmin compared with the optical observations which have milli-arcsecond resolution. With this caveat, for the three objects with sound CaII K detections, we find that the ionic abundance of CaII K relative to HI, A = log( N( CaK)/ N( HI)) for low- velocity Galactic gas ranges from - 8.3 to - 8.8 dex, with HI column densities varying from 3- 6 x 10(20) cm(-2). For Magellanic Bridge gas, the values of A are similar to 0.5 dex higher, ranging from similar to- 7.8 to - 8.2 dex, with N( HI) = 1- 5 x 1020 cm(-2). Higher values of A correspond to lower values of N( HI), although numbers are small. For the sightline towards B 0251 - 675, the Bridge gas has two different velocities, and in only one of these is CaII tentatively detected, perhaps indicating gas of a different origin or present-day characteristics ( such as dust content), although this conclusion is uncertain and there is the possibility that one of the components could be related to the Magellanic Stream. Higher signal-to-noise CaII K data and higher resolution H. data are required to determine whether A changes with N( HI) over the Bridge and if the implied difference in the metalicity of the two Bridge components towards B 0251-675 is real.

HI observations of the high-velocity cloud in the direction of M 92

We present wide-field neutral hydrogen (H I) Lovell telescope multibeam, and Dominion Radio Astrophysical Observatory Hi synthesis observations, of the high velocity cloud (HVC) located in the general direction of the globular cluster M92. This cloud is part of the larger Complex C and lies at velocities between similar to -80 and -130 km s(-1) in the Local Standard of Rest. The Lovell telescope observations, of resolution 12 arcmin spatially and 3.0 km s(-1) in velocity, fully sampling a 3.1 degrees x 12.6 degrees RA-Dec grid, have found that this part of HVC Complex C comprises two main condensations, lying approximately north-south in declination, separated by similar to2 degrees and being parallel to the Galactic plane. At this resolution, peak values of the brightness temperature and Hi column density of similar to1.4 K and similar to5 x 10(19) cm(-2) are determined, with relatively high values of the full width half maximum velocity (FWHM) of similar to 22 km s(-1) being observed, equivalent to a gas kinetic temperature, in the absence of turbulence and geometric effects of similar to 10 000 K. Each of these properties, as well as the sizes of the clouds, are similar in the two components. The DRAO observations, towards the Northern HVC condensation, are the first high-resolution Hi spectra of Complex C. When smoothed to a resolution of 3 arcmin, they identify several Hi intensity peaks with column densities in the range 4-7 x 10(19) cm(-2). Further smoothing of these data to 6 arcmin resolution tentatively indicates that parts of the HVC consist of two velocity components, of similar brightness temperature, separated by similar to7 km s(-1) in velocity, and with FWHM velocity widths of similar to5-7 km s(-1). No IRAS 60 or 100 micron flux is associated with the M92 HVC. Cloud properties are briefly discussed and compared to previous observations of HVCs.

CaIIKinterstellar observations towards early-type disc and halo stars, abundances and distances of intermediate- and high-velocity clouds

We present Ca II K (lambda(air) = 3933.661 angstrom) interstellar observations towards 20 early-type stars, to place lower distance limits to intermediate- and high-velocity clouds (IHVCs) in their lines of sight. The spectra are also employed to estimate the Ca abundance in the low-velocity gas towards these objects, when combined with Leiden-Dwingeloo 21-cm HI survey data of spatial resolution 0 degrees.5. Nine of the stars, which lie towards IHVC complexes H, K and gp, were observed with the intermediate dispersion spectrograph on the Isaac Newton Telescope at a resolution R = lambda/Delta lambda of 9000 (similar to 33 km s(-1)) and signal-to-noise ratio (S/N) per pixel of 75-140. A further nine objects were observed with the Utrecht Echelle Spectrograph on the William Herschel Telescope at R = 40 000 (similar to 7.5 km s(-1)) and S/N per pixel of 10-25. Finally, two objects were observed in both Ca II K and Na I D lines using the 2D COUDE on the McDonald 2.7-m telescope at R = 35 000 (similar to 8.5 km s(-1)). The abundance of Ca II K {log(10)(A) = log(10)[N(Ca II K)]-log(10)[N(HI)]} plotted against HI column density for the objects in the current sample with heights above the Galactic plane (z) exceeding 1000 pc is found to obey the Wakker & Mathis (2000) relation. Also, the reduced column density of Ca II K as function of z is consistent with the larger sample taken from Smoker et al. (2003). Higher S/N observations than those previously taken towards HVC complex H stars HD 13256 and HILT 190 reinforce the assertion that this lies at a distance exceeding 4000 pc. No obvious absorption is detected in observations of ALS 10407 and HD 357657 towards IVC complex gp. The latter star has a spectroscopically estimated distance of similar to 2040 pc, although this was derived assuming the star lies on the main sequence and without any reddening correction being applied. Finally, no Ca II K absorption is detected towards two stars along the line of sight to complex K, namely PG 1610+529 and PG 1710+490. The latter is at a distance of similar to 700 pc, hence placing a lower distance limit to this complex, where previously only an upper distance limit of 6800 pc was available.