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th1 Ori E is a young, moderate mass binary system, a rarely observed case of spectral-type G-giants of about 3 solar masses, which are still collapsing toward the main sequence, where they presumably become X-ray faint. We have obtained high-resolution X-ray spectra with Chandra and find that the system is very active and similar to coronal sources, having emission typical of magnetically confined plasma. It has a broad temperature distribution with a hot component and significant high energy continuum; narrow emission lines from H- and He-like ions, as well as a range of Fe ions, and relative luminosity, Lx /L bol = 10-3, at the saturation limit. Density, while poorly constrained, is consistent with the low density limits, our upper limits being ne < 1013 cm-3 for Mg XI and ne < 1012 cm-3 for Ne IX. Coronal elemental abundances are sub-solar, with Ne being the highest at about 0.4 times solar. We find a possible trend in Trapezium hot plasmas toward low relative abundances of Fe, O, and Ne, which is hard to explain in terms of the dust depletion scenarios of low-mass young stars. Variability was unusually low during our observations relative to other coronally active stars. Qualitatively, the emission is similar to post-main-sequence G-stars. Coronal structures could be compact or comparable to the dimensions of the stellar radii. From comparison to X-ray emission from similar mass stars at various evolutionary epochs, we conclude that the X-rays in th1 Ori E are generated by a convective dynamo, present during contraction, but which will vanish during the main-sequence epoch possibly to be resurrected during post-main-sequence evolution.

We present Chandra HETGS observations of the Herbig Ae star HD 104237 and the associated young stars comprising lower mass stars, in the 0.15-1.75 M mass range, in their pre-main-sequence phase. The brightest X-ray source in the association is the central system harboring the Herbig Ae primary and a K3 companion. Its X-ray variability indicates modulation possibly on timescales of the rotation period of the Herbig Ae star, and this would imply that the primary significantly contributes to the overall emission. The spectrum of the Herbig Ae+K3 system shows a soft component significantly more pronounced than in other K-type young stars. This soft emission is reminiscent of the unusually soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and therefore we tentatively attribute it to the Herbig Ae of the binary system. The HETGS spectrum shows strong emission lines corresponding to a wide range of plasma temperatures. The He-like triplet of Mg XI and Ne IX suggests the presence of plasma at densities of about 1012 cm[?]3, possibly indicating an accretion-related X-ray production mechanism. The analysis of the zeroth-order spectra of the other sources indicates X-ray emission characteristics typical of pre-main-sequence stars of similar spectral type, with the exception of the T Tauri HD 104237-D, whose extremely soft emission is very similar to the emission of the classical T Tauri star TW Hya and suggests X-ray production by shocked accreting plasma.

Massive stars rarely show intrinsic X-ray variability. One exception is th2 Ori A, which has shown strong variability over the last 5 years. We observed a large outburst of the X-ray source with the High Energy Transmission Grating Spectrometer on board Chandra and compare the emissivity and line properties in states of low and high flux. The low state indicates temperatures well above 25 MK. In the high state we find high emissivities in the range from 3 to over 100 MK. The outburst event in stellar terms is one of the most powerful ever observed and the most energetic one in the ONC, with a lower total energy limit of 1.5 x 1037 ergs. The line diagnostics indicate that the line-emitting regions in the low states are as close as within 1-2 stellar radii from the O star's photosphere, whereas the hard states suggest a distance of 3-5 stellar radii. We discuss the results in the context of stellar flares, magnetic confinement, and binary interactions. By matching the dates of all observations with the orbital phases of the spectroscopy binary orbit, we find that outbursts occur very close to the periastron passage of the stars. We argue that the high X-ray states are possibly the result of reconnection events from magnetic interactions of the primary and secondary stars of the spectroscopic binary. Effects from wind collisions seem unlikely for this system. The line properties in the low state seem consistent with some form of magnetic confinement. We also detect Fe fluorescence indicative of the existence of substantial amounts of neutral Fe in the vicinity of the X-ray emission.

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (ne < 2 x 1010 cm-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

In mid-September of 1999, a multiwavelength campaign was carried out on the coronally active RS Canum Venaticorum binary HR 1099 (K1 IV + G5 V; P= 2.84 days), during commissioning of the High Energy Transmission Grating Spectrometer (HETGS) of the Chandra X-ray Observatory(CXO). The coordinated program involved the Extreme Ultraviolet Explorer(EUVE), the Hubble Space TelescopeImaging Spectrograph (STIS), and the Very Large Array (VLA). The objective was to study the system in quiescence, across as much of the electromagnetic spectrum as practical, and to catch any flares that might occur.