R. Mewe

Line ratios for helium-like ions: Applications to collision-dominated plasmas ?

The line ratios R and G of the three main lines of He-like ions (triplet: resonance, intercombination, forbidden lines) are calculated for C v ,N vi ,O vii ,N eix ,M gxi, and Si xiii. These ratios can be used to derive electron density ne and temperatureTe of hot late-type stellar coronae and O, B stars from high-resolution spectra obtained with Chandra (LETGS, HETGS) and XMM-Newton (RGS). All excitation and radiative processes between the levels and the eect of upper-level cascades from collisional electronic excitation and from dielectronic and radiative recombination have been considered. When possible the best experimental values for radiative transition probabilities are used. For the higher-Z ions (i.e. Ne ix ,M gxi ,S ixiii) possible contributions from blended dielectronic satellite lines to each line of the triplets were included in the calculations of the line ratios R and G for four specic spectral resolutions: RGS, LETGS, HETGS-MEG, HETGS-HEG. The influence of an external stellar radiation eld on the coupling of the 2 3 S (upper level of the forbidden line) and 2 3 P levels (upper levels of the intercombination lines) is taken into account. This process is mainly important for the lower-Z ions (i.e. C v ,N vi ,O vii) at moderate radiation temperature (Trad). These improved calculations were done for plasmas in collisional ionization equilibrium, but will be later extended to photo-ionized plasmas and to transient ionization plasmas. The values for R and G are given in extensive tables, for a large range of parameters, which could be used directly to compare to the observations.

High resolution X-ray spectroscopy of zeta Puppis with the XMM-Newton reflection grating spectrometer

We present the rst high resolution X-ray spectrum of the bright O4Ief supergiant star Puppis, obtained with the Reflection Grating Spectrometer on-board XMM-Newton. The spectrum exhibits bright emission lines of hydrogen-like and helium-like ions of nitrogen, oxygen, neon, magnesium, and silicon, as well as neon-like ions of iron. The lines are all signicantly resolved, with characteristic velocity widths of order 1000{1500 km s 1 . The nitrogen lines are especially strong, and indicate that the shocked gas in the wind is mixed with CNO-burned material, as has been previously inferred for the atmosphere of this star from ultraviolet spectra. We nd that the forbidden to intercombination line ratios within the helium-like triplets are anomalously low for N VI, O VII, and Ne IX. While this is sometimes indicative of high electron density, we show that in this case, it is instead caused by the intense ultraviolet radiation eld of the star. We use this interpretation to derive constraints on the location of the X-ray emitting shocks within the wind that are consistent with current theoretical models for this system.

First Light Measurements of Capella with the Low-Energy Transmission Grating Spectrometer aboard the Chandra X-Ray Observatory

We present the first X-ray spectrum obtained by the Low-Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-Ray Observatory. The spectrum is of Capella and covers a wavelength range of 5-175 Å (2.5-0.07 keV). The measured wavelength resolution, which is in good agreement with ground calibration, is Deltalambda approximately 0.06 Å (FWHM). Although in-flight calibration of the LETGS is in progress, the high spectral resolution and unique wavelength coverage of the LETGS are well demonstrated by the results from Capella, a coronal source rich in spectral emission lines. While the primary purpose of this Letter is to demonstrate the spectroscopic potential of the LETGS, we also briefly present some preliminary astrophysical results. We discuss plasma parameters derived from line ratios in narrow spectral bands, such as the electron density diagnostics of the He-like triplets of carbon, nitrogen, and oxygen, as well as resonance scattering of the strong Fe xvii line at 15.014 Å.

First light measurements with the XMM-Newton reflection grating spectrometers: Evidence for an inverse first ionisation potential effect and anomalous Ne abundance in the Coronae of HR 1099

The RS CVn binary system HR 1099 was extensively observed by the XMM-Newton observatory in February 2000 as its first-light target. A total of 570 ks of exposure time was accumulated with the Reflection Grating Spectrometers (RGS). The integrated X-ray spectrum between 5-38A is of unprecedented quality and shows numerous features attributed to transitions of the elements C, N, O, Ne, Mg, Si, S, Fe. Ni, and probably others. We perform an in-depth study of the elemental composition of the average corona of this system, and find that the elemental abundances strongly depend on the first ionisation potential (FIP) of the elements. But different from the solar coronal case, we find an inverse FIP effect, i.e., the abundances (relative to oxygen) increase with increasing FIP. Possible scenarios, e.g., selective enrichment due to Ne-rich flare-like events, are discussed.

The XMM-Newton view of stellar coronae: X-ray spectroscopy of the corona of AB Doradus

M. A. acknowledges support from the Swiss National Science Foundation (grants 2100-049343 and 2000-058827), from the Swiss Academy of Sciences, and from the Swiss Commission for Space Research. H. M., K. B, and J. P. acknowledge financial support from PPARC. A. M. and R. P. acknowledge support from the Italian Space Agency. SRON is supported financially by NWO.

High-resolution X-ray spectroscopy of Procyon by Chandra and XMM-Newton

We report the analysis of the high-resolution soft X-ray spectrum of the nearby F-type star Procyon in the wavelength range from 5 to 175 A obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra and with the Reflection Grating Spectrometers (RGS) and the EPIC-MOS CCD spectrometers on board XMM-Newton. Line fluxes have been measured separately for the RGS and LETGS. Spectra have been tted globally to obtain self-consistent temperatures, emission measures, and abundances. The total volume emission measure is 4:1 10 50 cm 3 with a peak between 1 and 3 MK. No indications for a dominant hot component (T > 4 MK) were found. We present additional evidence for the lack of a solar-type FIP-eect, conrming earlier EUVE results.

A study of coronal abundances in RS CVn binaries

XMM-Newton has been performing comprehensive studies of X-ray luminous RS CVn binary systems in its calibration and guaranteed time programs. We present results from ongoing investigations in the context of a systematic study of coronal emission from RS CVns. We concentrate here on coronal abundances and investigate the abundance pattern in RS CVn binaries as a function of activity and average temperature. We find a transition from an Inverse First Ionization Potential (FIP) effect towards an absence of a clear trend (no FIP) in intermediately active RS CVn systems. This scheme fits well into the long-term evolution from an IFIP to a FIP effect found in solar analogs. We further study variations in the elemental abundances during a large flare.

Coronal activity in F-, G-, and K-type stars. III - The coronal differential emission measure distribution of Capella, Sigma-squared CrB, and Procyon

EXOSAT soft X-ray spectra of three binary systems of cool stars are analyzed: Capella (G6 III + F9 III), Sigma-squared CrB (F8 V + G1 V), and Procyon (F5 IV-V + DF). The EXOSAT transmission grating spectrometer permits the study of individual spectral lines and line complexes between 10 and 200 A with approximately 3 A resolution. First it is demonstrated that the spectra can be described reasonably well by a two-temperature model corona. Then the assumption that only two temperatures exist in the stellar coronas is relaxed and differential emission measure distributions are derived from the three spectra. The results from the multithermal modeling are consistent with those of the two-temperature models: emission from the coronas of each of the three stars is dominated by plasma in two relative narrow temperature intervals. These intervals are centered on 5 MK and 25 MK in the cases of Capella and Sigma-squared CrB, and 0.6 MK and 3 MK in the case of Procyon. The implications of the results for the structure of stellar coronas are briefly discussed. 41 refs.

Helium-like triplet density diagnostics - Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon

Electron density diagnostics based on the triplets of helium-like C v ,N vi ,a nd Ovii are applied to the X-ray spectra of Capella and Procyon measured with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f), intercombination (i), and the resonance (r) lines of the helium-like triplets are used. The (logarithmic) electron densities (in cgs units) derived from the f=i ratios for Capella are < 9:38 cm 3 for O vii (2 upper limit) (f=i =4 :0 0:25), 9:86 0:12 cm 3 for N vi (f=i =1 :78 0:25), and 9:42 0:21 cm 3 for C v (f=i =1 :48 0:34), while for Procyon we obtain 9:28 +0:4 9:28 cm 3 for O vii (f=i =3 :28 0:3), 9:96 0:23 cm 3 for N vi (f=i =1 :33 0:28), and < 8:92 cm 3 for C v (f=i =0 :48 0:12). These densities are quite typical of densities found in the solar active regions, and also pressures and temperatures in Procyons and Capellas corona at a level of T 10 6 K are quite similar. We nd no evidence for densities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little dierence in the physical properties of the layers producing the C v ,N vi ,a nd Ovii emission. Assuming the X-ray emitting plasma to be conned in magnetic loops, we obtain typical loop length scales of LCapella 8 LProcyon from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite dierent. The total mean surface fluxes emitted in the helium- and hydrogen-like ions are quite similar for Capella and Procyon, but exceed typical solar values by one order of magnitude. We thus conclude that Procyons and Capellas coronal lling factors are larger than corresponding solar values.

The XMM-Newton view of stellar coronae: Flare heating in the coronae of HR 1099

The RS CVn system HR 1099 was observed by XMM-Newton for a time span of 25 days during the commissioning phase. Rotational modulation in the RGS light curve has been detected with maximum flux when the active K1 IV star is in front. Following a quiescent phase of emission, the rise and peak of a large flare were observed. Time-dependent spectroscopy has been performed to derive elemental abundances and their variations during the various stages of the flare. Emission measure distributions have been reconstructed. Two different components are detected: a very hot plasma (up to 100 MK) that evolves rapidly, and a stable quiescent plasma. Low first-ionization-potential elemental (such as Fe and Si) abundances increase significantly during the flare, while the abundance of the high-FIP element Ne stays constant at the quiescent value. We report direct detection of a flux increase in the Fexxiv X-ray lines during a stellar flare.