The Coronae of AR Lacertae

Abstract

We observed the coronally active eclipsing binary AR Lac with the High Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV spectra and photometry were also obtained with the Extreme Ultraviolet Explorer. Significant variability in both X-ray and EUV fluxes were observed, dominated by at least one X-ray flare and one EUV flare. We saw no evidence of primary or secondary eclipses, but exposures at these phases were short and intrinsic variability compromised detection of any geometric modulation. X-ray flux modulation was largest at high temperature, indicative of flare heating of coronal plasma rather than changes in emitting volume or global emission measure. Analysis of spectral line widths interpreted in terms of Doppler broadening suggests that both binary stellar components are active. On the basis of line fluxes obtained from total integrated spectra, we have modeled the emission measure and abundance distributions. The EUV spectral line fluxes were particularly useful for constraining the parameters of the '' cool '' ( 2 Â 10 6 K) plasma. A strong maximum was found in the differential emission measure, characterized by two apparent peaks at log T ¼ 6:9 and 7.4, together with a weak but significant cooler maximum near log T ¼ 6:2 and a moderately strong hot tail from log T ¼ 7:6 8:2. Coronal abundances have a broad distribution and show no simple correlation with first ionization potential. While the resulting model spectrum generally agrees very well with the observed spectrum, there are some significant discrepancies, especially among the many Fe L lines. Both the emission measure and abundance distributions are qualitatively similar to prior determinations from other X-ray and ultraviolet spectra, indicating some long-term stability in the overall coronal structure.

X‐Ray Modeling of Very Young Early‐Type Stars in the Orion Trapezium: Signatures of Magnetically Confined Plasmas and Evolutionary Implications

“…The results also confirm a similar behavior for  1 Ori A and E. This means the bulk of the X-ray emission in the bright Orion Trapezium stars is not compatible with any form of instabilities in a line-driven wind. In fact, these stars seem to be of a hybrid nature where only a small fraction of the X-rays are produced in wind shocks; a larger fraction shows a magnetic origin and bears striking similarities to the hard X-ray emission pattern observed in stars with active coronae (Huenemoerder et al , 2003.…”

“…This may not be necessary. As an interesting analogy, the DEMs recently deduced from X-ray emission form active coronae in cool stars such as AR Lac (Huenemoerder et al 2003) and II Peg ) appear very similar during flares, where in addition to a small low-temperature peak one or more strong high-temperature peaks evolve. The nature of the spectra in these cases are of striking similarity to the ones that we observe in the Orion Trapezium with strong continua and unresolved lines.…”

The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin‐up

“…This conclusion is reinforced by the much more favorable comparison of the results of the recent Chandra and XMMNewton studies of AB Dor-this work compared with that of Güdel et al (2001) and Sanz-Forcada et al (2003). Audard et al (2004), Huenemoerder et al (2003), and SanzForcada et al (2003 have recently reported, for V824 Ara, AR Lac, and AB Dor, respectively, higher values for abundances of Al, Ca, and Na than for elements with slightly higher FIPs, suggesting a schism in the strictly inverse FIP effect scheme. In particular, Sanz-Forcada et al (2003) obtained different values of Ca abundance for AB Dor, including a very high Mewe et al (1996); (2) value of ½Ca=H ¼ 0:62 derived from the high-resolution RGS spectrum.…”

A 33 yr CONSTANCY OF THE X-RAY CORONAE OF AR Lac AND ECLIPSE DIAGNOSIS OF SCALE HEIGHT

“…Griffiths & Jordan (1998) estimated a coronal plasma density of 5 × 10 11 cm −3 based on Fe XXI lines. Huenemoerder et al (2003) also found tentative evidence for high plasma densities on the order of log n e ∼ 11 cm −3 using lines from the He-like ions of O and Ne formed around 2-4 × 10 6 K. This result was confirmed by Testa et al (2004a), who found log n e = 12.5 ± 0.5 cm −3 from He-like Mg formed at slightly hotter temperatures of ∼ 6×10 6 K (see also Ness et al 2004). Testa et al (2004a) also analysed the Chandra spectra of several other active binaries and found similar high densities to generally characterise the coronae of very active stars.…”