Photospheric and coronal abundances in solar-type stars: the peculiar case ofτ Bootis

Abstract

Aims. Chemical abundances in solar-type stars are still a much debated topic in many respects. In particular, planet-hosting stars are known to be metal-rich, but whether or not this peculiarity applies also to the chemical composition of the outer stellar atmospheres is still to be clarified. More in general, coronal and photospheric abundances in late-type stars appear to be different in many cases, but understanding how chemical stratification effects work in stellar atmospheres requires an observational base larger than currently available. Methods. We obtained XMM-Newton high-resolution X-ray spectra of τ Bootis, a well known nearby star with a Jovian-mass close-in planet. We analyzed these data with the aim to perform a detailed line-based emission measure analysis and derive the abundances of individual elements in the corona with two different methods applied independently. We compared the coronal abundances of τ Bootis with published photospheric abundances based on high-resolution optical spectra and with those of other late-type stars with different magnetic activity levels, including the Sun. Results. We find that the two methods provide consistent results within the statistical uncertainties for both the emission measure distribution of the hot plasma and for the coronal abundances, with discrepancies at the 2σ level limited to the amount of plasma at temperatures of 3-4 MK and to the O and Ni abundances. In both cases, the elements for which both coronal and photospheric measurements are available (C, N, O, Si, Fe, and Ni) result systematically less abundant in the corona by a factor 3 or more, with the exception of the coronal Ni abundance which is similar to the photospheric value. Comparison with other late-type stars of similar activity level shows that these coronal/photospheric abundance ratios are peculiar to τ Bootis and possibly related to the characteristic over-metallicity of this planet-hosting star.

The stellar wind cycles and planetary radio emission of the τ Boo system

“…While we have modelled the stellar wind that flows along the large‐scale coronal magnetic field lines, the environment surrounding a star is likely to be much more dynamic, especially for a star such as τ Boo that is more active than the Sun (Maggio et al 2011). We note that the radio flux estimated here is expected to increase if the planet is hit by powerful ejections of coronal material, e.g.…”

A Chandra/LETGS Survey of Main-sequence Stars

“…For τ Boo A and π 1 UMa the solution is to simply use published [Fe/H] measurements from XMM (Telleschi et al 2005;Maggio et al 2011). Observations from XMM are well suited for continuum measurement, with the high sensitivity of RGS and the possibility of considering spectra from both RGS and from the EPIC instrument.…”

Coronae of stars with supersolar elemental abundances

“…The coronal abundances of τ Boo and α Cen A, B were extensively analyzed by Maggio et al (2011) and by Raassen et al (2003) respectively. Maggio et al (2011) found a flat FIP dependence, while Raassen et al (2003) measured a solar-like FIP behavior.…”

“…Maggio et al (2011) analyzed the FIP behavior of this object. No clear FIP (or IFIP) trend was observed.…”

“…The observations of 11 LMi, τ Boo, α Cen A, and B have sufficiently long exposures that result in high-quality grating spectra (see Maggio et al 2011;Drake et al 2010;Raassen et al 2003, for details), making for reliable abundance measurements. On the other hand, ι Hor and HR 7291 were both observed intermittently during long observation sequences.…”