EPIDEMIOLOGÍA Y GEOGRAFÍA

troscopy. Early observations of HR 1099 and AB Dor with the XMM-Newton Reflection Grating Spectrometer uncovered a new, systematic FIP-related bias in magnetically ac- tive stars: in contrast to the solar case, low-FIP abundances are systematically depleted with respect to high-FIP elements (Brinkman et al. 2001; Güdel et al. 2001b; Audard et al. 2001a; Fig. 35), a trend that has been coined the “inverse FIP effect” (IFIP). As a consequence of this anomaly, the ratio between the abundances of Ne (highest FIP) and Fe (low FIP) is unusually large, of order 10, compared to solar photospheric conditions.

Fig. 35. Inverse FIP effect in the corona of HR 1099. The coronal element abundance ratios with

respect to oxygen and normalized to the solar photospheric ratios are plotted as a function of the FIP of the respective element (after Brinkman et al. 2001)

These trends have been widely confirmed for many active stars and from the various gratings available on XMM-Newton or Chandra (e.g., Drake et al. 2001; Huenemoerder et al. 2001, 2003; Raassen et al. 2003b; van den Besselaar et al. 2003, to name a few). With respect to the hydrogen abundance, most elements in active stars remain, however, depleted (Güdel et al. 2001a,b; Audard et al. 2001a), and this agrees with the overall findings reported previously from low-resolution spectroscopy. Strong Ne enhancements can be seen, in retrospect, also in many low-resolution data discussed in the earlier lit- erature, and the IFIP trend has now also been traced into the pre-main sequence domain by Imanishi et al. (2002).

When stellar spectra covering a wide range of magnetic activity are compared, only highly active stars show the presence of an IFIP pattern. In intermediately active stars, flat abundance distributions are recovered (Audard et al. 2003a). The abundances revert to a normal, solar-type FIP anomaly for stars at activity levels of log LX/Lbol ∼< − 4

(Güdel et al. 2002c; Telleschi et al. 2004, Fig. 36). Whenever the IFIP pattern is present, all abundances appear to be sub-solar, but the Fe/H abundance ratio gradually rises with decreasing coronal activity. The transition from an IFIP to a solar-like FIP abundance pattern and from very low Fe abundances to mild depletion seems to coincide with i) the transition from coronae with a prominent hot (T _∼> 10 MK) component to cooler coronae, and ii) with the transition from prominent non-thermal radio emission to the absence thereof (Güdel et al. 2002c).

In order to illustrate the above trends more comprehensively, Table 6 summarizes a few important parameters from recent abundance determinations based on high-resolut- ion spectroscopy. The table gives absolute Fe abundances, ratios of the high-FIP ele- ments O and Ne with respect to Fe, and the ratios between the two low-FIP element abundances of Mg and Fe and between the two high-FIP element abundances of Ne and O. Direct comparison of reported abundances should generally be treated with caution because various solar photospheric standards have been adopted. As far as possible, I

Fig. 36. Coronal abundance determination for solar analogs. Left: 47 Cas, a very active near-

ZAMS star; right: χ1Ori, an intermediately active solar analog. Abundances are given relative to Fe, and refer to solar photospheric abundances as given by Anders and Grevesse (1989) and Grevesse and Sauval (1999). Filled circles refer to determinations that used selected line fluxes of Fe for the DEM reconstruction; open circles show values found from a full spectral fit (figures courtesy of A. Telleschi, after Telleschi et al. 2004)

Table 6. Element abundancesafrom high-resolution spectroscopy

Star Ib ¯T Fe Ne/Fe O/Fe Mg/Fe Reference

Procyon L 1.4 0.66 1.5 1.0 1.1 Raassen et al. (2002) Procyon L 1.45 0.98 1.08 0.37 1.66 Sanz-Forcada et al. (2004) Procyon R 1.8 1.1 1.04 0.68 ... Raassen et al. (2002)

αCen A L 1.5 1.36 0.37 0.3 1.01 Raassen et al. (2003a)

αCen B L 1.8 1.43 0.38 0.23 1.12 Raassen et al. (2003a) Prox Cen R 3.7 0.51 1.6 0.6 2.1 Güdel et al. (2004)

Eri L 4.0 0.74 1.35 0.53 0.95 Sanz-Forcada et al. (2004)

χ1Ori R 4.4 0.87 0.73 0.33 1.12 Telleschi et al. (2004)c

κ1Cet R 4.5 1.18 0.95 0.39 1.94 Telleschi et al. (2004)c

π1UMa R 4.5 0.81 0.62 0.32 1.24 Telleschi et al. (2004)c AD Leo R 5.8 0.34 2.5 1.21 1.13 van den Besselaar et al. (2003) AD Leo L 6.1 0.39 3.43 1.64 0.6 van den Besselaar et al. (2003) Capella R 5.0 0.92 0.64 0.32 1.22 Audard et al. (2003a)

Capella R 6.5 0.84 0.50 0.50 1.08 Audard et al. (2001b) Capella L 6.5 1.0 0.5 0.48 0.91 Argiroffi et al. (2003) YY Gem R 7.6 0.21 3.62 1.42 0.81 Güdel et al. (2001a)

σ2CrB H 9 0.46 1.40 0.55 0.99 Osten et al. (2003) EK Dra R 9.1 0.72 1.01 0.51 1.54 Telleschi et al. (2004)c AT Mic R 9.2 0.30 4.8 3.2 1.4 Raassen et al. (2003b) 47 Cas R 10.6 0.50 1.68 0.70 2.21 Telleschi et al. (2004)c AB Dor R 10.0 0.40 4.8 2.23 0.95 Sanz-Forcada et al. (2003) AB Dor R 11.4 0.33 3.04 1.22 0.83 Güdel et al. (2001b) V851 Cen R 11 0.56 5.5 1.76 1.6 Sanz-Forcada et al. (2004)

λAnd H 11 0.37 2.23 1.35 1.56 Sanz-Forcada et al. (2004)

λAnd R 13.2 0.2 5.3 1.75 2.95 Audard et al. (2003a) VY Ari R 11.3 0.18 7.0 2.2 1.83 Audard et al. (2003a) Algol L 12 0.25 2.61 0.99 1.37 Schmitt and Ness (2004) HR 1099 H 13 0.30 10 3.0 2.5 Drake et al. (2001) HR 1099 R 14 ... 15.6 3.9 3.7 Brinkman et al. (2001) HR 1099 R 14.4 0.22 4.2 1.55 0.45 Audard et al. (2001a) HR 1099 R 14.8 0.20 6.6 2.75 0.9 Audard et al. (2003a) AR Lac H 15 0.74 2.16 0.81 0.95 Huenemoerder et al. (2003) UX Ari R 15.1 0.14 13.4 4.0 2.21 Audard et al. (2003a) II Peg H 16 0.15 14.9 7.4 2.7 Huenemoerder et al. (2001)

a_{All abundance relative to solar photospheric values: Anders and Grevesse (1989)} except for Fe: Grevesse and Sauval (1999)

b_{Instrument: R = XMM-Newton RGS; H = Chandra HETGS; L = Chandra LETGS} c_{Based on their method 2 using the SPEX database}

transformed the abundances to refer to the solar values of Anders and Grevesse (1989) except for Fe, for which I adopted the value given by Grevesse and Sauval (1999). No attempt has been made to quote error estimates. Errors are extremely difficult to assess and include systematics from calibration problems and from the inversion method, and most importantly uncertainties in the atomic parameter tabulations. It is unlikely that any measurement represents its “true” value within better than 20%. The average coro-

0 5 10 15 Average temperature (MK) 0.1 1.0 Abundance Fe Ne 0 5 10 15 Average temperature (MK) 1 10 Abundance ratio Ne/Fe 0 5 10 15 Average temperature (MK) 0.1 1.0 10.0 Abundance ratio Mg/Fe O/Ne 0 5 10 15 Average temperature (MK) 0.1 1.0 10.0 Abundance ratio O/Fe

Fig. 37. Abundances as a function of the average coronal temperature from high-resolution spec-

troscopy with XMM-Newton and Chandra. Top left: Abundances of Fe (filled circles) and Ne (open circles), relative to solar photospheric values for the sample reported in Table 6. Top right: Simi- lar, but the ratio between the Fe and Ne abundance is shown. Lower left: Similar, for the Mg/Fe (filled) and O/Ne ratios (open circles). Lower right: Similar, for the O/Fe ratio. Lines connect points referring to the same star analyzed by different authors, or based on different observations

nal temperature was estimated either from the logarithmic EMD, EM(log T ), or was calculated as the EM-weighted mean of log T in the case of numerically listed EMDs or results from multi-T fits. Figure 37 shows the relevant trends. Clearly, the low-FIP elements (Mg, Fe) vary in concert, and so do the high-FIP elements (O, Ne). But the absolute Fe abundance significantly drops with increasing activity, and the Ne/Fe ratio sharply increases as a consequence. The trend for O/Fe is very similar, with ratios that are lower by typically a factor of two. The transition from the FIP to the IFIP pattern for O/Fe occurs at average temperatures of about 7–10 MK.10

10 _{The trends are independent of the spectral inversion method used to determine the abundances} and the EMD; 17 spectra were fitted as a whole, while 17 spectra were analyzed with various iterative inversion methods using extracted line fluxes; both subsamples show identical trends.