5.9 The solar coronal composition in time

From a naive point of view, the coronal and solar-wind material is expected to show the same overall composition as that of the underlying star and, in particular, its photosphere from where the material has been brought up. However, measurements of the composition of the solar corona and the solar wind have revealed what is commonly known as the “First Ionization Potential (FIP) Effect”; this abundance anomaly results in elements with a FIP below ≈ 10 eV (e.g., Si, Mg, Ca, Fe) being enriched by factor of a few with respect to hydrogen when compared to the photospheric mixture, while elements with a higher FIP (e.g., C, N, O, Ne, Ar) show the photospheric composition. The solar measurements have been summarized comprehensively in the extensive work by Meyer (1985a,b) and Feldman (1992Jump To The Next Citation Point). The same FIP effect is in fact also present in solar energetic particles and – unexpectedly – in cosmic rays.3

A discussion of the solar FIP effect is not within the scope of this review, nor are the various physical models that have been proposed for more than two decades. I refer the interested reader to the presentations by Hénoux (1995), Jordan et al. (1998Jump To The Next Citation Point), Drake et al. (1995), and Laming et al. (1995Jump To The Next Citation Point), and also the series of papers in Space Science Reviews, vol. 85 (pp. 283–418, 1998). In short, a fractionation process, probably involving electric and/or magnetic fields or pressure gradients, occurs at chromospheric levels where low-FIP elements are predominantly ionized and high-FIP elements are predominantly neutral. Because ions and neutrals are affected differently by electric and magnetic fields, acceleration or drift of elements with different FIP may occur at different speed or with different efficiency. The FIP effect varies considerably in strength between various types of coronal structure; see, e.g., Jordan et al. (1998). Although it is also expressed in full-disk solar spectral data, it is confined to coronal heights where T ∼> 1 MK, while material at lower temperatures shows the photospheric composition (Laming et al., 1995).

Why are abundance anomalies interesting for the study of the young Sun? The study of solar analogs at different ages has, in fact, shown systematics in the coronal composition that a successful, future model for the generation and heating of stellar coronae must explain. I briefly review the various findings.

5.9.1 Abundances in stellar coronae

Although detailed studies of coronal composition require high-resolution X-ray spectra, anomalies in the coronal composition of magnetically active stars have been recognized already in the early days of X-ray astronomy. Most notably, a significant deficiency of coronal Fe, by factors of a few when compared to the solar composition, has been noted (e.g., Swank et al. 1981White et al. 1994). There are two caveats with regard to early abundance measurements: i) low-resolution X-ray spectroscopy does not isolate individual spectral lines of any element with the exception of Fe, which produces a line system at 6.7 keV that can be measured by low-resolution (e.g., CCD-type) detectors. This line, however, forms at very high temperatures (formation temperature ≈ 60 – 70 MK), and knowledge of the thermal structure at those high temperatures is required if the element abundances are to be trusted; ii) more severely, for most stars no reliable photospheric abundances are known (again, the exception is mostly Fe). Because the photospheric composition of stars may significantly differ from the solar photosphere, any coronal abundance anomaly in stars should really be defined with respect to the underlying photosphere. In any case, before the advent of spectroscopy with XMM-Newton and Chandra, a sufficient body of data clearly showed that the most active stars reveal strong depletion of most heavy elements, with little FIP-related systematics (Singh et al., 1999).

Early observations of the extremely active subgiant HR 1099 (Brinkman et al., 2001) and the MS K-type ZAMS star AB Dor (Güdel et al., 2001) with the XMM-Newton Reflection Grating Spectrometer uncovered a new, systematic FIP-related bias in magnetically active stars: In contrast to the solar case, low-FIP abundances are systematically depleted with respect to high-FIP elements, a trend now known as the “inverse FIP effect” (IFIP). The ratio between the abundances of Ne (highest FIP) and Fe (low FIP) is therefore unusually large, of order 10 in the most extreme cases (when compared to the solar photospheric mixture). This pattern has been confirmed for many further active stars (e.g., Drake et al. 2001Huenemoerder et al. 20012003 etc). Only highly active stars show the IFIP pattern. Toward intermediately active stars, the effect weakens, until eventually flat abundance distributions are recovered (Audard et al., 2003bJump To The Next Citation Point).

5.9.2 The composition of the young solar corona

Solar analogs provide the best basis for a systematic study of abundance trends because they directly link to the solar case. Fortuitously, many well-studied solar analogs with different ages and activity levels are available for study in the solar vicinity. Again, the “Sun in Time” sample is outstanding in that the photospheric mix has been well documented, often for several elements apart from Fe. The principal result is that all of them show photospheric abundances close to solar (see summary table in Telleschi et al. 2005Jump To The Next Citation Point). The only independent parameter for this sample with regard to coronal abundance anomalies is therefore again the activity level or, equivalently, age.

The sample shows a clear systematic development from an inverse FIP effect in the youngest examples (with underabundances of Fe and Si) to a marked FIP effect in objects at ages of about 300 Myr and older (see Figure 27View Image). As long as 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 abundance pattern reverts to a normal, solar-type FIP anomaly for stars at activity levels of log LX/Lbol < ∼ –4 (Telleschi et al., 2005Jump To The Next Citation Point). Unexpectedly, this transition also 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 (Telleschi et al., 2005Jump To The Next Citation Point).

View Image

Figure 27: Coronal element abundances normalized to the Fe abundance, with respect to the solar photospheric mixture as a function of FIP; the total X-ray luminosities are, from top to bottom: log LX = 30.06 (BP Tau), 30.66 (V410 Tau), 30.39 (47 Cas B), 30.08 (EK Dra), 29.06 (π1 UMa), 28.95 (χ1 Ori), 28.95 (κ1 Cet), 28.26 (β Com) (after Telleschi et al., 2005Jump To The Next Citation Point2007bJump To The Next Citation Point).

5.9.3 The Ne/O abundance ratio: Subject to evolution?

Figure 28View Image shows an anomaly for the O/Ne ratio which is found at values of 0.3 – 0.7 times the solar ratio, apparently regardless of the stellar activity level. Because both O and Ne are high-FIP elements, their abundance ratio could reflect the photospheric ratio. But then, the Sun’s composition would be anomalous.

The tabulations of several solar element abundances have recently been revised (Asplund et al., 2005b), resulting in significant discrepancies between solar models and helioseismological results (see Antia and Basu, 2005, and references therein) unless further solar abundances were also different. A Ne abundance higher by a factor of at least 2.5 than hitherto assumed would be needed. Therefore, Telleschi et al. (2005Jump To The Next Citation Point) were the first to point out that the systematically non-solar O/Ne abundance ratio (by a similar factor) calls for a revision of the solar Ne abundance tabulation which at the same time would solve the solar helioseismology problem. This was further elaborated by Drake and Testa (2005Jump To The Next Citation Point) who suggested a factor of 2.7 upward revision of the adopted solar Ne abundance.

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Figure 28: Coronal abundances of Fe (low FIP) and Ne (high FIP – left plot), and ratios of Mg/Fe (low FIP) and O/Ne (high FIP – right plot) for various stars, shown as a function of the average coronal temperature (from Güdel, 2004Jump To The Next Citation Point,  Springer Verlag, reprinted with permission).

Two directions should be taken to verify this suggestion. First, further coronal abundances of low-activity solar-analog stars should be derived. So far, agreement with solar values has in fact been reported for α Cen A (Raassen et al., 2003), Procyon (Raassen et al., 2002), and for β Com, the latter with large error bars, however (Telleschi et al., 2005Jump To The Next Citation Point). There seems to be a trend toward higher O/Ne ratios in Figure 28View Image indeed. New results for α Cen have been reported by Liefke and Schmitt (2006), placing the Ne/O abundance ratio significantly lower than values for active stars, albeit not as low as the presently accepted solar photospheric value. Second, verification of the solar Ne/O ratio is needed. Although Ne cannot be measured directly in the photosphere, the Ne/O ratio can be derived from coronal measurements the same way as done for stellar observations. Recent analysis of solar active region X-ray spectra and of EUV spectra from transition-region levels (Schmelz et al., 2005Young, 2005) both report the standard Ne/O abundance ratios, rejecting an upward correction of Ne. The issue should therefore be considered to be open for the time being. A FIP-related, i.e., “evolutionary” enrichment of Ne in active corona remains a viable possibility (Asplund et al., 2005a).

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