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Variability of a Stellar Corona on a Time Scale of Days Evidence for Abundance Fractionation in an Emerging Coronal Active Region

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Variability of a Stellar Corona on a Time Scale of Days Evidence for Abundance Fractionation in an Emerging Coronal Active Region A&A 550, A22 (2013) Astronomy DOI: 10.1051/0004-6361/201220491 & c ESO 2013 Astrophysics Variability of a stellar corona on a time scale of days Evidence for abundance fractionation in an emerging coronal active region R. Nordon1,2,E.Behar3, and S. A. Drake4 1 Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312 85741 Garching, Germany 2 School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, 69978 Tel-Aviv, Israel e-mail: [email protected] 3 Physics Department, Technion Israel Institute of Technology, 32000 Haifa, Israel e-mail: [email protected] 4 NASA/GSFC, Greenbelt, 20771 Maryland, USA e-mail: [email protected] Received 3 October 2012 / Accepted 1 December 2012 ABSTRACT Elemental abundance effects in active coronae have eluded our understanding for almost three decades, since the discovery of the first ionization potential (FIP) effect on the sun. The goal of this paper is to monitor the same coronal structures over a time interval of six days and resolve active regions on a stellar corona through rotational modulation. We report on four iso-phase X-ray spectroscopic observations of the RS CVn binary EI Eri with XMM-Newton, carried out approximately every two days, to match the rotation period of EI Eri. We present an analysis of the thermal and chemical structure of the EI Eri corona as it evolves over the six days. Although the corona is rather steady in its temperature distribution, the emission measure and FIP bias both vary and seem to be correlated. An active region, predating the beginning of the campaign, repeatedly enters into our view at the same phase as it rotates from beyond the stellar limb. As a result, the abundances tend slightly, but consistently, to increase for high FIP elements (an inverse FIP effect) with phase. We estimate the abundance increase of high FIP elements in the active region to be of about 75% over the coronal mean. This observed fractionation of elements in an active region on time scales of days provides circumstantial clues regarding the element enrichment mechanism of non-flaring stellar coronae. Key words. stars: atmospheres – stars: coronae – stars: abundances – X-rays: stars – stars: individual: EI Eridani 1. Introduction micro-flaring, which is observed on the sun, but perhaps with different statistics (e.g., Caramazza et al. 2007). The best studied cool-star corona is that of our own sun, which Solar coronal chemical abundances are different from the also provides the only spatially resolved stellar coronal source solar photospheric composition. The solar corona itself is non- of extreme ultraviolet (EUV) and X-rays. From X-ray images uniform in that respect. While in coronal holes, the abundances of the sun we know that the solar corona in its active state is are close to photospheric, abundances in active regions and in non-uniform and that a significant fraction of the emission in the solar wind are significantly different. The abundance pat- these bands comes from magnetic loop structures concentrated terns depend on the first ionization potential (FIP; Meyer 1985; in active regions around photospheric sun-spots. Evidently, these Feldman 1992; Sheeley 1995). Elements with FIP < 10 eV structures evolve in brightness on time scales of hours. The mor- (low FIP) are enriched in the corona relative to elements with phology as well as the metal abundances vary on longer time FIP > 12 eV (high FIP). Averaging over the entire solar disk, scales of days. Coronal structures on the Sun may survive for the enrichment factor is about 4 (Feldman & Widing 2002,and a few weeks, although tracking them continuously from the references therein). Such a FIP bias, in varying intensities, has ground is difficult due to the ∼24.5 day solar rotation period. been reported for many coronal structures, from closed magnetic Active coronae of other cool stars can be up to five orders loops to streamers (see reviews by Kohl et al. 2006; Doschek & of magnitude more luminous and an order of magnitude hot- Feldman 2010). Currently there is no generally accepted model ter than the solar corona. Although unresolved, they show many that explains the FIP bias. characteristics similar to those of the solar corona, and are there- The FIP effect became even more puzzling when high reso- fore believed to be structured and to evolve in a similar way. For lution X-ray spectra from XMM-Newton and Chandra revealed the most part, late-type stars have envelopes in which convec- that active stellar coronae do not follow the solar FIP pattern. In tion and differential rotation interact to drive a dynamo, which some cases, the high-FIP elements are even enriched compared produces magnetic flux tubes. The flux tubes float into the up- to the low-FIP ones, an effect labeled the inverse-FIP (IFIP) ef- per atmosphere and eventually the corona. It is generally un- fect (Brinkman et al. 2001). Later studies revealed that FIP and clear, however, whether stellar coronae actually have larger cov- IFIP biases are correlated with coronal activity (and age): Highly ering of their stellar photosphere, or even qualitatively different active stars show an IFIP effect, while less active coronae have a morphologies, such as inter-binary magnetic structures. Stellar solar FIP bias (Audard et al. 2003; Telleschi et al. 2005), imply- coronae show high variability, often attributed to continuous ing a transition on stellar-evolution time scales of Gyr. Article published by EDP Sciences A22, page 1 of 9 A&A 550, A22 (2013) Table 1. Observing log. Obs. ID Start time End time Exposure [ks] 060317201 2009-08-01T00:02:50 2009-08-01T09:13:05 32.84 060317301 2009-08-02T23:06:16 2009-08-03T09:24:46 36.92 060317401 2009-08-04T22:20:49 2009-08-05T07:42:41 33.54 060317501 2009-08-06T20:59:39 2009-08-07T07:31:30 37.48 On the face of it, the correlation with activity supported pre- M4-5 secondary, with a possible 19-year orbit of a third body vious suggestions that abundances may be linked to flares (e.g., (Washuettl et al. 2009a). Güdel et al. 1999, 2004; Osten et al. 2003; Nordon et al. 2006). Long term photometric monitoring of EI Eri seemed to indi- However, direct systematic measurements of abundance varia- cate a ∼11 yr brightness cycle that was conjectured to be an ana- tions during flares show that regardless of coronal FIP/IFIP bias, log of the solar activity cycle (Oláh & Strassmeier 2002), but un- during large flares the abundances tend to photospheric values like the sun, long term Doppler imaging of EI Eri by Washuettl (Nordon & Behar 2007, 2008). The mechanism for FIP/IFIP is, et al. (2009b) shows little variability of its cool starspots therefore, related either to the long-term evolution of coronal over 11 yr. As for the hot corona, Osten et al. (2002) observed structures, or to the continuous micro-flaring that produces the EI Eri with ASCA (and EUVE) and fitted its spectrum with a quiescence emission, but not to large flares. 2T model of kT = 0.72 keV and 2.2 keV, but the abundances In recent years, a model has been proposed in which abun- could not be tightly constrained with ASCA. The XMM-Newton dance fractionation is driven by magneto hydrodynamics(MHD) CCD spectra of a short flare on EI Eri on 2009-08-06 during the waves (Laming 2004, 2009, 2012). This “pondermotive” force is present campaign was analyzed by Pandey & Singh (2012), who controlled by the energy density of MHD waves at the bases of suggested the flare originated from a loop the size of ∼R∗ (pri- the coronal loops. Wood et al. (2012) studied a sample of X-ray mary), which is about a tenth of the binary separation distance. 29 −1 faint (LX < 10 erg s ) dwarfs and found no correlation with X-ray luminosity or surface flux, but a good correlation with spectral type instead. According to this relation M dwarfs are 2. Observations, data, and analysis IFIP biased while early G dwarfs are FIP biased. Wood et al. In Table 1 we list the log of the four XMM-Newton observations (2012) claim that this shift from FIP to IFIP can be explained of EI Eri. Note the start times for each observation, which are by the inversion of the pondermotive force at certain conditions. separated by approximately the EI Eri orbital period of 168.5 ks. The latter study is limited to X-ray faint dwarfs and may not be applicable to the typical highly active binaries from other stud- ies that are more than two order of magnitude more luminous in 2.1. Data reduction X-rays, emitted mostly by a giant or sub-giant star (RSCVn type XMM-Newton data were reduced using the Science Analysis systems). Software (SAS – ver. 10.0.0) in the standard processing chains Despite the assumed similarity between solar and stellar as described in the ABC Guide to XMM-Newton Data Analysis1, coronae, we have lacked a study of the evolution of a stellar and using the most updated calibration files (date 22.06.2010). corona on time scales of days, which is the relevant time for Pileup was checked using the epatplot routine of the SAS soft- coronal loop evolution on the sun. The goal of the observations ware package. No significant pileup was detected in either of the presented in this paper is to seek and measure the variability of EPIC instruments and thus events from single to quadruple were temperatures and abundances in an active corona over several retained.
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