Volume 532, August 2011
|Number of page(s)||18|
|Section||Planets and planetary systems|
|Published online||12 July 2011|
X-ray flux (0.12 − 2.48 keV) and fits of stars with exoplanets (XMM-Newton and Chandra data).
Stars with exoplanets. XUV luminosity predicted in different bandsa.
The determination of the EMD in the transition region (log T [K] ~ 4.2−5.8) usually benefits from the information provided by UV lines. For the sources without UV spectroscopic observations we need to develop a method to calculate the EMD in this region. We extrapolate the values of the EMD at those temperatures based on the coronal counterpart, for which a general proportionality seems to be present. Both transition region and coronal material are supposed to be part of the same geometrical structures (loops). In the coronal EMD of all sources we can identify material at log T [K] ~ 6.3, the typical temperature of the solar corona, despite of their activity level. We use the EM level at that peak, averaged over three values of T, to calibrate the relation to the lower temperature EMD. We used a sample of objects with a well calculated EMD over the whole range, using same technique in all cases (Sanz-Forcada et al. 2002; Sanz-Forcada & Micela 2002; Sanz-Forcada et al. 2003a; Huenemoerder et al. 2003; Sanz-Forcada et al. 2004), and adding α Cen B (Sect. B). We separated the sample in three groups according to the level of activity (interpreted from the amount of EMD found at the highest temperatures): low activity stars (group 1: Procyon, α Cen B), moderately active stars (group 2: ϵ Eri, ξ UMa B), and active stars (group 3: VY Ari, σ2 CrB, AR Lac, FK Aqr, AD Leo, UX Ari, V711 Tau, II Peg, AB Dor).
The lower temperature EMD can be defined using three parameters (see Fig. A.1, Table A.1). Two come from the fitting of the EMD with a straight line: the slope of this line and the difference between the minimum EM (at Tmin) and the local maximum at log T [K] = 6.2−6.4 (ΔEM1). The fit makes use of values in the temperature range log T [K] = 4.2 − Tmin. Since groups 1 and 2 have only four objects between them, we applied the same model to all of them.
We also need a way to account for the different sampling of the EMD in T, from the 0.1 dex binning used in the EMD to the 3-temperature fit typical in low-resolution spectra. The fits with one or two temperatures are assumed to be like the 3-T fits with the remaining temperatures considered as negligible. The third parameter needed in our model accounts for this binning in the form of a vertical shift of the EM (ΔEM2) to be added to ΔEM1. This parameter shows a dependence on the level of activity, according to the distribution of mass in temperature. We used a representative star for each group, all of them of spectral type K2V: α Cen B (group 1), ϵ Eri (group 2), and AB Dor (group 3). The ΔEM2 of each case is listed in Table A.1.
Linear fits (dashed lines) applied to the cool side of the EMD (solid lines) of well known coronal models.
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Depending on the temperature and EM found in the targets in our sample, we use one of the three groups and extrapolate the EM of the transition region using the value of EM at the temperature closer to log T (K) = 6.3 (EM6.3): we first determine the EM of Tmin (using log Tmin (K) = 5.7): EMmin = EMlog T ~ 6.3 − ΔEM1 − ΔEM2. Then we extend the EM at lower temperatures with a straight line with the slope in Table A.1, resulting in the values listed in Table A.2. Uncertainties in the lower temperature EMD are calculated based on those from Table A.1.
We tested the accuracy of the calculation with this method. We used the same three representative stars (α Cen B, ϵ Eri, and AB Dor) with a complete EMD calculated using UV lines and compared this to the flux in same spectral ranges using 3-T model combined with the extrapolated EM at lower temperatures. The values measured from both models (Table A.3) are very similar, so we are confident that the approach followed is correct.
Finally, we compared the calculation of the EUV flux of ϵ Eri with the direct EUVE spectrum. The luminosity in the band 80 − 170 Å in the observed spectrum was 3.2e+27 erg s-1, in the model based on the whole EMD was 2.7e+27, and in the model based on the 3T+extrapolated EMs we obtain 1.9e+27. These differences are very small considering the process followed to obtain the synthetic spectra. We are confident that the method can be safely applied to all late-type stars (late F to mid M spectral types).
Transition region EMD. Fit parameters.
Emission measure distribution in the transition region.
We calculated the EMD of the K2V star α Cen B, needed to test the extrapolation of the lower EMD temperature and the synthesis of the EUV spectra. We used the UV lines fluxes measured by Sanz-Forcada et al. (2003a) and the XMM-Newton/RGS lines fluxes listed in Table B.1, from an observation taken on Jan. 2009 (Fig. B.1). The coronal model (the EMD) was constructed following Sanz-Forcada et al. (2003b). The resulting EMD (Table B.2) is displayed in Fig. B.2, with coronal abundances as listed in Table B.3. A global fit to the Chandra/LETG spectrum was applied by Raassen et al. (2003), with similar results in the corona.
Emission measure distribution of α Cen B.
Coronal abundances of α Cen B (solar unitsa).
Upper panel: EMD of α Cen B. Thin lines represent the relative contribution function for each ion (the emissivity function multiplied by the EMD at each point). Small numbers indicate the ionization stages of the species. Lower panel indicates the observed-to-predicted line flux ratios for the ion stages in the upper figure. The dotted lines denote a factor of 2.
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The data server X-Exoplanets2 provides information on the planet-bearing stars that have been observed with XMM-Newton or Chandra. In the near future, synthetic spectra covering the EUV range (Sanz-Forcada et al. 2010a) and EUVE data will also be available. The system contains reduced, science-ready data and was set up to facilitate the analysis of the effects of coronal radiation on exoplanets atmospheres.
The data server X-Exoplanets is accessed by means of a web-based fill-in form that permits queries by list of objects and coordinates and radius. Searches can be customized to include physical parameters of the stars and planets as well as light curves and reduced spectra obtained from XMM-Newton and Chandra data.
An example of the result of a query is given in Fig. C.1. Light curves and reduced spectra can be visualized by clicking on the corresponding link (Fig. C.2). The system incorporates multidownload and preview capabilities. Links to SIMBAD and the Extrasolar Planet Encyclopaedia are also provided.
VO-compliance of an astronomical archive constitutes an added value of enormous importance for the optimum scientific exploitation of their datasets. The X-Exoplanet service has been designed following the IVOA standards and requirements. In particular, it implements the SSA (Simple Spectral Access) protocol and its associated data model, a standard defined for retrieving 1D data.
The data server X-Exoplanets. Light curve (left) and reduced spectrum (right). In the light curve of HD 189733 we mark the orbital phase (Winn et al. 2007) of HD 189733 b in the upper axis, as well as the interval when the transit takes place (partial in dotted line, total in solid line).
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© ESO, 2011
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