A&A 383, 210-217 (2002)
A. Marino1 - G. Micela2 - G. Peres1 - S. Sciortino2
1 - Dipartimento di Scienze Fisiche e Astronomiche, Sez. di Astronomia, Università di Palermo,
Piazza del Parlamento 1, 90134 Palermo, Italy
2 - Osservatorio Astronomico G. S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
Received 27 August 2001 / Accepted 27 November 2001
We have analyzed the X-ray variability of dF7-dK2 stars in the solar neighborhood detected with the pointed ROSAT-PSPC observations. Our data base is the sample of all stars listed in the CNS3 catalog (Gliese & Jahrei 1991) having a B-V color between 0.5 and 0.9; it includes 70 pointed observations of 40 distinct stars or multiple systems. We have applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time series of our sample: only 10 observations relative to 8 distinct stars are variable at a confidence level greater than 99 and 4 of them belong to multiple systems. For the subsample of 9 stars observed both at the beginning and at the end of the mission, we can study the variability on time scale of years and compare amplitude variations at short and long time scales. Our analysis suggests that, for these stars, the X-ray variability is more likely on longer time scale. All the stars variable on long time scale, and not on short time scale, are relatively quiet and similar to the Sun, suggesting that the variations may be due to cycles. The comparison of our results with those previously obtained for dM stars shows that the amplitude of variability of X-ray emission from dF7-dK2 stars is smaller than that observed in dM stars.
Key words: stars: coronae - stars: late-type - X-rays: stars - galaxy: solar neighbourhood
The Sun is the only star for which X-ray variability is extensively studied on virtually all time scales (e.g. Vaiana et al. 1973; Vaiana & Tucker 1974; Kreplin et al. 1977; Zombeck et al. 1978; Withbroe et al. 1985). The soft X-ray emission of the Sun is highly variable on time scales ranging from minutes to years. Moreover, observations of the Sun have high spatial, spectral and temporal resolution, and span a much longer time interval than stellar observations. Resolving the structure of the solar corona, presently down to 700 km, allows us to identify directly the sources of X-ray emission and to relate variations of the global solar X-ray flux (e.g. Kreplin et al. 1977; Kahler & Kreplin 1991; Donnelly & Bouwer 1981; Peres et al. 2000; Orlando et al. 2001) to the originating structures on the Sun.
Since no equivalent studies are possible for other stars, X-ray variability studies provide a powerful tool to test the properties of coronae of late-type stars and to infer the presence of solar-like phenomena. We know that magnetic phenomena largely determine the level of solar coronal activity; by analogy, we expect the same to hold for solar-type stars. Solar observations are the benchmark against which to test dynamo models: these models are calibrated on the Sun. While the Sun offers unique observational details and it is a very fruitful test for theory, it suffers the difficulty that relevant stellar parameters that are supposed to affect model prediction cannot be varied. Hence comparing the Sun with late-type stars with different masses, rotation rates and evolutionary states will help us in understanding how the dynamo process depends on basic stellar parameters.
The systematic analysis of the X-ray variability of nearby dM stars as observed with ROSAT-PSPC (Marino et al. 2000, hereafter Paper I), showed that variability is a general property of these stars on all time scales we have explored. The amplitudes of these variations are independent from both stellar X-ray and visual luminosity. Compared to properties of solar X-ray variability our results suggest that the amplitude distribution of X-ray variability in dM stars is consistent with the analogous distribution for solar flares. The comparison of our data with those obtained with Einstein IPC showed that long term variability (on time scales longer than 10 years), if present, must be of smaller amplitudes than the short term variations observed in the ROSAT X-ray light curves (Paper I).
Coronal and chromospheric activity are known to be related (Schrijver et al. 1992). The observations of the Ca II H and K lines has allowed to monitor the chromospheric activity of solar-type stars in detail (Wilson 1978; Baliunas & Vaughan 1985; Baliunas et al. 1995). These data show a variety of behaviors which can be basically reduced to three different types: mostly old quiet stars like the Sun show a cyclic behavior, with periods ranging from about 3 to 15 years or more; young active stars usually show a chaotic behavior with no obvious periodicities; the remaining stars appear to be constant, with no indication at all of an activity cycle. The X-ray emission of the stars in the spectral range dF7-dG9 observed with EINSTEIN Observatory (Maggio et al. 1987) resulted well correlated with their cromospheric Ca II H-K line emission.
In this paper we present a study of X-ray variability of nearby dF7-dK2 stars detected with ROSAT-PSPC (Pfeffermann et al. 1987; Trümper 1992) pointed observations. Since, the sensitivity of ROSAT-PSPC is much higher than that of the Einstein IPC, we can study variability of smaller amplitudes and in more X-ray quiet stars. Furthermore, a better time coverage is reached since the typical observation live times with ROSAT are longer than with Einstein. ROSAT observations (similary to Einstein ones) are fragmented in time segments of typically a few thousand seconds.
Our paper is organized as follows: in Sect. 2 we present our sample of dF7-dK2 stars, the X-ray data, and their analysis. The basic results are given in Sect. 3. In Sect. 4, we draw our conclusions.
|GL17.3||1835||G2 V||0.66||20.39||S||BE Cet|
|GL68||10476||K1 V||0.84||7.47||S||107 Psc|
|GL117||17925||K2 V||0.87||10.38||S||EP Eri|
|GL139||20794||G5 V||0.71||6.06||S||82 Eri|
|GL434||101501||G8 Ve||0.72||9.54||S||61 UMa|
|GL506||115617||G6 V||0.71||8.53||S||61 Vir|
|GL559.1||129333||dG0 e||0.61||33.94||S||EK Dra|
|GL566A||131156||G8 Ve||0.73||6.70||B||Boo A|
|GL566B||131156||K4 Ve||1.16||6.70||B||Boo B|
|GL567||131511||K2 V||0.84||11.54||S||DE Boo|
|GL575A||133640||F9 V||0.65||12.76||B||44 Boo A|
|GL575B||G2||12.76||B||44 Boo B|
|GL635A||150680||G0 IV||0.65||10.79||B||Her A|
|GL635B||K0 V||0.75||10.79||B||Her B|
|GL663A||155886||K1 Ve||0.85||5.33||B||36 OphA|
|GL663B||155885||K1 Ve||0.86||5.33||B||36 OphB|
|GL779||190406||G1 V||0.61||17.67||S||15 Sge|
|GJ1255||197433||K0 V||0.86||27.65||B||VW Cep|
|GL882||217014||G4 V||0.67||15.36||S||51 Peg|
|Name||Obs. seq.||Exposure||Elapsed Time||Observing||HR||Rate Err.||Results of|
|(s)||Time (s)||Dates||[cnt/s]||[erg/s]||the K-S test|
|Name||Obs. seq.||Live-time||Elapsed Time||Observing||HR||Rate Err.||Results of|
|(s)||(s)||Dates||[cnt/s]||[erg/s]||the K-S test|
|name||HD||K-S results||K-S results||Elapsed time||Elapsed time|
|range||on short-time||on long-time||of each obs.||of all obs.|
For each star we evaluated the number of photon counts in a circular region centered on the average position of the observed photons in the (0.1-2.4) keV range and with a radius R, ranging from 2 arcmin for sources positioned on the optical axis, up to 5 arcmin, for sources at large off-axis positions. Count rates of the off-axis sources were corrected for vignetting. The radius R has been determined as described in Paper I.
We computed X-ray luminosities from the obtained fluxes and the distances reported in Table 1. Since each observation consists of a set of temporal segments typically obtained during
different satellite orbits we estimated count rate, HR, flux and X-ray luminosity for each
temporal segment with at least 30 counts. In Fig. 1 we show the scatter diagram of X-ray
luminosity versus B-V. The segments connecting symbols indicate multiple observations of the same
star, filled diamonds mark the 10 short-term variable observations with a confidence level
>99%, filled circles mark the observations showing variability with confidence level between
the position of the Sun is also shown (square symbol), with the range of
expected solar luminosities between periods of minimum and maximum activity indicated by a
vertical line. There are very few sources with X-ray luminosity similar to that of the solar
minimum. This is due to a selection effect because of our choice of selecting the sample as
described in Sect. 2.1.
|Figure 1: Scatter plot of X-ray luminosities vs. B-V. The segments connecting symbols indicate multiple observations of the same star, filled diamonds mark the 10 short-term variable observations with confidence level >99%, filled circles mark observations showing variability with a confidence level between 90 and 99, the position and the luminosity variation of the Sun over the solar cycle, is also shown (square symbol). The (statistical) errors of the measured X-ray luminosities are typically 15%.|
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For each star in our sample, we obtained light curves in the (0.11-2.4) keV band.
In order to have a statistical evaluation of the X-ray variability we applied the unbinned Kolmogorov-Smirnov (K-S) test on all X-ray photon time series of our sample stars. We used the procedure implemented in the pros.xtiming package of IRAF after having removed data gaps. This method does not allow us to distinguish stochastic variability, or other forms of variability, from periodic ones (see also Collura et al. 1987; Haisch & Schmitt 1994 for a more sophisticated treatment of the gaps), but it can detect variability of any kind.
For each observation, we ran the K-S test on the source counts as well as on the counts detected in the background regions, the latter being carried out to monitor possible background variability. In three cases the background is variable with Confidence Level (CL) >99; GL 231 (Obs. seq. 180172N00), GL 475 (Obs. seq. 900137N00), GL 1255) however, in these cases, the number of counts in the variable background is much lower than the counts attributed to the source, hence we assume that the test results for these sources are reliable. In Col. 9 of Table 2 we report the results in terms of the confidence level at which we can reject the hypothesis that the source in the given observation is constant. In Table 4 we present a summary of K-S test results. Only during 10 observations relative to 8 distinct stars we have found the emission to be variable at a CL greater than 99%. This finding suggests that X-ray variability on short time scale is not common among these stars.
Furthermore, we have analyzed the K-S results for stars of different X-ray luminosities (see Table 5). The Table suggests that the X-ray variabilty changes with X-ray luminosity level,
and in particular that a larger fraction of variable stars are more X-ray luminous. However we show
below that this result is very likely due to the different count statistics distribution in the
|Results of the K-S||Number of observations|
In Fig. 3 we also compare the Time XLD for our sample, having , with the distribution obtained for dM stars (Fig. 4, Paper I). Using K-S two-sample test, we have tested the null hypothesis that the M XLD and the dF7-dK2 XLD are drawn from the same population, finding that, the two distribution are different, with dM stars more variable than dF7-dK2 . This finding may be a strong indication that magnetic activity on dM stars and solar type stars is due to different processes, as it appears to be the case, for example, in Brown Dwarfs as suggested by Berger et al. (2001).
|Figure 2: The cumulative X-ray luminosity functions for variable ( ; dashed line) and non variable (; solid line) stars, having at least 500 counts. The two luminosity functions are indistinguishable, according to a K-S two sample test, suggesting that variability is independent on X-ray luminosity.|
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|Figure 3: The dashed line shows the cumulative time X-ray luminosity distribution for dF7-dK2 stars having more than 500 counts and log( ; the solid line shows the cumulative time amplitude X-ray luminosity distribution for dM stars with more than 500 counts.|
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Nine stars of our sample were multiply observed with a similar off-axis, and with a time separation of at least six months. Using a procedure implemented in the pros.xdataio package of IRAF, we applied the K-S test on all available data for each star, just appending the various observations one after the other.
We find indications that the variability increases with the time scales. Five stars, not variable on time scales ranging from a few hours to less than 1 week, result variable on time scales of months (see Table 3). We note that the only two stars, out of the nine stars multiply observed, that show short term variability are: EK Dra, the most active star of our sample, and Cen, the one observed with the highest statistics. On the contrary the stars that do not show long term variability are Wo9158, observed over a time scale of only 6 months, and GL 620.1, the only star, together with EK Dra, of the subsample observed on long time scale, having erg/s. Below we discuss the evidence that very X-ray luminous G-type stars do not appear variable on longer time scale. The amplitude of the variations ranges between a factor 2 and 4 for variable stars on long time scales.
A few stars in our sample have been observed with Einstein-IPC in 1978-1981 allowing the study of possible variability on a time scale comparable with that of the solar cycle. We show in Fig. 4 the log() measured with the PSPC (one value for each observing time interval) versus log() measured with the IPC (one value averaged on the IPC observation (Maggio et al. 1987; Barbera et al. 1993). We excluded the upper limit IPC observations consistent with the PSPC observation and the PSPC observations spanning less than 500 s because in this case it is very difficult to disintangle flare emission from quiescent emission. Given the difference of the two instruments and the cross calibration uncertainties, we do not consider as significant any variability within a factor two i.e. inside the region of the plot enclosed by the dotted lines. Long term variability seems to exist for three stars of our sample (GL 86, GL 124, GL 598); we note that these stars have X-ray luminosity similar to the solar one. In literature we found information on cyclic variability only for one of these three stars (GL 598), it is found to have a cyclic period >30 yr (Baliunas et al. 1995). Saar & Brandenburg (1999) find for GL 598 a cyclic period >25 yr.
|Figure 4: Scatter plot of log() observed by Einstein-IPC (corrected using hipparcos distances) and ROSAT-PSPC. The solid line represents the line of equality and the dashed line represents variation of factors two.|
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|Figure 5: Scatter plot of X-ray luminosity vs. the Ca II emission index (Henry et al. 1996; Soderblom 1985; Radick et al. 1998) for the stars in our sample quoted in the same papers. Filled symbols are the stars variable on long time scale but not on short time scale, vertical lines show the range of luminosity spanned by stars having more than one temporal segment, among them the filled crossed symbols are the three stars variable between IPC and PSPC observations and not variable on short term scale (for them, the vertical lines indicate variations between IPC and PSPC observations). Crosses indicate the stars variable on short time scale with a , the Sun is shown as a square symbol. For the Sun vertical line spans the luminosity variation over the solar cycle (Peres et al. 2000).|
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Stellar magnetic cycles manifest themselves in a long term variability of their chromospheric (Wilson 1978; Baliunas et al. 1995) and possibly also coronal emission (Hempelmann et al. 1996). We therefore studied the relation between X-ray luminosity and Ca II H-K emission as parameterized by the Ca II index ( measures the intensity of the line corrected for the contribution of the photospheric continuum and scaled for the bolometric flux). Using the Ca II data published by Henry et al. (1996), taken between 1992-1993, integrated with data from Soderblom (1985) and Radick et al. (1998), we show in Fig. 5 the scatter plot of log() versus for the stars in our sample mentioned in the same papers. Figure 5 also shows the best fit power-law relation obtained by Maggio et al. (1987). Our data are in good agreement with this relation. The Ca II and X-ray data are not taken symultaneonsly, introducing some spread in the relation; in particular the star GL 449 (see Fig. 5) shows an anomalous value of Ca H-K. In Fig 5 vertical segments show also the spread introduced by X-ray variability Our data indicate that at least at low activity levels ( ) long-term variability can also account for the observed spread.
We have searched information on cyclic variability (Wilson 1978; Baliunas et al. 1995) of the stars in our sample. Chromospheric observations of GL 137 and GL 663 analyzed by Wilson (1978) indicate cyclic variations while GL 434 and GL 502 do not seem to show any cyclical behavior. Baliunas et al. (1995) find indication of long term variations (on time scales longer than about 20 yr) for GL 17.3, GL 137, GL 434, GL 663A; GL 502 analyzed by Baliunas et al. (1995) presents a 16.6 yr activity cycle superposed on a 9.6 yr cycle. GL 598 has rotation and mass similar to the Sun, but weak chromospheric activity and a variation in activity that appears to be longer than 30 yr. Data are too sparse to compare the time scales of variations in calcium and X-ray.
This finding is not in contrast with the hypothesis that the X-ray variability may be due to a cyclic behavior of relatively quiet stars.
The authors acknowledge financial support MURST-COFIN 99. This research made use of the Simbad database, operated at CDS. We also thank the anonymous referee for useful comments.