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Astron. Astrophys. 318, 215-230 (1997)

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1. Introduction

The existence of a tenuous corona around the Sun has been known for more than one hundred years. Through identification of forbidden lines detected at optical wavelengths it was realized (cf., Grottian 1939) that the temperature of the coronal gas exceeds [FORMULA] K, thus giving rise to the still unsolved coronal heating problem. Plasma at temperatures of a few million degrees loses the bulk of its radiative energy at soft X-ray wavelengths, and hence X-ray studies of the solar corona have become a vital tool for coronal and solar physics in general. In particular, at X-ray wavelengths the solar corona can be readily imaged with modern solar X-ray telescopes from disk center out to a few solar radii, while optical studies utilizing forbidden lines are usually restricted to regions near the solar limb.

In the context of the solar-stellar connection, one naturally expects other stars also to be surrounded by coronae. Stellar observations, however, do not permit the luxury of angular resolution, and all attempts to detect forbidden coronal lines at optical wavelengths have failed (Wallerstein et al. 1991). In the last couple of years an increasing number of radio detections of active stellar coronae has been reported (cf., Güdel 1994), but it is fair to say that stellar coronae at large can - at least at present - only be detected and studied at X-ray wavelengths. While the first generation of non-imaging X-ray missions was not sensitive enough to detect emission even from the nearest stars, the advent of imaging soft X-ray telescopes on board the Einstein Observatory (launched in 1978), EXOSAT (launched in 1983), and ROSAT (launched in 1990) allowed the study of stellar coronae at large. The first break-through in stellar X-ray astronomy was accomplished with the help of the Einstein Observatory (cf., Vaiana et al. 1981; Rosner, Golub and Vaiana 1985; Vaiana et al. 1992) by giving hundreds of examples of X-ray emission from almost all classes of stars. The ROSAT all-sky survey (RASS) has produced complete and unbiased samples of X-ray sources of all source classes: approximately one third of its [FORMULA] 80 000 sources are of coronal origin, thus providing the largest sample of coronal X-ray emitters known to date. However, as pointed out by Schmitt et al. (1995), solar-like coronal X-ray sources - at least from the point of view of energetics - can only be detected in the close vicinity of the Sun: For a star with a soft X-ray luminosity of [FORMULA] erg/sec, a typical value for the Sun under maximum conditions, one readily calculates a maximal distance of 9 pc, at which it can still be detected as a RASS source, given a characteristic limiting RASS X-ray flux of [FORMULA] erg/cm2 /sec. Under solar minimum conditions, the characteristic solar soft X-ray flux is reduced at least by one order of magnitude, emphasizing my point that stars emitting at solar levels or even below can be studied only if they are sufficiently close. There is considerable interest in the question whether and to what extent phenomena observed in the solar corona also occur on other stars. After all, within the context of solar physics one can deal with only one star, while in the stellar context one can study the coronal properties of stars with different ages, masses, radii, rotation rates etc. in comparison to what is known from the Sun. However, to this end the full range of stellar X-ray emission must be observed, but X-ray selected stellar samples are by necessity biased towards the intrinsically luminous X-ray emitters. Therefore the question to what extent we can consider the Sun as a typical star from the coronal point of view, cannot be satisfactorily answered with previously existing data.

Nearby solar-like stars have been systematically studied with the Einstein Observatory. Specifically, Maggio et al. (1987) studied all late F and G dwarf stars within a volume of 25 pc and observed with the Einstein Observatory, in order to assess the X-ray properties of solar-like "normal stars", and Schmitt et al. (1985) studied the X-ray properties of stars with shallow convection zones in the color range 0.1 [FORMULA] B-V [FORMULA] 0.5 and again within a distance of 25 pc, in an effort to investigate whether the onset of significant surface convection zones manifests itself in a detectable change of the coronal properties of these stars. Both of these studies were incomplete in the sense that by necessity they had to be restricted to those stars which were actually observed by the Einstein Observatory as a target or serendipitously; further, the sensitivity of these Einstein observations fell only occasionally below [FORMULA] erg/sec, with a more typical value being [FORMULA] erg/sec. Specifically, only 18 stars out of the sample of 61 objects studied by Maggio et al. (1987) are included in our 13 pc sample; all of these 18 stars except one were detected by ROSAT, while seven could not be detected with the Einstein Observatory. For the somewhat hotter stars studied by Schmitt et al. (1985) the improvement of the new ROSAT data is less significant. The sample in this paper has eight objects in common with the sample of shallow convection zone stars discussed by Schmitt et al. (1985). Seven of these eight objects were detected already with the Einstein Observatory and are now confirmed by ROSAT, [FORMULA] Peg (= HR 8430 = Gl 848) was added to the list of known X-ray sources. I do wish to point out, however, that some of my upper limits for A-type stars are significantly below those obtained from the Einstein data.

Considering the fact that the solar soft X-ray luminosity is typically in the range [FORMULA] to [FORMULA] erg/sec, a study of stars exhibiting solar-like levels of activity is quite difficult with the Einstein Observatory data. As a consequence, it is next to impossible to decide on the basis of pre-ROSAT data to what extent other stars really represent solar analogs. Specifically, from the Einstein observations it is clear that about one third of the G stars emit at levels below [FORMULA] erg/sec, but it is not clear whether there is any minimally attained X-ray luminosity and one cannot assess whether there are any X-ray dark solar-like stars.

Using new ROSAT observations, the difficulties of previous studies can be overcome, and the purpose of this paper is to present an X-ray study of a complete volume-limited sample of nearby stars of spectral type A, F and G. I primarily use data obtained during the ROSAT all-sky survey; those stars not detected in the all-sky survey, were systematically studied in the ROSAT pointing program. With this observing strategy I can improve on the Einstein observations in two important respects: First, completeness is guaranteed through the use of RASS data, and second, the required sensitivity is obtained through ROSAT pointings on those objects where deeper exposure time was needed.

The structure of this paper is as follows: In Sect. 2 I present the sample of my program stars, the new X-ray data and their analysis. The basic results are given in Sect. 3, while in Sect. 4 I discuss the detection thresholds and sensitivity, the detection completeness of my observations, construct X-ray luminosity distribution functions, demonstrate the correlation between X-ray luminosity and spectral hardness, and consider X-ray surface fluxes as well as the relationship between coronal and kinematic properties of my sample stars. In Sect. 5 I present the interpretation of my findings as well as my conclusions, which I briefly summarize in Sect. 6.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998