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Astron. Astrophys. 361, 581-593 (2000)

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2. Observations

This study is based on the list of potential members of the Tucanae association provided by Zuckerman & Webb (2000). We analyze the ROSAT observations of all stars from their Table 1, i.e. both `probable' members (Table 1a) and stars termed `improbable' members (Table 1b).

The ROSAT satellite is equipped with two X-ray detectors in the focus of its telescope, the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI). See Trümper (1983), Pfeffermann et al. (1988) and David et al. (1999) for a description of the instrumentation for the ROSAT mission. The raw data of all observations can be retrieved from public archives. Our cross-correlation of Table 1 from Zuckerman & Webb (2000) with the archive showed that none of the stars was in the field of any HRI exposure. However, three stars have been observed in pointed observations of the PSPC. All stars in Tucanae have been observed during the ROSAT All Sky Survey (RASS).

In this section we describe our analysis of the PSPC data (in both pointed and survey mode) using the Extended Scientific Analysis System (EXSAS; Zimmermann et al. 1995). The results of source detection are presented and the properties of detected and undetected stars are summarized.

2.1. PSPC pointed observations

Three stars from the Tucanae membership lists are in the field of view of a pointed PSPC observation (HIP 92680, HIP 100751, and HIP 103438). The ROSAT observation request numbers (ROR) are 201597p, 200099p, and 200404p, respectively. After extracting the raw data from the archive we have performed source detection on these exposures using a combined local and map detection algorithm based on a maximum likelihood technique (Cruddace et al. 1988). For the cross-correlation of detected X-ray sources with the membership list introduced above we allow a maximum distance between X-ray and optical position of [FORMULA] (shown by Neuhäuser et al. 1995 to minimize the contamination by background sources). All stars are clearly identified with an X-ray source at an offset less than [FORMULA] from the optical position.

The number of counts in the background map at the source location is scaled to the photon extraction area and subtracted from the total number of counts. Count rates are computed using the information in the exposure maps, and transformed into luminosities applying the individual distances of the stars (derived from the Hipparcos parallax) and an energy-conversion-factor (ECF) determined from the hardness ratio.

Hardness ratios computed from the standard energy bands of the PSPC provide spectral information in case of low signal-to-noise. The PSPC hardness ratios are defined as follows



where H, S, [FORMULA], and [FORMULA] substitute the number of counts in the respective energy band: [FORMULA], [FORMULA], [FORMULA], and [FORMULA].

As discussed by Fleming et al. (1995) for late-type stars the ECF can be computed from [FORMULA] and is given by


For early-type stars (HIP 100751 has spectral type B) this conversion may not be appropriate. In this case we have assumed thermal emission at [FORMULA] (see Bergh"ofer et al. 1996) and negligible absorption and computed the ECF following the Technical Appendix F to the ROSAT Call for Proposals. Alternatively, count rates have been converted to fluxes using standard EXSAS routines. The luminosities derived by the two methods were found to be in good agreement, showing that the Fleming relation holds for the Tucanae stars.

The pointed PSPC observations of stars from our membership list are summarized in Table 1. Next to the Hipparcos number, X-ray position, and offset between optical and X-ray position [FORMULA] (Column 4) we give the maximum likelihood of existence (Column 5), the exposure time (Column 6), and the broad band count rate (Column 7). To derive the X-ray luminosity (shown in Column 8) we have divided the count rate by the number of components in the system (HIP 103438 is a binary, the other two stars are singles; see also Table 2  and Table 3 for the multiplicity of the Tucanae stars). This implies that all members contribute the same amount of X-ray emission and was shown by K"onig et al. (2000) to be acceptable in almost all cases according to ROSAT HRI observations of resolved young binary stars in Taurus. Finally, Columns 9 and 10 contain the hardness ratios.


Table 1. X-ray properties of stars in the Tucanae region as observed during pointed PSPC observations. Only three stars have been observed including HIP 103438, a star from the list of improbable members. The ROSAT request numbers of the respective observations are 201597p (HIP 92680), 200099p (HIP 100751), and 200404p (HIP 103438). Note, that HIP 103438 is a binary, and we have divided the observed count rate by two to obtain [FORMULA]. See text for a description of the entries in individual columns.

2.2. RASS observations

During the first months of its operation ROSAT performed an All-Sky Survey. In the course of this program the whole sky was scanned by the [FORMULA] field of view of the PSPC. The FOV was shifted by [FORMULA] per scan, such that the total number of scans of a specific location in the sky is [FORMULA] 25. The exposure times depend on the ecliptic latitude [FORMULA] and scale with [FORMULA]. For the Tucanae stars they range from [FORMULA] s.

Owing to the short exposures ([FORMULA] 30 s per scan) the sensitivity of the RASS is limited to [FORMULA] at a distance of 45 pc. Its main advantage is therefore the spatial completeness. In contrast to the pointed observations, where only three Tucanae candidates have been observed, X-ray data for all potential Tucanae stars are available from the RASS.

We have retrieved the RASS raw data from the Public Archive. For the source detection we proceed in a similar way as described in Sect. 2.1 for pointed PSPC observations. The maximum offset allowed between optical and X-ray position is again [FORMULA]. As a consequence of the scanning mode a given source has a different off-axis angle in each RASS scan. The compilation of count rates and luminosities follows the description outlined in Sect. 2.1. Since for non-detections no information about the spectral hardness is available, we use the mean of the ECF of detected Tucanae members for the conversion of upper limit counts to upper limit [FORMULA].

The detection fraction is much larger among the likely members of Tucanae (13/22) than in the group of improbable members (1/15). The X-ray properties of all detected and undetected stars are given in Table 2  and Table 3. The meaning of Columns 1 to 7 in Table 2 (detected sources) is the same as in Table 1. We provide the distance derived from the Hipparcos parallax in Column 8. The multiplicity of the stellar system is given in Column 9, and the spectral type in Column 10. The multiplicity is used to derive the X-ray luminosity (Column 11) as described in Sect. 2.1. The last three columns show the ratio of X-ray to bolometric luminosity, and the PSPC hardness ratios. [FORMULA] was derived from the V magnitude and spectral type (as given by Zuckerman & Webb 2000) using the bolometric correction of Schmidt-Kaler (1982) and assuming negligible absorption. This is justified because of the small distance and lack of intervening gas in the line of sight. The low ([FORMULA]) hardness ratios of the Tucanae stars also show that absorption is small if not negligible. Note, that no individual V magnitudes are available for the components in close multiple systems. Therefore, we have used the combined [FORMULA] as well as the combined [FORMULA] (i.e. without scaling to the number of components) in order to compute [FORMULA].


Table 2. RASS X-ray data for candidate members of the Tucanae association, i.e. stars from Table 1 in Zuckerman & Webb (2000). We give the designations from the Hipparcos catalogue. One star has no Hipparcos parallax and we list its PPM number. The X-ray position (Columns 2 and 3), distance to optical position [FORMULA] (Column 4), maximum likelihood of existence (Column 5), exposure time (Column 6), and broad band count rate (Column 7) are given. The distance (Column 8) is derived from the Hipparcos parallax. (For PPM366328 no parallax has been measured and the guess by Zuckerman & Webb 2000 has been used for the distance.) The spectral types listed in Column 10 are from Zuckerman & Webb (2000). [FORMULA] in Column 11 is computed under the assumption that all components in multiples emit the same amount of X-rays, i.e. we have divided the counts by the multiplicity (given in Column 9). Since for close multiples only one value of the combined V magnitude is available, the X-ray luminosity used to compute [FORMULA] (Column 12) is the observed value, i.e. the combined luminosity without taking account of the number of components in the system. Columns 13 and 14 contain the PSPC hardness ratios.


Table 3. RASS X-ray data for undetected candidate members of the Tucanae association. Designations in Column 1 are the Hipparcos numbers. Upper limits have been measured at the optical position of the stars (see Columns 2 and 3). Column 4 contains the exposure time, and Column 5 the upper limit to the broad band count rate. The distance derived from the Hipparcos parallax is given in Column 6. The spectral types listed in Column 8 are from Zuckerman & Webb (2000). The multiplicity given in Column 7 was used to compute the X-ray luminosity (Column 9) for the individual components in multiples as described in the text. Note, that HIP 2484 and HIP 2487 build a triple but are listed separately because their optical position is different. For the compilation of the [FORMULA] ratio given in Column 10 we have used the observed count rates, i.e. the combined luminosity of all components in case of multiples since only combined V magnitudes are available.

For undetected sources (Table 3) we list the Hipparcos number, the optical position (Columns 2 and 3), exposure time (Column 4), upper limit to the broad band count rate (Column 5), the distance to the star (Column 6), multiplicity (Column 7), spectral type (Column 8), and upper limits to the X-ray luminosity (Column 9) and to the [FORMULA]-ratio (Column 10). As for detected sources [FORMULA] has been computed by dividing the observed count rate by the multiplicity.

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Online publication: October 2, 2000