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Astron. Astrophys. 363, 958-969 (2000)

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2. Observations

2.1. The sample

The target stars are listed in Table 1, where we basically repeat some of the information on the sources given by RSP; namely, we list a running number, the ROSAT ID RXJ number, the coordinates of the X-ray position, the HUBBLE Guide Star Catalog (GSC) number and V magnitude of the optical counterparts. We recall from RSP that none of the 12 X-ray sources had a counterpart in the SIMBAD catalog nor in any other catalogs available at the time apart from the GSC. Also note that, whereas RSP listed only one counterpart to the X-ray source #2 (since only one object is indeed listed in the GSC catalog), two very close objects are visible in the finding chart with an angular separation of [FORMULA]" (see Fig. 6 in RSP); we renamed them as objects #2a and #2b and observed both of them photometrically and spectroscopically. Within a distance of 2 arcmin from the star #1 there are two bright sources which will be also discussed later in this paper. They were measured as either secondary targets or for comparison purposes. This group of three stars will be referred to as #1a, #1b, and #1c (from brighter to fainter). We have obtained optical photometry for the three of them, but spectroscopy is available only for the X-ray candidate (#1a). In addition, the spectroscopy revealed that two of the objects (#6 and #9) were indeed AGNs, as confirmed by the Hamburg RASS Catalog of Opt. IDs (HRASSCAT; Bade et al. 1998) that became available after we had already carried out our observations. Although we have obtained photometry and spectroscopy for both objects, we obviously exclude them from the following discussion.


Table 1. Coma Berenices candidate members.

2.2. VRI photometry

CCD photometry in the Johnson VRI filters has been obtained for our target stars with the 0.8-m IAC80 1 telescope (Teide Observatory) on February 28, 1997. We used the Thomson (1024 [FORMULA] 1024 pixel) detector mounted at the Cassegrain focus of the telescope, providing [FORMULA]-sized pixels and a field of view of 54.4 arcmin2. Exposure times were typically 30 s in all bands for the brighter stars and 60 s for the fainter ones. Raw frames were processed with usual techniques within the IRAF 2 environment, which included bias subtraction, flat-fielding and correction for bad pixels by interpolation with values from the nearest-neighbour pixels. We performed the aperture photometric analysis using routines within DAOPHOT and fixing circular apertures at 5-6 times the full width half maximum of each image (the seeing was stable around [FORMULA] along the night). Instrumental magnitudes were corrected for atmospheric extinction and transformed into the VRI Cousins system using photometric standard stars from the Landolt (1992) list. Special care was taken in observing standards of different colours in order to ensure a reliable transformation for the reddest targets in our sample. Data were collected under photometric sky conditions resulting in small rms values for the final photometric calibrations (0.02, 0.03 and 0.04 mag for the V, R and I filters, respectively). We present in Table 2 our measurements where 1[FORMULA] uncertainties in the V magnitude and colours account for the average rms of the calibration and the instrumental error as derived in IRAF. Fig. 1 illustrates the colour-magnitude diagrams for our data.

[FIGURE] Fig. 1. Colour-magnitude diagrams for our targets (Cousins system). The stellar X-ray candidate members of the Coma Berenices cluster are indicated with filled circles, while three other stars also investigated in this paper are plotted with triangles. Objects are labelled as in the first column of Table 2. Error bars indicate 2[FORMULA] uncertainties in the photometry; errors in the y-axis are of the same size than the symbols. The cluster photometric sequence is given by previously known members represented with open circles in the figures. Overplotted to the data are the main sequence (solid line) from late-A to mid-M type field stars and the 600 Myr-isochrone (dashed line) by D'Antona & Mazzitelli (1998).


Table 2. Optical photometry, spectral types, H[FORMULA] equivalent widths, and estimated S (Ca II H&K) index (see Sects. 3.1 and 3.2).
[FORMULA]) Positive EW values correspond to H[FORMULA] lines in absorption, negative values to H[FORMULA] emission.
[FORMULA]) These values are the average of six measurements. Their error bars also account for the observed variability. The complete lists are given in Table 5.

The star #1a (RXJ 1212.0+2232) turned out to be variable ([FORMULA]mag [FORMULA] 0.8) within intervals of minutes. We performed eight repeated observations of this star during the night of February 28; listed in Table 2 are the average magnitudes and colours determined that night. Unfortunately we cannot establish the possible photometric variable nature of the other star candidates (#2-12) in our sample because we obtained single exposures for them. With the goal of deriving the period and amplitude of the light curve of the candidate #1a we re-observed it with the same instrumental setup at the IAC80 telescope on March 10, 1997, and using the Thomson CCD camera (1024 [FORMULA] 1024 pixel, [FORMULA]-sized pixels and a field of view of 26.2 arcmin2) at the Cassegrain focus of the 1-m OGS 3 telescope (Teide Observatory) on April 19, 1999. Filters used were VI and only I for the IAC80 and OGS campaigns, respectively. Raw images were processed as previously described. We obtained relative aperture photometry between the target star and the two nearby, bright comparison stars (#1b and #1c) indicated in Fig. 2, and no photometric calibration has been evaluated for any of the nights. Exposure times were set so as to generally attain a count level for the variable star and the comparison stars which would enable photometric accuracies of 0.01 mag. Differencing the two comparisons confirmed that both stars were constant on the timescale of the observations and that the photometry had the desired accuracy. Consecutive images every 1.1 min during 7.4 hours were taken at the OGS telescope allowing us to measure a well-sampled light curve of the candidate. The nature of the star #1a will be discussed in Sect. 3.4.

[FIGURE] Fig. 2. Finding chart (I-band, OGS telescope) for the three stars studied in the field of the X-ray candidate #1a (RXJ 1212.0+2232). The three stars present high proper motion (not consistent with membership in the Coma Berenices cluster). Stars #1a (W-type W UMa binary) and #1b form a multiple stellar system.

2.3. Optical spectroscopy

Spectroscopic observations were carried out on March 14-16, 1997, with the Intermediate Dispersion Spectrograph at the Isaac Newton telescope (2.5m) in the Roque de los Muchachos Observatory (La Palma). Spectra were recorded using a 1024 [FORMULA] 1024 pix (24 µm) Tektronix CCD attached to the low resolution 235 mm camera. A window of 300 pix in the spatial direction was selected to match a slit length of 39". Two wavelength settings were used to cover from 3800 to 5500 Å (grating R632V, [FORMULA] Å, slit width of 1.6") and from 5300 to 7000 Å (grating R600R, [FORMULA] Å, slit width of 1.7"), respectively, providing a spectral purity [FORMULA] Å in both cases (corresponding to [FORMULA] and 2000).

Data reduction (bias subtraction, flat-field correction, sky subtraction, and extraction of one-dimensional spectra) was performed by standard procedures using routines included in the IRAF suite of programs. Wavelength calibration was carried out by comparison with exposures of CuAr and CuNe lamps obtained for the blue and red setups, respectively. Second-order polynomial fits where applied using 24 CuAr and 28 CuNe lines, providing a rms scatter [FORMULA] Å in both cases, and final dispersions of 1.6 and 1.7 Å pix-1, respectively. The standard star Feige 66 (Oke 1990), observed under the same conditions, was used to correct for atmospheric extinction and the instrumental sensitivity along the spectral direction. Fig. 3 shows a sample of the final spectra for the 11 stars observed. They have been ordered from earlier to later spectral types (see Sect. 3.2). Blue and red spectra overlap smoothly in the 5400 Å region, with the exception of the star #1a (K6V) for which the blue and red spectra shown correspond to different phases: 0.27 and 0.66, respectively. A detailed analysis of the spectroscopic properties of this sample will be presented in Sect. 3.2.

[FIGURE] Fig. 3. Final spectra for the 11 stars observed. The spectra are ordered from earlier to later spectral types (see Sect. 3.2) and correspond, from top to bottom, to the following objects (indicated in parentheses): #5, #8, #4, #12, #11, #10, #1a, #2b, #3, #2a and #7. Blue and red spectra overlap smoothly in the 5400 Å region (indicated by an arrow), with the exception of the star #1a for which the blue and red spectra shown correspond to different phases.

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Online publication: December 5, 2000