A Survey of Chromospheric Activity in the Solar-Type Stars in M67
Mark S. Giampapa (National Solar Observatory)

2. Observational Approach

We are utilizing the 3.5-m WIYN telescope in conjunction with the Hydra multi-fiber positioner to perform multi-object spectroscopy over a 1 degree field. We use a bench-mounted spectrograph with the T2KC CCD, Simmons camera and the 1200 l mm-1 grating in second order. Two blocking filters (BG38 & BG39) must be employed to suppress red leak at the sacrifice of some throughput; these filters will soon be replaced in this program by a single CuSO4 filter, thus increasing our throughput. This configuration yields a spectral resolution of about 0.80 A in the 3950 A region between the Ca II K and H lines, as measured by the FWHMs of the CuAr comparison lines. We display in Figure 1 a diagram from the Hydra simulator illustrating the placement of the fiber positioner for target stars in the M67 field. While 100 blue fiber cables are available, only about half of them can be positioned at targets because of the disallowed proximity of fibers that inevitably occurs (i.e., fiber-fiber interactions) in centrally condensed fields. Thus multiple Hydra configurations are required to obtain spectra of the entire sample. At approximately a distance of 870 pc (Montgomery et al. 1993), solar-type stars in M67 are relatively faint with V ~14.5. It is really the advent of the Hydra multi-object spectrograph on a 4-m class telescope that enables the efficient conduct of a survey of ~100 solar-type stars based on K-line spectroscopy. Nevertheless, spectroscopy of cool stars in the blue is still challenging even with a relatively large-aperture telescope such as the WIYN telescope. A raw Hydra frame comprised of the sum of 4 x 3600 sec frames is displayed in Figure 2. Differences in intensity are due both to differences in V-magnitude and to variations in fiber throughput which can change by up to a factor of ~3 from one fiber to the next.

FIGURE 1:Hydra simulator showing the positioning of fibers for a target field in M67.

FIGURE 2:An example of a raw Hydra frame consisting of the sum of 4 x 3600 sec individual frames for solar-type stars in M67. The dispersion direction is vertical in the figure. The Ca II H and K lines are evident as prominent dark bands in the frame.

2.1. Stellar Sample

The stars are selected from the Girard et al. (1989) proper motion study combined with the CCD photometry given by Montgomery et al. (1993). For those objects from the Girard et al. study where photometry from Montgomery et al. is not available, we adopt the photometry given by Sanders (1977). We include only stars that have a membership probability of >= 90% as given by Girard et al. (1989). Our photometric criteria include stars in the range of apparent brightness of 14 <= V <= 15 and intrinsic color of +0.55 <= (B-V) <= +0.78, where we adopt a value of E(B-V)=0.05 for the color excess in M67 (Montgomery et al. 1993). The resulting spectral range of the sample is ~ F8 - K0. A histogram in intrinsic (B-V) color of the sample is given in Figure 3. In this way, we can ensure that we are observing stars that are very close to the Sun in their photospheric properties.

FIGURE 3:A histogram in (B-V) color of the stars observed in this program. The stellar sample emphasizes solar analogs where we adopt (B-V) = +0.65 for the Sun.

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