Publications of the Astronomical Society of the Pacific 87: 17-36, 1975 February
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Publications of the Astronomical Society of the Pacific 87: 17-36, 1975 February DDO INTERMEDIATE-BAND PHOTOMETRY OF MOVING-GROUP STARS ROBERT J. BOYLE AND ROBERT D. McCLURE* Yale University Observatory Received 1974 October 2 G and Κ giant stars in several of Eggen's moving groups have been observed on the DDO system. The cyanogen strength and absolute magnitude calibrations of the system are used to discuss the assignments of the observed stars to moving groups. We are able to confirm the assignment of some 50% of the members of the Hyades moving group and thus to segregate out a more homogeneous sample of Hyades group stars. These stars can be used to recalibrate intrinsic Hyades giant sequences. Less firm conclusions are reached concerning the other, older, groups discussed. Key words: moving groups — abundances — late-type stars I. Introduction yrs), contain many groups, among them the Wolf During the past 16 years, Eggen has presented 630, 61 Cygni, ζ Herculis, σ Puppis, e Indi, and an extensive and impressive body of evidence for η Cephei moving groups. The old-disk stars the existence of moving stellar groups. These share the velocity distribution of the evolved F streams of stars possessing similar space motions and G stars lying between the Hyades and are thought to result from the breakup of clusters NGC 188 sequences in the {Mv,B—V) plane and associations of stars sharing a common origin (Eggen 1973c). Near the boundary between the in space and time (Eggen 1958). Eggen has iso- old-disk stars and the halo population lies the lated moving groups in four stellar kinematic Arcturus moving group with an age comparable populations labeled as very young, young, old- to NGC 188, whose stars travel in galactic orbits disk, and halo. The youngest of these popula- of mean eccentricity e = 0.45 (Eggen 1971α, tions, that of the very young-disk stars, is defined 1974α). The halo population stars are those by the velocity distribution of the early Β stars which travel in orbits with e > 0.5. Among the (Eggen 1973ö) and contains two groups, the halo population, the Groombrige 1830 group of Pleiades group of stars that share V = — 25 km high-velocity stars, whose galactic orbits have a sec-1 with the Pleiades cluster, and a group with mean e = 0.83, has been isolated by Eggen and V = —18 km sec-1 (here U, V, W are the usual Sandage (1959). components of space motion with respect to the To the extent that the group assignments can sun, V being the velocity component in the direc- be made reliably, the concept of moving groups tion of galactic rotation). Stars of this very young- is one of great utility, yielding distance moduli, disk population are thought to be from 2 to space motions, and population information for 7 X 107 years old. The young-disk stars are large numbers of "field" stars. For example, those with ages between that of the very young- Eggen has used the red giants of the Hyades disk population and that of the Hyades cluster moving group to define intrinsic sequences in 8 (5 to 8 X 10 yrs, Eggen 1973b). Among the the (Mbol, R—I) plane and the {U~B, B— V) and moving groups of this population are the Hyades ((7—B, R—l) two-color diagrams (Eggen 1966, and Sirius moving groups. The old-disk-popula- 1972α, 1974&). The (Mbol,R—I) calibrations for tion stars, with ages between that of the Hyades the Hyades and old-disk groups have been used cluster and that of NGC 188 (some ~10 X 109 by Eggen {1973b,c) to obtain distances in dis- cussions of the kinematics of the immediate solar ♦Visiting Astronomer, Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory, which neighborhood, while the (C7—Β,Β—V) relation are operated by the Association of Universities for Re- has been employed to obtain ultraviolet excesses search in Astronomy, Inc., under contract with the Na- for giant stars (e.g., Eggen 1972b; McClure 1970, tional Science Foundation. 1974). Group membership has been used to dis- © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 18 BOYLE AND McCLURE cuss the absolute magnitudes of variable and II. Observational Data and peculiar stars such as δ Scuti, RR Lyrae and R Reduction Techniques Coronae Borealis variables (Eggen 1969α, 1970α; Eggen and Sandage 1959; Bessel 1967), Am and For G and Κ giants, observations through the Fp stars (Eggen 1970a), and Bau and R-type 48, 45, 42 and 41 filters of the original DDO stars (Eggen 1972c). Eggen has also made use intermediate-band system (McClure and van of moving groups in a discussion of the empirical den Bergh 1968) yield information on tempera- mass-luminosity relationship (Eggen 1963α). ture, luminosity, and cyanogen abundance (Mc- It is the aim of this paper to investigate the Clure 1973). The original cyanogen index, δ Cm, reliability of group assignments for G and Κ was found to have a residual dependence on giants through the use of the metallicity and abso- luminosity by Janes (1972,1974a) and was re- lute magnitude calibrations of the David Dunlap calibrated by him, yielding a new index δ CN, Observatory (DDO) photometric system (Mc- used here, δ CN is defined as the difference be- Clure 1973). Breger (1968) has used Strömgren tween the observed cyanogen band strangth, four-color photometry to conclude that from C(41-42) and the mean value for solar neighbor- 40% to 60% of the main-sequence A and F stars of hood stars of similar C(42-45) and C(45-48) the Hyades moving group may be incorrectly indices (that is, similar surface temperatures and assigned to the group. Eggen (1970α), however, gravities). Janes (1974a) has also been able to disputes this result, attributing the large spread use the colors C(45-48) and C(42-45) to cali- in πΐγ found by Breger to the presence of Am and brate DDO photometry in terms of absolute Fp stars in the group and to a luminosity effect visual magnitude, Mv. In the process of the in ml for some stars. Williams (1971) has also δ CN and Mv calibrations, the Copenhagen inter- uséd narrow-band spectrophotometer indices to mediate-band indices of Dickow et al. (1970) investigate the homogeneity and metal abun- were transformed to DDO indices (Janes 1972, dance in moving groups. He found that a major- 1974a) for a large number of stars, and we shall ity of Hyades stars studied had [Fe/H] values use these transformed indices to supplement similar to the Hyades cluster, but that other directly observed DDO data. groups were less homogeneous. We believe The new DDO observations obtained for this DDO photometry is well suited to a reinvestiga- investigation were made with the Yale 40-inch tion of this question for Hyades and perhaps (102-cm) reflector at Bethany, Conneticut, with a other group giants. two-channel pulse-counting photometer, and at In the discussion of the metallicity of the group either Kitt Peak National Observatory with 16- red giants we will implicitly assume a chemical inch and 36-inch reflectors and a one-channel homogeneity comparable to that found in clus- current-integrating photometer, or at Cerro ters as a requisite for group membership. It Tololo Inter-American Observatory with a 36- should be bom in mind, however, that the G and inch reflector and a one-channel pulse-counting Κ giants of some clusters may show evidence of photometer. Standard stars from McClure and a significant spread in metallicities through spec- van den Bergh (1968) (for Kitt Peak and Bethany tral peculiarities (e.g., Ban stars in NGG 2420 observations) or from a new list of equatorial (McClure, Forrester, and Gibson 1974), carbon standards being prepared by McClure (for Cerro stars in ω Centauri (Wing and Stock 1973)) and Tololo observations) were also observed each perhaps through a scatter shown by the color- night so that transformations to the original magnitude diagram of the giant branch. For DDO system were obtained. Mean extinction example, see discussions of NGC 188 (McClure coefficients were applied. In averaging observa- 1974) and ω Gen (Cannon and Stobie 1973). tions of the same star, a few results on nights of Also see the discussion of variations in G-band poor quality at Bethany were given half weight. strengths among giant stars in M 92 by Zinn Table I lists the sources of the data and standard (1973). With this reservation in mind, the range errors of the mean for the three colors of the of the DDO metallicity parameter, δ CN, to be DDO system for a star observed on three nights, expected in the general field, in clusters, and in both for new observations and observations from moving groups will be discussed in section III. previous investigations. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System PHOTOMETRY OF MOVING-GROUP STARS 19 TABLE I SUMMARY OF DDO PHOTOMETRY Standard Errors (3 Observations) Rpf Reference ^ 0(45-48) 0(42-45) 0(41-42) McOlure and van den Bergh (1968) 1 0^007 0^006 0^007 McOlure (1970) 2 0.004 0.005 0.005 Janes and McClure (1971) 3 0.006 0.005 0.005 Goodenough (1969) 4 0.012 0.016 0.013 Janes and McClure (unpublished) 5 0.007 0.005 0.005 Janes (1972) 6 0.005 0.005 0.005 Bethany (this paper) Β 0.008 0.009 0.007 Kitt Peak (this paper) KP 0.005 0.00M- O.OOM- Cerro Tololo (this paper) CT 0.002 0.002 0.002 Dickow, et al.