.1603 The Astrophysical Journal, 346:160-167,1989 November 1 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .346. 9ApJ. 198 ROTATIONAL VELOCITIES OF NEWLY DISCOVERED, LOW-MASS MEMBERS OF THE ALPHA PERSEI CLUSTER12 John R. Stauffer3 University of California at Santa Cruz and NASA/Ames Research Center Lee W. Hartmann3 Harvard-Smithsonian Center for Astrophysics AND Burton F. Jones University of California at Santa Cruz Received 1988 July 11 ; accepted 1989 April 28 ABSTRACT We have identified ~ 30 new, low-mass members of the young (age ^ 50 million yr) open cluster a Persei via a proper-motion study and subsequent photometric and spectroscopic observations. We have also con- firmed membership in the cluster for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox. Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show perculiar Ha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the a Per cluster shows both a large number of very slowly rotating stars and a significant number of stars with rotational velocities greater than 100 km s-1. Comparison of this rotational velocity distribution with the recently derived rotational velocity distributions for T Tauri stars places strong constraints on the mechanisms for angular momentum loss during pre-main- sequence evolution. Subject headings: clusters: open — photometry — stars: pre-main-sequence — stars: rotation I. INTRODUCTION older Pleiades cluster (Stauffer et al 1984; hereafter SHSB; The a Per open cluster is a relatively rich, relatively nearby, Stauffer and Hartmann 1987; hereafter SH). The Pleiades quite young assemblage of stars. Despite that, the cluster is not cluster also has rapidly rotating K and M dwarfs, but all the G well known, primarily because its low surface density and low dwarfs in the Pleiades have v sin i < 25 km s-1. This suggests galactic latitude combine to make it difficult to discern that a that G dwarfs spin down rapidly upon arrival on the main cluster is present when one views photographic plates of the sequence, in agreement with models for the evolution of solar region. Proper-motion surveys do not provide clear-cut mem- mass stars developed by Endal and Sofia (1981). The presence bership lists because the cluster mean motion is not far from of rapidly rotating K and M dwarfs in both clusters indicates the reflex solar motion. High-mass stars in the cluster can be that the spin-down time scale is longer for lower mass stars. identified without great difficulty because the field density of Rotational velocity distributions for T Tauri stars have also bright blue stars is not too great. However, if one excludes the become available recently (Hartmann et al 1986; hereafter bright cluster members (V < 8), there is no obvious surface HHSM; Bouvier, Bertout, and Mayor 1986; Hartmann and density enhancement at the cluster position, and hence no Stauffer 1988). It is possible to estimate pre-main-sequence cluster members later than about G5 were known prior to angular momentum loss via winds by evolving the T Tauri 1984. rotational velocity distributions to the main sequence with no In Stauffer et al (1985; SHBJ), we presented results of a faint assumed angular momentum loss and comparing this with the proper-motion survey of the a Per region and provided photo- observed rotational velocity distribution for a young cluster metry, radial velocities, and rotational velocities for the likely such as a Per. This was not practicable with just the data in cluster members. The photometry showed that late-type SHBJ because of the relatively small number of faint members. members of a Per are still contracting to the zero-age main In order to improve the statistics for low-mass members of a sequence (ZAMS) and that the positions in an H-R diagram for Per, we have conducted a new proper-motion survey covering the low-mass cluster members are consistent with the nuclear a wider area on the sky and going somewhat fainter than age for the cluster of ~5 x 107 yr (Mermilliod 1981). The before. In § II we report the results of that survey. As before, we echelle spectroscopy demonstrated that a significant fraction of have also obtained photometry and echelle spectroscopy of the the G, K, and M dwarfs in a Per are rapid rotators (v sin proper-motion candidates, and these data are also presented in i ~ 100 km s"1), as predicted from observations of the slightly § II. A discussion of the stars identified as members on the basis of the combined data set is provided in § III. 1 Lick Observatory Bulletin, No. 1129. 2 Research reported here was accomplished using the Multiple Mirror Tele- scope Observatory, a joint facility of the Smithsonian Institution and the II. OBSERVATIONS University of Arizona. a) Proper-Motion Survey 3 Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under Heckman, Dieckvoss, and Kox (1956) obtained proper contract with the National Science Foundation. motions for stars brighter than V ~ 11.5 in the vicinity of a 160 © American Astronomical Society • Provided by the NASA Astrophysics Data System .1603 ROTATIONAL VELOCITIES OF a PER CLUSTER 161 .346. Per, with slightly more than 10% of the 1400 stars surveyed b) Photometry identified as probably cluster members. The relative propor- tion of cluster members to field stars decreases for fainter stars All of the stars identified as possible cluster members via the 9ApJ. (the galactic latitude for the cluster is only 6°), making new proper-motion survey were observed with the KPNO 1.3 198 unbiased proper-motion surveys to fainter limits difficult. Of m telescope in 1985 and 1986. The Mk II computer-controlled the ~4000 stars with 10 < V < 15 measured in the proper- photometer, a GaAs phototube, and the Kitt Peak “special” motion survey by SHBJ, only ~1% appear to be cluster BVRI filter set were used. Most of the program stars were members. observed only once, although a few stars were monitored for In our original proper-motion survey, we only measured photometric variability (the results of the monitoring program stars in the inner few degrees of the cluster, and we did not will be reported elsewhere). Instrumental magnitudes were attempt to be complete even in that region. For our new converted to Johnson (1964) V and B — V values and Kron survey, we first obtained rough photographic photometry on (Kron, Gascoigne, and White 1957; hereafter, KGW) V— R red and blue plates for all stars in the region of the cluster using and R — I colors via the same techniques reported by us in our both the MADRAF measuring engine and the PDS-CCD previous open cluster papers (Stauffer 1982a, b; Stauffer 1984). hybrid (the “ Monet machine ”) at Kitt Peak. We then plotted Standard stars were generally selected from Moffett and these stars in a color-magnitude diagram and overplotted the Barnes (1979) and KGW with assumed standard colors as pro- previously identified cluster members. By excluding stars with vided in Stauffer (1982a). The 1 a accuracy of the photometry is colors incompatible with cluster membership, we were able to ~ 0.015 mag, based on the scatter of the derived standard star decrease the number of stars needing proper motions to ~ 10% magnitudes about their assumed true values. of the total. We further imposed a magnitude limit for the Table 1 provides a list of the photometry we have obtained proper-motion survey corresponding to R ^ 16.1, and a region for the stars which we conclude are possible or probable cluster on the sky where we deemed the search would be most fruitful. members. The stars labeled “ He ” are from Heckman, In all, the new survey includes ~ 4500 stars. Dieckvoss, and Kox (1956), while the newly identified possible The proper-motion measurements were made with the Lick cluster members are labeled “ AP ”. The AP identification Observatory Automatic Measuring Engine (AME), using the numbers start at AP 89 and are a continuation of the list same plate pairs as the survey reported in SHBJ. Even with the provided in SHBJ. The coordinate positions are for 1950. photometric selection procedures, the vast majority of the stars Finding charts for all of the AP stars are provided in Figure 1. measured appear not to be cluster members. The small number Only one of the stars monitored for variability was observed of cluster members combined with the nearness of the cluster sufficiently to determine a good rotation period. A light proper motion to the reflex solar motion make it impossible to curve for that star (AP 95) is shown in Figure 2. The rotation obtain formal proper-motion membership probabilities for the period we derive is 8.4 hr, which implies an equatorial rotation stars in the sample. Therefore, we simply identified all stars rate of order 125 km s- L The light curve shape and period are with proper motions compatible with cluster membership and comparable to light curves for the more rapidly rotating K derived 1950 coordinates for these stars. dwarfs in the Pleiades. TABLE 1 a Persei Cluster Photometry and Positions STAR V B-V V-R R-I V-I 1950 HE 299 11.19 0.64 0.28 0.28 0.56 3 12 24.7 50 13 16 HE 373 11.50 0.77 0.38 0.31 0.69 3 14 58.8 47 10 22 HE 828 11.59 0.71 0.34 0.34 0.68 3 25 27.2 48 03 49 HE 992 10.89 0.72 0.33 0.31 0.64 3 29