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1. Introduction 2. Observations THE ASTROPHYSICAL JOURNAL, 535:959È964, 2000 June 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. IDENTIFICATION OF A NEARBY STELLAR ASSOCIATION IN THE HIPPARCOS CATALOG: IMPLICATIONS FOR RECENT, LOCAL STAR FORMATION B. ZUCKERMAN1 AND R. A. WEBB1 Department of Physics and Astronomy, University of California, Los Angeles, 8371 Math Science Building, Box 951562, Los Angeles, CA 90095-1562; ben=astro.ucla.edu; webbr=astro.ucla.edu Received 1999 July 6; accepted 2000 January 20 ABSTRACT The TW Hydrae Association (D55 pc from Earth) is the nearest known region of recent star forma- tion. Based primarily on the Hipparcos catalog, we have identiÐed a group of nine or 10 comoving star systems at a common distance (D45 pc) from Earth that appear to comprise another, somewhat older association (““ the Tucanae Association ÏÏ). Together with ages and motions recently determined for some nearby Ðeld stars, the existence of the Tucanae and TW Hydrae Associations suggests that the Sun is now close to a region that was the site of substantial star formation only 10È50 Myr ago. The TW Hydrae Association represents a Ðnal chapter in the local star formation history. Subject headings: open clusters and associations: general È open clusters and associations: individual (TW Hydrae) È stars: formation È stars: preÈmain-sequence 1. INTRODUCTION group or Local Association proposed by Eggen (e.g., see Je†ries 1995 and references therein). For many years, the only known star clusters within 70 2. OBSERVATIONS pc of Earth were the rich Hyades (at D45 pc) and the sparse UMa nucleus (at D25 pc). Then a group of stars D55 pc Excepting TW Hya itself, the Ðrst T Tauri stars in the from Earth was established as a bona Ðde association of T TWA were identiÐed in a study of IRAS sources at high Tauri stars of age D10 Myr (Webb et al. 1999; Sterzik et al. Galactic latitude (de la Reza et al. 1989; Gregorio-Hetem et 1999; R. A. Webb, I. N. Reid, & B. Zuckerman 2000, in al. 1992). It has been shown that, on the main sequence, preparation). This TW Hydrae Association (hereafter young stars are more likely 60 km IRAS sources than are TWA) was unrecognized for many decades in spite of its old stars (e.g., Jura et al. 1993, 1998; M. D. Silverstone et al. being the nearest region of recent star formation (Kastner et 2000, in preparation), and young stars are also more apt to al. 1997). A ubiquitous signpost of newly formed stars has be members of associations than are older stars. Therefore, been a nearby molecular cloud. But no interstellar cloud we interrogated the Hipparcos catalog within a 6¡ radius of has been found near TW Hya despite multiple searches. two dozen stars detected by IRAS at 60 km and scattered Therefore, we ask: do additional unrealized young associ- around the sky. The stars were those with, in our opinion, ations far from molecular clouds exist near Earth? reliable excesses at 60 km listed by Mannings & Barlow Di†erent teams have used the Hipparcos catalog to (1998), Backman & Paresce (1993), and/or M. D. Silver- search for previously unrecognized stellar associations. For stone et al. (2000, in preparation). We searched for Hip- example, Platais et al. (1998) undertook a ““ search for star parcos stars with similar proper motions and distances to clusters from the Hipparcos Data ÏÏ and listed basic data for the IRAS stars; results for our most interesting regions are Ðve ““ very likely ÏÏ new clusters and associations as well as 15 presented in Figure 1, Tables 1 and 2 and are discussed ““ possible ÏÏ ones. Yet, in their own words, ““ At distances less below. Results at additional IRAS stars will be discussed in than 100 pc, the survey is incomplete as a result of the a later paper. chosen search strategy.ÏÏ Of the 20 potential new groupings To verify or deny common space motions of Table 1 stars in their Table 1, the closest, which contains 11 members with similar distances and proper motions in the Hipparcos from Hipparcos, is 132 pc away. catalog, we measured radial velocities with the Bench By contrast, the ““ Tucanae Association ÏÏ we propose in Mounted Echelle (BME) spectrograph on the 1.5 m tele- the present paper is only D45 pc from Earth and much scope at the Cerro Tololo Interamerican Observatory younger than the UMa and Hyades clusters. Just as the (CTIO). The spectra cover from D5000 to 8200A at a sparse UMa cluster nucleus is accompanied by more typical measured resolution of 0.15A . The data, obtained numerous UMa stream stars, we suggest that the Sun is 1999 August 21È26 (UT), were reduced and calibrated in embedded in a stream of stars with similar space motions IRAF. (the ““ Tucanae Stream ÏÏ), with the Tucanae Association Six radial velocity standard stars were observed during playing a role analogous to the UMa nucleus. Figures 1a the run with spectral types ranging from F6 to M1.5. Radial and 1b depict the Tucanae Association and some stream velocity was determined by cross-correlating the spectrum stars, respectively. These stars likely represent some of the of the target and a standard star of similar spectral type younger, nearer, members of the more extensive Pleiades observed close in time to the target. Approximately 15 echelle orders, chosen to produce strong correlations and have few atmospheric features, were used to compute the 1 Visiting Astronomer, Cerro Tololo Inter-American Observatory correlation. The accuracy of these measurements is strongly (CTIO), which is operated by AURA, Inc., under contract to the National dependent on the signal-to-noise ratio (S/N) of the spectra Science Foundation. and the spectral type and rotational velocity of the target 959 TABLE 1 POTENTIAL MEMBERS OF THE TUCANAE ASSOCIATION PROPER MOTION CATALOG NUMBER J2000.0 (mas yr~1) NUCLEAR ROSAT n EMBER [ HIP HD HR M ? Right Ascension Declination mv B V SpT (cps) (mas) ad A. Probable and Possible Members of the Tucanae Nucleus or Stream 1481 .................. 1466 . Y 00 18 26.0 [63 28 38 7.5 0.54 F8 0.26 24.42 90 [56 1910 .................. ... ... Y 0024 08.9 [62 11 04 11.3 1.39 M0 . 21.59 93 [46 1993 .................. ... ... Y 0025 14.6 [61 30 48 11.3 1.35 K7 . 26.69 85 [63 2484 .................. 2884 126 Y 00 31 32.6 [62 57 29 4.37 [0.07 B9 . 23.35 92 [59 2487 .................. 2885 127 Y 00 31 33.4 [62 57 56 4.54 0.15? A2]A7 . 18.95 98 [51 2578a ................. 3003 136 Y 00 32 43.8 [63 01 53 5.09 0.04 A0 . 21.52 94 [51 2729 .................. 3221 . Y 00 34 51.1 [61 54 58 9.6 1.05 K4 0.5 21.78 87 [59 92680 ................. 174429 . 18 53 05.9 [50 10 49 7.8(8.5) 0.78 K0 1.0 20.14 17 [85 93815 ................. 177171 7213 . 19 06 19.9 [52 20 26 5.2 0.53 F7 2.08 19.07 26 [117 95261a ................ 181296 7329 . 19 22 51.2 [54 25 25 5.05 0.02 A0 . 20.98 28 [83 95270a ................ 181327 . 19 22 58.9 [54 32 16 7.0 0.48 F5 . 19.77 28 [82 99803b ................ 191869 . 20 14 56.1 [56 58 34 7.24 0.49 F6.5 . 15.32 33 [92 100751 ............... 193924 7790 . 20 25 38.8 [56 44 06 1.94 [0.12 B7 . 17.60 7 [87 104308 ............... 200798 . 21 07 51.2 [54 12 59 6.7 0.24 A5 . 15.05 28 [82 105388 ............... 202917 . 21 20 49.9 [53 02 02 8.6 0.69 G5 0.62 21.81 30 [94 105404 ............... 202947 . 21 20 59.8 [52 28 39 8.9 0.85 K0 0.67 21.72 34 [105 107345 ............... ... ... ... 2144 30.1 [60 58 38 11.7 1.4 M1 0.137 23.66 41 [92 107947 ............... 207575 . 21 52 09.7 [62 03 08 7.2 0.51 F6 0.42 22.18 44 [94 108195 ............... 207964 8352 . 21 55 11.3 [61 53 11 5.9 0.39 F3 0.19 21.49 48 [93 PPM 366328b,c ...... ... ... ? 2315 01.2 [63 34 25 9.8 0.80 K0 0.15 20(guess) 116 [44 116748 ............... 222259 . Y 23 39 39.4 [69 11 44 8.2 0.78 G5/G8 IV 0.81 21.64 79 [60 118121 ............... 224392 9062 Y 23 57 35.0 [64 17 53 5.0 0.06 A1 . 20.53 83 [58 B. Improbable Members 459 ................... 67 ... ... 0005 28.3 [61 13 32 8.8 0.67 G5 . 18.57 87 [78 1399 .................. ... ... ... 0017 30.3 [59 57 04 11.3 1.4 M0 . 22.54 113 [40 93096 ................. 175531 . 18 57 56.6 [44 58 06 9.8 . G8/K0 . 15.51 27 [75 94051 ................. 177720 . 19 08 51.1 [54 02 17 8.7 0.56 G0 . 14.60 [1 [66 94858 ................. 180134 7297 . 19 18 09.8 [53 23 13 6.4 0.5 F7 . 21.94 25 [81 94997 ................. ... ... ... 1919 49.6 [53 43 13 12.1 1.58 M3 . 16.67 14 [90 95302 ................. 181516 . 19 23 20.5 [50 41 20 9.0 0.74 G6 IV . 13.24 31 [87 97705 ................. 187101 . 19 51 23.6 [58 30 34 8.0 0.58 F8/G0 . 14.73 36 [85 101636 ............... 195818 . 20 36 02.3 [54 56 28 8.6 0.58 G0 . 15.16 50 [84 101844 ..............
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