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Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters Ic 2391 and Ic 2602 John R.Stauffer,1 Lee W

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Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters Ic 2391 and Ic 2602 John R.Stauffer,1 Lee W THE ASTROPHYSICAL JOURNAL, 479:776È791, 1997 April 20 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. ROTATIONAL VELOCITIES AND CHROMOSPHERIC/CORONAL ACTIVITY OF LOW-MASS STARS IN THE YOUNG OPEN CLUSTERS IC 2391 AND IC 2602 JOHN R.STAUFFER,1 LEE W. HARTMANN,PAND CHARLES F. ROSSER Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 SOFIA RANDICH2 European Southern Observatory, Karl-Schwarzschild Strasse 2, D-85748 Garching, Germany SUCHITRA BALACHANDRAN1 Astronomy Department, University of Maryland, College Park, MD 20742 BRIAN M. PATTEN1 Smith College, Five Colleges Astronomy Department, Northampton, MA 01063 THEODORE SIMON Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 AND MARK GIAMPAPA National Optical AstronomyObservatories,3 National Solar Observatory, Tucson, AZ 85726 Received 1996 September 16; accepted 1996 November 12 ABSTRACT We have obtained high-resolution, moderate signal-to-noise ratio spectra for approximately 80 candi- date low-mass members of the nearby, very young open clusters IC 2391 and IC 2602. Most of the stars observed are conÐrmed as cluster members based on a combination of photometric and spectroscopic criteria. We provide radial velocities, rotational velocities, and Ha equivalent widths for these stars. From comparison to theoretical preÈmain-sequence (PMS) evolutionary isochrones from DÏAntona and Mazzitelli, we derive an estimated age of the two clusters of D25 Myr. By contrast, the usually quoted upper main-sequence turno† age for the clusters is D35 Myr. We do not believe that this provides evi- dence for noncoeval star formation within these clusters, but rather that the best age estimate for them given the uncertainties is D30 ^ 5 Myr. In principle, the scatter of stars about the PMS isochrone pro- vides a measure of the age spread among the low-mass stars in these clusters; however, with the data presently available, we are able to derive only a relatively uninteresting upper limit for an age spread of order 20 Myr. We compare the rotational velocity distribution for IC 2391/2602 to that observed for the Pleiades. For the G dwarfs in the IC clusters, we resolve rotation in all but one of the probable cluster members, and thus except for inclination e†ects, our data provide the complete distribution of rotational velocities for solar mass stars on their arrival on the ZAMS. The projected rotational velocities (v sin i) of the G dwarfs in the two IC clusters span the range from D8toD200 km s~1. Comparison of the distribution of rotational velocities for the G dwarfs of the Pleiades and the IC clusters indicates that both the slow and the rapid rotators lose of order half their angular momentum during the Ðrst D35 Myr on the main sequence if they rotate as solid bodies. The low-mass stars in these two clusters exhibit a similar correlation between rotation and coronal activity as is found in several other young open clusters. That is, there is a large spread in coronal activ- ity for stars with v sin i \ 25 km s~1, where we assume there is an intrinsic link between increasing rotation and increasing activity superimposed upon which are a variety of observational and physical mechanisms that act to smear out this relation; above v sin i D 25 km s~1, all of the low-mass stars have log (L /L ) D [3.0, the canonical ““ saturation ÏÏ limit. Our measurements of the Ha equivalent widths are consistentX bol with a similar relationship holding for chromospheric activity. One and possibly two of our spectra for M dwarf members of the IC clusters show broad wings for the Ha proÐle, which we attribute to a Ñare event or to microÑares. Since spectra of a small sample of late-type M dwarfs in the Pleiades also showed similarly broad Ha wings, this suggests that Ñare frequencies for very young M dwarfs may be quite high. Subject headings: open clusters and associations: individual (IC 2391, IC 2602) È stars: coronae È stars: chromospheres È stars: evolution È stars: low-mass, brown dwarfs È stars: rotation 1 Visiting Astronomer at the Cerro Tololo Inter-American Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 2 Partially based on observations carried out at the European Southern Observatory, La Silla. 3 The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 776 YOUNG OPEN CLUSTERS IC 2391 AND IC 2602 777 1. INTRODUCTION telescope of the European Southern Observatory in 1994 April. At the 4 m telescope, we used the red, long-camera Open clusters provide an empirical database with which echelle spectrograph with a 31.6 line mm ~1 grating blazed to study the time evolution of a variety of stellar properties. in the red and a 120 km slit width(0A.8 on the sky) providing In particular, open clusters can be used to (1) provide color- a 2 pixel resolution of about 0.15Ó at Ha. The detector was magnitude diagrams that can be compared to theoretical a Tektronix 2048 ] 2048 CCD, read out with four ampli- isochrones and thus enable tests of the theoretical evolu- Ðers using the ARCON controller, allowing a spectral tionary models (see, e.g.,Iben & Talbot 1966; Adams, coverage of roughly 5800È8200Ó. We used the IRAF Strom, & Strom1983; Prosser et al. 1994); (2) determine the QUAD package to bias-subtract and Ñat-Ðeld individually distribution of rotational velocities for low-mass stars as a each of the CCD quadrants and then used standard pack- function of age and thus place constraints on the role of ages within IRAF to extract the echelle orders and preÈmain-sequence (PMS) circumstellar disks and stellar wavelength-calibrate the spectra. Comparison lamp spectra winds in regulating rotational velocities(Cameron & Li were obtained only at the beginning and end of the night, 1994; Keppens,MacGregor, & Charbonneau 1995); (3) and hence the wavelength calibration for each target spec- study internal mixing mechanisms in low-mass stars via the trum is slightly in errorÈmostly as a zero-point shiftÈ mass and age dependence of the surface abundances of owing to Ñexure and thermal variations during the night beryllium, boron, and lithium(Garc•a Lopez et al. 1994; (we correct for this in a subsequent step, as discussed Boesgaard 1991; Chaboyer, Demarque, & Pinsonneault below). At the 3.6 m telescope, we used the CASPEC echelle 1995); and (4) study the age and mass dependence of the spectrograph(Pasquini & Gilliotte 1993) with a Tektronix coronal and chromospheric activity of low-mass stars 512 ] 512 CCD, a 31.6 line mm~1 grating, the short camera (Caillault 1996). Implicit in the use of open clusters for this and the red cross-disperser. A 280 km(2Aon the sky) slit purpose is the availability of accurate membership lists to width was used. The wavelength range for the ESO spectra identify sets of stars that can be conÐdently assumed to was 5500È8300Ó, and the 2 pixel resolution was D0.35 Ó. belong to the cluster(s) under study. Usually, the primary Separate Ñat-Ðeld and comparison lamp spectra were component of such membership studies has been a proper- obtained for each program object. IRAF routines were motion survey. However, in some casesÈparticularly for again used for the standard processing and wavelength cali- clusters in the southern hemisphereÈappropriate Ðrst- bration of the spectra. epoch photographic plates have not been available, and Radial and rotational velocities for the program stars therefore it has been necessary to consider alternate means were determined using cross-correlation routines (see Tonry to identify cluster members. Because young, low-mass stars & Davis1979; Hartmann et al. 1986). The stars observed at are usually strong X-ray sources, one means to accomplish ESO were generally G and K dwarfs, and we were able to this goal is to use ROSAT PSPC images of the cluster choose a single echelle order (jj6410È6495) that has a region to sort the relatively small number of active cluster number of moderately strong iron and calcium absorption members from the vastly larger number of background/ lines for the cross-correlation analysis. The template for the foreground Ðeld stars. This technique has recently been ESO cross-correlations was a high signal-to-noise ratio used for the D35 Myr old clusters IC 2391(Patten & Simon (S/N) spectrum of the daytime sky, and the radial velocity 1996)and IC 2602 (Randich et al. 1995). zero point was set via observation of the radial velocity The identiÐcation of cluster members using ROSAT standards HD 126053, HD 136202, HD 154417, and HD imaging data is not a foolproof techniqueÈthe X-ray error 187691(Latham & Stefanik 1991). By comparison of results circles often contain more than one optical candidate, and obtained using di†erent radial velocity standards as the thus the wrong star can be associated with the X-ray source, template, we estimate that for the ESO spectra the radial or in some cases the optical identiÐcation is made correctly velocity accuracies are 1È2kms~1 for the slow rotators but the star is not in fact a cluster member but is instead a (v sin i ¹ 20 km s~1), increasing to several km s~1 for stars Ðeld star whose photometric characteristics approximately with v sin i [ 50 km s~1. match those expected for members of the cluster. For this The stars observed at CTIO covered a wider spectral type reason, it is necessary to supplement the ROSAT data and range, including G, K, and M dwarfs, and so it was not optical photometry with other data that provide additional possible to use a single order for all of the analysis.
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