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Beryllium and Iron Abundances of the Solar Twins 16 Cygni a and B Constantine P

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Beryllium and Iron Abundances of the Solar Twins 16 Cygni a and B Constantine P View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Clemson University: TigerPrints Clemson University TigerPrints Publications Physics and Astronomy 5-1-2000 Beryllium and Iron Abundances of the Solar Twins 16 Cygni A and B Constantine P. Deliyannis Indiana University Katia Cunha Obervatorio Nacional, CNPq, Rua General Jose Cristano Jeremy R. King Clemson University, [email protected] Ann M. Boesgaard University of Hawaii Follow this and additional works at: https://tigerprints.clemson.edu/physastro_pubs Recommended Citation Please use publisher's recommended citation. This Article is brought to you for free and open access by the Physics and Astronomy at TigerPrints. It has been accepted for inclusion in Publications by an authorized administrator of TigerPrints. For more information, please contact [email protected]. THE ASTRONOMICAL JOURNAL, 119:2437È2444, 2000 May ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. BERYLLIUM AND IRON ABUNDANCES OF THE SOLAR TWINS 16 CYGNI A AND B CONSTANTINE P. DELIYANNIS1,2 Department of Astronomy, Indiana University, 319 Swain West, 727 East Third Street, Bloomington, IN 47405-7105; con=athena.astro.indiana.edu KATIA CUNHA Observato rioNacional, CNPq, Rua General Jose Cristino 77, 20921-400Sa8 o Cristo va8 o , RJ, Brazil; katia=baade.physics.utep.edu JEREMY R. KING1 Department of Physics, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154-4002; jking=bartoli.physics.unlv.edu AND ANN M. BOESGAARD1 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; boes=galileo.ifa.hawaii.edu Received 1999 May 24; accepted 1999 November 9 ABSTRACT Red (signal-to-noise ratio of S/N D 1000 pixel~1) and ultraviolet(S/N Z 100 pixel~1) Keck High Resolution Echelle Spectrograph (HIRES) spectra (R D 45,000 \ 3 pixels) are used to derive the iron (Fe) and beryllium (Be) abundances in each of the solar twins 16 Cygni A and B. Self-consistent spectroscopic \ ^ ^ \ ^ solutions yield, for 16 Cyg A and B, respectively,Teff 5795 20 and 5760 20 K, log g 4.30 0.06 and 4.40 ^ 0.06, m \ 1.25 ^ 0.05 and 1.12 ^ 0.05 km s~1, and [Fe/H] \ 0.04 ^ 0.02 and 0.06 ^ 0.02. If Fe is used as a surrogate for metallicity, this represents an average metallicity of 11% ^ 5% above solar. These are in excellent agreement with other recent studies of this (wide) binary. Whereas it can be argued that no single study is conclusive, the consistent Ðndings of these various studies o†er compelling evidence that these stars have just barely supersolar metallicity, that 16 Cyg A is just hotter than the Sun, and that 16 Cyg B is just cooler. We have previously reported (based on Keck HIRES data) a di†erence in the lithium (Li) abundances of these stars of at least a factor of 4.5; for 16 Cyg A we detected a Li abundance of a factor of D2 above solar, and for 16 Cyg B we placed a conservative upper limit of a factor of D3 below solar. We detect Be in both stars and Ðnd that, if there is any di†erence between them, it must be much smallerÈconservatively no more than 0.2 dex. Evidence suggests that solar-type stars deplete their surface Li abundance during the main sequence, a feat that the standard stellar evolution theory has, thus far, been unable to accomplish. Whatever physical mechanism depletes the surface Li abundance must create far less of a spread in the Be abundances than it does in the Li abundances. We Ðnd that our Li and Be results are consistent with the predictions of Yale models that include rotationally induced mixing driven by angular momentum loss. Our results provide no evidence for a small (D0.05 dex) enhancement in the 9Be abundance of the A component relative to the B com- ponent expected if the starsÏ Li abundance di†erence was due to accretion of planetary material by the A component. Given the errors, however, neither are we able to Ðrmly preclude such a signature. Key words: binaries: general È circumstellar matter È planetary systems È stars: abundances È stars: evolution È stars: fundamental parameters È stars: individual (16 Cygni A, 16 Cygni B) È stars: late-type 1. INTRODUCTION We shall therefore refer to these stars as solar twins.3 These stars have become of greater interest because Cochran et al. The binary system 16 Cygni comprises two early G (1997) have recently detected aZ1.5 MJ (Jupiter mass) com- dwarfs (the A component HR 7503 and the B component panion to 16 Cyg B from precision radial velocity measures. HR 7504), which have recently been the subject of great scrutiny. Cayrel de Strobel (1990) proposed that 16 Cyg B was one of only a few nearby genuine ““ solar twins.ÏÏ The spectroscopic analysis of Friel et al. (1993) conÐrmed that ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ ^ 3 both components had aTeff within 20 K of the Sun and a It has sometimes been suggested that the terms ““ solar twins ÏÏ be metallicity only barely higher (]0.05 dex) than the SunÏs. reserved for stars that are not in any way measurably di†erent from the Sun, the next most general category being solar analogs. However, this implies an unsettling degree of impermanency, as breakthroughs in tech- nology can imply the sudden and sad demotion of the status of a star from solar twin to solar analog. Furthermore, in practice, the category of solar ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ analogs encompasses, in our view, a rather broad range of spectral charac- 1 Visiting Astronomer, W. M. Keck Observatory, jointly operated by teristics (F to K stars). Keeping in mind also that, like human twins, the California Institute of Technology, the University of California, and whoÈupon close examinationÈshow subtle di†erences, we propose that the National Aeronautics and Space Administration. 16 Cyg A and B are sufficiently similar to the Sun and to each other that 2 Hubble Fellow. they can be designated ““ solar twins.ÏÏ 2437 2438 DELIYANNIS ET AL. Vol. 119 King et al. (1997b) carried out detailed analysis of the Li I Z4.5 di†erence in the Li abundances of these stars, the j6707 region in the Sun and in 16 Cyg A and B. Despite the precise parameters are then used to derive Be abundances near-identical parametersÈand, presumably, initial via spectrum syntheses, to see whether 16 Cyg A and B compositionÈof 16 Cyg A and 16 Cyg B, their current demonstrate any di†erence in their Be abundances. The photospheric lithium (Li) abundances di†er by a factor of implications of our results are discussed in ° 4. Z4.5. As King et al. (1997b) discuss, this represents a funda- mental (and nontrivial) failure of standard stellar models, 2. DATA whose Li abundances are determined uniquely by mass, age, The optical spectroscopy of 16 Cyg A and B we employ and composition. Li-abundance di†erences are also for stellar parameter determination is that presented by observed in otherwise identical solar-type stars in open King et al. (1997b), to which the reader is referred for fuller clusters, as well. Notable examples include the young details. BrieÑy, the High Resolution Echelle Spectrograph Pleiades (Soderblom et al. 1993b), the Hyades-age Praesepe (HIRES) spectrograph on the 10 m Keck I Telescope at the (Soderblom et al. 1993a), and NGC 6633 (Je†ries 1997) clus- W. M. Keck Observatory was used to obtain high- ters and the old M67 cluster (see, e.g., Deliyannis et al. 1994; resolution (R D 45,000 \ 3 pixels) spectra with incomplete Pasquini, Randich, & Pallavicini 1997; Jones, Fischer, & coverage from 4480 to 6760A . The Poisson-based signal- Soderblom 1999). Even the cluster with the tightest known to-noise ratio (S/N) measured near 6700A is 750 and 1050 Li-Teff relation, the Hyades, shows clear examples of Li dis- pixel~1 for 16 Cyg A and B, respectively. Samples of the persion (Thorburn et al. 1993; see their Fig. 11). spectra in wavelength regions containing Fe I and Fe II lines These examples of Li dispersion are one part of an accu- used in our analysis are shown in Figure 1. mulating body of evidence revealing the incompleteness of We derive Be abundances from near-UV spectroscopy standard stellar models. Other examples include (1) evi- obtained with Keck HIRES on 1994 July 4 (UT) using the dence for main-sequence Li depletion (see, e.g., Soderblom blue optics and a 2048 ] 2048 Tektronix CCD, in the et al. 1993c; Je†ries 1997), (2) higher Li abundances often region of the Be II resonance doublet at 3130.42 and 3131.06 observed in short-period binaries of various stellar popu- A . The measured (3 pixel FWHM) resolution is again lations (Ryan & Deliyannis 1995), (3) theLi-Teff morphol- R D 45,000. Two 10 minute exposures of 16 Cyg A and ogy of and signiÐcant star-to-star scatter in the Li three 10 minute exposures of 16 Cyg B yielded S/N of D110 abundances of solar-age subgiants in the open cluster M67 and 115 pixel~1, respectively, near the Be II lines. Multiple (Deliyannis, King, & Boesgaard 1997), and (4) the corre- internal Ñat-Ðeld lamp, ThAr lamp, and bias-frame expo- lated depletion of Li and beryllium (Be) observed in late F sures were acquired during the night. After preliminary pro- and early G stars (Deliyannis et al. 1998). All of these exam- cessing carried out with standard IRAF4 tasks, Ñat-Ðelding, ples support the argument for the need to consider the order identiÐcation, tracing, and one-dimensional extrac- action of additional physical mechanisms in low-mass stars, tion were performed using the specialized suite of FIGARO possibly slow mixing related to rotation.
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