A&A manuscript no. (will be inserted by hand later) ASTRONOMY AND Your thesaurus codes are: ASTROPHYSICS 08 (08.02.4; 08.03.2; 08.05.3; 08.18.1; 08.18.1) 6.3.2018 Are metallic A-F giants evolved Am stars? Rotation and rate of binaries among giant F stars ⋆, ⋆⋆ M. K¨unzli and P. North Institut d’Astronomie de l’Universit´ede Lausanne, CH-1290 Chavannes-des-Bois, Switzerland Received 3.12.96; accepted 25.5.97 Abstract. We test the hypothesis of Berthet (1992) 1. Introduction which foresees that Am stars become giant metallic A and Main sequence A and F stars show various interesting phe- F stars (defined by an enhanced value of the blanketing nomena: chemical peculiarities (Am,Fm,Ap...) on the one parameter ∆m of the Geneva photometry) when they 2 hand and/or pulsation on the other hand. Numerous stud- evolve. ies have been devoted to this part of the HR diagram near If this hypothesis is right, Am and metallic A-FIII stars the ZAMS, but few have explored more evolved stars. In need to have the same rate of binaries and a similar dis- this paper, we examine the rate of binaries and rotation tribution of v sin i. From our new spectroscopic data and of giant A and F stars. from v sin i and radial velocities in the literature, we show In a study of 132 giant A and F stars, Hauck (1986) that it is not the case. The metallic giant stars are of- showed that 36 % of them have an enhanced value of the ten fast rotators with v sin i larger than 100 kms−1, while blanketing parameter ∆m2 (defined roughly speaking by the maximum rotational velocity for Am stars is about − −1 m2(observed) m2(Hyades)) characterising Am and δ Del 100 kms . The rate of tight binaries with periods less stars. He showed also that this parameter could probably than 1000 days is less than 30 % among metallic giants, be interpreted in terms of metallicity for giant A and F which is incompatible with the value of 75 % for Am stars stars. This interpretation was confirmed by Berthet (1990, (Abt & Levy 1985). Therefore, the simplest way to explain 1991) from a detailed abundance analysis of such objects. the existence of giant metallic F stars is to suggest that His study pointed out that these stars have chemical prop- all normal A and early F stars might go through a short erties similar to those of δ Del stars, i.e. an overabundance ”metallic” phase when they are finishing their life on the of iron peak elements and especially of heavier elements main sequence. such as Sr and Ba, and solar composition for Ca and Sc. Besides, it is shown that only giant stars with spec- Am stars have similar characteristics for iron peak ele- tral type comprised between F0 and F6 may have a really ments, but Ca and Sc are deficient by factors of about enhanced ∆m2 value, while all A-type giants seem to be 10. normal. Based on chemical abundances, position in the (β,Mv) arXiv:astro-ph/9710226v1 21 Oct 1997 plane of Str¨omgren photometry, rotation and duplicity of Key words: Stars: binaries: spectroscopic - stars: chem- δ Del stars, Kurtz (1976) suggested that these stars are ically peculiar - stars: evolution - stars: rotation - stars: δ evolved Am stars. The fact that δ Del and giant metallic A Scuti and F stars harbour similar chemical properties suggests also a link between Am and giant metallic A and F stars. Thus, a star could begin its life on the main sequence as an Am star, then, as abundances of Ca and Sc increase to quasi-solar values with evolution, it would become a δ Del star and finally a giant metallic A-F where Ca and Sc are solar (Berthet 1992). Send offprint requests to: P. North To explain the emergence of chemical anomalies in Am ⋆ Based on observations collected at Observatoire de Haute stars, one generally invokes the radiative diffusion theory Provence (OHP), France. developed by Michaud et al (1983). This theory predicts ⋆⋆ Tables 2, 3, 6 are also available in electronic form at CDS that, in slow rotators, helium is no longer sustained and via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via flows inside the star and gradually disappears from the at- http://cdsweb.u-strasbg.fr/Abstract.html mosphere. The diffusion process could therefore take place 2 M. K¨unzli et al.: Are metallic giants evolved Am stars? just below the thin H convective zone where the diffu- measured at OHP some of Hauck’s stars having no ra- sion time is short with respect to the stellar lifetime; the dial velocities or v sin i, or only old determinations. These chemical elements whose radiative acceleration is larger data should allow to strengthen a preliminary work (North than gravity become overabundant and, in the opposite 1994) which casts some doubts on the validity of the sce- case, underabundant. The convective zone becomes deeper nario advocated by Berthet (1992). with evolution and so leads to normalisation of the surface abundance. This theory explains the chemical anomalies 2. The sample and the observations in Am stars and the increase of Ca and Sc abundances with time. We defined the sample to be observed from Table 3 and The chemical anomalies predicted by the diffusion the- Table 4 of Hauck (1986). The reason why we considered ory are generally larger than the observed anomalies. Table 4 (containing stars classified spectroscopically as These differences come from uncertainties of the atomic dwarfs but photometrically as giants) was that the pho- data but also from some mechanism in the stellar atmo- tometric criterion of luminosity seems much more reliable sphere. The mass loss probably takes a prominent part: than the spectroscopic luminosity class. −15 a rate of about 10 M⊙/year is sufficient to reduce the The selection criteria were: theoretical overabundance of heavy elements to observed - Visibility from OHP (δ & −20o) abundances (Charbonneau & Michaud 1991). Fast rota- − - Insufficient V data or unknown v sin i tion (v sin i > 120 kms 1) generates meridional circu- r lation which prevents the disappearance of the helium By insufficient Vr, we mean that the star has less than ionisation zone, so the metallic anomalies cannot appear three published Vr values in the literature. Applying these (Michaud 1983). Observationally, the upper limit of ro- criteria, 50 A and F giant stars were selected, both nor- tation for Am stars is about 100 kms−1 (Abt & Moyd mal and metallic. It was judged useful to have a good 1973), while for normal stars, we observe projected rota- estimate of the binary freqency of normal A and F giants tional velocities larger than 100 km s−1, which is in agree- for reference purposes. 40 stars in the sample are non- ment with the diffusion theory. The meridional circulation metallic, while only ten are metallic according to the cri- cannot be invoked to reduce theoretical abundances to terion ∆m2 ≥ 0.013 (see section 5.1). Thus, the estimate observed ones: in some cases, they can increase the the- of binary frequency among metallic giants will mainly rely oretical abundances and qualitatively no relation exists upon old, published Vr data. The ∆m2 values of the sam- between v sin i and the chemical anomalies in the velocity ple stars range between -0.027 and 0.082. They have been −1 range characteristic of Am and Fm stars (0-100 kms ). updated using a new reference sequence in the m2/B2−V 1 Indeed, the time diffusion under the hydrogen convective diagram (Hauck et al. 1991), and sometimes by comple- zone is shorter than that of the meridional circulation. mentary photometric measurements. Eight stars belong to The rate of binaries is completely different in Am and the δ Scuti class, and nine are spectroscopically classified normal stars: Am stars are often members of tight bina- as dwarf but have a large ∆d parameter (∆d is the lu- ries (P ≤ 100 days), while normal stars in double systems minosity parameter of Geneva photometry, equivalent to often have larger periods (P > 100 days). So, we would Str¨omgren’s δc1) indicating that they very probably are be tempted to explain the low rotation of Am stars by real giants. All stars are bright, with V ≤ 7.0, implying tidal braking. Nevertheless, Zahn (1977) showed that this that interstellar reddening is insignificant. effect is really important only for tight binaries with a The observations were performed at the Observatoire period less than 7 days. On the other hand, Abt & Levy de Haute Provence (OHP) with the Aurlie spectrograph (1985) showed that 75 % of Am stars have periods below attached to the 1.52m telescope at the Coud focus (Gillet 1000 days, so they do not necessarily belong to tight bi- et al 1994) in 1994. Four runs of a few nights’ duration naries. Other mechanisms must contribute to reduce the each were made respectively at the beginning of May and rotational velocity to 100 km s−1 or less. One of them may June for 18 stars and at the beginning of November and be the evolutionary expansion of stars during their main December for 32 stars. These measurements should allow sequence lifetime. According to Abt & Levy (1985), dur- to detect binary stars with a small period (P . 100 days). ing this phase, the velocity decreases by a factor of 2 and The detector is a double barrette CCD Thomson consequently most of the normal A stars may become Am TH7832 with 2048 pixels having a size of 750 x 13 µm.