R E P O RT S F R O M O B S E RV E R S A Study of the Activity of G and K Giants Through Their Precise Radial Velocity; Breaking the 10-m/sec Accuracy with FEROS J. SETIAWAN 1, L. PASQUINI 2, L. DA SILVA3, A. HATZES4, O. VON DER LÜHE1, A. KAUFER2, L. GIRARDI 5, R. DE LA REZA3, J.R. DE MEDEIROS6 1Kiepenheuer-Institut für Sonnenphysik, Freiburg (Breisgau), Germany; 2European Southern Observatory 3Observatorio Nacional, Rio de Janeiro, Brazil; 4Thüringer Landessternwarte Tautenburg, Tautenburg, Germany 5Dipartimento di Astronomia, Università di Padova, Padova, Italy 6Universidade Federal do Rio Grande do Norte, Natal, Brazil 1. Scientific Background giant stars and their progenitors. In par- ible stellar surface (as a result of stellar ticular, when combining accurate dis- rotation), they will also induce variabili- Asteroseismology is an indispensa- tances (e.g. from HIPPARCOS), the ty in the core of deep lines, as the Ca II ble tool that uses the properties of stel- spectroscopic determination of the H and K, which are formed in the chro- lar oscillations to probe the internal chemical composition and gravity along mosphere (see e.g. Pasquini et al. structure of stars. This can provide a di- with the oscillation spectrum, the stellar 1988, Pasquini 1992). Since FEROS rect test of stellar structure and evolu- evolutionary models will be required to allows the simultaneous recording of tion theory. Precise stellar radial veloc- fit all these observations. This could the most relevant chromospheric lines, ity (RV) measurements made in recent provide an unprecedented test bench it will be possible to test directly from years have not only discovered the first for the theories of the stellar evolution. our spectra the rotational modulation extra-solar planets, but have also un- Before this can be done, however, one hypothesis. In Figure 1, CaII K line covered new classes of low-amplitude must derive the full oscillation spectrum spectra of one target star taken at dif- variable stars. One such is represented (periods and amplitudes) for a signifi- ferent epochs are over-imposed: no ev- by the K giant stars which exhibit RV cant number of giant stars. idence for strong variability in the line variations with amplitudes in the range Although the short-period variations core is detected. of 50–300 m/s (Walker et al. 1989, in giants can only arise from radial and The expected rotational periods for Hatzes & Cochran 1993,1994 ab). This non-radial pulsation, the nature of the some giants are consistent with the ob- variability is multi-periodic and occur- long-period RV variations is still open to served periods and some evidence for ring on two time- scales: less than 10 debate. Possibilities include planetary equivalent-width variations of activity days and several hundreds of days. companions, rotational modulation by indicators accompanying the RV varia- In the most detailed study of a K gi- surface structure, or non-radial pulsa- tions have been found in two K giants ant star, Merline (1996) found 10 pulsa- tion. Each of these hypotheses has a (Larson et al. 1993; Lambert 1987). If tion modes with periods ranging from high astrophysical impact, and there the RV variations were caused by sur- 2–10 days in Arcturus. These modes are strong arguments to support each face inhomogeneities, then these were equally spaced in frequency, the of them. would be large enough to be resolved characteristic signature of p-mode os- When surface structures, such as ac- by future ground-based interferometry. cillations analogous to the solar 5- tive regions and spots, move on the vis- These stars will thus make excellent minute oscillations. The relatively large number of modes that may exist in gi- ant stars means that these objects may be amenable to asteroseismic tech- niques. Asteroseismology is one of the next milestones in astrophysics, and presently there are relatively few class- es of stars on which these techniques can be applied, so it is important when more such objects are discovered. Asteroseismology can be used to de- rive such fundamental stellar parame- ters as mass and radius. This is partic- ularly useful for giant stars as these oc- cupy a region of the H–R diagram where it is difficult to obtain accurate stellar parameters. Furthermore, the evolutionary tracks of main-sequence stars in the spectral range A-G, all con- verge on the giant branch, so it is im- possible to establish the nature of the progenitor star only from these theoret- ical tracks. Asteroseismology may play Figure 1: Ca II K line observations of one of the target stars. Observations taken in different a key role in understanding the stellar nights are overlapped. In case of rotational modulation induced by surface inhomogeneities, properties, structure, and evolution of the chromospheric core of this line will show detectable variations. 13 (2) its high effi- proaching that of other, state-of-art ciency permitted techniques. We will outline our method- the acquisition of ology in the data reduction and provide data on a large some tips for the potential users. sample of stars in one night, and fi- 3. Pushing the FEROS nally, Capabilities (3) the radial velocity accuracy Although not conceived explicitly for of 23 m/s, which very accurate radial-velocity measure- was shown during ments (original requirements were set the commission- to an accuracy of 50 m/sec), FEROS ing period (Kaufer has been equipped with a double-fibre et al. 1999) was at system, where the second fibre can be a level adequate used either for recording the sky or for for the study of gi- obtaining simultaneous calibration ant variability. We spectra to the science exposure (see also were confi- Fig. 3, which shows a part of a frame dent that the containing the stellar and simultaneous FEROS perform- calibration spectra), following the tech- ances could be nique pioneered by the Geneva group improved with a with ELODIE and CORALIE (Baranne targeted strategy et al. 1996). Figure 2: H-R diagram for the 63 stars observed so far for our pre - and by upgrading By using a simple cross-correlation cise radial velocity survey with FEROS. The sample well covers the the software and (Kaufer et al., 1999, see also FEROS Red Giant Branch and the “clump” region. the data analysis. user manual) in the commissioning As part of the time, it was shown that FEROS could targets for VLTI observations (von der ESO time on FEROS we obtained in obtain a radial-velocity accuracy of 23 Lühe et al. 1996). Periods 64 and 65 a total of 5 full and 6 m/sec by observing every night the G The non-radial g-mode oscillations half nights. An additional 12 half nights star HD10700 (Tau Cet), an object seem unlikely, since these are not ex- were allocated as part of the Brazilian which has been demonstrated to have pected to propagate through the deep time. Early in our programme, after sur- a radial velocity constant to better than outer convection zone in these stars, veying only 27 stars, we had indications 5 m/s (Butler et al. 1996). but nature is always full of surprises. that 80% of our target objects showed A “constant’’ star is extremely useful Alternatively, these long-period varia- variations on the time-scales of a few to to estimate the long-term accuracy of tions may represent more exotic pulsa- hundreds of days. This is consistent FEROS and to provide a standard for tions such as toroidal modes. The plan- with the results of earlier, more limited optimising the radial velocity analysis. et hypothesis is supported by these pe- surveys. Variability in G and K giants Demonstrating a lack of RV variability riods being long lived (for more than 12 may indeed be ubiquitous. In Period 65 in a standard star would make us more years) and coherent. In the case of the we could enlarge the sample which confident of our results on variable star Aldebaran, the long-period RV vari- now consists of 63 stars covering a stars in the programme. For this pur- ations were not accompanied by line- large fraction of the upper HR-diagram. profile variations (Hatzes & Cochran The H-R diagram of the observed stars 1998). Although these seem to exclude is shown in Figure 2. From this figure it rotations and pulsations, conclusive ev- is clear that the sample spans the idence in support of the planetary hy- whole Red Giant Branch as well as the pothesis has proved elusive. region of the “clump”. The targets were selected on the basis of accurate HIP- 2. Observations PARCOS parallaxes in order to ensure a reliable determination of the basic K giant RV variability is still a largely stellar parameters. The high resolution unexplored area. Not only is the nature and high S/N ratio of the FEROS spec- of the long-period variations unknown, tra will also allow the spectroscopic de- but a knowledge of the characteristics termination of effective temperatures, of the short-period (p-mode) oscilla- gravities and metallicities. With such an tions as a function of a giant’s position extensive data base it will be possible in the H–R diagram is also lacking. to investigate the dependence of the ra- For these reasons, in ESO Period 64 dial velocity variability on a wide variety (October 1999 – April 2000), we started of stellar parameters, including the es- an unprecedented survey of precise ra- timated stellar mass and evolutionary dial-velocity measurements as a Euro- status. This is important for calibrating pean-Brazilian collaboration aimed at theoretical evolutionary tracks.