Solar twins in M67 Based on observations collected at the ESO VLT, Paranal Observatory, Chile, program 278.D-5027(A).

Solar twins in M67

Key Words.:
stars: fundamental parameters – open clusters and associations: individual: M67 – stars: late-type
2

Abstract

Context:The discovery of true solar analogues is fundamental for a better understanding of the Sun and of the solar system. Despite a number of efforts, this search has brought only to limited results among field stars. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very similar to those of the Sun.

Aims:We analyze FLAMES spectra of a large number of M67 main sequence stars to identify solar analogues in this cluster.

Methods:We first determine cluster members which are likely not binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on the determination of stellar effective temperature, using analyses of line-depth ratios and H wings, making a direct comparison with the solar spectrum obtained with the same instrument. We also compute the lithium abundance for all the stars.

Results:Ten stars have both the temperature derived by line-depth ratios and H wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening , the solar colour and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars have also a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution.

Conclusions:We find several candidates for the best solar analogues ever. These stars are amenable to further spectroscopic investigations and planet search. The solar colours are determined with rather high accuracy with an independent method, as well as the cluster distance modulus.

1 Introduction

The specificity of the Sun and of our solar system have been the subject of active investigation over the last 5 decades. How typical is the Sun for a star of its age, mass, and chemical composition? How typical is that solar-type stars host planetary systems? Are they similar at all to ours?

The quest to find stellar analogues to the Sun has been going on for a long time (for an extensive review see, e.g., Cayrel de Strobel 1996), and it stems from the poor knowledge we have of the Sun when seen ‘as a star’ and from how typical the Sun is for a G2 type star, for its age, chemical composition, population. It is, however, after the discovery of the first exo-planets (Mayor & Queloz 1995) that this quest became even more compelling, because to find stars similar to our own would allow us to answer to fundamental questions related to the origin of the solar system, the frequency of planetary systems similar to ours, and eventually the formation of life in other exo-planetary systems (Cayrel de Strobel 1996). The need to identify in the night sky solar proxies to be used for spectroscopic comparison is also diffuse, in particular for the analysis of small solar system bodies (Böhnhardt, private communication).

Among the most recent results in this research, Meléndez et al. (2006) used high resolution, high signal-to-noise ratio Keck spectra to show that HD 98618 is a very close solar twin, and King et al. (2005) proposed HD 143436 after analyzing 4 stars pre-selected from literature. These stars seem to compare well with the best known solar twin, HR 6060, first analyzed by Porto de Mello & da Silva (1997), and subsequently confirmed by Soubiran & Triaud (2004), who made a comparative study of several hundreds of ELODIE spectra. Finally, Meléndez & Ramírez (2007) have shown HIP 56948 to be the best solar twin known to date both in stellar parameters and in chemical composition, including a low lithium abundance.

The open cluster M67 is a perfect target to search for solar analogues. Recent chemical analyses (Tautvaišiene et al. 2000; Randich et al. 2006; Pace et al. 2008), show that this cluster has a chemical composition (not only Fe, but also all the other elements) extremely similar to the solar one, as close as allowed by the high precision of the measurements. The analysis resulted in [Fe/H]=0.030.03 for Tautvaišiene et al. (2000), [Fe/H]=0.030.01 for Randich et al. (2006), and [Fe/H]=0.030.03 for Pace et al. (2008).

There are other two additional characteristics which make M67 strategical. The first one is that all the determinations of age give for this cluster an age encompassing that of the Sun (3.5-4.8 Gyr; Yadav et al. 2008), while the age determination for field stars is always uncertain. The second characteristic is that M67 is among the very few clusters showing Li depleted G stars (Pasquini et al. 1997). This is an important point because, as pointed out by Cayrel de Strobel (1996), even if many stars appear to have most characteristics similar to the Sun, their Li abundance is usually 10 times higher than in our star. Since Li is likely an indicator of the complex interaction taking place in the past between the stellar external layers and the hotter interior, the choice of stars which also share the same Li abundance with the Sun is an additional property to pinpoint the true analogues.

In our opinion, the search of analogues to the Sun and to the solar system can be well performed in open clusters (OCs), which show a homogeneous age and chemical composition, common birth and early dynamical environment. As a consequence, they provide an excellent laboratory for investigating the physics of solar stars and of planetary system evolution, besides being excellent probes of the structure and evolution of the Galactic disk.

M67 is a rich cluster, therefore it provides us with the opportunity to find many stars candidates sharing similar characteristics, and not only one. This is fundamental to obtain some meaningful statistics, and the cluster hosts many main sequence (MS) stars of mass around the solar mass, which form a continuous distribution (Fig. 1).

Finding several solar analogues in M67 will also help in providing an independent estimate of the solar colors, a quantity which still suffers of some relevant uncertainty (see, e.g., Holmberg et al. 2006), as well as an independent estimate of the distance modulus of the cluster.

The present paper is the culmination of a work, which involved the chemical determination of this cluster (Randich et al. 2006; Pace et al. 2008), photometry and astrometry (Yadav et al. 2008) to obtain membership, and FLAMES/GIRAFFE high resolution spectroscopy to clean this sample from binaries, and to look for the best solar analogues using the line-depth ratios method (Gray & Johanson 1991; Biazzo et al. 2007) and the wings of the H line (Cayrel de Strobel & Bentolila 1989) to determine accurate temperatures with respect to the Sun. In addition, the Li line is used to separate Li-rich from Li-poor stars.

Figure 1: Portion of the colour-magnitude diagram of M67. Our selected targets encompass the solar colour, and are high probability proper motion (Yadav et al. 2008) and radial velocity single members. The red points refer to the stars observed with FLAMES/GIRAFFE in three nights.

2 Observations and data reduction

We obtained 2.5 hours in three observing nights in service mode with the DDT program 278.D-5027(A). Observations were carried out with the multi-object FLAMES/GIRAFFE spectrograph at the UT2/Kueyen ESO-VLT (Pasquini et al. 2002) in MEDUSA mode3; we were able to observe 90 targets (Table 3). The employed setting was HR15N with central wavelength 665.0 nm, which with a wavelength range between 644 and 682 nm covers simultaneously the H and the Li i resonance doublet at 670.8 nm with a resolution of R17 000. Three separate exposures were obtained to be able to identify short and intermediate period binaries by comparing the radial velocities at different epochs; the combined spectra have a typical signal-to-noise () ratio of 80-110/pixel.

We selected from the catalog of Yadav et al. (2008) the main sequence stars () with close to that of the Sun (0.60–0.75) with the best combination of proper motions parameters, that is a membership probability superior to 60%, and exclusion of candidates with a proper motion larger than 6 mas/yr with respect to the average cluster members. Full details about proper motion errors and selection criteria can be found in the original Yadav et al. (2008) work.

The log of the observations is given in Table 1. The observations were reduced using the ESO-GIRAFFE pipeline.

Radial velocities were measured using the IRAF4 package FXCOR, which cross-correlates the observed spectrum with a template. As a template we used a solar spectrum acquired with FLAMES/GIRAFFE. Finally, the heliocentric correction was applied. The typical error for our single measurement is around 0.4 . The three spectra/star were finally co-added to perform the spectroscopic determination of temperature and lithium abundance (see Sections 3.2 and 3.3).

We note that the GIRAFFE solar spectrum5, taken with the same setup of our observations, is used through this work for spectroscopic comparison with the stars and the synthetic spectra. The solar spectrum has been obtained by averaging most of the GIRAFFE spectra (some show clear flat field problems and have not been used) and it has a nominal ratio above 400.

Date UT DIMM seeing
(d/m/y) (h:m:s) (s) (arc sec)
132.875 11.833 06/02/2007 06:24:29 2200 0.6
132.875 11.833 11/02/2007 04:01:08 2200 1.1
132.875 11.833 23/02/2007 01:34:55 2100 0.9
Table 1: Log of the observations.

3 Data analysis and membership

3.1 Radial velocity

Out of the 90 stars observed, all selected on the proper motion and membership criteria given above, we found that 59 of them are probable single radial velocity (RV) members. We have retained all the stars which show RV variations smaller than 1 km s in the three exposures acquired and which have a mean velocity within 2 sigma (1.8 km s) from the median cluster RV. In Fig. 2 the histogram of the radial velocity distribution of these stars is shown, together with a Gaussian fit with =32.90 km s and a km s. In Table 4 the RV values are listed for the stars of the final sample, while in Table 5 the values of the single RV measurements are given for the stars we discarded.

In Fig. 3 we show the enlarged portion of the colour-magnitude diagram CMD containing the original sample; in this Figure the discarded and the retained stars are indicated with different colours. Many of the discarded stars tend to occupy the brighter side of the main sequence, where binaries are indeed expected to be present. On the other hand, our procedure still leaves several stars which are apparently above the photometric main sequence. This is because the radial velocity measurements are not of superb quality and because the time span by the observations is of only 18 days. Long period binaries will not be discovered by our three radial velocity observations. We shall see as seven stars clearly stand up also in the Magnitude – Temperature diagram (see Fig. 5) and they are best candidates for binaries of similar mass. We have kept them in the sample, and we anticipate that their presence does not influence our analysis or conclusions.

Figure 2: Histogram of the radial velocity distribution of the 59 single members selected in M67 (continuous line). A Gaussian fit to the member stars distribution is also displayed (dashed line).
Figure 3: Zoom of the M67 CMD, centered on the targets observed. In blue the 59 retained single member candidates are shown, in red the stars discarded. As expected by binary contamination, most of the discarded stars lay on the bright side of the MS.

3.2 Effective temperature

Given that our targets are on the main sequence of a cluster of solar metallicity and age, the critical astrophysical parameters for the selection of the best solar analogues is the effective temperature. We have used two spectroscopic methods to compute the stellar effective temperature: the line-depth ratios and the H wings. To calibrate these methods we have used a grid of synthetic spectra, computed with SYNTHE from a grid of 1D LTE model atmospheres computed with version 9 of the ATLAS code (Kurucz 1993a, b) in its Linux version (Sbordone et al. 2004; Sbordone 2005). All the models have been computed with the “NEW” Opacity Distribution Functions (Castelli & Kurucz 2003) which are based on solar abundances from Grevesse & Sauval (1998) with 1  micro-turbulence, a mixing-length parameter  of 1.25 and no overshooting. The grid of synthetic spectra covers the temperature range 5450–6300 K with [Fe/H]=0, =4.4377, =1 km s and was degraded to the resolution of the FLAMES/GIRAFFE spectra. We stress that for both methods these models are used to quantify the difference between the stellar spectra and the solar spectrum. Zero point shifts are most likely present, due, for instance, to limitations in the atmospheric models or to not perfect treatment of the H lines. While these inaccuracies will reflect in a wrong temperature for the Sun, the difference between the stars and the Sun will be much less affected.

LDR method

It has been demonstrated that for stars with line-depth ratios (LDRs) are a powerful temperature indicator, capable to resolve temperature differences lower than 10 K (Gray & Johanson 1991; Catalano et al. 2002; Biazzo et al. 2007). Since our stars are within this range, we have applied the LDR method to the members previously selected by radial velocity measurements (see Table 4). To convert the line-depth ratios of our stars into effective temperature we need to calibrate a temperature scale for the measured LDRs. To this purpose we have considered an initial sample of about 100 lines of iron group elements (which are usually temperature sensitive) present in the spectral range covered by our observations, from which we selected lines with the following characteristics: weak (to avoid saturation effects), sensitive to temperature variations, and at the same time well measurable in our spectra. The final selection contains six line pairs suitable to apply the LDR method; we measured them in the synthetic spectra and then derived an LDR calibration for each pair. In Table 2 we list for these six line pairs the wavelength, the element and the excitation potential, as taken from the NIST6 Atomic Spectra Database Lines, while in Fig. 4 we show an example of one LDR calibration. The methods for the measurement of the line depth and the related uncertainties are described in Catalano et al. (2002) and Biazzo et al. (2007). In a summary, the lowest seven points in the core of each measured line were fitted with a cubic spline and the minimum of this cubic polynomial was taken as the line depth. Given the limited ratio and resolution, the errors in each line depth are dominated by the uncertainty of the signal in the continuum. We have then measured the LDRs and derived for each line pair the temperature; in Table 4 the averaged values for all line pairs are given, where the associated uncertainty reflects the scatter obtained by the six measurements.

LDR
(Å) (eV)
6469.210/6456.380 Fei/Feii 4.84/3.90
6498.937/6516.050 Fei/Feii 0.96/2.89
6608.024/6597.557 Fei/Fei 2.28/4.80
6608.024/6627.540 Fei/Fei 2.28/4.55
6646.932/6627.540 Fei/Fei 2.61/4.55
6646.932/6653.850 Fei/Fei 2.61/4.15
Table 2: List of the six line pairs used to derive the stellar temperature.
Figure 4: Example of LDR calibration obtained with synthetic spectra. The line-depth ratio is between 6498.937 Å and 6516.05 Å.

With this method the effective temperature of the observed GIRAFFE solar spectrum results in 579227 K, i.e. 15 K higher than the synthetic one (5777 K is the theoretical effective temperature of the solar atmosphere; Wilson & Hudson 1991).

We computed the temperature difference between the FLAMES/GIRAFFE targets and the Sun (as obtained from the six line-depth ratios and the summed spectra of the targets) as a function of the de-reddened colour (; Taylor 2007). The relationship for our targets is well described by a linear fit, which gives and an rms of 100 K.

In Fig. 5 the temperature-magnitude diagram is shown. The two colours of the symbols are referred to stars with lower and higher presence of lithium. Seven stars clearly stand out of the main sequence, suggesting a parallel binary sequence. They most likely are long period binaries with components of similar mass not detected as RV variable by our observations, because of the limited time base of our observations.

Figure 5: vs. of the probable candidates. The stars have different colours according to the LTE lithium abundance. Seven stars depart from the main sequence; they are most likely long period binaries which escaped the detection in our observations.

H line

The wings of the H line profile are very sensitive to temperature, like all the Balmer lines, and depend only slightly on metallicity and gravity (Cayrel et al. 1985; Fuhrmann et al. 1993; Barklem et al. 2002). In particular, the spectral region in the range between 3 and 5 Å from the H line center is a good effective temperature diagnostic (Cayrel de Strobel & Bentolila 1989). With respect to the higher members of the series, the H line has considerable less blending in the wings, making easier the placement of the continuum. A further advantage, over other members of the series, is that it is rather insensitive to convection and in particular to the adopted mixing length parameter  in 1D model atmospheres (Fuhrmann et al. 1993). Thus, we selected this region as temperature indicator and for each star we have compared the H line profile outside the core in our real spectrum against the synthetic profile. The hydrogen line profiles are computed in SYNTHE by routine HPROF4, which, for resonance broadening uses essentially the Ali & Griem (1965, 1966) theory, and for Stark broadening it calls routine SOFBET, written by Deane Peterson, which uses essentially the theory of Griem (1960), with modified parameters, so as to provide a good approximation to the Vidal et al. (1973) profiles (F. Castelli, private communication). For further details on the computations of hydrogen lines in SYNTHE, see Castelli & Kurucz (2001) and Cowley & Castelli (2002). The dominant broadening for H is resonance broadening, while Stark broadening becomes relevant for higher members of the series. In order to minimize the subjectiveness of the measurement, we have quantified the comparison between the synthetic profile and the observed one minimizing the rms of the subtraction. Continuum normalization is not easy for such a broad line, however, the fact of using a fiber instrument with a large coverage, minimize the subjectiveness of the process and makes it quite reproducible. Given the limited ratio of the observations, it is however very difficult to provide a realistic estimate of the involved uncertainties. The systematic errors in the effective temperature obtained from the H wings is given by Gratton et al. (2001), and the errors associated to the method have been discussed by, e.g., Bonifacio et al. (2007), where the dominant source of error for échelle spectra has been identified in residuals in the correction of the blaze function. The GIRAFFE spectra are fiber-fed and the flat-field is obtained through the same optical path as the stellar spectra, thus flat-fielding allows a better removal of the blaze function than it is possible for slit spectra. We have estimated for our stars an average error of 100 K. With this method the effective temperature of the Sun results in 5717100 K, i.e. 60 K lower than the solar real value. We note that the absolute temperature determined in this way depends critically on a number of assumptions in the model, and on the adopted broadening theory for H, and these produce a zero point shift of the Sun. The relative measurements, which are made with respect to the observed solar spectrum, are instead rather insensitive to all the assumptions used to build the synthetic profile.

A linear fit well describes the relationship between the temperature difference of the FLAMES/GIRAFFE targets and the Sun as obtained from the H wings, and the de-reddened colour: with an rms of 81 K.

The calibrations vs. obtained with the two methods agree quite well, as shown in Fig. 7; they have slightly different slopes which produce a maximum difference at the red edge () of the sample of 40 K (H temperatures are cooler). These relationships can be used to calibrate stars with metallicity close to solar. Our LDR vs. relationship has almost exactly the same slope of the Alonso et al. (1996) relationship, but it is hotter than this by 60 degrees. As a reference, the Alonso et al. (1996)’s scale produce an effective temperature for the Sun of 5730 K for a .

3.3 Lithium

Lithium is an important element because it is easily destroyed in stellar interiors, and its abundance indicates the amount of internal mixing in the stars. Lithium in Pop I old solar stars varies by a factor 10 (Pasquini et al. 1994) and M67 is one of the few clusters which clearly shows this spread among otherwise similar stars (Pasquini et al. 1997).

Equivalent widths (EWs) of the lithium line at  nm were computed using the IRAF task SPLOT; from measured EWs we derived Li abundances using the curves of growth (COGs) of Soderblom et al. (1993). At the GIRAFFE resolution the Li i lines are blended with the Fe i 670.744 nm line, whose contribution to the lithium blend was subtracted using the empirical correction of the same authors. Lithium abundances were then corrected for the NLTE effects using the prescriptions of Carlsson et al. (1994).

Fig. 6 shows the lithium abundance for the summed spectra of the 59 targets as a function of the effective temperature, as derived by the LDR method. The filled symbols refer to the LTE abundance, while the empty ones represent the non-LTE abundance. The difference between LTE and non-LTE values is minor. The blue points are the solar twins (see Section 4). The position of the Sun is shown at and K (see Section 3.2). The lithium abundance is listed in Table 4. Several solar twin candidates have Li abundances which are comparable with the Sun, whose value is 0.84 as measured by us on the GIRAFFE spectrum (see above), and 1.0 as measured in high resolution solar atlas (Müller et al. 1975). In most investigations the error associated to the effective temperature is usually the dominant one (100 K correspond to about 0.1 dex in ), but in this case, since we have good determination, but limited resolution and ratio, the uncertainty in the abundance associated to the equivalent width measurements is not negligible. The expected uncertainty in the measured lithium equivalent widths has been estimated from Cayrel (1988)’s formula:

where is the signal-to-noise ratio per pixel, is the full width of the line at half maximum, and the pixel size. The predicted accuracy, , is 3.0 mÅ for a typical ratio of 80 and of 1.6 mÅ for a ratio of 150. However, it should be noted that this formula neglects the uncertainty on the continuum placement. We estimate that, using homogeneous procedures for the determination of the continuum and the line widths, the statistical error for the weak lithium line is of the order of 2-3 mÅ, depending on the ratio of the co-added spectrum. This will correspond to an asymmetric error dex for a star with a line as weak as the Sun, and of dex for a star with a =2.2. Since the line is weak, the error (in percentage) is inversely proportional to the line strength. Given the errors, all the stars with upper limits in our sample may have a Li comparable to the solar one.

After the early works on M67, several Li surveys have been carried out of additional clusters well sampling the age metallicity space (Randich 2008 and references therein): out of nine clusters older than the Hyades with available Li measurements, only two, besides M67, show a significant dispersion. The latter seems to be an exception, rather than a rule and its occurrence does not depend on age, nor on metallicity, nor on global cluster parameters.

In this context, the novel result of our analysis and, in particular, of the careful selection and cleaning of the sample as well as of the precise effective temperature determination, is that the large spread is clearly present only for stars cooler than 6000 K. Stars warmer than 6200 K seem to show a decay, probably indicating the red side of the “Li-gap”, while stars in the 6000 K do not show any major scatter.

It is now well ascertained on empirical grounds that, in order to explain the MS Li depletion in solar-type stars, an extra or non-standard mixing mechanism must be at work. No consensus so far has been found on the nature of this mechanism; it is nevertheless clear that, whatever this process is, it must be driven by an additional stellar parameter besides mass and chemical composition. The presence of the Li spread indeed indicates that this parameter must vary from star to star, and that, depending on it, some stars (including the Sun) undergo a much more efficient mixing than others, while the absence of a dispersion for stars warmer than 6000 K suggests that this parameter is more uniform among F-type stars.

Recent modeling have had some success in reproducing the solar Li abundance by using fairly complex models which include internal gravity waves (Charbonnel & Talon 2005), however, those models are not able to reproduce the observed evolution of Li with age, and, in particular, the “plateau” in Li abundances at old ages (Randich 2008). We are not aware of similar models for a grid of masses, to be compared to our observations; since the number of possible parameters which influence the Li evolution is very large (depth of convective zone, initial rotation, magnetic field, mass losses and torques just to mention a few), we cannot really predict at present why the extra mixing takes places at a given in M67 stars.

Figure 6: Lithium abundance versus for the most probable single stars observed in M67. Filled circles: LTE abundances. Open circles: NLTE abundances. The blue points indicate the best solar analogues candidates. The position of the Sun is also shown at and K.

4 Solar analogues

With the aim to find the best solar analogues, we have compared to (Fig. 7).

Figure 7: as a function of for the 59 probable single member stars.

There are in our sample 10 stars for which both the and are within 100 K of the solar values (285, 637, 1101, 1194, 1303, 1304, 1315, 1392, 1787, 2018). We will use these stars to find the best solar analogues and our best evaluation for the solar colours and the cluster distance. These stars are indicated in bold face in Table 4.

The average difference between these 10 stars and the solar is of 13 K (with a sigma of 60 K), while the average is =9 K, with a sigma of 58 K. The average characteristics of these 10 stars should therefore well represent the solar values.

The average of the ten analogues is (), their average magnitude is mag (), and the (). The spread is larger than the formal errors in the photometry, indicating a possible real spread in the stellar characteristics. This is not surprising because, formally, these stars may span a range of up to 200 K in temperature.

If we take our results of Table 3, two stars (637 and 1787) have both determinations within 50 K from the solar values, and three additional ones (285, 1101, 1194) within 60 K; these 5 stars are overall the closest to the Sun, with nominal effective temperatures derived with both methods differing less than 60 K from the solar one. Their average magnitude (14.557 mag) is very similar to what found for the full subsample of 10, as well as their average colour (0.688) just 0.007 magnitude bluer than the whole subsample.

All the data in our possession indicate that some of these stars have a metallicity very close (within 0.03 dex; note that also their Li abundance is comparable; see previous Section) to the Sun, they have a very similar temperature (within 50 K), as well as a comparable age to the Sun, and they are true main sequence stars. To the best of our knowledge they are the best candidates in M67 to be the closest analogues to our star.

In Figure 8 we compare the GIRAFFE spectrum of the Sun with the sum of the spectra of the 10 best stars analogues and of the 5 best analogues in a portion of the spectra which includes H and in another including the Li lines. The extremely small difference between the solar spectrum and these co-added spectra confirm quantitatively the very close resemblance of these stars to the Sun.

At the request of the referee we performed also a direct comparison between the solar spectrum and the spectrum of our solar analogs. We used a minimization using a Doppler shift and a re-adjustment of the continuum of the stars of M67 as free parameters, in order to match the observed spectra to the observed GIRAFFE solar spectrum. The reduced of the fit, or the associated probability, then provides a mean to rank the M67 stars. We restricted the comparison to a range of 10 Å centered on H. While in the fitting of the synthetic spectra the core of the line was excluded from the fitting range, it was here included. The LTE synthetic spectra fail to reproduce the core of H due to the presence of a chromosphere (absent in the model atmospheres employed by us) and to NLTE effects. Instead, the sought-for solar analogs must behave exactly like the Sun, including in the core of H. With this method the three M67 stars which are most similar to the Sun are the stars 1194, 1101 and 637. The result is thus very similar to what obtained by comparing the observed spectra to synthetic spectra, confirming that the stars we selected are very similar to the Sun. We prefer the method based on synthetic spectra, since the direct comparison to the solar spectrum is affected by the noise present in the latter.

Figure 8: Comparison between the GIRAFFE solar spectrum and the spectra of the 10 and 5 best solar analogues. The H and lithium regions are shown in the upper and bottom panels, respectively.

5 Solar Colour

We would like finally to use the observed colour of the solar analogues to derive in an independent way the colour of the Sun, and this requires to evaluate the cluster reddening.

The reddening towards M67 has been evaluated by many authors in the last 50 years, and a thorough discussion is given by Taylor (2007). M67 reddening is evaluated by this author in , which is accidentally the same value obtained by An et al. (2007) as the average point of the traditionally accepted range for the cluster. We will therefore adopt this value. This implies that the de-reddened colour for the average of our 10 solar analogues is .

The value is in excellent agreement with what found by inverting the fit of all the stars using the , which would have predicted and the value obtained by inverting the fit of , which would give .

It is not simple to evaluate a realistic error estimate for this colour. This should include: the spread (0.020 mag) around our determination, the uncertainty in the cluster reddening, plus other systematics originating from stellar evolution and photometry. We evaluate the evolutionary effects by investigating the expected variations of the solar color with age and metallicity by using evolutionary models. Photometric uncertainties are estimated by comparing Yadav et al. (2008) photometry with what obtained for this cluster by other groups.

We use the tracks from Girardi et al. (2000) for analyzing differential evolutionary effects. Because our stars have a similar effective temperature to the Sun, age has no influence: for stars younger than 5 Gyr of solar , the visual absolute magnitudes and colours do not change in any appreciable way. If M67 were younger than the Sun, the only effect would be that the masses of our stars were higher, by about 1%, than the solar one, but no difference is predicted in magnitude or colours. The other source of systematic uncertainty is the possibility that metallicity is not exactly solar. In this case, for a fixed effective temperature, we do expect that a star more metal rich by 0.05 dex would be slightly brighter (0.08 magnitude in ) and slightly redder (0.01 mag) than the Sun.

Our photometry is taken by Yadav et al. (2008), which was calibrated on Sandquist et al. (2004). In order to check for photometric systematic errors, we have compared our values for the 10 best analogues with Montgomery et al. (1993), finding that using their photometry an average =0.650 would have been found, i.e. only 1 mmag bluer than our value. Sandquist et al. (2004) made on the other hand a general comparison between his colours and those of Montgomery et al. (1993), finding an overall zero point shift in of 8 mmag (the Sandquist’s are bluer than the Montgomery ones). The fact that the agreement for these 10 solar stars is better than this systematic shift might be due to a statistical fluctuation, it is on the other hand quite common that calibrations agree at best for solar stars. We will nevertheless consider a 0.008 uncertainty in the colour as introduced by the adopted photometry.

The simple average of the most recent estimate of the M67 metallicity gives [Fe/H]=0.01. This would require a correction of 2 mmag towards the blue for the solar colors derived from the M67 stars to compensate for their higher metallicity. We conclude that a solar is our present best estimate. The uncertainties associated are 0.006 magnitudes given by the spread of our solar analogues divided by the square root of number of our solar analogues (namely 10); a 0.007 magnitudes given by a generous uncertainty in the cluster metallicity (0.03). Zero points uncertainties in photometry are the dominant source and they account for 0.008. All these errors are summed quadratically. To this, the uncertainty in the cluster reddening determination, which is assumed to be 0.004 mag (Taylor 2007; An et al. 2007), is linearly added.

An additional hidden source of systematic effects, which might add a bias towards redder colours, might be present, and this is the presence of unidentified binaries. A typical red, faint companion will make the stars to appear slightly brighter and slightly redder than what they should be, still influencing very little the spectroscopic determination. Given our radial velocity selection, only a few binaries should be left in our sample, and with low mass objects. We cannot quantitatively account for their presence, but we shall keep this possibility in mind.

The value found is somewhat in the middle between the majority of the ‘old’ determinations (see Table 2 of Barry et al. 1978, which found =0.667 averaging most of previous measurements), and the most recent determinations, which, as summarized, for instance, by Holmberg et al. (2006), tend to find in the range between 0.62 and 0.64. None of these results are formally in disagreement with ours, but we can exclude the estimates at the edges of the distribution.

We think that this estimate is very robust, because our results are based on a very few steps and assumptions. We assume that the metallicity of M67 is essentially solar, and this fact is agreed on by all latest works. We determine the in a differential way from the Sun, on spectra taken with the same instrument, and using two sensitive methods (line-depth ratios and H wings). We prove that the stars are indeed very close to the Sun showing how their spectra overlap with the solar one. The stars observed are still on the main sequence.

6 Cluster distance

The average magnitude of the 10 solar analogues is 14.583 mag, which must be corrected for reddening: =3.10.041=0.127, implying a de-reddened magnitude of 14.456. With a solar absolute magnitude of 4.81 (Bessell et al. 1998), the distance modulus of M67 is of 9.65. As mentioned in the previous Section, a correction might be needed if the metallicity differs substantially from the solar one (of up to 0.08 magnitudes for [Fe/H]=0.05, but we consider such a large difference in metallicity very unlikely). A correction of 0.002 mag, corresponding to a metallicity of [Fe/H]=0.01 (used in the previous Section) would bring to a distance modulus of 9.63.

This determination is in excellent agreement with two recent determinations: An et al. (2007) who estimate a distance modulus of 9.61 and Sandquist et al. (2004), who find 9.60, both using the same reddening we adopted.

The associated error given by the spread around the average magnitude is of 0.060 (i.e. 0.19/) magnitudes. Other sources of uncertainty in the distance modulus will be given by the error in reddening, which accounts for 0.012 magnitudes, and by the uncertainty on [Fe/H]. If we assume an error on [Fe/H] of 0.03 dex, this accounts for 0.05 magnitudes in the distance modulus. Summarizing, our best estimate of the distance modulus is: 9.63.

A full comparison of our estimate with those present in literature is beyond the scope of this work. We find however remarkable the agreement between our distance estimate and the ones of Sandquist et al. (2004) and An et al. (2007) in particular when considering that our method is independent with respect to theirs.

7 Conclusion

By using selected observations with FLAMES/GIRAFFE at the VLT, we have made a convincing case that the open cluster M67 hosts a number of interesting potential solar twins, and we have identified them. We have computed spectroscopic accurate effective temperatures for all the stars with two methods. The color-temperature relationships we derive can be used to determine temperatures for MS solar-metallicity stars.

By computing the average solar twin colours, we have obtained a precise estimate of the solar : .

By averaging the magnitude of the solar twins, we have determined an accurate distance modulus for M67: 9.63, which is in excellent agreement with the most recent estimates, which were based on different, independent methods and data sets.

We have determined for all the stars Li abundances, confirming the presence of a large Li spread among the solar stars of this cluster, but showing for the first time, that the Li extra-depletion appears only in stars cooler than 6000 K. The candidate solar twins have Li abundance similar to that of our star, indicating that they also share with the Sun a similar mixing history.

Acknowledgements.
We are grateful to F. Castelli for helping us to understand how hydrogen profiles are computed in SYNTHE. KB has been supported by the ESO DGDF 2006, and by the Italian Ministero dell’Istruzione, Università e Ricerca (MIUR) fundings. PB acknowledges support from EU contract MEXT-CT-2004-014265 (CIFIST). This research has made use of SIMBAD and VIZIER databases, operated at CDS (Strasbourg, France).

Appendix A On line material

Object Name
(°) (°) (mag) (mag) (mag) (mas/yr) (mas/yr)
Obj219 132.893537 11.632623 13.7150.009 13.1000.008 12.4250.005 1.961.31 8.633.03 S1197
Obj266 132.860339 11.643584 14.2120.006 13.6010.006 12.9390.007 0.242.38 0.183.45 S944
Obj285 132.849450 11.647816 15.1650.006 14.4610.000 13.7130.002 0.421.78 2.801.37 S945
Obj288 132.900657 11.648980 14.4940.013 13.8570.004 13.1600.005 0.000.77 1.612.44 S1201
Obj291 132.937203 11.649712 14.0900.010 13.4780.009 12.8070.018 0.181.13 7.263.03 S1202
Obj342 132.823311 11.660013 14.1180.004 13.5030.005 12.8410.008 0.772.14 5.594.22 S948
Obj349 132.979809 11.661163 14.9780.006 14.3010.002 13.6140.007 2.928.87 2.924.82 MMJ6241/S1423
Obj350 132.836621 11.661145 14.2260.005 13.6240.010 12.9550.006 1.961.07 0.181.49 S950
Obj364 132.794912 11.664138 15.2880.005 14.5840.010 13.8090.006 1.371.55 0.241.43 S951
Obj401 132.829348 11.671034 14.2680.006 13.6610.007 13.0090.003 3.812.56 1.311.96 S954
Obj437 132.827827 11.676879 14.6000.006 13.9980.008 13.3310.002 2.980.71 2.023.69 S956
Obj455 132.971225 11.681574 13.9300.007 13.3010.004 12.6080.018 5.305.65 5.772.80 S1428
Obj473 132.809740 11.685891 15.1420.011 14.4430.008 13.7310.004 1.311.13 1.553.57 S958
Obj571 132.951947 11.706357 14.6860.012 14.0210.014 13.3090.004 0.891.31 0.303.21 S1211
Obj574 133.024207 11.706858 14.3090.016 13.6530.007 12.9320.005 1.731.01 0.241.07 MMJ6362/S1431
Obj587 132.911415 11.710387 14.7530.006 14.1070.006 13.4050.020 0.890.83 1.313.15 S1213
Obj613 132.825417 11.715202 13.9070.015 13.2540.006 12.5940.022 1.782.62 0.893.69 S964
Obj637 132.840991 11.721590 15.1910.017 14.4890.010 13.7510.011 0.650.54 0.422.50 S966
Obj673 132.722266 11.727791 15.0620.002 14.3560.009 13.5690.001 0.000.77 0.242.86 S746
Obj681 132.773982 11.729705 14.7150.006 14.0180.016 13.2540.008 1.191.31 1.492.38 S747
Obj689 132.875550 11.730521 13.7830.012 13.1200.011 12.4360.010 3.212.86 0.363.87 S1219
Obj713 132.840699 11.734734 14.8860.017 14.1720.009 13.4210.017 1.551.37 0.481.31 S969
Obj750 132.722677 11.742964 14.2150.007 13.5760.014 12.8650.005 0.182.08 1.072.20 S750
Obj756 132.947493 11.745099 15.3930.011 14.6950.008 13.9230.009 3.030.89 5.061.73 S1222
Obj769 132.959247 11.749185 14.1190.004 13.4780.003 12.7710.005 0.361.13 0.122.98 S1224a
Obj778 132.836684 11.750684 13.7160.010 13.0930.011 12.4110.011 1.371.78 2.984.40 S976
Obj809 132.758539 11.755306 15.6960.007 14.9590.015 14.1620.004 1.131.61 1.491.84 S754
Obj851 132.854122 11.761931 14.7300.007 14.1130.004 13.4490.007 0.713.09 2.981.07 S982
Obj880 132.770112 11.765793 14.2020.009 13.5440.014 12.8410.002 0.301.07 1.071.61 S757
Obj905 132.746798 11.770272 14.0380.004 13.4340.010 12.7480.015 1.011.73 1.732.74 S758
Obj911 132.791312 11.771367 15.2200.006 14.5470.010 13.7850.010 0.361.31 1.191.31 S991
Obj917 133.021288 11.772611 15.4970.007 14.7550.008 13.9230.006 0.542.56 2.442.62 S1442
Obj971 132.884919 11.779352 15.2870.005 14.5920.004 13.7930.000 0.120.59 0.360.30 S1246
Obj986 132.905212 11.782141 14.6460.008 14.0070.003 13.2830.001 0.241.25 1.190.54 S1247
Obj988 132.892171 11.782156 14.8190.005 14.1800.001 13.4750.002 0.650.65 0.241.49 S1248
Obj1010 132.850486 11.785927 14.1040.007 13.4780.009 12.7810.021 0.360.95 3.571.43 S2209
Obj1032 132.985778 11.790265 14.9970.005 14.3580.003 13.6490.001 3.333.33 0.240.48 S1449
Obj1036 132.851246 11.791211 15.6780.018 14.9470.003 14.1640.005 0.360.24 1.011.25 S1004
Obj1051 132.922899 11.793371 14.7260.007 14.0900.004 13.3820.004 1.550.54 1.961.07 S1252
Obj1062 132.900316 11.796392 15.1440.001 14.4770.008 13.7450.004 1.251.07 0.122.68 S1255
Obj1067 133.014592 11.796693 15.2010.005 14.5590.009 13.8240.002 0.242.08 0.301.25 S1452
Obj1075 132.912716 11.798715 14.3860.000 13.7120.006 12.9920.004 1.191.96 2.803.33 S1256
Obj1088 132.953942 11.800602 15.1510.004 14.4920.001 13.7600.007 0.771.90 3.992.56 S1258
Obj1090 132.869575 11.800607 14.4500.004 13.8000.005 13.0400.010 0.181.73 1.551.07 S1011
Obj1091 132.853518 11.801110 15.2370.011 14.5130.007 13.6690.008 0.120.83 0.481.55 S1012
Obj1101 133.035214 11.802908 15.3770.001 14.6750.004 13.9030.001 0.061.31 2.082.50 MMJ6384
Obj1108 132.855293 11.803762 14.8780.005 14.1770.001 13.3860.006 0.300.95 0.420.71 S1014
Obj1129 132.885730 11.806694 14.7950.005 14.1710.006 13.4820.002 1.130.30 1.551.31 S1260
Obj1137 132.800104 11.807413 15.5710.002 14.8730.008 14.1070.009 1.900.89 0.831.19 S2213
Obj1161 132.981615 11.810602 14.5490.004 13.8830.010 13.1490.006 0.894.46 6.842.86 S1457
Obj1163 132.787098 11.810483 14.6880.009 13.9920.004 13.2210.002 0.420.65 8.031.19 S1022
Obj1194 132.753356 11.814655 15.2810.002 14.6140.010 13.8760.002 0.301.01 0.420.65 S770
Obj1197 132.877372 11.815215 13.9210.006 13.3150.003 12.6180.024 0.181.07 1.780.77 MMJ5882/S1264b
Obj1247 132.812708 11.822522 14.7530.008 14.1440.008 13.4700.005 0.651.25 2.801.96 S1033
Obj1303 132.736097 11.831844 15.3180.008 14.6410.008 13.8990.009 0.062.26 0.891.25 S779
Obj1304 132.858515 11.832075 15.4540.015 14.7310.009 13.9160.006 0.121.31 0.121.90 S1041
Obj1315 132.994897 11.834025 14.9900.013 14.2970.011 13.5440.008 1.194.40 0.482.20 MMJ6306/S1462
Obj1334 132.866453 11.836636 15.0830.007 14.4030.007 13.6690.000 0.771.61 1.840.89 S1048
Obj1342 132.826093 11.838787 14.9350.006 14.2850.005 13.5470.001 1.371.49 0.360.06 S1050
Obj1387 132.874854 11.852505 14.7240.004 14.0980.002 13.3980.000 0.593.75 1.557.97 S1283
Obj1392 132.749200 11.853500 15.5270.002 14.8110.004 14.0470.000 1.492.08 1.610.77 S785
Obj1397 132.882992 11.854616 14.6310.001 14.0090.003 13.3040.001 1.842.32 3.035.89 S1287
Obj1424 132.890226 11.862493 13.8250.009 13.2030.005 12.5010.000 1.492.50 1.253.87 Check
Obj1458 132.762475 11.873808 15.7160.010 14.9770.005 14.1860.004 1.011.84 0.121.49 S795
Obj1480 132.916988 11.878716 14.3830.008 13.7830.008 13.1250.002 3.992.50 1.252.32 S1300
Obj1496 132.781325 11.882262 14.4860.013 13.8790.004 13.2140.002 0.950.12 0.300.65 S2224
Obj1504 132.864557 11.884028 14.7960.001 14.1710.011 13.4740.000 1.130.95 1.311.31 S1078
Obj1514 132.753181 11.886527 15.4980.003 14.7770.004 14.0080.000 0.121.13 1.731.37 S802
Obj1587 132.846438 11.901394 14.8040.015 14.1630.004 13.4690.006 0.951.73 1.011.78 S1087
Obj1622 132.801216 11.906389 14.7880.004 14.1560.002 13.4590.004 1.431.25 0.950.65 S1089
Obj1680 132.883591 11.919069 14.2910.015 13.6460.006 12.9510.004 1.011.13 0.181.07 S1314
Obj1706 133.010317 11.926166 15.4680.005 14.7450.017 13.9810.006 0.540.95 0.300.54 MMJ6341/S1481
Obj1716 132.866205 11.928044 13.9180.010 13.2990.009 12.6250.005 1.961.84 4.823.15 S1092
Obj1722 132.828014 11.930478 14.7310.002 14.1300.006 13.4490.009 3.5726.24 2.442.20 S1093
Obj1735 133.001728 11.935288 14.9930.012 14.3320.010 13.6170.007 1.961.78 2.921.55 S1483
Obj1758 132.746824 11.943570 13.8600.056 13.2070.015 12.5450.007 0.890.48 3.451.67 MMJ5342/S816
Obj1768 132.906577 11.945718 15.0600.003 14.4040.004 13.6840.004 0.771.13 1.010.89 MMJ6028/S1320
Obj1778 132.737778 11.947424 15.6790.004 14.9480.004 14.1550.003 0.831.96 0.650.65 MMJ5310/S820
Obj1787 132.788102 11.950097 15.2140.006 14.5470.004 13.8130.003 1.611.73 2.262.26 MMJ5484
Obj1788 132.804106 11.950254 15.1040.008 14.4410.004 13.7090.000 1.781.13 4.641.84 MMJ5541
Obj1842 132.786897 11.964913 14.8440.007 14.2370.002 13.5570.005 0.361.01 1.611.49 MMJ5479/S1102
Obj1852 133.013763 11.967948 14.5750.011 13.9620.008 13.2860.005 1.012.32 1.673.51 S1486
Obj1862 132.743111 11.970785 15.1260.001 14.4830.003 13.7530.004 0.061.37 0.891.13 MMJ5331
Obj1903 132.783150 11.981489 15.4220.004 14.7330.003 13.9710.001 1.671.25 0.830.83 MMJ5469
Obj1948 132.928314 11.991953 14.6270.009 14.0150.004 13.3270.002 0.42 1.67 3.452.98 S1330
Obj1955 132.743729 11.994304 14.8420.001 14.2120.004 13.4830.002 0.59 0.54 2.983.39 MMJ5338/S829
Obj1957 132.888539 11.994779 14.4060.007 13.7890.004 13.0850.008 1.191.90 0.183.69 MMJ5962/S1331
Obj2016 132.931975 12.015297 14.1580.002 13.5530.007 12.8410.000 0.77 2.38 0.302.44 S1333
Obj2017 132.943039 12.015413 15.5090.003 14.8570.001 14.1090.001 2.682.62 6.903.87 S1334
Obj2018 132.914220 12.015883 15.2370.000 14.5650.005 13.8320.007 2.501.55 1.672.02 MMJ6055
Table 3: Object ID, coordinates (Equinox J2000, Epoch J2000.13), photometry, and proper motions of the targets (see Yadav et al. 2008 for details). In the last column the name of the stars according to Sanders (1977) or to Montgomery et al. (1993) are given. For a few stars two names are given, because both stars are within 1 arc sec circle, according to SIMBAD.
Object
(km s) (K) (K) (mÅ) (mÅ)
Sun 577727 5777 12.1 0.3 0.8 0.8
219 33.850.28 624354 6110 22.7 1.2 2.1 2.1
266 32.950.46 614763 6060 53.2 1.7 2.6 2.5
285 33.720.67 583667 5777 1.5 0.0 0.6 0.6
288 34.260.40 600467 6010 42.4 1.5 2.3 2.3
291 32.130.29 617757 6160 59.6 1.7 2.6 2.6
349 34.280.42 595278 5917 10.4 0.0 0.7 0.7
350 32.560.31 602452 6010 68.2 1.8 2.6 2.6
401 32.640.37 616564 6110 44.9 1.6 2.5 2.4
473 34.740.38 591976 5807 2.7 0.0 0.7 0.7
587 33.060.39 607765 6060 37.2 1.4 2.3 2.3
613 33.050.29 620245 6110 61.3 1.7 2.7 2.6
637 34.000.50 580665 5777 17.8 0.9 1.4 1.4
673 32.440.57 563963 5747 19.1 0.9 1.4 1.4
689 32.880.24 609341 6110 42.3 1.5 2.4 2.3
750 33.130.28 591848 5927 16.4 0.9 1.5 1.5
769 34.470.29 598446 6010 29.4 1.3 2.1 2.0
778 33.450.24 611439 6060 15.6 0.8 1.7 1.7
809 31.870.48 566778 5537 6.9 0.0 0.4 0.5
851 33.470.31 594861 6060 12.9 0.6 1.3 1.3
911 32.080.35 588567 5837 13.4 0.6 1.2 1.2
988 32.070.28 593553 6060 28.2 1.3 2.0 2.0
1032 34.020.47 595560 6010 17.8 0.9 1.6 1.6
1036 33.390.48 561266 5537 20.9 1.0 1.4 1.4
1051 32.210.29 608155 6060 34.7 1.4 2.2 2.2
1062 32.640.45 592655 5867 21.0 1.1 1.7 1.7
1067 33.370.35 592969 5917 11.4 0.4 1.0 1.0
1075 33.130.24 587148 5917 10.7 0.0 0.6 0.7
1088 32.870.29 589059 5867 9.3 0.0 0.6 0.7
1090 33.340.32 608654 6010 41.2 1.5 2.3 2.3
1101 32.720.34 575660 5717 6.9 0.0 0.5 0.6
1129 33.920.30 595951 6010 32.4 1.4 2.1 2.1
1137 33.590.48 574169 5627 5.3 0.0 0.5 0.6
1194 33.300.40 576664 5837 5.3 0.0 0.5 0.6
1197 34.260.28 620744 6110 28.1 1.3 2.2 2.2
1247 32.440.51 599460 6010 30.2 1.3 2.1 2.1
1303 32.650.41 571664 5717 10.1 0.0 0.5 0.6
1304 33.600.39 570464 5717 7.9 0.0 0.5 0.5
1315 32.550.34 587458 5867 15.6 0.8 1.4 1.4
1334 32.370.45 595757 5957 29.2 1.3 2.0 2.0
1387 33.350.24 609058 6060 37.5 1.5 2.3 2.3
1392 33.680.57 571663 5687 6.3 0.0 0.5 0.6
1458 32.550.56 564065 5567 6.1 0.0 0.4 0.5
1496 34.220.48 617354 6160 63.1 1.7 2.7 2.6
1504 33.390.48 593456 6060 40.7 1.5 2.2 2.2
1514 33.330.49 561367 5597 25.6 1.2 1.6 1.6
1587 32.380.62 597555 6010 28.6 1.3 2.0 2.0
1622 33.260.61 604357 6010 34.6 1.4 2.2 2.2
1716 33.840.59 603040 6060 40.8 1.5 2.3 2.3
1722 33.870.52 600762 6010 38.1 1.5 2.3 2.2
1735 33.180.59 595959 5960 10.9 0.2 0.9 0.9
1758 33.780.67 622152 6160 36.3 1.4 2.4 2.4
1768 33.980.57 584457 5927 40.1 1.5 2.1 2.1
1787 33.440.57 576870 5807 12.9 0.5 1.1 1.0
1788 33.380.80 588660 5867 23.4 1.1 1.8 1.8
1852 32.300.40 600963 6010 32.2 1.4 2.1 2.1
1903 32.760.42 560972 5687 6.8 0.0 0.4 0.5
1948 32.860.29 616463 6010 43.0 1.5 2.4 2.4
1955 32.630.60 596176 5837 35.8 1.4 2.2 2.2
2018 31.780.43 569374 5777 8.7 0.0 0.4 0.5
Table 4: Radial velocities, effective temperatures, and lithium abundances of the 59 stars retained as possible single members. The values in Table are derived by adding the difference of between the stars and the Sun to the solar temperature (5777 K). The solar temperature derived from the GIRAFFE spectrum and our calibrations is of 5792 and 5717 K for the LDR and H methods, respectively. The 10 best solar twins candidates are indicated in bold face. The ratio/pixel of the co-added spectra varies between 80 and 110, depending on the magnitude of the stars.
Object
342 17.74 0.72 57.20 1.45 40.75 0.73
364 36.93 0.43 36.58 1.18 36.48 0.74
437 45.87 0.65 45.54 0.83 45.68 0.52
455 38.07 0.50 14.62 1.24 33.41 0.47
571 31.73 0.49 31.07 0.66 31.47 0.56
574 42.88 0.46 42.91 0.85 42.08 0.62
681 24.14 0.80 56.70 1.22 59.70 0.85
713 35.34 0.46 35.03 0.64 34.91 0.33
756 46.24 0.50 45.54 0.77 45.47 0.41
880 39.62 6.67 / / 33.55 5.81
905 34.78 0.35 37.30 1.02 74.65 0.89
917 18.66 0.86 15.59 1.36 / /
971 36.13 0.51 36.13 0.70 36.22 0.63
986 16.54 0.53 27.69 0.84 49.08 0.68
1010 17.22 0.97 30.77 0.63 34.29 0.75
1091 27.59 3.27 27.03 0.62 25.77 0.78
1108 15.92 1.00 / / / /
1161 35.53 0.65 35.49 1.21 22.45 1.55
1163 10.10 0.66 65.99 1.32 70.80 1.24
1342 37.97 0.69 28.40 1.06 34.93 0.65
1397 35.43 1.12 34.91 0.85 34.88 0.54
1424 54.44 1.62 57.22 1.31 56.04 1.58
1480 22.86 1.02 25.56 1.09 28.27 0.48
1680 37.65 1.29 35.65 0.96 32.27 0.81
1706 35.79 1.05 35.40 0.90 35.77 0.61
1778 29.59 1.30 29.92 2.96 31.19 1.26
1842 30.09 0.69 29.88 1.20 30.52 0.47
1862 31.31 0.63 30.29 2.03 31.27 0.43
1957 21.17 0.52 20.15 0.83 21.21 0.57
2016 21.64 0.46 25.96 0.94 65.63 0.66
2017 126.58 0.84 125.09 1.62 126.01 0.63
Table 5: Radial velocities of likely binaries or non members. For these stars each of the three RV measurements is given. Fields with blanks indicate that problems were present with the cross-correlation profile of these objects.

Footnotes

  1. thanks: Based on observations collected at the ESO VLT, Paranal Observatory, Chile, program 278.D-5027(A).
  2. offprints: L. Pasquini,
  3. This is the observing mode in FLAMES in which 132 fibers with a projected diameter on the sky of 12 feed the GIRAFFE spectrograph. Some fibers are set on the target stars and others on the sky background.
  4. IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of the Universities for Research in Astronomy, inc. (AURA) under cooperative agreement with the National Science Foundation.
  5. http://www.eso.org/observing/dfo/quality/GIRAFFE/pipeline/solar.html
  6. National Institute of Standards and Technology.

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