Spectra of composite systems (e.g., spectroscopic binaries) contain spatial information that can be retrieved by measuring the radial velocities (i.e., Doppler shifts) of the components in four observations with the slit rotated by 90 degrees in the
sky. By using basic concepts of slit spectroscopy we show that the geometry of composite systems can be reliably retrieved by measuring only radial velocity differences taken with different slit angles. The spatial resolution is determined by the precision with which differential radial velocities can be measured. We use the UVES spectrograph at the VLT to observe the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum expected separation of 23 milli-arcseconds. We measure an astrometric signal in radial velocity of 276 ms, which corresponds to a separation between the two components at the time of the observations of 18 $pm2$ milli-arcseconds. The stars were aligned east-west. We describe a simple optical device to simultaneously record pairs of spectra rotated by 180 degrees, thus reducing systematic effects. We compute and provide the function expressing the shift of the centroid of a seeing-limited image in the presence of a narrow slit.The proposed technique is simple to use and our test shows that it is amenable for deriving astrometry with milli-arcsecond accuracy or better, beyond the diffraction limit of the telescope. The technique can be further improved by using simple devices to simultaneously record the spectra with 180 degrees angles.With tachoastrometry, radial velocities and astrometric positions can be measured simultaneously for many double line system binaries in an easy way. The method is not limited to binary stars, but can be applied to any astrophysical configuration in which spectral lines are generated by separate (non-rotational symmetric) regions.
The Li-rich turn-off star in the globular cluster NGC 6397 could represent the smoking gun for some very rare episode of Li enrichment in globular clusters. We aim to understand the nature of the Li enrichment by performing a spectroscopic analysis o
f the star, in particular of its beryllium (Be) abundance, and by investigating its binary nature. We observe the near UV region where the Beii resonance doublet and the NH bands are located. We could not detect the Beii lines and derive an upper limit of log (Be/H)< -12.2, that is consistent with the Be observed in other stars of the cluster. We could detect a weak G-band, which implies a mild carbon enhancement [C/Fe]$+0.4pm0.2$. We could not detect the UV NH band, and we derive an upper limit [N/Fe]$< 0.0$. For oxygen we could notdetect any of the near UV OH lines, which implies that oxygen cannot be strongly enhanced in this star. This is consistent with the detection of the Oi triplet at 777nm, which is consistent with [O/Fe]~0.5. Combining the UVES and Mike data, we could not detect any variation in the radial velocity greater than 0.95 kms$^{-1}$ over 8 years. The chemical composition of the star strongly resembles that of `first generation NGC6397 stars, with the huge Li as the only deviating abundance. Not detecting Be rules out two possible explanations of the Li overabundance: capture of a substellar body and spallation caused by a nearby type II SNe. Discrepancies are also found with respect to other accretion scenarios,except for contamination by the ejecta of a star that has undergone the RGB Li-flash.
For the past six years we have carried out a search for massive planets around main sequence and evolved stars in the open cluster (OC) M67, using radial velocity (RV) measurements obtained with HARPS at ESO (La Silla), SOPHIE at OHP and HRS at HET.
Additional RV data come from CORALIE at the Euler Swiss Telescope. We aim to perform a long-term study on giant planet formation in open clusters and determine how it depends on stellar mass and chemical composition. We report the detection of three new extrasolar planets: two in orbit around the two G dwarfs YBP1194 and YBP1514, and one around the evolved star S364. The orbital solution for YBP1194 yields a period of 6.9 days, an eccentricity of 0.24, and a minimum mass of 0.34 Mj. YBP1514 shows periodic RV variations of 5.1 days, a minimum mass of 0.40 Mj, and an eccentricity of 0.39. The best Keplerian solution for S364 yields a period of 121.7 days, an eccentricity of 0.35 and a minimum mass of 1.54 Mj. An analysis of H_alpha core flux measurements as well as of the line bisectors spans revealed no correlation with the RV periods, indicating that the RV variations are best explained by the presence of a planetary companion. Remarkably, YBP1194 is one of the best solar twins identified so far, and YBP1194b is the first planet found around a solar twin that belongs to a stellar cluster. In contrast with early reports and in agreement with recent findings, our results show that massive planets around stars of open clusters are as frequent as those around field stars.
Lithium is a fundamental element for studying the mixing mechanisms acting in the stellar interiors, for understanding the chemical evolution of the Galaxy and the Big Bang nucleosynthesis. The study of Li in stars of open clusters (hereafter OC) all
ows a detailed comparison with stellar evolutionary models and permits us to trace its galactic evolution. The OC NGC 2243 is particularly interesting because of its low metallicity ([Fe/H]=$-0.54 pm0.10$ dex). We measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243. The first aim is to determine whether the Li dip extends to such low metallicities, the second is to compare the results of our Li analysis in this OC with those present in 47 Tuc, a globular cluster of similar metallicity. We performed a detailed analysis of high-resolution spectra obtained with the multi-object facility FLAMES at the ESO VLT 8.2m telescope. Lithium abundance was derived through line equivalent widths and the OSMARCS atmosphere models. We determine a Li dip center of 1.06 $M_odot$, which is much smaller than that observed in solar metallicity and metal-rich clusters. This finding confirms and strengthens the conclusion that the mass of the stars in the Li dip strongly depends on stellar metallicity. The mean Li abundance of the cluster is $log n{rm (Li)}=2.70$ dex, which is substantially higher than that observed in 47 Tuc. We estimated an iron abundance of [Fe/H]=$-0.54 pm0.10$ dex for NGC 2243, which is similar (within the errors) to previous findings. The [$ alpha$/Fe] content ranges from $0.00pm0.14$ for Ca to $0.20pm0.22$ for Ti, which is low when compared to thick disk stars and to Pop II stars, but compatible with thin disk objects. We found a mean radial velocity of 61.9 $pm$ 0.8 kms for the cluster.
Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their CMD can be interpreted in terms of differences in the Helium content of the stars belonging to the sub-po
pulations. Differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10830 {AA} lines. The He 10830 {AA} line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and H$alpha$, and simulate the corresponding He I 10830 line profiles. We show that, at a given value of He abundance, the He I 10830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10830 {AA} region, and the VLT-UVES to obtain spectra of the Ca II and H$alpha$ lines of our target stars. The two target stars have very similar Ca II and H$alpha$ lines, but different appearances in the He region. One line, blueshifted by 17 km s$^{-1}$ with respect to the He 10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. The difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of $Delta Y ge$ 0.17.We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means.
Gravitational redshifts in solar-type main-sequence stars are expected to be some 500 ms$^{-1}$ greater than those in giants. Such a signature is searched for between groups of open-cluster stars which share the same average space motion and thus hav
e the same average Doppler shift. 144 main-sequence stars and cool giants were observed in the M67 open cluster using the ESO FEROS spectrograph, obtaining radial velocities by cross correlation with a spectral template. M67 dwarf and giant radial-velocity distributions are well represented by Gaussian functions, sharing the same apparent average radial velocity within $simeq$ 100 ms$^{-1}$. In addition, dwarfs in M67 appear to be dynamically hotter ($sigma$ = 0.90 kms$^{-1}$) than giants ($sigma$ = 0.68 kms$^{-1}$). Explanations for the lack of an expected signal are sought: a likely cause is the differential wavelength shifts produced by different hydrodynamics in dwarf and giant atmospheres. Radial-velocity differences measured between unblended lines in low-noise averaged spectra vary with line-strength: stronger lines are more blushifted in dwarfs than in giants, apparently compensating for the gravitational redshift. Synthetic high-resolution spectra are computed from 3-dimensional hydrodynamic model atmospheres for both giants and dwarfs, and synthetic wavelength shifts obtained. In agreement with observations, 3D models predict substantially smaller wavelength-shift differences than expected from gravitational redshift only. The procedures developed could be used to test 3D models for different classes of stars, but will ultimately require high-fidelity spectra for measurements of wavelength shifts in individual spectral lines.
Elemental correlations and anti-correlations are known to be present in globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its fragility Li is destroyed at the temperature at which the CNO cycling occurs, and this makes Li a cru
cial study for the nature of the contaminating stars. We observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES at the VLT to investigate the presence and the extent of a Li-O correlation. This correlation is expected if there is a simple pollution scenario. Li (670.8 nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to a very narrow region of the color-magnitude diagram, so they have similar stellar parameters (Teff, log g). We find that O and Li correlate, with a high statistical significance that confirms the early results for this cluster. At first glance this is what is expected if a simple pollution of pristine gas with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship, however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one is the value predicted for a pure contamination model. We confirm an extended Li-O correlation in non evolved stars of NGC 6752. At the same time the characteristic of the correlation shows that a simple pollution scenario is not sufficient to explain the observations. Within this scenario the contaminant gas must have been enriched in Li. This would rule out massive stars as main polluters, and favor the hypothesis that the polluting gas was enriched by intermediate or high-mass AGB stars, unless the former can be shown to be able to produce Li. According to our observations, the fraction of polluting gas contained in the stars observed is a considerable fraction of the stellar mass of the cluster.
Chromospheric activity is widely used as an age indicator for solar-type stars based on the early evidence that there is a smooth evolution from young and active to old and inactive stars. We analysed chromospheric activity in five solar-type stars i
n two open clusters, in order to study how chromospheric activity evolves with time. We took UVES high-resolution, high S/N ratio spectra of 3 stars in IC 4756 and 2 in NGC 5822, which were combined with a previously studied data-set and reanalysed here. The emission core of the deep, photospheric Ca II K line was used as a probe of the chromospheric activity. All of the 5 stars in the new sample, including those in the 1.2 Gyr-old NGC 5822, have activity levels comparable to those of Hyades and Praesepe. A likely interpretation of our data is that solar-type-star chromospheric activity, from the age of the Hyades until that of the Sun, does not evolve smoothly. Stars change from active to inactive on a short timescale. Evolution before and after such a transition is much less significant than cyclical and long-term variations. We show that data presented in the literature to support a correlation between age and activity could be also interpreted differently in the light of our results.
[abridged] Beryllium is a pure product of cosmic ray spallation. This implies a relatively simple evolution in time of the beryllium abundance and suggests its use as a time-like observable. We study the evolution of Be in the early Galaxy and its de
pendence on kinematic and orbital parameters. We investigate the formation of the halo and the thick disk of the Galaxy and the use of Be as a cosmochronometer. Beryllium abundances are determined from high resolution, high signal to noise UVES spectra with spectrum synthesis in the largest sample of halo and thick disk stars analyzed to date. We present our observational results in various diagrams. 1) In a log(Be/H) vs [Fe/H] diagram we find a marginal statistical detection of a real scatter, above what expected from measurement errors, with a larger scatter among halo stars. The detection of the scatter is further supported by the existence of pairs of stars with identical atmospheric parameters and different Be abundances. 2) In an log(Be/H) vs [alpha/Fe] diagram, the halo stars separate into two components; one is consistent with predictions of evolutionary models, while the other has too high alpha and Be abundances and is chemically indistinguishable from thick disk stars. This suggests that the halo is not a single uniform population where a clear age-metallicity relation can be defined. 3) In diagrams of Rmin vs [alpha/Fe] and log(Be/H) the thick disk stars show a possible decrease of [alpha/Fe] with Rmin, whereas no dependence of Be with Rmin is seen. This anticorrelation suggests that the star formation rate was lower in the outer regions of the thick disc, pointing towards an inside-out formation. The lack of correlation for Be indicates that it is insensitive to the local conditions of star formation.
Solar analogues are fundamental targets for a better understanding of our Sun and our Solar System. Usually, this research is limited to field stars, which offer several advantages and limitations. In this work, we present the results of a research o
f solar twins performed for the first time in a open cluster, namely M67. Our analysis allowed us to find five solar twins and also to derive a solar colour of (B-V)0=0.649+/-0.016 and a cluster distance modulus of 9.63+/-0.08. This study encourages us to apply the same method to other open clusters, and to do further investigations for planet search in the solar twins we find.
