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.
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.
The discovery of true solar analogues is fundamental for a better understanding of the Sun and of the solar system. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very simil
ar to those of the Sun. We analyze FLAMES spectra of a large number of M67 main sequence stars to identify solar analogues in this cluster.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$alpha$ wings, making a direct comparison with the solar spectrum obtained with the same instrument. We also compute the lithium abundance for all the stars.Ten stars have both the temperature derived by line-depth ratios and H$alpha$ wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening $E(B-V)=0.041$, the solar colour $(B-V)_odot=0.649pm0.016$ 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. 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.
We measure the nitrogen abundance in 5 Turn Off(TO) stars of the Globular Clusters NGC 6397 and NGC 6752, and compare the cluster abundances with those of field stars of comparable metallicity. We determine the nitrogen abundance from the band head s
ystem at 3360 AA, using spectra of resolution R=45000 obtained with the UVES spectrograph on the VLT. We apply the same method previously used on field stars, to allow a direct comparison of the results. Nitrogen is found to have the same abundance in two of the NGC 6397 stars, in spite of a difference of one order of magnitude in oxygen abundance between them. In a third star of NGC 6397 the value is slightly lower, but compatible with the other two, within the uncertainties. All the stars in NGC 6397 are N-rich with respect to field objects of similar metallicity. The two stars in NGC 6752 show a difference in nitrogen abundance by over one order of magnitude. The same stars differ in the abundances of other elements such as Na, O and Li, only by a factor 3-4. The behaviour of N is different in the two clusters: no variation is observed NGC 6397, while a large variation is observed in NGC 6752. This is consistent with a picture in which the stars in NGC 6752 have been formed by a mixture of ``pristine material and material which has been processed by an early generation of stars, referred to as ``polluters. The N abundances here reported will help to constrain the properties of the polluters. In the case of NGC 6397 a simple pollution history is probably not viable, since the observed variations in O abundances are not accompanied by corresponding variations in N or Li.
