Thesis Open Access
Casamiquela Floriach, Laia
This thesis has as a main purpose the determination of the chemical composition of Open Clusters for the study of the chemical gradients in the Galactic disc with: Galactocentric radius, position above the plane, and age.
In 3 years and a half we have acquired high-resolution spectra of stars in 18 Open Clusters as a part of the OCCASO survey. We have used three fiber fed high-resolution spectrographs in the Observatorio del Roque de los Muchachos (La Palma, Spain) and the CAHA observatory (Almería, Spain). After a very accurate data reduction we have determined radial velocities, atmospheric parameters (effective temperature, surface gravity) and chemical abundances.
Using the derived radial velocities and proper motions from the literature we have done a kinematic study of these clusters in the context of the Galactic disc and the position near the spiral arms. We have obtained radial velocities for OC never studied before with high-resolution spectroscopy: NGC 1907 (vr=2.3+- 0.5 km/s), NGC 6991 (vr=-12.3+-0.6 km/s) and NGC 7245 (vr=-74.0+-1.4km/s). We have computed the possible orbits that the clusters have followed using two models of the Galactic potential: an axysimmetric one, and a model with a bar and spiral arms featuring those of the Milky Way. With this, we have recovered the position of the clusters at birth.
We have calculated temperatures, gravities and iron abundances using two different methodologies widely used in the literature. We have made an exhaustive comparison of the behaviour of both methods and the differences obtained among them. We obtained no systematics in effective temperature and surface gravity within the quoted errors, though with a large dispersion in surface gravity. As a sanity check we derived atmospheric parameters from BVI Johnson photometry for the stars from NGC 2420 and NGC 6791. We found systematic differences between spectroscopic and photometric determinations which change with slight variations of the assumed reddening, distance, age and metallicity to compute photometric parameters. Mean uncertainties in the final adopted in temperature and gravity are around 40 K and 0.1 dex. The comparison with literature values gives mean offsets well within uncertainties and dispersions. All these checks provide a study of the precision and accuracy of the obtained results.
We have measured abundances of iron-peak elements (Fe, Ni, Cr) and the so-called α-elements (Si, Ca, Ti, Mg, O). From member stars we derive mean Fe cluster abundances with the two methods. We do an extensive star-by-star comparison with literature, showing good agreement. We also derived mean cluster Ni, Cr, Si, Ca and Ti abundances, and its abundance ratios respect to Fe. We see that all the clusters present small dispersions in abundance. The larger ones are 0.03 ([Ni/Fe]), 0.06 ([Cr/Fe]), 0.05 ([Si/Fe]), 0.07 ([Ca/Fe]), 0.05 ([Ti/Fe]) dex, excluding NGC~6791 (for which we have larger errors)
We have compared the Galactocentric trend seen with OCCASO clusters with different theoretical models obtaining that the results for the oldest clusters favour a chemo-dynamical model instead of a pure chemical evolution model. Using OCCASO and two complementary samples (40 clusters in total), we have determined new values for the Galactocentric Fe gradient in three age bins, and the age-metallicity relation in four ranges of Galactocentric distance.
In particular we have studied NGC 6705 in detail. We have seen that this cluster presents an unexpected α-enhancement for its location in the disc and its young age. We have derived the birth location of this cluster computing the possible orbits that it could have followed in the disc using different models. It seems that its α-enhancement cannot be explained by a very different place of birth of this cluster (i.e. the inner Galaxy), which from our calculations would be at most from a radii of 6.5 kpc.