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Tesi etd-06192017-122536

Thesis type
Tesi di laurea magistrale
Near-Infared Photometry of the Galactic globular cluster M30
Corso di studi
relatore Prof. Bono, Giuseppe
relatore Prof. Prada Moroni, Pier Giorgio
Parole chiave
  • Adaptive Optics
  • Near-Infrared Photometry
  • M30
Data inizio appello
Riassunto analitico
Galactic Globular Clusters (GGCs) are gravitationally bound stellar clusters present in the halo and the bulge of the Milky Way. The stars of the clusters have the same metal abundance, the distance from us and very similar ages (within few 10^8 yr with respect to the cluster ages of the order of 10 Gyr).
They constitute a benchmark for stellar evolution and dynamics of stellar system theories. Moreover, they provide a robust constraint to the formation time scale and to the early chemical and dynamical history of the Milky Way.
Among the Galactic Globular Clusters, M30 is particularly interesting because is a very metal-poor GGC ([Fe/H]~-2.33) and therefore it is one of the oldest GCCs.
My thesis project is divided in two part. In the main part, I have reduced and analysed the near-infrared photometric data of the Globular Cluster M30 (NGC 7099) in J and Ks bands, never analysed before. The data were collected with three different instruments: SOFI (Son OF ISAAC, Infrared Spectrometer And Array Camera) mounted on the NTT telescope (3.58m) located at La Silla in Chile, HAWKI (High Acuity Wide field K-band Imager) and MAD (Multi-conjugate Adaptive optics Demonstrator) installed at the UT4 (8.2m) and the UT3 (8.2m) telescopes of the VLT located at Paranal in Chile, respectively. The data collected with MAD are taken with the Adaptive Optics (AO), a technology which was developed in order to overcome the impact of the atmospheric turbulence on the image formation for ground-based measurements.
The stellar photometry in globular clusters is quite difficult because they are very crowded stellar systems. Indeed, the stellar density can reach 10^5 stars/pc^3. In order to perform a very accurate and detailed photometry, I used Point Spread Function Photometry method by means of DAOPHOT/ALLSTAR and ALLFRAME software and I obtained accurate and deep near-infrared colour-magnitude diagrams.
To fully characterize the stellar system, I also cross-correlated the near-infrared catalogue with the optical ones available in the literature, in particular, data collected from the space by ACS (Advanced Camera for Surveys), mounted on HST (Hubble Space Telescope) and from an ensemble of the ground-based telescopes.
For the second part, the comparison between theoretical models and observed color-magnitude diagrams (CMD) was performed. The distance modulus, the reddening and the age of M30 were estimated by means of recent isochrones (the theoretical models which describe the stellar population of a cluster) compared with observed data.
The age suggested from the theory-observations comparison provided a value of 12+/-1 Gyr, in good agreement with the recent literature.
Among the evolutionary features, I focussed on the RGB bump, a star clustering along the red giant branch on the color-magnitude diagram, due to the crossing of the same luminosity range for three times by the stars during their evolution. This happens when the H-burning shell of a red giant star reaches the discontinuity in the chemical composition left by the convective envelope which had sunk inside the star in previous phases. The analysis of the bump position reveals a discrepancy between theory and observations, particularly evident for low-metallicity clusters. This fact, already discussed in the literature, is probably due to the uncertainties of the predicted external convective transport, mechanism for which we still have a limited knowledge.
From the data, it was possible to estimate the RGB bump magnitude from the luminosity function in different photometric bands. Since the RGB bump position is affected by the uncertainties of the distance modulus and the reddening, the parameter DeltaV^{bump}_{HB}=V_{bump}-V_{HB} is used, because it is independent of them. DeltaV^{bump}_{HB}=V_{bump}-V_{HB} is the difference between the apparent visual magnitude of the bump and the apparent visual magnitude of the horizontal branch at the luminosity level of RR Lyrae.
Comparing the predicted and the observed parameter, I found that the predicted values are ~ 0.41 mag fainter the observed one, suggesting a clear discrepancy between predictions and observations, independently of the used optical band (this discrepancy is investigated only for visual band in literature).
This confirm previous results found in the literature for other globular clusters which seems to indicate a problem in theoretical models to be analysed in much more details.