Recent observational evidence suggests the existence of a new class of White Dwarfs in the populations of several Globular Clusters, deviating from the standard cooling sequence due to what is thought to be residual hydrogen burning in their envelopes. The presence of such WDs seems correlated with the horizontal branch morphology of their host cluster, as they have so far only been detected in clusters with HBs featuring an extended blue tail. This work examines two galactic globular clusters known for their distinct HB morphologies, NGC 288 and NGC 362, using data collected from Hubble WFC3-UVIS in the F275W and F336W ultraviolet bands. Starting from the FITS *.flc files, I used state-of-the art programs to reduce the images and derive the cluster color-magnitude diagrams through single-pass and multi-pass photometry. Said diagrams are then used to re-derive cluster ages, metallicities, distance moduli and reddening coefficients by means of by isochrone fitting. The obtained values are then checked against the literature, and a morphological comparison of the two cluster CMDs is performed. Finally, using the information thus gained, the White Dwarf Cooling Sequences of both clusters are analyzed, with a focus on their luminosity functions. The luminosity function of NGC 288 WDs significantly deviates from theoretical expectations for standard cooling, as found in M13 and NGC 6752, while no such anomaly is detected in the WD luminosity function of NGC 362, in line with what observed in M3 and M5. In the conclusions I then explore a number of possible explanations for the observed differences in the WD sequence between the two cluster types.
Recenti analisi osservative suggeriscono l'esistenza, in alcuni ammassi globulari, di una nuova tipologia di Nane Bianche, il cui processo di raffreddamento presenta anomalie dovute, secondo recenti modelli teorici, ad un residuo bruciamento dell'idrogeno nei loro starti esterni. La presenza di queste NB sembra essere correlata alla morfologia del Ramo Orizzontale dell'ammasso di appartenenza, essendo state finora rilevate solo in presenza di una sequenza molto estesa nel blu. Questo lavoro esamina due ammassi globulari della Via Lattea noti per avere rami orizzontali molto diversi tra loro: NGC 288 e NGC 362, usando dati raccolti dalla WFC3-UVIS del telescopio spaziale Hubble nei filtri ultravioletti F275W e F336W. Partendo dai file FITS *.flc, riduciamo le immagini tramite programmi allo stato dell'arte e otteniamo i diagrammi Colore-Magnitudine dei due cluster tramite fotometria single- e multi-pass. I diagrammi vengono quindi usati per ri-derivare età, metallicità, moduli di distanza e coefficienti di reddening dei due ammassi tramite fit di isocrone. I valori ottenuti vengono quindi confrontati con la letteratura, e si procede a un confronto morfologico tra i diagrammi dei due ammassi. Infine, con le informazioni ottenute, si procede all'analisi delle rispettive sequenze di raffreddamento delle NB, con particolare attenzione rivolta alle loro funzioni di luminosità. Si constata come la funzione di luminosità di NGC 288 differisca in modo sostanziale dalle previsioni teoriche per un normale raffreddamento, al pari di M13 e NGC 6752, mentre lo stesso fenomeno non si presenti in NGC 362, in linea con quanto osservato in M3 e M5. Infine nelle conclusioni esploriamo alcune possibili spiegazioni per le differenze osservate tra le sequenze NB dei due tipi di ammassi.
Smoldering Embers - The Search for Slowly Cooling White Dwarfs in Globular Clusters NGC0288 and NGC0362
LIONETTO, SOFIA
2023/2024
Abstract
Recent observational evidence suggests the existence of a new class of White Dwarfs in the populations of several Globular Clusters, deviating from the standard cooling sequence due to what is thought to be residual hydrogen burning in their envelopes. The presence of such WDs seems correlated with the horizontal branch morphology of their host cluster, as they have so far only been detected in clusters with HBs featuring an extended blue tail. This work examines two galactic globular clusters known for their distinct HB morphologies, NGC 288 and NGC 362, using data collected from Hubble WFC3-UVIS in the F275W and F336W ultraviolet bands. Starting from the FITS *.flc files, I used state-of-the art programs to reduce the images and derive the cluster color-magnitude diagrams through single-pass and multi-pass photometry. Said diagrams are then used to re-derive cluster ages, metallicities, distance moduli and reddening coefficients by means of by isochrone fitting. The obtained values are then checked against the literature, and a morphological comparison of the two cluster CMDs is performed. Finally, using the information thus gained, the White Dwarf Cooling Sequences of both clusters are analyzed, with a focus on their luminosity functions. The luminosity function of NGC 288 WDs significantly deviates from theoretical expectations for standard cooling, as found in M13 and NGC 6752, while no such anomaly is detected in the WD luminosity function of NGC 362, in line with what observed in M3 and M5. In the conclusions I then explore a number of possible explanations for the observed differences in the WD sequence between the two cluster types.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/65402