Academic literature on the topic '47Tuc'

AbstractWe show that the current data of the HB branch of 47 Tuc show a particular feature that cannot be explained if a single population with an unique mechanism of mass loss is considered. We find that a spread in helium abundance among the stars is necessary, of ~0.02. We indicate that the same variation in helium is present among the sub giant branch stars and suggest that is responsible of the spread in luminosity of the bright sub giant branch, while only a small part of the second generation is characterized by C+N+O increase and gives the faint sub giant branch.

AbstractThe ideal distance indicator would be a standard candle abundant enough to provide many examples within reach of parallax measurements and sufficiently bright to be seen out to Local Group galaxies. The red clump stars closely match this description. These are the metal rich equivalent of the better known horizontal branch stars, and their brightness dispersion is only 0.2 mag (one sigma) in the Solar neighborhood. Using Hipparcos to calibrate a large, local sample, the red clump method has been used to measure accurate distances to the Galactic center (Paczyński & Stanek 1998), M31 (Stanek & Garnavich 1998), LMC (Udalski et al. 1998; Stanek et al. 1998; Udalski 1999) and some clusters in our Galaxy (e.g. 47Tuc: Kaluzny et al. 1998). As with all the distance indicators, the main worry lies in the possible systematics of the method, in particular, the brightness dependence on the stellar metallicity and age. These dependences have come under close scrutiny and, indeed, the population effects on the red clump brightness appear small and calibratable. Perhaps the most controversial result from the red clump method is the estimation of a “short” distance to the Large Magellanic Cloud (Udalski et al. 1998; Stanek, Zaritsky & Harris 1998; Udalski 2000). This distance to the LMC is shorter by 12% than the “standard” value, and has very important implications for the Cepheid distance scale and the determination of the Hubble constant.

This article discusses methodological issues of mathematical microworld development integrated with generating innovation in the school setting. This is done by means of vignettes of key episodes in our eight-year-long experience of developing a component architecture for educational software based on Logo as a scripting language. The vignettes touch on the problems of collaboration between organizations and people of different expertise. They also address issues to do with the school and the classroom as social systems, with the method for implementing innovation and with curriculum design, teaching, and learning. A set of issues that emerged as problematic are outlined and discussed; the different priority systems involved, the amount of investment in collaboration, the differing discourses and epistemologies, the notion of a product, the interdependencies, and the contrast between reform and innovation versus instant fit. It is suggested that awareness needs to be raised as well as methods for dealing with these factors in order to generate cultures developing and using exploratory software.

Over the last decade, 135 extrasolar planets have been discovered, the vast majority found by ongoing radial velocity searches. Of the stars sampled in these searches, 1% have `Hot Jupiter' planets associated with them. Having masses equivalent to Jupiter yet orbital periods of only a few days, this new class of planet is clearly unlike anything in our Solar System.¶ Hot Jupiters present us with an intriguing prospect. If the orientation of the planetary orbit is close to edge-on, the planet will periodically transit across the face of its star, resulting in a small drop in brightness. This transit phenomenon has been successfully used for planet detection over the last couple of years, allowing determination of the planetary radius and accurate mass estimates when coupled with radial velocity observations.¶ To aid understanding of the effect stellar environment plays on Hot Jupiter formation and survivability, this thesis presents the results of a wide-field search for transiting Hot Jupiters in the globular cluster 47 Tucanae. This cluster presents many thousands of stars in a moderate field of view and provides the perfect target for a search of this nature. One previous transit search has been made in the central core of 47 Tuc; using the HST for 8.3 continuous days, Gilliland et.al (2000) expected 17 transits yet found none. This null result suggests that either system metallicity or stellar density may be inhibiting Hot Jupiter formation or survivability in the cluster.¶ This thesis presents a search for transits with a field of view 250 times larger than the HST search and samples the uncrowded outer halo of the cluster (previously unsampled for transits), providing important constraints on the effect of environment on Hot Jupiter formation. If planets are found, then stellar density would seem responsible for the Gilliland et.al (2000) core null result. If no planets are found to a significant level, the survey would provide strong evidence that system metallicity is the dominant factor. Using the ANU 40'' (1m) telescope at Siding Spring Observatory, a 30.4 night observing run was executed and photometry was derived via differential imaging. The dataset numbers 109,000 cluster (and field) stars for photometric analysis, of which 22,000 are suitable for the transit search. With a custom-written transit detection algorithm and extensive Monte Carlo simulations to model the dataset, seven planets should be detectable if the occurrence rate of Hot Jupiters is the same in the cluster as in the Solar Neighbourhood.¶ Despite a detailed search, no transit signatures were identified. This result strongly indicates that the low metallicity of the cluster is the dominant factor inhibiting planet formation in 47 Tuc. Current results in the Solar Neighbourhood show that planet frequency is strongly biased towards stars of high metallicity. This thesis shows that the metallicity trend is likely a universal phenomenon, not only limited to the immediate Solar Neighbourhood and raises questions of whether planets were much rarer in the earlier Universe.¶ As a side result of the search, 100 variable stars were identified in the field, 69 of which are new discoveries. Subsequent analysis reveals a strong period segregation among the cluster eclipsing binaries, indicating previously unobserved dynamical effects in the cluster. Distance estimates for both 47 Tuc and the SMC are in agreement with previously published values and an independent identification of the binary period-colour relation was observed. Two binaries seem to have low-luminosity companions worthy of followup and one variable is likely a star in the early phases of planetary nebula formation. All of the results presented in this thesis have been published in three separately refereed research papers.

Gli ammassi globulari rappresentano i laboratori ideali nei quali studiare la dinamica di sistemi ad N-corpi ed i suoi effetti sull’evoluzione stellare. Infatti, gli ammassi globulari sono gli unici sistemi astrofisici che, entro il tempo scala dell’età dell’Universo, sperimentano quasi tutti i processi di dinamica stellare noti. Questo lavoro di tesi si inserisce in un progetto a lungo termine volto a fornire una dettagliata caratterizzazione delle proprietà dinamiche degli ammassi globulari galattici. In questa ricerca, strumenti di fondamentale importanza sono il profilo di dispersione di velocità del sistema e la sua curva di rotazione. Per determinare le componenti radiali di questi profili cinematici in ammassi globulari galattici è necessario misurare la velocità lungo la linea di vista di un ampio campione di stelle membre, a differenti distanze dal centro. Seguendo un approccio multi-strumentale, è possibile campionare l’intera estensione radiale dell’ammasso utilizzando spettrografi multi-oggetto ad alta risoluzione spettrale nelle regioni intermedie/esterne, e spettrografi IFU con ottiche adattive per le regioni centrali (pochi secondi d’arco dal centro). Questo lavoro di tesi è volto a determinare il profilo di dispersione di velocità dell’ammasso globulare 47 Tucanae, campionando un’estensione radiale compresa tra circa 20'' e 13' dal centro. Per questo scopo sono state misurate le velocità radiali di circa un migliaio di stelle nella direzione di 47 Tucanae, utilizzando spettri ad alta risoluzione ottenuti con lo spettrografo multi-oggetto FLAMES montato al Very Large Telescope dell’ESO. Le velocità radiali sono state misurate utilizzando la tecnica di cross-correlazione tra gli spettri osservati e appropriati spettri teorici, e sono state ottenute accuratezze inferiori a 0.5km/s. Il campione così ottenuto (complementare a quello raccolto con strumenti IFU nelle regioni centrali) è fondamentale per costruire il profilo di dispersione di velocità dell’ammasso e la sua eventuale curva di rotazione. Questi dati, combinati col profilo di densità dell’ammasso precedentemente determinato, permetteranno di vincolare opportunamente modelli teorici come quelli di King (1966) o di Wilson (1975), e di arrivare così alla prima solida determinazione dei parametri strutturali e dinamici (raggi di core e di metà massa, tempo di rilassamento, parametro collisionale, etc.) e della massa totale e distribuzione di massa del sistema.

Over the last decade, 135 extrasolar planets have been discovered, the vast majority found by ongoing radial velocity searches. Of the stars sampled in these searches, 1% have `Hot Jupiter' planets associated with them. Having masses equivalent to Jupiter yet orbital periods of only a few days, this new class of planet is clearly unlike anything in our Solar System. ¶ Hot Jupiters present us with an intriguing prospect. If the orientation of the planetary orbit is close to edge-on, the planet will periodically transit across the face of its star, resulting in a small drop in brightness. This transit phenomenon has been successfully used for planet detection over the last couple of years, allowing determination of the planetary radius and accurate mass estimates when coupled with radial velocity observations. ¶ ...