Reference Metals

X CANARY ISLANDS WINTER SCHOOL OF ASTROPHYSICS "Globular Clusters"

Course: CHEMICAL ABUNDANCES OF STARS IN GLOBULAR CLUSTERS EARLY NUCLEOSYNTHESIS

Prof. Raffaele Gratton
Astronomical Observatory of Padua ITALY

REFERENCE METALS

The metal content of a star is an indication of its age. When they are formed stars "feed" on the material of their surroundings; when a star is "metal poor" it is considered to be old since it has been formed in a medium unenriched by supernova ejecta from stars of earlier generations. To determine the metallicity of an object and its abundance of heavy elements, astronomers normally use hydrogen as a reference element, although they also have recourse to iron. By studying the ratio of heavier elements, such as calcium and oxygen, with respect to the metallicity of the object valuable information can be obtained concerning the enrichment of the interstellar medium, which in turn aids the study of the physical processes that might have occurred in the galaxy hosting the object. It is precisely at this point that globular clusters reveal themselves as ideal locations for such investigations, because they provide homogenous samples of more or less metal-poor stars, as explained during the Winter School by Professor Raffaele Gratton of the Astronomical Observatory of Padua (Italy).

What do you think are the most important contributions of the study of globular clusters to our knowledge of the Universe? What type of key information may be found when studying globular clusters exclusively?

"Perhaps the most important data concern ages of globular clusters, since they are the oldest individual objects that may be dated with some accuracy, and the provide a stringent lower limit to the age of the Universe. The age dispersion and distribution provide important test for the epoch and the mechanisms of galaxy formation. Clusters are the best approximation to pure (single age, single chemical composition) stellar populations: they then provide invaluable tests for stellar evolution. They are dense stellar systems, providing basic comparisons for our understanding of the dynamics of dense stellar environments and even on formation and evolution of stars in dense environments. They are rather bright objects, that may be observed at large distances, and may then be used as standard candles."

What is the minimum age dispersion in globular clusters? Is it correlated with their physical features? What kind of information does it provide in regard to the duration to the collapse of the protogalactic cloud?

"If I understand correctly, this question is about the cluster-to-cluster age spread. About twelve years ago, Sergio Ortolani and myself (and nearly at the same time Peter Stetson too) found that Pal 12 is significantly younger than the bulk of the globular clusters, with an age about 2/3 that of 47 Tuc. This was the first clear evidence breaking the Sandage, Eggen & Lynden-Bell paradigma of very fast formation of the galactic halo (although an alternative scenario was proposed earlier by Searle and Zinn). A few other clusters were later found to be young. However, on the whole they are a small minority. Recently, it has been shown that most (and perhaps all) of these clusters were likely accreted from dwarf satellites of the Milky Way. On the other hand, there is a lively debate about the spread of ages for the bulk of galactic globular clusters."

What is the reason for the absence, in our Milky Way, of clusters of metallicity [Fe/H]<-3 in other galaxies?

"The absence of very metal-poor clusters is sometimes considered as an evidence that clusters formed their own metals: a single 20 Mo SN would eject enough metals to pollute a 105 Mo proto-cluster at [Fe/H]<-3; as noticed by Andy McWilliam, this is likely related to the observed cosmic scatter in the abundances of the n-capture elements below [Fe/H]<-2. However, several years ago Vittorio Castellani observed that if the fraction of stars with meatllicity below a given value of Z is roughly proportional to Z (a quite reasonable assumption) we should expect less than cluster whose metallicity is less than a thousandth of that of the Sun, since there are les than 200 clusters in the galaxy. So maybe there is nothing special in the absence of clusters with [Fe/H]<-3."

In relation to the topic of this school, which is at present the most interesting problem in your research?

"The distance scale for both population I and II objects. This exemplified by the case of the LMC, where values of the true distance modulus ranging from 18.09 to 18.70 have been proposed in the last year: the shortest value has been obtained by application of the "red giant clump" method by Udalski et al., while the longest has been obtained by Feast and Catchpole using Cepheids. Furthermore there are a number of determinations spread over this broad range. Personally, I obtained two values of 18.60 and 18.37 using the subdwarf fitting method and the absolute magnitude of the horizontal branch respectively. Still both results are based on Hipparcos parallaxes of local calibrations. This range in distances implies an enormous range in ages or globular clusters (from about 11 to more than 20 Gyrs)."

PROFILE

Raffaele Gratton was born in La Plata (Argentina), on November 21, 1956. Italian, he studied at University "La Sapienza" in Rome where he got his Degree in 1979. He worked at Cerro Tololo Inter-American Observatory, La Serena, Chile, 1980-1981, Asiago Astrophysical Observatory, Italy, 1981-1984 Astronomer; Rome Astronomical Observatory, Italy, 1984-1989, and he is presently Associate Astronomer at Padua Astronomical Observatory, Italy. Gratton is an observational astronomer, but he is now working also on technological aspects (spectrograph design and construction). His speciality is abundance analysis, and his main area of interest are globular clusters and chemical evolution of the Galaxy. He also works on surface abundances as signatures of stellar evolution. Gratton is PI of the High Resolution Spectrograph for the Galileo National Telescope, under construction at the Roque de Los Muchachos Observatory (La Palma).