Resolved stellar populations : watching galaxy evolution in real time

One of the main issues in astrophysics is to understand how galaxies form and evolve. Deep photometric studies help the investigation of the evolution of resolved stellar contents of nearby systems. Hence the properties of these regions represent an archaeological record of the processes that shape a galaxy over cosmic time. So one can interpret from the star formation history the evolution of the star formation rate throughout the galaxy and the evolution of the mass and metallicity distributions. The system that has been studied in this project is the nearby galaxy M33, located in the Local Group. The photometric data was taken in the Canada-France-Hawaii Telescope with the MegaPrime/MegaCam wide-field mosaic imager and it is available for the filters g’, r’ and i’. The data analysis is presented in this work with the purpose of recovering its star formation history. Over one million point sources were identified in each filter. The program chosen for this process is DAOPHOT (Stetson, 1987). PSF-fitting photometry was performed using a few hundreds of point sources, selected from non-crowed areas, to fit the point-spread functions. This process, however, was repeated a couple of times in order to get a well adjusted point-spread function with the least residuals possible. The instrumental magnitude was then determined. A selection cut enabled spurious sources to be discarded based on the photometric errors (σ), residuals scatter (Χ2) and image quality (sharpness). Aperture and offset corrections were applied in the magnitudes before the transformation to the standard photometric system. A completeness test to examine the effects of crowding in the images was conducted in each photometric filter. The bias in the observed magnitudes and in the stellar counts due to high stellar density affects the final star formation history, resulting in the miss-assumption of the stellar age, metallicity and initial mass function. The artificial stars test (Williams et al., 2009) is a standard technique used to that end and consists of inserting synthetic stars in the images, with the routine ADDSTAR (Stetson, 1987), and performing again the photometric reductions in those synthetic images to compare the known inserted brightness with the recovered ones. The completeness is given by the ratio of the number of retrieved artificial stars over the number of added ones. Stars of all evolutionary stages lose mass and the mass recycled in the interstellar medium will be part of the next generation of stars and planets. The study of mass loss is quite well understood for metal-rich stars populating the asymptotic giant branch, though there is still a lot to be understood about the metal-poor stars losing mass during the red giant phase. The understanding of the mass loss process that happens in red giant stars of globular clusters might help us to better estimate the post-main sequence stellar evolutionary stages and the intra-cluster gas enrichment. Since the 70’s it has been known that the Galactic globular cluster ω Centauri shows an extremely complex stellar chemistry, with a wide variation in metallicity, [Fe/H] ≈ -2 to [Fe/H] ≈ -0.6, and light elements (like He, C, N...). Indeed, the properties of ω Cen favours the hypothesis that this is a remnant of a dwarf galaxy orbiting the Milky Way and tidal interactions partially disrupted it. With observations from the Infra-Red Array Camera aboard of the Spitzer telescope, investigations on red giant stars in ω Centauri are carried out to identify infrared colour excess originating from the emission of a circumstellar envelope surrounding the stars (e.g. Frogel & Elias, 1988; Origlia et al., 1996). This study is based on a proper combination of ground-based and original Spitzer photometric data as well as results from previous spectroscopic surveys. Prior to the selection of the dust excess stars, the magnitudes from the SDSS photometric system are converted to the TCS system based on the colour relations of Carpenter (2001) and Alonso et al. (1998) as the colour-temperature equations used to calculate the effective temperature are in different photometric filter systems. Bolometric corrections and the effective temperature are needed for comparisons between observations and theory and both parameters were derived according to Alonso et al. (1999). After selection, 34 giant stars presented colour excess in (K - 8) with metallicities ranging from -1.9 < [Fe=H] < -0.7; metallicities that were interpolated from PARSEC isochrones (Bressan et al., 2012). Field stars were rejected based on the proper motions from GAIA, which reduced to 18 the number of mass-losing candidates. The large amount of field stars excluded from the sample is due to the difference in spatial coverage from GAIA and Spitzer. The stellar synthetic spectral distribution of those stars is modelled and used to calculate its mass loss rate, using a modified version (Origlia et al., 2007) of the radiative transfer code DUSTY (Ivezic et al., 1999; Elitzur & Ivezic, 2001). The mass loss rates derived from our sample are in the range of 10−8 to 10−7 M yr−1, which is slightly off the values proposed by Origlia et al. (2002) and Boyer et al. (2008). The mass loss rates seem to increase with increasing luminosities and its dependency with metallicity is minimal. Only a fraction of red giant stars are losing mass indicating an episodic mass loss.