Found 24 talks width keyword accretion, accretion discs
AbstractMost studies of the stellar and substellar populations of star forming regions rely on the identification of the signatures of accretion, outflows, circumstellar dust, or activity characteristic of the early stages of stellar evolution. However, the decay of these observational signatures with time limits our ability to understand the complete star forming history of young aggregates, and to obtain unbiased samples of young stellar objects at different stages of disk evolution. I will present the results of a wide-area study of the stellar population of selected clouds in the nearby Lupus star forming region, initially defined to complement the data obtained by the Spitzer Space Observatory Legacy Program “From molecular cores to planet-forming disks”. When combined with 2MASS photometry, our data allow us to fit the spectral energy distributions of well over 150,000 sources seen in that direction, and to identify possible new members based on their photospheric fluxes alone, with independence of the display of signposts of youth. In this way we identify a very clear signature of the existence of a surprisingly numerous and thus far unrecognized population of cool members of Lupus 1 and 3, which is absent from Lupus 4.
The approximately 130 new members that we identify show that Lupus 1 and 3 have been forming low mass stars in numbers comparable to, or even exceeding in Lupus 1, those revealed by recent sensitive surveys based on the signposts of youth. We hypothesize on several possibilities for the origin of this population that may account for its puzzling properties of general lack of disks, coevality with the disk-bearing population, and preferential off-cloud location, which hint at a picture more complex and interesting than the quiescent formation inside dense molecular clouds.
I will review the status of our understanding of galaxy formation in the prevailing cold dark matter paradigm. After reviewing the successes and failures of the most natural predictions of this scenario I will focus on the consequences of two of its main predictions: the presence of large numbers of low-mass dark matter halos and the prevalence of accretion events during the formation of normal galaxies. In particular, I will discuss the interpretation of the recent discovery of a population of ultra-faint galaxies in the Local Group, and its relation to the profuse cold dark matter substructure expected in the Galactic halo. I will also discuss the importance that accretion events might have had in shaping not only the stellar halo but also the disk component(s) of the Milky Way.
Based on observations with the Advanced Camera for Surveys (ACS), I will present accurate relative ages for a sample of 64 Galactic globular clusters. This Hubble Space Telescope (HST) Treasury program has been designed to provide a new large, deep and homogeneous photometric database. Relative ages have been obtained using a main sequence fitting procedure between clusters in the sample. Relative ages are determined with an accuracy from 2% to 7%. It has been proved that derived ages are independent of the assumed theoretical models. The existence of two well defined Galactic globular cluster groups is found. A group of old globular clusters with an age dispersion of 6% and showing no age-metallicity relation, and, on the other hand, a younger group showing a clear age-metallicity relation similar to that found in the globular clusters associated to the Sagittarius dwarf galaxy. Roughly 1/3 of the clusters belong to the younger group. Considering these new results, it is very tempting to suggest a Milky Way's halo formation scenario in which two differentiated phases took place. A very fast collapse, where the old and coeval globular clusters where formed, followed by accretions of Milky Way's satellite galaxies.
AbstractStarbursts and AGNs are frequently coupled in the central kiloparsecs of Seyfert galaxies, where molecular gas plays a critical role in fueling nuclear starburst activity and feeding the central black hole. Unveiling the dusty nuclear regions with high-spatial resolution techniques in the near-infrared (NIR) permits us to disentangle the AGN and the stellar clusters, characterizing both sources separately. In this context, a small sample of nearby galaxies have been observed with VLT/NaCo adaptive optics in the NIR. These observations were completed with similar high-spatial resolution data in the mid-infrared (VLT/VISIR), optical (HST) and radio wavelengths (VLA). A new alignment for the starburst galaxy NGC 253 was found based on NIR and radio data, due to the high-spatial resolution in both spectral regions, finding NIR counterparts for 8 known radio sources. It is remarkable the lack of any optical or IR counterpart for the radio core, proposed as a low luminosity AGN, which presents an IR-to-radio emission ratio similar (or even lower) than Sgr. A*. Using the high-spatial resolution aligned dataset from optical-IR to radio wavelengths we derived a representative spectral energy distribution (SED) based on 37 young dust embedded clusters resolved in the inner 0.4 kpc. The template is characterized by a maximum at 20 μ and a gentle bump in the 1-2 μ range. These features, absent in lower spatial resolution templates, can be well reproduced by considering an important contribution of very young stellar objects to the IR, and are thus associated with hot dust surrounding the protostars. The average SED was then compared with the nuclear star forming regions found in the Seyfert 2/starburst galaxy NGC 7582.
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