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Punta Gorda, United States

Beccari G.,European Southern Observatory | De Marchi G.,Keplerlaan | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the Ha emission properties ofmain-sequence stars in the globular cluster 47 Tucanae. Using a combination of multiband observations in the F606W, F814W and F658N bands, we search for stars showing Ha excess emission. An accurate photometric measurement of theirHa equivalent width allows us to identify objectswith largeHa emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources, and their location in the colour-magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs).We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterize candidate CVs. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd | Sabbi E.,US Space Telescope Science Institute
Astrophysical Journal | Year: 2011

We have studied the properties of the stellar populations in the field of the NGC346 cluster in the Small Magellanic Cloud, using the results of a novel self-consistent method that provides a reliable identification of pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The 680 identified bona fide PMS stars show a bimodal age distribution, with two roughly equally numerous populations peaked, respectively, at 1Myr and 20Myr. We use the age and other physical properties of these PMS stars to study how star formation has proceeded across time and space in NGC346. We find no correlation between the locations of young and old PMS stars, nor do we find a correspondence between the positions of young PMS stars and those of massive OB stars of similar age. Furthermore, the mass distribution of stars with similar age shows large variations throughout the region. We conclude that, while on a global scale it makes sense to talk about an initial mass function, this concept is not meaningful for individual star-forming regions. An interesting implication of the separation between regions where massive stars and low-mass objects appear to form is that high-mass stars might not be "perfect" indicators of star formation and hence a large number of low-mass stars formed elsewhere might have so far remained unnoticed. For certain low surface density galaxies this way of preferential low-mass star formation may be the predominant mechanism, with the consequence that their total mass as derived from the luminosity may be severely underestimated and that their evolution is not correctly understood. © 2011. The American Astronomical Society. All rights reserved. Source


De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd | Girardi L.,National institute for astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2014

We have studied the interstellar extinction in a field of ∼ 3 arcmin × 3 arcmin located about 6 arcmin southwest of 30 Doradus in the Large Magellanic Cloud (LMC). Hubble Space Telescope observations in the U, B, V, I and Hα bands reveal patchy extinction in this field. The colour-magnitude diagram (CMD) shows an elongated stellar sequence, almost parallel to the main sequence (MS), which is in reality made up of stars of the red giant clump (RC) spread across the CMD by the uneven levels of extinction in this region. Since these objects are all at the same distance from us and share very similar physical properties, we can derive quantitatively both the extinction law in the range 3000-8000 Å and the absolute extinction towards about 100 objects, setting statistically significant constraints on the dust grains properties in this area. We find an extinction curve considerably flatter than the standard Galactic one and than those obtained before for the LMC. The derived value of RV = 5.6 ± 0.3 implies that in this region larger grains dominate. Upper MS stars span a narrower range of E(B - V) values than RC objects, at variance with what has been found elsewhere in the LMC. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Scarlata C.,University of Minnesota | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd
Astrophysical Journal | Year: 2015

We present a semi-analytical line transfer model, (SALT), to study the absorption and re-emission line profiles from expanding galactic envelopes. The envelopes are described as a superposition of shells with density and velocity varying with the distance from the center. We adopt the Sobolev approximation to describe the interaction between the photons escaping from each shell and the remainder of the envelope. We include the effect of multiple scatterings within each shell, properly accounting for the atomic structure of the scattering ions. We also account for the effect of a finite circular aperture on actual observations. For equal geometries and density distributions, our models reproduce the main features of the profiles generated with more complicated transfer codes. Also, our SALT line profiles nicely reproduce the typical asymmetric resonant absorption line profiles observed in starforming/starburst galaxies whereas these absorption profiles cannot be reproduced with thin shells moving at a fixed outflow velocity. We show that scattered resonant emission fills in the resonant absorption profiles, with a strength that is different for each transition. Observationally, the effect of resonant filling depends on both the outflow geometry and the size of the outflow relative to the spectroscopic aperture. Neglecting these effects will lead to incorrect values of gas covering fraction and column density. When a fluorescent channel is available, the resonant profiles alone cannot be used to infer the presence of scattered re-emission. Conversely, the presence of emission lines of fluorescent transitions reveals that emission filling cannot be neglected. © 2015. The American Astronomical Society. All rights reserved. Source


Spezzi L.,European Space Agency | De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,Supernova Ltd | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present a multiwavelength study of three star-forming regions, spanning the age range 1-14Myr, located between the 30 Doradus complex and supernova SN 1987A in the Large Magellanic Cloud (LMC). We reliably identify about 1000 pre-main-sequence (PMS) star candidates actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (M). Our measurements represent the largest M data set of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical M of PMS stars in the LMC is higher than for galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and M estimates, the difference in M between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disc dispersal and find indications that typically higher M are to be expected in low-metallicity environments. However, many issues of this scenario need to be clarified by future observations and modelling. We also find that, in the mass range 1-2M ⊙, M of PMS stars in the LMC increases with stellar mass as M ∝ M * b, with b≈ 1, i.e. slower than the second power law found for galactic PMS stars in the same mass regime. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

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