Punta Gorda, United States
Punta Gorda, United States

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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.


De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

Attention is given to a population of 110 stars in the NGC 6611 cluster of the Eagle nebula that have prominent near-infrared excess and optical colours typical of pre-main-sequence (PMS) stars older than 8Myr. At least half of those for which spectroscopy exists have a Ha emission line profile revealing active accretion. In principle, the V - I colours of all these stars would be consistent with those of young PMS objects (<1Myr) whose radiation is heavily obscured by a circumstellar disc seen at high inclination and in small part scattered towards the observer by the back side of the disc. However, using theoretical models it is shown here that objects of this type can only account for a fewper cent of this population. In fact, the spatial distribution of these objects, their X-ray luminosities, their optical brightness, their positions in the colour- magnitude diagram and the weak Li absorption lines of the stars studied spectroscopically suggest that most of them are at least eight times older than the ~1 Myr-old PMS stars already known in this cluster and could be as old as ~30 Myr. This is the largest homogeneous sample to date of Galactic PMS stars considerably older than 8 Myr that are still actively accreting from a circumstellar disc and it allows us to set a lower limit of 7 per cent to the disc frequency at ~16 Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~6Myr for disc dissipation. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


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.


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.


Maoz D.,Tel Aviv University | Mannucci F.,National institute for astrophysics | Li W.,University of California at Berkeley | Filippenko A.V.,University of California at Berkeley | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

Recovery of the supernova (SN) delay-time distribution (DTD) - the SN rate versus time that would follow a hypothetical brief burst of star formation - can shed light on SN progenitors and physics, as well as on the time-scales of chemical enrichment. Previous attempts to reconstruct the DTD have been based either on comparison of mean SN rates versus redshift to cosmic star-formation history (SFH), or on the comparison of SN rates among galaxies with different mean ages. Here, we present an approach to recover the SN DTD that avoids the averaging and loss of information of other schemes. We compare the SFHs of individual galaxies to the numbers of SNe discovered by a survey in each galaxy (generally zero, sometimes one SN, rarely a few). We apply the method to a subsample of 3505 galaxies, hosting 82 type-Ia SNe (SNe Ia) and 119 core-collapse supernovae (CC SNe), from the Lick Observatory Supernova Search (LOSS), that have SFHs reconstructed from Sloan Digital Sky Survey (SDSS) spectra. We find a >2σ SN Ia DTD signal in our shortest-delay, 'prompt' bin at <420 Myr. We identify and study a systematic error, due to the limited aperture of the SDSS spectroscopic fibres, that causes some of the prompt signal to leak to the later bins of the DTD. After accounting for this systematic error, we demonstrate that a prompt SN Ia contribution is required by the data at the >99 per cent confidence level. We further find a 4σ indication of SNe Ia that are 'delayed' by >2.4 Gyr. Thus, the data support the existence of both prompt and delayed SNe Ia. We measure the time integral over the SN DTD. For CC SNe we find a total yield of 0.010 ± 0.002 SNe per M⊙ formed, in excellent agreement with expectations, if all stars more massive than 8 M⊙ lead to visible SN explosions. This argues against scenarios in which the minimum mass for core-collapse SNe is ≳10 M⊙, or in which a significant fraction of massive stars collapse without an accompanying explosion. For SNe Ia, the time-integrated yield is 0.0023 ± 0.0006 SNe per M⊙ formed, most of them with delays <2.4 Gyr. Finally, we show the robust performance of the method on simulated samples, and demonstrate that its application to already existing SN samples, such as the full LOSS sample, but with complete and unbiased SFH estimates for the survey galaxies, could provide an accurate and detailed measurement of the SN Ia DTD. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


De Marchi G.,European Space Agency | Beccari G.,European Southern Observatory | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd
Astrophysical Journal | Year: 2013

We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Hα bands. We have identified about 300 pre-main-sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass, and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognize at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100″ north of the center of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of the two episodes appears to be comparable, but the episodes occurring more than 30 Myr ago might have been even stronger than the current one. We have investigated the evolution of the mass accretion rate, Ṁacc as a function of the stellar parameters finding that log Ṁacc ≃ -0.6 logt + logm + c, where t is the age of the star, m is its mass, and c is a decreasing function of the metallicity. © 2013. The American Astronomical Society. All rights reserved.


De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd | Romaniello M.,European Southern Observatory
Astrophysical Journal | Year: 2010

We have developed and successfully tested a new self-consistent method to reliably identify pre-main-sequence (PMS) objects actively undergoing mass accretion in a resolved stellar population, regardless of their age. The method does not require spectroscopy and combines broadband V and I photometry with narrowband Hα imaging to (1) identify all stars with excess Hα emission, (2) convert the excess Hα magnitude into Hα luminosity L(Hα), (3) estimate the Hα emission equivalent width, (4) derive the accretion luminosity Lacc from L(Hα), and finally (5) obtain the mass accretion rate from Lacc and the stellar parameters (mass and radius). By selecting stars with an accuracy of 15% or better in the Hα photometry, the statistical uncertainty on the derived is typically ≲17% and is dictated by the precision of the Hα photometry. Systematic uncertainties, of up to a factor of 3 on the value of , are caused by our incomplete understanding of the physics of the accretion process and affect all determinations of the mass accretion rate, including those based on a spectroscopic Hα line analysis. As an application of our method, we study the accretion process in a field of 9.16 arcmin2 around SN1987A, using existing Hubble Space Telescope photometry. We identify as bona fide PMS stars a total of 133 objects with a Hα excess above the 4σ level and a median age of 13.5Myr. Their median mass accretion rate of 2.6 × 10-8M yr-1 is in excellent agreement with previous determinations based on the U-band excess of the stars in the same field, as well as with the value measured for G-type PMS stars in the Milky Way. The accretion luminosity of these PMS objects shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the ultraviolet radiation of the latter is rapidly eroding the circumstellar disks around PMS stars. © 2010 The American Astronomical Society. All rights reserved.


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.


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

We have studied the interstellar extinction in a field of ~3 arcmin × 3 arcmin at the core of the 30 Doradus nebula, including the central R 136 cluster, in the Large Magellanic Cloud. Observations at optical and near-infrared wavelengths, obtained with the WFC 3 camera on board the Hubble Space Telescope, show that the stars belonging to the red giant clump are spread across the colour-magnitude diagrams because of the considerable and uneven levels of extinction in this region. Since these stars share very similar physical properties and are all at the same distance, they allow us to derive the absolute extinction in a straightforward and reliable way. Thus, we have measured the extinction towards about 180 objects and the extinction law in the range 0.3-1.6 μm. At optical wavelengths, the extinction curve is almost parallel to that of the diffuse Galactic interstellar medium. Taking the latter as a template, the value of RV = 4.5 ± 0.2 that we measure indicates that in the optical there is an extra grey component due to a larger fraction of large grains. At wavelengths longer than ~1 μm, the contribution of this additional component tapers off as λ -1.5, like in the MilkyWay, suggesting that the nature of the grains is otherwise similar to those in our Galaxy, but with a ~2.2 times higher fraction of large grains. These results are consistent with the addition of 'fresh' large grains by supernova explosions, as recently revealed by Herschel and ALMA (Atacama Large Millimeter Array) observations of SN 1987A. © 2014 The Authors.


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.

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