arXiv subjects

Alessandro Bressan

Publications and source records attributed to Alessandro Bressan.

At least 109 records · Page 6Linked to original sources

arXiv2013

Evolution of Thermally Pulsing Asymptotic Giant Branch Stars I. The COLIBRI Code

We present the COLIBRI code for computing the evolution of stars along the TP-AGB phase. Compared to purely synthetic TP-AGB codes, COLIBRI relaxes a significant part of their analytic formalism in favour of a detailed physics applied to a complete envelope model, in which the stellar structure equations are integrated from the atmosphere down to the bottom of the hydrogen-burning shell. This allows to predict self-consistently: (i) the effective temperature, and more generally the convective envelope and atmosphere structures, correctly coupled to the changes in the surface chemical abundances and gas opacities; (ii) sphericity effects in the atmospheres; (iii) the core mass-luminosity relation and its break-down due to hot bottom burning (HBB) in the most massive AGB stars, (iv) the HBB nucleosynthesis via the solution of a complete nuclear network (pp chains, and the CNO, NeNa, MgAl cycles), including also the production of 7Li via the Cameron-Fowler beryllium transport mechanism; (v) the chemical composition of the pulse-driven convective zone; (vi) the onset and quenching of the third dredge-up, with a suitable temperature criterion. At the same time COLIBRI pioneers new techniques in the treatment of the physics of stellar interiors. It is the first evolutionary code ever to use accurate on-the-fly computation of the equation of state for roughly 800 atoms, ions, molecules, and of the Rosseland mean opacities throughout the deep envelope. Another distinguishing aspect of COLIBRI is its high computational speed. This feature is necessary for calibrating the uncertain parameters and processes that characterize the TP-AGB phase, a step of paramount importance for producing reliable stellar population synthesis models of galaxies up to high redshift. (abridged)

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arXiv2013

An extended main sequence turn-off in the Small Magellanic Cloud star cluster NGC411

Based on new observations with the Wide Field Camera 3 onboard the Hubble Space Telescope, we report the discovery of an extended main sequence turn-off (eMSTO) in the intermediate-age star cluster NGC411. This is the second case of an eMSTO being identified in a star cluster belonging to the Small Magellanic Cloud (SMC), after NGC419. Despite the present masses of these two SMC clusters differ by a factor of 4, the comparison between their colour--magnitude diagrams (CMD) shows striking similarities, especially regarding the shape of their eMSTOs. The loci of main CMD features are so similar that they can be well described, in a first approximation, by the same mean metallicity, distance and extinction. NGC411, however, presents merely a trace of secondary red clump as opposed to its prominent manifestation in NGC419. This could be due either to the small number statistics in NGC411, or by the star formation in NGC419 having continued for 60 Myr longer than in NGC411. Under the assumption that the eMSTOs are caused by different generations of stars at increasing age, both clusters are nearly coeval in their first episodes of star formation. The initial period of star formation, however, is slightly more marked in NGC419 than in NGC411. We discuss these findings in the context of possible scenarios for the origin of eMSTOs.

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arXiv2013

Red Giant evolution and specific problems

In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties still remain due to our poor knowledge of some complex physical processes that still require an empirical calibration, such as the efficiency of convective heat transport and interior mixing. Here we will review the impact of these uncertainties on the evolution of red giant stars.

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arXiv2013

The star formation history of the Large Magellanic Cloud star clusters NGC1846 and NGC1783

NGC1846 and NGC1783 are two massive star clusters in the Large Magellanic Cloud, hosting both an extended main sequence turn-off and a dual clump of red giants. They present similar masses but differ mainly in angular size. Starting from their high-quality ACS data in the F435W, F555W and F814W filters, and updated sets of stellar evolutionary tracks, we derive their star formation rates as a function of age, SFR(t), by means of the classical method of CMD reconstruction which is usually applied to nearby galaxies. The method confirms the extended periods of star formation derived from previous analysis of the same data. When the analysis is performed for a finer resolution in age, we find clear evidence for a 50-Myr long hiatus between the oldest peak in the SFR(t), and a second prolonged period of star formation, in both clusters. For the more compact cluster NGC1846, there seems to be no significant difference between the SFR(t) in the cluster centre and in an annulus with radii between 20 and 60 arcsec (from 4.8 to 15.4 pc). The same does not occur in the more extended NGC1783 cluster, where the outer ring (between 33 and 107 arcsec, from 8.0 to 25.9 pc) is found to be slightly younger than the centre. We also explore the best-fitting slope of the present-day mass function and binary fraction for the different cluster regions, finding hints of a varying mass function between centre and outer ring in NGC1783. These findings are discussed within the present scenarios for the formation of clusters with multiple turn-offs.

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arXiv2012

The Panchromatic Hubble Andromeda Treasury I: Bright UV Stars in the Bulge of M31

As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle program, we observed a 12' \times 6.5' area of the bulge of M31 with the WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample of \sim4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and AGB-manqué stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manqué (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or α abundances when the mass loss on the RGB is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch stars (EHB) and their progeny. We construct the first radial profiles of these stellar populations, and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UVX. We calculate that only a few percent of MS stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population.

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arXiv2011

Tracing rejuvenation events in nearby S0 galaxies

With the aim of characterizing rejuvenation processes in early-type galaxies, we analyzed five barred S0 galaxies showing prominent outer ring in ultraviolet (UV) imaging. We analyzed GALEX far- (FUV) and near- (NUV) UV and optical data using stellar population models and estimated the age and the stellar mass of the entire galaxies and of the UV-bright ring structures. Outer rings consist of young (<200 Myr old) stellar populations, accounting for up to 70% of the FUV flux but containing only a few % of the total stellar mass. Integrated photometry of the whole galaxies places four of these objects on the green valley, indicating a globally evolving nature. We suggest such galaxy evolution is likely driven by bar induced instabilities, i.e. inner secular evolution, that conveys gas to the nucleus and to the outer rings. At the same time, HI observations of NGC 1533 and NGC 2962 suggest external gas re-fueling can play a role in the rejuvenation processes of such galaxies.

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arXiv2009

Mid-UV Narrow-Band Indices of Evolved Simple Stellar Populations

We explore the properties of selected mid-ultraviolet (1900-3200 angstrom) spectroscopic indices of simple stellar populations (SSPs). We incorporate the high resolution UVBLUE stellar spectral library into an evolutionary population synthesis code, based on the most recent Padova isochrones. We analyze the trends of UV indices with respect to age and chemical composition. As a first test against observations, we compare our results with the empirical mid-UV spectral indices of Galactic globular clusters, observed with the International Ultraviolet Explorer (IUE). We find that synthetic indices exhibit a variety of properties, the main one being the slight age sensitivity of most of them for ages>2 Gyr. However, for high metallicity, two indices, Fe II 2332 and Fe II 2402, display a remarkably different pattern, with a sharp increase within the first two Gyr and, thereafter, a rapid decline. These indices clearly mark the presence of young (~1 Gyr) metal rich (Z > Z_sun) stellar populations. We complement existing UV indices of Galactic globular clusters with new measurements, and carefully identify a sub-sample of ten indices suitable for comparison with theoretical models. The comparison shows a fair agreement and, in particular, the strong trend of the indices with metallicity is well reproduced. We also discuss the main improvements that should be considered in future modelling concerning, among others, the effects of alpha-enhancement in the spectral energy distributions.

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arXiv2009

Extreme UV QSOs

We present a sample of spectroscopically confirmed QSOs with FUV-NUV color (as measured by GALEX photometry) bluer than canonical QSO templates and than the majority of known QSOs. We analyze their FUV to NIR colors, luminosities and optical spectra. The sample includes a group of 150 objects at low redshift (z $<$ 0.5), and a group of 21 objects with redshift 1.7$<$z$<$2.6. For the low redshift objects, the "blue" FUV-NUV color may be caused by enhanced Ly$α$ emission, since Ly$α$ transits the GALEX FUV band from z=0.1 to z=0.47. Synthetic QSO templates constructed with Ly$α$ up to 3 times stronger than in standard templates match the observed UV colors of our low redshift sample. The H$α$ emission increases, and the optical spectra become bluer, with increasing absolute UV luminosity. The UV-blue QSOs at redshift about 2, where the GALEX bands sample restframe about 450-590A (FUV) and about 590-940A(NUV), are fainter than the average of UV-normal QSOs at similar redshift in NUV, while they have comparable luminosities in other bands. Therefore we speculate that their observed FUV-NUV color may be explained by a combination of steep flux rise towards short wavelengths and dust absorption below the Lyman limit, such as from small grains or crystalline carbon. The ratio of Ly$α$ to CIV could be measured in 10 objects; it is higher (30% on average) than for UV-normal QSOs, and close to the value expected for shock or collisional ionization. FULL VERSION AVAILABLE FROM AUTHOR'S WEB SITE: http://dolomiti.pha.jhu.edu/papers/2009_AJ_Extreme_UV_QSOs.pdf

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arXiv2008

Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models

We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000). Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the several peculiarities present in these TP-AGB tracks. Theoretical isochrones are then converted to about 20 different photometric systems -- including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., -- by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry by the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd.inaf.it/cmd) that provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.

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arXiv2007

PopStar: A new grid of Evolutionary Synthesis Models in the Virtual Observatory

We present a new set of theoretical evolutionary synthesis models, PopStar. This grid of Single Stellar Populations covers a wide range in both, age and metallicity. The models use the most recent evolutionary tracks together with the use of new NLTE atmosphere models for the hot stars (O, B, WR, post-AGB stars, planetary nebulae) that dominate the stellar cluster's ionizing spectra. The results of the models in VO format can be used through VOSpec.

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arXiv2007

A new grid of evolutionary synthesis models

We present new evolutionary synthesis models for Single Stellar Populations covering a wide range in age and metallicity. The most important difference with existing models is the use of NLTE atmosphere models for the hot stars (O, B, WR, post-AGB stars, and planetary nebulae) that have an important impact in the stellar cluster's ionizing spectra.

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arXiv2007

Near UV properties of Early-Type Galaxies at z~1

We have used spectral fits to SSP-based atmosphere models to derive an estimate of the average stellar age for an almost complete sample of 15 Early-Type Galaxies (ETG) at 0.88<z<1.3. The results are in only partial agreement with the age estimates previously obtained for the same objects from an analysis of the M/L_B ratio, derived from the Fundamental Plane (FP) parameters. In particular spectral fits seem to underestimate the age of the most luminous ETG, and therefore do not reproduce the downsizing effect, which is clear for the FP ages. We also analyse the relationship between the spectral-fit ages and various near-UV spectral indices.

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arXiv2006

The Ages of Early-Type Galaxies at z~1

The study of the ages of early-type galaxies and their dependence on galaxy mass and environment is crucial for understanding the formation and early evolution of galaxies. We review recent works on the M/L ratio evolution, as derived from an analysis of the Fundamental Plane of early-type galaxies at z~1 both in the field and in the clusters environment. We use the M/L ratio to derive an estimate of the galaxy age. We also use a set of high-S/N intermediate-resolution VLT spectra of a sample of early-type galaxies with 0.88<z<1.3 from the K20 survey to derive an independent estimate of their age by fitting SSP model spectra. Taking advantage of the good leverage provided by the high sample redshift, we analyse the results in comparison with the ages obtained for the same sample from the analysis of the M/L ratio, and with the predictions of the current hierarchical models of galaxy formation.

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arXiv2005

A Physical Model for the joint evolution of high-z QSOs and Spheroids

We summarize our physical model for the early co-evolution of spheroidal galaxies and of active nuclei at their centers. Our predictions are in excellent agreement with a number of observables which proved to be extremely challenging for all the current semi-analytic models, including the sub-mm counts and the corresponding redshift distributions, and the epoch-dependent K-band luminosity function of spheroidal galaxies. Also, the black hole mass function and the relationship between the black hole mass and the velocity dispersion in the galaxy are nicely reproduced. The mild AGN activity revealed by X-ray observations of SCUBA sources is in keeping with our scenario, and testify the build up of SMBH triggered by intense star formation.

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arXiv2004

Photometric Estimates of Stellar Masses in High-Redshift Galaxies

We present a new tool for the photometric estimate of stellar masses in distant galaxies. The observed SEDs are fitted by combining single stellar populations, with different SFRs and amounts of dust extinction. This approach gives us the best flexibility when dealing with the widest variety of physical situation for the target galaxies. In particular we tested the code on three classes of sources: dusty ISO-selected starbursts, K-band selected ellipticals/S0s and z=2-3 Lyman-break galaxies. We pay particular attention in evaluating the uncertainties in the stellar mass estimate, due to degeneracies in the physical parameters, different SFHs or metallicities. Based on optical-NIR photometric data, the stellar masses are found to have overall uncertainties of a factor of ~2 for E/S0s, ~2-5 for the starbursts population, and up to 10 for Ly-break galaxies. In any case the latter appear to correspond to a galaxy population significantly less massive than those observed at lower redshifts, possibly indicating substantial stellar build-up at z~1-2 in the field galaxy population. Using simulated deep SIRTF/IRAC observations of starbursts and Lyman-break galaxies, we investigate how an extension of the wavelength dynamic range will decrease the uncertainties in the stellar mass estimate, and find that they will reduce for both classes to factors of 2-3, good enough for statistically reliable determinations of the galaxy evolutionary mass functions.

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arXiv2003

A Physical Model for the Co-evolution of QSOs and of their Spheroidal Hosts

We present a physically motivated model for the early co-evolution of massive spheroidal galaxies and active nuclei at their centers. Within dark matter halos, forming at the rate predicted by the canonical hierarchical clustering scenario, the gas evolution is controlled by gravity, radiative cooling, and heating by feedback from supernovae and from the growing active nucleus. Supernova heating is increasingly effective with decreasing binding energy in slowing down the star formation and in driving gas outflows. The more massive proto-galaxies virializing at earlier times are thus the sites of the faster star-formation. The correspondingly higher radiation drag fastens the angular momentum loss by the gas, resulting in a larger accretion rate onto the central black-hole. In turn, the kinetic energy carried by outflows driven by active nuclei can unbind the residual gas, thus halting both the star formation and the black-hole growth, in a time again shorter for larger halos. For the most massive galaxies the gas unbinding time is short enough for the bulk of the star-formation to be completed before type Ia supernovae can substantially increase the $Fe$ abundance of the interstellar medium, thus accounting for the $α$-enhancement seen in the largest galaxies. The feedback from supernovae and from the active nucleus also determines the relationship between the black-hole mass and the mass, or the velocity dispersion, of the host galaxy, as well as the black-hole mass function. These and other model predictions are in excellent agreement with observations for a number of observables which proved to be extremely challenging for all the current semi-analytic models, including the sub-mm counts and the corresponding redshift distributions, and the epoch-dependent K-band luminosity function of spheroidal galaxies.

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arXiv2003

Spatially-resolved spectrophotometric analysis and modelling of the Superantennae

We have performed spatially-resolved spectroscopy of the double-nucleated Ultra-Luminous Infrared Galaxy IRAS 19254-7245, ``the Superantennae'', along the line connecting the two nuclei. These data are analysed with a spectral synthesis code, to derive the star formation and extinction properties of the galaxy. The star formation history (SFH) of the two nuclei is similarly characterized by two different main episodes: a recent burst, responsible of the observed emission lines, and an older one, occurred roughly 1 Gyr ago. We tentatively associate this bimodal SFH with a double encounter in the dynamical history of the merger. We have complemented our study with a detailed analysis of the broad band spectral energy distribution of the Superantennae, built from published photometry, providing the separate optical-to-mm SEDs of the two nuclei. Our analysis shows that: a) the southern nucleus is responsible for about 80% of the total infrared luminosity of the system, b) the L-band luminosity in the southern nucleus is dominated by the emission from an obscured AGN, providing about 40 to 50% of the bolometric flux between 8 and 1000 microns; c) the northern nucleus does not show evidence for AGN emission and appears to be in a post-starburst phase. As for the relative strengths of the AGN and starburst components, we find that, while they are comparable at FIR and sub-mm wavelengths, in the radio the Sy2 emission dominates by an order of magnitude the starburst.

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arXiv2002

Far infrared and Radio emission in dusty starburst galaxies

We revisit the nature of the FIR/Radio correlation by means of the most recent models for star forming galaxies. We model the IR emission with our population synthesis code, GRASIL (Silva et al. 1998). As for the radio emission, we revisit the simple model of Condon & Yin (1990). We find that a tightFIR/Radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electrons cooling time is shorter than the fading time of the supernova rate. Observations indicate that both these conditions are met in star forming galaxies. However since the radio non thermal emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. This delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation indicators. We suggest to complement the analysis of the deviations from the FIR/Radio correlation with the radio slope to obtain characteristic parameters of the burst. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q- radio slope diagram to asses the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. We discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.

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