arXiv subjects

S. Degl'Innocenti

Publications and source records attributed to S. Degl'Innocenti.

At least 73 records · Page 4Linked to original sources

arXiv2012

The Pisa Stellar Evolution Data Base for low-mass stars

The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition. With this aim we built the new "Pisa Stellar Evolution Database" of stellar models and isochrones by adopting a well-tested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. A total of 32646 models have been computed in the range of initial masses 0.30 - 1.10 Msun for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and alpha-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8 - 15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader's convenience.

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arXiv2011

The Pisa pre-main sequence tracks and isochrones. A database covering a wide range of Z, Y, mass, and age values

In recent years new observations of pre-main sequence stars (pre-MS) with Z <= Zsun have been made available. To take full advantage of the continuously growing amount of data of pre-MS stars in different environments, we need to develop updated pre-MS models for a wide range of metallicity to assign reliable ages and masses to the observed stars. We present updated evolutionary pre-MS models and isochrones for a fine grid of mass, age, metallicity, and helium values. We use a standard and well-tested stellar evolutionary code (i.e. FRANEC), that adopts outer boundary conditions from detailed and realistic atmosphere models. In this code, we incorporate additional improvements to the physical inputs related to the equation of state and the low temperature radiative opacities essential to computing low-mass stellar models. We make available via internet a large database of pre-MS tracks and isochrones for a wide range of chemical compositions (Z = 0.0002 - 0.03), masses (M = 0.2 - 7.0 Msun), and ages (1 - 100 Myr) for a solar-calibrated mixing length parameter α(i.e. 1.68). For each chemical composition, additional models were computed with two different mixing length values, namely α= 1.2 and 1.9. Moreover, for Z >= 0.008, we also provided models with two different initial deuterium abundances. The characteristics of the models have been discussed in detail and compared with other work in the literature. The main uncertainties affecting theoretical predictions have been critically discussed. Comparisons with selected data indicate that there is close agreement between theory and observation.

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arXiv2010

On the radial extent of the dwarf irregular galaxy IC10

We present new deep and accurate space (Advanced Camera for Surveys -- Wide Field Planetary Camera 2 at the Hubble Space Telescope) and ground-based (Suprime-Cam at Subaru Telescope, Mega-Cam at Canada-France-Hawaii Telescope) photometric and astrometric data for the Local Group dwarf irregular IC10. We confirm the significant decrease of the young stellar population when moving from the center toward the outermost regions. We find that the tidal radius of IC10 is significantly larger than previous estimates of $r_t \lesssim$ 10\min. By using the $I$,\vmi\ Color Magnitude Diagram based on the Suprime-Cam data we detect sizable samples of red giant (RG) stars up to radial distances of 18-23$'$ from the galactic center. The ratio between observed star counts (Mega-Cam data) across the tip of the RG branch and star counts predicted by Galactic models indicate a star count excess at least at a 3$σ$ level up to 34-42\min\ from the center. This finding supports the hypothesis that the huge H{\size{I}} cloud covering more than one degree across the galaxy is associated with IC10 \citep{huchtmeier79,cohen79}. We also provide new estimates of the total luminosity ($L_V\sim9\times$$10^7$ $L_\odot$, $M_V$$\sim$-15.1 mag) that agrees with similar estimates available in the literature. If we restrict to the regions where rotational velocity measurements are available (r$\approx13'$), we find a mass-to-light ratio ($\sim$10 $M_\odot$ $L_\odot$) that is at least one order of magnitude larger than previous estimates. The new estimate should be cautiously treated, since it is based on a minimal fraction of the body of the galaxy.

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arXiv2010

Pre-Main Sequence stars in the star forming complex Sh 2-284

Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age determination we concluded that triggered star formation is in act in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disk/envelopes, in contrast with what is found in other recent studies of low-metallicity star forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified 8 stars which are good candidates to pulsators of the delta Scuti type.

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arXiv2010

On the absolute age of the Globular Cluster M92

We present precise and deep optical photometry of the globular M92. Data were collected in three different photometric systems: Sloan Digital Sky Survey (g',r',i',z'; MegaCam@CFHT), Johnson-Kron-Cousins (B, V, I; various ground-based telescopes) and Advanced Camera for Surveys (ACS) Vegamag (F475W, F555W, F814W; Hubble Space Telescope). Special attention was given to the photometric calibration, and the precision of the ground-based data is generally better than 0.01 mag. We computed a new set of α-enhanced evolutionary models accounting for the gravitational settling of heavy elements at fixed chemical composition ([α/Fe]=+0.3, [Fe/H]=-2.32 dex, Y=0.248). The isochrones -- assuming the same true distance modulus (μ=14.74 mag), the same reddening (E(B-V)=0.025+-0.010 mag), and the same reddening law -- account for the stellar distribution along the main sequence and the red giant branch in different Color-Magnitude Diagrams (i',g'-i' ; i',g'-r' ; i',g'-z' ; I,B-I ; F814W,F475W-F814W). The same outcome applies to the comparison between the predicted Zero-Age-Horizontal-Branch (ZAHB) and the HB stars. We also found a cluster age of 11 +/- 1.5 Gyr, in good agreement with previous estimates. The error budget accounts for uncertainties in the input physics and the photometry. To test the possible occurrence of CNO-enhanced stars, we also computed two sets of α- and CNO-enhanced (by a factor of three) models both at fixed total metallicity ([M/H]=-2.10 dex) and at fixed iron abundance. We found that the isochrones based on the former set give the same cluster age (11 +/- 1.5 Gyr) as the canonical α-enhanced isochrones. The isochrones based on the latter set also give a similar cluster age (10 +/- 1.5 Gyr). These indings support previous results concerning the weak sensitivity of cluster isochrones to CNO-enhanced chemical mixtures.

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arXiv2010

On the stellar content of the Carina dwarf spheroidal galaxy

We present U,B,V,I photometry of the Carina dwarf spheroidal galaxy, based on more than 4,000 CCD images. Special attention was given to the photometric calibration, and the precision for the B,V,I bands is better than 0.01 mag. We compared in the V,B-V and V,B-I color-magnitude diagrams (CMDs) Carina with three Globular Clusters (GCs, M53, M55, M79). We find that only the more metal-poor GCs (M55, [Fe/H]=-1.85; M53, [Fe/H]=-2.02 dex) provide a good match with the Carina giant branch. We performed a similar comparison in the V,V-I CMD with three SMC intermediate-age clusters (IACs, Kron3, NGC339, Lindsay38). We find that the color extent of the SGB of the two more metal-rich IACs (Kron3, [Fe/H]=-1.08; NGC339, [Fe/H]=-1.36 dex) is smaller than the range among Carina's intermediate-age stars. However, the ridge line of the more metal-poor IAC (Lindsay38, [Fe/H]=-1.59 dex) agrees quite well with the Carina intermediate-age stars. These findings indicate that Carina's old stellar population is metal-poor and seems to have a limited spread in metallicity (Delta [Fe/H]=0.2--0.3 dex). Carina intermediate-age stars can hardly be more metal-rich than Lindsay38 and its spread in metallicity appears modest. We also find that the synthetic CMD constructed assuming a metallicity spread of 0.5 dex for intermediate-age stars predicts evolutionary features not supported by observations. The above results are at odds with recent spectroscopic investigations suggesting that Carina stars cover a broad range in metallicity (Delta [Fe/H]~1--2 dex). We present a new method to estimate the metallicity of complex stellar systems using the difference in color between the red clump and the middle of the RR Lyrae instability strip. The observed colors of Carina's evolved stars indicate a metallicity of [Fe/H]=-1.70+-0.19 dex, which agrees quite well with spectroscopic measurements.

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arXiv2010

On the Delta V_HB_bump parameter in Globular Clusters

We present new empirical estimates of the Delta V_HB_bump parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16<=[M/H]<=-0.58 dex). Adopting the homogeneous metallicity scale provided either by Kraft & Ivans (2004) or by Carretta et al. (2009), we found that the observed Delta V_HB_bump parameters are larger than predicted. In the metal-poor regime ([M/H]<=-1.7, -1.6 dex) 40% of GCs show discrepancies of 2sigma (~0.40 mag) or more. Evolutionary models that account either for alpha- and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations. The outcome is the same if we use the new Solar heavy-element mixture. The comparison between alpha- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed Delta V_HB_bump parameters, in the metal-rich regime ([M/H]=>0), might be systematically smaller than predicted.

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arXiv2010

Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark

We present a novel approach to derive the age of very young star clusters, by using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram (CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS luminosity function (LF) of the cluster, the TOn is identified as a peak followed by a dip. We propose that by combining the CMD analysis with the monitoring of the spatial distribution of MS stars it is possible to reliably identify the TOn in extragalactic star forming regions. Compared to alternative methods, this technique is complementary to the turn-off dating and avoids the systematic biases affecting the PMS phase. We describe the method and its uncertainties, and apply it to the star forming region NGC346, which has been extensively imaged with the Hubble Space Telescope (HST). This study extends the LF approach in crowded extragalactic regions and opens the way for future studies with HST/WFC3, JWST and from the ground with adaptive optics.

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arXiv2009

On a new Near-Infrared method to estimate the absolute ages of star clusters: NGC3201 as a first test case

We present a new method to estimate the absolute ages of stellar systems. This method is based on the difference in magnitude between the main sequence turn-off (MSTO) and a well defined knee located along the lower main sequence (MSK). This feature is caused by the collisionally induced absorption of molecular hydrogen and it can be easily identified in near-infrared (NIR) and in optical-NIR color-magnitude diagrams of stellar systems. We took advantage of deep and accurate NIR images collected with the Multi-Conjugate Adaptive Optics Demonstrator temporarily available on the Very Large Telescope and of optical images collected with the Advanced Camera for Surveys Wide Field Camera on the Hubble Space Telescope and with ground-based telescopes to estimate the absolute age of the globular NGC3201 using both the MSTO and the Delta(MSTO-MSK). We have adopted a new set of cluster isochrones and we found that the absolute ages based on the two methods agree to within one sigma. However, the errors of the ages based on the Delta(MSTO-MSK) method are potentially more than a factor of two smaller, since they are not affected by uncertainties in cluster distance or reddening.Current isochrones appear to predict slightly bluer (~0.05mag) NIR and optical-NIR colors than observed for magnitudes fainter than the MSK.

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arXiv2009

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

The expected distribution of Cepheids within the instability strip is affected by several model inputs, reflecting upon the predicted Period-Luminosity relation. On the basis of new and updated sets of evolutionary and pulsational models, we quantitatively evaluated the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud and on several physical assumptions adopted in the evolutionary models. We analysed how the different factors influence the evolutionary and pulsational observables and the resulting PL relation. As a result, we found that present uncertainties on the most relevant H and He burning reaction rates do not influence in a relevant way the loop extension in temperature. On the contrary, current uncertainties on the LMC chemical composition significantly affect the loop extension and also reflect in the morphology of the instability strip; however their influence on the predicted pulsational parameters is negligible. We also discussed how overshooting and mass loss influence the ML relation and the pulsational parameters. In summary, the present uncertainties on the physical inputs adopted in the evolutionary codes and in the LMC chemical composition are negligible for the prediction of the main pulsational properties; the inclusion of overshooting in the previous H burning phase and/or of mass loss is expected to significantly change the resulting theoretical pulsational scenario for Cepheids, as well as the calibration of their distance scale. These systematic effects are expected to influence the theoretical Cepheid calibration of the secondary distance indicators and in turn the resulting evaluation of the Hubble constant.

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arXiv2009

Stromgren photometry of Galactic Globular Clusters. II Metallicity distribution of red giants in Omega Centauri

We present new intermediate-band Stroemgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster Omega Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J,H,K_s images). The final optical-NIR catalog covers a region of more than 20*20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Stroemgren metallicity indices we estimate the photometric metallicity for ~4,000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]_phot=-1.73+/-0.08,-1.29+/-0.03,-1.05+/-0.02,-0.80+/-0.04,-0.42+/-0.12 and -0.07+/-0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H]_phot<=-2 dex) that agree quite well with spectroscopic measurements. We identify four distinct sub-populations,namely metal-poor (MP,[Fe/H]<=-1.49), metal-intermediate (MI,-1.49<[Fe/H]<=-0.93), metal-rich (MR,-0.95<[Fe/H]<=-0.15) and solar metallicity (SM,[Fe/H]~0). The last group includes only a small fraction of stars (~8+/-5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19+/-1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Stroemgren metallicity indices display a robust correlation with alpha-elements ([Ca+Si/H]) when moving from the metal-intermediate to the metal-rich regime ([Fe/H]>-1.7 dex).

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arXiv2009

On the stellar content of the starburst galaxy IC10

We investigate the stellar content of the starburst dwarf galaxy IC10 using accurate and deep optical data collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The comparison between theory and observations indicates a clear change in age distribution when moving from the center toward the external regions. Moreover, empirical calibrators and evolutionary predictions suggest the presence of a spread in heavy element abundance of the order of one-half dex. The comparison between old and intermediate-age core He-burning models with a well defined overdensity in the color-magnitude diagram indicates the presence of both intermediate-age, red clump stars and of old, red horizontal branch stars.

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arXiv2009

On the radial distribution of horizontal branch stars in ngc2808

We present accurate new ultraviolet and optical BVI photometry for the Galactic globular cluster ngc2808, based on both ground-based and archival HST imagery. From this we have selected a sample of ~2,000 HB stars; given the extensive wavelength range considered and the combination of both high-angular-resolution and wide-field photometric coverage, our sample should be minimally biased. We divide the HB stars into three radial bins and find that the relative fractions of cool, hot and extreme HB stars do not change radically when moving from the center to the outskirts of the cluster: the difference is typically smaller than ~2sigma. These results argue against the presence of strong radial differentiation among any stellar subpopulations having distinctly different helium abundances. The ratio between HB and RG stars brighter than the ZAHB steadly increases when moving from the innermost to the outermost cluster regions. The difference is larger than ~4sigma and indicates a deficiency of bright RGs in the outskirts of the cluster.

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arXiv2009

Star formation history in the SMC: the case of NGC602

Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 \times 10^{-3} M_\odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.

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arXiv2008

On the distance and reddening of the starburst galaxy IC10

We present deep and accurate optical photometry of the Local Group starburst galaxy IC10. The photometry is based on two sets of images collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We provide new estimates of the Red Giant Branch tip (TRGB) magnitude, m_{F814W}^{TRGB}=21.90+-0.03, and of the reddening, E(B-V)=0.78+-0.06, using field stars in the Small Magellanic Cloud (SMC) as a reference. Adopting the SMC and two globulars, Omega Centauri and 47 Tucanae, as references we estimate the distance modulus to IC10: independent calibrations give weighted average distances of mu=24.51+-0.08 (TRGB) and mu=24.56+-0.08 (RR Lyrae). We also provide a new theoretical calibration for the TRGB luminosity, and using these predictions we find a very similar distance to IC10 (mu~24.60+-0.15). These results suggest that IC10 is a likely member of the M31 subgroup.

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arXiv2008

MAD@VLT: Deep into the Madding Crowd of Omega Centauri

We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) Omega Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in Omega Cen is limited.

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arXiv2007

On the radial distribution of white dwarfs in the Galactic globular cluster Omega Cen

We present deep and accurate photometry (F435W, F625W, F658N) of the Galactic Globular Cluster Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We identified ~ 6,500 white dwarf (WD)candidates and compared their radial distribution with that of Main Sequence (MS) stars. We found a mild evidence that young WDs (0.1 < t < 0.6 Gyr) are less centrally concentrated when compared to MS stars in the magnitude range 25 < F435W < 26.5.

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arXiv2007

On the white dwarf cooling sequence of the globular cluster Omega Centauri

We present deep and precise photometry (F435, F625W, F658N) of Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We have identified ~ 6,500 white dwarf (WD) candidates, and the ratio of WD and Main Sequence (MS) star counts is found to be at least a factor of two larger than the ratio of CO-core WD cooling and MS lifetimes. This discrepancy is not explained by the possible occurrence of a He-enhanced stellar population, since the MS lifetime changes by only 15% when changing from a canonical (Y=0.25) to a He-enhanced composition (Y=0.42). The presence of some He-core WDs seems able to explain the observed star counts. The fraction of He WDs required ranges from 10% to 80% depending on their mean mass and it is at least five times larger than for field WDs. The comparison in the Color Magnitude Diagram between theory and observations also supports the presence of He WDs. Empirical evidence indicates that He WDs have been detected in stellar systems hosting a large sample of extreme horizontal branch stars, thus suggesting that a fraction of red giants might avoid the He-core flash.

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