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We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertained for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating massive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to AGB stars as producers of the material for next generation stars encounters severe difficulties, specifically concerning the mass budget problem and the detailed chemical composition of second generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revision of the cross section of a critical reaction rate destroying sodium, or alternatively by a more extensive exploration of the vast parameter space controlling the evolutionary behavior of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

With the NEOWISE portion of the Wide-field Infrared Survey Explorer (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 µm, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniform sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981±19 NEAs larger than 1 km and 20,500±3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32±0.14 below 1.5 km. This power law slope produces ∼ 13, 200±1,900 NEAs with D >140 m. Although previous studies predict another break in the cumulative size distribution below D ∼50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100-1000 m is lower than previous estimates. The numbers of near-Earth comets and potentially hazardous NEOs will be the subject of future work.Road,

As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [absolute] magnitudes of (M 475 , M 814 ) (29.0 [+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the Big Bang; a possible burst at the oldest ages cannot be claimed with high confidence.The peak SFR occurred ≈1.5-4 Gyr ago, at a level 5-10 times the current value. Our modelling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to ≈8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.

The Updated BaSTI Stellar Evolution Models and Isochrones Hidalgo, S. L.; Pietrinferni, A.; Cassisi, S.; Salaris, M.; Mucciarelli, A.; Savino, A.; Aparicio, A.; Aguirre, V. Silva; Verma, K. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. , with helium to metal enrichment ratio dY/dZ=1.31. The isochrones cover an age range between 20 Myr and 14.5 Gyr, consistently take into account the pre-main-sequence phase, and have been translated to a large number of popular photometric systems. Asteroseismic properties of the theoretical models have also been calculated. We compare our isochrones with results from independent databases and with several sets of observations to test the accuracy of the calculations. All stellar evolution tracks, asteroseismic properties, and isochrones are made available through a dedicated web site.

We use archival Hubble Space Telescope observations of resolved stellar populations to derive the star formation histories (SFHs) of eighteen nearby starburst dwarf galaxies. In this first paper we present the observations, color-magnitude diagrams, and the SFHs of the eighteen starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D< 8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long > 100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the shortlived nature of starbursts based on fuel limitations. Additionally, we find the strength of the Hα emission usually correlates with the CMD based SFR during the last 4−10 Myr. However, in four cases, the Hα emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the Hα emission limits identifying galaxies as

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy LGS-3 (Pisces) based on deep photometry obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope. Our observations reach the oldest main-sequence turnoffs at high signal to noise, allowing a time resolution at the oldest ages of 1.1 Gyr. Our analysis, based on three different SFH codes, shows that the SFH of LGS-3 is dominated by a main episode 11.7 Gyr ago with a duration of 1.4 Gyr. Subsequently, LGS-3 continued forming stars until the present, although at a much lower rate. Roughly 90% of the stars in LGS-3 were formed in the initial episode of star formation. Extensive tests of self-consistency, uniqueness, and stability of the solution have been performed together with the IAC-star/IAC-pop/MinnIAC codes, and these results are found to be independent of the photometric reduction package, the stellar evolution library, and the SFH recovery method. There is little evidence of chemical enrichment during the initial episode of star formation, after which the metallicity increased more steeply reaching a present-day value of Z 0.0025. This suggests a scenario in which LGS-3 first formed stars mainly from infalling fresh gas, and after about 9 Gyr ago, from a larger fraction of recycled gas. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of LGS-3 with expectations from cosmological models. Most or all the star formation was produced in LGS-3 after the reionization epoch, assumed to be completed at z 6 or 12.7 Gyr ago. The total mass of the galaxy is estimated to be between 2 and 4 × 108 M corresponding to circular velocities between 28 km s-1 and 36 km s-1. These values are close to but somewhat above the limit of 30 km s-1 below which the UV background is expected to prevent any star formation after reionization. Feedback from supernovae (SNe) associated with the initial episode of star formation (mechanical luminosity from SNe Lw = 5.3 × 1038 erg s-1) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of LGS-3; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.

We use deep HST/ACS observations to calculate the star formation history (SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches below the oldest main sequence turn-offs, which allows us to estimate the age and duration of the main episode of star formation in Cetus. This is well approximated by a single episode that peaked roughly 12±0.5 Gyr ago and lasted no longer than about 1.9±0.5 Gyr (FWHM). Our solution also suggests that essentially no stars formed in Cetus during the past 8 Gyrs. This makes Cetus' SFH comparable to that of the oldest Milky Way dSphs. Given the current isolation of Cetus in the outer fringes of the Local Group, this implies that Cetus is a clear outlier in the morphology-Galactocentric distance relation that holds for the majority of Milky Way dwarf satellites. Our results also show that Cetus continued forming stars through z ≃ 1, long after the Universe was reionized, and that there is no clear signature of the epoch of reionization in Cetus' SFH. We discuss briefly the implications of these results for dwarf galaxy evolution models. Finally, we present a comprehensive account of the data reduction and analysis strategy adopted for all galaxies targeted by the LCID (Local Cosmology from Isolated Dwarfs 15 ) project. We employ two different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three different SFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellar evolution libraries (BaSTI and Padova/Girardi), allowing for a detailed assessment of the modeling and observational uncertainties.