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Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron and on carbon, nitrogen, and oxygen. LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.
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Many results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of HI selected galaxies in the light of H-alpha and the far-ultraviolet (FUV) challenge this notion. The flux ratio H-alpha/FUV from these two star formation tracers shows strong correlations with the surface-brightness in H-alpha and the R band: Low Surface Brightness (LSB) galaxies have lower ratios compared to High Surface Brightness galaxies and to expectations from equilibrium star formation models using commonly favored IMF parameters. Weaker but significant correlations of H-alpha/FUV with luminosity, rotational velocity and dynamical mass are found as well as a systematic trend with morphology. The correlated variations of H-alpha/FUV with other global parameters are thus part of the larger family of galaxy scaling relations. The H-alpha/FUV correlations can not be due to dust correction errors, while systematic variations in the star formation history can not explain the trends with both H-alpha and R surface brightness. LSB galaxies are unlikely to have a higher escape fraction of ionizing photons considering their high gas fraction, and color-magnitude diagrams. The most plausible explanation for the correlations are systematic variations of the upper mass limit and/or slope of the IMF at the upper end. We outline a scenario of pressure driving the correlations by setting the efficiency of the formation of the dense star clusters where the highest mass stars form. Our results imply that the star formation rate measured in a galaxy is highly sensitive to the tracer used in the measurement. A non-universal IMF also calls into question the interpretation of metal abundance patterns in dwarf galaxies and star formation histories derived from color magnitude diagrams. Abridged.
The dominant systematic uncertainty in the age determination of galactic globular clusters is the depth of the convection envelope of the stars. This parameter is partially degenerate with metallicity which is in turn degenerate with age. However, if the metal content, distance and extinction are known, the position and morphology of the red giant branch in a color-magnitude diagram are mostly sensitive to the value of the depth of the convective envelope. Therefore, using external, precise metallicity determinations this degeneracy and thus the systematic error in age, can be reduced. Alternatively, the morphology of the red giant branch of globular clusters color magnitude diagram can also be used to achieve the same. We demonstrate that globular cluster red giant branches are well fitted by values of the depth of the convection envelope consistent with those obtained for the Sun and this finding is robust to the adopted treatment of the stellar physics. With these findings, the uncertainty in the depth of the convection envelope is no longer the dominant contribution to the systematic error in the age determination of the oldest globular clusters, reducing it from $0.5$ to $0.23$ or $0.33$ Gyr, depending on the methodology adopted: i.e., whether resorting to external data (spectroscopic metallicity determinations) or relying solely on the morphology of the clusterss color-magnitude diagrams. This results in an age of the Universe $t_{rm U}=13.5^{+0.16}_{-0.14} {rm (stat.)} pm 0.23(0.33) ({rm sys.})$ at 68% confidence level, accounting for the formation time of globular clusters and its uncertainty. An uncertainty of 0.27(0.36) Gyr if added in quadrature. This agrees well with $13.8 pm 0.02$ Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the $Lambda$CDM model.
We analyze a complete spectroscopic sample of galaxies ($sim$600,000 ) drawn from Sloan Digital Sky Survey (SDSS, DR7) to look for evidence of galactic winds in the local Universe. We focus on the shape of the [OIII]$lambda$5007 emission line as a tracer of ionizing gas outflows. We stack our spectra in a fine grid of star formation rate (SFR) and stellar mass to analyze the dependence of winds on the position of galaxies in the SFR versus mass diagram. We do not find any significant evidence of broad and shifted [OIII]$lambda$5007 emission line which we interpret as no evidence of outflowing ionized gas in the global population. We have also classified these galaxies as star-forming or AGN dominated according to their position in the standard BPT diagram. We show how the average [OIII]$lambda$5007 profile changes as function of nature of the dominant ionizing source. We find that in the star-forming dominated source the oxygen line is symmetric and governed by the gravitational potential well. The AGN or composite AGN$setminus$star-formation activity objects, in contrast, display a prominent and asymmetric profile that can be well described by a broad gaussian component that is blue-shifted from a narrow symmetric core. In particular, we find that the blue wings of the average [OIII]$lambda$5007 profiles are increasingly prominent in the LINERs and Seyfert galaxies. We conclude that, in the low-redshift Universe, pure star-formation activity does not seem capable of driving ionized-gas outflows, while, the presence of optically selected AGN seems to play a primary role to drive such winds. We discuss the implications of these results for the role of the quenching mechanism in the present day Universe.
We present observations of a region of the Galactic plane taken during the Early Science Program of the Australian Square Kilometre Array Pathfinder (ASKAP). In this context, we observed the SCORPIO field at 912 MHz with an uncompleted array consisting of 15 commissioned antennas. The resulting map covers a square region of ~40 deg^2, centred on (l, b)=(343.5{deg}, 0.75{deg}), with a synthesized beam of 24x21 and a background rms noise of 150-200 {mu}Jy/beam, increasing to 500-600 {mu}Jy/beam close to the Galactic plane. A total of 3963 radio sources were detected and characterized in the field using the CAESAR source finder. We obtained differential source counts in agreement with previously published data after correction for source extraction and characterization uncertainties, estimated from simulated data. The ASKAP positional and flux density scale accuracy were also investigated through comparison with previous surveys (MGPS, NVSS) and additional observations of the SCORPIO field, carried out with ATCA at 2.1 GHz and 10 spatial resolution. These allowed us to obtain a measurement of the spectral index for a subset of the catalogued sources and an estimated fraction of (at least) 8% of resolved sources in the reported catalogue. We cross-matched our catalogued sources with different astronomical databases to search for possible counterparts, finding ~150 associations to known Galactic objects. Finally, we explored a multiparametric approach for classifying previously unreported Galactic sources based on their radio-infrared colors.
The basic processes of the formation of the first stars in the primordial Universe are outlined and the implications for cosmological structure formation discussed. By employing theoretical and numerical models of cosmic structure evolution embedded within N-body hydrodynamical chemistry simulations, predictions for the production of the first heavy elements in the Universe are given. These results are then compared against measured data of UV luminosities and metal abundances in different kinds of observations in order to draw conclusions on the chemical and thermal state of the cosmic medium at different cosmological epochs.
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