Oxygen abundance in local disk and bulge: chemical evolution with a strictly universal IMF

The empirical differential oxygen abundance distribution (EDOD) is deduced from subsamples related to two different samples involving solar neighbourhood (SN) thick disk, thin disk, halo, and bulge stars. The EDOD of the SN thick + thin disk is determined by weighting the mass, for assumed SN thick to thin disk mass ratio within the range, 0.1-0.9. Inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted, to an acceptable extent, by their TDOD counterparts provided (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case. Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to a selected reference case) can also reproduce the data. The existence of a strictly universal IMF makes similar chemical enrichment within active (i.e. undergoing star formation) regions placed in different environments, but increasing probability of a region being active passing from SN halo to SN thick + thin disk, SN thin disk, SN thick disk, and bulge. On the basis of the results, it is realized that the chemical evolution of the SN thick + thin disk as a whole cannot be excluded.Comment: 26 pages, 10 tables, and 5 figures; tables out of page are splitted in two parts in Appendix B; sects.4 and 5 rewritten for better understanding of the results; further references added. Accepted for publication in Astrophysics & Space Scienc

Allan Sandage and the Cosmic Expansion

This is an account of Allan Sandage's work on (1) The character of the expansion field. For many years he has been the strongest defender of an expanding Universe. He later explained the CMB dipole by a local velocity of 220 +/- 50 km/s toward the Virgo cluster and by a bulk motion of the Local supercluster (extending out to ~3500 km/s) of 450-500 km/s toward an apex at l=275, b=12. Allowing for these streaming velocities he found linear expansion to hold down to local scales (~300 km/s). (2) The calibration of the Hubble constant. Probing different methods he finally adopted - from Cepheid-calibrated SNe Ia and from independent RR Lyr-calibrated TRGBs - H_0 = 62.3 +/- 1.3 +/- 5.0 km/s/Mpc.Comment: 12 pages, 11 figures, 1 table, Submitted to Astrophysics and Space Science, Special Issue on the Fundamental Cosmic Distance Scale in the Gaia Er