Introduction to the Theory of Stellar Evolution

In this chapter we briefly review the basics of the theory of stellar evolution.Comment: Chapter 1, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G. Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe

Binarity as the solution to the stellar evolution enigma posed by NGC 6791

Binary evolution is investigated as the source for the extreme horizontal branch (EHB) stars in the old and metal rich open cluster NGC~6791. Employing an updated version of our binary stellar evolution code we demonstrate that EHB stars naturally emerge from the common envelope phase. In sum, the binary model reproduces the observed ($T_{\rm{eff}}$, $\log{g}$) and temporal properties of the EHB over-density tied to NGC 6971, without needing an ad-hoc and anomalous mass-loss prescription.Comment: 9 pages, 5 eps figures, accepted for publication in the Astrophysical Journal Letter

The spiral potential of the Milky Way

Context. The location of young sources in the Galaxy suggests a four-armed spiral structure, whereas tangential points of spiral arms observed in the integrated light at infrared and radio wavelengths indicate that only two arms are massive. Aims: Variable extinction in the Galactic plane and high light-to-mass ratios of young sources make it difficult to judge the total mass associated with the arms outlined by such tracers. The current objective is to estimate the mass associated with the Sagittarius arm by means of the kinematics of the stars across it. Methods: Spectra of 1726 candidate B- and A-type stars within 3\ub0 of the Galactic center (GC) were obtained with the FLAMES instrument at the VLT with a resolution of 486000 in the spectral range of 396-457 nm. Radial velocities were derived by least-squares fits of the spectra to synthetic ones. The final sample was limited to 1507 stars with either Gaia DR2 parallaxes or main-sequence B-type stars having reliable spectroscopic distances. Results: The solar peculiar motion in the direction of the GC relative to the local standard of rest (LSR) was estimated to U 99 = 10.7 \ub1 1.3kms-1. The variation in the median radial velocity relative to the LSR as a function of distance from the sun shows a gradual increase from slightly negative values near the sun to almost 5 km s-1 at a distance of around 4 kpc. A sinusoidal function with an amplitude of 3.4 \ub1 1.3kms-1 and a maximum at 4.0 \ub1 0.6 kpc inside the sun is the best fit to the data. A positive median radial velocity relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius arm, can be excluded at a 99% level of confidence. A marginal peak detected at this distance may be associated with stellar streams in the star-forming regions, but it is too narrow to be associated with a major arm feature. Conclusions: A comparison with test-particle simulations in a fixed galactic potential with an imposed spiral pattern shows the best agreement with a two-armed spiral potential having the Scutum-Crux arm as the next major inner arm. A relative radial forcing dFr 48 1.5% and a pattern speed in the range of 20-30 km s-1 kpc-1 yield the best fit. The lack of a positive velocity perturbation in the region around the Sagittarius arm excludes it from being a major arm. Thus, the main spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an inter-arm feature with only a small mass perturbation associated with it

A Parallel Tree-SPH code for Galaxy Formation

We describe a new implementation of a parallel Tree-SPH code with the aim to simulate Galaxy Formation and Evolution. The code has been parallelized using SHMEM, a Cray proprietary library to handle communications between the 256 processors of the Silicon Graphics T3E massively parallel supercomputer hosted by the Cineca Supercomputing Center (Bologna, Italy). The code combines the Smoothed Particle Hydrodynamics (SPH) method to solve hydro-dynamical equations with the popular Barnes and Hut (1986) tree-code to perform gravity calculation with a NlogN scaling, and it is based on the scalar Tree-SPH code developed by Carraro et al(1998)[MNRAS 297, 1021]. Parallelization is achieved distributing particles along processors according to a work-load criterion. Benchmarks, in terms of load-balance and scalability, of the code are analyzed and critically discussed against the adiabatic collapse of an isothermal gas sphere test using 20,000 particles on 8 processors. The code results balanced at more that 95% level. Increasing the number of processors, the load-balance slightly worsens. The deviation from perfect scalability at increasing number of processors is almost negligible up to 32 processors. Finally we present a simulation of the formation of an X-ray galaxy cluster in a flat cold dark matter cosmology, using 200,000 particles and 32 processors, and compare our results with Evrard (1988) P3M-SPH simulations. Additionaly we have incorporated radiative cooling, star formation, feed-back from SNae of type II and Ia, stellar winds and UV flux from massive stars, and an algorithm to follow the chemical enrichment of the inter-stellar medium. Simulations with some of these ingredients are also presented.Comment: 19 pages, 14 figures, accepted for publication in MNRA

Star formation and chemical evolution in SPH simulations: a statistical approach

In Smoothed Particles Hydrodynamics (SPH) codes with a large number of particles, star formation as well as gas and metal restitution from dying stars can be treated statistically. This approach allows to include detailed chemical evolution and gas re-ejection with minor computational effort. Here we report on a new statistical algorithm for star formation and chemical evolution, especially conceived for SPH simulations with large numbers of particles, and for parallel SPH codes. For the sake of illustration, we present also two astrophysical simulations obtained with this algorithm, implemented into the Tree-SPH code by Lia & Carraro (2000). In the first one, we follow the formation of an individual disc-like galaxy, predict the final structure and metallicity evolution, and test resolution effects. In the second one we simulate the formation and evolution of a cluster of galaxies, to demonstrate the capabilities of the algorithm in investigating the chemo-dynamical evolution of galaxies and of the intergalactic medium in a cosmological context.Comment: 17 pages, 20 figures, accepted for publication on MNRA

On the existence of young embedded clusters at high Galactic latitude

Careful analyses of photometric and star count data available for the nine putative young clusters identified by Camargo et al. (2015, 2016) at high Galactic latitudes reveal that none of the groups contain early-type stars, and most are not significant density enhancements above field level. 2MASS colours for stars in the groups match those of unreddened late-type dwarfs and giants, as expected for contamination by (mostly) thin disk objects. A simulation of one such field using only typical high latitude foreground stars yields a colour-magnitude diagram that is very similar to those constructed by Camargo et al. (2015, 2016) as evidence for their young groups as well as the means of deriving their reddenings and distances. Although some of the fields are coincident with clusters of galaxies, one must conclude that there is no evidence that the putative clusters are extremely young stellar groups.Comment: Accepted for publication (MNRAS

Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66

We report on high resolution spectroscopy of four giant stars in the Galactic old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck telescope. We find that $[Fe/H]=-0.32\pm0.19$ and $[Fe/H]=-0.48\pm0.24$ for Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise the fundamental parameters of the clusters, and then discuss them in the context of the Galactic disk radial abundance gradient. We found that both clusters nicely obey the most updated estimate of the slope of the gradient from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical Journa

A photometric study of the young open cluster NGC 1220

We present UBV CCD observations obtained in the field of the northern open cluster NGC 1220, for which little information is available. We provide also BV CCD photometry of a field 5$^{\prime}$ northward of NGC 1220 to take into account field star contamination. We argue that NGC 1220 is a young compact open cluster, for which we estimate a core radius in the range $1.5-2.0$ arcmin. We identify 26 likely candidate members with spectral type earlier than $A5$, down to $V_o$=15.00 mag on the basis of the position in the two-colour Diagram and in the Colour Magnitude Diagrams (CMDs). By analyzing the distribution of these stars in the colour-colour and CMDs, we find that NGC 1220 has a reddening E$(B-V)=0.70\pm0.15$ mag, is placed $1800\pm200$ pc distant from the Sun, and has an age of about 60 Myrs. The cluster turns out to be located about 120 pc above the Galactic plane, relatively high with respect to its age.Comment: 7 pages, 8 eps figures, accepted for publication in A&

Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk

We perform a detailed photometric study of the stellar populations in a Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We present the first U,B,V,I photometry of the old open cluster Auner1 and determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the background of the cluster, at more than 9 kpc, we detect a young population most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect the signature of an older population and identify its Turn Off and Red Giant Branch. This population is found to have a mean age of 7 Gyrs and a mean metallicity of Z = 0.006 . We reconstruct the geometry of the stellar distribution and argue that this older population - often associated to the Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in the Astronomical Journa

The Relative Age of the Thin and Thick Galactic Disks

We determine the relative ages of the open cluster NGC 188 and selected Hipparcos field stars by isochrone fitting, and compare them to the age of the thick disk globular cluster 47 Tuc. The best fit age for NGC 188 was determined to be $6.5 \pm 1.0$ Gyr. The solar metallicity Hipparcos field stars yielded a slightly older thin disk age, $7.5 \pm 0.7$ Gyr. Two slightly metal-poor (\feh = -0.22) field stars whose kinematic and orbital parameters indicate that they are members of the thin disk were found to have an age of $9.7\pm 0.6$ Gyr. The age for 47 Tuc was determined to be $12.5 \pm 1.5$ Gyr. All errors are internal errors due to the uncertainty in the values of metallicity and reddening. Thus, the oldest stars dated in the thin disk are found to be $2.8\pm 1.6$ Gyr younger than 47 Tuc. Furthermore, as discussed by \citet{Chb99} 47 Tuc has a similar age to three globular clusters located in the inner part of the Galactic halo, implying that star formation in the thin disk started within $2.8\pm 1.6$ Gyr of star formation in the halo.Comment: 26 pages, 11 figures, to appear in Ap