We present Color Magnitude Diagrams (CMDs) created by using our database of 43,340 synthetic CMFGEN spectra. For each calculated spectra we measure the absolute flux by Johnson and Gaia photometry filters and build synthetic CMDs.

The temperature independent part of the Humphreys-Davidson (HD) limit sets the boundary for evolutionary channels of massive stars that either end their lives as red supergiants (RSGs) or as the hotter blue supergiants (BSGs) and Wolf-Rayet stars. Recent downward revision of most luminous RSGs the Galaxy below log(L / L⊙) ≈ 5.5, more in line with the Magellanic Clouds, might hint towards a metallicity (Z)-independent HD limit. We present MESA single star models in the 15-40 M⊙ range and study the different Z-dependent processes that could potentially affect the location of the upper luminosity limit of RSGs.

Since massive stars form preferentially as members of close binary systems, we use dense grids of detailed binary evolution models to explore how binary evolution shapes the main-sequence morphology of young star clusters. We propose that binary mergers might be the origin of the blue main sequence stars in young star clusters. Our results imply that stars may either form by accretion, or through a binary merger, and that both paths lead to distinctly different spins, magnetic fields, and stellar mass distributions.

The empirical upper limit to Red Supergiant (RSG) luminosity, known as the Humphreys-Davidson (HD) limit, has been commonly explained as being caused by the stripping of stellar envelopes by metallicity-dependent, line-driven winds. As such, the theoretical expectation is that the HD limit should be higher at lower metallicity, where weaker mass-loss rates mean that higher initial masses are required for an envelope to be stripped. In this work, we test this prediction by measuring the luminosity function of RSGs in M31 and comparing to those in the LMC and SMC. We find that $\[\log ({L_{{\rm{m}}ax}}/{L_ \odot }) = 5.53 \pm 0.03\]$ in M31 (Z ≳ Z⊙), consistent with the limit found for both the LMC (Z ∼ 0.5 Z⊙) and SMC (Z ∼ 0.25 Z⊙), while the RSG luminosity distributions in these 3 galaxies are consistent to within 1σ. We therefore find no evidence for a metallicity dependence on both the HD limit and the RSG luminosity function, and conclude that line-driven winds on the main sequence are not the cause of the HD limit.

Evolution of post-AGB stars is extremely fast. They cross the HR diagram vertically on a timescale of hundreds to some ten thousands of years to reach maximum temperature in their lifetime. This is reflected in an increasing excitation of planetary nebulae on a timescale of years and decades. Since evolutionary timescale of post-AGB stars is very sensitive to their mass, observed changes can be used to determine model dependent central star masses. If an additional parameter is determined (e.g. luminosity or dynamic age), the observed evolution of planetary nebulae can be utilized for observational verification of theoretical models.

By comparing two age indicators of high-mass white dwarfs (WDs) derived from Gaia data, two discoveries have been made recently: one is the existence of a cooling anomaly that produces the Q branch structure on the Hertzsprung–Russell diagram, and the other is the existence of high-mass WDs as double-WD merger products. The former poses a challenge for WD cooling models, and the latter has implications on binary evolution and type-Ia supernovae.

Taking advantage of the Gaia DR1, we combined TGAS parallaxes with the Tycho-2 and APASS photometry to calculate the star formation history (SFH) of the solar neighbourhood within 250 pc using the colour-magnitude diagram fitting technique. Our dynamically-evolved SFH is in excellent agreement with that calculated from the Hipparcos catalogue within 80 pc of the Sun, showing an enhanced star formation rate (SFR) in the past ~4 Gyr. We then correct the SFR for the disc thickening with age to obtain a SFR that is representative of the whole solar cylinder, and show that even with an extreme correction our results are not consistent with an exponentially decreasing SFR as found by recent studies. Finally, we discuss how this technique can be applied out to ~5 kpc thanks to the next Gaia data releases, which will allow us to quantify the SFH of the thin disc, thick disc and halo in situ.

The intermediate mass Herbig Ae/Be stars are young stars approaching the Main Sequence and are key to understanding the differences in formation mechanisms between magnetic low mass stars and non-magnetic high mass stars. A large fraction of known Herbig Ae/Be stars have TGAS parallaxes, which were used to derive luminosities and place 107 of these objects in the HR diagram, increasing the number of objects using directly determined parallaxes by a factor of 5. We also studied the characteristics of the infrared excesses of this set of Herbig Ae/Be stars and we linked our results to an evolutionary analysis.

Barium stars are formed via binary interaction with a former AGB companion. Observations are needed to constrain theoretical models and better understand their evolution and surface composition. We present the HR diagram of Ba and related stars, using the recently released TGAS parallaxes, and the mass distribution of the Ba giants that we derived from it.

We took advantage of the Gaia DR1 to combine TGAS parallaxes with Tycho-2 and APASS photometry to calculate the star formation history (SFH) of the solar neighbourhood within 250 pc using the colour-magnitude diagram fitting technique. We present the determination of the completeness within this volume, and compare the resulting SFH with that calculated from the Hipparcos catalogue within 80 pc of the Sun. We also show how this technique will be applied out to ~5 kpc thanks to the next Gaia data releases, which will allow us to quantify the SFH of the thin disc, thick disc and halo in situ, rather than extrapolating based on the stars from these components that are today in the solar neighbourhood.

Binarity is now a well-established quality affecting a large fraction of stars, and recent studies have shown that the fraction of binaries is a function of the spectral type of the primary star, with most massive stars being member of a close binary system. By cross-matching TGAS with SB9, we went one step further and derived the mass ratio distribution of binary systems as a function of the spectral type of the primary star. This, combined with the binary fraction, provides very strong constraints on star formation and critical input for stellar population models.

The recent discovery of a gravitational wave from the merging of two black holes of about 30 solar masses each challenges our incomplete understanding of massive stars and their evolution. Critical ingredients comprise mass-loss, rotation, magnetic fields, internal mixing, and mass transfer in close binary systems. The imperfect knowledge of these factors implies large uncertainties for models of stellar populations and their feedback. In this contribution we summarize our empirical studies of Wolf-Rayet populations at different metallicities by means of modern non-LTE stellar atmosphere models, and confront these results with the predictions of stellar evolution models. At the metallicity of our Galaxy, stellar winds are probably too strong to leave remnant masses as high as ~30 M⊙, but given the still poor agreement between evolutionary tracks and observation even this conclusion is debatable. At the low metallicity of the Small Magellanic Cloud, all WN stars which are (at least now) single are consistent with evolving quasi-homogeneously. O and B-type stars, in contrast, seem to comply with standard evolutionary models without strong internal mixing. Close binaries which avoided early merging could evolve quasi-homogeneously and lead to close compact remnants of relatively high masses that merge within a Hubble time.

Faint dwarf galaxies such as those found around the Milky Way (MW) display the largest known dynamical mass-to-light ratios (up to several 100s M⊙/L⊙). However, tidal interaction with the MW may impact the dynamical equilibrium in the outer parts of some of these objects, and partly affect the derived dynamical M/L. Assessing this is crucial for the study of the dark matter content of these galaxies. A clear sign of ongoing tidal disturbance would be the presence of tidal tails. These are expected to be low surface brightness features, hence difficult to detect from star counts in systems where contamination is also present, e.g. from foreground MW stars. At present we have searched for these sorts of tidal features in the Sextans dwarf spheroidal galaxy (dSph), by adopting the Matched Filter Method (e.g. Rockosi et al. 2002), a very efficient technique to decontaminate stellar density maps with a high ratio of contamination versus source density (dwarf galaxies outer regions or ultra faint dwarf galaxies). We also calculate structural parameters from the position of stars without requiring spatial binning (Richardson et al. 2011), through a Bayesian MCMC (Foreman-Mackey et al. 2013).

As the opening review to the focus meeting “Stellar Behemoths: Red Supergiants across the Local Universe”, I here provide a brief introduction to red supergiants, setting the stage for subsequent contributions. I highlight some recent activity in the field, and identify areas of progress, areas where progress is needed, and how such progress might be achieved.

Our knowledge of the formation and early evolution of globular clusters (GCs) has been totally shaken with the discovery of the peculiar chemical properties of their long-lived host stars. Therefore, the interpretation of the observed Colour Magnitude Diagrams (CMD) and of the properties of the GC stellar populations requires the use of new stellar models computed with relevant chemical compositions. In this paper we use the grid of evolution models for low-mass stars computed by Chantereau et al. (2015) with the initial compositions of second-generation stars as predicted by the fast rotating massive stars scenario to build synthesis models of GCs. We discuss the implications of the assumed initial chemical distribution on 13 Gyr isochrones. We build population synthesis models to predict the fraction of stars born with various helium abundances in present day globular clusters (assuming an age of 13 Gyr). With the current assumptions, 61 % of stars on the main sequence are predicted to be born with a helium abundance in mass fraction, Yini, smaller than 0.3 and only 11 % have a Yini larger than 0.4. Along the horizontal branch, the fraction of stars with Yini inferior to 0.3 is similar to that obtained along the main sequence band (63 %), while the fraction of very He-enriched stars is significantly decreased (only 3 % with Yini larger than 0.38).

Stability boundaries of radial pulsations in massive stars are compared with positions of variable and non-variable blue-supergiants in the spectroscopic HR (sHR) diagram (Langer & Kudritzki 2014), whose vertical axis is 4 log Teff - log g(= log L/M). Observational data indicate that variables tend to have higher L/M than non-variables in agreement with the theoretical prediction. However, many variable blue-supergiants are found to have values of L/M below the theoretical stability boundary; i.e., surface gravities seem to be too high by around 0.2-0.3 dex.

Globular clusters (GCs) are spheroidal concentrations typically containing of the order of 105 to 106, predominantly old, stars. Historically, they have been considered as the closest counterparts of the idealized concept of “simple stellar populations.” However, some recent observations suggest than, at least in some GCs, some stars are present that have been formed with material processed by a previous generation of stars. In this sense, it has also been suggested that such material might be enriched in helium, and that blue horizontal branch stars in some GCs should accordingly be the natural progeny of such helium-enhanced stars. In this contribution we show that, at least in the case of M3 (NGC 5272), the suggested level of helium enrichment is not supported by the available, high-precision observations.

We discuss the age of the stellar disks in the solar neighborhood. After reviewing the various methods for age dating, we discuss current estimates of the ages of both the thin- and the thick disks. We present preliminary results for kinematically-selected stars that belong to the thin- as well as the thick disk. All of these dwarf and sub-giant stars have been studied spectroscopically and we have derived both elemental abundances as well as ages for them. A general conclusion is that in the solar neighborhood, on average, the thick disk is older than the thin disk. However, we caution that the exclusion of stars with effective temperatures around 6500 K might result in a biased view of the full age distribution for the stars in the thick disk.

The Bologna Open Cluster Chemical Evolution (BOCCE) project is a photometric and spectroscopic survey of open clusters, to be used as tracers of the Galactic disk properties and evolution. The clusters parameters (age, distance, reddening, metallicity, and detailed abundances) are derived in a precise and homogeneous way. This will contribute to a solid, reliable description of the disk: the clusters parameters will be used, for instance, to determine the metallicity distribution in the Galactic disk and how it has evolved with time. We have concentrated on old open clusters and we have presently in our hands data for about 40 open clusters; we have fully analyzed the photometric data for about one half of them and the spectra for one quarter of them.