arXiv subjects

Cyril Georgy

Publications and source records attributed to Cyril Georgy.

At least 37 records · Page 2Linked to original sources

arXiv2021

Blue supergiants as tests for stellar physics

Massive star evolution is still poorly understood, and observational tests are required to discriminate between different implementations of physical phenomenon in stellar evolution codes. By confronting stellar evolution models with observed properties of blue supergiants, such as pulsations, chemical composition and position in the Hertzsprung-Russell diagram, we aim at determining which of the criterion used for convection (Schwarzschild or Ledoux) is best able to explain the observations. We compute state-of-the-art stellar evolution models with either the Schwarzschild or the Ledoux criterion for convection. Models are for $14$ to $35\,M_\odot$ at solar or Large Magellanic Cloud metallicity. For each model, we compute the pulsation properties to know when radial modes are excited. We then compare our results with the position of blue supergiants in the Hertzsprung-Russell diagram, with their surface chemical composition, and with their variability. Our results at Large Magellanic Cloud metallicity shows only a slight preference for the Ledoux criterion over the Schwarzschild one in reproducing at the same time the observed properties of blue supergiants, even if the Schwarzschild criterion cannot be excluded at this metallicity. We check that changing the overshoot parameter at solar metallicity does not improve the situation. We also check that our models are able to reproduce the position of Galactic blue supergiant in the flux-weighted-gravity -- luminosity relation. We confirm that overall, models computed with the Ledoux criterion are slightly better in matching observations. Our results also support the idea that most Galactic $α$ Cyg variables are blue supergiants of the group 2, i.e. stars that have been through a previous red supergiant phase where they have lost large amount of mass.

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arXiv2021

Massive stars in the Small Magellanic Cloud : Evolution, rotation and surface abundances

We study the evolutionary and physical properties of evolved O stars in the Small Magellanic Cloud (SMC), with a special focus on their surface abundances to investigate the efficiency of rotational mixing as a function of age, rotation and global metallicity. We analyse the UV + optical spectra of thirteen SMC O-type giants and supergiants, using the stellar atmosphere code CMFGEN to derive photospheric and wind properties. We compare the inferred properties to theoretical predictions from evolution models. For a more comprehensive analysis, we interpret the results together with those we obtained for O-type dwarfs in a former study. Most dwarfs lie in the early phases of the main-sequence. For a given initial mass, giants are farther along the evolutionary tracks, confirming that they are more evolved than dwarfs. Supergiants have higher initial masses and are located past the terminal age main-sequence. We find no clear trend of a mass discrepancy, regardless of the diagram that was used to estimate the evolutionary mass. CNO abundances are consistent with nucleosynthesis from the CNO cycle. Comparisons to theoretical predictions reveal that the initial mixture is important when the observed trends in the N/C versus N/O diagram are to be reproduced. A trend for stronger chemical evolution for more evolved objects is observed. More massive stars, are on average, more chemically enriched at a given evolutionary phase, qualitatively consistent with evolutionary models. Abundance ratios supports the theoretical prediction that massive stars at low metallicity are more chemically processed than their Galactic counterparts. Finally, models including rotation generally reproduce the surface abundances and rotation rates when different initial rotational velocities are considered. Nevertheless, there are objects for which a stronger braking and/or more efficient mixing is required.

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arXiv2021

Is GW190521 the merger of black holes from the first stellar generations?

GW190521 challenges our understanding of the late-stage evolution of massive stars and the effects of the pair-instability in particular. We discuss the possibility that stars at low or zero metallicity could retain most of their hydrogen envelope until the pre-supernova stage, avoid the pulsational pair-instability regime and produce a black hole with a mass in the mass gap by fallback. We present a series of new stellar evolution models at zero and low metallicity computed with the Geneva and MESA stellar evolution codes and compare to existing grids of models. Models with a metallicity in the range 0-0.0004 have three properties which favour higher BH masses as compared to higher metallicity models. These are (i) lower mass-loss rates during the post-MS phase, (ii) a more compact star disfavouring binary interaction and (iii) possible H-He shell interactions which lower the CO core mass. We conclude that it is possible that GW190521 may be the merger of black holes produced directly by massive stars from the first stellar generations. Our models indicate BH masses up to 70-75 Msun. Uncertainties related to convective mixing, mass loss, H-He shell interactions and pair-instability pulsations may increase this limit to ~85 Msun.

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arXiv2020

Grids of stellar models with rotation: V. Models from 1.7 to 120 Msun at zero metallicity

Understanding the nature of the first stars is key to understanding the early universe. With new facilities such as JWST we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (Mini) from 1.7 Msun to 120 Msun, focusing on massive models of 9 Msun < Mini < 120 Msun. Our grid includes models with and without rotation, with rotating models having an initial velocity of 40% of the critical velocity. We find that rotation strongly impacts the evolution of the first stars, mainly through increased core size and stronger H-burning shells during core He-burning. Without radiative mass loss, angular momentum builds at the surface in rotating models, thus models of initial masses Mini > 60 Msun reach critical rotation on the main sequence and experience mass loss. We find that rotational mixing strongly affects metal enrichment, but does not always increase metal production as we see at higher metallicities. This is because rotation leads to an earlier CNO boost to the H shell during He-burning, which may hinder metal enrichment depending on initial mass and rotational velocity. Electronic tables of this new grid of Population III models are publicly available.

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arXiv2020

Massive black holes regulated by luminous blue variable mass loss and magnetic fields

We investigate the effects of mass loss during the main-sequence (MS) and post-MS phases of massive star evolution on black hole (BH) birth masses. We compute solar metallicity Geneva stellar evolution models of an 85 $M_{\odot}$ star with mass-loss rate ($\dot{M}$) prescriptions for MS and post-MS phases and analyze under which conditions such models could lead to very massive BHs. Based on the observational constraints for $\dot{M}$ of luminous stars, we discuss two possible scenarios that could produce massive BHs at high metallicity. First, if a massive BH progenitor evolves from the observed population of massive MS stars known as WNh stars, we show that its average post-MS mass-loss rate has to be less than $1\,\times10^{-5}\,M_{\odot}$/yr. However, this is lower than the typical observed mass-loss rates of luminous blue variables (LBV). Second, a massive BH progenitor could evolve from a yet undetected population of $80-85$ $M_{\odot}$ stars with strong surface magnetic fields, which could quench mass loss during the evolution. In this case, the average mass-loss rate during the post-MS LBV phase has to be less than $5\,\times10^{-5}\,M_{\odot}$/yr to produce 70 $M_{\odot}$ BHs. We suggest that LBVs that explode as SNe have large envelopes and small cores that could be prone to explosion, possibly evolving from binary interaction (either mergers or mass gainers that do not fully mix). Conversely, LBVs that directly collapse to BHs could have evolve from massive single stars or binary-star mergers that fully mix, possessing large cores that would favor BH formation.

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arXiv2020

Relative Importance of Convective Uncertainties in Massive Stars

In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called `overshoot', namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses $15$, $20$ and $25\,\rm{M}_\odot$. We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main-sequence with more CBM, which is more in agreement with observations. Furthermore during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves red-wards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the $^{12}$C to $^{16}$O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduce and we find differences of up to $70\%$ for the core masses and the total mass of the star.

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arXiv2020

SNAPSHOT: Connections between Internal and Surface Properties of Massive Stars

We introduce SNAPSHOT, a technique to systematically compute stellar structure models in hydrostatic and thermal equilibrium based on 3 structural properties - core mass $M_{\rm core}$, envelope mass $M_{\rm env}$ and core composition. This approach allows us to connect these properties of stellar interiors to the luminosity and effective temperature $T_{\rm eff}$ in a more systematic way than with stellar evolution models. For MS models, we derive an analytical relationship between $M_{\rm core}$, $M_{\rm total}$ and central H abundance that can be used in rapid stellar evolution algorithms. Core-He burning models with $M_{\rm core}/M_{\rm total}$ from 0.2 to 0.8 have convective envelopes, low $T_{\rm eff}$ and will appear as red supergiants. For a given $M_{\rm core}$, they exhibit a small variation in luminosity (0.02 dex) and $T_{\rm eff}$ ($\sim 400\,\mathrm{K}$) over a wide range of $M_{\rm env}$ ($\sim 2 - 20\,\mathrm{M}_{\odot}$). This means that it is not possible to derive red supergiant masses from luminosities and $T_{\rm eff}$ alone. We derive the following relationship between $M_{\rm core}$ and the total luminosity of a red supergiant during core He burning: $\log M_{\rm core} \simeq 0.44 \log L/L_{\odot} - 1.38$. At $M_{\rm core}$/$M_{\rm total} \approx 0.2$, our models exhibit a bi-stability and jump from a RSG to a BSG structure. Our models with $M_{\rm core}/M_{\rm total} > 0.8$, which correspond to stripped stars produced by mass loss or binary interaction, show that $T_{\rm eff}$ has a strong dependence on $M_{\rm env}$, $M_{\rm core}$ and the core composition. We find the mass of one of these stripped stars in a binary system, HD 45166, to be less than its dynamical mass. When a large observational sample of stripped stars becomes available, our results can be used to constrain their masses and the physics of binary interaction.

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arXiv2020

How stellar rotation shapes the colour magnitude diagram of the massive intermediate-age star cluster NGC 1846

We present a detailed study of stellar rotation in the massive 1.5 Gyr old cluster NGC 1846 in the Large Magellanic Cloud. Similar to other clusters at this age, NGC 1846 shows an extended main sequence turn-off (eMSTO), and previous photometric studies have suggested it could be bimodal. In this study, we use MUSE integral-field spectroscopy to measure the projected rotational velocities (vsini) of around 1400 stars across the eMSTO and along the upper main sequence of NGC 1846. We measure vsini values up to ~250 km/s and find a clear relation between the vsini of a star and its location across the eMSTO. Closer inspection of the distribution of rotation rates reveals evidence for a bimodal distribution, with the fast rotators centred around vsini = 140 km/s and the slow rotators centred around vsini = 60 km/s. We further observe a lack of fast rotating stars along the photometric binary sequence of NGC 1846, confirming results from the field that suggest that tidal interactions in binary systems can spin down stars. However, we do not detect a significant difference in the binary fractions of the fast and slowly rotating sub-populations. Finally, we report on the serendipitous discovery of a planetary nebula associated with NGC 1846.

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arXiv2019

The Luminosity Function of Red Supergiants in M31

The mass-loss rates of red supergiant stars (RSGs) are poorly constrained by direct measurements, and yet the subsequent evolution of these stars depends critically on how much mass is lost during the RSG phase. In 2012 the Geneva evolutionary group updated their mass-loss prescription for RSGs with the result that a 20 solar mass star now loses 10x more mass during the RSG phase than in the older models. Thus, higher mass RSGs evolve back through a second yellow supergiant phase rather than exploding as Type II-P supernovae, in accord with recent observations (the so-called "RSG Problem"). Still, even much larger mass-loss rates during the RSG phase cannot be ruled out by direct measurements of their current dust-production rates, as such mass-loss is episodic. Here we test the models by deriving a luminosity function for RSGs in the nearby spiral galaxy M31 which is sensitive to the total mass loss during the RSG phase. We carefully separate RSGs from asymptotic giant branch stars in the color-magnitude diagram following the recent method exploited by Yang and collaborators in their Small Magellanic Cloud studies. Comparing our resulting luminosity function to that predicted by the evolutionary models shows that the new prescription for RSG mass-loss does an excellent job of matching the observations, and we can readily rule out significantly larger values.

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arXiv2019

Massive stars evolution: feedbacks in low-Z environment

Massive stars are the drivers of the chemical evolution of dwarf galaxies. We review here the basics of massive star evolution and the specificities of stellar evolution in low-Z environment. We discuss nucleosynthetic aspects and what observations could constrain our view on the first generations of stars.

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arXiv2019

Grids of stellar models with rotation IV. Models from 1.7 to 120 Msun at a metallicity Z = 0.0004

The effects of rotation on stellar evolution are particularly important at low metallicity, when mass loss by stellar winds diminishes and the surface enrichment due to rotational mixing becomes relatively more pronounced than at high metallicities. Here we investigate the impact of rotation and metallicity on stellar evolution. Using a similar physics as in our previous large grids of models at Z=0.002 and Z=0.014, we compute stellar evolution models with the Geneva code for rotating and nonrotating stars with initial masses (Mini) between 1.7 and 120 Msun and Z=0.0004 (1/35 solar). This is comparable to the metallicities of the most metal poor galaxies observed so far, such as I Zw 18. Concerning massive stars, both rotating and nonrotating models spend most of their core-helium burning phase with an effective temperature higher than 8000 K. Stars become red supergiants only at the end of their lifetimes, and few RSGs are expected. Our models predict very few to no classical Wolf-Rayet stars as a results of weak stellar winds at low metallicity. The most massive stars end their lifetimes as luminous blue supergiants or luminous blue variables, a feature that is not predicted by models with higher metallicities. Interestingly, due to the behavior of the intermediate convective zone, the mass domain of stars producing pair-instability supernovae is smaller at Z=0.0004 than at Z=0.002. We find that during the main sequence phase, the ratio between nitrogen and carbon abundances (N/C) remains unchanged for nonrotating models. However, N/C increases by factors of 10-20 in rotating models at the end of the MS. Cepheids coming from stars with Mini > 4-6 Msun are beyond the core helium burning phase and spend little time in the instability strip. Since they would evolve towards cooler effective temperatures, these Cepheids should show an increase of the pulsation period as a function of age.

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arXiv2019

A Novel Approach to Constrain Rotational Mixing & Convective-Core Overshoot in Stars Using the Initial-Final Mass Relation

The semi-empirical initial-final mass relation (IFMR) connects spectroscopically analyzed white dwarfs in star clusters to the initial masses of the stars that formed them. Most current stellar evolution models, however, predict that stars will evolve to white dwarfs $\sim$0.1 M$_\odot$ less massive than that found in the IFMR. We first look at how varying theoretical mass-loss rates, third dredge-up efficiencies, and convective-core overshoot may help explain the differences between models and observations. These parameters play an important role at the lowest masses (M$_{\rm initial}$ $<$ 3 M$_\odot$). At higher masses, only convective-core overshoot meaningfully affects white dwarf mass, but alone it likely cannot explain the observed white dwarf masses nor why the IFMR scatter is larger than observational errors predict. These higher masses, however, are also where rotational mixing in main sequence stars begins to create more massive cores, and hence more massive white dwarfs. This rotational mixing also extends a star's lifetime, making faster rotating progenitors appear like less massive stars in their semi-empirical age analysis. Applying the observed range of young B-dwarf rotations to the MIST or SYCLIST rotational models demonstrates a marked improvement in reproducing both the observed IFMR data and its scatter. The incorporation of both rotation and efficient convective-core overshoot significantly improves the match with observations. This work shows that the IFMR provides a valuable observational constraint on how rotation and convective-core overshoot affect the core evolution of a star.

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arXiv2018

3D Simulations and MLT: I. Renzini's Critique

Renzini (1987) wrote an influential critique of mixing-length theory (MLT) as used in stellar evolution codes, and concluded that three-dimensional (3D) fluid dynamical simulations were needed to clarify several important issues. We have critically explored the limitations of the numerical methods and conclude that they are approaching the required accuracy. Implicit large eddy simulations (ILES) automatically connect large scale turbulence to a Kolmogorov cascade below the grid scale, allowing turbulent boundary layers to remove singularities that appear in the theory. Interactions between coherent structures give multi-modal behavior, driving intermittency and fluctuations. Reynolds averaging (RA) allows us to abstract the essential features of this dynamical behavior of boundaries which are appropriate to stellar evolution, and consider how they relate static boundary conditions (Richardson, Schwarzschild or Ledoux). We clarify several questions concerning when and why MLT works, and does not work, using both analytical theory and 3D high resolution numerical simulations. The composition gradients and boundary layer structure which are produced by our simulations suggest a self-consistent approach to boundary layers, removing the need for ad hoc procedures for 'convective overshooting' and `semi-convection'. In a companion paper we quantify the adequacy of our numerical resolution, determine of the length scale of dissipation (the `mixing length') without astronomical calibration, quantify agreement with the four-fifths law of Kolmogorov for weak stratification, and extend MLT to deal with strong stratification.

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arXiv2018

Impact of binary interaction on the evolution of blue supergiants

A large fraction of massive stars evolve in interacting binary systems, which dramatically modifies the outcome of stellar evolution. We investigated the properties of blue supergiants in binary systems and whether they are suitable for extragalactic distance determinations using the flux-weighted gravity luminosity relationship (FGLR). This is a relationship between the absolute bolometric magnitude $M_{\rm bol}$ and the spectroscopically determined flux-weighted gravity $g_{\rm F} = g/T^4_{\rm eff}$. We computed a grid of binary stellar evolution models with MESA and use the v2.1 BPASS models to examine whether they are compatible with the relatively small scatter shown by the observed relationship. Our models have initial primary masses of 9 - 30 $M_\odot$, initial orbital periods of 10 - 2511 days, mass ratio $q$ = 0.9, and metallicity $Z$ = 0.02. We find that the majority of primary stars that produce blue supergiant stages are consistent with the observed FGLR, with a small offset towards brighter bolometric magnitudes. In between 1\% - 24\% of cases, binary evolution may produce blue supergiants after a mass transfer episode, that lie below the observed FGLR. A small number of such stars have been found in extragalactic FGLR studies, suggesting that they may have evolved through binary interaction. Some models with shorter periods could resemble blue hypergiants and LBVs. We used CMFGEN radiative transfer models to investigate the effects of unresolved secondaries on diagnostics for $T_{\rm eff}$ and $g$, and the biases on the determination of interstellar reddening and $M_{\rm bol}$. We find that the effects are small and within the observed scatter, but could lead to a small overestimate of the luminosity, of $T_{\rm eff}$ and of $g$ for extreme cases. We conclude that the observed FGLR can, in principle, be well reproduced by close binary evolution models.

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arXiv2018

Relative Importance of Convective Uncertainties

Convection plays a key role in the evolution of stars due to energy transport and mixing of composition. Despite its importance, this process is still not well understood. One longstanding conundrum in all 1D stellar evolution codes is the treatment of convective boundaries. In this study we compare two convective uncertainties, the boundary location (Ledoux versus Schwarzschild) and the amount of extra mixing, and their impact on the early evolution of massive stars. With increasing convective boundary mixing (CBM), we find a convergence of the two different boundary locations, a decreasing blue to red super giant ratio and a reduced importance of semiconvection.

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arXiv2018

Massive Star Evolution: Binaries as Two Single Stars

Binary stars are of course more than two stars, but they are also at least two stars. In this chapter we will review some aspects of the physics governing the evolution of single massive stars. We will also review the uncertainties of key physical ingredients: mass loss, rotation and convection.

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arXiv2018

Star-planet interactions. V. Dynamical and equilibrium tides in convective zones

When planets are formed from the protoplanetary disk and after the disk has dissipated, the evolution of their orbits is governed by tidal interactions, friction, and gravitational drag, and also by changes in the mass of the star and planet. These interactions may change the initial distribution of the distances between the planets and their host star by expanding the original orbit, by contracting it (which may cause an engulfment of the planet by the star), or by destroying the planet. We study the evolution of the orbit of a planet orbiting its host star under the effects of equilibrium tides, dynamical tides, drag (frictional and gravitational), and stellar mass loss. We used the Geneva stellar evolution code to compute the evolution of stars with initial masses of 1 and 1.5 solar mass with different rotation rates at solar metallicity. The star is evolved from the pre-main-sequence (PMS) up to the tip of the red giant branch. We used these models as input for computing the evolution of the planetary orbits. We explored the effects of changing the planet masses (of 1 Earth mass up to 20 Jupiter masses), the distance between the planet and the star (of 0.015 and more than 3 au), the mass, and the spin of the star. We present results when only the equilibrium tide was accounted for and when both equilibrium and dynamical tides were accounted for. The expression for the dynamical tide is a frequency-averaged dissipation of tidally excited inertial waves, obtained from a piecewise homogeneous two-layer stellar model. Gravity wave damping was neglected. Dynamical tides in convective zones have a significant effect on planetary orbits only during the PMS phase and only for fast-rotating stars. They have no significant effects during the PMS phase for initially slow-rotating stars and during the red giant branch phase, regardless of the initial rotation (abridged).

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arXiv2018

A Runaway Yellow Supergiant Star in the Small Magellanic Cloud

We recently discovered a yellow supergiant (YSG) in the Small Magellanic Cloud (SMC) with a heliocentric radial velocity of ~300 km/s which is much larger than expected for a star in its location in the SMC. This is the first runaway YSG ever discovered and only the second evolved runaway star discovered in a different galaxy than the Milky Way. We classify the star as G5-8I, and use de-reddened broad-band colors with model atmospheres to determine an effective temperature of 4700+/-250K, consistent with what is expected from its spectral type. The star's luminosity is then L/Lo ~ 4.2+/-0.1, consistent with it being a ~30Myr 9Mo star according to the Geneva evolution models. The star is currently located in the outer portion of the SMC's body, but if the star's transverse peculiar velocity is similar to its peculiar radial velocity, in 10Myr the star would have moved 1.6 degrees across the disk of the SMC, and could easily have been born in one of the SMC's star-forming regions. Based on its large radial velocity, we suggest it originated in a binary system where the primary exploded as a supernovae thus flinging the runaway star out into space. Such stars may provide an important mechanism for the dispersal of heavier elements in galaxies given the large percentage of massive stars that are runaways. In the future we hope to look into additional evolved runaway stars that were discovered as part of our other past surveys.

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