arXiv subjects

Sylvia Ekstrom

Publications and source records attributed to Sylvia Ekstrom.

At least 19 recordsLinked to original sources

arXiv2025

pySTARBURST99: The Next Generation of STARBURST99

STARBURST99 is a population synthesis code tailored to predict the integrated properties or observational characteristics of star-forming galaxies. Here we present an update to STARBURST99 where we port the code to python, include new evolutionary tracks both rotating and non-rotating at a range of low metallicity environments. We complement these tracks with a corresponding grid of new synthetic SEDs. Additionally we include both evolutionary and spectral models of stars up to 300-500Msol. Synthesis models made with the python version of the code and new input stellar models are labelled pySTARBURST99. We make new predictions for many properties, such as ionising flux, SED, bolometric luminosity, wind power, hydrogen line equivalent widths and the UV beta-slope. These properties are all assessed over wider coverage in metallicity, mass and resolution than in previous versions of STARBURST99. A notable finding from these updates is an increase in H I ionising flux of 0.3 dex in the first 2Myr when increasing the upper mass limit from 120 to 300Msol. Changing metallicity has little impact on H I in the first 2Myr (range of 0.015 dex from Z = 0.02 to 0.0) but lower metallicities have higher H I by 1 dex (comparing Z = 0.02 to 0.0004) at later times, with Z = 0.0 having even higher H I at later times. Rotating models have significantly higher H I than their equivalent non-rotating models at any time after 2Myr. Similar trends are found for He I and He II, bolometric luminosity and wind momentum, with more complex relations found for hydrogen line equivalent widths and UV beta-slopes.

astro-ph.GA↗

arXiv2024

Rapidly rotating Population III stellar models as a source of primary nitrogen

The first stars might have been fast rotators. This would have important consequences for their radiative, mechanical and chemical feedback. We discuss the impact of fast initial rotation on the evolution of massive Population III models and on their nitrogen and oxygen stellar yields. We explore the evolution of Population III stars with initial masses in the range of 9Msol < Mini < 120Msol starting with an initial rotation on the Zero Age Main Sequence equal to 70% of the critical one. We find that with the physics of rotation considered here, our rapidly-rotating Population III stellar models do not follow a homogeneous evolution. They lose very little mass in case mechanical winds are switched on when the surface rotation becomes equal or larger than the critical velocity. Impact on the ionising flux appears modest when compared to moderately-rotating models. Fast rotation favours, in models with initial masses above ~20Msol, the appearance of a very extended intermediate convective zone around the H-burning shell during the core He-burning phase. This shell has important consequences on the sizes of the He- and CO-cores and thus impacts the final fate of stars. Moreover, it has a strong impact on nucleosynthesis boosting the production of primary 14N. Fast initial rotation impacts significantly the chemical feedback of Population III stars. Observations of extremely metal-poor stars and/or starbursting regions are essential to provide constraints on the properties of the first stars.

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arXiv2022

The Time-Averaged Mass-Loss Rates of Red Supergiants As Revealed by their Luminosity Functions in M31 and M33

Mass-loss in red supergiants (RSGs) is generally recognized to be episodic, but mass-loss prescriptions fail to reflect this. Evolutionary models show that the total amount of mass lost during this phase determines if these stars evolve to warmer temperatures before undergoing core collapse. The current Geneva evolutionary models mimic episodic mass loss by enhancing the quiescent prescription rates whenever the star's outer layers exceed the Eddington luminosity by a large factor. This results in a 20 solar-mass model undergoing significantly more mass loss during the RSG phase than it would have otherwise, but has little effect on models of lower masses. We can test the validity of this approach observationally by measuring the proportion of high-luminosity RSGs to that predicted by the models. To do this, we use our recent luminosity-limited census of RSGs in M31 and M33, making modest improvements to membership, and adopting extinctions based on the recent panchromatic M31 and M33 Hubble surveys. We then compare the proportions of the highest luminosity RSGs found to that predicted by published Geneva models, as well as to a special set of models computed without the enhanced rates. We find good agreement with the models which include the supra-Eddington enhanced mass loss. The models with lower mass-loss rates predict a larger fraction of high-luminosity RSGs than observed, and thus can be ruled out. We also use these improved data to confirm that the upper luminosity limit of RSGs is log L/Lo~5.4, regardless of metallicity, using our improved data on M31 and M33 plus previous results on the Magellanic Clouds.

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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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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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arXiv2015

Impact of rotation on stellar models

After a brief recall of the main impacts of stellar rotation on the structure and the evolution of stars, four topics are addressed: 1) the links between magnetic fields and rotation; 2) the impact of rotation on the age determination of clusters; 3) the exchanges of angular momentum between the orbit of a planet and the star due to tides; 4) the impact of rotation on the early chemical evolution of the Milky Way and the origin of the Carbon-Enhanced-Metal-Poor stars.

astro-ph.SR↗

arXiv2014

The Effects of Stellar Rotation. II. A Comprehensive Set of Starburst99 Models

We present a new set of synthesis models for stellar populations obtained with Starburst99, which are based on new stellar evolutionary tracks with rotation. We discuss models with zero rotation velocity and with velocities of 40% of the break-up velocity on the zero-age main-sequence. These values are expected to bracket realistic rotation velocity distributions in stellar populations. The new rotating models for massive stars are more luminous and hotter due to a larger convective core and enhanced surface abundances. This results in pronounced changes in the integrated spectral energy distribution of a population containing massive stars. The changes are most significant at the shortest wavelengths where an increase of the ionizing luminosity by up to a factor of 5 is predicted. We also show that high equivalent widths of recombination lines may not necessarily indicate a very young age but can be achieved at ages as late as 10 Myr. Comparison of these two boundary cases (0 and 40% of the break-up velocity) will allow users to evaluate the effects of rotation and provide guidance for calibrating the stellar evolution models. We also introduce a new theoretical ultraviolet spectral library built from the Potsdam Wolf-Rayet (PoWR) atmospheres. Its purpose is to help identify signatures of Wolf-Rayet stars in the ultraviolet whose strength is sensitive to the particulars of the evolution models. The new models are available for solar and 1/7th solar metallicities. A complete suite of models can be generated on the Starburst99 website (www.stsci.edu/science/starburst99/). The updated Starburst99 package can be retrieved from this website as well.

astro-ph.GA↗

arXiv2014

The evolution of massive stars and their spectra I. A non-rotating 60 Msun star from the zero-age main sequence to the pre-supernova stage

For the first time, the interior and spectroscopic evolution of a massive star is analyzed from the zero-age main sequence (ZAMS) to the pre-supernova (SN) stage. For this purpose, we combined stellar evolution models using the Geneva code and atmospheric models using CMFGEN. With our approach, we were able to produce observables, such as a synthetic high-resolution spectrum and photometry, aiding the comparison between evolution models and observed data. Here we analyze the evolution of a non-rotating 60 Msun star and its spectrum throughout its lifetime. Interestingly, the star has a supergiant appearance (luminosity class I) even at the ZAMS. We find the following evolutionary sequence of spectral types: O3 I (at the ZAMS), O4 I (middle of the H-core burning phase), B supergiant (BSG), B hypergiant (BHG), hot luminous blue variable (LBV; end of H-core burning), cool LBV (H-shell burning through the beginning of the He-core burning phase), rapid evolution through late WN and early WN, early WC (middle of He-core burning), and WO (end of He-core burning until core collapse). We find the following spectroscopic phase lifetimes: 3.22e6 yr for the O-type, 0.34e5 yr (BSG), 0.79e5 yr (BHG), 2.35e5 yr (LBV), 1.05e5 yr (WN), 2.57e4 yr (WC), and 3.80e4 yr (WO). Compared to previous studies, we find a much longer (shorter) duration for the early WN (late WN) phase, as well as a long-lived LBV phase. We show that LBVs arise naturally in single-star evolution models at the end of the MS when the mass-loss rate increases as a consequence of crossing the bistability limit. We discuss the evolution of the spectra, magnitudes, colors, and ionizing flux across the star's lifetime, and the way they are related to the evolution of the interior. [abridged]

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arXiv2013

Progenitors of supernova Ibc: a single Wolf-Rayet star as the possible progenitor of the SN Ib iPTF13bvn

Core-collapse supernova (SN) explosions mark the end of the tumultuous life of massive stars. Determining the nature of their progenitors is a crucial step towards understanding the properties of SNe. Until recently, no progenitor has been directly detected for SN of type Ibc, which are believed to come from massive stars that lose their hydrogen envelope through stellar winds and from binary systems where the companion has stripped the H envelope from the primary. Here we analyze recently reported observations of iPTF13bvn, which could possibly be the first detection of a SN Ib progenitor based on pre-explosion images. Very interestingly, the recently published Geneva models of single stars can reproduce the observed photometry of the progenitor candidate and its mass-loss rate, confirming a recently proposed scenario. We find that a single WR star with initial mass in the range 31-35 Msun fits the observed photometry of the progenitor of iPTF13bvn. The progenitor likely has a luminosity of log (L/Lsun)~5.55, surface temperature ~45000 K, and mass of ~10.9 Msun at the time of explosion. Our non-rotating 32 Msun model overestimates the derived radius of the progenitor, although this could likely be reconciled with a fine-tuned model of a more massive (between 40 and 50 Msun), hotter, and luminous progenitor. Our models indicate a very uncertain ejecta mass of ~8 Msun, which is higher than the average of the SN Ib ejecta mass that is derived from the lightcurve (2-4 Msun). This possibly high ejecta mass could produce detectable effects in the iPTF13bvn lightcurve and spectrum. If the candidate is indeed confirmed to be the progenitor, our results suggest that stars with relatively high initial masses (>30 Msun) can produce visible SN explosions at their deaths and do not collapse directly to a black hole.

astro-ph.SR↗

arXiv2013

Four open questions in massive star evolution

We discuss four questions dealing with massive star evolution. The first one is about the origin of slowly rotating, non-evolved, nitrogen rich stars. We propose that these stars may originate from initially fast rotating stars whose surface has been braked down. The second question is about the evolutionary status of alpha-Cygni variables. According to their pulsation properties, these stars should be post red supergiant stars. However, some stars at least present surface abundances indicating that they should be pre red supergiant stars. How to reconcile these two contradictory requirements? The third one concerns the various supernova types which are the end point of the evolution of stars with initial masses between 18 and 30 Msun, i.e. the most massive stars which go through a red supergiant phase during their lifetime. Do they produce types IIP, IIL, IIn, IIb or Ib supernovae or do they end without producing any SN event? Finally, we shall discuss reasons why so few progenitors of type Ibc supernovae have yet been detected?

astro-ph.SR↗

arXiv2013

Fundamental properties of core-collapse Supernova and GRB progenitors: predicting the look of massive stars before death

We investigate the fundamental properties of core-collapse Supernova (SN) progenitors from single stars at solar metallicity. We combine Geneva stellar evolutionary models with initial masses of Mini=20-120 Msun with atmospheric/wind models using CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at the pre-SN stage. For Mini=9-20 Msun, we supplement our analysis using publicly available MARCS model atmospheres of RSGs. We employ observational criteria of spectroscopic classification and find that massive stars, depending on Mini and rotation, end their lives as red supergiants (RSG), yellow hypergiants (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtain the following types of SN progenitors: WO1-3 (Mini <= 32 Msun), WN10-11 (25 < Mini < 32 Msun), LBV (20 <= Mini < 25 Msun), G1 Ia+ (18 < Mini < 20 Msun), and RSGs (9 <= Mini <= 18 Msun). For non-rotating models, we find spectral types WO1-3 (Mini>40 Msun), WN7-8 (25 < Mini <= 40 Msun), WN11h/LBV (20 < Mini <= 25 Msun), and RSGs (9 <= Mini <= 20 Msun). Our rotating models indicate that SN IIP progenitors are all RSG, SN IIL/b progenitors are 56% LBVs and 44% YHGs, SN Ib progenitors are 96% WN10-11 and 4% WOs, and SN Ic progenitors are all WO stars. We find that not necessarily the most massive and luminous SN progenitors are the brighter ones in a given filter. We show that SN IIP progenitors (RSGs) are bright in the RIJHK_S filters and faint in the UB filters. SN IIL/b progenitors (LBVs and YHGs), and SN Ib progenitors (WNs) are relatively bright in optical/IR filters, while SN Ic progenitors (WOs) are faint in all optical filters. We argue that SN Ib and Ic progenitors from single stars should be undetectable in the available pre-explosion images with the current magnitude limits, in agreement with observational results.[abridged]

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arXiv2013

Evolution and fate of very massive stars

There is observational evidence that supports the existence of Very Massive Stars in the local universe. First, very massive stars (Mini<=320 M) have been observed in the Large Magellanic Cloud . Second, there are observed SNe that bear the characteristics of Pair Creation Supernovae which have very massive stars as progenitors. The most promising candidate to date is SN2007bi. In order to investigate the evolution and fate of nearby very massive stars, we calculated a new grid of models for such objects, for solar, LMC and SMC metallicities, which covers the initial mass range from 120 to 500M. Both rotating and non-rotating models were calculated using the Geneva stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since very massive stars have very large convective cores during the Main-Sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO) type stars. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M . All VMS stars in the metallicity range studied here produce either a type Ib or a type Ic SN. We estimate that the progenitor of SN2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M . None of models presented in this grid produce GRBs or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a BH by producing a completely disruptive PCSN.

astro-ph.SR↗

arXiv2013

The past and future evolution of a star like Betelgeuse

We discuss the physics and the evolution of a typical massive star passing through an evolutionary stage similar to that of Betelgeuse. After a brief introduction recalling various observed parameters of Betelgeuse, we discuss the Pre-Main-Sequence phase (PMS), the Main-Sequence (MS) phase, the physics governing the duration of the first crossing of the HR diagram, the red supergiant stage (RSG), the post-red supergiant phases and the final fate of solar metallicity stars with masses between 9 and 25 M$_\odot$. We examine the impact of different initial rotation and of various prescriptions for the mass loss rates during the red supergiant phase. We show that, whatever the initial rotation rate (chosen between 0 and 0.7$\times\upsilon_{\rm crit}$, $\upsilon_{\rm crit}$ being the surface equatorial velocity producing a centrifugal acceleration balancing exactly the gravity) and the mass loss rates during the RSG stage (varied between a standard value and 25 times that value), a 15 M$_\odot$ star always ends its lifetime as a RSG and explodes as a type II-P or II-L supernova.

astro-ph.SR↗

arXiv2012

The Effects of Stellar Rotation. I. Impact on the Ionizing Spectra and Integrated Properties of Stellar Populations

We present a sample of synthetic massive stellar populations created using the Starburst99 evolutionary synthesis code and new sets of stellar evolutionary tracks, including one set that adopts a detailed treatment of rotation. Using the outputs of the Starburst99 code, we compare the populations' integrated properties, including ionizing radiation fields, bolometric luminosities, and colors. With these comparisons we are able to probe the specific effects of rotation on the properties of a stellar population. We find that a population of rotating stars produces a much harder ionizing radiation field and a higher bolometric luminosity, changes that are primarily attributable to the effects of rotational mixing on the lifetimes, luminosities, effective temperatures, and mass loss rates of massive stars. We consider the implications of the profound effects that rotation can have on a stellar population, and discuss the importance of refining stellar evolutionary models for future work in the study of extragalactic, and particularly high-redshift, stellar populations.

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arXiv2011

Population Synthesis at the Crossroads

The current state-of-the-art of population synthesis is reviewed. The field is currently undergoing major revisions with the recognition of several key processes as new critical ingredients. Stochastic effects can artificially enhance or suppress certain evolutionary phases and/or stellar mass regimes and introduce systematic biases in, e.g., the determination of the stellar initial mass function. Post-main-sequence evolution is often associated with irregular variations of stellar properties on ultra-short time-scales. Examples are asymptotic giant branch stars and luminous blue variables, both of which are poorly treated in the models. Stars rarely form in isolation, and the fraction of truly single stars may be very small. Therefore, stellar multiplicity must be accounted for since many systems will develop tidal interaction over the course of their evolution. Last but not least, stellar rotation can drastically increase stellar temperatures and luminosities, which in turn leads to revised mass-to-light ratios in population synthesis models.

astro-ph.CO↗

arXiv2011

Massive Be and Oe stars at low metallicity and long gamma ray bursts

According to recent theoretical studies, the progenitors of Long Gamma Ray Bursts should be very fast rotating stars, massive enough but not so for collapsing into a black hole. In addition, recent observations seem to show that stars of about 20 solar masses could be at the origin of LGRBs. At low metallicity B-type stars rotate faster than at higher metallicity. We found with the ESO-WFI an occurrence of Be/Oe stars, that are quasi critical rotators, 3 to 5 times larger in the SMC than in the Galaxy. According to our results, and using observational clues on the SMC WR stars, as well as the theoretical predictions of the characteristics must have the LGRB progenitors, we have identified the low metallicity massive Be/Oe stars as potential LGRB progenitors. To support this identification, the expected rates and the numbers of LGRB were then calculated and compared to the observed ones: 3 to 6 LGRBs were found in the local universe in 11 years while 8 were actually observed.

astro-ph.SR↗

arXiv2010

Constraints on rotational mixing from surface evolution of light elements in massive stars

Context. Light elements and nitrogen surface abundances together can constrain the mixing efficiencies in massive stars on the main sequence, because moderate mixing at the surface leads to a depletion of light elements but only later to an enrichment in nitrogen. Aims. We want to test the rotational mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars. Methods. The GENEC is a 1D code containing sophisticated prescriptions for rotational mixing. We implemented an extended reaction network into this code including the light elements Li, Be and B, which allowed us to perform calculations testing the rotation induced mixing. Results. We followed 9, 12 and 15 solar mass models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of light isotopes for faster rotating stars and more massive stars. Conclusions. We find that the mixing prescriptions used in the present rotating models for massive single stars can account for most of the observations. However the uncertainties are quite large making it hard to draw a firm conclusion on the mixing scenario.

astro-ph.SR↗

arXiv2010

Evolution of massive Be and Oe stars at low metallicity towards the Long Gamma Ray bursts\footnote{Based on ESO runs 069.D-0275(A), 072.D-0245(A) and (C).}

Several studies have shown recently that at low metallicity B-type stars rotate faster than in environments of high metallicity. This is a typical case in the SMC. As a consequence, it is expected that a larger number of fast rotators is found in the SMC than in the Galaxy, in particular a higher fraction of Be/Oe stars. Using the ESO-WFI in its slitless mode, the data from the SMC open clusters were examined and an occurrence of Be stars 3 to 5 times larger than in the Galaxy was found. The evolution of the angular rotational velocity at different metallicities seems to be the main key to understand the specific behavior and evolution of these stars. According to the results from this WFI study, the observational clues obtained from the SMC WR stars and massive stars, and the theoretical predictions of the characteristics must have the long gamma-ray burst progenitors, we have identified the low metallicity massive Be and Oe stars as potential LGRB progenitors. To this end, the ZAMS rotational velocities of the SMC Be/Oe stars were determined and compared to models. The expected rates and the numbers of LGRB were then calculated and compared to the observed ones. Thus, a high probability was found that low metallicity Be/Oe stars can be LGRB progenitors. In this document, we describe the different steps followed in these studies: determination of the number of Be/Oe stars at different metallicities, identification of the clues that lead to suppose the low metallicity Be/Oe stars as LGRB progenitors, comparison of models with observations.

astro-ph.SR↗