Featured

The ASSESS team

At IAASARS/NOA I am part of the ASSESS project – Episodic MAss LoSS in Evolved MaSsive Stars. This project, funded by an ERC Consolidator grant (2018-2023, PI Bonanos), investigates the impact of episodic mass loss on the evolution of massive stars, and its role in the early Universe. The team is led by dr. Alceste Bonanos, and currently consists of three post-docs (dr. Grigoris Maravelias, dr. Ming Yang, and myself), and one PhD student (Stephan de Wit).

Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II

Murchikova, Lena; Murphy, Eric J.; Lis, Dariusz C.; Armus, Lee; de Mink, Selma; Sheth, Kartik; Zakamska, Nadia; Tramper, Frank; Bongiorno, Angela; Elvis, Martin; Kewley, Lisa; Sana, Hugues

The Astrophysical Journal, 903 29 (2020)

Abstract

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: ${L}_{\nu }\propto {\nu }^{-4.5\pm 0.4}.$ We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at $\lambda \leqslant 912\mathring{{\rm{A}}}$ , and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization.

Keywords: Astrophysicists;Interstellar medium;Star-forming regions;Star formation;Radiative recombination;Compact H II region;105;847;1565;1569;2057;286;Astrophysics – Astrophysics of Galaxies;Astrophysics – Solar and Stellar Astrophysics

Evolved massive stars at low metallicity. II. Red supergiant stars in the Small Magellanic Cloud

Yang, Ming; Bonanos, Alceste Z.; Jiang, Bi-Wei; Gao, Jian; Gavras, Panagiotis; Maravelias, Grigoris; Wang, Shu; Chen, Xiao-Dian; Tramper, Frank; Ren, Yi; Spetsieri, Zoi T.; Xue, Meng-Yao

Astronomy & Astrophysics, Volume 639, A116 (2020)

Abstract

We present the most comprehensive red supergiant (RSG) sample for the Small Magellanic Cloud (SMC) to date, including 1239 RSG candidates. The initial sample was derived based on a source catalog for the SMC with conservative ranking. Additional spectroscopic RSGs were retrieved from the literature, and RSG candidates were selected based on the inspection of Gaia and 2MASS color-magnitude diagrams (CMDs). We estimate that there are in total ∼1800 or more RSGs in the SMC. We purify the sample by studying the infrared CMDs and the variability of the objects, though there is still an ambiguity between asymptotic giant branch stars (AGBs) and RSGs at the red end of our sample. One heavily obscured target was identified based on multiple near-IR and mid-IR (MIR) CMDs. The investigation of color-color diagrams shows that there are fewer RSGs candidates (∼4%) showing PAH emission features compared to the Milky Way and LMC (∼15%). The MIR variability of RSG sample increases with luminosity. We separate the RSG sample into two subsamples (risky and safe), and identify one M5e AGB star in the risky subsample based on simultaneous inspection of variabilities, luminosities, and colors. The degeneracy of mass loss rate (MLR), variability, and luminosity of the RSG sample is discussed, indicating that most of the targets with high variability are also the bright ones with high MLR. Some targets show excessive dust emission, which may be related to previous episodic mass loss events. We also roughly estimate the total gas and dust budget produced by entire RSG population as ∼1.9-1.1+2.4 × 10-6 M yr-1 in the most conservative case, according to the derived MLR from IRAC1-IRAC4 color. Based on the MIST models, we derive a linear relation between Teff and observed J – KS color with reddening correction for the RSG sample. By using a constant bolometric correction and this relation, the Geneva evolutionary model is compared with our RSG sample, showing a good agreement and a lower initial mass limit of ∼7 M for the RSG population. Finally, we compare the RSG sample in the SMC and the LMC. Despite the incompleteness of LMC sample in the faint end, the result indicates that the LMC sample always shows redder color (except for the IRAC1-IRAC2 and WISE1-WISE2 colors due to CO absorption) and higher variability than the SMC sample, which is likely due to a positive relation between MLR, variability and the metallicity.

Keywords: infrared: stars;Magellanic Clouds;stars: late-type;stars: massive;stars: mass-loss;stars: variables: general;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – Astrophysics of Galaxies

Properties of OB star-black hole systems derived from detailed binary evolution models

Langer, N.; Schürmann, C.; Stoll, K.; Marchant, P.; Lennon, D. J.; Mahy, L.; de Mink, S. E.; Quast, M.; Riedel, W.; Sana, H.; Schneider, P.; Schootemeijer, A.; Wang, C.; Almeida, L. A.; Bestenlehner, J. M.; Bodensteiner, J.; Castro, N.; Clark, S.; Crowther, P. A.; Dufton, P. Evans, C. J.; Fossati, L.; Gräfener, G.; Grassitelli, L.; Grin, N.; Hastings, B.; Herrero, A.; de Koter, A.; Menon, A.; Patrick, L.; Puls, J.; Renzo, M.; Sander, A. A. C.; Schneider, F. R. N.; Sen, K.; Shenar, T.; Simón-Días, S.; Tauris, T. M.; Tramper, F.; Vink, J. S.; Xu, X. -T.

Astronomy & Astrophysics, Volume 638, A39 (2020)

Abstract

Context. The recent gravitational wave measurements have demonstrated the existence of stellar mass black hole binaries. It is essential for our understanding of massive star evolution to identify the contribution of binary evolution to the formation of double black holes.
Aims: A promising way to progress is investigating the progenitors of double black hole systems and comparing predictions with local massive star samples, such as the population in 30 Doradus in the Large Magellanic Cloud (LMC).
Methods: With this purpose in mind, we analysed a large grid of detailed binary evolution models at LMC metallicity with initial primary masses between 10 and 40 M, and identified the model systems that potentially evolve into a binary consisting of a black hole and a massive main-sequence star. We then derived the observable properties of such systems, as well as peculiarities of the OB star component.
Results: We find that ∼3% of the LMC late-O and early-B stars in binaries are expected to possess a black hole companion when stars with a final helium core mass above 6.6 M are assumed to form black holes. While the vast majority of them may be X-ray quiet, our models suggest that these black holes may be identified in spectroscopic binaries, either by large amplitude radial velocity variations (≳50 km s-1) and simultaneous nitrogen surface enrichment, or through a moderate radial velocity (≳10 km s-1) and simultaneous rapid rotation of the OB star. The predicted mass ratios are such that main-sequence companions can be excluded in most cases. A comparison to the observed OB+WR binaries in the LMC, Be and X-ray binaries, and known massive black hole binaries supports our conclusion.
Conclusions: We expect spectroscopic observations to be able to test key assumptions in our models, with important implications for massive star evolution in general and for the formation of double black hole mergers in particular.

Keywords: stars: evolution;stars: massive;binaries: close;stars: black holes;stars: early-type;stars: rotation;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – High Energy Astrophysical Phenomena

The Tarantula Massive Binary Monitoring. IV. Double-lined photometric binaries

Mahy, L.; Almeida, L. A.; Sana, H.; Clark, J. S.; de Koter, A.; de Mink, S. E.; Evans, C. J.; Grin, N. J.; Langer, N.; Moffat, A. F. J.; Schneider, F. R. N.; Shenar, T.; Tramper, F.

Astronomy & Astrophysics, Volume 634, A119 (2020)

Abstract

Context. A high fraction of massive stars are found to be binaries but only a few of them are reported as photometrically variable. By studying the populations of double-lined spectroscopic binaries in the 30 Doradus region, we found a subset of them that have photometry from the OGLE project and that display variations in their light curves related to orbital motions.
Aims: The goal of this study is to determine the dynamical masses and radii of the 26 binary components in order to investigate the mass-discrepancy problem and to provide an empirical mass-luminosity relation for the Large Magellanic Cloud (LMC).
Methods: We use the PHOEBE programme to perform a systematic analysis of the OGLE V and I light curves obtained for 13 binary systems in the 30 Doradus region. We adopt the effective temperatures, and orbital parameters derived previously to obtain the inclinations of the systems and the parameters of the individual components.
Results: Three systems display eclipses in their light curves, while the others only display ellipsoidal variations. We classify two systems as over-contact, five as semi-detached, and four as detached. The two remaining systems have uncertain configurations due to large uncertainties on their inclinations. The fact that systems display ellipsoidal variations has a significant impact on the inclination errors. From the dynamical masses, luminosities, and radii, we provide LMC-based empirical mass-luminosity and mass-radius relations, and we compare them to other relations given for the Galaxy, the LMC, and the Small Magellanic Cloud. These relations differ for different mass ranges, but do not seem to depend on the metallicity regimes. We also compare the dynamical, spectroscopic, and evolutionary masses of the stars in our sample. While the dynamical and spectroscopic masses agree with each other, the evolutionary masses are systematically higher, at least for stars in semi-detached systems. This suggests that the mass discrepancy can be partly explained by past or ongoing interactions between the stars.

Keywords: stars: early-type;binaries: eclipsing;open clusters and associations: individual: 30 Doradus;stars: fundamental parameters;Astrophysics – Solar and Stellar Astrophysics

The Tarantula Massive Binary Monitoring. III. Atmosphere analysis of double-lined spectroscopic systems

Mahy, L.; Sana, H.; Abdul-Masih, M.; Almeida, L. A.; Langer, N.; Shenar, T.; de Koter, A.; de Mink, S. E.; de Wit, S.; Grin, N. J.; Evans, C. J.; Moffat, A. F. J.; Schneider, F. R. N.; Barbá, R.; Clark, J. S.; Crowther, P.; Gräfener, G.; Lennon, D. J.; Tramper, F.; Vink, J. S.

Astronomy & Astrophysics, Volume 634, A118 (2020)

Abstract

Context. Accurate stellar parameters of individual objects in binary systems are essential to constrain the effects of binarity on stellar evolution. These parameters serve as a prerequisite to probing existing and future theoretical evolutionary models.
Aims: We aim to derive the atmospheric parameters of the 31 double-lined spectroscopic binaries in the Tarantula Massive Binary Monitoring sample. This sample, composed of detached, semi-detached and contact systems with at least one of the components classified as an O-type star, is an excellent test-bed to study how binarity can impact our knowledge of the evolution of massive stars.
Methods: In the present paper, 32 epochs of FLAMES/GIRAFFE spectra are analysed by using spectral disentangling to construct the individual spectra of 62 components. We then apply the CMFGEN atmosphere code to determine their stellar parameters and their helium, carbon, and nitrogen surface abundances.
Results: Among the 31 systems that we study in the present paper, we identify between 48 and 77% of them as detached, likely pre-interacting systems, 16% as semi-detached systems, and between 5 and 35% as systems in or close to contact phase. Based on the properties of their components, we show that the effects of tides on chemical mixing are limited. Components on longer-period orbits show higher nitrogen enrichment at their surface than those on shorter-period orbits, in contrast to expectations of rotational or tidal mixing, implying that other mechanisms play a role in this process. For semi-detached systems, components that fill their Roche lobe are mass donors. They exhibit higher nitrogen content at their surface and rotate more slowly than their companions. By accreting new material, their companions spin faster and are likely rejuvenated. Their locations in the N – v sin i diagram tend to show that binary products are good candidates to populate the two groups of stars (slowly rotating, nitrogen-enriched objects and rapidly rotating non-enriched objects) that cannot be reproduced through single-star population synthesis. Finally, we find no peculiar surface abundances for the components in (over-)contact systems, as has been suggested by evolutionary models for tidal mixing.
Conclusions: This sample, consisting of 31 massive binary systems, is the largest sample of binaries composed of at least one O-type star to be studied in such a homogeneous way by applying spectral disentangling and atmosphere modelling. The study of these objects gives us strong observational constraints to test theoretical binary evolutionary tracks.

Keywords: stars: early-type;binaries: spectroscopic;open clusters and associations: individual: 30 Doradus;stars: fundamental parameters;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – Astrophysics of Galaxies