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).

The ASSESS Team (from left to right): Frank Tramper, Grigoris Maravelias, Alceste Bonanos (PI), Ming Yang, and Stephan de Wit.

Evolved massive stars at low-metallicity. IV. Using the 1.6 μm H-bump to identify red supergiant stars: Case study of NGC 6822

Yang, Ming; Bonanos, Alceste Z.; Jiang, Biwei; Lam, Man I.; Gao, Jian; Gavras, Panagiotis; Maravelias, Grigoris; Wang, Shu; Chen, Xiao-Dian; Tramper, Frank; Ren, Yi; Spetsieri, Zoi T.

Astronomy & Astrophysics, 647, 167 (2021)

ADS – Journal – arXiv


We present a case study in which we used a novel method to identify red supergiant (RSG) candidates in NGC 6822 based on their 1.6 μm H-bump. We collected 32 bands of photometric data for NGC 6822 ranging from the optical to the mid-infrared, derived from Gaia, PS1, LGGS, VHS, UKIRT, IRSF, HAWK-I, Spitzer, and WISE. Using the theoretical spectra from MARCS, we demonstrate that there is a prominent difference around 1.6 μm (H-bump) between targets with high and low surface gravity (HSG and LSG). Taking advantage of this feature, we identify efficient color-color diagrams of rzH (r ‒ z vs. z ‒ H) and rzK (r ‒ z vs. z ‒ K) to separate HSG (mostly foreground dwarfs) and LSG targets (mainly background red giant stars, asymptotic giant branch stars, and RSGs) from crossmatching of optical and near-infrared (NIR) data. Moreover, synthetic photometry from ATLAS9 gives similar results. We further separated RSG candidates from the remaining LSG candidates as determined by the H-bump method by using semi-empirical criteria on NIR color-magnitude diagrams, where both the theoretic cuts and morphology of the RSG population are considered. This separation produced 323 RSG candidates. The simulation of foreground stars with Besançon models also indicates that our selection criteria are largely free from the contamination of Galactic giants. In addition to the H-bump method, we used the traditional BVR method (B ‒ V vs. V ‒ R) as a comparison and/or supplement by applying a slightly aggressive cut to select as many RSG candidates as possible (358 targets). Furthermore, the Gaia astrometric solution was used to constrain the sample, where 181 and 193 targets were selected with the H-bump and BVR method, respectively. The percentages of selected targets in the two methods are similar at ∼60%, indicating a comparable accuracy of the two methods. In total, there are 234 RSG candidates after combining targets from the two methods, and 140 (∼60%) of them are in common. The final RSG candidates are in the expected locations on the mid-infrared color-magnitude diagram with [3.6]‒[4.5] ≲ 0 and J ‒ [8.0] ≈ 1.0. The spatial distribution is also coincident with the far-ultraviolet-selected star formation regions, suggesting that the selection is reasonable and reliable. We indicate that our method can also be used to identify other LSG targets, such as red giants and asymptotic giant branch stars, and it can also be applied to most of the nearby galaxies by using recent large-scale ground-based surveys. Future ground- and space-based facilities may promote its application beyond the Local Group.

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

Evolved massive stars at low-metallicity. III. A source catalog for the Large Magellanic Cloud

Yang, Ming; Bonanos, Alceste Z.; Jiang, Biwei; Gao, Jian; Gavras, Panagiotis; Maravelias, Grigoris; Wang, Shu; Chen, Xiao-Dian; Lam, Man I.; Ren, Yi; Tramper, Frank; Spetsieri, Zoi T.

Astronomy & Astrophysics, 646, 141 (2021)

ADS – Journal – arXiv


We present a clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog for the Large Magellanic Cloud (LMC). The catalog was built by crossmatching (1″) and deblending (3″) between the source list of Spitzer Enhanced Imaging Products and Gaia Data Release 2, with strict constraints on the Gaia astrometric solution in order to remove the foreground contamination. It is estimated that about 99.5% of the targets in our catalog are most likely genuine members of the LMC. The catalog contains 197 004 targets in 52 different bands, including two ultraviolet, 21 optical, and 29 infrared bands. Additional information about radial velocities and spectral and photometric classifications were collected from the literature. We compare our sample with the sample from Gaia Collaboration (2018, A&A, 616, A12), indicating that the bright end of our sample is mostly comprised of blue helium-burning stars (BHeBs) and red HeBs with inevitable contamination of main sequence stars at the blue end. After applying modified magnitude and color cuts based on previous studies, we identified and ranked 2974 red supergiant, 508 yellow supergiant, and 4786 blue supergiant candidates in the LMC in six color-magnitude diagrams (CMDs). The comparison between the CMDs from the two catalogs of the LMC and Small Magellanic Cloud (SMC) indicates that the most distinct difference appears at the bright red end of the optical and near-infrared CMDs, where the cool evolved stars (e.g., red supergiant stars (RSGs), asymptotic giant branch stars, and red giant stars) are located, which is likely due to the effect of metallicity and star formation history. A further quantitative comparison of colors of massive star candidates in equal absolute magnitude bins suggests that there is essentially no difference for the BSG candidates, but a large discrepancy for the RSG candidates since LMC targets are redder than the SMC ones, which may be due to the combined effect of metallicity on both spectral type and mass-loss rate as well as the age effect. The effective temperatures (Teff) of massive star populations are also derived from reddening-free color of (J – KS)0. The Teff ranges are 3500 < Teff < 5000 K for an RSG population, 5000 < Teff < 8000 K for a YSG population, and Teff > 8000 K for a BSG population, with larger uncertainties toward the hotter stars.

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

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)

ADS – Journal – arXiv


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)

ADS – Journal – arXiv


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)

ADS – Journal – arXiv


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

Probability of OB stars of a given mass to have a BH companion as a function of the mass of the OB star, according to our population synthesis model. The initial mass function, initial binary parameter distributions, and the lifetimes of the OB+BH systems have been considered. A initial binary fraction of 100% has been assumed.