Massive stars in extremely metal-poor galaxies: a window into the past

Garcia, M. ; Evans, C. J. ; Bestenlehner, J. M. ; Bouret, J. C. ; Castro, N. ; Cerviño, M.; Fullerton, A. W. ; Gieles, M. ; Herrero, A. ; de Koter, A. ; Lennon, D. J. ; van Loon, J. Th. ; Martins, F. ; de Mink, S. E. ; Najarro, F. ; Negueruela, I. ; Sana, H. ; Simón-Díaz, S. ; Szécsi, D. ; Tramper, F..; Vink, J. ; Wofford, A.

Experimental Astronomy, 114 (2021)

ADS – Journal – arXiv


Cosmic History has witnessed the lives and deaths of multiple generations of massive stars, all of them invigorating their host galaxies with ionizing photons, kinetic energy, fresh material and stellar-mass black holes. Ubiquitous engines as they are, Astrophysics needs a good understanding of their formation, evolution, properties and yields throughout the history of the Universe, and with decreasing metal content mimicking the environment at the earliest epochs. Ultimately, a physical model that could be extrapolated to zero metallicity would enable tackling long-standing questions such as “What did the First, very massive stars of the Universe look like?” or “What was their role in the re-ionization of the Universe?”. Yet, most our knowledge of metal-poor massive stars is drawn from one single point in metallicity. Massive stars in the Small Magellanic Cloud (SMC, ∼1/5Z⊙∼1/5Z⊙) currently serve as templates for low-metallicity objects in the early Universe, even though significant differences with respect to massive stars with poorer metal content have been reported. This White Paper summarizes the current knowledge on extremely (sub-SMC) metal poor massive stars, highlighting the most outstanding open questions and the need to supersede the SMC as standard. A new paradigm can be built from nearby extremely metal-poor galaxies that make a new metallicity ladder, but massive stars in these galaxies are out of reach to current observational facilities. Such task would require an L-size mission, consisting of a 10m-class space telescope operating in the optical and the ultraviolet ranges. Alternatively, we propose that ESA unites efforts with NASA to make the LUVOIR mission concept a reality, thus continuing the successful partnership that made Hubble Space Telescope one of the greatest observatories of all time.

Keywords: Astrophysics – Instrumentation and Methods for Astrophysics;Astrophysics – Solar and Stellar Astrophysics

The young massive SMC cluster NGC 330 seen by MUSE. II. Multiplicity properties of the massive-star population

Bodensteiner, J.; Sana, H.; Wang, C.; Langer, N. ; Mahy, L.; Banyard, G.; de Koter, A. ; de Mink, S. E. ; Evans, C. J. ; Götberg, Y.; Patrick, L. R.; Schneider, F. R. N.; Tramper, F.

Astronomy & Astrophysics, 652, A70, 18 (2021)

ADS – Journal – arXiv


Context. Observations of massive stars in open clusters younger than ∼8 Myr have shown that a majority of them are in binary systems, most of which will interact during their life. While these can be used as a proxy of the initial multiplicity properties, studying populations of massive stars older than ∼20 Myr allows us to probe the outcome of these interactions after a significant number of systems have experienced mass and angular momentum transfer and may even have merged.
Aims: Using multi-epoch integral-field spectroscopy, we aim to investigate the multiplicity properties of the massive-star population in the dense core of the ∼40 Myr old cluster NGC 330 in the Small Magellanic Cloud in order to search for possible imprints of stellar evolution on the multiplicity properties.
Methods: We obtained six epochs of VLT/MUSE observations operated in wide-field mode with the extended wavelength setup and supported by adaptive optics. We extracted spectra and measured radial velocities for stars brighter than mF814W = 19. We identified single-lined spectroscopic binaries through significant RV variability with a peak-to-peak amplitude larger than 20 km s−1. We also identified double-lined spectroscopic binaries, and quantified the observational biases for binary detection. In particular, we took into account that binary systems with similar line strengths are difficult to detect in our data set.
Results: The observed spectroscopic binary fraction among stars brighter than mF814W = 19 (approximately 5.5 M on the main sequence) is fSBobs = 13.2 ± 2.0%. Considering period and mass ratio ranges from log(P) = 0.15−3.5 (about 1.4 to 3160 d), q = 0.1−1.0, and a representative set of orbital parameter distributions, we find a bias-corrected close binary fraction of fcl = 34−7+8%. This fraction seems to decline for the fainter stars, which indicates either that the close binary fraction drops in the B-type domain, or that the period distribution becomes more heavily weighted toward longer orbital periods. We further find that both fractions vary strongly in different regions of the color-magnitude diagram, which corresponds to different evolutionary stages. This probably reveals the imprint of the binary history of different groups of stars. In particular, we find that the observed spectroscopic binary fraction of Be stars (fSBobs = 2 ± 2%) is significantly lower than that of B-type stars (fSBobs = 9 ± 2%).
Conclusions: We provide the first homogeneous radial velocity study of a large sample of B-type stars at a low metallicity ([Fe/H] ≲ −1.0). The overall bias-corrected close binary fraction (log(P) < 3.5 d) of the B-star population in NGC 330 is lower than the fraction reported for younger Galactic and Large Magellanic Cloud clusters in previous works. More data are needed, however, to establish whether the observed differences are caused by an age or a metallicity effect.

Keywords: stars: massive;stars: emission-line;Be;binaries: spectroscopic;blue stragglers;open clusters and associations: individual: NGC 330;Magellanic Clouds;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – Astrophysics of Galaxies

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