A dearth of young and bright massive stars in the Small Magellanic Cloud

Abstract

Context. Massive star evolution at low metallicity is closely connected to many fields in high-redshift astrophysics, but is poorly understood so far. Because of its metallicity of ∼0.2 Z⊙, its proximity, and because it is currently forming stars, the Small Magellanic Cloud (SMC) is a unique laboratory in which to study metal-poor massive stars. Aims. We seek to improve the understanding of this topic using available SMC data and a comparison to stellar evolution predictions. Methods. We used a recent catalog of spectral types in combination with Gaia magnitudes to calculate temperatures and luminosities of bright SMC stars. By comparing these with literature studies, we tested the validity of our method, and using Gaia data, we estimated the completeness of stars in the catalog as a function of luminosity. This allowed us to obtain a nearly complete view of the most luminous stars in the SMC. We also calculated the extinction distribution, the ionizing photon production rate, and the star formation rate. Results. Our results imply that the SMS hosts only ∼30 very luminous main-sequence stars (M ≥ 40 M⊙; L ≳ 3 ⋅ 105 L⊙), which are far fewer than expected from the number of stars in the luminosity range 3 ⋅ 104 < L/L⊙ < 3 ⋅ 105 and from the typically quoted star formation rate in the SMC. Even more striking, we find that for masses above M ≳ 20 M⊙, stars in the first half of their hydrogen-burning phase are almost absent. This mirrors a qualitatively similar peculiarity that is known for the Milky Way and Large Magellanic Cloud. This amounts to a lack of hydrogen-burning counterparts of helium-burning stars, which is more pronounced for higher luminosities. We derived the H I ionizing photon production rate of the current massive star population. It agrees with the H α luminosity of the SMC. Conclusions. We argue that a declining star formation rate or a steep initial mass function are unlikely to be the sole explanations for the dearth of young bright stars. Instead, many of these stars might be embedded in their birth clouds, although observational evidence for this is weak. We discuss implications for the role that massive stars played in cosmic reionization, and for the top end of the initial mass function.

A uniform stellar origin for binary black holes revealed by lensing

“…It should also be appreciated that the relative fluxes of close, highly magnified images of lensed QSO's and SNe [42], typically differ by a factor of 2 in brightness, implying substructure in lensing galaxies is common on sub-arcsecond scales [43,44], in which case the weaker counter image will typically fall below the GW detection threshold, as most GW events are near the detection limit, SNR 8, with few exceeding a level of SNR > 16 required for detection of the weaker event to overcome a factor 2 flux anomaly, with credible examples proposed [40]. Irrespective of lensing, the viability of the high masses inferred for BBH events above 50M is challenged theoretically by the physical limit from pair instability [39] and also empirically there is a claimed deficit of high mass stars > 40M that are metal poor [37] in the SMC and similar shortages in the LMC and Milky Way, which if general would disfavour larger stellar progenitors required at low redshift to account for the reported high mass of BBH events. Instead, our conclusions appear qualitatively aligned with simulations of high mass star formation, made well before the GW detections, that predict a well defined process of fragmentation and accretion for generating close multiples of higher mass stars [38], qualitatively supporting the uniformity of the observed BBH component masses and our lensing based conclusion that most BBH events comprise pairs of conventional stellar mass black holes formed at early times.…”

ULLYSES and Complementary Surveys of Massive Stars in the Magellanic Clouds

“…The scarcity of SMC early O-type (super)giants in Fig. 2 arises from its modest star formation rate (see also Schootemeijer et al 2021).…”

Red supergiant stars in binary systems. I. Identification and characterisation in the Small Magellanic Cloud from the UVIT ultraviolet imaging survey

“…For our targets, this results in an average extinction value of 𝐴 V = 0.17 mag 3 with a typical dispersion of 0.15. We chose to retain the extinction value of 𝐴 V = 0.35 mag (Schootemeijer et al 2021), rather than use the extinction values determined by Skowron et al (2021), as the extinction values determined from red clump stars are more appropriate for lower mass stars. However, we argue that the dispersion determined from these extinction maps is an accurate indicator of the local spread of extinction values for each target, given that the SMC contains little intrinsic dispersion.…”

“…where 𝜇 is the SMC distance modulus, namely 𝜇 = 18.95 (Graczyk et al 2014, as for the RSG companions), and 𝐴 𝐹 172𝑀 is the extinction in the UVIT F172M filter, which is defined as 𝐴 𝐹 172𝑀 = 𝐴 V × 4.013 for the SMC bar (Gordon et al 2003), where 𝐴 V = 0.35 mag (Schootemeijer et al 2021). While a tailored approach for each star may lead to more precise estimates of the ambient extinction, for example, taking account of detailed extinction maps or estimates of SMC and MW contributions to the total extinction, we estimate that the uncertainties introduced by neglecting such effects are well within our adopted uncertainties of ±0.14 dex in log 𝐿/𝐿 .…”

“…With increasing companion mass, the age assumption becomes more important and, as a result, the most massive companions in the sample are those that are potentially most affected by uncertainties in the RSG age. The choice of the extinction for the SMC is guided by the results of Schootemeijer et al (2021), see Section 3.1. A fixed extinction value for all of our sources neglects the effects of variable and circumbinary extinction.…”