Spectral properties and detectability of supermassive stars in protoglobular clusters at high redshift

Abstract

Context. Globular clusters (GCs) contain multiple stellar populations with peculiar chemical compositions. Pollution of the intracluster gas by an early population of fast-evolving stars is the most common scenario for explaining the observations. Stars with masses in excess of 1000 M have recently been suggested as potential polluters. Aims. We investigate the spectral properties of proto-GCs that would host a supermassive star (SMS). Our main goal is to quantify how such a star would affect the integrated light of the cluster, and to study the detectability of such objects. Methods. We computed nonlocal thermal equilibirum atmosphere models for SMS with various combinations of stellar parameters (luminosity, effective temperature, and mass) and metallicities appropriate for GCs, and we predict their emergent spectra. Using these spectra, we calculated the total emission of young proto-GCs with SMS as predicted in a previously reported scenario, and we computed synthetic photometry in UV, optical, and near-IR bands, in particular for the James Webb Space Telescope (JWST).Results. At an effective temperature of 10000 K, the spectrum of SMSs shows a Balmer break in emission. This feature is due to strong nonlocal thermal equilibrium effects (implied by the high luminosity) and is not observed in "normal" stars. The hydrogen lines also show a peculiar behavior, with Balmer lines in emission while higher series lines are in absorption. At 7000 K, the Balmer break shows a strong absorption. At high effective temperatures, the Lyman break is found in emission. Cool and luminous SMSs are found to dominate the integrated spectrum of the cluster, except for the UV range. The predicted magnitudes of these proto-GCs are mag AB ∼28-30 between 0.7 and 8 µm and for redshifts z ∼ 4 − 10, which is detectable with the JWST. The peculiar observational features of cool SMSs imply that they might in principle be detected in color-color diagrams that probe the spectral energy distribution below and above the Balmer break. Conclusions.Our results show that SMSs should be detectable in proto-GCs if they are luminous and relatively cool. They may be found through deep imaging with the JWST.

Formation of supermassive black hole seeds in nuclear star clusters via gas accretion and runaway collisions

“…Even though current Euclid and WFIRST detection limits (26 and 28, respectively) are well below the H band magnitudes of both stars at 𝑧 ∼ 6 − 20, both of them would be able to detect blue SMSs as even a slight amount of gravitational lensing will boost the fluxes of these blue SMSs above the detection limits of these two missions. Martins et al (2020) found some important spectral features in the observational properties of SMSs with mass range ∼ 10 3 − 5 × 10 4 M . They have computed the spectra of SMSs for non-local thermal equilibrium spherical stellar atmosphere models.…”

“…Upcoming next generation telescopes e.g. James Webb Space Telescope (JWST), Euclid and WFIRST will be able to detect SMSs at 𝑧 ∼ 6 − 20 (Surace et al 2018(Surace et al , 2019Woods et al 2020;Martins et al 2020). The number of SMSs per unit redshift per unit solid angle, which are expected to be detected at a redshift 𝑧, is given by:…”

Star Clusters Near and Far

“…On the theoretical side, evolutionary synthesis models are developed to predict the characteristics of proto-GCs in the early Universe (Renzini 2017;Pozzetti et al 2019). In this context, Martins et al (2020) developed the first synthetic models of proto-GCs hosting multiple stellar populations and a SMS. They compute theoretical combined spectra and synthetic photometry in UV, optical, and near-IR bands over a wide range of redshift (1 to 10), and predict the detectability of cool SMS in proto-GCs through deep imaging with JWST NIR-CAM camera.…”

The Physics of Star Cluster Formation and Evolution

“…It may come as a surprise that SMS would be difficult to find observationally. Martins et al (2020) predicted the detectability of cool SMS in proto-GCs at high redshift through deep imaging with JWST NIRCAM camera. One problem at low redshift however is that clusters that would be massive enough are not found in the Milky Way.…”