Some First Stars Were Red: Detecting Signatures of Massive Population III Formation through Long-term Stochastic Color Variations

Summary

Abstract

Woods and collaborators study whether unusually massive Population III stars could be identified through changes in their color. Simulations show that some rapidly accreting stars expand and temporarily cool to about 5,000 K, shifting as much as 20 percent of their emission into optical wavelengths. These irregular periods of reddening may allow the James Webb Space Telescope to identify massive Population III populations at redshifts of roughly 10–13 using NIRCam colors.

1. Introduction

• Population III stars are normally expected to be very hot, blue, and difficult to observe individually.
• Delaying star formation until a halo becomes more massive can produce a larger and brighter population of primordial stars.
• Rapid accretion can inflate some stars, making them cooler and redder than the usual picture of Population III stars.

2.1 Halo Models

• Halo A experiences delayed star formation because of moderate Lyman-Werner radiation and heating caused by rapid halo growth.
• Halo B is a more typical Population III minihalo and serves as the comparison model.

2.2 Massive-Star Formation

• Halo A forms about 70 times more stellar mass than Halo B, with some stars reaching several thousand solar masses.
• Stars that encounter dense gas undergo brief periods of rapid accretion, which expands their photospheres and lowers their temperatures.

2.3 Spectral Modeling

• Normally accreting stars are modeled as hot objects with temperatures near 100,000 K, while rapidly accreting stars are assigned temperatures near 5,000 K.
• The CLOUDY code is used to calculate how surrounding gas processes the stars’ radiation and changes the combined spectrum.

3.1 Synthetic Spectra

• Halo A becomes about 100 times more luminous than the smaller comparison halo as its stellar population grows.
• Rapid-accretion episodes move as much as 20 percent of the stellar emission from ultraviolet into optical wavelengths.
• During a strong episode, the optical continuum can increase by about a factor of five.

3.2 JWST Magnitudes and Colors

• With long exposures, JWST could detect a Halo A-like object out to a redshift of approximately 12.
• At redshifts around 10.5–13, rapidly accreting Population III stars appear about 0.1–0.3 magnitudes redder than typical simulated galaxies.
• Their NIRCam colors and Lyman-limit dropout could help separate them from lower-redshift objects.

4. Discussion

• These halos may be uncommon, so wide surveys could first identify promising overdense regions for deeper follow-up observations.
• Gravitational lensing would make the objects easier to detect and allow shorter observing times.
• Their actual abundance depends on how often radiation, mergers, and baryonic streaming delay early star formation.

5. Conclusion

• Rapid accretion can make some massive Population III stars temporarily red instead of consistently hot and blue.
• The resulting color changes may provide a way to find and study primordial stellar populations using JWST imaging.

Best Figures

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