The Controversial Star-Formation History and Helium Enrichment of the Milky Way Bulge

TL;DR

The paper reviews conflicting evidence on the Milky Way bulge's star-formation history, highlighting discrepancies between observational data suggesting intermediate ages and models indicating predominantly old stars, with helium-enrichment proposed as a potential resolution.

Contribution

It critically examines various observational and theoretical studies, proposing helium-enrichment as a key factor to reconcile conflicting age estimates of the bulge stars.

Findings

Spectroscopic data suggest a significant intermediate-age population.

Photometric studies indicate mostly old stars with ages over 8 Gyr.

Helium-enrichment could resolve discrepancies in age estimates.

Abstract

The stellar population of the Milky Way bulge is thoroughly studied, with a plethora of measurements from virtually the full suite of instruments available to astronomers. It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history. Cosmological models predict the bulge to host the Galaxy's oldest stars for [Fe/H]$\lesssim -1$, and this is demonstrated by RR Lyrae stars and globular cluster observations. There is consensus that bulge stars with [Fe/H]$\lesssim0$ are older than $t \approx10$ Gyr. However, at super-solar metallicity, there is a substantial unresolved discrepancy. Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch, the abundances of asymptotic giant branch stars, the period distribution of Mira variables, the chemistry and central-star masses of planetary nebulae, all suggest a substantial intermediate-age population ($t \approx 3$ Gyr). This is in conflict with predictions from cosmologically-motivated chemical evolution models and photometric studies of the main-sequence turnoff region, which both suggest virtually no stars younger than $t \approx 8$ Gyr. A possible resolution to this conflict is enhanced helium-enrichment, as this would shift nearly all of the age estimates in the direction of decreasing discrepancy. Enhanced helium-enrichment is also arguably suggested by measurements of the red giant branch bump and the R-parameter.