The WFC3 Galactic Bulge Treasury Program: Metallicity Estimates for the Stellar Population and Exoplanet Hosts

TL;DR

This study uses HST photometry to analyze the metallicity distribution of the Galactic bulge, revealing a population of old, metal-rich stars and a correlation between high metallicity and exoplanet presence.

Contribution

It introduces reddening-free photometric indices for metallicity estimation in the bulge and provides new insights into the metallicity gradient and exoplanet host characteristics.

Findings

Bulge populations are old (~10 Gyr) with a wide metallicity range.

Detected a metallicity gradient with fewer super-solar metallicity stars farther from the Galactic center.

Exoplanets are more common around high-metallicity stars in the bulge, similar to the solar neighborhood.

Abstract

We present new UV-to-IR stellar photometry of four low-extinction windows in the Galactic bulge, obtained with the Wide Field Camera 3 on the Hubble Space Telescope (HST). Using our five bandpasses, we have defined reddening-free photometric indices sensitive to stellar effective temperature and metallicity. We find that the bulge populations resemble those formed via classical dissipative collapse: each field is dominated by an old (~10 Gyr) population exhibiting a wide metallicity range (-1.5 < [Fe/H] < 0.5). We detect a metallicity gradient in the bulge population, with the fraction of stars at super-solar metallicities dropping from 41% to 35% over distances from the Galactic center ranging from 0.3 to 1.2 kpc. One field includes candidate exoplanet hosts discovered in the SWEEPS HST transit survey. Our measurements for 11 of these hosts demonstrate that exoplanets in the distinct bulge environment are preferentially found around high-metallicity stars, as in the solar neighborhood, supporting the view that planets form more readily in metal-rich environments.