Spectral Energy Distribution Analysis for Habitable Worlds Observatory Target Stars

Abstract

The Habitable Worlds Observatory (HWO) will directly image and characterize planets potentially similar to Earth orbiting nearby stars. Accurate stellar properties, particularly effective temperatures ($T_\mathrm{eff}$), angular diameters ($θ_\star$), and bolometric fluxes ($F_\mathrm{bol}$), are essential for reliable Habitable Zone (HZ) calculations, exoplanet yield predictions, and coronagraph design trade studies. We present a spectral energy distribution (SED) analysis for the 164 stars from the HWO Exoplanet Exploration Program (ExEP) list, using $χ^2$-minimization fits of empirical spectral templates from the Pickles stellar spectral flux library to broadband photometric data. The SED fits provide direct measurements of $F_\mathrm{bol}$. We adopt spectroscopic $T_\mathrm{eff}$ values from the PASTEL catalog for 127 of the 164 stars, with the remainder drawn from the HWO ExEP catalog, and use these in combination with $F_\mathrm{bol}$ to calculate $θ_\star$ via the Stefan-Boltzmann equation. As a consistency check, we compare the Pickles template $T_\mathrm{eff}$ with the adopted spectroscopic values and find agreement to within $\sim$3% for 65% of the sample, with a systematic tendency for the templates to yield cooler values. We identify 48 stars with discrepancies exceeding 200~K, including close binary systems where photometric contamination compromises the SED solution. We discuss the implications for HZ boundary calculations, atmospheric retrieval of directly imaged planets, and target prioritization for HWO. This SED catalog constitutes a uniform set of empirically determined $F_\mathrm{bol}$ values and associated stellar parameters for the HWO target sample.