Publications

Refereed Journal Articles

1. Taylor, A. R., Koskinen, T., Huang, C., Arfaux, A., & Lavvas, P. 2026, ApJ, 999, 214, "Helium Escape in Context: Comparative Signatures of Four Close-in Exoplanets"
   doi:10.3847/1538-4357/ae41b5
   ▶ Show abstract
   Observations of escaping atmospheres on close-in exoplanets show a wide range in the strength and morphology of He I 10830 Å and H I absorption. We employ a 1D hydrodynamic modeling framework to investigate four systems — HD 209458b, HD 189733b, HD 149026b, and GJ 1214b — and examine how stellar activity, diffusive separation, eddy diffusion, metallicity, and molecular chemistry shape their escape signatures. For HD 209458b, our model reproduces the observed helium and hydrogen transit depths without invoking subsolar helium abundances, as diffusive separation naturally accounts for the modest absorption. HD 189733b requires approximately 12 km s⁻¹ of nonthermal broadening to match observed line widths despite comparable helium depths. The high surface gravity of HD 149026b suppresses escape and enhances diffusive separation, yielding extremely weak He I absorption consistent with its nondetection. For GJ 1214b, including molecular hydrogen substantially reduces predicted helium signatures relative to H/He-only models. Together, these results demonstrate that interpreting He I and Hα absorption requires first-principles models that include self-consistent temperature and velocity profiles, multispecies transport, and molecular chemistry.
2. Taylor, A. R., Koskinen, T., Argenti, L., Lewis, N., Huang, C., Arfaux, A., & Lavvas, P. 2025, ApJ, "A Multi-Species Atmospheric Escape Model with Excited Hydrogen and Helium: Application to HD 209458b"
   doi:10.3847/1538-4357/ade3c9
   ▶ Show abstract
   We present a comprehensive multispecies atmospheric escape model for HD 209458b that incorporates excited hydrogen and metastable helium. Our coupled models span from the lower atmosphere through the exosphere and include updated excitation rates and self-consistent treatment of diffusive separation. The model reproduces the observed He I 10830 Å and Hα absorption features, finding that strong diffusive separation is required to explain the relatively weak helium signal from this archetypal hot Jupiter. A photoelectron heating efficiency of 20%–40% corresponds to mass-loss rates of 1.9–3 × 10¹⁰ g s⁻¹. We find that helium absorption is sensitive to both stellar activity and diffusion, while Hα absorption is relatively insensitive owing to its dependence on Lyα excitation. These results suggest that multiple competing processes — beyond simple XUV flux scaling — control He I 10830 Å transit depths, and that combined observations of helium, Hα, and stellar activity indicators provide powerful constraints on upper-atmosphere dynamics and composition.
3. Taylor, A. R., Dunn, A., Peacock, S., Youngblood, A., & Redfield, S. 2024, ApJ, 964, 80, "Correlating Intrinsic Stellar Parameters with Mg II Self-reversal Depths"
   doi:10.3847/1538-4357/ad22da
   ▶ Show abstract
   The Mg II h&k emission lines are powerful diagnostics of stellar chromospheres and transition regions. Their cores commonly exhibit self-reversals — intensity dips arising from non-LTE effects — whose depths vary across stars of different types and activity levels. We examine a sample of 135 FGKM main-sequence stars with high-resolution near-ultraviolet spectra from the Hubble Space Telescope, using the PHOENIX atmosphere code to derive self-consistent stellar parameters (effective temperature, surface gravity, metallicity, mass, radius, and age) guided by archival photometry. We model the observed Mg II k profiles to isolate intrinsic self-reversal depths from interstellar medium attenuation. We find weak but significant correlations between self-reversal depth and stellar age, rotation period, Mg II luminosity, temperature, and mass. All stars older than ~2 Gyr or rotating slower than ~10 days exhibit self-reversals, while younger, rapidly rotating stars — indicators of higher magnetic activity — are more likely to lack deep reversals. Cooler stars consistently display self-reversals regardless of age, suggesting that magnetic activity is the dominant factor shaping self-reversal morphology.