Wolf-Rayet Stars in M33: The Impact of Stellar Crowding on Spectral Classification and Mass-Loss Diagnostics
Abstract
The metallicity dependence of Wolf-Rayet (WR) mass-loss rates is a critical input for massive star evolutionary models, chemical yield predictions, and ionizing radiation budgets of young stellar populations. The Triangulum Galaxy (M33), with 213 spectroscopically confirmed WR stars spanning a ∼0.5 dex radial metallicity gradient, is the premier laboratory for this calibration. However, the ground-based fiber spectra on which the current census rests blend each target with neighboring OB stars, diluting diagnostic emission-line equivalent widths and biasing spectral classifications, inferred mass-loss rates, and surface hydrogen abundances. Using 0farcs1-resolution HST/PHATTER survey imaging, we quantify this contamination for 123 WR stars within the survey footprint, finding that ∼27% suffer moderate-to-severe dilution from unresolved neighbors with WR continuum flux contributions FWR < 0.85. We provide crowding classifications, FWR fractions, and six-band photometry in the F275W, F336W, F475W, F814W, F110W, and F160W bands for each identified counterpart. We also present Keck/DEIMOS slit spectra of five WR stars, which are substantially less contaminated than the discovery MMT/Hectospec fiber spectra. A direct equivalent-width comparison of these targets confirms the predicted dilution. The slit spectra yield blue-edge velocities of He I absorption (vblue ≈ 700--$1500$ km s−1), comparable to WR stars at similar metallicity in the LMC. The FWR correction factors, together with the slit-spectrum benchmarks, provide the inputs needed to place M33 on the Ṁ--Z relation as a calibration anchor between the LMC and the Galaxy.
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BibTeX
@misc{raj2026wolfrayet,
title = {{Wolf-Rayet Stars in M33: The Impact of Stellar Crowding on Spectral Classification and Mass-Loss Diagnostics}},
author = {Rohit Raj and Puragra Guhathakurta and Robert E. Williams and Benjamin F. Williams and Julianne J. Dalcanton and Meredith J. Durbin},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.33163},
eprint = {2610.33163},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.33163},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Wolf-Rayet Stars in M33: The Impact of Stellar Crowding on Spectral Classification and Mass-Loss Diagnostics
AU - Rohit Raj
AU - Puragra Guhathakurta
AU - Robert E. Williams
AU - Benjamin F. Williams
AU - Julianne J. Dalcanton
AU - Meredith J. Durbin
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.33163
UR - https://arxiv.org/abs/2610.33163
AB - The metallicity dependence of Wolf-Rayet (WR) mass-loss rates is a critical input for massive star evolutionary models, chemical yield predictions, and ionizing radiation budgets of young stellar populations. The Triangulum Galaxy (M33), with 213 spectroscopically confirmed WR stars spanning a ∼0.5 dex radial metallicity gradient, is the premier laboratory for this calibration. However, the ground-based fiber spectra on which the current census rests blend each target with neighboring OB stars, diluting diagnostic emission-line equivalent widths and biasing spectral classifications, inferred mass-loss rates, and surface hydrogen abundances. Using 0farcs1-resolution HST/PHATTER survey imaging, we quantify this contamination for 123 WR stars within the survey footprint, finding that ∼27% suffer moderate-to-severe dilution from unresolved neighbors with WR continuum flux contributions F_WR < 0.85. We provide crowding classifications, F_WR fractions, and six-band photometry in the F275W, F336W, F475W, F814W, F110W, and F160W bands for each identified counterpart. We also present Keck/DEIMOS slit spectra of five WR stars, which are substantially less contaminated than the discovery MMT/Hectospec fiber spectra. A direct equivalent-width comparison of these targets confirms the predicted dilution. The slit spectra yield blue-edge velocities of He I absorption (v_blue ≈ 700--$1500$ km s^−1), comparable to WR stars at similar metallicity in the LMC. The F_WR correction factors, together with the slit-spectrum benchmarks, provide the inputs needed to place M33 on the Ṁ--Z relation as a calibration anchor between the LMC and the Galaxy.
ER -