Effects of rotation on the spectroscopic observables of massive stars

Context. Rotation is ubiquitous among massive stars and with rotation comes a deformation to the surface geometry. This, in turn leads to alterations in the distribution of parameters across the surface including surface gravity, temperature, and ionization balance of the surface elements. These 3D effects are often neglected when analyzing the spectra of rapidly rotating massive stars. Aims. We aim to determine whether neglecting the 3D deformations resulting from rapid rotation has an impact on the final spectroscopic observables, and if so to what degree. Methods. Using the SPAMMS code, we generated a grid of synthetic spectra that account for the 3D geometry of rapidly rotating stars and compared them to synthetic spectra generated assuming spherical geometry. Using equivalent width (EW) and full width half maximum (FWHM) measurements as proxies, we determined how the measured temperature, helium abundance, and projected rotation rates of individual lines in different ionization states vary with rotation rates and inclinations. Results. We find that the 3D geometry can have a significant impact on the measured parameters. We show that the temperature is highly dependent on both the rotation rate and the inclination, and that the same system viewed at different inclinations can have measured temperatures that differ by as much as 10%. We also find that the helium abundance can be underestimated by as much as 60% and that lines in different ionization states can have measurable differences in rotation rates. We demonstrate that these differences in rotation rates can be seen in observed data and show that this could allow for an inclination-independent measurement of the rotational velocity. Conclusions. Our results indicate that neglecting the 3D effects of rotation can cause significant biases in the measured spectroscopic parameters and that in many cases, the measured values are more than 3 sigma away from the true values.