We present mid-infrared Spitzer -IRS The Spitzer Space Telescope is operated by the Jet Propulsion Laboratory , California Institute of Technology under NASA contract 1407 .
Support for this work was partly provided by NASA through an award issued by JPL/Caltech .
PAC acknowledges financial support from the Royal Society .
The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center . spectroscopy of the prototype WN star EZ CMa ( HD 50896 , WN4b ) .
Numerous stellar wind lines of He ii are revealed , plus fine-structure lines of [ Ne iii ] 15.5 \mu m and [ O iv ] 25.9 \mu m. We carry out a spectroscopic analysis of HD 50896 allowing for line blanketing and clumping , which is compared to the mid-IR observations .
We make use of these stellar properties to accurately derive Ne/He=1.2–1.8 \times 10 ^ { -4 } and O/He=4.2–4.8 \times 10 ^ { -5 } by number , for the first time in an early WN star .
In addition , we obtain N/C \sim 40 and N/O \sim 50 by number , values in perfect agreement with current predictions for rotating massive stars at the end of interior hydrogen burning .