A Bipolar Outflow of the M-Type Giant IRC -10414 Traced by H2O Maser Emission

We report on results of five-epoch VLBI observations of H₂O maser emission in the M-type star IRC -10414, carried out with the VLBI Exploration of Radio Astrometry (VERA). The maser distribution extends over an area of 70mas×260mas and exhibits a bipolar structure. The relative proper motions of 17 maser features were measured during the monitoring observations spanning nine months. The distance to IRC -10414 was re-estimated on the basis of the statistical treatment of parallax and the model-fitting method, yielding D = 3.1±0.4kpc and D = 2.0±0.2kpc, respectively. The estimated distance, D = 2-3kpc, is much larger than that previously adopted. The stellar luminosity of IRC -10414 is also re-estimated to be L_* ∼9 × 10⁴ (D/2kpc) ²L_odot, much brighter by a factor of 10-20 than previously adopted (L_* ∼ 10000L_odot). The maser motions exhibit not only a spherically expanding flow with a velocity of ∼10 (D/2kpc) km s^-1, but also a faster bipolar outflow with a major axis in the north-south direction and at a small inclination angle with respect to the celestial sphere. These characteristics of the star and the circumstellar envelope seen in IRC -10414 are very similar to those in some supergiants exhibiting bipolar stellar mass loss.