The 2011 outburst of the recurrent nova T Pyxidis. Evidence for a face-on bipolar ejection

Aims: T Pyx is the first recurrent nova ever historically studied. It was seen in outburst six times between 1890 and 1966 and then not for 45 years. We report on near-IR interferometric observations of the recent outburst of 2011.
Methods: We obtained near-IR observations of T Pyx at dates ranging from t = 2.37 d to t = 48.2 d after the outburst, with the CLASSIC recombiner located at the CHARA array and with the PIONIER and AMBER recombiners located at the VLTI array. These data are supplemented with near-IR photometry and spectra obtained at Mount Abu, India. We compare expansion of the H and K band continua and the Brγ emission line, and infer information on the kinematics and morphology of the early ejecta.
Results: Slow expansion velocities were measured (≤300 km s^-1) before t = 20 d. From t = 28 d on, the AMBER and PIONIER continuum visibilities (K and H band, respectively) are best simulated with a two-component model consisting of an unresolved source plus an extended source whose expansion velocity onto the sky plane is lower than ~700 km s^-1. The expansion of the Brγ line-forming region, as inferred at t = 28 d and t = 35 d, is slightly larger, implying velocities in the range 500-800 km s^-1, which is still strikingly lower than the velocities of 1300-1600 km s^-1 inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brγ differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an inclination of i = 15°, and a position angle PA = 110° provides a good match to the AMBER observables. At t = 48 d, a PIONIER dataset confirms the two-component nature of the H band emission, consisting of an unresolved stellar source and an extended region whose appearance is circular and symmetric within error bars.
Conclusions: These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This finding has profound implications for interpreting past, current, and future observations of the expanding nebula.