Single Rotating Stars and the Formation of Bipolar Planetary Nebula

TL;DR

This study models rotating stars to determine if single stars can produce the high envelope rotation speeds needed to form bipolar planetary nebulae, concluding they likely cannot under current models.

Contribution

The paper provides new stellar evolution models including rotation and magnetic effects, testing their ability to produce necessary envelope rotation for bipolar PNe formation.

Findings

Single stars cannot reach required envelope rotation speeds for bipolar PNe formation.

Magnetic braking reduces core rotation, preventing high envelope spin.

Single stellar rotators are unlikely progenitors of bipolar PNe.

Abstract

We have computed new stellar evolution models that include the effects of rotation and magnetic torques under different hypothesis. The goal is to test if a single star can sustain in the envelope the rotational velocities needed for the magneto hydrodynamical (MHD) simulations to shape bipolar Planetary Nebulae (PNe) when the high mass-loss rates take place. Stellar evolution models with main sequence masses of 2.5 and 5 Mo, and initial rotational velocities of 250 km/s have been followed all the way to the PNe formation phase. We find that stellar cores have to be spun down using magnetic torques in order to reproduce the rotation rates observed for white dwarfs. During the asymptotic giant branch phase and beyond, the magnetic braking of the core has a practically null effect in increasing the rotational velocity of the envelope since the stellar angular momentum is removed efficiently by the wind. We have, as well, tested best possible case scenarios in rather non-physical contexts to give enough angular momentum to the envelope. We find that we cannot get the envelope of a single star rotating at the speeds needed by the MHD simulations to form bipolar PNe. We conclude that single stellar rotators are unlikely to be the progenitors of bipolar PNe under the current MHD model paradigm.