We study the conditions under which a main sequence binary companion to the central ionizing star of a planetary nebula (PN) might become magnetically active and thereby display strong X-ray luminosity, Lx~ > 5×10^29 erg s^−1. Since most PNe are older than few billion years, any main sequence companion will rotate too slowly to have magnetic activity and hence bright X-ray emission, unless it is spun-up. We demonstrate that if the orbital separation during the AGB phase of the PN progenitor is a~< 30 − 60 AU, main sequence companions in the spectral type range F7 to M4 (mass range 0.3M⊙~< M2 ~< 1.3M⊙) will accrete enough angular momentum from the AGB wind to rotate rapidly, become magnetically active, and exhibit X-ray luminosities Lx< 5×10^29 erg s^−1. Lower mass M stars and brown dwarfs can also become magnetically active, but they should have small orbital separations and hence are less likely to survive the AGB phase of the progenitor. For orbital separation of a~< 0.3 AU, i.e., for a binary systems that went through a common envelope phase, the fast wind from the central WD star will interact with (and potentially disrupt) the companion’s corona on the side facing the central star, while for a~< 6R⊙, i.e., an orbital period of Porb ~< 30 hours, the WD’s fast wind will compress a dense small region near the surface of the companion. This region may thermally emit X-rays with nonnegligible luminosity. We estimate that 20 − 30% of elliptical PNe and 30 − 50% of bipolar PNe are likely to have magnetically active companions which will reveal themselves in X-ray observations. Re-analysis of Chandra X-ray Observatory spectroscopy of the compact central source of NGC 7293 indicates that the emitting region of this object possesses abundance anomalies similar to those of coronally active main-sequence stars. High-resolution X-ray spectroscopy of this and other compact sources in PNe are necessary to confirm a coronal origin for the X-ray emission (Refer to PDF file for exact formulas).
Department, Program, or Center
Chester F. Carlson Center for Imaging Science (COS)
Astrophys.J. 570 (2002) 245-251
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