Binary system consisting of a white dwarf and a main sequence star or a brown dwarf
HD 101584 is a suspected post-common envelope binary. The engulfed companion triggered an outflow of gas, creating the nebula seen byALMA.Key stages in a common envelope phase. Top: A star fills itsRoche lobe. Middle: The companion is engulfed; the core and companion spiral towards one another inside acommon envelope. Bottom: The envelope is ejected and forms a PCEB or the two starsmerge.
Apost-common-envelope binary (PCEB) orpre-cataclysmic variable is abinary system consisting of awhite dwarf orhot subdwarf and amain-sequence star or abrown dwarf.[1] The star or brown dwarf shared acommon envelope with the white dwarf progenitor in thered-giant phase. In this scenario, the star or brown dwarf losesangular momentum as it orbits within the envelope, eventually leaving a main-sequence star and white dwarf in a short-period orbit. A PCEB will continue to lose angular momentum viamagnetic braking andgravitational waves and will eventually begin mass transfer, resulting in acataclysmic variable. While there are thousands of PCEBs known, there are only a feweclipsing PCEBs, also called ePCEBs.[2] Even more rare are PCEBs with a brown dwarf as the secondary.[1] A brown dwarf with a mass lower than 20MJ mightevaporate during the common-envelope phase, so the secondary is supposed to have a mass higher than 20MJ.[3]
The material ejected from the common envelope forms aplanetary nebula. One in five planetary nebulae are thought to be ejected from common envelopes, but this might be an underestimate. A planetary nebula formed by a common-envelope system usually shows a bipolar structure.[4]
The suspected PCEBHD 101584 is surrounded by a complex nebula. During the common-envelope phase, the red-giant phase of the primary was terminated prematurely, avoiding a stellar merger. The remaining hydrogen envelope of HD 101584 was ejected during the interaction between the red giant and the companion, and it now forms the circumstellar medium around the binary.[5]
Manyeclipsing post-common-envelope binaries show variations in the timing of eclipses, the cause of which is uncertain. While orbitingexoplanets are often proposed as the causes of these variations, planetary models often fail to predict subsequent changes in eclipse timing. Other proposed causes, such as theApplegate mechanism, often cannot fully explain the observed eclipse timing variations either.[6]
^Casewell, S. L.; Burleigh, M. R.; Wynn, G. A.; Alexander, R. D.; Napiwotzki, R.; Lawrie, K. A.; Dobbie, P. D.; Jameson, R. F.; Hodgkin, S. T. (November 2012). "WD0837+185: The Formation and Evolution of an Extreme Mass-ratio White-dwarf-Brown-dwarf Binary in Praesepe".The Astrophysical Journal.759 (2): L34.arXiv:1210.0446.Bibcode:2012ApJ...759L..34C.doi:10.1088/2041-8205/759/2/L34.ISSN0004-637X.S2CID53545021.
