Abstract
Asynchronous polars (APs) are accreting white dwarfs (WDs) that have different WD and orbital angular velocities, unlike the rest of the known polars, which rotate synchronously (i.e., their WD and orbital angular velocities are the same). Past nova eruptions are the predicted cause of the asynchronicity, in part due to the fact that one of the APs, V1500 Cyg, was observed to undergo a nova eruption in 1975. We used the Southern African Large Telescope 10m class telescope and the MDM 2.4m Hiltner telescope to search for nova shells around three of the remaining four APs (V1432 Aql, BY Cam, and CD Ind) as well as one Intermediate Polar with a high asynchronicity (EX Hya). We found no evidence of nova shells in any of our images. We therefore cannot say that any of the systems besides V1500 Cyg had nova eruptions, but because not all post-nova systems have detectable shells, we also cannot exclude the possibility of a nova eruption occurring in any of these systems and knocking the rotation out of sync.
Highlights
Polars are accreting white dwarf (WD) binaries characterized by the presence of a strong magnetic field (∼ 7 − 230 MG; Ferrario et al 2015), which prevents the formation of an accretion disc and instead channels the accreted material directly on to the poles of the white dwarfs (WDs)
CD Ind was observed on 2013 June 28 for a total of 1560s across 13 exposures, and EX Hya on 2013 July 11 for 1800s spread over 15 different exposures. (There is effectively no guide camera that can be used with SALTICAM, the large number of short exposures.) we observed BY Cam in Hα using the OSMOS instrument on the MDM Observatory 2.4m Hiltner telescope
The ejected shell will expand until it dissipates into the circumstellar medium
Summary
Polars are accreting white dwarf (WD) binaries characterized by the presence of a strong magnetic field (∼ 7 − 230 MG; Ferrario et al 2015), which prevents the formation of an accretion disc and instead channels the accreted material directly on to the poles of the WD. EX Hya is an Intermediate Polar (IP), with a magnetic field of ∼1 MG and an accretion disc that has been observed to have dwarf nova outbursts, which has a very high asynchronicity (Hellier 1996), and was included in this study as well The cause of this asynchronicity is theorized to be a nova eruption: accreted hydrogen builds up on the surface of the WD until reaching a critical temperature/pressure and igniting a thermonuclear runaway in the accreted layer, the ejection of which causes the WD to spin with a higher angular velocity than before the nova (Campbell & Schwope 1999). V1432 Aql, is under-synchronous, with a negative ω/Ω and percent difference, which may indicate a different formation mechanism, at this point the details of the theory are poorly understood in general, and when it comes to explaining how to obtain an under-synchronous AP These systems do not remain asynchronous indefinitely; instead, they likely start returning to a synchronous state quickly after being knocked out of sync. Sahman et al (2015) searched for shells around just two of our targets, V1432 Aql and BY Cam, using the Auxiliary Port on the 4.2 m William Herschel Telescope on La Palma and did not find evidence for any shells
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