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TT Ari continues its curious activity

Posted by Matthew Templeton on October 21, 2010 - 6:06pm

Observations of TT Ari seem to be showing a partial recovery from its deep minimum of 2008-2009, but this star still hasn't quite reached its normal high state yet.  Researchers also still haven't figured out why it's doing what it's doing, but your observations are going to help solve the puzzle.

TT Ari is a novalike variable that has been under regular observations by AAVSO observers since the 1970s.  For most of the observational history, the star has remained in its high state -- bright at around V=10.5, and with a high mass accretion rate -- but on at least two occasions the star's brightness has plummeted to below V=16.0 for more than a year.  My suspicion is that the fadings are caused by the mass accretion stopping somehow, but why that would happen isn't clear.

Mass transfer occurs when two stars come closer together than their Roche limit.  The Roche limit looks like a three dimensional figure-eight, and is an imaginary surface defined by the point at which a third body would feel an equal gravitational pull from both stars.  If the secondary expands to fill this surface, then matter outside the surface will become detached from the secondary and flow onto the white dwarf.  This is how all cataclysmic variables work.  But then the question becomes why would mass transfer stop?  Is the secondary star changing in size, shrinking below the Roche limit temporarily?  If so, why?

Whether my suspicion is right or wrong, the answers are going to come in part from the observations of AAVSO observers, so please continue to observe this star!  The light curve of this current event has been wonderful, clearly uncovering for the first time the large-amplitude flares that occur during the extended minimum (whose cause is also as yet unproven).

And beyond the great scientific interest, it's really a fun star to follow.

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484