A new class of Roche lobe-filling hot subdwarf binaries

Thomas Kupfer, Evan B. Bauer, Kevin B. Burdge, Jan van Roestel, Eric C. Bellm, Jim Fuller, JJ Hermes, Thomas R. Marsh, Lars Bildsten, Shrinivas R. Kulkarni, E. S. Phinney, Thomas A. Prince, Paula Szkody, Yuhan Yao, Andreas Irrgang, Ulrich Heber, David Schneider, Vik S. Dhillon, Gabriel Murawski, Andrew J. Drake, Dmitry A. Duev, Michael Feeney, Matthew J. Graham, Russ R. Laher, S. P. Littlefair, A. A. Mahabal, Frank J. Masci, Michael Porter, Dan Reiley, Hector Rodriguez, Ben Rusholme, David L. Shupe, Maayane T. Soumagnac

July 2020

Abstract

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T{rm eff}=33,700pm1000 $K a n d a s u r f a c e g r a v i t y o f$ log(g)=5.54pm0.11 $. T h e G T C + H i P E R C A M l i g h t c u r v e i s d o m i n a t e d b y t h e e l l i p s o i d a l d e f o r m a t i o n o f t h e H e - s d O B s t a r a n d s h o w s a n e c l i p s e o f t h e H e - s d O B b y a n a c c r e t i o n d i s k a s w e l l a s a w e a k e c l i p s e o f t h e W D . W e i n f e r a H e - s d O B m a s s o f$ M{rm sdOB}=0.41pm0.04 $M$ odot $a n d a W D m a s s o f$ M{rm WD}=0.68pm0.05 $M$ odot $. T h e w e a k e c l i p s e s i m p l y a W D b l a c k - b o d y t e m p e r a t u r e o f$ 63,000pm10,000 $K a n d a r a d i u s$ R{rm WD}=0.0148pm0.0020 $M$ _odot $a s e x p e c t e d f o r a W D o f s u c h h i g h t e m p e r a t u r e . T h e H e - s d O B s t a r i s l i k e l y u n d e r g o i n g h y d r o g e n s h e l l b u r n i n g a n d w i l l c o n t i n u e t r a n s f e r r i n g m a s s f o r$ approx1 $M y r s a t a r a t e o f$ 10^{-9} Modot {rm yr}^{-1} $w h i c h i s c o n s i s t e n t w i t h t h e h i g h W D t e m p e r a t u r e . T h e h o t s u b d w a r f w i l l t h e n t u r n i n t o a W D a n d t h e s y s t e m w i l l m e r g e i n$ approx30 $M y r s . W e s u g g e s t t h a t G a l a c t i c r e d d e n i n g c o u l d b i a s d i s c o v e r i e s t o w a r d s p r e f e r e n t i a l l y f i n d i n g R o c h e l o b e - f i l l i n g s y s t e m s d u r i n g t h e s h o r t - l i v e d s h e l l b u r n i n g p h a s e . S t u d i e s u s i n g r e d d e n i n g c o r r e c t e d s a m p l e s s h o u l d r e v e a l a l a r g e p o p u l a t i o n o f h e l i u m c o r e - b u r n i n g h o t s u b d w a r f s w i t h$ T{rm eff}approx25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD.

Type

Journal article

http://arxiv.org/abs/2007.05349v1