* Astronomy

Title: PTF1 J082340.04+081936.5: A hot subdwarf B star with a low mass white dwarf companion in an 87 minute orbit
Author: Thomas Kupfer (1), Jan van Roestel (2), Jared Brooks (3), Stephan Geier (4), Tom R. Marsh (5), Paul J. Groot (2), Steven Bloemen (2), Thomas A. Prince (1), Eric Bellm (1), Ulrich Heber (6), Lars Bildsten (3,7), Adam A. Miller (8,9), Martin J. Dyer (10), Vik S. Dhillon (10,11), Matthew Green (5), Puji Irawati (12), Russ Laher (13), Stuart P. Littlefair (10), David L. Shupe (14), Charles C. Steidel (1), Somsawat Rattansoon (10,12), Max Pettini (15), ((1) California Institute of Technology, (2) Radboud University Nijmegen, (3) University of California, Santa Barbara, (4) University of Tuebingen, (5) University of Warwick, (6) University of Erlangen-Nuremberg, (7) Kavli Institute for Theoretical Physics, Santa Barbara, (8) Northwestern University, (9) The Adler Planetarium, (10) University of Sheffield, (11) Instituto de Astrofsica de Canarias, (12) National Astronomical Research Institute of Thailand, (13) Spitzer Science Center, Caltech, (14) Infrared Processing and Analysis Center, Caltech, (15) University of Cambridge)

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period P_orb=87.49668(1) min (0.060761584(10) days), making it the second-most compact sdB binary known. The lightcurve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass M_sdB = 0.45^{+0.09}_{-0.07} solar masses, a companion white dwarf mass M_WD = 0.46^{+0.12}_{-0.09} solar masses and a mass ratio q= (M_WD)/(M_sdB) =1.03^{+0.10}_{-0.08}. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of M_sdB=0.47 solar masses, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 min at the exit from the last common envelope phase. For longer common envelope exit periods the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived log(g) and T_eff with our MESA models, we find that an sdB model with a hydrogen envelope mass of 5 x 10^-4 solar masses matches the measurements at a post-common envelope age of 94 Myr, corresponding to a post-common envelope orbital period of 109 min which is close to the limit to start accretion while the sdB is still burning helium.

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