Predicted Separation of Regulus and Companion During the Occultation by Erigone

ATel #5917; D. R. Gies (Georgia State Univ.), E. Kambe (Okayama Ap. Obs.), R. Chini (Ruhr-Univ. Bochum)
on 24 Feb 2014; 22:07 UT
Credential Certification: Douglas R. Gies (gies@chara.gsu.edu)

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The bright star Regulus (alpha Leo) will be occulted by the asteroid Erigone for observers in the New York and Ontario area on the night of 2014 Mar 20 (http://occultations.org/regulus2014/). This may be the first opportunity to detect the flux of the suspected white dwarf companion of Regulus (Gies et al. 2008, ApJ, 682, L117). Here we present the predicted separation and position angle of the companion at the time of the occultation. We collected high dispersion spectroscopy of Regulus over 17 nights between 2009 and 2013 at Okayama Observatory (Kambe et al. 2013, PASJ, 65, 15) and over three nights between 2010 and 2013 at Cerro Armazones Observatory (Chini et al. 2012, MNRAS, 424, 1925). We measured radial velocities from these spectra and combined them with previous measurements from Gies et al. (2008) to produce a revised orbital solution with P = 40.112 +/- 0.012 d and T(epoch of maximum velocity) = HJD 2,452,348.18 +/- 0.24. The orbital phase at the time of the occultation (HJD 2,456,736.755) will then be 0.408 +/- 0.031. The orbital plane should be coincident with the star's equatorial plane that appears at an inclination of 90 deg (Che et al. 2011, ApJ, 732, 68), so that the projected separation is equal to a cos(2 pi phi), where a is the angular semimajor axis and phi is orbital phase. From the estimated stellar masses (Regulus mass = 4.15, Che et a. 2011; white dwarf mass = 0.32, Gies et al. 2008) and parallax (41.13 mas, Van Leeuwen 2007, A&A, 474, 653), we estimate a = 15.54 +/- 0.15 mas. Thus, the projected separation at the time of the occultation is estimated to be 13.0 +/- 1.7 mas at a position angle above the stellar equator at 168 or 348 deg east from north (Che et al. 2011). An observer at the center of the occultation track may observe the occultation of the white dwarf 1.7 seconds after that of Regulus or the reappearance of the white dwarf 1.7 seconds before that of Regulus (depending on the actual position angle of the white dwarf), while an observer near one side of the track may see the flux of white dwarf for the full duration of the Regulus occultation. Although the companion is probably faint, it should be significantly brighter than the asteroid Erigone (V = 12.4), so we encourage high speed photometric observations of this occultation to detect the flux of the white dwarf companion.