A Planet Orbiting the Star rho Coronae Borealis

AFOE Observations

High precision radial velocity measurements of the star rho CrB with the AFOE (Advanced Fiber Optic Echelle spectrometer) show evidence of a planet orbiting the star rho Coronae Borealis (rhoCrB).

plot of the radial velocity measurements

Top panel:
Radial velocity measurements, and their associated uncertainties, as a function of heliocentric Julian date. The dashed line represents the orbital fit. There are 65 AFOE data points included. The units are meters per second (1 m/s is 2.237 mph).
Bottom panel:
Residuals from the orbital fit as a function of heliocentric Julian date.
The radial velocity measurements and parameters of the orbital fit, as described in our 1997 Ap.J. Letter (483, L111) are available here.

Combinded AFOE and Lick Observations

plot of the radial velocity measurements

Top panel:
Radial velocity measurements, including the 14 observations taken at the Lick observatory (in yellow; Butler, Marcy, and Fisher, 1998, personal communication).
Middle panel:
Residuals from the orbital fit as a function of heliocentric Julian date.
Bottom panel:
Phase diagram.

All data points fit within a 3.0-sigma rejection criterion. The RMS of the residuals to the fit is 9.3 m/s.

For more details on these results, please refer to a recent poster (700k postscript) presented at the IAU Colloquium #170 (Noyes, et al., 1998). The original announcements of the discovery of the planet orbiting rho CrB can be found in Noyes, et al., 1997 ApJ. 483, L111 (and 487, L195).

More on the Planetary Companion to rho CrB

The orbital parameters are (based on combined AFOE and Lick observations):
   Period:     39.90  +/- 0.08   days
   K1:         66.68  +/- 3.00   m/s
   e:           0.13  +/- 0.05
   omega:      306.9  +/- 19.2   degrees  (longitude of periastron)
   T:    2,450,524.1  +/- 2.07            (time of periastron, HJD)

   Chi^2:       1.31
   RMS{resid}:   9.3  m/s

   a1 sin(i): (36.28  +/- 0.87)  x 1E+6 m
   f1(m):     (1.195  +/- 0.085) x 1E-9 M(Sun)
   m2 sin(i):  1.11 M(Jup)

More on the star rho CrB

rho CrB is the star `rho' in the Corona Borealis constellation (i.e., the Northern Crown). The physical parameters of the star rho CrB are (from the scientific literature):
    R.A.:          16:01:03.39
    Dec.:         +33:18:51.5  (2000.0)
    Vis Mag.:      5.40
    aka:           HD 143761, HR 5968

    Spectral Type: G0V or G2V
    T(eff):        ~ 5800 K
    Parallax:      57.4 +/- 0.7 mas
    Distance:      17.4 +/- 0.2 pc, or 56.8 +/- 0.7 ly
    Luminosity:    1.77 L(Sun)
    Age:           ~ 10 Gyr
    Mass:          ~ 1.0 M(Sun)
    P(rotation):   ~ 20 d
    log(g):        ~ 4.2

Note also that G. Henry (1998, personal communication) reports that six years of precise photometric data for show no significant periodicities between 1 and 50 days, and constant seasonal means to within 0.00005 magnitudes. (Henry, Baliunas, Donahue, in prep.)

Formation and Migration

There are a variety of suggestions for the origin and possible migration of the companion planet. The orbital period and amplitude imply a mass of around 1.1 Jupiter masses, and semi-major axis of around .23 AU, which is roughly half of the distance between the Sun and Mercury.

In situ formation of such a planet is unlikely (Lin and Ida 1997). It is more plausible that the planet formed at several AU from the parent star by means of gas accretion onto a rocky core, and then migrated inward. This could have happened by interactions with another giant gas planet which was ejected in the process (Weidenschilling and Marzari 1996), through interactions with the protoplanetary gas disk (Lin et al. 1996, Ward 1997, Trilling et al. 1998), or by interactions with planetessimals in mean-motion resonances (Murray et al. 1998). More observations--and new discoveries--of other planetary systems will help to address these possibilities.

AFOE Logo There is more on the AFOE's planet detection program.

Sylvain G. Korzennik (skorzennik@cfa.harvard.edu)
Adam R. Contos (acontos@cfa.harvard.edu)
Last modified: Fri Apr 30 11:38:11 1999