Circulaires IAU sur 51 PEGASI

1995 October 25 (IAUC 6251)

M. Mayor and D. Queloz, Geneva Observatory, have reported the discovery of a Jupiter-mass object in orbit around the solar-type star 51 Peg. The announcement was made in Florence on Oct. 6 at the Ninth "Cambridge" Workshop on "Cool Stars, Stellar Systems, and the Sun". The claim is based on 18 months of precise Doppler measurements made with the ELODIE spectrograph of the Observatoire de Haute-Provence. The parameters of the orbital motion are as follows: P = 4.2293 +/- 0.0011 days, e = 0 (assumed), K = 0.059 +/- 0.003 km/s, T0 = 2449797.773 +/- 0.036. The minimum mass of the companion is 0.47 +/- 0.02 Jupiter mass. Alternative explanations for the radial-velocity variation (pulsation or spot rotation) seem to be ruled out by the absence of any significant corresponding photometric variation.

Following the Oct. 6 announcement, confirmation of the 4.2-day radial-velocity variation was obtained in mid-October by G. Marcy and P. Butler (San Francisco State University, University of California atBerkeley) at the Lick Observatory, as well as by a joint team from the Harvard-Smithsonian Center for Astrophysics (R. Noyes, S. Korzennik, M. Krockenberger and P. Nisenson), the High Altitude Observatory (T. Brown, T. Kennelly and C. Rowland) and Pennsylvania State University (S. Horner).

G. Burki, M. Burnet and M. Kuenzli, Geneva Observatory and Lausanne University, communicate: "Intensive photometric monitoring of 51 Peg has been carried out at the European Southern Observatory. There is no evidence for eclipses in the system. The rms of the V magnitude (on 17 nights) is 0.037, two comparison stars being used. A 4.2-day photometric variability larger than 0.002 mag can be ruled out."

1995 October 26 (IAUC 6253)

N. Houk, University of Michigan, has determined a spectral type of G2-3 V for 51 Peg from a 1-min IIa-O 10-deg objective-prism plate taken on 1981 Aug. 6 with the Burrell Schmidt telescope at Kitt Peak. The high-quality spectrum (resolution about 0.1 nm; dispersion 10.8 nm/mm, i.e., normal classification dispersion), together with the use of several standard spectra, revealed no duplicity or chemical peculiarities. The spectrum is definitely not as luminous as IV (quoted in the 1982 Bright Star Catalogue from Keenan and Pitts 1980, Ap.J. Suppl. 42, 561), or even 'IV-V'. It is also earlier than the temperature type of G5 given in several older publications.

Corrigendum. On IAUC 6251, line 24, for 0.037 read 0.0037

1995 November 14 (IAUC 6261)

E. Guinan, Villanova University, writes: "High-precision photoelectric photometry of 51 Peg has been carried out at Mount Hopkins by R. Dukes, H. Nations, and D. Buzasi (College of Charleston) and by E. Guinan and G. McCook since Oct. 14. No significant light variations have been detected for 51 Peg to the level of the photometric precision (0.0018 mag). Some of the photometry was carried out near the expected times of the companion's transit eclipse. Several nearby comparison and check stars have been used for this differential photometry. As a cautionary note to other observers, one of the most logical primary comparison stars for the photometric study of 51 Peg, HD 217813 (V = 6.60, B-V = +0.61; spectral type G5 V), was found by both groups to be a variable star, showing quasi-sinusoidal light variations with a period of about 8.5 days and brightness ranges of 0.012 mag in V and 0.020 mag in U; the period and wavelength dependence are consistent with the rotational modulation in brightness by starspots from a Hyades-age solar-type star. Another possible photometric reference star near the position of 51 Peg, HD 218396 (= HR 8799; A7-F0 V; V = 5.99, B-V = +0.25), is also a small- amplitude variable (see Rodriguez and Zerbi, IBVS No. 4170). HD 217924 (V = 7.3, B-V = +0.6; G0 V) shows no indications of photometric variations greater than about 0.002 mag and seems to be suitable as a comparison star for the study of 51 Peg."

1995 November 30 (IAUC 6268)

W. B. Landsman, Hughes STX; T. Simon, University of Hawaii; M. Mayor, Geneva Observatory; and I. Skillen, VILSPA Observatory, communicate: "We obtained an International Ultraviolet Explorer (IUE) low-dispersion, short-wavelength observation of 51 Peg on Nov. 25 (exposure time 360 min). The spectrum is that of a normal G dwarf, with no continuum detected shortward of 165.0 nm. At 160.0 nm, we place an upper limit on the continuum flux of 2.43 x 10E-15 erg cmE-2 sE-1 AE-1. From this limit, we infer that if a normal, 0.6-solar-mass white dwarf exists in the system, then it must be cooler than 10~000 K (for a distance of 17.4 pc). The spectrum between 170.0 and 195.0 nm is a very close match to an archival IUE spectrum of alpha Cen A, scaled by the ratio of the V magnitudes. Very weak C II and C IV emission in 51 Peg is detected with integrated fluxes of 2.5 and 4.0 x 10E-14 erg cmE-2 sE-1, respectively -- consistent with a solar-activity level and a rotation period of between 23 and 30 days."

51 Peg