The following was issued as part of AAVSO Alert Notice 281:

THE AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS

25 Birch Street, Cambridge, MA 02138 USA

INTERNET: aavso@aavso.org

Tel. 617-354-0484 Fax 617-354-0665

AAVSO ALERT NOTICE 281 (May 11, 2001)

POSSIBLE PLANETARY TRANSITS OF 2248-14 IL AQUARII = GLIESE 876

AAVSO member and observer Frederick West, Hanover, PA, gave a presentation
at the 89th Annual Meeting of the AAVSO in October 2000 in which he
suggested that amateur astronomers monitor the variable star IL Aqr =
Gliese 876 in order to observe transits by either or both of its
recently-discovered Jovian planets. [The text of his presentation, with
calculations, will appear in June as a Letter to the Editor in Journal
AAVSO, Vol. 29, No. 2.]

High-precision (CCD or photoelectric photometry) transit observations
could improve our information about the radii, mass, density, and orbital
elements of both Gliese 876 and its planets and about the limb darkening
of the star. The first window of opportunity to look for these transits
is May 16 – 27, 2001.

Gliese 876 = IL Aqr is a red dwarf star (RA = 22h 53m 20s, Decl. = -14deg
13.2min (2000), M4V, range 10.15-10.19 V, (B-V)=1.58, (R-I)=1.22),
possible periods = 20.2 and 28.7 days). Geoffrey Marcy, University of
California at Berkeley, and his colleagues (Marcy et al. 1998; Cowen 2001)
found two giant planets orbiting Gliese 876: Gliese 876b and Gliese 876c.
Using the information in these references and in Delfosse et al. (1998),
MacRobert (2001), Burrows et al. (1997), Sterne (1960), and West (1996),
West calculated approximate dates and times of predicted transits of
Gliese 876 by its planets; they are given in the table below.

Uncertainties in the orbital elements make the times uncertain by several
days, so Gliese 876 should be monitored for transits during “opportunity
windows” lasting ten days or longer. These windows will occur on the same
days of the calendar year for many years. Gliese 876 rises at sidereal
time 17:42 and sets at 4:04 for an observer at 40 N latitude. Thus,
morning observations are feasible from late May to September and evening
observations September to late January. For Gliese 876b a transit may
last as long as 3.53 hours and have an amplitude as much as 0.20
magnitude. For Gliese 876c a transit could last as long as 2.2 hours. If
the planets simultaneously transit, Gliese 876 could be dimmed by as much
as 0.45 magnitude. 

 Transit Number    ----Predicted Time of Transit----     Opportunity Window
  876b    876c        JD            Date and UT
           58      2452050.8     2001 May  21  07:12       2001 May  16-27
   29              2452079.97    2001 Jun. 19  11:17       2001 Jun. 14-26
           59      2452080.9     2001 Jun. 20  09:36       2001 Jun. 14-26
           60      2452111.0     2001 Jul. 20  12:00       2001 Jul. 15-26
   30              2452140.97    2001 Aug. 19  11:17       2001 Aug. 14-25
           61      2452141.1     2001 Aug. 19  14:24       2001 Aug. 14-25
           62      2452171.2     2001 Sep. 18  16:48       2001 Sep. 13-24
           63      2452201.3     2001 Oct. 18  19:12       2001 Oct. 13-25
   31              2452201.97    2001 Oct. 19  11:17       2001 Oct. 13-25
           64      2452231.4     2001 Nov. 17  21:36       2001 Nov. 12-23
           65      2452261.5     2001 Dec. 18  00:00       2001 Dec. 12-24
   32              2452262.97    2001 Dec. 19  11:17       2001 Dec. 12-24
           66      2452291.6     2002 Jan. 17  02:24       2002 Jan. 11-23

Observations should be made every 5 minutes. Observers should have access
to an accurate time source and record the time of their observations to
the nearest second, if possible. Accompanying is an AAVSO Preliminary ‘d’
scale chart of Gliese 876 with comparison stars suitable for CCD or
photoelectric observing. Please use this chart to make your observations
and REPORT ALL OBSERVATIONS DIRECTLY TO Dr. Frederick West, 520 Diller
Road, Hanover, PA 17331-4805, USA. (Do not report observations to the
AAVSO.)

References

Burrows, A., et al. 1997, in: Planets Beyond the Solar System and The Next
Generation of Space Missions (D.R. Soderblom,ed.), Ast. Soc. Pacific Conf.
Series, Vol. 119, pp. 9-17.

Cowen, R. 2001, Science News, 159, No. 2, 22.

Delfosse, X., et al. 1998. Astron. and Astrophys. Lett., 338, L67.

Marcy, G.W., et al. 1998, Astrophys. J. Lett., 505, L147.

MacRobert, A. 2001, Sky & Telescope, 101, No. 4, 20.

Sterne, T.E. 1960, An Introduction to Celestial Mechanics, Interscience,
New York, pp. 8-14.

Weis, E.W. 1994, Astron. J., 107, 1135.

West, F.R. 1996, J. Amer. Assoc. Var. Star Obs., 24, 19.

West, F.R. 1999, J. Amer. Assoc. Var. Star Obs., 27, 77.

CHARTS AVAILABLE ON AAVSO WEB AND FTP SITES

Electronic copies of the charts for Gliese 876 mentioned in this Alert
Notice are available through our web site at the following address:

http://www.aavso.org

They may also be obtained directly from our FTP site:

ftp.aavso.org (198.116.78.2), in /alerts/alert281

The answering machine at AAVSO Headquarters is on nights and weekends for
your convenience. Please call our charge-free number (888-802-STAR =
888-802-7827) to report your observations. We also encourage observers to
send observations by fax to 617-354-0665 or by e-mail through the Internet
to observations@aavso.org.

Many thanks for your valuable astronomical contributions and your efforts.

Good observing!

Janet A. Mattei

Director

Elizabeth O. Waagen

Senior Technical Assistant