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PHOTOELECTRIC MAGNITUDE MEASUREMENTS OF SATURN IN 1998-99.

ABSTRACT

A total of 52 photoelectric magnitude measurements of Saturn were made between Sept. 27, 1998 and Jan. 5, 1999. The V-filter measurements match up well with values predicted in the Astronomical Almanac. The selected normalized magnitudes for Saturn are: B(1,0) = -8.45[+ or -]0.02, V(1,0) = -9.44[+ or -]0.01, R(1,0) = -10.10[+ or -]0.02 and 1(1,0) = -10.35[+ or -]0.02 while the selected solar phase angle coefficients for the B, V, R and I filters are: 0.047[+ or -]0.008 0.035[+ or -]0.002, 0.038[+ or -]0.009 and 0.040[+ or -]0.009 respectively. Results over the last three years show that the solar phase angle coefficient in the V-filter changes with the changing tilt of the rings.

INTRODUCTION

There are several factors which can cause the brightness and color of Saturn to change over time; a few of these are: changes in the brightness of disc and ring features [1-3], the bi-colored aspect of the rings [4], the Seeliger effect [5-7], changing ring tilt angle [8] and large meteorological changes on Saturn [9]. Members of the British Astronomical Association, for example, have observed the A-ring [5] and the B-ring [6-7] get brighter as Saturn reached opposition which is the Seeliger effect. In another study, the rings were shown to reflect more blue light than the disc of Saturn [8] and consequently the color of Saturn+rings should change as the ring tilt changes. By measuring the brightness and color of Saturn over several years, one can learn more about the brightness and color of the rings and disc of Saturn; furthermore, one may be able to learn more about the large outbreak of white spots that occur on Saturn such as those which were imaged in November 1998 [9]. It is for these reasons that pho toelectric magnitude measurements of Saturn have been made with color filters. In this report, whole disc photoelectric magnitude measurements of Saturn are reported through filters that have been calibrated to the Johnson B V R and I system. A preliminary report of this work has been presented at the 1999 Georgia Academy of Science meeting in Lawrenceville, GA [10].

MATERIALS AND METHODS

An SSP-3 solid state photometer along with a 0.51 meter f/4.5 Newtonian telescope was used in measuring the brightness and color of Saturn during the 1998 opposition. The telescope aperture was reduced to 0.05 meters to prevent saturation of the detector. Filters that have been transformed to the Johnson B V R I system were also used in this study.

Transformation coefficients were evaluated using the two star method as described by Hall and Genet [11]. The two stars used in measuring the transformation coefficients were chi-Pegasus and gamma-Pegasus; both stars are listed as U B V R I standard stars in the Astronomical Almanac [12]. The magnitudes used for these two stars were taken directly from the Astronomical Almanac [12].

The comparison star used for all photoelectric measurements was alpha-Aries. The magnitudes for this star were taken from the Astronomical Almanac [12]. A check star, eta-Pisces, was measured every night that Saturn was measured. The average measured magnitudes for eta-Pisces (corrected for extinction and transformation) are: B = 4.59[+ or -]0.02, V = 3.61[+ or -]0.01, R = 2.90[+ or -]0.02 and I = 2.36[+ or -]0.01; the corresponding magnitudes listed in the Astronomical Almanac are: B = 4.59, V = 3.62, R = 2.90 and I = 2.40. The agreement is good except for the I filter where there is a 0.04 magnitude discrepancy.

RESULTS

The measured magnitudes, solar phase angles ([alpha]), ring tilt angles (B) and other relevant data are listed in Table I. The ring tilt angle and the solar phase angles were taken from the Astronomical Almanac (12, 13). All magnitudes have been corrected for both atmospheric extinction and transformation. Extinction corrections were made in the same way as described in Hall and Genet [11]; [k.sub.b]" was assumed to equal -0.03 while [k.sub.v]", [k.sub.r]", and [k.sub.i]" were assumed to equal 0.00. All k' values were measured on the same night as the photoelectric measurements. The dates listed in Table I correspond to the V-filter measurement of Saturn; other filter measurements in the same row were made within about 7 minutes of the V-filter measurements.

The solar phase angle coefficients and the normalized magnitudes extrapolated to a solar phase angle of zero degrees for a ring tilt angle of 15[degrees] are summarized in Table II. The plots of X(1,[alpha]) versus [alpha] are shown in Figure 1. The method used in obtaining these values were the same as used in Schmude [14]. The uncertainties in Table II were determined in the same was as in Schmude [15].

DISCUSSION

The average difference between the measured V-filter magnitudes and the V-magnitudes in the Astronomical Almanac [12, 13] was less than 0.01 magnitude. The normalized magnitudes in 1998, X(1,0), at a ring tilt angle of 15.0[degrees] were about 0.2 to 0.3 magnitudes brighter than the corresponding values in 1997 at a ring tilt angle of 90[degrees]. The color indexes at B = 15[degrees] and [alpha] = 0[degrees] in 1998 are: B-V = 0.99[+ or -]0.02, V-R = 0.66[+ or -]0.02 and R-I = 0.25[+ or -]0.03 which are a little different than the corresponding values in 1997 at B = 9[degrees] and [alpha] = 0[degrees] (B-V = 1.06[+ or -]0.03 and V-R = 0.70[+ or -]0.03) and 1996 at B = 9[degrees] and [alpha] = 0[degrees] (B-V 1.06[+ or -]0.04 V-R = 0.68[+ or -]0.04 and R-I = 0.10[+ or -]0.04). The lower B-V value in 1998 may be due to the fact that the rings are more blue [8] than the disc and that the rings were more open, resulting in the rings having a larger influence on the color of Saturn in 1998 than 1997. One signific ant difference is that the R-I value was much higher in 1998 than in 1996 and earlier [14, 16-18]. No infrared measurements were made in 1997.

The solar phase angle coefficient of Saturn (magnitudes/degree) in the V filter in 1998 (B = 15[degrees]) was 0.035[+ or -]0.02 which is higher than the values in 1997 (B = 9[degrees] 0.029[+ or -]0.002 and 1996 (B = 4[degrees]) 0.020[+ or -]0.007. The trend here suggests that at ring tilt angles of less than B = 15[degrees] the solar phase angle coefficient increases with increasing values of B.

ACKNOWLEDGMENTS

The author would like to thank the Atlanta Astronomy Club for maintaining the Walter Barber Observatory.

REFERENCES

(1.) McKim, RJ and Blaxall KW: Saturn 1943-1981: A Visual Photometric Study-I. J Brit Astron Assoc 94: 145-151, 1984.

(2.) Mckim RJ and Blaxall KW: Saturn 1943-1981: A Visual Photometric Study-II. J Brit Astron Assoc 94: 211-220, 1984.

(3.) McKim RJ and Blaxall KW: Saturn 1943-1981: A Visual Photometric Study-III. J Brit Astron Assoc 94: 249-255, 1984.

(4.) Benton JR, Jr: Observations of Saturn During the 1994-95 Apparition. J Assoc Lunar & Planet Obs 40: 1-13, 1998.

(5.) Heath AW: Saturn, 1984. J Brit Astron Assoc 96: 170-175, 1986.

(6.) Heath AW: Saturn, 1980-1981. J Brit Astron Assoc 95: 1-7, 1984.

(7.) Heath AW: Saturn, 1981-1982. J Brit Astron Assoc 95: 143-150, 1985.

(8.) Heath AW: Saturn 1974-1975. J Brit Astron Assoc 87: 294-299, 1977.

(9.) Parker D: Private Communication 1999.

(10.) Schmude RW Jr: Wideband Photometry of Saturn in 1998 (abstract). Ga J Sci 57: 67, 1999.

(11.) Hall DS and Genet RM: Photoelectric Photometry of Variable Stars, second edition, Willmann-Bell Inc., Richmond, VA 1988.

(12.) Astronomical Almanac for the Year 1999, U.S. Govt. Printing Office, Washington, D.C. 1998.

(13.) Astronomical Almanac for the Year 1998, U.S. Govt. Printing Office, Washington D.C., 1997.

(14.) Schmude RW Jr: Photoelectric Magnitudes of Saturn in 1996, Ga J Sci 56: 175-181, 1998.

(15.) Schmude RW Jr: Observations of Saturn in 1997, Ga J Sci 57: 187-191, 1999.

(16.) Schmude RW Jr: Observations of Saturn in 1995, Ga J Sci 55: 175-179, 1997.

(17.) Schmude RW Jr: Wideband Photometry of Saturn in 1994, Ga J Sci 54: 183-187, 1995.

(18.) Schmude RW Jr and Bruton D: Saturn in Late 1993, Tx J Sci 47: 13-20, 1995.
 Photoelectric Magnitude Measurements of
 Saturn Along With Other Relevant Data.
Date [*] Magnitude [alpha] B
 B V R I (Degrees) (Degrees)
1998
Sep.27.198 1.14 0.12 -0.49 -0.75 3.0 16.0
Sep.27.236 1.16 0.08 -0.47 -0.80 3.0 16.0
Sep.27.282 1.20 0.09 -0.58 -0.75 3.0 16.0
Oct.17.188 0.99 1.10 --- --- 0.8 15.5
Oct.17.210 1.05 0.00 --- --- 0.8 15.5
Oct.25.112 1.01 -0.01 -0.66 -0.94 0.4 15.2
Oct.25.133 0.97 0.00 -0.71 -0.95 0.4 15.2
Oct.25.155 0.91 0.01 -0.67 -0.91 0.4 15.2
Nov.29.195 1.24 0.20 -0.46 -0.68 3.9 14.4
Nov.29.225 1.24 0.20 -0.47 -0.68 3.9 14.4
Nov.29.261 1.26 0.19 -0.46 -0.61 3.9 14.4
1999
Jan.5.108 1.47 0.41 -0.25 -0.52 6.0 14.2
Jan.5.135 1.42 0.41 -0.25 -0.50 6.0 14.2
Jan.5.164 1.43 0.38 -0.27 -0.51 6.0 14.2
Date [*] Air Mass
 Saturn Comp.Star
1998
Sep.27.198 1.37 1.22
Sep.27.236 1.20 1.10
Sep.27.282 1.11 1.05
Oct.17.188 1.18 1.09
Oct.17.210 1.13 1.05
Oct.25.112 1.31 1.21
Oct.25.133 1.22 1.13
Oct.25.155 1.16 1.07
Nov.29.195 1.23 1.09
Nov.29.225 1.38 1.20
Nov.29.261 1.68 1.35
1999
Jan.5.108 1.30 1.12
Jan.5.135 1.47 1.22
Jan.5.164 1.76 1.44
(*.)All dates are Universal Time.
 Normalized Magnitudes and Solar Phase Angle
 Coefficients of Saturn Measured During the 1998 Opposition.
Filter X(1,0) [c.sub.x]
 B -8.45[+ or -]0.02 0.047[+ or -]0.008
 V -9.44[+ or -]0.01 0.035[+ or -]0.002
 R -10.10[+ or -]0.02 0.038[+ or -]0.009
 I -10.35[+ or -]0.02 0.040[+ or -]0.009


[Graph omitted]
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Author:Schmude Jr., Richard W.
Publication:Georgia Journal of Science
Date:Dec 22, 1999
Words:1822
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