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SHORTER COMMUNICATIONS: WIDEBAND PHOTOELECTRIC MAGNITUDE MEASUREMENTS OF SATURN IN 2000.

ABSTRACT

Between Sep. 16 and Dec. 8, 2000 the brightness and color of Saturn was measured with a photoelectric photometer and color filters. During this time the rings were tilted at an angle of 24[degrees] with respect to Earth. The selected normalized magnitudes of Saturn are: B(1, 0) = -8.67[+ or -]0.05, V(1, 0) = -9.69[+ or -]0.02, R(1, 0) = -10.36[+ or -]0.02 and 1(1, 0) = -10.61[+ or -]0.02, while the selected solar phase angle coefficients are: [c.sub.B] = 0.050[+ or -]0.019, [c.sub.V] = 0.036 [+ or -]0.006, [c.sub.R] = 0.035[+ or -]0.006 and [c.sub.1] = 0.038[+ or -]0.009. On Dec. 8, 2000 the brightness of Saturn was measured over a 5 hour period and it was found that Saturn's magnitude ranged from -0.21 to -0.24.

Key words: Saturn, Photoelectric Photometry, Color.

INTRODUCTION

Several recent studies and discoveries related to Saturn have been made. Gladmann and co-workers [1], for example, discovered several new moons of Saturn. Meanwhile, Nicholson, French and co-workers [2, 3] carried out an intensive study of the occultation of the star 28 Sgr by the rings of Saturn in 1989, and report: 1) the rings were stable at the [sim]18 kilometer scale during the 1980s, 2) there are not many microscopic sized particles in the main rings of Saturn and 3) the optical depth of the central portion of the B ring is 2.3[+ or -]0.1.5 at a wavelength of 3.9 microns. Estrada and Cuzzi [4], furthermore, report that the rings are "extremely reddish in color." Finally, Graham [5] reports that Don Parker preformed intensity scans on Saturn's rings in red and blue light but found no difference in brightness between the preceding and following ansae on five nights in 1993.

In this paper, we report new magnitude measurements of Saturn made through different color filters in late 2000. These measurements are part of an ongoing program to monitor Saturn's brightness and color as that planet's rings open up. Furthermore, this data is being collected to understand such phenomena as the bicolored aspect of Saturn's rings and large scale changes that take place on Saturn's globe [5, 6].

METHODS AND MATERIALS

An SSP-3 solid-state photometer combined with filters that are transformed to the Johnson B, V, Rand I system and a 0.090 meter Maksutov telescope were all used in making magnitude measurements. Due to the large field of view of the telescope-photometer system (13.4 arc-minutes) Saturn was allowed to drift inside of the field of view during measurements. The comparison star used for all measurements was Epsilon Taurus which is a G9.5 star; this is one of the 107 standard stars listed in the Astronomical Almanac. The B, V, R and I magnitudes for this star were taken from the Astronomical Almanac [7].

The transformation coefficients for the 2000 apparition were measured using the two star method which is summarized by Hall and Genet [8]. The two stars used were Gamma Ophichus and Beta Ophichus. The resulting transformation coefficients in the B, V, R and I filters for the photometer and the telescope used in this study are respectively: 0.092, -0.051, -0.021 and -0.095.

The extinction coefficients were usually measured by taking measurements of two different stars at two different altitudes. Extinction coefficients did vary somewhat and the coefficients measured on a particular day were used for just that day's observations. Average values of the extinction coefficients in the B, V, Rand I filters in late 2000 were respectively: 0.46, 0.29, 0.19 and 0.18 magnitudes/air mass.

Each magnitude measurement of Saturn was made using the sequence CSCSCSC where C is a comparison star value and S is a Saturn value. Typically a C value consisted of at least three sky/dark current photometer readings followed by three comparison star readings. The C value was obtained by subtracting the average sky/dark current reading from the average comparison star reading. The S value was computed in a similar manner. In the end, three raw magnitude values of Saturn were determined from each CSCSCSC sequence. Each value was corrected for extinction, transformation, and the k" term as described by Hall and Genet [8] and then averaged into one measurement.

RESULTS

The magnitude measurements of Saturn are listed in Table 1. As in previous studies, the normalized magnitude of Saturn at solar phase angle [alpha], X(1, [alpha]), was evaluated as in Schmude and Hallsworth [9] except that the "0.743" in equation [2] of that paper has been replaced by 0.851 which corresponds to a ring tilt of 24[degrees] instead of 20[degrees]. The X(1, [alpha]) values were plotted versus [alpha] as in the 1999-2000 study [9] and the resulting slopes (solar phase angle coefficients-[c.sub.X]) and y-intercepts (normalized magnitudes at a solar phase angle of 0 degrees - X(1, ,0)) were determined from a least squares routine. The resulting values are listed in Table 2. Uncertainties in the X(1 ,0) and [c.sub.X] values were computed in the same way as in Schmude [10].

Figure 1 shows the V-filter magnitude measurements of Saturn made on the evening of Dec. 8, 2000 over a 5 hour period; this corresponds to one half of Saturn's rotational period. Each point in figure 1 is an average of three magnitude values. The uncertainties for each point were evaluated by dividing the standard deviation of the three values by the square root of three. In cases where the uncertainty was less than 0.005 magnitude, no error bars were drawn since the points have a height of [+ or -]0.005 magnitudes. Saturn's magnitude remained near -0.21 between 1:30 and 3:45 (U.T) but increased to -0.24 by 4:15 and then decreased to -0.21 by 6:30.

DISCUSSION

The R-I values of Saturn at a phase angle of 0[degrees] remained high in late 2000, R-I = 0.25[+ or -]0.03 which is similar to values in 1998 (0.25[+ or -]0.03) and 1999-2000 (0.20[+ or -]0.04). The high R-I values are probably due to the opening up of Saturn's rings. The R-I value dropped to near zero in 1995 when the ring tilt was 0[degrees] but increased at larger tilts in the late 1990s. The authors predict that the RI value of Saturn will remain in the 0.2 to 0.3 magnitude range until around 2006. During 2007-2011, the R-I value is expected to fall back to 0.0 to 0.1 magnitude.

The higher uncertainty in the B [1,0] value is probably due to the large extinction coefficient in late 2000 (average value of 0.46 magnitude/air mass for B). In spite of this uncertainty, a consistent trend of brighter B [1,0] values with the increasing ring tilt angle is evident between 1995 and 2000; similar trends are evident for the V, R and I filters [9-13].

The measured V-filter magnitudes of Saturn were, on average, 0.02 magnitudes fainter than the values listed in the Astronomical Almanac [14] which is similar to results in 1998 and 1999-2000. This agreement suggests that the magnitude model used in the Astronomical Almanac is satisfactory for predicting the V-filter magnitude of Saturn at ring tilt angles of 15[degrees] - 24[degrees].

REFERENCES

(1.) Talcott R: Six More Moons for Saturn. Astronomy 29 (No. 2): 26, 2001.

(2.) Nicholson PD et al: Saturn's Rings I Optical Depth Profiles from the 28 Sgr Occultation. Icarus 145: 474-501, 2000.

(3.) French RG and Nicholson PD: Saturn's Rings II Particle Sizes Inferred from Stellar Occultation Data. Icarus 145: 502-523, 2000.

(4.) Estrada PR and Cuzzi JN: Voyager Observations of the Color of Saturn's Rings. Icarus 122: 251-272, 1996.

(5.) Graham DL: Saturn 1993. J. Brit. Astron. Assoc. 110: 207-213, 2000.

(6.) Alexander AFO'D: "The Planet Saturn." New York: Dover Publications Inc., 1962.

(7.) Astronomical Almanac for the Year 2001, U.S. Govt. Printing Office, Washington DC 2000.

(8.) Hall DS and Genet RM: "Photoelectic Photometry of Variable Stars." second edition, Richmond: Willmann-Bell, Inc., 1988.

(9.) Schmude RW Jr and Hallsworth W Jr: Wideband Photometry of Saturn: 1999-2000. Ga J Sci 58: 91-94, 2000.

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

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

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

(13.) Schmude RW Jr: Photoelectric Magnitude Measurements of Saturn in 1998-99. Ga J Sci 57: 240-245, 1999.

(14.) Astronomical Almanac for the Year 2000, U.S. Govt Printing Office, Washington DC 1999.
Table I
Photoelectric Magnitude Measurements of Saturn and Other Relevant
Data Collected During the 2000 Opposition.
 Magnitude Air Mass
Date (U.T.) Filter Measured X(1, [alpha]) [a] Saturn
Sep. 16.34, 2000 B 1.05 -8.44 1.13
Sep. 16.29 V -0.01 -9.50 1.33
Sep. 16.37 R -0.65 -10.15 1.06
Sep. 16.41 I -0.90 -10.39 1.04
Oct. 3.34 B 1.04 -8.40 1.04
Oct. 3.37 V -0.08 -9.51 1.04
Oct. 3.44 R -0.78 -10.21 1.14
Oct. 3.41 I -1.01 -10.44 1.08
Oct. 5.20 R -0.76 -10.19 1.61
Oct. 27.29 B 0.92 -8.45 1.04
Oct. 27.17 V -0.20 -9.58 1.38
Oct. 27.21 R -0.87 -10.25 1.31
Oct. 27.25 I -1.08 -10.46 1.08
Nov. 21.16 B 0.64 -8.72 1.10
Nov. 21.21 V -0.35 -9.71 1.05
Nov. 21.25 R -1.01 -10.38 1.06
Nov. 21.28 I -1.26 -10.63 1.06
Nov. 28.13 B 0.76 -8.60 1.12
Nov. 28.09 V -0.27 -9.64 1.25
Nov. 28.18 R -0.93 -10.30 1.05
Nov. 28.22 I -1.19 -10.56 1.05
Dec. 8.17 V -0.22 -9.59 1.12
Date (U.T.) [epsilon]-Tau [alpha]([degrees]) B([degrees])
Sep. 16.34, 2000 1.19 5.8 24.3
Sep. 16.29 1.46 5.8 24.3
Sep. 16.37 1.09 5.8 24.3
Sep. 16.41 1.04 5.8 24.3
Oct. 3.34 1.07 4.9 24.2
Oct. 3.37 1.04 4.9 24.2
Oct. 3.44 1.08 4.9 24.2
Oct. 3.41 1.04 4.9 24.2
Oct. 5.20 1.93 4.7 24.2
Oct. 27.29 1.05 2.7 23.9
Oct. 27.17 1.60 2.7 23.9
Oct. 27.21 1.19 2.7 23.9
Oct. 27.25 1.13 2.7 23.9
Nov. 21.16 1.16 0.4 23.6
Nov. 21.21 1.06 0.4 23.6
Nov. 21.25 1.03 0.4 23.6
Nov. 21.28 1.11 0.4 23.6
Nov. 28.13 1.23 1.0 23.5
Nov. 28.09 1.45 1.0 23.5
Nov. 28.18 1.08 1.0 23.5
Nov. 28.22 1.03 1.0 23.5
Dec. 8.17 1.14 2.2 23.3
(a)Computed for a ring tilt angle of 24[degrees]
Table II
Normalized Magnitudes, X(1, 0) and Solar Phase Angle Coefficients
([c.sub.x]) of Saturn Measured During Late 2000.
Filter X(1, 0) at B = [c.sub.x] at B
 24[degrees] = 24[degrees]
 B -8.67 [+ or -] 0.05 0.050 [+ or -] 0.019
 V -9.69 [+ or -] 0.02 0.036 [+ or -] 0.006
 R -10.36 [+ or -] 0.02 0.035 [+ or -] 0.006
 I -10.61 [+ or -] 0.02 0.038 [+ or -] 0.009
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Author:Schmude Jr., Richard W.; Hallsworth Jr., William
Publication:Georgia Journal of Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Sep 22, 2001
Words:2024
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