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Comet tempel-tuttle and the recent Leonid meteor shower
CHINESE ASTRONOMY AND ASTROPHYSICS PERGAMON
Chinese Astronomy
and Astrophysics
26 (2002) 40-48
Comet Tempel-Tuttle and the Recent Leonid Meteor Shower+* WU Guang-jie Yunnan Astronomica National
Astronomical
Observatory, Chinese Academy of Sciences, Observatories,
Kwaming 650011
Chinese Academy of Sciences,
Beijing 100012
Abstract
As one of the famous meteor showers the Leonid meteor shower had the most spectacular bursts in history. Since 1998 a new series of activity of the Leonids has been coming. What will it be in recent years and in the future? According to different references, the levels of the showers in history are reviewed and discussed in this paper. Especially, the diagram of CEOS (the Comet-Earth Orbits Separation) versus TE_C (the time gap of the Earth following the Comet) is analyzed. It is found that almost all the meteor storms are located in an inclined square, and the strongest storms are in a curved strip. The boundaries of the square have a slope of about 15 m s- l. The sizes of meteoroids calculated from this velocity are also reasonable. This means that the particles colliding with the Earth have a limited velocity to drift off its original orbit. With larger CEOS, the longer time interval after the comet passage would be needed for a possible storm. The width of this square indicates that the stronger Leonid showers would never be seen on more than about four consecutive years. In one cometary return, the percentage of bright meteors will decrease year by year. These facts could be explained by a model of the formation of a cometary dust tail, including the motion, dispersion, and breaking up of dust particles. Prediction could be made with this square region. We may have Leonid showers in 1998-2000 with a peak in 1999, and after AD 2000, one has to wait another century before significant displays will once again appear. Key words:
meteoroid
-
Leonids
-
comet
t Supported by National Natural Science Foundation Received 2000-07-24; revised version 2001-02-05 * A translation of Acto Astmn. Sin. Vol. 42, No. 2, pp. 125-133, 0275-1062/01/$-see front matter PII: SO275-1062(02)00041-3
2001
@ 2001 Elsevier Science B. V. All rights reserved.
WU Guang-jie
/ Chinese
Astronomy
and Astrophysics
26 (2002)
40-48
41
1. INTRODUCTION The November Leonid meteor showers and the occasional Leonid meteor storms are far better known than the Leonid parent body, i.e., Comet 55P/Tempel-Tuttle. The modern meteor astronomy began in the early morning of November 13, 1833, when people were woken up by a storm of Leonid meteors. In late 1865 and early 1866 Comet 55P/Tempel-Tuttle was discovered, and it had a very similar orbit to that of the Leonids with an orbital period of 33.5 yr. Le Verrier (1867) first connected the meteoroid stream to the newly discovered comet. Every year around November 17 the Earth orbiting around the Sun carries us through the Leonid meteoroid stream. The Earth would take a few days to pass through it. Since at least AD 902 the return of the Leonids in every 33 years has sometimes brought about a great meteor shower, but not always. Four of the most spectacular meteor storms have been witnessed in the past 200 yrs, namely, in 1799, 1833, 1866 and 1966. Now, we know that most storms appear to occur when the node of the cometary orbit is inside the Earth’s orbit and the Earth reaches the closest point after the comet has passed through the node, and that no storms take place more than three years from the perihelion passage date of the comet (Yeomans 1981, Williams 1997). For understanding the connection between the Leonids and its parent comet, and making predictions of future meteor showers and storms, it is necessary to study the stream and the comet appearance in more details and from different angles.
2. PREDICTIONS
AND THE LEONIDS
1998-1999
In 1998 the comet had its new return to the inner solar system (Rao 1998, Sanderson 1998). About the meteor shower astronomers gave various predictions. They predicted that “few will be seen” (Wu & Williams 1996, or WW96 for short hereafter), “modest strong” (Brown & Jones 1993, or BJ93 hereafter), or up to 40 a second for about an hour (Rae 1998, 1999). In 1998, a substantial Leonid activity occurred over ten hours earlier than predicted. $‘rom the visual records of 217 observers all over the world, Arlt (1998) got a strong, broad background component, having its maximum at X = 234.“52 with ZHR=340, and a “storm component” very close to the prediction time reaching ZHR=180. A joint observation of Chinese and Dutch astronomers in Qinghai also gave a similar result (Zhao et al. 2000). According to our observations on 5 days (Wu & Zhang 2000), the activity of the 1998 Leonids increased suddenly, but decreased slowly. We found that on November 15 and 16 the ZHR was very low (much lower than lo), but it increased steadily in the morning of November 17. We obtained a maximum number of meteor showers above 400, and it is a little larger than the result of Arlt (1998). Considering the tendency in our observations and the observations by Fitzsimmons et al. (Zay 1998, Fitzsimmons 1998), who reported about 500-700 hr-l around November 17-19, the maximum of the 1998 Leonids (supposed to be at lh40m UT according to Arlt, or 3 hours after our observations) may be up to 600-800. An intense outburst of Leonid meteors over Europe and the Middle East on 1999 November 18 was reported. Experienced visual observers in Spain and France witnessed a distinctive peak with a ZHR of around 5,000, which is considerably more than the 500 to 1,500 per hour predicted by most experts (Gyssens 1999, Phillips 1999). On the other hand, the maximum
42
WU Guang-jie / Chinese Astronomy and Astrophysics 26 (2002) 40-48
was witnessed at 2h 04m f 5m UT and coincided very well with the time of 2h 08”’ UT predicted by McNaught and Asher (1999).
3. DIAGRAM
OF TE_c
VERSUS
CEOS
AND FUTURE
PREDICTION
Major Leonids events in history and their comparison are listed in Table 1. The data mostly come from Yeomans (1981) and Jenniskens (1995), and for the ancient shower events, no quantitative data are given as to the shower rates, only subjective judgment. Table 2 Date(UT) 1999 Nov.17.8 1998 Nov.17.5 1969 Nov.17.0 1966 Nov.17.4 1965 Nov.17.0 1961 Nov.17.3 1932 Nov.16.5 1903 Nov.16.4 1901 Nov.15.9 1900 Nov.15.7 1899 Nov.15.4 1898 Nov. 1868 Nov.13.2 1867 Nov.13.8 1866 Nov.13.7 1836 Nov.13.2 1833 Nov.13.4 1832 Nov.13.2 1831 Nov.13.9 1799 Nov.12.5 1798 Nov.12.2 1698 Nov. 7.8 1666 Nov. 8.0 1625 Nov. 7.5 1602 Nov. 6.7 1601 Nov. 6.3 1566 Oct.26.4 1554 Oct.26.3 1538 Oct.25.0 1533 Oct.24.6 1532 Oct.24.5 1498 Oct.25.0 1466 Oct.24.9 1366 Oct.24.9 1238 Oct.20.4 1237 Oct.20.0 1202 Oct.18.7 1037 Oct.12.7 1035 Oct.13.2 1002 oct.13.7 967 Oct.14.0 934 Oct.13.5 931 Oct.13.6 902 Oct.12.7
CEOS(AU) -0.0080 -0.0080 -0.0032 -0.0032 -0.0032 -0.0032 -0.0062 -0.0117 -0.0117 -0.0117 -0.0117 -0.0117 -0.0065 -0.0065 -0.0065 -0.0013 -0.0013 -0.0013 -0.0013 -0.0032 -0.0032 -0.0162 -0.0043 +0.0025 +0.0102 +0.0102 -0.0024 -0.0024 -0.0065 -0.0065 -0.0065 +0.0054 +0.0107 +0.0027 -0.0031 -0.0031 -0.0059 -0.0249 -0.0249 -0.0129 -0.0064 -0.0064 -0.0064 -0.0113
Basic data of the Leonids in history G-idday) f622.3 +257.0 +1656.5 +561.0 +195.5 -1265.0 +121.4 +1591.4 +861.4 +495.8 +130.4 -235.0 +1029.9 +664.4 +299.4 +1403.7 +307.9 -50.7 -416.0 -116.9 -482.3 -345.5 +147.0 -2790.7 +830.2 +464.7 -148.5 -4531.5 +2056.2 +229.7 -135.3 -328.2 +73.5 -7.0 +1456.0 +1090.5 +612.8 +1078.1 +347.6 +633.8 +43.5 +116.4 -979.6 +597.4
Rate 5,000, St jl;OOO;sh 250, sh 150,000, St 5,000, St 156, sh 240, sh il,OOO,sh 144,000? St il,OOO, sh 40,sh sim1,500, 5,000, i2,000, 300, 50,000, 20,000,
sh St St sh St St Sh i5,000, St St sh sh sh stg stg sh St St sh sh stg sh St sh st$ sh sh sh St
pax
--
235.286 234.610 234.567 234.468
JM-N
+0.131 +0.032
(233.46) (233.46) (233.46) 233.122 232.713 232.627
<+0.550 = +0.141 +0.055
232.45 iO.02 (232.1)
43
WU Guang-jie / Chinese Astronomy and Astrophysics 26 (2002) 40-48
r
’
.s E
g*
B
a
, ‘(Y2ow .X\
‘0
l
-.
z
J
g
,St
. .
150+0
o-
s
1000
Fig. 1
<1,000 -=.. . . @ B
/.
’ :2,000 -.
. .
* . .
w
,
$000. * . k.’ .
.
--.,’
5,000
8
I
*.
-
;’
-
Q.-L_
I’
. .
‘w .
;
St
E
. .
*50>02-
. .
/ *.
/
. . .
. . ,k-._
.
l/
St
.
. 5,00?.* H
,
/
”
G
.
D
. .
A
*=
1’ \
-.
~5,000 *s;_y
R
* 20,ooo
. .
. . .x
0 500 Date after the Comet crossed the plane of Earthorbit@ay)
,
. . St*
/
/
/
/
,v ’
-500
Belt of shower distribution on the diagram of TE-c versus CEOS
In Table 1 CEOS represents Comet-Earth Orbits Separation, that is, the distance (in AU) between the descending node of the comet and the nodal point where the Earth passes the comet’s orbit plane, and minus sign indicates the comet passing inside the Earth’s orbit. T&C stands for the time gap in days of the Earth following (+) or leading (-) the comet at their corresponding nodal passages. In columns 4 and 5 the time of the observed maximum shower and visual rate per hour are given. X,MAX denotes the peak position of the showers in solar longitude, and SM_N, the angle (in degrees) between the position of maximum activity and the node of the comet. By putting the historical records of Leonids into a T&C versus CEOS diagram, one can find that the maximum likelihood of a shower occurs when the Earth runs into particles outside and behind the comet (Yeomans 1981). A similar diagram Of T&c versus CEOS was used by many authors (Yeomans 1981, Sekanina 1974, McIntosh 1973). From the diagram, it can be seen clearly that the storms are concentrated not only in quadrant II, but still more in a smaller area. We find that most of the Leonid showers, especially storms, are concentrated in an inclined square, except a few events (Wu 1999). For those exceptional dates, for instance, AD 1002, 1238 and 1601, there is no quantitative information, and some events may be doubtful. But even if these three events could indeed be called =Al=BOstorm=Al=Bl, it will not affect our discussion of the concentrated region. According to the analysis and the data summarized in Table 1, and for the sake of clarity, only the square part is plotted in Fig. 1. In the figure, a “dot” denotes a shower, a “star”, a storm with ZHR >2,000. For those “storms” before 1799 based only on subjective judgement, they are marked with hollow stars. This square region is further divided into 3x3 blocks of about equal size. The ZHR, whenever available, is marked alongside the symbol. Block A: Two storms in AD 1532 and 1798, but no ZHR estimate.
44
WU Gang-jie
/ Chinese Astronomy and Astrophysics
26 (2002) 40-48
Block B: No storm, only 4 showers. Block C: One storm in AD 902 and one possible storm in AD 1002, but no ZHR estimate. Block D: Three storms. The ZHR values could be up to 5,000, even 20,000, except the storm in AD 1566. Block E: Three storms. The ZHR recorded in two events could be 2,000-5,000. Block F: Four storms. In two events the ZHR is about 5,000. For the AD 1901 storm, a very high value of 144,000 was reported (Ferrin thought it seems to be a misprint, see Ferrin 1999), but 7,000 or less than 2,000 were also reported. Block G: One storm, without ZHR estimate. Block H: Two storms. The ZHR values are extraordinarily large as 50,000-150,000. Block I: No storm, but one shower in AD 1868. In addition, the probability of strongest storms is greater in the middle strip (more accurately, in the DHF curved strip) than on either side. Moreover, in history, the showers in the upper-right strip ABC may have a larger percentage of bright meteors than others. The width of the square indicates that the stronger Leonid showers appear at most in four consecutive years and, in general, only for two or three consecutive years. We note that the upper-right and lower-left boundaries have a slope of about 14.7 m s-l. Yeomans (1981) thought that the CEOS gives a distance measure of shower particles from the comet’s orbit at the time of the particle-Earth collision. If the particles traveled 0.008 AU in 257.0 d for the 1998 shower, the average velocity would be 54 m s-i. They are both in the same order of magnitude as the velocity of ejection of the particles, Veject=36 m s-l (Brown & Jones 1998). Of course, if the particles came from the ejection in an older cometary return, the off-orbit velocity would be much smaller than 54 m s-l. In virtue of the location of the year AD 1999 (in Block F), we predicted that Leonids in 1999 would be 3 to 5 times stronger than that in 1998 with ZHR=2,000-5,000 (Wu 1999). It is shown that the method is successful for ZHR prediction. Because the location of AD 2000 is at the outer part of Block I, we estimate that there will be a ZHR of 800-1,500, i.e. only a shower. As for AD 2001 and after, the points are far away from this inclined square. Also, the CEOS for this cometary return is 2.7 times that of AD 1238. Therefore, we believe that the Leonid showers have passed their peak, and that there can be no more large bursts.
4. DISCUSSION Some authors calculated the gravitational resonance of ejected particles with Jupiter, which can keep the trails undispersed over many orbital revolutions. The peak time they predicted for Leonids 1999 is surprisingly good (Asher et al.1999, McNaught & Asher 1999). In this paper, we can see that the TE_C N CEOS diagram could also give us some interesting results, especially about the strength of the showers. From Fig.3 of Yeomans et al. (Yeomans, Yau, and Weissman 1996, or YYW96 hereafter), in 934, 1238, 1566 and 1833, the CEOS indeed have very small minus values, and the smallest distance of the node of the cometary orbit from the Earth’s orbit does coincide with the greatest storm. On the other hand, the level of a shower can not be determined only by the value of CEOS. The inclined square tells us that if CEOS has a large value, we
WU Guang-jie / Chinese Astronomy and Astrophysics
Fig. 2
26 (2002) 40-48
45
Trajectories of particles and formation of the dust tail
may need to wait a longer time for a storm as the meteoroids have a limited velocity. This square can also explain why people had to wait till AD 1900 for a shower, since the point 1899 was outside the square. In the standard model for the formation of meteoroid streams, the ejection of the meteoroids from the cometary nucleus depends on the particular ejection model adopted. We think that the ejection from the comet is not isotropic, especially, the ejected material can form a dust tail due to the effect of the solar wind. The fact that quadrants III and IV are nearly empty may imply that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the particles. Figure 2 gives a sketch of the formation of a dust tail. It is obvious that the orbit of particles is much different from that of the comet. According to the theory of syndynames and synchrones of a dust tail, small particles will have a larger ratio of radiation pressure to gravitational force (1 - p). The principal effect is to “weaken” gravity so that the quantity GM is replaced by GM(l - p), where (1 - p) has the value of 5.75~1O-~/rp,r being the radius and p the bulk density of the grain, both in cgs units. This means that small particles have large velocities than large particles. The larger particles tend to hang around the comet because they leave its nucleus at a lower speed than their small brethren (Krishna Swamy 1986, Williams 1997). In this paper, we obtain a slope with a speed of 14.7m s-l. Suppose that the dust grains have an off-orbit velocity increment of 15 m s-l in the radial direction from the Sun under the action of solar radiation pressure after traveling for time t, then we have the
46
WV Guang-jie / Chinese Astronomy and Astrophysics
momentum
26 (2002) 40-48
relation
m(V,+v-mv,
=Ft
(1)
and F=(l-v)ma&
(2)
where V, is the off-orbit component of the ejection velocity of the particles, a is the gravitational acceleration on the solar surface, and I& is the solar radius in units of AU. Then we have
(1-CL)= &
=5.75 x lo-s/rp
and rcxt/v.
(4)
It is shown that smaller particles would get larger acceleration and meet the Earth earlier. Considering the bulk density p = 800 kgme3 (Brown & Jones 1998), the radius of the dust grain can be expressed as , r = 2.470 x 10-3t,
(5)
where t is in units of day, and r in centimeter. If we suppose that it is the particles ejected in the present return that form the Leonids of 1998-2000, that is, if t is equal to TE-C, then AD 1998: t = 257 d, r = 0.635 cm , it corresponds to a meteoroid of -1 magnitude, AD 1999: t = 622d r = 1.536cm, it corresponds to a meteoroid of -5 magnitude, AD 2000: t = 98 d, r = 2.440 cm, it corresponds to a meteoroid of -8 magnitude. These results are acceptable. If the time t is very long, it might seem that the meteoroids should be very large. However, in the long time traveling to the Earth, particles of big size will break up into many smaller pieces. It was proved that dust grains with a mass as small as lo-‘* g were detected at a distance of lo5 - lo6 km from the nucleus. Breaking-up has been verified for Comet Halley, and the resulting number density falls off as r-r rather than rA2, but what process leads to the break-up is unknown (Utterback & Kissel 1990). If the ejection point is very far away from the descending node, or the CEOS itself is large, the process of breaking-up of particles may become very serious. In addition, the process of dispersion of the particle stream may also become serious. In this situation, it is not easy to see a meteor storm by naked eye. This may be the reason why the Leonid shower could not be seen in more than three or four consecutive years, in general. As for the peak time of a shower, it can not be predicted from the diagram of TE-_C versus CEOS, but we can estimate it from historical records. Using the data in Table 1, the long term variation of Xrax may be expressed as , max
&l
= 203.“14 + O.“0159yr-r x T
(6)
WU Guang-jie
/ Chinese
Astronomy
and Astrophysics
26 (2002)
47
40-48
where T is the year of calculation. The rate, O.“0159yr-‘, could be thought to be due to the variation of comet’s orbit. Indeed, it agrees well with the variation of the “Node” value calculated by YYW96, which varies by about 28 degrees from AD 604 to 2530 (or O.“0145/yr ). This comparison also gives a result that, in general, the shower maximum happens about lh14”after the Earth passing through the comet’s orbital plane. In this paper, our discussion mainly focuses on the inclined square, in which Leonid storms are concentrated. Indeed, there are few storms outside the square. Even in the square area, though, the distribution of the showers and storms is not uniform. This inclined area can provide us some useful relations and single out some dominating factors. Ferrin (1999) has drawn out a particle density distribution around the comet and isolines in a similar diagram. His prediction for the Leonids 1999 (ZHR=3.5KflK) is also successful. As for AD 2000, he predicted that the ZHR would be up to 5K-20K, very different from our prediction. For the future, YYW96 gave the orbital solution and elements for 55P/TempelTuttle. It is easy to see that at least both the angle w and the longitude of the descending node increase continuously from AD 604 to 2530, except for small fluctuations in the former. In their Fig. 3, it is obvious that CEOS has a tendency of moving from negative to positive values over the years AD O-2500 while under rather large fluctuations. Before AD 600 the CEOS was too large, so it is possible that nobody could see a Leonid meteor shower then. In addition, the Leonid shower may vanish forever after AD 2500. Good Leonids shower displays could be seen only from about AD 895 to 2170. Because of planetary perturbations, it will be another century after the 1998-2000 events before significant Leonid meteor displays are once again visible. References Arlt R., WGN J.IMO,
1998, 26, 239
Asher D. J., Beiley M. E., Emel’yanenko
V. V., MNRAS,
1999, 304: = L53
Brown P., Jones J., Evolution of the Leonid Meteor Stream. oroids and Their Parent Bodies. J.. Icarus,
Bratislava:
Astronomical
In=AJ=BAStohl
J., Williams I. P., eds., Mete-
Inst, Slovak Acad.
Sci., 1993, 57 Brown P., Jones
1998, 133, 36
Ferrin I., AAp, 1999, 348, 295 Fitzsimmons
A., http://abob.libs.uga.edu/bobk/ccc/ccll2398.html,
Gyssens M., http://www.imo.net/leo99/leo99index.html, Krishna Swamy K. S., Physics of comets,
= 1998
November = 18-20, 1999
Singapore:
World Scientific Publishing Co Pte Ltd, 1986, 151
Jenniskens P., AAp, 1995, 295, 206 Le Verrier U. J. J., Comptes Rendus, 1867, 64, 94 McIntosh B. A., Origin and evolution of recent Leonid meteor = shower, In: Hemenway C. L., Millman P. M., Cook A. F., eds., Evolutionary
and Physical
Properties
of Meteoroids
(NASA SP-319),
NASA:
1973,
193 McNaught R. H., Asher D. J., WGN J.IMO,
1999, 27, 85
Phillips T., NASA, = http://science.nasa.gov/newhome/he~lines/~tlSnov99_l.htm, Telescope,
Rao J. Sky and Telescope,
1999, 11, 28
Sanderson R., Sky and Telescope, Sekanina Z., Meteoric
storms
Milet B., eds., Asteroids, Utterback
1999 Rao J., Sky and
1998, 11, 38 1998, 11, 30
and the formation
of meteor stream,
Comets and Meteoric = Matter,
N. G., Kissel J., AJ, 1990, 100, 1315
In: Cristescu
C., Klepczynski
New York: Schelium International,
W. J.,
1974, 239
48
WU Guang-jie / Chinese
Williams I. P., MNRAS, Wu G. J., CAA (Letters),
Astronomy
and Astruphysies
26 (2002)
40-48
1997, 292, L37 1999, 23, 268
Wu G. J., Zhang Z. S., Earth, Moon and Planets, 1998 (published in = 2000), 81, 193 Wu Z., Williams I. P., MNBAS,
1996, 280, 1210
Yeomans D. K., Icarus, 1981, 47, 492 Yeomans D. K., Yau K. K., Weissman P. R., Icarus, 1996, 124, 407 Zay G., Langbroeck
M., Zhao H. B., et al. IAUC, 1998, 7052
Zhao H. B., Li G. Y., Xu P. X., et al., KXTB=A3=AC2000=A3=AC45(5),
484
Chinese Astronomy
and Astrophysics
26 (2002) 40-48
Comet Tempel-Tuttle and the Recent Leonid Meteor Shower+* WU Guang-jie Yunnan Astronomica National
Astronomical
Observatory, Chinese Academy of Sciences, Observatories,
Kwaming 650011
Chinese Academy of Sciences,
Beijing 100012
Abstract
As one of the famous meteor showers the Leonid meteor shower had the most spectacular bursts in history. Since 1998 a new series of activity of the Leonids has been coming. What will it be in recent years and in the future? According to different references, the levels of the showers in history are reviewed and discussed in this paper. Especially, the diagram of CEOS (the Comet-Earth Orbits Separation) versus TE_C (the time gap of the Earth following the Comet) is analyzed. It is found that almost all the meteor storms are located in an inclined square, and the strongest storms are in a curved strip. The boundaries of the square have a slope of about 15 m s- l. The sizes of meteoroids calculated from this velocity are also reasonable. This means that the particles colliding with the Earth have a limited velocity to drift off its original orbit. With larger CEOS, the longer time interval after the comet passage would be needed for a possible storm. The width of this square indicates that the stronger Leonid showers would never be seen on more than about four consecutive years. In one cometary return, the percentage of bright meteors will decrease year by year. These facts could be explained by a model of the formation of a cometary dust tail, including the motion, dispersion, and breaking up of dust particles. Prediction could be made with this square region. We may have Leonid showers in 1998-2000 with a peak in 1999, and after AD 2000, one has to wait another century before significant displays will once again appear. Key words:
meteoroid
-
Leonids
-
comet
t Supported by National Natural Science Foundation Received 2000-07-24; revised version 2001-02-05 * A translation of Acto Astmn. Sin. Vol. 42, No. 2, pp. 125-133, 0275-1062/01/$-see front matter PII: SO275-1062(02)00041-3
2001
@ 2001 Elsevier Science B. V. All rights reserved.
WU Guang-jie
/ Chinese
Astronomy
and Astrophysics
26 (2002)
40-48
41
1. INTRODUCTION The November Leonid meteor showers and the occasional Leonid meteor storms are far better known than the Leonid parent body, i.e., Comet 55P/Tempel-Tuttle. The modern meteor astronomy began in the early morning of November 13, 1833, when people were woken up by a storm of Leonid meteors. In late 1865 and early 1866 Comet 55P/Tempel-Tuttle was discovered, and it had a very similar orbit to that of the Leonids with an orbital period of 33.5 yr. Le Verrier (1867) first connected the meteoroid stream to the newly discovered comet. Every year around November 17 the Earth orbiting around the Sun carries us through the Leonid meteoroid stream. The Earth would take a few days to pass through it. Since at least AD 902 the return of the Leonids in every 33 years has sometimes brought about a great meteor shower, but not always. Four of the most spectacular meteor storms have been witnessed in the past 200 yrs, namely, in 1799, 1833, 1866 and 1966. Now, we know that most storms appear to occur when the node of the cometary orbit is inside the Earth’s orbit and the Earth reaches the closest point after the comet has passed through the node, and that no storms take place more than three years from the perihelion passage date of the comet (Yeomans 1981, Williams 1997). For understanding the connection between the Leonids and its parent comet, and making predictions of future meteor showers and storms, it is necessary to study the stream and the comet appearance in more details and from different angles.
2. PREDICTIONS
AND THE LEONIDS
1998-1999
In 1998 the comet had its new return to the inner solar system (Rao 1998, Sanderson 1998). About the meteor shower astronomers gave various predictions. They predicted that “few will be seen” (Wu & Williams 1996, or WW96 for short hereafter), “modest strong” (Brown & Jones 1993, or BJ93 hereafter), or up to 40 a second for about an hour (Rae 1998, 1999). In 1998, a substantial Leonid activity occurred over ten hours earlier than predicted. $‘rom the visual records of 217 observers all over the world, Arlt (1998) got a strong, broad background component, having its maximum at X = 234.“52 with ZHR=340, and a “storm component” very close to the prediction time reaching ZHR=180. A joint observation of Chinese and Dutch astronomers in Qinghai also gave a similar result (Zhao et al. 2000). According to our observations on 5 days (Wu & Zhang 2000), the activity of the 1998 Leonids increased suddenly, but decreased slowly. We found that on November 15 and 16 the ZHR was very low (much lower than lo), but it increased steadily in the morning of November 17. We obtained a maximum number of meteor showers above 400, and it is a little larger than the result of Arlt (1998). Considering the tendency in our observations and the observations by Fitzsimmons et al. (Zay 1998, Fitzsimmons 1998), who reported about 500-700 hr-l around November 17-19, the maximum of the 1998 Leonids (supposed to be at lh40m UT according to Arlt, or 3 hours after our observations) may be up to 600-800. An intense outburst of Leonid meteors over Europe and the Middle East on 1999 November 18 was reported. Experienced visual observers in Spain and France witnessed a distinctive peak with a ZHR of around 5,000, which is considerably more than the 500 to 1,500 per hour predicted by most experts (Gyssens 1999, Phillips 1999). On the other hand, the maximum
42
WU Guang-jie / Chinese Astronomy and Astrophysics 26 (2002) 40-48
was witnessed at 2h 04m f 5m UT and coincided very well with the time of 2h 08”’ UT predicted by McNaught and Asher (1999).
3. DIAGRAM
OF TE_c
VERSUS
CEOS
AND FUTURE
PREDICTION
Major Leonids events in history and their comparison are listed in Table 1. The data mostly come from Yeomans (1981) and Jenniskens (1995), and for the ancient shower events, no quantitative data are given as to the shower rates, only subjective judgment. Table 2 Date(UT) 1999 Nov.17.8 1998 Nov.17.5 1969 Nov.17.0 1966 Nov.17.4 1965 Nov.17.0 1961 Nov.17.3 1932 Nov.16.5 1903 Nov.16.4 1901 Nov.15.9 1900 Nov.15.7 1899 Nov.15.4 1898 Nov. 1868 Nov.13.2 1867 Nov.13.8 1866 Nov.13.7 1836 Nov.13.2 1833 Nov.13.4 1832 Nov.13.2 1831 Nov.13.9 1799 Nov.12.5 1798 Nov.12.2 1698 Nov. 7.8 1666 Nov. 8.0 1625 Nov. 7.5 1602 Nov. 6.7 1601 Nov. 6.3 1566 Oct.26.4 1554 Oct.26.3 1538 Oct.25.0 1533 Oct.24.6 1532 Oct.24.5 1498 Oct.25.0 1466 Oct.24.9 1366 Oct.24.9 1238 Oct.20.4 1237 Oct.20.0 1202 Oct.18.7 1037 Oct.12.7 1035 Oct.13.2 1002 oct.13.7 967 Oct.14.0 934 Oct.13.5 931 Oct.13.6 902 Oct.12.7
CEOS(AU) -0.0080 -0.0080 -0.0032 -0.0032 -0.0032 -0.0032 -0.0062 -0.0117 -0.0117 -0.0117 -0.0117 -0.0117 -0.0065 -0.0065 -0.0065 -0.0013 -0.0013 -0.0013 -0.0013 -0.0032 -0.0032 -0.0162 -0.0043 +0.0025 +0.0102 +0.0102 -0.0024 -0.0024 -0.0065 -0.0065 -0.0065 +0.0054 +0.0107 +0.0027 -0.0031 -0.0031 -0.0059 -0.0249 -0.0249 -0.0129 -0.0064 -0.0064 -0.0064 -0.0113
Basic data of the Leonids in history G-idday) f622.3 +257.0 +1656.5 +561.0 +195.5 -1265.0 +121.4 +1591.4 +861.4 +495.8 +130.4 -235.0 +1029.9 +664.4 +299.4 +1403.7 +307.9 -50.7 -416.0 -116.9 -482.3 -345.5 +147.0 -2790.7 +830.2 +464.7 -148.5 -4531.5 +2056.2 +229.7 -135.3 -328.2 +73.5 -7.0 +1456.0 +1090.5 +612.8 +1078.1 +347.6 +633.8 +43.5 +116.4 -979.6 +597.4
Rate 5,000, St jl;OOO;sh 250, sh 150,000, St 5,000, St 156, sh 240, sh il,OOO,sh 144,000? St il,OOO, sh 40,sh sim1,500, 5,000, i2,000, 300, 50,000, 20,000,
sh St St sh St St Sh i5,000, St St sh sh sh stg stg sh St St sh sh stg sh St sh st$ sh sh sh St
pax
--
235.286 234.610 234.567 234.468
JM-N
+0.131 +0.032
(233.46) (233.46) (233.46) 233.122 232.713 232.627
<+0.550 = +0.141 +0.055
232.45 iO.02 (232.1)
43
WU Guang-jie / Chinese Astronomy and Astrophysics 26 (2002) 40-48
r
’
.s E
g*
B
a
, ‘(Y2ow .X\
‘0
l
-.
z
J
g
,St
. .
150+0
o-
s
1000
Fig. 1
<1,000 -=.. . . @ B
/.
’ :2,000 -.
. .
* . .
w
,
$000. * . k.’ .
.
--.,’
5,000
8
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-
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. .
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;
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. .
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.
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. 5,00?.* H
,
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.
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*=
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-.
~5,000 *s;_y
R
* 20,ooo
. .
. . .x
0 500 Date after the Comet crossed the plane of Earthorbit@ay)
,
. . St*
/
/
/
/
,v ’
-500
Belt of shower distribution on the diagram of TE-c versus CEOS
In Table 1 CEOS represents Comet-Earth Orbits Separation, that is, the distance (in AU) between the descending node of the comet and the nodal point where the Earth passes the comet’s orbit plane, and minus sign indicates the comet passing inside the Earth’s orbit. T&C stands for the time gap in days of the Earth following (+) or leading (-) the comet at their corresponding nodal passages. In columns 4 and 5 the time of the observed maximum shower and visual rate per hour are given. X,MAX denotes the peak position of the showers in solar longitude, and SM_N, the angle (in degrees) between the position of maximum activity and the node of the comet. By putting the historical records of Leonids into a T&C versus CEOS diagram, one can find that the maximum likelihood of a shower occurs when the Earth runs into particles outside and behind the comet (Yeomans 1981). A similar diagram Of T&c versus CEOS was used by many authors (Yeomans 1981, Sekanina 1974, McIntosh 1973). From the diagram, it can be seen clearly that the storms are concentrated not only in quadrant II, but still more in a smaller area. We find that most of the Leonid showers, especially storms, are concentrated in an inclined square, except a few events (Wu 1999). For those exceptional dates, for instance, AD 1002, 1238 and 1601, there is no quantitative information, and some events may be doubtful. But even if these three events could indeed be called =Al=BOstorm=Al=Bl, it will not affect our discussion of the concentrated region. According to the analysis and the data summarized in Table 1, and for the sake of clarity, only the square part is plotted in Fig. 1. In the figure, a “dot” denotes a shower, a “star”, a storm with ZHR >2,000. For those “storms” before 1799 based only on subjective judgement, they are marked with hollow stars. This square region is further divided into 3x3 blocks of about equal size. The ZHR, whenever available, is marked alongside the symbol. Block A: Two storms in AD 1532 and 1798, but no ZHR estimate.
44
WU Gang-jie
/ Chinese Astronomy and Astrophysics
26 (2002) 40-48
Block B: No storm, only 4 showers. Block C: One storm in AD 902 and one possible storm in AD 1002, but no ZHR estimate. Block D: Three storms. The ZHR values could be up to 5,000, even 20,000, except the storm in AD 1566. Block E: Three storms. The ZHR recorded in two events could be 2,000-5,000. Block F: Four storms. In two events the ZHR is about 5,000. For the AD 1901 storm, a very high value of 144,000 was reported (Ferrin thought it seems to be a misprint, see Ferrin 1999), but 7,000 or less than 2,000 were also reported. Block G: One storm, without ZHR estimate. Block H: Two storms. The ZHR values are extraordinarily large as 50,000-150,000. Block I: No storm, but one shower in AD 1868. In addition, the probability of strongest storms is greater in the middle strip (more accurately, in the DHF curved strip) than on either side. Moreover, in history, the showers in the upper-right strip ABC may have a larger percentage of bright meteors than others. The width of the square indicates that the stronger Leonid showers appear at most in four consecutive years and, in general, only for two or three consecutive years. We note that the upper-right and lower-left boundaries have a slope of about 14.7 m s-l. Yeomans (1981) thought that the CEOS gives a distance measure of shower particles from the comet’s orbit at the time of the particle-Earth collision. If the particles traveled 0.008 AU in 257.0 d for the 1998 shower, the average velocity would be 54 m s-i. They are both in the same order of magnitude as the velocity of ejection of the particles, Veject=36 m s-l (Brown & Jones 1998). Of course, if the particles came from the ejection in an older cometary return, the off-orbit velocity would be much smaller than 54 m s-l. In virtue of the location of the year AD 1999 (in Block F), we predicted that Leonids in 1999 would be 3 to 5 times stronger than that in 1998 with ZHR=2,000-5,000 (Wu 1999). It is shown that the method is successful for ZHR prediction. Because the location of AD 2000 is at the outer part of Block I, we estimate that there will be a ZHR of 800-1,500, i.e. only a shower. As for AD 2001 and after, the points are far away from this inclined square. Also, the CEOS for this cometary return is 2.7 times that of AD 1238. Therefore, we believe that the Leonid showers have passed their peak, and that there can be no more large bursts.
4. DISCUSSION Some authors calculated the gravitational resonance of ejected particles with Jupiter, which can keep the trails undispersed over many orbital revolutions. The peak time they predicted for Leonids 1999 is surprisingly good (Asher et al.1999, McNaught & Asher 1999). In this paper, we can see that the TE_C N CEOS diagram could also give us some interesting results, especially about the strength of the showers. From Fig.3 of Yeomans et al. (Yeomans, Yau, and Weissman 1996, or YYW96 hereafter), in 934, 1238, 1566 and 1833, the CEOS indeed have very small minus values, and the smallest distance of the node of the cometary orbit from the Earth’s orbit does coincide with the greatest storm. On the other hand, the level of a shower can not be determined only by the value of CEOS. The inclined square tells us that if CEOS has a large value, we
WU Guang-jie / Chinese Astronomy and Astrophysics
Fig. 2
26 (2002) 40-48
45
Trajectories of particles and formation of the dust tail
may need to wait a longer time for a storm as the meteoroids have a limited velocity. This square can also explain why people had to wait till AD 1900 for a shower, since the point 1899 was outside the square. In the standard model for the formation of meteoroid streams, the ejection of the meteoroids from the cometary nucleus depends on the particular ejection model adopted. We think that the ejection from the comet is not isotropic, especially, the ejected material can form a dust tail due to the effect of the solar wind. The fact that quadrants III and IV are nearly empty may imply that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the particles. Figure 2 gives a sketch of the formation of a dust tail. It is obvious that the orbit of particles is much different from that of the comet. According to the theory of syndynames and synchrones of a dust tail, small particles will have a larger ratio of radiation pressure to gravitational force (1 - p). The principal effect is to “weaken” gravity so that the quantity GM is replaced by GM(l - p), where (1 - p) has the value of 5.75~1O-~/rp,r being the radius and p the bulk density of the grain, both in cgs units. This means that small particles have large velocities than large particles. The larger particles tend to hang around the comet because they leave its nucleus at a lower speed than their small brethren (Krishna Swamy 1986, Williams 1997). In this paper, we obtain a slope with a speed of 14.7m s-l. Suppose that the dust grains have an off-orbit velocity increment of 15 m s-l in the radial direction from the Sun under the action of solar radiation pressure after traveling for time t, then we have the
46
WV Guang-jie / Chinese Astronomy and Astrophysics
momentum
26 (2002) 40-48
relation
m(V,+v-mv,
=Ft
(1)
and F=(l-v)ma&
(2)
where V, is the off-orbit component of the ejection velocity of the particles, a is the gravitational acceleration on the solar surface, and I& is the solar radius in units of AU. Then we have
(1-CL)= &
=5.75 x lo-s/rp
and rcxt/v.
(4)
It is shown that smaller particles would get larger acceleration and meet the Earth earlier. Considering the bulk density p = 800 kgme3 (Brown & Jones 1998), the radius of the dust grain can be expressed as , r = 2.470 x 10-3t,
(5)
where t is in units of day, and r in centimeter. If we suppose that it is the particles ejected in the present return that form the Leonids of 1998-2000, that is, if t is equal to TE-C, then AD 1998: t = 257 d, r = 0.635 cm , it corresponds to a meteoroid of -1 magnitude, AD 1999: t = 622d r = 1.536cm, it corresponds to a meteoroid of -5 magnitude, AD 2000: t = 98 d, r = 2.440 cm, it corresponds to a meteoroid of -8 magnitude. These results are acceptable. If the time t is very long, it might seem that the meteoroids should be very large. However, in the long time traveling to the Earth, particles of big size will break up into many smaller pieces. It was proved that dust grains with a mass as small as lo-‘* g were detected at a distance of lo5 - lo6 km from the nucleus. Breaking-up has been verified for Comet Halley, and the resulting number density falls off as r-r rather than rA2, but what process leads to the break-up is unknown (Utterback & Kissel 1990). If the ejection point is very far away from the descending node, or the CEOS itself is large, the process of breaking-up of particles may become very serious. In addition, the process of dispersion of the particle stream may also become serious. In this situation, it is not easy to see a meteor storm by naked eye. This may be the reason why the Leonid shower could not be seen in more than three or four consecutive years, in general. As for the peak time of a shower, it can not be predicted from the diagram of TE-_C versus CEOS, but we can estimate it from historical records. Using the data in Table 1, the long term variation of Xrax may be expressed as , max
&l
= 203.“14 + O.“0159yr-r x T
(6)
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/ Chinese
Astronomy
and Astrophysics
26 (2002)
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40-48
where T is the year of calculation. The rate, O.“0159yr-‘, could be thought to be due to the variation of comet’s orbit. Indeed, it agrees well with the variation of the “Node” value calculated by YYW96, which varies by about 28 degrees from AD 604 to 2530 (or O.“0145/yr ). This comparison also gives a result that, in general, the shower maximum happens about lh14”after the Earth passing through the comet’s orbital plane. In this paper, our discussion mainly focuses on the inclined square, in which Leonid storms are concentrated. Indeed, there are few storms outside the square. Even in the square area, though, the distribution of the showers and storms is not uniform. This inclined area can provide us some useful relations and single out some dominating factors. Ferrin (1999) has drawn out a particle density distribution around the comet and isolines in a similar diagram. His prediction for the Leonids 1999 (ZHR=3.5KflK) is also successful. As for AD 2000, he predicted that the ZHR would be up to 5K-20K, very different from our prediction. For the future, YYW96 gave the orbital solution and elements for 55P/TempelTuttle. It is easy to see that at least both the angle w and the longitude of the descending node increase continuously from AD 604 to 2530, except for small fluctuations in the former. In their Fig. 3, it is obvious that CEOS has a tendency of moving from negative to positive values over the years AD O-2500 while under rather large fluctuations. Before AD 600 the CEOS was too large, so it is possible that nobody could see a Leonid meteor shower then. In addition, the Leonid shower may vanish forever after AD 2500. Good Leonids shower displays could be seen only from about AD 895 to 2170. Because of planetary perturbations, it will be another century after the 1998-2000 events before significant Leonid meteor displays are once again visible. References Arlt R., WGN J.IMO,
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