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Increasing Geminid meteor shower activity

Increasing Geminid meteor shower activity MNRAS 475, L77–L80 (2018) doi:10.1093/mnrasl/slx205 Advance Access publication 2018 January 4 1‹ 2 G. O. Ryabova and J. Rendtel Research Institute of Applied Mathematics and Mechanics of Tomsk State University, Lenin pr. 36, 634050 Tomsk, Russian Federation Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany Accepted 2017 December 14. Received 2017 December 8; in original form 2017 November 11 ABSTRACT Mathematical modelling has shown that activity of the Geminid meteor shower should rise with time, and that was confirmed by analysis of visual observations 1985–2016. We do not expect any outburst activity of the Geminid shower in 2017, even though the asteroid (3200) Phaethon has a close approach to Earth in December of 2017. A small probability to observe dust ejected at perihelia 2009–2016 still exists. Key words: methods: data analysis – methods: numerical – meteorites, meteors, meteoroids – minor planets, asteroids: individual: (3200) Phaethon. the results with the modelling, and to explain why we expect the 1 INTRODUCTION increase in activity. The Geminid meteor shower is an annual major shower with the maximum activity near December 14. In 2017, asteroid (3200) 2MODEL Phaethon (the parent body of the stream) had a close encounter with the Earth on December 16, and the minimum distance was 2.1 Activity calculated to be 0.0689 au (Galushina, Ryabova & Skripnichenko 2015). Ryabova (2016) presented the Geminid meteoroid stream models, When a comet approaches the Sun, it is expected that its me- consisting of 30 000 meteoroids with fixed masses (0.02, 0.003, teor shower (if it exists) will display enhanced activity. The young and 0.0003 g). The stream was generated around starting epoch stream, ejected from the comet on the previous revolution, had in- JD 1720165.2248 (perihelion passage) using the cometary scenario sufficient time to spread around the orbit, so the swarm located in of ejection. For details of the model, method, and references, see the vicinity of the nucleus can produce outburst meteor activity on Ryabova (2016). We used one of these models with meteoroids of the Earth. However, this is not necessarily the case, and with high the ‘visual’ mass of 0.02 g and extended it until 2025 January 1. We probability is not applicable to the Phaethon–Geminid complex. tracked the encounters with the Earth assuming that model mete- A mathematical model, which has been developed over the last oroids approaching the Earth to a distance ≤0.02 au are recorded at 20 yr [see Ryabova (2007, 2016) and references therein], gives the the Earth. following scenario of the Geminid stream formation. About two Fig. 1 demonstrates that the encounter rate increases. In the last thousand years ago, comet Phaethon was captured on an orbit with 125 yr, it can be approximated by the linear equation perihelion distance 0.10–0.12 au. The catastrophic release of par- N (yr) = 0.041 × yr − 53.2, (1) enc ticles and volatiles caused a dramatic transformation of the orbit. The Geminids were generated during a short time and had no re- where N is the encounter rate, and yr is the year. However, the enc plenishment after that. This scenario does not imply an increase complete period is fitted best by the polynomial or the power-law of the Geminid shower activity during Phaethon’s encounter with fit: the Earth. Phaethon approaching the Earth does not mean that the −5 2 N (yr) = 25.93 − 0.056 × yr + 2.88 × 10 yr , (2) enc Geminid stream core approaches the Earth. Analysis of 60 yr of visual observations (1944–2003) has shown −20 6.375 N (yr) = 3.18 × 10 yr . (3) enc that the shower activity is rather stable. Meanwhile, the modelling fulfilled in anticipation of the Phaethon encounter has shown that the shower activity should grow. The aim of our research was to 2.2 Why activity increases revisit the previous analysis of the visual observations (Rendtel 2004) adding 13 yr of observations made since then, to compare The model Geminid stream undergoes strong anisotropic disper- sion (Ryabova 2007, section 3.1 and fig. 7). Its intersection with the ecliptic plane progressively stretches (Fig. 2) and at the same time it moves away from the Sun. Phaethon’s node and the mean E-mail: goryabova@gmail.com orbit of the stream (i.e. the densest part of the stream) gradually 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L78 G. O. Ryabova and J. Rendtel approach the Earth’s orbit. So the Geminid shower activity slowly increases. We should mention that the Poynting–Robertson drag makes meteoroids (but not the asteroid) spiral slowly towards the Sun, hence the node of the mean stream orbit (=the densest part or the stream core) also shifts towards the Sun. That is why the cyan cross separates from the red cross in Fig. 2 with time. However, the drift in the evolution from gravitational effects is much stronger. Phaethon’s node should intersect the Earth’s orbit about 2200, and the model Geminid stream core some time later. After that, the Geminid shower activity should decrease. In 2200, the activity could reach 140 per cent of its current rate according to fit (2). Studying the evolution of the minimal distance between the nom- inal orbit of Phaethon and the Earth’s orbit, Jakub´ık & Neslusan ˇ (2015) have already implied that ‘the activity of this shower is cur- rently increasing’ (i.e. Geminid shower). In essence, this is correct, but the process is more complicated, as we see. 3 OPTICAL OBSERVATIONS OF GEMINIDS 1985–2016 The present analysis differs from the previous one (Rendtel 2004) in three essential features. First, here we use only homogeneous ob- servations. The global data used for this study are stored in the Visual Meteor Data Base (VMDB) of the International Meteor Organization (IMO). It is available from the website www.imo.net. The analysis presented in Rendtel (2004) ends with the 2003 return. Right now, we can add another 13 yr at the end of the series. The second difference is that here we concentrate on the temporal evolution of the Geminid peak activity. So we calculated Zenithal ◦ ◦ Hourly Rate (ZHR) only for the peak period between 261 –262 in solar longitude. Thirdly, we used a constant population index r = 2.4. Since we are interested in the long-term evolution, we omitted a separate r determination per return. This last point needs some explanation. The ZHR is the number of shower meteors N seen per hour by Figure 1. Model meteoroid encounters (up to 0.02 au) with the Earth as a function of time. (a) For the entire period of the stream’s existence. one observer under standard conditions, i.e. radiant in the zenith The cyan line shows the polynomial fit (2), and the blue line shows the (elevation h = 90 ), limiting stellar magnitude LM =+6.5 mag, power fit (3). (b) For XX and XXI centuries. The red line shows the linear and unlimited field of view. The latter translates into an effective fit (1). ◦ field of view of 52 radius (Koschack & Rendtel 1990). For the correction, the population index r describing the increase of the number of meteors in subsequent magnitude classes is essential: (6.5−LM) c Nr ZHR = , (4) T sin h with c being the correction for the geometrical field obstruction. Hence, the knowledge of the population index r needs to be derived for each return of the shower. It has been found that r varies during the Geminid activity period. For most of the activity period of the Geminids, we find r = 2.6, but towards the peak it decreases to 2.5–2.1. Detailed analyses have been published for the well- documented returns in 1993 (Arlt & Rendtel 1994), 1996 (Rendtel &Arlt 1997), and 2004 (Arlt & Rendtel 2006). So for our search of the peak ZHR, we applied r = 2.4 which is typical for the main peak section of the activity profile. The resulting ZHR for all returns is shown in Fig. 3. The increase Figure 2. The model Geminid stream (100 particles, mass = 0.02 g) cross- of the peak ZHR with time is obvious in the interval under study sections in the ecliptic plane at the descending node in the years 1000, 1500, and can be described by a fit like 1700, and 2000. Nodes of the meteoroids are designated by black dots. The small cyan cross (+) is the node of Phaethon’s orbit, the large red cross ZHR(yr) = 1.79 × yr − 3431.41. (5) is the Geminid’s mean orbit node, and the blue line is the Earth’s orbit. The horizontal line shows that the cross-sections are aligned. We used the Observations under poor conditions, such as bright moonlight, standard heliocentric ecliptic system as reference system. may suffer from a small sample and from an undercorrection. The MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Increasing Geminid meteor shower activity L79 so the activity increase became apparent. Only during the last 17 yr (i.e. 2000–2016) activity has increased by 20 per cent! 4.2 Modelling and observations Model shower activity increase is obvious in Fig. 1(a) and not so obvious in Fig. 1(b). During 2000–2017, the activity growth comes to only 2.3 per cent, if we use the linear fit (1), 3.2 per cent for the power fit (3), and 5.2 per cent for the polynomial fit (2). Why is there such a striking difference with observations? The quality of the fit (1) is not good (coefficient of determination here is only 0.07), and the reason could be too small an amount of data points used (namely, 126). However, for equations (2) and (3), the coefficients of determination are 0.89 and 0.83, respectively. So the reason of the difference is not the quality of the fit, but the model itself. The model by Ryabova (2007, 2016)isa qualitative model. Figure 3. Activity level of the Geminids in the period 1985–2016. We show ◦ ◦ It has two significant (and probably unremovable) discrepancies the peak value of the visual ZHR for each year in the period 261 –262 in solar longitude as well as the maximum flux obtained from the IMO Video – in the location and in the width – with the real stream. The Meteor Network data since 2011. The line shows a fit of the visual ZHR discrepancies and their reason were discussed in detail in Ryabova derived for the period 1985–2016. (2007, 2016), and also in earlier papers. Being a qualitative model, it shows trends and patterns in the stream behaviour and structure, latter was found in several analyses of different meteor showers and and explains them. It is not intended for numerical estimations of is probably due to perception changes. In fact, the Geminid returns the stream parameters though. of 1997, 2003, and 2005 (below the fitted line) occurred with bright moonlight. Other low values (1986, 1987, 2015) are not affected in 4.3 Could the ‘thermal’ Geminids be observed in 2017? the same way (no moonlight). Another example is the 2008 return which coincided with the full Moon, but deviated in the opposite di- Phaethon was discovered in 1983 (as 1983TB) and no activity was rection. This suggests rather fluctuations between individual stream observed till in 2009 Phaethon brightened by 2 mag just after peri- encounters and needs further investigation. The lower ZHR in 2015 helion (Jewitt & Li 2010). The similar two-days brightening around found in both video and visual data independently also supports that perihelion was also observed in 2012 (Li & Jewitt 2013) and 2016 we are able to detect real stream features. (Hui & Li 2017). The most plausible reason of this phenomenon Another supporting and independent data sample is the collection is an ejection of dust generated by thermal fracture (Jewitt & Li of video flux data derived from the data stored by the IMO Video 2010). Very soon, it became clear that these perihelion outbursts are Meteor Network (Molau & Barentsen 2014). The flux can be calcu- not the main component of the stream (Li & Jewitt 2013, Ryabova lated via the access to meteorflux.io for each year since 2011. 2015,Hui &Li 2017). Ryabova (2012) found that 0.03 per cent We added the respective flux values given as number of meteors of the model dust swarm ejected in 2009 approached the Earth to intersecting the normal area in 1000 km per hour. The peak values distances 0.018–0.03 au in 2017. We made similar modelling [using of the visual and video data show the same pattern, supporting that the technique described in Ryabova (2012)] for ejections in 2012 the variations are not caused by observing conditions or selection and 2016. In all three cases, particles approach the Earth at solar lon- ◦ ◦ effects but are real. gitudes 262.45 ± 0.005 to distances down to 0.019 au. The radius of the Earth’s sphere of influence is about 0.03 au, so considering possible variations in ejection models and additional gravitational 4 DISCUSSION scattering close to Earth, there is a small probability that these me- teoroids reach Earth to become observable. It is difficult to estimate 4.1 The previous and the current analyses of observations whether we can distinguish them from the ‘regular’ Geminids. Despite the fact that the Geminids are currently the most active meteor shower, it gained rather little attention in the past. Like with 4.4 The previous Phaethon close encounters with the Earth many other meteor showers, our knowledge of the activity level more than about 30–40 yr back is scarce. So our previous analysis In the XX century, Phaethon had two close encounters with the for the Geminids (Rendtel 2004) was based on two kinds of observa- Earth: 1931 December 13 (0.038 39 au) and 1974 December 16 tions. Several series were taken from the literature and re-processed (0.054 74 au). Though we believe that the Geminid activity does to find ZHR (1944–1984). Other series (1985–2003) were taken not depend on Phaethon’s position, it is worth checking the old from IMO VMDB, and they were collected and processed accord- observations. ing to the standardized procedure. The Geminid shower activity The situation turned out to be unpromising for both times. There was found to be rather stable. A slight increase was noticed and are descriptions of single observations in 1933 and 1934 which do mentioned, but within the error margins. For the new analysis, we not allow us to derive an activity level (Millman 1934, 1935). This used only homogeneous series of visual observations from VMDB also holds for reports of some 1974 Geminid visual observations (1985–2016). compiled in the magazine Sky & Telescope (anon. 1975, p. 194). The previous analysis also included data for all the shower ac- A calibration is not possible because we have information neither tivity period. That led to the large scatter, and the trend in ZHR about the observing conditions nor on activity observed in the adja- had substantial noise. Confining ourselves to the peak period and cent years nor on the number of sporadic meteors recorded during constant population index r, we eliminated some part of this noise, these observations. The fact that a high activity was mentioned may MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L80 G. O. Ryabova and J. Rendtel mean that the maximum itself was not observed in other years. Arlt R., Rendtel J., 2006, MNRAS, 367, 1721 Berthier J., Carry B., Vachier F., Eggl S., Santerne A., 2016, MNRAS, 458, There may be still treasures in some archives. The last point applies also to the asteroid (3200) Phaethon. Now, Galushina T. Yu., Ryabova G. O., Skripnichenko P. V., 2015, Planet. Space when many astronomical archives are digitized and some useful Sci., 118, 296 tools for old asteroid observation searches, like SKYBOT(Berthier Hui M.-T., Li J., 2017, AJ, 153, 23 et al. 2016; IMCCE VO 2017), are accessible for the research IMCCE VO, 2017, SkyBoT – The Virtual Observatory Sky Body Tracker. community, there is a chance to find pre-discovery observations Available at: http://vo.imcce.fr/webservices/skybot/ of Phaethon. Jakub´ık M., Neslusan ˇ L., 2015, MNRAS, 453, 1186 Jewitt D., Li J., 2010, AJ, 140, 1519 Koschack R., Rendtel J., 1990, WGN, 18, 44 5 CONCLUDING REMARKS Li J., Jewitt D., 2013, ApJ, 145, 154 Millman P. M., 1934, JRASC, 28, 35 We analysed visual observations of the Geminid shower in Millman P. M., 1935, JRASC, 29, 115 1985–2016 around the shower maximum using homogeneous se- Molau S., Barentsen G., 2014, in Jopek T. J., Rietmeijer F. J. M., Watanabe ries of visual observations. It was found that the shower activity J., Williams I. P., eds, Proc. Astronomical Conference held at A.M. slowly increases. The same was obtained for video observations University, Meteoroids 2013. A.M. University Press, Poznan, ´ Poland, (2011–2016). These results were supported and explained by math- p. 297 ematical modelling. Activity of the shower increases because the Rendtel J., 2004, Earth Moon Planets, 95, 27 core of the Geminid stream moves towards the Earth. Rendtel J., Arlt R., 1997, WGN, 25, 75 A small probability exists that particles ejected in 2009 around Ryabova G. O., 2007, MNRAS, 375, 1371 perihelion of Phaethon’s orbit could be observed at solar longitudes Ryabova G. O., 2012, MNRAS, 423, 2254 ◦ ◦ 262.45 ± 0.005 (J2000.0). Ryabova G. O., 2015, Could the Geminid meteoroid stream be the result of long-term thermal fracture? Vol. 10 in European Planetary Space We do not expect any outburst Geminid activity in 2017, but Science Congress 2015. Nantes, France, EPSC2015-754, available at: only a wide observational campaign could validate or disprove our http://meetingorganizer.copernicus.org/EPSC2015/EPSC2015-754.pdf theoretical expectations. Ryabova G. O., 2016, MNRAS, 456, 78 ACKNOWLEDGEMENTS GOR was supported by Ministry of Education and Science of the Russian Federation project no. 9.9063.2017/BCh. This research has made use of NASA’s Astrophysics Data System. The authors thank the anonymous referee, who put forward several very relevant ques- tions. We are grateful to David Asher for reading and discussing the manuscript. REFERENCES Anon, 1975, Sky Telesc., March, 1975, 193 Arlt R., Rendtel J., 1994, WGN, 22, 167 This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Letters Oxford University Press

Increasing Geminid meteor shower activity

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Abstract

MNRAS 475, L77–L80 (2018) doi:10.1093/mnrasl/slx205 Advance Access publication 2018 January 4 1‹ 2 G. O. Ryabova and J. Rendtel Research Institute of Applied Mathematics and Mechanics of Tomsk State University, Lenin pr. 36, 634050 Tomsk, Russian Federation Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany Accepted 2017 December 14. Received 2017 December 8; in original form 2017 November 11 ABSTRACT Mathematical modelling has shown that activity of the Geminid meteor shower should rise with time, and that was confirmed by analysis of visual observations 1985–2016. We do not expect any outburst activity of the Geminid shower in 2017, even though the asteroid (3200) Phaethon has a close approach to Earth in December of 2017. A small probability to observe dust ejected at perihelia 2009–2016 still exists. Key words: methods: data analysis – methods: numerical – meteorites, meteors, meteoroids – minor planets, asteroids: individual: (3200) Phaethon. the results with the modelling, and to explain why we expect the 1 INTRODUCTION increase in activity. The Geminid meteor shower is an annual major shower with the maximum activity near December 14. In 2017, asteroid (3200) 2MODEL Phaethon (the parent body of the stream) had a close encounter with the Earth on December 16, and the minimum distance was 2.1 Activity calculated to be 0.0689 au (Galushina, Ryabova & Skripnichenko 2015). Ryabova (2016) presented the Geminid meteoroid stream models, When a comet approaches the Sun, it is expected that its me- consisting of 30 000 meteoroids with fixed masses (0.02, 0.003, teor shower (if it exists) will display enhanced activity. The young and 0.0003 g). The stream was generated around starting epoch stream, ejected from the comet on the previous revolution, had in- JD 1720165.2248 (perihelion passage) using the cometary scenario sufficient time to spread around the orbit, so the swarm located in of ejection. For details of the model, method, and references, see the vicinity of the nucleus can produce outburst meteor activity on Ryabova (2016). We used one of these models with meteoroids of the Earth. However, this is not necessarily the case, and with high the ‘visual’ mass of 0.02 g and extended it until 2025 January 1. We probability is not applicable to the Phaethon–Geminid complex. tracked the encounters with the Earth assuming that model mete- A mathematical model, which has been developed over the last oroids approaching the Earth to a distance ≤0.02 au are recorded at 20 yr [see Ryabova (2007, 2016) and references therein], gives the the Earth. following scenario of the Geminid stream formation. About two Fig. 1 demonstrates that the encounter rate increases. In the last thousand years ago, comet Phaethon was captured on an orbit with 125 yr, it can be approximated by the linear equation perihelion distance 0.10–0.12 au. The catastrophic release of par- N (yr) = 0.041 × yr − 53.2, (1) enc ticles and volatiles caused a dramatic transformation of the orbit. The Geminids were generated during a short time and had no re- where N is the encounter rate, and yr is the year. However, the enc plenishment after that. This scenario does not imply an increase complete period is fitted best by the polynomial or the power-law of the Geminid shower activity during Phaethon’s encounter with fit: the Earth. Phaethon approaching the Earth does not mean that the −5 2 N (yr) = 25.93 − 0.056 × yr + 2.88 × 10 yr , (2) enc Geminid stream core approaches the Earth. Analysis of 60 yr of visual observations (1944–2003) has shown −20 6.375 N (yr) = 3.18 × 10 yr . (3) enc that the shower activity is rather stable. Meanwhile, the modelling fulfilled in anticipation of the Phaethon encounter has shown that the shower activity should grow. The aim of our research was to 2.2 Why activity increases revisit the previous analysis of the visual observations (Rendtel 2004) adding 13 yr of observations made since then, to compare The model Geminid stream undergoes strong anisotropic disper- sion (Ryabova 2007, section 3.1 and fig. 7). Its intersection with the ecliptic plane progressively stretches (Fig. 2) and at the same time it moves away from the Sun. Phaethon’s node and the mean E-mail: goryabova@gmail.com orbit of the stream (i.e. the densest part of the stream) gradually 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L78 G. O. Ryabova and J. Rendtel approach the Earth’s orbit. So the Geminid shower activity slowly increases. We should mention that the Poynting–Robertson drag makes meteoroids (but not the asteroid) spiral slowly towards the Sun, hence the node of the mean stream orbit (=the densest part or the stream core) also shifts towards the Sun. That is why the cyan cross separates from the red cross in Fig. 2 with time. However, the drift in the evolution from gravitational effects is much stronger. Phaethon’s node should intersect the Earth’s orbit about 2200, and the model Geminid stream core some time later. After that, the Geminid shower activity should decrease. In 2200, the activity could reach 140 per cent of its current rate according to fit (2). Studying the evolution of the minimal distance between the nom- inal orbit of Phaethon and the Earth’s orbit, Jakub´ık & Neslusan ˇ (2015) have already implied that ‘the activity of this shower is cur- rently increasing’ (i.e. Geminid shower). In essence, this is correct, but the process is more complicated, as we see. 3 OPTICAL OBSERVATIONS OF GEMINIDS 1985–2016 The present analysis differs from the previous one (Rendtel 2004) in three essential features. First, here we use only homogeneous ob- servations. The global data used for this study are stored in the Visual Meteor Data Base (VMDB) of the International Meteor Organization (IMO). It is available from the website www.imo.net. The analysis presented in Rendtel (2004) ends with the 2003 return. Right now, we can add another 13 yr at the end of the series. The second difference is that here we concentrate on the temporal evolution of the Geminid peak activity. So we calculated Zenithal ◦ ◦ Hourly Rate (ZHR) only for the peak period between 261 –262 in solar longitude. Thirdly, we used a constant population index r = 2.4. Since we are interested in the long-term evolution, we omitted a separate r determination per return. This last point needs some explanation. The ZHR is the number of shower meteors N seen per hour by Figure 1. Model meteoroid encounters (up to 0.02 au) with the Earth as a function of time. (a) For the entire period of the stream’s existence. one observer under standard conditions, i.e. radiant in the zenith The cyan line shows the polynomial fit (2), and the blue line shows the (elevation h = 90 ), limiting stellar magnitude LM =+6.5 mag, power fit (3). (b) For XX and XXI centuries. The red line shows the linear and unlimited field of view. The latter translates into an effective fit (1). ◦ field of view of 52 radius (Koschack & Rendtel 1990). For the correction, the population index r describing the increase of the number of meteors in subsequent magnitude classes is essential: (6.5−LM) c Nr ZHR = , (4) T sin h with c being the correction for the geometrical field obstruction. Hence, the knowledge of the population index r needs to be derived for each return of the shower. It has been found that r varies during the Geminid activity period. For most of the activity period of the Geminids, we find r = 2.6, but towards the peak it decreases to 2.5–2.1. Detailed analyses have been published for the well- documented returns in 1993 (Arlt & Rendtel 1994), 1996 (Rendtel &Arlt 1997), and 2004 (Arlt & Rendtel 2006). So for our search of the peak ZHR, we applied r = 2.4 which is typical for the main peak section of the activity profile. The resulting ZHR for all returns is shown in Fig. 3. The increase Figure 2. The model Geminid stream (100 particles, mass = 0.02 g) cross- of the peak ZHR with time is obvious in the interval under study sections in the ecliptic plane at the descending node in the years 1000, 1500, and can be described by a fit like 1700, and 2000. Nodes of the meteoroids are designated by black dots. The small cyan cross (+) is the node of Phaethon’s orbit, the large red cross ZHR(yr) = 1.79 × yr − 3431.41. (5) is the Geminid’s mean orbit node, and the blue line is the Earth’s orbit. The horizontal line shows that the cross-sections are aligned. We used the Observations under poor conditions, such as bright moonlight, standard heliocentric ecliptic system as reference system. may suffer from a small sample and from an undercorrection. The MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Increasing Geminid meteor shower activity L79 so the activity increase became apparent. Only during the last 17 yr (i.e. 2000–2016) activity has increased by 20 per cent! 4.2 Modelling and observations Model shower activity increase is obvious in Fig. 1(a) and not so obvious in Fig. 1(b). During 2000–2017, the activity growth comes to only 2.3 per cent, if we use the linear fit (1), 3.2 per cent for the power fit (3), and 5.2 per cent for the polynomial fit (2). Why is there such a striking difference with observations? The quality of the fit (1) is not good (coefficient of determination here is only 0.07), and the reason could be too small an amount of data points used (namely, 126). However, for equations (2) and (3), the coefficients of determination are 0.89 and 0.83, respectively. So the reason of the difference is not the quality of the fit, but the model itself. The model by Ryabova (2007, 2016)isa qualitative model. Figure 3. Activity level of the Geminids in the period 1985–2016. We show ◦ ◦ It has two significant (and probably unremovable) discrepancies the peak value of the visual ZHR for each year in the period 261 –262 in solar longitude as well as the maximum flux obtained from the IMO Video – in the location and in the width – with the real stream. The Meteor Network data since 2011. The line shows a fit of the visual ZHR discrepancies and their reason were discussed in detail in Ryabova derived for the period 1985–2016. (2007, 2016), and also in earlier papers. Being a qualitative model, it shows trends and patterns in the stream behaviour and structure, latter was found in several analyses of different meteor showers and and explains them. It is not intended for numerical estimations of is probably due to perception changes. In fact, the Geminid returns the stream parameters though. of 1997, 2003, and 2005 (below the fitted line) occurred with bright moonlight. Other low values (1986, 1987, 2015) are not affected in 4.3 Could the ‘thermal’ Geminids be observed in 2017? the same way (no moonlight). Another example is the 2008 return which coincided with the full Moon, but deviated in the opposite di- Phaethon was discovered in 1983 (as 1983TB) and no activity was rection. This suggests rather fluctuations between individual stream observed till in 2009 Phaethon brightened by 2 mag just after peri- encounters and needs further investigation. The lower ZHR in 2015 helion (Jewitt & Li 2010). The similar two-days brightening around found in both video and visual data independently also supports that perihelion was also observed in 2012 (Li & Jewitt 2013) and 2016 we are able to detect real stream features. (Hui & Li 2017). The most plausible reason of this phenomenon Another supporting and independent data sample is the collection is an ejection of dust generated by thermal fracture (Jewitt & Li of video flux data derived from the data stored by the IMO Video 2010). Very soon, it became clear that these perihelion outbursts are Meteor Network (Molau & Barentsen 2014). The flux can be calcu- not the main component of the stream (Li & Jewitt 2013, Ryabova lated via the access to meteorflux.io for each year since 2011. 2015,Hui &Li 2017). Ryabova (2012) found that 0.03 per cent We added the respective flux values given as number of meteors of the model dust swarm ejected in 2009 approached the Earth to intersecting the normal area in 1000 km per hour. The peak values distances 0.018–0.03 au in 2017. We made similar modelling [using of the visual and video data show the same pattern, supporting that the technique described in Ryabova (2012)] for ejections in 2012 the variations are not caused by observing conditions or selection and 2016. In all three cases, particles approach the Earth at solar lon- ◦ ◦ effects but are real. gitudes 262.45 ± 0.005 to distances down to 0.019 au. The radius of the Earth’s sphere of influence is about 0.03 au, so considering possible variations in ejection models and additional gravitational 4 DISCUSSION scattering close to Earth, there is a small probability that these me- teoroids reach Earth to become observable. It is difficult to estimate 4.1 The previous and the current analyses of observations whether we can distinguish them from the ‘regular’ Geminids. Despite the fact that the Geminids are currently the most active meteor shower, it gained rather little attention in the past. Like with 4.4 The previous Phaethon close encounters with the Earth many other meteor showers, our knowledge of the activity level more than about 30–40 yr back is scarce. So our previous analysis In the XX century, Phaethon had two close encounters with the for the Geminids (Rendtel 2004) was based on two kinds of observa- Earth: 1931 December 13 (0.038 39 au) and 1974 December 16 tions. Several series were taken from the literature and re-processed (0.054 74 au). Though we believe that the Geminid activity does to find ZHR (1944–1984). Other series (1985–2003) were taken not depend on Phaethon’s position, it is worth checking the old from IMO VMDB, and they were collected and processed accord- observations. ing to the standardized procedure. The Geminid shower activity The situation turned out to be unpromising for both times. There was found to be rather stable. A slight increase was noticed and are descriptions of single observations in 1933 and 1934 which do mentioned, but within the error margins. For the new analysis, we not allow us to derive an activity level (Millman 1934, 1935). This used only homogeneous series of visual observations from VMDB also holds for reports of some 1974 Geminid visual observations (1985–2016). compiled in the magazine Sky & Telescope (anon. 1975, p. 194). The previous analysis also included data for all the shower ac- A calibration is not possible because we have information neither tivity period. That led to the large scatter, and the trend in ZHR about the observing conditions nor on activity observed in the adja- had substantial noise. Confining ourselves to the peak period and cent years nor on the number of sporadic meteors recorded during constant population index r, we eliminated some part of this noise, these observations. The fact that a high activity was mentioned may MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L80 G. O. Ryabova and J. Rendtel mean that the maximum itself was not observed in other years. Arlt R., Rendtel J., 2006, MNRAS, 367, 1721 Berthier J., Carry B., Vachier F., Eggl S., Santerne A., 2016, MNRAS, 458, There may be still treasures in some archives. The last point applies also to the asteroid (3200) Phaethon. Now, Galushina T. Yu., Ryabova G. O., Skripnichenko P. V., 2015, Planet. Space when many astronomical archives are digitized and some useful Sci., 118, 296 tools for old asteroid observation searches, like SKYBOT(Berthier Hui M.-T., Li J., 2017, AJ, 153, 23 et al. 2016; IMCCE VO 2017), are accessible for the research IMCCE VO, 2017, SkyBoT – The Virtual Observatory Sky Body Tracker. community, there is a chance to find pre-discovery observations Available at: http://vo.imcce.fr/webservices/skybot/ of Phaethon. Jakub´ık M., Neslusan ˇ L., 2015, MNRAS, 453, 1186 Jewitt D., Li J., 2010, AJ, 140, 1519 Koschack R., Rendtel J., 1990, WGN, 18, 44 5 CONCLUDING REMARKS Li J., Jewitt D., 2013, ApJ, 145, 154 Millman P. M., 1934, JRASC, 28, 35 We analysed visual observations of the Geminid shower in Millman P. M., 1935, JRASC, 29, 115 1985–2016 around the shower maximum using homogeneous se- Molau S., Barentsen G., 2014, in Jopek T. J., Rietmeijer F. J. M., Watanabe ries of visual observations. It was found that the shower activity J., Williams I. P., eds, Proc. Astronomical Conference held at A.M. slowly increases. The same was obtained for video observations University, Meteoroids 2013. A.M. University Press, Poznan, ´ Poland, (2011–2016). These results were supported and explained by math- p. 297 ematical modelling. Activity of the shower increases because the Rendtel J., 2004, Earth Moon Planets, 95, 27 core of the Geminid stream moves towards the Earth. Rendtel J., Arlt R., 1997, WGN, 25, 75 A small probability exists that particles ejected in 2009 around Ryabova G. O., 2007, MNRAS, 375, 1371 perihelion of Phaethon’s orbit could be observed at solar longitudes Ryabova G. O., 2012, MNRAS, 423, 2254 ◦ ◦ 262.45 ± 0.005 (J2000.0). Ryabova G. O., 2015, Could the Geminid meteoroid stream be the result of long-term thermal fracture? Vol. 10 in European Planetary Space We do not expect any outburst Geminid activity in 2017, but Science Congress 2015. Nantes, France, EPSC2015-754, available at: only a wide observational campaign could validate or disprove our http://meetingorganizer.copernicus.org/EPSC2015/EPSC2015-754.pdf theoretical expectations. Ryabova G. O., 2016, MNRAS, 456, 78 ACKNOWLEDGEMENTS GOR was supported by Ministry of Education and Science of the Russian Federation project no. 9.9063.2017/BCh. This research has made use of NASA’s Astrophysics Data System. The authors thank the anonymous referee, who put forward several very relevant ques- tions. We are grateful to David Asher for reading and discussing the manuscript. REFERENCES Anon, 1975, Sky Telesc., March, 1975, 193 Arlt R., Rendtel J., 1994, WGN, 22, 167 This paper has been typeset from a T X/LT X file prepared by the author. E E MNRASL 475, L77–L80 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/475/1/L77/4788794 by Ed 'DeepDyve' Gillespie user on 16 March 2018

Journal

Monthly Notices of the Royal Astronomical Society LettersOxford University Press

Published: Jan 4, 2018

Keywords: methods: data analysis; methods: numerical; meteorites, meteors, meteoroids; minor planets, asteroids: individual: (3200) Phaethon

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