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Context. Bright stellar positions are now known with an uncertainty below 1 mas thanks to Gaia DR2. Between 2019-2020, the Galactic plane will be the background of Jupiter. The dense stellar background will lead to an increase in the number of occultations, while the Gaia DR2 catalogue will reduce the prediction uncertainties for the shadow path. Aims. We observed a stellar occultation by the Galilean moon Europa (J2) and propose a campaign for observing stellar occultations for all Galilean moons. Methods. During a predicted period of time, we measured the light flux of the occulted star and the object to determine the time when the flux dropped with respect to one or more reference stars, and the time that it rose again for each observational station. The chords obtained from these observations allowed us to determine apparent sizes, oblatness, and positions with kilometre accuracy. Results. We present results obtained from the first stellar occultation by the Galilean moon Europa observed on 2017 March 31. The apparent fitted ellipse presents an equivalent radius of 1561.2 $\pm$ 3.6 km and oblatenesses 0.0010 $\pm$ 0.0028. A very precise Europa position was determined with an uncertainty of 0.8 mas. We also present prospects for a campaign to observe the future events that will occur between 2019 and 2021 for all Galilean moons. Conclusions. Stellar occultation is a suitable technique for obtaining physical parameters and highly accurate positions of bright satellites close to their primary. A number of successful events can render the 3D shapes of the Galilean moons with high accuracy. We encourage the observational community (amateurs included) to observe the future predicted events.
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Astronomy &Astrophysics manuscript no. 35500corr c
ESO 2019
May 30, 2019
Letter to the Editor
First stellar occultation by the Galilean moon Europa and
upcoming events between 2019 and 2021
B. Morgado1,2,?, G. Benedetti-Rossi1,2, A. R. Gomes-Júnior1,2,3,4, M. Assafin2,3, V. Lainey5,6, R. Vieira-Martins1,2,3,
J. I. B. Camargo1,2, F. Braga-Ribas7,2,8, R. C. Boufleur1,2, J. Fabrega9, D. I. Machado10,11, A. Maury9,
L. L. Trabuco11, J. R. de Barros12, P. Cacella13, A. Crispim7, C. Jaques12 , G. Y. Navas14, E. Pimentel12,
F. L. Rommel1,2,7, T. de Santana4, W. Schoenell15, R. Sfair4and O. C. Winter4
(Aliations can be found after the references)
Received XX; accepted XX
ABSTRACT
Context. Bright stellar positions are now known with an uncertainty below 1 mas thanks to Gaia DR2. Between 2019-2020, the
Galactic plane will be the background of Jupiter. The dense stellar background will lead to an increase in the number of occultations,
while the Gaia DR2 catalogue will reduce the prediction uncertainties for the shadow path.
Aims. We observed a stellar occultation by the Galilean moon Europa (J2) and propose a campaign for observing stellar occultations
for all Galilean moons.
Methods. During a predicted period of time, we measured the light flux of the occulted star and the object to determine the time when
the flux dropped with respect to one or more reference stars, and the time that it rose again for each observational station. The chords
obtained from these observations allowed us to determine apparent sizes, oblatness, and positions with kilometre accuracy.
Results. We present results obtained from the first stellar occultation by the Galilean moon Europa observed on 2017 March 31. The
apparent fitted ellipse presents an equivalent radius of 1561.2 ±3.6 km and oblatenesses 0.0010 ±0.0028. A very precise Europa
position was determined with an uncertainty of 0.8 mas. We also present prospects for a campaign to observe the future events that
will occur between 2019 and 2021 for all Galilean moons.
Conclusions. Stellar occultation is a suitable technique for obtaining physical parameters and highly accurate positions of bright
satellites close to their primary. A number of successful events can render the 3D shapes of the Galilean moons with high accuracy.
We encourage the observational community (amateurs included) to observe the future predicted events.
Key words. Methods: observational – Techniques: photometry – Occultations – Planets and satellites: individual: Europa
1. Introduction
A stellar occultation occurs when a solar system object passes in
front of a star from the point of view of an observer on Earth,
causing a temporary drop in the observed flux of the star. This
technique allows the determination of sizes and shapes with kilo-
metre precision and to obtain characteristics of the object, such
as its albedo, the presence of an atmosphere, rings, jets, or other
structures around the body (Sicardy et al. 2011,2016;Braga-
Ribas et al. 2013,2014;Dias-Oliveira et al. 2015;Benedetti-
Rossi et al. 2016;Ortiz et al. 2015,2017;Leiva et al. 2017;
Bérard et al. 2017), or even the detection of topographic features
(Dias-Oliveira et al. 2017).
Stellar occultations can also provide very accurate astromet-
ric measurements of the occulted body, with uncertainties that
can be as low as 5 to 10 km or even smaller for some objects
(Leiva et al. 2017;Desmars et al. 2019). Compared with other
methods in the context of the Galilean moons, classical CCD
astrometry enables us to obtain positions with uncertainties in
the 300-450 km level (Kiseleva et al. 2008), and relative posi-
tions between two close satellites achieve uncertainties of 100
km (Peng et al. 2012). Positions obtained using mutual phenom-
ena have uncertainties at a level of 15-60 km (Saquet et al. 2018;
Arlot et al. 2014;Dias-Oliveira et al. 2013;Emelyanov 2009),
?e-mail: morgado.fis@gmail.com
but they only occur at every equinox of the host planet (every six
years in the case of Jupiter). The technique of mutual approxi-
mations also provides positions with uncertainties between 15-
60 km, and this does not depend on the equinox of Jupiter (see
Morgado et al. (2016,2019) for details). Stellar occultation is
then the only ground-based technique that can furnish astromet-
ric measurements that are comparable with space probes, which
usually have uncertainties smaller than 5 km (Tajeddine et al.
2015). In addition, the positions and sizes that can be obtained
with stellar occultations are independent of reflectance models,
which may have systematic errors due to variations on the satel-
lite surface (Lindegren 1977).
In the context of the Galilean satellites, stellar occultations
can provide shapes and sizes with uncertainties that are compa-
rable with those of space probe images, but that are not aected
by albedo variations on the satellite surface or by limb fitting,
which is highly aected by the solar phase angle. From an astro-
metric point of view, these events can provide the best ground-
based astrometry of these moons, with uncertainties smaller than
1 mas. This is at least one order of magnitude better than other
methods.
Between 2019 and 2020, Jupiter will be in a very dense star
region, the Galactic centre will be its background. This will only
occur again in 2031. The probability of a stellar occultation by
the Jovian moons increases dramatically (Gomes-Júnior et al.
Article number, page 1 of 10
arXiv:1905.12520v1 [astro-ph.EP] 29 May 2019
A&A proofs: manuscript no. 35500corr
2016). This is a great opportunity to observe stellar occultations
by the Galilean moons, determine their positions, improve their
ephemerides, and measure their shapes independently of probes.
Accurate orbits help to prepare space missions targeting the Jo-
vian system (Dirkx et al. 2016,2017), and can help in the study
of tides (Lainey et al. 2009;Lainey 2016). In the near future, we
will have the ESA JUICE1and the NASA Europa Clipper2mis-
sions, which are scheduled for launch in the next decade (2020s).
Moreover, only two stellar occultations by Ganymede have been
observed in the past: one in 1972 (Carlson et al. 1973) and an-
other in 2016 (D’Aversa et al. 2017). No occultations by Io, Eu-
ropa, or Callisto are reported. Here we present the results ob-
tained by the first observation of a stellar occultation by Europa,
which occurred on 2017 March 31. These results serve as a proof
of concept for the 2019-2021 campaign that is being organised.
The prediction method and the observational campaign are
outlined in Section 2, while the data analysis and the occultation
results are given in Section 3. The upcoming events for the pe-
riod 2019 to 2021 can be found in Section 4. Our final remarks
are contained in Section 5.
2. Prediction and observational campaign
The passage of Jupiter in a region that has the Galactic plane
as background, as seen from Earth in the period between 2019
– 2020, creates a number of opportunities for observing stellar
occultations by its satellites because this region has a high den-
sity of stars (Gomes-Júnior et al. 2016). This passage motivates
the search for events involving the Galilean satellites because the
satellites need stars with a magnitude of G =11.5 or lower, so
that the magnitude drop (mag) can be higher than 0.005 mag.
This drop is otherwise very hard to observe with current equip-
ment and techniques.
Following similar procedures as described in Assafin et al.
(2010,2012), we predicted that Europa would occult a star (mag.
G=9.5) on 2017 March 31 at 06:44 UTC. The shadow path
would be crossing South America with a velocity of 17.78 km/s.
The prediction was made using the Europa ephemerides jup3103
with DE435 furnished by the Jet Propulsion Laboratory (JPL),
and the stellar position was obtained from the Gaia DR1 cata-
logue (Clementini et al. 2016) because the occultation occurred
before the launch of Gaia DR2 in April 2018 (Lindegren et al.
2018). The star was then identified in Gaia DR2. Its position
(right ascension α, declination - δ, and errors) was updated us-
ing its proper motion and parallax for the occultation time (2017
March 17 06:44:00 UTC), and its G mag,
αstar =13h1201500.5430 ±0.16 ma s,
δstar =055604800.7526 ±0.12 ma s,
Gmag =9.5065.(1)
This occultation was favourable for several potential ob-
servers in several countries in South America (Fig. 1). We then
organised a campaign to observe this event, and a total of nine
stations tried to observe the occultation. They are listed in Ta-
ble 1. However, due to bad weather conditions, only four were
able to obtain data (OPD/B&C, OPD/ROB, FOZ, and OBSPA)4.
The observation made at Itajubá with the 40 cm telescope was
1Website: http://sci.esa.int/juice/
2Website: https://www.nasa.gov/europa
3Website: https://naif.jpl.nasa.gov/naif/toolkit.html
4Alias, defined in Table 1.
Fig. 1. Occultation by Europa on 2017 March 31. The centre of the body
at the time of closest approach between the shadow and the geocentre
(CA), at 06:44:36 UTC, is represented by the large red dot. Blue lines
represent the size limit of Europa (1561.2 ±3.6 km). The small red
dots represent the centre of the body, each separated by 1 minute from
the reference instant CA. The blue dots are the observers with positive
detection, and the yellow dots are observers that were unable to obtain
the light curve due to bad weather conditions. The oset between our
result and the prediction (using the JPL jup310 ephemeris) was -0.90 ±
3.1 km for fcand -12.3 ±2.0 km for gc, see Sect. 3. The black arrow at
the bottom represents the direction of motion of the shadow.
discarded because the signal-to-noise ratio (S/N) was very low,
while the other three (OPD/B&C, FOZ, and OBSPA) obtained
positive detection.
Fig. 1presents the post-occultation map. It shows the shadow
path of Europa through South America, and the positions of the
observation stations. The blue lines represent the size of Europa
obtained after the fit (See Section 3).
3. Data analysis and results
Images from all data sets were corrected for bias and flat-field us-
ing standard iraf5procedures. A stacking image procedure6de-
veloped by us in python (Astropy Collaboration et al. 2013), fol-
lowing similar procedures as Zhu et al. (2018), was used in FOZ
images in order to obtain an S/N that was high enough to empha-
sise the magnitude drop. The low time-resolution at OPD/ROB
(cycle 7 s) did not allow the use of this technique, and it was
not possible to determine the magnitude drop.
Dierential aperture photometry was applied with the praia
package (Assafin et al. 2011). The calibrator and target apertures
were automatically chosen to maximise the S/N for each frame.
Io (J1) was set as calibrator in all sets of images because there
were no other star in the field of view with a good S/N that could
be used for callibration. The light fluxes of Europa and of the
occulted star were measured together in the same aperture. The
normalised light curves are displayed in Fig. 2, where the black
lines are the observations and the red lines are the modelled fitted
light curves.
5Website: http://iraf.noao.edu/
6More details in Appendix A
Article number, page 2 of 10
B. Morgado, G. Benedetti-Rossi, A. R. Gomes-Júnior et al.: Stellar occultation by Europa
Table 1. Observational stations, technical details, and circumstance.
Site Longitude (E) Observers Telescope aperture Exposure Time Light-curve
Alias Latitude (N) CCD Cycle rms
Status Altitude (m) Filter (s)
Itajubá/MG-Brazil -45o34’ 57.5" G. Benedetti-Rossi 60 cm 0.60 0.013
OPD/B&C -22o32’ 07.8" M. Assafin Andor/IXon-EM 0.63
Positive Detection 1864 Methane1
Foz do Iguaçu/PR-Brazil -54o35’ 37.0" D. I. Machado 28 cm 1.00 0.016
FOZ -25o26’ 05.0" L. L. Trabuco Raptor/Merlin 1.00
Positive Detection 184 Clear
San Pedro de Atacama/Chile -68o10’ 48.0" A. Maury 40 cm 0.40 0.017
OBSPA -22o57’ 08.0" J. Fabrega ProLine/PL16803 1.41
Positive Detection 2397 Red2
Mérida/Venezuela -70o52’ 21.6" G. R. Navas 100 cm
CIDA +08o47’ 25.8" ProLine/PL4240
Weather Overcast 26 Red2
Brasília/DF-Brazil -47o54’ 39.9" P. Cacella 50 cm
DHO -15o53’ 29.9" ASI174MM
Weather Overcast 1064
Itajubá/MG-Brazil -45o34’ 57.5" W. Schoenell 40 cm
OPD/ROB -22o32’ 07.8" ASCOM/KAF16803
Low S/N data 1864 Red2
Guaratinguetá/SP-Brazil -45o11’ 25.5" R. Sfair 40 cm
FEG -22o48’ 05.5" T. de Santana Raptor/Merlin
Weather Overcast 543 O. C. Winter
Curitiba/PR-Brazil -49o11’ 45.8" F. Braga-Ribas 25 cm
OACEP -25o28’ 24.6" A. Crispim Watec/910HX
Weather Overcast 861 F. Rommel Clear
Oliveira/MG-Brasil -43o59’ 03.1" C. Jaques 45 cm
SONEAR -19o52’ 55.0" E. Pimentel ML FLI16803
Weather Overcast 982 J. R. de Barros
1Methane filter is a narrow-band filter centred on 889 nm and λ=15 nm
2Red filter from the Johnson system.
The ingress (ti) and egress (te) times were determined with
a standard χ2procedure between the observational light curve
and the model. The model considers a sharp-edge occultation
model convolved with Fresnel diraction, stellar diameter (pro-
jected at the body distance, 0.633 km in our case (Bourges et al.
2017)), CCD bandwidth, and finite integration time (more de-
tails in Braga-Ribas et al. (2013)). Table 2contains the ingress
and egress UTC times and the errors for each station with pos-
itive detection. Their respective uncertainties are given in sec-
onds and kilometres (calculated using the event velocity of 17.78
km/s). More information about the light-curve fit procedure is
presented in Appendix B.
Each of the ingress and egress times were converted into a
star position ( f,g) regarding the body centre, with fand gbe-
ing measured positively toward local celestial east and celestial
north, respectively. Each pair of positions, from the same site, is
a chord, and each position is a point of which we can fit the five
parameters that defines an ellipse: (i and ii) the ellipse centre ( fc,
gc), (iii) apparent semi-major axis (a0), (iv) the apparent oblate-
ness (0=(a0b0)/a0, where b0is the apparent semi-minor
axis), and (v) the position angle of the pole Ppof b0. This posi-
tion angle was fixed as 24.2534, derived from the pole position
reported by Archinal et al. (2018). In Fig. 3we show the three
chords obtained from the occultation and the fitted ellipse. The
ellipse parameters are
Table 2. Occultation ingress (ti) and egress (te) times.
Station tite
(hh mm ss.ss ±s) (hh mm ss.ss ±s)
(km) (km)
OPD/B&C 06:38:28.55 ±0.75 06:41:06.15 ±0.24
(13.3) (04.3)
FOZ 06:39:26.07 ±0.77 06:41:21.59 ±1.07
(13.7) (19.0)
OBSPA 06:40:38.19 ±1.20 06:42:22.85 ±0.34
(21.3) (06.0)
fc=0.9±3.1km,
gc=12.3±2.0km,
a0=1562.0±3.6km,
0=0.0010 ±0.0028,
Pp=24.2534.(2)
It is important to highlight that the centre position ( fc,gc)
and the apparent size and shape of the ellipse (a0, 0) are cor-
related. This correlation decreases with the number of obtained
Article number, page 3 of 10
A&A proofs: manuscript no. 35500corr
Fig. 2. Three normalised light curves obtained with positive detection.
They are shifted vertically for better viewing. Observational circum-
stances are given in Table 1. The dierence in the depth of the curves is
due to the use of dierent filters.
chords, and moreover, with the distribution of these chords in the
shadow. Chords in both hemispheres are required to achieve the
best results. In our case, the higher correlation was 0.81 between
gcand 0, which is expected because we only had chords in the
southern hemisphere of the shadow.
We can compare our results with the values determined by
Galileo mission images (Nimmo et al. 2007). Our apparent semi-
major axis (a0=1562.0 ±3.6 km) was between the axis a
(1562.6 km) and b(1560.3 km) of the fitted ellipsoid. Our appar-
ent semi-minor axis (b0=1560.4 ±5.7 km) is nearly the caxis of
the ellipsoid (1559.5 km). The expected oblateness was between
0.0019 and 0.0005, in comparison with 0.0010 ±0.0028 that we
obtained. The equivalent radius (Req =a010=1561.2±3.6
km) agrees with the nominal equivalent radius of 1560.8 ±0.3
km of Europa. According to Kay et al. (2019), the topographic
features of Europa are at a level of some hundreds of metres,
therefore it is not possible to infer any topography from our ob-
servations.
With the limb fitting, we also obtained a geocentric position
for the Europa centre on 2017 March 31 at 06:44:00 UTC of
αJ2=13h1201500.548372 ±0.96 mas,
δJ2=055604800.687034 ±0.62 mas.(3)
This position has an oset with respect to the JPL jup310
ephemerides of +0.28 mas and +3.81 mas for αcos δand δ,
respectively, while the oset with respect to the IMCCE NOE-
5-2010-GAL (Lainey et al. 2009) is 2.59 mas and 6.01 mas,
respectively, both in the sense of observation minus ephemeris.
More details can be found in Appendix C.
4. Future Events
Observing very many occultations by the Galilean satellites is
required for better understanding their 3D shape (oblatness and
Fig. 3. Best elliptical limb fit (in black) using the three occultation
chords. Blue, cyan, and green lines are the chords from the positive
detection obtained at OPD/B&C, FOZ, and OBSPA, respectively. The
small red segments at the end of each segment indicate the 1σuncer-
tainty on each chord extremity, derived from the time uncertainties pro-
vided in Table 2. The position of the centre of the fitted ellipse is showed
as an X, with values of fc=0.9±3.1 km and gc=12.3±2.0 km,
and represents the oset with respect to the JPL geocentric ephemeris.
The fitted parameters of the ellipse are listed in Table 2.
pole positions). This in turn enables obtaining a highly accu-
rate absolute position (with the help of GAIA DR2 catalogue,
which furnishes the star positions) that are to be used in dynami-
cal models and with which high-quality ephemerides can be ob-
tained.
We conducted a search for occultation events that would oc-
cur between 2017 and 2021, using the GDR2 catalogue and the
JPL jup310 and DE435 ephemerides. In Table 3we summarise
the occultation predictions between 2019 and 2021. The occulta-
tion presented here is highlighted. We also list some parameters
that are necessary to produce occultation maps such as the clos-
est approach instant (UTC) of the prediction; C/A, the apparent
geocentric distance between the satellite and the star at the mo-
ment of the geocentric closest approach, in arcseconds; P/A, the
satellite position angle regarding the star to be occulted at C/A,
in degrees (zero at north of the star, increasing clockwise); mag-
nitude G of the occulted GDR2 star; and finally, the expected
magnitude drop (mag) for each event. For more information
about the definition and use of these stellar occultation geomet-
ric elements, see Assafin et al. (2010,2012). We present in Table
3three events by Io, four by Europa, one by Ganymede, and
three by Callisto with mag higher than 0.005 magnitude. The
prediction maps are shown in the Appendix D.
We draw the attention to the occultation by Io of a mag. G
5.8 star on 2012 April 2. The magnitude drop will be about 0.740
magnitude. This event can be observed in the South of North
America, Central America, and in the North of South America
(Fig. 4).
This type of observation is challenging because of the low
magnitude drop. We suggest the use of red filters (or a narrow-
band methane filter, centred on 889 nm with a width of 15 nm)
Article number, page 4 of 10
B. Morgado, G. Benedetti-Rossi, A. R. Gomes-Júnior et al.: Stellar occultation by Europa
Table 3. Predicted stellar occultations by the Galilean moons between
2019 and 2021.
Sat. Time (UTC) C/A P/AGmag?mag
502 2017-03-31 06:44 0.09 19.98 9.5 0.030
502 2019-05-06 20:32 0.58 183.07 10.9 0.008
502 2019-06-04 02:26 0.12 4.46 9.1 0.037
504 2019-06-05 23:12 0.90 182.86 10.2 0.020
501 2019-09-09 02:33 0.26 189.87 11.0 0.008
501 2019-09-21 13:08 1.22 7.97 11.3 0.007
504 2020-06-20 16:03 1.45 348.44 10.9 0.012
502 2020-06-22 02:09 2.07 348.23 11.3 0.005
501 2021-04-02 10:24 1.02 344.64 5.8 0.740
503 2021-04-25 07:55 0.44 160.79 11.1 0.005
504 2021-05-04 23:01 0.90 342.43 10.4 0.027
Note: See text for the definitions of geometric elements.
Fig. 4. Prediction of the occultation by Io on 02 April 2021 10:24 UTC,
using JPL DE435 and jup310 ephemeris. The blue lines represent the
expected size limit of Europa. The blue dashed lines are an uncertainty
of 20 mas in Europa’s position. The red dots represents the centre of the
body for a given time, each separated by 1 minute. The arrow represents
the sense of the shadow’s motion. The dark grey and the light grey area
represent the night and the day part of the globe, respectively.
in these observations. Jupiter should be placed outside the field
of view of the camera when possible, which will reduce noise
eects caused by scattered light from Jupiter. It is very important
to test the equipment configuration (gain, exposure time, etc.)
some days before the occultation in order to ensure the best S/N
possible.
We also suggest the use of fast cameras with high-cadence
images and minimum readout time. Stacking images techniques,
such as the one applied here (detailed in Appendix A), can be
used to increase the S/N of the images. Telescopes as small as 20
cm (8”) can provide useful data. Adaptive optics is also a possi-
bility, with adequate exposure time. For a telescope with a large
aperture and a sensitive camera, this could result in the direct
detection of a thin atmosphere around a Galilean satellite. Ob-
servations with resolutions higher than the Fresnel scale (about
0.44 km at Jupiter) can result in the direct measurement of re-
fraction spikes by a thin atmosphere such as the spikes observed
in occultations by Pluto (Dias-Oliveira et al. 2015;Sicardy et al.
2016;Meza et al. 2019).
5. Discussion
Stellar occultation is a ground-based technique capable of deter-
mining positions, sizes, and shapes with uncertainties of some
kilometres. These values are comparable with data obtained with
space probes. In this work we took advantage of the beginning of
the Jupiter passage through a very dense stellar region, with the
Galactic centre as background, and organised an observational
campaign to obtain data from a predicted stellar occultation by
the Galilean moon Europa. Its ephemeris indicates that it would
pass in front of a mag. G =9.5 GDR2 star on 2017 March 31.
Out of the nine stations in eight sites across South America, three
obtained a positive detection of the event.
The fitted ellipse for Europa gives an area equivalent radius
of 1561.2 ±3.6 km. Its absolute position uncertainty is 0.80 mas
(2.55 km), representing an oset with respect to the JPL jup310
ephemeris of 3.82 mas and of 6.54 mas with respect to the NOE-
5-2010-GAL ephemeris. This position for Europa at the time of
the occultation took advantage of the high-precision position of
Gaia DR2 stars and the fact that stellar occultations render accu-
rate body-star relative positions.
This is the first reported observation of a stellar occultation
by Europa. The favourable configuration of Jupiter, which has
the Galactic plane as background, increases the chances of ob-
serving other bright star occultations by its main satellites, as
discussed in Sect. 4. This configuration will only occur again
in 2031. For the future events predicted in the 2019 and 2021,
campaigns will be organised in due time.
We encourage the observational community to observe the
predicted future stellar occultations. Even the amateur commu-
nity with telescopes as small as 20 cm (8”) can help provide
useful data. A stellar occultation provides an apparent size and
shape at a specific moment. Combined with the observations of
the upcoming events, a 3D shape of these moons can be ac-
quired with kilometre precision and complement space mission
data. This information can aid in the study of planetary forma-
tion, evolution, and the influence of tides raised by the moons’
primary in their orbit. This highlights the importance of these
events.
Finally, all this information can also be used together with
dynamical models to ensure highly accurate orbits for these
moons. These orbits can be helpful for future space probes aimed
at the Jovian system, such as JUICE and the Europa Clipper mis-
sion.
Acknowledgements. This study was financed by the Coordenação de Aperfeiçoa-
mento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.
Part of this research is suported by INCT do e-Universo, Brazil (CNPQ grants
465376/2014-2). Based in part on observations made at the Laboratório Nacional
de Astrofísica (LNA), Itajubá-MG, Brazil. BM thanks the CAPES/Cofecub-
394/2016-05 grant. G.B.R. acknowledges the support of the CAPES and
FAPERJ/PAPDRJ (E26/203.173/2016) grants. ARGJ thanks FAPESP proc.
2018/11239-8. MA thanks CNPq (Grants 427700/2018-3, 310683/2017-3 and
473002/2013-2) and FAPERJ (Grant E-26/111.488/2013). VLs research was
supported by an appointment to the NASA Postdoctoral Program at the
NASA Jet Propulsion Laboratory, California Institute of Technology, ad-
ministered by Universities Space Research Association under contract with
NASA. RVM acknowledges the grants CNPq-306885/2013, CAPES/Cofecub-
2506/2015, FAPERJ/PAPDRJ-45/2013 and FAPERJ/CNE/05-2015. JIBC ac-
knowledges CNPq grant 308150/2016-3. FBR acknowledges CNPq support,
proc. 309578/2017-5. RS e OCW acknowledges Fapesp proc. 2016/24561-0,
CNPq proc. 312813/2013-9 and 305737/2015-5.
Article number, page 5 of 10
A&A proofs: manuscript no. 35500corr
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1Observatório Nacional/MCTIC, R. General José Cristino 77, Rio de
Janeiro, RJ 20.921-400, Brazil
2Laboratório Interinstitucional de e-Astronomia - LIneA & INCT do
e-Universo, Rua Gal. José Cristino 77, Rio de Janeiro, RJ 20921-
400, Brazil
3Observatório do Valongo/UFRJ, Ladeira Pedro Antonio 43, Rio de
Janeiro, RJ 20080-090, Brazil
4UNESP - São Paulo State University, SP 12516-410, Guaratinguetá,
São Paulo, Brazil
5Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Drive, Pasadena, CA 91109-8099, United States
6IMCCE, Observatoire de Paris, PSL Research University, CNRS-
UMR 8028, Sorbonne Universités, UPMC, Univ. Lille 1, 77
Av. Denfert-Rochereau, 75014 Paris, France
7Federal University of Technology - Paraná (UTFPR/DAFIS), Av.
Sete de Setembro, 3165, CEP 80230-901, Paraná, Brazil
8LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne
Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules
Janssen, 92195 Meudon, France
9San Pedro de Atacama Celestial Explorations, Casilla 21, San Pedro
de Atacama, Chile
10 Unioeste, Avenida Tarquínio Joslin dos Santos 1300, Foz do Iguaçu,
PR 85870-650, Paraná, Brazil
11 Polo Astronômico Casimiro Montenegro Filho/FPTI-BR, Avenida
Tancredo Neves 6731, Foz do Iguaçu, PR 85867-900, Paraná, Brazil
12 Observatório SONEAR, Brazil
13 Dogsheaven Observatory, SMPW Q25 CJ1 LT10B, Brasilia, Brazil
14 Research Center of Astronomy, Francisco J. Duarte (CIDA). 264,
5101-A Mérida, Venezuela.
15 Instituto de Física/UFRGS, Porto Alegre, RS 91501-970, Brazil,
Appendix A: Stacking image procedure
The stacking image procedure was developed with the aim to at-
tenuate the image noise, thereby increasing the image S/N when
is not possible to just integrate over more time. This is the case
when the Galilean moons are observed without a filter, as the
brightness of Jupiter or even the satellite itself would quickly
saturate the CCD image before we achieve an adequate S/N.
The first step in this procedure is to chose a well-sampled
object in the images that is to be used as reference to align the
images for the stacking. This object should not have a significant
motion (in the sky plane) between the first and last image. In our
case, in the absence of stars, we used Europa to align the images.
Each individual image had an exposure of 0.05 seconds and the
same cycle, and the motion of Europa was 5.39 mas/s in the sky
plane, corresponding to a motion of 0.0074 pixels/s on the CCD.
We measured the object centroid (x,y) with a 2D circular
symmetric Gaussian fit over pixels within one full-width at half-
maximum (FWHM seeing) from the centre. This was done
using the PRAIA package (Assafin et al. 2011). The uncertainty
in the centroid measurement was about 36 mas (0.05 pixels).
The alignment consists of vertical and horizontal shifts for each
image (x,y) relative to a chosen reference image; in our case,
the first image.
We performed a stack procedure over each of the 20 images,
which resulted in a temporal resolution of 1.0 second. Fig. A.1
contains the light curve we obtained using all individual frames
(light gray line), the light curve using the stacked images (black
line), and the modelled light curve after the fit (red line). Before
our procedure, the normalised light curve rms was 0.073, while
after the stacking, we were able to achieve 0.016. It is important
to highlight that the expected drop would be about 0.021 in flux.
Without the stacking image procedure, it would be impossible to
determine the ingress and/or the egress times.
Article number, page 6 of 10
B. Morgado, G. Benedetti-Rossi, A. R. Gomes-Júnior et al.: Stellar occultation by Europa
Fig. A.1. Normalised light flux from FOZ images with (black line) and
without (light gray line) the stacking image procedure. The red line is
the modelled light curve. Each stacked image contains the data of 20
single images stacked together.
Appendix B: Light-curve fitting procedure
The most common geological model of Europa is an ice surface
on top of a global ocean (Anderson et al. 1998). The presence of
plumes of water vapour (Roth et al. 2014;Jia et al. 2018;Sparks
et al. 2016,2019) is a strong evidence of this model.
The OPD/B&C light curve presents an apparent smooth flux
decay on the ingress and egress times during the passage of the
body in front of the star, and this slope would be compatible with
the common picture of the Europa geology and water vapour ev-
idence. These features must be analysed with caution, however,
because the measurements show some level of correlated noise.
To objectively assess the significance of this slope, we tested two
scenarios: one simple (sharp-edge model) scenario in which we
assumed that there are no slopes during the ingress and egress
times, and one more complex scenario (trapezoidal model) with
a smooth ingress and egress. It is also straightforward to show
that the sharp-edge model is a particular case of a trapezoidal
model, allowing us to analyse the decrease in residuals when the
number of fitted parameters is increased.
Because the more complex model has fewer degrees of free-
dom, it should at least fit the observables as well as the more
straightforward case. Having set the characteristics of the prob-
lem, we used the Fisher-Snedecor F-test (Chow 1960;Seber &
Lee 2003) to estimate the probability that the trapezoidal model
fits the data better than a more simple sharp-edge model. Un-
der the null hypothesis, the trapezoidal case does not produce a
better fit than the standard case, or it could be argued that the
slope coecients are similar. The test follows an F-distribution,
and the null hypothesis is rejected when the calculated F value is
higher than the expected critical value (or significance).
As an example, we discuss the light curve obtained by
OPD/B&C. The normalised chi-square statistics obtained from
the sharp edge model was 1.06, whereas the value obtained with
the trapezoidal model was 1.04. These models were applied to
the data within an interval of five minutes centred at the central
Fig. B.1. Normalised light curve obtained at OPD/B&C with a zoom-in
at the moment of the occultation. The black dots show the data, while
the red lines represent the sharp-edge model for each observation. Tthe
dotted blue line shows the trapezoidal model.
instant for each station. Using the F distribution, we can pre-
dict that within a 95% confidence level, the computed chi-square
value for the plume model should lie around 0.74. In this case,
increasing the number of model parameters leads only to a better
fit of the noise because we found out that the probability of the
trapezoidal model to explain the data better is about 17%. Our
measurements therefore do not support any further information
about the detection of a thin atmosphere (or a plume). The FOZ
and OBSPA light curves have lower cadences and therefore a
lower time resolution during ingress and egress of the event. The
probabilities for these sites are even lower, about 1% and 3%
respectively, as expected.
Figures B.1,B.2, and B.3 contain the individual light curves
for each observational site (OPD/B&C, FOZ, and OBSPA, re-
spectively) with a zoom (bottom graphs) of two minutes centred
on the ingress and egress times. The x-axes are the time in min-
utes relative to 06:44:00 UTC, and the y-axes are the normalised
light flux. The red lines are the sharp-edge model for each obser-
vation, and the dotted blue lines are the trapezoidal model.
Appendix C: Comparison between ephemeris
The absolute position of Europa obtained here has an uncertainty
smaller than 1 mas (3 km). This level of accuracy is of the
same order of magnitude as those obtained from space mission
images (Tajeddine et al. 2015). However, our position is com-
pletely independent of any variation in the satellite albedo and is
not aected by phase angle eects. Our geocentric position for
Europa is given in Eq. (C.1),
αJ2=13h1201500.548372 ±0.96 mas,
δJ2=055604800.687034 ±0.62 mas.(C.1)
Here we compare this position (αJ2, δJ2) with dierent geo-
centric ephemeris (αephem , δephem ) using Eq. (C.2),
Article number, page 7 of 10
A&A proofs: manuscript no. 35500corr
Fig. B.2. Same as Fig. B.1 for the light curve obtained at FOZ.
Fig. B.3. Same as Fig. B.1 for the light curve obtained at OBSPA.
α=αJ2αephem ,
δ=δJ2δephem ,
s'sα2cos2 δJ2+δephem
2!+ ∆δ2.(C.2)
In Table C.1 we present the oset between the Europa
position as derived based on this stellar occultation and the
ephemeris. We compare our result with 12 ephemerides combi-
nations. We use 6 planetary ephemerides (from the JPL, DE438s,
DE436s, DE435, and DE430; and from the IMCCE, INPOP17a
and INPOP13c) and two satellite ephemerides (from the JPL,
jup310; and from the IMCCE, NOE-5-2010-GAL).
Table C.1. Oset between the geocentric position of Europa obtained
from the stellar occultation on 2017 March 31 at 06:44 UTC and dier-
ent ephemerides in the sense observation minus ephemeris.
JPL jup310 IMCCE NOE-5-2010-GAL
(mas) (mas)
α- 7.94 - 5.07
DE438s δ-10.91 - 1.08
s13.46 5.15
α- 6.68 - 3.80
DE436s δ- 7.59 +2.23
s10.09 4.39
α- 0.27 +2.60
DE435 δ- 3.81 +6.01
s3.82 6.54
α- 1.67 +1.19
DE430 δ- 3.69 +6.13
s4.06 6.24
α+0.63 - 2.23
INPOP17a δ- 8.59 +1.23
s8.61 2.54
α- 2.64 - 5.50
INPOP13c δ-10.28 - 0.46
s10.61 5.49
Appendix D: Future event maps
In this appendix we present the stellar occultation maps for the
predicted events that will occur between 2019 and 2021 (See Ta-
ble 3for more details). We indicate in each figure the satellite
that will occult the star, and the date and time of the occultation
in UTC. The blue lines represent the expected size limit of the
occulting satellite. The red dots are the centre of the body for a
given time, each separated by 1 minute. The large red dot repre-
sents the centre of the body at the time of closest approach (CA)
indicated in the figure label. The arrow represents the direction
of motion of the shadow. The dark grey and light grey areas rep-
resent the night and day part of the globe, respectively.
Fig. D.1. Occultation of a mag. G 10.9 star by Europa on 2019 May 5,
20:32 UTC. The predicted relative velocity of the event is 23.1 km/s.
Article number, page 8 of 10
B. Morgado, G. Benedetti-Rossi, A. R. Gomes-Júnior et al.: Stellar occultation by Europa
Fig. D.2. Occultation of a mag. G 9.1 star by Europa on 2019 June 4,
02:26 UTC. The predicted relative velocity of the event is 27.3 km/s.
Fig. D.3. Occultation of a mag. G 10.2 star by Callisto on 2019 June 5,
23:12 UTC. The predicted relative velocity of the event is 21.0 km/s.
Fig. D.4. Occultation of a mag. G 11.0 star by Io on 2019 September 9,
02:33 UTC. The predicted relative velocity of the event is 18.1 km/s.
Fig. D.5. Occultation of a mag. G 11.3 star by Io on 2019 September 21,
13:08 UTC. The predicted relative velocity of the event is 26.5 km/s.
Fig. D.6. Occultation of a mag. G 10.9 star by Callisto on 2020 June 20,
16:03 UTC. The predicted relative velocity of the event is 18.9 km/s.
Fig. D.7. Occultation of a mag. G 11.3 star by Europa on 2020 June 22,
02:09 UTC. The predicted relative velocity of the event is 26.1 km/s.
Article number, page 9 of 10
A&A proofs: manuscript no. 35500corr
Fig. D.8. Occultation of a mag. G 5.8 star by Io on 2021 April 2, 10:24
UTC. The predicted relative velocity of the event is 16.5 km/s.
Fig. D.9. Occultation of a mag. G 11.1 star by Ganymede on 2021 April
25, 07:55 UTC. The predicted relative velocity of the event is 27.7 km/s.
Fig. D.10. Occultation of a mag. G 10.4 star by Callisto on 2021 May
4, 23:01 UTC. The predicted relative velocity of the event is 16.3 km/s.
Article number, page 10 of 10
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Article
Full-text available
Haumea-one of the four known trans-Neptunian dwarf planets- is a very elongated and rapidly rotating body1-3. In contrast to other dwarf planets4-6, its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system7, and the Centaur Chiron was later found to possess something similar to Chariklo's rings8,9. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multichord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea's equator and the orbit of its satellite Hi'iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea's spin period-that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea's largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates1,10,11. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Article
The results of photographic observations of Jupiter’s Galilean satellites made with the 26-inch refractor at the Pulkovo Observatory from 1986 to 2005 are given. Satellite coordinates with respect to Jupiter and the mutual distances between the satellites have been determined. A scale-trale technique that does not require reference stars for the astrometric reduction of measurements has been used. The effect of the Jupiter phase has been taken into account in the jovicentric coordinates. The observation results have been compared with a modern theory of the Galilean satellites’ motions. Systematic observation errors depending on the observation technique have been studied. The intrinsic observation accuracy in the random quotient is characterized by the values 0.041″ over X and Y. The external accuracy of the relative Galilean satellite coordinates determined by comparing the observations with modern ephemerides turned out to be equal to 0.165″, 0.213″ for the Jovicentric coordinates and 0.134″, 0.170″ for the “satellite-satellite” coordinates. The highest accuracy of the relative satellite coordinates is reached at small distances between the satellites which are less than 100″: the corresponding mean-square errors of one observation are equal in to the external convergence to 0.050″, 0.070″. The results of photographic observations have been compared with the first CCD observations of the Jupiter satellites made in 2004 with the 26-inch refractor.
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