Astronomical Science

Simultaneous HARPS and HARPS-N Observations of the Earth Transit of 2014 as Seen from Jupiter: Detection of an Inverse Rossiter–McLaughlin Effect

Paolo Molaro1 the astronomical unit was obtained with Moon will also produce a transit on the Lorenzo Monaco2 Halley’s method. Solar surface, but with an even smaller Mauro Barbieri3 RM of ~ 2 cm s–1. Simone Zaggia4 On 6 June 2012 using the Moon as a mir- Christophe Lovis5 ror, we detected the Rossiter–McLaughlin The timing of the Earth transit differs (RM) effect due to the eclipse by Venus slightly between the moons and Jupiter of the Solar disc with a precision of few itself and varies from one moon to 1 INAF – Osservatorio Astronomico di cm s–1 (Molaro et al., 2013). When a body another: the moons arrive at the alignment ­Trieste, Italy passes in front of a star the occultation of slightly before the planet, about 30 min- 2 Departamento de Ciencias Fisicas, a small area of the rotating stellar surface utes in the case of Europa and about ­Universidad Andres Bello, Santiago, produces a distortion of the stellar line one hour for Ganymede. The view of the Chile profiles, which can be measured as a drift Jovian system on 5 January 2104 from 3 Departamento de Fisica, Universidad de in the radial velocity. The phenomenon an observer on the Sun is illustrated in Atacama, Copiapo, Chile was first observed in eclipsing binaries by Figure 2. The moon Io was behind Jupiter 4 INAF – Osservatorio Astronomico di McLaughlin (1924) and Rossiter (1924). during part of the event. Padova, Italy The RM effect has now been observed in 5 Geneva Observatory, University of almost one hundred exoplanets, pro­ Unfortunately in January 2014 there was Geneva, Versoix, Switzerland viding important information on orbital no suitable observing site on Earth where geometry and showing that several exo- Jupiter could have been observed during planets have very tilted orbits. The Venus the entire length of the transit. The transit Although there will not be another transit observations demonstrated that the RM could not be observed at all from Mauna of Venus visible from the Earth until 2117, effect can be measured even for transits Kea and the high spectral resolution facil- transits of other planets, including Earth, of exoplanets of Earth size or smaller, ities that could deliver very precise radial will be visible from other planets. We and that transits in the Solar System can velocity measurements for the duration describe high resolution spectroscopic be studied with reflected sunlight. of the transit were not available at other observations of the transit of Earth suitable astronomical sites. La Palma and ­visible from Jupiter in January 2014 The observation of the transit of Venus La Silla were the only observatories where made with the HARPS North and South via the Moon implies that other transits a fraction of the phenomenon could be spectrographs. A ­Rossiter–McLaughlin can also eventually be seen in the Solar followed with high resolution spectro- effect was ob­­served but with reverse System, since transits from other planets graphs that are able to deliver the required sign and ~ 200 times larger. The result, occur each time heliocentric conjunctions radial velocity precision. presented in Molaro et al. (2015), was take place near one of the nodes of their not at all what was expected and the orbits, with the obvious exception of The two High Accuracy Radial velocity explanation eventually led to the dis- the innermost planet, Mercury. These Planetary Spectrographs (HARPS) at covery of a new effect, a sort of inverse rare events have been studied in detail by La Silla and La Palma are twins. They are Rossiter–McLaughlin effect that has Meeus (1989), who found that the Earth both housed in a vacuum, are thermally never been observed before. will be seen transiting the Sun from Mars isolated, stable and equipped with an in 2084 and from Jupiter on 5 January image scrambler that provides the uniform 2014, and again in 2026. spectrograph–pupil illumination that is Transits in the Solar System essential for high precision radial velocity observations. The HARPS-N (La Palma) Transits of Venus and Mercury in front Observations of the 2014 Earth transit and HARPS (La Silla) observations that of the Sun are major historical events. we made comprise a series of hundreds Johannes Kepler in 1627 predicted the During transits, the integrated Solar light of spectra of Europa and Ganymede transit of Venus of 1631, but died one can be recorded as it is reflected by ­covering the range from 380 to 690 nm. year before the event, and Pierre the planet from which the Earth is seen to At the epoch of the observations, Europa ­Gassendi, who tried to observe the be transiting in front of the Sun, thus offer- and Ganymede had visual magnitudes ­transit, missed it since it was not visible ing a surrogate for a direct view. Jupiter of 5.35 and 4.63 mag and apparent from Europe. Jeremiah Horrocks realised itself is not a good sunlight reflector due diameters of 1.02 and 1.72 arcseconds, that transit of Venus occur in pairs sepa- to its high rotational velocity and to the respectively. The integration times of rated by eight years and he successfully turbulent motions of its atmosphere, but the observations were 60 or 120 s and observed the transit in 1639. Since then its major rocky moons are better reflec- delivered a signal-to-noise ratio of ~ 200 only six other transits of Venus have tive mirrors. In Figure 1 the Earth and the each at 550 nm with a resolving power taken place. In 1716 Sir Edmund Halley Moon are shown as they would appear of R ~ 115 000. The observations are suggested the use of the transits of to an observer on Jupiter on 5 January described in Molaro et al. (2015). Venus to find a value for the distance of 2014. Due to the small angular size the Sun, the astronomical unit. Major of the Earth, the predicted RM effect is We started observing Ganymede on the expeditions to the most remote parts of extremely small, only about ~ 20 cm s–1. night preceding the transit in order to the world were organised and a value for Interestingly, together with Earth, the determine the pre-transit characteristic

20 The Messenger 161 – September 2015 IO Europa Ganymede Callisto

A radial velocity anomaly Figure 2. View of the Jovian system on 5 January 2014 from an observer on the Sun or on the Earth. From Jupiter the Earth transit started at MJD (Modi- The whole set of corrected Solar radial fied Julian Date) 56662.70, while its moons arrived velocities obtained from the spectra of somewhat in advance of the alignment with Jupiter. Jupiter moons taken over the course of Moon Earth the three nights from both sites, after subtraction of the radial velocity baseline shift of the lines. On the contrary, we of 107.5 m s–1, is shown in Figure 3. observed a redshift change of 37 m s–1, i.e. ~ 400 times greater than expected. The La Palma observations show a sud- Also the radial velocity drift lasted well Figure 1. Composite image of the Sun with the Earth den drop of ~ 7 m s–1 after about one beyond the end of the transit. The anom- and Moon as seen from Europa at 19:00 UT on 5 Jan- hour. Moreover, at the start there is a dif- aly in radial velocity cannot have an uary 2014. The sizes are to scale with the Earth's –1 size of 4.2 arcseconds and the diameter of the Solar ference of about 4 m s between the instrumental origin. This is demonstrated disc of 369 arcseconds. The Solar image is from a two spectrographs, with the La Silla by the fact that the two observatories Solar Dynamics Observatory (SDO)/NASA Helio­ sequence slightly lower. In the following give consistent results and such large seismic and Magnetic Imager (HMI) Intensitygram at night at about mid-transit the radial RV anomalies have never been observed 617.3 nm on 5 January 2014 and shows prominent sunspots in the approaching Solar hemisphere. The velocity rose very quickly until it reached with these spectrographs. Thus the –1 total duration of the passage was 9 h 40 m. a peak of 37 m s and then declined anomaly must have a physical origin. almost monotonically, with a persistent small difference between the two spectro­ It is known that Solar activity could affect Solar radial velocity. The following night graphs. In the night after the transit, the the radial velocity of the Solar lines and we selected Europa to cover the second radial velocities are back to the values indeed in Figure 1, the Solar image of fraction of the transit as much as possi- preceding the transit. 5 January 2014, does show the presence ble. In the night following the transit, we of several sunspots. However, the char- made observations of both Europa and The observed pattern was completely acteristic change in radial velocities is on Ganymede to determine the post-transit at odds with our expectations: when a timescale of the Solar rotation and characteristic Solar radial velocity. observed, the Earth was eclipsing the therefore no effect is expected during a receding Solar hemisphere and the RM relatively short period of ~ 10 hours. We The radial velocities were obtained with effect should have produced a small blue inspected the Birmingham Solar Oscilla- the HARPS and HARPS-N pipelines, but we computed the proper kinematical 40 corrections. These included not only the motions of the observer relative to Jupi- ter’s moons at the instant when the light received by the observer was reflected by the moons, but also the radial velocity components of the motion of the moons relative to the Sun at the instant the light 20 ) was emitted by the Sun. All these quan­ –1 s tities have been computed using Jet Pro- (m pulsion Laboratory (JPL) Horizon Ephe- 1 RV merides following the recipies of Molaro Δ and Monai (2012). 0 Figure 3. Radial velocities measured with HARPS on 4–6 January 2014. Open black circles are observa- tions of Europa from La Palma while colour squares are the observations of Ganymede (cyan) and Europa (red) from La Silla. A constant offset of 107.5 m s–1, as measured far from the transit, is taken as the instru- mental baseline and was subtracted from the data. 5666256662.55666356663.5 56664 The vertical dashed lines mark the expected ingress and egress of the Earth's transit as seen from Europa. MJD

The Messenger 161 – September 2015 21 Astronomical Science Molaro P. et al., Earth Transit of 2014 as Seen from Jupiter

tions Network (BiSON) archives containing 6 Solar radial velocities for January 2014 Las Campanas, Chile to check whether short-term strong Solar Izana, Tenerife activity coincided with the transit. The Carnarvon, Western Australia BiSON data were captured from several Narrabi, New South Wales, Australia 4 sites and provided continuous monitoring of the Solar activity proximate to the tran- sit. The BiSON velocity residuals are shown in Figure 4 and do not show any 2 peculiarity, thus ruling out definitively the ) –1

possibility that the anomalous radial s

velocities depend on a peak of activity of (m the Sun. s 0

An inverse RM effect induced by the Residual opposition surge –2

Thus the effect is real, and it took us a whole year to understand that what is observed was due to an entirely new –4 phenomenon produced by the opposition surge on the icy moon Europa (Molaro et al., 2015). –6

The opposition surge is the brighten- 4 4.555.566.57 ing of a rocky celestial surface when it UTC (Days since 1 January 2014) is observed at opposition. The precise physical origin is not yet completely under- Figure 4. Archival BiSON Solar observations contain- enhancement as a function of the angular stood and “shadow hiding” or coherent ing velocity residuals on the same days as our transit distance from the position of the transit- observations. The data are from: Narrabri, New South backscatter have been proposed. The Wales, Australia (red points); Carnarvon, Western ing Earth. However, a simplified model, former stems from the fact that when Australia (black); Izana, Tenerife (red); and Las which accounts for the asymmetric emis- light hits a rough surface at a small phase ­Campanas, Chile (green). Courtesy of Steven Hale. sion from the two rotating Solar hemi- angle, all the shadows decrease and the spheres, can explain most of the features object is illuminated to its largest extent. of the RV curve that we observed. In the coherent backscatter theory, the across the Sun, the Earth acts as an increase in brightness is due to a con- effective lens and the light magnification In a simplified model we considered a structive combination of the light reflected produces a radial velocity drift which is ­circular region centred on the Earth from the surface and by dust particles. opposite in sign to that expected from a of radius 6 arcminutes and with uniformly The constructive combination is achieved Rossiter–McLaughlin effect, but of identi- enhanced emission, and computed the when the size of the scatterers in the cal physical origin. effect in RV as if it were due to the RM ­surface of the body is comparable to the effect, but reversing the sign to simulate wavelength of light. At zero phase, the Thus, instead of receiving less radiation the emission rather than the eclipse. The light paths will constructively interfere, from the Solar hemisphere that the theoretical RM during the transit is com- resulting in an increase in the intensity, Earth is crossing, we are receiving more puted using the formalism of Gimenez while as the phase angle increases con- radiation because of the enhancement (2006). Since there is a degeneracy structive interference decreases. produced by the effect of the opposition between the area and the intensity surge of the reflecting body. This effect of emission, we simply scaled the radial A characteristic feature of the opposition not only compensates for the partial velocity to the observed one, but pre- surge is the brightening of the planet as Solar eclipse by the Earth, but is able to served the shape. the phase angle decreases. During the produce an opposite and much stronger transit, Solar photons which graze the radial velocity drift. The result is plotted in Figure 5. The Earth have smaller angles than photons ­predicted radial velocity rise follows the coming from regions of the Solar disc far observations quite well. The peak is away from the Earth’s edge. Thus they A toy model reached when the transiting Earth is at produce an effective increase in the radi- about three quarters of the Solar reced- ation coming from the region of the Sun The opposition surge is not fully under- ing hemisphere. This is the position just behind the Earth as it moves across stood and we cannot make a quantitative where we expect the strongest effect on the face of the Sun. During its passage prediction of the distribution of the light the radial velocity due to the combined

22 The Messenger 161 – September 2015 40 Figure 5 we have shifted the data points from La Silla by this time difference and it is possible to see that they provide a much better continuity and overlap with the values measured at La Palma, regard- 20 less of the fact that the alignment has slightly changed in the meantime. This finding indeed suggests that the intensity ) –1

s of the opposition surge is very sensitive

(m 0 to the location of the observer on Earth, and in particular to the distance to the RV Δ Earth’s projected edges.

There is also the possible presence of a –20 double peak around the position of the maximum of the radial velocity, which is suggestive of the presence of two com- ponents. While the broad component –40 could be associated with a diffuse area of 56662 56662.55666356663.5 56664 enhanced emission, the narrower com- MJD ponent could be due to a peak of emis- sion localised in proximity to the Earth. Figure 5. Model of the inverse Rossiter–McLaughlin city decreases smoothly while the Earth The result of simulated emission originat- RV drift induced by an increase in the Solar emis­ is moving away. ing in a relatively small area around the sivity in the region behind the Earth’s transit trajec- tory due to the opposition surge. The thin black line Earth is plotted as a thin line in Figure 5 shows the drift expected from a small area moving The RVs are still high many hours after and it reproduces the peak quite well. with the Earth, while the thick line shows a drift the end of the transit and it is only during ­coming from a larger area with a radius of ~ 10 arc- the following night that we again meas- minutes required to match the start of the radial velocity drift. The gap between the two models is ured a constant normal radial velocity. Prospects for another transit due to the numerical impossibility of computing an Assuming the opposition surge is quite RM effect for the total eclipse. The data points from symmetric, it probably started and ended The next Earth transit from Jupiter will La Silla (blue) are delayed by 0.1468 MJD to compen- ~ 15 hours before and after the transit, occur in 2026, but it will be a grazing sate for the longitude difference between the obser- vatories of La Palma and La Silla. The blue dotted when the Earth was at a projected dis- transit. However, since we have observed line shows the expected Rossiter–McLaughlin effect tance of about 10 arcminutes from the the effect of the opposition surge when amplified by a factor of 50. The inverse RM effect Solar limbs. the Earth was at an angle as high as detected is about 400 times larger than the expected ~ 10 arcminutes, we can predict that this RM due to the Earth’s transit. As we have already noted, the almost unique phenomenon can be observed simultaneous observations from the two again, although with a lower amplitude in observatories give slightly different radial radial velocity. We really hope to have a effect of the almost tangential rotational velocities. The RV values from La Silla are very high resolution spectrograph (e.g., velocity and of the limb darkening of always lower by 4 to 10 m s–1 than those HIRES) at the European Extremely Large the Sun. During the decline, a break in from La Palma, and in particular during Telescope (E-ELT) to follow this new the slope with a more gentle decrease is the transit event. This difference is too Earth transit. observed in proximity to the Earth’s large to be explained only by systematics egress. The region with enhanced emis- and it is quite probable that the different sion has been enlarged to 10 arcminutes locations on Earth of the two observato- References to allow the RV anomaly to extend well ries experience a slightly different opposi- Gimenez, A. 2006, ApJ, 650, 408 outside the transit. tion surge. In other words, the distance McLaughlin, D. B. 1924, ApJ, 60, 22 from the Earth’s edge could have been Meeus, J. 1989, Transits (Richmond, Virginia: It should be noted that the model simu­ relevant in determining the intensity of Willmann-Bell) lation does not end abruptly with the the opposition surge and therefore the Molaro, P. et al. 2013, MNRAS, 429, L79 Molaro, P. et al. 2013, The Messenger, 153, 22 end of the Earth transit. This is because radial velocity value. The time difference Molaro, P. & Monai, S. 2012, A&A, 544, 125 the opposition surge is also present between the longitudes of La Palma Molaro, P. et al. 2015, MNRAS, 453, 1684 when the Earth has just left the face of and La Silla is 0.1468 MJD (modified Rossiter, R. A. 1924, ApJ, 60, 15 the Sun. For many hours after the end of Julian date), while they have similar dis- the transit, the opposition surge makes tances from the equator. This implies Links the Solar hemisphere that has just been that, after this time interval, La Silla will left by the transiting Earth brighter than be at approximately the same distance 1 Solar System Dynamics Group, Horizons Web the more distant one and the radial velo­ from the Earth’s edge as La Palma. In ­Ephemerides, JPL: http://ssd.jpl.nasa.gov

The Messenger 161 – September 2015 23