Wednesday, 25 March 2020

EPIC 249893012: An evolved star with three transiting planets.

With the advent of space-based transit-search missions, the detection and characterization of exoplanets have undergone a fast-paced revolution. First the CoRoT and then the Kepler space telescopes marked a major leap forward in understanding the diversity of planets in our Galaxy. With the failure of its second reaction wheel, Kepler embarked on an extended mission, named K2, which surveyed di erent stellar fields located along the ecliptic. Their high-precision photometry has allowed the Kepler and K2 missions to dramatically extended the parameter space of exoplanets, bringing the transit detection threshold down to the Earth-sized regime. The Transiting Exoplanet Survey Satellite is currently extending this search to cover almost the entire sky. Although super-Earths (planets with radiuses 1-2 times that of the Earth and masses 1-10 tines that of the Earth ) and Neptune-sized planets (planets with radiuses 2-4 times that of the Earth and masses 10-40 times that of the Earth ) are ubiquitous in our Galaxy, we still have much to learn about the formation and evolution processes of small planets. Observations have led to the discovery of peculiar patterns in the parameters of small exoplanets. The radius–period diagram shows a dearth of short-period Neptune-sized planets, the so-called Neptunian Desert, while small planets cluster around radiuses of 1.3 and 2.6 times that of the Earth, with very few planets having radiuses around 1.8 times that of the Earth, the so-called radius gap. Atmospheric erosion by high-energy stellar radiation (also known as photoevaporation) is believed to play a major role in shaping both the Neptunian Desert and the bimodal distribution of planetary radii. A gap in the mass distribution of planets with a mass lower than about 20 times that of the Earth periods shorter than 20 days has also been observed, so far without any apparent physical explanation.

In a paper published in the journal Astronomy and Astrophysics, and on the online arXiv database at Cornell University Library, on 5 February 2020, a team of astronomers led by Diego Hidalgo of the Instituto de Astrofisica de Canarias and the Departamento Astrofísica at the Universidad de La Laguna, describe the K2 photometry of the star EPIC 249893012, together with the detection of three transiting planets, follow-up observations from ground-based telescopes.

EPIC 249893012 was observed during K2 Campaign 15, which lasted 88 days, from 23 August 2017 to 20 November 2017, observing a patch of sky toward the constellations of Libra and Scorpius. During Campaign 15, the Sun emitted 27 M-class and four X-class flares and released several powerful coronal mass ejections. This a ected the measured dark current levels for all K2 channels. Hildago et al. found three planetary signals in the EPIC 249893012 light curve, with periods of 3.59, 15.63, and 35.75 days.

Customized K2 image of EPIC 249893012. North is to the left and east at the bottom. The field of view is 43.78 x 51.74 arcsecond (3.98 arcsecond  per pixel). The red line marks the customized aperture for light-curve extraction. Green annotations are the Kepler magnitude of EPIC 249893012. Hildago et al (2020).

On 14 July 2019, Hildago et al. performed adaptive-optics imaging for EPIC 249893012 with the InfraRed Camera and Spectrograph on the Subaru 8.2m telescope to search for faint nearby sources that might contaminate the K2 photometry.

InfraRed Camera and Spectrograph 4 x 4 arcsecond image of EPIC 249893012. Hildago et al. (2020).

Visual inspection of the saturated image suggests no nearby companion within 5 arcseconds from EPIC 249893012, but it exhibits a faint source separated by 8.3 arcseconds in the southeast. that is, inside the aperture for light-curve extraction on the K2 image. Checking the Gaia DR2 catalog, we ound that this faint source is the Gaia DR2 6259260177825579136 star with a magnitude of 17.9. The transit signal with depth observed in the K2 could potentially be mimicked by an equal-mass eclipsing binary that is 9.29 magnitude fainter, i.e. a magnitude of 20.65 (in astronomy higher magnitudes indicate fainter objects). Taking into account the close similarity between the Gaia and Kepler bandpasses, Hildago et al. therefore cannot exclude Gaia DR2 6259260177825579136 as a source of a false positive for one of the three transit signals.

Adaptive-optics image of EPIC 249893012 obtained with the Subaru/IRCS instrument. North is up and east is to the left. Field of view of is 21 arcseconds in both directions. Because this image was created after median-combining the aligned frames, background levels as well as flux scatters in the corners are di fferent from those of the central part of the detector. Hildago et al. (2020).

Hildago et al. collected 74 high-resolution spectra of EPIC 249893012 using the High Accuracy Radial velocity Planet Searcher spectrograph mounted at the ESO-3.6m telescope of the La Silla Observatory in Chile. The observations were carried out between April 2018 and August 2019 as part of our radial velocity follow-up of K2 and Transiting Exoplanet Survey Satellite  planets conducted with the High Accuracy Radial velocity Planet Searcher spectrograph. Between April 2018 and March 2019, Hildago et al. also secured 11 high-resolution spectra with the HARPS-N spectrograph mounted at the 3.58m Telescopio Nazionale Galileo at Roque de Los Muchachos Observatory in La Palma, Spain. 

Between 6 May 2018 and 21 June 2018, Hildago et al. also collected 25 spectra of EPIC 249893012 with the Calar Alto high-Resolution search for Mdwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs instrument installed at the 3.5m telescope of Calar Alto Observatory in Spain.

Based upon these observations, Hildago et al. calculate that EPIC 249893012 is a G8 IV/V type star (yellow dwarf star), 324.7 parsecs (1059 light years) from Earth, with an effective surface temperature of 5430 K (compared to 5778 K for our Sun), a mass 1.05 times that of the Sun, a radius 1.71 times that of the Sun, a luminosity 2.26 times that of the Sun, and an age of about 9 bikkion years. Such a star would be coming to the end of its life as a main sequence star, and would be begining to expand.

The faint star detected in the Subaru/IRCS AO image and identified as Gaia DR2 6259260177825579136 cannot be excluded as a possible source of one of the transit signals detected in the K2 light curve of EPIC 249893012. The parallax (the extent to which an object's apparent position is changed by the Earth's motion around the Sun) of Gaia DR2 6259260177825579136 and its proper motion (extent to which an object apparently moves over time; a distant object will appear to move more slowly than a close object moving at the same speed) suggest that this is a distant background star, between 1.92 and 4.45 kpc (6262-14 514 light years) away. This makes a false-positive scenario with the Gaia DR2 6259260177825579136 star as an equal-mass eclipsing binary highly improbable for any of three transit signals of EPIC 249893012.

A planet can be detected by the tidal effect it exerts upon its star; a large planet orbiting close to a small star exerts a considerable gravitational pull, causing the star to wobble back and forth as the planet orbits it. This can be detected by sensitive telescopes using the Doppler effect, as the star moves towards us the light waves it emits are slightly compressed, making it appear slightly blueish (to very sensitive spectrometers, not human astronomers), as it moves away the light waves are expanded, making it appear slightly reddish. 

In order to search for the Doppler reflex motion induced by the three transiting planets and unveil the presence of possible additional signals in their time-series Doppler data, Hildago et al. performed a frequency analysis of the radial velocity measurements and their activity indicators. To this end, they used only the High Accuracy Radial velocity Planet Searcher data taken in 2019. This allowed them to avoid spurious peaks introduced by the one-year sampling and avoid having to account for radial velocity o ffsets between different instruments. The 60 High Accuracy Radial velocity Planet Searcher radial velocity measurements taken in 2019 cover a time baseline of about 171 days.

Hildago et al. found a significant peak at the orbital frequency of the inner transiting planet, EPIC 249893012 b,  3.6 days. Subtracting this from the overall radial velocity of EPIC 249893012 enabled Hildago et al. to detect a further signal at 15.6 days, attributed to a second planet, EPIC 249893012 c. Subtracting both these signals from the overall radial velocity revealed a third peak at 35.7 days, attributed to a third planet EPIC 249893012 d. No further signals were detected in this way.

By combining K2 photometry with high-resolution imaging and highprecision Doppler spectroscopy, Hildago et al. confirmed the three planets and determined their masses, radii, and mean densities. With an orbital period of 3.6 days, the inner planet, EPIC 249893012 b, is calculated to have a mass 8.75 times that of the Earth, and a radius 1.95 times that of the Earth. yielding a mean density of 6.39 grams per cm³ (for comparison the mean density of the Earth is 5.51 grams per cm³). With an orbital period of 15.6 days, EPIC 249893012 c is calculated to have a mass 14.67 times that of the Earth and a radius 3.67 times that of the Earth, yielding a mean density of 1.62 grams per cm³. The outer planet, EPIC 249893012 d, resides on a 35.7-day orbit, and has a mass 10.18 times that of the Earth and a radius 3.94 times that of the Earth, yielding a mean density of 0.91 grams per cm³.

EPIC 249893012 b is a super-Earth with a density compatible with a pure silicate composition. However, a more realistic configuration would be a nickel-iron core and a silicate mantle. It lies above the model for 50% iron - 50% silicate, which probably means that it still has some residual H₂-He atmosphere, which enlarges its radius but does not significantly contribute to the total planet mass. As has previously been noted, small planets follow a bimodal distribution with a valley at about 1.5-2.2 times the radius of the Earth, and peaks at approximately 1.3 times the radius of the Earth (super-Earths) and 2.4 times the radius of the Earth (sub-Neptunes). Planet EPIC 249893012 b lies in the transition zone and might have lost most of its atmosphere through di erent mechanisms. The first is photoevaporation, which suggests a past atmosphere principally composed of hydrogen, which mainly occurs in the first 100 million years of the stellar life, when it is more choromospherically active. An alternative possible mechanism to explain a relatively thin atmosphere works by delaying gas accretion into the planet until the gas in the protoplanetary disk is almost dissipated. Planetesimal impacts during planet formation can also encourage atmospheric loss, but it is unclear if impacts alone could produce the observed properties of EPIC 249893012 b. Hildago et al. estimate that given the proximity of EPIC 249893012 b to its star (about 0.05 AU, or 0.05 times the distance at which the Earth orbits the Sun), the influence of photoevaporation has been one of the most likely causes in the loss of its majority primordial hydrogen atmosphere.

EPIC 249893012 c and d are Neptune-sized planets, but with lower masses and hence lower mean densities (1.62 grams per cm³ and 0.91 grams per cm³ for planets c and d, respectively, compared to 1.95 grams per cm³ for Neptune), which suggest the presence of thicker atmospheres. EPIC 249893012 c has a stellar irradiation of 220 000 000 ergs per cm² per second, slightly above the threshold of 200 000 000 ergs per cm² per second above which planets might inflate their atmospheres and be subject of photoevaporation. In contrast, EPIC 249893012 d has a stellar irradiation of 72 000 000 ergs per cm² per second and should in principle not be subjected to photoevaporation processes. The radius of c may therefore be compared to models of gas giant planets, based on which, Hildago . derive a core mass of about 10 times that of the Earth. The density, radius mass of EPIC 249893012et al d suggest a relatively small but heavy core with a thick atmosphere. EPIC 249893012

Based on the study of three planetary systems, it has been proposed that close-in planets orbiting evolved stars tend to reside on eccentric orbits. If this scenario is correct, the nearly circular orbits of EPIC 249893012 b, c, and d may be the result of the planets orbiting a star that is not evolved enoughfor a fair comparison to be made. Hildago . conclude that et alEPIC 249893012 b orbits the star at a distance of 0.047 AU, EPIC 249893012 c at 0.13 AU, and EPIC 249893012 d at 0.22 AU. As such they consider the three planets of the system EPIC 249893012 as close-in planets, with circular orbits, although for EPIC 249893012EPIC 249893012EPIC 249893012 d a wide range of eccentricities from 0.04 to 0.36 is possible. Because the system is at an early stage of its evolution after leaving the main sequence, it is a good candidate for a detailed study of its dynamical evolution, to shed light on the formation of close-in giant planets and test the hypothesis that giant planets form a dynamical barrier that confines super-Earths to an inward-migrating evolution.EPIC 249893012

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