Cloud of the most frequent
words appearing in my papers. Done with ADS Bumblebee
Characterisation of transiting exoplanets
Radius of exoplanets can be measured throught high-precision photometry, from the ground or using space-based telescope likeKepler or CoRoT. To measure their mass, one technique consists in searching for the reflex motion of the star orbited by the planet using the Doppler effect, so-called radial velocity. This requires high-resolution spectrographs like SOPHIE, HARPS, HARPS-N.
Since 2010, I am leading a large spectroscopic programme with the SOPHIE spectrograph at the Haute-Provence Observatory (France) to characterise giant exoplanets and brown dwarfs found in transit by the Kepler / K2 mission. Here is a sample of characterised systems:
- "SOPHIE velocimetry of Kepler transit candidates. II. KOI-428b: a hot Jupiter transiting a subgiant F-star", by Santerne, Díaz, Bouchy et al., 2011a, A&A, 528, A63.
- "SOPHIE velocimetry of Kepler transit candidates. IV. KOI-196b: a non-inflated hot Jupiter with a high albedo", by Santerne, Bonomo, Hébrard et al, 2011b, A&A, 536, A70.
- "SOPHIE velocimetry of Kepler transit candidates. VI. An additional companion in the KOI-13 system", by Santerne, Moutou, Barros, et al., 2012a, A&A, 544, L12.
- "SOPHIE velocimetry of Kepler transit candidates. XII. KOI-1257 b: a highly eccentric three-month period transiting exoplanet", by Santerne, Hébrard, Deleuil, et al., 2014, A&A, 57, A37.
- "K2-29 b/WASP-152 b: An Aligned and Inflated Hot
Jupiter in a Young Visual Binary", by Santerne, Hébrard, Lillo-Box
et al., 2016b, ApJ, 824, 55.
The solution against astrophysical false positives: PASTIS
Several configurations of stellar systems can mimic
the transit of a
plane and are limiting the interpretation of large photometric surveys
like Kepler. The planetary
nature of transiting candidates must be confirmed. This can be done
e.g. by measuring the mass of the transiting object to determine if it
is in the planet’s mass range. Following up transiting candidates with
radial velocity instruments is thus in important step in the discovery
and characterization of a new transiting exoplanet.(a)
(b) (c)
(d)
Different astrophysical scenarios that can mimic a planetary transit. (a) a transit of a planet ; (b) an eclipse of a low-mass star or a brown dwarf with radius similar to that of Jupiter ; (c) an eclipse of a binary in the foreground/background, aligned with the target star, as seen from the Earth (so-called background eclipsing binary) or physically bound with the target star (so-called triple system) ; (d) a transit of a planet on a star aligned with the target star, as seen from the Earth, in the foreground/background (so-called background transiting planet) or physically bound with the target star (so-called companion transiting planet).
Different astrophysical scenarios that can mimic a planetary transit. (a) a transit of a planet ; (b) an eclipse of a low-mass star or a brown dwarf with radius similar to that of Jupiter ; (c) an eclipse of a binary in the foreground/background, aligned with the target star, as seen from the Earth (so-called background eclipsing binary) or physically bound with the target star (so-called triple system) ; (d) a transit of a planet on a star aligned with the target star, as seen from the Earth, in the foreground/background (so-called background transiting planet) or physically bound with the target star (so-called companion transiting planet).
Current spectrographs are not sensitive enough to detect the mass of most of the small-planet candidates discovered by Kepler and CoRoT. To establish their planetary nature, the planet-validation technique consists in evaluating the probability of each scenario to mimic the observed data (photometry, radial velocity, spectroscopy). If the planet scenario has a significantly higher probability than all other false-positive scenarios, then the candidate is considered as validated. The PASTIS tool (Planet Analysis and Small Transit Investigation Software) has been developped, mainly at the Laboratoire d’Astrophysique de Marseille, to perform such statistical comparison of the various aforementionned scenarios and then, to validate planets as small as the Earth.
The PASTIS tool is fully described in the papers:
- "PASTIS: Bayesian extrasolar planet validation - I. General framework, models, and performance" by Díaz, Almenara, Santerne et al., 2014, MNRAS, 441, 983.
- "PASTIS: Bayesian extrasolar planet validation - II. Constraining exoplanet blend scenarios using spectroscopic diagnoses" by Santerne, Díaz, Almenara, et al., 2015, MNRAS, 451, 2337.
- CoRoT-16 b: "Transiting exoplanets from the CoRoT space mission. XXII. CoRoT-16b: a hot Jupiter with a hint of eccentricity around a faint solar-like star", by Ollivier, Gillon, Santerne, et al., 2012, A&A, 541, A149.
- CoRoT-24 b & c: "Transiting exoplanets from the CoRoT space mission.
XXVI. CoRoT-24: a transiting multiplanet system", by Alonso,
Moutou, Endl, et al., 2014, A&A, 567, A112.
- KOI-1257 b / Kepler-420 b: "SOPHIE velocimetry of Kepler transit candidates. XII. KOI-1257 b: a highly eccentric three-month period transiting exoplanet", by Santerne, Hébrard, Deleuil, et al., 2014, A&A, 57, A37.
- CoRoT-22 b: "CoRoT-22 b: a validated 4.9 R⊕ exoplanet in 10-d orbit", by Moutou, Almenara, Díaz, et al., 2014, MNRAS, 444, 2783.
- K2-19 b & c: "One of the closest exoplanet pairs to the 3:2 mean motion resonance: K2-19b and c", by Armstrong, Santerne, Veras, et al., 2015, A&A, 582, A33.
- WASP-121 b: "WASP-121 b: a hot Jupiter close to tidal disruption
transiting an active F star", by Delrez, Santerne, Almenara, et
al., 2016, MNRAS, 458, 4025.
Evaluation of Kepler's
false-positive rate
The main
objective of the Kepler
prime mission was to measure the occurrence rate of exoplanets (i.e.
the
fraction of stars with planets), down to Earth-size planets in the
habitable zone (i.e. the distance from the star where liquid water
might be at the surface of the planet) of their host stars. To derive
robust occurrence rates, it is mandatory to evaluate the reliability of
the planetary candidate sample, or the false-positive probability/rate
(FPP / FPR), i.e. the probability that a transit candidate is not a
planet. While the FPP was initially estimated to be very low, the
observations with the SOPHIE spectrograph that I led on a complete
sample of Kepler's
short-period giant-planet candidates revealed that the false-positive
rate in this planet population was much higher, with an observed value
of 34.8 ± 6.5%. I then revised this value on the sample of Kepler's giant-planet candidates
transiting up to an orbital period of 400 days to a value of 54.6 ± 6.5
%. This last value includes the detections based on the entiere data
set
collected by Kepler (Q1 -
Q17).I led a study to evaluate how eccentric binairies for which only the secondary eclipse could be seen from the Earth might mimic exoplanetary transits. This study also estimated how many of these cases should be in the exoplanet candidate list of Kepler.
My papers on the Kepler false-positive rate:
- "SOPHIE velocimetry of Kepler transit candidates. VII. A false-positive rate of 35% for Kepler close-in giant candidates" by Santerne, Díaz, Moutou, et al., 2012b, A&A, 545, A76.
- "The contribution of secondary eclipses as
astrophysical false positives to exoplanet transit surveys" by
Santerne, Fressin, Díaz, et al., 2013, A&A, 557, A139.
- "SOPHIE velocimetry of Kepler transit candidates. XVII.
The physical properties of giant exoplanets within 400 days of period"
by Santerne, Moutou, Tsantaki, et al., 2016a, A&A, 578, A64.
Statistics of the giant planet populations in the Kepler field
Using the giant-planet candidates detected in the Kepler data (Q1 - Q17) for which all false positives were screened out with the SOPHIE spectrograph (over a 6-year observing programme), I led a paper deriving the occurrence rates of giant planets, in different ranges of orbital periods, from a day up to 400 days. By identifying one by one the false positives in the candidate sample, the derived statistics are very robust. The main surprise was the detection of the valley in the period distribution of giant exoplanets. This valley was first clearly identifiied since the beginning of spectroscopic surveys. This valley was first unconfirmed in the previous occurrence rate studies based on the Kepler data because of not-perfect correction for false positives. Hence, this valley that reveals two different formation procesess for giant planets was for the first time confirmed in this paper.This study also confirmed the depression of hot jupiters in the Kepler field compared to other photometric and spectroscopic surveys. This is not observed for giant planets with longer orbital periods, although few photometric surveys have explored this domain. The reason for the Kepler field to be hot-jupiter-poor is still not understood.
References:
- "SOPHIE velocimetry of Kepler transit candidates. XVII. The physical properties of giant exoplanets within 400 days of period" by Santerne, Moutou, Tsantaki, et al., 2016a, A&A, 578, A64.