Exoplanet Imitators: A Test of Stellar Activity Behavior in Radial Velocity Signals

Nava, Chantanelle; López‐Morales, Mercedes; Haywood, R. D.; Giles, H.

doi:10.3847/1538-3881/ab53ec

Cited by 59 publications

(45 citation statements)

References 51 publications

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“…ñightly) RV time-series at orbital periods longer than the stellar rotation period, as is the case for AU Mic b. However, for more sparsely sampled RV cadences such as ours, stellar activity can introduce apparent periodicities at time-scales longer than the stellar rotation period that can persist for several seasons 41 . The long-term magnetic activity evolution of AU Mic on timescales >100 days is also neither constrained nor modeled.…”

Section: Additional Methodsmentioning

confidence: 76%

A planet within the debris disk around the pre-main-sequence star AU Microscopii

Plavchan

Barclay

Gagné

et al. 2020

Nature

212

124

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AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79 parsecs and with an age of 22 million years 1 . AU Mic possesses a relatively rare 2 and spatially resolved 3 edge-on debris disk extending from about 35 to 210 astronomical units from the star 4 , and with clumps exhibiting non-Keplerian motion 5-7 . Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic 'activity' on the star 8,9 . Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46 days, an orbital distance of 0.07 astronomical units, a radius of 0.4 Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.

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Section: Additional Methodsmentioning

confidence: 76%

A planet within the debris disk around the pre-main-sequence star AU Microscopii

Plavchan

Barclay

Gagné

et al. 2020

Nature

212

124

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“…We note that this feature at P rot /2 is similar to the first harmonic of P rot observed on the Sun that has been shown to have either a comparable amount or at times more power than at P rot (Mortier & Collier Cameron 2017;Milbourne et al 2019). However, we note that simulated RV time series with injected quasi-periodic magnetic activity signals have been shown to produce spurious, and sometimes long-lived, periodogram signals that can masquerade as rotation signatures (Nava et al 2020). But given that the 22day signal is nearly identical to the first harmonic of the measured rotation period, we proceed with treating the 22-day signal as stellar activity and opt to simultaneously fit the HARPS-N and HIRES RVs with model components for TOI-1235 b, in the form of a Keplerian orbit, plus a quasi-periodic GP regression model of stellar activity whose covariance kernel as a function of time t takes the form…”

Section: Precise Rv Analysissupporting

confidence: 70%

“…In our GP activity model, we measure an exponential timescale of λ=115±20 days, indicating that active regions are relatively stable over a few rotation cycles. According to detailed investigations of periodogram signals in simulated RV time series, the persistence of the maximum RV activity peak at P rot /2 is consistent with active region lifetimes on TOI-1235 exceeding P rot (Nava et al 2020).…”

Section: Precise Rv Analysissupporting

confidence: 58%

TOI-1235 b: A Keystone Super-Earth for Testing Radius Valley Emergence Models around Early M Dwarfs

Cloutier

Rodriguez

Irwin

et al. 2020

Self Cite

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Small planets on close-in orbits tend to exhibit envelope mass fractions of either effectively zero or up to a few percent depending on their size and orbital period. Models of thermally driven atmospheric mass loss and of terrestrial planet formation in a gas-poor environment make distinct predictions regarding the location of this rocky/ nonrocky transition in period-radius space. Here we present the confirmation of TOI-1235 b (P=3.44 days, =-+ r 1.738 p 0.076 0.087 Å R), a planet whose size and period are intermediate between the competing model predictions, thus making the system an important test case for emergence models of the rocky/nonrocky transition around early M dwarfs (R s =0.630±0.015 R  , M s =0.640±0.016 M ). We confirm the TESS planet discovery using reconnaissance spectroscopy, ground-based photometry, high-resolution imaging, and a set of 38 precise radial velocities (RVs) from HARPS-N and HIRES. We measure a planet mass of-+ 6.91 0.85 0.75 Å M , which implies an iron core mass fraction of-+ 20 12 15 % in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like, and we constrain an H/He envelope mass fraction to be <0.5% at 90% confidence. Our results are consistent with model predictions from thermally driven atmospheric mass loss but not with gas-poor formation, suggesting that the former class of processes remains efficient at sculpting close-in planets around early M dwarfs. Our RV analysis also reveals a strong periodicity close to the first harmonic of the photometrically determined stellar rotation period that we treat as stellar activity, despite other lines of evidence favoring a planetary origin (=-+ P 21.8 0.8 0.9 days, =-+ m i sin 13.0 p 5.3 3.8 Å M) that cannot be firmly ruled out by our data.

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“…Signals at 12 and 17.4 d verify the five points listed above and are thus considered as planets. We, however, point out that the predicted rotation period of the star is 18 ± 5 d. This means that signals could be present in this period range, not necessarily at the stellar rotation period or its harmonic (Nava et al 2020). Furthermore, in Appendix B, we show that the 17.4 d signal is less significant and that certain noise models favor 17.7 d over 17.4 d. It appears that fitting signals at 17.4 or 17.7 d does not completely remove the other.…”

Section: Periodicity Originmentioning

confidence: 76%

The SOPHIE search for northern extrasolar planets

Hara

Bouchy

Stalport

et al. 2020

A&A

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Aims. Since 2011, the SOPHIE spectrograph has been used to search for Neptunes and super-Earths in the northern hemisphere. As part of this observational program, 290 radial velocity measurements of the 6.4 V magnitude star HD 158259 were obtained. Additionally, TESS photometric measurements of this target are available. We present an analysis of the SOPHIE data and compare our results with the output of the TESS pipeline. Methods. The radial velocity data, ancillary spectroscopic indices, and ground-based photometric measurements were analyzed with classical and ℓ1 periodograms. The stellar activity was modeled as a correlated Gaussian noise and its impact on the planet detection was measured with a new technique. Results. The SOPHIE data support the detection of five planets, each with m sin i ≈ 6 M⊕, orbiting HD 158259 in 3.4, 5.2, 7.9, 12, and 17.4 days. Though a planetary origin is strongly favored, the 17.4 d signal is classified as a planet candidate due to a slightly lower statistical significance and to its proximity to the expected stellar rotation period. The data also present low frequency variations, most likely originating from a magnetic cycle and instrument systematics. Furthermore, the TESS pipeline reports a significant signal at 2.17 days corresponding to a planet of radius ≈1.2 R⊕. A compatible signal is seen in the radial velocities, which confirms the detection of an additional planet and yields a ≈2 M⊕ mass estimate. Conclusions. We find a system of five planets and a strong candidate near a 3:2 mean motion resonance chain orbiting HD 158259. The planets are found to be outside of the two and three body resonances.

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Exoplanet Imitators: A Test of Stellar Activity Behavior in Radial Velocity Signals

Cited by 59 publications

References 51 publications

A planet within the debris disk around the pre-main-sequence star AU Microscopii

A planet within the debris disk around the pre-main-sequence star AU Microscopii

TOI-1235 b: A Keystone Super-Earth for Testing Radius Valley Emergence Models around Early M Dwarfs

The SOPHIE search for northern extrasolar planets

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