Recovering planet radial velocity signals in the presence of starspot activity in fully convective stars

Barnes, John; Jeffers, S. V.; Anglada-Escudé, G.; Haswell, C. A.; Jones, H. R. A.; Tuomi, Mikko; Feng, Fabo; Jenkins, James S.; Petit, P.

doi:10.1093/mnras/stw3170

Cited by 46 publications

(34 citation statements)

References 42 publications

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“…Second, our analysis also relies on the assumption that a GP model is an adequate model for stellar activity. Studies of other starspot-dominated convective M dwarfs 38 suggest this is adequate, but additional future observations and modelling efforts are needed, particularly for stars as active as AU Mic. From Kepler photometric time series of main-sequence stars, we demonstrated 40 that stellar activity should not introduce substantial power in densely sampled (approximately nightly) RV time series at orbital periods longer than the stellar rotation period, as is the case for AU Mic b.…”

Section: Radial-velocity Analysismentioning

confidence: 99%

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

Plavchan

Barclay

Gagné

et al. 2020

Nature

185

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: Radial-velocity Analysismentioning

confidence: 99%

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

Plavchan

Barclay

Gagné

et al. 2020

Nature

185

124

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“…Infrared polarimetry will be particularly important for assessing the optical Stokes V observations, which do not enable reliable estimation of the magnetic field inside the spots we recover with brightness imaging. In addition to providing information about the underlying dynamo mechanisms, an understanding of starspot distributions on fully convective stars, which may possess significant rotation on average (with v sin i ∼ 10 km s −1 , Jenkins et al 2009;Reiners et al 2012), is needed if we are to effectively deal with radial velocity jitter in radial velocity searches for planets (Barnes et al 2017). Both CRIRES+ and and uves will be important instruments in this respect, offering the opportunity for high resolution observations.…”

Section: Discussionmentioning

confidence: 99%

Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB

Barnes

Jeffers

Haswell

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We aim to understand how stellar parameters such as mass and rotation impact the distribution of starspots on the stellar surface. To this purpose, we have used Doppler imaging to reconstruct the surface brightness distributions of three fully convective M dwarfs with similar rotation rates. We secured high cadence spectral time series observations of the 5.5 AU separation binary GJ 65, comprising GJ 65A (M5.5V, P rot = 0.24 d) and GJ 65B (M6V, P rot = 0.23 d). We also present new observations of GJ 791.2A (M4.5V, P rot = 0.31 d). Observations of each star were made on two nights with uves, covering a wavelength range from 0.64 − 1.03 µm. The time series spectra reveal multiple line distortions, which we interpret as cool starspots and which are persistent on both nights suggesting stability on the timescale of 3 days. Spots are recovered with resolutions down to 8.3 • at the equator. The global spot distributions for GJ 791.2A are similar to observations made a year earlier. Similar high latitude and circumpolar spot structure is seen on GJ 791.2A and GJ 65A. However, they are surprisingly absent on GJ 65B, which instead reveals more extensive, larger, spots concentrated at intermediate latitudes. All three stars show small amplitude latitude-dependent rotation that is consistent with solid body rotation. We compare our measurements of differential rotation with previous Doppler imaging studies and discuss the results in the wider context of other observational estimates and recent theoretical predictions.

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“…Such precision would mean that activity-induced RV variations are expected to be larger than the RV uncertainties for most of the M dwarfs, most of the time. Under these circumstances, extending our picture of planetary occurrence rates to lower-mass planets and lower-mass stars will require a paradigm shift, from treating activity-induced RV variations as an additional noise source (e.g., Wright 2005) to modeling RV time series with both orbital-and activity-induced signals (e.g., Hébrard et al 2016;Barnes et al 2017). Farther in the future, one could even consider forward modeling the spectra themselves with the same ingredients.…”

Section: Summary and Future Workmentioning

confidence: 99%

The CARMENES search for exoplanets around M dwarfs

Tal-Or

Zechmeister

Reiners

et al. 2018

A&A

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Context. Previous simulations predicted the activity-induced radial-velocity (RV) variations of M dwarfs to range from ~1 cm s−1 to ~1 km s−1, depending on various stellar and activity parameters. Aims. We investigate the observed relations between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing CARMENES high-resolution visual-channel spectra (0.5–1μm), which were taken within the CARMENES RV planet survey during its first 20 months of operation. Methods. During this time, 287 of the CARMENES-sample stars were observed at least five times. From each spectrum we derived a relative RV and a measure of chromospheric Hα emission. In addition, we estimated the chromatic index (CRX) of each spectrum, which is a measure of the RV wavelength dependence. Results. Despite having a median number of only 11 measurements per star, we show that the RV variations of the stars with RV scatter of >10 m s−1 and a projected rotation velocity v sin i > 2 km s−1 are caused mainly by activity. We name these stars “active RV-loud stars” and find their occurrence to increase with spectral type: from ~3% for early-type M dwarfs (M0.0–2.5 V) through ~30% for mid-type M dwarfs (M3.0–5.5 V) to >50% for late-type M dwarfs (M6.0–9.0 V). Their RV-scatter amplitude is found to be correlated mainly with v sin i. For about half of the stars, we also find a linear RV–CRX anticorrelation, which indicates that their activity-induced RV scatter is lower at longer wavelengths. For most of them we can exclude a linear correlation between RV and Hα emission. Conclusions. Our results are in agreement with simulated activity-induced RV variations in M dwarfs. The RV variations of most active RV-loud M dwarfs are likely to be caused by dark spots on their surfaces, which move in and out of view as the stars rotate.

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Recovering planet radial velocity signals in the presence of starspot activity in fully convective stars

Cited by 46 publications

References 42 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

Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB

The CARMENES search for exoplanets around M dwarfs

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