# Publications by Suzanne Aigrain

## A self-lensing binary massive black hole interpretation of quasi-periodic eruptions

Monthly Notices of the Royal Astronomical Society Oxford University Press 503 (2021) 1703-1716

A Ingram, S Motta, S Aigrain, A Karastergiou

Binary supermassive black hole (SMBH) systems result from galaxy mergers, and will eventually coalesce due to gravitational wave (GW) emission if the binary separation can be reduced to . 0.1 pc by other mechanisms. Here, we explore a gravitational self-lensing binary SMBH model for the sharp (duration ⇠ 1 hr), quasi-regular X-ray flares – dubbed quasiperiodic eruptions – recently observed from two low mass active galactic nuclei: GSN 069 and RX J1301.9+2747. In our model, the binary is observed ⇠edge-on, such that each SMBH gravitationally lenses light from the accretion disc surrounding the other SMBH twice per orbital period. The model can reproduce the flare spacings if the current eccentricity of RX J1301.9+2747 is n0 & 0.16, implying a merger within ⇠ 1000 yrs. However, we cannot reproduce the observed flare profiles with our current calculations. Model flares with the correct amplitude are ⇠ 2/5 the observed duration, and model flares with the correct duration are ⇠ 2/5 the observed amplitude. Our modelling yields three distinct behaviours of self-lensing binary systems that can be searched for in current and future X-ray and optical time-domain surveys: i) periodic lensing flares, ii) partial eclipses (caused by occultation of the background mini-disc by the foreground mini-disc), and iii) partial eclipses with a very sharp in-eclipse lensing flare. Discovery of such features would constitute very strong evidence for the presence of a supermassive binary, and monitoring of the flare spacings will provide a measurement of periastron precession.

## TESS Re-observes the Young Multi-planet System TOI-451: Refined Ephemeris and Activity Evolution

Research Notes of the AAS American Astronomical Society 5 (2021) 51-51

O Barragán, S Aigrain, E Gillen, F Gutiérrez-Canales

## Planet Hunters TESS II: Findings from the first two years of TESS

Monthly Notices of the Royal Astronomical Society Oxford University Press 501 (2020) staa3739

N Eisner, O Barragán, C Lintott, S Aigrain, B Nicholson, T Boyajian, S Howell, C Johnston, B Lakeland, G Miller, A McMaster, H Parviainen, E Safron, M Schwamb, L Trouille, S Vaughan, N Zicher, C Allen, S Allen, M Bouslog, C Johnson, M Simon, Z Wolfenbarger, E Baeten, T Hoffman

We present the results from the first two years of the Planet Hunters TESS (PHT) citizen science project, which identifies planet candidates in the TESS (Transiting Exoplanet Survey Satellite) data by engaging members of the general public. Over 22 000 citizen scientists from around the world visually inspected the first 26 sectors of TESS data in order to help identify transit-like signals. We use a clustering algorithm to combine these classifications into a ranked list of events for each sector, the top 500 of which are then visually vetted by the science team. We assess the detection efficiency of this methodology by comparing our results to the list of TESS Objects of Interest (TOIs) and show that we recover 85 per cent of the TOIs with radii greater than 4 R⊕ and 51 per cent of those with radii between 3 and 4 R⊕. Additionally, we present our 90 most promising planet candidates that had not previously been identified by other teams, 73 of which exhibit only a single-transit event in the TESS light curve, and outline our efforts to follow these candidates up using ground-based observatories. Finally, we present noteworthy stellar systems that were identified through the Planet Hunters TESS project.

## Pleiades or Not? Resolving the Status of the Lithium-rich M Dwarfs HHJ 339 and HHJ 430

Astronomical Journal American Astronomical Society 160 (2020) 30

J Stauffer, D Barrado, T David, LM Rebull, LA Hillenbrand, EE Mamajek, R Oppenheimer, S Aigrain, H Bouy, J Lillo-Box

Oppenheimer et al. discovered two M5 dwarfs in the Pleiades with nearly primordial lithium. These stars are not low enough in mass to represent the leading edge of the lithium depletion boundary at Pleiades age (~125 Myr). A possible explanation for the enhanced lithium in these stars is that they are actually not members of the Pleiades but instead are members of a younger moving group seen in projection toward the Pleiades. We have used data from Gaia DR2 to confirm that these two stars, HHJ 339 and HHJ 430, are indeed not members of the Pleiades. Based on their space motions, parallaxes, and positions in a Gaia-based color–magnitude diagram, it is probable that these two stars are about 40 parsecs foreground to the Pleiades and have ages of ~25 Myr. Kinematically they are best matched to the 32 Ori moving group.

## Mon-735: a new low-mass pre-main-sequence eclipsing binary in NGC 2264

Monthly Notices of the Royal Astronomical Society Oxford University Press 495 (2020) 1531-1548

E Gillen, LA Hillenbrand, J Stauffer, S Aigrain, L Rebull, AM Cody

We present Mon-735, a detached double-lined eclipsing binary (EB) member of the ∼3 Myr old NGC 2264 star-forming region, detected by Spitzer. We simultaneously model the Spitzer light curves, follow-up Keck/HIRES radial velocities, and the system’s spectral energy distribution to determine self-consistent masses, radii, and effective temperatures for both stars. We find that Mon-735 comprises two pre-main-sequence M dwarfs with component masses of M = 0.2918 ± 0.0099 and 0.2661 ± 0.0095 M⊙, radii of R = 0.762 ± 0.022 and 0.748 ± 0.023 R⊙, and effective temperatures of Teff = 3260 ± 73 and 3213 ± 73 K. The two stars travel on circular orbits around their common centre of mass in P = 1.9751388 ± 0.0000050 d. We compare our results for Mon-735, along with another EB in NGC 2264 (CoRoT 223992193), to the predictions of five stellar evolution models. These suggest that the lower mass EB system Mon-735 is older than CoRoT 223992193 in the mass–radius diagram (MRD) and, to a lesser extent, in the Hertzsprung–Russell diagram (HRD). The MRD ages of Mon-735 and CoRoT 223992193 are ∼7–9 and 4–6 Myr, respectively, with the two components in each EB system possessing consistent ages.

## Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit

Monthly Notices of the Royal Astronomical Society Oxford University Press 494 (2020) 750-763

N Eisner, O Barragan Villanueva, S Aigrain, C Lintott, G Miller, N Zicher, TS Boyajian, C Briceño, EM Bryant, JL Christiansen, AD Feinstein, LM Flor-Torres, M Fridlund, D Gandolfi, J Gilbert, N Guerrero, JM Jenkins, K Jones, M Christensen, A Vanderburg, AR López-Sánchez, C Ziegler, DM Bundy, LD Melanson, I Terentev

We report on the discovery and validation of TOI 813b (TIC 55525572 b), a transiting exoplanet identified by citizen scientists in data from NASA's Transiting Exoplanet Survey Satellite (TESS) and the first planet discovered by the Planet Hunters TESS project. The host star is a bright (V = 10.3 mag) subgiant (USDR_\star=1.94\,R_\odotUSD, USDM_\star=1.32\,M_\odotUSD). It was observed almost continuously by TESS during its first year of operations, during which time four individual transit events were detected. The candidate passed all the standard light curve-based vetting checks, and ground-based follow-up spectroscopy and speckle imaging enabled us to place an upper limit of USD2 M_{Jup}USD (99 % confidence) on the mass of the companion, and to statistically validate its planetary nature. Detailed modelling of the transits yields a period of USD83.8911_{ - 0.0031 } ^ { + 0.0027 }USD days, a planet radius of USD6.71 \pm 0.38R_{\oplus}$, and a semi major axis of$0.423_{ - 0.037 } ^ { + 0.031 }USD AU. The planet's orbital period combined with the evolved nature of the host star places this object in a relatively under-explored region of parameter space. We estimate that TOI-813b induces a reflex motion in its host star with a semi-amplitude of USD\sim6USD msUSD^{-1}USD, making this system a promising target to measure the mass of a relatively long-period transiting planet.

## The spectral impact of magnetic activity on disc-integrated HARPS-N solar observations: exploring new activity indicators

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 494 (2020) 4279-4290

A Thompson, C Watson, R Haywood, J Costes, E de Mooij, AC Cameron, X Dumusque, D Phillips, S Saar, A Mortier, T Milbourne, S Aigrain, H Cegla, D Charbonneau, R Cosentino, A Ghedina, D Latham, M Lopez-Morales, G Micela, E Molinari, E Poretti, A Sozzetti, S Thompson, R Walsworth

&#xA9; 2020 Oxford University Press. All rights reserved. Stellar activity is the major roadblock on the path to finding true Earth-analogue planets with the Doppler technique. Thus, identifying new indicators that better trace magnetic activity (i.e. faculae and spots) is crucial to aid in disentangling these signals from that of a planet's Doppler wobble. In this work, we investigate activity related features as seen in disc-integrated spectra from the HARPS-N solar telescope. We divide high-activity spectral echelle orders by low-activity master templates (as defined using both logR _ HK and images from the Solar Dynamics Observatory, SDO), creating 'relative spectra'.With resolved images of the surface of the Sun (via SDO), the faculae and spot filling factors can be calculated, giving a measure of activity independent of, and in addition to, logR ' HK.We find pseudo-emission (and pseudoabsorption) features in the relative spectra that are similar to those reported in our previous work on &#x3B1; Cen B. In &#x3B1; Cen B, the features are shown to correlate better to changes in faculae filling factor than spot filling factor. In this work, we more confidently identify changes in faculae coverage of the visible hemisphere of the Sun as the source of features produced in the relative spectra. Finally, we produce trailed spectra to observe the radial velocity component of the features, which show that the features move in a redward direction as one would expect when tracking active regions rotating on the surface of a star.

## Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

Monthly Notices of the Royal Astronomical Society Royal Astronomical Society 493 (2020) 4342-4354,

J Taylor, V Parmentier, P Irwin, S Aigrain, G Lee, J Krissansen-Totton

Exoplanet emission spectra are often modelled assuming that the hemisphere observed is well represented by a horizontally homogenized atmosphere. However, this approximation will likely fail for planets with a large temperature contrast in the James Webb Space Telescope (JWST) era, potentially leading to erroneous interpretations of spectra. We first develop an analytic formulation to quantify the signal-to-noise ratio and wavelength coverage necessary to disentangle temperature inhomogeneities from a hemispherically averaged spectrum. We find that for a given signal-to-noise ratio, observations at shorter wavelengths are better at detecting the presence of inhomogeneities. We then determine why the presence of an inhomogeneous thermal structure can lead to spurious molecular detections when assuming a fully homogenized planet in the retrieval process. Finally, we quantify more precisely the potential biases by modelling a suite of hot Jupiter spectra, varying the spatial contributions of a hot and a cold region, as would be observed by the different instruments of JWST/NIRSpec. We then retrieve the abundances and temperature profiles from the synthetic observations. We find that in most cases, assuming a homogeneous thermal structure when retrieving the atmospheric chemistry leads to biased results, and spurious molecular detection. Explicitly modelling the data using two profiles avoids these biases, and is statistically supported provided the wavelength coverage is wide enough, and crucially also spanning shorter wavelengths. For the high contrast used here, a single profile with a dilution factor performs as well as the two-profile case, with only one additional parameter compared to the 1D approach.

## Transiting exoplanets from the CoRoT space mission XXIX. The hot Jupiters CoRoT-30 b and CoRoT-31 b

ASTRONOMY & ASTROPHYSICS 635 (2020)

PJ Borde, RF Diaz, O Creevey, C Damiani, H Deeg, P Klagyiviki, G Wuchterl, D Gandolfi, M Fridlund, F Bouchy, S Aigrain, R Alonso, J-M Almenara, A Baglin, CC Barros, AS Bonomo, J Cabrera, S Csizmadia, M Deleuil, A Erikson, S Ferraz-Mello, EW Guenther, T Guillot, S Grziwa, A Hatzes, G Hebrard, T Mazeh, M Ollivier, H Parviainen, M Paetzold, H Rauer, D Rouan, A Santerne, J Schneider

## A robust, template-free approach to precise radial velocity extraction

Monthly Notices of the Royal Astronomical Society Oxford University Press 492 (2020) 3960-3983

VM Rajpaul, S Aigrain, LA Buchhave

Doppler spectroscopy is a powerful tool for discovering and characterizing exoplanets. For decades, the standard approach to extracting radial velocities (RVs) has been to cross-correlate observed spectra with a weighted template mask. While still widely used, this approach is known to suffer numerous drawbacks, and so in recent years increasing attention has been paid to developing new and improved ways of extracting RVs. In this proof-of-concept paper, we present a simple yet powerful approach to RV extraction. We use Gaussian processes to model and align all pairs of spectra with each other; we combine the pairwise RVs thus obtained to produce accurate differential stellar RVs, without constructing any template. Doing this on a highly localized basis enables a data-driven approach to identifying and mitigating spectral contamination, even without the input of any prior astrophysical knowledge. We show that a crude implementation of this method applied to an inactive standard star yields RVs with comparable precision to and significantly lower rms variation than RVs from industry-standard pipelines. Though amenable to numerous improvements, even in its basic form presented here our method could facilitate the study of smaller planets around a wider variety of stars than has previously been possible.

## Erratum: “An 11 Earth-mass, Long-period Sub-Neptune Orbiting a Sun-like Star” (2019, AJ, 158, 165)

The Astronomical Journal American Astronomical Society 159 (2020) 34-34

AW Mayo, VM Rajpaul, LA Buchhave, CD Dressing, A Mortier, L Zeng, CD Fortenbach, S Aigrain, AS Bonomo, AC Cameron, D Charbonneau, A Coffinet, R Cosentino, M Damasso, X Dumusque, AFM Fiorenzano, RD Haywood, DW Latham, M López-Morales, L Malavolta, E Molinari, L Pearce, G Piotto, E Poretti, A Sozzetti

## The rotation of low mass stars at 30 Myr in the cluster NGC 3766

Proceedings of the International Astronomical Union (2020) 200-203

J Roquette, J Bouvier, E Moraux, H Bouy, J Irwin, S Aigrain, R Lachaume

© International Astronomical Union 2020. Together with the stellar rotation, the spotted surfaces of low-mass magnetically active stars produce modulations in their brightness. These modulations can be resolved by photometric variability surveys, allowing direct measurements of stellar spin rates. In this proceedings, we present results of a multisite photometric survey dedicated to the measurement of spin rates in the 30 Myr cluster NGC 3766. Inside the framework of the Monitor Project, the cluster was monitored during 2014 in the i-band by the Wide Field Imager at the MPG/ESO 2.2-m telescope. Data from Gaia-DR2 and grizY photometry from DECam/CTIO were used to identify cluster members. We present spin rates measured for â?"200 cluster members.

## Radial velocity confirmation of K2-100b: a young, highly irradiated, and low-density transiting hot Neptune

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY Oxford University Press (OUP) 490 (2019) 698-708

D Kubyshkina, D Gandolfi, J Livingston, MCV Fridlund, L Fossati, J Korth, H Parviainen, L Malavolta, E Palle, HJ Deeg, G Nowak, VM Rajpaul, N Zicher, G Antoniciello, N Narita, S Albrecht, LR Bedin, J Cabrera, WD Cochran, J de Leon, P Eigmueller, A Fukui, N Kusakabe, M Libralato, P Montanes-Rodriguez

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society We present a detailed analysis of HARPS-N radial velocity observations of K2-100, a young and active star in the Praesepe cluster, which hosts a transiting planet with a period of 1.7 d. We model the activity-induced radial velocity variations of the host star with a multidimensional Gaussian Process framework and detect a planetary signal of 10.6 ± 3.0 m s−1, which matches the transit ephemeris, and translates to a planet mass of 21.8 ± 6.2 M. We perform a suite of validation tests to confirm that our detected signal is genuine. This is the first mass measurement for a transiting planet in a young open cluster. The relatively low density of the planet, 2.04+−006661 g cm−3, implies that K2-100b retains a significant volatile envelope. We estimate that the planet is losing its atmosphere at a rate of 1011–1012 g s−1 due to the high level of radiation it receives from its host star.

## The K2 Bright Star Survey. I. Methodology and Data Release

Astrophysical Journal Supplement American Astronomical Society 245 (2019) 8

BJS Pope, TR White, WM Farr, J Yu, M Greklek-McKeon, D Huber, C Aerts, S Aigrain, TR Bedding, T Boyajian, OL Creevey, DW Hogg

While the Kepler mission was designed to look at tens of thousands of faint stars (V gsim 12), brighter stars that saturated the detector are important because they can be and have been observed very accurately by other instruments. By analyzing the unsaturated scattered-light "halo" around these stars, we retrieved precise light curves of most of the brightest stars in K2 fields from Campaign 4 onward. The halo method does not depend on the detailed cause and form of systematics, and we show that it is effective at extracting light curves from both normal and saturated stars. The key methodology is to optimize the weights of a linear combination of pixel time series with respect to an objective function. We test a range of such objective functions, finding that lagged Total Variation, a generalization of Total Variation, performs well on both saturated and unsaturated K2 targets. Applying this to the bright stars across the K2 Campaigns reveals stellar variability ubiquitously, including effects of stellar pulsation, rotation, and binarity. We describe our pipeline and present a catalog of the 161 bright stars, with classifications of their variability, asteroseismic parameters for red giants with well-measured solar-like oscillations, and remarks on interesting objects. These light curves are publicly available as a High Level Science Product from the Mikulski Archive for Space Telescopes (footnote 17).

## The K2 Bright Star Survey. I. Methodology and data release

Astrophysical Journal Supplement American Astronomical Society 245 (2019) 8

BJS Pope, TR White, WM Farr, J Yu, M Greklek-McKeon, D Huber, C Aerts, S Aigrain, TR Bedding, T Boyajian, OL Creevey, DW Hogg

While the Kepler mission was designed to look at tens of thousands of faint stars (V gsim 12), brighter stars that saturated the detector are important because they can be and have been observed very accurately by other instruments. By analyzing the unsaturated scattered-light "halo" around these stars, we retrieved precise light curves of most of the brightest stars in K2 fields from Campaign 4 onward. The halo method does not depend on the detailed cause and form of systematics, and we show that it is effective at extracting light curves from both normal and saturated stars. The key methodology is to optimize the weights of a linear combination of pixel time series with respect to an objective function. We test a range of such objective functions, finding that lagged Total Variation, a generalization of Total Variation, performs well on both saturated and unsaturated K2 targets. Applying this to the bright stars across the K2 Campaigns reveals stellar variability ubiquitously, including effects of stellar pulsation, rotation, and binarity. We describe our pipeline and present a catalog of the 161 bright stars, with classifications of their variability, asteroseismic parameters for red giants with well-measured solar-like oscillations, and remarks on interesting objects. These light curves are publicly available as a High Level Science Product from the Mikulski Archive for Space Telescopes (footnote 17).

## A ghost in the toast: TESS background light produces a false “transit” across τ Ceti

Research Notes of the AAS American Astronomical Society 3 (2019) 145-

N Eisner, B Pope, S Aigrain, O Barragan Villanueva, TR White, CX Huang, C Lintott, A Volkov

## An 11 Earth-mass, Long-period Sub-Neptune Orbiting a Sun-like Star

ASTRONOMICAL JOURNAL American Astronomical Society 158 (2019) ARTN 165

AW Mayo, VM Rajpaul, LA Buchhave, CD Dressing, A Mortier, L Zeng, CD Fortenbach, S Aigrain, AS Bonomo, AC Cameron, D Charbonneau, A Coffinet, R Cosentino, M Damasso, X Dumusque, A Martinez Fiorenzano, RD Haywood, DW Latham, M Lopez-Morales, L Malavolta, E Molinari, L Pearce, F Pepe, D Phillips, G Piotto

&#xA9; 2019. The American Astronomical Society. All rights reserved.. Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with M&#x2217; =0.892-0.035+0.051 and R&#x2217; = 0.8717-0.0061+0.0064 R o. Kepler-538b is a 2.215-0.034+0.040 R &#x2295; sub-Neptune with a period of P = 81.73778 &#xB1; 0.00013 days. It is the only known planet in the system. We collected radial velocity (RV) observations with the High Resolution Echelle Spectrometer (HIRES) on Keck I and High Accuracy Radial velocity Planet Searcher in North hemisphere (HARPS-N) on the Telescopio Nazionale Galileo (TNG). We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross-correlation function FWHM observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of K = 1.68-0.38+0.39 m s-1 and a planet mass of Mp = 10.6-2.4+2.5 M &#x2295;. Kepler-538b is the smallest planet beyond P = 50 days with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

## An 11 Earth-mass, long-period Sub-Neptune orbiting a Sun-like star

Astronomical Journal American Astronomical Society 158 (2019) 165-165

AW Mayo, VM Rajpaul, LA Buchhave, CD Dressing, CD Fortenbach, S Aigrain, AC Cameron, D Charbonneau, A Coffinet, R Cosentino, M Damasso, X Dumusque, RD Haywood, DW Latham, M López-Morales, G Micela, E Molinari, L Pearce, F Pepe, D Phillips, E Poretti, G Piotto, K Rice, A Sozzetti, S Udry

Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with = - M + 0.892 0.035 0.051* Me and R =* - + 0.8717 0.0061 0.0064 Re. Kepler-538b is a - + 2.215 0.034 0.040 R⊕ sub-Neptune with a period of P = 81.73778 ± 0.00013 days. It is the only known planet in the system. We collected radial velocity (RV) observations with the High Resolution Echelle Spectrometer (HIRES) on Keck I and High Accuracy Radial velocity Planet Searcher in North hemisphere (HARPS-N) on the Telescopio Nazionale Galileo (TNG). We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross-correlation function FWHM observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of = -+ K 1.68 0.38 0.39 m s−1 and a planet mass of = - M + p 10.6 2.4 2.5 M⊕. Kepler-538b is the smallest planet beyond P = 50 days with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

## Constraining the properties of HD 206893 B A combination of radial velocity, direct imaging, and astrometry data (vol 627, L9, 2019)

ASTRONOMY & ASTROPHYSICS 629 (2019) ARTN C1

A Grandjean, A-M Lagrange, H Beust, L Rodet, J Milli, P Rubini, C Babusiaux, N Meunier, P Delorme, S Aigrain, M Bonnefoy, J-L Baudino, M Bonavita, A Boccaletti, A Cheetham, J Hagelberg, M Janson, J Lannier, C Lazzoni, R Ligi, A-L Maire, D Mesa, C Perrot, D Rouan, A Zurlo

© A. Grandjean et al. 2019. The distance d was missing in the denominator of the first equation of Appendix D.2. The corrected equation is: (Formula Presented).

## Constraining the properties of HD 206893 B. A combination of radial velocity, direct imaging, and astrometry data

Astronomy and Astrophysics EDP Sciences 627 (2019) L9

A Grandjean, A-M Lagrange, H Beust, L Rodet, J Milli, P Rubini, C Babusiaux, N Meunier, P Delorme, S Aigrain, N Zicher, M Bonnefoy, BA Biller, J-L Baudino, M Bonavita, A Boccaletti, A Cheetham, JH Girard, J Hagelberg, M Janson, J Lannier, C Lazzoni, R Ligi, A-L Maire, C Perrot

<p><em>Context.</em> High contrast imaging enables the determination of orbital parameters for substellar companions (planets, brown dwarfs) from the observed relative astrometry and the estimation of model and age-dependent masses from their observed magnitudes or spectra. Combining astrometric positions with radial velocity gives direct constraints on the orbit and on the dynamical masses of companions. A brown dwarf was discovered with the VLT/SPHERE instrument at the Very Large Telescope (VLT) in 2017, which orbits at ∼11 au around HD 206893. Its mass was estimated between 12 and 50 <em>M</em><sub>J</sub> from evolutionary models and its photometry. However, given the significant uncertainty on the age of the system and the peculiar spectrophotometric properties of the companion, this mass is not well constrained.</p> <p><em>Aims</em>. We aim at constraining the orbit and dynamical mass of HD 206893 B.</p> <p><em>Methods</em>. We combined radial velocity data obtained with HARPS spectra and astrometric data obtained with the high contrast imaging VLT/SPHERE and VLT/NaCo instruments, with a time baseline less than three years. We then combined those data with astrometry data obtained by HIPPARCOS and <em>Gaia</em>with a time baseline of 24 yr. We used a Markov chain Monte Carlo approach to estimate the orbital parameters and dynamical mass of the brown dwarf from those data.</p> <p><em>Results.</em> We infer a period between 21 and 33 yr and an inclination in the range 20−41° from pole-on from HD 206893 B relative astrometry. The RV data show a significant RV drift over 1.6 yr. We show that HD 206893 B cannot be the source of this observed RV drift as it would lead to a dynamical mass inconsistent with its photometry and spectra and with HIPPARCOS and <em>Gaia</em> data. An additional inner (semimajor axis in the range 1.4–2.6 au) and massive (∼15 <em>M</em><sub>J</sub>) companion is needed to explain the RV drift, which is compatible with the available astrometric data of the star, as well as with the VLT/SPHERE and VLT/NaCo nondetection.</p>