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A&A
Volume 649, May 2021
Article Number L12
Number of page(s) 4
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202140786
Published online 07 May 2021

© ESO 2021

1. Introduction

Recently, Vedantham et al. (2020) reported the detection of circularly polarised radio emission in the LOw-Frequency ARray (LOFAR; van Haarlem et al. 2013) Two-Metre Sky Survey (LoTSS) data release I (Shimwell et al. 2019). It was detected over a relatively long duration (> 8 h) and at a low frequency (∼150 MHz) at the position in the sky of the M4.5-type (Lépine et al. 2013) star GJ 1151, for which we calculated a mass of 0.170 ± 0.010 M⊙ (Schweitzer et al. 2019). The star is a slow rotator with v sin i < 2 km s−1 (Reiners et al. 2018), and with a photometric rotation period estimated at Prot = 117.6 d (Newton et al. 2016) and Prot = 125 ± 23 d (Díez Alonso et al. 2019). All available evidence, including a measured pseudo-equivalent width of the Hα line pEW(Hα) = +0.342 ± 0.008 Å (following Schöfer et al. 2019), points at a very low magnetic activity in the star.

Vedantham et al. (2020) discuss that the Poynting flux required to produce the detected radio signal cannot be generated by a star with such characteristics and hence suggest that its origin is rather related to the interaction with a companion. The possibility for it to be a long-period substellar massive object was already ruled out by FastCam observations (Cortés-Contreras et al. 2017), at least for separations > 1 au. Thus, the authors suggest the existence of a short-period (P = 1−5 d) Earth-like planet with an orbit interior to the habitable zone of the star. Then, they argue that the radio signal could originate from the sub-Alfvénic interaction of this planet with the plasma of the stellar magnetosphere inducing electron cyclotron maser instability (Melrose & Dulk 1982). Recent results of XMM-Newton X-ray data (Foster et al. 2020) seem to strengthen this assumption. Since this effect is expected to be very weak, the detection of an exoplanet at radio wavelengths is very intriguing.

The existence of such a planet was initially evaluated by Pope et al. (2020) using 19 epochs of High Accuracy Radial velocity Planet Searcher of the Northern hemisphere (HARPS-N; Cosentino et al. 2012) radial velocity (RV) data. The authors did not find any significant signal but they placed an upper limit of M sin i < 5.6 M⊕ on the minimum mass of any possible close-in planet, assuming a stellar mass of 0.167 ± 0.025 M⊙ (Newton et al. 2016), and conclusively ruled out close-in stellar or gas-giant companions. More recently, Mahadevan et al. (2021) analysed the same HARPS-N RVs together with 50 epochs from newly obtained Habitable-zone Planet Finder (HPF; Mahadevan et al. 2012) near-infrared RVs. The authors report a significant Doppler signal compatible with an M sin i = 2.5 ± 0.5 M⊕ planet on a 2.02-day orbit, inducing an RV semi-amplitude of K = 4.1 ± 0.8 m s−1.

Here, we report on the combined analysis of the published HARPS-N and HPF RVs, together with an additional data set consisting of 70 epochs of Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs (CARMENES; Quirrenbach et al. 2020) RVs of GJ 1151.

2. Radial velocity analysis

We used the 20 publicly available HARPS-N spectra of GJ 1151 and calculated RVs with the TERRA pipeline (Anglada-Escudé & Butler 2012). The measurements (see Table A.1) show significantly smaller variations and uncertainties than the 19 RVs (see Table 1) from both Pope et al. (2020) and Mahadevan et al. (2021), which were derived with the wobble code (Bedell et al. 2019). The observations were acquired from December 2018 to February 2019, with occasional dense sampling (top panel of Fig. 1). In addition, Mahadevan (priv. comm.) kindly provided us with 25 RVs from the HPF near-infrared observations obtained between March 2019 and June 2020 which were computed with the SERVAL code (Zechmeister et al. 2018). These RVs correspond to nightly averages of the 50 individual measurements presented in their paper. The data points show significantly larger individual uncertainties than the RVs from the HARPS-N instrument.

thumbnail Fig. 1.

RV data as observed from HARPS-N (green triangles), HPF (red squares), and CARMENES (black dots) in time series (top panel), and phase-folded (bottom panel) to the 2.02-day period of the planet candidate announced by Mahadevan et al. (2021). Top panel: we include the best-fitting linear trend of 1.73 m s−1 and the long-period signal of ∼500 d represented by the black dashed line. Bottom panel: we also show the HARPS-N data as derived by the wobble code represented by magenta triangles.

Table 1.

Statistics of the different RV data sets.

GJ 1151 was observed on 70 occasions from 2016 to 2020 with CARMENES. The RVs of the visible channel were extracted with the SERVAL code (see Table A.1, for statistics see Table 1). The RVs of the near-infrared channel show average uncertainties of ∼8 m s−1 for this star and they were not used in this study. The data are separated in two blocks, one consisting of seven measurements from February to June 2016 and one of 62 measurements, with more intensive monitoring periods from February to December 2020 (top panel of Fig. 1). There is also one single measurement in between the two blocks. An apparent global upward trend is clearly visible, and also some modulation with a period > 300 d. The data do not overlap with the HARPS-N measurements, but they do with most of the HPF data. Both sets clearly show a quite steep downward trend around BJD = 2 458 900 d.

As an initial test, we phase-folded the full dataset to the 2.02-d period of the announced planet candidate by fitting individual offsets for each instrument and using the best-fit semi-amplitude for the RVs of the present work. This is graphically shown in the bottom panel of Fig. 1. While the HPF data seem to favour such a fit, therefore reproducing the results of Mahadevan et al. (2021), the HARPS-N data show phases with poor coverage, which then make the data compatible with the Keplerian signal found by the HPF data. In clear contrast, the CARMENES data do not confirm the modulation nor do they show any sign of periodic variability at 2.02 d. This can also be seen in the periodograms of Fig. 2 for the combined HARPS-N and HPF sets (top panel), the individual CARMENES RVs (middle panel), and the full data set (bottom panel).

thumbnail Fig. 2.

Periodograms of the RVs as observed of the combined HARPS-N and HPF data sets (top panel), the individual CARMENES data set (middle panel), and the combined full RV data set (bottom panel). The orange vertical line highlights a period of 2.02 d, and the horizontal blue dashed lines indicate analytical false-alarm probabilities of 10, 1, and 0.1% (from bottom to top). The y axis is shown up to the largest GLS power or the 0.1% FAP level.

The combined RV time series of the three instruments suggests the presence of a linear trend and a long-term modulation. Thus, we considered these two effects and optimised (maximum likelihood) their parameters together with the RV offsets amongst the different data sets. As a result, we find a linear trend of 1.73 m s−1 yr−1 and a highly significant signal with a period of ∼500 d and a semi-amplitude of K = 4.2 m s−1 in the combined data. Because of seasonal gaps, this long-period signal was not fully sampled in phase. The combined fit of the trend and the long-term signal is shown in the top panel of Fig. 1. The residuals from subtracting this fit from the data are shown as a time series in the top panel of Fig. 3, and they are folded to the 2.02-day period of the candidate planet in the bottom panel of Fig. 3. It is readily seen that the individual data sets, including HARPS-N and HPF, no longer support the existence of a significant periodicity. This is also observed in the periodograms of the residuals in Fig. 4 of the combined HARPS-N and HPF data sets (top panel), the individual CARMENES data set (middle panel), and the combined full RV data set (bottom panel). No prominent periodic signals are visible, including a lack of significant periodicity at 2.02 d. The signal identified in the HPF data was removed by the fit of the linear trend and the long-period signal. We therefore conclude that it was most likely a spurious signal caused by the dominant downward trend of the HPF data during the densely sampled epoch.

thumbnail Fig. 3.

Same as Fig. 1, but for the RV residuals that result after subtracting a linear trend and a long-period signal.

thumbnail Fig. 4.

Same as Fig. 2, but for the RV residuals that result after subtracting a linear trend and a long-period signal.

We followed the procedure described in Bonfils et al. (2013) to calculate the detection limits for the RV dataset and the limit for the minimum masses of planets with 1- to 5-d orbital periods. We employed a significance threshold at a false-alarm probability of 0.1%. We firstly considered the RV time series as observed, that is, without subtracting the trend and long-term modulation, and we obtained a flat detection limit of K = 2.21 ± 0.15 m s−1 for the RV semi-amplitude of circular orbits with periods between 1 and 5 d. When we ran the same calculations on the residuals after performing the correction, we derived a mean limiting RV semi-amplitude of K = 1.50 ± 0.07 m s−1, which translates into minimum planet masses of 0.72, 0.91, and 1.23 M⊕ for orbital periods of 1, 2.02, and 5 days, respectively. A graphical representation of the experiment is shown in Fig. 5. Since the scatter of the RV residuals (Table 1) is of the order of 3.3 m s−1, the simulations show that we would be able to detect planets with semi-amplitudes some 2.2 times smaller than such velocity scatter.

thumbnail Fig. 5.

Detection limits of the RV residuals (after correcting for a trend and long-term modulation) of the combined RVs of GJ 1151 of 20 HARPS-N, 25 HPF, and 70 CARMENES observations. We show the minimum planetary mass for which we detect (bootstrap false-alarm probability < 0.1%) an injected planetary companion for each period (black line). Coloured lines show constant semi-amplitudes of 1.25, 1.75 m s−1 (blue lines), and the average detection limit of 1.50 m s−1 (red line).

3. Conclusions

We analysed published HARPS-N and HPF RVs of the low-mass star GJ 1151 together with 70 new CARMENES RVs, following up on the recent announcement of a possible planet being responsible for low-frequency radio emission detected by LOFAR. The full combined data set shows a linear trend of 1.73 m s−1 yr−1 and contains a long-period signal > 300 d. We are not yet able to unambiguously derive the parameters and to assess the nature of the suggestive, potentially planetary, long-period signal, but observations are still ongoing and will be investigated in an upcoming article.

If we subtract a trend and a long-period signal from the observations, effectively applying a high-pass frequency filter, the resulting residual RVs show no signs of the 2.5 M⊕ planet in a 2.02-day orbit proposed by Mahadevan et al. (2021), which would induce a periodic RV signal with a semi-amplitude of 4.1 m s−1. We find that the reported periodic signal may rather be produced by the long-period signal that is not accounted for and the free offset used when combining both HPF and HARPS-N datasets.

In our study of the full RV data, we place an new upper limit on the semi-amplitude of a possible exoplanet orbiting GJ 1151 at 1.50 m s−1. A putative planetary companion with an orbit below 5 days, as put forward to explain the LOFAR data, would need to have a minimum mass lower than 1.2 M⊕ to remain compatible with the available RV dataset.

Acknowledgments

CARMENES is an instrument at the Centro Astronómico Hispano-Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Junta de Andalucía and the Instituto de Astrofísica de Andalucía (CSIC). The authors wish to express their sincere thanks to all members of the Calar Alto staff for their expert support of the instrument and telescope operation. CARMENES was funded by the Max-Planck-Gesellschaft (MPG), the Consejo Superior de Investigaciones Científicas (CSIC), the Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (ERDF) through projects FICTS-2011-02, ICTS-2017-07-CAHA-4, and CAHA16-CE-3978, and the members of the CARMENES Consortium (Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Andalucía, Landessternwarte Königstuhl, Institut de Ciències de l’Espai, Institut für Astrophysik Göttingen, Universidad Complutense de Madrid, Thüringer Landessternwarte Tautenburg, Instituto de Astrofísica de Canarias, Hamburger Sternwarte, Centro de Astrobiología and Centro Astronómico Hispano-Alemán), with additional contributions by the MINECO, the Deutsche Forschungsgemeinschaft through the Major Research Instrumentation Programme and Research Unit FOR2544 “Blue Planets around Red Stars”, the Klaus Tschira Stiftung, the states of Baden-Württemberg and Niedersachsen, and by the Junta de Andalucía. This work was based on data from the CARMENES data archive at CAB (CSIC-INTA). We acknowledge financial support from the Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades and the ERDF through projects PID2019-109522GB-C5[1:4]/AEI/10.13039/501100011033, PGC2018-098153-B-C33, AYA2018-84089, ESP2017-87676-C5-1-R, and the Centre of Excellence “Severo Ochoa” and “María de Maeztu” awards to the Instituto de Astrofísica de Canarias (SEV-2015-0548), Instituto de Astrofísica de Andalucía (SEV-2017-0709), and Centro de Astrobiología (MDM-2017-0737), and the Generalitat de Catalunya/CERCA programme.

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All Tables

Table 1.

Statistics of the different RV data sets.

In the text

All Figures

thumbnail Fig. 1.

RV data as observed from HARPS-N (green triangles), HPF (red squares), and CARMENES (black dots) in time series (top panel), and phase-folded (bottom panel) to the 2.02-day period of the planet candidate announced by Mahadevan et al. (2021). Top panel: we include the best-fitting linear trend of 1.73 m s−1 and the long-period signal of ∼500 d represented by the black dashed line. Bottom panel: we also show the HARPS-N data as derived by the wobble code represented by magenta triangles.
In the text
thumbnail Fig. 2.

Periodograms of the RVs as observed of the combined HARPS-N and HPF data sets (top panel), the individual CARMENES data set (middle panel), and the combined full RV data set (bottom panel). The orange vertical line highlights a period of 2.02 d, and the horizontal blue dashed lines indicate analytical false-alarm probabilities of 10, 1, and 0.1% (from bottom to top). The y axis is shown up to the largest GLS power or the 0.1% FAP level.
In the text
thumbnail Fig. 3.

Same as Fig. 1, but for the RV residuals that result after subtracting a linear trend and a long-period signal.
In the text
thumbnail Fig. 4.

Same as Fig. 2, but for the RV residuals that result after subtracting a linear trend and a long-period signal.
In the text
thumbnail Fig. 5.

Detection limits of the RV residuals (after correcting for a trend and long-term modulation) of the combined RVs of GJ 1151 of 20 HARPS-N, 25 HPF, and 70 CARMENES observations. We show the minimum planetary mass for which we detect (bootstrap false-alarm probability < 0.1%) an injected planetary companion for each period (black line). Coloured lines show constant semi-amplitudes of 1.25, 1.75 m s−1 (blue lines), and the average detection limit of 1.50 m s−1 (red line).
In the text

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