Towards an understanding of YSO variability: A multi-wavelength analysis of bursting, dipping, and symmetrically varying light curves of disc-bearing YSOs

This is the author accepted manuscript.Data availability: Data availability is not applicable to this article as no new data were created or analysed in this study.Using simultaneous optical and infrared light curves of disc-bearing young stars in NGC 2264, we perform the first multi-wavelength structure function study of YSOs. We find that dippers have larger variability amplitudes than bursters and symmetric variables at all timescales longer than a few hours. By analysing optical-infrared colour time-series, we also find that the variability in the bursters is systematically less chromatic at all timescales than the other variability types. We propose a model of YSO variability in which symmetric, bursting, and dipping behaviour is observed in systems viewed at low, intermediate, and high inclinations, respectively. We argue that the relatively short thermal timescale for the disc can explain the fact that the infrared light curves for bursters are more symmetric than their optical counterparts, as the disc reprocesses the light from all rotational phases. From this model, we find that the accretion variability onto these YSOs roughly follows a random-walk.Science and Technology Facilities Council (STFC

The mean longitudinal magnetic field and its uses in radial-velocity surveys

Funding: This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40_182901 and 51NF40_205606. XD acknowledges the support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE No 851555) and from the Swiss National Science Foundation under the grant SPECTRE (No 200021_215200). RDH is funded by the UK Science and Technology Facilities Council (STFC)’s Ernest Rutherford Fellowship (grant number ST/V004735/1). SD acknowledges support from the STFC consolidated grant number ST/V000721/1. BSL is funded by a UK Science and Technology Facilities Council (STFC) studentship (ST/V506679/1). XD acknowledges funding by the French National Research Agency in the framework of the Investissements d’Avenir program (ANR-15-IDEX-02), through the funding of the ‘Origin of Life’ project of the Grenoble-Alpes University. ACC acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1. BK acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 865624, GPRV) SHS gratefully acknowledges support from NASA XRP grant 80NSSC21K0607 and NASA EPRV grant 80NSSC21K1037. NP acknowledges the Scholarship program funded by the Knut and Alice Wallenberg Foundation.This work focuses on the analysis of the mean longitudinal magnetic field as a stellar activity tracer in the context of small exoplanet detection and characterization in radial-velocity (RV) surveys. We use Solar Dynamics Observatory/Helioseismic and Magnetic Imager filtergrams to derive Sun-as-a-star magnetic field measurements, and show that the mean longitudinal magnetic field is an excellent rotational period detector and a useful tracer of the solar magnetic cycle. To put these results into context, we compare the mean longitudinal magnetic field to three common activity proxies derived from HARPS-N Sun-as-a-star data: the full width at half-maximum, the bisector span, and the S-index. The mean longitudinal magnetic field does not correlate with the RVs and therefore cannot be used as a one-to-one proxy. However, with high cadence and a long baseline, the mean longitudinal magnetic field outperforms all other considered proxies as a solar rotational period detector, and can be used to inform our understanding of the physical processes happening on the surface of the Sun. We also test the mean longitudinal magnetic field as a ‘stellar proxy’ on a reduced solar data set to simulate stellar-like observational sampling. With a Gaussian Process regression analysis, we confirm that the solar mean longitudinal magnetic field is the most effective of the considered indicators, and is the most efficient rotational period indicator over different levels of stellar activity. This work highlights the need for polarimetric time series observations of stars.Peer reviewe

Trio of super-Earth candidates orbiting K-dwarf HD 48948 : a new habitable zone candidate

Funding: SD acknowledges support from the STFC consolidated grant no. ST/V000721/1. FR is funded by the University of Exeter’s College of Engineering, Maths and Physical Sciences, UK. ACC acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant no. ST/R003203/1. KR is grateful for support from UK STFC via consolidated grant no. ST/V000594/1. RDH is funded by the UK Science and Technology Facilities Council (STFC)’s Ernest Rutherford Fellowship (grant number ST/V004735/1). MC acknowledges the SNSF support under grant P500PT 211024. AS, ASB, and MD gratefully acknowledge support from the ‘Programma di Ricerca Fondamentale INAF 2023’ of the National Institute of Astrophysics (Large Grant 2023 EXODEMO). MP acknowledges support from the Italian Space Agency (ASI) under contract 2018-24-HH.0 ‘The Italian participation to the Gaia Data Processing and Analysis Consortium (DPAC)’ in collaboration with the Italian National Institute of Astrophysics. MP also acknowledges support from the European Union – NextGenerationEU (PRIN MUR 2022 20229R43BH) and the ‘Programma di Ricerca Fondamentale INAF 2023’. FPE and CLO would like to acknowledge the Swiss National Science Foundation (SNSF) for supporting research with HARPS-N through the SNSF grant numbers 140649, 152721, 166227, 184618, and 215190.We present the discovery of three super-Earth candidates orbiting HD 48948, a bright K-dwarf star with an apparent magnitude of mV = 8.58 mag. As part of the HARPS-N Rocky Planet Search programme, we collect 189 high-precision radial velocity measurements using the HARPS-N spectrograph from 2013 October 6, to 2023 April 16. Various methodologies are applied to extract the radial velocities from the spectra, and we conduct a comprehensive comparative analysis of the outcomes obtained through these diverse extraction techniques. To ensure the robustness of our findings, we employ several methods to address stellar variability, with a focus on Gaussian Process regression. To account for the impact of stellar variability and correlated noise in the radial velocity data set, we include activity indicators, such as logR′HK and bisector span, in the multidimensional Gaussian Process regression. Our analysis reveals three planetary candidates with orbital periods of 7.3, 38, and 151 d, and minimum masses estimated at 4.88±0.21 M ⊕ , 7.27±0.70 M ⊕ , and 10.59±1.00 M ⊕ , respectively. The outermost planet resides within the (temperate) habitable zone, positioned at a projected distance of 0.029arcsec from its star. Given the close proximity of this planetary system, situated at a distance of 16.8 parsecs, HD 48498 emerges as a promising target (closest super-Earth around FGK stars) for future high-contrast direct imaging and high-resolution spectroscopic studies.Peer reviewe

Trio of super-Earth candidates orbiting K-dwarf HD 48948:a new habitable zone candidate

We present the discovery of three super-Earth candidates orbiting HD 48948, a bright K-dwarf star with an apparent magnitude of mv = 8.58 mag. As part of the HARPS-N Rocky Planet Search programme, we collect 189 high-precision radial velocity measurements using the HARPS-N spectrograph from 2013 October 6, to 2023 April 16. Various methodologies are applied to extract the radial velocities from the spectra, and we conduct a comprehensive comparative analysis of the outcomes obtained through these diverse extraction techniques. To ensure the robustness of our findings, we employ several methods to address stellar variability, with a focus on Gaussian Process regression. To account for the impact of stellar variability and correlated noise in the radial velocity data set, we include activity indicators, such as logR'HK and bisector span, in the multidimensional Gaussian Process regression. Our analysis reveals three planetary candidates with orbital periods of 7.3, 38, and 151 d, and minimum masses estimated at 4.88 ± 0.21 M⊕, 7.27 ± 0.70 M⊕, and 10.59 ± 1.00 M⊕, respectively. The outermost planet resides within the (temperate) habitable zone, positioned at a projected distance of 0.029 arcsec from its star. Given the close proximity of this planetary system, situated at a distance of 16.8 parsecs, HD 48498 emerges as a promising target (closest super-Earth around FGK stars) for future high-contrast direct imaging and high-resolution spectroscopic studie

Trio of super-Earth candidates orbiting K-dwarf HD 48948:a new habitable zone candidate

We present the discovery of three super-Earth candidates orbiting HD 48948, a bright K-dwarf star with an apparent magnitude of mv = 8.58 mag. As part of the HARPS-N Rocky Planet Search programme, we collect 189 high-precision radial velocity measurements using the HARPS-N spectrograph from 2013 October 6, to 2023 April 16. Various methodologies are applied to extract the radial velocities from the spectra, and we conduct a comprehensive comparative analysis of the outcomes obtained through these diverse extraction techniques. To ensure the robustness of our findings, we employ several methods to address stellar variability, with a focus on Gaussian Process regression. To account for the impact of stellar variability and correlated noise in the radial velocity data set, we include activity indicators, such as logR'HK and bisector span, in the multidimensional Gaussian Process regression. Our analysis reveals three planetary candidates with orbital periods of 7.3, 38, and 151 d, and minimum masses estimated at 4.88 ± 0.21 M⊕, 7.27 ± 0.70 M⊕, and 10.59 ± 1.00 M⊕, respectively. The outermost planet resides within the (temperate) habitable zone, positioned at a projected distance of 0.029 arcsec from its star. Given the close proximity of this planetary system, situated at a distance of 16.8 parsecs, HD 48498 emerges as a promising target (closest super-Earth around FGK stars) for future high-contrast direct imaging and high-resolution spectroscopic studie

A hot mini-Neptune and a temperate, highly eccentric sub-Saturn around the bright K-dwarf TOI-2134

Funding: ACC and TGW acknowledge support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant number ST/R003203/1. RDH is funded by the UK Science and Technology Facilities Council (STFC)’s Ernest Rutherford Fellowship (grant no. ST/V004735/1). SD is funded by the UK Science and Technology Facilities Council (grant no. ST/V004735/1). BSL is funded by a UK Science and Technology Facilities Council (STFC) studentship (ST/V506679/1). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE no. 851555).We present the characterisation of an inner mini-Neptune in a 9.2292005±0.0000063 day orbit and an outer mono-transiting sub-Saturn planet in a 95.50+0.36-0.25 day orbit around the moderately active, bright (mv = 8.9 mag) K5V star TOI-2134. Based on our analysis of five sectors of TESS data, we determine the radii of TOI-2134b and c to be 2.69±0.16 R⊕ for the inner planet and 7.27±0.42 R⊕ for the outer one. We acquired 111 radial-velocity spectra with HARPS-N and 108 radial-velocity spectra with SOPHIE. After careful periodogram analysis, we derive masses for both planets via Gaussian Process regression: 9.13+0.78-0.76 M⊕ for TOI-2134b and 41.89+7.69-7.83 M⊕ for TOI-2134c. We analysed the photometric and radial-velocity data first separately, then jointly. The inner planet is a mini-Neptune with density consistent with either a water-world or a rocky core planet with a low-mass H/He envelope. The outer planet has a bulk density similar to Saturn’s. The outer planet is derived to have a significant eccentricity of 0.67+0.05-0.06 from a combination of photometry and RVs. We compute the irradiation of TOI-2134c as 1.45±0.10 times the bolometric flux received by Earth, positioning it for part of its orbit in the habitable zone of its system. We recommend further RV observations to fully constrain the orbit of TOI-2134c. With an expected Rossiter-McLaughlin (RM) effect amplitude of 7.2±1.3 m-1, we recommend TOI-2134c for follow-up RM analysis to study the spin-orbit architecture of the system. We calculate the Transmission Spectroscopy Metric, and both planets are suitable for bright-mode NIRCam atmospheric characterisation.Publisher PDFPeer reviewe

An investigation into the physical processes of stellar variability: from accretion outbursts to the quiet Sun.

Understanding stellar variability offers a wealth of information about the surfaces and nearby environments of stars. In this Thesis I explore stellar variability in two principal contexts. The first is a study of the photometric variability of young stars which still host their protoplanetary discs. Photometric variability, especially aperiodic variability, is a widespread phenomenon within these systems, and is known to reflect changes on the stellar surface, in the protoplanetary disc, or with the interaction between the disc and the stellar magnetic field.\\ Using simultaneous optical and infrared light curves of disc-bearing young stars in NGC 2264, I perform the first multi-wavelength structure function study of YSOs. I find that dippers are more variable than bursters and symmetric variables at all timescales longer than a few hours. By analysing optical-infrared colour time-series, I also find that the variability in the bursters is systematically less chromatic at all timescales than the other variability types. Finally, I propose a model of YSO variability in which symmetric, bursting, and dipping behaviour is observed in systems viewed at low, intermediate, and high inclinations, respectively. I argue that the relatively short thermal timescale for the disc can explain the fact that the infrared light curves for bursters are more symmetric than their optical counterparts, as the disc reprocesses the light from all rotational phases. %more symmetric infrared light curves of bursters, From this model, I find that the accretion variability onto these YSOs roughly follows a random-walk.\\ The second context in which I explore stellar variability is in the field of radial-velocity searches for exoplanets. Exoplanet detection is a rapidly evolving field with a push to detect ever lower-mass planets. I present an example planet search and mass confirmation of two low-mass transiting planets around the solar-type star TOI-5788. The goal of the next generation of radial-velocity surveys is the robust detection of Earth-twins. As instrumental precision has improved, it has become clear that intrinsic stellar variability poses the most significant challenge to detecting Earth-twins. Although a range of techniques exist and are constantly being developed to mitigate the effects of isolated magnetically active regions, comparatively little attention is paid to the effect of the magnetically quiet stellar surface.\\ Using images from the Helioseismic and Magnetic Imager aboard the \textit{Solar Dynamics Observatory} (SDO/HMI), I extract the radial-velocity (RV) signal arising from the suppression of convective blue-shift and from bright faculae and dark sunspots transiting the rotating solar disc. I remove these rotationally modulated magnetic-activity contributions from simultaneous radial velocities observed by the HARPS-N solar feed to produce a radial-velocity time series arising from the magnetically quiet solar surface (the \lq inactive-region radial velocities\rq). I find that the level of variability in the inactive-region radial velocities remains constant over the almost 7 year baseline and shows no correlation with well-known activity indicators. With an RMS of roughly 1 \mps, the inactive-region radial-velocity time series dominates the total RV variability budget during the decline of solar cycle 24. % The roughly 1 \mps variability shown by the inactive-region RVs is, for large parts of solar cycle 24, significantly larger than the variability in the \lq magnetic activity\rq\ RVs we calculate from the SDO/HMI images. Finally, I compare the variability amplitude and timescale of the inactive-region radial velocities with simulations of supergranulation. I find consistency between the inactive-region radial-velocity and simulated time series, indicating that supergranulation is a significant contribution to the overall solar radial velocity variability, and may be the main source of variability towards solar minimum. This work highlights supergranulation as a key barrier to detecting Earth twins.Science and Technology Facilities Counci