ABSTRACT
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.
ABSTRACT
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 ${\rm m\, s^{-1}}$, 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, $2.04 _{ - 0.61 } ^ { + 0.66 }$ ${\rm g\, cm^{-3}}$, implies that K2-100b retains a significant volatile envelope. We estimate that the planet is losing its atmosphere at a rate of $10^{11}\!-\!10^{12}\, {\rm g\, s^{-1}}$ due to the high level of radiation it receives from its host star.
ABSTRACT
We present a comprehensive analysis of 10 yr of HARPS radial velocities (RVs) of the K2V dwarf star HD 13808, which has previously been reported to host two unconfirmed planet candidates. We ...use the state-of-the-art nested sampling algorithm PolyChord to compare a wide variety of stellar activity models, including simple models exploiting linear correlations between RVs and stellar activity indicators, harmonic models for the activity signals, and a more sophisticated Gaussian process regression model. We show that the use of overly simplistic stellar activity models that are not well-motivated physically can lead to spurious ‘detections’ of planetary signals that are almost certainly not real. We also reveal some difficulties inherent in parameter and model inference in cases where multiple planetary signals may be present. Our study thus underlines the importance both of exploring a variety of competing models and of understanding the limitations and precision settings of one’s sampling algorithm. We also show that at least in the case of HD 13808, we always arrive at consistent conclusions about two particular signals present in the RV, regardless of the stellar activity model we adopt; these two signals correspond to the previously reported though unconfirmed planet candidate signals. Given the robustness and precision with which we can characterize these two signals, we deem them secure planet detections. In particular, we find two planets orbiting HD 13808 at distances of 0.11, 0.26 au with periods of 14.2, 53.8 d, and minimum masses of 11, 10 M⊕.
ABSTRACT
To date, only 18 exoplanets with radial velocity (RV) semi-amplitude <2 m s−1 have had their masses directly constrained. The biggest obstacle to RV detection of such exoplanets is ...variability intrinsic to stars themselves, e.g. nuisance signals arising from surface magnetic activity such as rotating spots and plages, which can drown out or even mimic planetary RV signals. We use Kepler-37 – known to host three transiting planets, one of which, Kepler-37d, should be on the cusp of RV detectability with modern spectrographs – as a case study in disentangling planetary and stellar activity signals. We show how two different statistical techniques – one seeking to identify activity signals in stellar spectra, and another to model activity signals in extracted RVs and activity indicators – can each enable a detection of the hitherto elusive Kepler-37d. Moreover, we show that these two approaches can be complementary, and in combination, facilitate a definitive detection and precise characterization of Kepler-37d. Its RV semi-amplitude of 1.22 ± 0.31 m s−1 (mass 5.4 ± 1.4 M⊕) is formally consistent with TOI-178b’s $1.05^{+0.25}_{-0.30}$ m s−1, the latter being the smallest detected RV signal of any transiting planet to date, though dynamical simulations suggest Kepler-37d’s mass may be on the lower end of our 1σ credible interval. Its consequent density is consistent with either a water-world or that of a gaseous envelope ($\sim 0.4{{\ \rm per\ cent}}$ by mass) surrounding a rocky core. Based on RV modelling and a re-analysis of Kepler-37 TTVs, we also suggest that the putative (non-transiting) planet Kepler-37e should be stripped of its ‘confirmed’ status.
ABSTRACT
We report on the discovery and validation of a two-planet system around a bright (V = 8.85 mag) early G dwarf (1.43 R⊙, 1.15 M⊙, TOI 2319) using data from NASA’s Transiting Exoplanet ...Survey Satellite (TESS). Three transit events from two planets were detected by citizen scientists in the month-long TESS light curve (sector 25), as part of the Planet Hunters TESS project. Modelling of the transits yields an orbital period of $11.6264 _{ - 0.0025 } ^ { + 0.0022 }$ d and radius of $3.41 _{ - 0.12 } ^ { + 0.14 }$ R⊕ for the inner planet, and a period in the range 19.26–35 d and a radius of $5.83 _{ - 0.14 } ^ { + 0.14 }$ R⊕ for the outer planet, which was only seen to transit once. Each signal was independently statistically validated, taking into consideration the TESS light curve as well as the ground-based spectroscopic follow-up observations. Radial velocities from HARPS-N and EXPRES yield a tentative detection of planet b, whose mass we estimate to be $11.56 _{ - 6.14 } ^ { + 6.58 }$ M⊕, and allow us to place an upper limit of 27.5 M⊕ (99 per cent confidence) on the mass of planet c. Due to the brightness of the host star and the strong likelihood of an extended H/He atmosphere on both planets, this system offers excellent prospects for atmospheric characterization and comparative planetology.
To date, the radial velocity (RV) method has been one of the most productive techniques for detecting and confirming extrasolar planetary candidates. Unfortunately, stellar activity can induce RV ...variations which can drown out or even mimic planetary signals – and it is notoriously difficult to model and thus mitigate the effects of these activity-induced nuisance signals. This is expected to be a major obstacle to using next-generation spectrographs to detect lower mass planets, planets with longer periods, and planets around more active stars. Enter Gaussian processes (GPs) which, we note, have a number of attractive features that make them very well suited to disentangling stellar activity signals from planetary signals. We present here a GP framework we developed to model RV time series jointly with ancillary activity indicators (e.g. bisector velocity spans, line widths, chromospheric activity indices), allowing the activity component of RV time series to be constrained and disentangled from e.g. planetary components. We discuss the mathematical details of our GP framework, and present results illustrating its encouraging performance on both synthetic and real RV data sets, including the publicly available Alpha Centauri B data set.
The GAPS Programme at TNG Damasso, M.; Lanza, A. F.; Benatti, S. ...
Astronomy and astrophysics (Berlin),
10/2020, Volume:
642
Journal Article
Peer reviewed
Open access
Context.
Detecting and characterising exoworlds around very young stars (age ≤10 Myr) are key aspects of exoplanet demographic studies, especially for understanding the mechanisms and timescales of ...planet formation and migration. Any reliable theory for such physical phenomena requires a robust observational database to be tested. However, detection using the radial velocity method alone can be very challenging because the amplitude of the signals caused by the magnetic activity of such stars can be orders of magnitude larger than those induced even by massive planets.
Aims.
We observed the very young (~2 Myr) and very active star V830 Tau with the HARPS-N spectrograph between October 2017 and March 2020 to independently confirm and characterise the previously reported hot Jupiter V830 Tau b (
K
b
= 68 ± 11 m s
−1
;
m
b
sin
i
b
= 0.57 ± 0.10
M
Jup
;
P
b
= 4.927 ± 0.008 d).
Methods.
Because of the observed ~1 km s
−1
radial velocity scatter that can clearly be attributed to the magnetic activity of V830 Tau, we analysed radial velocities extracted with different pipelines and modelled them using several state-of-the-art tools. We devised injection-recovery simulations to support our results and characterise our detection limits. The analysis of the radial velocities was aided by a characterisation of the stellar activity using simultaneous photometric and spectroscopic diagnostics.
Results.
Despite the high quality of our HARPS-N data and the diversity of tests we performed, we were unable to detect the planet V830 Tau b in our data and cannot confirm its existence. Our simulations show that a statistically significant detection of the claimed planetary Doppler signal is very challenging.
Conclusions.
It is important to continue Doppler searches for planets around young stars, but utmost care must be taken in the attempt to overcome the technical difficulties to be faced in order to achieve their detection and characterisation. This point must be kept in mind when assessing their occurrence rate, formation mechanisms, and migration pathways, especially without evidence of their existence from photometric transits.
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The GAPS Programme at TNG Nardiello, D.; Malavolta, L.; Desidera, S. ...
Astronomy and astrophysics (Berlin),
08/2022, Volume:
664
Journal Article
Peer reviewed
Open access
Context.
Great strides have been made in recent years in the understanding of the mechanisms involved in the formation and evolution of planetary systems. Despite this, many observational findings ...have not yet been corroborated by astrophysical explanations. A fine contribution to the study of planetary formation processes comes from the study of young, low-mass planets, with short orbital periods (<100 days). In the last three years, the NASA/TESS satellite has identified many planets of this kind and their characterization is clearly necessary in order to understand how they formed and evolved.
Aims.
Within the framework of the Global Architecture of Planetary System (GAPS) project, we performed a validation and characterization (radius and mass) of the ultra-short period planet TOI-1807 b, which orbits its young host star BD+39 2643 (~300 Myr) in only 13 h. This is the youngest ultra-short period planet discovered so far.
Methods.
Thanks to a joint modeling of the stellar activity and planetary signals in the TESS light curve and in new HARPS-N radial-velocity measurements, combined with accurate estimation of stellar parameters, we validated the planetary nature of TOI-1807 b and measured its orbital and physical parameters.
Results.
By using astrometric, photometric, and spectroscopic observations, we found that BD+39 2643 is a young, active K dwarf star and a member of a 300 ± 80 Myr old moving group. Furthermore, it rotates in
P
rot
= 8.8 ± 0.1 days. This star hosts an ultra-short period planet, exhibiting an orbital period of only
P
b
= 0.54937 ± 0.00001 days. Thanks to the exquisite photometric and spectroscopic series, along with the accurate information on its stellar activity, we measured both the radius and the mass of TOI-1807 b with high precision, obtaining
P
P,b
= 1.37 ± 0.09
R
⊕
and
M
P,b
= 2.57 ± 0.50
M
⊕
.
These planet parameters correspond to a rocky planet with an Earth-like density (
ρ
b
= 1.0 ± 0.3
ρ
⊕
) and no extended H/He envelope. From the analysis of the age-
R
P
distribution for planets with well measured ages, we inferred that TOI-1807 b may have already lost a large part of its atmosphere over the course of its 300 Myr lifetime.
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A number of governance mechanisms address socio-environmental challenges associated with commodity agriculture in tropical forested countries. Governance mechanisms that prove effective in one ...agricultural sector are often applied to other sectors as well. For example, voluntary certification programs have been adopted by producers of commodities as diverse as beef, coffee, palm oil, and soy. However, there are substantial differences in the extent to which governance mechanisms scale up and achieve impact in different sectors. This paper analyzes how the potential for scaling up a particular governance mechanism is influenced by environmental, market, and social geographies that differ between sectors. Through stakeholder interviews, farm-level surveys, and a literature review, we examine two types of voluntary governance mechanisms (third-party certification, and sustainable intensification programs) in the coffee and cattle sectors in Brazil, to understand why the two governance mechanisms have scaled differently between these two sectors. We find that third-party certification programs have scaled up relatively well in Brazil's coffee sector, more so than its cattle sector, in part owing to differences in sustainability priorities, market orientations, supply chain traceability, and social networks between the two sectors. We also find that pilot sustainable intensification programs in the cattle sector have had more success than certification in engaging farmers, in part because they involve less investment from participating farmers. We conclude that the distribution and quality of environmental resources, markets, knowledge, actors, and networks can play an important role in the ability of a governance mechanism to effectively take root.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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