Context. M-dwarf stars are promising targets for identifying and characterizing potentially habitable planets. K2-3 is a nearby (45 pc), early-type M dwarf hosting three small transiting planets, the ...outermost of which orbits close to the inner edge of the stellar (optimistic) habitable zone. The K2-3 system is well suited for follow-up characterization studies aimed at determining accurate masses and bulk densities of the three planets. Aims. Using a total of 329 radial velocity measurements collected over 2.5 years with the HARPS-N and HARPS spectrographs and a proper treatment of the stellar activity signal, we aim to improve measurements of the masses and bulk densities of the K2-3 planets. We use our results to investigate the physical structure of the planets. Methods. We analysed radial velocity time series extracted with two independent pipelines using Gaussian process regression. We adopted a quasi-periodic kernel to model the stellar magnetic activity jointly with the planetary signals. We used Monte Carlo simulations to investigate the robustness of our mass measurements of K2-3 c and K2-3 d, and to explore how additional high-cadence radial velocity observations might improve these values. Results. Even though the stellar activity component is the strongest signal present in the radial velocity time series, we are able to derive masses for both planet b (Mb = 6.6 ± 1.1 M⊕) and planet c (Mc = 3.1−1.2+1.3 M⊕) $M_{\textrm{c}}=3.1^{+1.3}_{-1.2}$Mc=3.1−1.2+1.3. The Doppler signal from K2-3 d remains undetected, likely because of its low amplitude compared to the radial velocity signal induced by the stellar activity. The closeness of the orbital period of K2-3 d to the stellar rotation period could also make the detection of the planetary signal complicated. Based on our ability to recover injected signals in simulated data, we tentatively estimate the mass of K2-3 d to be Md = 2.7 −0.8+1.2 M⊕ $_{\textrm{-0.8}}^{\textrm{+1.2}}$-0.8+1.2 M⊕. These mass measurements imply that the bulk densities and therefore the interior structures of the three planets may be similar. In particular, the planets may either have small H/He envelopes (<1%) or massive water layers, with a water content ≥50% of their total mass, on top of rocky cores. Placing further constraints on the bulk densities of K2-3 c and d is difficult; in particular, we would not have been able to detect the Doppler signal of K2-3 d even by adopting a semester of intense, high-cadence radial velocity observations with HARPS-N and HARPS.
Multi-planet systems exhibit remarkable architectural diversity. However, short-period giant planets are typically isolated. Compact systems like TOI-5398, with an outer close-orbit giant and an ...inner small-size planet, are rare among systems containing short-period giants. TOI-5398’s unusual architecture coupled with its young age (650 ± 150 Myr) make it a promising system for measuring the original obliquity between the orbital axis of the giant and the stellar spin axis in order to gain insight into its formation and orbital migration. We collected in-transit (plus suitable off-transit) observations of TOI-5398 b with HARPS-N at TNG on March 25, 2023, obtaining high-precision radial velocity time series that allowed us to measure the Rossiter-McLaughlin (RM) effect. By modelling the RM effect, we obtained a sky-projected obliquity of λ = 3.0 −4.2 +6.8 deg for TOI-5398 b, consistent with the planet being aligned. With knowledge of the stellar rotation period, we estimated the true 3D obliquity, finding ψ = (13.2 ± 8.2) deg. Based on theoretical considerations, the orientation we measure is unaffected by tidal effects, offering a direct diagnostic for understanding the formation path of this planetary system. The orbital characteristics of TOI-5398, with its compact architecture, eccentricity consistent with circular orbits, and hints of orbital alignment, appear more compatible with the disc-driven migration scenario. TOI-5398, with its relative youth (compared with similar compact systems) and exceptional suitability for transmission spectroscopy studies, presents an outstanding opportunity to establish a benchmark for exploring the disc-driven migration model.
Context.
The transiting exoplanetary system WASP-174 was reported to be composed by a main-sequence F star (
V
= 11.8 mag) and a giant planet, WASP-174b (orbital period
P
orb
= 4.23 days). However ...only an upper limit was placed on the planet mass (<1.3
M
Jup
), and a highly uncertain planetary radius (0.7−1.7
R
Jup
) was determined.
Aims.
We aim to better characterise both the star and the planet and precisely measure their orbital and physical parameters.
Methods.
In order to constrain the mass of the planet, we obtained new measurements of the radial velocity of the star and joined them with those from the discovery paper. Photometric data from the HATSouth survey and new multi-band, high-quality (precision reached up to 0.37 mmag) photometric follow-up observations of transit events were acquired and analysed for getting accurate photometric parameters. We fit the model to all the observations, including data from the TESS space telescope, in two different modes: incorporating the stellar isochrones into the fit, and using an empirical method to get the stellar parameters. The two modes resulted to be consistent with each other to within 2
σ
.
Results.
We confirm the grazing nature of the WASP-174b transits with a confidence level greater than 5
σ
, which is also corroborated by simultaneously observing the transit through four optical bands and noting how the transit depth changes due to the limb-darkening effect. We estimate that ≈76% of the disk of the planet actually eclipses the parent star at mid-transit of its transit events. We find that WASP-174b is a highly-inflated hot giant planet with a mass of
M
p
= 0.330 ± 0.091
M
Jup
and a radius of
R
p
= 1.435 ± 0.050
R
Jup
, and is therefore a good target for transmission-spectroscopy observations. With a density of
ρ
p
= 0.135 ± 0.042 g cm
−3
, it is amongst the lowest-density planets ever discovered with precisely measured mass and radius.
We report the discovery and characterisation of two Earth-mass planets orbiting in the habitable zone of the nearby M-dwarf GJ 1002 based on the analysis of the radial-velocity (RV) time series from ...the ESPRESSO and CARMENES spectrographs. The host star is the quiet M5.5 V star GJ 1002 (relatively faint in the optical,
V ~
13.8 mag, but brighter in the infrared,
J ~
8.3 mag), located at 4.84 pc from the Sun. We analyse 139 spectroscopic observations taken between 2017 and 2021. We performed a joint analysis of the time series of the RV and full-width half maximum (FWHM) of the cross-correlation function (CCF) to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with the stellar activity. We detect the signal of two planets orbiting GJ 1002. GJ 1002 b is a planet with a minimum mass
m
p
sin
i
of 1.08 ± 0.13
M
⊕
with an orbital period of 10.3465 ± 0.0027 days at a distance of 0.0457 ± 0.0013 au from its parent star, receiving an estimated stellar flux of 0.67
F
⊕
. GJ 1002 c is a planet with a minimum mass
m
p
sin
i
of 1.36 ± 0.17
M
⊕
with an orbital period of 20.202 ± 0.013 days at a distance of 0.0738 ± 0.0021 au from its parent star, receiving an estimated stellar flux of 0.257
F
⊕
. We also detect the rotation signature of the star, with a period of 126 ± 15 days. We find that there is a correlation between the temperature of certain optical elements in the spectrographs and changes in the instrumental profile that can affect the scientific data, showing a seasonal behaviour that creates spurious signals at periods longer than ~200 days. GJ 1002 is one of the few known nearby systems with planets that could potentially host habitable environments. The closeness of the host star to the Sun makes the angular sizes of the orbits of both planets (~9.7 mas and ~15.7 mas, respectively) large enough for their atmosphere to be studied via high-contrast high-resolution spectroscopy with instruments such as the future spectrograph ANDES for the ELT or the LIFE mission.
ABSTRACT
A growing number of eclipsing binary systems of the ‘HW Virginis’ (HW Vir) kind (i.e. composed by a subdwarf-B/O primary star and an M dwarf secondary) show variations in their orbital ...period, also called eclipse time variations (ETVs). Their physical origin is not yet known with certainty: While some ETVs have been claimed to arise from dynamical perturbations due to the presence of circumbinary planetary companions, other authors suggest that the Applegate effect or other unknown stellar mechanisms could be responsible for them. In this work, we present 28 unpublished high-precision light curves of one of the most controversial of these systems, the prototype HW Vir. We homogeneously analysed the new eclipse timings together with historical data obtained between 1983 and 2012, demonstrating that the planetary models previously claimed do not fit the new photometric data, besides being dynamically unstable. In an effort to find a new model able to fit all the available data, we developed a new approach based on a global-search genetic algorithm and eventually found two new distinct families of solutions that fit the observed timings very well, yet dynamically unstable at the 105-yr time-scale. This serves as a cautionary tale on the existence of formal solutions that apparently explain ETVs but are not physically meaningful, and on the need of carefully testing their stability. On the other hand, our data confirm the presence of an ETV on HW Vir that known stellar mechanisms are unable to explain, pushing towards further observing and modeling efforts.
ABSTRACT
We present broad-band photometry of 30 planetary transits of the ultra-hot-Jupiter KELT-16 b, using five medium-class telescopes. The transits were monitored through standard $B,\, V,\, R,\, ...I$ filters and four were simultaneously observed from different places, for a total of 36 new light curves. We used these new photometric data and those from the TESS space telescope to review the main physical properties of the KELT-16 planetary system. Our results agree with previous measurements but are more precise. We estimated the mid-transit times for each of these transits and combined them with others from the literature to obtain 69 epochs, with a time baseline extending over more than 4 yr, and searched for transit time variations. We found no evidence for a period change, suggesting a lower limit for orbital decay at 8 Myr, with a lower limit on the reduced tidal quality factor of $Q^{\prime }_{\star }\gt (1.9 \pm 0.8) \times 10^5$ with $95{{\rm {per\ cent}}}$ confidence. We built up an observational, low-resolution transmission spectrum of the planet, finding evidence of the presence of optical absorbers, although with a low significance. Using TESS data, we reconstructed the phase curve finding that KELT-16 b has a phase offset of 25.25 ± 14.03 °E, a day- and night-side brightness temperature of 3190 ± 61 K and 2668 ± 56 K, respectively. Finally, we compared the flux ratio of the planet over its star at the TESS and Spitzer wavelengths with theoretical emission spectra, finding evidence of a temperature inversion in the planet’s atmosphere, the chemical composition of which is preferably oxygen-rich rather than carbon-rich.
ABSTRACT
We present rotation period measurements for 107 M dwarfs in the mass range $0.15\!-\!0.70 \, \mathrm{M}_\odot$ observed within the context of the APACHE photometric survey. We measure ...rotation periods in the range 0.5–190 d, with the distribution peaking at ∼30 d. We revise the stellar masses and radii for our sample of rotators by exploiting the Gaia DR2 data. For ${\sim}20{{\ \rm per\ cent}}$ of the sample, we compare the photometric rotation periods with those derived from different spectroscopic indicators, finding good correspondence in most cases. We compare our rotation periods distribution to the one obtained by the Kepler survey in the same mass range, and to that derived by the MEarth survey for stars in the mass range $0.07\!-\!0.25 \, \mathrm{M}_\odot$. The APACHE and Kepler periods distributions are in good agreement, confirming the reliability of our results, while the APACHE distribution is consistent with the MEarth result only for the older/slow rotators, and in the overlapping mass range of the two surveys. Combining the APACHE/Kepler distribution with the MEarth distribution, we highlight that the rotation period increases with decreasing stellar mass, in agreement with previous work. Our findings also suggest that the spin-down time scale, from fast to slow rotators, changes crossing the fully convective limit at ${\approx}0.3 \, \mathrm{M}_\odot$ for M dwarfs. The catalogue of 107 rotating M dwarfs presented here is particularly timely, as the stars are prime targets for the potential identification of transiting small planets with TESS and amenable to high-precision mass determination and further atmospheric characterization measurements.
Context.
Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, ...recent discoveries are revealing a new group of low-density planets that are inconsistent with that description.
Aims.
We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright (
K
= 7.97 mag), nearby (
d
= 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days.
Methods.
We used Markov chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. Our model includes a planetary component and Gaussian processes aimed at modeling the correlated stellar and instrumental noise.
Results.
We find TOI-244 b to be a super-Earth with a radius of
R
p
= 1.52 ± 0.12
R
⊕
and a mass of
M
p
= 2.68 ± 0.30
M
⊕
. These values correspond to a density of
ρ
= 4.2 ± 1.1 g cm
−3
, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a 479
−96
+128
km thick hydrosphere over a 1.17 ± 0.09
R
⊕
solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition.
Conclusions.
With a 8% precision in radius and 12% precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations.
ABSTRACT
A comprehensive multiwavelength campaign has been carried out to probe stellar activity and variability in the young Sun-like star ι-Horologii. We present the results from long-term ...spectropolarimetric monitoring of the system by using the ultra-stable spectropolarimeter/velocimeter HARPS at the ESO 3.6-m telescope. Additionally, we included high-precision photometry from the NASA Transiting Exoplanet Survey Satellite (TESS) and observations in the far- and near-ultraviolet spectral regions using the STIS instrument on the NASA/ESA Hubble Space Telescope (HST). The high-quality data set allows a robust characterization of the star’s rotation period, as well as a probe of the variability using a range of spectroscopic and photometric activity proxies. By analysing the gradient of the power spectra (GPS) in the TESS light curves, we constrained the faculae-to-spot driver ratio ($\rm S_{fac}/S_{spot}$) to 0.510 ± 0.023, which indicates that the stellar surface is spot dominated during the time of the observations. We compared the photospheric activity properties derived from the GPS method with a magnetic field map of the star derived using Zeeman–Doppler imaging (ZDI) from simultaneous spectropolarimetric data for the first time. Different stellar activity proxies enable a more complete interpretation of the observed variability. For example, we observed enhanced emission in the HST transition line diagnostics C iv and C iii, suggesting a flaring event. From the analysis of TESS data acquired simultaneously with the HST data, we investigate the photometric variability at the precise moment that the emission increased and derive correlations between different observables, probing the star from its photosphere to its corona.
PDF
