Context.
The observed scarcity of brown dwarfs in close orbits (within 10 au) around solar-type stars has posed significant questions about the origins of these substellar companions. These questions ...not only pertain to brown dwarfs but also impact our broader understanding of planetary formation processes. However, to resolve these formation mechanisms, accurate observational constraints are essential. Notably, most of the brown dwarfs have been discovered by radial velocity surveys, but this method introduces uncertainties due to its inability to determine the orbital inclination, leaving the true mass – and thus their true nature – unresolved. This highlights the crucial role of astrometric data, helping us distinguish between genuine brown dwarfs and stars.
Aims.
This study aims to refine the mass estimates of massive companions to solar-type stars, mostly discovered through radial velocity measurements and subsequently validated using Gαìα DR3 astrometry, to gain a clearer understanding of their true mass and occurrence rates.
Methods.
We selected a sample of 31 sources with substellar companion candidates validated by
Gaia
Data Release (DR3) and with available radial velocities. Using the
Gaia
DR3 solutions as prior information, we performed an MCMC fit with the available radial velocity measurements to integrate these two sources of data and thus obtain an estimate of their true mass.
Results.
Combining radial velocity measurements with
Gaia
DR3 data led to more precise mass estimations, leading us to reclassify several systems initially labeled as brown dwarfs as low-mass stars. Out of the 32 analyzed companions, 13 have been determined to be stars, 17 are substellar, and two have inconclusive results with the current data. Importantly, using these updated masses, we reevaluated the occurrence rate of brown dwarf companions (13–80
M
Jup
) on close orbits (<10 au) in the CORALIE sample, determining a tentative occurrence rate of 0.8
−0.2
+0.3
%.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.
We present precise radial-velocity measurements of five solar-type stars observed with the HARPS Echelle spectrograph mounted on the 3.6-m telescope in La Silla (ESO, Chile). With a time ...span of more than 10 yr and a fairly dense sampling, the survey is sensitive to low mass planets down to super-Earths on orbital periods up to 100 days.
Aims.
Our goal was to search for planetary companions around the stars HD 39194, HD 93385, HD 96700, HD 154088, and HD 189567 and use Bayesian model comparison to make an informed choice on the number of planets present in the systems based on the radial velocity observations. These findings will contribute to the pool of known exoplanets and better constrain their orbital parameters.
Methods.
A first analysis was performed using the Data & Analysis Center for Exoplanets online tools to assess the activity level of the star and the potential planetary content of each system. We then used Bayesian model comparison on all targets to get a robust estimate on the number of planets per star. We did this using the nested sampling algorithm P
OLY
C
HORD
. For some targets, we also compared different noise models to disentangle planetary signatures from stellar activity. Lastly, we ran an efficient Markov chain Monte Carlo algorithm for each target to get reliable estimates for the planets’ orbital parameters.
Results.
We identify 12 planets within several multiplanet systems. These planets are all in the super-Earth and sub-Neptune mass regime with minimum masses ranging between 4 and 13
M
⊕
and orbital periods between 5 and 103 days. Three of these planets are new, namely HD 93385 b, HD 96700 c, and HD 189567 c.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Exoplanet surveys have shown that systems with multiple low-mass planets on compact orbits are common. Except for a few cases, however, the masses of these planets are generally unknown. At the very ...end of the main sequence, host stars have the lowest mass and hence offer the largest reflect motion for a given planet. In this context, we monitored the low-mass (0.13 M⊙) M dwarf YZ Cet (GJ 54.1, HIP 5643) intensively and obtained radial velocities and stellar-activity indicators derived from spectroscopy and photometry, respectively. We find strong evidence that it is orbited by at least three planets in compact orbits (POrb = 1.97, 3.06, 4.66 days), with the inner two near a 2:3 mean-motion resonance. The minimum masses are comparable to the mass of Earth (M sin I = 0.75 ± 0.13, 0.98 ± 0.14, and 1.14 ± 0.17 M⊕), and they are also the lowest masses measured by radial velocity so far. We note the possibility for a fourth planet with an even lower mass of M sin I = 0.472 ± 0.096 M⊕ at POrb = 1.04 days. An n-body dynamical model is used to place further constraints on the system parameters. At 3.6 parsecs, YZ Cet is the nearest multi-planet system detected to date. Based on observations made with the HARPS instrument on the ESO 3.6 m telescope under the program IDs 180.C-0886(A), 183.C-0437(A), and 191.C-0873(A) at Cerro La Silla (Chile).Radial velocity data (Table B.4) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/605/L11
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
We study the stability of mean-motion resonances (MMR) between two planets during their migration in a protoplanetary disk. We use an analytical model of resonances and describe the effect of the ...disk by a migration timescale (Tm,i) and an eccentricity damping timescale (Te,i) for each planet (i = 1,2 for the inner and outer planets, respectively). We show that the resonant configuration is stable if Te,1/Te,2> (e1/e2)2. This general result can be used to put constraints on specific models of disk-planet interactions. For instance, using classical prescriptions for type-I migration, we show that when the angular momentum deficit (AMD) of the inner orbit is greater than the outer’s orbit AMD, resonant systems must have a locally inverted disk density profile to stay locked in resonance during the migration. This inversion is very atypical of type-I migration and our criterion can thus provide an evidence against classical type-I migration. That is indeed the case for the Jupiter-mass resonant systems HD 60532b, c (3:1 MMR), GJ 876b, c (2:1 MMR), and HD 45364b, c (3:2 MMR). This result may be evidence of type-II migration (gap-opening planets), which is compatible with the high masses of these planets.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Close-in planetary systems detected by the Kepler mission present an excess of period ratios that are just slightly larger than some low order resonant values. This feature occurs naturally when ...resonant couples undergo dissipation that damps the eccentricities. However, the resonant angles appear to librate at the end of the migration process, which is often believed to be an evidence that the systems remain in resonance. Here we provide an analytical model for the dissipation in resonant planetary systems valid for low eccentricities. We confirm that dissipation accounts for an excess of pairs that lie just aside from the nominal period ratios, as observed by the Kepler mission. In addition, by a global analysis of the phase space of the problem, we demonstrate that these final pairs are non-resonant. Indeed, the separatrices that exist in the resonant systems disappear with the dissipation, and remains only a circulation of the orbits around a single elliptical fixed point. Furthermore, the apparent libration of the resonant angles can be explained using the classical secular averaging method. We show that this artifact is only due to the severe damping of the amplitudes of the eigenmodes in the secular motion.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
The spin–orbit angle, or obliquity, is a powerful observational marker that allows us to access the dynamical history of exoplanetary systems. For this study, we have examined the distribution of ...spin–orbit angles for close-in exoplanets and put it in a statistical context of tidal interactions between planets and their host stars. We confirm the previously observed trends between the obliquity and physical quantities directly connected to tides, namely the stellar effective temperature, the planet-to-star mass ratio, and the scaled orbital distance. We further devised a tidal efficiency factor
τ
combining critical parameters that control the strength of tidal effects and used it to corroborate the strong link between the spin–orbit angle distribution and tidal interactions. In particular, we developed a readily usable formula
θ
(
τ
) to estimate the probability that a system is misaligned, which will prove useful in global population studies. By building a robust statistical framework, we reconstructed the distribution of the three-dimensional spin–orbit angles, allowing for a sample of nearly 200 true obliquities to be analyzed for the first time. This realistic distribution maintains the sky-projected trends, and additionally hints toward a striking pileup of truly aligned systems. In fact, we show that the fraction of aligned orbits could be underestimated in classical analyses of sky-projected obliquities due to an observational bias toward misaligned systems. The comparison between the full population and a pristine subsample unaffected by tidal interactions suggests that perpendicular architectures are resilient toward tidal realignment, providing evidence that orbital misalignments are sculpted by disruptive dynamical processes that preferentially lead to polar orbits. On the other hand, star–planet interactions seem to efficiently realign or quench the formation of any tilted configuration other than for polar orbits, and in particular for antialigned orbits. Observational and theoretical efforts focused on these pristine systems are encouraged in order to study primordial mechanisms shaping orbital architectures, which are unaltered by tidal effects.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
We study the spin evolution of close-in planets in compact multi-planetary systems. The rotation period of these planets is often assumed to be synchronous with the orbital period due to tidal ...dissipation. Here we show that planet-planet perturbations can drive the spin of these planets into non-synchronous or even chaotic states. In particular, we show that the transit timing variation (TTV) is a very good probe to study the spin dynamics, since both are dominated by the perturbations of the mean longitude of the planet. We apply our model to KOI-227 b and Kepler-88 b, which are both observed undergoing strong TTVs. We also perform numerical simulations of the spin evolution of these two planets. We show that for KOI-227 b non-synchronous rotation is possible, while for Kepler-88 b the rotation can be chaotic.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Transit timing variations (TTVs) can provide useful information for systems observed in this way, putting constraints on the masses and eccentricities of the observed planets, and in some cases even ...revealing the existence of non-transiting companions. However, TTVs can also prevent the detection of small planets in transit surveys, or bias the recovered planetary and transit parameters. Here we show that Kepler-1972 c, initially the ‘not transit-like’ false positive KOI-3184.02, is an Earth-sized planet whose orbit is perturbed by Kepler-1972 b (initially KOI-3184.01). The pair is locked in a 3:2 mean-motion resonance, each planet displaying TTVs of more than 6h of amplitude over the duration of the
Kepler
mission. The two planets have similar masses m
b
/m
c
= 0.956
−0.051
+0.056
and radii R
b
= 0.802
−0.041
+0.042
R
Earth
, R
c
= 0.868
−0.050
+0.051
R
Earth
, and the whole system, including the inner candidate KOI-3184.03, appears to be coplanar. Despite the faintness of the signals (signal-to-noise ratio (S/N) of 1.35 for each transit of Kepler-1972 b and 1.10 for Kepler-1972 c), we recovered the transits of the planets using the RIVERS method, which is based on recognition of the tracks of planets in river diagrams using machine learning, and a photo-dynamic fit of the light curve. Recovering the correct ephemerides of the planets is essential to obtaining a complete picture of the observed planetary systems. In particular, we show that in Kepler-1972, not taking into account planet-planet interactions yields an error of ~30% on the radii of planets b and c, in addition to generating in-transit scatter, which is why KOI3184.02 was mistaken for a false positive. Alleviating this bias is essential for an unbiased view of
Kepler
systems, some of the TESS stars, and the upcoming PLATO mission.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
29.
Gaia Data Release 3 Holl, B.; Sozzetti, A.; Sahlmann, J. ...
Astronomy and astrophysics (Berlin),
06/2023, Volume:
674
Journal Article
Peer reviewed
Open access
Context.
The astrometric discovery of sub-stellar mass companions orbiting stars is exceedingly hard due to the required sub-milliarcsecond precision, limiting the application of this technique to ...only a few instruments on a target-per-target basis and to the global astrometry space missions H
IPPARCOS
and
Gaia
. The third
Gaia
data release (
Gaia
DR3) includes the first
Gaia
astrometric orbital solutions whose sensitivity in terms of estimated companion mass extends down to the planetary-mass regime.
Aims.
We present the contribution of the exoplanet pipeline to the
Gaia
DR3 sample of astrometric orbital solutions by describing the methods used for fitting the orbits, the identification of significant solutions, and their validation. We then present an overview of the statistical properties of the solution parameters.
Methods.
Using both a Markov chain Monte Carlo and a genetic algorithm, we fitted the 34 months of
Gaia
DR3 astrometric time series with a single Keplerian astrometric-orbit model that had 12 free parameters and an additional jitter term, and retained the solutions with the lowest
χ
2
. Verification and validation steps were taken using significance tests, internal consistency checks using the
Gaia
radial velocity measurements (when available), as well as literature radial velocity and astrometric data, leading to a subset of candidates that were labelled “validated”.
Results.
We determined astrometric-orbit solutions for 1162 sources, and 198 solutions were assigned the “Validated” label. Precise companion-mass estimates require external information and are presented elsewhere. To broadly categorise the different mass regimes in this paper, we use the pseudo-companion mass
M̃
c
assuming a solar-mass host and define three solution groups: 17 (9 validated) solutions with companions in the planetary-mass regime (
M̃
c
< 20
M
J
), 52 (29 validated) in the brown dwarf regime (20
M
J
≤
M̃
c
≤ 120
M
J
), and 1093 (160 validated) in the low-mass stellar companion regime (
M̃
c
> 120
M
J
). From internal and external verification and validation, we estimate the level of spurious and incorrect solutions in our sample to be ∼5% and ∼10% in the ‘OrbitalAlternative’ and ‘OrbitalTargetedSearch’ candidate sample, respectively.
Conclusions.
We demonstrate that
Gaia
is able to confirm and sometimes refine the orbits of known orbital companions and to identify new candidates, providing us with a positive outlook for the expected harvest from the full mission data in future data releases.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Comparisons of the alignment of exoplanets with a common host star and each other can be used to distinguish among concurrent evolution scenarios for the star and the planets. However, multi-planet ...systems usually host mini-Neptunes and super-Earths, whose sizes make orbital architecture measurements challenging. We introduce the Rossiter-McLaughlin effect Revolutions (RMR) technique, which can access the spin-orbit angle of small exoplanets by exploiting the full extent of information contained in spectral transit time series. We validated the technique through its application to published HARPS-N data of the mini-Neptune HD 3167c (
P
= 29.8 days), refining its high sky-projected spin-orbit angle (−108.9
−5.5
+5.4°
), and we applied it to new ESPRESSO observations of the super-Earth HD 3167 b (
P
= 0.96 days), revealing an aligned orbit (−6.6
−7.9
+6.6°
). Surprisingly different variations in the contrast of the stellar lines occulted by the two planets can be reconciled by assuming a latitudinal dependence of the stellar line shape. In this scenario, a joint fit to both datasets constrains the inclination of the star (111.6
−3.3
+3.1°
) and the 3D spin-orbit angles of HD 3167b (29.5
−9.4
+7.2°
) and HD 3167c (107.7
−4.9
+5.1°
). The projected spin-orbit angles do not depend on the model for the line contrast variations, and so, with a mutual inclination of 102.3
−8.0
+7.4°
, we can conclude that the two planets are on perpendicular orbits. This could be explained by HD 3167b being strongly coupled to the star and retaining its primordial alignment, whereas HD 3167c would have been brought to a nearly polar orbit via secular gravitational interactions with an outer companion. Follow-up observations of the system and simulations of its dynamical evolution are required to search for this companion and explore the likelihood of this scenario. HD 3167 b (
R
= 1.7
R
Earth
) is the smallest exoplanet with a confirmed spectroscopic Rossiter-McLaughlin signal. The RMR technique opens the way to determining the orbital architectures of the super-Earth and Earth-sized planet populations.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
PDF