ABSTRACT Studying the properties of young planetary systems can shed light on how the dynamics and structure of planets evolve during their most formative years. Recent K2 observations of nearby ...young clusters (10-800 Myr) have facilitated the discovery of such planetary systems. Here we report the discovery of a Neptune-sized planet transiting an M4.5 dwarf (K2-25) in the Hyades cluster (650-800 Myr). The light curve shows a strong periodic signal at 1.88 days, which we attribute to spot coverage and rotation. We confirm that the planet host is a member of the Hyades by measuring the radial velocity of the system with the high-resolution near-infrared spectrograph Immersion Grating Infrared Spectrometer. This enables us to calculate a distance based on K2-25's kinematics and membership to the Hyades, which in turn provides a stellar radius and mass to 5%-10%, better than what is currently possible for most Kepler M dwarfs (12%-20%). We use the derived stellar density as a prior on fitting the K2 transit photometry, which provides weak constraints on eccentricity. Utilizing a combination of adaptive optics imaging and high-resolution spectra, we rule out the possibility that the signal is due to a bound or background eclipsing binary, confirming the transits' planetary origin. K2-25b has a radius ( R⊕) much larger than older Kepler planets with similar orbital periods (3.485 days) and host-star masses (0.29 M ). This suggests that close-in planets lose some of their atmospheres past the first few hundred million years. Additional transiting planets around the Hyades, Pleiades, and Praesepe clusters from K2 will help confirm whether this planet is atypical or representative of other close-in planets of similar age.
Abstract
Star formation theories have struggled to reproduce binary brown dwarf population demographics (e.g., frequency, separation, and mass ratio). Kernel-phase interferometry is sensitive to ...companions at separations inaccessible to classical imaging, enabling tests of these theories at new physical scales below the hydrogen burning limit. We analyze the detections and sensitivity limits from our previous kernel-phase analysis of archival HST/NICMOS surveys of field brown dwarfs. After estimating physical properties of the 105 late-M to T dwarfs using Gaia distances and evolutionary models, we use a Bayesian framework to compare these results to a model companion population defined by log-normal separation and power-law mass-ratio distributions. When correcting for Malmquist bias, we find a companion fraction of
F
=
0.11
−
0.03
+
0.04
and a separation distribution centered at
ρ
=
2.2
−
1.0
+
1.2
au, smaller and tighter than seen in previous studies. We also find a mass-ratio power-law index that strongly favors equal-mass systems:
γ
=
4.0
−
1.5
+
1.7
−
11
−
3
+
4
depending on the assumed age of the field population (0.9–3.1 Gyr). We attribute the change in values to our use of kernel-phase interferometry, which enables us to resolve the peak of the semimajor axis distribution with significant sensitivity to low-mass companions. We confirm the previously seen trends of decreasing binary fraction with decreasing mass and a strong preference for tight and equal-mass systems in the field-age substellar regime; only
0.9
−
0.6
+
1.1
% of systems are wider than 20 au and
<
1.0
−
0.6
+
1.4
% of systems have a mass ratio
q
< 0.6. We attribute this to turbulent fragmentation setting the initial conditions followed by a brief period of dynamical evolution, removing the widest and lowest-mass companions, before the birth cluster dissolves.
Abstract
Identifying rocky planets in or near the habitable zones of their stars (near-Earth analogs) is one of the key motivations of many past and present planet-search missions. The census of ...near-Earth analogs is important because it informs calculations of the occurrence rate of Earth-like planets, which in turn feed into calculations of the yield of future missions to directly image other Earths. Only a small number of potential near-Earth analogs have been identified, meaning that each planet should be vetted carefully and then incorporated into the occurrence rate calculation. A number of putative near-Earth analogs have been identified within binary-star systems. However, stellar multiplicity can bias measured planetary properties, meaning that apparent near-Earth analogs in close binaries may have different radii or instellations than initially measured. We simultaneously fit unresolved optical spectroscopy, optical speckle and near-IR adaptive optics contrasts, and unresolved photometry and retrieved revised stellar temperatures and radii for a sample of 11 binary Kepler targets that host at least one near-Earth-analog planet, for a total of 17 planet candidates. We found that 10 of the 17 planets in our sample had radii that fell in or above the radius gap, suggesting that they are not rocky planets. Only two planets retained super-Earth radii and stayed in the habitable zone, making them good candidates for inclusion in rocky-planet occurrence rate calculations.
Abstract
In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small ...planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5
R
⊕
) and sub-Neptunes (between 2.0 and 4.0
R
⊕
), with few planets in between (the “radius gap”). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited
M
⋆
range of 0.8–1.4
M
⊙
. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8
M
⊙
. We found that the radius gap follows
R
p
∝
P
m
with
m
= −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that
m
varies with
M
⋆
. We observed a correlation between the average sub-Neptune size and
M
⋆
. Over 0.5–1.4
M
⊙
, the average sub-Neptune grows from 2.1 to 2.6
R
⊕
, following
R
p
∝
M
⋆
α
with
α
= 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These
M
⋆
–
R
p
trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of
M
c
, which grows linearly with stellar mass according to
M
c
≈ 10
M
⊕
(
M
⋆
/
M
⊙
). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.
Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting EPIC 247589423, a late-K dwarf in the Hyades ( 800 Myr) ...cluster, and robust detection limits for additional planets in the system. The planets were identified from their K2 light curves as part of our survey of young clusters and star-forming regions. The smallest planet has a radius comparable to Earth ( ), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of and , respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m s−1, achievable with modern precision RV spectrographs. Because the target star is bright (V = 11.2) and has relatively low-amplitude stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best known planets in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.
We report the selection and spectroscopic confirmation of 129 new late-type (SpT = K3-M6) members of the Tucana-Horologium moving group, a nearby (d ~ 40 pc), young (tau ~ 40 Myr) population of ...comoving stars. We have used radial velocities, H alpha emission, and Li sub(6708) absorption to distinguish between contaminants and bona fide members. Our expanded census of Tuc-Hor increases the known population by a factor of ~3 in total and by a factor of ~8 for members with SpT > or =, slanted K3, but even so, the K-M dwarf population of Tuc-Hor is still markedly incomplete. We find that 60% of K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would have been omitted in X-ray-selected samples. In contrast, GALEX UV-selected samples using a previously suggested criterion for youth achieve completeness of 77% and purity of 78%, and we suggest new SpT-dependent selection criteria that will yield >95% completeness for tau ~ 40 Myr populations with GALEX data available.
Abstract
Young planets (<1 Gyr) are helpful for studying the physical processes occurring at the early stage of planet evolution. TOI-251 b is a recently discovered sub-Neptune orbiting a young G ...dwarf, which has an imprecise age estimation of 40–320 Myr. We select TOI-251 sibling candidates based on kinematics and spatial proximity to TOI-251 and further use the color–magnitude diagram to refine the list and to compare to multiple open clusters. We report the stellar rotational period for 321 sibling candidates in a 50 pc radius around TOI-251 by analyzing their stellar light curves and find a color–rotational period sequence that lies in between the Group X (300 Myr) and Pleiades (120 Myr) members, suggesting an age ∼ 200 Myr. A quantitative age analysis using gyrochronology relations gives 204 ± 45 Myr, consistent with the average Li age of selected siblings (238 ± 38 Myr) and the Gaia variability age (193
−
54
102
Myr). The detection fraction of comoving candidates that have a short rotational period is 68.1%, much higher than the typical value in the field (14%–16% from Kepler). The overdensity of young stars and consistency in age of stellar siblings suggest a potential young association candidate in the Phoenix–Grus constellation. Though TOI-251 b has a radius larger than most of its field-age counterparts, we are uncertain whether TOI-251 is inflated, due to a lack of knowledge on the planet’s mass.
Abstract
Young stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To ...identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars and use it to identify ∼3 × 10
4
candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis of all groups with substructure, using age estimates to reveal each region’s star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ∼4 km s
−1
(∼4 pc Myr
−1
). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales.
Eclipsing binaries in star clusters offer more stringent tests of stellar evolution theory than field binaries because models must not only match the binary properties, but also the radiative ...properties of all other cluster members at a single chemical composition and a single age. Here we report new spectroscopic observations of the G-type, detached eclipsing binary EPIC 219394517 in the open cluster Ruprecht 147 (Fe/H = +0.10), which was observed in late 2015 by the K2 mission. A joint analysis of our radial-velocity measurements and the K2 light curve shows the 6.5 day orbit to be nearly circular. We derive highly precise masses of and , radii of 1.055 0.011 and 1.042 0.012 , and effective temperatures of 5930 100 K and 5880 100 K for the primary and secondary, respectively. The distance we infer, pc, corresponds to a parallax in good agreement with the Gaia/DR2 value for the star. Current stellar evolution models from the MIST and PARSEC series match the above physical properties well at ages of 2.48 and 2.65 Gyr. Isochrones for these same ages and the measured composition, along with our reddening estimate for EPIC 219394517, also show generally good agreement with the optical and near-infrared color-magnitude diagrams of the cluster, which can be constructed with no free parameters as the distances of all member stars are known from Gaia.
The occurrence rate of hot Jupiters from the Kepler transit survey is roughly half that of radial velocity surveys targeting solar neighborhood stars. One hypothesis to explain this difference is ...that the two surveys target stars with different stellar metallicity distributions. To test this hypothesis, we measure the metallicity distribution of the Kepler targets using the Hectochelle multi-fiber, high-resolution spectrograph. Limiting our spectroscopic analysis to 610 dwarf stars in our sample with > 3.5, we measure a metallicity distribution characterized by a mean of , in agreement with previous studies of the Kepler field target stars. In comparison, the metallicity distribution of the California Planet Search radial velocity sample has a mean of , and the samples come from different parent populations according to a Kolmogorov-Smirnov test. We refit the exponential relation between the fraction of stars hosting a close-in giant planet and the host star metallicity using a sample of dwarf stars from the California Planet Search with updated metallicities. The best-fit relation tells us that the difference in metallicity between the two samples is insufficient to explain the discrepant hot Jupiter occurrence rates; the metallicity difference would need to be 0.2-0.3 dex for perfect agreement. We also show that (sub)giant contamination in the Kepler sample cannot reconcile the two occurrence calculations. We conclude that other factors, such as binary contamination and imperfect stellar properties, must also be at play.
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