234 research outputs found
Near-Infrared Spectroscopy of 2M0441+2301 AabBab: A Quadruple System Spanning the Stellar to Planetary Mass Regimes
We present Keck/NIRC2 and OSIRIS near-infrared imaging and spectroscopy of
2M0441+2301 AabBab, a young (1--3 Myr) hierarchical quadruple system comprising
a low-mass star, two brown dwarfs, and a planetary-mass companion in Taurus.
All four components show spectroscopic signs of low surface gravity, and both
2M0441+2301 Aa and Ab possess Pa emission indicating they each harbor
accretion subdisks. Astrometry spanning 2008--2014 reveals orbital motion in
both the Aab (0.23" separation) and Bab (0.095" separation) pairs, although the
implied orbital periods of 300 years means dynamical masses will not be
possible in the near future. The faintest component (2M0441+2301 Bb) has an
angular -band shape, strong molecular absorption (VO, CO, HO, and FeH),
and shallow alkali lines, confirming its young age, late spectral type (L1
1), and low temperature (1800~K). With individual masses of
200 Mjup, 35 5 Mjup, 19 3 Mjup, and 9.8 1.8
Mjup, 2M0441+2301 AabBab is the lowest-mass quadruple system known. Its
hierarchical orbital architecture and mass ratios imply that it formed from the
collapse and fragmentation of a molecular cloud core, demonstrating that
planetary-mass companions can originate from a stellar-like pathway analogous
to higher-mass quadruple star systems as first speculated by Todorov et al.
More generally, cloud fragmentation may be an important formation pathway for
the massive exoplanets that are now regularly being imaged on wide orbits.Comment: Added an additional mention of an already cited reference, upon
indirect reques
Dynamical Mass of the Young Brown Dwarf Companion PZ Tel B
Dynamical masses of giant planets and brown dwarfs are critical tools for
empirically validating substellar evolutionary models and their underlying
assumptions. We present a measurement of the dynamical mass and an updated
orbit of PZ Tel B, a young brown dwarf companion orbiting a late-G member of
the Pic moving group. PZ Tel A exhibits an astrometric acceleration
between Hipparcos and Gaia EDR3, which enables the direct determination of the
companion's mass. We have also acquired new Keck/NIRC2 adaptive optics imaging
of the system, which increases the total baseline of relative astrometry to 15
years. Our joint orbit fit yields a dynamical mass of , semi-major axis of ,
eccentricity of , and inclination of
. The companion's mass is consistent within
of predictions from four grids of hot-start evolutionary models.
The joint orbit fit also indicates a more modest eccentricity of PZ Tel B than
previous results. PZ Tel joins a small number of young () systems with benchmark substellar companions that have dynamical
masses and precise ages from moving group membership.Comment: 14 pages, 5 figures, accepted to A
Constraints on the Spin Evolution of Young Planetary-Mass Companions
Surveys of young star-forming regions have discovered a growing population of
planetary-mass (<13 M_Jup) companions around young stars. There is an ongoing
debate as to whether these companions formed like planets (that is, from the
circumstellar disk), or if they represent the low-mass tail of the star
formation process. In this study we utilize high-resolution spectroscopy to
measure rotation rates of three young (2-300 Myr) planetary-mass companions and
combine these measurements with published rotation rates for two additional
companions to provide a look at the spin distribution of these objects. We
compare this distribution to complementary rotation rate measurements for six
brown dwarfs with masses <20 M_Jup, and show that these distributions are
indistinguishable. This suggests that either that these two populations formed
via the same mechanism, or that processes regulating rotation rates are
independent of formation mechanism. We find that rotation rates for both
populations are well below their break-up velocities and do not evolve
significantly during the first few hundred million years after the end of
accretion. This suggests that rotation rates are set during late stages of
accretion, possibly by interactions with a circumplanetary disk. This result
has important implications for our understanding of the processes regulating
the angular momentum evolution of young planetary-mass objects, and of the
physics of gas accretion and disk coupling in the planetary-mass regime.Comment: 31 pages, 10 figures, published in Nature Astronomy,
DOI:10.1038/s41550-017-0325-
Planet Hunters. VIII. Characterization of 41 Long-Period Exoplanet Candidates from Kepler Archival Data
The census of exoplanets is incomplete for orbital distances larger than 1 AU. Here, we present 41 long-period planet candidates in 38 systems identified by Planet Hunters based on Kepler archival data (Q0βQ17). Among them, 17 exhibit only one transit, 14 have two visible transits, and 10 have more than three visible transits. For planet candidates with only one visible transit, we estimate their orbital periods based on transit duration and host star properties. The majority of the planet candidates in this work (75%) have orbital periods that correspond to distances of 1β3 AU from their host stars. We conduct follow-up imaging and spectroscopic observations to validate and characterize planet host stars. In total, we obtain adaptive optics images for 33 stars to search for possible blending sources. Six stars have stellar companions within 4''. We obtain high-resolution spectra for 6 stars to determine their physical properties. Stellar properties for other stars are obtained from the NASA Exoplanet Archive and the Kepler Stellar Catalog by Huber et al. We validate 7 planet candidates that have planet confidence over 0.997 (3Ο level). These validated planets include 3 single-transit planets (KIC-3558849b, KIC-5951458b, and KIC-8540376c), 3 planets with double transits (KIC-8540376b, KIC-9663113b, and KIC-10525077b), and 1 planet with four transits (KIC-5437945b). This work provides assessment regarding the existence of planets at wide separations and the associated false positive rate for transiting observation (17%β33%). More than half of the long-period planets with at least three transits in this paper exhibit transit timing variations up to 41 hr, which suggest additional components that dynamically interact with the transiting planet candidates. The nature of these components can be determined by follow-up radial velocity and transit observations
Population-level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels
The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. We combine new high-contrast imaging observations of substellar companions obtained primarily with Keck/NIRC2 together with astrometry from the literature to test for differences in the population-level eccentricity distributions of 27 long-period giant planets and brown dwarf companions between 5 and 100 au using hierarchical Bayesian modeling. Orbit fits are performed in a uniform manner for companions with short orbital arcs; this typically results in broad constraints for individual eccentricity distributions, but together as an ensemble, these systems provide valuable insight into their collective underlying orbital patterns. The shape of the eccentricity distribution function for our full sample of substellar companions is approximately flat from e = 0β1. When subdivided by companion mass and mass ratio, the underlying distributions for giant planets and brown dwarfs show significant differences. Low mass ratio companions preferentially have low eccentricities, similar to the orbital properties of warm Jupiters found with radial velocities and transits. We interpret this as evidence for in situ formation on largely undisturbed orbits within massive extended disks. Brown dwarf companions exhibit a broad peak at e β 0.6β0.9 with evidence for a dependence on orbital period. This closely resembles the orbital properties and period-eccentricity trends of wide (1β200 au) stellar binaries, suggesting that brown dwarfs in this separation range predominantly form in a similar fashion. We also report evidence that the "eccentricity dichotomy" observed at small separations extends to planets on wide orbits: the mean eccentricity for the multi-planet system HR 8799 is lower than for systems with single planets. In the future, larger samples and continued astrometric orbit monitoring will help establish whether these eccentricity distributions correlate with other parameters such as stellar host mass, multiplicity, and age
Population-level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels
The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. We combine new high-contrast imaging observations of substellar companions obtained primarily with Keck/NIRC2 together with astrometry from the literature to test for differences in the population-level eccentricity distributions of 27 long-period giant planets and brown dwarf companions between 5 and 100 au using hierarchical Bayesian modeling. Orbit fits are performed in a uniform manner for companions with short orbital arcs; this typically results in broad constraints for individual eccentricity distributions, but together as an ensemble, these systems provide valuable insight into their collective underlying orbital patterns. The shape of the eccentricity distribution function for our full sample of substellar companions is approximately flat from e = 0β1. When subdivided by companion mass and mass ratio, the underlying distributions for giant planets and brown dwarfs show significant differences. Low mass ratio companions preferentially have low eccentricities, similar to the orbital properties of warm Jupiters found with radial velocities and transits. We interpret this as evidence for in situ formation on largely undisturbed orbits within massive extended disks. Brown dwarf companions exhibit a broad peak at e β 0.6β0.9 with evidence for a dependence on orbital period. This closely resembles the orbital properties and period-eccentricity trends of wide (1β200 au) stellar binaries, suggesting that brown dwarfs in this separation range predominantly form in a similar fashion. We also report evidence that the "eccentricity dichotomy" observed at small separations extends to planets on wide orbits: the mean eccentricity for the multi-planet system HR 8799 is lower than for systems with single planets. In the future, larger samples and continued astrometric orbit monitoring will help establish whether these eccentricity distributions correlate with other parameters such as stellar host mass, multiplicity, and age
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