1,818 research outputs found
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-
An Excess of Jupiter Analogs in Super-Earth Systems
We use radial velocity observations to search for long-period gas giant
companions in systems hosting inner super-Earth (1-4 R_Earth, 1-10 M_Earth)
planets to constrain formation and migration scenarios for this population. We
consistently re-fit published RV datasets for 65 stars and find 9 systems with
statistically significant trends indicating the presence of an outer companion.
We combine these RV data with AO images to constrain the masses and semi-major
axes of these companions. We quantify our sensitivity to the presence of
long-period companions by fitting the sample with a power law distribution and
find an occurrence rate of 39+/-7% for companions 0.5-20 M_Jup and 1-20 AU.
Half of our systems were discovered by the transit method and half were
discovered by the RV method. While differences in RV baselines and number of
data points between the two samples lead to different sensitivities to distant
companions, we find that occurrence rates of gas giant companions in each
sample are consistent at the 0.5 level. We compare the frequency of
Jupiter analogs in these systems to the equivalent rate from field star surveys
and find that Jupiter analogs are more common around stars hosting
super-Earths. We conclude that the presence of outer gas giants does not
suppress the formation of inner super-Earths, and that these two populations of
planets instead appear to be correlated. We also find that the stellar
metallicities of systems with gas giant companions are higher than those
without companions, in agreement with the well-established metallicity
correlation from RV surveys of field stars.Comment: published in A
Qatar-2: A K Dwarf Orbited by a Transiting Hot Jupiter and a more Massive Companion in an Outer Orbit
We report the discovery and initial characterization of Qatar-2b, a hot Jupiter transiting a V = 13.3 mag K dwarf in a circular orbit with a short period, P_b = 1.34 days. The mass and radius of Qatar-2b are M_P = 2.49 M_J and R_P = 1.14 R_J, respectively. Radial-velocity monitoring of Qatar-2 over a span of 153 days revealed the presence of a second companion in an outer orbit. The Systemic Console yielded plausible orbits for the outer companion, with periods on the order of a year and a companion mass of at least several M_J. Thus, Qatar-2 joins the short but growing list of systems with a transiting hot Jupiter and an outer companion with a much longer period. This system architecture is in sharp contrast to that found by Kepler for multi-transiting systems, which are dominated by objects smaller than Neptune, usually with tightly spaced orbits that must be nearly coplanar
Lurking in the Shadows: Wide-Separation Gas Giants as Tracers of Planet Formation
Over the past two decades, thousands of planets with an extraordinary diversity of properties have been discovered orbiting nearby stars. Many of these exoplanetary systems challenge our narrative for how planets form and evolve, motivating the search for observational clues to the underlying mechanisms that led to this diversity. In this quest, gas giant analogs to our own Jupiter and Saturn immediately stand out as the most visible relics of the planet formation process. They are products of their birth environment, with properties such as atmospheric and interior compositions, masses, and formation locations sculpted by protoplanetary disk and host star properties. They also actively shape their surroundings; early in their lifetimes, gas giants can alter the structure of the gas disk from which additional planetary bodies may coalesce and affect the transport of rocky and icy materials to the inner disk. After the gas has dissipated these same behemoths can push smaller planets around, causing them to migrate or even ejecting them from the system. Thus to explain the observed diversity of exoplanet systems, we must first understand how gas giant planets form and evolve. This thesis presents four studies that harness multiple observational techniques to explore this question of how gas giant planets outside our solar system form and evolve
Myosin VI contributes to synaptic transmission and development at the Drosophila neuromuscular junction
Abstract
Background
Myosin VI, encoded by jaguar (jar) in Drosophila melanogaster, is a unique member of the myosin superfamily of actin-based motor proteins. Myosin VI is the only myosin known to move towards the minus or pointed ends of actin filaments. Although Myosin VI has been implicated in numerous cellular processes as both an anchor and a transporter, little is known about the role of Myosin VI in the nervous system. We previously recovered jar in a screen for genes that modify neuromuscular junction (NMJ) development and here we report on the genetic analysis of Myosin VI in synaptic development and function using loss of function jar alleles.
Results
Our experiments on Drosophila third instar larvae revealed decreased locomotor activity, a decrease in NMJ length, a reduction in synaptic bouton number, and altered synaptic vesicle localization in jar mutants. Furthermore, our studies of synaptic transmission revealed alterations in both basal synaptic transmission and short-term plasticity at the jar mutant neuromuscular synapse.
Conclusions
Altogether these findings indicate that Myosin VI is important for proper synaptic function and morphology. Myosin VI may be functioning as an anchor to tether vesicles to the bouton periphery and, thereby, participating in the regulation of synaptic vesicle mobilization during synaptic transmission
The Young Substellar Companion ROXs 12 B: Near-Infrared Spectrum, System Architecture, and Spin-Orbit Misalignment
ROXs 12 (2MASS J16262803-2526477) is a young star hosting a directly imaged
companion near the deuterium-burning limit. We present a suite of
spectroscopic, imaging, and time-series observations to characterize the
physical and environmental properties of this system. Moderate-resolution
near-infrared spectroscopy of ROXs 12 B from Gemini-North/NIFS and Keck/OSIRIS
reveals signatures of low surface gravity including weak alkali absorption
lines and a triangular -band pseudo-continuum shape. No signs of Pa
emission are evident. As a population, however, we find that about half (46
14\%) of young (15 Myr) companions with masses 20
possess actively accreting subdisks detected via Pa
line emission, which represents a lower limit on the prevalence of
circumplanetary disks in general as some are expected to be in a quiescent
phase of accretion. The bolometric luminosity of the companion and age of the
host star (6 Myr) imply a mass of 17.5 1.5
for ROXs 12 B based on hot-start evolutionary models. We identify a wide (5100
AU) tertiary companion to this system, 2MASS J16262774-2527247, which is
heavily accreting and exhibits stochastic variability in its light curve.
By combining sin measurements with rotation periods from , we
constrain the line-of-sight inclinations of ROXs 12 A and 2MASS
J16262774-2527247 and find that they are misaligned by
60. In addition, the orbital axis of ROXs 12 B is likely
misaligned from the spin axis of its host star ROXs 12 A, suggesting that ROXs
12 B formed akin to fragmenting binary stars or in an equatorial disk that was
torqued by the wide stellar tertiary.Comment: AJ, accepte
Exterior Companions to Hot Jupiters Orbiting Cool Stars Are Coplanar
The existence of hot Jupiters has challenged theories of planetary formation since the first extrasolar planets were detected. Giant planets are generally believed to form far from their host stars, where volatile materials like water exist in their solid phase, making it easier for giant planet cores to accumulate. Several mechanisms have been proposed to explain how giant planets can migrate inward from their birth sites to short-period orbits. One such mechanism, called Kozai–Lidov migration, requires the presence of distant companions in orbits inclined by more than ~40° with respect to the plane of the hot Jupiter's orbit. The high occurrence rate of wide companions in hot-Jupiter systems lends support to this theory for migration. However, the exact orbital inclinations of these detected planetary and stellar companions is not known, so it is not clear whether the mutual inclination of these companions is large enough for the Kozai–Lidov process to operate. This paper shows that in systems orbiting cool stars with convective outer layers, the orbits of most wide planetary companions to hot Jupiters must be well aligned with the orbits of the hot Jupiters and the spins of the host stars. For a variety of possible distributions for the inclination of the companion, the width of the distribution must be less than ~20° to recreate the observations with good fidelity. As a result, the companion orbits are likely well aligned with those of the hot Jupiters, and the Kozai–Lidov mechanism does not enforce migration in these systems
Hormigones elaborados con distintos tipos de agregado expuestos a altas temperaturas
En este trabajo se presentan los resultados obtenidos en experiencias realizadas sobre hormigones elaborados con matrices de similares características y diferente tipo de agregado grueso, expuestos a altas temperaturas (500 y 700 °C) y enfriados bajo distintas condiciones (lento: al aire y rápido: mediante chorros de agua). Estos hormigones fueron evaluados antes y después del tratamiento mediante diversos ensayos no destructivos. Posteriormente se realizó un análisis exhaustivo del comportamiento mecánico bajo solicitaciones de compresión y tracción de las muestras sometidas a temperatura el que es comparado con el de otras muestras que no fueron expuestas a ningún tipo de tratamiento.This work presents results obtained in experiences carried out over concretes elaborated with similar matrix characteristics and different types of coarse aggregate, exposed at high temperatures (500 and 700SC) and cooled down under different conditions (slowly: in air and fast: through water spurts). These concretes were evaluated before and after the treatment by means of divers non-destructive tests. Subsequently, an exhaustive analysis of the mechanical behavior under compression and tensile loads has been carried out over the samples submitted to temperature, which has been compared with the one accomplished on other samples which were not exposed to any kind of treatment at all
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