First steps of planet formation around very low mass stars and brown dwarfs

Brown dwarfs and very low mass stars are a significant fraction of stars in our galaxy and they are interesting laboratories to investigate planet formation in extreme conditions of low temperature and densities. In addition, the dust radial drift of particles is expected to be a more difficult barrier to overcome during the first steps of planet formation in these disks. ALMA high-angular resolution observations of few protoplanetary disks around BDs and VLMS have shown substructures as in the disks around Sun-like stars. Such observations suggests that giant planets embedded in the disks are the most likely origin of the observed substructures. However, this type of planets represent less than 2% of the confirmed exoplanets so far around all stars, and they are difficult to form by different core accretion models (either pebble or planetesimal accretion). Dedicated deep observations of disks around BDs and VLMS with ALMA and JWST will provide significant progress on understanding the main properties of these objects (e.g., disk size and mass), which is crucial for determining the physical mechanisms that rule the evolution of these disks and the effect on the potential planets that may form in these environments

Fingerprints of giant planets in the photospheres of Herbig stars

Around 2% of all A stars have photospheres depleted in refractory elements. This is hypothesized to arise from a preferential accretion of gas rather than dust, but the specific processes and the origin of the material -- circum- or interstellar -- are not known. The same depletion is seen in 30% of young, disk-hosting Herbig Ae/Be stars. We investigate whether the chemical peculiarity originates in a circumstellar disk. Using a sample of systems for which both the stellar abundances and the protoplanetary disk structure are known, we find that stars hosting warm, flaring group I disks typically have Fe, Mg and Si depletions of 0.5 dex compared to the solar-like abundances of stars hosting cold, flat group II disks. The volatile, C and O, abundances in both sets are identical. Group I disks are generally transitional, having radial cavities depleted in millimetre-sized dust grains, while those of group II are usually not. Thus we propose that the depletion of heavy elements emerges as Jupiter-like planets block the accretion of part of the dust, while gas continues to flow towards the central star. We calculate gas to dust ratios for the accreted material and find values consistent with models of disk clearing by planets. Our results suggest that giant planets of ~0.1 to 10 M_Jup are hiding in at least 30% of Herbig Ae/Be disks.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter

Dust trapping by spiral arms in gravitationally unstable protostellar discs

In this paper we discuss the influence of gravitational instabilities in massive protostellar discs on the dynamics of dust grains. Starting from a Smoothed Particle Hydrodynamics (SPH) simulation, we have computed the evolution of the dust in a quasi-static gas density structure typical of self-gravitating disc. For different grain size distributions we have investigated the capability of spiral arms to trap particles. We have run 3D radiative transfer simulations in order to construct maps of the expected emission at (sub-)millimetre and near-infrared wavelengths. Finally, we have simulated realistic observations of our disc models at (sub-)millimetre and near-infrared wavelengths as they may appear with the Atacama Large Millimetre/sub-millimetre Array (ALMA) and the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) in order to investigate whether there are observational signatures of the spiral structure. We find that the pressure inhomogeites induced by gravitational instabilities produce a non-negligible dynamical effect on centimetre sized particles leading to significant overdensities in spiral arms. We also find that the spiral structure is readily detectable by ALMA over a wide range of (sub-)millimetre wavelengths and by HiCIAO in near-infrared scattered light for non-face-on discs located in the Ophiucus star-forming region. In addition, we find clear spatial spectral index variations across the disc, revealing that the dust trapping produces a migration of large grains that can be potentially investigated through multi-wavelenghts observations in the (sub-)millimetric. Therefore, the spiral arms observed to date in protoplanetary disc might be interpreted as density waves induced by the development of gravitational instabilities.Comment: 14 pages, 12 figures. Accepted for publication in MNRA

Can dead zones create structures like a transition disk?

[Abridged] Regions of low ionisation where the activity of the magneto-rotational instability is suppressed, the so-called dead zones, have been suggested to explain gaps and asymmetries of transition disks. We investigate the gas and dust evolution simultaneously assuming simplified prescriptions for a dead zone and a magnetohydrodynamic (MHD) wind acting on the disk. We explore whether the resulting gas and dust distribution can create signatures similar to those observed in transition disks. For the dust evolution, we included the transport, growth, and fragmentation of dust particles. To compare with observations, we produced synthetic images in scattered optical light and in thermal emission at mm wavelengths. In all models with a dead zone, a bump in the gas surface density is produced that is able to efficiently trap large particles ($\gtrsim 1$ mm) at the outer edge of the dead zone. The gas bump reaches an amplitude of a factor of $\sim5$, which can be enhanced by the presence of an MHD wind that removes mass from the inner disk. While our 1D simulations suggest that such a structure can be present only for $\sim$1 Myr, the structure may be maintained for a longer time when more realistic 2D/3D simulations are performed. In the synthetic images, gap-like low-emission regions are seen at scattered light and in thermal emission at mm wavelengths, as previously predicted in the case of planet-disk interaction. As a conclusion, main signatures of transition disks can be reproduced by assuming a dead zone in the disk, such as gap-like structure in scattered light and millimetre continuum emission, and a lower gas surface density within the dead zone. Previous studies showed that the Rossby wave instability can also develop at the edge of such dead zones, forming vortices and also creating asymmetries.Comment: Minor changes after language edition. Accepted for publication in A&

Observational diagnostics of elongated planet-induced vortices with realistic planet formation timescales

Gap-opening planets can generate dust-trapping vortices that may explain some of the latest discoveries of high-contrast crescent-shaped dust asymmetries in transition discs. While planet-induced vortices were previously thought to have concentrated shapes, recent computational work has shown that these features naturally become much more elongated in the gas when simulations account for the relatively long timescale over which planets accrete their mass. In this work, we conduct two-fluid hydrodynamical simulations of vortices induced by slowly-growing Jupiter-mass planets in discs with very low viscosity ($\alpha = 3 \times 10^{-5}$). We simulate the dust dynamics for four particle sizes spanning 0.3 mm to 1 cm in order to produce synthetic ALMA images. In our simulations, we find that an elongated vortex still traps dust, but not directly at its center. With a flatter pressure bump and disruptions from the planet's overlapping spiral density waves, the dust instead circulates around the vortex. This motion (1) typically carries the peak off-center, (2) spreads the dust out over a wider azimuthal extent $\geq 180^{\circ}$, (3) skews the azimuthal profile towards the front of the vortex, and (4) can also create double peaks in newly-formed vortices. In particular, we expect that the most defining observational signature, a peak offset of more than $30^{\circ}$, should be detectable $>30\%$ of the time in observations with a beam diameter of at most the planet's separation from its star.Comment: Accepted to MNRAS. 13 pages, 8 figures. Movies available at: https://lavinia.as.arizona.edu/~mhammer/vortex_signatures.htm

A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap

We combined hydrodynamical simulations of planet-disk interactions with dust evolution models that include coagulation and fragmentation of dust grains over a large range of radii and derived observational properties using radiative transfer calculations. We studied the role of the snow line in the survival of the inner disk of transition disks. Inside the snow line, the lack of ice mantles in dust particles decreases the sticking efficiency between grains. As a consequence, particles fragment at lower collision velocities than in regions beyond the snow line. This effect allows small particles to be maintained for up to a few Myrs within the first astronomical unit. These particles are closely coupled to the gas and do not drift significantly with respect to the gas. For lower mass planets (1$M_{\rm{Jup}}$), the pre-transition appearance can be maintained even longer because dust still trickles through the gap created by the planet, moves invisibly and quickly in the form of relatively large grains through the gap, and becomes visible again as it fragments and gets slowed down inside of the snow line. The global study of dust evolution of a disk with an embedded planet, including the changes of the dust aerodynamics near the snow line, can explain the concentration of millimetre-sized particles in the outer disk and the survival of the dust in the inner disk if a large dust trap is present in the outer disk. This behaviour solves the conundrum of the combination of both near-infrared excess and ring-like millimetre emission observed in several transition disks.Comment: Accepted for publication in A&A (including acknowledgments

Dust evolution in protoplanetary disks

Planet formation models rely on knowledge of the physical conditions and evolutionary processes in protoplanetary disks, in particular the grain size distribution and dust growth timescales. In theoretical models, several barriers exist that prevent grain growth to pebble sizes and beyond, such as the radial drift and fragmentation. Pressure bumps have been proposed to overcome such barriers. In the past decade ALMA has revealed observational evidence for the existence of such pressure bumps in the form of dust traps, such as dust rings, gaps, cavities and crescents through high-resolution millimeter continuum data originating from thermal dust emission of pebble-sized dust grains. These substructures may be related to young protoplanets, either as the starting point or the consequence of early planet formation. Furthermore, disk dust masses have been measured for complete samples of young stars in clusters, which provide initial conditions for the solid mass budget available for planet formation. However, observational biases exist in the selection of high-resolution ALMA observations and uncertainties exist in the derivation of the disk dust mass, which both may affect the observed trends. This chapter describes the latest insights in dust evolution and disk continuum observations. Specifically, disk populations and evolutionary trends are described, as well as the uncertainties therein, and compared with exoplanet demographics.Comment: submitted, invited chapter for the "Handbook of Exoplanets". Comments welcom

An Inner Disk in the Large Gap of the Transition Disk SR 24S

We report new Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 3 observations at 2.75 mm of the TD around SR 24S with an angular resolution of $\sim$0.11''$\times$ 0.09'' and a peak signal-to-noise ratio of $\sim24$. We detect an inner disk and a mostly symmetric ring-like structure that peaks at $\sim$0.32'', that is $\sim$37 au at a distance of $\sim$114.4 pc. The full width at half maximum of this ring is $\sim$28 au. We analyze the observed structures by fitting the dust continuum visibilities using different models for the intensity profile, and compare with previous ALMA observations of the same disk at 0.45 mm and 1.30 mm. We qualitatively compare the results of these fits with theoretical predictions of different scenarios for the formation of a cavity or large gap. The comparison of the dust continuum structure between different ALMA bands indicates that photoevaporation and dead zone can be excluded as leading mechanisms for the cavity formation in SR 24S disk, leaving the planet scenario (single or multiple planets) as the most plausible mechanism. We compared the 2.75 mm emission with published (sub-)centimeter data and find that the inner disk is likely tracing dust thermal emission. This implies that any companion in the system should allow dust to move inwards throughout the gap and replenish the inner disk. In the case of one single planet, this puts strong constraints on the mass of the potential planet inside the cavity and the disk viscosity of about $\lesssim$5 $M_{\rm{Jup}}$ and $\alpha\sim10^{-4}-10^{-3}$, respectively.Comment: Accepted to Ap

Diverse protoplanetary disk morphology produced by a Jupiter-mass planet

Combining hydrodynamic planet-disk interaction simulations with dust evolution models, we show that protoplanetary disks having a giant planet can reveal diverse morphology in (sub-)millimeter continuum, including a full disk without significant radial structure, a transition disk with an inner cavity, a disk with a single gap and a central continuum peak, and a disk with multiple rings and gaps. Such a diversity originates from (1) the level of viscous transport in the disk which determines the number of gaps a planet can open; (2) the size and spatial distributions of grains determined by the coagulation, fragmentation, and radial drift, which in turn affects the emmisivity of the disk at (sub-)millimeter wavelengths; and (3) the angular resolution used to observe the disk. In particular, our results show that disks having the same underlying gas distribution can have very different grain size/spatial distributions and thus appearance in continuum, depending on the interplay among coagulation, fragmentation, and radial drift. This suggests that proper treatments for the grain growth have to be included in models of protoplanetary disks concerning continuum properties and that complementary molecular line observations are highly desired in addition to continuum observations to reveal the true nature of disks. The fact that a single planet can produce diverse disk morphology emphasizes the need to search for more direct, localized signatures of planets in order to confirm (or dispute) the planetary origin of observed ringed substructures.Comment: 12 pages, 8 figures, Accepted for publication in the ApJ Letter

Diferencia alvéolo arterial de oxígeno como predictor de requerimiento de ventilación mecánica en pacientes mayores de 65 años con neumonía adquirida en la comunidad

46 Páginas.Buscamos si la diferencia alvéolo arterial de oxígeno es un predictor de ventilación mecánica en pacientes mayores de 65 años con neumonía. Metodología: Los pacientes ingresan si cumplían los criterios de inclusión y de exclusión, se toman los gases arteriales y se observa el requerimiento de ventilación a las 48 horas. Resultados: De 215 pacientes, se recolectaron 89 pacientes. La distribución según el género fue 53.93% hombres y 46.06% mujeres, una edad promedio de 78 años. 4.49% requirieron ventilación mecánica. El 50% de los pacientes ventilados vs 57.65% del grupo no ventilados, presentaron una DAaO2 ≥ a 19mmHg. Conclusiones: No encontramos diferencias significativas para la DAaO2. Es necesario completar la muestra para verificar si las tendencias encontradas serán las definitivas