The characteristics of pulmonary arterial hypertension (PAH), including pathology, symptoms, diagnosis and treatment are reviewed in children and adults. The histopathology seen in adults is also ...observed in children, although children have more medial hypertrophy at presentation. Both populations have vascular and endothelial dysfunction. Several unique disease states are present in children, as lung growth abnormalities contribute to pulmonary hypertension. Although both children and adults present at diagnosis with elevations in pulmonary vascular resistance and pulmonary artery pressure, children have less heart failure. Dyspnoea on exertion is the most frequent symptom in children and adults with PAH, but heart failure with oedema occurs more frequently in adults. However, in idiopathic PAH, syncope is more common in children. Haemodynamic assessment remains the gold standard for diagnosis, but the definition of vasoreactivity in adults may not apply to young children. Targeted PAH therapies approved for adults are associated with clinically meaningful effects in paediatric observational studies; children now survive as long as adults with current treatment guidelines. In conclusion, there are more similarities than differences in the characteristics of PAH in children and adults, resulting in guidelines recommending similar diagnostic and therapeutic algorithms in children (based on expert opinion) and adults (evidence-based).
High levels of dust have been detected in the immediate vicinity of many stars, both young and old. A promising scenario to explain the presence of this short-lived dust is that these analogues to ...the zodiacal cloud (or exozodis) are refilled in situ through cometary activity and sublimation. As the reservoir of comets is not expected to be replenished, the presence of these exozodis in old systems has yet to be adequately explained. It was recently suggested that mean-motion resonances with exterior planets on moderately eccentric (ep ... 0.1) orbits could scatter planetesimals on to cometary orbits with delays of the order of several 100 Myr. Theoretically, this mechanism is also expected to sustain continuous production of active comets once it has started, potentially over Gyr time-scales. We aim here to investigate the ability of this mechanism to generate scattering on to cometary orbits compatible with the production of an exozodi on long time-scales. We combine analytical predictions and complementary numerical N-body simulations to study its characteristics. We show, using order of magnitude estimates, that via this mechanism, low-mass discs comparable to the Kuiper belt could sustain comet scattering at rates compatible with the presence of the exozodis which are detected around Solar-type stars, and on Gyr time-scales. We also find that the levels of dust detected around Vega could be sustained via our proposed mechanism if an eccentric Jupiter-like planet were present exterior to the system's cold debris disc. (ProQuest: ... denotes formulae/symbols omitted.)
Exocomets: A spectroscopic survey Rebollido, I.; Eiroa, C.; Montesinos, B. ...
Astronomy & astrophysics,
07/2020, Letnik:
639
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context.
While exoplanets are now routinely detected, the detection of small bodies in extrasolar systems remains challenging. Since the discovery of sporadic events, which are interpreted to be ...exocomets (falling evaporating bodies) around
β
Pic in the early 1980s, only ∼20 stars have been reported to host exocomet-like events.
Aims.
We aim to expand the sample of known exocomet-host stars, as well as to monitor the hot-gas environment around stars with previously known exocometary activity.
Methods.
We have obtained high-resolution optical spectra of a heterogeneous sample of 117 main-sequence stars in the spectral type range from B8 to G8. The data were collected in 14 observing campaigns over the course of two years from both hemispheres. We analysed the Ca
II
K&H and Na
I
D lines in order to search for non-photospheric absorptions that originated in the circumstellar environment and for variable events that could be caused by the outgassing of exocomet-like bodies.
Results.
We detected non-photospheric absorptions towards 50% of the sample, thus attributing a circumstellar origin to half of the detections (i.e. 26% of the sample). Hot circumstellar gas was detected in the metallic lines inspected via narrow stable absorptions and/or variable blue- and red-shifted absorption events. Such variable events were found in 18 stars in the Ca
II
and/or Na
I
lines; six of them are reported in the context of this work for the first time. In some cases, the variations we report in the Ca
II
K line are similar to those observed in
β
Pic. While we do not find a significant trend in the age or location of the stars, we do find that the probability of finding CS gas in stars with larger
v
sin
i
is higher. We also find a weak trend with the presence of near-infrared excess and with anomalous (
λ
Boo-like) abundances, but this would require confirmation by expanding the sample.
Context . Ground-based large-aperture telescopes, interferometers, and future extremely large telescopes equipped with adaptive optics (AO) systems provide angular resolution and high-contrast ...performance superior to space-based telescopes at thermal infrared wavelengths. Their sensitivity, however, is critically limited by the high thermal background inherent to ground-based observations in this wavelength regime. Aims . We aim to improve the subtraction quality of the thermal infrared background from ground-based observations using principal component analysis (PCA). Methods . We used data obtained with the Nulling-Optimized Mid-Infrared Camera on the Large Binocular Telescope Interferometer as a proxy for general high-sensitivity AO-assisted ground-based data. We applied both a classical background subtraction – using the mean of dedicated background observations – and a new background subtraction based on a PCA of the background observations. We compared the performances of these two methods in both high-contrast imaging and aperture photometry. Results . Compared to the classical approach for background subtraction, PCA background subtraction delivers up to two times better contrasts down to the diffraction limit of the LBT’s primary aperture (i.e., 350 mas in N -band), that is, in the case of high-contrast imaging. An improvement factor between two and three was obtained over the mean background retrieval within the diffraction limit in the case of aperture photometry. Conclusions . The PCA background subtraction significantly improves the sensitivity of ground-based thermal infrared imaging observations. When apply to LBTI’s nulling interferometry data, we expect the method to improve the sensitivity by a similar factor of two to three. This study paves the way to maximizing the potential of future infrared ground-based instruments and facilities, such as the future 30m-class telescopes.
Context. Planet perturbations have been often invoked as a potential explanation for many spatial structures that have been imaged in debris discs. So far this issue has been mostly investigated with ...pure N-body numerical models, which neglect the crucial effect collisions within the disc can have on the disc’s response to dynamical perturbations. Aims. We numerically investigate how the coupled effect of collisions and radiation pressure can affect the formation and survival of radial and azimutal structures in a disc perturbed by a planet. We consider two different set-ups: a planet embedded within an extended disc and a planet exterior to an inner debris ring. One important issue we want to address is under which conditions a planet’s signature can be observable in a collisionally active disc. Methods. We use our DyCoSS code, which is designed to investigate the structure of perturbed debris discs at dynamical and collisional steady-state, and derive synthetic images of the system in scattered light. The planet’s mass and orbit, as well as the disc’s collisional activity (parameterized by its average vertical optical depth τ0) are explored as free parameters. Results. We find that collisions always significantly damp planet-induced spatial structures. For the case of an embedded planet, the planet’s signature, mostly a density gap around its radial position, should remain detectable in head-on images if Mplanet ≥ MSaturn. If the system is seen edge-on, however, inferring the presence of the planet is much more difficult, as only weak asymmetries remain in a collisionally active disc, although some planet-induced signatures might be observable under very favourable conditions. For the case of an inner ring and an external planet, planetary perturbations cannot prevent collision-produced small fragments from populating the regions beyond the ring. The radial luminosity profile exterior to the ring is in most cases close to the one it should have in the absence of the external planet. The most significant signature left by a Jovian planet on a circular orbit are precessing azimutal structures that can be used to indirectly infer its presence. For a planet on an eccentric orbit, we show that the ring becomes elliptic and that the well known pericentre glow effect is visible despite of collisions and radiation pressure, but that detecting such features in observed discs is not an unambiguous indicator of the presence of an outer planet.
ABSTRACT
The time-scales on which astronomical dust grows remain poorly understood, with important consequences for our understanding of processes like circumstellar disc evolution and planet ...formation. A number of post-asymptotic giant branch (AGB) stars are found to host optically thick, dust- and gas-rich circumstellar discs in Keplerian orbits. These discs exhibit evidence of dust evolution, similar to protoplanetary discs; however, since post-AGB discs have substantially shorter lifetimes than protoplanetary discs, they may provide new insights on the grain-growth process. We examine a sample of post-AGB stars with discs to determine the far-infrared and sub-millimetre spectral index by homogeneously fitting a sample of data from Herschel, the Submillimeter Array (SMA), and the literature. We find that grain growth to at least hundreds of micrometres is ubiquitous in these systems, and that the distribution of spectral indices is more similar to that of protoplanetary discs than debris discs. No correlation is found with the mid-infrared colours of the discs, implying that grain growth occurs independently of the disc structure in post-AGB discs. We infer that grain growth to ∼millimetre sizes must occur on time-scales <<105 yr, perhaps by orders of magnitude, as the lifetimes of these discs are expected to be ≲105 yr and all objects have converged to the same state. This growth time-scale is short compared to the results of models for protoplanetary discs including fragmentation and may provide new constraints on the physics of grain growth.
Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their counterparts in the solar system are the asteroid and ...Edgeworth-Kuiper belts. Aims. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. Methods. The full sample of 177 FGK stars with d ≤ 20 pc proposed for the DUst around NEarby Stars (DUNES) survey is presented. Herschel/PACS observations at 100 and 160 μm were obtained, and were complemented in some cases with data at 70 μm and at 250, 350, and 500 μm SPIRE photometry. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the Disc Emission via a Bias-free Reconnaissance in IR and Sub-mm (DEBRIS) consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. Results. The subsample of 105 stars with d ≤ 15 pc containing 23 F, 33 G, and 49 K stars is complete for F stars, almost complete for G stars, and contains a substantial number of K stars from which we draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type are 0.26+0.21-0.14 (6 objects with excesses out of 23 F stars), 0.21+0.17-0.11 (7 out of 33 G stars), and 0.200.14-0.09 (10 out of 49 K stars); the fraction for all three spectral types together is 0.22+0.08-0.07 (23 out of 105 stars). The uncertainties correspond to a 95% confidence level. The medians of the upper limits of Ldust/L∗ for each spectral type are 7.8 × 10-7 (F), 1.4 × 10-6 (G), and 2.2 × 10-6 (K); the lowest values are around 4.0 × 10-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.
Context. Debris disks are commonly considered to be a by-product of planet formation. Structures in debris disks induced by planet-disk interaction are promising to provide valuable constraints on ...the existence and properties of embedded planets. Aims. We investigate the observability of structures in debris disks induced by planet-disk interaction with future facilities in a systematic way. High-sensitivity, high angular resolution observations with large (sub-)mm interferometers and large space-based telescopes operating in the near- to mid-infrared wavelength range are considered. Methods. The observability of debris disks with the Atacama Large Millimeter/submillimeter Array (ALMA) is studied on the basis of a simple analytical disk model. Furthermore, N-body simulations are used to model the spatial dust distribution in debris disks under the influence of planet-disk interaction. From these simulations, images at optical scattered light to millimeter thermal re-emission are computed. Available information about the expected capabilities of ALMA and the James Webb Space Telescope (JWST) are used to investigate the observability of characteristic disk structures with these facilities through spatially resolved imaging. Results. Our simulations show that planet-disk interaction can result in prominent structures in the whole considered wavelength range. The exact result depends on the configuration of the planet-disk system and on the observing wavelength which provides the opportunity of detecting and characterizing extrasolar planets in a range of masses and radial distances from the star that is not accessible to other techniques. Facilities that will be available in the near future at both considered wavelength ranges are shown to provide the capabilities to spatially resolve and characterize structures in debris disks that arise because of planet-disk interaction. Limitations are revealed and suggestions for possible instrument setups and observing strategies are given. In particular, ALMA is limited by its sensitivity to surface brightness, which requires a trade-off between sensitivity and spatial resolution. Space-based mid-infrared observations will be able to detect and spatially resolve regions in debris disks even at a distance of several tens of AU from the star, where the emission from debris disks in this wavelength range is expected to be low. Conclusions. Both ALMA and the planned space-based near- to mid-infrared telescopes will provide unprecedented capabilities to study planet-disk interaction in debris disks. In particular, a combination of observations at both wavelengths will provide very strong constraints on the planetary/planetesimal systems.
ABSTRACT Debris discs are typically revealed through the presence of excess emission at infrared wavelengths. Most discs exhibit excess at mid- and far-infrared wavelengths, analogous to the solar ...system's Asteroid and Edgeworth-Kuiper belts. Recently, stars with strong (∼1%) excess at near-infrared wavelengths were identified through interferometric measurements. Using the HIgh Precision Polarimetric Instrument, we examined a sub-sample of these hot dust stars (and appropriate controls) at parts-per-million sensitivity in SDSS g′ (green) and r′ (red) filters for evidence of scattered light. No detection of strongly polarized emission from the hot dust stars is seen. We, therefore, rule out scattered light from a normal debris disk as the origin of this emission. A wavelength-dependent contribution from multiple dust components for hot dust stars is inferred from the dispersion (the difference in polarization angle in red and green) of southern stars. Contributions of 17 ppm (green) and 30 ppm (red) are calculated, with strict 3- upper limits of 76 and 68 ppm, respectively. This suggests weak hot dust excesses consistent with thermal emission, although we cannot rule out contrived scenarios, e.g., dust in a spherical shell or face-on discs. We also report on the nature of the local interstellar medium (ISM), obtained as a byproduct of the control measurements. Highlights include the first measurements of the polarimetric color of the local ISM and the discovery of a southern sky region with a polarization per distance thrice the previous maximum. The data suggest that λmax, the wavelength of maximum polarization, is bluer than typical.
AU Microscopii’s debris disc is one of the most famous and best-studied debris discs and one of only two resolved debris discs around M stars. We perform in-depth collisional modelling of the AU Mic ...disc including stellar radiative and corpuscular forces (stellar winds), aiming at a comprehensive understanding of the dust production and the dust and planetesimal dynamics in the system. Our models are compared to a suite of observational data for thermal and scattered light emission, ranging from the ALMA radial surface brightness profile at 1.3 mm to spatially resolved polarisation measurements in the visible. Most of the data are shown to be reproduced with dust production in a belt of planetesimals with an outer edge at around 40 au and subsequent inward transport of dust by stellar winds. A low dynamical excitation of the planetesimals with eccentricities up to 0.03 is preferred. The radial width of the planetesimal belt cannot be constrained tightly. Belts that are 5 au and 17 au wide, as well as a broad 44 au-wide belt, are consistent with observations. All models show surface density profiles that increase with distance from the star up to ≈40 au, as inferred from observations. The best model is achieved by assuming a stellar mass loss rate that exceeds the solar one by a factor of 50. The models reproduce the spectral energy distribution and the shape of the ALMA radial profile well, but deviate from the scattered light observations more strongly. The observations show a bluer disc colour and a lower degree of polarisation for projected distances <40 au than predicted by the models. These deviations may be reduced by taking irregularly shaped dust grains which have scattering properties different from the Mie spheres used in this work. From tests with a handful of selected dust materials, we favour mixtures of silicate, carbon, and ice of moderate porosity. We also address the origin of the unresolved central excess emission detected by ALMA and show that it cannot stem from an additional inner belt alone. Instead, it should derive, at least partly, from the chromosphere of the central star.
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