Herbig Ae/Be stars are intermediate-mass pre-main sequence stars surrounded by circumstellar dust disks. Some are observed to produce jets, whose appearance as a sequence of shock fronts (knots) ...suggests a past episodic outflow variability. This “jet fossil record” can be used to reconstruct the outflow history. We present the first optical to near-infrared (NIR) spectra of the jet from the Herbig Ae star HD 163296, obtained with VLT/X-shooter. We determine the physical conditions in the knots and also their kinematic “launch epochs”. Knots are formed simultaneously on either side of the disk, with a regular interval of ~16 yr. The velocity dispersion versus jet velocity and the energy input are comparable between both lobes. However, the mass-loss rate, velocity,and shock conditions are asymmetric. We find Ṁjet/Ṁacc ~ 0.01−0.1, which is consistent with magneto-centrifugal jet launching models. No evidence of any dust is found in the high-velocity jet, suggesting a launch region within the sublimation radius (<0.5 au). The jet inclination measured from proper motions and radial velocities confirms that it is perpendicular to the disk. A tentative relation is found between the structure of the jet and the photometric variability of the central source. Episodes of NIR brightening were previously detected and attributed to a dusty disk wind. We report for the first time significant optical fadings lasting from a few days up to a year, coinciding with the NIR brightenings. These are very likely caused by dust lifted high above the disk plane, and this supports the disk wind scenario. The disk wind is launched at a larger radius than the high-velocity atomic jet, although their outflow variability may have a common origin. No significant relation between outflow and accretion variability could be established. Our findings confirm that this source undergoes periodic ejection events, which may be coupled with dust ejections above the disk plane.
Context. HD 181327 is a young main sequence F5/F6 V star belonging to the β Pictoris moving group (age ~12 Myr). It harbors an optically thin belt of circumstellar material at radius ~90 AU, ...presumed to result from collisions in a population of unseen planetesimals. Aims. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. Methods. We obtained far-infrared photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory⋆, complemented by new 3.2 mm observations carried with the ATCA⋆⋆ array. The geometry of the belt is constrained with newly reduced HST/NICMOS scattered light images that allow the degeneracy between the disk geometry and the dust properties to be broken. We then use the radiative transfer code GRaTeR to compute a large grid of models, and we identify the grain models that best reproduce the spectral energy distribution (SED) through a Bayesian analysis. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo. Results. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the OI and CII line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of ~0.05 M⊕ (in grains up to 1 mm). We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than ~17 M⊕. Conclusions. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.
Abstract
HST UV spectra were obtained to study whether the
φ
Leo system has one or more stellar companions, as shown for the A-star dwarf, HR10, stellar pulsation, or transiting exocomets. During ...five partial HST observation sets spread over 13 months, redshifted gas with radial velocities up to +245 km s
−1
was observed on one occasion, with lower velocity infalling material seen twice. Blueshifted gas features with velocities as high as −150 km s
−1
with up to three features in two spectra were also seen. Low-mass stellar companions to late A stars are coronal sources. Assuming an age of 450 Myr, G-K companions to
φ
Leo, and multiple M-star companions would have been detected in the ROSAT All-Sky Survey data, but are not seen. Further, the star shows only 1 major pulsation component, suggesting it is single, excluding an A- or early F-star companion. Low covering factor absorption features with
∣
v
∣
≤ 200 km s
−1
have been reported in optical spectra with variation on timescales consistent with nonradial pulsation. However, the highest velocity infall feature has a broader, continuous absorption profile extending from the stellar velocity to +245 km s
−1
, with a covering factor of ∼25% which is more consistent with a transit by a body on a star-grazing orbit.
The Eroding Disk of AU Mic Grady, C. A.; Wisniewski, J. P.; Schneider, G. ...
Astrophysical journal. Letters,
01/2020, Letnik:
889, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We report Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph imaging of AU Mic's debris disk from 2017 and archival data. Outward motion of the features in the southeast arm continues. ...At least three features have reached or exceeded projected escape velocity in the past decade, yielding a combined feature mass-loss rate of ∼1.2 × 10−7 MEarth yr−1, or ∼1.2 × 10−13 M yr−1, ∼10% of AU Mic's stellar wind mass-loss rate, and similar to the ratio of coronal mass ejection mass loss to the stellar wind mass-loss rate. We confirm the 2018 finding of feature height changes for one feature (B/SE4), but the HST data are too sparse to compare (yet) with the stellar activity cycle. Detection of what appears to be a chain of features in a second system suggests that the disk of AU Mic is not unique, although a larger sample of disks is required to quantify how common the phenomenon is.
BACKGROUND: In the face of climate change, shifts in genetic structure and composition of terrestrial plant species are occurring worldwide. Because different genotypes of these plant species support ...different soil biota and soil processes, shifts in genetics are likely to have cascading effects on ecosystems. SCOPE: We explore plant genetic effects on soil function in the context of climate change, and selection by soils, soil biota and plant-soil feedbacks. We propose categories of genetically-based plant traits that should be prioritized in research on genetic-based effects on soil processes including plant productivity and C allocation, tissue quality, plant water-use, and rhizosphere mutualisms. Additionally, we posit that soil community responses to climate change should be considered in concert with plant genotype because of sensitivity of soil communities to climate. We use two case studies to highlight these points. CONCLUSIONS: We argue that the effects of climate change as an agent of selection on plants may cascade to affect soils, and ultimately the structure, composition and function of ecosystems. Understanding the ecological and evolutionary potential of plant-soil linkages may help us understand and mitigate the extended consequences of global change for ecosystems worldwide. Accordingly, we conclude with experimental approaches for examining genetically-based plant-soil interactions across climate change gradients.
We observed a sample of 20 representative Herbig Ae/Be stars and 5 A-type debris discs with PACS onboard Herschel, as part of the GAS in Protoplanetary Systems (GASPS) project. The observations were ...done in spectroscopic mode, and cover the far-infrared lines of O i, C ii, CO, CH+, H2O, and OH. We have a O i 63 μm detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. The O i 145 μm line is only detected in 25% and CO J = 18-17 in 45% (and fewer cases for higher J transitions) of the Herbig Ae/Be stars, while for C ii 157 μm, we often find spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. First seen in HD 100546, CH+ emission is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and either the stellar or disc parameters, such as stellar luminosity, ultraviolet and X-ray flux, accretion rate, polycyclic aromatic hydrocarbon (PAH) band strength, and flaring. We find that the stellar ultraviolet flux is the dominant excitation mechanism of O i 63 μm, with the highest line fluxes being found in objects with a large amount of flaring and among the largest PAH strengths. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of O i 63 μm and O i 145 μm, CO J = 18-17 and O i 6300 Å, and between the continuum flux at 63 μm and at 1.3 mm, while we find weak correlations between the line flux of O i 63 μm and the PAH luminosity, the line flux of CO J = 3-2, the continuum flux at 63 μm, the stellar effective temperature, and the Brγ luminosity. Finally, we use a combination of theO i 63 μm and 12CO J = 2-1 line fluxes to obtain order of magnitude estimates of the disc gas masses, in agreement with the values that we find from detailed modelling of two Herbig Ae/Be stars, HD 163296 and HD 169142. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
As solar photovoltaic power generation becomes more commonplace, the inherent intermittency of the solar resource poses one of the great challenges to those who would design and implement the next ...generation smart grid. Specifically, grid-tied solar power generation is a distributed resource whose output can change extremely rapidly, resulting in many issues for the distribution system operator with a large quantity of installed photovoltaic devices. Battery energy storage systems are increasingly being used to help integrate solar power into the grid. These systems are capable of absorbing and delivering both real and reactive power with sub-second response times. With these capabilities, battery energy storage systems can mitigate such issues with solar power generation as ramp rate, frequency, and voltage issues. Beyond these applications focusing on system stability, energy storage control systems can also be integrated with energy markets to make the solar resource more economical. Providing a high-level introduction to this application area, this paper presents an overview of the challenges of integrating solar power to the electricity distribution system, a technical overview of battery energy storage systems, and illustrates a variety of modes of operation for battery energy storage systems in grid-tied solar applications. The real-time control modes discussed include ramp rate control, frequency droop response, power factor correction, solar time-shifting, and output leveling.
We present four new epochs of Ks-band images of the young pre-transitional disk around LkCa 15 and perform extensive forward modeling to derive the physical parameters of the disk. We find ...indications of strongly anisotropic scattering (Henyey-Greenstein g = 0.67-0.11+0.18) and a significantly tapered gap edge (“round wall”) but see no evidence that the inner disk, whose existence is predicted by the spectral energy distribution, shadows the outer regions of the disk visible in our images. We marginally confirm the existence of an offset between the disk center and the star along the line of nodes; however, the magnitude of this offset (x = 27-20+19 mas) is notably lower than that found in our earlier H-band images. Intriguingly, we also find an offset of y=69-25+49 mas perpendicular to the line of nodes at high significance. If confirmed by future observations, this would imply a highly elliptical – or otherwise asymmetric – disk gap with an effective eccentricity of e ≈ 0.3. Such asymmetry would most likely be the result of dynamical sculpting by one or more unseen planets in the system. Finally, we find that the bright arc of scattered light we see in direct imaging observations originates from the near side of the disk and appears brighter than the far side because of strong forward scattering.
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