Herbig Stars Brittain, Sean D.; Kamp, Inga; Meeus, Gwendolyn ...
Space science reviews,
02/2023, Letnik:
219, Številka:
1
Journal Article
Recenzirano
Herbig Ae/Be stars are young contracting stars on the radiative track in the HR diagram on their way to the main sequence. These stars provide a valuable link between high and low mass stars. Here we ...review the progress that has been made in our understanding of these fascinating objects and their disks since the last major review on this topic published in 1998. We begin with a general overview of these stars and their properties. We then discuss the accretion of circumstellar material onto these stars. Next we discuss the dust and gas properties of the circumstellar disk before exploring the evidence for planet formation in these disks. We conclude with a brief discussion of future prospects for deepening our understanding of these sources and propose a new working definition of Herbig Ae/Be stars.
Dusty wind of W Hydrae Khouri, T; Waters, L B F M; de Koter, A ...
Astronomy and astrophysics (Berlin),
05/2015, Letnik:
577
Journal Article
Recenzirano
Odprti dostop
Low- and inter-mediate-mass stars go through a period of intense mass-loss at the end of their lives, during the asymptotic giant branch (AGB) phase. While on the AGB a significant part, or even ...most, of their initial mass is expelled in a stellar wind. We study the dusty wind of the oxygen-rich AGB star W Hya to understand its composition and structure and shed light on the mass-loss mechanism. We modeled the dust envelope of W Hya using an advanced radiative transfer code. We analyzed our dust model in the light of a previously calculated gas-phase wind model and compared it with measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Alsub 2Osub 3 emission. In our model, the silicates need to have substantial near-infrared opacities to be visible at such large distances.
We conducted a detailed petrologic study on six crystal-poor (< 12 vol%) dacites from South Sister volcano (OR) to determine the conditions that favor the formation and extraction (i.e., eruption) of ...intermediate magmas in a system that features prominent Daly Gaps. We present whole rock compositions for six dacites (63.2–65.1 wt% SiO
2
) that form the base of South Sister and six mafic flows (51.3–56.1 wt% SiO
2
) that erupted in the peripheral volcanic field. The dacites are saturated in plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite; hornblende is not observed. Evaluation of trace element concentrations in whole rock analyses reveals that end member mixing (basalt with rhyolite) is not the primary petrogenetic mechanism for generating the dacites, and comparison of compositions of minerals in the dacites with those equilibrated in experiments suggests that the majority of crystals in the dacites are plausible phenocrysts. Application of multiple thermometers to pyroxene and Fe–Ti oxide compositions in the dacites shows that crystallization in the dacites initiated at elevated temperatures (980–1045 °C) and that modest zonation in the phenocryst assemblage can be attributed to the effects of minor closed system crystallization ± degassing ± cooling. When pre-eruptive temperatures, plagioclase and whole rock compositions from the dacites are incorporated into the plagioclase hygrometer, we obtain maximum pre-eruptive H
2
O contents that range from 2.1 to 3.3 wt%; these H
2
O contents are consistent with those associated with plagioclase-in curves determined from phase equilibrium experiments. Through review of glasses equilibrated in experimental run products in the literature, we find that dacitic melt compositions with temperatures similar to the South Sister dacites (i.e., 980–1045 °C) are restricted to partial melting experiments on MgO-rich (> 5 wt% MgO) basalts and that dacitic melt compositions most common in partial melting experiments conducted under low H
2
O fugacities. Partial melting of basalt resolves the compositional gaps between the basalts and dacites in the South Sister volcanic record because, during partial melting, dacites (1) can be generated without requiring the formation of a compositional intermediate, (2) are produced in eruptible melt fractions (e.g., 20–25%), (3) are favorably erupted if they occur in fluid undersaturated conditions (owing to the positive Clapeyron slope in the absence of a fluid phase). Though the efficiency of partial melting is debated because of the amount of heat required to generate melt, it is a viable process beneath South Sister because of the elevated geothermal gradient, high pre-eruptive temperatures of primitive basalts erupting in the surrounding areas and the recent history of volcanism in the region. Our results suggest that eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.
Selective molecular recognition of hydrophilic guests in water plays a fundamental role in a vast number of biological processes, but synthetic mimicry of biomolecular recognition in water still ...proves challenging both in terms of achieving comparable affinities and selectivities. This Review highlights strategies that have been developed in the field of supramolecular chemistry to selectively and non‐covalently bind three classes of biologically relevant molecules: nucleotides, carbohydrates, and amino acids. As several groups have systematically modified receptors for a specific guest, an evolutionary perspective is also provided in some cases. Trends in the most effective binding forces for each class are described, providing insight into selectivity and potential directions for future work.
Hydrophilic guests are particularly challenging to selectively bind in water, but selective recognition of hydrophilic molecules is vital in biological processes. In this Review, strategies to develop synthetic receptors that address this challenge are highlighted, with application to nucleotides, carbohydrates, and amino acids. This body of work provides general insights as well as future directions for the field.
Context. Two protoplanets have recently been discovered within the PDS 70 protoplanetary disk. JWST/NIRCam offers a unique opportunity to characterize them and their birth environment at wavelengths ...that are difficult to access from the ground. Aims. We image the circumstellar environment of PDS 70 at 1.87 μm and 4.83 μm, assess the presence of Pa- α emission due to accretion onto the protoplanets, and probe any IR excess indicative of heated circumplanetary material. Methods. We obtained noncoronagraphic JWST/NIRCam images of PDS 70 within the MIRI mid-INfrared Disk Survey (MINDS) program. We leveraged the Vortex Image Processing (VIP) package for data reduction, and we developed dedicated routines for optimal stellar point spread function subtraction, unbiased imaging of the disk, and protoplanet flux measurement in this type of dataset. A radiative transfer model of the disk was used to separate the contributions from the disk and the protoplanets. Results. We redetect both protoplanets and identify extended emission after subtracting a disk model, including a large-scale spiral-like feature. We interpret its signal in the direct vicinity of planet c as tracing the accretion stream that feeds its circumplanetary disk, while the outer part of the feature may rather reflect asymmetric illumination of the outer disk. We also report a bright signal that is consistent with a previously proposed protoplanet candidate enshrouded in dust near the 1:2:4 mean-motion resonance with planets b and c . The 1.87 μm flux of planet b is consistent with atmospheric model predictions, but the flux of planet c is not. We discuss potential origins for this discrepancy, including significant Pa- α line emission. The 4.83 μm fluxes of planets b and c suggest enshrouding dust or heated CO emission from their circumplanetary environment. Conclusions. The use of image-processing methods that are optimized for extended disk signals on high-sensitivity and high-stability from JWST can uniquely identify signatures of planet–disk interactions and enable accurate photometry of protoplanets at wavelengths that are difficult to probe from the ground. Our results indicate that more protoplanets can be identified and characterized in other JWST datasets.
Active galactic nuclei (AGNs) display many energetic phenomena-broad emission lines, X-rays, relativistic jets, radio lobes-originating from matter falling onto a supermassive black hole. It is ...widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.
Context. Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds, and outflows. The ...formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale, and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. Aims. We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozen individual objects, both photometrically and spectroscopically, looking for signs of multiple generations of young stars and investigating the role of the massive stars in this process. Methods. We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate the ages and masses of individual objects. We identify the population of embedded young stellar objects (YSOs) by their infrared colors and emission line spectra. Results. RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 ± 0.6 Myr. Of the stars that could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Conclusions. Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps and two exposed protostellar jets.
Context
. Stars in the solar neighbourhood have refractory element ratios slightly different from that of the Sun. It is unclear how much the condensation of solids and thus the composition of ...planets forming around these stars is affected.
Aims
. We aim to understand the impact of changing the ratios of the refractory elements Mg, Si, and Fe within the range observed in solar-type stars within 150 pc of the Sun on the composition of planets forming around them.
Methods
. We use the GGchem code to simulate the condensation of solids in protoplanetary disks with a minimum mass solar nebula around main sequence G-type stars in the solar neighbourhood. We extract the stellar elemental composition from the Hypatia Database.
Results
. We find that a lower Mg/Si ratio shifts the condensation sequence from forsterite (Mg
2
SiO
4
) and SiO to enstatite (MgSiO
3
) and quartz (SiO
2
); a lower Fe/S ratio leads to the formation of FeS and FeS
2
and few or no Fe-bearing silicates. Ratios of refractory elements translate directly from the gas phase to the condensed phase for T < 1000 K. However, ratios with respect to volatile elements (e.g. oxygen and sulphur) in the condensates – the building blocks of planets – differ from the original stellar composition.
Conclusions
. Our study shows that the composition of planets crucially depends on the abundances of the stellar system under investigation. Our results can have important implications for planet interiors, which depend strongly on the degree of oxidation and the sulphur abundance.
The chemistry of astronomical hydrocarbons, responsible for the well-known infrared emission features detected in a wide variety of targets, remains enigmatic. Here we focus on the group of young ...intermediate-mass Herbig Ae stars. We have analyzed the aliphatic and polycyclic aromatic hydrocarbon (PAH) emission features in the infrared spectra of a sample of 53 Herbig Ae stars, obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. We confirm that the PAH-to-stellar luminosity ratio is higher in targets with a flared dust disk. However, a few sources with a flattened dust disk still show relatively strong PAH emission. Since PAH molecules trace the gas disk, this indicates that gas disks may still be flared, while the dust disk has settled due to grain growth. There are indications that the strength of the 11.3 {mu}m feature also depends on dust disk structure, with flattened disks being less bright in this feature. We confirm that the CC bond features at 6.2 and 7.8 {mu}m shift to redder wavelengths with decreasing stellar effective temperature. Moreover, we show that this redshift is accompanied by a relative increase of aliphatic CH emission and a decrease of the aromatic 8.6 {mu}m CH feature strength. Cool stars in our sample are surrounded by hydrocarbons with a high aliphatic/aromatic CH ratio and a low aromatic CH/CC ratio, and vice versa for the hot stars. We conclude that, while the overall hydrocarbon emission strength depends on the dust disk's geometry, the relative differences seen in the IR emission features in disks around Herbig Ae stars are mainly due to chemical differences of the hydrocarbon molecules induced by the stellar UV field. Strong UV flux reduces the aliphatic component and emphasizes the spectral signature of the aromatic molecules in the IR spectra.
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