Aims.
In this paper, we aim to constrain the dust mass and grain sizes in the interaction regions between the stellar winds and the interstellar medium (ISM) around asymptotic giant branch (AGB) ...stars. By describing the dust in these regions, we aim to shed light on the role of evolved low-mass stars in the origin of dust in galaxies.
Methods.
We use images in the far-infrared (FIR) at 70 and 160 µm to derive dust temperatures and dust masses in the wind-ISM interaction regions around a sample of carbon-rich and oxygen-rich AGB stars. The dust temperature and mass are determined in two ways: first, directly from the data using the ratio of the measured fluxes and assuming opacities for dust with a constant grain size of 0.1 µm, and then using three-dimensional dust-radiative transfer models spatially constrained by the observations. Each of the radiative transfer models contains one constant grain size, which is varied between 0.01 and 5.0 µm.
Results.
We find that the observed dust mass in the wind-ISM interaction regions is consistent with mass accumulated from the stellar winds. For the carbon-rich sources, adding the spatial constraints in the radiative transfer models results in preferentially larger grain sizes (≈2 µm). For the oxygen-rich sources, the spatial constraints result in overly high temperatures in the models, making it impossible to fit the observed FIR ratio irrespective of the grain size used, indicating a more complex interplay of grain properties and the stellar radiation field.
Conclusions.
Our results have implications for how likely it is for the grains to survive the transition into the ISM, and the properties of dust particles that later act as seeds for grain growth in the ISM. However, our results for the oxygen-rich sources show that the derivation of dust properties is not straight forward, requiring more complex modelling.
Context.
Stars on the asymptotic giant branch lose considerable amounts of matter through their dust-driven stellar winds. A number of such sources have been imaged by
Herschel
/PACS, revealing a ...diverse sample of different morphological types. Among them are a few examples which show geometrically thin, spherically symmetric shells which can be used to probe the mass loss history of their host stars.
Aims.
We aim to determine the physical properties of the dust envelope around the two carbon stars U Hya and W Ori. With the much-improved spatial constraints from the new far-infrared maps, our primary goal is to measure the dust masses contained in the shells and see how they fit the proposed scenarios of shell formation.
Methods.
We calculated the radiative transfer of the circumstellar dust envelope using the 1D code
More of DUSTY
(MoD). Adopting a parametrised density profile, we obtained a best-fit model in terms of the photometric and spectroscopic data, as well as a radial intensity profile based on
Herschel
/PACS data. For the case of U Hya, we also computed a grid of circumstellar envelopes by means of a stationary wind code and compare the results of the two modelling approaches.
Results.
The
Herschel
/PACS maps show U Hya surrounded by a detached shell of 114′′ (0.12 pc) in radius, confirming the observations from previous space missions. The dust masses calculated for the shell by the two approaches are consistent with respect to the adopted dust grain properties. In addition, around W Ori, we detect for the first time a weak spherically symmetric structure with a radius of 92′′ (0.17 pc) and a dust mass of (3.5 ± 0.3) × 10
−6
M
⊙
.
Aims.
The carbon-rich asymptotic giant branch (AGB) star TX Piscium (TX Psc) has been observed multiple times during multiple epochs and at different wavelengths and resolutions, showing a complex ...molecular CO line profile and a ring-like structure in thermal dust emission. We investigate the molecular counterpart in high resolution, aiming to resolve the ring-like structure and identify its origin.
Methods.
Atacama Large Millimeter/submillimeter Array (ALMA) observations have been carried out to map the circumstellar envelope (CSE) of TX Psc in CO(2–1) emission and investigate the counterpart to the ring-like dust structure.
Results.
We report the detection of a thin, irregular, and elliptical detached molecular shell around TX Psc, which coincides with the dust emission. This is the first discovery of a non-spherically symmetric detached shell, raising questions about the shaping of detached shells.
Conclusions.
We investigate possible shaping mechanisms for elliptical detached shells and find that in the case of TX Psc, stellar rotation of 2 km s
−1
can lead to a non-uniform mass-loss rate and velocity distribution from stellar pole to equator, recreating the elliptical CSE. We discuss the possible scenarios for increased stellar momentum, enabling the rotation rates needed to reproduce the ellipticity of our observations, and come to the conclusion that momentum transfer of an orbiting object with the mass of a brown dwarf would be sufficient.
Context.
On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar ...material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars.
Aims.
We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass.
Methods.
We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of
Herschel
/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass.
Results.
The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 × 10
−10
M
⊙
yr
−1
and a detached shell dust mass of (2.9 ± 0.3) × 10
−5
M
⊙
. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin.
Context.
Hot subdwarfs experienced strong mass loss on the red giant branch (RGB) and are now hot and small He-burning objects. These stars constitute excellent opportunities for addressing the ...question of the evolution of exoplanetary systems directly after the RGB phase of evolution.
Aims.
In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes (
Kepler
, K2, TESS, and CHEOPS) with our custom-made pipeline SHERLOCK in order to determine the occurrence rate of planets around these stars as a function of orbital period and planetary radius. We also aim to determine whether planets that were previously engulfed in the envelope of their red giant host star can survive, even partially, as a planetary remnant.
Methods.
For this first paper, we performed injection-and-recovery tests of synthetic transits for a selection of representative
Kepler
, K2, and TESS light curves to determine which transiting bodies in terms of object radius and orbital period we will be able to detect with our tools. We also provide estimates for CHEOPS data, which we analyzed with the pycheops package.
Results.
Transiting objects with a radius ≲1.0
R
⊕
can be detected in most of the
Kepler
, K2, and CHEOPS targets for the shortest orbital periods (1 d and shorter), reaching values as low as ~0.3
R
⊕
in the best cases. Sub-Earth-sized bodies are only reached for the brightest TESS targets and for those that were observed in a significant number of sectors. We also give a series of representative results for larger planets at greater distances, which strongly depend on the target magnitude and on the length and quality of the data.
Conclusions.
The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The
Kepler
, K2, and CHEOPS data will allow us to search for planetary remnants, that is, very close and small (possibly disintegrating) objects.
Recent studies based on photometry from the Transiting Exoplanet Survey Satellite (TESS) have suggested that the dayside of KELT-1b, a strongly irradiated brown dwarf, is significantly brighter in ...visible light than what would be expected based on
Spitzer
observations in the infrared. We observed eight eclipses of KELT-1b with CHaracterising ExOPlanet Satellite (CHEOPS) to measure its dayside brightness temperature in the bluest passband observed so far, and we jointly modelled the CHEOPS photometry with the existing optical and near-infrared photometry from TESS, LBT, CFHT, and
Spitzer.
Our modelling has led to a self-consistent dayside spectrum for KELT-1b covering the CHEOPS, TESS,
H
,
Ks
, and
Spitzer
IRAC 3.6 and 4.5 µm bands, where our TESS,
H
,
Ks
, and
Spitzer
band estimates largely agree with the previous studies. However, we discovered a strong discrepancy between the CHEOPS and TESS bands. The CHEOPS observations yield a higher photometric precision than the TESS observations, but they do not show a significant eclipse signal, while a deep eclipse is detected in the TESS band. The derived TESS geometric albedo of 0.36
−0.13
+0.12
is difficult to reconcile with a CHEOPS geometric albedo that is consistent with zero because the two passbands have considerable overlap. Variability in cloud cover caused by the transport of transient nightside clouds to the dayside could provide an explanation for reconciling the TESS and CHEOPS geometric albedos, but this hypothesis needs to be tested by future observations.
Aims
. DE Boo is a unique system, with an edge-on view through the debris disk around the star. The disk, which is analogous to the Kuiper belt in the Solar System, was reported to extend from 74 to ...84 AU from the central star. The high photometric precision of the Characterising Exoplanet Satellite (CHEOPS) provided an exceptional opportunity to observe small variations in the light curve due to transiting material in the disk. This is a unique chance to investigate processes in the debris disk.
Methods
. Photometric observations of DE Boo of a total of four days were carried out with CHEOPS. Photometric variations due to spots on the stellar surface were subtracted from the light curves by applying a two-spot model and a fourth-order polynomial. The photometric observations were accompanied by spectroscopic measurements with the 1m RCC telescope at Piszkésteto and with the SOPHIE spectrograph in order to refine the astrophysical parameters of DE Boo.
Results
. We present a detailed analysis of the photometric observation of DE Boo. We report the presence of nonperiodic transient features in the residual light curves with a transit duration of 0.3–0.8 days. We calculated the maximum distance of the material responsible for these variations to be 2.47 AU from the central star, much closer than most of the mass of the debris disk. Furthermore, we report the first observation of flaring events in this system.
Conclusions
. We interpreted the transient features as the result of scattering in an inner debris disk around DE Boo. The processes responsible for these variations were investigated in the context of interactions between planetesimals in the system.
We present ALMA observations of the circumstellar envelope around the AGB carbon star TX Psc in molecular CO(2–1) emission, and detect a previously unknown detached shell with filamentary structure ...and elliptical shape. Up to now, all observed detached shells are found around carbon AGB stars and are of remarkable spherical symmetry. The elliptical shell around TX Psc is the first clear exception to that rule, with TX Psc being classified as rather ”fresh” carbon star, that most likely has only experienced very few thermal pulses yet. We investigate and discuss the 3D structure of the CSE and its most likely formation scenarios, as well as the link of this peculiar detached shell to the AGB evolutionary status of TX Psc.
Abstract
Based on hydrostatic models we discuss the effects of molecular opacities and abundance changes concerning C, N or O on the atmospheric structures, spectra and photometric properties of C/M ...AGB giants.
Detached circumstellar dust shells are detected around three carbon variables using Herschel-PACS. Two of them are already known on the basis of their thermal CO emission and two are visible as ...extensions in IRAS imaging data. By model fits to the new data sets, physical sizes, expansion timescales, dust temperatures, and more are deduced. A comparison with existing molecular CO material shows a high degree of correlation for TT Cyg and U Ant but a few distinct differences with other observables are also found.
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