arXiv subjects

Leen Decin

Publications and source records attributed to Leen Decin.

At least 91 records · Page 5Linked to original sources

arXiv2018

An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii

High-resolution interferometric observations of the circumstellar environments of AGB stars show a variety of morphologies. Guided by the unusual carbon monoxide line profile of the AGB star EP Aquarii, we have observed its circumstellar environment with ALMA band 6 in cycle 4. We describe the morphological complexity of the CO, SiO and SO2 molecular emission. The CO emission exhibits the characteristics of a bi-conical wind with a bright nearly face-on spiral feature around the systemic velocity. This is the first convincing detection of a spiral morphology in an O-rich wind. Based on the offsets of the centers of the two bi-conical wind hemispheres, we deduce the position angle of the inclination axis to be ~150 deg measured counterclockwise from north. Based on the velocity width of the spiral signature we estimate the inclination angle of the system to be between 4 deg and 18 deg. The central emission zone exhibits a morphology that resembles simulations modelling the spiral-inducing wind-Roche-lobe-overflow mechanism. Though the spiral may be a (companion-induced) density enhancement in the stellar outflow, the extremely narrow width of the spiral signature in velocity space suggests that it may be a hydrodynamical perturbation in a face-on differentially rotating disk. The SiO emission does not show the spiral, but exhibits a local emission void approximately 0.5'' west of the continuum brightness peak. We hypothesise that this may be a local environment caused by the presence of a (stellar) companion with a mass of at most 0.1 Msol, based on its non-detection in the continuum...

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arXiv2018

ALMA detection of a tentative nearly edge-on rotating disk around the nearby AGB star R~Doradus

A spectral scan of the circumstellar environment of the asymptotic giant branch (AGB) star R~Doradus was taken with ALMA in cycle 2 at frequencies between 335 and 362 GHz and with a spatial resolution of $\sim$150 milliarcseconds. Many molecular lines show a spatial offset between the blue and red shifted emission in the innermost regions of the wind. The position-velocity diagrams of this feature, in combination with previous SPHERE data and theoretical work point towards the presence of a compact differentially rotating disk, orientated nearly edge-on. We model the $^{\rm 28}$SiO ($v=1,~J=8\to7$) emission with a disk model. We estimate the disk mass and angular momentum to be $3 \times 10^{-6}$ Solar masses and $5 \times 10^{40}\ {\rm m^2 kg/s}$. The latter presents an `angular momentum problem' that may be solved by assuming that the disk is the result of wind-companion interactions with a companion of at least 2.5 earth masses, located at 6 AU, the tentatively determined location of the disk's inner rim. An isolated clump of emission is detected to the south-east with a velocity that is high compared to the previously determined terminal velocity of the wind. Its position and mean velocity suggest that it may be associated with a companion planet, located at the disk's inner rim.

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arXiv2018

Simplified models of circumstellar morphologies for interpreting high-resolution data. Analytical approach to the equatorial density enhancement

Equatorial density enhancements (EDEs) are a very common astronomical phenomenon. Studies of the circumstellar environments (CSE) of young stellar objects and of evolved stars have shown that these objects often possess these features. These are believed to originate from different mechanisms, ranging from binary interactions to the gravitational collapse of interstellar material. Quantifying the effect of the presence of this type of EDE on the observables is essential for a correct interpretation of high-resolution data. We seek to investigate the manifestation in the observables of a circumstellar EDE, to assess which properties can be constrained, and to provide an intuitive bedrock on which to compare and interpret upcoming high-resolution data (e.g. \emph{ALMA} data) using 3D models. We develop a simplified analytical parametrised description of a 3D EDE, with a possible substructure such as warps, gaps, and spiral instabilities. In addition, different velocity fields (Keplerian, radial, super-Keplerian, sub-Keplerian and rigid rotation) are considered. The effect of a bipolar outflow is also investigated. The geometrical models are fed into the 3D radiative transfer code {\tt LIME}, that produces 3D intensity maps throughout velocity space. We investigate the spectral signature of the $J$=3$-$2 up to $J$=7$-$6 rotational transitions of CO in the models, as well as the spatial aspect of this emission by means of channel maps, wide-slit position-velocity (PV) diagrams, stereograms, and spectral lines. Additionally, we discuss methods of constraining the geometry of the EDE, the inclination, the mass-contrast between the EDE and the bipolar outflow, and the global velocity field. Finally, we simulated \emph{ALMA} observations...

astro-ph.SR↗

arXiv2018

ALMA observations of the nearby AGB star L$_{\rm 2}$ Puppis. II. Gas disk properties derived from $^{\rm 12}$CO and $^{\rm13}$CO $J=$3$-$2 emission

The circumstellar environment of the AGB star L$_{\rm 2}$ Puppis was observed with ALMA in cycle 3, with a resolution of $15 \times 18 \rm\ mas$. The molecular emission shows a differentially rotating disk, inclined to a nearly edge-on position. In the first paper in this series (paper I) the molecular emission was analysed to accurately deduce the motion of the gas in the equatorial regions of the disk. In this work we model the optically thick $^{\rm 12}$CO $J=$3$-$2 and the optically thin $^{\rm 13}$CO $J=$3$-$2 rotational transition to constrain the physical conditions in the disk. To realise this effort we make use of the 3D NLTE radiative transfer code {\tt LIME}. The temperature structure and velocity structure show a high degree of complexity, both radially and vertically. The radial H$_{\rm 2}$ density profile in the disk plane is characterised by a power law with a slope of $-3.1$. We find a $^{\rm 12}$CO over $^{\rm 13}$CO abundance ratio of 10 inside the disk. Finally, estimations of the angular momentum in the disk surpass the expected available angular momentum of the star, strongly supporting the indirect detection of a compact binary companion reported in paper I. We estimate the mass of the companion to be around 1 Jupiter mass.

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arXiv2017

Stratosphere circulation on tidally locked ExoEarths

Stratosphere circulation is important to interpret abundances of photo-chemically produced compounds like ozone that we aim to observe to assess habitability of exoplanets. We thus investigate a tidally locked ExoEarth scenario for TRAPPIST-1b, TRAPPIST-1d, Proxima Centauri~b and GJ 667 C~f with a simplified 3D atmosphere model and for different stratospheric wind breaking assumptions. These planets are representatives for different circulation regimes for orbital periods: $P_{orb}=1-100$~days. The circulation of exoplanets with $P_{orb} \leq $ 25~days can be dominated by the standing tropical Rossby wave in the troposphere and also in the stratosphere: It leads to a strong equatorial eastward wind jet and to 'Anti-Brewer-Dobson'-circulation that confines air masses to the stratospheric equatorial region. Thus, the distribution of photo-chemically produced species and aerosols may be limited to an 'equatorial transport belt'. In contrast, planets with $P_{orb}>25$~days, like GJ~667~C~f, exhibit efficient thermally driven circulation in the stratosphere that allows for a day side-wide distribution of air masses. The influence of the standing tropical Rossby waves on tidally locked ExoEarths with $P_{orb} \leq 25$~days can, however, be circumvented with deep stratospheric wind breaking alone - allowing for equator-to-pole transport like on Earth. For planets with $3 \leq P_{orb} \leq 6$~days, the extratropical Rossby wave acts as an additional safe-guard against the tropical Rossby wave in case of shallow wind breaking. Therefore, TRAPPIST-1d is less prone to have an equatorial transport belt in the stratosphere than Proxima~Centauri~b. Even our Earth model shows an equatorial wind jet, if stratosphere wind breaking is inefficient.

physics.ao-ph↗

arXiv2017

The close circumstellar environment of Betelgeuse - V. Rotation velocity and molecular envelope properties from ALMA

We observed Betelgeuse using ALMA's extended configuration in band 7 (f~340 GHz, λ~0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the 28SiO(v=2,J=8-7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of v_eq sin i = 5.47 +/- 0.25 km/s at the equivalent continuum angular radius R_star = 29.50 +/- 0.14 mas. This corresponds to an angular rotation velocity of ω sin i = (5.6 +/- 1.3) x 10^(-9) rad/s. The position angle of its north pole is PA = 48.0 +/- 3.5°. The rotation period of Betelgeuse is estimated to P/sin i = 36 +/- 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ~10 au (45 mas or 1.5x the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong "rogue" convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.

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arXiv2017

Radiation-pressure-driven sub-Keplerian rotation of the disc around the AGB star L2 Pup

We study the sub-Keplerian rotation and dust content of the circumstellar material around the asymptotic giant branch (AGB) star L$_2$ Puppis. We find that the thermal pressure gradient alone cannot explain the observed rotation profile. We find that there is a family of possible dust populations for which radiation pressure can drive the observed sub-Keplerian rotation. This set of solutions is further constrained by the spectral energy distribution (SED) of the system, and we find that a dust-to-gas mass ratio of $\sim10^{-3}$ and a maximum grain size that decreases radially outwards can satisfy both the rotation curve and SED. These dust populations are dynamically tightly coupled to the gas azimuthally. However grains larger than $\sim0.5μ$m are driven outward radially by radiation pressure at velocities $\sim5$kms$^{-1}$, which implies a dust replenishment rate of $\sim3\times10^{-9}$M$_\odot$yr$^{-1}$. This replenishment rate is consistent with observational estimates to within uncertainties. Coupling between the radial motion of the dust and gas is weak and hence the gas does not share in this rapid outward motion. Overall we conclude that radiation pressure is a capable and necessary mechanism to explain the observed rotation profile of L$_2$ Pup, and offers other additional constraints on the dust properties.

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arXiv2016

Influence of Stellar Flares on the Chemical Composition of Exoplanets and Spectra

More than 3000 exoplanets have been detected so far, and more and more spectroscopic observations of exoplanets are performed. Future instruments are eagerly awaited as they will be able to provide spectroscopic data with a greater accuracy and sensitivity than what is currently available. An important aspect to consider is temporal stellar atmospheric disturbances that can influence the planetary composition, and hence spectra, and potentially can lead to incorrect assumptions about the steady-state atmospheric composition of the planet. We focus on perturbations that come from the host star in the form of flare events that significantly increase the photon flux impingement on the exoplanet atmosphere. In some cases, and particularly for M stars, this sudden increase may last for several hours. We aim at answering the question to what extent a stellar flare is able to modify the chemical composition of the planetary atmosphere and, therefore influence the resulting spectra. We use a 1D thermo-photochemical model to study the neutral atmospheric composition of two hypothetic planets located around the star AD Leo. This active star has already been observed during a flare. We use the spectroscopic data from this flare event to simulate the evolution of the chemical composition of the atmospheres of the two hypothetic planets. We compute synthetic spectra to evaluate the implications for observations. The increase of the incoming photon flux affects the chemical abundances of some important species down to altitudes associated with an atmospheric pressure of 1 bar, that can lead to variations in planetary spectra if performed during transit.

astro-ph.EP↗

arXiv2015

Connecting the dots II: Phase changes in the climate dynamics of tidally locked terrestrial exoplanets

We investigate 3D atmosphere dynamics for tidally locked terrestrial planets with an Earth-like atmosphere and irradiation for different rotation periods ($P_{rot}=1-100$ days) and planet sizes ($R_P=1-2 R_{Earth}$) with unprecedented fine detail. We could precisely identify three climate state transition regions that are associated with phase transitions in standing tropical and extra tropical Rossby waves. We confirm that the climate on fast rotating planets may assume multiple states ($P_{rot}\leq 12$ days for $R_P=2 R_{Earth}$). Our study is, however, the first to identify the type of planetary wave associated with different climate states: The first state is dominated by standing tropical Rossby waves with fast equatorial superrotation. The second state is dominated by standing extra tropical Rossby waves with high latitude westerly jets with slower wind speeds. For very fast rotations ($P_{rot}\leq 5$~days for $R_P=2 R_{Earth}$), we find another climate state transition, where the standing tropical and extra tropical Rossby wave can both fit on the planet. Thus, a third state with a mixture of the two planetary waves becomes possible that exhibits three jets. Different climate states may be observable, because the upper atmosphere's hot spot is eastward shifted with respect to the substellar point in the first state, westward shifted in the second state and the third state shows a longitudinal 'smearing' of the spot across the substellar point. We show, furthermore, that the largest fast rotating planet in our study exhibits atmosphere features known from hot Jupiters like fast equatorial superrotation and a temperature chevron in the upper atmosphere.

astro-ph.EP↗

arXiv2015

Simplified models of stellar wind anatomy for interpreting high-resolution data: Analytical approach to embedded spiral geometries

Recent high-resolution observations have shown stellar winds to harbour complexities which strongly deviate from spherical symmetry, generally assumed as standard wind model. One such morphology is the archimedean spiral, generally believed to be formed by binary interactions, which has been directly observed in multiple sources. We seek to investigate the manifestation in the observables of spiral structures embedded in the spherical outflows of cool stars. We aim to provide an intuitive bedrock with which upcoming ALMA data can be compared and interpreted. By means of an extended parameter study, we model rotational CO emission from the stellar outflow of asymptotic giant branch stars. To this end, we develop a simplified analytical parametrised description of a 3D spiral structure. This model is embedded into a spherical wind, and fed into the 3D radiative transfer code LIME, which produces 3D intensity maps throughout velocity space. Subsequently, we investigate the spectral signature of rotational transitions of CO of the models, as well as the spatial aspect of this emission by means of wide-slit PV diagrams. Additionally, the potential for misinterpretation of the 3D data in a 1D context is quantified. Finally, we simulate ALMA observations to explore the impact of interefrometric noise and artifacts on the emission signatures. The spectral signatures of the CO rotational transition v=0 J=3-2 are very efficient at concealing the dual nature of the outflow. Only a select few parameter combinations allow for the spectral lines to disclose the presence of the spiral structure. The inability to disentangle the spiral from the spherical signal can result in an incorrect interpretation in a 1D context. Consequently, erroneous mass loss rates would be calculated...

astro-ph.SR↗

arXiv2015

The EChO science case

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? What causes the exceptional diversity observed as compared to the Solar System? EChO (Exoplanet Characterisation Observatory) has been designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large and diverse planet sample within its four-year mission lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300K-3000K) of F to M-type host stars. Over the next ten years, several new ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets. Placing the satellite at L2 provides a cold and stable thermal environment, as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. A 1m class telescope is sufficiently large to achieve the necessary spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16 micron) and SNR to be achieved by EChO, thanks to its high stability and dedicated design, would enable a very accurate measurement of the atmospheric composition and structure of hundreds of exoplanets.

astro-ph.EP↗

arXiv2015

New chemical scheme for studying carbon-rich exoplanet atmospheres

Atmospheres with a high C/O ratio are expected to contain an important quantity of hydrocarbons, including heavy molecules (with more than 2 carbon atoms). To study correctly these C-rich atmospheres, a chemical scheme adapted to this composition is necessary. We have implemented a chemical scheme that can describe the kinetics of species with up to 6 carbon atoms. This chemical scheme has been developed with specialists of combustion and validated through experiments on a wide range of T and P. This chemical network is available on the online database KIDA. We have created a grid of 12 models to explore different thermal profiles and C/O ratios. For each of them, we have compared the chemical composition determined with a C0-C2 chemical scheme (species with up to 2 carbon atoms) and with the C0-C6 scheme. We found no difference in the results obtained with the two schemes when photolyses are not included in the model, whatever the temperature of the atmosphere. In contrast, when there is photochemistry, differences can appear in the upper atmosphere. These differences are found for all the tested PT profiles in the case that the C/O ratio is above 1. When the C/O ratio of the atmosphere is solar, differences are only found at temperatures lower than 1000K. The differences linked to the use of different chemical schemes do not have important influence on the synthetic spectra. However, we have confirmed that C2H2 and HCN as possible tracers of warm C-rich atmospheres. The use of this new chemical scheme is mandatory to model atmospheres with a high C/O ratio and, in particular, if one is interested in studying in details the photochemistry. If one is just interested in the synthetic spectra, the use of a smaller scheme may be sufficient.

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arXiv2014

Influence of different parameters on the chemical composition of warm neptunes

We developed a 1D photo-thermochemical model to study the atmosphere of warm exoplanets. The chemical scheme used in this model is completely new in planetology and has been constructed in collaboration with specialists of combustion. It has been validated as a whole through experiments on a large range of temperature (300 - 2500 K) and pressure (1 mbar - 100 bar), allowing to study a wide variety of exoplanets. We have used this chemical model to study the atmosphere of two warm Neptunes, GJ3470b and GJ436b, and the influence of different parameters (vertical mixing, metallicity, temperature, . . . ) on their chemical composition. We present here the results obtained in these studies.

astro-ph.EP↗

arXiv2014

A stubbornly large mass of cold dust in the ejecta of Supernova 1987A

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 micron [O I] line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 micron flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5+-0.1 Msun of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 Msun of amorphous carbon and 0.5 Msun of silicates, totalling 0.8 Msun of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies.

astro-ph.SR↗

arXiv2014

Connecting the dots: A versatile model for the atmospheres of tidally locked Super-Earths

Radiative equilibrium temperatures are calculated for the troposphere of a tidally locked Super-Earth based on a simple greenhouse model, using Solar System data as a guideline. These temperatures provide in combination with a Newtonian relaxation scheme thermal forcing for a 3D atmosphere model using the dynamical core of the Massachusetts Institute of Technology global circulation model (MITgcm). Our model is of the same conceptional simplicity than the model of Held & Suarez (1994) and is thus computationally fast. Furthermore, because of the coherent, general derivation of radiative equilibrium temperatures, our model is easily adaptable for different planets and atmospheric scenarios. As a case study relevant for Super-Earths, we investigate a Gl581g-like planet with Earth-like atmosphere and irradiation and present results for two representative rotation periods of $P_{rot}$ = 10 days and $P_{rot}$ = 36.5 days. Our results provide proof of concept and highlight interesting dynamical features for the rotating regime 3 < $P_{rot}$ < 100 days, which was shown by Edson et al. (2011) to be an intermediate regime between equatorial superrotation and divergence. We confirm that the $P_{rot}$ = 10 days case is more dominated by equatorial superrotation dynamics than the $P_{rot}$ = 36.5 days case, which shows diminishing influence of standing Rossby- Kelvin waves and increasing influence of divergence at the top of the atmosphere. We argue that this dynamical regime change relates to the increase in Rossby deformation radius, in agreement with previous studies. However, we also pay attention to other features that are not or only in partial agreement with other studies, like, e.g., the number of circulation cells and their strength, the role and extent of thermal inversion layers, and the details of heat transport

astro-ph.EP↗

arXiv2014

Using numerical models of bow shocks to investigate the circumstellar medium of massive stars

Many massive stars travel through the interstellar medium at supersonic speeds. As a result they form bow shocks at the interface between the stellar wind. We use numerical hydrodynamics to reproduce such bow shocks numerically, creating models that can be compared to observations. In this paper we discuss the influence of two physical phenomena, interstellar magnetic fields and the presence of interstellar dust grains on the observable shape of the bow shocks of massive stars. We find that the interstellar magnetic field, though too weak to restrict the general shape of the bow shock, reduces the size of the instabilities that would otherwise be observed in the bow shock of a red supergiant. The interstellar dust grains, due to their inertia can penetrate deep into the bow shock structure of a main sequence O-supergiant, crossing over from the ISM into the stellar wind. Therefore, the dust distribution may not always reflect the morphology of the gas. This is an important consideration for infrared observations, which are dominated by dust emission. Our models clearly show, that the bow shocks of massive stars are useful diagnostic tools that can used to investigate the properties of both the stellar wind as well as the interstellar medium.

astro-ph.SR↗

arXiv2013

Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?

Many evolved stars travel through space at supersonic velocities, which leads to the formation of bow shocks ahead of the star where the stellar wind collides with the interstellar medium (ISM). Herschel observations of the bow shock of $α$-Orionis show that the shock is almost free of instabilities, despite being, at least in theory, subject to both Kelvin-Helmholtz and Rayleigh-Taylor instabilities. A possible explanation for the lack of instabilities lies in the presence of an interstellar magnetic field. We wish to investigate whether the magnetic field of the interstellar medium (ISM) in the Orion arm can inhibit the growth of instabilities in the bow shock of $α$-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic simulations of a circumstellar bow shock, using the wind parameters derived for $α$-Orionis and interstellar magnetic field strengths of $B\,=\,1.4,\, 3.0$, and $5.0\, μ$G, which fall within the boundaries of the observed magnetic field strength in the Orion arm of the Milky Way. Our results show that even a relatively weak magnetic field in the interstellar medium can suppress the growth of Rayleigh-Taylor and Kelvin-Helmholtz instabilities, which occur along the contact discontinuity between the shocked wind and the shocked ISM. The presence of even a weak magnetic field in the ISM effectively inhibits the growth of instabilities in the bow shock. This may explain the absence of such instabilities in the Herschel observations of $α$-Orionis.

astro-ph.SR↗

arXiv2013

Herschel/HIFI search for H2-17O and H2-18O in IRC+10216: constraints on models for the origin of water vapor

We report the results of a sensitive search for the minor isotopologues of water, H2-17O and H2-18O, toward the carbon-rich AGB star IRC+10216 (a.k.a. CW Leonis) using the HIFI instrument on the Herschel Space Observatory. This search was motivated by the fact that any detection of isotopic enhancement in the H2-17O and H2-18O abundances would have strongly implicated CO photodissociation as the source of the atomic oxygen needed to produce water in a carbon-rich circumstellar envelope. Our observations place an upper limit of 1/470 on the H2-17O/H2-16O abundance ratio. Given the isotopic 17O/16O ratio of 1/840 inferred previously for the photosphere of IRC+10216, this result places an upper limit of a factor 1.8 on the extent of any isotope-selective enhancement of H2-17O in the circumstellar material, and provides an important constraint on any model that invokes CO photodissociation as the source of O for H2O production. In the context of the clumpy photodissociation model proposed previously for the origin of water in IRC+10216, our limit implies that 12C-16O (not 13C-16O or SiO) must be the dominant source of 16O for H2O production, and that the effects of self-shielding can only have reduced the 12C-16O photodissociation rate by at most a factor ~ 2.

astro-ph.GA↗