arXiv subjects

Leen Decin

Publications and source records attributed to Leen Decin.

At least 73 records · Page 4Linked to original sources

arXiv2022

Grid of pseudo-2D chemistry models for tidally locked exoplanets -- II. The role of photochemistry

Photochemistry is expected to change the chemical composition of the upper atmospheres of irradiated exoplanets through the dissociation of species, such as methane and ammonia, and the association of others, such as hydrogen cyanide. Although primarily the high altitude day side should be affected by photochemistry, it is still unclear how dynamical processes transport photochemical species throughout the atmosphere, and how these chemical disequilibrium effects scale with different parameters. In this work we investigate the influence of photochemistry in a two-dimensional context, by synthesizing a grid of photochemical models across a large range of temperatures. We find that photochemistry can strongly change the atmospheric composition, even up to depths of several bar in cool exoplanets. We further identify a sweet spot for the photochemical production of hydrogen cyanide and acetylene, two important haze precursors, between effective temperatures of 800 and 1400 K. The night sides of most cool planets (effective temperature < 1800 K) are shown to host photochemistry products, transported from the day side by horizontal advection. Synthetic transmission spectra are only marginally affected by photochemistry, but we suggest that observational studies probing higher altitudes, such as high-resolution spectroscopy, take photochemistry into account.

astro-ph.EP↗

arXiv2021

Bottom-up dust nucleation theory in oxygen-rich evolved stars I. Aluminium oxide clusters

Aluminum oxide (alumina, Al$_{2}$O$_{3}$) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina \textit{seed} particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood.} {To shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments by a quantum-chemical bottom-up approach.} {Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network describing the formation and destruction of aluminium-bearing molecules and dust-forming (Al$_{2}$O$_{3}$)$_n$ clusters up to the size of dimers ($n$=2) coagulating to tetramers ($n=$4). Intermediary species include the prevalent gas-phase molecules AlO and AlOH, and Al$_x$O$_y$ clusters with $x=$1$-$5, $y=$1$-$6. The resulting extensive network is applied to two model stars, representing a semi-regular variable and a Mira-type, and to different circumstellar gas trajectories including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al$_{2}$O$_{3}$)$_n$ clusters with $n=$4$-$10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated by density functional theory. We provide energies, bond characteristics, electrostatic properties and vibrational spectra of the clusters as a function of size $n$ and compare these to corundum corresponding to the crystalline bulk limit ($n\rightarrow \infty$). {The circumstellar aluminium gas-phase chemistry in oxygen-rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H$_2$, AlO+H$_2$O, and their reverse. Models of ...}

astro-ph.SR↗

arXiv2021

The Ionization Energies of Dust-Forming Metal Oxide Clusters

Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigated the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They included clusters of magnesia (MgO)$_n$, silicon monoxide (SiO)$_n$, alumina (Al$_2$O$_3$)$_n$, and titania (TiO$_2$)$_n$ with stoichiometric sizes of $n$=1$-$8. The magnesia, alumina, and titania clusters showed relatively little variation in their ionization energies with respect to the cluster size n: 7.1$-$8.2 eV for (MgO)$_n$, 8.9$-$10.0 eV for (Al$_2$O$_3$)$_n$, and 9.3$-$10.5 eV for (TiO$_2$)$_n$. In contrast, the (SiO)$_n$ ionization energies decrease with size $n$, starting from 11.5 eV for $n$=1, and decreasing to 6.6 eV for $n$=8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral-neutral reactions.

astro-ph.GA↗

arXiv2021

Atomium: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae. I. Morpho-kinematical interpretation of CO and SiO emission

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the Atomium large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with ALMA. The morphology of the inner wind of R Hya, which has a known companion at ~3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of ~10'' that is oriented along the north-south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of ~115 deg. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is ~70 deg in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ~0.65 Msol on the basis of the highest measured speeds in the disc and the location of its inner rim at ~6 au from the AGB star.

astro-ph.SR↗

arXiv2020

Indications for very high metallicity and absence of methane for the eccentric exo-Saturn WASP-117b

We investigate the atmospheric composition of the long period ($P_{\rm orb}=$ 10 days), eccentric exo-Saturn WASP-117b. WASP-117b could be in atmospheric temperature and chemistry similar to WASP-107b. In mass and radius WASP-117b is similar to WASP-39b, which allows a comparative study of these planets. We analyze a near-infrared transmission spectrum of WASP-117b taken with Hubble Space Telescope/WFC3 G141, which was reduced with two independent pipelines. High resolution measurements were taken with VLT/ESPRESSO in the optical. We report the robust ($3σ$) detection of a water spectral feature. Using a 1D atmosphere model with isothermal temperature, uniform cloud deck and equilibrium chemistry, the Bayesian evidence of a retrieval analysis of the transmission spectrum indicates a preference for a high atmospheric metallicity ${\rm [Fe/H]}=2.58^{+0.26}_{-0.37}$ and clear skies. The data are also consistent with a lower-metallicity composition ${\rm [Fe/H]}<1.75$ and a cloud deck between $10^{-2.2} - 10^{-5.1}$ bar, but with weaker Bayesian preference. We retrieve a low CH$_4$ abundance of $<10^{-4}$ volume fraction within $1 σ$ and $<2\cdot 10^{-1}$ volume fraction within $3 σ$. We cannot constrain the equilibrium temperature between theoretically imposed limits of 700 and 1000~K. Further observations are needed to confirm quenching of CH$_4$ with $K_{zz}\geq 10^8$~cm$^2$/s. We report indications of Na and K in the VLT/ESPRESSO high resolution spectrum with substantial Bayesian evidence in combination with HST data.

astro-ph.EP↗

arXiv2020

MARVEL, a four-telescope array for high-precision radial-velocity monitoring

Since the first discovery of a planet outside of our Solar System in 1995, exoplanet research has shifted from detecting to characterizing worlds around other stars. The TESS (NASA, launched 2019) and PLATO mission (ESA, planned launch 2026) will find and constrain the size of thousands of exoplanets around bright stars all over the sky. Radial velocity measurements are needed to characterize the orbit and mass, and complete the picture of densities and composition of the exoplanet systems found. The Ariel mission (ESA, planned launch 2028) will characterize exoplanet atmospheres with infrared spectroscopy. Characterization of stellar activity using optical spectroscopy from the ground is key to retrieve the spectral footprint of the planetary atmosphere in Ariel's spectra. To enable the scientific harvest of the TESS, PLATO and Ariel space missions, we plan to install MARVEL as an extension of the existing Mercator Telescope at the Roque De Los Muchachos Observatory on La Palma (SPAIN). MARVEL consists of an array of four 80 cm telescopes linked through optical fibers to a single high-resolution echelle spectrograph, optimized for extreme-precision radial velocity measurements. It can observe the radial velocities of four different stars simultaneously or, alternatively, combine the flux from four telescopes pointing to a single faint target in one spectrum. MARVEL is constructed by a KU Leuven (Belgium) led collaboration, with contributions from the UK, Austria, Australia, Sweden, Denmark and Spain. In this paper, we present the MARVEL instrument with special focus on the optical design and expected performance of the spectrograph, and report on the status of the project.

astro-ph.IM↗

arXiv2020

Magritte, a modern software library for 3D radiative transfer: II. Adaptive ray-tracing, mesh construction and reduction

Radiative transfer is a notoriously difficult and computationally demanding problem. Yet, it is an indispensable ingredient in nearly all astrophysical and cosmological simulations. Choosing an appropriate discretization scheme is a crucial part of the simulation, since it not only determines the direct memory cost of the model but also largely determines the computational cost and the achievable accuracy. In this paper, we show how an appropriate choice of directional discretization scheme as well as spatial model mesh can help alleviate the computational cost, while largely retaining the accuracy. First, we discuss the adaptive ray-tracing scheme implemented in our 3D radiative transfer library Magritte, that adapts the rays to the spatial mesh and uses a hierarchical directional discretization based on HEALPix. Second, we demonstrate how the free and open-source software library Gmsh can be used to generate high quality meshes that can be easily tailored for Magritte. In particular, we show how the local element size distribution of the mesh can be used to optimise the sampling of both analytically and numerically defined models. Furthermore, we show that when using the output of hydrodynamics simulations as input for a radiative transfer simulation, the number of elements in the input model can often be reduced by an order of magnitude, without significant loss of accuracy in the radiation field. We demonstrate this for two models based on a hierarchical octree mesh resulting from adaptive mesh refinement (AMR), as well as two models based on smoothed-particle hydrodynamics (SPH) data.

astro-ph.IM↗

arXiv2020

Evolution and Mass Loss of Cool Ageing Stars: a Daedalean Story

The chemical enrichment of the Universe; the mass spectrum of planetary nebulae, white dwarfs and gravitational wave progenitors; the frequency distribution of Type I and II supernovae; the fate of exoplanets ... a multitude of phenomena which is highly regulated by the amounts of mass that stars expel through a powerful wind. For more than half a century, these winds of cool ageing stars have been interpreted within the common interpretive framework of 1-dimensional (1D) models. I here discuss how that framework now appears to be highly problematic. * Current 1D mass-loss rate formulae differ by orders of magnitude, rendering contemporary stellar evolution predictions highly uncertain. These stellar winds harbour 3D complexities which bridge 23 orders of magnitude in scale, ranging from the nanometer up to thousands of astronomical units. We need to embrace and understand these 3D spatial realities if we aim to quantify mass loss and assess its effect on stellar evolution. We therefore need to gauge * the 3D life of molecules and solid-state aggregates: the gas-phase clusters that form the first dust seeds are not yet identified. This limits our ability to predict mass-loss rates using a self-consistent approach. * the emergence of 3D clumps: they contribute in a non-negligible way to the mass loss, although they seem of limited importance for the wind-driving mechanism. * the 3D lasting impact of a (hidden) companion: unrecognised binary interaction has biased previous mass-loss rate estimates towards values that are too large. Only then will it be possible to drastically improve our predictive power of the evolutionary path in 4D (classical) spacetime of any star.

astro-ph.SR↗

arXiv2020

Atomium: A high-resolution view on the highly asymmetric wind of the AGB star Pi1 Gruis. I. First detection of a new companion and its effect on the inner wind

The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch star Pi1 Gruis, which has a known companion at ~440 au and is thought to harbour a second, closer-by (<10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04'' (6 au). The CO data unambiguously reveal that Pi1 Gru's circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of 38+/-3 deg with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04'' from the asymptotic giant branch star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ~6. The CO emission suggests that the spiral is approximately Archimedean within 5'', beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5'' exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ~1.1 Msol.

astro-ph.SR↗

arXiv2020

A detailed view on the circumstellar environment of the M-type AGB star EP Aquarii I. High-resolution ALMA and SPHERE observations

Cool evolved stars are known to be significant contributors to the enrichment of the interstellar medium through their dense and dusty stellar winds. High resolution observations of these outflows have shown them to possess high degrees of morphological complexity. We observed the asymptotic giant branch (AGB) star EP Aquarii with ALMA in band 6 and VLT/SPHERE/ZIMPOL in four filters the visible. Both instruments had an angular resolution of 0.025''. These are follow-up observations to the lower-resolution 2016 ALMA analysis of EP Aquarii, which revealed that its wind possesses a nearly face-on, spiral-harbouring equatorial density enhancement, with a nearly pole-on bi-conical outflow. At the base of the spiral, the SiO emission revealed a distinct emission void approximately 0.4'' to the west of the continuum brightness peak, which was proposed to be linked to the presence of a companion. The new ALMA data better resolve the inner wind and reveal that its morphology as observed in CO is consistent with hydrodynamical companion-induced perturbations. Assuming that photodissociation by the UV-field of the companion is responsible for the emission void in SiO, we deduced the spectral properties of the tentative companion from the size of the hole. We conclude that the most probable companion candidate is a white dwarf with a mass between 0.65 and 0.8 Msol , though a solar-like companion could not be definitively excluded. The radial SiO emission shows periodic, low-amplitude perturbations. We tentatively propose that they could be the consequence of the interaction of the AGB wind with another much closer low-mass companion. The polarised SPHERE/ZIMPOL data show a circular signal surrounding the AGB star with a radius of 0.1''...

astro-ph.SR↗

arXiv2020

Equatorial anti-rotating day side wind flow in WASP-43b elicited by deep wind jets?

We present WASP-43b climate simulations with deep wind jets (down to 700~bar) that are linked to retrograde (westward) flow at the equatorial day side for $p<0.1$~bar. Retrograde flow inhibits efficient eastward heat transport and naturally explains the small hotspot shift and large day-night-side gradient of WASP-43b ($P_{\text{orb}}=P_{\text{rot}}=0.8135$~days) observed with Spitzer. We find that deep wind jets are mainly associated with very fast rotations ($P_{\text{rot}}=P_{\text{orb}}\leq 1.5$~days) which correspond to the Rhines length smaller than $2$ planetary radii. We also diagnose wave activity that likely gives rise to deviations from superrotation. Further, we show that we can achieve full steady state in our climate simulations by imposing a deep forcing regime for $p>10$~bar: convergence time scale $τ_{\text{conv}}=10^6-10^8$~s to a common adiabat, as well as linear drag at depth ($p\geq 200$~bar), which mimics to first order magnetic drag. Lower boundary stability and the deep forcing assumptions were also tested with climate simulations for HD~209458b ($P_{\text{orb}}=P_{\text{rot}}=3.5$~days). HD~209458b simulations always show shallow wind jets (never deeper than 100~bar) and unperturbed superrotation. If we impose a fast rotation ($P_{\text{orb}}=P_{\text{rot}}=0.8135$~days), also the HD~209458b-like simulation shows equatorial retrograde flow at the day side. We conclude that the placement of the lower boundary at $p=200$~bar is justified for slow rotators like HD~209458b, but we suggest that it has to be placed deeper for fast-rotating, dense hot Jupiters ($P_{\text{orb}}\leq 1.5$~days) like WASP-43b. Our study highlights that the deep atmosphere may have a strong influence on the observable atmospheric flow in some hot Jupiters.

astro-ph.EP↗

arXiv2019

Magritte, a modern software library for 3D radiative transfer: I. Non-LTE atomic and molecular line modelling

Radiative transfer is a key component in almost all astrophysical and cosmological simulations. We present Magritte: a modern open-source software library for 3D radiative transfer. It uses a deterministic ray-tracer and formal solver, i.e. it computes the radiation field by tracing rays through the model and solving the radiative transfer equation in its second-order form along a fixed set of rays originating from each point. Magritte can handle structured and unstructured input meshes, as well as smoothed-particle hydrodynamics (SPH) particle data. In this first paper, we describe the numerical implementation, semi-analytic tests and cross-code benchmarks for the non-LTE line radiative transfer module of Magritte. This module uses the radiative transfer solver to self-consistently determine the populations of the quantised energy levels of atoms and molecules using an accelerated Lambda iteration (ALI) scheme. We compare Magritte with the established radiative transfer solvers Ratran (1D) and Lime (3D) on the van Zadelhoff benchmark and present a first application to a simple Keplerian disc model. Comparing with Lime, we conclude that Magritte produces more accurate and more precise results, especially at high optical depth, and that it is faster.

astro-ph.IM↗

arXiv2019

Measurements of Low Temperature Rate Coefficients for the Reaction of CH with CH$_{2}$O and Application to Dark Cloud and AGB Stellar Wind Models

Rate coefficients have been measured for the reaction of CH radicals with formaldehyde, CH$_{2}$O, over the temperature range 31 - 133 K using a pulsed Laval nozzle apparatus combined with pulsed laser photolysis and laser induced fluorescence spectroscopy. The rate coefficients are very large and display a distinct decrease with decreasing temperature below 70 K, although classical collision rate theory fails to reproduce this temperature dependence. The measured rate coefficients have been parameterized and used as input for astrochemical models for both dark cloud and AGB stellar outflow scenarios. The models predict a distinct change (up to a factor of two) in the abundance of ketene, H$_{2}$CCO, which is the major expected molecular product of the CH + CH$_{2}$O reaction.

astro-ph.SR↗

arXiv2019

Developing a self-consistent AGB wind model: II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture

Unravelling the composition and characteristics of gas and dust lost by asymptotic giant branch (AGB) stars is important as these stars play a vital role in the chemical life cycle of galaxies. The general hypothesis of their mass loss mechanism is a combination of stellar pulsations and radiative pressure on dust grains. However, current models simplify dust formation, which starts as a microscopic phase transition called nucleation. Various nucleation theories exist, yet all assume chemical equilibrium, growth restricted by monomers, and commonly use macroscopic properties for a microscopic process. Such simplifications for initial dust formation can have large repercussions on the type, amount, and formation time of dust. By abandoning equilibrium assumptions, discarding growth restrictions, and using quantum mechanical properties, we have constructed and investigated an improved nucleation theory in AGB wind conditions for four dust candidates, TiO$_2$, MgO, SiO and Al$_2$O$_3$. This paper reports the viability of these candidates as first dust precursors and reveals implications of simplified nucleation theories. Monomer restricted growth underpredicts large clusters at low temperatures and overpredicts formation times. Assuming the candidates are present, Al$_2$O$_3$ is the favoured precursor due to its rapid growth at the highest considered temperatures. However, when considering an initially atomic chemical mixture, only TiO$_2$-clusters form. Still, we believe Al$_2$O$_3$ to be the prime candidate due to substantial physical evidence in presolar grains, observations of dust around AGB stars at high temperatures, and its ability to form at high temperatures and expect the missing link to be insufficient quantitative data of Al-reactions.

astro-ph.SR↗

arXiv2018

A global optimisation study of the low-lying isomers of the alumina octomer (Al$_2$O$_3$)$_8$

We employ the Monte-Carlo Basin-Hopping (MC-BH) global optimisation technique with inter- atomic pair potentials to generate low-energy candidates of stoichiometric alumina octomers ((Al$_2$O$_3$)$_8$). The candidate structures are subsequently refined with density functional theory calculations employing hybrid functionals (B3LYP and PBE0) and a large basis set (6-311+G(d)) including a vibrational analysis. We report the discovery of a set of energetically low-lying alumina octomer clusters, including a new global minimum candidate, with shapes that are elongated rather than spherical. We find a stability limit for these and smaller-sized clusters at a temperature of $T\simeq1300-1450$ K corresponding to a phase transition in liquid alumina.

cond-mat.mtrl-sci↗

arXiv2018

Developing a self-consistent AGB wind model: I. Chemical, thermal, and dynamical coupling

The material lost through stellar winds of Asymptotic Giant Branch (AGB) stars is one of the main contributors to the chemical enrichment of galaxies. The general hypothesis of the mass loss mechanism of AGB winds is a combination of stellar pulsations and radiative pressure on dust grains, yet current models still suffer from limitations. Among others, they assume chemical equilibrium of the gas, which may not be justified due to rapid local dynamical changes in the wind. This is important as it is the chemical composition that regulates the thermal structure of the wind, the creation of dust grains in the wind, and ultimately the mass loss by the wind. Using a self-consistent hydrochemical model, we investigated how non-equilibrium chemistry affects the dynamics of the wind. This paper compares a hydrodynamical and a hydrochemical dust-free wind, with focus on the chemical heating and cooling processes. No sustainable wind arises in a purely hydrodynamical model with physically reasonable pulsations. Moreover, temperatures are too high for dust formation to happen, rendering radiative pressure on grains impossible. A hydrochemical wind is even harder to initiate due to efficient chemical cooling. However, temperatures are sufficiently low in dense regions for dust formation to take place. These regions occur close to the star, which is needed for radiation pressure on dust to sufficiently aid in creating a wind. Extending this model self-consistently with dust formation and evolution, and including radiation pressure, will help to understand the mass loss by AGB winds.

astro-ph.SR↗

arXiv2018

The Transiting Exoplanet Community Early Release Science Program for JWST

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

astro-ph.EP↗

arXiv2018

On the detection of CO and mass loss of Bulge OH/IR stars

We report on the succesful search for CO (2-1) and (3-2) emission associated with OH/IR stars in the Galactic Bulge. We observed a sample of eight extremely red AGB stars with the APEX telescope and detected seven. The sources were selected at sufficient high galactic latitude to avoid interference by interstellar CO, which hampered previous studies of inner galaxy stars. To study the nature of our sample and the mass loss we constructed the SEDs from photometric data and Spitzer IRS spectroscopy. In a first step we apply radiative transfer modelling to fit the SEDs and obtain luminosities and dust mass loss rates (MLR). Through dynamical modelling we then retrieve the total MLR and the gas-to-dust ratios. We derived variability periods of our stars. The luminosities range between approximately 4000 and 5500 Lsun and periods are below 700 days. The total MLR ranges between 1E-5 and 1E-4 Msun/yr. Comparison with evolutionary models shows that the progenitor mass is approximately 1.5 Msun, similar to the Bulge Miras and are of intermediate age (3 Gyr). The gas-to-dust ratios are between 100 and 400 and are similar to what is found for OH/IR stars in the galactic Disk. One star, IRAS 17347-2319, has a very short period of approximately 300 days which may be decreasing further. It may belong to a class of Mira variables with a sudden change in period as observed in some Galactic objects. It would be the first example of an OH/IR star in this class and deserves further follow-up observations.

astro-ph.SR↗