arXiv subjects

Keiichi Ohnaka

Publications and source records attributed to Keiichi Ohnaka.

At least 19 recordsLinked to original sources

arXiv2026

A phoenix rises from the ashes: WOH G64 is still a red supergiant, for now

For a long time, WOH G64 was known as the most extreme red supergiant outside our Galaxy. However, in a matter of years it has faded, its pulsations have become suppressed and the spectrum has become dominated by emission lines from ionised gas, a far cry from the Mira-like pulsation and late M-type spectrum it used to display. Around the same time, a hot dust cloud was discovered using the VLT interferometer. WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution. Here we present spectra of WOH G64 obtained with the Southern African Large Telescope (SALT) between November 2024 and December 2025. Molecular absorption bands from TiO are seen at all times. This implies that WOH G64 is currently a red supergiant, and may never have ceased to be. However, the shallow, resolved bands and possible detection of VO hint at a highly extended atmosphere. The continuum appears to be varying, while the line emission shows a different behaviour, suggesting two separate components in the system. Meanwhile, atomic absorption lines are deepening. This places important constraints on scenarios for the dramatic events that are unfolding.

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arXiv2025

Identification and characterization of nascent planetary nebulae with OH and H$_2$O masers

Stars like the Sun expel their outer layers and form planetary nebulae (PNe) as they evolve into white dwarfs. PNe exhibit diverse morphologies, the origins of which are not fully understood. PNe with OH (OHPNe) and H$_{2}$O (H$_{2}$OPNe) masers are thought to be nascent PNe. However, the number of known OHPNe and H$_{2}$OPNe remains small, and only in eight cases the position of the maser emission has been found to coincide with the PN, using the high astrometric accuracy of interferometric observations. In order to identify more OHPNe and H$_{2}$OPNe, we used public databases and our own ATCA/VLA observations to match the positions of OH and H$_{2}$O masers with known PNe and radio continuum emitters, considering radio continuum emission as a possible tracer of the photoionized gas that characterizes PNe. Here we report the confirmation of positional coincidence of maser emission with one more PN, and 12 PN candidates. Moreover, we have confirmed three evolved stars as `water fountains' (WFs) hosting H$_2$O masers. These WFs are associated with radio continuum emission, but their possible nature as PNe has not yet been confirmed. Although a final characterization of maser-emitting PNe as a group still requires confirmation of more objects, their distribution in the infrared color-color diagrams suggests that they are a heterogeneous group of PNe. In particular, the new OHPN IRAS 07027$-$7934 has been reported to contain a late [WC]-type central star, while the maser emission implies an O-rich envelope. This property is found in only one other known maser-emitting PN, although we found evidence that other confirmed and candidate OHPNe may also have mixed chemistry, since they show emission from polycyclic aromatic hydrocarbons. The new WF IRAS 18443$-$0231 shows radio continuum that is dominated by strong and variable non-thermal emission, as in magnetized outflows.

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arXiv2025

Low- and high-velocity `water fountains': different evolutionary stages

`Water fountains' (WFs) are optically obscured evolved stars, most of them thought to be in the post-asymptotic giant branch (post-AGB) phase, characterized by H$_{2}$O maser emission tracing molecular jets. Interestingly, four WFs (IRAS 15445$-$5449, IRAS 18019$-$2216, IRAS 18443$-$0231, and IRAS 18464$-$0140) and one WF candidate (IRAS 18480+0008) are potential planetary nebulae (PNe) because they exhibit radio continuum emission, suggesting the presence of a photoionized region characteristic of PNe. To classify these objects, we obtained K-band (2.0-2.3 $μ$m) spectra of these WFs, including the only WF PN known (IRAS 15103$-$5754) for comparison. Our spectra reveal two group of sources: (i) `low-velocity' WFs with H$_2$O maser velocity spread of $\lesssim$50 km s$^{-1}$ (IRAS 18019$-$2216, IRAS 18464$-$0140, and IRAS 18480+0008) showing the CO band at 2.29 $μ$m in absorption, typical of cool giant stars, and no emission lines; and (ii) `high-velocity' WFs, velocity spread of $\gtrsim$50 km s$^{-1}$ (IRAS 15103$-$5754, IRAS 15445$-$5449, and IRAS 18443$-$0231), exhibiting emission lines of Br$γ$, He I, and H$_2$, consistent with hotter central stars and/or shock-excited emission. The emission line ratios of these lines in IRAS 18443$-$0231 indicates that it may be a nascent PN. The spectrum of IRAS 15445$-$5449 also shows a CO band and Na I doublet in emission, suggesting the presence of a compact circumstellar disk and/or active mass loss. These results favor the previously suggested notion that the difference between low- and high-velocity WFs is not simply a projection effect but reflects intrinsically different evolutionary stages. Moreover, the results are also consistent with the idea of an increase in the jet ejection velocity as the post-AGB evolution proceeds.

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arXiv2025

Red Giant Winds Driven by Alfvén Waves with Magnetic Diffusion

We investigate the driving mechanism of Alfvén wave-driven stellar winds from red giant stars, Arcturus ($α$ Boo; K1.5 III) and Aldebaran ($α$ Tau; K5 III), with nonideal MHD simulations in 1D super-radially open flux tubes. Since the atmosphere is not fully ionized, upward propagating Alfvénic waves excited by surface convection are affected by ambipolar diffusion. Our fiducial run with the nonideal MHD effect for $α$ Boo gives a time-averaged mass-loss rate, $\dot{M}=3.3\times 10^{-11}M_{\odot}$/yr, which is more than one order of magnitude reduced from the result in the ideal MHD run and nicely explains the observational value. Magnetized hot bubbles with $T\gtrsim 10^6$ K are occasionally present simultaneously with cool gas with a few $10^3$ K in the atmosphere because of the thermal instability triggered by radiative cooling; there coexist fully ionized plasma emitting soft X-rays and molecules absorbing/emitting infrared radiations. The inhomogeneity in the atmosphere also causes large temporal variations in $\dot{M}$ within an individual magnetic flux tube. We also study the effect of magnetic field strength and metallicity, and find that the wind density, and accordingly the mass-loss rate, positively and sensitively depends on both of them through the ambipolar diffusion of Alfvénic waves. The nonideal MHD simulation for $α$ Tau, which is slightly more evolved than $α$ Boo and has weaker magnetic field, results in weaker wind with $\dot{M}=1.5\times 10^{-12}M_{\odot}$/yr with $T\lesssim 10^5$ K throughout the simulation time. However, given the observations implying the presence of locally strong magnetic fields on the surface of $α$ Tau, we also conduct a simulation with a field strength twice as strong. This results in $\dot{M}=2.0\times 10^{-11}M_{\odot}$/yr - comparable to the observed value - with transient magnetized hot bubbles.

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arXiv2024

Spatially resolving the AGB star V3 in the metal-poor globular cluster 47 Tuc with VLTI/GRAVITY

Mass loss at the asymptotic giant branch (AGB) plays an important role not only in the final fates of stars, but also in the chemical evolution of galaxies. Nevertheless, the metallicity effects on AGB mass loss are not yet fully understood. We present spatially resolved observations of an AGB star, V3, in the metal-poor globular cluster 47 Tuc (NGC 104). The AGB star 47 Tuc V3 was observed using the GRAVITY instrument at ESO's Very Large Telescope Interferometer (VLTI) at 2-2.45 micron with a projected baseline length of up to 96 m. The object 47 Tuc V3 has been spatially resolved and stands as the first to attempt to spatially resolve an individual star in a globular cluster. The uniform-disk fit to the observed data results in an angular diameter of ~0.7 mas. Our modeling of the spectral energy distribution and near-infrared interferometric GRAVITY data suggests that the observed data can be explained by an optically thin dust shell with a 0.55 micron optical depth of 0.05-0.25, consisting of metallic iron grains, likely together with effects of the extended atmosphere of the central star. The dust temperature at the inner shell boundary is 500-800 K (corresponding to 23-90 stellar radii), significantly lower than observed in nearby oxygen-rich AGB stars. Radiation pressure on small (< 0.05 micron) iron grains is not sufficient to drive stellar winds. Therefore, iron grains may grow to larger sizes, even in the metal-poor environment. Alternatively, it is possible that the observed iron grain formation is a result of the mass outflow initiated by some other mechanism(s). The sensitivity and angular resolution of VLTI provides a new window onto spatially resolving individual stars in metal-poor globular clusters. This allows us to improve subsequent studies of the metallicity dependence of dust formation and mass loss.

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arXiv2020

Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula

The asymptotic giant branch (AGB) star HR3126, associated with the arcminute-scale bipolar Toby Jug Nebula, provides a rare opportunity to study the emergence of bipolar structures at the end of the AGB phase. We carried out long-baseline interferometric observations with AMBER and GRAVITY (2--2.45 micron) at the Very Large Telescope Interferometer, speckle interferometric observations with VLT/NACO (2.24 micron), and imaging with SPHERE-ZIMPOL (0.55 micron) and VISIR (7.9--19.5 micron). The images reconstructed in the continuum at 2.1--2.29 micron from the AMBER+GRAVITY data reveal the central star surrounded by an elliptical ring-like structure with a semimajor and semiminor axis of 5.3 and 3.5 mas, respectively. The ring is interpreted as the inner rim of an equatorial dust disk viewed from an inclination angle of ~50 degrees, and its axis is approximately aligned with the bipolar nebula. The disk is surprisingly compact, with an inner radius of a mere 3.5 Rstar (2 au). Our 2-D radiative transfer modeling shows that an optically thick flared disk with silicate grains as large as ~4 micron can reproduce the observed continuum images and the spectral energy distribution. The images obtained in the CO first overtone bands reveal elongated extended emission around the central star, suggesting the oblateness of the star's atmosphere or the presence of a CO gas disk inside the dust cavity. The object is unresolved with SPHERE-ZIMPOL, NACO, and VISIR. If the disk formed together with the bipolar nebula, the grain growth from sub-micron to a few microns should have taken place over the nebula's dynamical age of ~3900 yrs. The non-detection of a companion in the reconstructed images implies that either its 2.2 micron brightness is more than ~30 times lower than that of the red giant or it might have been shredded due to binary interaction.

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arXiv2019

Infrared interferometric three-dimensional diagnosis of the atmospheric dynamics of the AGB star R Dor with VLTI/AMBER

The mechanism of mass loss in late evolutionary stages of low- and intermediate-mass stars is not yet well understood. Therefore, it is crucial to study the dynamics of the region within a few stellar radii, where the wind acceleration is considered to take place. We present three-dimensional diagnosis of the atmospheric dynamics of the closest asymptotic giant branch (AGB) star R Dor from the low photospheric layers to the extended outer atmosphere--for the first time for a star other than the Sun. The images reconstructed with a spatial resolution of 6.8 mas--seven times finer than the star's angular diameter of 51.2 mas in the continuum--using the AMBER instrument at the Very Large Telescope Interferometer show a large, bright region over the surface of the star and an extended atmosphere. The velocity-field maps over the star's surface and atmosphere obtained from the Mg and H2O lines near 2.3 micron forming at atmospheric heights below ~1.5 stellar radii show little systematic motion beyond the measurement uncertainty of 1.7 km/s. In marked contrast, the velocity-field map obtained from the CO first overtone lines reveals systematic outward motion at 7--15 km/s in the extended outer atmosphere at a height of ~1.8 stellar radii. Given the detection of dust formation at ~1.5 stellar radii, the strong acceleration of material between ~1.5 and 1.8 stellar radii may be caused by the radiation pressure on dust grains. However, we cannot yet exclude the possibility that the outward motion may be intermittent, caused by ballistic motion due to convection and/or pulsation.

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arXiv2019

Constraining stellar parameters and atmospheric dynamics of the carbon AGB star V Oph

Molecules and dust produced by the atmospheres of cool evolved stars contribute to a significant amount of the total material found in the interstellar medium. To understand the mechanism behind the mass loss of these stars, it is of pivotal importance to investigate the structure and dynamics of their atmospheres. Our goal is to verify if the extended molecular and dust layers of the carbon-rich asymptotic giant branch (AGB) star V Oph, and their time variations, can be explained by dust-driven winds triggered by stellar pulsation alone, or if other mechanisms are operating. We model V Oph mid-infrared interferometric VLTI-MIDI data ($8$-$13~μ$m), at phases $0.18$, $0.49$, $0.65$, together with literature photometric data, using the latest-generation self-consistent dynamic atmosphere models for carbon-rich stars: DARWIN. We determine the fundamental stellar parameters: $T_\text{eff} = 2600~$K, $L_\text{bol} = 3585~$L$_{\odot}$, $M = 1.5~$M$_{\odot}$, $C/O = 1.35$, $\dot{M} = 2.50\cdot10^{-6}$M$_{\odot}$/yr. We calculate the stellar photospheric radii at the three phases: $479$, $494$, $448$ R$_{\odot}$; and the dust radii: $780$, $853$, $787$ R$_{\odot}$. The dynamic models can fairly explain the observed $N$-band visibility and spectra, although there is some discrepancy between the data and the models, which is discussed in the text. We discuss the possible causes of the temporal variations of the outer atmosphere, deriving an estimate of the magnetic field strength, and computing upper limits for the Alfvén waves velocity. In addition, using period-luminosity sequences, and interferometric modeling, we suggest V Oph as a candidate to be reclassified as a semi-regular star.

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arXiv2019

Exploring the innermost dust formation region of the oxygen-rich AGB star IK~Tau with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Low- and intermediate-mass stars at the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with SPHERE-ZIMPOL (phase 0.27) as well as interferometric observations with AMBER. IK Tau was observed with ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the H$α$ line at 656.3 nm, and in the TiO band at 717 nm. The AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 $μ$m (R=12 000). The polarimetric imaging capabilities of SPHERE-ZIMPOL have allowed us to spatially resolve clumpy dust clouds at 20--50 mas from the central star, which corresponds to 2--5 $R_{\star}$ when combined with a central star's angular diameter of 20.7$\pm$1.53 mas measured with AMBER. The diffuse, asymmetric dust emission extends out to $\sim$73 $R_{\star}$. We find that the TiO emission extends to 150 mas (15 $R_{\star}$). The AMBER data in the CO lines also suggest a molecular outer atmosphere extending to $\sim$1.5 $R_{\star}$. The results of our 2-D Monte Carlo radiative transfer modelling of dust clumps suggest that the polarized intensity and degree of linear polarization can be reasonably explained by small-sized (0.1 $μ$m) grains of Al$_{2}$O$_{3}$, MgSiO$_{3}$, or Mg$_{2}$SiO$_{4}$ in an optically thin shell ($τ_{550\mathrm{nm}}$=0.5$\pm$0.1) with an inner boundary radius of 3.5 $R_{\star}$. IK Tau's mass-loss rate is 20 to 50 times higher than the previously studied AGB stars W Hya, R Dor, and $o$ Cet. Nevertheless, our observations of IK Tau revealed that clumpy dust formation occurs close to the star as seen in those low mass-rate AGB stars.

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arXiv2019

Cool, evolved stars: results, challenges, and promises for the next decade

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g.~Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch and red super giant stars, and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae and their relation with the parent asymptotic giant branch stars.

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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

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

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

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arXiv2016

Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER II. Time variations between pre-maximum and minimum light

Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ~2 Rstar. We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) in the continuum (645, 748, and 820 nm), in the Halpha line (656.3 nm), and in the TiO band (717 nm) as well as high-spectral resolution long-baseline interferometric observations in 2.3 micron CO lines with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). The high-spatial resolution polarimetric images have allowed us to detect clear time variations in the clumpy dust clouds as close as 34--50~mas (1.4--2.0 Rstar) to the star. We detected the formation of a new dust cloud and the disappearance of one of the dust clouds detected at the first epoch. The Halpha and TiO emission extends to ~150 mas (~6 Rstar), and the Halpha images reveal time variations. The degree of linear polarization is higher at minimum light (13--18%) than that at pre-maximum light. The power-law-type limb-darkened disk fit to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1+/-1.5 mas and a limb-darkening parameter of 1.16+/-0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling suggests the predominance of small (0.1 micron) grains of Al2O3, Mg2SiO4, and MgSiO3 at minimum light, in marked contrast to the predominance of large (0.5 micron) grains at pre-maximum light. The variability phase dependence of the grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of efficient grain growth.

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arXiv2016

Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

We present visible polarimetric imaging observations of the well-studied AGB star W Hya taken with VLT/SPHERE-ZIMPOL as well as high spectral resolution long-baseline interferometric observations with the AMBER instrument of the Very Large Telescope Interferometer (VLTI). We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Halpha line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 micron with a spectral resolution of 12000. Taking advantage of the polarimetric imaging capability of SPHERE-ZIMPOL combined with the superb adaptive optics performance, we have succeeded in spatially resolving three clumpy dust clouds located at ~50 mas (~2 Rstar) from the central star, revealing dust formation very close to the star. The AMBER data in the individual CO lines suggest a molecular outer atmosphere extending to ~3 Rstar. Furthermore, the SPHERE-ZIMPOL image taken over the Halpha line shows emission with a radius of up to ~160 mas (~7 Rstar). We found that dust, molecular gas, and Halpha-emitting hot gas are coexisting within 2--3 Rstar. Our modeling suggests that the observed polarized intensity maps can reasonably be explained by large (0.4--0.5 micron) grains of Al2O3 or Mg2SiO4 or MgSiO3 in an optically thin shell with an inner boundary radius of 1.9--2.0 Rstar. The observed clumpy structure can be reproduced by a density enhancement by a factor of 4 +/- 1. The grain size derived from our polarimetric images is consistent with the prediction of the hydrodynamical models for the mass loss driven by the scattering due to micron-sized grains. The detection of the clumpy dust clouds close to the star lends support to the dust formation induced by pulsation and large convective cells as predicted by the 3-D simulations for AGB stars.

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arXiv2015

AMBER-NACO aperture-synthesis imaging of the half-obscured central star and the edge-on disk of the red giant L2 Pup

The red giant L2 Pup started a dimming event in 1994, which is considered to be caused by the ejection of dust clouds. We present near-IR aperture-synthesis imaging of L2 Pup achieved by combining data from VLT/NACO speckle observations and long-baseline interferometric observations with the AMBER instrument of the Very Large Telescope Interferometer (VLTI). We also extracted an 8.7 micron image from the mid-IR VLTI instrument MIDI. Our aim is to spatially resolve the innermost region of the circumstellar environment. The diffraction-limited image at 2.27 micron obtained by bispectrum speckle interferometry with NACO with a spatial resolution of 57 mas shows an elongated component. The aperture-synthesis imaging combining the NACO speckle data and AMBER data (2.2--2.29 micron) with a spatial resolution of 5.6x7.3 mas further resolves not only this elongated component, but also the central star. The reconstructed image reveals that the elongated component is a nearly edge-on disk with a size of ~180x50 mas lying in the E-W direction, and furthermore, that the southern hemisphere of the central star is severely obscured by the equatorial dust lane of the disk. The angular size of the disk is consistent with the distance that the dust clouds that were ejected at the onset of the dimming event should have traveled by the time of our observations, if we assume that the dust clouds moved radially. This implies that the formation of the disk may be responsible for the dimming event. The 8.7 micron image with a spatial resolution of 220 mas extracted from the MIDI data taken in 2004 (seven years before the AMBER and NACO observations) shows an approximately spherical envelope without a signature of the disk. This suggests that the mass loss before the dimming event may have been spherical.

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arXiv2014

Properties of the CO and H$_2$O MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations

Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is well suited for studying the complex processes in its atmosphere that lead to the chemical enrichment of the interstellar medium. Aims. We intend to investigate the shape and composition of the close molecular layer (also known as the MOLsphere) that surrounds the star. This analysis is part of a wider program that aims at understanding the dynamics of the circumstellar envelope of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral resolution of the instrument (R $\sim$ 1500), we were able to investigate the carbon monoxide band heads and the water-vapor bands. We used two different approaches to analyse our data: a model fit in both the continuum and absorption lines and then a fit with a Radiative HydroDynamics (RHD) simulation. Results. Using the continuum data, we derive a uniform disk diameter of $41.01 \pm 0.41$~mas, a power law type limb-darkened disk diameter of $42.28 \pm 0.43$~mas and a limb-darkening exponent of $0.155 \pm 0.009$. Within the absorption lines, using a single layer model, we obtain parameters of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in the visibilities. Conclusions. We derived a new value of the angular diameter of Betelgeuse in the K band continuum. Our observations in the absorption lines are well reproduced by a molecular layer at 1.2 stellar radii containing both CO and H$_2$O. The visibilities at higher spatial frequencies are matching a convection pattern in a RHD simulation.

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arXiv2014

Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR

We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar radii = 136--306 AU) away from the star. We also detected compact emission within a radius of 0.5" from the star. Comparison of our VISIR image taken in 2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals the outward motions of four dust clumps. The proper motions of these dust clumps amount to 0.2"--0.6" in 12 years. This translates into expansion velocities (projected onto the plane of the sky) of 13--40 km/s with an uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained by a simple accelerating outflow, implying the possible random nature of the dust cloud ejection mechanism. The inner compact emission seen in the 2010 VISIR image is presumably newly formed dust, because it is not detected in the image taken in 1998. The mass of the dust clouds is estimated to be (3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount of dust ejected in one year estimated from the (gas+dust) mass-loss rate of 2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on the clumpy dust cloud ejection.

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