The RCB star V854 Cen is surrounded by a hot dusty shell

Aims : The hydrogen-deficient supergiants known as R Coronae Borealis (RCB) stars might be the result of a double-degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a planetary nebula central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures, is of great importance. Methods : We obtained near-IR observations of V854 Cen with the AMBER recombiner located at the Very Large Telescope Interferometer (VLTI) array with the compact array (B$\leq$35m) in 2013 and the long array (B$\leq$140m) in 2014. At each time, V854 Cen was at maximum light. The $H$- and $K$-band continua were investigated by means of spectrally dependant geometric models. These data were supplemented with mid-IR VISIR/VLT images. Results : A dusty slightly elongated over density is discovered both in the $H$- and $K$-band images. With the compact array, the central star is unresolved ($\Theta\leq2.5$\,mas), but a flattened dusty environment of $8 \times 11$ mas is discovered whose flux increases from about $\sim$20% in the $H$ band to reach about $\sim$50% at 2.3\micron, which indicates hot (T$\sim$1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126$\pm$29$\deg$. Adding the long-array configuration dataset provides tighter constraints on the star diameter ($\Theta\leq1.0$ mas), a slight increase of the overdensity to $12 \times 15$ mas and a consistent P.A. of 133$\pm$49$\deg$. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3% of the total flux in 2013 and 2014. The VISIR images exhibit a flattened aspect ratio at the 15-20% level at larger distances ($\sim$1\arcsec) with a position angle of 92$\pm$19$\deg$, marginally consistent with the interferometric observations. Conclusions : This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for a bipolar structure proposed for this star in the literature, which were mainly based on polarimetric and spectroscopic observations.Comment: Accepted by A\&A, new version after language editing, Astronomy and Astrophysics (2014

Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163-3907

Through CO mm-line and optical spectroscopy, we investigate the properties of the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be one of the rare members of the yellow hypergiant class. The CO J=2-1 and J=3-2 emission arises from a region within 20" of the star and is clearly associated with the circumstellar material. The CO lines show a multi-component asymmetrical profile, and an unexpected velocity gradient is resolved in the east-west direction, suggesting a bipolar outflow. This is in contrast with the apparent symmetry of the dust envelope as observed in the infrared. The optical spectrum of IRAS 17163-3907 between 5100 and 9000 {\AA} was compared with that of the archetypal yellow hypergiant IRC+10420 and was found to be very similar. These results build on previous evidence that IRAS 17163-3907 is a yellow hypergiant.Comment: 14 pages including appendix, accepted for publication in A&

V838 Monocerotis: the central star and its environment a decade after outburst

Aims. V838 Monocerotis erupted in 2002, brightened in a series of outbursts, and eventually developed a spectacular light echo. A very red star emerged a few months after the outburst. The whole event has been interpreted as the result of a merger. Methods. We obtained near-IR and mid-IR interferometric observations of V838 Mon with the AMBER and MIDI recombiners located at the Very Large Telescope Interferometer (VLTI) array. The MIDI two-beam observations were obtained with the 8m Unit Telescopes between October 2011 and February 2012. The AMBER three-beam observations were obtained with the compact array (B$\leq$m) in April 2013 and the long array (B$\leq$140m) in May 2014, using the 1.8m Auxiliary Telescopes. Results. A significant new result is the detection of a compact structure around V838 Mon, as seen from MIDI data. The extension of the structure increases from a FWHM of 25 mas at 8 {\mu}m to 70 mas at 13 {\mu}m. At the adopted distance of D = 6.1 $\pm$ 0.6 kpc, the dust is distributed from about 150 to 400 AU around V838 Mon. The MIDI visibilities reveal a flattened structure whose aspect ratio increases with wavelength. The major axis is roughly oriented around a position angle of -10 degrees, which aligns with previous polarimetric studies reported in the literature. This flattening can be interpreted as a relic of the 2002 eruption or by the influence of the currently embedded B3V companion. The AMBER data provide a new diameter for the pseudo-photosphere, which shows that its diameter has decreased by about 40% in 10yrs, reaching a radius R$_*$ = 750 $\pm$ 200 R$_{\odot}$ (3.5 $\pm$ 1.0 AU). Conclusions. After the 2002 eruption, interpreted as the merging of two stars, it seems that the resulting source is relaxing to a normal state. The nearby environment exhibits an equatorial over-density of dust up to several hundreds of AU.Comment: Astronomy and Astrophysics (2014) Will be set by the publishe

Intense Mass Loss from C-rich AGB Stars at low Metallicity?

We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have a lower mass-loss rate, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.Comment: 4 pages, 3 figures. Accepted for publication in A&A. Updated after language editing. Additional typos fixe

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy

‘The definitive version is available at: www3.interscience.wiley.com '. Copyright Blackwell / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14736.xWe present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C2H2 and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range 1.0–3.3 × 10−8 M⊙ yr−1 . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.Peer reviewe

The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample

The mass-loss process from evolved stars is a key ingredient for our understanding of many fields of astrophysics, including stellar evolution and the chemical enrichment of the interstellar medium via stellar yields. One the main unsolved questions is the geometry of the mass-loss process. Taking advantage of the results from the Herschel Mass loss of Evolved StarS (MESS) programme, we initiated a coordinated effort to characterise the geometry of mass loss from evolved red giants at various spatial scales. For this purpose we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this contribution we present an overview of the interferometric data collected within the frame of our Large Programme, and we also add archive data for completeness. We studied the geometry of the inner atmosphere by comparing the observations with predictions from different geometric models. Asymmetries are detected for five O-rich and S-type, suggesting that asymmetries in the N band are more common among stars with such chemistry. We speculate that this fact is related to the characteristics of the dust grains. Except for one star, no interferometric variability is detected, i.e. the changes in size of the shells of non-mira stars correspond to changes of the visibility of less than 10%. The observed spectral variability confirms previous findings from the literature. The detection of dust in our sample follows the location of the AGBs in the IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in region II and in the carbon stars in region VII. The SiC dust feature does not appear in the visibility spectrum of UAnt and SSct, which are two carbon stars with detached shells. This finding has implications for the theory of SiC dust formation.Comment: 43 pages, 31 figures; accepted for publication in Astronomy & Astrophysics. Abstract shortened for compilation reasons. Metadata correcte

A close look into the carbon disk at the core of the planetary nebula CPD-568032

We present high spatial resolution observations of the dusty core of the Planetary Nebula with Wolf-Rayet central star CPD-568032. These observations were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode providing a typical 300 mas resolution and in interferometric mode using UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. The visible HST images exhibit a complex multilobal geometry dominated by faint lobes. The farthest structures are located at 7" from the star. The mid-IR environment of CPD-568032 is dominated by a compact source, barely resolved by a single UT telescope in a 8.7 micron filter. The infrared core is almost fully resolved with the three 40-45m projected baselines ranging from -5 to 51 degree but smooth oscillating fringes at low level have been detected in spectrally dispersed visibilities. This clear signal is interpreted in terms of a ring structure which would define the bright inner rim of the equatorial disk. Geometric models allowed us to derive the main geometrical parameters of the disk. For instance, a reasonably good fit is reached with an achromatic and elliptical truncated Gaussian with a radius of 97+/-11 AU, an inclination of 28+/-7 degree and a PA for the major axis at 345+/-7 degree. Furthermore, we performed some radiative transfer modeling aimed at further constraining the geometry and mass content of the disk, by taking into account the MIDI dispersed visibilities, spectra, and the large aperture SED of the source. These models show that the disk is mostly optically thin in the N band and highly flared.Comment: Paper accepted in A&

Developing a tool to assess trainees during crisis management training for major risks

International audienceOften based on simulation exercises, crisis management training helps prepare decision-makers to manage crises better. However, this training has definite limits in terms of trainee assessment and the feed-back given during the debriefing phase. This paper presents a method for better organising the assessment of trainees involved in a real time crisis management training exercises and for giving them feedback during the debriefing phase. The approach presented is based on creating a typology of training objectives in order to or-ganise the assessment. The assessment includes expected outcomes techniques as well as the human and or-ganisational factors that can be observed within a group. The assessment tools developed were then experi-mented within crisis management exercises completed with trainees. Beyond the basic results, these tools helped redefine the basic roles played by observers and trainers during training exercises