Imaging the expanding knotty structure in the close environment of the LBV star $eta$ Carinae

$eta$~Car is one of the most massive stars in the Galaxy. It underwent a massive eruption in the 19th century, which produced the impressive bipolar Homunculus nebula now surrounding it. The central star is an eccentric binary with a period of 5.54,years. Although the companion has not been detected directly, it causes time-variable ionization and colliding-wind X-ray emission. By characterizing the complex structure and kinematics of the ejecta close to the star, we aim to constrain past and present mass loss of $eta$~Car. $eta$~Car is observed with the extreme adaptive optics instrument SPHERE at the Very Large Telescope, using its polarimetric mode in the optical with the ZIMPOL camera. A spatial resolution of 20,mas was achieved, i.e. very close to the presumed 13 mas apastron separation of the companion star. We detect new structures within the inner arcsecond to the star (2,300,au at a 2.3,kpc distance). We can relate these structures to the eruption near 1890 by tracking their proper motions derived from our new images and historical images over a 30,years time span. Besides, we find a fan-shaped structure in the inner 200~au to the star in the H$alpha$ line, that could potentially be associated with the wind collision zone of the two stars.

A thin shell of ionized gas explaining the IR excess of classical Cepheids

Despite observational evidences, InfraRed (IR) excess of classical Cepheids are seldom studied and poorly understood, but probably induces systematics on the Period-Luminosity (PL) relation used in the calibration of the extragalactic distance scale. This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids : RS Pup (P=41.46d), zeta Gem (P=10.15d), eta Aql (P=7.18d), V Cen (P=5.49d) and SU Cyg (P=3.85d). A time series of atmospheric models along the pulsation cycle are fitted to a compilation of data, including optical and near-IR photometry, Spitzer spectra (secured at a specific phase), interferometric angular diameters, effective temperature and radial velocity measurements. Herschel images in two bands are also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual is found in the SED, whatever the pulsation phase, and for wavelengths larger than about $1.2mu$m, which corresponds to the so-determined infrared excess of Cepheids. This IR excess is then corrected from interstellar medium absorption in order to infer or not the presence of dust shells, and is finally used in order to fit a model of a shell of ionized gas. For all Cepheids, we find a continuum IR excess increasing up to about -0.1 magnitudes at 30$mu$m, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). We show, for the first time, that the IR excess of Cepheids can be explained by free-free emission from a thin shell of ionized gas, with a thickness of about 15% of the star radius, a mass of $10^{-9}-10^{-7}$ Msol and a temperature ranging from 3500 to 4500K. This result has to be tested with interferometers operating in visible, in the mid-IR or in the radio domain. The impact of such CSEs of ionized gas on the PL relation of Cepheids needs also more investigations.

ALMA Compact Array observations of the Fried Egg nebula: Evidence for large-scale asymmetric mass-loss from the yellow hypergiant IRAS 17163-3907

Yellow hypergiants are rare and represent a fast evolutionary stage of massive evolved stars. That evolutionary phase is characterised by a very intense mass loss, the understanding of which is still very limited. Here we report ALMA Compact Array observations of a 50$$-mosaic toward the Fried Egg nebula, around one of the few Galactic yellow hypergiants IRAS 17163-3907. The emission from the $^{12}$CO J=2-1 line, H30$alpha$ recombination line, and continuum is imaged at a resolution of $sim$8$$, revealing the morphology of the molecular environment around the star. The continuum emission is unresolved and peaks at the position of the star. The radio recombination line H30$alpha$ shows unresolved emission at the star, with an approximately gaussian spectrum centered on a velocity of 21$pm$3~km/s with a width of 57$pm$6~km/s. In contrast, the CO 2-1 emission is complex and decomposes into several components beyond the contamination from interstellar gas in the line of sight. The CO spectrum toward the star is a broad plateau, centered at the systemic velocity of +18 km/s and with an expansion velocity of 100$pm$10 km/s. Assuming isotropic and constant mass-loss, we estimate a mass-loss rate of 8$pm$1.5 $times10^{-5}$~M$_odot$ yr$^{-1}$. At a radius of 25$$ from the star, we detect CO emission associated with the dust ring previously imaged by {it Herschel}. The kinematics of this ring, however, is not consistent with an expanding shell, but show a velocity gradient of $v_{sys} pm$20 km/s. In addition, we find a puzzling bright feature radially connecting the star to the CO ring, at a velocity of +40 km/s relative to the star. This spur feature may trace a unidirectional ejection event from the star. Our ACA observations reveal the complex morphology around IRAS 17163 and illustrate the breakthroughs that ALMA will bring to the field of massive stellar evolution.

Witnessing the Emergence of a Carbon Star

During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Most of the carbon in the Universe is believed to originate from stars with masses up to a few solar masses. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. Two models have been proposed to explain this dual chemistry: one postulates that a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch changed the surface chemistry of the star. The other model postulates that oxygen-rich material exists in stable keplerian rotation around the central star. The two models make contradictory, testable, predictions on the location of the oxygen-rich material, either located further from the star than the carbon-rich gas, or very close to the star in a stable disk. Using the Faint Object InfraRed CAmera (FORCAST) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) Telescope, we obtained images of the carbon-rich planetary nebula (PN) BD+30 3639 which trace both carbon-rich polycyclic aromatic hydrocarbons (PAHs) and oxygen-rich silicate dust. With the superior spectral coverage of SOFIA, and using a 3D photoionisation and dust radiative transfer model we prove that the O-rich material is distributed in a shell in the outer parts of the nebula, while the C-rich material is located in the inner parts of the nebula. These observations combined with the model, suggest a recent change in stellar surface composition for the double chemistry in this object. This is evidence for dredge-up occurring ~1000yr ago.

Photometric properties of carbon stars in the Small Magellanic Cloud

The Optical Gravitational Lensing Experiment identified over 1,800 carbon-rich Mira and semi-regular variables in the Small Magellanic Cloud. Multi-epoch infrared photometry reveals that the semi-regulars and Miras follow different sequences in color-color space when using colors sensitive to molecular absorption bands. The dustiest Miras have the strongest pulsation amplitudes and longest periods. Efforts to determine bolometric magnitudes reveal possible systematic errors with published bolometric corrections.

Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds

Infrared spectra of carbon-rich objects which have evolved off the asymptotic giant branch reveal a range of dust properties, including fullerenes, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and several unidentified features, including the 21 um emission feature. To test for the presence of fullerenes, we used the position and width of the feature at 18.7-18.9 um and examined other features at 17.4 and 6-9 um. This method adds three new fullerene sources to the known sample, but it also calls into question three previous identifications. We confirm that the strong 11 um features seen in some sources arise primarily from SiC, which may exist as a coating around carbonaceous cores and result from photo-processing. Spectra showing the 21 um feature usually show the newly defined Class D PAH profile at 7-9 um. These spectra exhibit unusual PAH profiles at 11-14 um, with weak contributions at 12.7 um, which we define as Class D1, or show features shifted to ~11.4, 12.4, and 13.2 um, which we define as Class D2. Alkyne hydrocarbons match the 15.8 um feature associated with 21 um emission. Sources showing fullerene emission but no PAHs have blue colors in the optical, suggesting a clear line of sight to the central source. Spectra with 21 um features and Class D2 PAH emission also show photometric evidence for a relatively clear line of sight to the central source. The multiple associations of the 21 um feature to aliphatic hydrocarbons suggest that the carrier is related to this material in some way.

The expanding dusty bipolar nebula around the nova V1280 Sco

V1280 Sco is one of the slowest dust-forming nova ever historically observed. We performed multi-epoch high-spatial resolution observations of the circumstellar dusty environment of V1280 Sco to investigate the level of asymmetry of the ejecta We observed V1280 Sco in 2009, 2010 and 2011 using unprecedented high angular resolution techniques. We used the NACO/VLT adaptive optics system in the J, H and K bands, together with contemporaneous VISIR/VLT mid-IR imaging that resolved the dust envelope of V1280 Sco, and SINFONI/VLT observations secured in 2011. We report the discovery of a dusty hourglass-shaped bipolar nebula. The apparent size of the nebula increased from 0.30 x 0.17 in July 2009 to 0.64 x 0.42 in July 2011. The aspect ratio suggests that the source is seen at high inclination. The central source shines efficiently in the K band and represents more than 56+/-5% of the total flux in 2009, and 87+/-6% in 2011. A mean expansion rate of 0.39+/-0.03 mas per day is inferred from the VISIR observations in the direction of the major axis, which represents a projected upper limit. Assuming that the dust shell expands in that direction as fast as the low-excitation slow ejecta detected in spectroscopy, this yields a lower limit distance to V1280 Sco of 1kpc; however, the systematic errors remain large due to the complex shape and velocity field of the dusty ejecta. The dust seems to reside essentially in the polar caps and no infrared flux is detected in the equatorial regions in the latest dataset. This may imply that the mass-loss was dominantly polar.

Carbon-rich dust production in metal-poor galaxies in the Local Group

We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax, and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. The spectra show significant quantities of dust around the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous comparisons of carbon stars with similar pulsation properties in the Galaxy and the Magellanic Clouds revealed no evidence that metallicity affected the production of dust by carbon stars. However, the more metal-poor stars in the current sample appear to be generating less dust. These data extend two known trends to lower metallicities. In more metal-poor samples, the SiC dust emission weakens, while the acetylene absorption strengthens. The bolometric magnitudes and infrared spectral properties of the carbon stars in Fornax are consistent with metallicities more similar to carbon stars in the Magellanic Clouds than in the other dwarf spheroidals in our sample. A study of the carbon budget in these stars reinforces previous considerations that the dredge-up of sufficient quantities of carbon from the stellar cores may trigger the final superwind phase, ending a stars lifetime on the asymptotic giant branch.