Observations of binaries in AGB, post-AGB stars and Planetary Nebulae

During the last years, many observational studies have revealed that binaries play an active role in the shaping of non spherical planetary nebulae. We review the different works that lead to the direct or indirect evidence for the presence of binary companions during the Asymptotic Giant Branch, proto-Planetary Nebula and Planetary Nebula phases. We also discuss how these binaries can influence the stellar evolution and possible future directions in the field.

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 ($Thetaleq2.5$,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$. Adding the long-array configuration dataset provides tighter constraints on the star diameter ($Thetaleq1.0$ 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.

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.

Optical interferometry and adaptive optics of bright transients

Bright optical transients (i.e. transients typically visible with the naked eye) are populated mainly by novae eruptions plus a few supernovae (among which the SN1987a event). One bright nova happen every two years, either in the North ot in the South hemisphere. It occurs that current interferometers have matching sensitivities, with typically visible or infrared limiting magnitude in the range 5--7. The temporal development of the fireball, followed by a dust formation phase or the appearance of many coronal lines can be sudied with the Very Large Telescope Interferometer. The detailed geometry of the first phases of novae in outburst remains virtually unexplored. This paper summarizes the work which has been done to date using the VLTI.

Stars with the B[e] phenomenon seen by long baseline interferometry

Thanks to the high spatial resolution provided by long baseline interferometry, it is possible to understand the complex circumstellar geometry around stars with the B[e] phenomenon. These stars are composed by objects in different evolutionary stages, like high- and low-mass evolved stars, intermediate-mass pre-main sequence stars and symbiotic objects. However, up to now more than 50% of the confirmed B[e] stars are not well classified, being called unclassified B[e] stars. From instruments like VLTI/AMBER and VLTI/MIDI, we have identified the presence of gaseous and dusty circumstellar disks, which have provided us with some hints related to the nature of these objects. Here, we show our results for two galactic stars with the B[e] phenomenon, HD 50138 and CPD-529243, based on interferometric measurements.

Interferometric studies of novae in the infrared

The Interferometric studies of novae in the optical and near-infrared is a nascent but fast emerging field which has begun to provide new and invaluable insights into the nova phenomenon. This is particularly so in the early stages of the eruption when all the relevant physical phenomena are on the scale of milli-arcseconds and thus are amenable to be studied only by interferometric techniques. In this review the instruments and arrays involved in this domain of work are briefly described, followed by a description of the major results obtained so far. A discussion is made of the physical aspects, where the application of interferometric techniques, can bring the most valuable information. Finally, prospects for the near future are discussed.

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.

The 2011 outburst of the recurrent novaT Pyx. Evidence for a face-on bipolar ejection

We report on near-IR interferometric observations of the outburst of the recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC recombiner, located at the CHARA array, and with the PIONIER and AMBER recombiners, located at the VLTI array. These data are supplemented with near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion velocities were measured (<300km/s) before t=20d (assuming D=3.5kpc). From t=28d on, the AMBER and PIONIER continuum visibilities (K and H band, respectively) are best simulated with a two component model consisting of an unresolved source plus an extended source whose expansion velocity onto the sky plane is lower than 700km/s. The expansion of the Brgamma line forming region, as inferred at t=28d and t=35d is slightly larger, implying velocities in the range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brgamma differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an inclination of i=15^{circ} and a position angle P.A.=110^{circ} provides a good match to the AMBER observables (spectra, differential visibilities and phases). At t=48d, a PIONIER dataset confirms the two component nature of the H band emission, consisting of an unresolved stellar source and an extended region whose appearance is circular and symmetric within error bars.These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This finding has profound implications for the interpretation of past, current and future observations of the expanding nebula.

High spatial resolution monitoring of the activity of BA supergiant winds

There are currently two optical interferometry recombiners that can provide spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands, and VEGA/CHARA, recently commissioned, operating in the visible. These instruments are well suited to study the wind activity of the brightest AB supergiants in our vicinity, in lines such as H$alpha$ or BrGamma. We present here the first observations of this kind, performed on Rigel (B8Ia) and Deneb (A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and 2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in 2008-2009 by VEGA. The extension of the Halpha and BrGamma line forming regions were accurately measured and compared with CMFGEN models of both stars. Moreover, clear signs of activity were observed in the differential visibility and phases. These pioneer observations are still limited, but show the path for a better understanding of the spatial structure and temporal evolution of localized ejections using optical interferometry.

Observations of dusty torii and compact disks around evolved stars: the high spatial resolution IR view

The recent high angular resolution observations have shown that the transition between a globally symmetrical giant and a source surrounded by a spatially complex environment occurs relatively early, as soon as the external layers of the stars are not tightly bound to the core of the star anymore. In this review, the emphasis will be put on the delineating the differences between the torus and disk classification through the presentation of many examples of near-IR and mid-IR high angular resolution observations. These examples cover the disks discovered in the core of some bipolar nebulae, post-AGB disks, the dusty environment around born-again stars and recent novae, and also the disks encountered around more massive evolved sources. We discuss the broad range of circumstances and time scales for which bipolar nebulae with disks are observed.