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- 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 8 m unit telescopes between October 2011 and February 2012. The AMBER three-beam observations were obtained with the compact array (B ≤ 35 m) in April 2013 and the long array (B ≤ 140 m) in May 2014, using the 1.8 m 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 μm to 70 mas at 13 μm. At the adopted distance of D = 6.1 ± 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°, which aligns with previous polarimetric studies reported in the literature. This flattening can be interpreted as a relic of the 2002 eruption or as caused 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 10 yr, reaching a radius R∗ = 750 ± 200 R⊙ (3.5 ± 1.0 AU). Conclusions. After the 2002 eruption, which was 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 overdensity of dust up to several hundred AU.
A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages ...could result from interactions with the companion during the common-envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 ± 0.59 kiloparsecs from the Sun.
Enterotoxins produced by
Staphylococcus aureus are responsible for staphylococcal food-poisoning outbreaks (SFPO). In France, SFPO are the second cause of food-borne diseases after
Salmonella. ...However, very little is known about the strains involved. The objective of this study was to characterize the staphylococcal strains related to these SFPO through phenotypic and genotypic analyses. A total of 178 coagulase-positive staphylococcal isolates recovered from 31 SFPO (1981–2002) were screened through biotyping. Thirty-three strains representative of the different biotypes in each SFPO were further examined for
SmaI macrorestriction-type, phage-type, resistance to various antimicrobial drugs, presence of staphylococcal enterotoxin (
se) genes
sea to
sei, and production of enterotoxins SEA to SED. All these 33 strains were identified as
S. aureus species: 27 were of human biotypes and six ovine or non-host-specific biotypes. Most (74.1%) strains reacted with group III phages. Eleven strains were resistant to at least two classes of antibiotics and among them, two were resistant to methicillin. Twenty-nine strains carried one or several of the eight
se genes tested; the gene
sea was most common (
n
=
23), and often linked to
sed (
n
=
12) or
seh (
n
=
5). The novel
se genes
seg-i were in all cases associated with
se genes
sea to
sed except for one strain which carried only
seg and
sei. Pulsed-Field Gel Electrophoresis (PFGE) of
SmaI macrorestriction digests of the 33 strains discriminated 32 PFGE patterns grouped into nine biotype-specific clusters. All five strains carrying
sea and
seh were grouped together into the same sub-cluster. Three of the four
se-gene-negative strains were in one PFGE cluster: all four should be tested for
se genes not included in this study and, if negative, be further investigated for the presence of unidentified SEs.
Aims. We study the close environment of nearby Cepheids using high spatial resolution observations in the mid-infrared with the VLTI/MIDI instrument, a two-beam interferometric recombiner. Methods. ...We obtained spectra and visibilities for the classical Cepheids X Sgr and T Mon. We fitted the MIDI measurements, supplemented by B,V,3,H,K literature photometry, with the numerical transfer code DUSTY to determine the dust shell parameters. We used a typical dust composition for circumstellar environments. Results. We detect an extended dusty environment in the spectra and visibilities for both stars, although T Mon might suffer from thermal background contamination. We attribute this to the presence of a circumstellar envelope (CSE) surrounding the Cepheids. This is optically thin for X Sgr (T sub(0.55 mu m) = 0.008), while it appears to be thicker for T Mon (T sub(0.55 mu m) = 0.15). They are located at about 15-20 stellar radii. Following our previous work, we derived a likely period-excess relation in the VISIR PAH1 filter, f sub(8.6) sub( mu m)%= 0.81(+ or -0.04)Pday. We argue that the impact of CSEs on the mid-IR period-luminosity (P - L) relation cannot be negligible because they can bias the Cepheid brightness by up to about 30%. For the K-band P - L relation, the CSE contribution seems to be lower (<5%), but the sample needs to be enlarged to firmly conclude that the impact of the CSEs is negligible in this band.
Context. It has been shown that convection in red supergiant stars (RSG) gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The resulting motion of ...the photocentre (on time scales ranging from months to years) could possibly have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the granulation on the photocentric and photometric variability. We quantify these effects in order to better characterise the error that could possibly alter the parallax. Methods. We use 3D radiative-hydrodynamics (RHD) simulations of convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps and spectra in the Gaia G band 325-1030 nm. Results. We provide astrometric and photometric predictions from 3D simulations of RSGs that are used to evaluate the possible degradation of the astrometric parameters of evolved stars derived by Gaia. We show in particular from RHD simulations that a supergiant like Betelgeuse exhibits a photocentric noise characterised by a standard deviation of the order of 0.1 AU. The number of bright giant and supergiant stars whose Gaia parallaxes will be altered by the photocentric noise ranges from a few tens to several thousands, depending on the poorly known relation between the size of the convective cells and the atmospheric pressure scale height of supergiants, and to a lower extent, on the adopted prescription for galactic extinction. In the worst situation, the degradation of the astrometric fit caused by this photocentric noise will be noticeable up to about 5 kpc for the brightest supergiants. Moreover, parallaxes of Betelgeuse-like supergiants are affected by an error of the order of a few percents. We also show that the photocentric noise, as predicted by the 3D simulation, does account for a substantial part of the supplementary "cosmic noise" that affects Hipparcos measurements of Betelgeuse and Antares.
Context. To progress in the understanding of evolution of massive stars one needs to constrain the mass-loss and determine the phenomenon responsible for the ejection of matter an its reorganization ...in the circumstellar environment Aims. In order to test various mass-ejection processes, we probed the geometry and kinematics of the dust and gas surrounding the Ae supergiant HD 62623. Methods. We used the combined high spectral and spatial resolution offered by the VLTI/AMBER instrument. Thanks to a new multi-wavelength optical/IR interferometry imaging technique, we reconstructed the first velocity-resolved images with a milliarcsecond resolution in the infrared domain. Results. We managed to disentangle the dust and gas emission in the HD 62623 circumstellar disc. We measured the dusty disc inner rim, i.e. 6 mas, constrained the inclination angle and the position angle of the major-axis of the disc. We also measured the inner gaseous disc extension (2 mas) and probed its velocity field thanks to AMBER high spectral resolution. We find that the expansion velocity is negligible, and that Keplerian rotation is a favoured velocity field. Such a velocity field is unexpected if fast rotation of the central star alone is the main mechanism of matter ejection. Conclusions. As the star itself seems to rotate below its breakup-up velocity, rotation cannot explain the formation of the dense equatorial disc. Moreover, as the expansion velocity is negligible, radiatively driven wind is also not a suitable explanation to explain the disc formation. Consequently, the most probable hypothesis is that the accumulation of matter in the equatorial plane is due to the presence of the spectroscopic low mass companion.
Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the ...thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System.
Context. The fast temporal evolution of the ejecta morphology of novae can be considered as an important test bench for studying the shaping of many kinds of nebulae. V1280 Sco is one of the slowest ...dust-forming nova ever historically observed that has experienced a particularly long common-envelope phase. Aims. 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. Methods. We observed V1280 Sco in 2009, 2010 and 2011 (from t = 877 days after discovery until t = 1664 d) 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. Results. We report the discovery of a dusty hourglass-shaped bipolar nebula. The apparent size of the nebula increased from 0.30′′ × 0.17′′ in July 2009 to 0.64′′ × 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 milliarcsec 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 ~1 kpc; 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. Conclusions.V1280 Sco is an excellent test case for studying the temporal evolution of dusty bipolar ejecta. As the nebula expands, observations will be easier and we advocate a yearly monitoring of the source using high angular resolution techniques.
Context. Luminous BA-type supergiants are the brightest stars in the visible that can be observed in distant galaxies and are potentially accurate distance indicators. The impact of the variability ...of the stellar winds on the distance determination remains poorly understood. Aims. Our aim is to probe the inhomogeneous structures in the stellar wind using spectro-interferometric monitoring. Methods. We present a spatially resolved, high-spectral resolution (R = 12 000) K-band temporal monitoring of the bright supergiant β Orionis (Rigel, B8 Iab) using AMBER at the Very Large Telescope Interferometer (VLTI). Rigel was observed in the Brγ line and its nearby continuum once per month over 3 months in 2006−2007, and 5 months in 2009−2010. These unprecedented observations were complemented by contemporaneous optical high-resolution spectroscopy. We analyse the near-IR spectra and visibilities with the 1D non-LTE radiative-transfer code CMFGEN. The differential and closure phase signals are evidence of asymmetries that are interpreted as perturbations of the wind. Results. A systematic visibility decrease is observed across the Brγ line indicating that at a radius of about 1.25 R∗ the photospheric absorption is filled by emission from the wind. During the 2006−2007 period the Brγ and likely the continuum forming regions were larger than in the 2009−2010 epoch. Using CMFGEN we infer a mass-loss rate change of about 20% between the two epochs. We also find time variations in the differential visibilities and phases. The 2006−2007 period is characterised by noticeable variations in the differential visibilities in Doppler position and width and by weak variations in differential and closure phase. The 2009−2010 period is much quieter with virtually no detectable variations in the dispersed visibilities but a strong S-shaped signal is observed in differential phase coinciding with a strong ejection event discernible in the optical spectra. The differential phase signal that is sometimes detected is reminiscent of the signal computed from hydrodynamical models of corotating interaction regions. For some epochs the temporal evolution of the signal suggests the rotation of the circumstellar structures.
Context. Studying the appearance and properties of bipolar winds is critical to understand the stellar evolution from the AGB to the planetary nebula (PN) phase. Many uncertainties exist regarding ...the presence and role of binary stellar systems, mainly due to the deficit of conclusive observational evidences. Aims. We investigate the extended equatorial distribution around the early bipolar planetary nebula M 2−9 (“Minkowski’s Butterfly Nebula”) to gather new information on the mechanism of the axial ejections. Methods. Interferometric millimeter observations of molecular emission provide the most comprehensive view of the equatorial mass distribution and kinematics in early PNe. Here we present subarcsecond angular-resolution observations of the 12CO J = 2−1 line and continuum emission with the Plateau de Bure interferometer. Results. The data reveal two ring-shaped and eccentric structures at the equatorial basis of the two coaxial optical lobes. The two rings were formed during short mass-loss episodes (~40 yr), separated by ~500 yr. Their positional and dynamical imprints provide evidence of the presence of a binary stellar system at the center, which yields critical information on its orbital characteristics, including a mass estimate for the secondary of ≲0.2 M⊙. The presence of a stellar system with a modest-mass companion at the center of such an elongated bipolar PN strongly supports the binary-based models, because these are more easily able to explain the frequent axisymmetric ejections in PNe.
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