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Wolf-Rayet Stars at the Highest Angular Resolution

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Wolf-Rayet Stars at the Highest Angular Resolution Astronomical Science Wolf-Rayet Stars at the Highest Angular Resolution Florentin Millour1 provide spatially resolved observations leading to these extraordinary events, Olivier Chesneau2 of massive stars and their immediate vicin- ]hpdkqcd pdeo eo jkp uap ànihu aop]^heoda`* Thomas Driebe1 ity. However, the number of Wolf-Rayet Only ~ 1% of core-collapse SNe are able Alexis Matter2 stars in the solar vicinity is very low (Van to produce a highly relativistic collimated Werner Schmutz3 der Hucht, 2001; Crowther, 2007). There- kqpâks ]j`( daj_a( ] CN>* Bruno Lopez2 fore, all Wolf-Rayet stars are too remote Romain G. Petrov2 to be spatially resolved by adaptive optics. WR stars are characterised by an extra- José H. Groh1 However, for a handful of objects, the kn`ej]nu du`nkcaj)`aà_eajp ola_pnqi( Daniel Bonneau2 geometry of the innermost circumstellar dominated by broad emission lines of 4 Luc Dessart structures (discs, jets, latitude-dependent highly ionised elements (such as He EE, C ER, 1 Karl-Heinz Hofmann winds, or even more complex features) N R or O RE). The dense stellar winds com- Gerd Weigelt1 can be directly probed with the highest pletely veil the underlying atmosphere spatial resolution available, through the so that an atmospheric analysis can only use of stellar interferometry. be done with dynamical, spherically 1 Max-Planck-Institut für Radioastronomie, extended model atmospheres, such as Bonn, Germany those developed by Schmutz et al. (1989), 2 Observatoire de la Côte dAzur, Nice, A closer look at Wolf-Rayet stars Hillier & Miller (1998), or Gräfener et al. France $.,,.%* Ej pda psk h]op `a_]`ao( oecjeà_]jp 3 Physikalisches-Meteorologisches Wolf-Rayet (WR) stars begin their life as progress has been achieved in this Observatorium Davos, World Radiation massive objects (usually O-type super- respect, so that WR stars can be placed Center, Switzerland giant stars) with at least 20 times the on the Hertzsprung-Russell Diagram 4 Observatoire de Paris, France mass of the Sun. Their life is brief, and $DN@% sepd okia _kjà`aj_a $oaa a*c*( they die hard, exploding as supernovae Crowther, 2007). and blasting vast amounts of heavy Interferometric observations of high- elements into space, which are recycled ?h]ooeà_]pekj kb SN op]no eo ^]oa` qlkj mass evolved stars provide new and in later generations of stars and planets. the appearance of optical emission very valuable information of their By the time these massive stars are near lines of different ions of helium, carbon, nature. With the unique capabilities of the end of their short life, during the nitrogen and oxygen. The nitrogen-rich, the VLTI, direct images of their closest _d]n]_paneope_ ÑSkhb)N]uapí ld]oa( pdau kn SJ)pula( eo `aàja` ^u ] ola_pnqi environment where mass loss and `arahkl ] àan_a opahh]n sej` § ] opna]i in which helium and nitrogen lines (He EEE dust formation occur, can be obtained. of particles ejected from the stellar sur- and N EEER) dominate. In the carbon-rich, The breakthrough of the VLTI in terms face by the radiative pressure expel - or WC-type, helium, carbon, and oxygen of angular resolution as well as spectral ling mass at a tremendous rate (up to lines (C EEER, He EEE and O EEERE) dominate 3 resolution allows competing theoretical 10 I/yr) while they are synthesising the emission-line spectrum. Castor, models, based on indirect constraints, elements heavier than hydrogen in their =^^kpp " Ghaej $-531% `aikjopn]pa` pd]p to be tested. The high angular resolu- cores. One of the characteristics of WR hot-star winds could be explained by tion made available by the VLTI shows stars is the low hydrogen content of the considering radiation pressure alone. In that there is still a lot to discover about atmosphere due to the stripping-off of this early model, each photon is scattered these massive stars. the outer layers as a result of the strong kj_a ]p ikop( ]j` pdeo eo oqbà_eajp pk mass loss. Before and during the WR `nera K>)op]n sej`o( sdeha ikna `ebà_qhpeao phase, such stars eject a large amount were encountered for WR stars. Model ,]ooera op]no opnkjchu ejâqaj_a pdaen oqn - of matter, more than 10 I, and with ]pikoldana opq`eao d]ra ]`r]j_a` oqbà - roundings due to their extreme tem- velocities up to 3000 km/s, which then ciently to enable the determination of perature, luminosity, and mass-loss rate. surrounds the central star in the form of stellar temperatures, luminosities, abun- In addition, they are short-lived, and gas and dust. `]j_ao( ekjeoejc âqtao ]j` sej` lnklan - their fate is to explode as core-collapse ties of WR stars. What remains uncertain supernovae. Among the extensive zool- Theoretically, the evolution of a WR star are the kinematics of the wind and the ogy of massive stars, Wolf-Rayet stars ends with the collapse of its core and, wind acceleration law. Furthermore, rota- probably represent the last stage of stellar as a rule, the formation of a black hole or pekj eo ranu `ebà_qhp pk ia]oqna ej SN evolution, just before the explosion as a a neutron star occurs. Energetic Type Ib stars, since photospheric features are supernova. So far, Wolf-Rayet stars have and Ic supernovae (SNe) are thought absent. This parameter is crucial when been mainly studied by means of spec- to be direct descendants of massive WR considering GRBs. troscopy and spectropolarimetry, based stars (see, for instance, Crowther, 2007). on spatially unresolved observations. It is now recognised that long-duration Atmospheric models of WR stars are Nonetheless, there is extensive evidence gamma-ray bursts (GRBs) are linked to parameterised by the inner boundary of binarity and geometrical complexity the collapse of massive stars. The merg- radius N , at high Rosseland optical of the nearby wind in many Wolf-Rayet ing of the components in a binary sys- depth (typically above 10), but only the stars. In this context, high angular resolu- tem, including a rapidly rotating WR, is optically thin part of the atmosphere pekj pa_djemqao àhh pda c]l( oej_a pdau _]j considered to be one of the channels is seen by the observer. Therefore, the .2 The Messenger 135 ä Iarch 2009 determination of N depends on the can be resolved by an interferometer, Probing the wind of the closest WR star: 2 assumptions made on the velocity law of provided that a minimum spectral resolu- G Vel the wind, considering that typical WN pekj kb 1,, eo ]r]eh]^ha* Pdanabkna( k^oanr- 2 ]MC S? sej`o d]ra na]_da` ] oecjeà_]jp ing the LFR of WR stars with long-baseline Among all WR stars, G Vel is by far the fraction of their terminal velocity before optical interferometry offers the opportu- closest, with a well-known distance today they become optically thin in the continu- nity to probe their winds in the following kb //2 Ω 4 l_ $Jknpd ap ]h*( .,,3%( sdehop um (especially in the near- and mid-IR). ways: all others are beyond 1 kpc. Owing to its relative proximity, G2 Vel is relatively bright Typical scales for N are 36 N, depend- ing on the spectral sub-type (Crowther, By measuring the extension of the LFR and has been studied in great detail, 2007). In the near-IR, the main opacity compared to the continuum. The spatial mainly using spectroscopy. Through 2 comes from freefree interactions, and and kinematical characteristics of the spectro scopic eyes, G Vel is shown to be the continuum-forming region is more wind can be better inferred and the wind ] ^ej]nu SN ' K ouopai $S?4 ' K3*1EEE( extended at longer wavelengths, reach- velocity law (especially with access to L 9 34*1/ `]uo%( pdqo kbbanejc ]__aoo pk shorter wavelength data) possibly con- the fundamental parameters of the WR ing 26 N . The core radius of WN stars is larger than of WC stars, but the wind strained. star, usually obtained only indirectly of WC stars is denser, and the continuum through the study of its dense and fast forms farther away from the core radius. By examining any deviation from wind. The presence of X-ray emission in As a consequence, the continuum diam- sphericity of these objects. If WR stars the system (Skinner et al., 2001) could be eter is about the same, of the order of were rapid rotators, one would expect explained by an X-ray-emitting bow-shock strong wavelength-dependent devia- between the O-star wind and the WR-star 1020 N, corresponding to a diameter of about 0.10.2 mas for sources at tions from spherical symmetry, as was wind (the so-called windwind collision 2 1 kpc. This implies that the continuum of detected for the luminous blue variable zone, [WWCZ]). G Vel was observed by both WN- and WC-type WR stars remains star H Car with AMBER/VLTI (Weigelt et the Narrabri intensity interferometer, oper- unresolved for the VLTI in the near-IR, al., 2007) for example. ]pejc ]p ]nkqj` ,*01 …i( ]o a]nhu ]o -524 even with the longest (130 m-scale) base- (Hanbury Brown et al., 1970), but since hejao* Uap( pdeo _kj_hqoekj `kao jkp dkh` By examining the deviations and per- the observations lacked spectral resolu- for the line-forming regions (LFR) that can turbations of the WR wind from purely tion, they could not resolve the WWCZ. radial motion. Such deviations may ^a hk_]pa` ]p pule_]hhu 1ä1, N and that originate from dust formation very close Our team observed G2 Vel in December to some of these hot stars.
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