Oxygen-rich disk in the V778 Cyg system resolved

a r X i v :a s t r o -p h /0504354v 2 20 M a r 2006Astronomy &Astrophysics manuscript no.3782cESO 2008February 2,2008Oxygen-rich disk in the V778Cyg system resolved.⋆R.Szczerba 1,M.Szymczak 2,N.Babkovskaia 3,J.Poutanen 3,A.M.S.Richards 4,and M.A.T.Groenewegen 51N.Copernicus Astronomical Center,Rabia´n ska 8,87-100Toru´n ,Poland2Toru´n Centre for Astronomy,Nicolaus Copernicus University,Gagarina 11,87-100Toru´n ,Poland 3Astronomy Division,P.O.Box 3000,University of Oulu,FIN-90014Oulu,Finland4Jodrell Bank Observatory,University of Manchester,Macclesfield,Cheshire SK119Dl,UK 5Instituut voor Sterrenkunde,PACS-ICC,Celestijnenlaan 200B,B-3001Leuven,BelgiumReceived /AcceptedABSTRACTAims.Various scenarios have been proposed to explain the presence of silicate features associated with carbon stars,such as V778Cyg.We have attempted to constrain these theories by means of mapping water maser mission from V778Cyg.Methods.The 22GHz water maser emission from this star has been mapped using MERLIN with an astrometric accuracy of 25mas.Results.The spatially-and kinematically-resolved maser complex is displaced by ∼190mas from the position of the C-star as measured 10years earlier using Tycho.Our simulations and analysis of available data show that this position di fference is unlikely to be due to proper motion if V778Cyg is at the assumed distance of 1.4kpc.The maser components seem to form a distorted S-shaped structure extended over ∼18mas with a clear velocity gradient.We propose a model which explains the observed water maser structure as an O-rich warped disk around a companion of the C-star in V 778binary system,which is seen almost edge-on.Conclusions.Analysis of observational data,especially those obtained with MERLIN,suggests that V778Cyg (and,by implication,other silicate carbon stars)are binary systems composed of a C-rich star and a companion which stores circumstellar O-rich material.Key words.stars:AGB and post-AGB –stars:carbon –stars:chemically peculiar –masers1.IntroductionThe silicate emission features at about 10and 18µm are char-acteristic of O-rich dust envelopes.Surprisingly,these features were also discovered in the IRAS LRS data for some opti-cally classified carbon stars (Little-Marenin 1986;Willems &de Jong 1986),later termed silicate carbon stars.The detec-tion of silicate emission from these stars suggests that their relatively close surroundings contain oxygen-based dust,in spite of their photospheric chemical composition which shows C /O >1.An additional argument for the persistence of O-rich material comes from the detection of water and OH maser lines towards some silicate carbon stars (e.g.Little-Marenin et al.1994;Engels 1994;Little-Marenin et al.1988).Little-Marenin (1986)proposed that silicate carbon stars are binaries consisting of C-rich and O-rich giants,but ex-tensive observations (mbert et al.1990;Engels &Leinert 1994)did not show any evidence for an O-rich giant in these sources.Willems &de Jong (1986)proposed that sil-icate carbon stars were formed very recently due to athermal2Szczerba et al.:O-rich disk in V778Cyg 2.Observations and data reductionThe observations were taken on2001October12/13undergood weather conditions,usingfive telescopes of MERLIN(Diamond et al.2003).The longest MERLIN baseline of217km gave a fringe spacing of12mas at22GHz.A band-width of2MHz was used divided into256spectral channelsper baseline providing a channel separation of0.105km s−1.The velocities(V LSR)were measured with respect to the lo-cal standard of rest.The continuum calibrator sources wereobserved in16MHz bandwidth.We used the phase referenc-ing method;4min scans on V778Cyg were interleaved with2min scans on the source2021+614(at3.◦8from the target)over11.5h.VLBA observations at8.6GHz resolve2021+614into two components separated by7mas along the position an-gle of33◦(Fey et al.1996).We detect only one unresolvedsource at22GHz.Its absolute position coincides within lessthan2mas with the VLBA position of the stronger componentat8.6GHz.Theflux density of2021+614of1.48Jy was de-rived from4C39.25,which had aflux density of7.5±0.3Jy atthe epoch of our observations(Terasranta2002,private com-munication).This source was also used for bandpass calibra-tion.After initial calibration with MERLIN software,the data were processed using the AIPS package(Greisen1994).To derive phase and amplitude corrections for atmospheric and instrumental effects the phase reference source was mapped and self-calibrated.These corrections were applied to the V778Cyg visibility data.The absolute position of the bright-est feature at−15.1km s−1was determined before further cali-bration.Finally,the clean components of this image were used as a model for phase self-calibration of this channel and the solutions were applied to all channels.Each channel was then mapped and cleaned using a12mas circular restoring beam. We present results for total intensity(Stokes I)images.The map noise of∼27mJy beam−1in a line-free channel was close to the predicted thermal noise level.To determine the position and the brightness of the maser components two dimensional Gaussian components werefit-ted to the emission in each channel maps.The position uncer-tainty depends on the channel signal to noise ratio(Condon et al.1998,Richards et al.1999and references therein)and is lower than1mas for about80%of the maser components towards V778Cyg.The absolute position of the phase ref-erence source is known within∼3mas.The uncertainties in the absolute positions of the maser components are dominated by errors in the telescope positions and tropospheric effects. Uncertainties in telescope positions of1−2cm cause a maser position error of∼10mas.The latter uncertainty was estimated by observing the phase rate on the point source4C39.25which appeared to introduce a position error of<∼9mas.We checked this using reverse phase referencing.Emission from15chan-nels around the reference feature at−15.1km s−1was averaged and mapped.The map obtained was used as a model to self-calibrate the raw target data and these solutions were then ap-plied to the raw data of2021+614.The position of the referencesource was shifted by only∼2mas with respect to the catalog position.These factors imply that the absolute position accu-Fig.1.Positions of the water maser components in V778Cyg relative to the reference component at−15.1km s−1.The sym-bols correspond to the velocity ranges given in the upper left corner.The size of each symbol is proportional to the logarithm of peak brightness of the corresponding component.Inset: MERLIN spectrum of the H2O maser emission in V778Cyg. The dotted line shows the spectrum magnified by a factor of four to enlarge the weak features.racy of the maser source is<∼25mas.All MERLIN coordinates are given in the ICRS system.3.Results and discussionA single,unresolved maser component brighter than150 mJy beam−1(∼5σ)was found in each of51spectral chan-nels.The overall distribution of the H2O maser components in V778Cyg is shown in Fig.1(parameters of maser compo-nents are listed in Table2).The total angular extent of the maser emission is about18mas.All observed maser compo-nents seem to form a distorted S-like shape at a position an-gle(P.A.)of about−10◦.However,the most spatially extended series of components(V LSR∼−17km s−1)are aligned along P.A.≈+18◦.There is a clear velocity gradient along the whole structure,blue-shifted in the south with respect to the brightest northern components.The inset in Fig.1shows the cross-correlation water maser spectrum towards V778Cyg.The emission was dominated by a−15km s−1feature.Weak emission of about200−250mJy was seen at−19and−22km s−1.The spectral shape is roughly similar to that of the single dish spectra observed by Engels &Leinert(1994)and by Nakada et al.(1987),apart from a −22km s−1featurefirst detected on11.3.95by D.Engels1.Szczerba et al.:O-rich disk in V778Cyg3 Table 1.Radio and optical coordinates(with errors)forV778Cyg.MERLIN200120h36m07.s3833(±0.s0008)60◦05′26.′′024(±0.′′025)Tycho2199120h36m07.s4022(±0.s0028)60◦05′26.′′154(±0.′′040)2The following parameters also enter themodel:a distance Sun-Galactic Centre of8.5kpc(Kerr&Lynden-Bell1986),Oort’s con-stants of A=14.4km s−1kpc−1,B=−12.0km s−1kpc−1,and higher order terms d2θdr3|R0=2.0km s−1kpc−3(Pont et al.1994),and a Solar motion of19.5km s−1in the direction l=56.◦0, b=23.◦0(Feast&Whitelock1997).Fig.2.Different realisations of the proper motions for an evolved star with a LSR velocity close to−20km s−1,which lies in the direction of V778Cyg at different distances(see text for details).The proper motion required to match optical C-star and radio maser position is marked by diamond.of V778Cyg,−20±5km s−1,(determined from the heliocen-tric radial velocity of about−35km s−1,reported by Barbier-Brossat&Figon2000).The distribution of points in Fig.2 demonstrate that it is unlikely that an evolved star with V LSR=-20±5km s−1(V778Cyg)at a distance between0.5and2.0kpc would have a proper motion sufficient to match the optical C-star position with the radio position.This would require a trans-verse velocity of order of(−10,−10)mas yr−1,shown by the diamond in Fig.2,which is outside all the predictions of our simulation.Note,that the distance to V778Cyg is estimated to be D≃1.4kpc(Peery et al.1975;Yamamura et al.2000).Therefore,we believe that existing observations and simu-lations support a binary system model discussed in detail by Yamamura et al.(2000)and that the water maser is associ-ated with a companion star.However,observational errors are not negligible and simultaneous optical and radio observations would be required to confirm this hypothesisfirmly.The water maser components at−15,−17and−19km s−1 have been detected several times during the last15years (Nakada et al.1987,Engels&Leinert1994,this paper). Comparison of these observations show that changes in their radial velocities,∆V,do not exceed0.5km s−1.If the velocity change is due to the orbital motion of the secondary(with its maser)around the carbon star(mass M c),the rate of change (independently of the companion mass)is given by:∆V2sin i,(1)4Szczerba et al.:O-rich disk in V778Cyg Fig.3.Relative positions(together with errors)of the C-starand water maser in V778Cyg.The assumed disk orientationis shown by the straight solid line.The shaded region marksthe allowed C-star locations which are inferred from the propermotion estimations(see text for details).The sector of a circlemarks the minimal allowed angular distance of the C-star fromthe radio MERLIN position(as suggested by Eq.2)assumingthat the distance to V778Cyg is1.4kpc.The straight dottedline shows the position of orbital plane in case of no propermotion,while the dashed line marks the position angle of theoverall structure seen in Fig.1.where∆t is the time between observations,G is the gravita-tional constant and i is the inclination angle.Hence,for i≃90◦,the distance d between the binary companions is given by:d>∼75 ∆V15years 1/2 M cSzczerba et al.:O-rich disk in V778Cyg5 Fig.4.The LSR velocity against the distance along the majoraxis for water maser components in V778Cyg.The major axisis assumed to be a line at P.A.=+18◦defined by the V LSR∼−17km s−1water maser components;the zero of angular offsetmarks the centre of this structure.peaks close to the Keplerian rotation velocity(see e.g.Grinin&Grigor’ev1983,Watson&Wyld2000).For a Keplerian ve-locity of2km s−1,the radius of the disk R is about1/3largerthan the impact parameter at which the amplification is maxi-mal x max≃7mas.This results in R≃(4/3)×7mas×D[kpc]=13AU(D/1.4)kpc.Assuming that the mass of the disk itself isnegligible in comparison with mass of the secondary star,wecan derive the central mass inside the disk using Kepler’s laws,giving M s≃0.06(D/1.4kpc)M⊙.Note,that an object of sucha small mass(brown dwarf?)would not be detected by speckleinterferometry as performed by Engels&Leinert(1994).The proposed model can easily explain components at−15,−17and−19km s−1.The variations in their relative strengthscould be due to changes in physical conditions inside the disk:e.g.radial temperature gradient(the temperature should behighest at the northern disk edge),sub-sonic turbulence orclumpiness.However,this model cannot explain the weakestcomponent at−22km s−1.It is possible that this feature isformed in material evaporating from the disk(see model ofYamamura et al.2000).On the other hand,non–detection ofan edge of the disk is also possible.As exponential,direc-tional maser amplification exaggerates underlying conditions,some asymmetry is not surprising:there could even be materialat corresponding red–shifted velocities where masing is tem-porarily disrupted.If we do not detect the extremities of thedisc,or if the angle of inclination is not quite90◦,the centralstar could be more massive(although still sub-Solar).Finally,note that the observed strength of the water maser could be ex-plained for the water concentration of about10−5cm−3and thegas temperature in the disk of about300K.Such a temperatureis possible to achieve by the heating of dust inside the disk bythe radiation from the C-star and the energy exchange betweendust and gas(Babkovskaia et al.in preparation).4.ConclusionsWe mapped the water maser emission from silicate carbon starV778Cyg using MERLIN.The radio position obtained fromMERLIN in2001is at an angular separation of190mas fromthe optical position of V778Cyg given in the1991Tycho2cat-alog.This cannot be explained by proper motion if V778Cygis at a distance of∼1.4kpc and instead(probably)provides ob-servational support for the binary system model of Yamamuraet al.(2000).Simultaneous radio and optical measurements areneeded to verify this model which will be developed in a fur-ther paper.The velocity changes of the main maser compo-nents over15years imply that the distance between the C-starand disk is at least75AU.The water maser components havean almost linear,S-shaped distribution as projected on the skyand in the variations of V LSR along the elongation of the maincomponent at about−17km s−1.We suggest that this can beinterpreted as an almost edge-on warped Keplerian disk lo-cated around a companion object and tilted by no more than20◦relative to the orbital plane.We estimate that the centralmass inside the disk is>∼0.06M⊙(for a distance of1.4kpc).Acknowledgements.This work has been supported by grant2.P03D017.25of the Polish State Committee for Scientific Research(RS),the Magnus Ehrnrooth Foundation,and the Finnish Graduate Schoolfor Astronomy and Space Physics(NB),and the Academy of Finland(JP).ReferencesBarbier-Brossat,M.,&Figon,P.2000,A&AS,142,217Colomer,F.,Reid,M.J.,Menten,K.M.,&Bujarrabal,V.2000,A&A,355,979Condon,J.J.,Cotton,W.D.,Greisen,E.W.,et al.,1998,AJ,115,1693Deguchi,S.,Kawabe,R.,Ukita N.,et 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106−14.8203607.38325600526.02407.1388E+00 2.57E−020.07 107−14.9203607.38327600526.02381.2418E+01 2.89E−020.04 108−15.0203607.38327600526.02391.9127E+01 3.18E−020.03 109−15.1203607.38326600526.02402.4397E+01 2.49E−020.02 110−15.2203607.38326600526.02392.1525E+01 3.30E−020.02 111−15.3203607.38326600526.02401.9169E+01 3.06E−020.03 112−15.4203607.38327600526.02411.3279E+01 2.93E−020.04 113−15.5203607.38327600526.02387.6574E+00 2.69E−020.07 114−15.6203607.38329600526.02413.9029E+00 2.63E−020.14 115−15.7203607.38325600526.02381.7063E+00 2.60E−020.32 116−15.8203607.38327600526.02408.9236E−01 2.63E−020.60 117−15.9203607.38338600526.02413.3061E−01 2.68E−02 1.64 118−16.1203607.38341600526.02282.8095E−01 2.54E−02 1.92 119−16.2203607.38341600526.02016.7754E−01 2.68E−020.80 120−16.3203607.38347600526.02111.0845E+00 2.63E−020.50 121−16.4203607.38341600526.02001.3044E+00 2.63E−020.41 122−16.5203607.38347600526.02042.0243E+00 2.56E−020.26 123−16.6203607.38345600526.01983.0483E+00 2.69E−020.18 124−16.7203607.38351600526.01973.4839E+00 2.54E−020.15 125−16.8203607.38341600526.01924.2444E+00 2.63E−020.13 126−16.9203607.38343600526.01935.0099E+00 2.68E−020.11 127−17.0203607.38340600526.01924.7547E+00 2.67E−020.11 128−17.1203607.38337600526.01864.2378E+00 2.67E−020.13 129−17.2203607.38336600526.01823.7844E+00 2.73E−020.14 130−17.3203607.38331600526.01762.8783E+00 2.62E−020.19 131−17.4203607.38327600526.01712.9090E+00 2.51E−020.19 132−17.5203607.38326600526.01682.8753E+00 2.66E−020.19 133−17.6203607.38326600526.01632.8295E+00 2.56E−020.19 134−17.7203607.38327600526.01642.4962E+00 2.68E−020.21 135−17.8203607.38327600526.01562.2390E+00 2.64E−020.24 136−17.9203607.38322600526.01552.1629E+00 2.66E−020.25 137−18.1203607.38325600526.01561.2451E+00 2.65E−020.43 138−18.2203607.38320600526.01528.3832E−01 2.59E−020.64 139−18.3203607.38334600526.01383.1351E−01 2.65E−02 1.72 141−18.5203607.38328600526.01561.7954E−01 2.60E−02 3.02 142−18.6203607.38362600526.01353.7605E−01 2.62E−02 1.44 143−18.7203607.38359600526.01283.9566E−01 2.65E−02 1.36 144−18.8203607.38366600526.01246.6483E−01 2.58E−020.81 145−18.9203607.38364600526.01235.6821E−01 2.55E−020.95 146−19.0203607.38365600526.00994.0059E−01 2.64E−02 1.35 147−19.1203607.38363600526.01152.1827E−01 2.58E−02 2.47 148−19.2203607.38317600526.01211.5076E−01 2.61E−02 3.60 149−19.3203607.38329600526.01353.2427E−01 2.55E−02 1.66 172−21.7203607.38363600526.00843.2280E−01 2.50E−02 1.67 173−21.8203607.38366600526.00776.2120E−01 2.66E−020.87 174−22.0203607.38367600526.00765.5633E−01 2.70E−020.97 175−22.1203607.38361600526.00796.0993E−01 2.56E−020.88 176−22.2203607.38365600526.00693.9179E−01 2.70E−02 1.37。