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mira ( o ceti ) is the prototype of the class of mira - type variable stars , characterized observationally by large visual photometric pulsation amplitudes and regular periods between 100 and 1000 days . mira - type variables represent one of the late stages of evolution for 1 - 2 solar mass stars , with luminosities of several thousand times solar and mass loss rates of order @xmath2 solar masses per year . the winds from mira - type variables are relatively slow ( typically @xmath310 kms@xmath4 ) , which means that the mass lost in these winds can be readily accreted onto any stellar companions . it is known that 57% of the progenitors of mira - type variables , g and f dwarfs , have a companion with a mass more than a tenth of the primary mass @xcite . however , most of the companions around mira - type primaries remain undetected because of the overwhelming flux of the mira - type primary and the difficulty in making precise radial velocity measurements due to the stellar pulsation . the most reliable ways to detect companions are through the signs of wind accretion : either a short - wavelength ( e.g. ultraviolet ) excess or rapid photometric variability @xcite . mira  b was first discovered visually in 1923 , when it was at a separation of 0.9  @xcite , and has since been imaged in x - rays , ultraviolet and the radio @xcite . observations of the mira  ab system with the infrared spatial interferometer also offered a tentative detection of flux from near the position of mira  b at 11.17@xmath1 m , interpreted as a clump of dust @xcite . previous attempts at deconvolution of mid - infrared images @xcite showed a coma - like elongation in the direction of mira  b and a feature at @xmath0twice the mira  ab separation , but quantitative interpretation of these data was difficult due to the difficulties in de - convolving seeing - limited images . the nature of mira  b has been and remains controversial , due to the radiation originating from mira  b being dominated by accretion . according to the arguments originally made by @xcite and reinforced by @xcite , mira b must be a low - mass main sequence star , as the x - ray and total luminosity of mira  b is far too weak for the companion to be a white dwarf . this argument relies on measurements of the mira  b wind , reasonable values for the true ( i.e. non projected ) separation of mira  ab and the bondi - hoyle accretion rate being correct within a factor of 10 . this paper presents novel non - redundant interferometry observations of mira  ab , enabling robust detection of mid - infrared flux from the vicinity of mira  b. these and other traditional imaging observations are presented in detail in section  [ sectobservations ] . an examination of possible models for this emission is presented in section  [ sectmodelling ] and the implications of the preferred ( disk ) model presented in section  [ sectdiscussion ] . in the mid - infrared , observations on a 10 m class of telescope can be nearly diffraction limited in good seeing conditions . with a 15 cm atmospheric coherence length at 550 nm , a 10 m aperture is two coherence lengths wide at 10@xmath1 m . in this regime , the image of a point source usually has a single bright core , but deconvolution is difficult due to changes in seeing and changes in low order aberrations with changing elevations . we first overcame these difficulties by individually re - pointing and re - phasing 4 sets of 6 segments out of the 36 separate segments that made up the primary mirror of the keck i telescope . as part of an observing program designed to resolve the dusty winds from mira variables , these `` segment tilting '' observations of @xmath5  ceti were made at 8 , 9.9 , 10.7 and 12.5@xmath1 m . these four sets of segments formed 4 interferograms on the long - wavelength spectrometer ( lws ) camera , each of which gave 15 visibility amplitude and 20 closure phase ( 10 independent ) measurements on the source . this technique is described in @xcite and @xcite . at 10.7@xmath1 m , data was also taken in a configuration with 3 sets of 7 segments with non - redundant spacings . calibration observations were made of @xmath6  cet and @xmath6  tau to calibrate visibility amplitude and subtract out any instrumental closure - phase . the macim algorithm : a monte - carlo technique for imaging from closure - phase and squared visibility @xcite was used to create images at all wavelengths . as part of this imaging process , we incorporated prior knowledge that a set fraction of the flux comes from a compact central source ( i.e. the central star mira  a ) with a 50mas diameter . this size comes from the longer - baseline observations of @xcite . we estimated the fraction of light from this 50mas source by assuming simple black - body emission , based on the source containing 75% of the 8@xmath1 m flux . this step had little effect on flux near the location of mira  b , but affected the deconvolution of the central source and dust shell , due to the documented tendency for techniques in the maximum - entropy family to spread flux over the image at the expense of the compact central source . unambiguous detections of flux near the location of mira  b were made at 12.5 and 10.7 microns , with a lower signal - to - noise detection at 9.9 microns . the 10.7 and 12.5 micron images are shown in figure  [ figkeckims ] . the separation and position angle of mira  b with respect to mira  a was obtained for the 10.7 and 12.5@xmath1 m data by searching for a peak in the difference image between the reconstructed image and the azimuthally - averaged reconstructed image with mira  b masked - out . flux measurements of mira  b were obtained from aperture - photometry on this differenced image . we also attempted to simultaneously reconstruct a dust shell image and perform a binary fit to the fourier data using macim , obtaining consistent results and an independent estimation of errors on the binary parameters directly from macim s markov chain . the average separation and position angle from these fits is @xmath7and @xmath8 degrees . we independently calibrated the camera plate scale and orientation by observing binaries xy per and @xmath6  her , giving results in agreement with previous calibrations @xcite . plate scale uncertainties are included in the above error estimates .    to obtain comparable astrometry for the ultraviolet position of mira  b , we extrapolate based on a linear fit between @xcite and the 2004 february hst stis o8wy02040 data sets . projected to jd2453248 , we find a separation of @xmath9  at @xmath10 degrees . in comparing the ultraviolet and mid - infrared detections , the position angles are found to be consistent . however , the difference in separation is significant , so the thermal ir and uv emission from mira  b must arise from different physical regions .    to confirm this result , we obtained additional data using an aperture - mask in the t - recs camera on gemini south ( program i d gs-2006b - q-15 ) . the mask is installed in the cold lyot stop wheel of t - recs , decreasing the background significantly from the keck experiment , where the full telescope pupil contributed to the background in all interferograms . this mask has 7 holes in a non - redundant configuration , amounting to 21 simultaneous visibility amplitudes and 35 closure - phases ( 15 independent ) . baseline lengths projected on the primary mirror range from 1.78 to 6.43 m , and sky rotation is used to increase the fourier coverage while maintaining the same relationship between positions on the mask and positions on the primary mirror . the masking observations of mira  ab used @xmath11  eri , @xmath6  ceti and @xmath6  tau as interferometric calibrators . these data were obtained in november 2006 at 7.7@xmath1 m , 9.7@xmath1 m , 12.3@xmath1 m and 18.3@xmath1 m . due to a greater instrumental stability , these data were of better quality than the keck data , although the resolution was slightly less and as mira  a was near minimum , the contrast between mira  a and the dust shell was lower . images reconstructed using macim are shown in figure  [ figgeminiims ] - again showing a clear feature in the vicinity of mira  b in three out of four filters . astrometry and flux estimates were obtained in a similar way to the keck data , with fits summarized in table  [ tabbinaryfits ] . flux ratios and astrometry were consistent between the keck and gemini data . in addition to these mid - infrared aperture - masking observations , we analyzed earlier keck / lws observations of @xmath5  ceti from september 1999 and august 2002 made without the use of a mask or segment - tilting . rotating the images 180 degrees and subtracting enabled most of the primary psf to be removed . again , a bright feature in the vicinity of mira  b was detected at a high level of confidence . the contrast ratios obtained from these data were 0.102@xmath120.022 in september 1999 ( jd 2451423.6 ) and @xmath13 in august 2002 ( jd 2452506.6 ) . the errors were estimated by applying the same analysis method to 6 calibrator star observations . other observations at wavelengths between 0.97 microns ( hst , nicmos data set n4rk02otq ) 3.08 microns ( aperture - masking ) and 4.67 microns ( direct imaging ) did not detect the companion , but were able to place 2-@xmath14 upper limits on the contrast ratio of 500:1 , 700:1 and 700:1 respectively . all observations reported in this paper as well as previously published hst observations , are summarized in table  1 , with the spectral - energy distribution shown in figure  [ figsed ] . in addition to these observations , the paper of @xcite provided model - fits to infrared spatial interferometer ( isi ) observations with a dust ` clump ' near the location of mira  b. they reported the separation and position angle of this clump to be 0.55@xmath15 and 120 degrees , but with no estimate of uncertainty due to the limited uv coverage of the array . the flux ratios of the clump were not given , although the dust has an optical - depth near unity and a linear size of 17 au . lllrrrrrr 0.346 & foc & 2450063.3 & 0.70 & 11.0@xmath121.6 & 578@xmath122 & 108.3@xmath120.1 & 10.14 m & 10.23 m + 0.501 & foc & 2450063.3 & 0.70 & 0.88@xmath120.12 & - & - & 10.48 m & 11.30 m + 0.374 & stis & 2453051.8 & 0.65 & 0.80@xmath120.04 & 546@xmath123 & 105.8@xmath120.3 & 11.07 m & 11.95 m + 0.97 & nicmos & 2451037.3 & 0.62 & @xmath30.002 & - & - & - & - + 3.08 & nirc & 2453252 & 0.24 & @xmath30.0014 & - & - & - & - + 4.67 & nirc2 & 2453909.5 & 0.22 & @xmath30.0014 & - & - & - & - + 18.75 & lws & 2451423.6 & 0.77 & 0.102@xmath120.022 & 476@xmath1220 & 99@xmath122 & 1050j & 107@xmath1228j + 17.65 & lws & 2452506.6 & 0.02 & 0.11@xmath120.03 & 511@xmath1234 & 96@xmath124 & 1020j & 112@xmath1235j + 10.7 & lws & 2453248 & 0.24 & 0.022@xmath120.003 & 470@xmath1214 & 102.7@xmath121.6 & 4190j & 92j + 12.5 & lws & 2453248 & 0.24 & 0.021@xmath120.003 & 489@xmath1214 & 105.2@xmath122.0 & 2450j & 51j + 7.9 & t - recs & 2454053.6 & 0.65 & @xmath160.005 & - & - & 1780j & - + 9.7 & t - recs & 2454053.6 & 0.65 & 0.025@xmath120.012 & 480@xmath1230 & 101@xmath123 & 2040j & 51j + 12.3 & t - recs & 2454053.6 & 0.65 & 0.027@xmath120.009 & 467@xmath1230 & 104@xmath123 & 1410j & 38@xmath1213j + 18.3 & t - recs & 2454055.6 & 0.65 & 0.065@xmath120.02 & 487@xmath1230 & 105@xmath123 & 1690j & 110@xmath1235j +   + [ tabbinaryfits ] in order to find a preferred model for the mid - infrared emission coming from the region around mira  b , we must explain the time - dependent astrometry and the total flux ( i.e. the energetics ) of the emission . we will now examine several hypotheses in turn . if the radiation near mira  b were re - processed radiation from mira  a , then the flux would change with the mira variability cycle . there is a decrease in flux at the most temperature - sensitive wavelength of 10@xmath1 m for the feature near mira  b occurring over the interval between the keck ( lws ) and gemini ( t - recs ) observations . this decrease co - incides with a decrease in the flux from mira  a due to the mira variability cycle ( see table  [ tabbinaryfits ] ) , consistent with the re - processsed radiation hypothesis . this is further supported by the modeling of @xcite , where a constant amount of dust was used to model observations at several epochs . one possible origin is therefore re - radiated flux from an unusually dense clump in the wind from mira  a.    the likelihood of a clump of dust in the wind causing the mid - infrared emission is not great , as an optical - depth of order unity is required to produce enough emission . in turn , this requires a wind over - density of at least 25 ( based on full silicate condensation at solar metallicity ) .    we can be slightly more sophisticated and model this dust clump as a uniform slab , assuming that the dust is made from a combination of fully - condensed corundum and olivine at solar metallicity . we take the optical constants for olivine from @xcite and those of corundum from @xcite and calculate dust opacities as in @xcite . from three - color photometry , we can derive a column density , a temperature and an angular extent as the radius of an equivalent uniformly - illuminated disk . for the near - maximum lws data , we obtain 0.28g@xmath17 , 310k and a disk radius of 59mas , and for the near - minimum t - recs data we obtain 0.2g@xmath17 , 240k and 96mas . these temperatures are also consistent with the assumption of heating of the dust by mira  a for moderate ( roughly 45 degree ) angles between the clump to the line - of - sight . these column densities can be compared to those of a 100au deep column in the time - averaged wind at the @xmath050au projected separation , which totals only 0.007g@xmath17 . these calculations are somewhat dependent on the assumed and unknown dust opacities , but they serve to provide plausible physical model parameters for the emission . the large required column densities are due to the emission in the 12@xmath1 m bandpasses being brighter than expected from optically - thin emission in the 10 and 18@xmath1 m bands . alternatively , one can think of the emission at 10 and 18@xmath1 m as being suppressed due to the dust being optically - thick . the modeling of the fourier data in @xcite put a dust ` clump ' near the location of mira  b between 1990 and 1994 . the position angle of 120 degrees and separation of 0.55@xmath15 places this emission inside the early-1990s position of mira  b but outside the position of the dust clump reported in this paper . therefore , not only is this clump of dust inexplicably over - dense , but its apparent motion is in the opposite direction to the outflow from mira  a , which should move outwards at 14mas per year according to the outflow measurements of @xcite with an assumed distance of 107pc . this would mean that if the dust ` clump ' seen by keck and gemini moved with the mira  a outflow , it would have been at @xmath18 during the @xcite epoch . despite the simple modeling in that paper , it is not plausible that the clump separation was overestimated by a factor of 1.8 , or that in addition to the 0.55@xmath15 clump , a ( brighter ) @xmath00.31@xmath15 clump was missed in the analysis .    therefore , due to both the anomalously high optical depth of dust near mira b and the motion of the emission counter to to the prevailing wind from mira  a , we conclude that the mid - infrared emission is inconsistent with a clump in the wind from mira  a.      although the variability of the mid - infrared flux near mira  b is consistent with re - processed radiation from mira  b , the phase coverage and signal - to - noise of the data in table  [ tabbinaryfits ] is clearly inadequate to exclude processes independent of mira  a s radiation field . therefore , we will check if the energy involved in the wind interaction region is enough to explain the mid - infrared emission . @xcite model the early 2004 outflow from mira  b as a 450kms@xmath4 wind losing @xmath19yr@xmath4 . this wind is variable , with evidence that it was weaker in the late 1990s and stronger in the iue era @xcite . as this wind encounters the more dense wind from mira  a , it will lose it s energy in a shock front . this shock luminosity ( @xmath20 ) will be @xmath21 , which is a tiny fraction of the mid - infrared luminosity coming from near mira  b , which is @xmath22 . thus , if the mid - infrared emission were to originate from the wind - interaction region , it would have to be due to a quasi - static increase in the density of mira  a s wind due to the wind from mira  b. according to the model of section  [ sectclump ] , this region has to have at least @xmath23 solar - masses of material , over a length scale of @xmath24au . 20au is roughly the maximum distance over which the flux would still be largely unresolved in our images , once projection effects are taken into account .    the simplest geometry in which to consider the outflow from mira  b is one where the wind from mira  b is largely perpendicular to the wind from mira  a. in this geometry , the mass excess due to the slowing of mira  a s wind by mira  b s wind is of order @xmath25 over length scales @xmath26 , which is @xmath27 , or 6 orders of magnitude too little material . the outflow from mira  b may be nearly spherically - symmetric , including a component as just described , as well as a region where the two winds collide nearly head - on , possibly forming a stagnation point . calculation of the stagnation point position is a simple balance of dynamic ( ram ) pressures @xmath28 , where @xmath29 is the wind density and @xmath30 the wind speed ( for either mira  a or mira  b ) . this stagnation point is significantly influenced by the gravitational field from mira  b : both by an change in density @xmath29 of mira  a s wind and an increase in velocity . we have taken these effects into account using newtonian physics , and calculated the stagnation point position . with mira  b wind parameters of @xmath31@xmath32 at 450kms@xmath4 , mira  a wind parameters of @xmath33@xmath32 at 6.7kms@xmath4 @xcite a mira  ab distance of 90au and a mira  b mass of 0.7m@xmath34 ( see section  [ sectdisk ] ) , this stagnation point occurs only 1au from mira  b ( projection effects not taken into account ) . an upper limit for this distance is the 3.7au calculated in @xcite for the stronger iue era wind , with different assumed wind and orbital parameters . therefore , a large build - up of dust at this stagnation point could not form a region geometrically large enough to provide the mid - infrared flux . t  tauri stars are well known to have significant mid - infrared excesses . as accretion onto mira b occurs through a disk , we can also ask if the emission is consistent with expectations from known and well - characterized accretion disks . by comparing the emission of the mira  b system to t  tauri stars ( i.e. systems with significant accretion luminosity ) as in figure [ figsed ] , it is clear that although the blue and ultraviolet accretion luminosity is typical , the very large mid - infrared flux from mira  b is anomalous by more than a factor of 50 . therefore , this emission must be due to a mechanism atypical of isolated systems with a disk and outflow . furthermore , the problem of the in - phase variation with the mira  a primary presents an additional stumbling block to this model . we will now examine a model of the mid - infrared emission near mira  b that is caused by illumination of an accretion disk around mira  b by radiation from mira  a. in this section we will not discuss the possibilities of this disk being around a white dwarf or a main sequence star . this disk will be supposed to be in the plane of the mira  ab orbit , so that one edge is illuminated and the opposite edge is in shadow . this co - planar geometry is expected if angular momentum is preferentially accreted in the plane of the orbit . it is illustrated in figure  [ figquickdiagram ] . first , we shall review the theory of wind accretion as it applies to mira  ab . the accretion of the wind from mira  a onto mira  b is given by the formulae of bondi - hoyle - lyttleton accretion ( e.g. * ? ? ? * ) . in its simplest form , the wind is gravitationally focused into a wake behind the accretor , and the wake is accreted wherever it is gravitationally bound to the accretor . this gives an accretion radius , which is the radius of a circular area that corresponds to the effective cross - section of the accretor :    @xmath35    here @xmath36 is the accretor ( i.e. mira  b ) mass , @xmath30 the wind velocity and @xmath37 the orbital velocity . this formula is most applicable to highly supersonic flows , which is the case for mira  b accreting the wind from mira  a. this accretion radius can be used to find an accretion rate :    @xmath38    the accretion luminosity is then given by the simple formula @xmath39 , where @xmath40 is the radius of the companion . this formula assumes an equilibrium situation , which may not be the case if material is accumulating within the disk .    in order to test hypotheses that the accretion disk around mira  b has been directly observed , we should first examine some basic parameters of the system . the best estimate for @xmath41 is @xmath42 per year , with @xmath43kms@xmath4 @xcite . this mass - loss rate is based on a co to h@xmath44 ratio of @xmath45 which is in turn based on a near - solar metallicity . if mira  a has sub - solar metallicity as expected from its kinematic association with the thick - disk population , then this mass - loss rate will be underestimated . the orbit of mira  b is poorly known due to it s long period , but if we assume a circular orbit and fix the total system mass to 1.5@xmath46 , then the astrometry of @xcite combined with the most recent hst epoch ( in table  [ tabbinaryfits ] ) gives a crude orbital radius of @xmath47 au , an inclination is 63 degrees and a period of 610 years . changing the assumed total system mass to 2.0@xmath46 only changes the orbital radius to 85 au and the inclination to 66 degrees . for the total mass 1.5@xmath46 orbit , we have an orbital velocity @xmath48kms@xmath4 and an accretion radius in au of 32@xmath49 according to equation  [ eqnbhradius ] . the maximum size of the accretion disk around mira  b will be approximately the bondi - hoyle accretion radius , as beyond this radius the wind from mira  a will have enough energy to both escape from the gravity of mira  b and transfer some momentum onto material orbiting around mira  b. conversely , the minimum accretion radius is the size of the disk measured from the difference in astrometry between mira  b and the mid - infrared emission . in the plane of the sky , the mid - infrared emission is separated from mira  b by @xmath50au . the error in this astrometric difference is difficult to quantify in individual images , particularly given the possibility that asymmetries in the atmosphere of mira  a could move its centroid around by of order 1au . however , even if we do not count the @xcite result as a single measurement , then we have 6 independent measurements of the astrometric difference in table  [ tabbinaryfits ] that give the same sign and a similar magnitude . determination of the average separation between mira  b ( from the uv ) and the mid - infrared emission therefore appears statistically robust with reliable errors . the true separation of mira  b and the mid - infrared emisson is then @xmath51au , given that the vector between mira  a and b is at 38 degrees to the line - of - sight from our preferred orbit . this is consistent with a disk @xmath52 au in size once the effect of the curved geometry of the disk edge is taken into account . this disk size is is consistent with bondi - hoyle accretion theory , predicting an accretion radius range from 16 to 22au for @xmath36 between 0.5 and 0.7 @xmath53 . importantly , the disk size shows that the accretion radius can not be under - predicted using bondi - hoyle accretion theory by more than a factor of @xmath02 . we must also establish that the amount of mid - infrared emission seen from the edge of the disk around mira  b is consistent with the observed emission . already in section  [ sectclump ] we have seen that the emission is consistent with a region of cross - sectional area about 100 square au at temperature 310k ( from the slab model with a radius of 59mas ) . if mira  b is on the far side of mira  a as viewed from earth , we see the edge of the disk that is directly illuminated by mira  a.    if the disk edge has a temperature of 310k , then its hydrostatic equilibrium density distribution is gaussian with full - width half - maximum ( fwhm ) of 1.5au at a 10au radius . this disk of diameter 20au would only give a 100 square au cross - sectional area if the disk remained optically - thick to radiation from mira  a out to several times the density fwhm . however , given the significant uncertainties and simplifications in the slab model ( including the opacities themselves ) , this emission is certainly consistent with an origin in a side - illuminated accretion disk .    note also that the spectral energy distribution shown in figure  [ figsed ] is not consistent with the optical depth of unity temperature being the same throughout the mid - infrared : the sillicate emission peak and the upper limit at 7.9@xmath1 m ( from the near - minimum gemini epoch ) demonstrate that the outer surface of the disk has to be heated . this is an essential part of any physical model of the disk , where radiative energy flows inwards . given that we have established that mira  b has a @xmath010au radius dusty disk around it , we should establish whether the term ` protoplanetary ' can be applied to it . this term implies that there is or will be enough material in the disk to form planets . according to the prescription of @xcite , the minimum - mass solar nebula had 2 jupiter masses out to a radius of 10au . the total mass accreted onto mira  b during the lifetime of mira  a is of order 7 jupiter - masses , assuming that mira  b has 0.7 solar - masses and mira  a loses 0.6 solar masses while on the asymptotic giant branch . the ability for such a disk to form planets clearly depends on a number of factors , such as the role of viscosity in the disk and the maximum accretion rate that will occur as mira  a s wind transitions to a radiatively driven wind with a @xmath020 times increase in mass - loss rate .    according to the viscous evolution prescription of @xcite with their ` typical ' disk parameters , the current equilibrium disk mass around mira  b is only 0.26@xmath54 based on a @xmath550.7@xmath46 , an accretion rate given by equation  [ eqnbhacc ] and a disk radius of 10au . the time scale for viscous evolution of the disk is @xmath56 years . as the time scale for mira  a s evolution is based on the time between thermal pulses which is of order @xmath57 years @xcite , the disk will likely maintain a near - equilibrium mass . at the time mira a becomes a white dwarf , and the mass - loss rate from mira  a is @xmath58 solar - masses per year , the equilibium disk mass will have increased to several jupiter - masses , certainly enough to justify the title ` protoplanetary disk ' . now that we have established that mira  b has a @xmath59au accretion disk around it , we should re - examine the idea that mira  b is a main - sequence star and not a white dwarf as often identified in the literature . the mira  ab system is quite different to symbiotics such as mwc  560 , where the accretion luminosity is 1000 instead of 0.1@xmath60 . however , accretion luminosity should vary by several orders of magnitude for symbiotics due to differing accretion rates , and as mira  ab is unique in its relatively well - known geometry and wind properties , it is difficult to make quantitative comparisons . a second general property of symbiotics such as mwc  560 is that there are spectral features with wind absorption and emission profiles with widths that are one to several thousand kms@xmath4 @xcite , comparable to or greater than the escape velocity from the surface of a white dwarf . despite the large number of lines observed for mira  b ( e.g. * ? ? ? * ) , there are no lines with half - widths of several thousand kms@xmath4 as one would expect if mira  b were a white dwarf . the analysis of section  [ sectdisk ] demonstrated that the observed size of the accretion disk around mira  b is consistent with bondi - hoyle accretion , and that this accretion theory can not underestimate the accretion rate onto mira  b by more than a factor of a few . on the other hand if we assume that mira  b is a 0.6@xmath46 white dwarf , then the same analysis gives an accretion rate of @xmath61/yr and an accretion luminosity of 9.4l@xmath62 . this is inconsistent with the measured accretion luminosity of mira  b , which is @xmath63 , depending on what fraction of the flux is from accretion and what the extinction is . the value of @xmath64 is an upper limit based on all the measured luminosity at @xmath65 m being due to accretion and @xmath66 , the extinction calculated from full silicate condensation in mira  a s wind . there is a possibility that the accretion luminosity is lower than expected for mira  b due to a non - equilibrium situation where mass is accreted into a disk but not onto mira  b itself . the disk evolution time - scale discussed in section  [ sectdisk ] makes this a highly unlikely scenario . if on the other hand , we assume that mira  b is a 0.7@xmath46 main - sequence star , then the accretion luminosity is 0.2@xmath67 , completely consistent with the measured luminosity of mira  b. finally , if mira  b were a main - sequence star , we could expect to see spectral features that indicate the presence of a main - sequence star , although these features could be very weak . the weak features are due to significant veiling by the accretion flux : a good example of this is for t  tauri stars can be found in the spectrum of bp  tau , which is significantly veiled for all wavelengths shorter than 0.5@xmath1 m @xcite . the faint object camera ( foc ) spectra partially published in @xcite do indeed show weak ( i.e. veiled ) spectral features of an early - mid k dwarf . we have examined the full wavelength range of this spectrum ( including that not published ) , derived from hst data set x31g010at , and displayed the long - wavelength end of it in figure  [ figkdwarf ] . we took the wavelength scale calibration from chapter  8 of the foc manual , referencing the zero point to the mg ii doublet near 2800prominent in the mira  b spectrum . prominent spectral features at 4250and 5100are both detected at @xmath68 , matching a veiled model @xmath04000k dwarf spectrum well . the @xmath060% veiling in v - band places the photospheric spectrum of mira  b at an absolute v magnitude of @xmath69 , also consistent with a 0.7@xmath46 k5 dwarf ( with @xmath70 mag of extinction at v band ) . all remaining possibilities for the nature of mira  b that could influence our conclusions consist of some kind of exotic geometries . an example could be a triple system with a radial velocity signature less than @xmath71kms@xmath4 ( so that it is not betrayed by ultraviolet spectral shifts ) and a close - system separation less than @xmath72au . in the absence of any data to support such a possibility , we will eliminate these possibilities with an appeal to occam s razor . therefore , we conclude that mira  b is indeed a main - sequence star , with a most probable mass of roughly 0.7@xmath46 .      from the standpoint of population synthesis , we can examine how common we expect mira  b like systems to be . our current galactic location in - between spiral arms means that the column - integrated star formation rate is about a quarter of what it was when mira  a s progenitor was born @xcite . in order to accrete at least a jupiter mass via wind - accretion , a wind - accreting binary should have log(p ) between 4 and @xmath06.5 in days ( the upper limit for p is dependent on primary and secondary mass ) . according to @xcite , @xmath023% of solar - mass stars have companions with log(p ) in this range . these results , taken together , imply that the birth - rate of mira  b - like disks around companions to 1 - 2@xmath53 stars should be similar to the local birth - rate of 1 - 2@xmath53 stars themselves . this immediately raises the question : where are the disks that formed by wind - accretion ? most debris disks in the solar neighborhood are around stars likely belonging to young associations @xcite . however , these debris disks are also generally cool and large , while a wind - accretion disk will be truncated by the dynamical pressure of the wind at roughly the bondi - hoyle accretion radius . the disks around secondaries in wind - accretion binaries should grow viscously after the primary dwindles into a white - dwarf , but clearly this process must leave debris disks much less often than the large primordial disks around young stars . the expected abundance of disks around companions to young white dwarfs is a clearly testable hypothesis of the bondi - hoyle truncation model of wind - accretion disks presented here . however , in order to test it , a sample of very young white dwarfs in binaries would have to be chosen . there are some such systems discovered through uv excesses : @xcite discusses many of them and some of the difficulties in forming a near - complete sample . other systems are discovered through infrared excesses to white dwarfs : many of these have been imaged with hst @xcite . disk fraction around the main sequence companions should approach unity for appropriate separations and ages less than several times the disk viscous evolution time - scale , i.e. a few @xmath73 years . this paper has presented evidence that mira  b is a @xmath00.7@xmath46 main sequence star surrounded by a @xmath59au radius disk . the new observations on which this is based were made possible by the high - contrast imaging capability of non - redundant interferometry on single - aperture telescopes . mira  ab is unique amongst wind accretion binaries in its well known binary and wind parameters , but this class of system is predicted to be relatively common as the end product of roughly 1 in 5 star systems with a solar - type primary . systems like mira  ab should produce the clear observational signatures of an accretion disk around the secondary for at least a few @xmath73 years after the primary becomes a white dwarf . we gratefully acknowledge the support of charles townes , the assistance of marc kassis in making the keck observations , the assistance of adwin boogert in planning the gemini observations and helpful discussions with nuria calvet and klaus pontoppidan . m.i . would like to acknowledge michelson fellowship support from the michelson science center and the nasa navigator program . this research has made use of the simbad database , the ines principle center and the iraf point source catalog . j.d.m . acknowledges support from the grant nasa - jpl 1267021 . some of the data presented herein were obtained at the w.m . keck observatory , which is operated as a scientific partnership among the california institute of technology , the university of california and the national aeronautics and space administration . the observatory was made possible by the generous financial support of the w.m . keck foundation . the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of mauna kea has always had within the indigenous hawaiian community . we are most fortunate to have the opportunity to conduct observations from this mountain . based in part on observations obtained at the gemini observatory , which is operated by the association of universities for research in astronomy , inc . , under a cooperative agreement with the nsf on behalf of the gemini partnership : the national science foundation ( united states ) , the particle physics and astronomy research council ( united kingdom ) , the national research council ( canada ) , conicyt ( chile ) , the australian research council ( australia ) , cnpq ( brazil ) and conicet ( argentina ) .                                          , b. , danchi , w.  c. , bester , m. , hale , d.  d.  s. , lipman , e.  a. , monnier , j.  d. , tuthill , p.  g. , townes , c.  h. , degiacomi , c.  g. , geballe , t.  r. , greenhill , l.  j. , cruzalebes , p. , lefevre , j. , mekarina , d. , mattei , j.  a. , nishimoto , d. , & kervin , p.  w. 1997 , , 488 , 807
the mira  ab system is a nearby ( @xmath0107pc ) example of a wind accreting binary star system . in this class of system 
 , the wind from a mass - losing red giant star ( mira  a ) is accreted onto a companion ( mira  b ) , as indicated by an accretion shock signature in spectra at ultraviolet and x - ray wavelengths . using novel imaging techniques , we report the detection of emission at mid - infrared wavelengths between 9.7 and 18.3@xmath1 m from the vicinity of mira  b but with a peak at a radial position about 10 au closer to the primary mira  a. we interpret the mid - infrared emission as the edge of an optically - thick accretion disk heated by mira  a. the discovery of this new class of accretion disk fed by m - giant mass loss implies a potential population of young planetary systems in white - dwarf binaries which has been little explored , despite being relatively common in the solar neighborhood .