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au  mic is a nearby ( @xmath4  pc , * ? ? ? * ) m1ve flare star @xcite with the young age of @xmath5  myr @xcite , a key epoch in the formation of planetary systems . the star is surrounded by a nearly edge - on circumstellar disk discovered in coronographic images of scattered starlight that extends to a radius of at least 210  au @xcite . since the small grains responsible for scattering should be removed rapidly by stellar pressure forces , the disk is thought to consist of the collisional debris of unseen planetesimals experiencing ongoing impacts ( for recent reviews of the debris disk phenomenon , see * ? ? ? * ; * ? ? ? because of its proximity , the au  mic debris disk has become one of the best studied examples at optical and infrared wavelengths , including detailed imagery of both intensity and polarization from the _ hubble space telescope _ @xcite . many observational properties of the au  mic debris disk bear striking similarities to the archetype debris disk surrounding @xmath2  pic , also viewed nearly edge - on and located in the same young moving group . in particular , the midplane optical surface brightness profiles of these disks are remarkably alike , with shallow inner slopes ( @xmath6 ) that steepen substantially ( @xmath7 ) in the outer regions , near 35  au and 100  au for au  mic and @xmath2  pic , respectively @xcite . these similarities inspired the development of a unified framework for debris disks based on a localized belt of planetesimals , or `` birth ring '' , that produces dust in a collisional cascade @xcite . the smallest grains are blown out from the belt by radiation and winds from the star , larger grains are launched into eccentric orbits with the same periastron as the belt , and the largest grains , which are minimally affected by non - gravitational forces , do not travel far from the belt before being ground down . the grains are therefore segregated according to their size , which gives rise to the characteristic scattered light profile . for these dynamics to prevail , the disks must be substantially free of gas @xcite , a property confirmed by measurements at ultraviolet wavelengths @xcite .    a `` birth ring '' , if present , would remain hidden in optical and infrared images dominated by small grains that populate an extended halo ( e.g. , * ? ? ? by contrast , observations at ( sub)millimeter wavelengths highlight thermal emission from the largest grains and hence the location of the dust - producing planetesimals @xcite . to date , the best case for a `` birth ring '' comes from millimeter observations of @xmath2  pic @xcite . while the optical disk of @xmath2  pic extends more than @xmath8  au from the star , the millimeter imaging reveals a much more compact belt of emission at @xmath9  au radius . this millimeter emission belt coincides with the change in the slope of the optical surface brightness , which in the models marks the outer extent of the colliding planetesimals . previous ( sub)millimeter - wave observations of au mic did not have sufficient angular resolution to reveal much about the emission morphology . a detection at 850  @xmath10 m using jcmt / scuba ( @xmath11 beam ) indicated a reservoir of cold dust with mass @xmath12  m@xmath13 , but did not resolve any structure @xcite . subsequent observations at 350  @xmath10 m using cso / sharc  ii ( @xmath14 beam ) marginally resolved an orientation compatible with the scattered light , but were otherwise limited @xcite . in this _ letter _ , we present imaging observations of au mic at 1.3  millimeters using the submillimeter array ( sma ) that obtain @xmath0 resolution and provide evidence for a planetesimal belt . we observed au  mic with the sma @xcite on mauna kea , hawaii at 1.3  millimeters wavelength using the compact and extended configurations of the array . table  [ tab : obs ] provides basic information about these observations , including the observing dates , baseline lengths , and atmospheric opacities . the weather conditions were good for all of these observations and best for the two compact configuration tracks when only 6 of the 8 array antennas were available . the phase center was @xmath15 , @xmath16 ( j2000 ) , offset by @xmath17 from the location of the star . the @xmath18 ( fwhm ) field of view is set by the primary beam size of the 6  meter diameter array antennas . the total bandwidth available was 8  ghz derived from two sidebands spanning @xmath19  to  @xmath20  ghz from the lo frequency . time dependent complex gains were calibrated using observations of two quasars , j2101 - 295 ( 3.9 degrees away ) and j1924 - 292 ( 17.4 degrees away ) , interleaved with observations of au mic in a 15  minute cycle . the passband shape for each track was calibrated using available bright sources , mainly j1924 - 292 . the absolute flux scale was set with an accuracy of @xmath21 using observations of callisto or ganymede in each track . all of the calibration steps were performed using the idl based mir software , and imaging and deconvolution were done with standard routines in the miriad package . we made a series of images with a wide range of visibility weighting schemes to explore compromises between higher angular resolution and better surface brightness sensitivity . lcccc array configuration & extended & compact & compact & compact + number of antennas & 8 & 6 & 6 & 7 + baseline lengths ( m ) & 10189 & 868 & 868 & 868 + lo frequency ( ghz ) & 235.6 & 225.4 & 225.4 & 235.6 + 225 ghz atm . opacity & 0.10 & 0.100.06 & 0.06 & 0.08 + figure  [ fig : image ] shows a contour image of the 1.3  millimeter emission overlaid on a _ hubble space telescope_/acs coronographic image of optical scattered light ( f606w filter ) from @xcite . the synthesized beam size for this 1.3  millimeter image is @xmath22 ( @xmath23  au ) , position angle @xmath24 , obtained with natural weighting and an elliptical gaussian taper @xmath25 ( fwhm ) oriented east - west to make the beam shape more circular . the rms noise in this image is 0.40  mjy  beam@xmath26 , and the individual peaks have a signal - to - noise @xmath27 . the star symbol marks the stellar position corrected for proper motion , offset by @xmath28 from the phase center . the image shows a resolved band of 1.3 millimeter emission that extends approximately symmetrically from the stellar position to the southeast and northwest . by fitting a line to the positions of the two peaks in the image , we estimate the position angle of the emission structure to be @xmath29 , in excellent agreement with the disk orientation inferred from scattered light observations @xcite . from its position angle and double peaked morphology , we identify the emission structure as a limb - brightened dust belt . note that thermal emission from the stellar photosphere contributes only @xmath30  mjy at this wavelength , and even the strongest synchrotron radio flares from stellar activity should remain well below the noise @xcite . the total flux density obtained by integrating over a box that surrounds the emission is @xmath31  mjy , where the uncertainty is estimated conservatively by evaluating boxes of the same size in emission - free regions of the image . no other significant features are detected in the field of view . an extrapolation of the measurements at shorter wavelengths indicates that missing flux due to the spatial filtering by the sma is not a significant issue for these observations . the spectral index between 350  @xmath10 m @xcite and 850  @xmath10 m @xcite is @xmath32 , which predicts @xmath33  mjy at 1.3  millimeters , in good agreement with the sma measurement . this shallow index is consistent , within the uncertainties , with values near 2.0 determined for a sample of debris disks by @xcite , notably for stars later than f - type where dust temperatures are so low that the rayleigh - jeans approximation in this part of the spectrum is invalid . we conclude that the compact structure detected by the sma accounts for all of the 1.3  millimeter emission in the au  mic system .      to characterize the millimeter emission structure , we use a simple parametric disk model , following closely the method used by @xcite for analyzing similar observations of the @xmath2  pic disk . we assume the emission arises from a geometrically thin , axisymmetric belt with a radial surface brightness profile given by @xmath34 . this functional form is intended to capture the essence of low optical depth emission from a disk with a spatially invariant dust emissivity , surface density profile @xmath35 , and temperature profile that falls off with radius as a power - law . we fix the power - law index @xmath36 to approximate radiative heating from the central star , and we consider two shapes for @xmath35 : ( 1 ) an annulus with power law slope , @xmath37 for @xmath38 , @xmath39 , and ( 2 ) a gaussian , @xmath40}$ ] . in each case , the model is described by three parameters : a center radius @xmath41 , width @xmath42 , and total flux density @xmath43 . we fix the disk inclination at @xmath44 and position angle at @xmath45 , as determined from scattered light data @xcite , and place the origin within @xmath46 of the nominal stellar position ( within the sma astrometric accuracy ) . note that the vertical thickness of the millimeter grain population is expected to be @xmath47 @xcite , negligible compared to the resolution of the data . the utility of this simple modeling approach is to provide basic constraints while avoiding the multitudinous assumptions about grain properties , dynamics , and radiative transfer required in more sophisticated treatments .    to estimate the model parameters and their uncertainties , we calculate three - dimensional grids spanning appropriate ranges of the parameter values . for the gaussian model , the grid covers @xmath48  au and @xmath49  au in 1  au steps , and @xmath50  mjy in 0.2  mjy steps . for each position in a model grid , we compute a set of synthetic sma visibilities and compare directly to the data with a @xmath51 value ( the sum of real and imaginary components over all spatial frequencies ) that uses natural weights modified by the gaussian taper used for the image in figure  [ fig : image ] . the rightmost panel of figure  [ fig : model ] shows the @xmath51 surface in the @xmath52 plane for the gaussian model after marginalizing over @xmath53 . the best - fit model is marked by a cross , and the contours delineate ( marginalized ) @xmath54 intervals in @xmath55 . the best - fit center radius is @xmath56  au , with corresponding width @xmath57  au ( or fwhm = @xmath58  au ) and flux density @xmath59  mjy . the three lefthand panels of figure  [ fig : model ] show the original 1.3  millimeter image from figure  [ fig : image ] , the gaussian model image obtained using the best - fit parameter values and the same visibility weighting scheme , and the imaged residuals ( where data@xmath60model subtraction is conducted on the visibilities ) . this comparison shows clearly that the best - fit model reproduces the main features of the image and leaves no systematic residuals . the power - law models for @xmath35 give similar best - fit parameter values : for @xmath61 , we find @xmath62  au and @xmath63  au ( where the upper limit is set by the computational grid boundary ) . for @xmath64 , the best - fit locus is shifted to roughly @xmath65 larger values of @xmath41 , a manifestation of the degeneracy between the radial gradient and emission extent . all of these best - fit models reproduce the observed sma visibilities equally well ( i.e. , have effectively the same minimum @xmath51 values ) . the modest signal - to - noise of the observations preclude placing tighter constraints on the model parameters . nonetheless , it is reassuring that consistent results are obtained from the various assumptions about the emission morphology . each gives a similar center radius , as well as an effectively cleared central cavity with size compatible with that inferred from scattered light ( @xmath66  au * ? ? ? * ) and the lack of mid - infrared emission ( @xmath67  au * ? ? ? the best - fit gaussian model reproduces better the contrast of the maxima to the extended emission of the disk in the image plane , but there is no clear preference for any of the functional forms . of course , none of these _ ad hoc _ models are perfect , and more sensitive data will be needed to constrain further the details of the emission distribution . we have spatially resolved the thermal emission at 1.3 millimeters from the au  mic debris disk , thereby revealing the distribution of its millimeter grain population . these large grains are expected to have dynamics similar to the unseen dust - producing planetesimals , and the emission at this long wavelength provides a direct link to their spatial distribution . the overall structure of the au  mic millimeter emission is reproduced well by a belt surrounding the star centered near a radius of @xmath68  au . the inferred location of this emission belt coincides with the region over which the scattered light brightness profile of the disk steepens markedly , the feature that provided impetus for developing the `` birth ring '' model . like au  mic s more massive sister star @xmath2  pic @xcite , the resolved multi - wavelength data are consistent with a scenario where destructive collisions of planetesimals within the belt create grains with a spectrum of sizes , and the effects of size - dependent dust dynamics generate the millimeter emission and spatially extended scattered light nebula . for @xmath2  pic , the intense stellar radiation pressure can account for the large halo of small grains in the outer disk . but for au  mic , which is about two orders of magnitude less luminous , the much weaker stellar radiation field is apparently augmented by a stellar wind to expel small grains @xcite . though the fractional radial extent of the au  mic millimeter emission is not strongly constrained , the best - fit models suggest that it could be broad , @xmath69 . for @xmath2  pic , the millimeter emission is characterized by @xmath70 , with the upper end favored for the width of the underlying dust - producing planetesimal belt from detailed models of the knee of the scattered light profile @xcite . such broad belts are not unusual features of debris disks , in particular among the few resolved at millimeter wavelengths @xcite . an inner cavity devoid of dust is plausibly maintained by the combination of grinding collisions in the belt and clearing by stellar radiation forces , though a tenable alternative is that the inner regions are swept clear by a planet . planets seem to be the most viable explanation for the millimeter cavities common to the gas - rich transition disks around younger , pre - main - sequence stars @xcite . while a planet has been imaged to orbit within the cavity in the disk around @xmath2  pic @xcite , direct evidence for planets in the au mic system remains elusive , both from high contrast imaging observations @xcite and from transit searches @xcite . the outer boundary of the emission belts may reflect the outer extent of successful planetesimal formation , or it may simply mark the edge of the region where the planetesimal stirring mechanism has been successful at initiating dust - producing collisions , as in the models of @xcite . these new millimeter observations of au  mic show the global structure of large grains in its debris disk , but they do not have sufficient resolution and sensitivity to reveal any significant departures from axisymmetry or substructure that might result from gravitational interactions with unseen planets . the higher resolution images of scattered light clearly show several radial and vertical inhomogeneities at subarcsecond scales ( features a - e , see * ? ? ? * ; * ? ? ? * ; * ? ? ? if these substructures have counterparts at millimeter wavelengths , then future observations with the atacama large millimeter / submillimeter array ( alma ) may be able to detect them , pinpoint the location of parent bodies , and constrain their origins .
we present imaging observations at 1.3  millimeters of the debris disk surrounding the nearby m - type flare star au  mic with beam size @xmath0 ( 30  au ) from the submillimeter array . 
 these data reveal a belt of thermal dust emission surrounding the star with the same edge - on geometry as the more extended scattered light disk detected at optical wavelengths . 
 simple modeling indicates a central radius of @xmath1  au for the emission belt . 
 this location is consistent with the reservoir of planetesimals previously invoked to explain the shape of the scattered light surface brightness profile through size - dependent dust dynamics . 
 the identification of this belt further strengthens the kinship between the debris disks around au  mic and its more massive sister star @xmath2  pic , members of the same @xmath3  myr - old moving group .