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t tauri stars are thought to be young stars that are still accreting gas through a disk . @xcite discovered t tauri star kh 15d ( k7 v , @xmath2 pc ) , which shows an eclipse for 1/3 of its period , implying that it is obscured by faint circumstellar material rather than a stellar companion ; earlier data showed the star rebrighten at mid - eclipse , although this has weakened in recent data @xcite . this indicates that the obscuring material subtends a large angle , which may herald the presence a low mass stellar companion to shepherd the material via resonant interaction or to create a distorted disk @xcite . in addition , the duration of the eclipse trough is lengthening over time , changing from 16 days to 18 days over 5 years , and data from 50 - 90 years ago showed no evidence for eclipses @xcite , which means that the obscuring material is rapidly evolving . during eclipse , residual flux is still present at a level of @xmath31/20 of the flux outside of eclipse indicating that either ( a ) the obscuring material is porous ( covering factor is @xmath4 ) , ( b ) the eclipse is total , but scattered light `` fills in '' the eclipse , creating some residual flux , ( c ) some flux is scattered and some transmitted . the scattering interpretation may be favored as the star becomes slightly bluer during eclipse as one would expect from scattered light , although the fluctuations in color are comparable to the color difference @xcite . in addition , the equivalent width of h@xmath5 increases dramatically from 2    to @xmath6  indicating a change from an unobscured `` weak - line '' t tauri star to a classical t tauri star !    the eclipse provides a natural `` coronagraph '' during which the suppression of stellar light allows any scattering by surrounding material to become more prominent . we attempted to distinguish the above possibilities by carrying out spectropolarimetry inside and outside of the eclipse . since the unpolarized flux of the star dilutes the percentage of polarization , we expected that the percentage polarization due to scattered light would increase by a factor of at least 20 during the eclipse if the scattering material lies exterior to the occulting material and if the starlight is intrinsically unpolarized . this scenario is analogous to type 1 active galactic nuclei ( agn ) which are unobscured and show low polarization versus type 2 agn which are obscured and show much higher polarization @xcite . some t tauri stars show an anti - correlation between total flux and polarization ( e.g. * ? ? ? * ; * ? ? ? * ) , which may be exaggerated in kh 15d as the flux variations are much more extreme . on the other hand , if the residual flux is only due to incomplete coverage by the occulting material , then the polarization will not change . interstellar foreground polarization , which is measured to be 0.10.4% in the v - band for ngc 2264 @xcite , will not change during eclipse . before our observations , the optical polarization of kh 15d had not yet been measured , but during eclipse it shows the characteristics of classical t tauri stars which have a mean polarization of 1.2% with a standard deviation of 0.8% and maximum polarization of 2.5% @xcite . in  2 we summarize the spectropolarimetric observations , in  3 we give the results of the observations , in  4 we discuss the implications of these results for models of this t tauri star , and in  5 we review our conclusions . the primary observational goal was to measure the difference in polarization between eclipse and non - eclipse . outside eclipse , the star is sufficiently bright to observe at the palomar 200-inch telescope ( @xmath7 ) , but during the eclipse the star is a factor of @xmath8 fainter which required using keck to obtain a similar number of photons in a reasonable observing time . the palomar observations were taken with the double spectrograph spectropolarimeter @xcite , while the keck i observations were taken with the low resolution imaging spectrograph polarimeter @xcite . both instruments add dual - beam polarimetry optics to a double spectrograph incorporating dichroics to separate the blue and red beams . the observations and reductions were done using standard spectropolarimetry techniques @xcite . the observations of kh 15d were performed in sequences of four 15-minute exposures at four different positions of a rotating half - wave plate . each four - exposure sequence allows measurement of the stokes parameters @xmath9 and @xmath10 . for each hour of observations , the position angle of the spectrograph slit was aligned with the parallactic angle for the midpoint of the hour . the instrumental setup is described in table 1 , while the observational summary is given in table 2 . null , polarized , and flux standards were observed during each night . since instrumental polarization can be present in the palomar 200-inch @xcite , we observed four null standards each at two perpendicular slit position angles . we used the measured polarization of the null standards as an estimate of the mean and uncertainty of the instrumental polarization , and we subtracted the mean instrumental stokes parameters @xmath9 and @xmath10 from the measured stokes parameters of kh 15d to correct for the instrumental polarization . at palomar the instrumental polarization was found to be @xmath11% over @xmath12   and @xmath13% over @xmath14  . at keck the instrumental polarization was negligible , @xmath15% . at palomar the observations were obtained through clouds , while the keck nights were mostly clear , but not photometric . ccc dbsp , red & 5620 - 8120 & 2.45 dbsp , blue & 3350 - 5750 & 3.38 lrisp , red & 3200 - 5850 & 2.0 lrisp , blue & 5650 - 9400 & 2.0    llccl 2003 jan 25 & palomar & 3 & 2@xmath160 & partly cloudy2003 feb 23 & keck & 3 & 1@xmath165 & partly cloudy2003 feb 24 & keck & 1 & 1@xmath160 & clear2003 feb 25 & keck & 2 & 1@xmath165 & clear    a 7th magnitude b2iii star , hd 47887 , is located 39@xmath17 away from kh 15d . in the keck observations a diffraction spike from this bright star periodically swept through the slit , passing within several arcseconds of kh 15d in several exposures . in these exposures sky subtraction was difficult , making it impossible to accurately measure the polarization and also adversely affecting the color of the extracted spectra for two of the six hours . figure 1 depicts the phase of our observations with respect to the 48.36 day period of kh 15d . [ lightcurve ] the continuum polarization in the keck and palomar data was measured over wavelength bins of width 1000  . we averaged the 3 measurements from palomar and subtracted off the mean polarization measured from observations of 4 null - standard stars . we computed the mean of 4 measurements from keck and the error bars were computed from the standard deviation of these measurements plus the statistical error of the individual measurements . the palomar data show a small enough polarization and large enough errors that the data are consistent with being unpolarized at the @xmath3 1 @xmath18 level ( see figure 2 ) . however , the keck data showed a much higher polarization , and are inconsistent with the palomar data at a high significance . tables 3 and 4 summarize the polarization measurements of the eclipsed and uneclipsed data . the eclipsed data show a slight increase in polarization from the blue ( 4600 - 5600  ) to the red ( 7000 - 8000  ) at 3 @xmath18 significance . most of this change is due to an increase in the @xmath9 stokes parameter ( figure 2 ) , while @xmath10 remains constant ; however , the errors are large enough that a change in the polarization angle is only significant at 1.5 @xmath18 . lccc 46005600 & 0.26@xmath190.25 & 31@xmath1927 & 1.00@xmath190.0460007000 & 0.17@xmath190.21&-19@xmath1935 & 170008000 & 0.16@xmath190.23&-22@xmath1941 & 1.05@xmath190.04    lcc 40004600 & 1.46@xmath190.70 & 23.9@xmath1914.46005600 & 1.63@xmath190.19 & 21.8@xmath193.360007000 & 1.85@xmath190.14 & 18.0@xmath192.270008000 & 2.25@xmath190.10 & 16.8@xmath191.280009000 & 2.24@xmath190.29 & 15.5@xmath193.7    figure 3 shows the total flux spectrum in and out of eclipse . the spectral shape appears nearly identical in and out of eclipse , despite the change in total flux of @xmath20 magnitudes . we measured the mean of the ratio of the uneclipsed to the eclipsed data in each of the three bands ( 46005600 , 6000 - 7000 , and 7000 - 8000  ) , excluding the emission lines , and computed the standard deviation of the ratio within each band , which is reported in table 3 . the flux ratios differ between the bluest ( 4600 - 5600  ) and reddest ( 7000 - 8000  ) bands by only 1 @xmath18 , which is statistically consistent with no color change . the absolute value of the uneclipsed to eclipsed flux ratios are not to be trusted as the nights were not photometric , so we report only the ratio of each band to the 60007000   band . for comparison with the photometric data of @xcite we also computed synthetic photometry of our spectra which gave @xmath21 mag and @xmath22 mag during eclipse and @xmath23 mag outside of eclipse , which are in agreement with the range of values reported by @xcite . we estimate the uncertainty on these colors to be @xmath24 mag as there appear to be differences between individual spectra on scales of @xmath25   which may be due to fluctuations in the flat - fields , difficulty of sky subtraction , overall systematic uncertainties in flux calibration , or other instrumental variations . there are a large number of stellar photospheric features which are seen in both spectra and are indicative of a k6v spectral type with @xmath26 k , somewhat hotter than the k7v type estimated by @xcite which may be due to the fact that our spectrum covers bluer wavelengths . almost all of these features show the same equivalent widths in both spectra , indicating that the eclipsed spectrum is a scaled version of the uneclipsed spectrum . we have measured the lick indices @xcite for both the eclipsed and uneclipsed spectra , plotted in figure 4 . the h@xmath27 indices are contaminated by the [ sii ] emission line in the keck data . the na d equivalent width is smaller during the eclipse , which we attribute to unobscured na d emission filling in the absorption line . the measured indices for kh 15d agree best with those of a k6v star ; the agreement is good enough that there is no evidence for optical veiling . the h@xmath5 emission line has a much higher flux in the eclipsed spectrum than in the uneclipsed spectrum by a factor of @xmath28 ( since our data were not photometric , this factor is probably @xmath29 ) . this may be partly due to variability and partly due to partial eclipse of the h@xmath5 line as @xcite found that the flux of h@xmath5 could decrease or increase during eclipse and the shape of h@xmath5 varied significantly as well . they measured an equivalent width of 2   outside eclipse and 40 - 70 in eclipse , compared with our measurements of 3   and 30  , respectively . in addition , four emission lines appear in the eclipsed spectrum that are not apparent in the uneclipsed spectrum . the fluxes and equivalent widths of the emission lines are given in table 5 , which we have computed by subtracting off a continuum template , with errors estimated from the standard deviation of the residuals . included is the na d absorption line at @xmath30    which is stronger in the uneclipsed spectrum than in the eclipsed spectrum ( we have used the uneclipsed spectrum as the continuum template for this line ) . lcc   (  ) & ( @xmath31 erg @xmath32 s@xmath33 & (  ) @xmath34 6563 & 5.88 @xmath190.71 & 3.2@xmath190.39 @xmath34 6563 & 1.74 @xmath190.19 & 29.7@xmath193.2 @xmath34 4069 & 0.362@xmath190.025 & 22.2@xmath191.5 @xmath34 5890 & 0.153@xmath190.015 & 3.6@xmath190.36@xmath35 6716,6731 & 0.093@xmath190.013 & 1.6@xmath190.23@xmath34 6300 & 0.397@xmath190.065 & 7.0@xmath191.1 @xmath34 6364 & 0.132@xmath190.017 & 2.3@xmath190.3 the variation of the polarization between eclipse and non - eclipse indicates that the _ scattering region is not completely obscured by the occulting material_. if the occulter completely blocks all the light from the star , if the scattering is nearly isotropic , and if the scattering region is nearly spherically symmetric , then the residual flux , @xmath36% , indicates that the optical depth of the unobscured scattering material is @xmath37 . since the spectra inside and outside eclipse are consistent with no change in the spectral shape , and since the spectrum within eclipse is consistent with very weak wavelength dependence of the polarization , we conclude that the scattering mechanism is nearly achromatic , implying that the scatterers may be electrons or large grains . we discuss each of these possibilities in turn . if the scattering mechanism were electrons , then the low measured polarization of 2% indicates that the scattering region need be only slightly asymmetric . for example , in the model of @xcite , the polarization caused by an axisymmetric thomson scattering envelope is @xmath38 , where @xmath5 is a `` shape factor '' which depends on the precise geometry of the scatterers . since the eclipsing region is likely in the equatorial plane of the circumstellar material , we can assume that @xmath39 . thus , @xmath40 for @xmath41% . for a scattering geometry which consists of a slightly oblate or prolate spheroid of axis ratio @xmath42 , then @xmath43 , so @xmath44 to create the observed polarization , i.e. only a 20% deviation from spherical symmetry is required to give the measured polarization . note that this example is simply illustrative since we have no preferred model for the scattering region . if we assume that all of the light during eclipse is due to electron scattering , then we can estimate the bound - free continuum emission expected from this gas . consider a nearly symmetric region of uniform electron density and temperature surrounding the star of size @xmath45 . we assume that the eclipsing material is located near the keplerian radius , @xmath46 , for a 48 day period . the low polarization implies that the electron scattering region must be at a larger radius , @xmath47 au to avoid becoming too asymmetric due to obscuration . since the material is nearly symmetric , the mean optical depth of the scattering region is @xmath48 , which implies an electron number density of @xmath49 @xmath50 ( assuming a uniform density ) . the bound - free emission expected at , say , 6000   must be less than @xmath51 erg @xmath32 s@xmath52 to avoid changing the strength of the lick indices . computing the emission from an optically - thin spherical region of plasma in lte , we find that @xmath53 k to be consistent with this upper limit . note that this is a strong lower limit since it is likely that the scattering region need be larger to give the small observed polarization . in addition , one can place a constraint on the temperature based on the strength of balmer line emission ; however , the interpretation is complicated by the unknown optical depth of the balmer lines . even so , the lower limit on the temperature based on the lack of detected bound - free emission is four times the virial temperature at @xmath46 , meaning that this gas would be strongly unbound and will outflow in less than a dynamical timescale , implying a mass loss rate of @xmath54 yr @xmath55 , which is rather large for a weak - line t tauri star . thus , it is more likely that the scattering is not due to electrons , but may be due to large dust grains , which we discuss next . if the scattering were due to dust , then the grain sizes would have to be fairly large to allow the weak wavelength dependence of the polarization _ and _ flux . this is true for the zodiacal light which reflects an optical spectrum that is identical to that of the sun @xcite . based on the achromaticity of the zodiacal scattering , the scatterers are thought to be large dust grains , 10 - 100 @xmath56 m in size , and there is further evidence that these grains might be quite `` fluffy '' ( i.e. low filling factor ) with densities as low as @xmath57 g @xmath50 .    for the estimation of the wavelength dependence of the scattering and polarization by dust particles we approximate the particles as spheres ( mie scattering ) and compute the polarized differential scattering cross section by single particles using a standard approach @xcite.swolf / miex - web / miex.htm - or contact the authors . ] we have chosen the distribution of refractive indices of the materials from @xcite : 62.5% astronomical silicate and 37.5% graphite . since graphite is crystalline , it was considered as two `` different '' materials in the computation : the first with a refractive index as measured perpendicular to the crystal rotation axis , 25% , and the second parallel , 12.5% . we assumed that distribution of the dust size , @xmath58 , follows a power - law @xmath59 for @xmath60 , and derived the scattering properties of the ensemble of particle species and sizes ( * ? ? ? * wolf & voshchinnikov , in prep . ) . we averaged the scattered polarized flux over the entire @xmath61 steradians divided by the total scattered flux to estimate the mean polarization as a function of wavelength for each dust model . we then compared this to the relative wavelength dependence of the measured polarization ( the absolute polarization depends on the exact geometry of the scattering region , which is unknown ; see the next section for one particular model ) . we also compared the relative wavelength dependence of the scattered flux by dividing the eclipsed spectrum by the uneclipsed spectrum . given the assumptions that ( a ) the wavelength dependence of the flux and polarization does not depend on the geometry of the scatterers , and ( b ) the scatterers can be described by a single power - law , we can then place a constraint on the parameters of the dust model . we find that the lower limit , @xmath62 , does not change the fit much , so we simply vary @xmath63m@xmath64 m and @xmath65 . the minimum @xmath66 is 12 for 5 data points for @xmath67 m and @xmath68 , with the confidence contours plotted in figure 5 . at 90% confidence , @xmath69 and @xmath70m@xmath71 m , which indicates that the scattering is dominated by large dust grains . unfortunately , the minimum reduced @xmath72 , which is rather large . this is due to the fact that the measured polarization increases toward longer wavelengths , while the dust scattering models predict a polarization that is nearly constant , or slightly decreasing , with wavelength . the opacity of large dust grains scales as @xmath73 @xmath74 g@xmath55 , where @xmath75 m . thus , the eddington limit for the smaller of the large dust grains scales as @xmath76 erg s@xmath55 , which is similar to the luminosity of kh 15d , @xmath77 erg s@xmath55 . thus , smaller dust grains will be removed from the system by radiation forces . in addition , radiation forces on the larger grains may be significant enough to modify the structure of the system . @xcite have proposed a model for the occulter which consists of a semi - infinite edge crossing the star . this model is attractive as the finite size of the star and limb darkening can account for the shape of the ingress and egress , and , if the occulter is at the keplerian radius , then it must be inclined to the orbital plane by 15@xmath78 to give the correct duration of ingress and egress . however , we have found that the ingress and egress of the eclipse ( figure 1 , near phase of 0.3 and 0.7 ) show a more gradual rolloff than expected for a semi - infinite occulter . this can be fit with an optical depth of the occulter which scales with position as @xmath79 , where @xmath80 is the coordinate perpendicular to the edge of the occulter , which gives an excellent fit to the phased lightcurve data ( solid line in figure 1 ) . this material causing extinction will scatter as well , and in the case of large grains there will be significant forward scattering @xcite . in fact , this may account for the scattered flux present during the eclipse ; however , to compute the scattered light requires a knowledge of the three - dimensional distribution of scattering and occulting material , as well as a knowledge of the scattering properties . we construct such a model by assuming that ( a ) the occulter is located at the keplerian radius and ( b ) the 15@xmath78 tilt of the occulter is due to a warped disk . we model the material as a succession of tilted circular annuli , creating a warped disk , with the tilt angle given by @xmath81},\ ] ] where @xmath82 is the radius of maximum tilt , @xmath83 is the maximum tilt , @xmath84 is the radius in the disk , and @xmath85 is the width of the tilted region , and we assume that the nodes of the warped annuli are aligned . the observer we place at an angle @xmath86 to the equatorial plane , and we rotate the warped disk as a solid body with a period equal to that of kh 15d . in addition , we cut the disk off at @xmath87 and @xmath88 . the opacity of the scattering material we distribute as @xmath89 ^ 2 $ ] , to produce the observed roll - off in intensity , where @xmath90 is the angular scale height , @xmath91 is the polar angle , and @xmath92 is a fiducial opacity . we find a good fit to the lightcurve for @xmath93 , @xmath94 , @xmath95 , @xmath96 au ( this is the keplerian radius ) , @xmath97 . when taking into account only extinction , this distribution produces the solid lightcurve in figure 1 after the addition of 4.5% of the uneclipsed flux during eclipse . however , part of the extinction is due to scattering , so we also have computed the singly - scattered component of this density distribution assuming an albedo of unity and using the scattering function for zodiacal light derived by @xcite which is strongly forward scattering and has a maximum polarization of 22% . we find that the polarized flux of the scattered light during eclipse is 0.1% of the unobscured total flux , which is the same as the observed polarized flux during eclipse of kh 15d ( figure 6 ) . figure 7 shows the appearance of the scattered light in this model at mid - eclipse ( an animated version of this figure which shows the appearance as a function of phase is available on - line ) . one prediction of this model is that the polarization angle should vary by @xmath98 during the eclipse . however , the total model flux during mid - eclipse is only 2.5% of the uneclipsed flux which is about half the observed mid - eclipse flux , indicating that there may be another component of scattered light which has low polarization , such as diffracted light , which is not taken into account in the model of @xcite , or back - scattered light . in a future study we may carry out more sophisticated modeling , varying the radius of the warp as well as the symmetry of the warp , and including multiple scattering and varying the dust grain model , to see how the scattered flux varies with dust composition and with wavelength of the observed light , and we will see whether diffraction , multiple - scattering , or back - scattering can explain the brightening at mid eclipse . there is evidence that the ingress and egress differ slightly in time between the even and odd eclipses , which may imply that the warp is an @xmath99 mode rather than @xmath100 as in our model . if so , the back warp will be partly eclipsed by the front warp , while the edges will be visible during mid - eclipse , which may explain the rebrightening . alternatively , the symmetry during mid - eclipse of the front warp may cause a brightening due to diffraction , which is likely to be wavelength dependent . if the albedo is less than unity , then mid - infrared observations may point toward an @xmath99 configuration if they also show a peak at mid - eclipse since the infrared light is emitted isotropically , similar to the back - scattered light . a summary of the results :    \1 ) near mid - eclipse the polarization is @xmath32% , and is consistent with zero outside of eclipse ( 0.2@xmath190.2% ) . combined with other photometric data , the polarized flux near mid - eclipse is @xmath24% of the flux outside of eclipse . this indicates that there is an unobscured scattering region surrounding the t tauri star which creates the polarization ; the polarization can not be solely due to foreground transmission effects ( not associated with the star ) as there would no variation . \2 ) the data are nearly consistent with no wavelength dependence of the polarization during eclipse . this indicates that the scatterers are approximately grey . in addition , electron scattering is unlikely to cause the observed polarization . \3 ) the spectrum within the eclipse is consistent with being a scaled version of the spectrum outside of eclipse with no wavelength dependence of the ratio except for the presence of stronger emission lines which also argues for grey scattering ; rayleigh scattering is ruled out . our mie dust scattering model implies @xmath101 and @xmath70m@xmath71 m at 90% confidence ; however , the best fit model is not an adequate fit . \4 ) the qualitative features of the eclipse can be reproduced by a model of a warped disk that orbits in front of the star , periodically eclipsing the star , having an extended atmosphere which causes a decrease in light before and after eclipse as well as scattered light within eclipse , which is consistent with the observed polarized flux . the polarization adds an extra clue that the eclipsing material completely occults the star since the maximum observed polarization of classical t tauri stars is @xmath102% @xcite and the kh 15d spectrum looks like that of a classical t tauri star during eclipse . this adds to the evidence that the eclipse is complete based on ( a ) the steep egress and ingress ; ( b ) the flat bottom of the trough of the eclipse ; ( c ) the fine tuning that would be required to arrange 95% covering . it is interesting that the trough of the eclipse does show some slight variations , namely an increase near mid - eclipse , which may indicate that the scattering region is partially eclipsed and this eclipse varies with time . it would be interesting to measure the polarization as a function of time within the eclipse to look for variation . we have modeled the eclipse with a warped disk since a discrete cloud would be sheared in ten orbits , while the eclipses have repeated with similar durations for @xmath3 50 orbits . however , this does not mean that the warped disk model is without its own problems : if the warped model is correct , what drives the warp at such a short period ? the interaction of the stellar magnetosphere is proposed to cause a similar occulting behavior in the t tauri star aa tauri @xcite ; however , this star has a much short period between occultations , 8 days , while the longer period of kh 15d is much longer than the spin period of any known t tauri star . one can place a lower limit on the warp radius of @xmath103 since inwards of this radius the warp would have to be at @xmath104 to create the correct duration of ingress due to the finite size of the star . at @xmath105 , the keplerian period is 0.8 days , while a warp precession timescale is typically much longer than the keplerian timescale . it may be that a planet excites a wave in the disk @xcite or that an elongated dusty vortex causes obscuration @xcite ; each of these models might be consistent with the observed polarization profile , which remains to be quantified with a scattering calculation similar to the one presented here . future observations of interest would be to ( 1 ) measure the magnitude and angle of polarization throughout ingress and/or egress to test the warp model ; ( 2 ) measure the polarization at higher spectral resolution within the h@xmath5 emission line to see whether it is unpolarized ; ( 3 ) look for variations of the flux during eclipse to determine whether the stellar variability is washed out by the time delays from scattering ( although this requires a rather extended scattering region ) . in addition , near- and mid - infrared observations may show fluctuations if the occulting region has a warped geometry , as we have suggested . we thank catrina hamilton , lynne hillenbrand , patrick ogle , and russel white for useful discussions and marshall cohen for exchange of observing time . we thank bill herbst for providing published photometric data . 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 . support for e. a. was provided by the national aeronautics and space administration through chandra postdoctoral fellowship award pf0 - 10013 issued by the chandra x - ray observatory center , which is operated by the smithsonian astrophysical observatory for and on behalf of the national aeronautics space administration under contract nas 8 - 39073 . s. wolf was supported by nasa through grant nag  5 - 11465 . antonucci , r. , 1993 , ara&a , 31 , 473 barge , p. & viton , m. , 2003 , apj , 593 , l117 bohren c.f . , huffman d.r . , 1983 , _ absorption and scattering of light by small particles , _ john wiley & sons , new york bouvier , j. et al . , 1999 , a&a , 349 , 619 breger , m. & dyck , h. m. , 1972 , apj , 175 , 127 brown , j. c. ; mclean , i. s. , 1977 , a&a , 57 , 141 bryden , g. , r@xmath106yczka , m. , lin , d. n. c. & bodenheimer , p. , 2000 , apj , 540 , 1091 goodrich , r. , 1991 , pasp , 103 , 1314 hamilton , c. m. , herbst , w. , shih , c. & ferro , a. j. , 2001 , apj , 554 , l201 hamilton , c. m. , herbst , w. , mundt , r. , bailer - jones , c. a. l. , johns - krull , c. m. , 2003 , astro - ph/0305477 herbst , w. et al . , 2003 , pasp , 114 , 1167 hong , s. s. , 1985 , a&a , 146 , 67 kartje , j. , 1995 , apj , 452 , 565 kearns , k. e. & herbst , w. , 1998 , apj , 116 , 261 leinert , ch . et al . , 1998 , a&as , 127 , 1 martin p.g . , 1978 , cosmic dust . its impact on astronomy , clarendon press , oxford mnard , f. , 1991 , in _ iau colloq . 129 _ , ed . bertout c. , collin - souffrin , s. , & lasota , j. p. ( gif - sur - yvette : editions frontieres ) , p. 59 mnard , f. , bouvier , j. , dougados , c. , melnikov , s. y. , & konstantin , n. g. , 2003 , a&a , in press , astro - ph/0306552 miller , j. s. , robinson , l. b. & goodrich , r. w. , 1988 , in _ instrumentation for ground - based optical astronomy _ , l. b. robinson ( new york : springer ) , p. 157 mekkaden , m. v. , 1999 , a&a , 344 , 111 ogle , p. m. , cohen , m. h. , miller , j. s. , tran , h. d. , goodrich , r. w. & martel , a. r. , 1999 , apjs , 125 , 1o oke , j. b. et al . , 1995 , pasp , 107 , 375 oke , j. b. & gunn , j. e. , 1982 , pasp , 94 , 586 prugniel , ph . & soubiran , c. , 2001 , a&a , 369 , 1048 van de hulst , h. c. , 1981 , _ light scattering by small particles _ , dover : new york voshchinnikov n.v . , 2002 , astrophys . space phys . , 12 , 1 weingartner , j. c. & draine , b. t. , 2001 , apj , 548 , 296 winn , j. n. , garnavich , p. m. , stanek , k. z. , sasselov , d. d. , 2003 , apj , 593 , l121 wiscombe w.j . , 1980 , appl . optics , 19 , 1505 wolf , s. , 2000 , computer physics communications , 150 , 99 wolf , s. , henning , th . & stecklum b. , 1999 , a&a , 349 , 839 worthey , g. & ottaviani , d. l. , 1997 , apjs , 111 , 377
kh 15d is a strongly variable t tauri star in the young star cluster ngc 2264 that shows a decrease in flux of 3.5 magnitudes lasting for 18 days and repeating every 48 days . 
 the eclipsing material is likely due to orbiting dust or rocky bodies in a partial ring or warped disk that periodically occults the star . 
 we measured the polarized spectrum in and out of eclipse at the keck and palomar observatories . 
 outside of the eclipse , the star exhibited low polarization consistent with zero . during eclipse , 
 the polarization increased dramatically to @xmath0% across the optical spectrum , while the spectrum had the same continuum shape as outside of eclipse and exhibited emission lines of much larger equivalent width , as previously seen . from the data , we conclude that ( a ) the scattering region is uneclipsed ; ( b ) the scattering is nearly achromatic ; ( c ) the star is likely completely eclipsed so that the flux during eclipse is entirely due to scattered light , a conclusion also argued for by the shape of the ingress and egress . 
 we argue that the scattering is not due to electrons , but may be due to large dust grains of size @xmath1 m , similar to the interplanetary grains which scatter the zodiacal light . 
 we construct a warped - disk model with an extended dusty atmosphere which reproduces the main features of the lightcurve , namely ( a ) a gradual decrease before ingress due to extinction in the atmosphere ( similar for egress ) ; ( b ) a sharper decrease within ingress due to the optically - thick base of the atmosphere ; ( c ) a polarized flux during eclipse which is 0.1% of the total flux outside of eclipse , which requires no fine - tuning of the model . 
 the inclination of the warp is set by the duration of ingress and egress assuming that the warp is located at the keplerian radius , and the inclination of the observer is then determined by the duration of the eclipse .