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the ultraviolet ( uv ) radiation plays a crucial role in many processes of the interstellar medium ( ism ) , such as photoelectric heating , grain charging , photoionization , and photo - dissociation of molecules @xcite , in which the far - ultraviolet ( fuv : 6ev @xmath11 13.6ev ) radiation from massive stars or interstellar radiation field ( isrf ) influences the structure , chemistry , thermal balance , and evolution of neutral interstellar medium @xcite . therefore , studying these influence helps to understand the process of star formation . the fuv emission selectively dissociates co rare isotopologues more effectively than co owing to different levels of self - shielding effects @xcite . for the fuv emission with energy high enough to photodissociate @xmath12co , it rapidly becomes optically thick when it penetrates into molecular clouds . in contrast , the self - shielding effect of c@xmath1o is relatively less significant because of the shift of its absorption lines and its low abundance . therefore , the fuv emission with energy above the c@xmath1o dissociation level is expected to penetrate relatively deeper in a molecular cloud , and the abundance ratio of @xmath0co and c@xmath1o , @xmath4 , will increase when the self - shielding effect of c@xmath1o does not yet dominate . when the self - shielding effect of both @xmath0co and c@xmath1o become important , the abundance ratio should decrease toward the intrinsic abundance value which derived from abundances of the elements , @xmath12c , @xmath0c , @xmath13o , and @xmath1c . the intrinsic value may vary as the distance to the galactic center @xcite . the @xmath4 has been observed in regions with various conditions . in the typical massive star - forming regions , where the ism is filled up with diffuse fuv flux from ob stars , the measurements of @xmath4 are usually higher than the intrinsic value . @xcite measured @xmath4 in the orion - a giant molecular cloud ( orion - a gmc ) and found that the @xmath4 of most regions are a factor of two greater than 5.5 , the typical value in the solar system . it is possible that besides the interstellar fuv radiation , embedded ob stars provide strong fuv radiation so that the distance of the penetration will increase . another possibility is that the higher temperature in the massive cores will also change the fractionation of @xmath12c and @xmath0c @xcite . in the intermediate - mass star - forming regions , @xcite showed that the intensity ratio of @xmath0co ( @xmath14=21 ) to c@xmath1o ( @xmath14=21 ) , which is equivalent to @xmath4 for optically thin case , rises to a peak up to @xmath240 at @xmath8 5mag then decreases to @xmath24.5 with increasing @xmath7 in the southeast of the california molecular cloud . although the trend is consistent with the theoretical expectations , the peak occurs at somewhat higher extinction ; @xcite showed that the peak is at @xmath813mag for regions with various density from 10@xmath15 to 10@xmath16@xmath17 . in the low - mass star - forming regions , @xcite observed a part of the ic 5146 filament and found that the @xmath4 value is considerably greater than 5.5 in the outer parts ( @xmath18 10mag ) . ldn 1551 ( hereafter l1551 ) is a relatively isolated nearby star - forming region located at a distance of 160pc @xcite at the south end of the taurus - auriga - perseus molecular cloud complex . two small clusters of young stellar objects ( ysos ) were detected in l1551 . one contains two embedded class i sources , l1551 irs 5 ( hereafter irs 5 ) and l1551 ne ( hereafter ne ) , and the other is a group of more evolved ysos ( hereafter hl tau group ) located at the north of irs 5 and ne . the irs 5/ne cluster co - host a parsec - scale bipolar outflow @xcite which is likely mixed from outflows of each source @xcite . several herbig - haro objects along the outflows have been identified with different origins @xcite . another east - west redshifted outflow was found later @xcite , but since irs 5 and ne are two binary systems @xcite the origin of this east - west outflow is still questionable @xcite . the hl tau group contains hl tau , xz tau , lkh@xmath19 358 , and v1213 tau . the jets and outflows of hl tau group were detected in optical observations @xcite . in addition to the protostars , a gravitationally bound starless core , l1551 mc , is found at the north - western side of the irs 5/ne cluster which may have started gravitational collapse or could be supported by magnetic fields of @xmath2160@xmath20 g @xcite . since l1551 dose not contain ob stars , l1551 is a suitable target for studying the fuv influence only from the interstellar radiation .    in this paper , we aim to study the variation of @xmath4 under the influence of the interstellar fuv radiation . in contrast to previous studies , we observed @xmath12co ( @xmath14=10 ) , @xmath0co ( @xmath14=10 ) , and c@xmath1o ( @xmath14=10 ) lines using the 25-beam array receiver system ( bears ) receiver equipped on the nobeyama radio observatory ( nro ) 45 m telescope to obtain data that could resolve the sub - parsec scale with a complete coverage from the outskirts of the cloud into its dense region . we also use @xmath6 archival data to obtain a visual extinction map up to @xmath870mag in order to examine the variation of @xmath4 with the fuv attenuation . in  [ sec_obs ] , we describe our nro observations and herschel data . in  [ sec_result ] , we present the @xmath12co , @xmath0co , and c@xmath1o maps of l1551 , and then derive the excitation temperature and optical depths of the @xmath0co and c@xmath1o lines and the column densities of these molecules . in  [ sec_discussion ] , we discuss the dependence of the abundance ratio of @xmath0co to c@xmath1o on visual extinction , @xmath7 , the influence of fuv radiation , and the x - factor in l1551 . at the end , we summarize our results in  [ sec_summary ] . we observed the l1551 star - forming region using the 45 m telescope at nro . three molecular lines were observed : @xmath12co ( @xmath14=10 ; 115.27120ghz ) , @xmath0co ( @xmath14=10 ; 110.20135ghz ) , and c@xmath1o ( @xmath14=10 ; 109.78218ghz ) . the @xmath12co data has been published in @xcite . the observations were carried out between 2007 to 2010 ( table 1 ) . the telescope has beam sizes ( hpbw ) , @xmath21 , of 15@xmath22 at 115ghz , and of 16@xmath22 at 110ghz . the main beam efficiencies , @xmath23 , were measured as 32@xmath24 in 20072008 season at 115ghz , 38@xmath24 in 20092010 season at 110ghz , and 43@xmath24 in 2009 season at 110ghz . these efficiencies were measured every season by nro staff with a superconductor - insulator - superconductor ( sis ) receiver , s100 , and an acousto - optical spectrometers ( aoss ) . the front end is bears , which has 25 beams configured in a 5-by-5 array with a beam separation of 41@xmath251 @xcite . bears is operated under double - sideband ( dsb ) mode with each beam connected to a set of 1024 channel auto - correlators ( acs ) as the back end . we set each set of acs to a bandwidth of 32mhz and a resolution of 37.8khz @xcite , corresponding to velocity resolutions of @xmath20.1kms@xmath26 at the rest frequencies of the three lines . we calibrated the variations in both beam efficiency and sideband ratio of the 25 beams with the values from the nro website . these values were measured by nro staff every season through measurements of bright sources with s100 in ssb mode .    for the observations , we used the on - the - fly ( otf ) mapping technique @xcite . the antenna is driven at a constant speed to continuously scan our observing region toward l1551 . to get rid of artificial scanning patterns in the results , we scanned the observed regions twice , one in ra and one in dec directions , and then combined the otf maps in the two orthogonal directions by plait algorithm @xcite . the antenna pointing was checked every @xmath26070 minutes by observing the sio maser source nml - tau , and the pointing accuracy was about 3@xmath22 during the observations . figure [ fig_intro ] ( a ) , ( b ) , and ( c ) show the observed areas of @xmath12co , @xmath0co , and c@xmath1o , covering 44@xmath27 @xmath3 44@xmath27 , 42@xmath27 @xmath3 43@xmath27 , and 30@xmath27 @xmath3 30@xmath27 , respectively . the data were converted in terms of the main - beam brightness temperature in units of k , @xmath28 , where @xmath29 is the antenna temperature in units of k. in order to maximize the energy concentration ratio , we applied a spheroidal function with @xmath30 and @xmath31 @xcite to convolve the otf data , and then obtained three dimensional @xmath32 final cube data . adopting nyquist sampling for the 45 m telescope , we set the spatial grid size to 7@xmath255 , and the final effective beam sizes , @xmath33 , are 21@xmath258 for @xmath12co and 22@xmath252 for @xmath0co and c@xmath1o . the rms noise 1@xmath34 levels are 1.23k for @xmath12co , 0.94k for @xmath0co , and 0.67k for c@xmath1o in @xmath35 for a velocity resolution of 0.1kms@xmath26 . to achieve higher signal - to - noise ratios , we further convolved all maps with gaussians to make channel maps , integrated intensity maps , mean velocity maps , and velocity dispersion maps , and the effective beam size of each map is written in its figure caption . we summarize the parameters for each line in table [ obs_parameter ] . we used the herschel science archival spire / pacs image data ( observation i d : 1342202250/1342202251 , quality : level 2 processed ) of 160 , 250 , 350 , and 500 @xmath20 m to make dust temperature ( @xmath36 ) and column density maps in a similar way to @xcite . since the 70@xmath20 m emission seems to be not detected except towards irs 5/ne and the hl tau group , we used only 4 bands ( 500 , 350 , 250 , and 160@xmath20 m ) .    at first , we made convolutions for all @xmath6 images ( other than the 500@xmath20 m image ) to smooth their resolutions to the 500@xmath20 m resolution of 36@xmath22 by using the idl package developed by @xcite . then , we resampled up or down all the images ( other than the 250@xmath20 m ) to the same grid size of the 250@xmath20 m image ( 6@xmath22 ) and derived a spectral energy distribution ( sed ) at each position of the 250@xmath20 m image . here , we adopted an area of @xmath37 , centered at ( ra@xmath38 , dec@xmath38 ) = ( 4@xmath3930@xmath4034@xmath419 , + 18@xmath4228@xmath2755@xmath22 ) , as an area of the zero point of the surface brightness for the l1551 cloud .    assuming a single temperature of the dust emission , the gray - body sed fitting was performed with a function of @xmath43 where @xmath44 infers the frequency , @xmath45 expresses planck s law , @xmath46 is the observed surface brightness , and @xmath47 is the dust optical depth . @xmath47 can be expressed as @xmath48 , where @xmath49 is the dust opacity per unit mass and @xmath50 is the surface mass density . we adopted @xmath51 where @xmath52 = 2 , following @xcite . we carried out the sed fitting at each point where the dust emission was detected above 3 times of the rms noise level , which was measured in the area of the zero point of the surface brightness at all these 4 bands . in the case that the sed fitting failed or the surface brightness was not high enough for sed fitting at some points , we set nan there . as the data weight of the sed fitting , we adopted @xmath53 , where @xmath54 is the square sum of the rms noise and the calibration uncertainties of surface brightness ( 15@xmath24 at 500 , 350 , and 250@xmath20 m from @xcite ; 20@xmath24 at 160@xmath20 m from @xcite ) . the dust temperature of each pixel was derived from the above sed fitting , and the result is shown in fig.[fig_intro ] ( f ) . we used @xmath55 statistic to determine the goodness of the sed fitting per pixel @xcite . for the sed fitting , each @xmath55 statistic has a @xmath55 distribution with two degrees of freedom , and then the probability q gives a quantitative measure for the goodness . in most of the inner region of the l1551 cloud , the sed fitting seems good ( the probability q @xmath2 0.8 ) , while in the region around ysos and toward the outskirts , the fitting seems bad with small probability q. the badness around ysos may be due to multiple - temperature components presenting in the sed .    to obtain a column density map with a higher resolution of 18@xmath22 ( fig.[fig_intro ] ( d ) ) , we derived the column density directly from the 250@xmath20 m surface brightness ( @xmath56 from previous fitting ) adopting the dust temperature of the sed fitting with a lower resolution of 36@xmath22 @xcite . figure [ fig_intro ] ( a ) shows the integrated intensity map of @xmath12co ( @xmath14=10 ) integrated in the @xmath57 range from @xmath5815kms@xmath26 to 20kms@xmath26 for pixels with its signal - to - noise ratio greater than 3 . the @xmath12co emission is distributed all over the observed area . a sharp edge at the southeast of the mapping area can be recognized . @xcite suggested that the stellar winds from massive stars , betelgeuse and rigel in the ori - eridanus supershell , possibly compressed the edge of the l1551 molecular cloud . two elongated structures which are extended in the southwest  northeast direction and centered on the class i source irs 5 are identified as the molecular outflows ejected from irs 5 and another embedded class i source ne @xcite . figure [ fig_intro ] ( b ) shows the integrated intensity map of @xmath0co ( @xmath14=10 ) integrated in the @xmath57 range from @xmath5815kms@xmath26 to 20kms@xmath26 for pixels with its signal - to - noise ratio greater than 3 . the overall distribution of @xmath0co is consistent with those in previous studies of @xmath0co carried out with position switching mode of the fcrao 14 m @xcite and nro 45 m @xcite telescopes . there are a cavity structure at the northeast of ne and a u - shaped wall structure at the southwest of irs 5 . in addition , two intensity peaks can be seen toward irs 5 and ne . these structures are not recognized in the @xmath12co maps . figure [ fig_intro ] ( c ) shows the integrated intensity map of c@xmath1o ( @xmath14=10 ) integrated in the @xmath57 range from @xmath5815kms@xmath26 to 20kms@xmath26 for pixels with its signal - to - noise ratio greater than 3 . the c@xmath1o emission line is likely to trace the inner part of the regions traced by @xmath12co and @xmath0co . the overall distribution of c@xmath1o is consistent with those in previous studies of c@xmath1o carried out with the otf mode of the kitt peak 12 m telescope @xcite and the position switching mode of the nro 45 m @xcite telescope .      in order to derive the optical depths and column densities of @xmath0co and c@xmath1o , we assume that ( 1 ) @xmath12co ( @xmath14=10 ) is optically thick , ( 2 ) @xmath12co and its rarer isotopic species trace the same component , and ( 3 ) these three lines reach local thermal equilibrium ( lte ) . thus , the temperature of @xmath12co ( @xmath14=10 ) can be treated as the excitation temperature , @xmath59 , of @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) for deriving the optical depths and column densities of @xmath0co and c@xmath1o . in order to make direct comparison between the different lines , we smooth all the data so that they have the same effective beam size of 30@xmath254 ( corresponding to 0.023pc at the distance of 160pc ) . then , we obtain their peak intensities and full widths at half maximum ( fwhm ) by applying gaussian fitting to spectra of the @xmath12co ( @xmath14=10 ) , @xmath0co ( @xmath14=10 ) , and c@xmath1o ( @xmath14=10 ) cube data for pixels with their signal - to - noise ratio greater than 5 . in general , the spectra of @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) show single - component velocity structures , and thus we can apply single - gaussian fitting as an appropriate method . however , since the spectra of @xmath12co ( @xmath14=10 ) often show multiple velocity components due to the prominent outflows , we adopt different fitting strategies to obtain the excitation temperature . first , if the @xmath12co spectrum has only one velocity component , which usually happen in quiescent ambient region , the single - gaussian fitting may gives an adequate result . if the residual of the fitting at the peak velocity is less than 3@xmath34 , we take the peak intensity to calculate the excitation temperature . second , if the single - gaussian fitting fails , we apply double - gaussian fitting to separate the ambient and outflow components . except the regions around ne , only one redshifted or blueshifted component appears . we identify the component of which the peak velocity is similar to the peak velocities of @xmath0co and c@xmath1o spectra as the ambient component and the other component as the outflow component , if ( 1 ) the residuals of the fitting at both the peak velocities are less than 3@xmath34 and ( 2 ) the velocity ranges of the two gaussian components at half maximum are not overlapped with each other . the later criterion is included , because the determination of the peak intensity of the ambient component could be affected by the close outflow component . in some cases , the peak @xmath12co intensity is even smaller than the peak @xmath0co intensity , which is not physical and is rejected . third , for the remaining spectra , the velocity structure seems to contain three or more components . since our goal is to obtain the peak intensity of the ambient gas , we only apply a single gaussian to one peak having the peak velocity closest to the ambient gas velocity determined from the @xmath0co and c@xmath1o spectra at the same position . we fix the peak velocity of this single gaussian as the mean value of the @xmath0co and c@xmath1o peak velocities and we only fit the gaussian within a narrow velocity range bracketed by the two local minima around the peak . following the above procedure , we obtain the estimate of the excitation temperature from the peak intensity of the fitted gaussian to the ambient @xmath12co emission . figure [ fig_intro ] ( e ) shows the excitation temperature map deriving from the following equation with the previous assumption ( 1 ) and the beam filling factor of 1 ( e.g. , * ? ? ? * ; * ? ? ? * ) @xmath60^{-1}\rm\,k\\ & = 5.53 \left[\log\left(1+\frac{5.53}{t_{\rm mb,^{12}co}+0.818}\right)\right]^{-1 } \rm\,k , \end{split}\ ] ] where @xmath61 is the peak intensity of @xmath12co ( @xmath14=10 ) in units of k from the above fitting , @xmath62 is the effective radiation temperature @xcite , and @xmath63=2.7k is the temperature of cosmic microwave background radiation . hereafter , we call this excitation temperature derived from eq.([equ_tex ] ) , @xmath64 . we then obtain the optical depths , @xmath65 , of the @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) emission and the column densities , @xmath66 , of the @xmath0co and c@xmath1o gas using the following equations ( e.g. , * ? ? ? * ; * ? ? ? * ) @xmath67^{-1}-0.164)}\right),\ ] ] @xmath68^{-1}-0.1666)}\right),\ ] ] @xmath69 and @xmath70 where @xmath71 and @xmath72 are the peak intensities in k , @xmath73 and @xmath74 are the fwhms in kms@xmath26 , and @xmath75 and @xmath76 are the beam filling factors . the beam filling factors can be expressed as @xmath77 , where @xmath78 and @xmath79 are the source size and the effective beam size , respectively . in molecular clouds , the c@xmath1o emission is usually considered to trace the dense cores , clumps , and/or filaments . on the other hand , the @xmath0co emission traces more extended regions than c@xmath1o . in the case of the taurus molecular cloud , @xcite and @xcite found that both the typical @xmath0co and c@xmath1o core size are @xmath20.1pc . since the effective beam sizes we used here are 30@xmath254 ( corresponding to 0.023pc at the distance of 160pc ) which is much smaller than the typical core size , we can assume the emission of our sources fill up the beam and the beam filling factors become @xmath20.95 . therefore , we simply adopt @xmath75 and @xmath76 as 1 . figures [ fig_tau_n ] ( a ) and [ fig_tau_n ] ( b ) show the optical depth maps of @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) , respectively . for the @xmath0co ( @xmath14=10 ) emission , its optical depth is more than 1.5 at the center of l1551 , and drops to less than 1 at the outer edge . for the c@xmath1o ( @xmath14=10 ) emission , its optical depth is less than 0.8 in the whole region , suggesting that the c@xmath1o ( @xmath14=10 ) emission is fully optically thin in l1551 . figures [ fig_tau_n ] ( c ) and [ fig_tau_n ] ( d ) show the column density maps of @xmath0co and c@xmath1o , respectively . the cavity structure and the u - shaped wall structure can be seen in the @xmath0co column density map . the c@xmath1o column density map shows that c@xmath1o concentrate in the region surrounded by irs 5 , ne , and l1551 mc . moreover , the u - shaped wall structure is also seen in the c@xmath1o column density map as a low column density part . we can derive the abundances from @xmath80 and @xmath81 with the h@xmath82 column density derived from the fir dust continuum images . because we adopt the dust temperature map with an effective beam size of 36@xmath22 ( see  [ sec_obs_her ] ) , we convolve the @xmath83 and @xmath84 maps with gaussians to degrade their resolutions down to 36@xmath22 ( corresponding to 0.027pc at the distance of 160pc ) . figures [ fig_xx ] ( a ) and [ fig_xx ] ( b ) show the abundance maps of @xmath0co and c@xmath1o , respectively . the mean and standard deviation of @xmath85 and @xmath86 are ( 3.1@xmath51.2)@xmath310@xmath87 and ( 3.1@xmath51.2)@xmath310@xmath88 , respectively . we can see @xmath89 peaks toward irs 5 , ne , and l1551 mc in fig.[fig_intro ] ( d ) and @xmath84 also concentrates in the above regions in fig.[fig_tau_n ] ( d ) but the peaks of @xmath84 are not as obvious as @xmath89 . as a result , the @xmath86 map shows low values in above three regions , which means the c@xmath1o depletion occurs at these regions . it is reasonable for the depletion at l1551 mc because we find that l1551 mc is dense and cold in figures [ fig_intro ] ( d ) and ( f ) . for irs 5 and ne , the dust temperature at the envelop of protostars are higher than other regions , but the dust temperature is still less than the co evaporation temperature of 2025k and the h@xmath82 column density is very high compared with other regions ; therefore the envelop of protostars also show c@xmath1o depletion . on the other hand , the low @xmath85 value also appears at the c@xmath1o depletion region ; however , the low @xmath85 value derived in this region may not show the real situation because @xmath0co and dust emission may not trace the exactly same layer in the line of sight ( see  [ temperature ] ) . the abundance ratio of @xmath0co to c@xmath1o can be directly derived from @xmath4 = @xmath90 . figure [ fig_xx ] ( d ) shows the spatial variation of @xmath4 . the @xmath4 value ranges from @xmath23 to @xmath227 in the whole region , and the mean and standard deviation of the abundance ratios are 8.1@xmath52.8 . spatially , the @xmath4 value in the outskirts of the cloud is more than @xmath28 and drops to less than 5.5 around l1551 mc except for the hl tau group and the southwest blueshifted outflow lobe with the brightest @xmath12co integrated intensity . to study the influence of interstellar fuv radiation on @xmath4 , we remove the outflow regions to avoid the influence from outflow activities for later discussion . we define the outflow regions by the regions where the @xmath12co integrated intensities in either blueshifted or redshifted parts are higher than 3@xmath34 ( see fig.[fig_xx ] ( f ) ) ; here we regard @xmath5815 to 5.5kms@xmath26 and 7.9 to 20kms@xmath26 as the blueshifted and redshifted outflow velocity ranges , respectively . moreover , to avoid the influence from underestimation of @xmath86 by c@xmath1o depletion , we also remove the region where the @xmath86 value is less than the ism standard value of 1.6@xmath310@xmath88 @xcite ( shown as the gray contour in fig . [ fig_xx ] ( b ) ) . in the other words , we remove the region where c@xmath1o depletion factor = @xmath91 1 . figure [ fig_xx ] ( e ) shows the abundance map excluding the outflow and c@xmath1o depletion regions . we can see that the spatial variation of the @xmath4 value is outside - in deceasing and the minimum of the @xmath4 value coincides with the dense part in l1551 ( see the h@xmath82 column density map in fig.[fig_intro ] ( d ) as a comparison ) . this spatial variation might be caused by the penetration capability of interstellar fuv radiation which is related to the visual extinction , @xmath7 . in order to investigate the dependence of the abundance ratio on the visual extinction within the molecular cloud , we derived the @xmath7 map from the h@xmath82 column density map with a relation from @xcite , @xmath92 figure [ fig_xx ] ( c ) shows the derived @xmath7 map . since we adopt the dust temperature map with an effective beam size of 36@xmath22 ( see  [ sec_obs_her ] ) , we convolve the abundance ratio maps with gaussians to degrade their resolutions down to 36@xmath22 ( corresponding to 0.027pc at the distance of 160pc ) . figure [ fig_av_xx ] ( a ) shows the correlation between the @xmath4 value and the @xmath7 value . the @xmath4 value is higher than the typical solar system value of 5.5 in the @xmath7 range less than 10mag , and then decreases down to @xmath25.5 with increasing @xmath7 value , suggesting the dependence of the abundance ratio on @xmath7 value is significant . although the sample of @xmath18 2mag is sparse because we derived the abundance ratio from the detections with both @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) more than 5@xmath34 , we still can see that the maximum of @xmath4 value occurs between @xmath8 14mag . to figure out the averaged trend of the @xmath4 value , we averaged @xmath4 value in 2.5mag @xmath7 bins . the averaged @xmath4 value reaches the maximum of 10.3 which is @xmath22 times more than 5.5 in the @xmath7 range less than 2.5mag . theoretical studies predict that in the mean isrf , the @xmath4 maximum occurs at @xmath8 13mag and reaches @xmath2510 times of the intrinsic abundance ratio , and then decreases toward the intrinsic abundance ratio when @xmath7 further increases @xcite ; that is , the selective photodissociation of c@xmath1o appears at @xmath8 13mag . our maximum of the averaged @xmath4 value is only @xmath22 times of 5.5 , and our maximum of the non - averaged @xmath4 value just barely reaches 27.7 which is @xmath25 times of 5.5 . since the model calculations in @xcite only considered non - turbulent uniform density model , our lower maximum @xmath4 value may be caused by non - uniform density distributions . another example is shown by @xcite who derived a similar correlation in a filament of ic 5146 ( see fig.19 in * ? ? ? * ) , which is also a low mass star - forming region . they found that the @xmath4 ratio at @xmath93 10mag is @xmath23 times of 5.5 and then decreases to @xmath25.5 at @xmath94 10mag . note that the @xmath4 peak positions in the @xmath7 axes in l1551 and ic 5146 are different . although @xcite used the near - infrared color excess ( nice ) technique to derive @xmath7 and we used the gray - body sed fitting and equ.[equ_av ] to derive @xmath7 , both the nice method and our method are based on the assumption that @xmath95 . thus these difference may be owing to that the difference beam size between our data or there are still some different physical condition between them .    as seen in fig.[fig_intro ] ( c ) , the c@xmath1o emission is rather weak or not detected at the periphery of the cloud , in which higher @xmath4 value is expected . on the other hand , the @xmath0co emission is strong at these regions . since we only calculated @xmath4 value as the signal - to - noise ratio of both @xmath0co and c@xmath1o greater than 5 ( see  [ result_col_abu ] ) , we may only sample the positions with high c@xmath1o abundance . in order to check this possibility , we can estimate the upper limit of @xmath4 value by using the c@xmath1o emission with the signal - to - noise ratio of 3 and assuming fwhm@xmath96 as the value at the outskirts in fig.[fig_intro ] ( c ) ( @xmath20.18 km s@xmath26 ) at the region where the c@xmath1o emission is weak ( @xmath97 5@xmath34 level noise ) but the @xmath0co emission is still strong ( @xmath98 5@xmath34 level noise ) . then @xmath4 value can reach @xmath245 at these regions , which is @xmath28 times of 5.5 .      in order to derive the optical depths and column densities , we assumed that the @xmath12co , @xmath0co , and c@xmath1o lines trace the same region , and derived the excitation temperature , @xmath64 , using the peak intensity of @xmath12co ( @xmath14=10 ) . however , the three lines may trace different regions , and @xmath64 could not be used as the excitation temperatures of @xmath0co ( @xmath14=10 ) and c@xmath1o ( @xmath14=10 ) . to investigate the influence of the excitation temperature , we re - estimate the abundance ratio by using the dust temperature ( shown in fig.[fig_intro ] ( f ) ) , @xmath36 , as the excitation temperature of @xmath0co or / and c@xmath1o . since the thermal emission from dust is usually optically thin and can traces inner regions , the @xmath0co or / and c@xmath1o may trace the same region as dust . we found that @xmath36 is generally smaller than @xmath64 in the inner region of l1551 and @xmath36 in the area we calculated abundance ratio has a mean value of 13.2@xmath50.7k ; thus , there is a possibility that the @xmath12co emission and the dust emission may trace different regions in the inner region of l1551 . the h@xmath82 column density traced by the dust emission is also a very useful measure to examine whether the co lines trace the same region as the dust or not . in fact , we can see the distributions of the @xmath89 map and the @xmath84 map are similar , compared to the @xmath83 map . thus , we assume the excitation temperature of c@xmath1o is the dust temperature , but we try both @xmath64 and @xmath36 for @xmath0co for comparison . on the other hand , since @xcite used the two rotational transitions of @xmath12co ( @xmath14=10 and 32 ) to derive a mean excitation temperature of the main ne / irs 5 outflow to be 16.5k , which is generally higher than @xmath64 , we also use this temperature for comparison . therefore , we compare the derived abundance ratios under the following four assumptions :    1 .   @xmath99 = @xmath64 and @xmath100 = @xmath64 . ( original)[tex_1 ] 2 . @xmath99 = @xmath64 and @xmath100 = @xmath36.[tex_2 ] 3 . @xmath99 = @xmath36 and @xmath100 = @xmath36.[tex_3 ] 4 . @xmath101.[tex_4 ]    each panel in fig.[fig_av_xx ] shows the correlation between @xmath4 and @xmath7 under one of the above four assumptions . the mean and standard deviation of the abundance ratios are 8.0@xmath52.8 , 8.4@xmath53.1 , 9.5@xmath53.2 , and 7.6@xmath52.9 under these four assumptions respectively . the dependence of the @xmath4 value on the @xmath7 value under the assumption ( [ tex_2 ] ) and ( [ tex_4 ] ) are consistent with that under the original assumption ( [ tex_1 ] ) . we notice that the @xmath4 value at @xmath102 5 under the assumption ( [ tex_4 ] ) is sightly lower than that in the assumption ( [ tex_1 ] ) , which reveals that a higher excitation temperature will derive lower abundance ratio . actually , @xmath64 used in fig.[fig_av_xx ] ( a ) has a mean value of 15.3@xmath51.9k . even if we use 16.5k as the excitation temperature in the assumption ( [ tex_4 ] ) , we still obtain almost the same trend in the correlation between @xmath4 and @xmath7 . thus , this derivation of abundance ratio is not sensitive to the excitation temperature . in the case of the assumption ( [ tex_3 ] ) , the trend of @xmath4 is similar to those under the other assumptions in the @xmath7 range less than 10mag , but the @xmath4 value increases in the @xmath7 range more than 10mag . in fact , the ambient region in which the @xmath7 value larger than 10mag is corresponding to l1551 mc in which the dust temperature is less than @xmath210k . since the only difference between the assumption ( [ tex_2 ] ) and ( [ tex_3 ] ) is the excitation temperature of @xmath0co , this behavior can be interpreted as follows : the excitation temperature of @xmath0co in l1551 mc is underestimated in the case of @xmath59 = @xmath36 , and thus the column density of @xmath0co is overestimated . this suggests that the @xmath0co line does not trace the dense region of l1551 mc traced by the dust emission : l1551 mc can not be recognized in the @xmath0co integrated intensity map of fig . [ fig_intro ] ( b ) . consequently , even when we take into account the uncertainties in the excitation temperature estimation , our results still lead to the same conclusion . that is , the @xmath4 value reaches @xmath211 at the low @xmath7 value ( @xmath8 14mag ) which is indeed larger than the solar system value , and decreases toward the solar system value in the high @xmath7 value ( @xmath8 520mag ) except the case of the low ( @xmath210 k ) excitation temperature for the @xmath0co line . orion - a gmc is located at a distance of 400pc @xcite and is under strong influence of the fuv emission from the trapezium cluster and nu ori @xcite . several photon dissociation regions ( pdrs ) have been identified from comparison of the distributions of the 8@xmath20 m , 1.1 mm , and @xmath12co ( @xmath14=10 ) line emission @xcite . @xcite found that the abundance ratios of @xmath0co to c@xmath1o in pdrs and non - pdrs are both higher than 5.5 , and concluded that these high abundance ratios are due to the selective fuv photodissociation of c@xmath1o .    in order to investigate the influence of the different environments on the abundance ratio of @xmath0co to c@xmath1o , we compare the abundance ratio between l1551 and orion - a . figure [ fig_n18_xx ] shows the @xmath4 value as a function of @xmath84 for l1551 and orion - a ( both the pdr and non - pdr regions ) . the data of orion - a are from @xcite . here , @xcite used the c@xmath1o column density to estimate the total column density . although the c@xmath1o column density and the @xmath7 value do not have a perfect linear relation because of the selective photodissociation @xcite , we can still consider that the higher column density is generally corresponding to the higher @xmath7 value and vice versa . our results show that the abundance ratio in l1551 is generally lower than that in orion - a . the mean @xmath4 value in l1551 is 8.0@xmath52.8 in contrast to 16.5@xmath50.07 in pdrs and 12.3@xmath50.02 in non - pdrs in orion - a @xcite . for the non - pdrs of orion - a , the @xmath4 value has a maximum in the low @xmath84 regime ( @xmath103 @xmath104 @xmath105 ) , and then decreases to @xmath210 in the high @xmath84 regime ( see fig.7 ( d ) , ( e ) , ( f ) , and ( g ) in * ? ? ? this trend is similar to the result of l1551 but the selective photodissociation of c@xmath1o in l1551 only occurs at the outskirts ( @xmath8 14mag , see fig.[fig_av_xx ] ( a ) ) . however , even at the high column density , the abundance ratio is still about two times higher than 5.5 in orion - a , because of the presence of the embedded ob stars in the cloud . in the other words , the fuv strength in orion - a is higher than that in l1551 , which is the environmental difference between l1551 and orion - a . in the orion - a , the fuv radiation from the embedded ob stars is likely to penetrate the whole cloud . for the pdrs of orion - a , the stronger fuv radiation causes the even higher convergence value of @xmath215 . measurements of the mass distribution in molecular clouds help to understand their physical and chemical characteristics . however , in ism , the most abundant molecular species , h@xmath82 , is hard to be observed because h@xmath82 lacks its electric - dipole moment and its quadruple transition is hard to occur in the typical molecular cloud environment . the secondary abundant molecular species , co , is not the case , because co is much easier to be observed and the @xmath12co ( @xmath14=10 ) emission is considered as the most available mass tracer . thus , the co - to - h@xmath82 conversion factor , also called  x - factor " , is defined as , @xmath106 where @xmath107 is the integrated intensity of the @xmath12co ( @xmath14=10 ) emission . @xcite showed that the averaged x - factor = 2@xmath310@xmath108@xmath105k@xmath26km@xmath26s with @xmath530@xmath24 uncertainty in the milky way disk . nevertheless , the x - factor can vary by a factor of @xmath2 100 in different regions , because @xmath12co ( @xmath14=10 ) is often optically thick @xcite . for example , @xcite measured x - factor @xmath2 ( 1.612)@xmath310@xmath108@xmath105k@xmath26km@xmath26s in the taurus molecular cloud , @xcite measured x - factor @xmath2 3@xmath310@xmath109@xmath105k@xmath26km@xmath26s in the perseus molecular cloud , and @xcite measured x - factor @xmath2 2.53@xmath310@xmath108@xmath105k@xmath26km@xmath26s in the southeastern part of the california molecular cloud . these discrepancies are also found in ism numerical simulations which show that x - factor is likely dependent on extinction , volume density , temperature , metallicity , turbulence , star formation feedback , and so on @xcite . even within a molecular cloud , the x - factor can still vary with a wide range . on the other hand , the variation of the x - factor of the @xmath0co and c@xmath1o can be smaller ( see appendix [ appendix_x ] ) . hereafter we calculate the x - factor for pixels in fig.[fig_intro ] ( a ) with the signal - to - noise ratio of the @xmath12co integrated intensity greater than 3 . figure [ fig_av_w12 ] shows the correlation between the @xmath107 value and the @xmath7 value , and the points are color - coded by @xmath110 . we can see that there are multiple trends . the averaged x - factor of the whole region is 1.08@xmath11110@xmath108@xmath105k@xmath26km@xmath26s . this is about a factor of two smaller than the average x - factor in milky way , but the dispersion still covers the average x - factor in milky way . in order to reveal these different trends , although we have already defined two regions , the ambient and outflow components , in the previous discussion , we divide into more regions based on the previous division here : ( a ) the diffuse component which is the ambient component excluding the l1551 mc component , ( b ) the l1551 mc component which is the green polygon in fig.[fig_xx ] ( f ) , and ( c ) the outflow component which is the same as defined in sec . 4.1 . in each panel , we use the chi - squre fitting method with errors in both coordinates to find the best fit line . the diffuse component ( fig.[fig_av_w12 ] ( a ) ) shows a relatively narrow distribution , compared with the other two regions , and the fitted x - factor = 1.24@xmath11210@xmath108@xmath105k@xmath26km@xmath26s is similar to the fitted x - factor of the whole region and is the same order of magnitude as the average x - factor in milky way . the l1551 mc component ( fig.[fig_av_w12 ] ( b ) ) , however , shows a more extended distribution and a larger fitted x - factor = 5.59@xmath11310@xmath108@xmath105k@xmath26km@xmath26s . this is due to the fact that the @xmath12co emission is optically thick and thus saturated in this dense starless core . the outflow component ( fig.[fig_av_w12 ] ( c ) ) has a fitted x - factor = 7.43@xmath11410@xmath109@xmath105k@xmath26km@xmath26s , which is smaller than that of the diffuse component , but shows three different distributions . two of the distributions are extended and have shallow slopes in the correlation , which belong to the outflow region overlapped with irs 5 and ne shown in fig.[fig_xx ] ( f ) as two green incomplete circles and marked in the panel ( c ) of fig.[fig_av_w12 ] . the @xmath12co emission from the irs 5 and ne region is likely saturated because the h@xmath82 column densities of irs 5 and ne are comparable to that of l1551 mc ( @xmath115@xmath105 ) . the other narrow distribution belongs to the remaining outflows and dominates the fitting of x - factor , which may be due to the fact that the entrained energy more strongly excites the @xmath12co emission and makes its high excitation temperature . consequently , in the low @xmath7 range ( @xmath116mag ) , the x - factor of the diffuse component is consistent with the milky way average value , and the x - factor of the outflow component is smaller . however , in l1551 mc ( @xmath117mag ) , the x - factor becomes larger and has a large dispersion . figure [ fig_av_x ] shows the correlation between the x - factor and the @xmath7 value , and the points are color - coded by @xmath59 . we can see that the x - factor of the whole region is dependent on @xmath7 with a fitted power law , x - factor @xmath9 @xmath118 . especially , the l1551 mc component has a well - correlated distribution ( less dispersion of the power index ) , x - factor @xmath9 @xmath119 . a similar distribution but a shallower power law , x - factor @xmath9 @xmath120 , was also found by @xcite in the southeastern part of the california molecular cloud excluding the hot region around the massive star lkh@xmath19 101 . however , in perseus , @xcite found two characteristic features : the x - factor steeply decreases at @xmath121mag and gradually increase at @xmath117mag . the steeply decreasing trend is likely due to the sharp transition from cii / ci to co , but this steeply decreasing trend does not appear in our results . the ism numerical simulations performed by @xcite suggests that the transition behavior is dependent on the volume density . they simulated molecular clouds with uniform density distribution from 10@xmath15 to 10@xmath122@xmath17 and initial turbulence , and showed that the low density case has the decreasing trend with a wider @xmath7 range compared to the high density case . for the highest density case in their simulation ( n @xmath2 10@xmath122@xmath17 ) , the result only shows one characteristic trend , x - factor @xmath9 @xmath89 , which means the @xmath12co emission is saturated ( optically thick ) to be constant and then the x - factor directly relates to @xmath89 , which is consistent with our results . in l1551 , we do not find the decreasing trend even at the low @xmath7 range , which indicates the fuv strength in l1551 is less than that in perseus . under the mean isrf , @xmath12co is self - shielded at @xmath123 0.5mag @xcite , which may be the case in l1551 . figure [ fig_tex_x ] shows the correlation between the x - factor and the @xmath59 value , and the points are color - coded by @xmath7 . our data do not show obvious correlated distributions . however , an inverse power law , x - factor = @xmath124 , was found in the southeastern part of california cloud @xcite , which is consistent with x - factor @xmath9 @xmath125 in simulations of @xcite . although this relation passes through the diffuse component of l1551 , we can not confirm this relationship due to the narrow temperature range of our data . figure [ fig_mom2_x ] shows the correlation between the x - factor and the @xmath12co velocity dispersion , @xmath126 , and the points are color - coded by @xmath59 . the simulations predict that x - factor @xmath9 @xmath127 @xcite . however , our data do not have a correlated distribution . @xcite also found no correlation in the southeastern part of california cloud . we have carried out wide - field otf observations in the @xmath12co , @xmath0co , and c@xmath1o ( @xmath14=10 ) lines toward the l1551 molecular cloud using the bears receiver of the nro 45 m telescope . the main results are summarized as follows :    1 . the abundance ratio of @xmath0co to c@xmath1o ( @xmath4 = @xmath90 ) is derived to be @xmath2327 . the mean value is 8.0@xmath52.8 . we can see a spatially decreasing trend in the @xmath4 map from the outskirts to the dense part of l1551 , the starless core l1551 mc . we found that the @xmath4 value reaches its maximum in @xmath8 14mag , and then decreases down to about the typical solar system value of 5.5 in the high @xmath7 value , suggesting that the selective photodissociation of c@xmath1o from the interstellar fuv radiation occurs in l1551 . pdr theoretical models with the mean isfr suggest that the selective photodissociation of c@xmath1o causes the increase of @xmath4 value up to @xmath2510 times of the intrinsic value in the low @xmath7 regions with @xmath2 13mag . however , in l1551 , the average @xmath4 value in 2.5mag bins reaches its maximum value of 10.3 which is @xmath22 times of 5.5 in the @xmath7 range less than 2.5mag . since the model calculations consider non - turbulent uniform cloud density , this lower maximum @xmath4 value may be caused by non - uniform cloud density . the mean of the excitation temperatures of @xmath0co and c@xmath1o derived from @xmath12co in the non - outflow regions is 15.3@xmath51.9k . even if we consider the higher excitation temperature of @xmath128 16.5k or assume @xmath129 , we also find a similar trends of @xmath4 . therefore , our results described in item 2 are not sensitive to the uncertainties in the excitation temperature determination . 4 .   the trends in the @xmath4 variations in l1551 and orion - a are similar to each other . that is , both @xmath4 values reach their maximums at low @xmath84 value , and then decrease as @xmath84 value increase ; however , the @xmath4 value in orion - a converges to a higher value than that in l1551 . this is due to the fuv radiation from the embedded ob stars . we calculated that the averaged x - factor = @xmath130 in l1551 is 1.08@xmath11110@xmath108@xmath105k@xmath26km@xmath26s which is somewhat smaller but consistent with the milky way average value @xmath2 2@xmath310@xmath108@xmath105k@xmath26km@xmath26s . the x - factor of the different regions in l1551 has a large variation . for the diffuse region , we found a x - factor = 1.24@xmath11210@xmath108@xmath105k@xmath26km@xmath26s which is similar to the milky way average value . for the outflow region , we found a smaller x - factor = 7.43@xmath11410@xmath109@xmath105k@xmath26km@xmath26s . the region of l1551 mc has a larger x - factor = 5.59@xmath11310@xmath108@xmath105k@xmath26km@xmath26s with a large dispersion because of the saturation of the @xmath12co emission . for the whole region , we found a correlation between the x - factor and the @xmath7 value with x - factor @xmath9 @xmath118 at @xmath7 = @xmath20.270mag which is due to the saturation of the @xmath12co emission . the 45 m radio telescope is operated by nobeyama radio observatory , a branch of national astronomical observatory of japan . part of this work was achieved using the grant of naoj visiting fellow program supported by the research coordination committee , national astronomical observatory of japan ( naoj ) . s.j.l . and s.p.l . acknowledge support from the ministry of science and technology of taiwan with grants most 102 - 2119-m-007 - 004-my3 . figure [ fig_ch_12co ] shows the velocity channel maps of @xmath12co ( @xmath14=10 ) . in the velocity range from @xmath588kms@xmath26 to 6kms@xmath26 , an extended elongated emission can be recognized at the southwest of irs 5 . this component is identified as the blueshifted lobe of the outflows ejected from irs 5 and ne @xcite . in the velocity range from 4kms@xmath26 to 10kms@xmath26 , a narrower elongated structure appears at the northeast of irs 5 and through ne , which could still be a mix of the outflows from both sources . in the velocity range from 2kms@xmath26 to 6kms@xmath26 , a clam - shaped emission shows up around the hl tau group , which are identified as the collective outflows from the hl tau group @xcite . this outflow structure extends to @xmath214kms@xmath26 in the redshifted lobe . in the velocity range from 10kms@xmath26 to 20kms@xmath26 , there is an emission between ne and the hl tau group . this component is located between the redshifted outflows from irs 5 and ne and the collective outflows from the hl tau group @xcite , which could be the results of the outflow interaction . in the velocity range from 8kms@xmath26 to 16kms@xmath26 , another narrow elongated emission is distributed in the east  west direction pointing back to irs 5 and ne , but the origin of this east  west outflow is still unclear @xcite . at 10kms@xmath26 , a diffused emission with an intensity of @xmath224k is distributed at the southeast of the l1551 molecular cloud , and this is a part of the ambient gas in taurus @xcite . at 8kms@xmath26 , two individual small cores with an extent of @xmath235@xmath27 at the northern side probably do not associate with the l1551 molecular cloud because their velocity of @xmath28 km s@xmath26 is different from the systemic velocity of @xmath26 km s@xmath26 in l1551 ( see the 6 km s@xmath26 panel in fig.[fig_ch_12co ] ) .    figures [ fig_mom ] ( a ) and ( d ) show @xmath12co mean velocity and velocity dispersion maps , respectively . the mean velocity over whole region is 6.6@xmath50.44kms@xmath26 @xcite . we calculate that the mean velocity dispersion toward all observed area is 0.44@xmath50.35kms@xmath26 . note that the velocity dispersion at the area of the outflows ( @xmath20.54.0kms@xmath26 ) is relatively higher than the mean value toward the overall observed area . figure [ fig_ch_13co ] shows the velocity channel maps of @xmath0co ( @xmath14=10 ) . in the velocity range from 4.25kms@xmath26 to 7.25kms@xmath26 , we can see the u - shaped wall clearly at the southwest of irs 5 which is distributed around the southwestern blueshifted lobe of the outflows traced in @xmath12co and extending to the outside of those outflows . note that although the u - shaped wall seems to surround the blueshifted outflows , its velocity covers both redshifted and blueshifted ranges , which may hint that the blueshifted outflow axis is almost parallel to the plane of sky . in the velocity range from 6.25kms@xmath26 to 7.25kms@xmath26 , we can see the cavity at the northeast of ne . at the velocity of 6.25kms@xmath26 , we can see a narrow filamentary structure sticking out from irs 5 and extending toward l1551 mc and beyond . at the velocity of 6.75kms@xmath26 , the @xmath0co emission is distributed over the whole region , which is consistent to the centroid velocity of 6.7@xmath50.21kms@xmath26 measured by @xcite . at the velocity of 7.75kms@xmath26 , we can barely see an east  west elongated structure corresponding to the east  west outflow seen in the @xmath12co map . figure [ fig_mom ] ( b ) shows the @xmath0co mean velocity map . the cavity is clearly seen in the blueshifted velocities with respect to the mean velocity of 6.7 km s@xmath26 . because the cavity is on top of the redshifted outflows from irs 5/ne , we speculate that the gas on the cavity region is pushed toward us by the redshifted outflows . there is a redshifted streamer ( @xmath131 , see the black dashed line in fig.[fig_mom ] ( b ) ) at the southwest of the narrow filamentary structure ( see the grey dashed line in fig.[fig_mom ] ( b ) ) and at the north of east  west outflow . however , the integrated intensity of this streamer is very low in fig.[fig_intro ] ( b ) , an thus this component is not discussed hereafter . figure [ fig_mom ] ( e ) shows the @xmath0co velocity dispersion map . the velocity dispersion of the u - shaped wall ( @xmath20.40.7kms@xmath26 ) is relatively higher than the mean value in the overall observed area ( 0.20@xmath50.09kms@xmath26 ) . figure [ fig_ch_c18o ] shows the velocity channel maps of c@xmath1o ( @xmath14=10 ) . these maps show a filamentary structure in the northwest  southeast direction in the velocity range from 6.25kms@xmath26 to 7.25 km , and l1551 mc traced by nh@xmath132 coincides with this filament @xcite . this filamentary structure can also be recognized in the @xmath0co channel maps ( the 6.25kms@xmath26 panel of fig.[fig_ch_13co ] ) and the h@xmath82 column density map ( fig.[fig_intro ] ( d ) ) . a u - shaped wall structure is present in the velocity range from 6.75kms@xmath26 to 7.05 km , which coincides with the redshifted part of the u - shaped wall traced in @xmath0co . another fainter filamentary structure can be seen in the velocity range from 7.05kms@xmath26 to 7.35kms@xmath26 , and its position is just above the upper arm of the u - shaped wall . figure [ fig_mom ] ( c ) and ( f ) are the c@xmath1o mean velocity and the velocity dispersion maps , respectively . within the u - shaped wall , the integrated intensity becomes low and the gas is mostly blueshifted ( see fig.[fig_intro ] ( c ) ) . figures [ fig_av_w13 ] and [ fig_av_w18 ] , respectively , show the correlation between the @xmath133 value , the integrated intensity of the @xmath0co ( @xmath14=10 ) emission , and the @xmath7 value , and the @xmath134 value , the integrated intensity of the c@xmath1o ( @xmath14=10 ) emission , and the @xmath7 value , in which the points are color - coded by @xmath110 . in order to make direct comparison with their column density , we only plot the same regions where the pixels have their signal - to - noise ratio greater than 5 ( see  [ result_col_abu ] ) . the fitted x - factor of the @xmath0co and c@xmath1o emission are 7.48@xmath13510@xmath108 and 7.23@xmath13610@xmath137@xmath105k@xmath26km@xmath26s , respectively . in fig.[fig_av_w13 ] and [ fig_av_w18 ] , the distribution of points in each panel is similar to that for @xmath12co but has a narrower distribution because their optical depths are smaller than that of the @xmath12co emission . especially for the c@xmath1o emission , since @xmath138 , even the distribution of l1551 mc has a linear correlation , compared with the @xmath12co and @xmath0co . alonso - 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shaped wall structure and the magenta one a cavity structure . the linear scale of 0.1pc is shown at the bottom - left corner of each panel . ( d ) the column density map of h@xmath82 in units of @xmath105 with an effective beam size of 18@xmath22 , derived from _ herschel _ data . ( e ) the excitation temperature map in units of k with an effective beam size of 30@xmath254 , derived from the @xmath12co ( @xmath14=10 ) peak intensities . ( f ) the dust temperature map in units of k with an effective beam size of 36@xmath22 . each beam size is denoted as the white ellipses at the bottom - right corner of each panel . ] co ( @xmath14=10 ) and ( b ) c@xmath1o ( @xmath14=10 ) and the column density maps of ( c ) @xmath0co and ( d ) c@xmath1o in units of @xmath105 . the effective beam sizes are 30@xmath254.[fig_tau_n ] note that these maps are made from the data with the signal - to - noise ratio larger than 5 . the symbols in the panels have the same meaning as in fig.1 . ]     and @xmath86 , the abundance ratio , @xmath4 , and the visual extinction , @xmath7 . ( a ) the @xmath85 map and ( b ) the @xmath86 map with effective beam sizes of 36@xmath22 . the gray contour in the panel ( b ) indicates the ism standard value , @xmath86 = 1.6@xmath310@xmath88 . ( c ) the @xmath140 map in units of mag , derived from _ herschel _ data . ( d ) the @xmath4 map and ( e ) the @xmath4 map without outflow and c@xmath1o depletion regions . the effective beam sizes are 36@xmath22 . the white , magenta , and black contours in the panels ( d ) and ( e ) indicate 5.5 , 8.25 , 11mag , respectively . ( f ) the @xmath12co ( @xmath14=10 ) integrated intensity contours with an effective beam size of 36@xmath22 superposed on the column density map of h@xmath82 with an effective beam size of 18@xmath22 . the contours indicate the blueshifted outflows ( blue , @xmath57 = [ @xmath5815kms@xmath26 , 5.5kms@xmath26 ] ) , the redshifted outflows ( red , @xmath57 = [ 7.9kms@xmath26 , 20kms@xmath26 ] ) , and the ambient component ( black , @xmath57 = [ 5.5kms@xmath26 , 7.9kms@xmath26 ] ) . the contour levels start from 3@xmath34 with a step of 6@xmath34 where the rms noise 1@xmath34 levels are 1.44k kms@xmath26 ( red ) , 2.68k kms@xmath26 ( black ) , 1.23k kms@xmath26 ( blue ) . the green polygon is a selected region of l1551 mc and the two incomplete green circles are selected regions of irs 5 and ne , respectively . the symbols in the panels have the same meaning as in fig.1 . ]    , and the abundance ratio , @xmath4 . the top x - axis is the column density of h@xmath82 , @xmath89 . the four panels ( a ) , ( b ) , ( c ) , and ( d ) correspond to the four cases of the excitation temperatures of the @xmath0co and c@xmath1o ( @xmath14=10 ) lines . the blue and green dots denote the undepleted ambient and rest ( depletion factor @xmath141 1 and outflow regions ) components , respectively . the black filled squares and their error - bars are the average values and the dispersion of @xmath4 in the 2.5mag @xmath7 bin . the yellow contours indicate the surface density of ambient points at the 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density of the undepleted ambient points . the horizontal dashed lines indicate the solar system value of 5.5 . ] o , @xmath84 , and the abundance ratio , @xmath4 in l1551 and orion a. the blue and green dots denote the undepleted ambient and rest components in l1551 , respectively . the gray dots denote the orion a data from shimajiri et al . the yellow and black contours indicate the surface density of the blue and grey points at the 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of their maximum surface density of the ambient component in l 1551 and orion a , respectively . the horizontal dashed line indicates the solar system value of 5.5 . ]    , and the integrated intensity of @xmath12co ( @xmath14=10 ) , @xmath107 . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the excitation temperature , @xmath59 . the black contours indicate the surface density of the color - coded points at the 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density in each panel . ]    , and the x - factor . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the excitation temperature , @xmath59 . the blue contours indicate the surface density of the color - coded points at the 5@xmath24 , 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density of all data . ]    , and the x - factor . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the visual extinction , @xmath7 . the blue contours indicate the surface density of the color - coded points at the 5@xmath24 , 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density of all data . ] co ( @xmath14=10 ) , @xmath126 , and the x - factor . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the excitation temperature , @xmath59 . the blue contours indicate the surface density of the color - coded points at the 5@xmath24 , 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density of all data . ] co ( @xmath14=10 ) in units of k. the numbers at the top - left corners denote the central lsr velocities of the individual channels in units of kms@xmath26 . the effective beam sizes are 30@xmath250 . the linear scale of 0.25pc is shown at the bottom - left corner of the first panel.[fig_ch_12co ] ]    co ( @xmath14=10 ) , ( b ) @xmath0co ( @xmath14=10 ) , and ( c ) c@xmath1o ( @xmath14=10 ) in units of kms@xmath26 with effective beam sizes of 28@xmath253 , 28@xmath256 , and 28@xmath256 , respectively . the gray and black dashed lines indicate in ( b ) the narrow filamentary structure and the redshifted streamer , respectively . the velocity dispersion maps of ( d ) @xmath12co ( @xmath14=10 ) , ( e ) @xmath0co ( @xmath14=10 ) , and ( f ) c@xmath1o ( @xmath14=10 ) in units of kms@xmath26 with effective beam sizes of 28@xmath253 , 28@xmath256 , and 28@xmath256 , respectively . the mean velocities and velocity dispersions of @xmath12co , @xmath0co , and c@xmath1o maps are calculated in @xmath57 = [ @xmath5815kms@xmath26 , 20kms@xmath26 ] , [ 3kms@xmath26 , 8kms@xmath26 ] , and [ 5.7kms@xmath26 , 7.5kms@xmath26 ] , respectively . the rest symbols in the panels have the same meaning as in fig.1 . ] co ( @xmath14=10 ) in units of k. the numbers at the top - left corners denote the central lsr velocities of the individual channels in units of kms@xmath26 . the effective beam sizes are 30@xmath253 . the linear scale of 0.25pc is shown at the bottom - left corner of the first panel.[fig_ch_13co ] ]    o ( @xmath14=10 ) in units of k. the numbers at the top - left corners denote the central lsr velocities of the individual channels in units of kms@xmath26 . the effective beam sizes are 30@xmath254 . the linear scale of 0.25pc is shown at the bottom - left corner of the first panel.[fig_ch_c18o ] ]    , and the integrated intensity of @xmath0co ( @xmath14=10 ) , @xmath133 . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the excitation temperature , @xmath59 . the black contours indicate the surface density of the color - coded points at the 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density in each panel . ]    , and the integrated intensity of c@xmath1o ( @xmath14=10 ) , @xmath134 . the gray dots denotes all data points in the map . the color - coded dots show the data points in the labeled regions , and the color represents the excitation temperature , @xmath59 . the black contours indicate the surface density of the color - coded points at the 15@xmath24 , 45@xmath24 , and 75@xmath24 levels of the maximum surface density in each panel . ] lccc molecular line & @xmath12co ( @xmath14=10 ) & @xmath0co ( @xmath14=10 ) & c@xmath1o ( @xmath14=10 ) + rest frequency [ ghz ] & 115.27120 & 110.20135 & 109.78218 + primary beam hpbw [ @xmath22 ] & @xmath215 & @xmath216 & @xmath216 + frequency resolution [ khz ] & 37.8 & 37.8 & 37.8 + main beam efficiency & 0.32 & 0.38 & 0.43 + observation & 2007 dec2008 jun & 2009 dec2010 feb & 2009 feb2009 may + scan mode & otf & otf & otf + mapping area [ arcmin@xmath15 ] & 44 @xmath3 44 & 42 @xmath3 43 & 30 @xmath3 30 + pixel grid size [ @xmath22 ] & 7.5 & 7.5 & 7.5 + effective beam size [ @xmath22 ] & 21.8 & 22.2 & 22.2 + velocity resolution [ kms@xmath26 ] & 0.1 & 0.1 & 0.1 + typical noise level in @xmath35 [ k ] & 1.23 & 0.94 & 0.67 +
we investigate the intrinsic abundance ratio of @xmath0co to c@xmath1o and the x - factor in l1551 using the nobeyama radio observatory ( nro ) 45 m telescope . 
 l1551 is chosen because it is relatively isolated in the taurus molecular cloud shielded from fuv photons , providing an ideal environment for studying the target properties . 
 our observations cover @xmath240@xmath340with resolution @xmath230 , which are the maps with highest spatial dynamical range to date . 
 we derive the @xmath4 value on the sub - parsec scales in the range of @xmath2327 with a mean value of 8.0@xmath52.8 . comparing to the visual extinction map derived from the @xmath6 observations 
 , we found that the abundance ratio reaches its maximum at low @xmath7 ( i.e. , @xmath8 14mag ) , and decreases to the typical solar system value of 5.5 inside l1551 mc . the high @xmath4 value at the boundary of the cloud is most likely due to the selective fuv photodissociation of c@xmath1o . 
 this is in contrast with orion - a where its internal ob stars keep the abundance ratio at a high level greater than @xmath210 . in addition 
 , we explore the variation of the x - factor , because it is an uncertain but widely used quantity in extragalactic studies . 
 we found that x - factor @xmath9 @xmath10 which is consistent with previous simulations . excluding the high density region , 
 the average x - factor is similar to the milky way average value .