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semi - regular variables ( srvs ) are a sub - class of asymptotic giant branch ( agb ) stars which also include long period variables called mira variables .
these objects show a large amplitude visual variability ( v@xmath1 2.5 ) with well defined periods of 300 - 400 days . on the other hand srvs have smaller variability amplitude ( v@xmath2 2.5 ) and periods range from 10 to 1000 days .
srvs are also less studied than mira variables .
there is a speculation that srvs could be the progenitors of mira variables , but needs confirmation ( bedding & zijlstra , 1998 ) .
the boundary between these two subclasses is not well defined .
several stars which were classified as mira variable , have been reclassified as srv ( srb ) after observing irregularity in there periods ( ragland et al . , 2006 ) .
asymmetry in agb variables , particularly in miras , have been observed earlier with high angular resolution techniques like lunar occultations ( lo ) , long baseline interferometry ( lbi ) , and aperture masking interferometry ( mondal & chandrasekhar , 2002 ; ragland et al .
, 2006 ; weiner et al . , 2006 ) .
the departure from spherical / circular symmetry may be due to stellar rotation , non - radial pulsation or bright hot / cold spots in the outer layers of the atmosphere .
multi - epoch angular diameter measurements of the same source using lo technique are important in this context as they provide angular diameter values at different position angles across the occulted stellar source .
any significant variation in the angular diameter values is an indication of asymmetric brightness distribution in the occulted source .
such asymmetric brightness distributions have been reported earlier in several miras . in a study using hst images , karovska et al .
( 1997 ) reported asymmetry in mira a atmosphere at 0.5 @xmath0 which they ascribed to bright spots on the surface of the star , while the asymmetry observed in uv - band is explained as mira a s interaction with mira b. asymmetric structure in a mira variable , u ori , has been seen using lo technique by mondal and chandrasekhar ( 2004 ) .
this spatial asymmetry was attributed to pulsational mass - loss in a preferred direction .
asymmetry in mira variables often changes over their pulsation periods which is because of directional mass - loss in different pulsation periods .    in an extensive study , ragland et al .
( 2006 ) observed 56 agb variables using the three - telescope infrared optical telescope array ( iota ) interferometer at near infrared h - band ( 1.65 @xmath0 ) to study asymmetry in the source brightness distribution .
their sample includes 35 mira variables , 18 srvs and 3 irregular variables .
asymmetric brightness distribution has been detected in 29% of the sources .
they also speculated that 75% of the mira variables in their sample could be asymmetric , but are beyond the spatial resolution of the detector system they used . among the 16 sources which show asymmetric brightness distributions , 3 are srvs , namely , uu aur , v hya and y cvn .
all of the srvs are carbon stars and have uniform disk ( ud ) angular diameters of 10.9 milliarcsecond ( mas ) , 21.2 mas and 14.1 mas respectively .
the cause of the asymmetry is not clearly known .
they explained that it is unlikely for these stars to have rotational asymmetry or non - radial pulsation unless they have a relatively massive companion ( @xmath3 0.1 m@xmath4 ) .
these stars are large in size and harbour massive envelopes around them .
hence , they can not have sufficient angular momentum to develop asymmetry without a binary companion . though convective cells can stimulate non - radial pulsations , but that will be on a smaller scale . however , they speculated that the asymmetry seen in mira variables is due to the formation of an inhomogeneous translucent molecular screen located at about 1.5 to 2.5 stellar radii .
agb variables generally develop molecular layers in the atmosphere which may cause different optical depth in different wavelengths .
wittkowski et al .
( 2011 ) performed spectro - interferometry on four mira variables in the near - infrared ( nir ) k - band and found wavelength dependent angular diameter .
they also found a significant deviation from the point symmetry in h@xmath5o band near 2.0 @xmath0 , which was interpreted as inhomogeneities or presence of clumps within water vapour .
another aspect to be investigated while studying asymmetry in srvs is the polarization of the source .
radiation from the luminous agb variables is often found to be linearly polarized , generally 1 - 5% .
asymmetric radiation field due to localized hot / cool spots or deformed shape of the envelope may provide an overall polarization . in case of agb variables polarization arises from purely scattering phenomena . in a recent paper , neilson , ignace & henson ( 2013 )
has shown that mira v - band polarization varies only 0 - 1% over last 40 years but the variation in position angle corresponding to it is from 0@xmath6 to 150@xmath6 .
they suggest that these variations in polarization as well as position angle of mira are due to convective hot spots . on the otherhand in a srv ( v cvn )
, they found variability in the polarization of 0 - 7% with a nearly constant position angle .
the constant position angle of polarization suggests a long term asymmetry along the direction of polarization .
polarization in very cool stars have been studied for a longtime ( serkowski , 1966 ; dyck , forbes & shawl , 1971 ; kruszewski & coyne , 1976 ) .
kruszewski & coyne ( 1976 ) reported polarization measurement of 18 cool stars ( including uz ari ) in the visible and nir bands in the spectral range of 0.3 - 1.0 @xmath0 .
they made polarization observations with the steward observatory 229 cm reflector on the kitt peak and the 154 cm reflector of the mt .
lemmon observatory .
they found a good correlation of degree of polarization with [ 3.5 - 11 @xmath0 ] colour which characterizes the dust envelope of the star . on the otherhand
, a weak correlation is found for polarization with nir ( i - k ) colour which characterizes the stellar photosphere .
in addition to polarization , maser emission also is not an uncommon phenomena among the late type giants .
circular polarization observations of these masers have revealed the strength and structure of the magnetic field through out the circumstellar shell .
kruszewski & coyne ( 1976 ) found a good correlations between the degree of polarization and the separation between 18 cm oh - maser line components .
the authors concluded that the polarization is produced in the extended stellar atmosphere where the infrared excess and oh - maser emission also originate .
however , the source of our interest uz ari , does not have any oh - maser emission detection till date , though water maser at 22 ghz has been seen sporadically ( engels , schmid - burgk & walmsley , 1988 ; takaba et al . , 2001 ) .
in this paper we present 3-epoch uniform disk angular diameter measurements of the semi - regular variable uz arietis using the lo technique in the nir k - band ( 2.2@xmath0 ) .
these measurements are the first reported high angular resolution observations of uz ari .
we have made a detailed study of the srv uz arietis based on our lo observations and optical polarimetric observations made at a latter epoch .    [
cols="^,<,^,^,^,^,^,^,^,^ " , ]     @xmath8d1970 : dyck , forbes & shawl , 1970 ; k1976 : kruszewski and coyne , 1976    [ pol_old ]
from a three epoch lo observations at 2.2 @xmath9 m we report the first uniform disk angular diameter measurements of semi - regular variable uz arietis .
three well resolved uniform disk angular diameters are obtained at three different position angles across the source .
optical linear polarization measurements made recently by us combined with earlier values suggest that source polarization value and direction has not changed over the last 40 years .
our lo measurements along with the polarization observations suggest a small asymmetry in the brightness profile of the source directed along or close to the polarisation axis .
this work was supported by dept of space , govt of india .
this research made use of the simbad data base operated at the cds , starsbourg , france and catalogues associated with it , aavso and gcvs for the optical light curve , data products from the wide - field infrared survey explorer , a joint project of the university of california , los angeles , and the jpl / caltech funded by nasa .
we thank the anonymous referee for constructive suggestions which have improved the quality of the paper .
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semi - regular variables ( srvs ) though closely related to mira variables , are a less studied class of agb stars . 
 while asymmetry in the brightness distribution of many mira variables is fairly well known , it is detected only in a few srvs . 
 asymmetry in the brightness distribution at the level of a few milliarcsecond ( mas ) can be detected by high angular resolution techniques like lunar occultations ( lo ) , long baseline interferometry , and aperture masking interferometry . 
 multi - epoch lo observations have the potential to detect a departure of brightness profile from spherical symmetry . 
 each lo event provides a uniform disk ( ud ) angular diameter along the position angle of the occultation . 
 any significant difference in the ud angular diameter values of multi - epoch lo observations signifies a brightness asymmetry . in this paper , we report for the first time three epoch ud angular diameter values of a srv uz arietis using the lo technique at 2.2 @xmath0 . 
 optical linear polarization of the source observed by us recently is also reported . 
 the asymmetric brightness distribution of uz ari suggested by a small difference in the fitted ud values for the three epochs , is discussed in the context of optical polarization exhibited by the source and the direction of polarization axis in the plane of the sky .    
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