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Paper text:
wolf - rayet ( wr ) stars are evolved massive o  type stars which are predicted to be the progenitors of type ibc core - collapse supernovae ( ccsne ) . however , to date there has been no direct confirmation of the wr - sne connection .
pre - sne broad - band images have failed to reveal the progenitor of @xmath010 type ib / c sne ( * ? ? ?
* ( smartt 2009 ) ) .
wr stars can be classified into two main subtypes , nitrogen  rich ( wn ) and carbon  rich ( wc ) stars , which reveal the products of cno burning and triple alpha reactions , respectively .
wr stars exhibit a unique emission line spectrum which is dominated by heii@xmath14686 emission lines for wn stars while wc spectra , which also show weaker heii@xmath14686 , are dominated by ciii@xmath14650 and civ@xmath15808 ( * ? ? ?
* ( crowther 2007 ) ) .
these strong emission lines can be 0.2 - 2.5magnitudes brighter than the adjacent continuum , making wr stars easy to detect using narrow - band imaging techniques .
type ib sne are hydrogen  poor , while type ic sne are both hydrogen and helium  poor .
the similarities between the observed type ib and ic sn spectra and the chemical composition of the wn and wc stars makes them strong progenitor candidates , respectively .
m101 is a grand - design spiral galaxy which lies face - on at a distance of 6.2mpc ( * ? ? ?
* ( shappee & stanek , 2011 ) ) .
it has a high star - formation rate of at least @xmath04.5m@xmath2yr@xmath3 based on the h@xmath4 flux or @xmath06.8m@xmath2yr@xmath3 from far uv imaging ( * ? ? ?
* ( lee et al . 2009 ) ) .
there is a wealth of hst / acs archival data for m101 which was obtained under the legacy program , however the most effective way to identify wr stars is via narrow - band imaging techniques centered on the wr emission lines .
we obtained 18 pointings of m101 using wfc3/f469n over 36 orbits in cycle 17 ( pi : shara ) .
identifying wr stars beyond the local group with ground - based imaging is challenging since we can only resolve them on scales of @xmath025pc .
the emission lines of wr stars , although strong , can easily be diluted by strong continuum from other ob stars in the unresolved region ( * ? ? ?
* ( bibby & crowther 2010 ) ) .
the high spatial resolution images allow us to resolve sources down to @xmath03pc , decreasing the contamination of the wr emission by continuum sources .    typically wr stars span an absolute magnitude range of m@xmath5=4 to 8mag ; however , our ground - based studies do not extend fainter than m@xmath5=5mag .
the improved sensitivity of hst allows us to identify the faintest and least massive wr stars , detecting stars to m@xmath6=3.5mag and m@xmath7=4mag .
the acs and wfc3 data were re - drizzled onto the same scale of 0.05 arcsec pix@xmath3 using multidrizzle .
photometry was performed with the stand - alone daophot package .
stars that had at least a 3 sigma excess in the f469n filter compared to the f435w and f555w filters were identified as wr candidates .
these candidates were then visually inspected using the `` blinking '' method ( * ? ? ?
* ( moffat & shara , 1983 ) ) .
four of the wfc3/f469n pointings have currently undergone analysis and have revealed 372 wr candidates with a heii @xmath14686 excess of at least 3 sigma ( fig .
[ magnitudes ] ) . the brightest candidates with m(f435w)-m(f469n )
excess of @xmath82.5mag are likely to be single wr stars , whereas the fainter candidates at m(f469n)@xmath025mag with small m(f435w)-m(f469n ) excesses @xmath80.6mag are more likely to host multiple wr stars in unresolved clusters .    from the photometric analysis we were also able to identify sources in the f469n image that were not identified in either the f435w or f555w images .
this is indicative of a faint wr star with a continuum which lies below the detection threshold of the broad - band images .
we identified an additional 269 wr candidates that were only detected in the narrow - band images , which were again checked using the blinking method ( fig.[blink ] ) .
we are currently awaiting execution of follow - up multi - object spectroscopy with gemini - north / gmos for a sample of these candidates ( @xmath035% ) which will allow us to ( i ) determine the multiplicity of the sources , ( ii ) assign a spectral classification of wn or wc to the candidate and ( iii ) infer the subtype of the remaining candidates for which we did not obtain spectroscopic confirmation .
we identified 641 wr candidates within four hst / wfc3 pointing of m101 , 42% of which are _ only _ detected in the narrow - band image .
this is much higher than the 25% found from ground - based studies of other nearby spiral galaxies , e.g. ( * ? ? ?
* bibby & crowther ( 2012 ) ) , which is most likely due to the improved sensitivity and spatial resolution of hst which allows us to detect fainter wr stars .
this work highlights the effectiveness of high , spatial resolution narrow - band observations in detecting wr stars .
if we hope to confirm or , equally important , rule out wr stars as the progenitors of type ib / c sne , then we require a _ complete _ sample of wr stars in several nearby galaxies .
\(i ) investigate how the number of wr stars varies with the number of o stars and red supergiants ( rsg ) across the galaxy as a function of metallicity and compare the results to predictions from theoretical evolutionary models .
\(iv ) expand our existing catalogue of wr stars which can then be referred to in the event that a type ib / c sne occurs in one of these galaxies .
this will allow strong , direct observational evidence for , or against , the wr - sne connection .
wolf - rayet ( wr ) stars are the evolved descendants of massive o - type stars and are considered to be progenitor candidates for type ib / c core - collapse supernovae ( sne ) . 
 recent results of our hst / wfc3 survey of wolf - rayet stars in m101 are summarised based on the detection efficiency of narrow - band optical imaging compared to broad - band methods . 
 we show that on average of 42% wr stars , increasing to @xmath085% in central regions , are _ only _ detected in the narrow - band imaging . 
 hence , the non - detection of a wr star at the location of @xmath010 type ib / c sne in broad - band imaging is no longer strong evidence for a non - wr progenitor channel .