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recent high - resolution images of many hii regions taken with the _ hubble space telescope _ and the _ spitzer space telescope _ show very complicated structures inside them .
one of such hii regions is ic 1848 (= w5 ) .
see , e.g. , figure 4 of koenig et al .
( 2008 ) , where we find a wealth of intricate structures inside / on its boundaries .
some of them are bright - rimmed clouds ( brcs ) cataloged in sugitani , fukui , and ogura ( 1991 ) .
but others are morphologically much different from usual brcs , suggesting that they are genetically different from brcs .
we discuss this point in sect . 4 , but briefly
, we suspect that , whereas brcs mostly originate from pre - existing cloud clumps left - over in evolved hii regions , some of them may have resulted from the hydrodynamical instability of the ionization front of the expanding hii region
. of particular interest is a concentration of thin and long elephant trunk - like structures ( hereafter abbreviated as _
etls_s ) in the southeastern part of ic 1848e .
figure 1 is a contrast - enhanced pseudo - color image of part of ic 1848e taken by the _
spitzer space telescope _
( blue : 3.6@xmath2 m , green : 8.0@xmath2 m , red : 24.0@xmath2 m ) . note that all these _ etls_s point to hd 18326 , the exciting o star of ic 1848e . very interestingly ,
some of them have a star / a few stars at their very tip , as marked in figure 1 .
this led us to suspect that they gave birth to these stars under the compressing effects of hii gas .
zavagno et al .
( 2007 ) found two intrusions of similar morphology with a star at their tip in rcw 120 ( see their fig .
we further suspect that the hydrodynamical instability of the ionization fronts creating _ etls_s makes a third mechanism of triggered star formation associated with hii regions , in addition to the _ collect - and - collapse _ process of the shell accumulated around an expanding hii region and _ radiation - driven implosion _ of brcs .    in order to examine the pre - main - sequence ( pms ) nature of the stars located at the tip of the _
etls_s , we carried out @xmath0 photometry of these stars and constructed a @xmath3 color - magnitude diagram ( cmd ) .
we also used near - infrared ( nir ) data from the _ two micron all sky survey _ ( 2mass ) to construct a nir color - color diagram as well as mid - infrared ( mir ) data from the _ spitzer space telescope _ to make spectral energy distribution ( sed ) curves .
brcs are small clouds apparently with both width and length of several arcminutes , corresponding to the physical size of a few _ parsecs _ typically ( see sugitani et al .
1991 , sugitani & ogura 1994 ) . in these papers brcs
are morphologically classified into _ types a , b _ and _ c _ according to their length - to - width ratios with _ type c _ being the most elongated .
but many of the _ etls_s found in figure 1 are much more elongated and of far smaller widths ( typically one tenth of _ pc _ ) than most of the _ type c _ brcs in sugitani et al .
( 1991 ) and sugitani and ogura ( 1994 ) .
we searched for such peculiar _ etls_s that have a star / stars at their tip on the _ spitzer _ 3.6 @xmath2 m , 4.5 @xmath2 m , and 8.0 @xmath2 m images .
table 1 gives the results , listing such stars with running numbers identified in figure 1 , the coordinates and some remarks .
we refer to these stars as _ etls _ stars .
some of them are listed in koenig et al .
( 2008 , their table 4 ) .
[ cols= " < , < , < , < " , ]     [ tab5 ]
we paid attention to the numerous , elephant trunk - like clouds in ic 1848e and carried out @xmath0 photometry of the optically visible stars located at the tip of several of them .
their positions on the @xmath3 cmd indicate that they are low - mass pms stars of ages of mostly one _ myr _ or less .
the physical parameters derived for these stars by using the sed fitting tools indicate that they are largely class i or class ii pms sources .
the pms nature of these stars strongly suggests that they must have formed from these _ etls_s . on the basis of the morphology , the size distributions , and the ages of the associated young stars
we conclude that the _ etls_s and brcs have different origins , and suspect that the former are created by the hydrodynamical instability of the if / sf of the expanding hii region .
we further hypothesize that , in addition to the _ collect - and - collapse _ process and rdi , this mechanism makes a third mode of triggered star formation associated with hii regions .
we are grateful to the anonymous referee for his / her useful comments that improved this paper .
we thank the staff of ioa , hanle and crest , hosakote for the assistance during the observations .
nc is thankful to the fellowship granted by dst and csir , india .
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the hii region ic 1848 harbors a lot of intricate elephant trunk - like structures that look morphologically different from usual bright - rimmed clouds ( brcs ) 
 . of particular interest is a concentration of thin and long elephant trunk - like structures in the southeastern part of ic 1848e . 
 some of them have an apparently associated star ( or two stars ) at their very tip . 
 we conducted @xmath0 photometry of several of these stars . 
 their positions on the @xmath1 color - magnitude diagram as well as the physical parameters obtained by sed fittings indicate that they are low - mass pre - main - sequence stars having ages of mostly one _ myr _ or less . 
 this strongly suggests that they formed from elongated , elephant trunk - like clouds . 
 we presume that such elephant trunk - like structures are genetically different from brcs , on the basis of the differences in morphology , size distributions , and the ages of the associated young stars . 
 we suspect that those clouds have been caused by hydrodynamical instability of the ionization / shock front of the expanding hii region . 
 similar structures often show up in recent numerical simulations of the evolution of hii regions . 
 we further hypothesize that this mechanism makes a third mode of triggered star formation associated with hii regions , in addition to the two known mechanisms , i.e. , _ collect - and - collapse _ of the shell accumulated around an expanding hii region and _ radiation - driven implosion _ of brcs originated from pre - existing cloud clumps .