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in recent years , eruptive objects with maximum luminosities intermediate between those of classical novae ( cne ) and supernovae ( sne ) have been discovered in increasing numbers , as surveys for galactic and extragalactic transients are made with greater depth and sky coverage by professional and amateur astronomers . the term `` supernova impostors '' was introduced by van dyk et al.(2000 ) following the outburst of sn  1997bs . sn  1997bs was classified as a sn  iin event , `` n '' denoting narrow emission lines during outburst , in contrast to the high ejection velocities of typical sne . however , it reached an absolute magnitude at maximum of only @xmath7 , more than 3  mag fainter than a typical core - collapse sn . van  dyk et al .  argued that sn  1997bs was instead a `` superoutburst '' of a massive luminous blue variable ( lbv ) , analogous to those experienced by the galactic objects p  cygni and @xmath8  carinae ( see humphreys , davidson , & smith 1999 ) . the subject of intrinsically faint sne  iin and @xmath8  car analogs has been summarized by van  dyk ( 2005 ) and smith et al.(2008 ) . more recently , sn  2008s , which had a sn  iin - type spectrum but reached only @xmath9 , has been discussed by thompson et al . ( 2008 ) and smith et al . prieto et al .  ( 2008 ) have identified the precursor of sn  2008s in pre - outburst mid - ir images obtained with the _ spitzer space telescope _ , demonstrating that the progenitor was a luminous star deeply embedded in circumstellar dust . v838  monocerotis is the prototype of an apparently separate class of transients also lying in the cn - sn gap . v838  mon erupted in 2002 , illuminating a spectacular light echo that provided a geometric distance ( sparks et al . 2008 ) implying an absolute magnitude at maximum of @xmath10 . unlike a cn , v838  mon became progressively redder during its outburst , eventually becoming enshrouded in circumstellar dust . its outburst light curve ( bond et al . 2003 ) was very different from that of a sn or lbv eruption , showing a series of four maxima separated by about a month each . the cause of this outburst remains uncertain , but a leading suggestion is that it was a stellar collision or merger ( e.g. , tylenda & soker 2006 , corradi & munari 2007 , and references therein ) . possible extragalactic analogs of v838  mon although this interpretation is controversial  include the `` m31  red variable '' ( `` m31  rv '' ) which reached @xmath11 ( bond & siegel 2006 and references therein ) , and a luminous red transient in m85 ( rau et al .  2007 ; kulkarni et al .  2007 ) , which attained @xmath12 . m31  rv and the m85 transient appear to have occurred in old populations , suggesting that they did not arise from very massive progenitors .    in this _ letter _ , we report observations of a luminous transient discovered in 2008 may in the nearby spiral galaxy ngc  300 . we obtained extensive ground - based imaging , photometry , and spectroscopy , and we also investigated pre- and post - outburst space - based optical and infrared images of the transient . this _ letter _ presents an overview of our results , along with a preliminary discussion of the nature of this remarkable object . subsequent publications will give fuller details . we designate the object as the `` ngc  300 optical transient , '' hereafter `` ngc  300  ot . '' the outburst of ngc  300  ot was discovered by monard ( 2008 ) during his sn search program with the bronberg observatory 0.3-m telescope equipped with a ccd camera . the ot was detected at broad - band magnitude 14.3 on a frame obtained on 2008 may 14 ( all dates in this paper are ut ) . subsequent inspection of an image from 2008 april 24 , taken in morning twilight shortly after ngc  300 had emerged from behind the sun , showed that the ot was already rising , at @xmath216.3  mag . it was fainter than 18  mag on 2008 february  8 , and on all previous bronberg observations . ngc  300 is an sad spiral in the sculptor group , located just outside the local group . the ot lies in a spiral arm with active star formation . we adopt a distance modulus of @xmath13 ( @xmath14  mpc ) , based on cepheids ( gieren et al . 2005 ) and the red - giant tip ( rizzi et  al .  2006 ) .    at maximum the ot was much fainter than a typical sn , but brighter than any cn . because of our interest in luminous red transients , we began a program of photometric and spectroscopic monitoring , using the 1.3- and 1.5-m telescopes at cerro tololo interamerican observatory operated by the smarts consortium . the photometric monitoring in particular was intensive at first , because of our expectation that the ot might exhibit periodic spikes , like those during the outburst of v838  mon ( bond et al . 2003 ) . we used the andicam optical / near - ir direct camera ( depoy et al . 2003 ) on the smarts 1.3-m with ccd and ir detectors , operated simultaneously , for the _ bvri _ and _ jhk _ photometry . the _ bvri _ magnitudes of the ot were determined differentially with respect to a nearby comparison star , calibrated on photometric nights using landolt ( 1992 ) standard fields . the _ jhk _ magnitudes are differential with respect to the same star , calibrated using its 2mass photometry . the j2000 position of the ot , based on astrometry of andicam frames calibrated against the usno - nomad catalog , is ra = 00:54:34.51 , dec = @xmath15:38:31.4 , with errors in each coordinate of about @xmath16@xmath17 . figure  1 shows the _ bvrijhk _ light curve . unfortunately , the rise to maximum was poorly covered , but it appears that it was more rapid than the subsequent slower decline . the brightest @xmath5 magnitude we measured was 14.69 , on the first night of smarts observations ( 2008 may 15 ) , corresponding to @xmath18 to @xmath19 for the adopted distance and a reddening lying in the range @xmath20 to 0.4 ( see  3.1 ) . following maximum light , the ot has declined smoothly in brightness at all wavelengths , while becoming steadily redder . for example , @xmath21 evolved from 3.1 in mid - may to 7.4 at the beginning of 2008 december . the rate of decline in the optical lessened around the beginning of 2008 september , but at this writing the brightness continues to decrease in both the optical and near - ir . the @xmath22 magnitudes are not fading as rapidly as in the other filters , due to strong h@xmath23 emission . we obtained low- and moderate - resolution optical spectroscopy of ngc  300  ot throughout its outburst , using the smarts 1.5-m cassegrain spectrograph . our first low - resolution spectrum was observed on 2008 may 15 ( bond , walter , & velsquez 2008 ) and is shown in figure  2 ( top spectrum ; resolution 17.2   ) . this spectrum exhibits strong emission at h@xmath23 , h@xmath24 , the triplet at 854284988662   , and the unusual forbidden [ ] doublet at 72917323   . the balmer lines are only slightly resolved at the velocity resolution ( @xmath25 ) of the spectrum . the underlying continuum resembles a high - luminosity f - type supergiant , with h and k seen in absorption along with several weaker luminosity - sensitive lines such as the triplet at 7774   . our photometry at maximum and shortly afterwards yields @xmath26 , implying that the inter- and circumstellar extinction could be as high as @xmath27 , since a mid- to late f - type supergiant normally has @xmath28 . the mean foreground reddening of ngc  300 is @xmath29 ( gieren et al . 2005 ) . this spectrum is clearly not that of a cn nor a typical sn . with strong and [ ] emission and an f - type absorption spectrum , it resembles that of the type  iin supernova impostor sn  2008s ( steele et al . 2008 ; smith et al . it is also closely similar to the spectra of the well - studied post - red supergiant irc  + 10420 ( jones et al . 1993 ; humphreys , davidson , & smith 2002 ) , variable  a in m33 in its recent quiescent state ( humphreys et al .  2006 ) , and v838  mon near its maximum ( e.g. , wisniewski et al . these f - type supergiant spectra with superposed narrow emission lines of relatively low excitation are indicative of eruptions that produce an optically thick wind or slowly expanding envelope . as the ot declined , its spectrum became dominated by the h@xmath23 and emission . one example , selected from our monitoring program , is the bottom spectrum shown in figure  2 , obtained on 2008 august  23 . by this date , the continuum had faded dramatically and reddened somewhat , but the strong emission remained . we also obtained moderate - resolution ( 3.1   ) spectra regularly with the smarts 1.5-m spectrograph , and on three occasions with the magellan 6.5-m telescope and mage echellette spectrograph ( marshall et al .  2007 ; resolution 1.5    at , 1.1    at h@xmath23 ) : 2008 july  6 , aug 30 , and sep  1 . these spectra show that the triplet , h@xmath23 , and h@xmath24 have double - peaked emission lines , with the blue component being the stronger . figure  3 shows the h@xmath23 and emission features from the magellan echellette spectra on 2008 july  6 ( top spectrum ) and the average of the august 30 and september  1 observations ( bottom spectrum ) , and illustrates the double structure of these lines . double emission is usually attributed either to a bipolar outflow or a rotating disk , but in the case of an eruption , the double lines are most likely formed in a bipolar wind . irc  + 10420 shows very similar double - peaked emission and has strong independent evidence for a bipolar outflow ( davies et al . 2007 ; patel et al .  2008 ) . the peak of the ot s primary blue component slowly increases in strength with time relative to the red component , supporting our conclusion that the outflow is bipolar , with the nearer , blue - shifted lobe expanding towards us . the doppler velocities measured from both the hydrogen and emission lines in the echellette spectra indicate expansion velocities for the primary wind of @xmath270@xmath30 with respect to the star s systemic velocity of about @xmath31 . there is evidence as well for additional emission components , appearing as shoulders or secondary bumps on the primary blue and red peaks at velocities of @xmath32@xmath33 . a wind velocity of @xmath2@xmath34 is typical of the winds of f - type supergiants , and is somewhat lower than the winds associated with lbvs in eruption . it is very similar to the expansion velocity of @xmath2@xmath35 of irc  + 10420 ( jones et al . 1993 ; humphreys et al .  2002 ) . by contrast , the [ ] lines , formed in a very low - density region , do not show a double - peaked structure ; they probably arise in more - distant and slower - moving material , likely formed before the current eruption . we imaged the site of the ot twice with the _ hubble space telescope _ ( ) , in the director s discretionary ( dd ) program go-11553 ( pi : bond ) . we used the wide field planetary camera  2 ( wfpc2 ) with the f450w and f814w filters ; observations were made on 2008 june  9 and september  1 . in addition , the archive contains two sets of deep observations made before the outburst , obtained with the wide field channel of the advanced camera for surveys ( acs ) and the f435w , f475w , f555w , f606w , and f814w filters . the acs observations were made on 2002 december  25 ( go-9492 , pi : f.  bresolin ) and 2006 november 810 ( go-10915 , pi : j.  dalcanton ) . we used these data to locate the ot precisely , search for a progenitor object , study the surrounding stellar population , and set limits on a light echo from the outburst . in addition to the data , we were awarded dd time on the eso very large telescope ( vlt ) to obtain near - ir imaging of the ot , using the adaptive - optics naco camera ( lenzen et al . 2003 ; rousset et al . the vlt / naco images were obtained on 2008 june  6 . the wfpc2 exposure times for our dd observations ranged from 5 to 260  s , chosen so as not to saturate the image of the ot , but to show enough of the surrounding star field for precise registration with the pre - outburst acs frames . the archival acs frames are considerably deeper , with total exposure times of 1080 , 2976 , 1080 , 3030 , and 4524  s in f435w , f475w , f555w , f606w , and f814w , respectively . the vlt / naco @xmath36-band frames were four 600  s exposures , combined in the eso pipeline to produce a single image . figure  4 shows a montage of post- and pre - outburst images . the  ot lies amid a rich field of resolved stars . remarkably , however , there is no significant detection of a progenitor object at the ot s precise location . the two nearest reliably detected sources lie about @xmath37 and @xmath38 from the site , well in excess of our registration error of about @xmath39 . these two stars are red giants with f606w magnitudes of about 26.3 and 27.0 . in the deepest stacked pre - outburst images there are suggestions of local maxima at the ot site , but these are all consistent with noise . from the three deepest acs stacked images , we find the following limiting magnitudes ( vega - mag scale , 3@xmath40 upper limits ) for the ot progenitor : 28.3 ( f475w ) , 28.5 ( f606w ) , and 26.6 ( f814w ) . similar limiting magnitudes were reported earlier , based on registering the 2006 acs images with ground - based magellan / clay 6.5-m images , by berger & soderberg ( 2008 ) , but with larger positional uncertainties . the ot was thus very unlike a typical core - collapse sn , for which luminous progenitor stars are being found relatively easily in archival images at distances greater than that of ngc  300 ( e.g. , smartt et al . 2008 and references therein ) , nor was it similar to erupting lbvs , which even in quiescence have high optical luminosities ( e.g. , humphreys & davidson 1994 ) . the acs images also provide deep cmds for the environment surrounding the  ot . these will be presented in detail in a subsequent publication , but we note that there is a rich population of young objects in the immediate vicinity of the ot . for example , within @xmath41 ( @xmath223  pc ) of the ot site there are over a dozen blue main - sequence stars brighter than about f606w magnitude 26 , reaching as bright as @xmath42 ( corresponding to b1 stars of @xmath32@xmath43 ) . the surrounding spiral - arm field is rich in blue clusters and associations . we compared the post - outburst images of 2008 june  9 and september  1 in search of an expanding light echo , similar to the one surrounding v838  mon ( e.g. , bond 2007 , sparks et al . 2008 , and references therein ) . although a light echo of the surface brightness and angular size of v838  mon s , moved to the distance of ngc  300 , would have been detectable at resolution , no echo is seen . the very faint limits on the pre - outburst optical brightness of ngc  300  ot initially suggested to us that there was a similarity with v838  mon , whose progenitor was also optically inconspicuous ( e.g. , afar & bond 2007 , and references therein ) . however , the very different light curve and spectroscopic behavior of the ot as its outburst proceeded casts doubt on this parallel . moreover , a striking difference emerged when one of us ( prieto 2008 ) reported that the progenitor of the ot _ was _ detected as a bright mid - ir source in archival observations obtained with _ spitzer _ on 2003 november  21 ( pi : g.  helou ) and 2007 december  28 ( pi : r.  kennicutt ) . the source was present in all four irac bands ( 3.6 , 4.5 , 5.8 , and 8  ) , and in the mips 24  band . a black - body fit to the 3.624  fluxes gives the following results for the luminosity , effective temperature , and radius of the ot progenitor : @xmath44 , @xmath45  k , and @xmath46  au . these are consistent with a heavily dust - enshrouded red supergiant of @xmath210@xmath6 ( e.g. , maeder & meynet 2000 ) . there was no variability of the progenitor between 2003 and 2007 . further detailed discussion of the _ spitzer _ observations is given by thompson et al.(2008 ) . a picture thus emerges in which a luminous massive star , optically inconspicuous because it was deeply embedded in a dusty envelope , underwent a sudden outburst that cleared most of the surrounding dust . in many respects , ngc  300 ot has behaved remarkably similarly to the supernova impostor sn  2008s ( see  1 ) . sn  2008s was also heavily enshrouded by dust prior to its outburst , with an intrinsic luminosity of @xmath47 based on its ir sed , implying an initial mass of @xmath2@xmath48 . the energy released in the ot s eruption was relatively modest . integrating over the extinction - corrected visual light curve , beginning with the initial detection in 2008 april , and assuming that the bolometric correction is zero , we find that the total energy released was @xmath2@xmath49@xmath50  ergs . this is similar to the energy radiated by sn  2008s , @xmath2@xmath51  ergs ( smith et al . 2008 ) , but considerably less than the @xmath2@xmath52 ergs emitted in @xmath8  car s great eruption , and of course the @xmath2@xmath53 ergs in true core - collapse sne .    given the large amounts of pre - outburst obscuring dust , both ngc  300  ot and sn  2008s must have experienced significant mass loss as post - main - sequence stars , and were most likely red supergiants ( rsgs ) , post - rsgs , or conceivably lower - mass stars at the tip of the agb just before the outbursts . thompson et al .  ( 2008 ) have suggested that these events may represent a new class of low - luminosity outbursts arising from stars in the mass range @xmath28@xmath54 . by comparison with multi - epoch _ spitzer _ irac observations in m33 , they have identified a group of rare , reddened sources that they call extreme - agb stars , which may be the progenitor class in a short - lived stage lasting only @xmath55@xmath56  yr immediately preceding the outbursts . there are also many enshrouded oh / ir stars in our own galaxy , including agb - tip stars and red supergiants with very red mid - ir colors ; some may be as red as the progenitors of the ot and sn  2008s . here we suggest that ngc 300 ot , and by analogy sn  2008s , were oh / ir stars before their eruptions . given both objects lack of obvious variability from _ spitzer _ observations several years apart , these ots were not mira variables just before outburst . they had presumably left the region of rsgs and agb stars in the hr diagram , and were on a blue loop back to warmer temperatures . the ngc  300 ot s bipolar wind at @xmath3275 km s@xmath57 also supports a warmer temperature for the progenitor . during this transition , both stars reached a state in which they exceeded the eddington limit for their luminosities and masses and suddenly initiated outflows , bipolar in the case of ngc  300  ot . exactly why this should happen remains uncertain : the eruptions may have resulted from some type of as - yet unexplained failed supernova , a binary merger , or a photospheric eruption .    continued observations of the ngc  300  ot as it fades are especially important . we currently observe the spectrum of its wind , but if it does not form another cocoon of dust , we may eventually observe the survivor , determine its true nature , and possibly gain some insight into the cause of its eruption .    partial support for this research was provided by nasa through grant go-11553 from the space telescope science institute , which is operated by the association of universities for research in astronomy , inc . , under nasa contract nas5 - 26555 . stsci s participation in the smarts consortium is supported by the stsci director s discretionary research fund . we thank m.  hernndez and j.  velsquez for conducting the 1.5-m observations , j.  espinoza , a.  miranda , and j.  vasquez for the 1.3-m observations , and michelle buxton and suzanne tourtellotte for scheduling the 1.3-m telescope . g.  pietrzyski kindly obtained one of the mage spectra . we thank kris davidson , chris kochanek , kris stanek , and todd thompson for useful discussions .
a luminous optical transient ( ot ) that appeared in ngc  300 in early 2008 had a maximum brightness , @xmath0 to @xmath1 , intermediate between classical novae and supernovae . 
 we present ground - based photometric and spectroscopic monitoring and adaptive - optics imaging of the ot , as well as pre- and post - outburst space - based imaging with and _ spitzer_. the optical spectrum at maximum showed an f - type supergiant photosphere with superposed emission lines of hydrogen , , and [ ] , similar to the spectra of low - luminosity type  iin `` supernova impostors '' like sn  2008s , as well as cool hypergiants like irc  + 10420 . 
 the emission lines have a complex , double structure , indicating a bipolar outflow with velocities of @xmath2@xmath3 . the luminous energy released in the eruption was @xmath2@xmath4 ergs , most of it emitted in the first 2 months . by registering new images with deep archival frames , we have precisely located the ot site , and find no detectable optical progenitor brighter than broad - band @xmath5 magnitude 28.5 . 
 however , archival _ spitzer _ images reveal a bright , non - variable mid - ir pre - outburst source . 
 we conclude that the ngc  300  ot was a heavily dust - enshrouded luminous star , of @xmath210@xmath6 , which experienced an eruption that cleared the surrounding dust and initiated a bipolar wind . 
 the progenitor was likely an oh / ir source which had begun to evolve on a blue loop toward higher temperatures , but the precise cause of the outburst remains uncertain .