The Diversity of Massive Star Outbursts I: Observations of SN 2009ip, UGC 2773 OT2009-1, and Their Progenitors
Ryan J. Foley, Edo Berger, Ori Fox, Emily M. Levesque, Peter J. Challis, Inese I. Ivans, James E. Rhoads, Alicia M. Soderberg
aa r X i v : . [ a s t r o - ph . S R ] F e b Draft version April 15, 2018
Preprint typeset using L A TEX style emulateapj v. 11/10/09
THE DIVERSITY OF MASSIVE STAR OUTBURSTS I: OBSERVATIONS OF SN 2009ip, UGC 2773 OT2009-1,AND THEIR PROGENITORS
Ryan J. Foley , Edo Berger , Ori Fox , Emily M. Levesque , Peter J. Challis , Inese I. Ivans ,James E. Rhoads , Alicia M. Soderberg Draft version April 15, 2018
ABSTRACTDespite both being outbursts of luminous blue variables (LBVs), SN 2009ip and UGC 2773 OT2009-1 have very different progenitors, spectra, circumstellar environments, and possibly physical mecha-nisms that generated the outbursts. From pre-eruption
HST images, we determine that SN 2009ipand UGC 2773 OT2009-1 have initial masses of &
60 and & M ⊙ , respectively. Optical spectroscopyshows that at peak SN 2009ip had a 10,000 K photosphere and its spectrum was dominated by narrowH Balmer emission, similar to classical LBV giant outbursts, also known as “supernova impostors.”The spectra of UGC 2773 OT2009-1, which also have narrow H α emission, are dominated by a forest ofabsorption lines, similar to an F-type supergiant. Blueshifted absorption lines corresponding to ejectaat a velocity of 2000 – 7000 km s − are present in later spectra of SN 2009ip — an unprecedentedobservation for LBV outbursts, indicating that the event was the result of a supersonic explosion,rather than a subsonic outburst. The velocity of the absorption lines increases between two epochs,suggesting that there were two explosions in rapid succession. A rapid fading and rebrightening eventconcurrent with the onset of the high-velocity absorption lines is consistent with the double-explosionmodel. A near-infrared excess is present in the spectra and photometry of UGC 2773 OT2009-1that is consistent with ∼ η Car, NGC 300 OT2008-1,and SN 2008S. This qualitative analysis suggests that massive star outbursts have many physicaldifferences which can manifest as the different observables seen in these two interesting objects.
Subject headings: circumstellar matter — stars: evolution — stars: individual (UGC 2773 OT2009-1,SN 2009ip) — stars: mass loss — stars: variable: other — stars: winds, outflows— supernovae: general INTRODUCTION
Very massive stars appear to go through a phase ofinstability and large mass loss; during this stage, a star isa member of the luminous blue variable (LBV) class (seeHumphreys & Davidson 1994 for a review). In additionto low-amplitude variability (called S Dor variabilityafter the prototypical LBV), where the star ejects massfrom its envelope but its bolometric luminosity remainsnearly constant, some LBVs have “giant eruptions.” Gi-ant eruptions can expel & M ⊙ of material, while havinga luminosity similar to the lowest-luminosity supernovae(SNe). The classical examples of giant eruptions areP Cygni in 1600 and η Car in 1843, and more recent, ex-tragalactic examples include SN 1961V (Goodrich et al.1989; Filippenko et al. 1995; Van Dyk et al. 2002; butsee Chu et al. 2004), V12/SN 1954J (Smith et al. 2001;Van Dyk et al. 2005), SN 1997bs (Van Dyk et al.2000), SN 2000ch (Wagner et al. 2004), andV37/SN 2002kg (Weis & Bomans 2005; Maund et al. Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, Cambridge, MA 02138, USA. Clay Fellow. Electronic address [email protected] . Department of Astronomy, University of Virginia, P.O. Box400325, Charlottesville, VA 22904, USA. Institute for Astronomy, University of Hawaii, 2680 Wood-lawn Dr., Honolulu, HI 96822, USA. Predoctoral Fellow, Smithsonian Astrophysical Observatory Department of Physics and Astronomy, University of Utah,Salt Lake City, UT 84112, USA. School of Earth and Space Exploration, Arizona State Uni-versity, P.O. Box 871404, Tempe, AZ 85287, USA
Spitzer images, but were undetected to deep limits in the opti-cal (Prieto et al. 2008; Berger et al. 2009b; Bond et al.2009), indicating significant reddening from circumstel-lar dust. The progenitor stars were originally believed tohave ZAMS masses of 8 – 20 M ⊙ , which is below thatof the least massive LBVs. Using the stars in the vicin-ity of NGC 300 OT2008-1, Gogarten et al. (2009) founda slightly higher mass range of 12 – 25 M ⊙ . A reason-able range for the initial mass of the progenitors is ∼
10– 25 M ⊙ , with NGC 300 OT2008-1, having a more lu-minous progenitor, being toward the upper end of thatrange.Recently, two transients were discovered with onebeing very similar to classical LBV giant eruptions(SN 2009ip), and another sharing characteristics of bothLBV giant eruptions and the outbursts of the dusty starsdiscussed above (UGC 2773 OT2009-1). SN 2009ip wasdiscovered by Maza et al. (2009) in NGC 7259 ( µ =32 . ± .
15 mag ; D ≈
24 Mpc) on 2009 August 26 (UT We use the distance modulus corresponding to the Hubbledistance for NGC 7259 with H = 73 km s − Mpc − and correctingfor the Virgo Infall and Great Attractor flow model of Mould et al. Foley et al.dates will be used throughout this paper). Miller et al.(2009) noted that SN 2009ip had been variable for severalyears and identified a possible progenitor with M F606W ≈− . Hubble Space Telescope ( HST ) image. The variability and high luminosity of the eventled Miller et al. (2009) to suggest that SN 2009ip was ei-ther an LBV or cataclysmic variable outburst. An earlyspectrum of the event showed a blue continuum withrelatively narrow (FWHM = 550 km s − ) H Balmerlines. The combination of the spectrum with the rel-atively low absolute magnitude ( R ≈ − . , fad-ing by at least 3 mag in 16 days and rebrightening by2 mag in the next 10 days (Li et al. 2009), reminiscent ofthe variability immediately before maximum of the 1843eruption of η Car (Frew 2004) and immediately aftermaximum in the LBV outburst SN 2000ch (Wagner et al.2004). Smith et al. (2009b, hereafter S09) presented ahistorical light curve of SN 2009ip that begins 5 yearsbefore maximum light, excluding the
HST image of theprogenitor. The star varied by at least 1.5 mag duringthis time. S09 presented additional data which led themto conclude that SN 2009ip was the giant eruption of a50 – 80 M ⊙ LBV.UGC 2773 OT2009-1 was discovered by Boles (2009) inUGC 2773 ( µ = 28 . ± .
17 mag; D ≈ − )H α emission, P-Cygni lines from the Ca II NIR triplet,and [Ca II ] emission lines (Berger & Foley 2009). Wealso noted that the spectrum was similar to that ofNGC 300 OT2008-1 and mentioned that it was possi-bly a very low-luminosity SN II or an LBV outburst,“but the strong [Ca II ] emission would be unexpected inthis case.” Berger & Foley (2009) also detected a po-tential progenitor star in archival HST images. S09presented a historical light curve of SN 2009ip that be-gins 9 years (excluding the
HST image of the progen-itor) before maximum light. The star slowly increasedfrom F814W = 22 .
22 mag to an unfiltered magnitude of17.70 mag at maximum light, corresponding to a linearincrease of ∼ − before outburst. S09 con-cluded from the progenitor identification, their historiclight curve, the peak luminosity, and optical light curvesthat UGC 2773 OT2009-1 was the outburst of a & M ⊙ LBV.This is the first in a series of papers where we investi-gate the diversity of massive star outbursts. In this paperwe demonstrate the heterogeneity of the class with ob-servations of SN 2009ip and UGC 2773 OT2009-1. In fu-ture papers, we will detail the properties of the class andthe links between observations and the physical mecha- (2000). Smith et al. (2009b) use a distance modulus of 31.55 mag,which differs by 0.50 mag from our assumed value, correspondingto the Hubble-flow distance modulus correcting only for the CMBdipole. To be consistent with S09, we adopt MJD = 55061 . .
75 as times of maximum light for UGC 2773OT2009-1 and SN 2009ip, respectively. However, we note thatthe objects may reach their true maximum later, which UGC 2773OT2009-1 already has (as shown by data presented in S09) nisms which cause the outbursts. In Section 2, we presentultraviolet (UV), optical, and near-infrared (NIR) pho-tometry and optical and NIR spectroscopy of UGC 2773OT2009-1 and SN 2009ip. In this section, we also re-fine previous identifications of the progenitors. In Sec-tion 3, we examine the progenitor masses, the spectro-scopic characteristics of the outbursts, and the spectral-energy distributions (SEDs) of the events. In Section 4,we discuss how these outbursts connect to previous mas-sive star outbursts and the mass loss history and ultimatefates of massive stars. We summarize our conclusions inSection 5. OBSERVATIONS AND DATA REDUCTION
Identification and HST Photometry of theProgenitors
UGC 2773 was observed with
HST /WFPC2 on 1999August 14 (Program 8192; PI Seitzer). The observationsincluded two exposures of 600 s each with the F606Wand F814W (roughly V and I ) filters. NGC 7259 wasobserved with HST /WFPC2 on 1999 June 29 (Program6359; PI Stiavelli). Exposures of 200 and 400 s wereobtained with the F606W filter.To determine whether the progenitors of SN 2009ipand UGC 2773 OT2009-1 are detected in the archival
HST observations, we performed differential astrometryusing optical observations of the transients. Observationsof UGC 2773 OT2009-1 were obtained with the Gem-ini Multi-Object Spectrograph (GMOS) on the Gemini-North 8-m telescope, and the astrometry was performedusing 55 objects in common with the
HST /WFPC2images resulting in an astrometric rms of σ GB → HST =24 mas in each coordinate. Observations of SN 2009ipwere obtained with the Inamori Magellan Areal Cameraand Spectrograph (IMACS) on the Magellan/Baade 6.5-m telescope, and the astrometry was performed using 10objects in common with the
HST /WFPC2 image result-ing in an astrometric rms of σ GB → HST = 38 mas in eachcoordinate.The positions of the two transients on the archival
HST images are shown in Figure 1. In both cases, we find aclear coincidence with objects in the archival
HST im-ages. For SN 2009ip we find an offset of 24 ±
38 mas rel-ative to the object in the WFPC2/F606W image, whilefor UGC 2773 OT2009-1 we find an offset of 32 ±
24 masrelative to the object in the WFPC2/F606W and F814Wimages.The measurements of the photometry for UGC 2773OT2009-1 and nearby stars were done using HSTphot 1.1(Dolphin 2000). HSTphot was run using a weighted PSF-fit, which is recommended for crowded fields, and a lo-cal sky determination, which is recommended for rapidlyvarying backgrounds. HSTphot performs the conversionfrom
HST /WFPC2 flight magnitudes to the Bessel mag-nitude system. Our astrometry and photometry for thenominal progenitor of UGC 2773 OT2009-1 and nearbystars are listed in Table 1.We performed photometry of the point source coinci-dent with SN 2009ip using a 0 . ′′ aperture and a zero-point of 22.47 mag appropriate for the F606W filter. Wefurther applied a correction of − .
29 mag to convert tothe Vega system, and applied a correction for the Galac-tic extinction of 0.05 mag. The resulting magnitude ofiverse Massive Star Outbursts I 3
UGC2773 OT2009−1
SN2009ip
Figure 1.
HST /WFPC2 F606W image at the position of UGC 2773 OT2009-1 (left) and SN 2009ip (right) obtained 10 years beforemaximum. Both images are 10 ′′ × ′′ , and North is up and East is left. The UGC 2773 OT2009-1 and SN 2009ip images have pixel scalesof 0.1 ′′ pixel − . The position of each transient is marked by the black circle whose radius corresponds to 10 σ uncertainty in the position. Table 1
HST
Photometry of Stars Near UGC 2773 OT2009-1Object R.A. Dec. F606W (mag) F814W (mag)1 a a Star 1 is identified as the progenitor of UGC 2773 OT2009-1. the source is 21 . ± .
25 mag. For reasonable colors (seeSection 3.1.1), this corresponds to M V = − . Ultraviolet, Optical, and Near-Infrared Photometry
We obtained optical photometry of UGC 2773OT2009-1 with the Gemini Multi-Object Spectrograph(GMOS) on the Gemini-North 8-m telescope in the gri filters. We performed the photometry using IRAF/ phot with the standard GMOS zero-points . Our results arepresented in Table 2.We obtained near-infrared (NIR) photometry ofUGC 2773 OT2009-1 with FanCam, a 1024 × outside of Charlottesville, VA (Kanneganti et al. 2009).Each epoch consists of fifteen minutes of integration in JHK s bands, which have detection limits at the 10 σ level of 0.066, 0.098, and 0.156 mJy (or 18.5, 17.5, and16.5 mag), respectively. Individual exposures are sky-background limited and have an integration time of ei-ther 30 or 60 s. Flat-field frames are composed of duskand dawn sky observations. We employed standard NIRdata reduction techniques in IRAF . Because of the rel-atively small galaxy size, it was possible to fit the en-tire galaxy in a single array quadrant. Empty quad-rants were efficiently utilized as sky exposures. Data IRAF is distributed by the National Optical Astronomy Ob-servatory, which is operated by the Association of Universities forResearch in Astronomy (AURA) under cooperative agreement withthe National Science Foundation.
Foley et al.
Table 2
UV and Optical Photometry of UGC 2773 OT2009-1 and SN 2009ipObject MJD Filter Mag TelescopeUGC 2773 OT2009-1 51404.13 F606W 22.82 (0.03)
HST
UGC 2773 OT2009-1 51404.14 F814W 22.29 (0.05)
HST
UGC 2773 OT2009-1 55078.38 J H K s g r i J H K s HST
SN 2009ip 55084.44 UVW2 21.09 (0.19)
Swift
SN 2009ip 55084.45 UVM2 20.92 (0.28)
Swift
SN 2009ip 55084.44 UVW1 20.69 (0.18)
Swift
SN 2009ip 55084.44 U Swift
SN 2009ip 55084.44 B Swift
SN 2009ip 55084.45 V Swift were taken with the galaxy placed in each quadrant andeach quadrant was reduced separately. Ultimately, allreduced quadrants were coadded. We performed pho-tometry with IRAF’s PSF package. For magnitude cal-ibration, the transient is compared to 2MASS referencestars located in the field of view. Table 2 lists our
JHK s photometry, which is similar to the single epoch JHK s data from S09.We obtained UV and optical observations of SN 2009ipwith the Swift
UV/optical telescope on 2009 September10. The data were processed using standard routineswithin the HEASOFT package. Photometry of the tran-sient in all filters, with the exception of UVW2, was per-formed using a 2 ′′ aperture to avoid contamination fromnearby objects. Aperture corrections to the standard 5 ′′ aperture were determined using isolated stars; photome-try of the source in the UVW2 filter was performed usinga 5 ′′ aperture. X-ray Observations
We observed SN 2009ip and UGC 2773 OT2009-1with the
Swift
X-ray Telescope on 2009 September 10for a total exposure time of 9.0 and 4.2 ks, respec-tively. No X-ray counterpart is detected at the posi-tion of either source to a limit of F X . . × − and . . × − erg s − cm − , respectively (95% limit). Inboth cases we assume a power law model with an elec-tron index of −
2, and account for the Galactic neutralhydrogen column. The corresponding limits on the lu-minosity are L X . . × and . . × erg s − .These limits are comparable to the X-ray emission fromSNe on a similar timescale (e.g., Soderberg et al. 2008). Radio Observations
We observed the both LBV candidates with the VeryLarge Array (VLA) following their optical discoveryto search for radio counterparts, under Rapid Responseprograms AS1001 and AS1002 (PI Soderberg). Our ra-dio observations were carried out at two frequencies, 8.46 The Very Large Array is operated by the National Radio As-tronomy Observatory, a facility of the National Science Foundationoperated under cooperative agreement by Associated Universities,Inc.
Table 3
VLA observations of UGC 2773 OT2009-1 and SN 2009ipObject Date F ν, . F ν, (UT) ( µ Jy) ( µ Jy)SN 2009ip 2009 Sep 7.36 · · · − ± · · · ± · · · UGC 2773 OT2009-1 2009 Sep 13.57 · · · − ± · · · ± · · · Note . — Uncertainties are 1 σ rms map noise. and 22.5 GHz, on dates spanning 2009 September 7.36- 16.51 in the C-array antenna configuration. All ob-servations were taken in standard continuum observingmode with a bandwidth of 2 ×
50 MHz. Phase referencingwas performed with calibrators J0325+469 and J2213-254, and we used 3C38 (J0137+331) for flux calibration.Data were reduced using standard packages within theAstronomical Image Processing System (AIPS).We detect no radio sources in positional coincidencewith either object and derive upper limits summarizedin Table 3. At 8.5 GHz, our upper limits corre-spond to L ν < . × erg s − Hz − and L ν < . × erg s − Hz − for SN 2009ip and UGC 2773OT2009-1, respectively. These limits are less luminousthan an extrapolation of the observed SN 1961V radioemission at t ≈
10 years, to a similarly early epoch as L ν ∝ t − . (Stockdale et al. 2001). We note, however,that the SN 1961V radio emission may have reached max-imum intensity significantly later than our observationsof UGC 2773 OT2009-1 and SN 2009ip, similar to the ra-dio evolution of SNe IIn which typically reach maximumlight several years after the explosion (van Dyk et al.1996).A comparison of these radio upper limits for the out-bursts to the observed properties of other core-collapseSNe places them among the least luminous events, 2-4 or-ders of magnitude less luminous than the most powerfulSNe IIn, and 4-200 times higher than the early radio sig-nal seen for SN 1987A (Ball et al. 1995). Through this Upper limits are calculated as the measured flux density atthe optical position summed with 2 × rms the off-source map noise. iverse Massive Star Outbursts I 5simple comparison we emphasize that radio data alonecannot distinguish between massive star outbursts andcatastrophic explosions. However, with the 10-fold in-crease in continuum sensitivity provided by the EVLAwe will begin to map out the radio properties for massivestar outbursts and enable direct comparisons with thoseof core-collapse SNe (e.g., NRAO Key Project AS1020,”Exotic Explosions,Eruptions,and Disruptions: A NewTransient Phase-Space”, PI Soderberg). Optical Spectroscopy
We obtained low- and medium-resolution spectra ofSN 2009ip and UGC 2773 OT2009-1 with the MagEspectrograph (Marshall et al. 2008) on the MagellanClay 6.5 m telescope, the Blue Channel spectrograph(Schmidt, Weymann, & Foltz 1989) on the MMT 6.5 mtelescope, and GMOS (Hook et al. 2004) on the Gemini-North 8 m telescope. A journal of our optical spectro-scopic observations can be found in Table 4.Standard CCD processing and spectrum extractionwere accomplished with IRAF. The data were extractedusing the optimal algorithm of Horne (1986). Low-order polynomial fits to calibration-lamp spectra wereused to establish the wavelength scale, and small adjust-ments derived from night-sky lines in the object frameswere applied. For the MagE spectra, the sky was sub-tracted from the images using the method described byKelson (2003). The GMOS data were reduced usingthe Gemini IRAF package (for details, see Foley et al.2006). We employed our own IDL routines to flux cali-brate the data and remove telluric lines using the well-exposed continua of the spectrophotometric standards(Wade & Horne 1988; Foley et al. 2003, 2009).Representative spectra of SN 2009ip and UGC 2773OT2009-1 are presented in Figure 2. Both objects havesimilar blue continua, but the line features are very dif-ferent. SN 2009ip has few line features besides strongH Balmer lines, Na D, and He I . Although UGC 2773OT2009-1 has a strong H α line, it is much narrower andweaker than that of SN 2009ip. UGC 2773 OT2009-1also displays many additional narrow line features, in-cluding lines from intermediate mass and Fe-group ele-ments. Blueward of ∼ II lines. Finally, UGC 2773OT2009-1 has very strong Ca II NIR triplet lines and[Ca II ] λλ II ] are rarely seen in classical LBV out-bursts, but were distinguishing features of SN 1999bw(Garnavich et al. 1999), SN 2008S (e.g., Smith et al.2008b), and NGC 300 OT2008-1 (e.g., Berger et al.2009b).The spectra from 2009 September 21 (corresponding todays 34 and 24 for UGC 2773 OT2009-1 and SN 2009ip,respectively) were obtained on the same night as theLRIS spectra shown by S09. Near-Infrared Spectroscopy
On 2009 September 9 (22 days after maximum), weobtained a 2400 s NIR spectrum of UGC 2773 OT2009-1with TripleSpec, a medium resolution NIR spectrographlocated at Apache Point Observatory. This spectrographis one of three NIR, cross-dispersed spectrographs cover-ing wavelengths from 1 – 2.4 µ m simultaneously at a res-olution of ∼ RESULTS
Progenitor Masses
SN 2009ip
We use the absolute magnitude, M V = − . V − I colors of − .
05 to 1 . E ( B − V ) = 0 .
019 mag ( A V = 0 .
05 mag;Schlegel, Finkbeiner, & Davis 1998). We adopt a dis-tance modulus of µ = 32 .
05 mag for NGC 7259 (seediscussion in Section 1), and assume no additional hostgalaxy or circumstellar extinction.In Figure 3, we compare the color of the SN 2009ipprogenitor to the non-rotating, standard mass-loss evolu-tionary tracks of the Geneva group (Schaller et al. 1992).From this plot we can place a lower initial mass limit of60 M ⊙ on the progenitor of SN 2009ip in the absenceof a color estimate for this progenitor, the higher-massevolutionary tracks all coincide with its estimated loca-tion on the color-magnitude diagram, precluding us fromplacing an upper limit on this initial mass estimate. Fig-ure 3 assumes a solar metallicity for these tracks; how-ever, we find that our progenitor mass prediction is con-sistent across the full range of metallicities accommo-dated by the Geneva evolutionary tracks ( Z = 0 . Z ⊙ to Z = 2 Z ⊙ ). It should be noted that an increased amountof extinction, from the host galaxy or circumstellar en-vironment, could also effectively increase the estimatedinitial mass of this progenitor.S09 estimated the initial mass for the progenitor ofSN 2009ip to be 50 – 80 M ⊙ . Although the HST pho-tometry from S09 is the same as that presented here,their assumed distance modulus is 0.50 mag smaller thanour assumed value. They also make no color correctionto transform the F606W measurements into V . Despitethese differences, the two mass ranges are similar. UGC 2773 OT2009-1
Using its M V and V − I color, we are able to deter-mine an estimate of the initial mass for the progenitorof UGC 2773 OT2009-1 (Figure 3). There is significantMilky Way extinction of E ( B − V ) = 0 .
564 mag ( A V =1 .
75 mag; Schlegel, Finkbeiner, & Davis 1998), which weconvert to E ( V − I ) = 0 .
902 mag (Schultz & Wiemer1975). We use a distance modulus of µ = 28 .
82 mag andinitially assume no host galaxy or circumstellar extinc-tion.From Figure 3, we find that the progenitor ofUGC 2773 OT2009-1 is consistent with an initial massof ∼ M ⊙ . Our progenitor mass prediction remains thesame across the full range of metallicities covered by theGeneva evolutionary tracks, and is consistent with thevalue found by S09. Foley et al. Table 4
Log of Optical Spectral ObservationsTelescope / Grating / ExposurePhase a UT Date Instrument Central Wavelength (˚A) (s) Observer b UGC 2773 OT2009-115.1 2009 Sep. 2.6 Gemini/GMOS R400/7000 2 × × ×
900 PC34.0 2009 Sep. 21.5 MMT/Blue Channel 832/4830 2 ×
900 PC34.0 2009 Sep. 21.5 MMT/Blue Channel 832/6563 3 ×
900 PC95.8 2009 Nov. 22.3 MMT/Blue Channel 300/5787 2 × · · · ×
900 II22.5 2009 Sep. 20.3 MMT/Blue Channel 300/5787 4 × × × × a Days since maximum, MJD 55,061.5 and 55,071.8 for UGC 2773 OT2009-1 and SN 2009ip (S09), respectively. b II = I. Ivans, KO = K. Olsen, PC = P. Challis, RM = R. McDermid S ca l e d f λ UGC 2773−OT−1 SN 2009ip
Hydrogen Balmer Series Ca IIFe IIHe I/Na D K ISc II Si II Mg II[Ca II]He IHe I O I
Figure 2.
Optical spectra of SN 2009ip and UGC 2773 OT2009-1. The spectrum of UGC 2773 OT2009-1 has been dereddened by E ( B − V ) = 0 .
564 mag. Several lines are identified and marked. .
We have also performed this procedure on several starsin the vicinity of the progenitor of UGC 2773 OT2009-1. Assuming that all of these stars are part of a clusterand were formed at the same time, they should placeadditional limits on the current maximum-mass stars ofthe cluster. These stars are all consistent with an initialmass of M ⊙ . M ⊙ . There is a single star that is par-ticularly blue (and therefore potentially very massive),but it is still consistent with an initial mass of 25 M ⊙ .The likely association of the progenitor of UGC 2773 OT2009-1 with this cluster and its upper mass limit of ∼ M ⊙ further supports the initial mass estimate forthe progenitor of UGC 2773 OT2009-1.Considering the blue colors of the stars in the clus-ter, it is unlikely that they are significantly reddened byhost galaxy dust. As shown in Figure 3, a relativelysmall amount of extinction could significantly increaseour initial mass estimate for UGC 2773 OT2009-1. InSection 3.4 we show that there was likely a significantamount of circumstellar dust existing before the out-iverse Massive Star Outbursts I 7 Figure 3.
Color-magnitude diagram ( V − I vs. M V ) for the pro-genitor of UGC 2773 OT2009-1 (star) and stars spatially locatedwithin the same star cluster (grey circles). The measurements havebeen corrected for the Milky Way extinction of A V = 1 .
75 magand E ( V − I ) = 0 .
902 mag, but no host or circumstellar extinctionis assumed for the stars. For comparison, solar metallicity, non-rotating, “standard” mass loss stellar evolution tracks are also plot-ted (Schaller et al. 1992). The progenitor of UGC 2773 OT2009-1has the same colors and absolute magnitude of a 20 M ⊙ model.The stars in the cluster are consistent with the models of starswith ZAMS masses ≤ M ⊙ , but a single star is also consistentwith a much higher mass. The arrow represents A V = 0 . R V = 3 . & M ⊙ . burst, indicating that the progenitor had an initial massmuch larger than the reddening-free estimate of 20 M ⊙ .The combination of the reddening-free initial mass es-timate for the progenitor of UGC 2773 OT2009-1, theinitial mass estimates of stars likely within the same clus-ter as the progenitor, and the probably circumstellar dustextinction give us a conservative lower limit on the ini-tial mass of the progenitor of UGC 2773 OT2009-1 of ∼ M ⊙ . Spectroscopic Comparisons
SN 2009ip
We present the 24 and 86 day spectra of SN 2009ipin Figure 4. In the upper panel of Figure 4, the 24 dayspectrum is compared to the 2 day spectrum of the LBVoutburst SN 1997bs (Van Dyk et al. 2000). Both objectshave blue continua, strong and narrow H Balmer lines,and He I and Fe II emission lines. Unlike SN 1997bs,SN 2009ip has a particularly strong He I λ I λ ∼ − (seeSection 3.3.1 for a detailed discussion of this high-velocityabsorption).Inspecting the 25 day spectrum from S09, we see someindication of the 3000 km s − absorption component,particularly for H β and He I λ S ca l e d f λ SN 2009ipSN 1997bs S ca l e d f λ + C on s t a n t SN 1998SSN 2009ip H α H β Fe II Sc II He I Si II
Figure 4.
Top panel: the 23 day optical spectrum of SN 2009ipcompared to the 2 day spectrum of SN 1997bs (Van Dyk et al.2000). Bottom panel: the 86 day optical spectrum of SN 2009ipafter smoothing and subtracting a 10,000 K blackbody (see text fordetails). For comparison, the 25 day spectrum of SN 1998S (aftersubtracting a 10,000 K blackbody) is also shown (Leonard et al.2000). Prominent, high-velocity lines have been marked. tures are present in all reductions. The same featureis present in all spectra taken with the MMT on days23 and 24, which were taken with different gratings andwavelength regions. It is also present on day 86, butwith a different velocity. The absorption is present forall Balmer lines and He I λ − ) has beenapplied to the spectrum of SN 2009ip (Blondin et al.2006). This filtering will smear out features with intrin-sic widths less than 1000 km s − , but will appropriatelysmooth features on larger scales. The high-velocity ab-sorption in the 86 day spectrum of SN 2009ip is at a higher velocity than at 24 days. At this epoch, the ve-locity of the fast-moving SN 2009ip ejecta are very simi-lar to that of SN 1998S. Although the H Balmer emissionlines are much stronger in SN 2009ip, most other featuresare similar in the two spectra. In particular, SN 2009ipshows the H Balmer, He I , Sc II , and Fe II features seenin SN 1998S. SN 2009ip is missing the strong absorp-tion at 6250 ˚A that is attributed to Si II in SN 1998S(Leonard et al. 2000). This feature may be the result ofa significant amount of nuclear burning, and thus notpresent in the ejecta of SN 2009ip. UGC 2773 OT2009-1
As discussed in Section 2.5, UGC 2773 OT2009-1 hasa spectrum with narrow H α emission, [Ca II ] emission,and P-Cygni absorption from many intermediate-massand Fe-group elements. Perhaps the most distinguishingfeature compared to other massive star outbursts is the[Ca II ] emission. In Figure 5, we compare the 15 day Foley et al. S ca l e d f λ NGC 300−OT−1SN 2008SUGC 2773−OT−1SN 1994W
Figure 5.
Optical spectra of UGC 2773 OT2009-1, NGC 300OT2008-1 (Berger et al. 2009b), SN 1994W (Chugai et al. 2004),and SN 2008S (Smith et al. 2008b). All spectra have narrow H α and [Ca II ] emission; however, NGC 300 OT2008-1 and SN 2008Slack the forest of lines (especially Fe II ) that UGC 2773 OT2009-1and SN 1994W display. spectrum of UGC 2773 OT2009-1 to spectra of the low-luminosity transients NGC 300 OT2008-1 (Berger et al.2009b) and SN 2008S (Smith et al. 2008b), as well asSN IIn 1994W (Chugai et al. 2004); all of these objectshave [Ca II ] emission in their spectra.All spectra in Figure 5 are relatively similar. The con-tinuum of each spectrum is well-described by a blackbodyspectrum, with all four objects having a similar temper-ature. Each object has a prominent H α emission line,with UGC 2773 OT2009-1 having a narrower line thanthe other objects. Additionally, SN 1994W has a strongH α absorption line blueward of its emission peak.NGC 300 OT2008-1 and SN 2008S are very simi-lar objects with massive (10 – 25 M ⊙ ), dusty progeni-tors (Prieto et al. 2008; Berger et al. 2009b; Bond et al.2009). Their spectra share many characteristics with theyellow hypergiant IRC+10240 (Smith et al. 2008b). Al-though UGC 2773 OT2009-1 shares some spectroscopicproperties with these two transients and IRC+10240 (seeS09 for additional discussion), the latter objects lack theforest of absorption lines in UGC 2773 OT2009-1. Theselines are reminiscent of an F-type supergiant. The P-Cygni profiles of these lines and the hydrogen Balmeremission are very similar to S Dor during a cool phase(e.g., Massey 2000).SN 1994W was very luminous at peak ( M V ≈−
19 mag), but generated at most 0.03 M ⊙ of Ni (Sollerman, Cumming, & Lundqvist 1998).Dessart et al. (2009) presented an alternative method ofproducing the photometric and spectroscopic propertiesof this object: the collision of two massive hydrogenshells ejected from the star with no core collapse.Spectra of SNe IIn are rather heterogeneous (seeFigure 5 of Smith et al. 2009a for a comparison ofvarious objects), and SN 1994W is relatively distinctfor its narrow absorption features. Given the spectralsimilarity between UGC 2773 OT2009-1 and SN 1994W,the strict upper limit of Ni mass in SN 1994W, and the alternative model of Dessart et al. (2009), one mustfurther question if SN 1994W destroyed its progenitorstar.
Contrasting SN 2009ip and UGC 2773 OT2009-1 At t = 0 days, the temperature of UGC 2773OT2009-1 is ∼ ∼ & ∼ Line Profiles
In this section, we examine the line profiles of H α andCa lines. These features provide an indication of thekinematics of the emitting material. The narrow linesare a tracer of the pre-shock circumstellar material, whilethe high-velocity absorption features in the spectra ofSN 2009ip probe the outburst ejecta. H α In Figure 6, we present the H α line profiles ofUGC 2773 OT2009-1 and SN 2009ip. Three separateepochs are shown for each object. Both objects haveasymmetric line profiles. There are absorption compo-nents at about −
350 km s − for UGC 2773 OT2009-1 andbetween − − − for SN 2009ip. Theline profile of SN 2009ip has a different shape and is muchbroader than that of UGC 2773 OT2009-1. We haveattempted to fit these line profiles, but because of theasymmetry of the profiles, we first fit only the redshiftedportion of each line profile and then add an absorptioncomponent to reproduce the blueshifted profile.For the 15 day spectrum of UGC 2773 OT2009-1, wefit a Gaussian with FWHM = 780 km s − to the redside of the feature. This value is twice that of the valuefound by S09 for a spectrum from day 22, but examina-tion of their figures suggest that they reported half-widthat half maximum (HWHM) or the standard deviation ofthe Gaussian fit (which is smaller by a factor of 2.35)rather than FWHM. The 34 day spectrum of UGC 2773OT2009-1 is contaminated by host-galaxy emission lines,iverse Massive Star Outbursts I 9 −1.0 −0.5 0.0 0.5 1.0Velocity (10 km s −1 )1101001000 N o r m a li ze d f λ UGC 2773 OT2009−196 d34 d15 d −8 −6 −4 −2 0 2 4Velocity (10 km s −1 ) SN 2009ip4 d24 d86 d
Figure 6.
Normalized spectra of UGC 2773 OT2009-1 andSN 2009ip near H α . The line profiles are fit with Gaussian andLorentzian profiles, respectively. A profile fit to the redshiftedportion of each profile is shown as a dashed line. The grey linescorrespond to the redshifted profile with a Gaussian absorptioncomponent added. Narrow [N II ] can be seen in the spectrum ofUGC 2773 OT2009-1. making a fit to the inner regions of the line profile prob-lematic. Ignoring this region, we were able to fit theredshifted portion of the line profile with a single Gaus-sian with FWHM = 590 km s − . The 96 day spectrumhas lower resolution, but is successfully fit by a Gaussianprofile with FWHM = 470 km s − .To account for the asymmetric profile, we add an ab-sorption component to the Gaussian line profiles. Fit-ting the full profile with two Gaussian functions, theemission component fit to the red side of the line andthe absorption component added to fit the blue side ofthe line, we find absorption minima at − − −
80 km s − for the 15, 34, and 96 day spectra, respec-tively. This is different from the value of the actual mini-mum ( −
350 km s − ) since the relatively strong emissionmasks the true minimum.The line profiles of the first two spectra (days 4 and24) of SN 2009ip are well fit by Lorentzian profiles withFWHM = 780 km s − and the third is best fit by aLorentzian profile with FWHM = 890 km s − , which arelarger than that found by S09, 550 km s − . A Lorentzianprofile of 550 km s − is not a particularly bad fit to ourdata, but we find that the larger velocities better rep-resent the data. One can also see in Figure 8 of S09,that the 550 km s − Lorentzian slightly underpredictsthe true FWHM of the line, so the data appear to beconsistent.In the 24 day spectrum of SN 2009ip, we see an ab-sorption feature with a minimum at a velocity of about − − . (This high-velocity absorption is seenfor all Balmer lines with varying instrument configura-tions and on two epochs; see Section 3.2.1.) This featureis well fit by including a Gaussian absorption componentwith a minimum at − − . Adding a componentwith this velocity also improves the fit to the 4 day H α profile slightly, but not in a significant way. The 86 day spectrum shows an even stronger high-velocity absorp-tion component with the minimum of the absorption ata larger velocity of − − . The blue wing of theabsorption component, representing the fastest movingmaterial, corresponds to a velocity of about − − − for the 24 and 86 day spectra, respectively.These velocities are significantly larger than the out-flow velocity of 550 km s − assumed by S09. They aremuch larger than the wind speed of LBVs and are largerthan the measured velocity for any LBV eruption withthe exception of the 1843 eruption of η Car, which hadsome material expelled at 3000 – 6000 km s − (Smith2008). The velocities measured for SN 2009ip are sim-ilar to that of the ejecta of typical core-collapse SNe(such as SN 1998S; see Figure 4 and Section 3.2.1) andare somewhat similar to that of Wolf-Rayet winds (e.g.,Abbott & Conti 1987). We discuss the implications ofthese features in Section 4.2. Permitted and Forbidden Ca II Only our first spectrum of SN 2009ip covers the Ca II NIR triplet, and no spectrum shows obvious [Ca II ] λλ II behavior in this object (other than the absent[Ca II ] lines).UGC 2773 OT2009-1, on the other hand, has strongCa II features. This can be seen in Figure 2. We examinethe Ca H&K, [Ca II ] λλ II NIR tripletline profiles in Figure 7. The Ca H&K lines show a broadabsorption extending from − − and aminimum at about −
50 km s − that does not appear tochange significantly between the two epochs. Each com-ponent of the Ca II NIR triplet shows a strong P-Cygniprofile with a minimum at approximately −
250 km s − ,slightly larger than the minima of Ca H&K.The [Ca II ] λλ II λ II ] lines have asymmetric profiles in allspectra; the peak is at zero velocity, but the emission ex-tends further to the red than to the blue. The lines fromall epochs have FWHMs of ∼
400 km s − , which is abouthalf the width of H α (see Section 3.3.1), similar to thatfound for NGC 300 OT2008-1 (Berger et al. 2009b). Spectral Energy Distribution and Dust Emission
Using our available photometry and spectroscopy, wecan examine the spectral energy distribution (SED) ofboth objects. We have only optical spectra of SN 2009ip,which limits our ability to examine multiple blackbodycomponents for this object. A 10,000 K blackbody fitsour optical spectra well, which is consistent with thatfound by S09.Our single epoch of
Swift photometry occurred duringthe dramatic fading of the light curve immediately fol-lowing maximum brightness (S09). In Figure 8, the
Swift photometry is combined with the unfiltered photometry(approximately R band) of S09 (with an uncertainty of0.5 mag to account for the 16 hour difference in the epochof the observations) during the minimum. We overplot0 Foley et al.
96 d34 d Ca H&K N o r m a li ze d f λ [Ca II]4 d34 d96 d −1.2 −0.6 0.0 0.6 1.2Velocity (10 km s −1 )0.61.01.52.02.4 Figure 7.
Normalized spectra of UGC 2773 OT2009-1 near CaH&K (top), [Ca II ] λλ II NIR triplet(bottom). Dashed lines indicate the continuum flux and zero ve-locity for each line. Each member of the multiplet is overplottedfor a given spectrum. f λ ( − e r g s s − c m − Å − ) Figure 8.
UV/Optical photometry of SN 2009ip during the fadingevent immediately after maximum brightness. The blue dotted,orange dashed, and red solid curves correspond to 10,000, 8000,and 7000 K blackbody spectra, respectively. The 23 day spectrumis also plotted to show the consistency with both the photometryand the 10,000 K blackbody. All photometry is consistent with the8000 and 10,000 K blackbody spectra. Ignoring the bluest (UVW2)filter, the data are also consistent with the 7000 K blackbody. the 23 day spectrum for comparison. The optical pho-tometry is consistent with the optical spectrum and a10,000 K blackbody. The UVW2 flux is also consistentwith this blackbody, however, the UVM2 and UVW1measurements fall well below this curve. Although thismay be the result of line blanketing, these data are alsoconsistent with a blackbody curve with a temperatureas low as 8000 K. If we ignore the UVW2 measurement,the data can be fit by a 7000 K blackbody. Althoughour data suggest a possible change in the SED duringthe fading event, the lack of necessary comparison UVdata from a different epoch prevent a clear indication ofa change.Using the 15 day optical spectrum and 22 day NIRspectrum, we are able to examine the SED of UGC 2773OT2009-1 over nearly a decade in wavelength. Betweenthese dates, the light curve of UGC 2773 OT2009-1 wasessentially constant, having the same magnitude (within1 σ ) (S09). Using the long wavelengths of the NIR spec-trum, our data are sensitive to any low-temperature ther-mal components.We fit a single blackbody to these data, ignoring re-gions with strong line features and simultaneously fittingthe scaling between the optical and NIR spectra. Doingthis results in a best-fit temperature of 6800 K. Thissingle blackbody consistently under-predicts the flux atNIR wavelengths. As a result, we have also attempted tofix the spectrum with a double blackbody model. Thismodel, which produces a much better fit, results withtemperatures T = 6950 K and T = 2100 K. The fullspectrum and associated fits are shown in Figure 9.S09 noted that UGC 2773 OT2009-1 had a (photo-metric) NIR excess, but could not distinguish betweencircumstellar extinction and dust emission. To test theformer case, we attempted to fit the spectrum with a sin-gle blackbody, but with an additional extinction term.With R V fixed to 3.1, this model did not fit the dataiverse Massive Star Outbursts I 11 f λ ( − e r g s s − c m − Å − ) Figure 9.
Optical/NIR spectrum of UGC 2773 OT2009-1. Single(6800 K; dashed orange line) and double (2100 and 6900 K; solidred line) blackbody fits to the spectrum are overplotted. The indi-vidual components of the double blackbody fit are shown as bluedotted lines. The double blackbody is a better fit to the data thanthe single blackbody. Green points show our photometry, whichalso shows a NIR excess. The g -band flux is below that of eithercurve, but that is likely the result of line blanketing. well. The model was able to sufficiently reproduce thedata if we allowed R V <
1, which is unphysical. Wetherefore conclude that the NIR excess is likely due todust emission.Scaling our spectrum to our broad-band photometry,we can calibrate the blackbody flux, which in turn con-strains the ratio
R/D , where R is the radius of the black-body radiation and D is the distance to the object. Us-ing D = 6 ± . . ± . × cm and(4 . ± . × cm (13 . ± . ± η Car.Following the prescription outlined bySmith, Foley, & Filippenko (2008a) (and referencestherein), we can measure the mass of the emitting dust.Specifically, M d = 400 πρR d T d , (1)where M d is the dust mass, R d is the radius of the dust, T d is the dust temperature, and ρ is the dust density.For the values obtained from the spectra, T d = 2100 Kand R d = 4 . × cm, and assuming a dust graindensity of ρ = 2 .
25 g cm − , we find a dust mass of M d ≈ × − M ⊙ . Since there could be a significant amountof dust emission at lower temperatures, this is a lowerlimit on the total dust mass; however, it is worth notingthat this measurement is orders of magnitude less thanthe dust created in some SNe (e.g., Kotak et al. 2009,and references therein). We note depending on the dustcomposition, the dust temperature may differ from theblackbody temperature by hundreds of degrees.The dust is very close to the star and its temperature isnear the limit of grain survival. Given these conditions,it is very likely that pre-existing circumstellar dust was heated and is emitting as it is being vaporized, ratherthan newly formed dust emitting as it cools. DISCUSSION
Different Massive Star Outbursts
UGC 2773 OT2009-1 and SN 2009ip provide excel-lent examples of the diversity of massive star outbursts.UGC 2773 OT2009-1 increased its optical brightness by ∼ II ] emission. Itoccurred near a star cluster containing stars with initialmasses of ∼ M ⊙ and shows evidence for a very cool( T ≈ η Car. UGC 2773OT2009-1, on the other hand, has spectral characteris-tics similar to that of S Dor at maximum. The relativelysmall increase in optical luminosity may indicate thatUGC 2773 OT2009-1 was the result of normal S Dor vari-ability, but that it was a particularly luminous maximum.While the largest normal variation of S Dor stars vary by ∼ ∼ II ] emission, but as suggested by S09, this may belinked to the circumstellar environment, and particu-larly dust destruction, rather than the event. Our ob-servations have shown that there is dust in the circum-stellar environment of the progenitor, and that it waslikely pre-existing dust that is in the process of being va-porized. Additionally, SN 1999bw had [Ca II ] emission(Garnavich et al. 1999) and had an IR excess consistentwith dust emission at late times (Sugerman et al. 2004).All four massive star outbursts with observed [Ca II ]emission (SN 1999bw, SN 2008S, NGC 300 OT2008-1,and UGC 2773 OT2009-1) have evidence of circumstel-lar dust. We do note that SN 2000ch had a ∼ II ] emission was detectedin the relatively low signal-to-noise spectra presented byWagner et al. (2004). It is therefore possible to have acool object and circumstellar dust yet not have [Ca II ]emission.Although UGC 2773 OT2009-1, NGC 300 OT2008-1,and SN 2008S have circumstellar dust and similar tem-peratures, other than the narrow H Balmer and [Ca II ](which is linked to the presence of circumstellar dust)emission, the spectra and progenitors are not particularlysimilar. Particularly, NGC 300 OT2008-1 and SN 2008Shad relatively featureless spectra and progenitors with2 Foley et al.initial masses of 10 – 25 M ⊙ , while UGC 2773 OT2009-1 had spectrum dominated by narrow lines and a moremassive progenitor ( & M ⊙ ). Additional data are nec-essary to determine if the outburst mechanisms in theseobjects are similar.Using SN 2009ip and UGC 2773 OT2009-1 as exam-ples, there appears to be two distinct elements that de-termine the observational properties of massive star out-bursts. The first is the temperature of the outburst,which may be related to the increase in luminosity, theinstability that causes the eruption, the width of theemission lines, and possibly the energetics of the out-burst and if there is an explosion (see Section 4.2). Thisdirectly determines the shape of the optical SED, the ion-ization, if there is a forest of absorption lines, and possi-bly the shape of the line profiles, if there is high-velocityabsorption. The other characteristic is the amount ofcircumstellar dust, which may cause strong Ca emission(and particularly [Ca II ] emission) and will determinethe shape of the SED at longer wavelengths. UGC 2773OT2009-1 and SN 2009ip would occupy very differentregions of the parameter space created by these two di-mensions. NGC 300 OT2008-1 and SN 2008S would beclose to UGC 2773 OT2009-1, while LBV giant eruptionssuch as SN 1997bs would be close to SN 2009ip.It remains to be seen if there are hot massive star out-bursts with a large amount of circumstellar dust or ifthere are cool massive star outbursts with little circum-stellar dust. η Car has 0 . M ⊙ of dust surroundingit (Smith et al. 2003); if it were to have another gianteruption today, would it be cool? SN 1999bw had [Ca II ]emission and displayed dust emission at late times; wasit hot? An IR survey of recent massive star outburstswith good spectroscopic coverage may provide these an-swers. In the future, optical and NIR observations maybe sufficient to determine these characteristics for othermassive star outbursts. SN 2009ip: A Supersonic Explosion
The spectra of SN 2009ip have absorption attributedto high-velocity (up to ∼ − ) material (see Sec-tion 3.3.1). Contrary to what is expected from a singleoutburst or explosion, the velocity of the absorption fea-ture increases with time. In a typical SN, the ejectanaturally follow a Hubble law with the highest velocitymaterial being the most distant from the explosion site.Spectral lines have a blueshifted absorption due to thescattering processes in the photosphere of the SN. Lowvelocity material is hidden behind the photosphere, onlyto be revealed at later times. As the photosphere recedes,the highest-velocity material becomes optically thin, re-sulting in the blueshifted velocity of a spectral line todecrease with time. Since the absorbing material mustbe at just slightly larger radii than the photosphere, thehigh-velocity material must have been ejected during theeruption. (If the absorbing material were from a previ-ous eruption, the ejecta from the more recent eruptionwould have had to be moving even faster.)It is possible that the high-velocity absorption is a com-ponent of P-Cygni features from the ejected material.The Lorentzian profile slightly underestimates the emis-sion flux in the 86 day spectrum (see Figure 6), whichmay be the result of P-Cygni emission contributing to theline. Since the high-velocity absorption is coming from the ejecta, the outburst of SN 2009ip must have beenextremely energetic, expelling a large amount of mate-rial at very high velocities. However, for the velocity to increase with time, either the ejecta must not follow aHubble expansion or the radius of the photosphere (invelocity space) must somehow increase with time.In a single explosion, the ejecta naturally follow a Hub-ble law; however, multiple explosions can change the ve-locity profile of the ejecta. If two explosions occurred inshort succession, one can produce the inverted velocitygradient seen in SN 2009ip. In this toy model, the photo-sphere would recede into the ejecta of the first explosion,but at some point the fastest-moving ejecta of the secondexplosion would overtake the photosphere, increasing thevelocity. If there are no other explosions, the velocity ofthe absorption would decrease from there.The photometric behavior of SN 2009ip is consistentwith this picture. The first explosion would produce thefast rise to maximum. As noted by S09, the timescale ofthe fading is much shorter than the timescales for manyphysical processes such as dust extinction. This behav-ior is very similar to that of SN 2000ch, which bright-ened by 2.1 mag in 9 days to maximum, then immedi-ately faded by 3.4 mag in 7 days, immediately followedby a 2.2 mag rise in 4 days, after which the magnitudestayed relatively constant (Wagner et al. 2004). Spectraof SN 2000ch taken during the fading and on its plateaushow no strong evidence for high-velocity ejecta, but thespectra may not be of high enough quality to see thesefeatures.S09 hypothesized that rapid fading may have beencaused by an optically thick shell being ejected after thefirst outburst. If this process did occur in SN 2009ip,then there are several implications: (1) the velocity ofthe absorption should eventually decrease, (2) the inter-action of the ejecta from the two explosions could bea significant source of X-ray and radio emission, and (3)the X-rays might excite certain elements producing high-excitation lines such as He II in optical spectra. Our X-ray limit of L X < . × ergs s − taken during theminimum is not particularly constraining. We do not de-tect any He II λ II λ CONCLUSIONS
We have presented extensive UV, optical, and NIRdata for two transients, SN 2009ip and UGC 2773OT2009-1. Although these events appear to be similarphenomena (luminous outbursts of massive stars), thedetails of the events show that there are many differ-ences. These differences provide examples of the diver-sity of such events.A previous study of these events, S09, provided an ini-tial analysis of the object. Although the two studiesagree on many points, our interpretation of the entire The calculation by (S09) for the time until dust formation forSN 2009ip assumes a velocity of 500 km s − . Although the ejectaare moving much faster than this assumed value, they would needto have a velocity of & ,
000 km s − to reach the sublimationradius at the time of fading. iverse Massive Star Outbursts I 13data set is somewhat different than that of S09. In par-ticular, we agree that based on pre-outburst HST imag-ing, historical light curves, and outburst spectroscopy,the progenitors of SN 2009ip and UGC 2773 OT2009-1are LBVs with masses of &
60 and & M ⊙ , respectively.We also agree that the spectra of the two events are sig-nificantly different, but consistent with known LBVs orLBV outbursts. While UGC 2773 OT2009-1 had a coolerspectrum with a forest of absorption lines reminiscent ofa F-type supergiant (similar to S Dor in its high state),SN 2009ip had a hot spectrum and exhibited mainly HBalmer emission (similar to other LBV giant eruptions).The spectral characteristics (particularly [Ca II ] emis-sion) and circumstellar dust link UGC 2773 OT2009-1to the lower-mass, dust-obscured progenitors of NGC 300OT2008-1 and SN 2008S. We agree that the progenitorsof these objects are all massive stars and may have manycharacteristics similar to those of the LBV class, whichcould extend the mass range for LBV-like activity to rel-atively low-mass stars.However, there are distinct differences between theanalyses of S09 and of that presented here. Specifically,the initial mass ranges for the progenitors are slightlydifferent in the two studies, with S09 estimating 50 –80 M ⊙ (instead of & M ⊙ ) and & M ⊙ (instead of & M ⊙ ) for SN 2009ip and UGC 2773 OT2009-1, re-spectively. The differences lie in the conversion from HST filters to Bessell filters and the adapted color range forthe progenitor of SN 2009ip, and the additional informa-tion provided by stars in the vicinity of the progenitor ofUGC 2773 OT2009-1.Our interpretation of the exact nature of UGC 2773OT2009-1 differs from that of S09. While S09 contendsthat this object is a true giant eruption of an LBV, wequestion this assertion and propose that it may be theresult of extreme S Dor variability.While S09 found an NIR excess for UGC 2773 OT2009-1, hypothesizing that there may be dust emission as itis vaporized, we find more conclusive evidence for thisscenario through our NIR spectroscopy. The NIR spec-trum is consistent with an additional blackbody with T ≈ HST imag-ing) is a lower limit, and that the true initial mass islikely much larger.In addition to what was discussed by S09, we havealso detected high-velocity absorption in the spectra ofSN 2009ip, indicative of an explosion (as opposed to sub-sonic outburst). The absorption has an inverse veloc-ity gradient suggesting multiple explosions in short suc-cession. The rapid fading and brightening shortly aftermaximum brightness noted by (S09) is consistent withmultiple explosions, where a second explosion ejects anoptically thick shell that temporarily dims the object.We also note the spectroscopic similarity of UGC 2773OT2009-1 and SN 1994W, which Dessart et al. (2009)has previously suggested was not a true SN that de-stroyed its progenitor star.SN 2009ip and UGC 2773 OT2009-1 are very differ-ent manifestations of a similar phenomenon: extremebrightening of massive stars. With these objects andsimilar events (such as η Car, NGC 300 OT2008-1, andSNe 1961V, 1954J, 1997bs, 2000ch, 2002kg, and 2008S), we show that luminous outbursts of massive stars arevery heterogeneous. Some of this diversity is likely linkedto the instability that causes the eruption, while some iscaused by the circumstellar environment. Additional ob-servations of new massive star eruptions are necessary todetermine the physical mechanisms of the eruptions, thecontent of the circumstellar environments, and whetherthe two are physically connected.
Facilities:
ARC (TripleSpec), FMO:31in (Fan-Cam), Gemini:Gillett (GMOS), HST (WFPC2), Mag-ellan:Baade (IMACS), Magellan:Clay (MagE), MMT(Blue Channel), Swift (UVOT, XRT)R.J.F. is supported by a Clay Fellowship. O.D.F.is grateful for support from NASA GSRP, ARCS, andVSGC. E.M.L. is supported in part by a Ford Founda-tion Predoctoral Fellowship.We are indebted to the staffs at the APO, Gemini,Magellan, and MMT Observatories for their dedicatedservices. We thank K. Olsen and R. McDermid for ob-taining some of the data presented in the paper. Wethank R. Chornock and R. Kirshner for stimulating dis-cussions about the transients.Based on observations made with the NASA/ESAHubble Space Telescope, obtained from the data archiveat the Space Telescope Science Institute. STScI is op-erated by the Association of Universities for Researchin Astronomy, Inc. under NASA contract NAS 5-26555.Based in part on observations obtained at the GeminiObservatory, which is operated by the Association of Uni-versities for Research in Astronomy, Inc., under a coop-erative agreement with the US National Science Foun-dation on behalf of the Gemini partnership: the NSF(United States), the Science and Technology FacilitiesCouncil (United Kingdom), the National Research Coun-cil (Canada), CONICYT (Chile), the Australian Re-search Council (Australia), Minist´erio da Ciˆencia e Tec-nologia (Brazil) and Ministerio de Ciencia, Tecnolog´ıa eInnovaci´on Productiva (Argentina); the 6.5 meter Mag-ellan Telescopes located at Las Campanas Observatory,Chile; the MMT Observatory, a joint facility of theSmithsonian Institution and the University of Arizona;the Fan Mountain Observatory 0.8 meter telescope; andthe Apache Point Observatory 3.5 meter telescope, whichis owned and operated by the Astrophysical ResearchConsortium. We acknowledge the use of public data fromthe
Swift data archive.
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