SN 2008jb: A “Lost” Core-Collapse Supernova in a Star-Forming Dwarf Galaxy at \sim 10 Mpc This paper includes data gathered with the 6.5 meter Magellan telescope at Las Campanas Observatory, Chile.

# SN 2008jb: A “Lost” Core-Collapse Supernova in a Star-Forming Dwarf Galaxy at ∼10 Mpc11affiliation: This paper includes data gathered with the 6.5 meter Magellan telescope at Las Campanas Observatory, Chile.

José L. Prieto22affiliation: Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA 33affiliation: Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 1111affiliation: Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland , J. C. Lee22affiliation: Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA 1212affiliation: Hubble, Carnegie-Princeton Fellow , A. J. Drake44affiliation: California Institute of Technology, 1200 E. California Blvd., CA 91225, USA , R. McNaught55affiliation: Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia , G. Garradd55affiliation: Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia , J. F. Beacom66affiliation: Department of Physics, The Ohio State University, 191 W. Woodruff Ave., Columbs, OH 43210, USA 77affiliation: Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA 88affiliation: Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210, USA , E. Beshore99affiliation: Department of Planetary Sciences, Lunar and Planetary Laboratory, The University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA , M. Catelan1010affiliation: Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuna Mackena 4860, 782-0436 Macul, Santiago, Chile , S. G. Djorgovski44affiliation: California Institute of Technology, 1200 E. California Blvd., CA 91225, USA 1414affiliation: Distinguished Visiting Professor, King Abdulaziz University, Jeddah 21589, Saudi Arabia , G. Pojmanski1111affiliation: Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland , K. Z. Stanek77affiliation: Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA 88affiliation: Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210, USA , and D. M. Szczygieł77affiliation: Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
###### Abstract

We present the discovery and follow-up observations of SN 2008jb, a core-collapse supernova in the southern dwarf irregular galaxy ESO 30214 ( mag) at  Mpc. This nearby transient was missed by galaxy-targeted surveys and was only found in archival optical images obtained by the Catalina Real-time Transient Survey and the All-Sky Automated Survey. The well sampled archival photometry shows that SN 2008jb was detected shortly after explosion and reached a bright optical maximum,  mag (). The shape of the light curve shows a plateau of  days, followed by a drop of  mag in -band to a slow decline with the approximate Co decay slope. The late-time light curve is consistent with  M of Ni synthesized in the explosion. A spectrum of the supernova obtained 2 years after explosion shows a broad, boxy H emission line, which is unusual for normal type IIP supernovae at late times. We detect the supernova in archival Spitzer and WISE images obtained  months after explosion, which show clear signs of warm ( K) dust emission. The dwarf irregular host galaxy, ESO 30214, has a low gas-phase oxygen abundance, ( Z), similar to those of the SMC and the hosts of long gamma-ray bursts and luminous core-collapse supernovae. This metallicity is one of the lowest among local ( Mpc) supernova hosts. We study the host environment using GALEX far-UV, -band, and H images and find that the supernova occurred in a large star-formation complex. The morphology of the H emission appears as a large shell ( pc) surrounding the FUV and optical emission. Using the H-to-FUV ratio, and FUV and -band luminosities, we estimate an age of  Myr and a total mass of  M for the star-formation complex, assuming a single-age starburst. These properties are consistent with the expanding H supershells observed in many well-studied nearby dwarf galaxies, which are tell-tale signs of feedback from the cumulative effect of massive star winds and supernovae. The age estimated for the star-forming region where SN 2008jb exploded suggests a relatively high-mass progenitor star with initial mass  M, and warrants further study. We discuss the implications of these findings in the study of core-collapse supernova progenitors.

galaxies: dwarf – supernovae: general – supernovae: individual (SN 2008jb)
1313affiliationtext: Carnegie Fellow

## 1 Introduction

Core-collapse supernovae are energetic explosions that mark the death of stars more massive than  M. They are extremely important in several areas of astrophysics, including the nucleosynthesis of chemical elements, energy feedback that affects the evolution of galaxies, the formation of compact object remnants, the production of high-energy particles, as tracers of recent star-formation in galaxies, and as test cases of massive stellar evolution.

Supernovae discovered in nearby galaxies have been particularly important for testing our physical understanding of the explosions and to establish direct links between the events and their progenitors, which puts tight constraints on stellar evolution theory. The two best-studied nearby core-collapse supernovae, the peculiar type II SN 1987A in the LMC and the type IIb SN 1993J in M81, had progenitors that were detected in pre-explosion images. Both were very interesting cases that challenged theoretical expectations based on stellar evolution models that predicted red supergiants as their progenitor stars: SN 1987A had a  M blue supergiant progenitor star (e.g., Arnett et al. 1989) and SN 1993J had a  M yellow supergiant progenitor (e.g., Aldering et al. 1994), which turned out to be a massive binary system (e.g., Maund et al. 2004). After more than a decade of work, Smartt et al. (2009) presented a thorough study of nearby ( Mpc) type II-Plateau supernovae with deep pre-explosion imaging (see also Li et al. 2005), which allowed them to constrain the progenitors of type IIP supernovae to be red supergiants with initial main-sequence masses in the range  M. This result is in conflict with well-studied red supergiant samples in the Galaxy and the Magellanic Clouds (e.g., Levesque et al. 2006), which also contain massive supergiants with  M, and a Salpeter Initial Mass Function (IMF) for high-mass stars.

This “red supergiant problem” can be alleviated if high-mass red or yellow supergiants produce other spectroscopic or photometric type II supernova sub-types (e.g., Smith et al. 2009; Elias-Rosa et al. 2009, 2010), or if high-mass supergiants experience strong winds due to pulsations (e.g., Yoon & Cantiello 2010). Very recently, Walmswell & Eldridge (2011) have proposed circumstellar dust as a possible solution. It can also be explained by failed supernovae, an hypothesis that would have several deep implications (e.g., Kochanek et al. 2008). In addition, there are known selection biases that have not been discussed in depth or accounted for. Of the 18 nearby type II supernovae with fairly secure progenitor (either single, binaries, or compact clusters) detections111These are: SN 1987A, 1993J; SN 1961V from Kochanek et al. (2011) and Smith et al. (2011c); SN 1999ev, 2003gd, 2004A, 2004am, 2004dj, 2004et, 2005cs, and 2008bk from the sample of Smartt et al. (2009); SN 2005gl, from Gal-Yam et al. (2007) and Gal-Yam & Leonard (2009); SN 2008ax from Crockett et al. (2009); SN 2008cn and 2009kr from Elias-Rosa et al. (2009, 2010); SN 2009md from Fraser et al. (2011); SN 2010jl from Smith et al. (2011b); and SN 2011hd from Maund et al. (2011), Szczygiel et al. (2011) and Van Dyk et al. (2011). We do not include here low-luminosity transients with type IIn-like spectral features and progenitor detections in the mid infrared, like SN 2008S and NGC 300-OT (e.g., Prieto et al. 2008b)., two of the 18 events were initially discovered by the galaxy-targeted Lick Observatory Supernova Search (LOSS; Filippenko et al. 2001), and 14 of the 18 were initially discovered by dedicated amateur astronomers that mostly concentrate their search efforts in big galaxies. Only 2 of these supernovae were found in dwarf galaxies (SN 1987A and 2008ax) and all the type IIPs (10 of the 18) were found in the disks of spiral galaxies, mostly in grand-design spirals.

The existing selection bias against finding nearby core-collapse supernovae in dwarf galaxies could be important because of the local star formation rate density is in galaxies with absolute magnitude  mag (e.g., James et al. 2008; Young et al. 2008; Williams et al. 2011). Since metallicity is a key parameter in massive stellar evolution and stellar death (e.g., Prieto et al. 2008a; Modjaz 2011, and references therein), changes in the fractions of different type II spectroscopic subtypes with metallicity (e.g., Arcavi et al. 2010) could affect our census of progenitors in local samples. Also, possible variations of the stellar IMF with environment (e.g., Meurer et al. 2009; but see, e.g., Myers et al. 2011 and references therein) can introduce complications to the progenitor analyses. Recently, Horiuchi et al. (2011) discussed in detail related effects of incompleteness in the existing supernova rate measurements and how they affect their use as star formation rate indicators.

In this paper we present the discovery, follow-up observations, and analysis of the nearby type II SN 2008jb (, ) in a southern dwarf-irregular galaxy at  Mpc (Prieto et al. 2011). This bright  mag supernova was missed by targeted southern supernova surveys, like CHASE (Pignata et al. 2009) and amateur searches, mainly because the host galaxy ESO 30214 is a fairly low-luminosity dwarf and was not included in the catalogs of galaxies that are surveyed for supernovae. We are able to recover the optical light curves of this supernova from two surveys that do not target individual galaxies, but rather scan large areas of the sky: the Catalina Real-time Transient Survey (CRTS; Drake et al. 2009; Djorgovski et al. 2011) and the All-Sky Automated Survey (ASAS; Pojmanski 1998). In Section §2 we discuss the observations, including the optical discovery and follow-up, archival mid-infrared imaging, optical spectroscopy, and other archival data. In Section §3 we present detailed analysis and discussion of the optical and mid-IR observations. In Section §4 we discuss the results and conclusions. We adopt a distance of 9.6 Mpc to ESO 30214 (Lee et al. 2009), estimated from the Virgo-inflow corrected recession velocity and  km s Mpc.

## 2 Observations

### 2.1 Optical Imaging

SN 2008jb was discovered in CRTS archival data from the Siding Spring Survey (SSS) 0.5m Schmidt telescope through the SNhunt project. SNhunt is an open survey for transients in nearby galaxies that uses image subtraction in images obtained by the CRTS. A new transient was recovered in 2010 Oct. 26 (ID SNhunt12) after running the difference imaging pipeline in archival data from the SSS. SN 2008jb is first detected in unfiltered SSS images obtained on 2008 Nov. 23, at  mag. Figure 1 shows the first detection of SN 2008jb in the SSS images. It was also detected in 10 images obtained between 2008 Dec. 3 and 2010 Jan. 11. We used the image subtraction software described in Freedman et al. (2009) in order to obtain clean images of the supernova (see Fig. 1). The image used as a template for the subtractions was a combination of images obtained in 2007. We performed PSF photometry using DAOPHOT (Stetson 1992) on the difference images. The final calibration of the supernova photometry was done relative to ASAS -band data of stars in the field (see discussion below). Table 1 gives the -band calibrated photometry from the SSS data. We also include 3 upper limits on the magnitudes of the supernova obtained from the images just before the first detection and after the last detection.

The supernova was also detected in archival images collected using the 7 cm ASAS South telescopes in Las Campanas Observatory, Chile. The first detection in ASAS South data is from 2008 Nov. 19 (4 days earlier than CRTS) at  mag in the -band. The ASAS images were processed with the reduction pipeline described in Pojmanski (1998, 2002). The and magnitudes are tied to the Johnson and Cousins scale using Tycho (Høg et al., 2000) and Landolt (Landolt, 1983) standard stars. We used aperture photometry (2 pixel radius) from the normal reduction pipeline to obtain the magnitudes of the supernova in ASAS images and estimated upper limits before and after the first and last detection epochs, respectively. Unlike the case of the SSS images where there is contamination from the nearby star-forming region at faint magnitudes, the much shallower ASAS data ( mag) does not require image subtraction. The ASAS and magnitudes of SN 2008jb are presented in Table 2. Figure 2 shows the light curve of the supernova from ASAS and CRTS data.

We obtained late-time observations of SN 2008jb on 2010 Nov. 7 using the Inamori-Magellan Areal Camera & Spectrograph (IMACS; Dressler et al. 2006) on the 6.5 m Magellan I (Baade) telescope at Las Campanas Observatory. We obtained  sec images of the field of SN 2008jb with the f/2 camera ( per pixel) using the -band filter under good weather and seeing conditions (). Since we lack the deep template image necessary to obtain the supernova flux at late times, these data are only used to study the host galaxy environment. We plan to obtain more late-time observations and the analysis of these data will be presented in a future publication.

### 2.2 Optical Spectroscopy

We gathered spectroscopic observations of SN 2008jb on 2011 Jan 6 using the IMACS on the 6.5 m Baade telescope at Las Campanas Observatory. The observations consisted of  sec spectra obtained using the 300 l/mm grism (range  Å) and a slit aligned at the parallactic angle, obtained under clear weather and good seeing (). This setup gives a FWHM resolution of  Å at 6500 Å. The data were reduced using standard techniques in IRAF, which included basic data reduction (overscan and bias subtraction, flat-fielding), 1D spectrum extraction, wavelength calibration using a HeNeAr arclamp obtained after the science observations, and flux calibration using a flux standard observed on the same night. The spectrum goes through two CCD detectors separated by a gap of  Å. We used a simple linear interpolation at the edges of the CCDs. The final spectrum of SN 2008jb is shown in Figure 3.

We also obtained the spectrum of an H ii region in the host galaxy at  pc () south of the supernova position on 2011 Jan. 2 with the Wide-Field Reimaging CCD Camera (WFCCD) on the 2.5-m du Pont telescope at Las Campanas Observatory. The spectrum consisted of  sec exposures with the 400 l/mm grism, which gives a FWHM resolution of 8 Å and continuous coverage in the  Å range. The data were reduced using standard tasks in IRAF similar to the reduction procedure of the IMACS supernova spectrum. The final spectrum of the H ii region is shown in Figure 4. We measured the fluxes of the most prominent emission lines in the spectrum using Gaussian profiles and a custom Perl Data Language (PDL) fitting routine, including the Balmer recombination lines (H  4862 and H  6563) and forbidden emission lines ([O iii]  4959, 5007, [S ii]  6713, 6731, and [N ii]  6583). The fluxes of the optical emission lines are presented in Table 3.

### 2.3 Mid-Infrared Imaging

The host galaxy of SN 2008jb, ESO 30214, was observed with Spitzer/IRAC (Fazio et al. 2004) using the 3.6 and 4.5 m channels in warm Spitzer observations obtained as part of the Spitzer Survey of Stellar Structure in Galaxies (S4G; Sheth et al. 2010). They obtained two epochs of imaging during Cycle 6 (PID 61060) separated by  month, in 2009 Sep. 2 and Oct. 10. We retrieved the post-BCD images of ESO 30214 from the Spitzer Heritage Archive, which are fully reduced and flux-calibrated. The supernova is clearly detected as a bright, variable mid-IR source in these images, at a position consistent with the optical coordinates. The mid-IR detections are shown in Figure 5. We measured the 3.6 and 4.5 m fluxes of the source using aperture photometry with a aperture radius ( annulus for sky determination) and applied the aperture corrections and flux conversion factors listed in the IRAC Instrument Handbook. The IRAC fluxes are presented in Table 4.

The Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) preliminary data release includes observations of the field of SN 2008jb obtained at 3.4, 4.6, 12, and 22 m between 2010 Jan. 23 and Jan. 30. We searched for the WISE data in the NASA/IPAC Infrared Science Archive (IRSA). The supernova is clearly detected at 3.4 and 4.6 m (see Figure 5), marginally detected at 12 m, and undetected at 22 m. We retrieved the photometry of source J035144.63-382700.6 from the WISE catalog and list the fluxes in Table 4. We did not attempt to measure photometry in individual WISE images, but rely on the catalog fluxes measured from the combined data. The effective date of the observations is 2010 Jan. 27.

### 2.4 Other Archival Data

We searched for other existing archival data of ESO 30214 obtained before the discovery of SN 2008jb to help characterize the progenitor and galaxy environment. The Survey for Ionization in Neutral Gas Galaxies (SINGG; Meurer et al. 2006) obtained -band and H narrow-band images of the host galaxy using the CTIO 1.5-m telescope on 2000 Oct. 28 (seeing was in ). We retrieved calibrated images from SINGG (including H-subtracted images) through the NASA Extragalactic Database (NED).

The Galaxy Evolution Explorer (GALEX; Martin et al. 2005) ultraviolet space telescope observed the field of ESO 30214 on 2004 Nov. 18. We retrieved calibrated NUV and FUV images (GR6 data release) from the GALEX online archive.

We also searched for pre-explosion imaging data in the HST, Gemini, and ESO archives. Unfortunately, there are no deep pre-explosion images obtained with these facilities.

## 3 Analysis

In the following sections we present analysis and discussion of the results from the light curves, spectra, mid-IR emission, and host galaxy environment. We present a summary of derived physical properties obtained from the optical light curves and host galaxy in Table 5 and Table 6, respectively.

### 3.1 Light Curve

In order to make light curve comparisons and derive intrinsic physical properties, we first need to estimate the total reddening along the line of sight to the supernova. For the Galactic reddening, we use a CCM reddening law with (Cardelli, Clayton & Mathis 1989) and  mag from the reddening maps of Schlegel et al. (1998). We estimate the intrinsic reddening in the host galaxy using the Balmer decrement measured from the spectrum of the H ii region in ESO 30214 (see Table 3). We assume an intrinsic case B recombination Balmer flux ratio , which is appropriate for an H ii region at a typical electron temperature and density (Storey & Hummer 1995). Then we assume an SMC reddening law from Gordon et al. (2003) for the host galaxy, which is appropriate given its star-formation rate, absolute magnitude, and metallicity (see Section 3.4). The resulting intrinsic color excess is  mag and extinction  mag, which we will use in all subsequent analysis. The values obtained using an LMC reddening law,  mag and  mag, and Galactic CCM reddening law,  mag and  mag, are consistent with our adopted reddening. We note that for H ii regions its usually assumed (Calzetti 2000), but we adopt conservatively because of the young age of the region (see Section 3.4).

The - and -band light curves of SN 2008jb resemble those of type II-Plateau supernovae, the most common kind of core-collapse events in nearby galaxies (e.g., Li et al. 2011). In Figure 6 we compare the absolute -band light curves of different core-collapse supernovae with SN 2008jb, including: the low-luminosity type IIP 2005cs (Pastorello et al. 2009a), the luminous type IIP 2004et (Maguire et al. 2010), and the low-luminosity type IIL 1999ga (Pastorello et al. 2009b). The absolute magnitudes of SN 2008jb lie in between SN 2005cs and SN 2004et, and appear consistent with SN 1999ga and normal type IIP supernovae (e.g., Li et al. 2011). In the comparison we have adopted the host distances, total extinctions, and explosion times presented in the published studies. We estimate an approximate explosion time for SN 2008jb as the midpoint between the last pre-discovery -band non-detection from CRTS and the first -band detection from ASAS, which gives (2008 Nov. 11).

The ASAS data samples well the initial plateau, which lasts  days in the -band. This plateau duration is in the observed range of plateaus in type IIP supernovae, typically between  days (e.g., Bersten & Hamuy 2009). The -band light curve shows a slow linear decay of  mag/day and the -band light curve declines faster at  mag/day in the initial phase, which is slightly faster than well-studied type IIP supernovae where the initial decay slope in the plateau is slower at redder bands (e.g., Poznanski et al. 2009; D’Andrea et al. 2010). It is interesting to note the similarities of the initial light curve decline of SN 2008jb with the light curve of the low-luminosity type IIL SN 1999ga (Pastorello et al. 2009b), as shown in Figure 6. The -band light curves of type IIL supernovae typically have faster initial decline slopes than SN 2008jb and SN 1999ga (e.g., Barbon et al. 1979; Fig. 3 of Pastorello et al. 2009b).

The absolute magnitudes close to the explosion date are  mag and  mag. At mid plateau ( days), the absolute magnitudes are  mag and  mag. The evolution of the color, from  mag close to explosion to  mag at mid plateau, is also fairly consistent with the evolution of well-studied type IIP supernovae, which traces small changes in the color temperature of the photosphere (e.g., Hamuy et al. 2001). It is worth noting that the unreddened color at mid-plateau of SN 2008jb is very close to the  mag ridgeline that Nugent et al. (2006) derived from a large sample of type IIP supernovae, supporting our reddening estimation.

After the  day plateau, the light curve drops  mag in 32 days (at  mag/day) to a late-time linear decay of  mag/day. We do not have -band observations that show the transition phase, but the ASAS non-detections at these epochs are consistent with the -band light curve shape. The -band drop to a late-time linear decay is seen in all type IIP supernovae, but it is typically stronger ( mag in the -band; e.g., Maguire et al. 2010). In Figure 6 we show that the late-time linear decay of SN 2008jb is similar to other type II supernovae and reasonably consistent with the Co to Fe radioactive decay slope.

Hamuy (2003) studied the physical properties of a sample of well-observed type II supernovae and derived the Ni masses produced in the explosions from their late-time light curves, assuming full trapping of -ray photons produced by the radioactive decay of Co. Using Eq. 1 in his study evaluated at day 115 after explosion (the first light curve point at the radioactive tail) we obtain a Ni mass of  M. This result assumes a bolometric correction of  mag in the -band, which is calculated from the well-studied SN 1987A and SN 1999em at nebular phases (Hamuy 2003). The estimated uncertainty does not include a systematic error in the distance to the host.

### 3.2 Spectrum

The late-time spectrum of SN 2008jb, obtained  years after the explosion, shows a prominent H emission feature (see Figure 3), supporting the classification of this transient as a type II. The H feature is broad ( Å), flat-topped, boxy, and blueshifted (centered at  km s), with the blue edge of the line at  km s and red edge at  km s after correcting for the host galaxy’s recession velocity. The spectrum does not show any other strong emission features characteristic of normal type II supernovae at late times (e.g., [O i] and [Ca ii]), except for a tentative low S/N feature centered at  Å with  km s. If the spectrum is heavily smoothed, there is a “bump” that shows up more clearly at  Å, which could correspond to low-level emission associated with the [Ca ii] doublet at  7291, 7323. There is an increase in flux to the blue of  Å, which could in part be explained by contamination from nearby sources, although the S/N in the blue part of the spectrum decreases also due to flatfielding errors.

The broad and boxy H emission feature detected in SN 2008jb is not typically observed in late-time spectra of type II supernovae. We used spectra of normal type IIPs in the SUSPECT database to measure the FWHM at  days after explosion, in the nebular phase, and measure H line widths of  km s, significantly lower velocities than SN 2008jb. The peculiar SN 2007od (Andrews et al. 2010; Inserra et al. 2011) is the only example we could find in the literature of a type IIP with a comparably broad and blueshifted H component at late-times. In this case, however, the line also showed multiple narrower peaks on top of the broad profile that indicated clear signs of interaction between the ejecta and the circumstellar medium (CSM) from the projenitor wind, which are not seen in SN 2008jb.

Other less common type II supernovae have shown very broad emission lines at late times. The type IIL SN 1979C in M100 (e.g., Branch et al. 1981) showed a broad  km s H profile one year after discovery (Cappellaro et al. 1995). Also the type IIL SN 1980K showed an H profile with  km s eight years after the explosion (Fesen & Becker 1990), although at earlier epochs comparable to our observations of SN 2008jb the line widths were significantly narrower (e.g., Capellaro et al. 1995). The well-studied type IIb SN 1993J showed a strong H emission feature at  year after explosion, with velocity and profile shape fairly similar to SN 2008jb (Filippenko et al. 1994; Matheson et al. 2000). The boxy feature observed in SN 1993J was interpreted as indication of circumstellar interaction in a spherical shell (Matheson et al. 2000), which supported the evidence from the radio and X-ray observations (e.g., Fransson et al. 1996 and references therein). Patat et al. (1995) had also shown that the luminosity of the H line  year after the explosion was in excess of the expectations from radioactive Co decay, and the most likely source of extra energy was the ejecta-CSM interaction.

Chugai (1991) presented models of the evolution of the H line luminosity as a function of time for type II supernovas assuming radioactive decay as the energy source. We used these models to test if the measured luminosity of the H emission line in SN 2008jb at  days after explosion is consistent with heating from radioactive decay or extra energy is needed to explain it. We use the parametrization of the models presented in Pastorello et al. (2009b; their Fig. 9), scaled to a nickel mass of  M, and extrapolated the curves linearly at late-times (in log L versus time). Figure 7 shows the models with different assumptions for the total ejected mass from the progenitor ( M) and the measured H luminosity of SN 2008jb after correcting for Galactic and internal extinction. There is a clear excess in H emission luminosity with respect to the models at  days by a factor of , suggesting the existence of an extra energy source (e.g., from ejecta interaction with the progenitor wind) or a more massive progenitor star ( M). Unfortunately, we do not have early spectra to trace the evolution in H emission luminosity with time to differentiate between these scenarios. Also, the gap between CCD chips is right at the wavelength of the H line, so we are unable to detect a narrow component that would be a clear indication of circumstellar interaction.

### 3.3 Mid-IR Emission

We clearly detect mid-IR emission from SN 2008jb in three Spitzer and WISE epochs obtained between 286 and 434 days after the first optical detection (see Figure 5). The red Spitzer color of  mag indicates a rising spectral energy distribution at mid-IR wavelengths, a clear sign of warm dust emission. In order to characterize the evolution of the optical and mid-IR emission, we fit the SED of the supernova using two black-bodies, a hot component to fit the optical -band data and a warm component to fit the mid-IR data. Since we only have single-band optical data at late-times, we assume an effective temperature of  K for the hot component, which is a typical temperature measured in normal type II supernovae in the nebular phase.

The results of the two component black-body fits are presented in Table 7, and Figure 8 shows the model fits to the SED as a function of time. The total integrated luminosity decreases by a factor of 6.8 in  days, which is equivalent to 0.014 mag/day. The luminosity contributed by the warm black-body component, which is better constrained from the observed SED, decreases by a factor of 5.5 in 148 days, or  mag/day. The consistency with the late-time -band decline and the Co decay slope is interesting and argues for radioactive decay as the dominant energy source of dust heating. The temperature of the warm black-body component is  K in the three epochs where we have mid-IR imaging. The WISE data from day 434 clearly shows that the SED peaks between 4.6 and 12 m, supporting our conclusion. We also find that the radius of the warm black-body component decreases by a factor of between the first and last epochs with mid-IR data, between 765 AU (day 286) and 237 AU (day 434). The total mid-IR luminosity can be used to estimate the amount of dust needed to explain it (e.g., Dwek et al. 1983; Prieto et al. 2009); we find a dust mass of  M assuming a black-body spectrum and carbon dust composition.

Several nearby type II supernovae have shown excess mid-IR emission at late times, which is interpreted as the presence of warm dust either newly formed or pre-existing in the progenitor CSM (or a combination). Some examples of nearby type IIP events with clear signs of warm dust emission include SN 2003gd (Sugerman et al. 2006; Meikle et al. 2007), SN 2004et (Kotak et al. 2009), SN 2004dj (Szalai et al. 2011; Meikle et al. 2011), SN 2007it (Andrews et al. 2011), and SN 2007od (Andrews et al. 2010). The classic, luminous type IIL SN 1979C and SN 1980K showed infrared excesses at late times (Dwek 1983; Dwek et al. 1983). Also, a large fraction of type IIn supernovae show mid-IR emission at late times, which has been associated with pre-existing CSM dust (e.g., Fox et al. 2011, and references therein). In the low-luminosity end, SN 2008S-like events have dusty massive star progenitors and also show signs of dust during the transients (e.g., Prieto et al. 2008b, 2009; Thompson et al. 2009; Kochanek 2011).

The typical dust masses needed to explain the mid-IR emission in normal type II supernovae are  M (but see Matsuura et al. 2011 for the discovery of a large reservoir of cold dust in SN 1987A) and fairly consistent with the range derived here for SN 2008jb. The observations for several of the well-studied type IIs have been usually explained by newly formed dust in the ejecta. It seems unlikely that this can explain SN 2008jb. The main reason for this is that the shock velocities inferred from the black-body fits to the mid-IR detections are in the range of  km s (depending on the epoch), which is quite low compared to the observed velocity of the H emission line. On the other hand, a decreasing black-body radius as a function of time has been seen in SN 2008S and NGC 300-OT (also the type IIP SN 2007it and SN 2007od), and has been explained by Kochanek (2011) using a model in which dust reforms in the progenitor wind. This model, however, requires very high densities and low velocities, which are not observed in SN 2008jb.

### 3.4 Host Galaxy Environment

The host of SN 2008jb, ESO 30214, is a star-forming irregular galaxy similar to the Magellanic Clouds. In particular, it has a total -band absolute magnitude  mag,  mag fainter than the SMC, with a low UV derived (total) current star-formation rate (SFR) of 0.03 M yr (Lee et al. 2009). The total stellar mass of the galaxy is  M, estimated from , , and the mass-to-light ratios as a function of presented in Bell & de Jong (2001), assuming a Salpeter IMF. The total neutral hydrogen (H i) mass is  M (Meurer et al. 2006, scaled by our adopted distance). These masses imply high SFRs per unit stellar mass, and , which are typical for local star-forming dwarf galaxies (e.g., Lee et al. 2011).

We use the spectrum we obtained of the H ii region at  pc from the position of SN 2008jb to measure the local oxygen abundance using the strong forbidden oxygen lines and hydrogen recombination lines. Using the emission line fluxes reported in Table 3 and correcting for host galaxy extinction, we estimated the [O iii] 5007/H and [N ii]/H line ratios. Then we used the and calibrations from Pettini & Pagel (2004; PP04) to estimate oxygen abundances of () and (). These uncertainties are statistical and do not include the  dex error in the oxygen abundance calibration (PP04). For comparison, the average oxygen abundance of H ii regions in the SMC from Russell & Dopita (1990) are () and ().

The environment of SN 2008jb in ESO 30214 has a low oxygen abundance compared to the environments of nearby type IIP supernovae ( Mpc) used to constrain progenitor properties from deep the pre-explosion observations (Smartt et al. 2009), although there are also normal core-collapse supernovae discovered in relatively nearby galaxies that have fairly low-metallicity environments (e.g., Prieto et al. 2008b; Anderson et al. 2010). The metallicity here is similar to the environments of long-duration GRB hosts (e.g., Stanek et al. 2006; Levesque et al. 2010) and the hosts of the most luminous core-collapse supernovae that are being discovered in galaxy-blind surveys (e.g., Kozlowski et al. 2010; Stoll et al. 2011), which are bound to contain a population of very massive stars.

By closely examing the local host galaxy properties we can constrain the progenitor properties (see, e.g., Anderson & James 2008, 2009). We have relatively deep optical images obtained at late times with Magellan/IMACS, archival UV data from GALEX and H from SINGG. Figure 9 shows a mosaic with the Magellan -band and GALEX FUV image, scaled to fit the whole galaxy (top panels) and the region where the supernova exploded (lower panels). We see that SN 2008jb exploded in a large star-formation complex, the brightest and highest surface brightness star-forming region within the galaxy in the optical and FUV. It is composed of at least two resolved clusters or stellar associations that are well separated in the Magellan -band image. Interestingly, the H emission (red contours in the lower panel of Fig. 9) is offset and outside the brightest optical and FUV emission, forming an apparent ring with a projected diameter of  pc.

We can estimate an approximate age for the star-forming complex in ESO 30214 using the ratio of H to FUV luminosities, which is a sensitive age indicator in an instantaneous burst of star formation (e.g., Stewart et al. 2000; Sánchez-Gil et al. 2011). We use the Starburst99 (Leitherer et al. 1999) models to generate a grid of single-age clusters with total masses between  M and  M, in steps of  M. We choose a Salpeter IMF with stellar masses between 0.1 and 100 M and Geneva stellar evolution models with , consistent with the measured oxygen abundance of the H ii region. Figure 10 shows the expected ratio of H-to-FUV luminosities as a function of the FUV luminosities for these models. We label the ages of the clusters between 1 Myr and 50 Myr. The total measured H-to-FUV ratio and FUV luminosity  erg s Hz of the star-forming complex, corrected by Galactic and host galaxy extinction, are shown in the figure. We obtain an age of  Myr and total mass of  M. We also used the integrated -band flux (extinction corrected) of the star-forming region to independently check this mass estimate. We derive a total mass of  M from the -band flux and the mass-to-light ratio obtained from the Starburst99 models, which is fully consistent with the FUV estimate.

If the progenitor star of SN 2008jb was formed in this star-formation episode 9 Myr ago, then we infer an initial main-sequence mass of  M for the progenitor from the Geneva stellar evolution models with extended mass-loss used in Starburst99 (Lejeune & Schaerer 2001). This estimate is obtained as the maximum main-sequence mass of a star from an isochrone of 9 Myr of age. We note that this estimate is based only in the total H and FUV fluxes of the region, and a more detailed analysis of the SEDs of the individual stellar clusters is in preparation.

The equivalent width (EW) of the H ( Å) and H ( Å) emission lines in the spectrum of the H ii region can also be used to estimate an age for the star-forming region using the Starburst99 models (e.g., Stasińska & Leitherer 1996; Schaerer & Vacca 1998; Leloudas et al. 2011). Essentially, the EW of the recombination lines gives an estimate of the young over the old stellar population (see Leitherer 2005, and references therein). We obtain an age of  Myr from H EW and  Myr from H EW. These estimates are  Myr ( using the statistical uncertainty) younger than the age derived from the H-to-FUV ratio of the whole star-forming region. The differences might imply multiple star-formation episodes within the star-forming region, however, this would need to be tested with a detailed study of the stellar populations within the region.

We can estimate the SFR of the star-forming region using the FUV luminosity with the calibrations of Lee et al. (2009), finding  M yr, or about 30% of the total SFR of the dwarf galaxy. We note that the H-estimated SFR is a factor of lower than the FUV value, a systematic difference seen in dwarf galaxies at low SFRs which lacks a satisfactory explanation (e.g., Lee et al. 2009; see also Fumagalli et al. 2011).

## 4 Summary and Conclusions

We have presented the discovery, follow-up observations, and analysis of SN 2008jb, a bright type II supernova in the metal-poor, southern dwarf irregular galaxy ESO 30214 at  Mpc. This  mag supernova was found in archival data obtained by the CRTS and ASAS all-sky surveys. This transient was missed by galaxy-targeted supernova surveys like CHASE and by amateur astronomers mainly because the host galaxy is a low-luminosity dwarf,  mag fainter than the SMC, and targeted surveys use catalogs that are incomplete for small galaxies (e.g., Leaman et al. 2011).

SN 2008jb has - and -band light curves similar to normal type IIP supernovae, with peak magnitude  mag, but it can also be classified as an intermediate case between type IIP and type IIL due to its faster initial decay, perhaps similar to SN 1999ga (Pastorello et al. 2009b). It shows a  day plateau and a fast  mag decline to a late-time decline slope of  mag/day in the -band. This decline is consistent with the radioactive decay of Co to Fe, and argues for  M of Ni synthesized in the explosion that is powering the lightcurve. We detect mid-IR emission from SN 2008jb months after explosion in three epochs of archival Spitzer and WISE data. The mid-IR emission has an SED with black-body temperature of  K, characteristic of the warm dust emission seen in some nearby core-collapse supernovae and luminous transients (e.g., Kotak et al. 2009; Prieto et al. 2009). The evolution of the mid-IR emission with time is consistent with the products of radioactive decay heating the dust. The characteristic mid-IR dust radius shrinks with time, an evolution that is not typically seen in normal type IIs (some exceptions are SN 2007it and SN 2007od; Andrews et al. 2010, 2011).

We obtained a spectrum of SN 2008jb about 2 years after the explosion. It displays a very broad ( km s), boxy, and flat-topped H emission line, leading to its type II supernova spectroscopic classification. The broad and boxy line profile seen in H is quite unusual for normal type IIP supernovae at late times, but has been seen in some objects like SN 1993J, SN 2007od, and also a few well-studied type IIL supernovae. We find that the H line luminosity is in excess of the expected luminosity from radioactive Co decay predicted by the models of Chugai (1991) for total ejected masses  M. This indicates that there is an external source of energy, like ejecta-CSM interaction, and/or the mass of the progenitor star is  M. We do not see clear signs of ejecta-CSM interaction like narrow lines or irregularities in the H profile. It would be interesting to obtain late time X-ray and radio observations in order to have independent constraints on the importance of ejecta-CSM interaction in SN 2008jb.

We studied the host galaxy environment of SN 2008jb in ESO 30214 with optical spectra and imaging. Using the spectrum of an H ii region at  pc from the supernova site we measure an oxygen abundance of (PP04  method) and (PP04  method) from the strong nebular emission lines, which is similar to the SMC and one of the lowest measured metallicities of local core-collapse supernovae environments (e.g., in the lower 3% of measured oxygen abundances of the core-collapse sample of Anderson et al. 2010). The supernova exploded in a large star-forming complex with strong optical and GALEX FUV emission which is surrounded by a large H ring with  pc. The H-to-FUV ratio of this region is consistent with a stellar population with an age of  Myr derived from Starburst99 modeling and single-age stellar population models. This age implies a supernova progenitor mass of  M, assuming a single star (but see, e.g., Smith et al. 2011a for the importance of binary progenitors), if the progenitor formed in this star-forming episode. The equivalent width of the H and H emission lines in the spectrum of the H ii region give another constraint on the age of the region of  Myr,  Myr younger than the estimate from H-to-FUV ratio. We note that the star-formation history could be more complicated than a single burst, and we plan to study the region in detail using multiwavelength data in a future study. In particular, it would be interesting to include high-resolution data from HST.

Large expanding H shells (supershells with radii  pc) have been observed and studied in many nearby star-forming dwarf galaxies and have typical dynamical ages of  Myr (e.g., Martin 1998), which is fairly consistent with the age we derive here from the H and FUV emission. These structures are produced by the combined effect of many supernova explosions and winds from massive stars (e.g., Chakraborti & Ray 2011), and are the likely precursors of galactic winds. In a sense, we may perhaps be witnessing supernova feedback in real-time in this star-forming region.

The bias to large star-forming galaxies is clearly present in the samples of nearby ( Mpc) core-collapse supernovae used for progenitor studies (e.g., Smartt et al. 2009), and has been discussed in detail as a possible explanation for the discrepancy between measured local supernova rates and predicted supernova rates from galaxy star-formation rates (e.g., Horiuchi et al. 2011). Since the environments of nearby core-collapse supernovae used for progenitor studies generally miss dwarf galaxies because they were not included in the original searches, the progenitor properties and conclusions drawn from these samples regarding stellar evolution are not complete. In particular, the dearth of high-mass ( M) progenitor stars of type IIP supernovae (“red supergiant problem”) could be alleviated if these progenitor stars prefer lower-metallicity environments. For example, this could be caused by evironmental variations in the stellar IMF (e.g., Meurer et al. 2009) or by changes in the fraction of type II spectroscopic subtypes as a function of metallicity due to stellar evolution (e.g., Arcavi et al. 2010). Indeed, we find that the properties of the spectrum of SN 2008jb are more consistent with a massive progenitor (but see discussion about supernova modeling in, e.g., Smartt et al. 2009; Bersten & Hamuy 2009; Bersten et al. 2011), and the star-forming region where it was found has a young age compared with the ages of detected type IIP progenitors ( Myr).

Interestingly, a strong preference for low-metallicity hosts is observed in long GRBs (e.g., Stanek et al. 2006) and luminous core-collapse supernovae (e.g., Neill et al. 2011; Stoll et al. 2011), which have been linked with massive star progenitors ( M). SN 2008jb offers the unique chance of studying in detail a nearby type II supernova with host properties similar to long GRBs and the most luminous core-collapse supernovae.

The mapping between different classes of massive stars and their supernovae is not yet fully understood. Special insights are expected to be obtained when unusual explosions can be connected to unusual progenitor stars and galaxy hosts. Nearby objects are especially useful in terms of larger fluxes for an extended time after explosion, better spatial resolution for progenitor studies, and improved prospects for detection by new messengers like gamma rays (e.g., Timmes & Woosley 1997; Horiuchi & Beacom 2010), neutrinos (e.g., Ando & Beacom 2005; Kistler et al. 2011), and gravitational waves (e.g., Ott 2009). In addition, data from these objects are needed for a comprehensive understanding of the nearby universe.

It is difficult to find the nearest supernovae in small host galaxies with searches that target individual (generally large) galaxies, like LOSS, CHASE, and also amateur efforts. And it is difficult to find them with volume-based searches such as PTF (Rau et al. 2009) and Pan-STARRS (Kaiser et al. 2002) that have a deep, but relatively small survey area with good cadence. A shallower all-sky survey with excellent cadence, like ASAS, will help us find nearby ( Mpc) supernovae in all kinds of environments, including low-metallicity dwarf galaxies like the host of SN 2008jb (see also Khan et al. 2011, Stoll et al. 2011, for other supernovae studied with ASAS). Upgrades that will significantly increase the sensitivity and response speed of ASAS are underway.

We thank John Mulchaey for obtaining one of the Magellan images presented in this work, Chris Kochanek for detailed comments, Rupali Chandar, Crystal Martin, and Linda Watson for discussions, and Chris Burns for providing his image subtraction code. We also thank the anonymous referee for a careful reading of the manuscript. We are indebted to the staff of Las Campanas Observatory for their assistance. JLP acknowledges support from NASA through Hubble Fellowship grant HF-51261.01-A awarded by STScI, which is operated by AURA, Inc. for NASA, under contract NAS 5-2655. JFB is supported by the National Science Foundation CAREER Grant PHY-0547102. GP is supported by the Polish MNiSW grant N203 007 31/1328. KSZ and DMS are supported in part by NSF grant AST-0908816. Support for MC is provided by the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant P07-021-F, awarded to The Milky Way Millenium Nucleus; by Proyecto Basal PFB-06/2007; by FONDAP Centro de Astrofísica 15010003; and by proyecto FONDECYT Regular #1110326. The CRTS is supported in part by the NSF grant AST-0909182. This research has made extensive use of the NASA/IPAC Extragalactic Database (NED) which is operated by the JPL, Caltech, under contract with NASA.

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