---
abstract: 'Here we present previously unpublished optical spectra of supernova (SN) 2001ig, a Type IIb SN, from about a week after explosion until nearly one year later. The earliest spectrum consists of only a few broad absorption features, but soon more common Type II SN features including hydrogen P-Cygni profiles and helium absorption become apparent. At later times, as the H features fade and the absorption becomes more prominent, we observe the SN to transition from a Type II to a Type Ib. Finally, observations after 250 days past explosion show a nebular-phase SN spectrum with one of the largest magnesium to oxygen intensity ratios ever seen. Additionally, we present models of the late-time spectra which indicate that the inner ejecta consist of [$\sim\!\!$ ]{}1.15 [M$_\sun$]{} of material, most of which (by mass) is in the form of oxygen, with [$\sim\!\!$ ]{}0.13 [M$_\sun$]{} of $^{56}$Ni and essentially no hydrogen.'
author:
- 'Jeffrey M. Silverman, Paolo Mazzali, Ryan Chornock, Alexei V. Filippenko, Alejandro Clocchiatti, Mark M. Phillips, Mohan Ganeshalingam, and Ryan J. Foley'
bibliography:
- 'astro\_refs.bib'
title: Optical Spectroscopy of the Somewhat Peculiar Type IIb Supernova 2001ig
---

Introduction {#s:intro}
============

It is thought that most high-mass stars ($\gtrsim8$ [M$_\sun$]{}) end their lives as core-collapse supernovae (SNe) [e.g., @Woosley05 and references therein]. These SNe are divided spectroscopically into subgroups based on the strength of H and He in their optical spectra. The progression from Type II to Ib to Ic corresponds to SNe having both H and He, to just He, to neither H nor He present in their spectra [for a review, see @Filippenko97]. The class of Type IIb SNe (SNe IIb) have spectra which undergo a transition from Type II to Type Ib as their H features fade with time. It is likely that SNe IIb, Ib, and Ic undergo significant mass loss, due either to stellar winds and eruptions or to mass transfer to a binary companion, before explosion.

[SN 2001ig]{} was discovered in the nearby late-type spiral galaxy by @Evans01 on 2001 Dec. 10.43 (UT dates are used throughout this paper). It is located at $\alpha_{\rm J2000} = 
22^{\rm h}57^{\rm m}30\fs 74$ and $\delta_{\rm J2000} = -41\degr02\arcmin26\farcs1$ [@Ryder01]. An explosion date of 2001 Dec. 3 has been estimated by @Ryder04 from modeling the radio light curve, and we use this as the reference date for our spectral observations throughout this paper. A spectrum was obtained within two days of detection of the SN and showed only a few broad absorption features. However, the spectrum from Las Campanas Observatory on 2001 Dec. 13 [@Phillips01] revealed similarities to the Type IIb SNe 1987K [@Filippenko88] and 1993J [@Filippenko93].

Over the following month, spectra from the European Southern Observatory [@Clocchiatti01; @Clocchiatti02] showed [SN 2001ig]{} to evolve in a manner similar to both SNe IIb 1987K and 1993J, as the hydrogen lines became weaker and showed the complex shape characteristic of the transitional stage [@Filippenko94]. By Oct. 2002, the predicted transition to a Type Ib SN in the nebular phase was complete [@Filippenko02]. At the time it was also noted that the \] $\lambda$4571 feature was the strongest (relative to \[\] $\lambda\lambda$6300, 6364) ever seen in a SN. Our data complement the optical spectropolarimetry and total-flux spectra that were presented and analyzed by @Maund07.

In addition to optical data, X-ray images were obtained with the Advanced CCD Imaging Spectrometer-S (ACIS-S) instrument on the [*Chandra*]{} X-ray Observatory on 2002 May 22, showing a 0.2–10.0 keV luminosity of [$\sim\!\!$ ]{}$10^{38}$ [ergs s$^{-1}$]{} [@Schlegel02]. [SN 2001ig]{}was also detected at radio frequencies; for further information on the radio data and analysis, see @Ryder04. Here we present and analyze optical spectroscopic data as well as two spectral models for [SN 2001ig]{} at late times. In §\[s:obs\] we describe our observations and data reduction, and in §\[s:analysis\] we discuss our analysis of the spectra. Our model spectra and their implications are presented in §\[s:model\]. We summarize our conclusions in §\[s:conclusions\].

Observations and Data Reduction {#s:obs}
===============================

Beginning one week after explosion, optical spectra were obtained using the LDSS-2 spectrograph [@Mulchaey01] mounted on the Magellan Baade 6.5-m telescope, the red arm of the dual imager/spectrograph EMMI [@Dekker86] mounted on the European Southern Observatory (ESO) 3.6-m New Technology Telescope, the Low-Resolution Imaging Spectrometer [LRIS; @Oke95] mounted on the Keck I 10-m telescope, and the dual imager/spectrograph EFOSC2 [@Buzzoni84] mounted on the ESO 3.6-m telescope. Our last spectral observation occurred on 2002 Nov. 8, approximately 340 days after explosion. All observations used a $0.7''$–$1.0''$ wide slit and were aligned along the parallactic angle to reduce differential light losses [@Filippenko82]. Table \[t:obs\] summarizes the spectral data of [SN 2001ig]{} presented in this paper.

[lrllrcr]{} 2001 Dec. 11 & 8 & LDSS2 & 3800–9000 & 13 & 1.07 & $2\times90$\
2001 Dec. 13 & 10 & LDSS2 & 5000–10000 & 13 & 1.18 & $2\times60$\
2001 Dec. 16 & 13 & EMMI & 3980–9400 & 9 & 1.64 & $3\times60$\
2001 Dec. 23 & 20 & LRIS & 3400–10300 & 7 & 2.49 & 60\
2002 Jan. 13 & 41 & EFOSC2 & 3360–10300 & 9 & 1.87 & $3\times90$\
2002 Jan. 17 & 45 & LRIS & 3520–10000 & 7 & 4.09 & 360\
2002 Jan. 18 & 46 & LRIS & 3500–5380 & 3 & 4.99 & 450\
2002 Jan. 18 & 46 & LRIS & 5710–7000 & 2 & 4.99 & 450\
2002 Jan. 29 & 57 & LDSS2 & 3600–10000 & 13 & 2.61 & 90\
2002 Aug. 16 & 256 & FORS1 & 4000–8600 & 12 & 1.04 & $4\times1200$\
2002 Oct. 8 & 309 & LRIS & 3150–9420 & 6 & 2.18 & $2\times600$\
2002 Nov. 8 & 340 & LRIS & 3100–9410 & 6 & 2.05 & 900\

All spectra were reduced using standard techniques [e.g., @Foley03]. Standard CCD processing and spectrum extraction for the LRIS data were completed with IRAF[^1], and the data were extracted with the optimal algorithm of @Horne86. We obtained the wavelength scale from low-order polynomial fits to calibration-lamp spectra. Small wavelength shifts were then applied to the LRIS data after cross-correlating a template sky to the night-sky lines that were extracted with the SN. Using our own IDL routines, we fit spectrophotometric standard-star spectra to the LRIS data in order to flux calibrate our spectra and to remove telluric lines [@Wade88; @Matheson00].

Each EMMI observation was reduced independently and the final spectra were combined with a pixel-rejection algorithm based on the mean.

Spectral Analysis {#s:analysis}
=================

Early-Time Spectra {#ss:early}
------------------

We present our early-time spectra of [SN 2001ig]{} in Figure \[f:early\_spec\]; all have had the recession velocity of removed [939 [km s$^{-1}$]{}, from @Koribalski04]. The first spectrum of [SN 2001ig]{}, taken on 2001 Dec. 11 (8 days past explosion), shows only a few strong, broad absorption features. We conjecture that the feature near 4000 Å is [H$\gamma$]{} and the feature near 4300 Å is a blend of lines (near a rest wavelength of 4570 Å) blueshifted by [$\sim\!\!$ ]{}22,500 [km s$^{-1}$]{} and [$\sim\!\!$ ]{}18,200 [km s$^{-1}$]{}, respectively. Unfortunately, this identification is somewhat dubious since the [H$\gamma$]{}feature appears to be unusually strong.

Instead, we note that this pair of features resembles the “W” feature seen in very early-time spectra of the Type II SN 2005ap [@Quimby07] and the Type Ib SN 2008D [@Malesani09; @Modjaz09 a spectrum of which is reproduced in Figure \[f:early\_spec\]]. The features in both objects were attributed to a combination of , , and at blueshifted velocities of [$\sim\!\!$ ]{}21,000 [km s$^{-1}$]{} and [$\sim\!\!$ ]{}30,000 [km s$^{-1}$]{}, respectively. If we assume that these are the correct line identifications, then they are blueshifted by [$\sim\!\!$ ]{}26,000 [km s$^{-1}$]{} in the day 8 spectrum of [SN 2001ig]{}. However, there is some uncertainty in the bluer of these two features since its left wing is distorted somewhat by the noise at the bluest end of our spectrum. It should also be noted that @Mazzali08 found no evidence for the “W” feature in their earliest spectrum of SN 2008D.

The third feature (near 4600 Å) can be decomposed into two Gaussian profiles centered at [$\sim\!\!$ ]{}4498 Å and [$\sim\!\!$ ]{}4596 Å, which could be a blend of [H$\beta$]{} and $\lambda\lambda$4921, 5015 blueshifted by [$\sim\!\!$ ]{}23,200 [km s$^{-1}$]{} and [$\sim\!\!$ ]{}23,300 [km s$^{-1}$]{}, respectively. At this epoch [SN 2001ig]{} defies simple classification [as pointed out by @Phillips01].

There is also weak, but broad, absorption at [$\sim\!\!$ ]{}5300 Å and 5420 Å, the latter of which could be $\lambda$5876 blueshifted by [$\sim\!\!$ ]{}24,100 [km s$^{-1}$]{}. In addition, the absorption near 6060 Å is most likely [H$\alpha$]{} blueshifted by [$\sim\!\!$ ]{}24,000 [km s$^{-1}$]{}.

We measure a blackbody-fit temperature of [$\sim\!\!$ ]{}14,000 K for the continuum of this observation. In comparison, the derived early-time blackbody temperatures of SNe 1993J and 2008D were [$\sim\!\!$ ]{}12,000 K [@Clocchiatti95] and 13,000 K [@Modjaz09], respectively, which match nicely with [SN 2001ig]{}.

In our highest-resolution spectrum, taken on 2002 Jan. 18 (46 days after explosion; see §\[ss:transition\] for more on this observation), we measure an upper limit to the equivalent width (EW) of the  D line in of [$\sim\!\!$ ]{}0.1 Å. This corresponds to a host-galaxy reddending of $E\left(B-V\right) \lesssim
0.02$ mag [@Munari97], which is comparable to the Galactic reddening of $E\left(B-V\right)=0.011$ mag from @Schlegel98. The low host reddening for [SN 2001ig]{} is reasonable since the SN occurred on the outskirts of the galaxy. Since these numbers are both quite small, we ignore reddening when plotting our spectra. We also note that @Maund07 required a total reddening somewhat in excess of the Galactic value plus the value we calculate for the host to explain the polarization they observe.

In all other early-time spectra, we detect $\lambda$5876 and P-Cygni profiles of [H$\alpha$]{} (with the absorption component greatly dominating over the emission component). The observation from 2001 Dec. 13, for example, shows the feature blueshifted by [$\sim\!\!$ ]{}16,000 [km s$^{-1}$]{}. In addition, @Phillips01 mention that there are broad absorption features at 4080 Å and 4580 Å in the observation from 2001 Dec. 13, and it is possible that they are [H$\gamma$]{} and [H$\beta$]{}, respectively. However, the observation lacked acceptable blue-side flatfields and proper wavelength-calibration files, so data at wavelengths $\lesssim$5000 Å cannot be completely trusted. We also identify the very weak, broad absorption near 7300 Å as $\lambda$7774. By 13 days after the explosion many more spectral features have appeared. Still present are the P-Cygni profile of [H$\alpha$]{}  and the $\lambda$5876 line. @Maund07 point out that [H$\alpha$]{} at this time is likely to be blended with $\lambda$6678. No longer visible in this spectrum is the aforementioned “W” feature that probably came from a combination of , , and . This is again similar to SN 2008D, whose “W” disappeared in the span of [$\sim\!\!$ ]{}1 day [@Modjaz09]. However, there [*are*]{} still features at about 4130 Å and 4287 Å, and we associate these with possible absorption due to [H$\gamma$]{}  and , respectively.

The feature near 4600 Å is most likely [H$\beta$]{}  while the noisy dips just below 4800 Å are probably due to a combination of $\lambda\lambda$4921, 5015 and $\lambda\lambda$4924, 5018. Also seen is weak absorption likely due to $\lambda$5169 near 5030 Å  and quite broad, weak absorption from the near-infrared (IR) triplet around 8150 Å. On 2001 Dec. 23, we acquired the fifth spectrum from the top of Figure \[f:early\_spec\]. Again, we detect the [H$\alpha$]{} P-Cygni line, although now it has a far more complex profile with a larger full width at half-maximum intensity (FWHM) than previously seen. This implies that the [H$\alpha$]{} feature is now likely becoming blended with $\lambda$6355. The feature near 5600 Å in this spectrum clearly shows two local minima which are probably due to $\lambda$5876 and  D. Also in this observation we detect distinct [H$\beta$]{}, $\lambda$4921, and $\lambda$5015 absorption, in addition to absorption from the near-IR triplet around 8200 Å. Finally, extremely strong absorption due to  H&K can now be seen near 3750 Å. It has been pointed out that in many ways the first few spectra of [SN 2001ig]{} resemble those of SNe 1987K and 1993J at early times [@Phillips01; @Clocchiatti01]. Examples of both at similar ages are plotted for comparison in Figure \[f:early\_spec\] and there do appear to be quite a few similarities among these objects. However, it should be noted that [H$\alpha$]{} (both the absorption and emission components) is weaker in [SN 2001ig]{}. In addition, the overall continuum shape of [SN 2001ig]{} does not change much during this two-week period.

Figure \[f:early\_spec\] also indicates that [SN 2001ig]{}has a bluer continuum than either SN 1987K or SN 1993J; however, this could be due to reddening. Both SNe 1987K and 1993J are plotted with no reddening correction, which is approximately consistent with observations [@Richmond94], but there is also evidence that the color excess of SN 1993J is as high as 0.25–0.32 mag [@Richmond94; @Clocchiatti95].

Transition Spectra {#ss:transition}
------------------

Our spectra from [$\sim\!\!$ ]{}5.5 weeks past explosion to [$\sim\!\!$ ]{}8 weeks past explosion are presented in Figure \[f:mid\_spec\]. The most obvious change in the spectra of [SN 2001ig]{} from Figure \[f:early\_spec\] to Figure \[f:mid\_spec\] is that the flux in the blue part of the continuum has decreased significantly; [SN 2001ig]{} now looks very much like SN 1993J in its overall continuum shape. In addition, SNe 2001ig and 1993J look closely similar in the range [$\sim\!\!$ ]{}4000–6000 Å, much of which is a forest of blends (with some hydrogen Balmer and lines as well). As noted by @Clocchiatti02, the main difference at this time between SNe 2001ig and 1993J is that [SN 2001ig]{} has stronger  H&K lines, weaker and lines, and faster expansion velocities.

Also mentioned by @Clocchiatti02, by 2002 Jan. 13 (the top spectrum in Figure \[f:mid\_spec\]), [SN 2001ig]{} has begun its transition from Type II to Type Ib. This transformation is marked by the decrease in strength of [H$\alpha$]{}and [H$\beta$]{} while the spectral features increase in strength. The most notable cases of this are the dramatic appearances of the $\lambda$6678 absorption feature, which appears as a notch on the red wing of the [H$\alpha$]{} absorption profile, and strong $\lambda$7065 absorption, which unfortunately coincides with a telluric absorption band in the day 41 and 57 spectra. This transformation continues during the subsequent two weeks of observations (i.e., the rest of the [SN 2001ig]{} spectra in Figure \[f:mid\_spec\]).

The minimum of the [H$\alpha$]{} absorption feature is becoming more complex and appears to have developed a doubled-troughed profile. This is to be expected since at this time [H$\alpha$]{} is being heavily blended with $\lambda$6355. In addition, as was pointed out by @Maund07, the $\lambda$5876 feature has become more prominent and the red wing of its absorption now shows a notch due to D absorption.

Also, by this time the  H&K feature has developed a P-Cygni profile with quite a strong emission component. In addition, the near-IR triplet absorption has strengthened dramatically. The previously identified absorption from near 4300 Å and 5000 Å  becomes stronger, broader, and more complex with time. This is likely due to line blanketing which was also seen in SN 2002ap [@Foley03].

The broad absorption near 9000 Å could be $\lambda$9266 at about $-9000$ [km s$^{-1}$]{}. In addition, strong, relatively broad absorption from $\lambda$7774 is again seen in these observations, but at roughly $-8000$ [km s$^{-1}$]{}. However, this feature coincides with a telluric absorption band in the day 41 and 57 spectra, making the velocity determination difficult and uncertain.

Late-Time Spectra {#ss:late}
-----------------

A late-time spectrum from @Maund07 (256 days past explosion) and our late-time spectra (309 and 340 days past explosion) are presented in Figure \[f:late\_spec\] and are compared with similar late-time spectra of SNe 1993J and 2002ap. [SN 2001ig]{} has reached the nebular phase by this time and has quite strong \[\] $\lambda\lambda$6300, 6364 and \] $\lambda$4571 emission. These and other common nebular SN features are marked in Figure \[f:late\_spec\]; the features of SNe 2001ig, 2002ap, and 1993J are all similar, but the relative strengths of some of the emission lines differ.

One of the most notable differences among the objects is that SN 1993J exhibits obvious [H$\alpha$]{}emission by day 298, appearing as a shoulder on the red wing of \[\] $\lambda\lambda$6300, 6364. @Filippenko94 and @Matheson00 showed that the [H$\alpha$]{} feature gradually strengthened from this time onward in SN 1993J, whereas in spectra at similar epochs of [SN 2001ig]{} shown in Figure \[f:late\_spec\] there is no obvious [H$\alpha$]{} emission. For SN 1993J the hydrogen at late times was attributed to interaction with circumstellar gas, but in [SN 2001ig]{} we see no evidence for such an interaction.

Figure \[f:late\_spec\] also shows that the blended \[\] emission near 5200 Å is weak, which indicates a relatively small $^{56}$Ni mass (see §\[s:model\] for more information on the mass of the ejecta). The line profiles of \] $\lambda$4571, \[\] $\lambda\lambda$6300, 6364, and \[\] $\lambda\lambda$7291, 7324 in the spectra of [SN 2001ig]{} in Figure \[f:late\_spec\] are quite strong and complex; Figure \[f:OMG\_comparison\] illustrates them in detail. The \] and \[\] lines at all epochs appear to have a broad peak, punctuated by multiple local maxima. \] has peaks near $-$2000 [km s$^{-1}$]{} and at $-$1000 [km s$^{-1}$]{} relative to the systemic velocity of [939 [km s$^{-1}$]{}; @Koribalski04]. This line may also have another bump near $-$3000 [km s$^{-1}$]{} in the blue wing of the main feature. The \[\] emission is even more complex, with local peaks near $-$3000 [km s$^{-1}$]{}, $-$1700 [km s$^{-1}$]{}, 150 [km s$^{-1}$]{}, and 1200 [km s$^{-1}$]{} (again relative to the systemic velocity of ). The \[\] $\lambda\lambda$7291, 7323 emission in Figure \[f:OMG\_comparison\] has a broad peak which is blueshifted by [$\sim\!\!$ ]{}1000 [km s$^{-1}$]{} in all three epochs.

Figure \[f:OMG\_comparison\] also shows the temporal evolution of the \] $\lambda$4571 and \[\] $\lambda\lambda$6300, 6364 line profiles. One of the most notable changes is the decrease in relative flux in the far wings of the \] line near $\pm$5000 [km s$^{-1}$]{}. As mentioned above, the feature near $-$3000 [km s$^{-1}$]{} appears to decrease in strength with time. This decrease of relative flux with time near $-$3000 [km s$^{-1}$]{} is also seen in the \[\] feature. Finally, we note that there is also an increase in the flux in the wings of the \[\] feature near $\pm$2000–5000 [km s$^{-1}$]{}.

The complex structure of the \] $\lambda$4571 and \[\] $\lambda\lambda$6300, 6364 lines has been seen previously in another well-studied Type IIb, SN 1993J [@Matheson00]. They found that the \[\] and \] line profiles resulted from clumps in the ejecta while the \[\] lines did not. They also pointed out that this is consistent with the explosion scenario first proposed for SN 1987A by @Li92 [@Li93], in which the emission from oxygen comes from clumps of newly synthesized material while the calcium emission originates mainly in preexisting uniformly distributed material. @Filippenko89 found similar irregularities in the \[\] $\lambda\lambda$6300, 6364 lines of the SN Ib 1985F. They suggested that the spectral peculiarities come either from Rayleigh-Taylor fingers of higher-speed material or from local density enhancements.

The \[\] doublet of [SN 2001ig]{} somewhat resembles that of the peculiar SN Ib 2005bf [@Modjaz08] in that [SN 2001ig]{}may also show a double-peaked profile with one of the peaks near zero velocity and the other blueshifted by a few thousand [km s$^{-1}$]{}. However, the \] line in [SN 2001ig]{} at all epochs has a broad profile with a complex top and does not seem to exhibit the same double-peaked nature of the \[\] emission. This differs from the results of @Foley03 for the energetic broad-lined SN Ic 2002ap, which showed identical \[\] and \] profiles. In addition, the flux in the \] line appears to be more centrally concentrated in [SN 2001ig]{} compared to SN 2002ap.

It has been proposed that the double-peaked \[\] profile of SN 2005bf was caused by a unipolar blob or jet [@Maeda07] while its polarization signature was attributed to a possible “tilted jet” passing through an asymmetric photosphere [@Maund07b; @Maund07]. Both our models of late-time spectra of [SN 2001ig]{} and the similarity of the profiles in our late-time spectra to those of SN 1993J [@Matheson00] indicate clumpy, oxygen-rich ejecta as well (see §\[s:model\] for more information regarding the models and ejecta). Thus, [SN 2001ig]{} probably has large-scale asymmetry in its ejected oxygen, either in the form of a blob or jet (tilted or otherwise), while the distributions of magnesium and calcium appear more uniform (though the magnesium seems only marginally more uniform than the oxygen). This is supported by @Fransson89, who predicted that magnesium and calcium should be found closer to the core of the explosion than oxygen. @Filippenko02 stated that the \] $\lambda$4571 feature was the strongest (relative to \[\] $\lambda\lambda$6300, 6364) ever seen in a SN. We investigate this claim more closely and the results are shown in Table \[t:lines\], where we list the relative line strengths for each nebular spectrum of [SN 2001ig]{} along with some of the values from @Foley03 [Table 6] and values calculated from other SN spectra in the literature. To compute the relative line strengths for [SN 2001ig]{} we subtracted a local continuum from each spectral feature and then integrated the flux in each line. This is the same procedure presented by @Foley03.

[lcccccc]{} [SN 2001ig]{}(IIb) & 256 & 0.38 & 1.77 && 0.21\
[SN 2001ig]{}(IIb) & 309 & 0.74 & 2.06 & 0.30 & 0.36\
[SN 2001ig]{}(IIb) & 340 & 0.83 & 1.99 & 0.57 & 0.42\
SN 2008D (Ib) & 109 & 0.34 & 1.02 & 5.61 & 0.33\
SN 2006aj (Ic-BL) & 153 & 1.19 & 4.39 & 1.37 & 0.27\
SN 2003jd (Ic-BL) & 367 & 0.73 & 1.58 & 0.80 & 0.46\
SN 1993J (IIb) & 298 & 0.27 & 1.79 & 0.36 & 0.15\
SN 2002ap (Ic) & 242 & 0.72 & 2.17 & 0.91 & 0.33\
SN 2002ap (Ic) & 274 & 0.63 & 2.01 & 0.66 & 0.31\
SN 2002ap (Ic) & 336 & 1.00 & 2.29 & 0.45 & 0.43\
SN 2002ap (Ic) & 386 & 0.87 & 2.19 & 0.46 & 0.40\
SN 1998bw (Ic) & 215 & 0.23 & 1.47 & 0.41 & 0.15\
SN 1994I (Ic) & 146 & 0.26 & 0.97 & 0.84 & 0.26\
SN 1985F (Ib/c) & [$\sim\!\!$ ]{}240 & 0.26 & 3.15 & 0.56 & 0.08\

Although it has been pointed out that this method of line-strength measurement is somewhat inaccurate [@Foley03], it does seem to indicate that the \] line of [SN 2001ig]{} grows with time with respect to the \[\] $\lambda\lambda$7291, 7324 doublet and the \[\] doublet. Relative to the \[\] feature, the \] emission from [SN 2001ig]{} is somewhat weaker than that of SN 2002ap at similar epochs. The other SNe that also appeared in @Foley03 [Table 6] all have about the same \] strength, which is lower that that of either SNe 2001ig or 2002ap. However, we also perform this measurement on a few SNe from the literature (see Table \[t:lines\]) and find that while SNe 2008D [@Modjaz09] and 1993J [@Matheson00] have comparable \] strength, this feature is quite strong in the broad-lined Type Ic (Ic-BL) SNe 2006aj [@Mazzali07:06aj] and 2003jd [@Valenti08].

Again comparing [SN 2001ig]{} to the SNe that appeared in @Foley03 [Table 6], it has a Mg/O ratio comparable to that of SNe 1994I and 2002ap, but larger than that of SNe 1998bw and 1985F. In addition, the Mg/O ratio of [SN 2001ig]{} is similar to that of SNe 2008D, 2006aj, and 2003jd (which in fact has quite a large ratio), but significantly greater than that of SN 1993J. This confirms the claim of @Filippenko02 that [SN 2001ig]{} had the strongest nebular O/Mg emission-line ratio of any SN observed at that time. Table \[t:lines\] also seems to indicate that it is not uncommon for core-collapse SNe to have relatively large Mg/O ratios, yet what distinguishes between the SNe that do or do not show this is unclear.

Spectral Models {#s:model}
===============

By several months after the explosion of a SN, the densities in the ejecta are sufficiently low from expansion that the opacity drops below unity and the gas obeys nebular physics. At this time it is heated by radioactive decay of $^{56}$Ni and cooled by line emission. The resulting line profiles map rather directly the distribution of the heating material and can be used to derive several properties including $^{56}$Ni mass, mass of the emitting elements, and morphology of the explosion [@Mazzali05].

In order to derive accurate models, we need our spectra to be as spectrophotometrically well calibrated as possible. While the relative fluxes of our data are accurate because of our flux-calibration routines (see §\[s:obs\]), the absolute fluxes may be off due to imperfect observing conditions. To address this, we obtained publicly available photometric data on [SN 2001ig]{} from ESO and the Space Telescope – European Coordinating Facility (ST-ECF) Science Archive Facility[^2]. The observations were single 60 s Bessell [@Bessell90] $R$-band images taken with the FORS1 spectrograph mounted on the ESO 8.2-m Very Large Telescope Melipal and Antu [@Appenzeller98].

We calibrated both images, observed on 2002 Oct. 12 and 2002 Dec. 8, to the United States Naval Observatory B1.0 (USNOB) Catalog with three comparison stars and measured magnitudes for [SN 2001ig]{} of 17.99$\pm$0.08 and 19.13$\pm$0.14, respectively. In addition to our stated statistical uncertainty, we adopt a systematic uncertainty of [$\sim\!\!$ ]{}0.25 mag based on comparisons of USNOB magnitudes to Bessell $R$-band magnitudes obtained as part of the Lick Observatory Supernova Search [LOSS; @Filippenko01] photometry follow-up program. We then synthesized Bessell $R$-band magnitudes from our late-time spectra (taken on 2002 Oct. 8 and 2002 Nov. 8) and compared them to the magnitudes derived from the photometry, assuming a linear decline in magnitude during the last three months of 2002. In order for our spectra to match the photometry, and thus for the absolute flux to be accurate, our spectra from 2002 Oct. 8 and 2002 Nov. 8 must be scaled up by factors of 4.41 and 1.29, respectively. Due to the photometric measurements alone, the uncertainty in the absolute flux of each of these two spectra is [$\sim\!\!$ ]{}25%.

We modeled both late-time spectra of [SN 2001ig]{} using a code that computes nebular-line emission from a gaseous cloud. The gas is heated by the deposition of $\gamma$-rays and positrons produced by the decay of radioactive $^{56}$Ni (which is produced in the SN explosion itself) into $^{56}$Co and then into stable $^{56}$Fe. The energy thus deposited in the gas is thermalized collisionally, which leads to excitation and ionization of the gas. Heating is balanced by cooling, which takes place via line emission. Most of the emission lines are forbidden, but some permitted transitions (e.g., the near-IR triplet) are also active. The code is based on the original description by @Axelrod80. The propagation of the $\gamma$-rays is followed with a Monte Carlo procedure [@Cappellaro97]. Level populations are computed assuming non-local thermodynamic equilibrium (non-LTE). A more detailed description is provided by @Mazzali07:02ap.

Modeling the spectra also requires knowledge of the reddening of, and distance to, the SN. As discussed in §\[ss:early\], the reddening within is negligible and we have only an upper limit from our data, so we adopt a total reddening of $E\left(B-V\right)=0.011$ mag from @Schlegel98 for the models. For the distance to [SN 2001ig]{}we have used a recession velocity of 939 [km s$^{-1}$]{}[$z=0.003132$; @Koribalski04] throughout this paper, yielding a distance modulus of $m-M=30.5$ mag with $H_0 = 73$ [km s$^{-1}$]{} [Mpc$^{-1}$]{} [@Riess05], assuming the entire recession velocity is due to the Hubble flow. However, there is some uncertainty in this value due to the relatively small distance to . A search of the literature reveals a range in distance moduli for this galaxy of 30.2–30.7 mag using various independent techniques [HyperLEDA[^3]; @Tully88; @Mould00; @Meurer06; @Davis09 private communication[^4]]. The value of 30.5 mag that we use here is a reasonable average as long as we keep in mind that it has an uncertainty of [$\sim\!\!$ ]{}0.3 mag which leads to an uncertainty of [$\sim\!\!$ ]{}30% in the absolute flux of each late-time spectrum. When this is combined in quadrature with the uncertainty from the photometric measurements, the total uncertainty in the absolute flux of each of our late-time spectra is [$\sim\!\!$ ]{}39%.

The late-time spectra and models are shown in Figure \[f:model\]; both models appear to fairly accurately reproduce the major features observed in the data. However, the emission near 4000 Åand the feature near 7700 Å seem somewhat suppressed in the models. In addition, the intensity of the feature near 8600 Å in the models is somewhat larger than that of the observed spectrum, leading to a small inconsistency in the position of the combined plus \[\] $\lambda$8727 emission.

Here we use the one-zone version of the model described by @Mazzali07:02ap. It requires an emitting nebula with an outer velocity of 4300 [km s$^{-1}$]{}, as determined from reproducing the width of the strongest emission lines (\[\], \], and \[\]). The spectrum is powered by [$\sim\!\!$ ]{}0.13 [M$_\sun$]{} of $^{56}$Ni. This is determined from the combination of the intensity of the \[\] emission feature near 5200 Å and the requirement that the energy emitted in the radioactive decay chain of $^{56}$Ni excites other ions and reproduces the observed emission in other lines. This is a slightly larger value than what was derived for SN 1993J [see @Swartz93 and references therein], but it is still fairly typical for core-collapse SNe.

The total mass enclosed within the outer velocity of 4300 [km s$^{-1}$]{} is [$\sim\!\!$ ]{}1.15 [M$_\sun$]{}. Interestingly, our models of [SN 2001ig]{} are quite similar to that of the SN Ic 1994I which underwent significant mass loss, most likely in a binary interaction, and which also ejected [$\sim\!\!$ ]{}1 [M$_\sun$]{} of material [@Sauer06]. @Nomoto94 showed the progenitor star of SN 1994I to be [$\sim\!\!$ ]{}15 [M$_\sun$]{} initially, which suggests that the progenitor of [SN 2001ig]{} was also initially [$\sim\!\!$ ]{}15 [M$_\sun$]{}.

Furthermore, @VanDyk02 and @Maund04 found that both the companion and the progenitor of SN 1993J had intial masses of [$\sim\!\!$ ]{}15 [M$_\sun$]{} and that the companion of SN 1993J is currently 22 [M$_\sun$]{}. @Ryder06 discovered that the companion of [SN 2001ig]{} is currently 10–18 [M$_\sun$]{}, so it seems likely that the progenitors of [SN 2001ig]{} and SN 1993J (and possibly even SN 1994I) had similar initial masses, and that their companions have similar masses as well. The differences in the companions’ masses (as well as physical separations from the progenitors) probably led to slightly different mass-loss and interaction histories which gave rise to the spectroscopic differences observed.

Table \[t:model\] shows the complete mass composition of the models. The upper and lower error bars represent the change in the derived abundance after increasing and decreasing the measured flux by 39% to reflect the flux-calibration uncertainties mentioned above. As in the nebular spectra of all other stripped-envelope SNe, and unlike the case in SNe Ia, \[\] lines are missing (in particular a strong feature near 4800 Å). This is best reproduced by assuming fairly significant clumping in the ejecta, which is a signature of an aspherical explosion [e.g., @Mazzali07:02ap]. @Maund07 use spectropolarimetric observations of [SN 2001ig]{} to come to the same conclusion regarding the asphericity.

The dominant element in the ejecta by mass is oxygen (0.81 [M$_\sun$]{}). The derived carbon abundance is quite small (0.04 [M$_\sun$]{}); otherwise, the emission near 8500 Å would become too strong. We point out that a small C/O ratio is not unusual in stripped-envelope SNe. In addition, hydrogen may be present only marginally at these low velocities, as deduced from the almost complete absence of any emission (as seen in Figure \[f:late\_spec\]).

As mentioned in §\[ss:late\], the late-time spectra of [SN 2001ig]{} have an unusually large Mg/O intensity ratio, comparable to that of SN 2002ap. However, the Mg/O ratio by mass of [SN 2001ig]{} is [$\sim\!\!$ ]{}0.014 (see Table \[t:model\]) while that of SN 2002ap is [$\sim\!\!$ ]{}0.032 [@Mazzali07:02ap]. This difference in masses is consistent with the fact that the envelope of SN 2002ap was extremely stripped before explosion, since it showed no spectroscopic evidence of hydrogen or helium [@Foley03]. On the other hand, the envelope of [SN 2001ig]{} was only partially stripped prior to core collapse since we observed a significant amount of hydrogen and helium at early times.

[lcc]{} C & $2.0^{+0.0}_{-0.8}\times10^{-2}$ & $5.0^{+0.0}_{-1.0}\times10^{-2}$\
O & $8.1^{+2.0}_{-2.1}\times10^{-1}$ & $8.2^{+1.8}_{-1.7}\times10^{-1}$\
Na & $7.0^{+0.0}_{-1.5}\times10^{-5}$ & $1.0^{+0.2}_{-0.0}\times10^{-4}$\
Mg & $1.1^{+0.2}_{-0.0}\times10^{-2}$ & $1.1^{+0.2}_{-0.0}\times10^{-2}$\
Si & $1.0^{+0.0}_{-0.0}\times10^{-1}$ & $1.0^{+0.0}_{-0.0}\times10^{-1}$\
S & $3.0^{+0.0}_{-0.0}\times10^{-2}$ & $3.0^{+0.0}_{-0.0}\times10^{-2}$\
Ca & $3.1^{+0.8}_{-0.9}\times10^{-2}$ & $4.0^{+1.2}_{-0.9}\times10^{-2}$\
$^{56}$Ni & $1.5^{+0.3}_{-0.3}\times10^{-1}$ & $1.1^{+0.2}_{-0.3}\times10^{-1}$\
Total & $1.2^{+0.3}_{-0.3}$ & $1.2^{+0.2}_{-0.2}$\

Conclusions {#s:conclusions}
===========

In this paper we have presented and analyzed optical spectra of [SN 2001ig]{}. One week after explosion, the SN defied simple spectral classification, but by two weeks after explosion [SN 2001ig]{}appeared to be part of the Type IIb subclass of SNe. This was confirmed as the SN went through a transition from Type II to Type Ib during the first few months of observations.

By nine months after explosion, [SN 2001ig]{} had entered the nebular phase, revealing some of the strongest \] $\lambda$4571 and \[\] $\lambda\lambda$6300, 6364 features ever observed in a SN; its Mg/O intensity ratio is one of the largest ever seen as well. We derive models of [SN 2001ig]{} from our spectra taken 309 and 340 days after explosion, showing that the majority of the inner ejecta (below 4300 [km s$^{-1}$]{}) were in the form of oxygen and a significant fraction (nearly 0.13 [M$_\sun$]{}) in the form of $^{56}$Ni. Additionally, there appears to be a distinct lack of hydrogen in the inner ejecta. We also find that the total mass of this inner ejecta was [$\sim\!\!$ ]{}1.15 [M$_\sun$]{}, suggesting that the progenitor of [SN 2001ig]{} was a relatively low-mass star ([$\sim\!\!$ ]{}15 [M$_\sun$]{}).

We are grateful to the staffs at the Keck Observatory, ESO, and Las Campanas Observatory for their support. We thank the following for their assistance with some of the observations and data reduction: F. Barrientos, R. Carlberg, M. Gladders, S. W. Jha, T. Matheson, J. L. Prieto, and B. Leibundgut. We especially thank J. Maund and his collaborators for allowing us to reproduce one of their spectra. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. ESO VLT data were acquired under programs 68.D-0571(A), 69.D-0438(A), and 170.A-0519(A), and La Silla data under program 68.A-0443. A.V.F.’s group is supported by the National Science Foundation through grant AST–0607485. A.C. is supported by grants P06-045-F ICM de MIDEPLAN, Basal CATA PFB 06/09, and FONDAP No. 15010003.

[^1]: IRAF: the Image Reduction and Analysis Facility is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.

[^2]: http://archive.eso.org/

[^3]: http://leda.univ-lyon1.fr/

[^4]: Using the 2MASS Redshift Survey (2MRS) galaxy catalog to model bulk flows in the local Universe.
