The extremely luminous Type Ia SN 2009dc

Early phase observations of extremely luminous Type Ia Supernova 2009dc

Abstract

We present early phase observations in optical and near-infrared wavelengths for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate of the light curve is , which is one of the slowest among SNe Ia. The peak -band absolute magnitude is mag even if the host extinction is mag. It reaches mag for the host extinction of mag as inferred from the observed Na i D line absorption in the host. Our -band photometry shows that the SN is one of the most luminous SNe Ia also in near-infrared wavelengths. These results indicate that SN 2009dc belongs to the most luminous class of SNe Ia, like SN 2003fg and SN 2006gz. We estimate the ejected Ni mass of for no host extinction case (or M for the host extinction of mag). The C ii 6580 absorption line keeps visible until a week after maximum, which diminished in SN 2006gz before its maximum brightness. The line velocity of Si ii 6355 is about 8000 km s around the maximum, being considerably slower than that of SN 2006gz, while comparable to that of SN 2003fg. The velocity of the C ii line is almost comparable to that of the Si ii. The presence of the carbon line suggests that thick unburned C+O layers remain after the explosion. SN 2009dc is a plausible candidate of the super-Chandrasekhar mass SNe Ia.

1. Introduction

Type Ia Supernovae (SNe Ia) have been believed to occur when the mass of the progenitor white dwarf (WD) reaches the Chandrasekhar’s limiting mass, by mass accretion from a companion star. The homogeneity in their light curves is explained by this scenario and the calibrated luminosity of SNe Ia has been used as an important tool for the constraints on the expansion rate and the dark energy content of the universe (Perlmutter et al. 1999; Riess et al. 1998). However, their progenitors and detailed explosion mechanism have not been confirmed yet (e.g., Nomoto et al. 1997; Hillebrandt & Niemeyer 2000).

Observationally, SNe Ia have been classified into three subclasses: normal SNe Ia, overluminous SNe Ia (SN 1991T-like), and faint SNe Ia (SN 1991bg-like) (Branch et al. 1993; Filippenko 1997; Li et al. 2001). The light curves of more luminous SNe Ia decline more slowly (Phillips 1993).

Recently, two extremely luminous SNe Ia 2003fg and 2006gz have been discovered (Howell et al. 2006; Hicken et al. 2007). Their absolute maximum magnitudes are mag for SN 2003fg and mag for SN 2006gz, and both SNe show the slowest luminosity evolution. Such an extreme brightness suggests that their progenitor’s masses exceed the Chandrasekhar limit (“super-Chandrasekar mass WD”) (Howell et al. 2006; Hicken et al. 2007). Interestingly, these SNe showed strong carbon absorptions in their early stages; the C ii line was seen in SN 2003fg around maximum, while the line diminished before maximum in SN 2006gz. The expansion velocity inferred from the Si ii 6355 line around maximum was slow in SN 2003fg ( 8000 km s), while it was typical ( 11000-12000 km s) in SN 2006gz.

SN 2009dc was discovered on 2009 Apr 9.31 UT at non-filter magnitude of 16.5 near the outer edge of an S0 galaxy UGC 10064 (Puckett et al. 2009, from the database; Falco et al. 1999). A follow-up observation on Apr 16.22 revealed spectroscopic similarity with SN 2006gz before maximum light, including the existence of conspicuous C ii features (Harutyunyan et al. 2009). The expansion velocity deduced from the Si ii 6355 line is about 8700 km s , which is slower than that of SN 2006gz (Hicken et al. 2007), but comparable to that of SN 2003fg (Howell et al. 2006).

In this Letter, we show our photometric and spectroscopic observations of this peculiar SN from days through days after the maximum. The observational results strongly suggest that SN 2009dc is a super-Chandrasekher SNe Ia having some peculiar properties compared with other candidates.

2. Observations and Reduction

We performed -band photometry of SN 2009dc on 30 nights from 2009 Apr 17.8 UT ( days after the -band maximum; see §3.1) through Jul 14.5 ( days), using HOWPol (Hiroshima One-shot Wide-field Polarimeter; Kawabata et al. 2008) installed to the 1.5 m KANATA telescope at Higashi-Hiroshima Observatory. The images were reduced according to a standard procedure of a CCD photometry. We performed point-spread-function fitting photometry using package in . The magnitude is calibrated with photometric standard stars in Landolt fields (Landolt 1992) observed on photometric nights. Additionally, we obtained -band photometric data on 10 nights from to +19.3 days, using MITSuME 0.5 m telescope (Multicolor Imaging Telescopes for Survey and Monstrous Explosions). The MITSuME -band magnitudes are consistent with the KANATA/HOWPol photometry within systematic differences less than mag.

Our near-infrared (NIR) photometric data were obtained from through days using the 1 m telescope in Kagoshima University and the 1.88 m telescope at Okayama Astrophysical Observatory of NAOJ equipped with ISLE (near-infrared imager and spectrograph; Yanagisawa et al. 2006). Their magnitude calibrations were performed with nearby stars in 2MASS catalogue.

The optical spectra were obtained using GLOWS (Gunma LOW dispersion Spectrograph) installed to the 1.5 m telescope at Gunma Astronomical Observatory on six nights from through +53.7 days. The wavelength coverage was 4200–8000 Å and its resolution was at 6000 Å. The flux was calibrated using several spectrophotometric standard stars taken in the same night. We have removed the strong telluric absorption features from the object spectra using the standard star spectra.

3. Results and Discussion

3.1. Light Curves

Figure 1.— (a) -band light curves of SN 2009dc, compared with a super-Chandrasekhar SN Ia 2006gz (Hicken et al. 2007) and an overluminous SN Ia 1991T (Lira et al. 1998) and a normal SN 2005cf (Wang et al. 2009). The Galactic extinction has been corrected for in SN 2009dc. For other SNe the light curves are shifted to match the maximum magnitude. (b) , , , color evolutions, compared with those of SNe 1991T, 2005cf and 2006gz. In SN 2009dc, only Galactic extinction ( mag and ) is corrected for. In other SNe Ia, the extinctions in both our and host galaxies are corrected for ( mag for SN 1991T, for SN 2005cf, for SN 2006gz). The color evolution of SN 2009dc is unique compared with those of the other SNe Ia.

We show optical and NIR light curves of SN 2009dc in Figure 1 (a). The Galactic extinction of mag and are corrected for (see §3.3). We derive -band maximum magnitude of 15.190.16 mag and its date of MJD (Apr 25.9 UT) by polynomial fitting to the light curve. In Figure 1, we compare SN 2009dc with the super-Chandrasekhar SN Ia 2006gz (Hicken et al. 2007; ), an over-luminous SN Ia 1991T (Lira et al. 1998; ) and a normal SN Ia 2005cf (Wang et al. 2009; ).

We notice that the brightness evolution across the maximum is slower than those of SNe 1991T and 2005cf in any band. We derive the decline rate of for SN 2009dc, which is similar to of SN 2006gz (Hicken et al. 2007). The decline rate of SN 2009dc is one of the smallest ones among SNe Ia which have ever been published.

The -band light curve (which would be the first NIR light curve of super-Chandrasekhar SNe Ia ever published) merginally shows a more significant dip between the first and second maximum than the I-band light curve. The -band light curves suggest the existence of more luminous secondary peak than the first one. These characteristics are likely typical for SNe Ia (Krisciunas et al. 2004; Wang et al. 2009)

3.2. Color Evolution

We show the evolution of color indices of SN 2009dc in Figure 1 (b), together with SNe 1991T, 2005cf and 2006gz for comparison. The evolution of of SN 2009dc is similar to those of SNe 1991T, 2005cf and 2006gz; this suggests that the Lira-Phillips relation (homogeneous evolution at 30–90 days, Phillips et al. 1999) also holds for SN 2009dc. We discuss it in §3.3. On the other hand, the evolutions of , and colors in SN 2009dc are somewhat different from those of SNe 1991T and 2005cf; the color indices of SN 2009dc become redder monotonously, while the other SNe Ia (except for SN 2006gz) have small troughs at 10–15 days after the -band maximum. SN 2006gz shows the color evolution similar to SN 2009dc, while it keeps bluer at through days and shows broad troughs in the and curves.

E (mag) host Ni mass
Galactic host (mag) (mag) (erg s) ()
36
Table 1Estimated maximum absolute magnitude, luminosity and the Ni mass in some extinction cases

3.3. Host Galactic Extinction and Absolute Magnitude

To confirm that SN 2009dc is one of the most luminous SNe Ia, it is important to determine the extinction toward this SN (Galactic + host).

The Galactic color excess is estimated to be mag (Schlegel et al. 1998), corresponding to an extinction of mag within our Galaxy (a typical selective extinction is assumed). On the other hand, the extinction within the host galaxy is somewhat uncertain. If the Lira-Phillips relation holds for SN 2009dc, it predicts a reddening of ) mag. However, this is likely an overestimation because the equivalent width (EW) of the Na i D absorption line in the host galaxy ( Å) is only twice the EW of Na i D in our Galaxy ( Å; Tanaka et al. 2009). If we assume that the extinction is simply proportional to the EW, mag is plausible for the host extinction. Additionally, the empirical relation between the color excess and the EW of Na i D (Turatto et al. 2003; we adopt their lower extinction case) predicts mag. These two values are consistent. This also suggests that the relation is consistent with our Galaxy’s values of mag and the EW of Å for the Galactic Na i D line. In Table 1 we summarize the estimated absolute magnitude for various cases of extinction parameters. There is an additional uncertainty for the extinction due to the diversity of (Wang et al. 2006; Krisciunas et al. 2006; Elias-Rosa et al. 2006). We adopt and following Hicken et al. (2006). Even if mag within the host galaxy is assumed, the absolute maximum magnitude is mag, indicating that SN 2009dc is one of the most luminous SNe Ia.

Krisciunas et al. (2004) pointed that the absolute maximum magnitude in NIR bands does not depend on the decline rate (except for faint SNe Ia) and derived the mean values of mag, mag and mag for , and -bands, respectively. We estimate the absolute maximum magnitudes of SN 2009dc as mag, mag, mag for zero host extinction case, which suggests that SN 2009dc is very luminous even in NIR wavelengths.

It is interesting to examine whether the maximum magnitude- relation (e.g. Altavilla et al. 2004) holds for this brightest SN Ia. If there is no host extinction, the absolute V magnitude derived from observations is roughly consistent () with the relation. On the other hand, it seems much brightter (by ) than the prediction of this relation if the host extinction is mag.

3.4. Bolometric Light Curve and Ni Mass

Figure 2.— Bolometric light curve of SN 2009dc. Filled and open circles show the cases of the host extinction mag with and , respectively The asterisk shows the case with no host extinction. The bolometric light curves of the normal SN Ia 2005cf (dashed line; Wang et al. 2009) and the super-Chandra SN 2006gz (thick line; Hicken et al. 2007) are shown for comparison.

We obtain the bolometric luminosity of SN 2009dc using our -band data, assuming that the optical luminosity occupies about 60% of the bolometric one around maximum brightness (Wang et al. 2009). Because of the uncertainty involved in this assumption, we consider that this bolometric luminosity may have a somewhat large systematic error (%). To confirm the reliability, we also calculate the bolometric luminosity from + data at days and check the consistency. We assume that the integrated luminosity is 80% of the total (Wang et al. 2009). They agree within an error of 12%. But this discrepancy includes an uncertainty of determining the maximum date of -band luminosity.

The obtained bolometric light curves are shown in Figure 2. We also calculate the bolometric luminosity of SN 2005cf with the same assumption and confirm the consistency with the results by Wang et al. (2009). Even if we assume that the host extinction is zero, the maximum bolometric luminosity is erg s, which is comparable to that of SN 2006gz, erg s for mag (Hicken et al. 2007). When we adopt the host extinction of mag and , erg s, which is likely to exceed that of SN 2003fg ( erg s; Howell et al. 2006).

The mass of ejected Ni can be roughly estimated from the peak luminosity (e.g., Arnett 1982). Stritzinger & Leibudgut (2005) suggested that the Ni mass depends approximately on the peak bolometric luminosity and its rising time ( days), as

(1)

The slow evolution of brightness in SN 2009dc around the maximum suggests that the rising time of the bolometric luminosity is comparable to or slightly longer than those of SN 2006gz ( days; Hicken et al. 2007) or typical SNe Ia ( days; e.g., Conley et al. 2006). Assuming days for SN 2009dc, we derive the Ni mass of for no host extinction case. It reaches if we assume the host extinction of mag and (Table 1). Although the derived and the Ni mass still include somewhat large uncertainties, the observational results suggest that the mass of the progenitor might exceed the Chandrasekhar-limit.

3.5. Spectral Evolution

Figure 3.— Spectra of SN 2009dc compared with those of other SNe Ia; subluminous object SN 1991bg, over-luminous object SN 1999aa, typical object SN 2003du and the super-Chandrasekhar SN Ia 2006gz. All spectra are calibrated to the Rest frame wavelength. The spectrum of SN 2009dc at days is from Tanaka et al. (2009). The telluric absorption feature have been removed.
Figure 4.— Si ii 6355 line velocity evolution of SN 2009dc and comparative SNe Ia, 2006gz (Hicken et al. 2007), 2006X (Yamanaka et al. 2009), 2004eo (Pastorello et al.2007), 2003du (Stanishev et al.2007), 2002bo (Benetti et al.2004). We also show the C ii 6580 line velocity of SN 2009dc with black open circles. The low expansion velocity of SN 2009dc is remarkable.

In Figure 3, we compare the spectra of SN 2009dc with those of the super-Chandrasekhar SN 2006gz at days and days (Hicken et al. 2007), the subluminous SN 1991bg (Filippenko et al.1992), the overluminous SN 1999aa (Garavini et al.2004) and the typical SN 2003du (Stanishev et al.2007) around maximum. The spectra of SN 2009dc around maximum show Si ii 6355 absorption, a W-shape S ii absorption feature and Fe-group multiple absorptions. Additionally, the absorption line of C ii 6580 is seen in SN 2009dc, even a few days after the maximum. This feature is seen only in a small fraction of SNe Ia at their earliest epochs (Tanaka et al. 2008). In the super-Chandrasekhar candidate SN 2006gz, the carbon feature also exists, while the feature is significant only in the earliest stages ( days; Hicken et al. 2007). In another, more distant super-Chandrasekhar SN 2003fg, the C ii 6580 feature is not significant at days, while there is a possible carbon feature around 4150 Å at the same epoch. These indicate that a massive C+O layer exists in the atmosphere of SN 2009dc.

In Figure 4, we show the line velocity of Si ii 6355 together with those in other SNe Ia. The Si ii line velocity of SN 2009dc is 8000 km s at days and then decreases to 6000 km s by days. This indicates that SN 2009dc is one of the most slowly expanding SNe Ia (except for faint SNe Ia). The line velocity is much lower than that of SN 2006gz, while comparable with that of SN 2003fg ( km s at days; Howell et al. 2006). The velocity of the C ii 6580 line in SN 2009dc evolves roughly as well as that of the Si ii line (Fig. 4).

4. Discussion and Conclusions

We summarize the observational characteristics of the peculiar SN Ia 2009dc as follows: (1) one of the slowest evolution of the light curve, i.e., , (2) one of the most luminous SNe Ia, i.e., or brighter, (3) a strong carbon feature in the early spectra, and (4) the lowest expansion velocity among normal and overluminous SNe Ia. The first three features are similar to another super-Chandrasekhar candidate SN 2006gz, while the last item is clearly different. Although the detailed data for the distant super-Chandrasekhar candidate SN 2003fg are lacking, its expansion velocity is comparable to that of SN 2009dc. The C ii 6580 feature is still present at days in SN 2009dc, although it diminishes by the similar epoch in SNe 2003fg and 2006gz. These facts suggests an existence of a massive unburned C+O layer in the ejecta of SN 2009dc. Additionally, we derive the amount of ejected Ni mass of even for no host extinction. If we assume the host extinction of mag and , it reaches . Therefore, we suggest that SN 2009dc is a SN Ia explosion with a super-Chandrasekhar mass WD.

This research has been supported in part by the Grant-in-Aid for Scientific Research from JSPS (20540226, 20740107, 21018007, 20840007) and MEXT (19047004, 20040004), and WPI Initiative, MEXT. M.T. has been supported by the JSPS Research Fellowship for Young Scientists.

Footnotes

  1. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  2. affiliation: Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
  3. affiliation: Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
  4. affiliation: Gunma Astronomical Observatory, Takayama, Gunma 377-0702, Japan
  5. affiliation: Department of Astronomy, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
  6. affiliation: Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata, Asakuchi-shi, Okayama 719-0232, Japan
  7. affiliation: Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Japan
  8. affiliation: Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Japan
  9. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  10. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  11. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  12. affiliation: Gunma Astronomical Observatory, Takayama, Gunma 377-0702, Japan
  13. affiliation: Gunma Astronomical Observatory, Takayama, Gunma 377-0702, Japan
  14. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  15. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  16. affiliation: National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
  17. affiliation: Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
  18. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  19. affiliation: Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata, Asakuchi-shi, Okayama 719-0232, Japan
  20. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  21. affiliation: National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
  22. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  23. affiliation: National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
  24. affiliation: Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
  25. affiliation: Department of Physics, Faculty of Science, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan
  26. affiliation: Department of Physics, Faculty of Science, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan
  27. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  28. affiliation: Toyota Technical Development, Corp., 1-21 Imae, Hanamoto-cho, Toyota, Aichi 470-0334, Japan
  29. affiliation: Gunma Astronomical Observatory, Takayama, Gunma 377-0702, Japan
  30. affiliation: Gunma Astronomical Observatory, Takayama, Gunma 377-0702, Japan
  31. affiliation: Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan; myamanaka@hiroshima-u.ac.jp
  32. affiliation: Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
  33. affiliation: National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
  34. affiliation: Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata, Asakuchi-shi, Okayama 719-0232, Japan
  35. affiliation: Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Kamogata, Asakuchi-shi, Okayama 719-0232, Japan
  36. footnotetext:

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