Spectroscopy of Bright Supernovae at DDO

Jozsef Vinko

Department of Optics & Quantum Electronics, University of  Szeged, Hungary


Supernovae (SNe) are exploding stars, which appear practically randomly in space and time. The number of discovered SNe per year has been constantly increasing over the last decade. Some of the brightest SNe have been succesfully observed with the DDO 1.88" telescope and Cassegrain spectrograph, both  in medium- and low-resolution mode. At DDO, SNe are observed as a Target-of-Opportunity (ToO) program with the approval both from the director and the scheduled observer.  This web page describes the past and present SNe observations made at DDO and gives the references of the publications based on these data.

SN 1998S (NGC 3877)

This SN was discovered on March 2.68, 1998 by Z. Wan. On March 16, 1998, shortly after discovery, Mike Gladders and Jim Thomson took a single spectrum of the SN, 'just for curiousity' as Jim often said, with the 150A grating in 2nd order, centered on 6000 Angstroms. The observed spectrum can be seen in Fig.1 (left panel) showing the hot, blue continuum. In the right panel, the continuum-normalized spectrum, enhancing the visibility of the H-alpha line, is presented. This SN turned out to be a IIn-type one, which is illustrated very well by the narrow H-alpha emission peak superimposed on a broader, but fainter P Cygni - profile. These SNe are thought to explode within a dense circumstellar medium (CSM), and the strong, narrow H-alpha emission during the early phases is formed when the CSM is heated and excited by the SN ejecta.

Fig.1. Left panel: observed spectrum of SN 1998 S. Right panel: contimuum-normalized spectrum

Fig.2. SN 1998S in NGC 3877 from Konkoly Observatory, Hungary

Fig.2 shows the location of SN 1998S in its host galaxy. NGC 3877 is a highly inclined Sc-type galaxy. SN 1998S occured in an outer spiral arm, where Type II SNe are usually found. The left image shows the galaxy plus the SN roughly one month after explosion, while the right image was made in Nov. 1998, more than 200 days after explosion. The position of the SN is marked. These CCD images were taken by Laszlo L. Kiss with the 90-cm Schmidt telescope at Konkoly Observatory, Hungary.

SN 1998aq (NGC 3982)

SN 1998aq was discovered by M. Armstrong on April 13, 1998, very shortly after explosion. It turned out to be a Type Ia SN, an exploding white dwarf in a binary system. SN 1998aq reached its maximum brightness on April 29, 1998.

Three spectra of this event have been obtained at DDO by Jim Thomson, Wen Lu, Stefan Mochnacki and Tom Bolton on April 22 (7 days before maximum), May 8 (9 days post-maximum) and May 27 (28 days post-maximum) using the same setup as for SN 1998S. Fig.3 displays the three spectra. In the first two spectra the strongest absorption feature at 6150 A is the Si II transition, the characteristics of Type Ia SNe. This SN evolved fast, as usual for Type Ia SNe: on the third spectrum, one month after maximum, the Si II absorption is engulfed by the broad Fe II and [Fe III] emission around 6500 A.

Fig.3. Spectra of the Type Ia SN 1998aq in NGC 3982.

Vinkó et al. (1999) measured  the line depth ratio of the Si II transitions at 5800 A and 6150 A,  R = 0.22 +/- 0.02 based on the first DDO spectrum. This parameter correlates with the intrinsic luminosity of SNe Ia. From this single epoch measurement  a distance of d = 15.1 +/- 4.4 Mpc has been calculated. The Cepheid distance to the host galaxy is d = 21 +/- 3 Mpc (Stetson & Gibson 2001; Saha et al. 2001; Riess et al. 2005).

SN 1999by (NGC 2841)

SN 1999by was discovered independently by R. Arbour, E. Berkó and LOSS on April 28.8, 1999. It was observed at DDO by Stefan Mochnacki, Laszlo L. Kiss and Jim Thomson on May 11, 14 and 21, 1999. The three spectra are plotted in Fig.4 (left panel). This SN is turned out to be a subluminous Type Ia event, similar to SN 1991bg, which is characterized by the relatively strong absorption through at 5800 A, beside the deep Si II line at 6150 A (Vinkó et al., 2001; Garnavich et al. 2004).

Fig.4. Left panel: Spectra of SN 1999by obtained at DDO. Right panel: The optical image of SN 1999by in NGC 2841.

The right panel in Fig.4 shows the location of SN 1999by in NGC 2841. This color-composite image (red: Cousins I, green: Johnson V, blue: Johnson B) was made by Róbert Szabó with the 1-m RCC telescope of Konkoly Observatory, Hungary on May 21, 1999. The red lines point to the SN.

SN 2002ap (M 74 = NGC 628)

This SN was discovered by Y.Hirose on Jan. 29, 2002, well before maximum. Between Feb.2 and Feb. 25, 2002, 7 spectra were collected at DDO by Mel Blake, Jim Thomson, Heide DeBond and Stefan Mochnacki with the Cassegrain spectrograph, applying the new 100H grating. This new low-dispersion grating enabled us to get high S/N low-resolution spectra covering the whole visible spectral range (4000 - 8000 A). SN 2002ap turned out to be a very interesting event, sometimes called "hypernova". Its first DDO spectrum is plotted in Fig.5 (left panel) with some spectral features identified. The term "hypernova" refers to the very broad spectral lines, implying expansion velocities as high as 30,000 km/s. Many of such SNe are connected with GRBs, such as SN 1998bw/GRB 980425. In spite of that, SN 2002ap was associated with no recorded GRB.

Fig. 5. Left panel: DDO spectrum of SN 2002ap taken on Feb.2, 2002. Right panel: The position of SN 2002ap in M74 (imaged by K. Sárneczky).

The right panel in Fig. 5 shows SN 2002ap in M74 near maximum light. It is apparent that the SN is very blue compared with some nearby field stars. This image was obtained by K. Sárneczky with the 90-cm Schmidt telescope at Konkoly Observatory.

Detailed analysis of this SN has been published by Vinkó et al. (2004) based on the data obtained at DDO and other observatories. The spectral evolution of this peculiar Type Ic SN is summarized in Fig.6, where the DDO spectra (red), and spectra from Wise Observatory (green; Gal-Yam et al., 2002) are plotted for comparison. Such SNe are thought to originate from the explosion of a Wolf-Rayet star, practically a stripped stellar core that lost most of its H-rich envelope prior to explosion. This is why there are no hydrogen lines in the spectra. The very broad lines may be due to an asymmetric explosion, where the high-velocity jet is pointing nearly to the line-of-sight, producing much higher velocities than in ordinary SNe.

Fig. 6.
Spectral evolution of SN 2002ap from data obtained at DDO (red) and Wise Obs. (green).

SN 2004dj (NGC 2403)

This bright, nearby SN was discovered by Itagaki on July 31, 2004. Soon after discovery, several spectra were obtained at DDO, starting from Aug. 17, 2004. Unfortunately, this Type II-P SN was discovered ~50 days after explosion, so the early-time spectra could not been studied. However, it was the brightest SN observed since SN 1987A, so its spectral evolution could be followed well into the nebular phase. The observations at DDO were continuing up to Nov. 14, 2004, ~140 days after explosion. The observers were Heide DeBond, Jason Grunhut, Jim Thomson, Stefan Mochnacki and Tuba Koktay.

In Fig.7 (left panel) the spectrum obtained on Aug.17, 2004 is plotted together with the identified spectral features. This is a textbook example of a Type II-P SN, a supergiant star with M > 8 solar mass, exploding after the gravitational collapse of its iron core (the letter "P" in the type refers to a constant luminosity "plateau" in the light curve). The most prominent lines are due to the Balmer-series of hydrogen, and several other metallic lines (mostly Fe II) are present. The right panel shows the position of the SN in one of the outer spiral arms of NGC 2403 (this image was made by K. Sárneczky with the 90-cm Schmidt telescope of Konkoly Obs. on Aug.8, 2004).

Left panel: Photospheric-phase spectrum of SN 2004dj observed at DDO on Aug.17, 2004. Right panel: SN 2004dj in NGC 2403 (from Konkoly Obs.).

In Fig.8 the last DDO spectrum (made on Nov.14, 2004) is plotted. This is a typical late-time spectrum of a SN that cooled and became optically thin due to the expansion. The spectrum is dominated by bright emission lines of H-alpha, Na D and several other other permitted and forbidden transitions of metals, mostly Fe.

Fig.8. A nebular spectrum of SN 2004dj obtained at DDO.

Combining the DDO spectra with photometric data from other observatories, we derived the distance to the host galaxy NGC 2403 as d = 3.5 Mpc by applying the Expanding Photosphere Method. Details of this analysis can be found in Vinkó et al. (2006).

This SN is particularly interesting, because its progenitor star was a member of a young compact stellar cluster, called Sandage-96, within NGC 2403. Pre-explosion photometry of this cluster and the analysis of its spectral energy distribution allowed us to determine the age of the cluster, providing tight constraint on the mass of the exploding star. The cluster age turned out to be very young, between 8 - 20 Myrs, resulting in the mass of the progenitor star M ~ 20 solar masses. This may be the most massive SN progenitor measured up to now.

SN 2007gr (NGC 1058)

This was the first SN brightening up to 13 mag in 2007. It was discovered by LOSS (CBET 1034) on Aug.15.51, 2007. Because of two bright foreground stars very nearby (~2 arcsec), the observation of this SN is difficult. Nevertheless, Heide DeBond and  Danny Wallbank took three spectra on Aug.16 and Aug 17, just after discovery! Meantime, SN 2007gr was classified as aType Ib/c by Chornock et al. (CBET 1036) caught very shortly after explosion. Later, this classification was refined by Crockett et al. (2007) as a peculiar Type Ic.

Fig. 9 (left panel) shows the similarity between the spectrum of SN 2007gr (Aug.17, 2007, DDO) and SN 2004aw (Taubenberger et al. 2006), another peculiar Type Ic. Similar spectral features appear in both spectra, although the line shapes are slightly different. Hydrogen is absent, so does Helium, that's why the type is Ic. Si II is present, but weak. There is a narrow emission near 6570 A that can be attributed to H-alpha from an underlying H II area (W. Li, personal communication). The spectral appearance of SN 2007gr suggests a very early photospheric phase of a Type Ic SN.

Fig. 9. Left panel: Optical spectra of SN 2007gr (DDO) and SN 2004aw (Taubenberger et al. 2006).
Right panel: Pre-explosion image of NGC 1058 taken with HST WFPC2 (P.I. S.Smartt; Crockett et al. 2007). SN 2007gr occured within the encircled region.

The right panel of Fig.9 displays the host galaxy, NGC 1058, imaged with the HST WFPC2 camera on July 3, 2001 (P.I. S.Smartt), downloaded from the HST archive at Canadian Astronomy Data Centre (CADC). SN 2007gr occured within the encircled region (between the two bright field stars) containing several resolved point sources. These are young massive stellar clusters (Crockett et al. 2007). The monitoring of SN 2007gr combined with pre-explosion data could provide unprecedented information on the progenitor and the explosion of Ic SNe. Collecting more spectra of this SN at DDO is underway.


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