Peremennye Zvezdy

Peremennye Zvezdy (Variable Stars) 34, No. 4, 2014

Received 1 December; accepted 10 December.

Article in PDF

Explosion of an LBV star in the galaxy UGC 8246

E.A. Barsukova1, V.P. Goranskij2, A.F. Valeev1,3, and S.S. Kaisin1

Special Astrophysical Observatory, Russian Academy of Science, Nizhnij Arkhyz, Karachai-Cherkesia, 369167, Russia; e-mail: bars@sao.ru

Sternberg Astronomical Institute, Lomonosov Moscow University, Universitetski pr., 13, Moscow, 199992, Russia

Kazan Federal University, Kremlevskaya 18, Kazan, 420008, Russia


We present the results of spectroscopy and CCD photometry of the intermediate-luminosity red transient PSN J13100734+3410514 in the galaxy UGC 8246 performed in February and June 2014 with the Russian 6-meter telescope and SCORPIO spectral camera. Our CCD photometry was continued with the Special Astrophysical Observatory's 1-m telescope till November 2014. The star was discovered in late December 2013 at visual brightness 176, which corresponded to the absolute magnitude , and was identified as a supernova impostor. Spectra taken at the visual brightness level of 195 show composite triple profiles in the H and H emission lines. We explain the main component of the profiles as radiation from a photoionized extensive gaseous envelope formed by the stellar wind of the progenitor before the outburst. The other two components are treated as radiation from bipolar ejecta. In Balmer line profiles, there is an evidence for a light echo propagating in the surrounding medium after the outburst. Our spectra contain emissions of He I, Na I, Mg I, numerous Fe II emissions, the strongest of which have P Cyg profiles. There are also [O II], [O III], and [S II] emissions of an H II region associated with the transient. The emissions of the region are superimposed on the star spectrum. The light curves show rapid decline and color reddening. Our observations confirm that the UGC 8246 transient was an explosion of a high-mass LBV star.



1. INTRODUCTION

Discoveries of a large number of supernovae using networks of automatic robotic telescopes and preliminary classification of their spectra fill the maximum-luminosity gap between classical novae and supernovae. The optical transients reaching in their outbursts the absolute visual magnitude from 8 to 17 and evolving their spectra to cooler subtypes during outbursts are called Intermediate-Luminosity Red Transients (ILRTs). This family of astrophysical objects is heterogeneous and contains luminous blue variable (LBV) stars with giant eruptions ( Car, low-luminosity SNe 1961V and 1954J with spectra resembling SNe IIn in outbursts), calcium transients like SN 2008S in NGC 6496 or NGC 300 OT having super-AGB stars as progenitors, and luminous red novae like V1006/7 in M31, V838 Mon in the Galaxy, and M85 OT2006-1. Van Dyk et al. (2000) called lower-luminosity SNe IIn "supernova impostors", but later this term was extended to other transients with peak luminosities in the gap (Berger et al. 2009; Smith et al. 2009).

Some LBVs and Galactic red novae turned out to be binaries or multiple systems. The most promising scenario for luminous red novae as merging events in binary systems was suggested by Tylenda & Soker (2006). Tylenda et al. (2011) observed a merging event in the contact binary before the outburst of V1309 Sco as a red nova. On the other hand, Martini et al. (1999) suggested that a red nova phenomenon is due to a nuclear event in a single star, in which a slow shock drives the photosphere outward. Barsukova et al. (2014) specify that this is the phenomenon connected with the adiabatic expansion of the star envelope after an episodic energy release in the star center. The release may be due to merging of stellar nuclei inside a massive common envelope or to instability of a single stellar nucleus of a young star.  Car, the LBV which experienced a Great Eruption in the mid-1800s accompanied by an episodic mass-loss event, is known as a binary (Corcoran & Ishibashi 2012). The companion is not seen in the spectrum, but it is found to be an O4-O6 giant having mass of 40-50  based on indirect evidence (Mehner et al., 2010). RXTE X-ray observations show cyclic variability with the 2024-day period that may be the signature of the star's motion in an elliptic orbit with and a semi-major axis  AU (Ishibashi et al. 1999). However, the role of this companion in the evolution of  Car and in the Great Eruption scenario is still unknown. Kashi & Soker (2010) and Kashi (2010) suggest that most outbursts of LBV-type systems are powered by gravitational energy released from either a vigorous mass transfer process from the evolved primary star to a main sequence secondary star or a merger of two stars. So, major LBV eruptions can be triggered by stellar companions, and in extreme cases, a short-duration event with a huge mass transfer rate can lead to a bright transient event on time scales of weeks to months. Alternative hypotheses explain LBV eruptions either with enhanced stellar wind episodes when a massive star luminosity sometimes exceeds the Eddington limit, or with interior explosions due to instabilities in stellar nuclei or envelopes that may be pulsational instabilities (Owocki & Shaviv 2012).

Note that one of LBVs in the galaxy NGC 7259, SN 2009ip, previously went through two typical impostor outbursts that reached in maxima and then became a true core-collapse supernova of -18th absolute magnitude in the peak of brightness (Mauerhan et al. 2013; Margutti et al. 2014).

Fig. 1. A color image of the UGC 8246 transient PSN J13100734+3410514 composed from BTA/SCORPIO , , and frames. The image size is . The comparison star is labeled "std". The straight lines marked 1, 2, and 3 are directions of the slit of BTA spectra.

A recent discovery of an LBV-type transient as a possible supernova in UGC 8246 on 2013 December 20.93 UT was reported by Wang and Gao (2013). The object had the  magnitude of approximately 17.75. UGC 8246 is an SB(s)cd type galaxy, its redshift is and distance,  Mpc; the Galactic interstellar absorption is (NED). With these parameters, the absolute magnitude of the transient at the time of discovery was . Additional photometry acquired in 2013 December was published by Elenin, Wang & Gao, Luppi & Buzzi (CBAT) and by Brimacombe (2014). -band observations by Wang & Gao, continued during 10 days, show no essential light decay, which strongly suggests that their observations cover the brightness maximum. However, this suggestion may be incorrect in the case of plateau shape of the light curve. Tartaglia et al. (2014a) carried out spectroscopic observations and classified the object as a supernova impostor similar to the prototypical SN 1997bs. SN 1997bs was classified by Van Dyk et al. (2000) as a superoutburst of a very massive luminous blue variable star, analogous to  Car. In the spectrum of the UGC 8246 transient taken on January 8, 2014, Tartaglia et al. (2014a) found H emission with an unresolved narrow component superimposed on broader wings (FWHM of about 1800 km s). Taking into account scientific interest to SN impostors, we continued spectroscopic and photometric observations of this object.

2. OBSERVATIONS AND DATA REDUCTION

We obtained medium-resolution optical spectra of the transient PSN J13100734+3410514 in UGC 8246 with the SCORPIO focal reducer (Afanasiev & Moiseev 2005) mounted on the 6-m BTA telescope of the Special Astrophysical Observatory (SAO) on 2014 February 8 and June 7. Seeing was measured as on February 8 and on June 7. Additional photometry was performed using the SAO 1-m Zeiss telescope and a CCD photometer with an EEV42-40 chip on 2014 April 3, November 14 and 22. November observations were carried out with the and filters at good weather conditions with seeing, and the total exposure times were 1200 s or longer.

In the photometric mode with the SCORPIO, we obtained CCD frames with standard , and Cousins filters. The color image generated using these frames is shown in Fig. 1. It demonstrates that the transient occurred in a spiral arm rich in star-forming regions.

To perform relative photometry, we used the comparison star marked "std" in Fig. 1, its SDSS magnitudes being . We transferred SDSS magnitudes to magnitudes by interpolation using and AB magnitudes of Lyr as described in our previous paper (Barsukova et al. 2012). As a result, the magnitudes of the standard star were derived as (18.95, 17.97, 17.25, 16.49). All currently available photometry is presented in Table 1. The light curves plotted using our data and all published observations in the and filters are shown in Fig. 2.

Fig. 2. and light curves of the transient PSN J13100734+3410514 in UGC 8246. Dates of BTA/SCORPIO spectroscopy are marked "sp".

On February 8, 2014, the spectroscopic camera functioned in the long-slit mode. First it was equipped with the VPHG 1200G grism (nominal spectral range  Å, resolution 5 Å, dispersion 0.88 Å pixel), and later it was replaced with the VPHG 550G grism (spectral range  Å, resolution 10 Å, dispersion 2.1 Å pixel). Actual resolution measured using emission lines of night sky was 5.4 Å for the VPHG 1200G grism and 14.6 Å for VPHG 550G. In the first spectrum, we found an essential contribution of a nearby galactic H II region in the brightest lines of the stellar spectrum, so we took spectra with different position angles of the slit. Slit locations at different telescope pointings are shown as straight lines in Fig. 1 and they are listed in Table 2 for individual spectra. On June 7, 2014, we used only the VPHG 550G grism. Spectra were reduced using standard ESO MIDAS procedures for the long-slit mode. Basic parameters of our spectra are collected in Table 2. The blue and green spectral regions are displayed in Fig. 3, and the whole optical spectra are presented in Fig. 4. The wavelengths in these figures have been corrected with respect to bright emission components of Balmer lines.

Fig. 3. BTA/SCORPIO spectra of the UGC 8246 transient in the blue and green regions on February 8, 2014. Both spectra are corrected for the redshift and presented for zero velocity. Top: the higher resolution spectrum; bottom: the same fragment of the lower resolution spectrum.

Fig. 4. A comparison of lower resolution spectra of the UGC 8246 transient taken with BTA/SCORPIO on February 8 and June 7, 2014. Both spectra are corrected for the redshift.

Fig. 5. Two-dimensional BTA/SCORPIO spectrograms of the UGC 8246 transient in the  Å wavelength range for the three slit locations. The numbers correspond to slit locations displayed in Fig. 1. [O III] emissions evidently belong to a close star-forming H II region, and the transient is located at its edge. At the same time, the photos show that the radiation of the blue component belongs just to the star, but the main component is contaminated by radiation of the star-forming region which has the same radial velocity as the star.

3. RESULTS

Analyzing published early non-homogeneous observations presented in Table 1, one can suggest that the star had a color index about zero at the outburst maximum. We do not know the interstellar reddening in the galaxy UGC 8246 and therefore can determine only the upper limit to the bolometric magnitude ( ) and the lower limit to the effective temperature ( ). Our February photometry taken 50 days after the maximum gave the following magnitudes and color indices: , , . Reddening continued in June; on the 169th day after the light maximum, was 086. In November, on the 336th day at the level of , the star became even redder, with . The star had a primary decay of in 50 days and a nonlinear secondary decay both in the and bands. By the 169th day after maximum, the speed of the decay decreased to about 0 in 100 days.

The spectrum of the UGC 8246 transient taken in February was rich in emission lines. It resembled spectra of Fe II-class novae in the fireball stage (e.g., see Williams 2012; we used the finding list of optical emission lines in his Table 2 for identification). The lines we have identified are listed in Table 3. The strongest emission lines in our spectra are Balmer lines, [O II], and [O III]. Oxygen lines are evidently radiated in the nearby H II regions in the spiral arm of the galaxy. As seen in Fig. 5, the regions of [O III] emission are superimposed on the stellar spectrum only partly. Otherwise, this figure shows that the main component of H emission mostly has a stellar origin. But oxygen lines are located at the same radial velocity, +920 km s, as the strongest H component. Direct images taken in the filter in November confirm the association of the transient with a compact H II region, the brightest part of which is located in 1 0 east and 0 5 south of the transient. These spectra and the direct images definitely provide evidence that the explosion of this transient has happened in the active star forming region rich in young massive stars.

The equivalent widths of Balmer lines on February 8 were the following:  Å;  Å;  Å;  Å. H, in a blend with Ca II H, form a deep absorption line. On June 7, the equivalent widths changed to -350 Å (H), -41 Å (H), and -7 Å (H).

With the best resolution of 5.4 Å, the H line profile looks complex (Fig. 6). The strongest narrow component has  km s after correction for the spectral resolution. The profile extends over  km s. Additionally, there is another weak blue narrow emission component in the profile, displaced by  km s with respect to the main component and having approximately the same half width, 320 km s. A dip between these components is at the velocity of -380 km s