The Yellow Supergiant Progenitor of the Type II Supernova 2011dh in M51

The Yellow Supergiant Progenitor of the Type II Supernova 2011dh in M51

J. R. Maund1,2,3, M. Fraser4, M. Ergon5, A. Pastorello6, S.J. Smartt4, J. Sollerman5, S. Benetti6, M.-T. Botticella6, F. Bufano7, I.J. Danziger8, R. Kotak4, L. Magill4,9, A.W. Stephens10, S. Valenti6
Abstract

We present the detection of the progenitor of the Type II SN 2011dh in archival pre-explosion Hubble Space Telescope images. Using post-explosion Adaptive Optics imaging with Gemini NIRI+ALTAIR, the position of the SN in the pre-explosion images was determined to within mas. The progenitor object was found to be consistent with a F8 supergiant star ( and ). Through comparison with stellar evolution tracks, this corresponds to a single star at the end of core C-burning with an initial mass of . The possibility of the progenitor source being a cluster is rejected, on the basis of: 1) the source is not spatially extended; 2) the absence of excess emission; and 3) the poor fit to synthetic cluster SEDs. It is unclear if a binary companion is contributing to the observed SED, although given the excellent correspondence of the observed photometry to a single star SED we suggest the companion does not contribute significantly. Early photometric and spectroscopic observations show fast evolution similar to the transitional Type IIb SN 2008ax, and suggest that a large amount of the progenitor’s hydrogen envelope was removed before explosion.

supernovae:general – supernovae:individual (2011dh)

,

1 Introduction

The search for the progenitors of core-collapse supernovae (CCSNe) has now become an integral part of the study and understanding of nearby supernovae (SNe). The last decade has shown that the global archives of high resolution images of nearby galaxies can provide definitive detections of progenitor stars on images of galaxies gathered before explosion (for a review, see Smartt 2009). The luminosity and inferred mass of progenitors give an insight into the origin of the SN types, explosion mechanisms and the last stages of stellar evolution.
The most common type by volume, the Type II Plateau (IIP) SNe (Li et al. 2011a) have been shown to originate from red supergiants (Van Dyk et al. 2003; Smartt et al. 2004; Maund et al. 2005; Li et al. 2006). The disappearance of these stars several years after explosion is further reassurance that they were indeed the stellar progenitors (Maund & Smartt 2009). However the low and high mass end of the distribution of progenitor masses are providing some unexpected results. The lack of high mass stellar progenitors is becoming statistically significant (Smartt et al. 2009), and the question of what SNe these high mass stars () produce is critical to understanding stellar evolution, black hole and neutron star formation and galactic chemical evolution. While a number of high-mass progenitors have been associated with Type IIn SNe (Gal-Yam & Leonard 2009; Smith et al. 2011), the frequency of high-mass progenitor detection is not high enough to comfortably match the expected numbers from initial mass function considerations (Smartt et al. 2009). At the low luminosity end, faint progenitors have been frequently detected (Maund et al. 2005; Li et al. 2006; Mattila et al. 2008; Fraser et al. 2010a). They are often associated with the sub-luminous, low kinetic energy and Ni-poor SNe which resemble SNe 2005cs and 1997D (Pastorello et al. 2009). The obvious interpretation is that these are low mass stars () producing explosions of low energy ergs.
Several observers independently reported discoveries of SN 2011dh in M51 (Reiland et al. 2011), with the earliest recorded epoch being May 31.954, at a position . Silverman et al. (2011) and Yamanaka et al. (2011) classified it as a young Type II SN. The Whirlpool galaxy has now hosted three modern SNe (1994I, 2005cs and 2011dh), and the Hubble Heritage images (Mutchler et al. 2005) provide deep multi-color images for a progenitor star search, as already done for SN 2005cs (Maund et al. 2005; Li et al. 2006). This letter reports the discovery of the progenitor of SN 2011dh using Gemini North NIRI+ALTAIR Adaptive Optics (AO) images of the SN and archival pre-discovery optical images. We adopt a distance of 7.1 Mpc (Takáts & Vinkó 2006), and a recessional velocity of from NED111http://ned.ipac.caltech.edu/. The foreground reddening towards M51 as quoted by NED, after Schlegel et al. (1998), is . Using the metallicity gradient derived by Bresolin et al. (2004) for H ii regions in M51, we adopt a metallicity at the SN position of .

2 The Progenitor Star

The site of SN 2011dh was observed, prior to explosion, with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) and the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) instruments, on 2005 Nov 13 and 2005 Jan 20-21. ACS observations were conducted with the , , and filters (with observations with each filter being composed of four dithered exposures) of total duration 2720, 1360, 2720 and 1360s, respectively. A WFPC2 observation from 2005 Nov 13 used the filter, composed of two equal exposures (1300s). Additional WFPC2 observations of the site, acquired on 2001 Jun 9, used the , and filters with total exposure times of 2000s per filter. For both sets of WFPC2 observations, the site of the SN fell on the WF2 chip with pixel scale 0.1.

The HST data were retrieved from the HST archive222http://archive.stsci.edu/hst/search.php. The ACS images were combined using the multidrizzle task running under PyRAF to correct for the geometric distortion of the ACS WFC chip and improve sampling of the point spread function (PSF), resulting in a final pixel scale of . Photometry of these images was conducted using DAOphot, with PSFs derived from the data themselves. Aperture corrections were derived to correct the photometry to an aperture of radius , with the additional correction to an infinite aperture using the tabulated values of Sirianni et al. (2005). No correction was applied for charge transfer inefficiency in the drizzled images. In parallel, the four distorted images for each filter were processed separately using the DOLphot package333http://americano.dolphinsim.com/dolphot/, which utilises pre-computed PSFs and does include corrections for charge transfer inefficiency. The photometry from these two techniques was found to agree to within the photometric errors. The WFPC2 images were processed and analysed using the HSTphot package (Dolphin 2000).

We observed SN 2011dh in the -filter with NIRI and the ALTAIR AO system on the Gemini North Telescope on 2011 Jun 6. The f/32 camera was used, which gives 0.022pixels over a 2222field of view. As the SN was bright, it was used as a natural guide star for ALTAIR. Separate off-source frames were taken to remove the sky background. The data were reduced using the IRAF gemini niri package. The final image has a coadded exposure time of 3000s, and a full-width at half-maximum of 0.2 (after binning by 2 pixels in both the x and y directions).

Geometric transformations between the post-explosion NIRI image and the pre-explosion HST ACS and WFPC2 images were calculated with the IRAF task geomap. 18 stars were identified in both the pre-explosion ACS image and post-explosion NIRI image, resulting in a transformation with rms error of 23mas. We measured the position of the SN in the NIRI image using the three different centering algorithms in phot. The standard deviation of the three measurements is 2 mas. Transforming the coordinates of the SN as measured in the NIRI image to the ACS image, we find the SN position to be coincident with a bright, compact source as shown on Figure 1 which we denote Source A. This progenitor candidate is located 3mas from the transformed position of the SN (within the uncertainties of the transformation).

In the WFPC2 pre-explosion image, the progenitor was detected at with (in flight system Vega magnitudes). In the drizzled ACS WFC images, the progenitor was measured to have , , and . The 2001 WFPC2 , and images give similar magnitudes, approximately magnitudes brighter than the ACS images (to be expected due to the blending of the progenitor with a source to the south). There is no evidence for any variability in the progenitor’s brightness before explosion. At the adopted distance to M51, after correction for foreground reddening, this implies .

To determine if Source A, observed at the SN position in the pre-explosion frames, was a cluster or a stellar object we employed the ishape package (Larsen 1999) to probe its spatial extent. Due to the low S/N nature of the detection of the source in the pre-explosion WFPC2 F336W image, as well as the subsampled nature of the PSF in this image, we only analysed the pre-explosion ACS/WFC images. After subtracting nearby, well-resolved stars from within a radius of 0.75(or 1.5 times the radius of the calculated PSF), ishape was run on the images with both delta and Moffat (order 1.5) functions convolved with the PSF. ishape was permitted to recalculate the coordinates of the source and the source was permitted to be elliptical. In all cases, the shape of Source A was found to be consistent with an unresolved, point source.

The observed photometry was compared with synthetic photometry for model spectral energy distributions (SEDs) for single stars. We used chorizos (Maíz-Apellániz 2004), an SED fitting package, with ATLAS synthetic spectra (Kurucz 1993)444CHORIZOS and the SED library were obtained from http://jmaiz.iaa.es/software/chorizos/chorizos.html assuming a solar metallicity and a Cardelli et al. (1989) reddening law. The observed SED is shown on Fig. 2. The parameters derived for Source A are , and with . The chorizos analysis was tested using a reddening-free index , which was selected due to its monotonic nature from 4000 to 8000K and being single valued for . The temperature and the associated uncertainties calculated from the reddening-free index, using the same ATLAS spectra, were identical to those calculated using chorizos. In addition, similar stellar parameters were calculated using MARCS synthetic spectra (Gustafsson et al. 2008).

The photometry was also compared with the synthetic SEDs of clusters generated using Starburst99 (Leitherer et al. 1999) yielding with . This age is significantly higher than the age of clusters observed around the SN location (; Scheepmaker et al. 2009). This age is also inconsistent with the lifetimes of massive stars that are expected to end their lives as SNe. A further deficiency in interpreting the observed SED as that of a cluster is the absence of excess, with the observed brightness in being consistent with stellar continuum (see Fig. 2).

We compared the and colors of the progenitor candidate to those of the observed population of Wolf-Rayet (WR) stars in M31 (Massey et al. 2006). All data have been corrected for foreground Milky Way extinction. As can be seen in Fig. 3, the progenitor color is not consistent with a WR star. Note that the progenitor colors are in the HST flight system, which for the , and ACS filters are consistent (to within our photometric uncertainties) with the Johnson-Cousins filters. In the case of the -band, however, the difference between the filter and the Johnson is non-negligible. Using synthetic photometry of Potsdam WR model SEDs (Gräfener et al. 2002), we find a color difference of mags. Applying this to the progenitor, the color increases to 1.5 mags, which makes the discrepancy between with the WR population even more marked.

Given the point-like nature and colors of Source A, we conclude that Source A is consistent with a F8 supergiant star with zero intrinsic reddening. Utilising a bolometric correction of () and a color correction Johnson , derived from ATLAS spectra, we infer a luminosity for Source A of . The location of the progenitor object on the Hertzsprung-Russell (HR) diagram is shown on Fig. 4 and compared with stars stellar evolution tracks (Eldridge & Tout 2004). In deriving the mass estimate, we use the final luminosities for progenitor stars at the end of core C-burning (see Smartt et al. 2009, and their Fig. 1) which yields an initial mass of . The object we have called Source A is very likely to be the same source identified by Li et al. (2011b), however they derive an initial mass of .

  Fig. 1.—: Pre-explosion observations of the site of SN 2011dh. Each panel has dimensions , and is oriented such that North is up and East is left. The progenitor candidate is denoted Source A and a nearby red star is denoted as Source B. From left to right, the panels are: Pre-explosion WFPC2 WF2 image, pre-explosion ACS WFC F555W image, pre-explosion ACS WFC image, and pre-explosion ACS WFC image.
  Fig. 2.—: The observed SED of the progenitor of SN 2011dh, as measured from pre-explosion HST WFPC2 () and ACS/WFC () images. An ATLAS synthetic spectrum for a star with and is shown in grey.
  Fig. 3.—: The observed and colors of the progenitors of SNe 2011dh () and 1993J (), compared with the colors of WR stars () in M33 (Massey et al. 2006). All colors have been corrected for foreground reddening.
  Fig. 4.—: Hertzsprung-Russell diagram showing the luminosities and temperatures of the progenitors of SNe 2011dh (), 1993J (; Maund et al. 2004; Aldering et al. 1994), 2008cn (; Elias-Rosa et al. 2009), and 2009kr (; Fraser et al. 2010b; Elias-Rosa et al. 2010). Overlaid are stars stellar evolution tracks for solar (red solid) and LMC (blue dashed) metallicities. At the end of each track the corresponding initial mass is indicated.

3 The early characteristics of SN 2011dh

  Fig. 5.—: Left panel) Pseudo-bolometric UBVRI lightcurves for SN 2011dh and comparison SNe calculated as described in Fraser et al. (2010a). Middle panel) Optical spectra for SN 2011dh and comparison SNe at 5d. Right panel) Optical spectroscopic evolution for SN 2011dh. To visualise the temporal evolution the spectra have been aligned to the time axis at the right border of the panel. Telluric lines are indicated with a symbol. All spectra have been corrected for redshift as given by NED. All spectra and photometry have been corrected for extinction using the extinction law of Cardelli et al. (1989) and =3.1. The phase is given in days relative to an assumed explosion date of 2011 May 31.5, estimated from reported detections and non-detections (Reiland et al. 2011).

Soon after the announcement of the SN discovery, a wide European collaboration started a monitoring campaign using several telescopes available to the collaboration. The results of the complete follow-up campaign will be published in a forthcoming paper (Ergon et al. in prep.). Here we present data obtained during the first 10 days after the explosion.

All images were bias, flat-field and overscan corrected. The SN photometry was measured using PSF-fitting and calibrated with reference to several stars of the stellar sequence presented in Pastorello et al. (2009). Spectroscopic data of SN 2011dh have been processed using standard techniques, using the QUBA pipeline (Valenti et al. 2011), and spectral fluxes were checked against the coeval photometry.

Our data are presented in Figure 5, where we compare SN 2011dh to the Type IIb SNe 2008ax (Pastorello et al. 2008; Taubenberger et al. 2011) and 1993J (Lewis et al. 1994), the peculiar Type IIP SN 1987A (Menzies et al. 1987; Phillips et al. 1988) and the normal Type IIP SN 1999em (Leonard et al. 2002; Baron et al. 2000). The distance and extinction of SN 1987A are from Suntzeff & Bouchet (1990); the explosion epoch and extinction of SN 1999em is from Elmhamdi et al. (2003), while its distance is taken from Leonard et al. (2003).

Contrary to the spectroscopic behavior of the Type IIb SN 1993J, the early-time spectra of SN 2011dh show prominent H lines and relatively weak He I features. The light curve of SN 2011dh, however, has a rise time markedly similar to that of the type IIb SN 2008ax (see also Arcavi et al. 2011). This is evidence against the presence of a massive, extended H envelope. From the available photometry we conclude that SN 2011dh is not a classical Type IIP SN, but further monitoring is necessary to definitely associate SN 2011dh with one of the other Type II subtypes (IIb, IIL, 1987A-like).

Our inference of negligible host galaxy reddening from our study of the SED of the progenitor is supported by the non-detection of NaI D, at the rest-wavelength of M51, in high resolution spectra acquired as part of our monitoring campaign (Ergon et al., in prep.).

4 Discussion & Conclusions

Despite the canonical prediction that Type II SNe arise from Red Supergiants, there is mounting evidence that some stars explode as Yellow Supergiants (YSGs). A handful of Type II SNe have been observed to arise from YSGs: SNe 1993J (Aldering et al. 1994; Maund et al. 2004), 2008cn (Elias-Rosa et al. 2009; Fraser et al., in prep.) and 2009kr (Fraser et al. 2010b; Elias-Rosa et al. 2010). The locations of the progenitors on the HR diagram shows clearly that these stars are not located on the predicted end points for single star stellar evolution tracks. In addition, despite arising from supposedly similar YSG progenitors, these SNe display a wide range of properties. While SN 1993J was a Type IIb SN (with most of its H envelope stripped by mass transfer onto a binary companion), SN 2008cn was a bright Type IIP SN. Elias-Rosa et al. (2010) present evidence that SN 2009kr is a Type IIL SN. While Arcavi et al. (2011) propose a Type IIb classification for SN 2011dh, its early photometric and spectroscopic properties (see Fig. 5) are not identical to SN 1993J.

The classification scheme for Type II SNe (IIP IIL IIb) may be interpreted as being due to increasing mass loss (i.e. stripping of the H envelope) from the progenitor. The effective temperatures derived for the YSG progenitors and the classification of the resulting SNe shows this scheme is not correlated with observed of the progenitors (Fig. 4).

It is clear from our observations that the SED of the progenitor of SN 2011dh is consistent with a late-F supergiant. While possible combinations of two stars in a binary might also be employed to fit the SED, any improvement in the quality of the fit is mitigated by two key factors: 1) in introducing a binary companion, three additional parameters (the temperature and gravity of the secondary and the ratio of the brightness of the two stars at a reference wavelength) are introduced into the fit; and 2) a companion star may not be contributing any measurable flux to observed SED, yielding a single star SED despite an underlying binary system. Unlike the case of SN 1993J, we do not observe a UV excess associated with a possible companion onto which mass from the progenitor of SN 2011dh may have been transferred. We propose that any binary companion is below the detection limit of the pre-explosion observations.

The necessary binary parameters to produce such a YSG progenitor are also unclear. For example, the binary models of Claeys et al. (2011) only produce progenitor stars with and in that extreme case the companion is of similar luminosity to the progenitor (with ) that would produce significant UV excess not observed in these pre-explosion images.

Despite the photometric and spectroscopic similarity to SN 2008ax, there are significant differences between the proposed progenitor scenarios for that SN (Crockett et al. 2008) and SN 2011dh. While Crockett et al. concluded the photometry of the progenitor of SN 2008ax required either a single WR star or two stellar components, our observations of the progenitor of SN 2011dh are consistent with a single normal stellar component.

In isolation, very massive stars can lose significant amounts of mass through stellar winds and eruptions (e.g. as Luminous Blue Variables (LBV) and WR stars). We have demonstrated, however, that the broad-band colors of the observed progenitor of SN 2011dh are inconsistent with those of WR stars. While some caution is required in the application of this analysis to all YSG progenitors of Type II SNe (e.g. the locus of the progenitor of SN 1993J, which was not a WR star (Maund & Smartt 2009), on Fig. 3), that caution is not justified for the progenitor of SN 2011dh. As noted by Fraser et al. (2010b) for SN 2009kr, there is similarly no evidence for previous LBV eruptions of the progenitor, as we do not detect the signatures of eruptive mass loss such as: excess emission in the progenitor photometry or the spectroscopic signature of the SN interacting with a dense circumstellar medium (Fig. 5).

The nature of Source A as a single star or binary will be ultimately confirmed at late-times, once the SN has disappeared and a surviving binary companion, if present, is revealed again (Maund et al. 2004; Maund & Smartt 2009).

Acknowledgements

The research of JRM is funded through the Sophie & Tycho Brahe Fellowship. The Dark Cosmology Centre is supported by the DNRF. Based on observations obtained at the Gemini Observatory (program GN-2011A-Q-22), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership. The data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. Our thanks go to the staff of the 3.58m Telescopio Nazionale Galileo (La Palma, Spain), and to the Asiago 1.82m Telescope (Asiago, Italy). SB and FB are partially supported by the PRIN-INAF 2009 with the project ”Supernovae Variety and Nucleosynthesis Yields”. Based in part on observations obtained with the Liverpool Telescope operated on the island of La Palma (prog. XIL10B01).

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