Hubble Space Telescope Cycle 11 General Observer Proposal Searching for the Progenitors of Subluminous Type Ia Supernovae with SN 2013bc Principal Investigator: Institution: Electronic mail: Maximilian Katz Stony Brook University USA/NY maxpkatz@gmail.com Scientific category: HOT STARS Scientific keywords: SPECTROSCOPY, SUPERNOVAE Instruments: STIS Proprietary period: 12 Cycle 11 primary orbits: 2 Cycle 11 parallel orbits: 0 Abstract Despite the success of Type Ia supernovae in discovering the accelerated expansion of the universe, their progenitors are still uncertain. Recent observations and theory suggest that systems with either one or two white dwarfs may contribute to the observed explosion rate. Spectroscopic observations of these systems can discriminate between progenitors. At late times, the spectrum reveals the content of the inner part of the progenitor, which will look very different depending on whether the progenitor is a single white dwarf or the simultaneous detonation of two white dwarfs. I propose STIS CCD spectroscopic observations of SN 2013bc, a subluminous Type Ia supernova in IC 4209 at z = 0.022 that was first detected in November 2013. Early observations suggest that it is similar to the subluminous SN 1991bg. These subluminous supernovae are the most promising candidates for the double white dwarf progenitor, and observations of the nebular spectra can be used to test the various hydrodynamic explosion models. Of particular interest is the recent discovery of the [O I] 6300 Å, 6364 Å line in the subluminous SN 2010lp. This strongly disfavors the classical near-chandrasekhar mass white dwarf progenitor but can be explained by a double white dwarf progenitor. Observations of similar systems should continue to provide evidence that can be used to determine the relative contributions of the progenitor channels.
Investigator Institution Country PI: Maximilian Katz Stony Brook University USA/NY Total number of investigators: 1 Observing Summary: Configuration,mode,aperture Total Target RA DEC V spectral elements orbits Flags SN-2013BC 13 10 21-07 10 24.1 17.9 STIS/CCD 2 SPECTROSCOPIC G750L, G430L Grand total orbit request 2 2
Scientific Justification Type Ia supernovae (SNe) are among the brightest transient events in the universe. They are widely used as cosmological distance indicators ( standardizable candles ) due to the uniformity of their light curves, and in this role were used to discover the accelerated expansion of the universe in the 1990s [8, 6]. Early on in the work, it was discovered that while most of the typical SNe Ia peak around absolute magnitude M V 19, there is a small but distinct variation in the peak brightness. The variation is strongly connected to the width of the lightcurve; generally speaking, brighter events take longer to decay [7]. This one-parameter Phillips relation is largely attributed to the specifics of the explosion mechanism. A SN Ia is thought to be the thermonuclear explosion of a white dwarf (WD), converting carbon and oxygen to silicon-group (IME) and iron-group (IGE) elements [2]. The brightness of the light curve is set by the amount of nickel produced in the explosion, as this radioactively decays to iron over the span of months [10]. The amount of IGEs produced also largely determines the opacity of the expanding fireball, thereby also controlling the time evolution of the light curve. While this general description is supported by a variety of observations, there is considerable debate about the mechanism that initiates the explosion. In particular, in the classical single degenerate (SD) model, a WD accretes mass from a main sequence or giant progenitor until it approaches the Chandrasekhar mass and ignites a thermonuclear runaway near the center of the star. This propagates throughout the star, possibly transitioning into a detonation that unbinds the entire star on the timescale of about one second. In the double degenerate (DD) model, a binary white dwarf system coalesces through the emission of gravitational waves, and the rapid accretion of one star (the secondary) onto the other (the primary) triggers a detonation in the primary [11, 3]. There are also variations on these mechanisms such as the double detonation model, where a helium detonation on the surface of a carbon-oxygen WD propagates a shock wave into the center of the star, triggering a carbon detonation. At present, no progenitor system has been directly detected, leaving us to use indirect means. Recent observations of both the supernova remnants and the host environments suggest that all of these channels are likely to contribute to the observed SN Ia rate, though the relative contributions may differ for different sub-classes of SNe Ia [5]. Determining the nature of the progenitor system is important for cosmological distance measurements: a primary contributor to the remaining uncertainty in the inferred distances is the variation due to the differences in progenitor classes. One class that warrants special attention is the subluminous SNe Ia. These are events that are significantly dimmer than the standard events used for cosmological measurements, and may contribute as much as 20% of the total number of Type Ia supernovae. The prototype of this class is SN 1991bg. Theoretical considerations disfavor the SD model, which ignites a Chandrasekhar mass of material and thus produces too many IGEs. The double degenerate or double detonation mechanisms can more naturally explain a sub-chandrasekhar mass explosion. To study the nature of the progenitor, spectroscopic observations are typically employed. At late times (approximately more than one month after the peak), the 3
outer regions of the supernova become optically thin and a nebular spectrum is measured. This penetrates to the inner regions of the newly-formed remnant. Nebular spectra of SNe Ia are dominated by forbidden transitions of iron, indicating that the central regions of these systems are mostly burned to IGEs. This is expected in all of the favored models. However, many other facets of the nebular spectra (such as geometrical asymmetry) can be used to constrain the nature of the progenitor. Recently, narrow [O I] lines at 6300 Å, 6364 Å were detected for the first time in the nebular spectrum of the subluminous SN 2010lp [9]. Neutral oxygen lines are often seen at early times after the explosion, indicating that the outer layers of the white dwarf are not completely burned. However, narrow oxygen emission lines in the nebular spectrum are an indication that unburned oxygen is present near the center of the progenitor. This is difficult to reconcile with the SD or double detonation models, but fits naturally with the DD model [4]. The central part of the secondary may be largely unburned in the latter, which would explain the presence of oxygen at the center. The prospect of confirming the DD model as a viable progenitor model warrants further observations of subluminous supernovae. SN 2013bc was detected by the Catalina Real-Time Transient Survey on March 31, 2013 and classified as a young Type Ia supernova shortly after by the Public ESO Spectroscopic Survey for Transient Objects (PESSTO) [1]. The early-time spectrum is a good match with the prototype of the subluminous SNe, SN 1991bg. It is located shortly outside IC 4209, a galaxy at redshift z = 0.022, and peaked around magnitude 17.5 in the IR. It is a prime target for follow-up observations of the nebular spectrum as it is relatively close and a member of a peculiar sub-class. STIS can be used to obtain a full optical spectrum and identify the prominent spectral lines. Detection of the neutral oxygen lines would strengthen support for the double degenerate model, but there are a number of other lines that can be used to infer the nature of the progenitor as well. After identification of the lines, the spectrum would be compared to the synthetic spectra generated from hydrodynamic explosion models. SN 2010lp was favorably compared to the violent merger of a 0.9 M and 0.76 M white dwarf binary system, and the same approach would be used here to infer the progenitor set-up. In the era of precision cosmology, it is increasingly important to understand the systematic uncertainties associated with cosmological distance indicators. Ascertaining the explosion mechanism of Type Ia supernova would play a key role in driving down these uncertainties and therefore increasing the precision in our measurements of quantities such as the Hubble constant and deceleration parameter. Additionally, understanding whether the double degenerate or single degenerate models predominate would give insight into the evolution of supernova properties with the age of the universe, as the DD model is expected to be associated with older stellar populations and the SD model with younger stellar populations. As type Ia supernovae play a crucial role in many areas of astrophysics, such as cosmology, galactic evolution, and star formation, it is important to identify their progenitor systems. 4
References [1] Hachingerm S. et al., PESSTO spectroscopic classification of optical transients, The Astronomer s Telegram #4940 (2013) [2] Hoyle, F., and Fowler, W. A., Nucleosynthesis in Supernovae, The Astrophysical Journal (1960), 132, 565 [3] Iben, I., Jr. and Tutukov, A. V., Supernovae of type I as end products of the evolution of binaries with components of moderate initial mass (M not greater than about 9 solar masses), The Astrophysical Journal Supplement Series (1984), 54, 335 [4] Kromer, M. et al., SN 2010lpa Type Ia Supernova from a Violent Merger of Two Carbon-Oxygen White Dwarfs, The Astrophysical Journal Letters (2013), 778, 18 [5] Maoz, D., and Mannucci, F., Type-Ia Supernova Rates and the Progenitor Problem: A Review, Publications of the Astronomical Society of Australia (2012), 29, 447 [6] Perlmutter, S. et al., Measurements of Omega and Lambda from 42 High-Redshift Supernovae, The Astrophysical Journal, 517, 565 [7] Phillips, M. M., The absolute magnitudes of Type IA supernovae, The Astrophysical Journal (1993), 413, L105 [8] Riess, A. G. et al., Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, The Astronomical Journal (1998), 116, 1009 [9] Taubenberger, S. et al., [O I] 6300, 6364 in the Nebular Spectrum of a Subluminous Type Ia Supernova, The Astrophysical Journal Letters (2013), 775, 43 [10] Truran, J. W., Arnett, W. D., and Cameron, A. G. W., Nucleosynthesis in supernova shock waves, Canadian Journal of Physics (1967), 45, 2315 [11] Webbink, R. F., Double white dwarfs as progenitors of R Coronae Borealis stars and Type I supernovae, The Astrophysical Journal (1984), 277, 355 Description of the Observations I propose two observations with the STIS CCD in spectroscopic mode, to obtain widerange spectral measurements of the target. Observation with the G750L and G430L grating will allow a wide range of the optical and near IR spectrum to be obtained, including the portion covering the neutral oxygen line. The target, SN 2013bc, was observed with the European Southern Observatory s Faint Object Spectrograph and Camera (EFOSC2) at La Silla, Chile in April 2013. This is mounted onto the New Technology Telescope (NTT), with 3.58m diameter. This was a 900s exposure that achieved a signal-to-noise ratio of 5
approximately 25. As the HST primary has a 2.4m diameter, the light collected will be approximately 45% that of the NTT, but the spectral resolution of STIS in this range is 5-10x higher than EFOSC2 in the relevant wavelength range. A single orbit is limited to approximately 54 minutes of observing time, leaving approximately 45 minutes for exposure time after target acquisition and overhead costs if a single exposure is done. At the same magnitude, a lower limit SNR of 65 would be obtained for a 20 minute exposure with each of the gratings. After accounting for the improved seeing and background, the SNR would be even higher. However, at the time of the observation the event will be significantly fainter. A SN Ia will decay in brightness in the V band at approximately half of a magnitude per month at late times. This means that the observations should be done as early in the cycle as possible to achieve the maximum possible brightness. For example, at eight months after the peak a SNR 10 is achievable before accounting for the improved background; this is a conservative estimate. Therefore I propose two orbits; each will be dedicated to a separate grating. Special Requirements Coordinated Observations Justify Duplications Previous HST Programs 6