Investigation of the Progenitors of the Type Ia Supernovae Associated With the LMC Supernova Remnants 0505-67.9 and 0509-68.7

Although Type Ia supernovae have been heavily scrutinized due to their use in making cosmological distance estimates, we are still unable to definitively identify the progenitors for the entire population. While answers have been presented for certain specific systems, a complete solution remains elusive. We present observations of two supernova remnants (SNRs) in the Large Magellanic Cloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center of the remnant and the 99.73% containment central region in which any companion star left over after the supernova must be located. Both remnants have a number of potential ex-companion stars near their centers; all possible single and double degenerate progenitor models remain viable for these two supernovae. Future observations may be able to identify the true ex-companions for both remnants.Comment: 8 pages, 4 figures, 4 tables, ApJ In Press; Table 2 truncated, full version available in published paper or directly from author

Identifying and Quantifying Recurrent Novae Masquerading as Classical Novae

Recurrent novae (RNe) are cataclysmic variables with two or more nova eruptions within a century. Classical novae (CNe) are similar systems with only one such eruption. Many of the so-called 'CNe' are actually RNe for which only one eruption has been discovered. Since RNe are candidate Type Ia supernova progenitors, it is important to know whether there are enough in our galaxy to provide the supernova rate, and therefore to know how many RNe are masquerading as CNe. To quantify this, we collected all available information on the light curves and spectra of a Galactic, time-limited sample of 237 CNe and the 10 known RNe, as well as exhaustive discovery efficiency records. We recognize RNe as having (a) outburst amplitude smaller than 14.5 - 4.5 * log(t_3), (b) orbital period >0.6 days, (c) infrared colors of J-H > 0.7 mag and H-K > 0.1 mag, (d) FWHM of H-alpha > 2000 km/s, (e) high excitation lines, such as Fe X or He II near peak, (f) eruption light curves with a plateau, and (g) white dwarf mass greater than 1.2 M_solar. Using these criteria, we identify V1721 Aql, DE Cir, CP Cru, KT Eri, V838 Her, V2672 Oph, V4160 Sgr, V4643 Sgr, V4739 Sgr, and V477 Sct as strong RN candidates. We evaluate the RN fraction amongst the known CNe using three methods to get 24% +/- 4%, 12% +/- 3%, and 35% +/- 3%. With roughly a quarter of the 394 known Galactic novae actually being RNe, there should be approximately a hundred such systems masquerading as CNe.Comment: 3 figures, 7 tables, accepted for publication in Ap

Recurrent novae and Type Ia supernova progenitors

We investigated two types of stellar explosions, recurrent novae (RNe) and Type Ia supernovae (SNe Ia). SNe Ia are the most useful distance markers in astrophysics, but we do not know the identity of their progenitor systems. RNe are good progenitor candidates that consist of a white dwarf (WD) that accretes material from a companion star. The material builds on the surface of the WD until a runaway thermonuclear eruption is triggered, which ejects the accreted material and causes the system to brighten dramatically. We studied the demographics of the nova population and concluded that approximately 25% of classical novae are actually RNe for which only one eruption has been discovered. Importantly, this means that there are enough RNe in our galaxy to provide a significant fraction of the SNe Ia. We present a list of good RN candidates; for one such system, V2487 Ophiuchi, we sought and found a previous eruption in the astronomical plate archives. We examined two known RNe in detail. T Pyxidis has a unique shell; we used observations of the shell and central star to produce a new model for the long-term evolution of the system, which will never become a supernova. U Scorpii erupted in 2010 as predicted. We led a worldwide collaboration of astronomers that discovered the eruption and comprehensively observed it from start to finish. We discovered three new phenomena and were able to make the best-yet measurement of the amount of mass ejected during the eruption. We searched the centers of nearby SN Ia remnants looking for ex-companion stars left behind after the WD exploded centuries ago. For one remnant, SNR 0509-67.5, we can definitively state that there are no ex-companion stars in the center of the remnant and therefore the system must have consisted of two WDs that collided to form the SN Ia. The other nearby remnants have possible ex-companion stars; more observations are needed to determine which, if any, are the true ex-companions. Some large fraction of the SNe Ia must come from double-WD systems, but there is a possibility that RNe provide a significant fraction as well

The Diffuse Source at the Center of LMC SNR 0509-67.5 is a Background Galaxy at z = 0.031

Type Ia supernovae (SNe Ia) are well-known for their use in the measurement of cosmological distances, but our continuing lack of concrete knowledge about their progenitor stars is both a matter of debate and a source of systematic error. In our attempts to answer this question, we presented unambiguous evidence that LMC SNR 0509-67.5, the remnant of an SN Ia that exploded in the Large Magellanic Cloud 400 +/- 50 years ago, did not have any point sources (stars) near the site of the original supernova explosion, from which we concluded that this particular supernova must have had a progenitor system consisting of two white dwarfs (Schaefer & Pagnotta 2012). There is, however, evidence of nebulosity near the center of the remnant, which could have been left over detritus from the less massive WD, or could have been a background galaxy unrelated to the supernova explosion. We obtained long-slit spectra of the central nebulous region using GMOS on Gemini South to determine which of these two possibilities is correct. The spectra show H-alpha emission at a redshift of z = 0.031, which implies that the nebulosity in the center of LMC SNR 0509-67.5 is a background galaxy, unrelated to the supernova.Comment: 2 figures, accepted for publication in Ap

On the progenitor of SNR 0509-67.5

We have used three independent methods to determine an accurate and precise geometric center of SNR 0509-67.5, at RA = 05:09:31.208, DEC = -67:31:17.48 (J2000). This supernova, which occurred approximately 400 years ago in the Large Magellanic Cloud, was confirmed to be a Type Ia by Rest et al. (2005), Rest et al. (2008) based on spectra of a light echo from the eruption. If this supernova had a single-degenerate progenitor system, we would see the leftover companion star within a certain distance of the remnant\u27s center. Accounting for an offset due to enhanced ISM in the west-southwest quadrant of the remnant, we find the eruption position to be at RA = 05:09:30.976, DEC = -67:31:17.90; the error circle which should contain any possible ex-companion star has a radius of 1.60″ for 99.73% (3-sigma) containment. This accounts for the proper motion of the stars, the possibility of kicks from the supernova, and asymmetries in the explosion and remnant expansion. We find no possible ex-companion stars within this ellipse, to a limiting magnitude of V = 26.9: there are no red giants, which precludes symbiotic progenitors, no subgiants, which when combined with the lack of red giants precludes recurrent nova progenitors, and no main sequence stars with mass greater than 1.16 solar masses (V brighter than 22.7 mag), which precludes persistent supersoft X-ray source progenitors. Indeed, all published SD models are eliminated, so we conclude that this particular Type Ia supernova had a double-degenerate progenitor. Copyright © International Astronomical Union 2013