On the M31 Nova Progenitor Population
aa r X i v : . [ a s t r o - ph . S R ] M a r STELLA NOVAE: FUTURE AND PAST DECADESASP Conference Series, Vol. **Volume Number**P. A. Woudt and V. A. R. M. Ribeiro, eds c (cid:13) On the M31 Nova Progenitor Population
S. C. Williams , M. J. Darnley , M. F. Bode , and A. W. Shafter Astrophysics Research Institute, Liverpool John Moores University, TwelveQuays House, Egerton Wharf, Birkenhead, CH41 1LD, UK Department of Astronomy, San Diego State University, San Diego, CA 92182,USA
Abstract.
We present a survey of M31 novae in quiescence. This is the first cat-alogue of extragalactic systems in quiescence and contains 37 spectroscopically con-firmed novae from 2006 to 2013. We used Liverpool Telescope and Faulkes Tele-scope North images taken during outburst to identify accurate positions for each sys-tem. These positions were then transformed to archival
Hubble Space Telescope (HST) images and we performed photometry on any resolvable source that was consistent withthe transformed positions. As red giants in M31 will be resolvable in the
HST images,we can detect systems with red giant secondaries. There are only a few confirmed ex-amples of such systems in our Galaxy (e.g. RS Oph and T CrB). However, we find amuch higher portion of the nova population in M31 may contain red giant secondaries.For some novae, coincident
HST images had been taken when the nova was still fad-ing, allowing us to produce light curves that go fainter than is possible to achieve formost extragalactic systems. Finally, we compare the M31 and Galactic quiescent novapopulations.
1. Introduction
Amongst the hundreds of known Galactic classical novae (CNe), there are only tenconfirmed recurrent novae (RNe; Schaefer 2010). Traditionally, RNe have only beenconfirmed by the observation of more than one outburst from the same system. Usingmultiple outbursts as the sole discriminator between recurrent and classical novae issubject to a number of significant selection e ff ects, as such the Galactic population often RNe is a lower limit and quite likely far from the true picture. Recently, moreindirect methods have been employed to introduce a small number of new candidateRNe in systems with only one recorded outburst (see paper by M. J. Darnley, theseproceedings).The Sun’s position in the Milky Way significantly hinders any attempt to study thenova population of the Galaxy. As such, traditionally, we turn to M31 which, whilstfar from ideal, provides us with the best opportunity to study the nova population ofan entire galaxy. With a nova rate of 65 + − yr − (Darnley et al. 2006), M31 presentsus with a potential sample size twice that of the Milky Way and far beyond the ∼ ff ects. Nonetheless, there have been a number of attempts to explore the RN popula-tion of M31 by searching for multiple outbursts, but these have only yielded a handfulof RN candidates (see paper by A. W. Shafter, these proceedings). This approach isfurther hampered by the misidentification of long period Mira variables as M31 novae(see, for example Darnley et al. 2004).When studying M31 novae, a number of the above problems can be overcome byusing a spectroscopically confirmed sample, with well determined astrometry, but thisessentially limits us to M31 novae since around 2006 (Shafter et al. 2011). Based onthe recurrence timescales of their Galactic counterparts, such a short baseline is notlong enough to recover a good sample of RNe using multiple outbursts alone.Here we propose a technique that can be used to recover the progenitor systems ofnovae containing red giant secondaries - those belonging to the RG-nova class (whichis dominated by confirmed and candidate RNe of the RS Oph sub-class, Darnley et al.2012). In some (exceptional) cases this technique may also be able to recover novaewith sub-giant secondaries (SG-novae; a class dominated by U Sco-like RNe) as wasrecently achieved for the (albeit significantly closer) LMC recurrent nova LMC 2009a(Bode et al., in prep).
2. Observations
A number of space-based and the larger ground-based optical telescopes are capableof resolving the red giant population of M31. As such, the progenitor systems of RG-novae can in principle be directly imaged. This was successfully carried out for the RNcandidate M31N 2007-12b (see Bode et al. 2009, on which this work is largely based).This technique relies upon accurate registration between images of the nova in outburstand deeper (typically archival) high spatial resolution images when the system is in qui-escence (either post- or pre-outburst). For this study, we use data taken by the LiverpoolTelescope (LT) and one of its sister telescopes, Faulkes Telescope North (FTN), to de-termine the outburst position of the novae and archival
Hubble Space Telescope (HST) data for the identification and photometry of the progenitor. The
HST observations aretaken from a mixture of the WFPC2, ACS / WFC and WFC3 / UVIS instruments, all ofwhich provide a very good overlap with the LT / FTN fields. A full description of theprogenitor recovery technique is given in Williams & Darnley et al. (in prep) and issummarised in Bode et al. (2009).
3. Progenitor Results
From a spectroscopically confirmed sample of 108 M31 nova from 2006 onward (mostfrom Shafter et al. 2011), 37 have accurate outburst astrometry from LT / FTN data cou-pled with quiescent archival
HST imaging. Initial analysis shows that of these 37, theposition of at least 23 novae are coincident (at the 3 σ level, or better) with at least oneresolved source in the archival data. Of this subset, a Monte-Carlo analysis indicatesthat 10 of these alignments would be expected to occur through chance less than 5%of the time. Figure 1 presents thumbnail HST images centred around four of the bet-ter progenitor candidates; M31N 2007-02b, 2007-10a, 2007-11d and 2009-11d all ofwhich have alignments ≤ σ and coincidence probabilities < NE1’’ NE1’’1’’ NE NE 1’’
Figure 1.
HST
ACS / WFC images of the ∼ ′′ × ′′ region surrounding the novae(from top-left, clockwise) M31N 2007-02b, 2007-10a, 2009-11d and 2007-11d. Theinner green circle indicates the 1 σ radius search region for the progenitor, the outercircle the 3 σ region, and the red × indicates the position of progenitor candidates. Williams &Darnley etal.Figure 2 shows an “M31” colour-magnitude diagram and presents the photome-try of six M31 nova progenitor candidates. Four of these systems are associated withthe red giant branch and would likely be classified as RG-novae. However, two ofthese systems are coincident with the quiescent positions of the SG-novae U Sco andV2491 Cyg, which implies that they may also be SG-novae. It is worth noting that theRG-nova candidate KT Eri has particularly large B -band emission and is also coincidentwith this pair of systems.The observed Galactic RG-nova population is ∼ −
3% that of the Galactic novapopulation (Darnley et al. 2012), our initial results for M31 indicate that as many as10 from a sample of 37 spectroscopically confimed novae are associated with a redgiant secondary. This indicates at first sight a M31 RG-nova population of ∼
30% theunderlying nova population, significantly higher than has been observed Galactically.
4. Extended Light-Curve Coverage
In a small number of chance cases, archival
HST data have been taken shortly followingthe outburst of M31 novae. As such this has allowed us to follow the evolution of M31novae to previously unprecedented deepness. In Figure 3 we present the light curve ofM31N 2009-08a (originally presented by Shafter et al. 2011). This very slow nova wasoriginally followed by a combination of telescopes (including LT / FTN) through ∼ HST
ACS / WFC archival data have allowed us tofollow this nova to ∼ References
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Figure 2. Colour-magnitude diagram showing stars from the
Hipparcos data set(Perryman & ESA 1997) moved to the position of M31 assuming ( m − M ) = . E B − V = . ⊙ solar-like star, the solid line a 1.4 M ⊙ solar-like star (Pietrinferni et al. 2004). Williams &Darnley etal.
Figure 3. Light curve of M31N 2009-08a, ground-based data from Shafter et al.(2011). The uncertainties in the photometric measurements are shown as verticalbars with the following colours representing the di ff erent band passes: B , blue; V ,green; R , dark grey; r ′ , red; i ′ , black; z ′ , light gray; HST
ACS //