The SWARMS Survey: Unveiling the Galactic population of Compact White Dwarf Binaries Weizmann Institute of Science / Tel-Aviv University Type Ia SN Progenitors Workshop Leiden, September 22 2010 Steve Bickerton (Princeton) Fergal Mullally, Susan Thompson (NASA/Ames) Mukremin Kilic (CfA), Tom Matheson (NOAO) Dan Maoz (TAU)
DD SN Ia Progenitors The merging of binary WDs due to gravitational wave emission was suggested as SN Ia progenitor scenario in the 1980s [Webbink 84 ApJ 277, 355; Iben & Tutukov 84 ApJS 54, 335]. The only scenario that naturally explains the lack of H in SN Ia spectra. K. Thorne T. Carnahan Can produce both short and long delay times [Yungelson & Livio 99, ApJ 528, 108]. Caveat: it is unclear whether WD mergers lead to SN Ia explosions or end in accretion induced collapse [Saio & Nomoto 85 A&A 150, 21] ignition if dm/dt>mcrit (~10-5 M yr-1). Depends on the final accretion phase multi-d simulations [Rosswog talk]. Might not need M1+M2=MCh [van Kerkwijk et al. 10, arxiv:1006.4391 ]. Loren-Aguilar et al. 09 A&A 500, 1193 2
Where are the DD SN Ia Progenitors? [Nelemans talk]: Theoretical uncertainties are large. Observations are crucial. Are there enough WD+WD systems with M1+M2 > MCh & tmerge < thubble? Not a single such system is known (a few claims have been disproved). SPY Survey [Napiwotzki et al. 01 AN 322, 411]: ~1000 known WDs with B<16.5 using ESO VLT. SPY found ~100 DD systems [Napiwotzki et al. 04 ASP Conf. Ser. 318, 402], masses and periods only published for 26 systems [Nelemans et al. 05 A&A 440, 1087]. Of these, only 5 pre-mergers. Now we can do better... 3
The SWARMS Survey SWARMS Sloan White dwarf Radial velocity data Mining Survey Strategy: Use the multiple exposures ( 3, ~15 min each) taken for all the spectra in the Sloan SN 1994D in Digital Sky Survey (SDSS) to look for radial velocity 4526 (P. Challis) (RV) shiftsngc in White Dwarfs ~15,000 WDs in the SDSS DR7 catalog [Eisenstein et al. 06, ApJS 167, 40 (E06); Kleinman et al. 10 in prep.]. Goals: Find the double degenerate WD (DDWD) progenitors of Type Ia SNe (WD binaries with MA+MB MCh and tmerge<thubble ) Characterize the population of (pre-merging) compact white dwarf binaries in the Galaxy binary fraction, mass ratio distribution, separation distribution... merger rates. First results: Badenes et al. 09, ApJ 707, 971; Mullally et al. 09, ApJL 707, L51. 4
Time Resolved Spectroscopy in SDSS SDSS was not designed to do timeresolved spectroscopy; this is a data mining project. Among the DR4 WDs, ~50% have 3 exposures and ~20% have 4. The maximum timescale that can be probed is below 6 hr in ~70% of the objects, but can go to weeks or longer in ~10% of the objects. Pre-merging DDWDs Expected RV shifts are large (~100 km s-1) and can be measured using SDSS. Excellent training and complement for present and forthcoming synoptic surveys (PTF, PanSTARRS, LSST). 5
SDSS 1257+5428: Discovery Classified as DA by E06; Mg=16.8. RV shift of ~8 Å (~490 km s-1) between exposures 0, 1 (taken 10/03/2003) and 2 (taken 10/04). 6
SDSS 1257+5428: Tight and Massive Follow-up observations: APO ARC 3.5m telescope on 02/2009. RV curve is well fit by a circular orbit with P=4.5550±0.0007 hr; KA=322.7±6.3 km s-1. System must be tight and/or have massive components. 7
SDSS 1257+5428: What is it? We can make a conservative mass estimate from high-order Balmer lines: MA=0.92±0.32 M Together with the orbital parameters, this yields: MBsin(i)=1.62±0.25 M The companion is too massive to be another WD probably a neutron star or black hole. At 29 D 58 pc, closest stellar remnant of a SN explosion. tcool=2.0±1.0 Gyr old system! A WD companion cannot be discarded by the data. 8
SDSS 1257+5428: WD+WD Binary! Companion is a WD [Marsh et al. 10 arxiv:1002.4677] 120 spectra (followed since 2008). Cold, low-mass WD (line cores) + Hot, rotating high mass WD (line wings) no double core! Effect is subtle, spectral 'disentangling' is very challenging. Marsh et al. 10, arxiv:1002.4677 Marsh et al. 10, arxiv:1002.4677 9
SDSS 1257+5428:... of some kind Will the system lead to a Type Ia explosion? Probably not (mass ratio is too low) 5 HST orbits in Cycle 18. Rotation (~1 rpm) implies accretion!! Keck spectra [Kulkarni & van Kerkwijk 10, ApJ 719, 1123] Magnetic WD? Marsh et al. 10, arxiv:1002.4677 Kulkarni & van Kerkwijk 10, ApJ 719, 1123 10
SDSS 1436+5010 Classified as DA by E06; Mg=18.2. RV shifts can be detected between consecutive SDSS exposures! 11
SDSS 1053+5200 Classified as DA by E06; Mg=18.9. Cross-correlation works well for noisy spectra. 12
The Two Shortest Period DDWDs SDSS 1436+5010 and 1053+5200 are the two detached DDWDs with the shortest periods known (1.15 and 0.96 hr). Their potential binarity had ben noted previously from their low masses (~0.19 M ) [Kilic et al. 07, ApJ 660, 1451]. Mullally et al. 09 ApJ 707, L51; Kilic et al. 10, ApJ 716, 122. SDSS 1436+5010 SDSS 1053+5200 13
SWARMS vs SPY SWARMS complements and improves on the SPY survey [Napiwotzki et al. 01, Ast. Nach. 322, 411]. No control over original observations luck is involved and completeness is hard to establish. Follow-up observations of individual candidates continue (4 nights on Kitt Peak 4m telescope in October). We anticipate more exciting discoveries, so stay tuned! SPY SWARMS S (Paranal, Chile) N (APO, NM) ~1,000 ~15,000 (DR7) 18,500 (~2 km s-1) 1,800 (~120 km s-1 w/cc) Limiting mag. B 16.5 g 19 Best suited for Systematic study of DDWDs Finding pre-mergers and SN Ia progenitors Location Nobjects Resolution 14
The SWARMS Treasure Chest: DDWD 15
The SWARMS Treasure Chest: DAH+M 16
The SWARMS Treasure Chest: DA+M 17
Characterizing the DDWD Population Beyond the identification of specific SN Ia progenitor candidates, SWARMS can be used to statistically characterize the DDWD population in the Milky Way. Distribution of RV shifts in the ~15,000 WDs Monte Carlo simulations of DDWDs with different period and mass ratio distributions (see also [Maxted & Marsh 99, MNRAS 307, 122]). Selection effects and observational biases have to be dealt with carefully. K. Thorne T. Carnahan Not binaries End result: rate of WD mergers as a function of mass. E R P Confirmed binaries Y R A N I M I L 18
Conclusions SWARMS is well placed to find MCh DD SN Ia progenitors (if they exist) in particular, and pre-merger WD binaries in general. Multi-year project, fun for all the family. SDSS 1257+5428 is definitely an interesting DD WD, but its nature (and final fate) have not been fully established yet. SDSS 1436+5010 and SDSS 1053+5200 are the two detached DDWDs with the shortest periods known. Individual system follow-up will continue. Great prospects to statistically characterize the DDWD population in the disk of the Milky Way. Stay tuned for more developments! 19
The Future: Spectral Variability in SDSS The techniques developed for SWARMS have a wider applicability time resolved spectroscopy with the largest spectroscopic data base! Probe temporal scales between a few hours and a few months: chromospheric M-dwarf variability, sdb binaries, close binaries across all spectral types, quasar variability... Excellent training for future missions and surveys like Gaia (RVS) or LSST: data mining techniques, spectral characterization of transients, etc. Largest data base for time resolved spectroscopy 20
SDSS Variability: CV? 21
SDSS Variability: Diverse stuff Hα (at z=0.18) Hα (at z=0.18) 22