Outline

1. Anyone Out There?Post-AGB Stars in the Galactic HaloS. Weston, R.Napiwotzki & S. CatalánUniversity of Hertfordshire, UK

2. Outline • Post-AGB overview • Spectroscopic search for post-AGB • Observed post-AGB population • Simulated post-AGB population • Conclusions

3. Low/Intermediate Mass Stellar Evolution • Low/Intermediate mass => 0.8 - 8 M⊙ • Mass loss during thermal pulse phases Figure using Blöcker(1995) data of a 3M⊙ star

4. How many post-AGBs are known? Torun catalogue Szczerba et al. (2007) Version 2.0 • 391 Very likely post-AGB objects • Few with halo implied galactic coordinates • 26 post-AGBs with |b|>30

5. How many Post-AGBs are expected in the Halo? • Drilling & Schönberner (1985) estimate that 97-99.8% of stars that evolve off the MS become post-AGBs. • 13 Gyr population – 0.8 Mʘ still on turn-off • IMF peaks at low masses (~0.6Mʘ) • Luminous, so can be observed up to 10kpc away In short, many!

6. Search for post-AGB stars and CSPN in complete SDSS DR7 spectroscopic sample Balmer line fitted all the SDSS spectra of blue objects

7. SDSS Spectroscopic Search We only found one candidate!!! • SDSS J145817.52+022806.6 Classified subdwarf (Eisenstein et al. 2006) Teff = 24581K logg=3.63 • l=359.3 b=+50.9 • ugriz - 18.51 18.58 18.96 19.23 19.61

8. SDSS Spectroscopic Search Possible selection bias? • Low priority as not extragalactic • Photometry for selection not unique • Some too bright - saturation

9. Where are all of these post-AGBs? • The next steps: • Complete SDSS photometric search • GALEX cross match where available • Look at another smaller but complete survey

10. Palomar-Green • UV Excess Stellar Object Catalogue • Photographic 10,000 square degree survey Saffer et al. (1997) complete sub-sample from PG • Complete for post-AGBs to BPG = 14.7 • Three regions of 1200 square degrees each

11. Saffer Post-AGB Sample

12. Saffer Post-AGB Sample

13. Saffer Sample Limits Teff limits:14,000 – 34,000K Magnitude limit: BPG<14.7 Coordinate limit: b>=70 OR 315<α<15 0<δ<20 OR 127.5<α<157.5 -10< δ<50

14. Simulated post-AGB populations • Monte Carlo simulation of thin disc, thick disc and halo stars (Napiwotzki 2009) • Given initial number of stars • Stars distributed randomly based on standard model of Galactic structure (Robin et al. 2003) • Stars are created with initial masses drawn from a SalpeterIMF

15. Simulated post-AGB populations • Metallicities derived from literature relations for each population • Detailed simulation of stars evolved to tip of AGB phase. (Padova group) • Post-AGB evolution from Schönberner (1983) & Blöcker (1995) tracks • Calibrated with observed WD population density Holberg et al. (2008) and normalised

16. 0.524 Mʘ post-AGB

17. 0.605 Mʘ post-AGB

18. 0.524 Mʘ post-AGB

19. Normalised and Monte-Carlo simulated post-AGB populations

20. Saffer Post-AGB Sample

21. Does metallicity have an affect? • Post-AGB tracks of Schönberner (1983) & Blöcker (1995) are solar metallicity • Vassiliadis & Wood (1993 & 1994) produce tracks with, Z= 0.016 (solar) 0.008(LMC), 0.004(SMC), 0.001 • Weiss & Ferguson (2009) recent tracks which also cover halo metallicity (Z=0.0005)

23. Has this been observed before? YES! M32 • Deep HST imaging with STIS (Brown et al. 2008) • Nearby elliptical galaxy • Metal-rich – solar to 0.3 solar metallicity • Significant hot HB population found • Age ~13Gyr

24. M32 UV CMD Figure taken from Brown (2004)‏

25. Low/Intermediate Mass Stellar Evolution • HB – core He burning • P-AGB – thermal pulses, mass loss • P-EAGB – no thermal pulses • AGB-Manqué – no He shell burning Figure taken from Dorman et al. (1993)

26. Conclusions • SDSS highly suggestive of a lack of post-AGBs • PG shows a real dearth in observations compared to population synthesis simulations • Stellar evolution for low masses and/or metallicities incorrect? • Significant fraction of older populations evolve through the EHB • Leading to a dominant AGB-manqué channel for low mass stars.

27. Any Questions? Gay

28. Why observe Post-AGBs and determine their birthrates? • Compared to WD birthrates to determine evolutionary channel preference (EHB/pAGB) • Direct study of pAGB evolutionary phase • Explain PN shaping and formation scenario

29. Why are few Post-AGBs known? • Short-lived phase of evolution • Star often shrouded by its own circumstellar shell or ejected nebula • Photometric colours similar to many objects • High resolution spectroscopy needed to confidently confirm classification

30. Need More Accuracy!!!

31. CSPN Formation Rates and Evolutionary Time-scales • Lifetime - ~104yrs • WD formation rate - 2.3×10-12 pc-3 yr-1(Weidemann, 1991)1.0±0.25×10-12pc-3 yr-1(Liebert et al, 2005) • 10,000-140,000 PNe in Milky Way • PNe formation rate – 3.0×10-12 pc-3 yr-1(Pottasch, 1996)5.1±1.0×10-12 pc-3 yr-1(Cahn & Wyatt, 1976)8.0×10-12 pc-3 yr-1(Ishida & Weinberger, 1987)1.1±0.5×10-12 pc-3 yr-1(Moe & De Marco, 2006)

32. SDSS and GALEX Filters

33. LEGACYSample SDSS Spectroscopic Search

34. SEGUE Sample

35. Calibration

36. Calibration • Used r’ magnitude as main calibrator • Calibrated other colours with respect r’ • Used WDs as initial Calibration • Calibration checked with post-AGB/CSPN • Only one standard CSPN in SDSS • Use SDSS Spectra to verify

37. 0.605 Mʘ post-AGB

38. Extinction Distances Using 3D Dust Maps

39. Mr-tkin Relation & Evolutionary tracks PN G148.4+57.0 • θ = 170" • Vexp. ~20km/s • Dist. - 200-1000pc • Mass - 0.6-1.0Mʘ

40. Starting point • Initially minimum reddening and crowding • Large survey area • Reliable photometry • Some spectra for sanity check

41. PNeBirthrate Aims WHY? Binary/single star scenario • Find the PN • Determine distance to each PN • Calculate a space density of PNe • With lifetime approximation, calculate birthrate Identify central star for known PNe within field

42. Finding Central Stars in Known PNe • Initially minimum reddening and crowding • SDSS (NGP) • Large Survey Area • 11,663 sq. deg. (SDSS)

43. LEGACYSample SDSS Spectroscopic Search

44. SDSS • Reliable Photometry • 0.01-0.04 (SDSS) • Some spectra for sanity check • SDSS has spectroscopic follow-up

45. Locating the CSPN One object has all SDSS colours consistent with a central star.

46. Need More Accuracy!!! • GALEX - 25,000 sq. deg. (All Sky UV survey) • Broadband photometry • UV -> Optical -> IR • 153 (FUV), 230 (NUV), 354(u’), 475(g’), 622(r’), 763(i’), 905(z’) • Need our own calibration (Weston et al. 2009, proceedings)

47. Calibration 10,000K 6.00cms-2 4.00cms-2 2.00cms-2 Increase logg Increase Teff 20,000K 30,000K 50,000K

48. Locate CSPN and post-AGBs • We can locate CSPN from the field of a known PN • With photometric calibration, we can determine atmospheric parameters using grid • Same grid can be used for post-AGB stars • We should be able to observe many halo post-AGB stars, some may have PN around them

49. Future Work • Produce paper based on comparisons with Saffersample. • Complete SDSS photometric calibration and carry out complete post-AGB photometric search • Apply to CSPN in SDSS • Write up and submit thesis • Publish photometric calibration work, if time.