Anyone Out There? Post-AGB Stars in the Galactic Halo S. Weston, R.Napiwotzki & S. Catalán University of Hertfordshire, UK

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

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

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

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

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

SDSS Spectroscopic Search We only found one candidate!!! SDSS J Classified subdwarf (Eisenstein et al. 2006) Teff = 24581K logg=3.63 l=359.3 b=+50.9 ugrizugriz

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

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

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 B PG = 14.7 Three regions of 1200 square degrees each

Saffer Post-AGB Sample

Saffer Sample Limits T eff limits:14,000 – 34,000K Magnitude limit: B PG <14.7 Coordinate limit: b>=70 OR 315<α<15 0<δ<20 OR 127.5<α< < δ<50

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 Salpeter IMF

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

0.524 M ʘ post-AGB

0.605 M ʘ post-AGB

0.524 M ʘ post-AGB

Normalised and Monte-Carlo simulated post-AGB populations ModelThin Disc post- AGBs Thick Disc post- AGBs Halo post- AGBs Total ±240±4175±11227± ±131±363±2108± ±1 14±116± ±00±117±4 Observed002(?)2

Saffer Post-AGB Sample

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

Vassilidas & Wood (1993)Weiss & Ferguson (2009) ModelMetallicityTotalModelMetallicityTotal ± alpha43± ± solar40± ± alpha36± ± alpha4±1

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

M32 UV CMD Figure taken from Brown (2004)‏

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

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.

Any Questions? Gay

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

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

Need More Accuracy!!! Photometry Teff and logg of central star Distances using magnitudes Distances from reddening (using 3D dust maps) Spectroscopy Properties of central stars (Teff, logg, metallicity) Distances using central star Use sample as photometric check

Lifetime - ~10 4 yrs WD formation rate - 2.3× pc -3 yr -1 (Weidemann, 1991) 1.0±0.25× pc -3 yr -1 (Liebert et al, 2005) CSPN Formation Rates and Evolutionary Time-scales 10, ,000 PNe in Milky Way PNe formation rate – 3.0× pc -3 yr -1 (Pottasch, 1996) 5.1±1.0× pc -3 yr -1 (Cahn & Wyatt, 1976) 8.0× pc -3 yr -1 (Ishida & Weinberger, 1987) 1.1±0.5× pc -3 yr -1 (Moe & De Marco, 2006)

SDSS and GALEX Filters

SDSS Spectroscopic Search LEGACY Sample

SEGUE Sample

Calibration FUVNUVu’ Mean σ mean g’i’z’ Mean σ mean

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

0.605 M ʘ post-AGB

Extinction Distances Using 3D Dust Maps

M r -t kin Relation & Evolutionary tracks PN G θ = 170" V exp. ~20km/s Dist pc Mass M ʘ

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

PNe Birthrate 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

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

SDSS Spectroscopic Search LEGACY Sample

SDSS Reliable Photometry  (SDSS) Some spectra for sanity check  SDSS has spectroscopic follow-up

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

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)

Calibration 10,000K 20,000K 50,000K 2.00cms cms cms -2 Increase logg Increase Teff 30,000K

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

Future Work Produce paper based on comparisons with Saffer sample. 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.