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 => 0.8 - 8 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 2.0

391 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 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!

Search for post-AGB stars and CSPN in complete SDSS DR7

spectroscopic sampleClass N(total) N(main) N(SEGUE)

All 1,640,960 1,374,080 266,880

Stars (except M) 380,214 150,748 229,466

Blue objects 21,031

Balmer line fitted all the SDSS spectra of blue objects

SDSS Spectroscopic SearchWe 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

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 BPG = 14.7

Three regions of 1200 square degrees each

Saffer Post-AGB Sample

Saffer Post-AGB Sample

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

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 Mʘ post-AGB

0.605 Mʘ post-AGB

0.524 Mʘ post-AGB

Normalised and Monte-Carlo simulated post-AGB

populationsModel Thin Disc

post-AGBsThick Disc post-AGBs

Halo post-AGBs

Total

0.524 12±2 40±4 175±11 227±10

0.546 14±1 31±3 63±2 108±1

0.565 1±1 1±1 14±1 16±3

0.605 0±0 0±1 17±4 17±4

Observed 0 0 2(?) 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= 0.016 (solar) 0.008(LMC), 0.004(SMC), 0.001

Weiss & Ferguson (2009) recent tracks which also cover

halo metallicity (Z=0.0005)

Vassilidas & Wood (1993) Weiss & Ferguson (2009)

Model Metallicity Total Model Metallicity Total

0.569 0.016 111±12 0.534 0.0005alpha 43±5

0.620 0.008 6±1 0.538 0.0005solar 40±4

0.593 0.004 72±6 0.551 0.0005alpha 36±3

0.623 0.001 43±5 0.599 0.0005alpha 4±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 EvolutionHB – core He burningP-AGB – thermal pulses, mass lossP-EAGB – no thermal pulsesAGB-Manqué – 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 - ~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)

CSPN Formation Rates and Evolutionary Time-scales

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)

SDSS and GALEX Filters

SDSS Spectroscopic SearchLEGACY Sample

SEGUE Sample

CalibrationFUV NUV u’

Mean 0.210 0.104 0.048

σmean 0.110 0.095 0.043

g’ i’ z’

Mean 0.004 -0.024 -0.023

σmean 0.027 0.011 0.032

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 Mʘ post-AGB

Extinction Distances Using 3D Dust Maps

Mr-t

kin Relation &

Evolutionary tracksPN G148.4+57.0

θ = 170"Vexp. ~20km/s

Dist. - 200-1000pcMass - 0.6-1.0Mʘ

Starting point Initially minimum reddening and crowding

Large survey area

Reliable photometry

Some spectra for sanity check

PNe Birthrate AimsWHY? 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 SearchLEGACY Sample

SDSS Reliable Photometry

0.01-0.04 (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-2

4.00cms-26.00cms-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.