Post on 25-Dec-2015
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 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
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
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
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.
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)
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
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 Reliable Photometry
0.01-0.04 (SDSS)
Some spectra for sanity check
SDSS has spectroscopic follow-up
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