Download Abundance anomalies in globular cluster (GC) stars

Li Abundance of TO
stars in globular
clusters
Zhixia Shen
Luca Pasquini
The Globular Cluster (GC)
• The same distance, the
same age and [Fe/H]:GCs
are good testbeds for
– stellar evolution
– Nucleosynthesis in old
stars
– Galaxy chemical
evolution
– The age of the universe
Outlines
• Chemical inhomogeneity of GCs
• Li variations of TO stars in GCs
– History
– Our work
Abundance Anomalies in
Globular clusters
• Homogeneous Fe abundance
• Homogeneous n-capture element
abundances
• Light element abundance anomalies
– C-N
– Na-O
– Mg-Al
– etc
Chemical Anomaly of GCs: Fe Group
• Most globular clusters
(GCs) have a very
uniform distribution of Fe
group elements - all the
stars have the same
[Fe/H].
• Several years ago people
believed that this
indicated that the cluster
was well-mixed when the
stars formed
• Now, no the 3rd dredgeup
Kraft, et al., 1992: M3, M13
Chemical Anomaly of GCs: Fe Group
--compared to field stars
Gratton et al., 2004
Chemical Anomaly of GCs: Fe Group
--compared to field stars
Gratton et al., 2004
Chemical Anomaly of GCs: n-capture
elements
Gratton et al., 2004
The C-N & C-L anticorrelation
• Large spread in Carbon and
Nitrogen in many GCs:
• The first negative
correlation
(anticorrelation) : C is
low when N is high.
• The anticorrelation is
explicable in terms of the
CN cycle, where C is burnt
to N14
 The C abundance decreases
with L on the RGB (and N
increases). This is known
as the C-L anticorrelation
 This is also observed in halo field
stars.
Cohen, Briley, & Stetson (2002)
M3, Smith 2002
O-Na Anticorrelation
Gratton et al., 2004
O-Na Anticorrelation
• This is readily explained by hot(ter) hydrogen burning, where
the ON and NeNa chains are operating - the ON reduces O,
while the NeNa increases Na (T ~ 30 million K)
• Where this occurs is still debatable.
• The amazing thing about this abundance trend is that it only
occurs in Globulars - it is not seen in field halo stars
Mg, Al…
• Mg-Al anticorrelation in (some)
GCs.
• This can also be explained
through high-temperature (T~
65 million K) proton capture
nucleosynthesis, via the MgAl
chain (Mg depleted, Al
enhanced).
• It does not occur in field stars...
• The light elements also show
various correlations among
themselves--->
(Kraft, et al, 1997. Giants)
Summary
• All these anticorellations point to hydrogen burning - the CN, ON, MgAl, NeNa cycles/chains -- at
various temperatures.
– CN, ON, NeNa: T~20 MK-40 MK(?)
– MgAl: T~40 MK-65 MK(?)
• Previously, the most popular site* for this is at the
base of the convective envelope in AGB stars - Hot
Bottom Burning
• And now, maybe winds from massive stars (WMS)
Summary
1) Heavy Elements are uniform throughout cluster
No the 3rd dredge-up
2) C and N (only) have been shown (conclusively) to vary
with evolution/luminosity.
Most likely ongoing deep mixing on RGB, but not very
deep mixing.
3) Light elements (C – Al) show spreads to varying
degrees, and are linked through the (anti)correlations.
Spreads are seen in non-evolved stars also.
Inhomogeneous light element pollution; could be
pre-formation: AGB? WMS?
intrinsic stellar pollution (i.e. deep mixing), Non-evolved star?
accretion (Bondi-Hoyle?, binaries?, planets?). Fe? Mass of
accretion material (O depletion to 1/10, 9:1 accretion mass?)?
Subgaints?
Li abundace in globular clusters
• Among the light elements
Li has a special role. Li is
produced in Big Bang
nucleosynthesis,enriched
during the galaxy
evolution,and destroyed
in the stellar interior
– WMAP: A(Li)=2.64
– Li-plaue: 2.1-2.3 (halo stars,
NGC 6397)
– Diffusion or extra-mixing
mechanism
Li abundance of TO stars in
GCs
• Indicator of globular
cluster chemical
evolution history
– The low temperature
for Li depletion (2.5
MK)
– CNO circle: ~30 MK
• TO stars: unevolved
• History
– M 92: can’t be trusted
– NGC 6397: Li abundance is an constant
– NGC 6752: Li-O correlation;Li-Na/N anticorrelation;
– 47 Tuc: Li-Na anti-correlation, lack of
correlation between Li and N.
M 92
•
One of the most metalpoor:
[Fe/H] = -2.2
•
One of the oldest:
16Gyr
(according to Grundahl et al 2000)
•
•
m-M=14.6
Distance = 27,000 ly
M 92
• Boesgaard et al.
1998
–
–
–
–
–
V ~ 18
Keck I
1.5-6.5 hr
R ~ 45,000
S/N: 20-40
• Reanalysis of
Bonifacio et al.
(2002): a
variation of only
0.18 dex
NGC 6397
• [Fe/H] ~ -2.0
• Age ~ 13-14 Gyr
• Distance ~ 7,200 ly
– One of the closest
• m-M ~ 12.5
• Li:
– Bonifacio et al. 2002
Something interesting…


For a long time, people believed that whereas
NGC6752 shows much variation, NGC6397 does not
(Gratton et al 2001)
 [O/Fe] = 0.21
 [Na/Fe] = 0.20
 Star-to-star  0.14 dex
 Can be explained by obs error and variance in
atmospheric parameters
Carretta et al. (2004): Na, O variations in NGC 6397
– Li?
– Lack of Li-N correlation?
NGC 6752
•
•
•
•
[Fe/H] ~－1.43
Age ~ 13 Gyr
Distance ~13,000 ly
Log (M/M0) = 5.1
(DaCosta’s thesis, 1977)
• m-M ~ 13.13
• Li:
– Pasquini et al. 2005
47 Tuc
•
•
•
•
•
[Fe/H] ~ -0.7
Age ~ 10 Gyr
Distance ~ 13,400 ly
m-M ~ 13.5
Li:
– Bonifacio et al. 2007
Our data
• TO stars:
– V = 17.0-17.3; (B-V)=0.40.51
– With the same temperature
and mass, at the same
stage
– VLT-FLAMES/GIRAFFE,
medusa mode
– For Li 6708Å, R~17,000,
S/N ~ 80-100
– For O 7771-7775Å,
R~18,400, S/N ~ 40-50
Results
Error:
Li: 0.09-0.14 dex
O: 0.17-0.26 dex
• Li variation: 1.7-2.5, 0.8 dex
– The upper bundary is consistent with the prediction of
WMAP
– Not all stars have Li
• Li-O correlation:
– Possibility > 99.9% (ASURV)
– Can’t be made by TO star themselves
• For CNO circle, Te > 30 MK
• In the center of TO: 20 MK
• Li depletion: 2.5 MK
• Large dispersion in Li-O correlation
Explanation
• The Li/O-rich stars, which are also Na poor,
have a composition close to the "pristine"
one, while the Li/O-poor and Na-rich stars
are progressively contaminated.
• The contamination gas is from
– the Hot bottom burning (HBB) of an AGB star
or
– Wind of massive stars.
The chemical component of
pollution gas
• If we assume a primordial Li abundance of
2.64, given the observed lower boundary
of 1.8, more than 80% of the gas should
be polluted for such stars.
• If primordial [O/Fe] = 0.4, [O/Fe] of the
most Li-poor stars are -0.3, then the
pollution gas should have O/H~6.6
• Pasquini et al. (2005) for pollution gas:
– A(Li) ~2.0, Na/H > 5.4, O/H<7.0, N/H~7.4
AGB or WMS: production
• The results of Pasquini et al. (2005) for
NGC 6752 is qualitatively consistent with
the AGB model of Venture et al. (2002)
• The lack of N in 47 Tuc: WMS is more
possible (Bonifacio et al. 2007)
– For metal-poor AGB stars, the reaction from O
to N is quite efficient (Denissenkov et al. 1997
etc)
AGB: production problem
• Quantatively, AGB can’t explain the abundance
variation for most GCs (Fenner et al. 2004)
– Too much or not enough Na while O is not depleted
enough
– When Mg needs to be burnt, it is produced
– C+N+O can’t be constant as observed
• AGB models depends on two uncertain factors:
– Mass loss rate
– Efficiency of convective transport
• Weiss et al. (200
０) for HBB
production
– When Al is
produced, too
much Na
• Denissenkov et al.
(2001): 23Na firstly
produced then
destroyed during
interpulse phase -> accurate period
for both Odepletion and
23Na production
WMS: production
• Decressin et al. (2007):
– Fast rotate models of metal-poor ([Fe/H]=-1.5)
massive stars from 20-120 solar mass
– Surface chemical composition changes with
mass loss
– Based on Li abundances:
• 30% primordial gas is added to the winds
• The model could reproduce C,N,O and Li variation
• But failed in Mg
Li: pollution scenario (Prantzos
& Charbonnel 2006) - AGB
• If IM-AGB (4-9 solar mass)
– 20-150 Myr
– Before that, M* > 9Msun --> SNe-->wind of
400km/s --> no Li-rich primordial gas left
• Li-production? Hard to get A(Li)=2.5
– After that, 2-4Msun stars eject almost the
same amount of material as IM-AGB
• Maybe no HBB, but the third dredge-up --> C and
s-process elements variation
WMS
• In 20 Myr, massive stars evolve and slowly
release gas through winds. The gas is
mixed with primordial material.
• The shock wave of SNe induce the
formation of the new stars
• After 20 Myr, wind ejecta from low mass
stars (<10 Msun) won’t form stars because
of no trigger.
Li abundance variations and
dynamics
• AGB: the ejecta will
concentrate to the
center of the GC
• In 47 Tuc, most CNrich stars near the
center
• However, in NGC
6752:
– Red: A(Li) < 2.0
– Green: 2.0 < A(Li) < 2.3
– Black: A(Li) > 2.3
Different GCs, different
abundace variations
• Bekki et al. (2007): GCs come from dwarf
galaxies in dark halo at early age. The
pollution gas is from outside IM-AGB field
stars
– The difference of GCs
– Can’t produce the abundance variation
pattern
– Supported by Gnedin & Prieto (2006): all GCs
10 kpc away from the Galaxy center are from
satellite galaxies.
Primordial Li abundance
• Are field stars also polluted by the first
generation stars?
Conclusions
• Li variation is exist in GCs
• Li abundance is correlated with Na and O
• A mixing of contamination gas and primordial
gas is needed
• The contamination gas may comes from WMS
• Next work:
– The large scatter in Li-O correlation
– New data of 47 Tuc
The scatter
Thank you!
Invitation for Lunch
Time: 11:30 am today
Place: The third floor of NongYuan
Everyone is welcomed!
Shen Zhixia & Wang Lan