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Observations of
Globular Clusters
(of relevance for the MODEST collaboration)
Giampaolo Piotto
Dipartimento di Astronomia
Universita’ di Padova
Collaborators: Jay Anderson, Luigi R. Bedin, Santi Cassisi,
Francesca De Angeli, Ivan R. King,
Yazan Momany, Marco Montalto, Alejandra
Recio Blanco, Sandro Villanova
Recent Instrumental Advances
New Observational Facilities
HST (!)
Astrometry
Photometry (?)
with ACS
WFI
Photometry
FLAMES@VLT
Astrometry
Pre-Flames
Velocities
Abundances
Proper Motion
New instruments for both imaging and spectroscopy have strongly
affected the research topics in globular cluster astronomy.
We have also started to take advantage of the 20-25 year baseline of
images on solid state digital imagers and, overall, of more than 10
year baseline of HST imaging for for high accuracy proper motions!
High Precision Astrometry on WFPC2/ACS HST Images
(Anderson and King 2002, 2003)
Just the random error remains
~0.02 pxl on the WFPC2
(~0.01 pxl on the ACS)
which corresponds
to 1 mas (PC) on a single imagewith N images:
N: ~1 mas /sqrt(N) (in the PC case)
Astrometry (1):
Identify cluster
members for
deep surveys
Hunting the
bottom of the
Main Sequence
down to the
hydrogen
burning
limit (HBL)
NGC6121=M4
(Bedin, Anderson, King, Piotto 2001, ApJL, 560, L75)
Luminosity-Radius Relation (LRR)
low [M/H]
intermediate [M/H]
The models
cannot fit the
main sequence
at intermediate
and high
metallicities
NGC 6397
King, Anderson,
Cool, Piotto, (1999)
M4
(Bedin et al. 2001)
Mass functions
in different
radial bins:
NGC 6121 = M4
Bedin et al. 2004, in prep
Observational
constraint
on mass
segregation.
Set constraints
on the cluster
dynamical
model.
Ongoing projects
Cluster
Camera
[Fe/H]
NGC6397*
NGC6121*
WFPC2
-2.2
WFPC2/ACS -1.2
NGC104
NGC6791
ACS
ACS
-0.7
+0.4
NGC5139
ACS
-1.6/-0.5
Example:
47 Tucanae
GO9444
CMD spanning
more than
17 magnitudes,
from the
RGB tip down
to Mv~15,
close to the
HBL
GO9648
Bedin, Piotto, King, Anderson, in prep.
ABSOLUTE MOTIONS
Astrometry (2):
Measure proper
motions
…of M4:
(U,V,W)LSR =
( 53+- 3,
-202+-20,
0+- 4)Km/s
(P, Q, Z)LSR =
(
54+- 3,
16+-20,
0+- 4)Km/s
Once corrected
m l cos b and mb
for
the Sun peculiar
motion we can get
Bedin, Piotto, King, Anderson 2003, AJ, 126, 247
Astrometry (3):
GEOMETRICAL DISTANCES
OF GLOBULAR CLUSTERS
This is our major project, at the moment
Globular cluster age measurement error is
dominated by uncertainty on distance, which is at
least ~10% => 0.2 mag distance modulus,
which translates in a >25% error in age!!!
Direct measurements of distances
are several years away (GAIA, SIM,…)
and we have to rely on standard
candles, whose luminosiy is still
poorly known, and sometimes strongly
dependent on other parameters
as metallicity (e.g. RR Lyrae).
We need reliable measures of distances
for as many GGCs as possible,
covering a wide range of metallicities
in order to measure accurate ages
Our method is very simple
(…in principle… )
we compare the dispersion of
the internal proper motions
(an angular quantity)
with the dispersion of
the radial velocity
(a linear quantity)
it is not a new idea, but now…
INTERNAL DYNAMICS
(Bedin et al. 2003)
…and
thanks to
instruments
like the
high
resolution
multifiber
spectrograph
FLAMES@VLT:
We get thousands
of radial velocities per night!!!
The main source of error is the
sampling error: 1/sqrt(2N).
For a typical sample of 3,000 stars
this implies an error of 1.3% in the
distance.
The distance scale obtained will not
be only sound, but its uncertainty
will no longer contribute to the
uncertainty on the age estimates.
NGC 2808
M4
(Formula from Pryor & Meylan 1993)
Error budget is very important!
Harris 2003: 2.2 kpc… Diff= 20% closer!!!
Better agreement with Peterson, Rees & Cudworth et al. d=1.72+/-0.14 kpc
This is a preliminary calculation!!!
The sources of systematic errors are:
- estimates of the observational errors
PLUS
- mass segregation
- rotation
- anisotropies
MODEL !!!
 Model predictions
O Data
We fit the observed radial
velocities and internal proper
motions with a superposition of
orbits constructed with an
axisymmetric dynamical model
(Schwarzschild models). The
orbit library is generated using
the code developped by
Gebhardt et al. (2000).
Preliminary results for 47Tuc
F. De Angeli PhD thesis
Ongoing work on proper motions: example
HST observations completed
last month
GO9899, PI: Piotto
Ongoing work on radial velocities: example
NGC2808: 2000 stars observed
(FLAMES@VLT, PI: Piotto)
In addition: NGC6121 (2600*)
NGC6397 (1700*)
NGC6752 (1500*)
Geometrical distance project
priority list
NGC
NGC
NGC
NGC
NGC
NGC
6121 Least model dependent!
2808
6752
6397
5139
104
plus 7 other clusters with at
least two epoch HST observations
Why should all this be
of interest for MODEST?
From the various proper motion projects we get:
1) Accurate proper motions AND radial velocities for up
to a few thousand stars, from the cluster center to many
core radii from the center;
2) Mass functions, in a few cases down to 0.1 solar
masses;
3) Mass segregation;
4) For a selected number of clusters, accurate distances
and ages;
5) In some cases, absolute motion
Accurate Reddening and Metallicity measurement
with GIRAFFE/UVES+FLAMES@VLT
Cluster
Giraffe
UVES
Ongoing ESO program
(PI Gratton)
Targets:
Metallicities with 0.03dex
uncertainties (UVES data)
Reddening with 0.005 mag.
uncertainty (GIRAFFE data
Coupled with the geometric distance project we should be
able to measure GC ages with a few 100 Myr uncertainties
• 74 GC cores observed with the
WFPC2 in the F439W and F555W
band [all clusters with (m-M)B<18];
•Data reduced with DAOPHOT and
ALLFRAME;
•Data calibrated to both HST Flight
and standard Johnson B, V systems
following Dolphin (2000);
•Completeness available for all the
CMD branches (7100 experiments with
more than 5 million artificial stars)
•Photometric data and completeness
are available at
http://dipastro.pd.astro.it/globulars
• The database has proven to be a mine of
information
Piotto et al. (2002), A&A, 391, 945
Relative Ages of Galactic Globular Clusters
Within each single
bin, GCs are coeval,
with an age
dispersion less than
1Gyr (smaller for
the most metal poor
clusters).
Clusters with
[Fe/H]<-1.5 appears
1.5-2 Gyr younger,
but this second
results is totally
model dependent.
Omega Centauri:the population puzzle goes deeper
Astrometry (4):
Omega Centauri.
Accurate
astrometry
implies
accurate
photometry!
The problem of the double MS and of
the multiple SGBs and TO
Bedin, Piotto et al. 2004, ApJL, 605, L125
While the multiple
TO could be
understood in terms
of a metallicity (and
age) spread,
the double
main sequence
represents a real
puzzle.
Bedin, Piotto, Anderson et al. 2004, ApJL, 605, L125
Is it a structure in
the background?
Leon, Meylan, & Combes 2000
Bedin et al. (2004) have proposed an alternative
explanation for the Omega Centauri double main sequence:
It represents a population of super-helium rich stars
(Y>0.30), which might be originated by material
polluted by intermediate mass (1.5-3 solar masses)
AGB star ejecta.
This would be consistent with:
1) The increase of s-process elements with metallicity
found by Smith et al. (2000)
2) The anomalously hot horizontal branch
3) The lack of correlation between period shift and metallicity
among RRLyr stars (Gratton et al. 1986)
ESO DDT project (PI Piotto) approved for 15hr at
FLAMES@VLT in order to verify this hypothesis
3 HST extra orbits allocated on DDT (GO10101, PI King)
17 blue main sequence
17 red main sequence
33 upper SGB
32 middle SGB
23 lower SG
FLAMES
+GIRAFFE
Observatios
in June2004
First results: the double main sequence
RedMS:
Rad. Vel.: 235+-11km/s
[Fe/H]=-1.56
17x12=204 hours i.t.
Piotto et al., ApJL, in preparation
BlueMS:
Rad. Vel.: 232+-6km/s
[Fe/H]=-1.27
It is more metal rich!
Other chemical elements:
Red Main Sequence:
Blue Main Sequence
[C/Fe]=0.0
[N/Fe]~1.0
[Ba/Fe]=0.4
[C/Fe]=0.0
[N/Fe]~1.0
[Ba/Fe]=0.7
The blue main sequence stars are richer in Ba (s-process element),
but NOT carbon rich. This is the second important result.
The fact that there is no significant radial velocity difference and no
significant difference in proper motion make the background object
explanation even more unlike.
The only other possibility is indeed a strong He overabundance
An overabundance of helium (Y~0.40) indeed can reproduce the
observed blue main sequence.
The fact that the more metal
rich, and possibly helium
richer stars are not carbon
rich seems to exclude that
the cloud has been
contaminated by AGB ejecta
According to Thielemann
et al. (1996) SNe from 8-12
solar mass stars should
produce a huge amount of
helium. Material polluted
by these SNe could in
principle originate stars
with the chemical content
of the blueMS stars in
Omega Centauri.
Future Plans:
Observations:
1) Reduce the new ACS/HST images (foreseen for June 2005)
to follow the two MSs in Omega Cen down to the hydrogen
burning limit; Use the first epoch of the same field for accurate
proper motions of the stars in the two MSs
2) With improved ACS photometry search for main sequence splits
and/or broadening in other globular clusters.
Theory (of interest for MODEST!)
1) Investigate the fraction of material ejected by SNe from 8-12
solar mass stars that can be retained within the cluster
(see also proposal at the end of the talk).
NEXT STEP FOR ASTROMETRY:
GROUND-BASED ASTROMETRY
Example: [email protected] ESO ~12 mas/frame
A post doc (Ramakant Singh Yadav)
full time dedicated in Padova
IN JUST ~2.8yr
Blue Stragglers from the snapshot catalog
•Blue stragglers (BS) are present in all of our 74 CMDs;
•Almost 3000 BSs have been extracted from 62 GCs;
•The location of BSs in the CMD depends on metallicity;
•The brightest BSs have always a mass less than 1.6 solar masses;
•In all GCs, BSs are significantly more concentrated than other cluster
stars.
Piotto, De Angeli et al. (2004, ApJL, 605, L125)
Ns represents the density
of stars in a cluster.
(i.e. the observed number
of stars has been divided by
the fraction of the cluster
light sampled by our
WPFC2 images, and then
divided by the total cluster
light).
There is a
significant correlation
between the BSS
frequency and the total
cluster luminosity (mass)
and a very mild
anticorrelation
with the central collision
rate.
Here, we plot the
estimated total number
of stars, obtained from
the observed counts,
divided by the fraction
of the cluster light
sampled by our images
Note that:
1) The total number
of HB stars scales
linearly with Mv,
or the total mass,
as expected.
2) The number of BS
is largely independent
from the total mass
and the collision rate.
Evolutionary pathway to produce Blue Stragglers in GCs
A more massive main sequence star
exchanges into a binary containing
two main sequence stars.
The primary evolves off the main
sequence and fills the Roche lobe.
The secondary gains mass and
becomes a blue straggler.
Blue stragglers will form earlier in
binaries containing more massive
stars, i.e. in high collision rate
clusters.
Given the finite lifetime of a blue
straggler, the blue straggler
population (from primordial binaries)
in the most crowded clusters today
could be lower than in very sparse
clusters.
Davies, Piotto, De Angeli 2004, MNRAS, 349, 129
Production of Blue Stragglers in GCs
Davies, Piotto, De Angeli 2004
Blue Straggler Luminosity Function
On the basis of our
model, we expect to
find predominantly
BS produced by
collisions in clusters
with Mv<-8.8.
These BS are
expected to be
brighter
(Bailyn and
Pinsonneault 1995)
This prediction
seems to be
confirmed by
the observed
luminosity function.
We have extended our investigation to open clusters…
NEW!!!!
GCs
Open clusters
The trends
continues into the
mass regimes of
(relatively) old
open clusters
(age>0.5Gyr).
(The high noise
for open clusters
is mainly due to
the small number
of red clump
stars.)
BSS in
Open Clusters
Log(age)
If we include the total
cluster sample, the
anticorrelation with the
total magnitude (mass) is
even more evident
(extending the trend
already observed for
GCs).
Apparently, there is also a
correletion with the
cluster age, with older
clusters having more BSS
Total Absolute Magnitude
Extended horizontal branches
22 out of the 74 clusters of the snapshot
database show a blue tail which extends
to Te>=20.000K, entering into the EHB
regime.
A number of these have been identified as
EHB clusters within the snapshot project.
In practice, 25-30% of the clusters of our
sample have a blue tail extended to
Te=20.000K or more.
EHB are not so rare, after all!
WHY?
Horizontal Branch Extension
For each cluster we
fitted a model to
obtain the
temperature of
its hottest stars, as
an index of the HB
extension.
Then we started by
exploring simple
pairwise correlations.
Metallicity: the first parameter
Clearly there is a
correlation between
the HB extension and
metallicity. The
metallicity is the first
parameter, afterall.
There is also a large
dispersion. Indeed,
The metallicity
explains only 32%
of the total variance.
Basically, this is the
“second parameter
problem.”
New important correlations: the total absolute magnitude
The total absolute
magnitude accounts
for 19% of the total
variance.
Note the if we
remove the most
metal rich clusters
(for which the
metallicity effect
dominates), the
correlation between
the HB extension
and the total absolute
magnitude (mass) is
much stronger.
No correlation
with the central
density or other
relevant cluster
parameters
Multicomponent Analysis
PCA analysis
confirms that
the HB extension
correlates
with
[Fe/H] and Mv
(i.e. total mass)
Why the dependence on the total cluster mass?
A possible explanation could be related to what we have found in
Omega Centauri:
IF a significant fraction of the material
self pollution!
lost by intermediate mass AGB stars
and/or SNe can be retained by the cluster
and contaminate the medium from which
less massive stars are still forming,we
would end with low mass stars richer in
helium. Stars richer in helium would
become bluer HB stars.
In this scenario:
more massive clusters
D’Antona et al. (2002)
would be able to retain material
from the AGB/SNe ejecta than
less massive ones, and therefore
would end with
more extended HBs
as observed!
A proposal for MODEST collaboration
The new results in Omega Centauri and on the extension of the
dependence of the horizontal branch in globular clusters on the
cluster total mass rise a number of questions.
1) Can the ejecta from SNe generated by 8-12 solar mass stars
be retained inside a globular cluster?
2) Can the ejecta from intermediate mass AGB stars be retained
inside a globular cluster?
3) Which is the fraction of retained ejecta as a function of the
cluster mass, mass function, etc.?
4) Which is the resulting chemical contamination?