Eugenio CarrettaINAF- Osservatorio Astronomico di Padova

L’anticorrelazione Na-O:

sonda per la formazione e la prima evoluzione degli ammassi

globulari?

Bologna, 12-13 giugno 2003 – riunione cofin2001

in collaboration with:

• Raffaele Gratton – INAF Oss.Astr. Padova

•Angela Bragaglia – INAF Oss.Astr. Bologna

• Angela Bragaglia – INAF Oss.Astr. Bologna

• Carla Cacciari – INAF Oss. Astr. Bologna

• Giacomo Mulas – INAF Oss. Astr. Cagliari

• Emanuel Rossetti – INAF Oss. Astr. Bologna

LP ESO 165.L-0263

NGC 6397

NGC 6809 – M 55

NGC 7099 – M 30

NGC 104 – 47 Tuc

FLAMES Science

Verification

NGC 2808

The Na-O anticorrelation

Signature of the action of high temperature proton capture fusion

The NeNa cycle which enhances Na, is expected to operate in the same fusion zones in which the ON part of the CNO cycle if fully operative (Langer et al. 1993).

Larger Na abundances should accompany larger N and smaller O abundances.

Ivans et al. (2001)

Gratton, Sneden, Carretta, Bragaglia (2000)

First important step: THE ENVIRONMENT

Field stars explained with:

• first dredge-up

• a second mixing episode after the RGB-bump

Large variations in Na and O restricted to the cluster environment

No more debate on the mechanism, while the site of the nuclear reshuffling is not as clear: either in the presently observed stars or in previous nucleosynthetic generations.

Maybe in both sites...

Consistent sets of light element abundances for globular cluster stars in a wide range of evolutionary states would obviously shed a great deal of light on the relative importance of primordial abundance variations and internal evolution in producing the abundance anomalies that are observed.

(Pilachowski et al. 1996, AJ, 112, 545)

New generation of efficient spectrographs

samples

deeper

larger

FL

AM

ES

U

VE

S

Second important step : Na, O abundances in GC dwarfs

Internal mixing is ruled out:

• negligible convective envelopes

• the Al-Mg anticorrelation in TO stars in NGC 6752 requires too high core temperatures

•dominant H-burning cycle is p-p not CNO

Two main questions:

•Are the stars we presently observe saving in their photospheres the ashes of a past, first generation of intermediate mass stars that polluted with their ejects the protocluster

medium or the newly formed stars?

•Did these stars with non-original composition save the memory of this different fate, showing up with different intrinsic properties on, e.g.,

the Horizontal Branch?

Apparently homogeneous cluster...

BUT...

poor statistics

Na-poor, O-rich stars are present in the enlarged SGB sample!!

so that…

Variations in elements

produced by proton capture fusion do exist also in NGC

6397

Hints of inhomogeneities in SGB stars in M 55 from the preliminary analysis of UVES data.

Carretta et al. (2003a)

Work in progress…

.

[Fe/H] = -2.17 0.02 =0.08 dex

from 8 SGB and 2 TO stars, with UVES data

47 Tuc

turn-off stars (UVES)

SGB stars (UVES)

Carretta et al. (2003b)

RGB stars

M 71 – RGB stars

Carretta (1994; PhD Thesis)

FLAMES SV: Mass loss in NGC 2808 (Cacciari & Bragaglia)

Test for the ESO 072.D-0507

• does it exist a second generation of stars in GC?

•is there any link between the Na-O anticorrelation and the cluster properties (mass, density, HB morphology)

UVES

Red Arm

GIRAFFE

HR 12

Preliminary analysis:

Variations in the abundances of Na and O seen for the first time in giants of NGC 2808

•O and Na measured in 13 RGB stars

•upper limit in O for another star

[Fe/H]=-1.14 from UVES spectra, Teff from Alonso et al.(1999), gravities from the position in CMD, Na abundances from D lines

Preliminary analysis shows large star-to-star scatter in [Na/Fe] at all positions on the giant branch, suggesting large proton capture abundance vatiations down to stellar luminosities comparable to the red HB!

Carretta, Bragaglia, Cacciari, Rossetti (2003d, A&A, submitted)

Statistically significant samples are within reach for detailed and accurate abundance studies

NGC 2808

M 13

Carretta et al. (2003d)

Pilachowski et al. (1996)

Suggestions of a larger weight of the evolutionary effects along the RGB seems to work well only in M 13...

MV

80-100 RGB stars

in ~ 25 GC

•homogeneous analysis

•less concerns related to model atmospheres

•better abundance indicators

[O I] 6300

Na I 5682-88

Na I 6154-60

Main goals:

• properly weight the relevance of primordial variations and overimposed evolutionary effects on abundances of elements produced by proton capture

• ascertain if there is a link between the distribution function of stars along the Na-O anticorrelation and global properties of the globular clusters, in primis the HB morphology.

Salaris, Cassisi, Weiss (2002)

Sweigart-like:

He enrichment in the envelope

D’Antona-like:

He enrichment in the whole stellar stucture

scenario

how happens

Rotation induced deep mixing in today evolving RGB stars

Cluster stars form out of material with enhanced helium, due to ejecta of AGB stars, rich of He from the second and partly the third dredge-up

He enrichment in the envelope

He enrichment in the whole stellar stucture

scenario

implications

Higher helium (Y=0.28):

•fainter MS (0.12 mag)

•smaller He Mc at the RGB tip (0.008 Mo)

•smaller mass (0.05 Mo) at the RGB tip

•larger mass loss (Reimers’ rate increases with decreasing MRGB)

Much bluer HB regions are populated

Rotation and deep mixing:

•He-flash delayed, hence larger He Mc

•greater mass loss

•Yenv increased

•increased L at the RGB tip