“why are massive o-rich agb stars in our galaxy not s-stars?” d. a. garcía-hernández...

“Why are massive O-rich AGB stars in our Galaxy not S-stars?”

D. A. García-Hernández (IDC-ESAC, Madrid, Spain)

In collaboration with P. García-Lario (IDC-ESAC), B. Plez (GRAAL, France), A. Manchado (IAC, Spain), F. D’Antona (OAR, Italy), J. Lub & H.

Habing (Sterrewacht Leiden, The Netherlands)

Gdansk, June 29 2005

AGB stellar nucleosynthesis• Main processes during the Thermal Pulsing phase

12C, s-element production (Rb, Zr, Ba, Tc, Nd, etc.) (3rd dredge-up)

• 3rd dredge-up increases C/O ratio forming M-, MS-, S-, SC-, C-type stars

Hot Bottom Burning (M>4 M)• When Tbce 2.107 K 12C 13C, 14N (CN-cycle) and

HBB prevents the carbon star formation• 7Li production and low 12C/13C ratios (Sackman &

Boothroyd 1992; Mazzitelli et al. 1999 )

D.A. García-Hernández

1

Previous works (MCs)

• HBB activation in massive AGB stars in the Magellanic Clouds (MCs) (e.g. Smith & Lambert 1989; Plez et al. 1993; Smith et al. 1995)

• Characteristics: 7 Mbol 6 ( M~ 4 8 M ) log (Li) ( ~ 1 4 dex) C/O < 1 ( ~ 0.5 ) 12C/ 13C ( < 10 ) (s-process) ( [s/Fe] > 0.5 dex)


2

Previous works (Milky Way)• Li-rich AGBs not so luminous (6 Mbol 3.5) S-, SC-, C-type stars with low mass (e.g. Abia & Isern 96, 97,

00; Abia & Wallerstein 98) not yet well understood!!• HBB models predicts log (Li) in massive (M>4 M) O-rich AGB

stars (e.g. Mazzitelli et al. 99)• Galactic candidates: OH/IR stars (L, C/O<1, Long Period

Variables) Optical observations very difficult due to strong mass-loss (~

104 106 M /yr) at present (Li) and (s-process) are unknown!


3

Massive Galactic O-rich AGBs

• Long Period Variables (P ~ 300 1000 days)• Large amplitude variability (8 10 mag in V)• Late-type stars (>M5) • OH maser emission emitters (Vexp(OH) < 25 km s-1)• Comparison stars plus 9 C-rich stars (18 objects)• Members of the galactic disk population with strong

IR excesses detected by IRAS


Selection of the sample (102 OH/IR stars):

4

Observations and data reduction

• Echelle spectra with UES (WHT, La Palma) and CASPEC (ESO 3.6m) in 1996-1997 at R ~ 50,000 (4 runs)

• Spectral range: ~ 5000 9000 Å. We were mainly interested in the Li I 6708 Å region

• Exposures times of ~ 10 30 minutes with S/N>100 in the Li I region

• Data reduction with the ECHELLE software package in IRAF


5

UES echelle spectra


“Blue example”

“Red example”

6

Overview • 25 stars detected in the Li I 6708 Å line• 32 stars non-detected in the Li I 6708 Å line• 45 stars too red at 6708 Å or without OPC• Extremely red spectra dominated by TiO bands• Absence of molecular bands of ZrO (YO, LaO, etc.)• From Vdoppler: Li I, Ca I, TiO (stellar) are formed deeper

than K I, Rb I (very probably of circumstellar origin)• Some stars also show H emission (shock-waves)


Li I 6708 Å region ZrO 6474 Å region

7

Progenitor masses• Period and Vexp(OH) as distance-independent mass

indicators (e.g. Chen 2001; Jiménez-Esteban 2004)• Different sources (masses) depending on P and Vexp(OH)


IRAS

Galaxy

8

Vexp(OH) (km s-1) Observed Li not detected Li detected Too red

< 6 13 12 (92%) 0 (0%) 1 (8%)

6 12 25 6 (24%) 11 (44%) 8 (32%)

> 12 53 10 (19%) 8 (15%) 35 (66%)

Period (days) Observed Li not detected Li detected Too red

< 400 12 10 (84%) 1 (8%) 1 (8%)

400 700 26 4 (15%) 12 (46%) 10 (39%)

> 700 18 1 (6%) 2 (11%) 15 (83%)

Chemical analysis

• Classical model atmospheres (HE, LTE, etc.) for cool stars (MARCS) and the “TURBOSPECTRUM” spectral synthesis code (Plez et al. 1992)

• TiO, ZrO are included and atomic lines from VALD-2• The whole machinery was tested on the high

resolution spectrum of the Sun and Arcturus• Spectral regions of interest (~60 Å): Li I 6708 Å ZrO 6474 Å K I 7699 Å; Rb I 7800 Å


9

Overall strategy• Initial range for Teff and log g from the VK photometry • Further constraints on the set of stellar parameters (M, Teff, C/O, log g, , z, (Zr), CNO,

12C/13C) using spectral synthesis


M=2 M

C/O=0.5

log g=0.5

=3 km s-1

(z, CNO,12C/13C)

Teff, FWHM

Li and Zr (s-elements) chemical abundances

( log (Li), log (Zr) )

Model vs. observations

2 test

10

Best fit in the Li I region


Zoom

Teff=3000 K,

log (Li) =+1.3

are needed to

fit the observations!

11

Best fit in the ZrO 6474 Å region


[Zr/Fe]=+0.0

is needed to


Comparison with a galactic S-star

12

IRAS 10436: a galactic S-star


[Zr/Fe]=+1.0

is needed to


13

Li and Zr abundances• Li detected stars show log (Li) ~ 1 3 dex • Li non-detected stars show log (Li) < 0.0 dex• Uncertainty of log (Li) ~ 0.4 0.6 dex (sensitivity to the

atmosphere parameters)• All stars show upper limits to the Zr abundance consistent

with no s-element overabundance


[Zr/Fe] < 0.0 0.25 dex for Teff > 3000 K [Zr/Fe] < 0.25 0.5 dex for Teff < 3000 K

14

P and Vexp(OH) vs. HBB


No clear correlation between log (Li) and P, Vexp(OH)But no Li-rich stars with P < 400 days and Vexp(OH) < 6 km s-1

Half of the stars with higher P and Vexp(OH) are Li-rich

15

Theory vs. observations- Stars with P<400 days and Vexp(OH)<6 km s-1 are non-

HBB stars (3 M < M < 4 M) non Li-rich - Stars with higher P and Vexp(OH) are HBB stars (M > 4

M) Li-rich but why only half of them are Li-rich? - Both type of stars experience strong mass loss and

only a few thermal pulses (and less efficient because of the high metallicity) no s-process enhancement

- The obscured stars must also be HBB stars and they represent the more massive AGB stars in the Galaxy

This scenario is consistent with the strong IR excess detected by IRAS and the HBB and nucleosynthesis model predictions!


16

Galaxy vs. Magellanic Clouds• Massive O-rich AGB stars in the MCs are S-stars and

~80 % of them are also Li-rich HBB stars• Why are these stars s-element enriched?Metallicity effect!- Theoretical models predict a higher efficiency of the

dredge-up in low metallicity environments (e.g. Busso et al. 1988; 2001; Straniero et al. 1995; 2000; Lugaro et al. 2003; Herwig 2004)

- Lower metallicity lower dust production (van Loon 00) less efficient mass loss longer AGB lifetime in the MCs compared to the Galaxy!


17

Conclusions- 25 stars detected in the Li I 6708 Å line, 32 stars non-

detected and 45 stars too red (or no OPC)- The chemical analysis revealed that half of the stars

with useful optical spectra are Li-enriched HBB- All stars in the sample are considerably massive (M > 3 M) but only the more massive ones (M > 4 M)

experience HBB. The lack of lithium in some HBB stars is a consequence of the timescale of the Li production phase (~104 years)


18

Conclusions- As a consequence of the different metallicity,

massive galactic O-rich AGB stars are not s-process enriched in strong contrast to Magellanic Cloud massive AGB stars Observational evidence that the chemical evolution during the AGB is strongly modulated by the metallicity!!

- Need of extending the analysis to other Galaxies in the Local Group with a wide variety of metallicities


19

Li I 6708 Å region


ZrO 6474 Å region


IRAS vs. P and Vexp


Galactic latitude vs. Vexp(OH)


Zoom in the Li I region


log (Li)=+1.3

is needed to


Other possible hypotheses?

• Are they more massive stars (M > 4 M)? - This is not consistent with the non-detection of Li

in any of them! • Are they lower mass stars (M < 1.5 M)? - This is not consistent with the lack of s-process

elements. Other low-mass stars of S- and C-type show strong s-process element enrichment

- A early stage as AGB stars is also not consistent with the strong IR excess observed by IRAS


Timescale of the Li production


HBB models (Mazzitelli et al. 1999) explain the lack of lithium in half of the massive O-rich AGB stars where the HBB is active!

The Li-rich phase is of the order of the interpulse time (~104 years)!

Li production at low metallicity


HBB models (Mazzitelli et al. 1999) explain the higher detection rate of Li-rich stars in the MCs because they predict a lower mass limit of only 3.03.7 M (in the LMC) for the HBB activation and a faster lithium production

“why are massive o-rich agb stars in our galaxy not s-stars?” d. a. garcía-hernández...

Documents