The chemical composition of
R-stars: Evolutionary status
O. ZAMORA, C. ABIA, B. PLEZ AND I. DOMINGUEZ
VIII Torino Workshop on Nucleosynthesis in AGB stars
Granada, 6-10 February 2006
1. INTRODUCTION
What are R stars?.
•R stars are carbon stars that present peculiarities with respect the normal
AGB carbon stars C(N).
•R stars are located in the galactic disk. In number, R stars are the 1% of
G, K giants and 10 times more numerous than C(N) stars.
•Spectroscopically, are classified in 2 types, with Teff = 2000-5000 K:
Early or hot R stars: R0-R4 (spectra like G9-K2 stars)
Late or cool R stars: R5-R9 (spectra like K-M stars)
•Early do not have significant circumstellar dust emission and are non
variable in general.
•Late have circumstellar dust emission indicative of mass loss (K-[12] >
0.7) and some of them are SR variables or Miras.
Why are R stars special?.
•Thanks to the data of satellite Hipparcos (ESA 1997) , was possible for
the 1st time to obtain directly the distances for giant carbon stars. It
allowed to Knapp et al. 2001 situate the R stars accurately in the 2 color
diagram. Early are located at the red clump (core He burning stars)
whereas Late are more scattered, have higher luminosities and some of
them are situated in the region of the C(N) stars.
•Dominy (1984) found chemical anomalies in hot R stars respect the
C(N) stars: no s-elements overabundances 12C/13C ~ 10
Other abundances derived:
No detectable Li
[C/H] ~ 0.46
[Fe/H] ~ -0.10
[N/H]~ 0.64
[O/H]~ -0.08
• Respect to the cool R stars, nothing is known...are they the evolutive
descendants of the hot stars? mark the beginning of the AGB? Are
similar or identical to the C(N) stars?
2 COLOR
DIAGRAM
Knapp et
al. 2001
Cool: R4-R9
H-K > 0.3
(V-K)0 > 4
Hot: R0-R3
H-K < 0.3
(V-K)0 < 4
What is the origin of R stars?. That is the question!!!.
There are at least 2 possibilities:
A) If R stars are intrinsic....are they in the HB? (C in the atmosphere
caused by a violent He flash) or maybe trace the beginning of the
AGB? (C transported by the TDU)...are the cool R stars the
evolutive descendants of hot R stars?
OK!, but...the canonical He flash cannot produce a carbon star =>
an anomalous flash is required to reach C/O > 1 in the surface
(Pacynsky & Tremaine 1977).
B) If R stars are extrinsic...are the R stars the final result of a merged
binary system?
OK!, but... McClure 1997 studied the variation of the radial
velocity of 22 R hot stars and found no evidence of binarity => all
R hot stars are single or all merged binaries.
New observational and theoretical treatment is required!!!!!!!.
2. OBSERVATIONS
We have observed a sample of 22 R stars selected from the
Hipparcos catalogue in the optical, using the echelle spectrograph
FOCES at the 2.2 m telescope of CAHA, Calar Alto (Spain).
The typical resolution is R ~ 35000 with S/N >> 100 at 7000 Å
and S/N ~ 50 at 4800 Å.
Quality of the spectra
HIP 36623
R9
HIP 84266
R2
3. ATMOSPHERIC PARAMETERS
We have estimated initially the atmospheric parameters in the following
form:
•We have used IR photometry JHK (from Knapp et al. 2001 and
2MASS) to estimate the Teff of the sample using the calibration proposed
by Bergeat et al. 2001. The interestellar extinction was corrected using
the model of Arenou et al. 1992 and BC was calculated in K according to
Costa & Frogel (1996) calibration for carbon stars. We estimate an error
of 300K in the temperature determination.
•The gravity was computed assuming 1M~, obtaining a typical value of
between logg ~ 2 - 2.5.
•A typical microturbulence parameter was set at 2 km/s (according to
Dominy 1984).
4. ANALYSIS
•We have derived abundances of interest using the spectral synthesis
code Turbospectrum and the grid of carbon stars atmosphere models
from Eriksson et al. (1984). When they were not available for a
particular Teff, we use O-rich MARCS models or Kurucz models
increasing the carbon abundance.
•We have found the right atmosphere model for each star fitting by eye
the observed spectrum with the synthetic one in the region near 8000
Å, starting from the estimated atmospheric parameters and solar
abundances [Fe/H] = 0 from Grevesse & Sauval (1998) except for
CNO abundances (from Asplund 2005). At 8000 Å, we have derived
C/O and 12C/13C increasing the amount of C properly.
•We analysed also:
Li (6700 Å), Rb (7800 Å), Tc (5920 Å), others s-elements (in
progress)…
•An important point is that the spectral classification for a given star
is very ambiguous in the literature.
For example, HIP 85750 has been classified as:
R2 by Vandervoort (1958)
CH by Yamashita (1975)
C(N) by Barnbaum et al. (1996).
This can rest ‘purity’ to the stars classified as R.
•Some of the stars are variable and the spectral temperature subtype
indicator could be wrong for them. In order to avoid this problem,
from the Teff derived in the analysis, we have separated the R- stars of
our sample with the following criterion:
Cool Teff ≤ 3800 K
Hot Teff > 3800 K
2 COLOR DIAGRAM
C(N) stars (Abia et al. 2001)
Late R-stars
Early R-stars
5. PRELIMINAR RESULTS
C/O and 12C/13C (8000 Å)
Isotopic C ratio below 25 for the most of R stars!!.
Hot stars show higher spread in C/O. Cool stars are located mainly
at C/O=1-1.1.
The 2 cool stars that have higher values of 12C/13C are locate in the
2 color diagram at the position of C(N) stars.
HIP 36623: R9 Teff = 3300 K C/O = 1.04
12C/13C
---No 13C ---Best fit: 12C/13C = 24
= 24
HIP 84266: R2 Teff = 4371 K C/O = 1.23
12C/13C
= 12
---No 13C ---Best fit: 12C/13C = 12
C/O vs 12C/13C
Early R stars
Late R stars
CH?
Lithium (6700 Å)
z
There is a clear separation between early and cool stars.
z
The mean value for the hot stars is 0.92 and for the cool
is –0.47, similar to C(N) stars.
z
HIP 62944 (hot) is a Li rich hot star, but is not a carbon
star C/O = 0.93 and also has been classified as K.
HIP 62944: R3 Teff = 4311 K log ε(Li) = 3.5
---log ε(Li)= 2.5 ---Best fit: log ε(Li) = 3.5
Li vs Teff
Early R stars
Late R stars
Rubidium (7800 Å)
z
The hot R stars have a higher Rb abundances and lower
metallicities than the cool ones. In cool R stars, [Rb/Fe] are close
to the solar values.
z
Cool R stars are locate in the region typical of C(N) stars.
However, Hot R stars are separated from them.
HIP 36623: Teff = 3300 K [Rb/Fe] = -0.1
---No Rb ---Best fit: [Rb/Fe] = -0.1
[Rb/Fe] vs [Fe/H]
Comparison to AGB models
for different efficiency of 13C pocket
Early R stars
Late R stars
C(N) stars
Models from Gallino et al. 1998
Metallicity (6700 and 8000 Å)
R-stars are around solar values, but there is more spread in hot stars
(some of them are a bit metal poor).
Red: Hot R stars
White: Cool R stars
CH?
Technetium (5920 Å)
z
Tc ‘detected’ in 2 stars (2 hot) and doubtful in one more (hot).
To be confirmed with line at 4260 Å (more stronger and
reliable) and other s-elements analysis. Histogram of R flux
ratio indicates main distribution around 0.1 but there are not
available F(12µm) data for stars with Tc ‘detection’.
R = F(12µm)/F(2.2 µm)
Jorissen et al. (1993):
R < 0.1 => extrinsic
R ≥ 0.1 => intrinsic
HIP 39118: R2 Teff = 4000 K logε (Tc) < 1.2
---Tc 1.2 ---Tc = 0.7 ---No Tc
Circumstellar envelope
The circumstellar envelope is an indicator of the possible mass
loss or binarity character. For our stars:
A Circumstellar envelope was found in 3 hot and in 2 cool
ones!!!. In 1 of them (hot), Tc was ‘detected’ as an upper limit
whereas 2 of them (cool and hot) show evidence of s-elements.
HIP 39118
R2 Teff = 4000 K
circumstellar
envelope
Na I
Na I
More s-elements...in progress
z
Preliminary results show probable s-elements
enhancement in 1 star (hot) and maybe in other one (cool).
Must be completed with the analysis at 4800 Å (in
progress).
Ba
R8
Teff = 3776 K
Comparison with previous Dominy´s analysis for hot R stars:
[Fe/H] ~ -0.25, [C/H] ~ 0.23 : according with Dominy values within
errors.
12C/13C < 25 in general: slightly higher from previous.
Li was detected with a mean value log ε(Li)= 0.92, similar to the
result predicted for a 2.0 solar mass star in the tip of the RGB
(Lambert et al. 1980, Luck & Lambert 1982).
CONCLUSIONS
FUTURE WORK
To finish the analysis of s-elements and to build new theoretical
models to reproduce the abundances obtained.
We will model for 2 possible scenarios:
A) Intrinsic origin of R stars: modify the canonical He flash with the
evolution code FRANEC to produce carbon enhancement in the
surface.
B) Extrinsic origin of R stars: simulate the merging of a binary
system with the hydrodynamic code SPH (Smoothed Particle
Hydrodynamics). This is been doing now with a system composed
by a MS star + AGB carbon rich star.