Thibaut Le Bertre, from Paris Observatory, and his collaborators studied hundreds of these stars and highlighted how this enrichment occurs, according to the distance from the galactic center. In particular the stars rich in oxygen are found closer to the Galactic center than the Sun, whereas the stars rich in carbon are found farther, this phenomenon being attributed to the metallicity gradient in the Galaxy.
Carbon stars are post main-sequence stars which develop an anomalous chemical composition where carbon dominates instead of oxygen. There are several categories of carbon stars, and this enrichment in carbon is not always well understood. Among them carbon-rich red giants have been the subject of many studies. In this case the carbon enrichment is thought to be due to the dredge-up of carbon, recently synthesized in the stellar core, to the stellar surface by convection. For red giants the stellar effective temperatures are rather low and molecules are forming in the atmosphere. When they are enriched in carbon, astronomers have observed molecules such as C2H2 (acetylene) or C2, CH, HCN, etc., which produce characteristic absorption bands in the optical and infrared range of the stellar spectra.
Figure 1. A schematic view of a mass-losing AGB star. The numbers are only indications on the orders of magnitude. ISM stands for interstellar medium, ISRF for interstellar radiation field, and HAC for hydrogenated amorphous carbon. For comparison, the solar radius is 7 1010 cm, the astronomical unit 1.5 1013 cm, and the parsec 3 1018cm From Le Bertre T., 1997, ``Cool Stars Winds and Mass Loss : Observations", Lecture Notes in Physics, Springer, Vol. 497, p. 133
The atmospheres of these luminous low-temperature stars are often unstable and they pulsate with time-scale of typically one year. Under the action of this pulsation the atmosphere is extended and, in the upper layers, the matter may reach a temperature low enough for some elements to condense into very small particles ("star dust"). In the case of a carbon star the dust is composed of carbonaceous material, possibly connected to the polycyclic aromatic hydrocarbons (PAH). Other components such as amorphous carbon and silicon carbide (SiC) are thought to be also present. When these dust particles appear in a stellar atmosphere they are submitted to radiation pressure which tends to eject them, but simultaneously they drag the gas with them. An outflow develops and the star is progressively surrounded by an expanding circumstellar shell of dust and gas. The star is thus losing material to interstellar space. An interesting aspect of mass losing carbon stars is that they are one of the most important contributors to the replenishment of the interstellar medium with material processed by stars, and of course they contribute most of the carbon-rich material. It is therefore of interest to find these carbon-rich sources, to locate them in our Galaxy and to determine at which rate they are losing matter to space. A difficulty is that this rate of mass loss may be so large that the stars get hidden into their circumstellars shells and completely disappear at optical wavelengths. One needs to search for them at infrared wavelengths which are less affected by dust absorption. A systematic search has been conducted by a team of Japanese and French astronomers. They used the results acquired by a small space infrared telescope, IRTS (Infrared Telescope in Space), constructed and operated by the Japanese Institute of Space and Astronautical Science (ISAS).
Figure 2. An image of the IRTS inside its cryogenic vessel, during the vibration test phase before launch. More details on the IRTS. Copyright : ISAS
The spectral range covered by the IRTS is particularly rich in features allowing to characterize late-type stars. The carbon stars can be easily identified in the spectra thanks to a deep molecular absorption band at 3.1 µm which is due to a blend of C2H2 and HCN.
Figure 3. An IRTS spectrum of a carbon star. Although the source is heavily reddened, one may identify easily an absorption band at 3.1 µm due to a blend of C2H2 and HCN, typical of such a star. Other molecular bands can be recognized at 1.6 (CO), 1.8 (C2), 2.3 (CO) and 3.9 (C2H2) microns.
In their sample of 700 sources they identified 126 carbon-rich stars. This sample allowed them to estimate the relative contribution of the 2 kinds of mass losing giants to the replenishment of the interstellar medium at different distances from the Galactic Centre (GC). They found a clear dependence on galactocentric distance with O-rich sources outnumbering C-rich sources inside the solar circle (dGC 8.5 kpc), and the reverse outside.
Figure 4. Space distribution of the mass losing red giants in the IRTS sample projected on the Galactic Plane. The carbon stars are represented with empty diamonds and the oxygen-rich stars with filled dots. The Sun is at the center of the figure. It is located at a distance of 8.5 kpc from the Galactic Center, which is at the top (at the centre of all dotted circles). One clearly sees that carbon stars are located preferentially outside the solar circle.
This result illustrates clearly that the chemical evolution is not proceeding uniformly throughout our Galaxy. The IRTS mission was unfortunately of a too short duration and only a small part of the sky could be observed. But this pioneering mission has demonstrated that a survey of the sky in the near-infrared range would aid our understanding of evolved stars in our Galaxy and of their rôle in its evolution. Other space missions of the same kind, such as the project RESPIRE, are presently under consideration in various institutes around the world.
Reference
- Le Bertre T., Tanaka M., Yamamura I., Murakami H. : 2003, "Galactic mass-losing AGB stars probed with the IRTS. II", Astron. and Astrophys., 403, 943
Contact
- Thibaut Le Bertre
(Observatoire de Paris, LERMA) - Masahiro Tanaka,
- Issei Yamamura, Hiroshi Murakami
Institute of Space and Astronautical Science->http://www.isas.ac.jp/e/index.html],
Sagamihara, Japan