Carbon star

Carbon stars ( english carbon stars ) are late giant stars, similar red giant or occasionally red dwarfs .

In contrast to the “normal” red giants, however, carbon stars contain more carbon than oxygen . The latter combines with the carbon in the cool outer layers to form carbon monoxide, which does not emit any spectral lines in the optical spectrum. Other carbon atoms form C₂-, C₃-, CH-, CN- and SiC₂- molecules or molecular fragments. The sooty outer shell of the stars adsorbs the blue and yellow light components from their spectra . For the observer, this leads to their distinctly red appearance.

In contrast, the majority of all stars - including our sun - contain more oxygen than carbon. Such stars are oxygen Star ( english oxygen stars ) called; they can be recognized by the predominant spectral lines of metal oxides, mostly titanium oxides .

Spectra

The spectrum of the carbon stars is characterized by the Swan bands from C ₂ . In addition, there are spectra of other heavy elements that were created as by-products of the helium burning (three-alpha process) and the s-process in the star's interior and that are transported to the surface as a result of mixing. These include in particular lithium and technetium , which cannot be detected in old oxygen stars.

For spectral classification usually is nowadays Morgan - Keenan system (see luminosity class ) used the carbon star parallel to the normal arranges red giant. A spectral of C5,4 (or C5, ₄ ) describes a carbon star C with a surface temperature 5 (see table below) and a thickness of the Swan-bands with the index. 4

Spectral type	C0	C1	C2	C3	C4	C5	C6	C7
Corresponds to	G4-G6	G7-G8	G9-K0	K1-K2	K3-K4	K5-M0	M1-M2	M3-M4
T eff	4500	4300	4100	3900	3650	3450	---	---

Origin and subdivision

There are no carbon stars in star formation regions or young open star clusters . Hence, it is believed that the carbon atoms form in the late stages of stellar evolution. An excess of carbon is observed in five classes of stars:

Red giants on the asymptotic giant branch

The classic carbon stars on the giant asymptotic branch are a by-product of the unstable burning of helium . In a later phase, an explosive ignition of the three-alpha process occurs episodically every 10,000 to 100,000 years in a shell around the core. The star becomes unbalanced by the additional energy and the newly created elements are transported to the star's surface by convection . The star expands and the helium burn goes out again. In addition to carbon, short-lived radioactive isotopes are also transported to the star's surface. The process is called helium flash .

Mass transfer in binary star systems

The second class of carbon stars are found in binary star systems. Here one partner goes through the explosive helium burn described above and expands. The material is transferred from its outer layer to the companion by the stellar wind , on which carbon accumulates. The donor appears as a faint white dwarf due to the loss of material . The barium stars and the CH stars belong to this group of carbon stars.

Low hydrogen and changeable carbon stars

This third group with HdC stars (abbreviation HdC = Hydrogen deficit carbon stars) and variables of the RCB type is poorly understood. They do not seem to be double stars and, unlike the RCB stars, they do not show any infrared excess. The variability of this class of stars is caused by clouds of soot that are ejected at irregular intervals from these hydrogen-poor stars. The soot clouds absorb the visible light, which is then emitted in the infrared.

J-type carbon stars

Unlike normal carbon stars, which are also called N-type carbon stars, J-type carbon stars show an enrichment of nitrogen, a low ¹² C / ¹³ C isotope ratio, above-average luminosity, a lack of s-process elements and are lithium -rich in their star atmospheres . They make up around 10 to 15% of all carbon stars in the Milky Way. All of these stars are single stars. Since over 50% of all stars are components of binary star systems, it is assumed that the J-type carbon stars have emerged from the merging of two stars. Their chemical composition can be simulated when a white dwarf and a red giant go through a common envelope phase, with the white dwarf sinking into the core of the red giant and merging with it.

DQ white dwarfs

If white dwarfs show signs of atomic carbon or carbon molecules in their spectra, they are assigned to the spectral type DQ. The carbon compounds got from the C / O core into the atmosphere of the degenerate stars through a mixing process . They reach temperatures of 5,000 and up to 24,000 K.

variability

Like all red giants, carbon stars with a spectral type later than C4 are variable. Compared to the oxygen stars, the amplitude is lower for a comparable spectral type, since the titanium oxide and zirconium oxide bands are more temperature-sensitive than the Swan bands. Typical representatives of the carbon stars are La Superba (= Y Canum Venaticorum), John Russell Hinds "Blood Red Star" (Crimson Star) (= R Leporis), IRC +10216 (= CW Leonis) and RU Camelopardalis, the former Cepheid .

See also

• Pecular star

literature

• James B. Kaler: Stars and their Spectra. Astronomical signals from light . Spectrum Academic Publishing House, Heidelberg u. a. 1994, ISBN 3-86025-089-2 .
• Harm J. Habing, Hans Olofson (Eds.): Asymptotic Giant branch stars . Springer, Berlin a. a. 2004, ISBN 0-387-00880-2 (Astronomy and Astrophysics Library).

Individual evidence

1. ^ RI Hynes et al .: CXOGBS ~ J173620.2--293338: A Candidate Symbiotic X-ray Binary Associated with a Bulge Carbon Star . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.2597v1 .
2. ↑ Xianfei Zhang and C. Simon Jeffery: White-dwarf red-giant mergers, early-type R stars, J stars and lithium . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.0766 .
3. ^ S. Sengupta, RG Izzard, HHB Lau: A nova re-accretion model for J-type carbon stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.1402v1 .
4. ^ Paul Green: Innocent Bystanders: Carbon Stars from the Sloan Digital Sky Survey . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.4264 .
5. ^ Richard Hinckley Allen: Star-names and their meanings. GE Stechert, New York 1899, p. 269.