R Coronae Borealis variable

R Coronae Borealis Stars ( GCVS nomenclature abbreviation: RCB) are stars whose brightness decreases strongly at irregular intervals.

R Coronae Borealis stars belong to the class of eruptive variables. They are water -poor yellow supergiants of spectral types F or G with a carbon-rich atmosphere. The brightness of waste are likely to result in irregular intervals emitted soot clouds that obscure the photosphere of the star.

Light change

All R Coronae Borealis stars show brightness drops of up to 8 mag. Hence, the time of a minimum is just as predictable as its depth. The waste from the normal light is steep 3 to 6 mag in 50 days. The following increase can be as fast as the waste or even much slower. The increase may be overlaid with new brightness declines. The average distance between minima of about 1100 days for all RBCs. During a minima an RCB star assumes a red color, which is considered a sign of extinction.

In normal light show all R Coronae Borealis star semi- regular brightness variations with an amplitude of a few tenths magnitudine and periods between 40 to 100 days. This semi- regular variability can be observed in the infrared where the dust is opaque, even in deep minima. In most, if not all cases, this semi- regular light changes a sequence of pulsations. In some RCB stars is a correlation between the phase of the semi-regular light variation and the beginning of the light loss was found. Therefore, it is speculated that the pulsations may be the trigger for the ejection of matter.

Spectrum

All R Coronae Borealis stars are yellow supergiants with spectral types F or G at an effective temperature 5000-7000 K. Furthermore, it is an extreme underabundance of hydrogen by a factor of 100 was observed ( 1% as opposed to 90% in the sun measured by the number of atoms ). Their atmospheres consist of 98 % helium. Compared to the solar composition are highly enriched carbon, sodium, sulfur, silicon, sulfur, nitrogen, nickel, and elements formed in the S- process. The isotope ratios of many elements, differs significantly from those of all other classes of stars. Some RCB stars show signs of lithium in their atmospheres. Since lithium is destroyed by thermonuclear reactions at low temperatures, it can be synthesized only recently.

All RCB stars are single stars and during the deep minima occur at any fundamental change in the spectrum. Before and at the beginning of the minima appear blue-shifted absorption lines at a speed of up to -400 km / s, which are interpreted as rapidly accelerated mass ejection. These lines can be detected over a period of three months and are interpreted as an accelerated dust and accelerates the gas to jolts. During the minima emission lines are visible and partly disappear again. This plays a chronological order again, the later they disappear in the emission lines from the spectrum, depending on their origin from the star away.

There is also a small group of hot R Coronae Borealis stars, which include in the Milky Way V348 Sgr, MV Sgr and DY Cen. Its effective temperature is in the range 15000-20000 K. Their spectra are also in hydrogen with a mass fraction of less than four percent, and they also show an infrared excess due to widespread dust shell.

Extremely cool RCB stars are referred to as DY -Per -star after the prototype DY Persei with an effective surface temperature of about 3500 K. Their spectra are also low in hydrogen and carbon- rich, but they show a slow and symmetrical light change. The circumstellar envelope of DY Per stars is both warmer and fainter than the RCB stars. In contrast to these, they show normal abundance carbon isotope C13, while a strong lower frequency or complete absence is a characteristic of RCB stars. Besides are DY Per stars only one-tenth as bright strong as normal RCB stars. Therefore, it could act to normal carbon stars also that occasionally pass through minima due to an ejection of a dust cloud, without being in a developmental sequence with the R Coronae Borealis stars. RCBs are among the supergiants with absolute magnitudes -3.5 to -5 MV.

Cause of the variability

A brightness minimum is the result of a discharge of matter from the star, which condenses at some distance from dust. The dust obscures the star in our line of sight. This assumption is supported by measurements of polarization, wherein the polarization at the beginning of the minima increases. In the course of radiation pressure accelerates the dust and transported him into the interstellar space. An ejected cloud does not embrace the entire star, but covers only a small solid angle. Therefore, the variation of the brightness in the infrared is not correlated with the minima in the optical.

At what distance from the R Coronae Borealis star condenses the dust, is open. The observations suggest a distance of only two stellar radii close. However, the temperature is too high there for the condensation of graphite. Only in 20 stellar radii, the conditions are suitable for the formation of dust.

R Coronae Borealis dust around stars

About a third of the optical radiation is absorbed by the circumstellar dust and re-emitted in the infrared. The infrared radiation is in first approximation, the two black bodies with temperatures of 400 to 900 Kelvin and 30 to 100 K. During the warmer temperature of the dust clouds we attributed that also causes the deep Helligkeitsminima, the cooler component is located at a large distance from the RCB star. It could be explained by condensed components from the stellar wind of the progenitor star of the R Coronae Borealis star. Furthermore, have been detected polycyclic aromatic hydrocarbons and simple Buckminster fullerene C60 in the infrared spectrum at DY Cen and V854 Cen.

Since the extinction coefficient differs between RCB stars and the interstellar matter is before a different composition. It is believed that this is mainly glassy or amorphous graphite. According polarimetric measurements on the prototype R CrB the dust is distributed in three components:

  • In a diffuse halo
  • In clouds that up to a few decades can have up to a lifetime resolution,
  • As well as in small wisps of cloud.

The clouds have distributed no preferred direction and are randomly around the star. In the clouds can grow with a larger diameter than in the halo, where ondensieren molecules from the stellar wind on dust formation due to the higher density graphite.

Development

R Coronae Borealis stars are rare. Despite a high probability of detection due to the large amplitude of the light variation, only some 100 RCBs are known and in the entire Milky Way should their number be less than 1000. They, therefore, either a rare process in the stellar evolution is or the phase is very short-lived. Furthermore, they are old and surrounded by a detectable in the infrared dust shell. This must have been repelled 105 years before the RCB stage. From the pulsations was closed on a mass 0.7 to 0.8 solar masses.

There are four hypotheses regarding the origin of the R Coronae Borealis stars in the discussion. It is the model of the final helium flash, the merger of two white dwarfs and the formation in a common envelope phase:

  • In the final helium flash is the last gasp of a single white dwarf before the final cooling. Thus, the helium- rich layer of the white dwarf ignites again and the outer shell puffs up. This later thermal pulse specified process has been repeatedly observed in V605 Aquilae, and FG Sagittae V4334 Sagittarii ( Sakurai's object). These stars showed only short Staubminima and have not developed (yet) in a RCB star.
  • When two white dwarfs, a helium and a carbon-oxygen white dwarf, the giant formed in a former binary star system. The two white dwarfs have converged under the emission of gravitational waves. The less massive star has been torn apart and a part is used as fuel for a helium- burning layer. The other part of the torn companion forms the shell of the supergiant. This hypothesis is supported by the frequency of 18O and fluorine in the atmospheres of R CrB stars. A tight pair of two white dwarfs that can merge within the Hubble time, created when two common- envelope phase is run. This meeting place of the companion star within the atmosphere of a developed Red Riesens its orbit. By doing friction occurring kinetic energy is effectively reduced.
  • Merging two helium white dwarfs, it should first create a sdO sub- dwarf. Computer simulations show that some of the resulting massive sub- dwarfs there is a helium burning in a shell around the core and the star evolved subsequently into a giant with the spectral type B, A or F. The chemical composition of the thus formed corresponds to the giants of R Coronae Borealis stars and other extreme helium stars. This development process is relevant only for a small part of the RCB. He provides an explanation for the RCBs found in some low- luminosity lithium that could have formed from 3He in the fusion.
  • DY Cen is a double star system with an orbital period of 39.6 days and a high orbital eccentricity. The emission lines in the spectrum can be interpreted as a sign of ongoing mass transfer to the RCB star and DY Cen could be an example of a common- envelope system in which swim lengths the two stars of the binary system in a common sheath. However DY Cen has an unusually high proportion of hydrogen and is not a typical R CrB star.

Extreme helium stars

Extreme helium stars ( marriage) share many similarities with the R Coronae Borealis stars. What they lack, however, is the semi- regular change of light, the deep minima and an infrared excess emitted by clouds of carbon. The span of their temperatures ranging 9000-35000 ° K and is thus higher than the RCB. The marriage still have a lower by a factor of 10 on average proportion of hydrogen in their atmospheres. It is believed that the marriage are the successors of the RCBs and evolve after losing its atmosphere to white dwarfs. Hydrogen poor carbon stars (HDC abbreviation of the term hydrogen deficient carbon stars ) correspond, however, in their chemical composition at lower temperatures usually rather the RCBs. They also show how the extreme helium stars no deep minima.

Dust -induced minima at other variable stars

In addition to the R Coronae Borealis stars are still observed in the following classes of stars by dust clouds in the line of sight minima generated:

  • Population I WC9 stars
  • Symbiotic stars
  • Some central stars of planetary nebulae such as V651 Mon
  • As a result of a late thermal pulse can result in a highly carbon-enriched atmosphere similar to Sakurai's Object, V605 Aql and FG Sge
  • Possibly in AGB stars with long secondary periods

These star ratings are either pulsations as in the R Coronae Borealis stars or wind - wind collisions in binary systems, the cause of the condensation of dust.

Known R Coronae Borealis stars

R CrB, RY Sgr, SU Tau, Z UMi

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