R Coronae Borealis Star

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typical light curve of an R Coronae Borealis star, here by RY Sagittarii in the period 1988-2015

R Coronae Borealis stars (after their prototype R Coronae Borealis ; GCVS systematic abbreviation : RCB ) are stars whose brightness decreases sharply at irregular intervals.

R Coronae Borealis stars belong to the class of the eruptive variable . You are hydrogen poor yellow supergiants of spectral types  F or G with a carbon -rich atmosphere . The drop in brightness is likely due to soot clouds that are ejected at irregular intervals and obscure the star's photosphere .

properties

spectrum

R Coronae Borealis stars are yellow Supergiants the spectral F or G with absolute magnitudes of -3.5 to -5 M V at an effective temperature 5000-7000  K . Furthermore, an extreme underfrequency of hydrogen by a factor of 100 is observed (1% in contrast to 90% for the sun , measured according to the number of atoms). 98% of their atmospheres are helium. Compared to the solar composition, carbon, sodium , sulfur , silicon , nitrogen , nickel and elements that are formed in the s-process are highly enriched . The isotope ratios of many elements also differ considerably from those of all other star classes . Some RCB stars are showing signs of lithium in their atmospheres. Since lithium is destroyed by thermonuclear reactions at low temperatures, it may have been synthesized only a short time ago.

RCB stars are single stars, and no fundamental changes in the spectrum occur during the deep minima. Before and at the beginning of the minima, blue-shifted absorption lines appear with a speed of up to −400 km / s, which are interpreted as strongly accelerated mass ejection. These lines can be detected over a period of three months and are interpreted as accelerated dust, which also accelerates the gas through collisions . During the minima, emission lines become visible and partially disappear again. This reflects a chronological sequence in which the emission lines disappear from the spectrum the later the further their origin is from the star.

There is also a small group of hot R-Coronae Borealis stars that include V348 Sgr, MV Sgr, and DY Cen in the Milky Way . Their spectra are also low in hydrogen with a mass fraction of less than four percent, and they also show an infrared excess due to an extensive dust cover, but their effective temperature is between 15,000 and 20,000 K.

Extremely cool RCB stars with an effective surface temperature of approx. 3500 K are referred to as DY Persei stars after the prototype DY Persei . Their spectra are also low in hydrogen and high in carbon, but they show a slow and symmetrical change in light. The circumstellar shell of the DY-Per stars is both warmer and fainter than that of the RCB stars. They show a normal abundance of the carbon isotope C 13 , while a strong underfrequency or complete absence is a characteristic of the RCB stars. In addition, DY-Per stars are only a tenth as bright as normal RCB stars. Therefore, they could also be normal carbon stars, which occasionally pass through minima due to the ejection of a dust cloud without being in an evolutionary sequence with the R-Coronae-Borealis stars.

Change of light

R Coronae Borealis stars show brightness drops of up to 8  mag . The time of a minimum is just as unpredictable as its depth. The drop from normal light is steep with 3 to 6 mag in 50 days. The following increase can be just as fast as the decrease or also considerably slower; it can be overlaid with new drops in brightness. The mean interval between minima is approximately 1100 days. During the minima, an RCB star takes on a red color, which is interpreted as an indication of an extinction .

In normal light, all R Coronae Borealis stars also show semi-regular changes in brightness with an amplitude of a few tenths of a magnitude and periods between 40 and 100 days; in the infrared , where the dust is opaque , this semi-regular variability can also be observed in the deep minima. In most, if not all, cases, the semi-regular light change is a result of pulsations .

For some RCB stars, a correlation was found between the phase of the semi-regular light change and the start of the deep decrease in brightness. It is therefore speculated that the pulsations could trigger the ejection of matter.

Cause of changeability

A minimum brightness of the star is the result of an ejection of matter, which condenses into dust at some distance . This obscures the star in our line of sight. This assumption is supported by measurements which show that the polarization increases at the beginning of the minima. In the further course, the radiation pressure accelerates the dust and transports it into the interstellar space . An ejected cloud does not envelop the entire star, but only covers a small solid angle . Therefore, the variation in brightness in the infrared is not correlated with the minima in the optical .

The distance from the R-Coronae-Borealis star at which the dust condenses is open; Observations suggest a distance of only two star radii, but the temperature there is too high for graphite particles to condense . The conditions are only suitable for dust formation in 20 star radii.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists just under 50 stars with the abbreviation RCB , which means that around 0.1% of all stars in this catalog belong to the class of R Coronae Borealis stars.

Dust around R Coronae Borealis stars

About a third of the optical radiation is absorbed by the circumstellar dust and emitted again in the infrared. As a first approximation, the infrared radiation is the radiation of two black bodies with temperatures from 400 to 900  Kelvin and from 30 to 100 K. While the warmer temperature is attributed to the dust clouds, which also cause the low brightness minima, the cooler component lies a long way from the RCB -Star; this could be condensed components from the stellar wind of the precursor star .

Since the extinction coefficient of RCB stars differs from the extinction coefficient of interstellar matter , the composition is different. Presumably, the dust from the RCB stars is mostly glass- like or amorphous graphite particles. According to polarimetric measurements on the R CrB prototype, the dust is distributed in three components:

  • in a diffuse halo
  • in clouds, which can have a lifespan of several decades before they dissolve
  • in small wisps of cloud.

The clouds have no preferred direction and are randomly distributed around the star. Due to their higher density, graphite particles with a larger diameter can grow in them than in the halo, where molecules from the stellar wind condense during the formation of dust.

In addition, polycyclic aromatic hydrocarbons and simple Buckminster fullerenes (C 60 ) have been detected in the infrared spectrum at DY Cen and V854 Cen .

development

R Coronae Borealis stars are rare. Despite a high probability of detection due to the large amplitude of the light change, only about 100 RCBs are known, and their number is likely to be less than 1000 in the entire Milky Way . They therefore either represent a rare process in stellar evolution , or this phase is very short-lived. Furthermore, they are old and surrounded by a dust cover that can be detected in the infrared. This must have been repelled 100,000 years before the RCB stage. A mass of 0.7 to 0.8 solar masses was concluded from the pulsations  .

Four hypotheses are discussed regarding the formation of the R Coronae Borealis stars :

  • The final helium flash is the final rearing of a single white dwarf before it finally cools down. Accordingly, the helium- rich layer of the white dwarf ignites again and the outer shell puffs up. This process, also later called thermal pulse , has already been observed several times in V605 Aquilae , FG Sagittae and V4334 Sagittarii (Sakurai's object). However, these stars only showed dust minima for a short time and have not (yet) evolved into an RCB star.
  • The supergiant is created when two white dwarfs of a former binary star system merge , a helium and a carbon- oxygen white dwarf. Before that, the two white dwarfs approached each other, radiating gravitational waves . The star with lower mass has been torn apart, part of it 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 abundance of 18 O and fluorine in the atmospheres of RCB stars.
Such a close pair of two white dwarfs that can merge into a supergiant within the Hubble time is created when a common envelope phase is passed through twice . Here the companion star moves its orbit within the atmosphere of a developed red giant . By thereby occurring friction is effectively kinetic energy reduced.
  • If two helium-white dwarfs merge, a sdO sub -dwarf should arise first. Simulation calculations show that some of the massive subdwarfs created in this way cause helium to burn in a shell around the core and that the star subsequently develops into a giant of the spectral type B, A or F. The chemical composition of these resulting giants corresponds to that of R-Coronae Borealis stars and other extreme helium stars.
This formation process is only relevant for some RCBs of low luminosity in which lithium was found. He provides the explanation for this that it could have formed from 3 He during the fusion .
  • DY Cen is a binary star system with an orbital period of 39.6 days and a high orbital eccentricity . The emission lines in the spectrum are interpreted as indications of an ongoing mass transfer to the RCB star. DY Cen could be an example of a common envelope system in which the two stars of the binary system move their orbits in a common envelope. However, DY Cen has an unusually high proportion of hydrogen and is therefore not a typical RCB star.

Extreme helium stars

Extreme helium (EHe) stars share a lot in common with the R-Coronae-Borealis stars. However, they lack the semi-regular light change, the deep minima and an infrared excess caused by ejected carbon clouds. At 9,000 to 35,000 K, their temperatures are higher than with the RCBs. Furthermore, the EHe have an average of 10 times less hydrogen in their atmosphere. Presumably, EHe are the successors of the RCBs and develop into white dwarfs after losing their atmosphere.

On the other hand, hydrogen-poor carbon stars (HdC after the English term hydrogen deficient carbon stars ) correspond more closely to the RCBs in their chemical composition at mostly lower temperatures. Like the extreme helium stars, they do not show any deep minima.

Dust-induced minima in other variable stars

Minima, which are generated by dust clouds in the line of sight, are observed in addition to the R Coronae Borealis stars in the following star classes :

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

Well-known R Coronae Borealis stars

literature

  • C GC Clayton: The R Coronae Borealis Stars . In: Publications of the Astronomical Society of the Pacific . tape 108 , 1996, pp. 225 , doi : 10.1086 / 133715 .
  • C. Hoffmeister , G. Richter, W. Wenzel: Variable stars . 3. Edition. JA Barth, Leipzig 1990, ISBN 3-335-00224-5 .
  • JR Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  • B. Miszalski, J. Mikołajewska, J. Köppen, T. Rauch, A. Acker, M. Cohen, DJ Frew, AFJ Moffat, QA Parker, AF Jones, A. Udalski: The influence of binarity on dust obscuration events in the planetary nebula M 2-29 and its analogues . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1101.4959 .

Web links

Individual evidence

  1. ↑ Jan. E. Staff, Athira Menon, Falk Herwig, Wesley Even, Chris L. Fryer, Patrick M. Motl, Tom Geballe, Marco Pignatari, Geoffrey C. Clayton, Joel E. Tohline: Do R Coronae Borealis Stars Form from Double White Dwarf Mergers? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.0732 .
  2. ↑ Geofrey C. Clayton et al: Variable Winds and Dust formation in R Coronae Borealis Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.5047v1 .
  3. ↑ Geofrey C. Clayton et al .: The Dust Properties of Two Hot R Coronae Borealis stars and a Wolf-Rayet Centralstar of a Planetary Nebula: in Search of a Possible Link . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1106.0563v1 .
  4. ^ P. Tisserand et al: New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0905.3224v1 .
  5. ↑ AA Miller, JW Richards, JS Bloom, SB Cenko, JM Silverman, DL Starr, KG Stassun: Discovery of Bright Galactic R Coronae Borealis and DY Persei Variables: Rare Gems Mined from ASAS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.4181 .
  6. ^ Lisa A. Crause, Warrick A. Lawson, Arne A. Henden: Pulsation-decline relationships in R Coronae Borealis stars . In: Monthly Notice of the Royal Astronomical Society . tape 375 , 2007, p. 301-306 , doi : 10.1111 / j.1365-2966.2006.11299.x .
  7. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved August 4, 2019 .
  8. ^ SV Jeffers et al .: Direct imaging of a massive dust cloud around R CrB . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.1265v1 .
  9. ↑ DA Garcia-Hernandez, N. Kameswara Rao, David L. Lambert: Dust around R Coronae Borealis stars: I. Spitzer / IRS observations . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1107.1185v1 .
  10. ↑ P. Tisserand, L. Wyrzykowski, PR Wood, A. Udalski, MK Szymański, M. Kubiak, G. Pietrzyński, I. Soszyński, O. Szewczyk, K. Ulaczyk, R. Poleski: New R Coronae Borealis stars discovered in OGLE-III Galactic Bulge fields from their mid- and near- infrared properties . In: Astronomy and Astrophysics, vol. 529, A118 . 2011.
  11. ^ Richard Longland, Pablo Loren-Aguilar, Jordi Jose, Enrique Garcıa-Berro, Leandro G. Althaus: Lithium production in the merging of white dwarf stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.2538v1 .
  12. ↑ Xianfei Zhang and C. Simon Jeffery: Can RCrB stars form from the merger of two helium white dwarfs? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.3907 .
  13. ↑ N. Kameswara Rao, David L. Lambert, DA Garcia-Hernandez, C. Simon Jeffery, Vincent M. Woolf, Barbara McArthur: The hot R Coronae Borealis star DY Centauri is a binary . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.4199 .
  14. ↑ P. Tisserand: Tracking down R Coronae Borealis stars from their mid-infrared WISE colors . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1110.6579v1 .