Symbiotic star

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The symbiotic star SS Leporis (17 Lep) captured by the VLT interferometer using PIONIER. The images were subsequently colored according to the known star temperatures.

A symbiotic star is an interacting binary star system consisting of a red giant and a hot blue companion, often a white dwarf , embedded in an ionized nebula .

It is characteristic of symbiotic stars that the spectrum is made up of an emission and an absorption spectrum . As with the cataclysmically variable , there is a transfer of matter to the hot companion, in contrast to these, the two stars are further away from each other, so that there is usually no transfer of matter over the Roche limit , only wind accretion .

The name "symbiotic star" goes back to Paul W. Merrill .

Symbiotic stars, which do not achieve the necessary conditions for a permanent thermonuclear reaction and in which the accreted matter burns explosively at irregular intervals, are called symbiotic novae .

definition

The definitions of the symbiotic star class are numerous. The oldest and still in use is based on the properties of the composite optical spectrum :

  • The spectrum shows the properties of a late giant with the spectral classes G, K or M, which belong neither to a main sequence star nor to a supergiant . These properties include the absorption lines of Ca I, Ca II, Na I, Fe I, H 2 O, CN, CO, TiO, VO and others.
  • In addition, the star spectrum shows emission lines of hydrogen or helium as well as either other emission lines with an ionization energy of more than 20 eV (e.g. O III) or an AF continuum with absorption lines of simply ionized metals.

The General Catalog of Variable Stars defines symbiotic stars as Z And stars. These are close binary stars consisting of a hot star, a star with a late spectrum and an extended shell excited by the hot star. The changes in brightness reach up to 4 mag . The class of symbiotic stars is described very heterogeneously. There around 50 stars are divided into the ZAND class , which corresponds to around 0.1% of all stars in this catalog.

In contrast, Joanna Mikolajewska describes symbiotic stars as interacting double stars consisting of a developed giant (a red giant or a Mira star ), which transfers mass to a hot and luminous white dwarf .

separations

The heterogeneous group of symbiotic stars is subdivided according to various criteria.

Infrared spectrum

A distinction is made in the infrared :

  • S-systems, which account for about 80% of the symbiotic stars and only a stellar in the infrared spectrum photosphere with an effective temperature of 3000 to 4000 K show
  • D systems which, in addition to the strongly reddened spectrum of a Mira star, show signs of an approximately 1000 K warm dust cover
  • D 'systems, which, unlike D systems, do not contain a Mira star, but an F to K giant

After the hot blue companion

The red giant's hot blue companion that releases ionizing radiation can be of the following types:

  • Main sequence star , as in the case of SS Lep
  • a white dwarf, as is the case with the majority of the symbiotic stars
  • a neutron star , as at GX1 + 4 = V2116 Oph. The ionizing radiation and the emission lines are caused by an accretion disk around the neutron star. The related term Symbiotic ray binary contrast, describes an X-ray binary stars , low mass, the companion, a Red or Yellow Giant is independent of the presence of emission lines in the optical spectrum.

According to the type of accretion

This classification distinguishes the way in which matter is supplied to the blue companion by the red giant.

  • Wind accretion . In accretion, also known as Bondi-Hoyle accretion, the blue companion collects matter with the help of its gravity from the undirected stellar wind given off by the red giant . This is the case with most symbiotic stars.
  • River across the Roche border . In a binary star system there is a maximum radius that the red giant can occupy. If the star expands beyond this limit, matter flows over the inner Lagrange point to the companion. The possible mass flow is considerably greater than with wind accretion.

In the manner of hydrogen burning

The accreted hydrogen and possibly also the helium can burn almost permanently on the surface of the white dwarf or in the main sequence star . If the temperature, pressure and material flow required for permanent combustion are not reached, explosive combustion, a thermonuclear runaway, occurs . Such double stars are also known as symbiotic novas.

properties

variability

All symbiotic stars belong to the variable stars . The changes in brightness can be assigned to various causes:

  • Cover variability when the bright blue component is behind the red giant from Earth. This form of variability is suitable for analyzing the geometric dimensions of the binary star system.
  • Reflection effect. The radiation of the bright blue companion heats the side of the red giant facing it and leads to a change in color and brightness with the period of the period of rotation .
  • Variability through the ellipsoidal shape of the red giant, which occurs due to its proximity to the blue companion. This variability also changes periodically with the period of rotation of the binary star system and can only be separated from the other forms of variability in the infrared.
  • Pulsations of the red giant, which occur either semi- regularly or almost regularly in the case of the Mira stars. The changes in brightness take place in periods of months to years
  • The period of rotation of the red giant can modulate the light curve over star spots or the variable intensity of the outflow of matter along disturbances of the magnetic field
  • Minima due to the absorption of light after the formation of dust from ejected matter such as R-Coronae-Borealis stars
  • Flickering with amplitudes of up to 0.5 mag within minutes. The Flickering appears only in symbiotic stars with river on the Roche limit occur
  • Quasi-periodic oscillations, probably similar to dwarf nova oscillations
  • a periodic signal due to the rotation of the white dwarf and the incidence of matter along the magnetic field lines of the white dwarf. The period is on the order of 10 minutes
  • Normal breakouts of type Z And. These bursts last months to years and show an increase in brightness up to 4 mag in the ultraviolet. The bolometric brightness remains almost constant, but the effective temperature of the blue companion drops from 100,000 to 10,000 K.
  • Symbiotic nova outbursts with changes in brightness of up to 10 mag within days to decades

Red giant component

The spectral type of the red giants in symbiotic binary star systems is mostly between M3 and M7. This is a very late spectral type compared to the general galactic field for red giants. Furthermore, the red giants show a strong stellar wind on average. It was determined by radio observations at more than 10 −7 solar masses per year. A strong stellar wind is a prerequisite for sufficient accretion to the blue companion and therefore a selection effect. With the stellar wind around symbiotic stars, stellar masers like the OH / IR stars have often been observed. These are lines of OH, SiO, H 2 O and CO. In the case of the symbiotic Nova V407 Cygni, it was possible to examine the origin of the maser radiation in detail, as the kinetic energy of the ejected shell interrupted the maser during the nova eruption, which requires a steady stellar wind. But just three months later, the stellar wind of the Mirastern was restored to such an extent that a stellar burl could be detected again.

Given the very heterogeneous structure of the symbiotic stars, it is not surprising that there is not necessarily a red giant in the binary star system. The mass-losing companion can also be yellow giants with the spectral types GK or carbon stars as in IPHAS J205836.43 + 503307.2.

Blue companion component

The blue companion in a symbiotic binary star system often shows a temperature in the ultraviolet of more than 100,000 K at 100 to 1000 times the luminosity of the sun. In the Hertzsprung-Russell diagram , the position of the central stars of planetary nebulae overlaps with those of symbiotic stars. The high luminosity cannot only be a result of accretion on the white dwarf, as this would require an accretion rate of at least 10 −6 solar masses per year. This would be higher than the entire undirected stellar wind from the red giant. Therefore, the high luminosity is probably the result of permanent hydrogen burning on the surface of the white dwarf. The luminosity of the accretion disk should only play a subordinate role, with the exception of symbiotic stars with a neutron star. Another exception is likely to be symbiotic stars with a massive white dwarf as a blue companion. At a mass close to the Chandrasekhar limit , hard X-rays and flickering with a large amplitude can be detected in quiescent light. Both phenomena are attributed to fluctuations in the accretion rate and are a direct consequence of the potential energy released during accretion. The blue companion is also the source of the classic Z-And outbursts and nova outbursts.

The mass stored in an accretion disk should be between 10 −5 and 10 −3 solar masses. Of these, between 50 and 80% fall on the white dwarf, while the remainder flows vertically from the accretion disk via a wind . Overall, the white dwarfs are likely to accrete only 0.1 solar masses in the symbiotic phase, which lasts several million years, whereby a not inconsiderable proportion of this matter is ejected back into interstellar space via nova outbursts.

Orbital parameters

The orbital period for symbiotic stars of type S is between 200 and 1000 days and for type D up to 44 years. Compared to other double stars, the orbits are almost circular, they have a low eccentricity of almost 0. Only the symbiotic stars, whose companion is a main sequence star, show a deviation from the circular shape on average. The low eccentricity in symbiotic stars with a white dwarf is a consequence of a previous common envelope phase . The white dwarf was previously a red giant that transferred part of its atmosphere to the current red giant. The then red giant had expanded so far that the orbit of the companion was temporarily within its extended atmosphere. Frictional forces then led to a disappearance of the eccentricity and a shrinkage of the orbital path.

Crowds

In general, the masses of the red giants are between 0.6 and 3.2 solar masses. The masses of the blue component are mostly between 0.4 and 0.8 solar masses for the classic symbiotic stars and between 1.1 and 1.3 solar masses for the repeating symbiotic novas. The mass of a blue main sequence star in a symbiotic binary star system can be up to 8 solar masses.

Classic symbiotic outbursts

The Z And type outbreaks last months to years and show an increase in brightness up to 4 mag in the ultraviolet. The bolometric brightness remains almost constant. However, there is a drop in the effective temperature of the blue companion from 100,000 to 10,000 K and thus a shift in the electromagnetic radiation from the far ultraviolet to the optical spectral range. Furthermore, the strength of the highly excited emission lines increases, the accretion disk is likely to expand and a bipolar discharge is formed from the white dwarf or the accretion disk. As the optical brightness increases, so does the hard X-ray radiation , which is likely caused by bremsstrahlung when the matter from the bipolar outflow collides with the stellar wind of the red giant. The ionization zone around the symbiotic double star expands during an outbreak.

The outbreak is explained as a consequence of an increased accretion rate due to a thermal instability of the accretion disc, which leads to an expansion of the hydrogen burning zone and thus to the formation of an A to F pseudophotosphere. The biggest problem for this model is the short interval between eruptions, which is sometimes only a few years. During this period, the deflated accretion disc cannot have refilled in the event of wind accretion.

Symbiotic stars with a neutron star show no bursts in the optical spectrum. Their outbreaks occur almost exclusively in the area of ​​hard X-rays and are also the result of an instability of the accretion disk, similar to the outbreak model of dwarf novae . The X-rays are produced when the accreted matter hits the crust of the neutron star, and this interpretation is supported by an accelerated rotation of the X-ray pulsar after the eruption has ended. No major eruptions are known of symbiotic stars with a main sequence star as a blue component.

Symbiotic Novae

A nova is the result of a thermonuclear runaway (an explosive ignition of thermonuclear reactions) on the surface of a white dwarf. The consequence of the sudden onset of hydrogen burning is a steep increase in brightness, the formation of a strong stellar wind combined with the ejection of a shell, an infrared excess due to the formation of dust at some distance from the nova by the ejected matter and the detection of a soft X-ray source after the fall of the optical brightness. The super soft X-ray source becomes visible when the X-ray radiation produced during hydrogen burning is no longer absorbed because the expanding shell has become transparent.

Symbiotic novas differ from the classical novas only in the mass donating companion of the white dwarf, which is a main sequence star or sub-giant in classical novas and a red giant in symbiotic novas. As a result, the amplitude of the outbreak of the symbiotic novae is apparently smaller, as the red giant contributes more light to the brightness of rest. Symbiotic novas are broken down into the repetitive symbiotic novas and the extremely slow novas. The repetitive symbiotic novae are rapid novae with an increase in brightness within days and they return to rest brightness within a few months. The masses of the white dwarfs are between 1.1 and 1.3 solar masses, and therefore the conditions for a reignition of a thermonuclear runaway are given again after a few decades. Their accretion rate is around 10 −7 solar masses per year.

The very slow symbiotic novas show an increase in brightness over months and take years to decades (AG Peg about 100 years) to return to the brightness of rest. The white dwarfs have a mass less than 0.6 times that of the Sun. In these novae a large part of the accreted hydrogen is lost by the stellar wind due to the slow reaction rate on the surface of the white dwarf. In the outbreaks of symbiotic novas z. B. in RS Oph and V407 Cyg high-energy gamma radiation has been detected in contrast to classic Novae. This is also interpreted as a consequence of the formation of a shock front between matter from the nova eruption and the stellar wind of the red giant.

Symbiotic Novae as possible precursors of a type Ia supernova

Repeating symbiotic novae are candidates for the precursors of Type Ia supernovae . These supernovae are the standard glowing candles in cosmology and have led to the discovery of the accelerated expansion of the universe . Although it is generally accepted that type Ia supernovae arise from the collapse of a CO white dwarf after crossing the Chandrasekhar mass limit , it has so far not been possible to detect a precursor of a supernova of this type, nor to show a development process that does not contradict other observations. Because repetitive symbiotic novae are home to white dwarfs with crowds near the Chandrasekhar boundary mass, they are promising candidates. It is not clear, however, whether the white dwarf does not lose more mass in the outbreaks than is gained by accretion. There is an unusual symbiotic star called J0757 that shows no signs of symbiotic activity between bursts, just the spectrum of a red giant. A flare in the 1940s with a ten-year duration with no evidence of mass discharge is interpreted as a calm hydrogen burn on the surface of the white dwarf. This type of symbiotic star could develop into a Type Ia supernova as the mass of the white dwarf increases in these. However, they are too rare to make a significant contribution to the observed rate of 0.003 supernovae Ia per year in the Milky Way. In contrast, in the light curve and the spectra of the supernova PTF 11kx of type Ia, indications of several circumstellar envelopes made of gas and dust have been found. The speed at which these envelopes move is too fast for a stellar wind and far too slow to come from the supernova itself. The distance between the envelopes combined with the rate of expansion makes nova eruptions appear to be the most likely source of the gas and dust envelopes with a few decades between eruptions. Such a short distance between nova bursts and the presence of a continuous component resembling the stellar wind of a red giant in the circumstellar envelope around the supernova indicate a symbiotic nova. However, type Ia supernovae with the properties observed with PTF 11kx are very rare and should therefore be responsible for a maximum of 10% of all cases of this supernova group.

Symbiotic fog

The ionized nebula around symbiotic stars is called a symbiotic nebula. Despite a different evolutionary history, it does not differ in many properties from those of the planetary nebula , since the blue component of symbiotic binary stars in the Hertzsprung-Russell diagram lies at the position of the central stars of the planetary nebula. It can therefore be assumed that many planetary nebulae are incorrectly classified.

Symbiotic nebulae are almost all asymmetrical and show at least 40% bipolarity. A binary nature of the central star is assumed to be the source of bipolarity in both symbiotic and planetary nebulae. The electron density is significantly higher at 10 6 to 10 10 per cubic centimeter and corresponds more closely to the solar corona . The electron temperature of 10,000 to 80,000 K is comparable to the planetary nebula. From spectral analyzes, chemical frequencies in symbiotic nebulae could be determined and the origin of the gas in the fog could be traced back to the red giant. The plasma processed by the blue companion when burning hydrogen is released into the nebula by the stellar wind and sometimes also by jets . However, this source plays a subordinate role in the rest phases, both in terms of the amount of material introduced and as a source of ionization. Only during the classic symbiotic outbursts does the kinetic energy of the star wind of the blue companion become an important source of energy in the symbiotic nebula.

The stellar wind that leads to the formation of the symbiotic nebulae is also a source of soft X-rays from the symbiotic systems. In the area where the stellar wind of the red giant collides with the wind emanating from the blue companion, the gas heats up to temperatures that lead to thermal emissions of up to 2.4 keV. The luminosity is 10 30–31 erg / s and requires a wind speed of the blue component of a few 100 km / s, as it is also derived from optical spectra.

Examples

Web links

Commons : Symbiotic Star  - collection of images, videos and audio files

Individual evidence

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