Be star

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A Be star or OeBeAe star is an "early" star of luminosity class V, IV, or III ( i.e. not a supergiant ), which at least occasionally shows emission lines in the Fraunhofer lines , which is indicated by the suffix e for English. emission lines after which B is given for the spectral class .

definition

Flattening of the Be star Achernar due to the high rotational speed

In about 15% of the stars with the "early" spectral classes (O to A), emission lines are embedded in the cores of the first Balmer lines and the lines of simply ionized elements of iron (Fe II), silicon (Si II) and magnesium (Mg II) . A central depression occurs within the emission at the hydrogen lines. Another characteristic of the Be stars is the great width of the photospheric absorption lines , which are the result of high rotational speeds on the star's surface with values ​​between 200 and 500 km / s. This corresponds to 70–80% of the speed at which the centrifugal forces exceed the gravitational force at the equator . The high speed of rotation also leads to an asymmetry of the stellar wind . Due to the strong flattening of the star, the poles are hotter than the equator and the outflowing matter is accelerated more strongly by the more intense radiation . All Be stars show a pronounced infrared excess . Linear polarization of up to 2% has been observed in the emission lines of some Be stars and is interpreted as a result of electron scattering in the non-spherical circumstellar shell.

Be stars are divided into classic Be stars and Be shell stars. While Be stars show no narrow absorption lines of the Balmer series and of metal lines, which are called envelope lines, these can be detected with Be envelope stars. The distinction is probably purely geometrically due to the inclination angle with which the observer looks at the axis of rotation of the blue stars.

interpretation

The emission lines arise in optical thin envelopes that form along the equator of the stars and thus lead to a ring that is stationary at times. The Be stars in the Hertzsprung-Russell diagram are in the area of ​​the Beta Cephei stars . In recent years, multiperiodic radial and non-radial pulsations have been found in the Be stars . The rings are a result of a resonance of closely spaced radial vibrations in combination with the high rotation speed of the early stars, so that both effects together lead to a separation of matter in the area of ​​the equator. Due to the greater distance from the star, the ring rotates more slowly than the star's surface (see Kepler's laws ) and therefore emission lines only form in the nuclei of the absorption lines. From the analysis of the spectrum of Be stars, an average density in the rings between 10 10 and 10 13 hydrogen atoms per cm 3 was derived with an envelope mass of 10 −10 solar masses .

The high speed of rotation of Be stars could be the result of a past interaction with a companion in a binary star system that is now a compact object in the form of a neutron star , white dwarf or black hole . This hypothesis would also explain the high number of X-ray binary stars among the Be stars. Alternatively, the Be stars may have been born with a rapid rate of rotation or acquired the rapid rotation during the main sequence phase . The relationship between the speed of rotation and the critical speed at which the centrifugal forces exceed gravity seems to be mass-dependent. Probably because of the high speed of rotation, Be stars have an extended main sequence phase as hydrogen is mixed into the core of the stars.

variability

Be stars show a pronounced variability in the strength of the emission lines, whereby the emission lines can no longer be detected at times as with Pleione in the years 1905 to 1938. A variability in the spectrum can also be accompanied by a change in the optical brightness of the star as in Example of Gamma Cassiopeiae . Since the early stars emit most of the radiation in the ultraviolet radiation , the cooler rings can absorb energy and emit it again in the optical .

Many variable Be stars are also known as shell stars or Gamma Cassiopeiae stars ( GCVS designation GCAS), whereby the brightness in the visual changes by up to 2.5 magnitudes . In the outbreak, the spectral type can cool down to F5. This corresponds to a surface temperature of around 6800 K , while an undisturbed B-star shows a surface temperature of 10,000 to 25,000 K. It is likely that much of the star's surface will be covered by a curved disk during the eruption. The curvature is the result of a compact companion (e.g. neutron star) not revolving in the plane of rotation and the resulting tidal effects. An asymmetrical supernova explosion during the formation of the neutron star is assumed to be the cause of the deviating orbital plane .

In the meantime it has been shown that not all variable Be stars can be classified as Gamma Cassiopeiae stars. In the GCVS these stars are currently grouped under the abbreviation BE . Others have already defined new types, for example the Lambda-Eridani stars.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists over 350 stars with the abbreviations BE or GCAS , which means that just over 0.5% of all stars in this catalog are classified as variable Be stars.

V / R variations

In many Be stars, variations in the strength of the blue (V) or red shifted (R) emission lines have been found. Correlated with the changes in the emission lines, the degree of polarization also fluctuates . The V / R variations are interpreted as a deviation from the axial symmetry in the ring around the Be star. The cause could be a vibration in the disk around the early star, triggered by gravitational resonance due to the orbit of a companion around the Be star.

Be stars in X-ray binary stars

Be stars are often the mass donor in HMXB (Engl. High Mass X-ray binaries , X-ray binaries high ground). A neutron star orbits the Be star in a mostly elliptical orbit. If the neutron star is far away, it accretes only a little matter, and when it hits its surface after falling through the gravitational field , only a small amount of X-ray radiation is released. If the neutron star comes into contact with the circumstellar ring around the Be star, this can increase or weaken the X-ray radiation through the transfer of more gas. Attenuation occurs when the matter in the ring is so dense that the X-ray radiation generated by the neutron star is absorbed again in the ring . The temporary formation of rings or envelopes around Be stars is often the cause of irregularly variable X-rays in HMXBs. During an accretion phase, the X-rays from the BeXB stars are usually pulsed with a period of a few seconds to a minute. This is a consequence of the flow of matter along the magnetic field lines of the neutron star, which means that, due to the rotation of the star, the regions emitting bremsstrahlung are periodically visible or covered over the magnetic poles.

Bursts are observed from Be / X X-ray binary stars, which are divided into type I and type II bursts based on the duration of the eruptions. Type I bursts last a small part of the orbital period of the binary star system and achieve X-ray brightnesses of up to 10 37 erg / s. Because it occurs near the periastron passage in an elliptical orbit , it is an accretion event when the neutron star dips into the circumstellar disk around the Be star. The type II bursts, on the other hand, can last several orbit revolutions and achieve X-ray brightnesses of more than 10 37 erg / s. Its cause is seen in the excitation of a disc inclined against the orbital axis around the Be star by the passing companion.

The Gamma Cassiopeiae analogs are differentiated from the BeXB (Be stars in X-ray binary stars ) . They show a weaker X-ray radiation than normal BeXB with a harder spectrum. This term describes a larger proportion of short-wave to long-wave X-rays. Furthermore, Gamma Cassiopeiae analogs do not show any X-ray pulsations that are interpreted as a result of a strong magnetic field on a neutron star . Due to the similarity of the X-ray spectrum of Gamma Cassiopeiae analogs with those of cataclysmic variables , it has been assumed that the optically undetectable companion is a white dwarf . However, it is difficult to understand how a white dwarf forms in orbit around a B star, since the more developed star must have been even more massive than the B star and therefore exploded as a supernova . A neutron star or black hole is formed from a supernova explosion . The alternative is that the Gamma Cassiopeiae analogs are not X-ray binaries. This assumption is also supported by the lack of modulation of the X-ray light curve with the orbit of the hypothetical white dwarf. However, all Gamma Cassiopeiae analogs show a spectral class between B0.5 and B1.5 with an intense Hα emission line with an equivalent width of 30 to 40 Angstroms . These very similar stellar parameters indicate a different cause for the origin of the X-rays. It is therefore assumed that magnetic short circuits occur in a magnetic field in the circumstellar disk of the Be star due to the differential rotation. The short circuits create flares similar to some solar flares , which also release X-rays.

Super-soft X-ray sources are white dwarfs in binary star systems, in which hydrogen burning occurs on the surface of the white dwarfs . Most Super Soft X-ray Sources arise in cataclysmic variables as a result of a nova eruption , in which the mass-donating companion star of the white dwarf is a red dwarf or a later subgiant . However, in rare cases a white dwarf from the stellar wind or from the disk around a Be star can accret enough matter to initially collect it on its surface. After a few years, when the matter has reached a suitable density, the nuclear reaction ignites and soft X-rays are emitted from the thin atmosphere of the white dwarf.

Analyzes of the optical light curves show a number of time scales in the variability of the Be / X binary stars. These include radial and non-radial oscillations of the atmosphere of the Be star with periods between 0.1 and 2 days. Overlapping of these pulsations can show periods of a few 100 days and are then difficult to separate from the orbital period in the range of 10 to 500 days. The latter fluctuations in brightness are interpreted as a disturbance of the circumstellar disk by the compact companion or the formation of a temporary accretion disk around the neutron star. There is also variability on the order of several years, which is likely due to irregularities in the circumstellar disk. An example of such a disturbance is a bending of the disk due to a resonance between the orbit of the neutron star and the orbital period in the circumstellar disk.

Development models

In the course of further development, the Be star expands, whereby the speed of rotation decreases while maintaining the angular momentum and the decretion disc disappears. However, the rapid rotation changed the chemical structure of the star, as more hydrogen-rich matter was transported into the core due to meridional currents. This leads to a higher nuclear mass of the former Be stars compared to slowly rotating stars of the spectral class B. It is assumed that because of this property, Be stars develop into bright blue variables . Massive Be stars could, according to the collapsar model, also be the forerunners of long-lasting gamma ray bursts .

B [e] stars

A distinction is made between the Be stars and the B [e] stars, which are sometimes also referred to as BQ stars or Bep stars. They are described as stars with early spectral types with low-excitation emission lines, forbidden lines, and signs of warm dust in the near and mid-infrared. Their characteristics are:

  • The presence of broad Balmer lines, some of which also show P-Cygni profiles , with half- widths of up to 1000 Angstroms
  • Emission lines with allowed transitions from simply ionized metals , mostly Fe II
  • Narrow emission lines with forbidden transitions from [FeII] and [OI]
  • A strong infrared excess due to circumstellar dust with temperatures around 1000 K.
  • In a multicolor diagram in the infrared, the stars form a separate group

The main difference between B [e] stars and Be stars is the absence of any evidence of warm dust around Be stars. B [e] stars are further divided into normal and B [e] supergiants . The former are likely to be low-luminosity objects that evolve from a AGB star to a planetary nebula . The outer atmosphere is still inflated and re-emits the radiation at temperatures that are not yet sufficient to stimulate the expelled shell. However, the B [e] phenomenon is also associated with the Herbig Ae / Be stars , which are only on their way to the age-zero main sequence. The B [e] stars are probably a very heterogeneous group.

The B [e] supergiants are stars in the post-main sequence stage with luminosity from 10 4 to 10 6 times that of the sun. They also show a high rotational speed, which is probably also the cause of the circumstellar disk made of dust and molecules. The shape of a disk was deduced from polarimetric measurements and the shadowing by a dense disk also explains the coexistence of ionizing radiation from the hot atmosphere of a B-star and the presence of dust and molecules. If these were directly exposed to such radiation, they should be destroyed by photoevaporation within a short time. B [e] supergiants are viewed as predecessors or, according to other sources, as successors of luminous blue variables or as the result of mergers in a binary star system. Also, many B [e] sg are variable with periods of a few 10 to 100 days and thus resemble interacting binary star systems such as the Beta Lyrae stars , the W Serpentis stars and the double period variables . The properties of the disk of some B [e] supergiants can best be interpreted as a Kepler disk around a binary star system. The companion of the supergiant can only be detected indirectly via changes in radial speed. A scientific consensus on the question of the formation of B [e] supergiants has not yet been reached.

B [e] stars with a luminosity class IV or V are also referred to as FS Canis Majoris stars. These are probably developed stars that are still close to the main sequence and mostly occur in a binary star system. Their properties are explained as the result of a slow equatorial and a fast polar stellar wind with a speed of a few 100 km / s, as a result of which a ring of dust forms around the equator. In the equatorial outflow disk, which flows off at a speed of only a few 10 km / s, the emission lines for hydrogen and low-excited metals are formed. The emission lines of the highly excited metals, on the other hand, arise in the polar regions, where the stellar wind absorbs less UV radiation due to its low density.

See also

Examples

Web links

Individual evidence

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  2. ↑ Masahiro Mon, Masakazu Suzuki, Yuki Moritani, Tomokazu Kogure: Spectroscopic Variations of the Be-shell Star EW Lac in the V / R Variation Periods . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.2511v1 .
  3. ↑ H. Scheffler, H. Elsässer: Physics of the sun and the stars . 2nd Edition. BI Wissenschaftsverlag, Mannheim 1990, ISBN 3-411-14172-7 .
  4. ^ Walter Lewin, Michael van der Klies: Compact Stellar X-ray Sources (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2010, ISBN 978-0-521-15806-0 .
  5. ↑ Amber N. Marsh Boyer, M. Virginia McSwain, Christina Aragona, and Benjamin Ou-Yang: Physical Properties of the B and Be Star Populations of h and χ Persei . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.5771 .
  6. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JABarth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
  7. ↑ Rebecca G. Martin, JE Pringle, Christopher A. Tout, Stephen H. Lubow: Tidal Warping of Be Star Decretion Discs . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1106.2591 .
  8. ↑ a b Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved July 28, 2019 .
  9. ↑ Christophe Martayan, Thomas Rivinius, Dietrich Baade, Anne-Marie Hubert, Jean Zorec: Review about populations of Be stars: stellar evolution of extreme stars . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1010.3341 .
  10. ^ WHG Lewin, J. van Paradijs, EPJ van den Heuvel: X-ray binaries . Cambridge University Press, Cambridge 1995, ISBN 0-521-59934-2 .
  11. ↑ Sachindra Naik, Chandreyee Maitra, Gaurava K. Jaisawal, Biswajit Paul: Timing and Spectral properties of Be / X-ray pulsar EXO 2030 + 375 during a Type I outburst . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.1112 .
  12. ↑ P. Maggi et al .: Discovery of a 168.8 s X-ray pulsar transiting in front of its Be companion star in the Large Magellanic Cloud . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.4665v1 .
  13. ↑ Pablo Reig: Be / X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1101.5036 .
  14. ↑ JM Torrejon et al .: HOT THERMAL X-RAY EMISSION FROM THE BE STAR HD119682 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.2913 .
  15. ↑ JM Torrejon, NS Schulz, MA Nowak: Chandra and Suzaku observations of the Be / X-ray star HD110432 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.1004v1 .
  16. ^ KL Li et al .: A Luminous Be + White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.5023v1 .
  17. ↑ AJBird, MJCoe, VAMcBride and A.Udalski: On the periodicities present in the optical light curves of SMC Be / X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.5426 .
  18. ↑ Th. Rivinius, AC Carciofi, C. Martayan: Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.3962v1 .
  19. ^ Zickgraf, F.-J .: Current Definition of B [e] Stars . In: B [e] stars: Proceedings of the Paris workshop held 9-12 June, 1997 . Kluwer Academic Publishers, Dordrecht; Boston 1998, ISBN 0-7923-5208-4 , pp. 1 .
  20. ↑ Stefan Kraus, Nuria Calvet, Lee Hartmann, Karl-Heinz Hofmann, Alexander Kreplin, John D. Monnier, and Gerd Weigelt: On the nature of the Herbig B [e] star binary system V921 Scorpii: Discovery of a close companion and relation to the large-scale bipolar nebula . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.2420v1 .
  21. ^ A. Aret, M. Kraus, MF Muratore and M. Borges Fernandes: A new observational tracer for high-density disc-like structures around B [e] supergiants . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.6808v1 .
  22. ↑ HEWheelwright, WJ de Wit, G. Weigelt, RD Oudmaijer, and JD Ilee: AMBER and CRIRES observations of the binary sgB [e] star HD 327083: evidence of a gaseous disc traced by CO bandhead emission . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.6252 .
  23. ^ PR Dunstall, M. Fraser, JS Clark, PA Crowther, PL Dufton, CJ Evans, DJ Lennon, I. Soszynski, WD Taylor, JS Vink: The VLT-FLAMES Tarantula Survey V. The peculiar B [e] -like supergiant , VFTS698, in 30 Doradus . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.6389v1 .
  24. ↑ J. cushion, D. Korčáková, V. Votruba, P. Skoda, M. Slechta, B. Kucerova, J. Kubat: Time-dependent spectral-feature variations of stars displaying the B [e] phenomenon; I. V2028 Cyg . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.4003 .