AM Canum Venaticorum Star

from Wikipedia, the free encyclopedia

AM-Canum-Venaticorum stars or AM-CVn stars are compact, close binary star systems , consisting of an accreting white dwarf and another degenerate companion. The components circulate for between 5 and 65 minutes. The difference from the cataclysmic variable stars is the lack of hydrogen in the companion's atmosphere and the accreted matter. This class of variable stars is named after the prototype AM Canum Venaticorum .

construction

Artist's impression of an AM-CVn system

The AM Canum Venaticorum stars consist of a white dwarf in a binary star system with a companion that is either also a white dwarf, a helium star or a developed main sequence star . The companion fills its Roche volume and transfers matter to the white dwarf. The matter flows along a stream towards the white dwarf and, due to the conservation of angular momentum, forms an accretion disk around the compact star. At the point where the flow of matter hits the accretion disc, the matter is slowed down; a bright, hot spot forms. This leads to a modulation of the light curve of the AM-CVn system with the period of the cycle duration. Another sign of the accretion of matter is flickering , a small, irregular brightness variation in the seconds range. The accreted matter loses angular momentum in the disk around the white dwarf and finally falls on it. Upon impact, the resulting thermal radiation is mainly emitted in the X-ray range . At ES Ceti, due to the small distance between the two degenerate stars, the matter could fall directly onto the white dwarf without going through an accretion disk.

Classification

The AM Canum Venaticorum stars are mainly classified according to the period of orbit:

  • In the long-period systems with a cycle time of more than 40 minutes, only a small mass exchange takes place. The accretion disks are optically thin and range the dominating emission lines of helium. The variability is often not pronounced and these AM-CVn stars are difficult to spot.
  • The short-period systems with a cycle time of less than 20 minutes are always in a state of high mass transfers with an optically thick accretion disk. Its spectrum shows prominently broad absorption lines for helium. These AM-CVn stars always or partially show superhumps . This is a sinusoidal variation of the light curve with a period that is a few percent longer than the orbital period of the binary star system and is probably caused by a rotating elliptical accretion disk. These systems correspond to the nova-like cataclysmic variables that dwarf novae are in a state of permanent eruption.
  • The erupting systems with a cycle time of 20 to 40 minutes. They show bursts with amplitudes between 3 and 5 mag , which correspond to those of dwarf novae in the cataclysmic variable stars . They can also experience superhumps. The outbreaks last a few weeks and recur irregularly over a period of months. Some AM-CVN stars in this group show a dip in brightness of unknown cause before the outbursts.

The dwarf nova-like eruptions can be explained with a disk instability model according to the model for hydrogen-rich cataclysmic variables. The crucial difference is the strong influence of a variable mass transfer rate, which dominates the development of super bursts, dips and standstills. The mass transfer rate probably fluctuates due to the different heating of the mass donor in previous outbreaks, which in turn is attributed to a precessing, curved accretion disk. The similarity to the hydrogen-rich cataclysmic variables is also shown in the light curve in the X-ray range. The X-rays in calm light at low accretion rates arise in the boundary layer between the white dwarf and the accretion disc. The temperature of the boundary layer, in which the plasma is decelerated from the Kepler speed in the accretion disk to the speed of rotation of the white dwarf, reaches values ​​around a few kilo-electron volts and only the stellar wind from the white dwarf absorbs part of the X-rays. In the outbreak at higher accretion rates, the temperature in the boundary layer continues to rise, but the boundary layer also absorbs the X-rays almost completely due to an increasing opacity . This behavior corresponds to the hydrogen-rich dwarf novae .

Thermonuclear eruptions

The normal bursts of AM-CVn stars correspond to those of dwarf novae . The accretion disk oscillates between two stable states. In the active state, the viscosity of the material increases and the disc heats up due to the increased friction. When the accretion disk is partially deflated, the eruption ends and it goes into the low state. Here, less matter is transferred to the white dwarf than flows into the accretion disk, which leads to a new outbreak after a while.

In addition, AM Canum Venaticorum stars could also have the equivalent of classical Novae . While novae produce explosive hydrogen burning , AM-CVn systems produce an unstable helium burning on the surface of the white dwarf. This type of breakout is expected in the short period AM-CVn systems. With low mass transfer rates from the companion to the white dwarf, there could even be an unstable helium flash with a mass of up to 0.1 solar masses involved. Due to the high pressure of the helium near the surface of the white dwarf, the thermonuclear reactions can produce heavy elements up to 56 Ni. These radioactive isotopes are also the energy source for the afterglow of supernovae , and a corresponding flash of helium would be perceived as a faint supernova of type Ia , which only achieves a tenth of the maximum brightness of its class. Archival images of the Chandra X-ray satellite before the outbreak of the 2007on supernova in NGC 1404 found a weak X-ray source with a spectrum similar to that of an AM-CVn star.

More recent studies, however, raise doubts as to whether the merging of two degenerate white dwarfs will result in a type Ia supernova. First, the total mass of a merging binary star system composed of two white dwarfs scatters between 1.4 and 2 solar masses and can hardly explain the uniformity in the energy release of these stellar explosions. And secondly, simulations show that in most cases either an accretion-induced collapse leads directly to the formation of a neutron star rather than a thermonuclear explosion or to a transformation into a massive white dwarf of the O-Ne-Mg type, which is transformed by means of electron capture also transformed into a neutron star. Therefore, type Ia supernovae are probably very rarely the product of a merger of two white dwarfs from an AM Canum Venaticorum star. In the case of a very thin helium-rich layer with a mass of less than 0.1 solar masses, if the helium burn is ignited in the case of massive white dwarfs in an AM-CVn system, a shock front can propagate, which runs at the speed of sound through the zone with convective energy transport . Instead of a nova eruption whose luminosity does not exceed the Eddington limit, the result could be an ignition of the unstable carbon burn in the core of the white dwarf. This type of supernova of type Ia should be detectable by a certain chemical composition of the expanding shell with little 52 Fe and 56 Ni as well as an increased proportion of 40 Ca, which is accompanied by a deviation from spherical symmetry.

It may only be a deflagration instead of a detonation if an unstable helium flash occurs in the boundary layer between the CO core and a helium atmosphere . This subspecies of thermonuclear supernovae is called type .Ia because the luminosity is only one tenth of the value of a normal Ia supernova. The faint supernova SN 2010X is counted among the supernovae .Ia.

development

Several development channels are known for the formation of the AM-CVn systems in order to get two degenerate stars into a close orbit :

  • In the so-called white dwarf canal, a pair of white dwarfs emerges as a result of a common shell phase . The white dwarf formed first is immersed in the atmosphere of its evolved companion, and the friction leads to both a shortening of the orbit axis and a discarding of the companion's atmosphere. A separate binary star system is created from two white dwarfs, which come into contact due to the radiation of gravitational waves and thus develop into an AM Canum Venaticorum star.
  • In the helium star channel, a white dwarf accretes from an initially not degenerate helium star. In the course of time enough matter is transferred from the companion to extinguish the helium burn . as a result, the binary star system develops into shorter orbital times of up to a minimum of 10 minutes due to the radiation of gravitational waves. At this point the internal structure of the companion changes in such a way that it expands, and as a result the orbit axis of the binary star system grows again. The AM-CVn star ends its active phase, leaving a separate pair of white dwarfs. The helium star channel is also referred to as the double common envelope channel, since in this evolutionary model both stars develop into a red giant , whose companions temporarily orbit in their expanded atmosphere.
  • In the developed-cataclysmic-variable-channel it concerns normal cataclysmic-variable , in which the mass exchange begins only when the companion of the white dwarf develops away from the main sequence and has already used up the hydrogen supply in its core. The hydrogen-rich shell of the companion is lost in the course of the development of the cataclysmic variable through mass transfer. What remains is a partially degenerate helium star as a companion of the accreting white dwarf, whereby the atmosphere, in contrast to the other two formation scenarios, still contains a few percent hydrogen.

In all scenarios the development of an AM-CVn star is controlled by the emission of gravitational waves . The gravitational waves transport the angular momentum of the double star system, and so the double star always remains a half-separated contact system. Due to the small distance between the two stars, the gravitational wave radiation is so strong that it can be detected with the help of LISA . When two white dwarfs merge, depending on the type of mass transfer, which can be stable or unstable, hot sub- dwarfs, massive white dwarfs, extreme helium stars , R-Coronae-Borealis stars or supernovae of type Ia or .Ia can arise.

In the area of ​​the orbital periods of the AM-CVN stars there are also separate double stars, which consist of two white dwarfs. These are known as double stars from white dwarfs with extremely low mass, the mass of the stars being at values ​​below 0.2 solar masses . According to the English term, these binary star systems are referred to as ELM (extremely low mass) helium white dwarfs. They only come into contact and the mass transfer begins with periods of circulation of less than 10 minutes. The separate ELM double stars are better suited for verifying the general theory of relativity and the gravitational waves derived from it than the AM-CVn stars, since the interaction between the components makes it difficult to determine their physical properties. J0651 + 2844 is the closest known eclipse double star system consisting of white dwarfs without mass exchange. The orbital period is only 765 seconds and decreases by 0.31 milliseconds per year in accordance with general relativity.

Examples

  • AM Canum Venaticorum
  • ES Ceti

Individual evidence

  1. ↑ G. Nelemans: AM CVn stars . In: Astrophysics. Solar and Stellar Astrophysics . 2005, arxiv : astro-ph / 0409676v2 .
  2. ↑ David Levitan et al: PTF1 J071912.13 + 485834.0: AN OUTBURSTING AM CVN SYSTEM DISCOVERED BY A SYNOPTIC SURVEY . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1107.1209v1 .
  3. ^ Edward M. Sion, Albert P. Linnell, Patrick Godon, Ronald-Louis Ballouz: The Hot Components of AM CVn Helium Cataclysmics . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1108.1388v1 .
  4. ↑ Lars Bildsten, Ken J. Shen, Nevin N. Weinberg, Gijs Nelemans: Faint Thermonuclear supernovae from AM Canum Venaticorum binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2007, arxiv : astro-ph / 0703578v2 .
  5. ↑ Gavin Ramsay, Thomas Barclay, Danny Steeghs, Peter J. Wheatley, Pasi Hakala, Iwona Kotko, Simon Rosen: The long-term optical behavior of helium-accreting AM CVn binaries . In: Monthly Notice of the Royal Astronomical Society . tape 419 , 2012, p. 2836–2843 , doi : 10.1111 / j.1365-2966.2011.19924.x .
  6. ^ Iwona Kotko, Jean-Pierre Lasota, Guillaume Dubus, and Jean-Marie Hameury: Models of AM CVn stars outbursts . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.5999v1 .
  7. ↑ Gavin Ramsay, Peter J. Wheatley, Simon Rosen, Thomas Barclay, Danny Steeghs: Suppression of X-rays during an optical outburst of the helium dwarf nova KL Dra . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.1222 .
  8. ↑ Gils Nelemans et al: The astrophysics of ultra-compact binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0902.2923v1 .
  9. ↑ Rasmus Voss & Gijs Nelemans: Discovery of the progenitor of the type Ia supernova 2007on . In: Nature . tape 451 , 2008, p. 802-804 , doi : 10.1038 / nature06602 .
  10. ↑ Bo Wanga, Zhanwen Hana: Progenitors of type Ia supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.1155v1 .
  11. ↑ DEAN M. TOWNSLEY, KEVIN MOORE, AND LARS BILDSTEN: LATERALLY PROPAGATING DETONATIONS IN THIN HELIUM LAYERS ON ACCRETING WHITE DWARFS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.6517v1 .
  12. ↑ Mukremin Kilic, JJ Hermes, A. Gianninas, Warren R. Brown, Craig O. Heinke, MA Agueros, Paul Chote, Denis J. Sullivan, Keaton J. Bell, Samuel T. Harrold: Found: The Progenitors of AM CVn and Supernovae .Ia . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.6359v1 .
  13. ↑ GHA Roelofs, G. Nelemans, and PJ Groot: The population of AM CVn stars from the Sloan Digital Sky Survey . In: Astrophysics. Solar and Stellar Astrophysics . 2007, arxiv : 0709.2951v1 .
  14. ↑ E. Breedt, BT Gänsicke, TR Marsh, D. Steeghs, AJ Drake, CM Copper Wheat: CSS100603: 112,253 to 111,037: A helium-rich dwarf nova with a 65 minute orbital period . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.3836v1 .
  15. ↑ MUKREMIN KILIC, WARREN R. BROWN, CARLOS ALLENDE PRIETO, SJ KENYON, CRAIG O. HEINKE, MA AGÜERO, SJ KLEINMAN: THE ELM SURVEY. IV. 24 WHITE DWARF MERGER SYSTEMS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.0028v1 .
  16. ↑ JJ Hermes, Mukremin Kilic, Warren R. Brown, DE Winget, Carlos Allende Prieto, A. Gianninas, Anjum S. Mukadam, Antonio Cabrera-Lavers, Scott J. Kenyon: Rapid Orbital Decay in the 12.75-minute WD + WD Binary J0651 + 2844 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.5051 .