Luminous red nova

Luminous Red Novae, short red novae, are a rare class of eruptive variable stars ( a subclass of the variable stars ). These star class developed in the context of an outbreak an intense red color.

Properties

The light next to the prototype Strengthening Red Novae V838 Mon, V4332 Sgr and V1309 Sco and extragalactic star M31RV, NGC300 OT2008, OT2006 M85 -1 and M99 PTF10FQS be counted. Although its optical light curve is similar to a steep rise and slow fall of classical novae, there are a number of differentiating features:

• The expansion velocity of the accelerated during the eruption shell is 100 km / s instead of at more than 1000 km / s
• The absence of highly ionized spectral lines
• In classical novae a part of the white dwarf is blasted in the eruption. A corresponding signature can luminous in the spectra of red novae not be detected.
• The low temperature of the cast-off shell a few weeks after the eruption of only 900 K in the case of V838 Mon and the occurrence of a strong infrared excess.
• In the late stage, shows the spectrum of an M- or L- supergiants
• The luminosity of luminous red nova of about one million solar luminosities is too large in relation to the speed of light loss for classical novae and too small for a supernova

Term

In addition to the terms Red Nova or Luminous Red Nova these stars are also called intermediate- luminosity transient, Intermediate - Luminosity Optical Transients or Intermediate - Luminosity Red Transients. These designations are somewhat wider than that of the term Red Nova, as they contain all outbreaks of stars whose maximum luminosity of a classical nova and a supernova lie between the. A Eruptionsart, which is counted to the Intermediate - Luminosity Optical Transients, but not to the light Strengthening Red Novae is a great eruption of a luminous Strengthening Blue Variables. The discovery of this new class of variable stars is the result of systematic searches for supernovae in nearby galaxies.

Models

In order to explain the eruptions, the following models have been proposed:

• An Atypical Nova outbreak based on a thermonuclear runaway on the surface of a white dwarf that has a small mass of 0.2 solar masses. In classical novae, the mass of the white dwarf is more than 0.5 solar masses.
• Stepping on or near the Asymptotic Giant Branch to thermal pulses when the core of the star ignites carbon explosively. The absence of a circumstellar envelope which is typical of stars on the Asymptotic Giant Branch, makes this scenario unlikely.
• In a helium flash in a solid star core ignites helium and the reaction produces carbon. This process of explosive helium burning, in some cases, lead to a strong mass loss with the ejection of a circumstellar envelope.
• An atypical type IIn supernova, which is exposed to strong circumstellar extinction could reproduce the light curve of the red novae.
• The capture of an exoplanet could have released the observed energy. In recent years, many exoplanets have been discovered in close orbits around their host stars. Come star and planet too close to begin their atmospheres to interact with each other. This increases the friction and the planet crashes into the star. The released energy is sufficient to ignite the deuterium burning in the star's atmosphere and thus the brightness increases as observed within a few days of strong.
• The merger of two stars in a binary star system is already longer viewed as a cause of rapidly rotating single star FK Comae Berenices type. As a precursor, the eclipsing contact systems are suspected of type W Ursae Majoris star. In these, the two stars have already come so close in a binary star system that they form a common envelope. Friction spirals of smaller companion into the larger component and the implementation of the kinetic energy leads to the ejection of an expanding shell.
• Depending on the radius and density of the stars in a close binary system can be a star torn apart by tidal forces and then forms an accretion disk around the more massive main sequence star. The energy released in the accretion gravitational energy is then observed as the Red Nova.
• Accretion of matter onto a main sequence star of an AGB star. The resulting accretion disk and jets could explain the shape of some bipolar planetary nebula.

Observations were confirmed by the models of the Merger bursts and the bursts of massive AGB stars with strong absorbance.

Merger bursts

The erupted in 2008 luminous red nova V1309 Sco is in a monitoring field of OGLE, and therefore are photometric data from this red nova from the years before the outbreak. In the years before the outbreak, there was a covering of light change with a period of 1.4 days. The light change was typical of a contact system. In these binary systems, the distance between the two stars is so low that they form a common envelope. The period has decreased exponentially in the six years prior to the outbreak and the light curve was highly variable. In the year before the outbreak no longer cover light change was detectable. The overall brightness of V1309 Sco rose in the years to continuously to in 2007 to 1 may decrease. The following year, the brightness is slowly increased, and then within a few weeks the maximum with an outburst amplitude of 10 may achieve.

The coverage change of light shows that V1309 Sco was a close binary star system before the outbreak, the components of which had already formed a common sheath. The exponential decrease of the orbital period by 1.2 percent within 6 years freed up enough energy to explain the slow increase in brightness before the outbreak. In the year before the eruption, the light curve changed. She was no longer typical for a light cover change, but for a Rotationallichtwechsel an ellipsoidal body. The drop in brightness before the outbreak suggests that V1309 Sco preferred threw out in the orbital plane due to the high angular momentum matter. This is likely to have formed a dusty circumstellar disk, as detected in V4332 Sgr due to the high degree of polarization measured. The trigger of the eruption in 2008 could be a Darwin instability. At the outbreak of angular momentum is converted to a small part in radiation, while most of the angular momentum is reduced by the ejection of matter. The predecessor of the Red Nova was according to simulations, a double star with a total mass of about 2 solar masses and an orbital period of 2.3 to 3.3 days.

The eruption mechanism is also referred to as a merger burst ( in German about merger outbreak ). Due to the different masses and the mass distribution in the binary systems involved and depending on the orbital inclination, a series of breakout light curves are modeled. Whether the Mergeburst is the only mechanism for the generation of luminous red nova or whether the other models listed above are also responsible for some outbreaks, can not be answered on the basis of the available data. Case of a merger between a star and a planet and a brown dwarf and a planet, it also comes to an outbreak with a smaller amplitude and duration. An eruption with a duration of only a few days could not be detected.

In particular, hot Jupiters are candidates for a merger burst. This gas planets are already in short-period orbits around their central star. The tracks can be unstable, for example, due to the Darwin instability. The fusion rate within the Milky Way is estimated to be 0.1 to 1 per year. The course of the outbreak is dependent on the ratio of the densities of the star and the planet, either because the planet occurs as an object in the atmosphere of the star and a stable mass flow from the planet adjusts to the star or the planet will be torn apart by tidal forces before the merger. An outburst in the optical, ultraviolet and in the field of X-rays should be a consequence of the thermal radiation of the accretion disk, and caused by the interaction of jets with circumstellar material.

A merger burst can lead to eruptions whose luminosities exceed the Reds of light Strengthening novae clearly. The Supernova Impostor SN 2009ip could be the result of a merger of a supermassive star with a mass of about 100 solar masses and a massive star of about 30 solar masses. The time scales in this outbreak are similar to those of a merger bursts as in V838 Mon, but the luminosities are higher by several orders of magnitude. However, may have been caused by an unusual core-collapse supernova or a large eruption of a luminous Strengthening Blue Variables the event.

The curves of the light bursts merger an alternate hypothesis has been proposed. Thus, an unrealistically high opacity is required for the observed period shortening before the outbreak and the slow rise to maximum brightness takes too long for a merger burst. Instead, it could be the beginning of a contact phase of a binary system, which a lot of material is dispensed from a star in a common non- co-rotating shell around the binary star. Within the common envelope, therefore, there is a binary star system with a smaller period still. .

Explosive processes in super- AGB stars

The group of Red Novae, which do not arise from a merger burst belong SN 2008S, NGC300 OT2008, OT2006 M85 -1, SN 2010da, SN 2010dn, PTF 10acbp and M99 PTF10FQS. They share the following characteristics:

• The predecessor is in the optical undetectable, since the optical radiation is absorbed by circumstellar dust
• Infrared luminosities place the progenitor star at the tip of the AGB- development
• The absolute visual magnitude during the outbreak reaches -13 to -15
• During eruption narrow emission lines with speeds show significantly smaller than 3000 km / s
• There is evidence of circumstellar dust in the near and mid-infrared, which probably consists of carbonates instead of silicates
• The progenitor star did not survive the outbreak

The cause for this subgroup the birth of a massive white dwarf, a faint core-collapse supernova or the eruption of a massive star was discussed. The observations correspond best to the interpretation of the birth of a massive white dwarf from a massive AGB star of about 10 solar masses. However, it is possible that there are former Red supergiants on the development path to warmer temperatures.

Failed Supernovae

An Un -Nova is a failed core-collapse supernova. A massive star at the end of his life can no longer generate energy through nuclear fusion further, since the synthesis of elements beyond iron are endothermic reactions that consume more energy than they produce after the nucleosynthesis of iron in its core. Thus, the radiation pressure around the star in a gravitational collapse to prevent missing and the core collapses into a proto- neutron star with the emission of neutrinos. In this case, a shock wave travels through the atmosphere of the star, the firm runs due to the dissociation of heavy elements. Collapse of the proto neutron star very quickly into a black hole, so there is little or no electromagnetic radiation is emitted. The collapse into a black hole can occur when falling back matter drives the proto- neutron star on the Tolman -Oppenheimer - Volkoff limit or the neutron star by cooling loses enough thermal energy and therefore the core collapse can not turn away. If the star is a red hyper giant, then its atmosphere is only weakly gravitationally bound. In core-collapse between 0.2 and 0.5 solar masses emitted in the form of neutrinos and on this mass loss, the star responded with an expansion. Here, a kinetic energy of 1039 erg / s free and the atmosphere is s thrown at a speed of 100 km /. The color index of the transients is extremely red and should be distinguished from a light-emitting Strengthening Red Nova hardly.