N Novae

- Close binary systems with orbital periods from ff!05 to 230?. One of the components of these systems is a hot dwarf star that suddenly, during a time interval from one to several dozen or several hundred days, increases it brightness by 7"'-19"' in V, then returns gradually to its former brightness over several months, years, or decades. Small changes at minimum light may be present. Cool components may be giants, subgiants, or dwarfs of K-M type. The spectra of novae near maximum light resemble A-F absorption spectra of luminous stars at first. Then broad emission lines (bands) of hydrogen, helium, and other elements with absorption components indicating the presence of a rapidly expanding envelope appear in the spectrum. As the light decreases, the composite spectrum begins to show forbidden lines characteristic of the spectra of gaseous nebulae excited by hot stars. At minimum light, the spectra of novae are generally continuous or resemble the spectra of Wolf-Rayet stars. Only spectra of the most massive systems show traces of cool components.

Observation Key f Mixed stars ft Large amplitudes

^ Mixed periods

S> Visual, CCD/PEP

Some novae reveal pulsations of hot components with periods of approximately 100s and amplitudes of about 0".'05 in V after an outburst. Some "novae" eventually turn out to be eclipsing systems. According to the features of their light variations, novae are subdivided into fast (NA), slow (NB), very slow (NC), and recurrent (NR) categories. NA (subgroup) - Fast novae displaying rapid light increases and then, having achieved maximum light, fading by 3m in 100 or fewer days. NB (subgroup) - Slow novae that fade after maximum light by 3"' in 150*. Here the presence of the well-known "dip" in the light curve of novae similar to T Aur and DQ Her is not taken into account: The rate of fading is estimated on the basis of a smooth curve, its parts before and after the "dip" being a direct continuation of one another. NC (subgroup) - Novae with a very slow development and remaining at maximum light for more than a decade, then fading very slowly. Before an outburst, these objects may show long-period light changes with amplitudes of l"'-2"' in V; cool components of these systems are probably giants or supergiants, sometimes semiregular variables, and even Mira variables. Outburst amplitudes may reach 10"'. High excitation emission spectra resemble those of planetary nebulae, Wolf-Rayet stars, and symbiotic variables. The possibility that these objects are planetary nebulae in the process of formation is not excluded. NR (subgroup) - Recurrent novae, which differ from typical novae by the fact that two or more outbursts (instead of a single one) separated by 10-80 years have been observed. GCVS

Together with dwarf novae and novalike variables, novae are interacting binary stars generally possessing a short orbital period. Novae consist of a massive white dwarf, the primary, and a cool dwarf star, the secondary. The secondary star overflows its Roche lobe and thereby loses mass to the primary star. This matter forms an accretion disk around the primary star and is finally accreted onto its surface. Instabilities in the accretion disk lead to short and long period photometric variability at the stage of minimum light.

The cause of the nova outburst is a thermonuclear runaway reaction that occurs in the accreted hydrogen-rich layer near the surface of the massive white dwarf into which C and O nuclei from the outer layers of the white dwarf are mixed. When the critical pressure is reached, hydrogen burning via the CNO cycle begins within the degenerate hydrogen-rich outer layer. A

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of Q Cyg (NA|. D^ provided by the v$Mff Used with permission rapid increase of the temperature leads to a lifting of the degeneracy and to the formation of a shock wave. This, in combination with radiation-driven mass-loss, produces an expanding atmosphere of large size and of high absolute magnitude, typically Mv = -6 to -9, at maximum light. Decreasing mass loss with an ongoing energy release causes a decline of the visual light output, a shrinking of the photosphere, and radiative heating of the ejected material, resulting in interesting spectroscopic phenomena in the course of the outburst (Figure 5.4).

The detailed light and spectral properties of novae are complex, and depend on the white dwarf mass and its chemical composition, and mixing of CO-rich nuclei into the accreted material as well as on dust formation in the ejected shell. Each nova has its own unique, characteristic photometric and spectroscopic evolution. In spite of this, novae can be broadly classified into several subgroups.

NA - fast novae which, after maximum light, decline three magnitudes in visual light in 100 days or less. They usually have fairly smooth light curves and generally have higher absolute magnitudes (see Figure 5.5).

NB - slow novae which decline three magnitudes from maximum in visual light in more than 100 days. They have usually fairly structured light curves and, as a rule, fainter absolute magnitudes. NC - very slow novae, which remain near maximum light for years or even decades. The bulk of these objects are symbiotic stars, accreting objects with late-type giant companions. They are often called symbiotic novae. NR - recurrent novae that show repeated outbursts at time intervals of decades. Recurrent novae are usually fast novae, often have giant companions,

Figure 5.5. Light curve o(VU94 Agl (Nova).

Chort provided by the AAVSO. Used with permission

Figure 5.5. Light curve o(VU94 Agl (Nova).

Chort provided by the AAVSO. Used with permission

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and the accreting white dwarfs are probably near the Chandrasekhar limit, a position that allows explosions in degenerate material under high pressure.

Members of the groups NA and NB are also called classical novae. Their absolute magnitude is correlated with the speed of the light-curve decay: faster novae are more luminous at maximum light. The fastness is measured by t2- or f3-time: a nova takes a certain time,

Figure 5.6. Light curve of RS Oph. DoIq provided bylfieVSNET. Used with Permission.

Figure 5.6. Light curve of RS Oph. DoIq provided bylfieVSNET. Used with Permission.

measured in days, to decline by 2 or 3 magnitudes from maximum light.

When first detected, novae are always a surprise and produce much excitement within the professional and amateur community. Recent nova discoveries include: Nova Sgr 2001 No. 3 = V4740 Sgr that reached a maximum brightness of ~6™8, Nova Sgr 2001 No. 2 = V4739 Sgr that reached a maximum brightness of ~6™4 and Nova Cyg 2001 No. 2 = V2275 Cyg that reached a maximum brightness of ~6nl6. These three novae were all within the observing capabilities of binocular observers or small telescopes.

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