Planetary nebulae and their central stars — origin and evolution

Abstract Approximately once every year a new star of mass $ 1M ⊙ is born in our Galaxy. Ninety nine times out of a hundred, this star is less massive than ≈ 10M⊙ and, if it is a single star or in a wide binary, it will become an asymptotic giant branch star with an electron-degenerate core and an extended hydrogen-rich envelope. Between the core, which is essentially a hot white dwarf, and the envelope, hydrogen and helium burn alternately in shells. Carbon and s-process isotopes are made in the helium-burning region and, following each helium-burning episode, these freshly made nuclei are carried by convection into the envelope. The envelope pulsates acoustically, and heating by shocks formed by the pulsations causes the stellar atmosphere to expand; dust grains are formed in the atmosphere, and radiation pressure on the grains leads to a wind which carries mass from the surface. Mass is lost from the surface more rapidly than mass is processed in the interior by nuclear burning. In some cases, the matter in the escaping wind amplifies the radiation impinging on it from below and the wind appears as a maser in various molecular lines — OH, H2O, SiO. Mass loss continues until the hydrogen-rich envelope is not massive enough to remain in equilibrium at giant dimensions. The system then separates into a contracting remnant and an ejected, expanding shell. As the remnant contracts at constant luminosity, its surface temperature increases until enough of the photons emitted from the surface are, at first, energetic enough to dissociate molecules in the shell and, then, energetic enough to ionize atoms in the shell. It is in this later stage that the shell appears as a classical planetary nebula, which shines brightly in forbidden lines of doubly ionized oxygen and in hydrogen Balmer lines. A planetary nebula remains visible for typically 104 yr. As the central star ages, photons from its surface become progressively harder, and the nebula shines more brightly. The central star can achieve surface temperatures of 100 000–300 000 K, depending upon its mass. After achieving its maximum surface temperature, the central star dims and cools, eventually becoming a white dwarf. Depending upon the details of its departure from the asymptotic giant branch, the central star can become a “born-again” asymptotic giant branch star or a “self-induced” nova. The born-again phase is initiated by a final helium shell flash which occurs after the planetary nebula has been formed, and sometimes after the central star has become a white dwarf. The self-induced nova phase is due to chemical diffusion which carries hydrogen inward and carbon outward until the product of the abundances of the two elements becomes large enough for the initiation of a thermonuclear runaway. The number of central stars which are burning helium as opposed to the number which are burning hydrogen has bearing on estimates of the rate at which planetary nebulae and white dwarfs are born in our Galaxy. Approximately two-thirds of all stars are born as pairs which are close enough that at least once in their lifetime they will interact by transferring mass. In perhaps half of such mass-transfer events, the rate at which the donor contributes mass is larger than the rate at which the companion can adjust to accept this mass; the transferred matter piles up around both stars to form a “common envelope” which is ejected from the system in consequence of a frictional interaction between the orbiting stellar cores and the matter in the common envelope. The energy used to eject the common envelope comes at the expense of orbital energy and the two stars spiral in towards one another. At the end of this phase, the remnant of the donor is a luminous, compact star which can be in a fairly tight orbit with its companion. In many instances, as the ejected matter expands outward, the compact remnant becomes hot enough to emit ionizing radiation in sufficient quantities to cause the ejected matter to fluoresce. The system then looks exactly like a planetary nebula formed by a single star, except that it may show a more pronounced axial symmetry which reflects the fact that matter in the common envelope is ejected preferentially in the plane of the orbit. Long after the planetary nebula stage, some binary systems evolve into cataclysmic variables which consist of a white dwarf with a low-mass main-sequence companion that transfers matter to the white dwarf at a relatively modest rate. Whenever the white dwarf accretes enough matter, the accreted layer erupts in consequence of a thermonuclear runaway in its interior, and the system appears as a classical nova. Approximately thirty novae appear in our Galaxy every year, although most of them are obscured by intervening dust; we actually see only about 10% of those formed. Novae evolve into systems which are conceptually very similar to planetary nebulae in that they consist of an expanding shell and a hot central star. However, the mass of the ejected shell is typically 10000 times less massive than the mass of a planetary nebula and the ejected shell expands 10–100 times more rapidly than a planetary nebula shell, so nova shells are visible for a much shorter time than the ≈ 104yr typical of planetary nebulae.