An Introduction to Planetary Nebulae by Jason J Nishiyama

Author:Jason J Nishiyama
Language: eng
Format: epub
ISBN: 9781681749600
Publisher: IOP Publishing
Published: 2018-05-21T00:00:00+00:00

2.4.1 White dwarf physical characteristics

As they come from the central stars of planetary nebulae, it is safe to assume that white dwarfs share many of the same characteristics. They are similar in mass and radius to central stars as they are composed of the same degenerate matter. Surveys of white dwarf stars show a similar distribution and clustering of masses around 0.6M⊙ as the central stars of planetary nebulae do [38]. The mass distribution extends to lighter and heavier than that of the central stars since as we have seen in section 2.3.2 stars with less mass than the Sun evolve directly into white dwarfs without forming a visible planetary nebula while stars greater that 6M⊙ evolve so quickly that they cause their nebular envelopes to glow for such a short time we fail to observe them.

As with central stars, the matter in white dwarfs is also electron degenerate and thus in terms of radius they have radii in the same range as central stars—that is on the order of 1 to 2 R⊕. They are as well limited at the upper end of masses by the Chandrasekhar limit to under 1.4M⊙ which corresponds neatly with both observation as well as the initial to final mass relation given in equation (2.30).

Unlike the central stars of planetary nebulae, white dwarfs exhibit a large range of temperatures, from 150 000 K down to 4000 K [2]. This is partially due to the difference in mass of the progenitors causing temperature differences in the core of the star but also because of cooling. As there is no longer an energy source to maintain the heat inside the white dwarf, as energy leaves the star in the form of photons there is nothing to replace it so all white dwarfs are in a state of cooling. This is a long process however, as cooling by blackbody radiation is not the most effective method of energy transfer and is the only method available to the white dwarf star. This leads to cooling times on the order of 1010 years for most white dwarf stars [45]. Given this rate of cooling it is likely that the Universe itself is not old enough for any but the most ancient white dwarfs to have cooled to invisibility.

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