ZZ Ceti Stars

About 80% of all known white dwarf stars have hydrogen atmospheres, showing only hydrogen absorption lines in their spectra. These have been assigned the white dwarf spectral type of DA (presumably D for dwarf and A for the first, or most common, type of white dwarf). Arlo Landolt (1935-) was the first to discover variability in a white dwarf by observing the mv=15.0 DA white dwarf star named HL Tau 76 (not to be confused with HL Tau!), in front of a dark nebula in Taurus (LDN 1521C = MLB 3-13), in December 1964. This star now has the standard variable star designation V411 Tauri.

A second DA white dwarf, mv=14.2 Ross 548 in Cetus, was discovered to be variable by Barry Lasker (1939-1999) and Jim Hesser in 1970, and in 1972 it was assigned the variable star designation ZZ Ceti.

By 1976, seven luminosity-variable DA white dwarfs had been discovered, and John T. McGraw and Edward L. Robinson stated in an ApJ paper

We suggest that the recently proposed ZZ Ceti class of variable stars be reserved for the DA variables in Table 1 and specifically exclude the DB variables since the mechanism of variation is almost certainly different.

McGraw & Robinson, Astrophysical Journal, Vol. 205, p. L155-L158 (1976)

So, why wasn’t this new class of luminosity-variable DA white dwarfs named after V411 Tau, the first of its class discovered? Why are they called ZZ Ceti stars, after the second such star discovered, instead? In each constellation, variable stars are given one- or two-letter designations in order of discovery, and when the letter designations run out, the letter “V” is used followed by a number. The first “V” star is V335, the 335th variable star to be discovered in a constellation. Well, V411 Tau was the 411th variable star discovered in Taurus, and as a matter of tradition, no class of variable stars is ever named after a “V” designation. So, the honor fell to the runner-up, ZZ Ceti. Besides, ZZ Cet is a little brighter than V411 Tau, so not a bad choice.

ZZ Ceti stars, also known as DAV stars (as in DA Variable), are multimodal pulsating white dwarfs having periods ranging from 70 seconds to 25 minutes. But the amplitude of the brightness variations is tiny to small, ranging from less than 0.001 magnitude up to 0.3 magnitudes.

V411 Tau has a dozen detected pulsation modes. In order of amplitude (in millimagnitudes), they are (without error bars):

Period (seconds)Amplitude (mmag)
540.95
30.89
382.4717.88
664.2116.22
596.7915.63
1064.9712.27
781.09.88
492.127.73
449.87.27
976.387.01
799.105.63
1390.844.26
933.642.61

ZZ Cet has eleven detected pulsation modes. In order of amplitude, they are (without error bars):

Period (seconds)Amplitude (mmag)
213.13267.119
274.25084.658
212.76844.448
274.77453.368
212.94631.477
274.52091.253
186.87400.9518
318.07630.7836
333.6447
0.6740
217.83360.3506
318.76570.3289

ZZ Ceti stars are not radial pulsators, that is they do not undergo radial oscillations (changes in size). White dwarf stars typically have diameters of only 0.9 to 2.2 that of the Earth, so they are much smaller than “normal” stars. As short as the pulsation periods are, they are not short enough if the cause were radial pulsations. Instead, the pulsations are due to shock waves traversing the atmosphere of the ultradense star. The slow rotation of these stars often causes closely spaced “double periods” such as we see in ZZ Cet.

The brightest (and closest) ZZ Ceti star yet discovered is DN Draconis, shining at visual magnitude 12.2. DN Dra pulsates with an amplitude of just 0.006 magnitude (6 millimagnitudes), and its pulsation period is 109 seconds.

What ZZ Ceti star has the largest amplitude? As they say, “it’s complicated”. Even though references to a maximum amplitude as high as 0.3m can be found in the literature, I was unable to find any ZZ Ceti stars with amplitudes greater than 0.12m. Moreover, pulsation modes of ZZ Ceti stars can come and go, so one observer may observe a higher amplitude but the next may not. Though pulsation modes can and do appear and disappear over time, there is also the changing additive nature of many pulsation modes to consider from one observing run to the next.

Patterson et. al (1991) report mv=13.0 ZZ Psc having a pulsation amplitude of 0.116m and period 614.9s in blue light. Mukadam et al. (2004) report mv=15.2 UCAC4 448-059643 (in the constellation Serpens) having a pulsation amplitude of 0.121m and period 873.2s in blue light.

One challenge in the literature is that pulsation amplitudes are variously given in units of milli-modulation amplitude (mma), milli-magnitudes (mmag), percent, or parts per thousand. Here are the unit conversions:

1\:mma = \frac{1}{2.5\log_{10}e}\:mmag = 0.1\% = 1\:ppt

The study of the pulsation modes of white dwarfs and other stars is called asteroseismology. I hope this article has piqued your interest in learning more about this rapidly developing and fascinating field!

References
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