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Suitable type for the pulsating helium dwarf?

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DDE's picture
Suitable type for the pulsating helium dwarf?

Hello VSX and AAVSO members!

I am in trouble characterizing the new variable star I have discovered while cross-matching GALEX and WISE data. It is definitely a helium white dwarf, since there are no hydrogen lines in its spectrum. And the light curve is asymmetric, with the faster rise than drop (rise time about 46 per cent). See the attached DDE156-phased_LC.gif for the CRTS light curve folded with the best period. The adopted type for pulsating hot white dwarfs is ZZO. The problem is that the known ZZO stars pulsate with the periods from 6 to 35 minutes (0.1-0.6 hr), and my recent discovery has a period of 16.75 hours (0.6977 d)!

Also, the amplitude of pulsation is quite large: 0.2m from maximum to minimum. Well known example of ZZO stars GW Vir is listed in VSX with the range of 14.87(0.03). That's 14.84-14.90 in classical notation, or a total of 0.06m. Interestingly, GW Vir has the (NUV-W1) color index very similar to my new variable: -2.81 versus -3.03.

I smell something extraordinary here. Is there a theoretical prediction of the instability strip in the 100+ thousand Kelvin hot helium dwarfs? Or can there be another reason for the helium dwarf to vary with such a long period and asymmetrical light curve?

I'm really very puzzled by this one!


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Sebastian Otero
Sebastian Otero's picture

Hi Denis,
Since you already submitted the star I'll give you my opinion here.
With such a long period, relating both classes is not an option. Also the repetitive nature of the variations makes it unlikely to be a ZZ whose frequencies may vary over time causing a phase plot to appear noisy.
This sounds more like orbital-related and thus, reflection. The problem one may see is the lack of detection of a companion both spectroscopically and in colors. However, a faint red dwarf might only show up when it is heated and that wouldn't affect the published mean combined colors (B-V= -0.3).
I go for R as the variability type. The period is typical.

Also, keep in mind that VSX lists amplitudes not semi-amplitudes, thus 0.03 is actually 0.03 mag. peak to peak and the magnitude given at maximum is a mean magnitude from a catalogue but not necessarily exactly in the middle of the range (e.g. for a detached eclipsing binary with an amplitude of 0.3 mag. the mean mangitude will probably be cloe to the maximum magnitude so if the magnitude is 14.35 the range wouldn't be 14.20 - 14.50 but something like 14.33 - 14.63 or so.).

Add the name EC 01017-1817 too.


DDE's picture
Re: Reflection?

Thanks Sebastian! I have revised the submission.

With the absolute magnitude of DO star M=7.6, the red dwarf component with M>12 would be indeed practically undetectable. Still the light curve is not sinusoidal, as one would expect for the reflection effect, and might be showing short term variations. The star is worth while checking for the pulsations when it comes to a better visibility in August.


DDE's picture
Whatever it is, it's

Whatever it is, it's extremely hot! I have plotted a diagram of (FUV-NUV) vs (NUV-W1) colors for R (reflection) type variables in VSX. Only 16 out of 50 variables have both GALEX and WISE magnitudes. DDE 156 is by far the hottest of them. The nearest one is XY Sex (NUV-W1=-1.72) with the period 0.073 d.

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