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Cataclysmic Variable Stars

Of the many types of variable stars, cataclysmic variables (CVs) exhibit some of the biggest changes in brightness over the shortest time scales. Although their behaviour is well understood, their times of sudden brightening, or outburst, are not known in advance, adding to the interest in observing them. This article gives details of my observations of two CVs and a list of other interesting targets. Reference [1] contains a list of the more interesting CVs.

What are CVs?

CVs are small, double star systems, comprising a red dwarf and a white dwarf in a tight orbit. Typically, the whole system would fit inside our Sun and the two stars orbit each other in a few hours. The stars are so close together that the intense gravity of the white dwarf pulls gas from the red dwarf to form an accretion disc. The disc gradually increases in density until it becomes unstable, heats up suddenly and causes an outburst. It then cools over days or weeks and the accretion process starts over again.

SS Cygni (RA 21h 42m 43s, Dec +43° 35' 08" (J2000))

Discovered in 1896, this is possibly the most famous and well-observed CV. It is well placed for observing from the northern hemisphere in the summer and autumn. Its outburst at the end of August 2019 was well observed over three weeks thanks to a spell of very good weather. SS Cyg is easy to find by star hopping from Deneb due west to the closest bright star to it, 75 Cyg. Figure 1 shows the area around SS Cyg; it is a rich starfield, having a chain of stars leading to three distant 15th magnitude galaxies. In colour images, the galaxies appear very red due to absorption by dust in the Milky Way.

Outbursts of SS Cyg occur on average every month or so. Figure 2 shows a lightcurve generated from data taken from the website of the American Association of Variable Star Observers (AAVSO) [2], for 2018. It shows three distinct types of outburst:

I first took images of SS Cyg on 13 August 2019 when it was still at minimum magnitude in its quiescent state. This is where it sits most of the time, near magnitude 12. A few days later there were signs that it was starting to brighten, having increased about one magnitude by 15 August. Unfortunately, the main part of the outburst occurred during daytime on 16 August and that night it was raining in Suffolk! By the time I was able to image the star again, on 19 August, it was at maximum brightness. Good weather then returned and I captured images at every opportunity revealing a plateau lasting approximately 10 days before the fade began on 28 August. Another week passed as the star faded to magnitude 12. Figure 3 summarises my images, plotted against the background of the lightcurve plotted by AAVSO.

Several observers captured spectra of SS Cyg during the fade; see e.g. [1] and [3]. I captured figure 4, showing a low resolution spectrum when SS Cyg was near magnitude 11, revealing hydrogen Balmer series emission lines. The spectrum corresponds to spectrum (e) in [3]. Emission lines are produced from tenuous gas in the system, probably material streaming from the red dwarf or a thin halo of hydrogen around the dense accretion disc excited by ultraviolet light from the white dwarf.

SS_Cyg_pos.jpg Fig. 1. Star field around SS Cyg.

AAVSO_SS_Cyg_2018.jpg Fig. 2. AAVSO lightcurve for SS Cyg.

20190907_SS_Cyg_MJH.jpg Fig. 3. SS Cyg.

2019_SS_Cyg_spectrum_MJH.jpg Fig. 4. Spectrum of SS Cyg.

U Geminorum (RA 07h 55m 06s, Dec +22° 00' 09" (J2000))

U Gem, discovered in 1855, is well placed for observation in winter and spring. It is situated a few degrees south of Castor and Pollux in Gemini so is relatively easy to find. Its outbursts occur less frequently than those of SS Cyg, averaging about 100 days between events, and the change in brightness is slightly greater, from magnitude 14 to near magnitude 8.5, although the outburst in March 2019 reached only magnitude 9.5.

My first image of U Gem, acquired on 23 March 2019, showed that it was in the outburst phase, and I was able to follow it for two weeks down to near minimum. Figure 5 summarises my observations, again with an AAVSO lightcurve for comparison. Gemini is hidden by glare from the Sun in summer so I did not capture the likely outburst in July 2019.

Z Camelopardalis (RA 08h 25m 13s, Dec +73° 06' 43" (J2000))

A bizarre sub-class of CVs, Z Cam stars turn off for weeks or months before resuming "normal" outburst activity. During the off state, they shine at a magnitude intermediate between maximum and minimum. At the time of writing, Z Cam has been stable for almost 350 days, since 12 October 2018, but could resume outbursts at any time! Figure 6, from the AAVSO, shows its lightcurve over the last two years.

AL Comae Berenices (RA 12h 32m 25s, Dec +14° 20' 44" (J2000))

AL Com is a faint object for most of the time, shining at near magnitude 19 or 20, presenting a challenge even for CCD imaging. But at intervals of over a year, it can suddenly increase to magnitude 13 (an increase in brightness of almost x250). Making it even more interesting is the fact that it is in the same field as M88, making a nice target for imaging as shown in figure 7.

20190404_U_Gem_MJH.jpg Fig. 5. U Gem.

AAVSO_Z_Cam_2018-19.jpg Fig. 6. AAVSO lightcurve for Z Cam.

AL_Com+M88.jpg Fig. 7. AL Com and M88.

RX Andromedae (RA 01h 04m 36s, Dec +41° 18' 00" (J2000))

RX And is another Z Cam star. It is currently actively increasing and decreasing in brightness on a weekly basis but could turn off at any time. It is notable by virtue of being only 4° from M31.

VW Hydri (RA 04h 09m 08s, Dec -71° 17' 40" (J2000))

And finally, for anyone looking for a remote telescope project, why not try VW Hyi, a far southern CV lying between the Magellanic clouds? It has been extensively studied by the Royal Astronomical Society of New Zealand variable star section and typically shows very short outbursts, lasting just a few days, separated by a few weeks.

Conclusion

CVs can show a dramatic increase in brightness over just a day or two. Unfortunately, they are intrinsically faint systems and none are close enough to us to be visible without using a telescope. But what if one was much closer? Imagine one of the seven bright stars of Ursa Major as a CV. Usually at magnitude 2, it could rise in as little as 24 hours to be as bright as Venus, stay there for a week or more and then fade back to magnitude 2. Such a CV would certainly receive a lot of attention!

References

[1]

Coel Hellier, "Cataclysmic Variable Stars: How and Why They Vary", Springer-Praxis Books in Astronomy and Space Sciences, 2001.

[2]

American Association of Variable Star Observers (AAVSO) light curve generator: https://www.aavso.org/lcg.

[3]

David Boyd, "Spectroscopic Observations of the Outburst of SS Cygni in 2013 September-October", JBAA vol. 123, no. 6, p.366, 2013, https://www.britastro.org/journal_item/3235.


Mike Harlow