Observing through the light
Gary Poyner
Reproduced with permission from the Journal of the British Astronomical Association (JBAA)
The George Alcock Memorial Lecture. BAA May 25th 2011 George Alcock MBE was an observational astronomer unsurpassed. During his long career he discovered five Comets and five Novae from his observing location in Peterborough, as well as hundreds of stellar omissions from popular star atlases of the day. He was rightly regarded by many of his piers as the finest visual observer of all time. He died in 2000.I was fortunate to meet George Alcock on several occasions, mainly TA meetings in the 1980’s, but one meeting sticks out more than the others. During my time as BAAVSS Director in the late 1990’s we held a small meeting in Cambridge which was attended by the VSS officers, Janet Mattei and George himself. This gave me the opportunity to chat with him about visual Nova patrolling, something I had been doing since 1980. It was a great moment for me, talking to this legend of a visual observer about something we had in common. George was always in my thoughts whilst I was patrolling for Nova, and the thought was always at the back of my mind that he was looking for Novae at the same time as me. What chance did I have!
George’s Novae discoveries provided me with some of the first ‘Variable Stars’ I observed when I took up VS observing in 1975. HR Del (Nova Del 1967) had faded to magnitude 11.5 in 1976 when I first turned my telescope onto it, and NQ Vul (Nova Vul 1976) was a timely new addition to my observing programme in the Summer of that year. I was also fortunate to observe George’s final Nova V838 Her (Nova Her 1991) in March 1991 – the fastest Nova on record and possibly the most remarkable discovery of all! I also observed George’s final comet IRAS-Araki-Alcock in 1983, and although large and bright, comets are far from ideal observing targets in the light polluted skies some five miles north of the centre of Birmingham , where I have lived all my life.
A good way to judge the quality of the sky is by ‘The Bortle Scale’ [1], first proposed over ten years ago by the veteran American comet and variable star observer John Bortle. Here he assigns a number to the quality of sky visible - 1 being an excellent dark sky with Zodiacal Band and Airglow apparent, magnitude 7-8 visible with the naked eye and the Milky Way casting shadows. An “Observers Nirvana” in Bortle’s own words. Class 9 is an inner city sky with magnitude 4 being the naked eye limit at the zenith and several of the fainter constellations not visible at all. My sky I judge to be ‘Bortle 7’ – “The entire sky background has a vague, greyish white hue. Strong light sources are evident in all directions. The Milky Way is totally invisible or nearly so. M44 or M31 may be glimpsed with the unaided eye but are very indistinct. Clouds are brilliantly lit. Even in moderate-size telescopes, the brightest Messier objects are pale ghosts of their true selves. The naked-eye limiting magnitude is 5.0 if you really try, and a 32-cm reflector will barely reach 14th magnitude”. This pretty much describes the sky I observe under, although I disagree with the limiting magnitudes given by Bortle in his scale, as these are very much ‘observer dependent’ and I think it’s counter productive to bias people into what they can and can’t see with various instruments. Try it for yourself is my motto!
Even from such poor quality skies, there is still a huge amount of visual observing which can be done, especially in the field of Variable Stars. The light dome from Birmingham stretches to about 40 degrees above the southern horizon. With a 35cm telescope my limiting visual magnitude is 13.3 at 20 degrees altitude, increasing to 15.0 at 40 degrees (a 51cm gives 15.6 at this altitude), and culminating under transparent skies at 16.5 at 80 degrees (or 17.1 with the 51cm). This means that hundreds of variable stars can be observed of all types and magnitudes within these limits. No need to look outside, see an orange sky and think about buying a CCD.
My observing programme now consists of 450 stars, nearly all of which are Cataclysmic in nature, with a dozen or so AGN thrown in as well as a couple of Mira stars which I’ve been observing for many years and haven’t the heart to drop. It’s from this list that I have chosen three interesting objects to discuss in this talk. The word ‘Nova’ is synonymous with George Alcock, so my subjects will be three old Novae, which are still active today decades after their initial discovery.
Let us first remind ourselves of what a Nova actually is. We have a binary system consisting of a White Dwarf (WD) of 1.0 to 1.4 solar mass, and a secondary main sequence star. The orbital period of the system is generally 98m to 460d, with the separation of the two stars in the order of 130-2000 solar radii. The WD supplies the secondary with a fresh source of Hydrogen, which over time forms a layer on the WD. This layer is compressed and heated until a thermonuclear runaway occurs. The outer shell of the WD ‘explodes’, the nova brightens and the expanding shell becomes visible sometime after the outburst has occurred.
The light curve generally follows a standard rise by ~9-11 magnitudes, followed by an early decline of 3.5 magnitudes. From here it reaches the ‘transition state’ where it’s likely to do one of three things – continue fading gradually to post nova state, show oscillations until a decline of a further three magnitudes have occurred, when it then continues on it’s final decline to a post nova state or dip by several magnitudes before rising again only to then fade back down to quiescence. These dips are dust events, and are by no means common in novae.
Nova Cygni 1876 (Q Cyg)
On the evening of November 24th 1876 Dr. Johanne F.J. Schmidt, then Director of Athens Observatory visually discovered a magnitude 3 star in the constellation Cygnus (quite an achievement, and one that G.E.D Alcock would have been proud). A discovery telegram was sent to Dr. Littrow in Vienna, but news didn’t reach the UK until the second week in December when a note appeared in Bulletin International. Astronomers at Greenwich were disappointed to miss eight clear nights where observations at the critical early stage could have been obtained. The first British observation was made by J. R Hind (discoverer of U Gem) on December 13th at ~6th magnitude. A detailed light curve constructed by Norman Lockyer appeared in Philosophical Transactions in 1891. [2] From this a t3 of 11d was established (t3 is the time a nova takes to fade by three magnitudes, and is the standard for classifying whether a novae is fast or slow).
Figure 1. Observations of Q Cyg obtained by J. G. Lohse with 'Mr. Wiggleworth's 15.5-inch Cooke refractor'.
Plotted by the author from observations reported in MNRAS Vol. 47, 1886-1887
Further observations from Jan 1879 to Mar 1882 (probably made by Ralph Copeland, Astronomer Royal for Scotland) showed the Nova to have faded to magnitude 14.0 [3]. Through 1885-1886 J.G. Lohse reported Nova Cygni to have reached magnitude 15.0 by September 1st 1885 [4] [fig. 1] It was at this time that it could be seen that the Nova was doing interesting things by varying between magnitude 14.0 and 15.0. Following these reports, Nova Cyg (or Q Cygni as it had been named) disappeared from the limelight for many years. My observations began in 1990 [fig. 2] and have continued up to present day.
Figure 2. Visual observations of Q Cygni. G. Poyner, 1990-2011
A quick glance at the plot shows what looks like ‘stunted outbursts’ at regular intervals, followed by intervals where they are totally absent. These ‘outbursts’ were first studied in the early 1980’s by Shugarov [5] who concluded that an outburst period of 60d was present. This was later refined to 50d by Honeycutt et al. in 1998 [6]. Analysis of my own observations during these outburst periods reveal a period of 51d. In 2003 Kafka [7] measured the orbital period to be 10.08h and announced the discovery of an intermittent wind outflow – only the second CV after BZ Cam to show this phenomenon by this time. As the outbursts appear to occur during a brighter state in the system, debate continues as to where they actually originate – disc, boundary layer or from mass transfer itself.
Nova Lyrae 1919 (HR Lyr)
Moving on 43 years, the first Nova to be discovered in the constellation Lyra was detected by Johanna Mackie on December 6th 1919, a discovery for which she was awarded the AAVSO Gold Medal. Mackie was a member of E.C. Pickering’s famous ‘Harem’ of lady ‘computers’ at Harvard College University, led by Annie Jump Canon – a team dedicated to classifying stars by their spectral signature [fig.3]. For this the women were pad 50c per hour, half the rate men were paid for doing the same job!
Figure 3. "Pickering's Harem". Johanna Mackie may be 2nd from left on the front row.
Isis 73 94, 1982 March
The Nova was discovered during a Harvard plate search, and was announced on Harvard College Observatory bulletin 705 on January 6, 1920. This bulletin remarks that “between December 4 and 6 it rose rapidly from the sixteenth magnitude or fainter, to a maximum of about 6.5. Since that time it has undergone marked fluctuations in brightness. It’s present magnitude is 8.5”. News reached Europe in January of 1920, which meant that few observations were made of the Nova at maximum light due to Lyra’s poor location in the sky. The first BAA observations were obtained by Felix de Roy in February and March 1920, and in later years W. H. Steavenson obtained observations. From 1921 until the late 1970’s, HR Lyr (as it had been designated) received scant attention.
A light curve has been constructed from observations from the AAVSO, AFOEV and BAAVSS database which provides us with a t3 of 97d. This plot reveals scatter during both transition phase and at minimum, with the presence of ‘intra day variations’ and flickering in evidence. The 1967 commission 27 of the IAU produced a list of 11 possible Recurrent Novae of which HR Lyr was one, and with this in mind it was added to the BAAVSS Recurrent Objects Programme in 1993. [8] BAAVSS observations from 1995 to 2010 reveal HR Lyr varying in excess of one magnitude , but not until September 2010 did anything really unusual happen. Quite unexpectedly, and without precedence, HR Lyr faded below magnitude 17.0 for the first time before rising again to the mid-15’s by the end of the year [9]. This unusual behaviour has continued into 2011, with HR Lyr still varying between 16.0-16.5, well below the limit one might call it’s ‘normal’ quiescence. As this is a new phenomenon observed in this system, one can only speculate as to the cause. Certainly a ‘disc cooling’ event is a likely cause, but further observations are needed to see if anything like this happens in the future.
Nova Herculis 1934 (DQ Her)
On the morning of December 12th 1934, J.P. Manning Prentice (Director of the BAA Meteor section) was observing the Geminid meteor shower, when he noticed that “there was something wrong with the head of Draco”. He estimated the new star to be of 3.4 magnitude, and knowing that the spectrum of the star should be photographed at the earliest possible moment, rang the RGO at 5am in the morning to report his discovery [10]. At the same time, Prentice’s friend and Meteor colleague G.E.D Alcock was also observing Geminids in Peterborough. Alcock remarks “Manning was just stretching his legs, after a long spell observing in one position, when he spotted a bright Nova in the constellation of Hercules. It was a magnificent Nova – one of the best this century. If I hadn’t finished early that night, I would have seen it when it moved into my observing field.” [11]
Nova Her 1934 or DQ Her as it became known, peaked at magnitude 1.5 on December 21st, and was well observed by BAA members. A light curve of maximum brightness appeared in the BAA Journal, compiled from observations by 36 members – including a number of notable well known names [12]. The following weeks saw DQ Her lose four magnitudes as it approached the transition stage, before it faded by a further 8 magnitudes to 13.0 before recovering to magnitude seven after which a very slow decline set in. [fig. 4] This was the first time a Nova had been seen to show this ‘dip’ in the transition phase, a phenomenon now usually termed the ‘DQ Her dip’. As previously stated, these events are uncommon in Nova, and the astronomical world had to wait until Nova Serpentis in 1970 to confirm that these strange dips were indeed due to dust in the outer shell!
Figure 4. BAAVSS observations of DQ Her fading following discovery.
DQ Her was first identified as a binary system in 1954 with an orbital period of 4.65h. Later associated with the magnetic systems called Polars (or AM Her stars), it was realised that unlike the AM Her stars whose binary are locked into a synchronous orbit (the WD spin is the same as the orbital period due to interlocking magnetic fields), the two stars in the DQ Her system were asynchronous. Other differences were later established. DQ Her objects had truncated accretion discs due to the WD magnetic field, whereas the stronger magnetic field of the WD in AM Her stars prevented the formation of any disc whatsoever. DQ Her stars also lack hard X-ray emission of AM Her stars due to the close proximity of the magnetic field to the primary.
The orbital inclination of the DQ Her system was shown to be 87°, which means that it displays eclipses every 4.65h. The duration of the eclipse is 56m, in which the star fades by 1.3 magnitudes. These can be very interesting to watch visually if you have a telescope large enough, or with a CCD camera on smaller instruments, and not all eclipses follow exactly the same profile. [fig. 5]. My own observations of DQ Her go back to 1988. In this time the Nova has been seen to vary (outside of eclipse) by one half magnitude between 14.0-14.5. Interestingly by applying a linear trend line to the data, it can be seen that DQ Her is still fading by a few tenths of a magnitude in that 23 year period.
Figure 5. Visual observations of an eclipse in DQ Her. G. Poyner
The interest in Nova Patrolling in the UK - especially visual patrolling - has declined in the last 20 years. This is somewhat surprising since the discovery of a Nova propels the discoverer to ‘instant fame’, and the discovery is of obvious value to science. It’s a sobering fact that the last Nova to be discovered from the UK was Nova Aquila (V1548 Aql) in 2001, which was a photographic discovery by Mike Collins. We have to go back a further 10 years for the last visual discovery – Nova Herculis (V838 Her) 1991. The discoverer? G.E. D Alcock MBE.
References
1: http://www.skyandtelescope.com/resources/darksky/3304011.html
2: Phil Tran. Vol 182A, 1891
3: Observations of Schmidt’s Nova Cygni. Copernicus Vol 2, 1882
4: MNRAS Vol. 47, 1886-1887
5: Astron. Tsirk, No. 1252, 6 1982
6: Unusual stunted outbursts in old Novae and Nova Like Cataclysmic Variables. Honeycutt et al, AJ, 115: 2527-2538 1998 June
7: Spectroscopic study of Q Cygni: Surprises from an old Nova. Kafka et al AJ, 126: 1472-1482, 2003 September
8: HR Lyr (Nova Lyr 1919): from outburst to active quiescence. Shears & Poyner JBAA Vol. 117, No.3, 2007
9: A deep fade of HR Lyr. Shears & Poyner. Observers Forum, JBAA 120, 6, 2010
10: JBAA Vol. 45, No. 3 1934-35
11: Under an English Heaven – The life of George Alcock. Kay Williams. Genesis Publications Ltd. 1996
12: JBAA Vol. 45, No. 4 1935