Ordinary meeting, 2010 January 27: held at the Royal Astronomical Society, Burlington House, Piccadilly, London WI.
Ron Johnson, Hazel Collett & Nick James, Secretaries
The President opened the third Ordinary Meeting of the 120th session. The audience approved the minutes of the meeting of 2009 December 12, and the election of 25 new members was confirmed by the meeting. The Papers Secretary said that, at the Council Meeting earlier that day, Council had accepted five papers for publication in the Journal:
Saturn during the 2006/2007 apparition, by Mike Foulkes
VSX J003909.7+611233: a new gamma Doradus variable in Cassiopeia, by David Boyd et al.
Superoutbursts of the SU UMa-type dwarf nova CP Draconis, by Jeremy Shears et al.
The Hampstead Observatory, 1910-2010: a century of service to the public, by Doug Daniels
A home-built, fully-automated observatory by Mike Beales
The President announced that the next meeting at Burlington House would be the Special General Meeting and Ordinary Meeting on March 31, at which the main speaker would be Prof. Linda French. He also mentioned the Deep Sky Section meeting on March 6, the Winchester Weekend on April 9-11 at Sparsholt College, Winchester, the Out of London meeting in Shrewsbury on April 24, on the theme Cosmic Bangs and Explosions, and the Exhibition Meeting on June 26.
The President then introduced Dr Mike Dworetsky of University College, London.
Chemically peculiar stars of the upper main sequence
Dr Dworetsky first explained the principle of the spectrograph, showing pictures of the grating spectrograph used on the 24inch (61cm) Allen Telescope at Mill Hill Observatory. The spectrum of the Sun was demonstrated with a high resolution image taken with the UCL echelle spectrograph before it was shipped to the Anglo-Australian Telescope.
Dr Dworetsky reflected that the chemical classification of stars according to their spectra has occurred for over 100 years. The basic system was pioneered at Harvard Observatory in the 1890s and its main features remain in use today. Many astronomers remember the sequence of spectral classes OBAFGKRNS from a mnemonic such as Oh Be A Fine Girl and Kiss Me Right Now or Sooner. Dr Dworetsky reviewed the main elemental lines characteristic of each of these types of stars and showed examples of their spectra. He pointed to the observation that the more luminous (hotter) stars have narrow lines, while less luminous (cooler) stars have broader lines, and also reviewed the effect that magnetic fields have on spectral lines, the Zeeman splitting observed when starlight is passed through a calcite crystal polariser, which separates circularly polarised and linearly polarised light. He further pointed out that stellar rotation broadens spectral lines due to the divergent Doppler shifting of light from opposite limbs of the star.
Dr Dworetsky went on to outline the four main types of chemically peculiar star. These are: Am stars, which show strong heavy metal lines, no magnetic field, and and are slow rotators; Ap stars, which show lines of strontium, chromium and rare earth elements, a strong magnetic field, and generally slow rotation; mercury-manganese stars, which show abundance of singly ionised Hg and Mn, but weak magnetic fields and very slow rotation rates; and finally, helium-weak stars, which have weaker helium lines than would be expected from their colour. In order to further demonstrate the peculiarity of the compositions of these stars, Dr Dworetsky showed a graph of the log of abundance versus atomic number for all elements, based on the composition of 'normal' stars, the Sun, and meteorites.
The speaker concentrated the rest of his talk on the Ap stars and the mercury-manganese stars. The Ap stars are thought to have 'frozen-in' magnetic fields arising from their slow rotation. Normal convective mixing does not occur in the outer layers of these stars, and abundance anomalies arise on areas of the star surfaces due to magnetic anomalies. 53 Cancri is a typical Ap star with rotation period of 8 days. On this star, a patchy distribution of elements on the surface is apparent, and starspots can be mapped. The mercury-manganese stars may be explained by a diffusion model. These are low rotation velocity stars so they are not well-mixed, and the normal He II convective zone is suppressed. Atoms in the gas migrate due to differential radiation pressure, and species may go up or down. The anomalous abundances observed are due to the 'settling-out' of certain elements in the surface layers, and internally, these stars' compositions are thought to be more normal. Some of them show emission as well as absorption lines.
In answer to a question from the President, Dr Dworetsky said that the anomalous characters of these stars do not seem to arise from their progenitor gas clouds. Theory says that the rotation velocities of stars should show a random (Maxwellian) distribution when they form from gas clouds, but this is not observed--there are too many slow ones. It is not known why.
The President then introduced Sheridan Williams, Director of the Computing Section.
The Indian total solar eclipses of 1898 and 2009
Mr Williams told the meeting that he had been invited to go to Delhi in 2009 January, by the Indian Science Popularisation Association of Communicators and Educators (SPACE), to take part in a solar eclipse workshop. There he had given talks to the many young delegates on The photography of eclipses and Indian eclipses 1800-2199. Mr Williams repeated for the meeting a presentation he had first given in India, that had come from his ongoing work to digitise all past issues of the Journal. He had discovered, through scanning old Journals, that the 1898 Indian eclipse had occasioned the first successful BAA eclipse expedition. Mr Williams showed a diagram which compared the paths of the 1898 and 2009 eclipses, and others from the 19th and 20th centuries which had crossed India.
In 1898 the vexed question had been the nature of the corona--was it connected to the Sun, the Moon, or even the Earth's atmosphere? It was believed it could not be entirely a solar atmospheric phenomenon due to the great solar gravity. BAA contingents went to three Indian locations. A Mr Newall took a spectroscope he had designed to try to determine whether the corona is rotating, and also to examine the chemical makeup of the corona. Photography was not sensitive enough at that time to record the corona well, but many people made drawings, and temperature measurements were also taken. It was thought at this time that there could be a correlation between eclipses and earthquakes, and this, Mr Williams commented, is an idea that is not entirely dead today.
The rest of Mr Williams' talk concerned his travels in India in 2009 (though in fact he observed the eclipse from China). He told us of the tremendous publicity the eclipse had received in Bihar. While there he had given advice to schoolchildren as to how, and from where, they should attempt to observe the eclipse, and had suggested experiments they could do. He also mentioned his visit to Jantar Mantar Royal Observatory at Jaipur. Built in the 18th century, this gives the time by the Sun accurate to 10 seconds.
Mr Williams said that, in the event, the weather had been favourable for viewing the eclipse in many locations in India. He showed photos of the event that had been supplied to him by his Indian contacts Ankush Maria and Raghu Kalra. One of them showed a train stopped so that passengers could watch the eclipse. One effect of the 2009 Indian eclipse had been to reduce the previously highly-prevalent superstitions about eclipses in that country.
In questions to the speaker, Nick James commented that there was no possible physical mechanism that could account for any eclipse-earthquake correlation, and the idea also had no statistical validity, so should be firmly scotched. Bob Marriott pointed out that the BAA had sent an expedition to Norway in 1896 to observe a total eclipse, but all the observers had been in the same place, and all had been clouded out. India in 1898 was the first successful BAA eclipse expedition.
The President then introduced Nick James to present the Sky Notes.
The January and February sky
Mr James began by noting that the weather had been terrible, with the UK almost entirely covered by snow in late January. He noted that sunspots are now on a rising trend, and showed an image of recent spots by Ron Johnson. An annular eclipse that occurred on January 15 had been observed from Africa and India, John Mason, Hazel McGee and Nick Hewitt being amongst those who had been on expeditions to Africa. A picture was shown of John Mason using a shower mat as an observing instrument. Baily's beads had been observed at the northern edge of the track in India, and Naimal Islam Opu had produced a sequence of images from Bangladesh. On 31 December there had been a small partial eclipse of Moon, and Mr James had taken a picture of this using his mobile phone.
The Geminid meteors had given a good shower. The maximum was on January 13, but many parts of the UK had been clouded. The ZHR had been about 140, close to predictions. Sheridan Williams had observed the meteors from Libya and had recorded three meteors in about 300 exposures. Mr James had experimented with using a Watec camera (a low-light video camera) to record meteors. He had picked up eight meteors, including a bolide.
The Moon will eclipse the Pleiades on February 21, and there will be some graze events visible from the UK. Jupiter is now low in the twilight. In December, Neptune had been very close to Jupiter in the sky, which Sheridan Williams had captured in a photograph. Mr James had observed the event visually.
Mercury might currently be seen in the morning sky. Mars has its opposition on January 30, between Castor and Pollux and Regulus. On this occasion it will attain an angular diameter of 14 arcseconds. Mr James showed recent images of the red planet by Damian Peach, showing the north polar cap rift, which is a dune field at the melted edge of the polar cap. Images of Mars by Richard McKim and Don Parker were also shown. The asteroid Vesta is currently to be found in Leo, an easy object with binoculars. Rising late in the evening is Saturn, in Virgo, the ringed planet being illustrated by an image taken by Paul Maxson from Australia, where the planet is seen higher than from the UK. Transits of Rhea occur on February 5/6, February 15, and February 24.
Amongst the comets, 81 P/Wild and 2007 Q3 (Siding Spring) are currently the best objects, both of 10th magnitude.
In the world of variable stars, attention is focused on epsilon Aurigae. This star undergoes eclipses, but the eclipsing object is not fully understood. An eclipse is now underway. From 3rd magnitude normally, the star has now faded to mag 3.8. For professional instruments it is too bright an object, hence the opportunity for amateur study.
Mr James then mentioned the highly-successful meeting that had been held at the Royal Institution in honour of Sir Patrick Moore's 75 years' membership of the Association. Roger and Gill Perry have produced a 4-DVD set documenting this historic occasion, and it is available to members for 14 [pounds sterling] a copy (nonmembers 18 [pounds sterling]). Included, as an extra, is a new interview with Sir Patrick by John Mason.
Finally, the President mentioned that a new BAA website had been launched that day. He thanked the speakers and adjourned the meeting to Wednesday March 31 at the same venue.
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|Publication:||Journal of the British Astronomical Association|
|Date:||Dec 1, 2010|
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