U Orionis light curve 2007-9 by Syeve Woodbridge – click to enlarge

Although in the constellation of Orion, this star is near the star Chi Orionis on the borders of Gemini, so actually has a declination of +20 and it is best found starting at Eta Geminorum. So it can be observed for quite a large part of the year from the UK (from mid-August to early May).The predicted maximum for 2009 was February 15th, and for 2010 is February 19th , so it is currently well placed for observation.

I have now been observing it for two seasons, starting in September/October and finishing around May. In the autumn, I have not been able to see it in my binoculars as it was too faint (these observations are marked in yellow on the diagram). I have then followed it right through maximum successfully.

The diagram below summarises my observations over the last 18 months.

2008 was a fairly typical maximum with a peak magnitude around 6.3. The rise to maximum is normally faster than the fall to minimum, as is typical with Mira stars, although this is not clear from my data. 2009 was a much fainter maximum, only about 7.4 and many observers (including myself) did not pick it up in binoculars until well into January.

In terms of the date of maximum, in 2008 it was later than predicted; the date of maximum for 2009 is not clear from my data..

I liaised with another member of the BAA Variable Star section and my early observations for 2007-8 were probably incorrect: I may have been getting confused with a close “companion” star – a common problem with certain variables with a large range.

I plan to continue observing this star until it disappears from my binoculars in the next couple of months, and then to pick it up again in November or December 2009, in advance of the 2010 maximum.

In many ways Long Period variable stars are fun to watch as they do change in brightness so much. We located this star and made observations at the Society meeting in February 2009. If society members (or others) want to join me in watching this interesting star, I will be happy to help advise and collate observations.

07-8 I started observing a couple of W Ursa Majoris Stars. These contact eclipsing binaries have very short periods (fractions of a day), and because they are so close together the stars themselves are distorted, leading to continuous variation rather than the sharp dip during eclipse that Algol-type eclipsing binaries show.

One of these stars is VW Cephei, which is one of the brightest of this class of star, varying between magnitude 7.2 and 7.7. The variation is small (making it a challenge to observe visually) although noticeable, and as the period, covering both a primary and secondary eclipse is around 6.5 hours, it is possible to see a lot of change in a single night.

Initially I faced a few problems in plotting magnitude variation, but then I noticed that the comparison star magnitudes as listed in Hipparcos and elsewhere were somewhat different from those on the BAA chart. When I starting using the Hipparcos magnitudes, it became easier to see a light curve: I have asked the BAA to re-examine the magnitudes on the chart!

The diagram below shows my most successful night’s observations last October:

vw Cehpei light curve - click to enlarge

I am continuing to observe this star (it has a high northerly declination and so is circumpolar), and a summary of my observations to date is shown in the first diagram below, mapping the observed magnitude against the plot of “phase” (the position in the orbital cycle as predicted by the current best known elements from Krakow university – 0.0 being the point of primary minimum).

As you can see there is quite a high degree of scattering in my visual observations. I am hoping with further observations this year to get a better plot.

This system is very amenable to CCD observations, and indeed to digital camera observations. Digital camera data from earlier this year from another BAA member is shown in the second diagram above and this indicates that eclipses are running 12 minutes early than predicted. It is not clear whether this indicates continuing mass-transfer between the stars as there is at least one other body (planet/brown dwarf) in the system. If other members with CCDs want to try observing this star, I am very happy to provide a chart and magnitudes. It is nice because every clear night you can trace out the fluctuations across much of a cycle, and it gives you a chance to put your CCD to use in another way that can help contribute to scientific study.

R Scuti (Download locator chart)

This RV Tauri variable (magnitude range 4.2 – 8.6) has now just come back into view in the dawn sky.   There have been two deep minima over the last 6 months, one around 3rd October 2008 and one around 22nd February 2009, with a shallow minimum occurring at some point when the star was too close to the sun for observation. A chart of my observations (yellow diamonds) against the worldwide observations is shown below. If anyone else has observations to add please let me know.

RZ Cassopeiae (Download locator chart):

I have not had any specific light curves from this star to report as yet. However as it is circumpolar it is still easily observable in the spring. It varies from magnitude 6.2 – 7.7 with the eclipse lasting almost 5 hours. The total period is 1.2 days, although the variation outside primary eclipse is very small. Key dates this spring to observe to catch eclipses are:

• The evenings of:
  • 21^st, 26^th March
  • 1^st, 7^th, 14^th, 20^th, 26^th April,
  • 2^nd, 8^th May
• Or the mornings of:
  • 20^th, 22^nd, 28^th March,
  • 3^rd, 9^th, 16^th, 22^nd, 28^th April,
  • 4th, 16th, 22nd, 28th May.

TX Draconis (Download locator chart)

Circumpolar semi-regular variable very close to Eta Draconis. The magnitude range is 6.6 – 8.4, and the average period is around 2 ½ months. Be careful you use the correct comparison stars. I have made a number of observations over the last few months and will try to provide a graph in the near future.

If you are interested in any stars in this campaign, please let me know so I can collate observations and we can put further results up on the website. Observations on U Orionis and Rho Cassiopeiae (which were observed at the recent observing evening) would also be welcome.

Rho Cassiopeiae is classified as an SRd star, which means it is an F-type supergiant and thus not as red as most of these stars. Nevertheless, I have found it shows some signs of the classic “Purkinje” effect which means that it starts to seem brighter if you look at it too long. It also suffers from having comparisons that are several degrees away, which can make estimates difficult or a little unreliable if Cassiopeia is a rising “W” so that a key comparison star is at a somewhat lower altitude than Rho itself and thus extincted by the atmosphere more (this may be a cause of some of the variation in my estimates, particularly in those around Julian date 54720).

The diagrams below summarises my observations over the last 2 years, with the AAVSO observations for the period.

Rho Cass light curve 2007-9 by Steve Woodbridge -click to enarge

 

Rho cas AAVSO Light Curve -click to enarge

My results broadly agree with the AAVSO’s – suggesting one maximum around Julian day 54330 (August 07), a second around Julian day 54560 (April 08 – around 8 months apart) and a third due imminently (Spring 09), with a minimum at the start of my observations and in between the maxima.

It is interesting to note the high spread of magnitude values in the AAVSO observations. This reflects the “personal equation” in how people see a particular star. Rho Cassiopeiae seems particularly prevalent to this, as in discussions with another UK variable star observer he remarked how different visual observers can give estimates varying by about half a magnitude on the same night.

This star has been observed over a long period, and at times shows significantly deep fades (eg in 1984 – see averaged plot below from BAA data). These deep fades in particular are unexplained (some have suggested an eclipse of some sort). It is thus an interesting star to observe, although it does need some patience, as the period of 10.5 months is quite long and the range fairly small.

Rho Cas BAA Average Plot 1984-1989

This star was part of our February 2009 Observing session. If society members or others want advice or to send observations to me, I will be happy to present some joint findings.