Review of
“Review of the 2010 Eruption of Recurrent Nova U Scorpii”
by Bradley E. Schaefer
The Proceedings of the Physics of Accreting Compact Binaries, 2010
http://arXiv:1009.31971
Motivation
Recurrent novae have always held a fascination for me due to their unpredictable nature and frequency of outburts.
They also have very interesting light curves, the analysis of which give good insights into their physical nature. U
Scorpii has special significance for me as at the 1999 outburst I had a very good run of weather and was able to make
an extensive series of visual observations which were published along with corresponding spectroscopic and
photometric observations. (“The 1999 outburst of the eclipsing and recurrent nova U Scorpii”, Munari et al, Letter to
the Editor, Astronomy and Astrophysics, 347, L39-L42, 1999)
Overview
U Scorpii is an eclipsing and recurrent nova that has undergone outbursts in 1863, 1906, 1936, 1979, 1987, 1999 and
recently in 2010. Looking at the frequency of the outbursts that have been detected, it is believed that approximately
2 may have been missed around the years 1927 and 1957. The paper makes an interesting point in that we have
probably been quite lucky to have seen as many outbursts as we have as U Sco lies very close to the ecliptic plane
and therefore the gap in observations is quite large (at least 60 days) compared to the typical duration for an outburst.
U Sco is the fastest known nova event of any type. The eruptions rise very fast, typically within 6 hours to rise to
magnitude 7.5. The decline is characterised by a similar fast decline with t 3 = 2.6 to 4.6 days (time to decline 3
magnitudes).
U Sco is an eclipsing binary with an orbital period of 1.23 days. The eclipses are deep and it has been possible to
measure these accurately enough to measure steady orbital period change due to mass transfer between eruptions
as well as the period change across eruptions allowing the mass ejected during an eruption to be determined.
The distance to U Sco is well known (around 12 kpc) and has probably one of the best known nova distances since
the orbital period as determined from the eclipse light curve is accurate and from there the size of the companion star
is known. Since the eclipse is total, it means the unirradiated hemisphere of the companion star only is seen during
an eclipse and the brightness at this minimum can be measured. With the size and brightness of the companion star,
the distance to the system can be determined relatively accurately. It’s impressive that key parameters such as the
size and distance can be determined based on the light curve characteristics.
The system consists of two components:
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A white dwarf (primary)
A red subgiant (secondary) with a high accretion rate onto the WD
The unpredictability of eruptions for recurrent nova has always been one of their interesting features! However back
in 2005 Schaefer proposed a method to predict the eruptions based on the quiescent brightness level between
eruptions. This is based on the premise that recurrent novae will erupt when a certain amount of mass has accreted
onto the white dwarf. When U Sco is comparatively bright in quiescence this implies a high accretion rate and the
eruption interval is short. When it is faint, the eruption interval is longer.
2010 Outburst
Based on the premise above, Schaefer predicted the next outburst would occur in 2009.3 give or take 1 year. AAVSO
members Barbara Harris and Shawn Dvorak detected the outburst on 28 th January 2010. At the time this was the
best observed nova event with a number of satellite observations as well as over 35,000 photometric observations!
The majority of these were from amateurs.
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The significant amount of observational data from the 2010 outburst led to several new phenomena being discovered
and I summarise a few of these in the light curve analysis below.
Analysis of the Light Curve
Schaefer produces a very good summary of the light curve and breaks it up into a discrete series of phases and then
outlines some of the physical processes going on related to the light curve. I have learned from this review that there
is a significant amount of information that can be gleaned from the light curve, some not immediately obvious.
Unfortunately Schaefer did not include the data pictorially so I include the following plots below.
The one on the left is all AAVSO visual and V observations from the 2010 outburst. The one on the right is from
Munari et al (1999) which shows the 1979, 1987 and 1999 visual light curves. The visual observations from 1999 are
generally ones made by myself. Schaefer provides a summary of the various phases as follows:
Day 0 to +9 – A very fast decline with t3 = 3.6 days (in 1999 t3 = 4.3 days). The speed of the decline suggests that the
white dwarf is near the Chandrasekhar limit which I have found out to be when a star gets to a mass of 1.44 x the Sun
and it is not able to balance gravity and will collapse under it’s own weight. This is exciting to me as it suggests that U
Sco may become a Type Ia supernova!
Day +9 to +15 – The curve transitions into the first plateau and in this period the 2010 outburst showed large
amplitude flares (amplitudes over 0.5 mag) which had not been seen before. These are not eclipses, which are not
seen at this stage as evidenced by the AAVSO light curve. The flares are not flickering or related to the white dwarf of
accretion disk and the paper concludes that their origin is not understood from theory or have been seen in other
systems.
Day +15 to +21 – This is the first part of the plateau period where the star is around magnitude 14 and is easily seen
on both graphs above. This is also when the first eclipses are visible as can be seen from the AAVSO graph. At this
point, the nova shell and winds have decreased such that the region near the white dwarf is visible.
Day +21 to +26 – This is the middle third of the plateau period where the eclipses deepen to around 0.8 mag. By Day
24-25, flickering is seen which confirms that much of the accretion disk around the white dwarf has reformed after the
outburst.
Day +26 to +32 - This is the latter third of the plateau period and the eclipses deepen further to 1.2 mag. Most of the
optical light is coming from a source which is elongated along the orbital plane which is interpreted as a forming
accretion disk where matter has not yet filled the interior.
Day +32 to +41 – There is a sharp drop in brightness and the eclipses deepen further to 1.4 mag.
Day +41 to +54 – The curve flattens out and the magnitude is around 17. The eclipses become more shallow.
Eclipse mapping shows that a normal sized accretion disk is in place ans that filling in of the inner part of the disk has
occurred. A new phenomena detected in the 2010 outburst was dips in the light curve outside of the eclipses with
depths around 0.5-0.7 mag. Many observers saw these and there was no precedent for these features from othe
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novae. Schaefer explained this as due to temporary high edges of the accretion disk (raised rims as the accretion
stream collides with the disk) which occults the bright inner regions and therefore the optical light decreases.
Day +54 to +67 – This is the final fading of U Sco. The optical dips discussed above stop at around Day +61 as the
accretion disk has settled down. The eclipses seen are the same as seen for the system at normal quiescence.
Conclusions
I found this a fascinating paper on a very interesting star. The main observations I came away with are:
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A lot of information on the physical nature and behaviour of nova can come from careful analysis of the light
curve
Schaefer’s theory on predicting recurrence intervals for recurrent nova outbursts based on brightness flux
levels at quiescence appears to be vindicated but could do with more test cases.
The importance of timely notification of outbursts by amateurs to alert professionals and the professional
quality photometric data amateurs can contribute during an outburst.
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