UNIVERSITY OF KENT
DEPARTMENT
OF
Pretty Good Novae:
V838 Monocerotis & Luminous Red
Transients in Nearby Galaxies
Colloquium 1
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12/15/2007
This colloquium observes Dr Howard Bond as he gives his presentation on V838 Mon, the
conclusions he draws about the mechanisms behind the outburst along with another Non Globular
Cluster (NGC300)
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A colloquium that discusses and answers the questions;
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V838 Mon and its observed light echo and how these occur
The origin and mechanism behind the outburst
NGC 300 and its outburst
Possible Mechanisms behind that outburst and its comparison to V838
Supernovae are some of the most exciting and brightest images taken in the night sky.
However the classifications of these explosively luminous objects can be difficult. Sydney Harris a
scientific cartoonist created an illustration mocking this classification system by drawing two
scientists peering through a telescope and describing the novae they saw as a “pretty good novae”
somewhere in between novae and supernovae. Howard Bond uses this illustration to introduce the
subject of his colloquium and the subject on which it is based, V838 Mon is a star on the very edge of
our galaxy’s spiral arms. One interesting observation with V838 Mon and the reason it is voted one
of the Hubble space telescopes most beautiful images, is that the stars outburst caused a light echo.
Light echoes are very rare events where specific conditions are required for a light echo to be
produced. One requirement is that the star varies rapidly in brightness, is intrinsically bright and
luminous and that circumstellar and interstellar dust is nearby. A light echo is caused when one path
of light reaches us directly on Earth and another reaches us indirectly due to reflection off dust.
The Mathematics behind Light Echoes.
The mathematics of the light echo can be shown to follow a parabolic path as seen below.
Considering a dust particle located at the coordinate (x,z).
Fig1: Mathematical approach to light echo and the light path length.
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The direct light path follows along z and reaches Earth at sometime t after emission. The
indirect path length however can be calculated by simply using Pythagoras’s theorem. Since the time
between images and the light cone expansion is known and the distance the light cone has
expanded, the distance to these objects can be calculated. The locust of dust illuminated at a time t
is hence given by;
𝑧=
𝑥2
2𝑐𝑡−
𝑐𝑡
2
(Equation 1)
Dr Howard Bond describes the light echo as if looking down a champagne glass with the
nearest point being the expansion of light and the bottom of the glass being the initial outburst.
The first recorded light cone was in 1901 and was of Persei Novae. The apparent expansion
of the light cone appeared to be travelling faster than the speed of light. J Kapteyn suggested that
the apparent super luminal speeds were due to propagation of light from the novae outburst into
nearby stellar dust. The dust is stationary and it is only the light that is moving. More recently a light
echo was used to measure the distance of a supernovae in the Messier 93 galaxy and accurately
predicted the distance of thirty six mega light years away.
V838 Mon Outburst
The first outburst of V838 Mon was in January 2002 and was initially an unknown variable
star which was discovered by amateur astronomer Nicholas Brown. V838 Mon rose to 10th
magnitude on the 6th January and then again another 4 magnitude in February and was unlike any
previously know nova, supernova or variable star. Light curves were taken of the stars outburst and
eventually the echo was discovered illuminating previously unseen circumstellar material that was
observable from Earth. With this discovery Bond requested the use of the Hubble telescope for
more accurate observations, direct images and polarimetry, which was accepted. Images were taken
from April 2002 through to February 2004. The images taken were some of the most spectacular
images from the Hubble Space Telescope and show the light echo expansion described by the
parabolic nature of the echo. Bond takes a light hearted tangent from the rigorous scientific
explanation and remarks on many of his colleague’s comparisons of the V838 to a Van Gough
painting and a blog found comparing V838 Mon to the Mozilla Firefox logo suggesting that the
technology used by the popular internet browser was created by extraterrestrial life forms.
V838 Mon continued to be imaged through to 2009. The collection of images shows the light
echo dispersing and continued to be imaged well into the x-ray spectrum. One of the exciting
opportunities that arise from analysing light echoes are that they provide a single tomographic slice
through the nebula revealing detailed structure. In the case of V838 Mon the echoes are only light
weeks old. This type of structure is rare when imaging astronomical objects since we see 3D images
projected down to 2D images. Over the seven years images were collected until Bond and other
members of his research team began to analyse the images in detail enabling properties of the star
and its outburst to be determined. The first thing Bond and his team did was analyse the apparent
superluminal expansion. Below is a graph showing the V838 Mon paraboloid expansion.
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ct
>ct
Fig 2: Behaviour of Light Echo over distance
At initial star outburst when z=0 and x=ct, and as the light expands the dust in front of the star can
be shown when x >ct. It can then be seen that the apparent expansion rate of the light echo can be
much greater than the speed of light! However this is not the case, the light reflects of stationary
dust and reaches us as observers on Earth indirectly.
The images are beautiful but what can the Hubble Space Telescope images actually tell us.
Well one thing the images can be used for are to directly measure the geometrical distance to the
star by using a method called polarimetry, V838 Mon was the first star to have its distance
calculated using this method which will be discussed in more detail shortly. A full 3D map was
created of the circumstellar dust including the overall morphology, and some of the questions such
as was the dust ejected from the star or was it bipolar morphology or interstellar dust can be
answered. V838 Mon also gave us information on dust physics the scattering angle and illuminating
spectrum was calculated.
Polarimetry
The distance to the outburst was calculated using the same V838 Mon paraboloid graph
(Fig 2). The maximum linear polarization occurs when the scattering angle lies at ninety degrees and
the dust lies at some distance x since light takes a specific travel time which is measurable, then the
corresponding angular separation from the star gives the distance to the star. This technique is a
remarkable way of directly measuring the distance to a star, but can only be applied in very specific
situations.
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Fig 3: The polarisation profiles for given light echoes over time.
The angular ring radii were set to the product of the speed of light and the time taken for
the light to travel; this will give the distance d to the star. The analysis of the polarised light received
on Earth from V838 Mon allowed many of its properties to be calculated. The distance to V838 Mon
was calculated to be 6.1 Kpc or roughly 20000 light years this places the star on the very outer
fringes of the Milky Way. V838 Mon was extremely luminous during its outburst, it was calculated to
have an absolute magnitude of -9.8 which implies that during its outburst the star was
approximately 600,000 times brighter than the sun and was brighter than all the very brightest
classical novae but less luminous than a type 1a and type 2 Super novae. Spectroscopy on V838 Mon
showed that the star was a large, cool red supergiant throughout its outburst. V838 Mon’s outburst
was very unlike classical novae which usually eject outer layers and rapidly become blue. V838 Mon
is also one of the coolest stars at type L and is still very luminous in the Infra red spectrum. Even
closer analysis of the spectroscopy revealed something amazing. That the star was in a Binary system
with another hot type B3V star. Subsequent spectra disclosed a further two type B stars which
indicates that V838 Mon was in a young stellar cluster! Spectral classification of the type B stars and
main sequence fitting confirmed the distance calculated using the polarimetry technique as around
6.1 Kpc. The cluster has an upper age limit of about 25Mlyr and has a reddening𝐸(𝐵 − 𝑉) = 0.85.
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Dr Howard Bond then goes on to discuss what he calls the dying of the light, as shown by Fig 4 below;
Fig 4: Shows the light curve of the two binary stars, V838 Mon and its companion dwarf. B
represents the Binary Star and V represents V838 Mon.
As can be seen by Fig 4 the magnitude of the companion star begins to drop which leads to
the assumption of stellar cannibalism and the possibility that V838 Mon has expanded and the force
of gravity pulled its companion into itself. Bond and his team also observed how the spectroscopy
changes in the red and blue wavelengths. This technique allows the composition of V838 Mon and
its companion star to be determined.
These were then compared to other recent extragalactic outbursts. In M31 a red variable
star outburst was detected (M31 RV) and in 1988 it reached a magnitude of -9.5 and remained at
that magnitude for several months. This area was reimaged in 1999 using the HST and only an old
bulge was present. Another outburst was observed in 2006 in the M85 galaxy which is an old
population in Virgo cluster and reached a maximum magnitude of -11.5.
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Discussing the phase space of V838 Mon shows that it is unlike many other novae outbursts
as shown by Fig 5
Fig 5: The Phase space diagram of cosmic explosive supernovae and eruptive novae and LBV transients
As can be seen from the above diagram the V838 Mon outburst is in an unexpected position
between novae and supernovae, because of this the mechanism behind the outburst is unknown.
Some of the mechanisms explored are;
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Thermonuclear Energy
However novae that occur from thermonuclear energy are usually binaries and occur when
Hydrogen is transferred from a normal star to a white dwarf which eventually ignites
thermonuclear runaway leading to an explosion. However V838 did not act like any other
previously seen Novae and the estimated age of the cluster is too short a time frame to
create a white dwarf, so this mechanism is less likely.
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Gravitational energy
Fig 7: Spiral of light curve of V838. Death spiral?
A stellar collision, a merge of stars or the swallowing of a planet in an unstable triple system could
provide the energy output observed. However a more detailed observation shows that the stars
should be rotating more rapidly if they were in a triple system, x-rays should be emitted.
Observations were undertaken in 2008 and x-rays were in fact detected so success! Planned Chandra
in 2010 observations may shed further light.
Next Dr Bond takes a change of subject with a rather charming picture of John Cleese with a
pig and discusses more recent observations undertaken of a cluster called NGC 300.
Fig 8: John Cleese with a Piglet
NGC 300 is in a nearby spiral galaxy which is at a distance of about 1.9Mpc and was
discovered by an amateur astronomer in 2008 when the outburst reached 14.2 magnitude on the
15th May and reached a maximum magnitude in the V spectrum of around -12.5. NGC 300 had a very
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similar outburst to V838 Mon and also to a supernovae type IIn although much fainter. The spectrum
was then analysed as shown below;
Fig 9: Spectrum of NGC 300.
The new team of astronomers and astrophysicists began the search from archived surveys
for the progenitor of the star and hence the source of the outburst. The site of the outburst was
analysed and no star greater than 28.5 magnitude was seen, but did the outburst at NGC 300
undergo the same type of outburst as V838 Mon?
Initially it was thought that the outburst of NGC 300 was very similar however when the light
curve was analysed it was very different.
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Fig 10: Showing the light curve of NGC 300.
The pre-outburst Spitzer observations detected an intra-red source longward of K that was
surrounded by dust of temperature 3
̴ 50K and a radius of 300 AU with a luminosity of a star between
10 and 15 solar masses.
So Dr Bond and his team began to search for an undetected progenitor. The theoretical
progenitor remained undetected in deep HST optical images but conspicuous in pre-outburst Spitzer.
So the team came to the conclusion that there was an outburst of a new kind on a heavily dust
enshrouded massive star, which was different to V838 Mon and was not a stellar merger. Later HST
images showed no light echo which concluded that it was a different type of outburst to V838 since
no light echo was detected. To describe the outburst detected new outburst mechanisms were
discussed. Proposals include:
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Electron capture –SN
EC occurs in 8-10 solar mass stars and occurs when a star burns C to make an ONeMg core,
which then collapses through electron capture. However no cases of EC have been detected
before and NGC300 may have had a mass larger than 10 solar masses.
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Co decay rate
Fig 11: Showing the decay rate of Co
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LBV eruption
The recognised LBV eruptions occur in stars with mass less than 20 solar masses and the
eruptions occur on optically conspicuous stars that are able to exceed the Eddington limit.
These outburst mechanisms are not very well understood so maybe the phenomenon
extends to lower mass stars and stars enshrouded by dust?
Extreme mass-transfer episode in a massive binary
These NGC 300 events are not rare, SN 2008S in the NGC 6946 was very similar.
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Summary
V838 Mon is a prototype of a new class of stars that rapidly expand to red
supergiants. Illuminates and provides the most spectacular light echo in astronomical history
however the cause of the outburst is still unclear, it is unlikely to be novae on a white dwarf
since the V838 Mon cluster is too young. It may however have occurred due to a stellar
merger or collision.
V838’S multi-peaked light curve and x-ray emission may support the merger theory.
The origin of the dust illuminated is unknown and the situation has been clouded by the
recent discovery of the NGC 300 and SN 2008S transients with heavily enshrouded massive
stars, mimicking the outburst of V838 Mon. There could potentially be two new classes of
transients with luminosities lying between those of Novae and Supernovae. Upcoming
surveys could find more in large numbers (Pan-STARRS, LSST).
Dr Bond ends his presentation back on the cartoon initially showed presenting that
there may in fact be Nomenclature for outburst lying between Novae and Supernovae.
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Questions:
Q: Do you expect the B star will reappear would it show evidence of material?
A: I assume that it is inside of a dusty shell as opposed to being inside the other star. So I would
expect it to emerge at some point yes. The light curve over the last few years has been increasing so
whether that means that it is beginning to emerge I don’t know.
Q: You implied that the B3 star in the [inaudiable] system would be consistent to account for what
we saw in V838 pre outburst.
A: That is somewhat controversial but the B3 star would account for all of the Light that we initially
imaged in V838 Mon, so there is no evidence of a cool star. Recently it has occurred to me since the
NGC 300 event that V838 Mon could have a deeply embedded star, but there are no existing surveys
that would be able to determine. If there is an embedded star then yes it would be able to account
for the light of both stars as a system, it is doubtful though due to the age of the cluster and the way
that the light graphs “act” with each other.
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Strengths:
Dr Howard Bond delivered the colloquium clearly and concisely. Although there was a large amount
of slides to go through (135!) the colloquium did not feel like it was slow paced. Throughout the
presentation Dr Bond used simple anecdotes along with the mathematical explanations which were
useful for members of the audience that did not have a familiarity with the subject or some of the
topics or objects being discussed. The presentation was very image and figure based which kept me
entertained and focused on what Dr Bond was talking about. He combined complicated scientific
ideas with humour which he used very well to keep the audience entertained.
Weaknesses:
Due to the short notice that Dr Bond had to prepare (this is stated at the beginning of the
colloquium) the content of his presentation is a little bit disjointed. It isn’t specifically aimed at one
particular subject although it discusses V838 Mon for most of it he also includes his current research
I suspect to “fill” out the time given the short notice he was given for his presentation. He does not
specifically pose questions at the beginning of the presentations that are later answered; instead it is
more of a presentation of what they found and some of the theories they had and how they were
proven or disproven.