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ABSTRACT

FG Sagittae is an extremely rare example of a "born-again star" and in
order to gain a greater understanding of it, observations obtained during
the last 50 years were re-examined. Using ultraviolet spectra obtained in
the 1980s and 1990s by the IUE, along with recent theoretical models, this
project checked the stars evolutionary pattern during this epoch.

The purpose of this project was to measure changes in UV flux and gradient
as a function of time. Another aim of the project was to re-examine
observations over the last 50 years (to assess the validity of Montesinos
et al. findings). The project was to discover the relationship between
effective temperature and date and to attempt to settle the controversial
hydrogen abundance of FG Sge.

To carry of this investigation different IUE spectrum of FG Sge were
analysed and their graphs were studied using a simple plotting programme
called 'DIPSO'. Temperatures were derived from these results by fitting the
flux distribution of 'STERNE' (model stellar atmospheres). On inspection of
the results and graphs, after running a programme called 'ffit' on the
data, the effective temperature was discovered to be too high. To solve
this problem, visual light data and infrared data from IUE were added to
the results and the temperature was corrected. This process took around
three weeks to complete and the interpretation was completed after.

The results verified that Montesinos et al. results were correct and a
reason for the high temperature was discovered, i.e. A discrepancy between
the UV, visual light and IR data caused the difference in temperature.

A trend was discovered, i.e. the temperature was decreasing therefore the
effective temperature appeared to fall with increasing time, this showed a
systematic offset and that the models fitted well with the spectrum. It was
recognised that a hydrogen abundance of 90% was correct, implying that as
the star cooled it maintained its normal surface. This has implications for
those astronomers who perceive FG Sge to be of RCB nature.

Future work should include an improvement of the model (red line in fig.5)
to make it a best fit to all the data. Improved model atmospheres may
provide a better fit at all wavelengths. The multidisciplinary tasks on the
spectrum of FG Sge should be repeated. More research and spectra are needed
to develop research on FG Sge.























[pic]


CONCLUSIONS


After repeating Montesinos et al. experiment, we concluded with the same
results. However another trend was discovered, i.e. the temperature was
decreasing. The effective temperature appeared to fall with increasing time
(see fig.6 and fig.4), this showed a systematic offset and that the models
were good fits with the spectrum.

The initial problem with high temperature was solved with the addition of
visible light and infrared data. Once this data was added, the temperature
was reduced to a more feasible one (i.e. closer to the 5500K model in
fig.5). The discrepancy between the flux of UV and infrared was the cause
of the difference in temperature. The reason for the improved results after
this additional data was used is because most of the energy for the star is
not in the ultraviolet data, but most is in visible light and infrared,
therefore the temperature improved.

However even with the addition of the visible and infrared data the model
(red line in fig.5) was not as accurate as preferred. Using a programme on
the 'UNIX' system the fit was corrected to almost match with the 5500K
model (blue line in fig.5).

Through the graphs of 'log Teff' against time ('Year') of the three
different hydrogen abundances, it was proven that the 90% hydrogen
abundance was correct (see fig.3 and fig.4). This means that as the star
cooled it maintained its normal surface, i.e. No convective turnover yet as
the turnover could possibly happen when the star is cooler.

This has implications for those who perceive FG Sge to be of RCB nature, as
one of the characteristics of RCB stars is their distinct lack of hydrogen.
The surfaces of RCB stars are unusually poor in hydrogen and are rich in
carbon and nitrogen (remnants of evolved stars).






EVALUATION AND FUTURE WORK

Strengths

. Found a decrease in temperature with time. The graphs of the spectrum
match well with the models, i.e. good fits (see fig.2) a consistent
decline is apparent as a systematic offset (although temperature was
originally too high, the trend of a decline in temperature was still
there).
. The aim of the project was to check that what Montesinos did was
correct and it was.
. Discovered why Montesinos et al. temperature was too high, this was
due to an inconsistency between UV and IR data.


Weaknesses and Shortcomings

. Originally there was an oddly high overall temperature but the
explanation for this was concluded to be an inconsistency between the
ultraviolet and infrared data. Even with the addition of IR and
visible light data the model (red line in fig.5) was not a perfect
match with the 5500K model.
. To find a star's total luminosity, it requires the measurement of its
flux (i.e. the amount of energy passing through each unit of area in a
second= brightness) over the complete range of the electromagnetic
spectrum. This may not be possible due to: - Detector may work within
a limited range.
- Earth's atmosphere absorbs some of the
energy.
- Matter in interstellar space may also absorb
some energy.
. FG Sge was first viewed in 1900, however the data used in this project
from the first IUE result of FG Sge was taken in 1978.
. It would have been helpful if more data of FG Sge's spectrum had been
recorded by the IUE.

Future work should include:
. The model (red line on fig.5) could still be improved to make it a
best fit to all data, not just magnitude.
. Improved model atmospheres may provide a better fit at all
wavelengths.
. Repeat multidisciplinary tasks on spectrum of FG Sge.
. More research and spectra are needed to develop research on FG Sge.


ACKNOWLEDGEMENTS


I would like to take this opportunity to acknowledge bursary support from
the Nuffield Trust and Sentinus U.U. The Queens University of Belfast N.
Ireland is also gratefully acknowledged.

My thanks to the research staff at Armagh Observatory for their support and
friendship during my bursary. I especially wish to thank my supervisor Dr
Simon Jeffery for his helpfulness (and continued support) and efforts he
made to teach me the skills of astroarchaelogical astronomers.

























METHODS AND MATERIALS



Outline of Methodologies

1. Observations of FG Sge were taken by IUE. This data was used to
compile a table of results (Table 1).
2. The different spectrum of IUE data were analysed and the relevant
graphs were studied using 'DIPSO'.
3. Temperatures were derived from these results by fitting the flux
distribution of STRENE (model stellar atmospheres).
N.B. These temperatures depended on certain chemical composition
assumptions.


Equipment constructed

This consisted of writing programmes similar to the example shown on the
following page, on a digital 'unix' system. It was necessary to use an
editor called 'nedit' in a simple plotting package called 'DIPSO'.

The programme 'ffit' was used to compare the theoretical models of stellar
energy distributions with that observed. 'Ffit' solves for theta, E(B-V)
for one or two stars by fitting theoretical flux distributions to observed
fluxes and/or photometry.

In order to interpret stellar spectra, it is necessary to construct models
that accurately represent the structure of a stellar atmosphere and the
transfer of radiation from the stellar interior into interstellar space
(using STERNE).

Models from STERNE (STERNE model atmospheres) were fitted to the observed
fluxes. STERNE is a programme that calculates the structure of stellar
atmospheres. (These programmes run on local workstations running Digital
Unix/Linux).

















Structure

| |






Programme to create fig.4


del 1-99
font 2
frame 15 10 5

alaslins 2 0, alascols 1 2, alasrd lw_flux.d
xsub 2440000, xdiv 365.242, xadd1968.390
ymult 1000, title "H01", logy, push

alaslins 2 0, alascols 1 4, alasrd lw_flux.d
xsub 2440000, xdiv 365.242, xadd1968.390
ymult 1000, title "H10", logy, push

alaslins 2 0, alascols 1 6, alasrd lw_flux.d
xsub 2440000, xdiv 365.242, xadd1968.390
ymult 1000, title "H90", logy, push

lweight 1, mark
xr 1955 2000, yr 3.6 4.1
xlab Year, ylab log Teff

mset 1 12, nofill, expand 1.0 M
cset 1, box, pop1, title " ", pm 0, nb
pwrite 1960 4.000 6 "H01"

mset 3 4, cset 2, nofill, pm 2
expand 1.0 M
pwrite 1960 3.925 6 "H10"

mark, cset 4, mset 1 4, fill, pm 3
pwrite 1960 3.850 6 "H90"

Steps involved in reaching final solution


1. Research about "born-again" stars.
2. Familiarisation with editor (nedit) and programmes for analysing data
(DIPSO).
3. Collect IUE data on FG Sge from internet from the "IUE Final Archive".
Using DIPSO and nedit, read in spectra and studied the graphs (see
example: fig.2)
4. Changes in UV flux measured as a function of time, gradient of graphs
were also calculated.
. Wave bands selected (2400-2700) Angstroms (A).
. For each LWP and LWR spectrum, the flux in each wave band was

measured.
. Each measurement recorded in Table 1 (f2550, f2850, f3150).
. File read into DIPSO and flux plotted in each waveband as a
function of DATE.

N.B. The dates are measured in 'Julian Dates' (JD) which are simply a
continuous count of days and fractions since noon Universal Time on
January 1st 4713 BCE (almost 2.5 million days have transpired since
this date). JD is widely used as time variables with astronomical
software. JD transforms: year, month, day, hour, minute and second
into one figure.

5. Created programme (see previous page) using nedit to create plots of :

Teff (H01) Theta (H01) Teff- Effective temperature
Teff (H10) Theta (H10) Theta- angle star is subtended
at
Teff (H90) Theta (H90) (see fig.4)
6. Created 'log Teff' plot against 'time' for H01 H10 H90. This resulted
in H90 (90% Hydrogen) being correct, therefore plot H90 showing its
gradient was created (see fig.3).
7. Ran 'ffit' to find temperature of FG Sge at different times (depending
on chemical composition, i.e. hydrogen abundance.
8. On inspection of results and graphs, after completing 'ffit' on the
data, the effective temperature was too high. To solve this problem,
data from the IUE (visual light and infrared data) were added to the
results (see Table 2.)
9. After fitting models to the new results, the temperature was
corrected. However the model (red line on fig. 5) could still be
improved. Improved model atmospheres may provide a better fit at all
wavelengths.





























INTRODUCTION


FG Sagittae is an extremely rare example of a star displaying "born-again"
nature. It is of great importance to astronomers as it is a star which has
returned from the 'white dwarf' stage by re-igniting its last reserves of
nuclear fuel and has developed into a yellow super giant. FG Sge is a
unique object in the sense that we have direct evidence of stellar
evolution for this star but in a timescale comparable with a human
lifetime.

To gain a greater understanding of this extraordinary star, observations
obtained during the last 50 years must be re-examined. Using ultraviolet
spectra obtained in the 1980s and 1990s by the IUE along with recent
theoretical models, this project will check the stars evolutionary pattern
during this epoch.

The International Ultraviolet Explorer (IUE) was used to obtain 104,470
ultraviolet spectra between Jan 26. 1978 and Sep 30. 1996, including many
spectral images of the star in question, FG Sge. The IUE was the longest
and most productive astronomical Space Observatory (fig.1 shows the IUE
'all sky survey' (1978-1992). For each spectrograph there were prime and
redundant cameras. The "IUE Final Archive" (including observations of FG
Sge) was a collaboration between NASA, ESA and PPARC, to generate a high
quality and uniform spectral archive during the final phase of the mission.

V4334 Sgr (Sakurai's Object) and V605 Aql are the only other stars known to
display the same born-again nature as FG Sge. However both of these stars
have a significantly shorter evolutionary timescale than the born-again
star FG Sge (which has been observed to undergo born-again behaviour for
more than 120 years).

The present understanding of the evolutionary progress of FG Sge is as
follows:
. 1900-1970: Brightened from magnitude 13.5 to 9.5
. 1955: Hot Super giant
. 1983: Cooled to a temperature similar to that of Sun
. 1992: Series of irregular and sheer drops in brightness (star bluer
and hotter)
. 1992-1996: Further drops occurred
FG Sge has been compared to a class of star known as R Coronae Borealis
Stars (RCB Stars). Characteristically these stars experience sudden deep
declines by fading dramatically and unpredictably by factors of up to one
thousand within a few weeks. Over the succeeding months however, they
gradually recover their original brightness. RCB stars tend to have an
excess of carbon and generally are deficient in Hydrogen.

"A strong case has been made that FG Sge, V605 Aql and Sakurai's Object
have become RCB stars in this century. All have increased in brightness in
a relatively short time and both FG Sge and Sakurai's Object have exhibited
sudden drastic declines in brightness.

To fully understand the relevance of FG Sge to astronomers, one must
primarily consider 'Normal Stellar Evolution'. This takes place over such
long ages that from the human stand-point, the Sun and stars seem to stay
the same forever. Most stars reside on the main sequence (the long and
stable phase) where they generate energy by fusing hydrogen into helium,
deep in their cores.

The Hertzsprung-Russel Diagram (1911) is used to help explain 'Normal
Stellar Evolution' (see diagram 1). This diagram plots intrinsic brightness
of stars against their temperatures. From the end of the last century FG
Sge has moved across the H R Diagram changing from a normal hot giant to a
cool star with significant changes in its' surface chemical composition.

[pic]

Early in the 20th Century the work of the astronomers Ejnar Hertzprung and
Henry Norris Russell led to the construction of what are now known as
Hertzprung-Russell (H-R) Diagrams, in which a star's absolute luminosity is
plotted against its surface temperature (or spectral type).


[pic] [pic]



. Star spends most of its life on the main sequence, until hydrogen
fuel begins to run out.
. Helium core shrinks and brightens (stars outer parts balloon
enormously).
. This giant phase does not last for long. Its' core is hot and dense
converting helium to carbon and oxygen (releasing energy).
. The outer envelope is ejected as the 'planetary nebula' which expands
and the central star heats up to over 100 000K. The star then cools
and the nebula disperses and the core is exposed as a white dwarf;
"its fate is to continue cooling forever".
. If the Giant is massive enough, an iron core is produced which
eventually collapses violently on itself producing a supernova and
eventually a neutron star.
. If Giant is even more massive it can punch an infinitely deep
gravitational well, i.e. a black hole.



Research and development to date

Significant research has been carried out by various different astronomers
on the evolution and origin of FG Sge. Montesinos et al. produced a report:
'Ultraviolet and Infrared monitoring of FG Sagittae during 1982-1989'. In
this project we are checking to confirm that this report was correct.

"UV IUE spectra and IR (JHKL) photometry of FG Sge obtained over the period
1982-1989 indicate that the cooling rate of the star observed until
approximately 1980 seems to have stopped and that an increase of the
effective temperature may have occurred, this being a significant change in
the evolutionary path traced so far." (Montersinos et.al.)

This project will contribute to on-going research by C.S. Jeffery in his
report, "Stellar Archaeology: the evolving spectrum of FG Sge". It will be
included in section '5.7' of the report "Teff in the 1980's".
"Of relevance to the evolutionary history of FG Sge are hydrogen-burning
AGB remnants with a thermal-pulse cycle phase sufficiently close to one,
i.e. those that experience the next (and last) thermal pulse while they are
contracting towards the white-dwarf stage. Three different possibilities
can be distinguished. . .In the first two cases, where the flash occurs far
off the AGB, the star experiences a rapid expansion back to the vicinity of
the AGB, sometimes called the AGB born again stage. The duration of this
red-ward excursion is controlled by the gravitational -thermal timescale of
the stellar envelope and is therefore rather short." (C.S.Jeffery).

Hawley and Millar (1978) found that:
"Taking all the existing analyses into account, it appears to us that there
is no observational evidence of any brief mixing episode that might have
changed the surface composition of FG Sge into a hydrogen-free and/or s-
process element enriched one."

There is still more room for research.

Objectives
. To measure changes in UV flux and gradient as a function of
time.
. To re-examine observations over the last 50 years (to assess the
validity of Montesinos et al. findings).
. To discover the relationship between effective temperature and
date.
. To attempt to settle the controversial hydrogen abundance of FG
Sge.
. To acquire knowledge of skills used by astronomers to
investigate the pattern of stellar evolution.
[pic]















RESULTS


. Prior to this study ultraviolet fluxes led to a high 6500K for the
effective temperature of FG Sge (Montesinos etal.). This temperature
can be eliminated by combining the UV data with the visual and infra
red data presented by the same authors, and by comparing these with
the theoretical flux distributions.
. While the combination of UV, visual and IR data does not produce a
quality of fit as good as for the UV data alone, they do provide a
much more reasonable account of the total flux from the star.
. By repeating Montesinos et al. procedure the same results were
produced but in addition, an overall decrease in temperature was
found.



Further explanation of Fig.5:
1. The temperature defines the shape of the flux distribution.
2. Theta defines the magnitude of flux (vertical scale).
3. Foe data from Oct. 1986 - compared fluxes for different temperatures
from models (5000K, 5500K, 7000K) with observations.
4. 7000K (original temperature) fits the UV (IUE) data best but fails to
fit visual and IR data.
5. Visual and IR data are best fitted with 5500K (5000K is just too low).
6. The model from 'ffit' in red (after alterations) now clearly matches
the 5500K model.



Table 1.Data table of Teff and Theta of three different hydrogen
abundances (H01, H10, H90) in relation to date (JD).







[The remainder of the results section was printed out in the observatory
and I have copies of it at home although I am unable to email them to you
as they are not saved on my disk]





















-----------------------
UV AND VISUAL SPECTROPHOTOMETRY

TEFF, (, E(B-V)

FFIT

MODEL FLUX GRID

STERNE

COMPOSITION

Diagram 1. The Hertzsprung Russel Diagram showing normal stellar evolution.

Diagram 2. This illustrates the direction of travel of a normal star and
maps onto the H-R Diagram shown as Diagram 1.

Diagram 3. An illustration of two born-again stars mapped onto the H-R
Diagram. FG Sge is arrowed.

Figure 1. This is an Aitoff projection of the positions of more than
80,000 images plotted in galactic co ordinates.
The SWP (Short Wavelength Prime) exposures are flagged as red.
The LWP/LWR exposures are flagged as yellow