Catch a Star! 2004  

Report N: 310

IK Pegasi (HR 8210)

 

member: Hristo Stavrev Stavrev - 16

member: Ivan Zhivkov Dimitrov - 18

leader: Svetlana Yordanova Tzekova

  BULGARIA

School: People's Astronomical Observatory of Yambol

Address: Bulgaria, Yambol 8600, P.O.B. 220

e-mail: ikpegasi@mail.ru

 

Special thanks to Margarita Sharina from the

 Special Astrophysical Observatory of the Russian Academy of Science,

for helping us gather scientific data

 

Introduction:

     Recently a shocking astronomical breakthrough has been made.

     HR 8210 is a binary star with a normal star for a main component ( IK Pegasi A) and a hot white dwarf for a secondary component ( IK Pegasi B). There are many systems like it, and such combination is not unusual. What makes the system interesting is the discovery of the student from Harvard University, Kathryn Sandstorm. Working on a college paper, she has discovered that there’s a hidden danger in that system.

    It is known that the dwarf has an amazingly high mass, and the main star is known to be an Am -type star in the end stages of evolution. That means that very soon, in cosmological meaning, the main star will die, ejecting its outer layers onto the dwarf, forcing it to pass the critical boundary of 1.4M and go off into a supernova.

    At first sight there is nothing unusual about this stellar association, such supernova explosions happen all the time all over the Universe.

    The main reason to consider the HR 8210 system dangerous is its distance to the Solar system. It is located only 150 ly away, which is a distance far under the safe for a supernova. If HR 8210 goes off at its present position it could wipe out life on earth.

    This gives reason for many speculations and mass panic.

    So in order to get a better understanding of the situation we must at first make a full review of each component in the HR 8210 system and make conclusions about the system as whole.

 

 

1. Basic parameters, Location, Position in the sky

 

1.1. Location in the sky, how can we find it!

 

    Because of its comparatively high luminosity (approximately 6 m ) HR 8210 has been known  since antiquity (of course it had been considered as a single star). In fact on a very dark night it’s visible to a person with a very sharp sight. Placed in the Pegasus constellation its

 

equatorial coordinates are:  J2000 RA:  21h26m26.70s   DE:+19°22'32.0"

 

Other basic parameters:

Alternative names: HR 8210 ( Harvard revised, Bright stars), HD 204188 (Henry Draper Catalog), IK Pegasi

m vis :  6.07 (Integral, The luminosity of the dwarf is very lower)

Color Index:   0.22 (the system seems white )

Spectral Class:   A8m (for the main component)               

Annual movement:   0.081 0.014

I.I.2005 RA: 21h26m36.22s DE: +19°23'25.8"

 

Picture 1

The star over the red arrow is not IK Pegasi, but a neighbor of it 1 Peg ( m = 4 m.08)          (see Picture 2).

We can see the constellations Pegasus, Cassiopeia, Lacerta Cygnus, Lyra, Delphinus, Sagittarius, Equuleus and parts of Aquila, Aquarius, Cepheus and others.

Other gut reference area is the Summer Triangle.

A good way to find it is to draw a straight line trough the star Altair, and the head of the Delphinus.

 

 

Picture 2

This picture shows the neighboring area of HR 8210. We can see   the constellations Delphinus and Equuleus.   The ends of “horns” of the Pegasus are also displayed.

A good landmark is the brighter star    1 Peg which masks IK Pegasus at the picture above.

Another way to find it is to imagine that the Dolphin is “diving” towards it.

 

 

 

    The object can be found with the naked eye on nights when its constellation is visible, but this task is not so easy.  For the Northern hemisphere the best observing period is September, when it is visible the whole night.

    Each observer can generate a map of the object area using computer or normal atlas, but observing without professional instrument is useless. However finding the star with a telescope isn't hard. Using the Dolphin, find the bright star 1 Pegasi, then slightly shifting the telescope towards Enif and away from the Dolphin, you will se HR 8210 in your field.

    To the amateur astronomer it seems only like a normal isolated star, only a professional spectrograph or long-term brightness observations can show any of the intriguing features of the object.

 

 

1.2. Orbital parameters of the components , Place of the system in the galaxy

 

    It is known that HR 8210 is a single-lined spectral binary system.

    The spectroscopic orbit shows that the velocity amplitude of the A star is 41 km s -1 , the orbital period is 21.7d, the distance between the main star and the center of mass is 1.24x10 7 km. Assuming a current mass ratio of ~1.5, we get a component separation of 3.1x10 7 km (which is 44R -solar radiuses), or an angular separation of 0”.0005at the present distance of 44 pc.

    Further studies have shown a component mass for the primary star – 1.7M (solar masses) and for the secondary – minimum 1.1M

    At present HR 8210 has its own motion through the galaxy, considered from the sun is: U= -17; V= -9; W= -6 km s -1.  The minuses in the speeds indicate moving away from the solar system. HR 8210 is a member of the young disk population.

 

 

2. The main star - IK Peg A

 

2.1. Although most of the attention is on the dwarf, IK Peg A has its own interesting features

 

IK Pegasi in visual    The main star in the HR 8210 system, known as IK Pegasi A, is a 6.07 magnitude star. It has been known to astronomers for a long time, because its brightness allows naked eye observations. But since the discovery of the hot white dwarf companion - IK Peg B, most of the attention is drawn to the exotic companion and less on the main star.    But IK Peg A certainly deserves our interest. The star is known to be a short-period, small-amplitude variable (from the type of Delta Scuti), also the spectra of the star shows the presence of some peculiarities that usually are associated with the metallic-lined stars from the type Am , but the spectral type of the star is still uncertain.

 The image in the right is a photograph of the IK Pegasi system in visual light. The dwarf is invisible in that spectral range.

 

 

 

2.2. Physical characteristics

 

    The picture in the left shows the position of an A -type star on the main sequence.

    Spectras of the star have been used to receive certain data about the star’s basic physical characteristics. Using the Ballmer line spectral method, which is suitable for all A-type stars, astronomers have estimated the star's surface temperature - T eff , surface gravity - g , the star's mass, surface rotation speed - v sin i , and the micro turbulence – ξi in the star’s atmosphere, which is a factor helping astronomers to better estimate a star's chemical composition. The results from the measurements are shown below:  

T eff 7770±100 K
Mass ~1.7 M
g ~1.78x10 4 m s -2
ξi 2.6±­0.2 km s -1
v sin i 32.5±2.5 km s -1   
Abs. Stellar Magnitude - M v 2 m.85

 

 

 

2.3. Delta Scuti pulsational activity, Frequency analysis

    It is known for a long time that IK Peg A shows to be a Delta Scuti variable.

Delta Scuti stars are pulsating variables of spectral types A to early F with luminosity classes V to III. Their pulsations can be either radial or nonradial modes with periods between about 0.5 to 8 hours. They also have low photometric amplitudes–up to 1 magnitude.

    They are called "Dwarf Cepheids" because they seem to be different from RR Lyrae stars because of their higher metallicity and different period-luminosity relation. In most cases, they oscillate in the fundamental mode or first harmonic and resemble closely classical positional variables, like Cepheids or RR Lyrae stars. However, it is not clear whether or not nonradial pulsations are excited in a number of high-amplitude Delta Scuti stars. Most of them do, however, show very small amplitudes. The smaller the amplitudes become, the more variables are found. From searches for Delta Scuti-like variability among open cluster stars it could be deduced that about 30% of all stars with spectral types of A2 to F0 on the Main Sequence are pulsators. The above two points make it quite reasonable that all stars in the lower instability strip could be pulsators; their discovery just requires higher photometric precision. This stars also have very interesting PLCR (period-luminosity-color relationship), but this topic is too extended, and we do not consider that its detailed view is not necessary here.

    The observation of pulsating stars enables scientists to derive their physical parameters like mass, temperature, luminosity, and composition and to test stellar evolution theory. The results can also be applied to other stars.

 

2.4. Spectral type of the star

    A lot of detailed chemical abundance analyses of IK Pegasi have been made by many astronomers over the years.

    These observations have shown not only, that the spectra indicates periodical changes in the radial velocity, a sure sign for a dim companion ( IK Pegasi B), but they have shown scientists some remarkable peculiarities.

    It is known for usual A-type stars to have chemical abundances similar to the solar, but IK Pegasi A shows unusually strong metallical absorption lines, witch indicate an overabundance of elements from the Fe -group. The presence of high metalicity in a star’s spectra specifies it as a metallic lined star, and puts a letter m in the spectral type indication.

    However it is not possible to classify IK Pegasi A as a classical Am star, as it has some odd features. An overabundance of Ba , Cu , Zn , Sr , Y , Zr is a characteristic of all classical Am stars, and their solar levels in the spectra of IK Pegasi A puzzle astronomers.

    The chemical abundances of IK Pegasi A are shown in the chart above.

    The high presence of Barium in the star’s atmosphere brings the idea that this is a Barium star. But the period of the variations do not agree with that suggestion. Barium stars usually have pulsation periods in the range of 80-2000 days, IK Pegasi A and its short period definitely does not fit to that class.

    IK Pegasi A fits better to the rare star type of F str  λ 4077. F str λ 4077 type stars have an  excess of Zn and Y , and could be found in short period binary systems. But however this type of stars is heterogenic and the hottest members may be considered as Am stars.

 

2.5. Theories, and models explaining the peculiarities

 

    The exhibition of strangely low levels of Ba and Sr in the star’s spectra can be explained with two theories:

·          One is that the Ba and Sr excesses are caused by radiative diffusion acting on normal abundances of these elements. It’s known that the star makes little-amplitude pulsations, causing turbulence and changing the balance of atmospheric elements.

·          T he other theory sais that the elements have been transferred between IK Peg B and IK Pegasi A, when they were in a Common Envelope phase – when the companion star, a dwarf at present, was in its red giant stage. Then, because of the small orbiting distance, and the low density of the stars’ atmospheres, The normal A star had dredged deep into the giant.

    During that common envelope phase many elements from the red giant might have been transferred between the stars. That transfer depends on the orbital distance of the stars and the radius of the red giant in its “end” stage. If the distance between the stars is the same as the radius of the giant, the stars’ photospheres would barely touch, and the element transfer would be rather insignificant, and it would not last long. Otherwise, if the radius of the orbit was smaller than the giants radius, the A star would have penetrated the giant’s atmosphere more deeply, making the interaction between the stars’ material more direct and efficient.

 

    However, the Common Envelope model is rather speculative, and the more probable reason for the peculiarities may be radiative diffusion. Even if in the system’s past there was a common envelope phase, it lasted a very short time, and probably its anomalies have already been masked by the effects of radiative diffusion.

 

2.6. Conclusion:

 

  The main star in the system HR 8210 is a star going to the final stages if its evolution. IK Pegasi A is a pulsating variable, and pulsators are known to be stars in their late evolutionary stages. Pulsations are a sure sign thermo-dynamical instability in a star's core.

 The metallic lines in the star's spectra could not be considered a sign of aging, but could be considered a sign for the intensity of the pulsations.

 

 

3. The hot white dwarf companion - IK Pegasi B

 

3.1. When and how the dwarf was discovered

 

    During the first all-sky survey in the Extreme Ultra Violet (EUV) range, with the Wide Field Camera of the ROSAT satellite many EUV sources were detected. Most of them were considered to be active stars, but some of the strong signals were from hot white dwarfs.

    The image in the left  is the original photograph with which the strong EUV source of the system IK Pegasi was found.  The image is 160x160 arc minutes in size. The green circle marks the center of the picture, and the purple circle marks the position of the source.

     It’s easy to see that this is a very strong signal for an A -type star, which are usually very weak in that spectral region. It’s been known for a long time that IK Pegasi is a single-line spectral binary.  So the visually weak companion easily became an explanation of that source.  The dwarf was given the number   WD2124+191

    But the companion that has a low brightness in the visual and a very high brightness in the EUV must have an extremely high temperature. It could only be a hot white dwarf (a neutron star would be bright in the X-ray region). It soon became clear that the system of HR 8210 contains a normal main-sequence star from the spectral class A8, and a hot white dwarf companion.

    In visible light the primary star is very bright, like all stars from that type, and the dwarf is practically invisible, because it gets masked by it. But in the UV the situation is different. The high surface temperature of the dwarf (T eff ~ 35 000 K) makes it extremely bright in the short wavelengths.

 

 

3.2. Further observations, and analyses , Physical characteristics of IK Peg B

 

     In the case of an isolated white dwarf, to get any certain information about the star’s mass and radius, its surface temperature - T eff and the surface gravity – g , are used  standard Ballmer-line methods.

    But in the case of HR 8210 we have a binary system with a bright A-type star. The main star totally masks all lines from the Ballmer series of the dwarf, with its own. So to get the needed data about the dwarf, spectral lines from the Lyman a  and Lyman b series in the far-ultraviolet range are used. But the Lyman series cannot be used to determine the physical parameters of the star as well as the Ballmer. Using only Lyman lines it’s not possible to get independent values of T and g , because different combinations of these two parameters may lead to similar line profiles. The only way to get any information is by matching the observed spectra with computer generated models.

Above the spectra from the satellite "International Ultraviolet Explorer" is matched with the "pure Hydrogen atmosphere" theoretical model.

 

The surface temperature of the dwarf T eff is extremely high. It s determined to be T = 34 500 ± 1 500 K.

 

The mass of the dwarf is its most intriguing and unexplainable feature. Spectral analyses lead to the extraordinary mass of  M=1.12M (with an accuracy of 10%). A classical dwarf has an average  mass of 0.6 M and s cientists have trouble explaining that unusually high mass of IK Peg B.

 

Its surface gravity - g - is estimated to be g ~ 8x10 8 m s -2 (for example on Earth it is 9.8 m s -2 ). Such high surface gravity is an result of the high density of the degenerated gas.

 

There are no direct data about the color of the dwarf. Measurements of the B-V index could not be made, because of the bright companion star. But the dwarf has a very high T , and its peak frequency is in the far UV so the star must seem blue .

 

Spectral analyses in the extreme ultra violet have shown a total absence of chemical elements other than Hydrogen in the dwarf's atmosphere.

 

3.3. Evolutionary past and future

 

    It is clear that in the past IK Peg B has had bigger mass than its present main-sequence companion. The A and B stars are formed in one system and at the same time. The way we see the system now is a result of the B-star's high mass. Because the B-star has been more massive than the A star in the past, it has “burned up” its fuel faster, and has gone to its final evolutionary stage earlier.  

    While the A star was still on the main sequence the secondary star, that has been much more massive at that time,  run out of hydrogen. It has started to “burn” Helium. Its atmosphere has expanded to an enormous size. If then the orbital distance between the components has been smaller than the radius of the expanded red giant, the system has went trough a common envelope phase.

    At the end of that phase, a helium-burst has blew away the upper lairs of the star, which  have flown away into space, forming a planetary nebula, of which no signs are found today. The naked core of the star that has left after the loss of the upper layers has collapsed and because it doesn’t have enough mass for a supernova it has turned into a white dwarf.

    The whole process is shown in the animation above.

    The lack of a planetary nebula today assumes that enough time has passed since the helium burst for the nebula to disintegrate. The system has been in its present condition for tens of thousands of years. If the helium-burst happened in the recent thousands of  years, maybe the pre-historic people have observed it.

    It is assumed, that a dwarf with mass ~1M originates from a main sequence B-type star with mass ~8M . The lifetime of a star with such mass is ~5x10 7 years, and a dwarf with parameters of IK Pegasi B would cool down for ~10 3 years. This information may help finding out from which stellar cluster does HR 8210 originate.

 

    There’s a theory, that says that at first HR 8210 may have been a triple system, and the high mass of IK Peg B may be explained with the collision, and merging of two white dwarfs gone trough similar evolutionary stages. This theory increases the age of the system, by lowering the hypothetical mass of the mother stars and increasing the duration their stable evolutionary period. But there is absolutely no proof for this theory and it is considered highly speculative. Also the merging of two white dwarfs assumes that the mass of the two merging dwarfs is lower than average, assuming a mass in their previous stages is even lower than the mass of the present A star. But if their masses were lower they'd probably live longer than IK Pegasi A, and we see that this is not true.

 

 

  4. The DANGEROUS system of HR 8210

 

4.1. Chandrasekhar limit (1.4M )

 

    It is known that the pressure in the core of white dwarfs comes from degenerated electron gas. Using that, the Indian scientist S. Chandrasekhar calculated that the size of a dwarf gets smaller as the mass gets higher. So when the mass gets critical the pressure of matter rises and the speed of the particles of degenerated electron gas gets higher. Simply because the speed of the electrons cannot be higher than the speed of light, the critical mass is been calculated to be M cr =1.4M

    In higher masses, the pressure would force the electrons and protons of the degenerated gas to merge to neutrons. If that happens there will no longer be a dwarf but a neutron star. 

 

4.2 . How little is needed for the catastrophe, How will it go

 

     There are two types of supernovas:
    II -type supernovas happen when a massive star runs out of hydrogen, and its mass produces a strong enough gravitational force to make an enormous gravity collapse, followed by an explosion.
    I -type supernovas are a result of a totally different process. They appear in binary systems, where one component is a white dwarf. When the dwarf receives matter, with some form of accretion, at one point in time it reaches the critical mass. Then the balance between the gravity force and the inner pressure is disturbed. The pressure in the star’s core could not resist the gravitational force, which becomes bigger from the fallen matter. The gravity force overcomes the pressure, and the star goes through a gravitational collapse, quickly followed by a supernova explosion.

HR 8210 is the best and closest candidate for an I-type supernova.

    In the case of HR 8210, the mass of the dwarf is 1.15M . It is also known that the primary star IK Peg A is going to the end stages of its evolution. In the near hundreds of millions of years the primary star is going to turn into a red giant, expanding to enormous size. The secondary star in the system, the dwarf, is going to be so close to the giant’s rare atmosphere, that with its enormous gravity field would “suck” it over itself. If the red-giant-stage lasts long enough, the dwarf could receive trough accretion enough mass to reach critical mass. And even if the dwarf hasn’t got enough mass at the end of the red-giant-stage, after the helium-burst the upper lairs of the A-star would fly into space, directly onto the dwarf. Once it reaches the critical mass, the dwarf will go off in a spectacular explosion of a supernova .

There are two main scenarios for the explosion.
Scenario 1: The dwarf reaches critical mass, while it “sucks” matter trough the point of Lagrange. (On the picture above.)
Scenario 2: It reaches critical mass while the freshly thrown out early planetary nebula, falls onto it.

    The first scenario does not suggest strong influence on the evolution of the giant. If the supernova goes off early enough, the outer layers of the giant might be dense enough to resist the shockwave of the supernova and be kept by the giant’s gravity.
    In the second case, there could not be a serious influence to the former giant, as it does not exist any more. The compact white dwarf, left after the red giant burst of IK Pegasi A, could not be affected by the blast of the older dwarf. Anyway the planetary nebula would be blasted into space with enormous speed. And it would not be considered a planetary nebula anymore but a supernova remain.
 

    But no matter how the explosion goes, the released energy would be the same and the amount of dangerous radiation reaching Earth would not change.
 

4.3. Distance from Earth, Ecological problems, Should we panic

 

     It is a characteristic of the supernovas from type I that they all throw into space the same amount of energy, and they all have the same peak absolute stellar magnitude (Because of this I-type supernovas are used to measure the distance to their far galaxies). They are known as “cosmological candles”.

    Knowing exactly how much energy HR 8210 will release, it is easy to forecast the implications that such an explosion may have on Earth. The results are horrifying.  If the supernova goes off at the present distance of 44 pc (~150 ly) it would practically wipe out life on Earth. The strong UV, X-ray, and g radiation that will reach Earth will destroy the protective ozone layer, leaving all life exposed to the deadly rays coming from space. The event may have effects even more deadly than the meteorite impact that killed dinosaurs. The picture is an artist’s view of the catastrophe.

    The safe distance for a supernova to go off without any serious implications on Earth is 200 ly. The present 150 ly are too close for comfort. But spectral observations indicate that the system HR 8210 is moving away from us with a high velocity. There’s a chance that at the time of the catastrophe the dangerous object might have traveled away from us to a safe distance. But considering the uncertainty in the forecast of stars’ evolution, it is possible that the Red Giant Burst in the system, and the supernova explosion following it, starts earlier than thought, causing the extinction of life on Earth. It is a matter of chance if the impact will be on a safe distance.

    But depending on chance would be an unacceptable option for the future generations. People of the future must find a way to protect their selves from the deadly radiation, and abandoning Earth isn’t a solution, as the deadly radiation will reach much further than the solar system.

 

 

  5. Practical exercise   -   Exploring a spectral binary star

 

5.1. Definition of a spectral binary .

 

    Double stars often orbit around each other in such configurations that they in some way, in their orbital motion, periodically move towards and away from Earth. That change in radial speed can be observed from Earth as a Doppler shifting of the spectral lines. With long-term observations of spectra of HR 8210 the periodical change in radial speed was noticed, and the presence of the dwarf companion - suggested.

 

5.2. How to "catch" a spectral-binary star

 

    The sky is full of stars, and spectral observations of all of them cloud not be done. In our galaxy there are many spectral-binary stars that have not been discovered yet. It is possible for students to discover a spectral-binary star even without leaving school.

    At the present moment there are so many observatories and space satellites gathering data for the universe that there are practically not enough time for scientists to deal with it. Everybody could become a helper of astronomers simply by entering the Internet. There are many observatories working online with students all over the world. You could apply for observational time or receive ready data. For example here’s the address for applying for observations on the 1-m telescope in the Special Astrophysical Observatory of the Russian Academy of Science. http://www.sao.ru/hq/racs/index.html.

    There are many other online observatories, all you have to do is search the web.

 

    In order to discover a spectral binary star, you must choose a star, or better a few stars to increase the chance, to study. It is better if you take a very faint stars, because the fainter the star the bigger the possibility for it to be unstudied. And if the star you choose is well explored, the things you discover won’t be new, and it won’t be any help to scientists. Check if somebody else has been studying the chosen star before in some astronomical database: http://adsabs.harvard.edu/abstract_service.html?nosetcookie=1

 

5.3. Practical exercise

 

    So if you have chosen a star, or better a few, checked that it is unexplored enough, and have managed to gather spectral observations you could start work.

    If your goal is to discover a binary star you would need no less than three spectral observations over a period of time. The goal is to detect changes in radial speed, by measuring the Doppler shifting of spectral lines. Every star has its own motion in the galaxy, so just detecting some radial speed is not a proof for a binary system. You must compare the length of a particular line in three spectras form three different moments of time, because two observations are not enough to conclude a lack of spectral shifting.

    If you don’t see any change in the spectra it is possible that either the star is not binary, either it is binary but its orbital period is too long to detect changes over such a short time. Another observation after an year may show changes.

    If you detect a change, make more observations, in a longer period of time, to be sure it is not a defect of the spectrograph. Start measuring the change of wavelength of a certain strong spectral line, and put the results in a chart, with one axis for Time and the other for .

    If you notice any regularities in the chart's movement contact any astronomical organization to announce your discovery. Then many other astronomers will repeat your observations, and if they confirm your results, your discovery will be announced in public!

 

 

 

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In our work we have used materials from these articles:

“The hot White-Dwarf Companions of 1608” by Wayne Landsman, Theodore Simon, P. Bergeron

“The chemical composition of IK Pegasi”, by B. Smalley, K. C. Smith, D. Wonnacott, and C.S. Allen

“Extreme ultra-violet spectrophotometry of HD 15638 and HR 8210 (IK Pegasi)” by M.A. Barstow, J. B. Holberg, and D. Koester

“The frequency analysis of low amplitude δ Scuti stars – I.HR 4591, and HR 8210” by D. W. Kurtz

“Pulsational activity on IK Pegasi” by  D. Wonnacott, B. J. Kellett, B. Smalley, C. Loyd

 

We have used images from:   

   The listed above articles.

   T he ROSAT Data Archive . -  http://rosat.gsfc.nasa.gov/docs/rosat/archive_access_961119.html

   The www.college.ru web astronomy course.

   www.yahoo.com greetings.

   The star charts are generated using the program "The sky"

   All the animations and some schemes are made by us using the programs "AC3D" and "Adobe Photoshop"