Information needed to evaluate possible research programs
Visual Binary Star Observations
Capabilities of the SSUO equipmentThe primary SSUO instrument is a computer controlled 10-inch aperture f/5 telescope with a ST-7 CCD camera system. The CCD system has a filter wheel and filters are available for BVRI photometry and for true color image reconstruction. The telescope system was custom built for SSU by Epoch Instruments, and the CCD is a commercial astronomy system from Santa Barbara Instrument Group (SBIG).The CCD system has a field of view of approximately 1/2 degree and the telescope will point reliably to within 1-3 arcminutes. The telescope will track to within about one arcsecond so that unguided exposures of up to 3-5 minutes are possible. Longer effective exposures are possible by stacking multiple successive images of the same field.
Stars as faint as 12th magnitude can easily be seen with exposures of a few seconds. Stars as faint as 14th magnitude can easily be recorded with unguided exposures. Stars as faint as 16th magnitude can be recorded on unguided exposures.
The ultimate photometric precision of the system for measuring stellar magnitudes has not yet been evaluated. A precision of 0.1 magnitude is certainly achievable, and a precision of up to 0.01 magnitude is to be expected. Positions may be determined from CCD images to a precision of about 1 arcsecond.
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Here is information that would be useful to sumarize…
Object type and objectiveBrief description of type of objectObservations or work/analysis needed
Scientific objective
Difficulties to be expected
Preliminary work needed
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Variable stars are stars in special stages of their evolution. They are often in advanced stages of evolution or are extremely young objects. For the most part, variable stars are not found in the vicinity of the mainsequence on the HR diagram.
The following illustration indicates the regions of the HR diagram where variability is encountered.
Variability in the HR diagram. The horizontal axis is the V-I color index and the vertical axis is the absolute magnitude. Blus indicates little or no variability and red indicates 100% of the objects are variable
The location of some types of variable stars on the HR diagram. Only Beta Cephei stars on the high mass end of the mainsequence and flare stars on the low mass end are relatively close to the mainsequence. T Tauri stars are pre-mainsequence objects, but all remaining types are in advanced stages of evolution.
For the geometric variables such as eclipsing binaries, these objects are variable, primarily, because of their special orientation in space with respect to the earth. If the orbital plane of each of these objects was slightly different, or if the solar system was in a slightly different location in the galaxy, these objects would not appear to be variable.
For all types of variables appropriately spaced or timed photometric observations will enable a light curve to be determined. This can establish or verify the type of variable star. Many variables are periodic so that accurate timing of observations can be used to determine a period. Periods for some pulsating variables can be related to intrinsic luminosity so distances to the objects may ultimately be determined. This is currently one of the more reliable and powerful techniques for determining distances in astronomy. Periods for eclipsing variables (eclipsing binaries) may ultimately be used to determine masses of the stellar components. Analysis of accurate light curves for eclipsing systems may be used to determine relative sizes, shapes, and temperatures for the component stars. This is currently one of the most reliable techniques for determining sizes and masses for stars.
Changes observed from previously determined light curves may indicate the effects of evolutionary processes. Changes in periods may also indicate evolutionary changes. Determining precise times of maximum for pulsating variables and times of minimum for eclipsing systems can be used to search for period changes. Period changes for massive binaries have also been suggested as a possible further test of general relativity.
The long period variables (LPVs or Mira variables) are red giant stars that are pulsating. Stars more massive than the sun can find themselves in this state during the advanced stages of their evolution off the main sequence. These objects are believed to be precursors of planetary nebulae, and ultimately the precursors for all objects that will evolve into white dwarfs. Mira variables are likely losing mass with each pulsational cycle and many are known to be producing dust shells. As a consequence of these dusty envelopes, many Miras are IR sources and also OH maser sources. The details of all these processes are at present only dimly understood.
Some variables have been determined to be X-ray sources, IR sources, or radio sources. In general, source catalogues in these wavelength regions have not been exhaustively searched for correspondences with known variable stars. For variables that have been detected, similar objects with similar optical brightness have often not been detected in these other wavelength regions. Monitoring such objects may enable us to determine why some objects are detectable in other wavelength regions and how this might be related to evolutionary processes. Monitoring such objects can also provide for opportunities for collaboration with astronomers and physicists from other institutions.
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While modern, accurate light curves (photometric observations) are available for selected bright or interesting examples of most of the types of variable stars, this probably represents fewer than 10% of the total of known variable stars. As of 1992, the number of officially designated variable stars was approximately 30,000. Furthermore, new and fainter variables continue to be discovered. In particular, current technology now makes it possible to routinely discover variables in other nearby galaxies. In reality, light curves and periods for most known variables are based on photographic images obtained in the 40's, 50's, and 60's. Furthermore, while amateur astronomers organized by the American Association of Variable Star Observers (AAVSO) have been carefully monitoring bright examples of selected types of variables for many decades, this data is based on visual estimates. The AAVSO program is focused on long period variables (LPVs, or Mira type variables).
In reality, our understanding of variable stars, and the information they can provide about stellar evolution and fundamental stellar properties, is based on accurate observations of fewer than 10% of the known sample. Most of the available data is decades old photographic measurements or estimates, or visual estimates from small telescopes. Thus, a modern accurate light curve for almost any variable can contribute important fundamental scientific data. Changes in period or light curve shape can indicate real evolutionary changes, or "complications" due to short-term interaction effects. Modern light curves for faint or poorly studied systems can verify or refine the existing classification of variable type, or possibly correct the original classification. Modern light curves for faint or poorly studied systems can identify those special objects that will improve our understanding of stellar evolution and stellar structure. Modern times of maxima or minima can be used to improve period determinations and to search for period changes.
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Some Specific Research Projects
Times of minima for selected long period eclipsing systems. Eclipsing systems are known with periods ranging from 6-8 hours to several decades. Most of the known systems have periods on the order of a few days. Eclipses tend to last for only a few hours, so an entire eclipse can often be observed in a single night… if the timing is good and the star is well placed for observation. A sequence of photometric observations that includes the minimum of the eclipse is needed to determine a time of minimum. Once a reliable period is known, and a reliable light curve has been determined, this light curve can be used to infer times of minima from fragmentary data obtained during an eclipse.
The following illustration shows several schematic light curves for some well studied eclipsing systems.
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Longer period systems with periods greater than 5 or 10 days are likely to be poorly studied. It is also likely that such systems may not have been checked for period changes in several decades. Actually, our entire knowledge of such systems may be based on photographic observations now several decades old. Furthermore, these longer period systems tend to get passed over when systems are selected for detailed modern light curve determination. It is the rule of thumb among eclipsing binary observers that the period in days will correspond to the time in years that should be required to obtain a complete and accurate light curve. That is, for a system with a period near one day, it will probably take about one year of observing to obtain a decent light curve. For longer period systems the time to be expected to obtain appropriate observational coverage increases accordingly.
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Times of minima for eclipsing systems known or suspected to have variable periods. A number of systems are known to have time varying periods. Abrupt period changes are likely due to a mass exchange event for a close binary system. Thus, active mass exchange systems can be identified and studied. Sinusoidal period changes may be due the presence of a third star in the system. Apparent period changes may also be produced by normal orbital changes (such as the advance of perihelion) in an elliptical orbit. Such changes may be used to infer the internal density distribution within the component stars. General relativity also predicts period changes for massive binary systems.
There are a large number of eclipsing systems which are reported to possibly have variable periods. Nearly all these reports require verification by determining times of minima for these systems.
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Light curves for poorly known or interesting eclipsing systems. Light curve data can be used in conjunction with eclipsing binary models to determine physical parameters for stellar components. The truly "interesting" eclipsing systems have light curves that cannot be perfectly fit by the current models. Complications to the light curves may result from interaction between the components, mass exchange events, an intrinisically variable star as one of the components, or the presence of additional light sources in the system.
There are many eclipsing systems known that are worthy of modern, accurate light curves. This data can only be obtained by dedicated observers.
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Monitoring selected LPVs. LPVs have periods between 80 and about 1000 days. More than 1000 such systems are known in the galaxy and most have periods near 255 days. Amplitudes of the light variation can be as small as 1 or 2 magnitudes, but can be as great as 10 magnitudes(!). Unlike eclipsing systems and Cepheid variables that tend to repeat their light curves faithfully like clock work, the LPVs do not repeat exactly from one cycle to the next. The light curve shapes can change, the amplitudes can change, and even the times of maximum light cannot be predicted with great confidence. Indeed the published periods are really little more than mean periods determined over a sequence of cycles.
Light curve shapes can be nearly sinusoidal, or the more classical Cepheid shape of a rapid rise to maximum brightness followed by a slow decline to minimum. Light curves can also exhibit bumps on either the rising or falling branch of the light curve. Of course, the bumps can vary, can move to different locations on the light curve, and can even disappear completely.
Thje following illustrations show light curves for several LPVs.
Examples of light curves for two typical LPVs. The upper lightcurve is for X Cep and the lower lightcurve is for SS Cas. The tic marks on the horizontal axis correspond to 250 days (nearly a year). The tic marks on the vertical axis (brightness) correspond to 1 magnitude.
Example of a LPV with a prominent bump. This object is R Cen. The horizontal tic marks correspond to 250 days and the vertical tic marks (brightness)correspond to 1 magnitude.
While there is no tightly correlated relationship between period and luminosity similar to the well established period-luminosity relationship for Cepheids, a mild trend can be seen for the LPVs. That is, the longer period systems tend to be more luminous on the average. It is also true that a mild trend can be seen in the amplitude data in that the longer period systems tend to have greater amplitudes Our ability to study any such relationships is surely affected by the observed variability from one cycle to the next. Under these circumstances it is often difficult to define an accurate period or amplitude.
These systems can be monitored to document departures from light curves observed at previous epochs. Times of maxima can be determined and compared with predictions. For poorly studied systems, the photometric monitoring can lead to the first accurate light curve and classification according to LPV subtypes. Systems might be selected because of the appearance of the known light curve; possible correspondences (or lack thereof) with radio, IR, or X-ray sources; or simply the absence of an accurate light curve.
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Monitoring selected SRs. Semiregular variables (SRs) are less regular in their behavior than the LPVs. It is sometimes extremely difficult to determine any kind of period. However, these systems are not totally irregular, and with the availability of sufficient observations a mean cycle length can be determined. However, the light curve from one cycle to the next can vary significantly. Even with these irregularities, a number of distinct subtypes have been identified.
SRs are known with "periods" between 30 and 190 days, with the most common periods clustering around 85 days. Amplitudes are generally less than 2 magnitudes. Thus, it appears that the SRs may be an extension of the LPVs to shorter periods and smaller amplitudes.
Light curve for the semi-regular variable RS Gem. The horizontal tic marks correspond to 500 days and the vertical tic marks correspond to 1 magnitude.
These systems can be monitored to document departures from light curves at previous epochs and to see if the cycle-to-cycle variations change in any systematic manner over longer periods of time. Determine current times of maxima (or minima?) and compare with predictions and previous attempts at period determination. Systems could be selected for study to examine possible differences between "long period" SRs and "short period" LPVs. For poorly studied systems, determine first accurate light curve and classify according to subtypes.
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Discovery of new variable stars. Numerous variable stars remain to be discovered in the galaxy. Successful searches are to be expected for regions that have not been exhaustively searched, for objects fainter than 10th magnitude (or so), for objects with amplitudes smaller than 0.5 magnitude (or so), or for objects that may have been over looked or missed.One such project is to survey parts of the galactic anti-center for new Cepheid variables. This project was actually initiated at SSUO in 1990.Since most variable star surveys concentrated on the galactic center, this is where most variables are now known. Looking toward the anti-center we are looking into the next outermost spiral arm and this region is expected to have a different chemical history. Thus, Cepheids in the outer regions of the galaxy may have different properties. For example, there may be differences in the shapes of the light curves. Accumulating data on such objects may help explain the so-called "anamolous Cepheids" that are occasionally discovered in other galaxies.
Additional discoveries of Mira variables (LPVs) in the anti-center region may also be productive since it is known that the Miras in the outer regions of the galaxy have a different period distribution. Systematic examination of anti-center Miras may help explain this phenomena.
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Supernovae observations (a special type of variable star)
Brightness and color observations of new discoveriesSupernova can be discovered at any time. There is currently no way to predict a discovery in advance. While supernovae can become as bright as the galaxy in which they exist, they generally stay bright enough to observe conveniently only for a few weeks or months. The shape of the light curve can be used to classify the type of supernova.
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Active Galactic Nuclei (AGN) all seem to be variable on some level. While the variability appears to be irregular or random, this has not really been documented over long time scales. There have been occasional reports of semi-regular or quasi-periodic light variations, but these tend not to be well documented or verified.The variations for the BL Lac objects (the blazars) can sometimes reach several magnitudes, but the variations reported for the other categories of AGN are generally less than a magnitude or so.
- BL Lac objects (blazars)
- Seyfert galaxies
- Quasars
SSUO had a photographic program to monitor several Seyfert galaxies for several years. However, the program encountered a difficulty in the anaylsis of the data. The light from the underlying galaxy would interfer with the stellar nucleus depending on the seeing or the sky brightness. To be successful, a monitoring program for Seyfert galaxies would need to use some stellar profile fitting technique to derive magnitudes. Special care would be needed to verify that any light from the underlying galaxy would not affect the brightness determinations for the nucleus of the galaxy.
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There are now several programs funded by NASA and other organizations to discover Near Earth Objects (NEOs). Several thousand such objects are now believed to exist in the vicinity of the earth, but only several hundred are known. Such objects can collide with the earth.Such objects tend to be rather faint and can move rather rapidly across the sky. They may be observable at all for only a few weeks. Such objects may not be conveniently placed for observation. Of course, there is no way to predict the characteristics or timing for any new discoveries. Several newly discovered NEOs are now being reported each month.
- Observations of newly discovered objects
- Recovery observations for known objects
Lists of newly discovered NEOs and other interesting objects along with their orbital ellements can be found at...
http://cfa-www.harvard.edu/iau/Ephemerides/Unusual/index.html
Lists of "unusual" objects along with their orbital ellements can be found at...
http://cfa-www.harvard.edu/iau/lists/Unusual.html
This site also contains lists of various types of minor planets, orbit diagrams, and general information which can be found at...
http://cfa-www.harvard.edu/iau/lists/MPLists.html
The basic reference for currently observable minor planets and comets of all types can be found at...
http://cfa-www.harvard.edu/iau/Ephemerides/
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One or two new comets are generally discovered each month. They are often faint and can be in locations on the sky that will not be convenient for observation. Systematic positional observations can be used to improve the preliminary orbits. Such observations must be reduced and reported in a timely fashion to be useful.At any given time there are likely to be a handful of known comets that may be observed. These are likely to be faint and may not be conviently placed for observation. Precise positional observations are always useful to document any possible out gasing events that may have altered the orbit of the comet. Positional observations can also be used to document orbital changes produced by gravitational perturbations by the major planets or close approaches to minor planets or other comets.
Brightness observations are always needed! Except for the brightest most dramatic comets, brightness variations and fluctuations for comets are poorly documented and poorly studied
Comets appear stellar when they are faint and beyond the orbit of Mars (or Jupiter). Under these circumstances normal stellar photometry would be adequate. When comets are in the inner solar system and have out gassed to produce a coma and/or a tail, special analysis techniques would be needed to measure the brightness.
- Observations of newly discovered objects
- Brightness observations of known objects
- Positional observations of known objects
The standard reference for lists of currently observable comets may be found at...http://cfa-www.harvard.edu/iau/Ephemerides/Comets/index.html This resource includes newly discovered comets as well as returning periodic comets.
Another very useful site for current information about comets may be found at...http://encke.jpl.nasa.gov/ This site is maintained by NASA and contains images, information about brightness determinations, as well as a great deal of general information about comets.
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Visual binary star observations
Visual binary systems provide us with our only source of fundamental determinations of stellar mass. While several thousand visual binaries are known, orbits are available only for a few hundred. For many of the known systems, the only observations may be the discovery observations made visually in the 18th or 19th century. Many systems have only two or three observations and may not have been observed at all in many decades.CCD images can be used to determine positions that can be used to determine or improve orbits. CCD images can provide the best data for "wide" systems that do not have a great difference in the magnitudes of the components.
- Positional observations for previously discovered systems
A good modern discussion of visual binaries is available from the Royal Observatory of Belgium...
http://www.oma.be/KSB-ORB/D2/index.html
The web site for the publication Double Star Observer also has links to many useful resources...
http://www.cshore.com/royce/dso/
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ggs
Oct 1999