Nightfall Documentation

Terminology

In Nightfall, the Primary is the star passing in front of the other one first, and the Secondary thus is the star first eclipsed.

The mass ratio is mass(Secondary)/mass(Primary), thus it is higher than one if the Secondary is more massive.

The inclination is the angle, under which the plane, in which the two stars orbit each other, is seen from earth. Only for inclinations near 90 deg (near edge-on view) an eclipse is possible.

The Roche lobe is the maximum size a star can have in a binary system. It depends on the mass ratio - the more massive star has the larger Roche lobe. If the stars overfill their Roche lobes, they form a common envelope or overcontact system.

To define the sizes of the two stars, you have to specify the Roche lobe filling factors, which are defined in units of the maximum polar radii (i.e. the polar radii of the Roche lobes).

The temperature is the surface temperature of the star. The luminosity of a star depends on its surface area (i.e. size) and the temperature.

Limiting values

To avoid numerical problems, parameters are limited to reasonable values. These are:
          mass ratio:              0.0001 - 10000 
          inclination:             0     - 90
          Roche lobe fill factor:  0.001 - 1.3
          temperature:             350   - 350000 
                                        (blackbody)
                                   3000 - 35000  
                                        (model atmosphere)
If a value is out of range, it will be reset to the lowest/highest allowed value. For Roche lobe fill factors above one, the mass ratio is restricted to 0.003 - 50.

Terminal Output

Warnings will cause Nightfall to print out warnings (like parameter out of range). Usually, this refers to problems Nightfall can cope with by itself. In case Nightfall refuses to do what you want, it might be of some help to use this option.

Verbose will print out some verbose output on what Nightfall is currently doing.

Busy will print out some status information (just enought to keep your screen busy) in case you are doing something rather lengthy, like, e.g., automatic fitting of parameters to observational data.

Menus

There are several menu entries located at the top of the application window.

File

Open Data This option allows to read in observational data from a file, which are required for fitting a model. For the format of the file, please see the manual or the files included in the distribution.

Clear Memory Before reading in data for a new binary star, you should use this button to delete all data for the current binary, otherwise you would have a mixture of data for different stars in memory. It is not possible to selectively delete only part of the data.

Open Configuration Allows you to read in a file defining the geometry of a binary system. For the format of the file, please see the manual or the files included in the distribution. Note that the latter include commands to read data, thus you should use Clear Memory before using this option.

Write Configuration This will write the current geometry of the binary system (including spots etc.) to a file for later use. Commands for reading in the data files currently in memory will be included as well.

Exit For closing the application.

Output

PlotCurve plots the computed lightcurve. If observational data are available for the selected filter, they will be overplotted. The output filter and the part of the lightcurve plotted can be selected from panel Plot Options. You need to compute a lightcurve first to use this option.

StarView gives you the opportunity to see the binary as a nearby observer would see it. The phase angle and the inclination of the orbital plane can be set interactively.

RocheSlicer allows you to view a slice through the Roche potential. You can interactively switch between the X-Y plane (the orbital plane) and the X-Z plane (perpendicular to the orbital plane). You can also choose interactively the height above the selected plane.

ViewGeometry some nifty plots (non-interactive version of StarView and RocheSlicer, basically).

DataSheet opens a window with the data on the binary star system, including things like individual masses of the stars, largest radius (Point Radius) and smallest radius (Polar Radius), mean temperatures etc. Basically, this is the header of the output file {NightfallCurve.dat}.

Profiler shows the profile of an absorption line in the spectrum of the star. You can change the phase interactively. You need to compute the line profile first to use this option.

Advanced Options

Asynchroneous rotation

Synchroneous rotation means that both stars rotate with their orbital period, and thus always face each other with the same side (like the moon does towards the earth). Asynchroneous rotation thus means that the star rotates faster or slower than the orbital period. The asynchroneous rotation rate F_Ratio is defined relative to the orbital period, i.e. a value of 1.0 is equivalent to synchroneous rotation. Note: the critical radius of a star depends on F_Ratio. For values larger 1.0 it shrinks, thus stars with a filling factor of 1.0 (both) are not in physical contact anymore. For values smaller 1.0 it expands, thus stars might 'intersect' even with filling factors smaller 1.0. (This condition is tested.)

Eccentric orbit

According to Keplers laws, the shape of the orbit is an ellipse. Often, it is close to a special case of an ellipse - a circle. However, sometimes binary orbits are markedly eccentric, i.e. non-circular. To define a non-circular elliptical orbit, two parameters are required: Eccentricity is defined as (r2 - r1)/(r2 + r1), where r1 is the smallest and r2 the largest distance. Obviously, an eccentricity of 0.0 corresponds to a circular orbit. The upper limit in Nightfall is 0.95 (somewhat arbitrary). Periastron length is the length (in degree) of the Periastron, i.e. the point of closest approach during the orbit.

Fractional Visibility

Calculate frational visibility (fractional eclipse) for individual surface elements on the shadow limb. Will yield a smoother lightcurve in case of problems ('spikes' in the curve during the eclipse).

Model Atmosphere

Use tabulated fluxes from model atmosphere calculations instead of the blackbody approximation for light fluxes.

Detailed reflection

By default, the irradiation by the companion star is treated by assuming that the companion is a point source. This may lead to incorrect results if the star(s) fill out a large fraction of the Roche lobe(s). With detailed reflection, the mutual irradiation of all pairs of surface elements is calculated. By iterating a few (2-3) times, the accuracy can be further improved. As this is an N^2 algorithm, it is somewhat slow.

Limb darkening

The limb of a star is darker than its centre. This effect can be accounted for by various approximations, which express the limb darkening as a function of the angular distance to the limb. Options include a simple linear law as well as quadratic and square root functions. As there is some dependency on the temperature (which is not constant over the surface), there is also an option to compute the limb darkening for each surface element individually.

Lightcurve steps

This option determines the number of steps, for which the lightcurve is computed.

Line profile

This option will compute the profile of an absorption line in the star's spectrum at each step in orpital phase. You can use the Profiler (in the
Output menue) to view the result.

Plot Options

PlotCurve Options

Note that all these options only take effect for the next plot.

Single phase plots the lightcurve for one full orbit.
Double phase plots the lightcurve for two consecutive orbits.
Zoom Eclipse (P) and Zoom Eclipse (S) will zoom on the eclipse of the primary or secondary, respectively. If observational data for the chosen filter (see next) are in memory, they will be overplotted.

Filters: Nightfall computes the lightcurve for several different filters commonly used in astronomy. Each of these filters is only sensitive for a certain range of wavelenghts (just like the human eye as well). Options include the UBVRIJHK broadband filters, which range from the ultraviolet (U) to the infrared (JHK), and the narrow-band Stroemgren uvby filters. The sensitivity of the human eye is best matched by the V and the Stroemgren v filters.

ViewGeometry options

These options include visualization of the Roche potential (actually a cross-section in the orbital plane), and visualization of the stars themselves (or the surface grid defining them).

In the Roche Potential Contours plot, contours are plotted for the stars, the Roche lobes, and the Lagrange 2 potential, which limits overcontact systems.

Postscript

If toggled, output from PlotCurve or will go to a postscript file named Nightfall.ps. This file will be overwritten by the next plot.

Data Fitting

Name

Give a name to the binary system (will be stored in an output configuration file, if you choose to write one.

Mass and Separation

For the calculation of a radial velocity curve, the orbital period, binary mass and the separation of the stars must be known in absolute units (here: days, solar mass and solar radius). Only two of these three are required -- set the third one to 0.0 and use 'Compute' to obtain its proper value.

Fit

This will use the Simplex algorithm to determine the best-fit binary geometry for your observational data. Note that this has some shortcomings - not related to this special algorithm - which are discussed in more detail in the manual.
After completion, the binary geometry is automatically reset to the best-fit values. For better results, restart then at least once.
All data currently in memory will be fitted simultaneously.
All parameters for which the option is toggled will be fitted. All else will remain constant.

Simulated Annealing

This will use the Simulated Annealing algorithm to determine the best-fit binary geometry for your observational data. Very time-consuming.

Map

This will create a two-dimensional map of the merit function (which is a measure of the goodness of a fit) with respect to two parameters. Start values are the current values, step values can be entered. The gridsize is fixed at compile time (default 16 x 16, i.e. 256 lightcurves will be evaluated).

Spots

The total number of spots on each star is a compile-time configuration option (the default is ten). However, only two spots per star can be defined interactively. Each of these two spots can be switched on/off seperately. (If you want more spots, you have to define them on the command line or in a configuration file.)

For the definition of a spot, the following four parameters are required:

Longitude in degrees, counted clockwise from the line pointing from the Primary to the Secondary.

Latitude in degrees, counted from the equator.

Radius in degrees.

Dimfactor is the (dimensionless) factor, by which the temperature in the spot region is higher or lower than the stellar surface temperature. Note: actual brightness is proprtional to the fourth power of the temperature !

Third Light

Here you can define the eventual contribution of a third star (third light) in the system, separately for each filter.

Third Light is assumed to be a star that is never eclipsed, i.e. no eclipse testing is done. Thus, this option only affects the contrast of an eclipse, i.e. its depth.

Third light is defined as L1 + L2 + L3 = 1.0, where L1, L2 is the contribution from the binary, and L3 is third light.

GNOME session management

Nightfall supports GNOME session management. Normally, only a few parameters are stored, as defined in the menu Options->Preferences. However, if you have Nightfall running on your GNOME desktop and save the current session when logging out of GNOME, then on re-start your current Nightfall session will be fully restored to the state at logout.