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.
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.
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.
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.
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.
Single phase plots the lightcurve for one full
orbit.
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.
In the Roche Potential Contours
plot, contours are plotted for the stars, the Roche
lobes, and the Lagrange 2 potential, which limits
overcontact systems.
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 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.
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.
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.
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.
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.
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).
Postscript
If toggled, output from PlotCurve or 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 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.)
Third Light
Here you can define the eventual contribution
of a third star (third light) in the system,
separately for each filter.
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.