Model: help

Distance step and range:

The distance range may be limited. Values are in kpc. This can help reducing the CPU time.
The distance step can be:

progressive:
if z<100 or r<200: step is 20 pc
elseif z<400 or r<800: step is 30 pc
otherwise step is 50 pc
linear: step > 0.5 pc
logarithmic: delta(r)/r (between 0.01 and 0.1)

Small field/large field:

In "small field " option, the simulated stars are all supposed to be at the same coordinates. This mode is valuable only if the density gradient across the field is negligible besides the Poisson noise for your solid angle. Longitudes and latitudes are given in degrees, between -180 and +360 for l and -90 and +90 for b.
In "large field" option, you are asked to define a region of sky and a step to cover this region, i.e. a range in galactique longitude and latitude (in degree). The step (in degree) should be chosen in order that within this step the density gradient is negligible. The step should be small enough in the plane in latitude, but can be large in longitude (except near the bulge). Be careful that a too small step could produce a lack of stars because of rounding errors when the volume is small.

Extinction distribution:

The present model of extinction is made of a disc of diffuse absorbing matter, following a Einasto ellipsoid of excentricity 0.014 (equivalent scale height of about 140 pc) a scale length of 7555 pc and an adjustable normalisation. The default normalisation, 0.7 mag/kpc in the V band, is suitable for medium and high latitudes. At low latitudes this value may be adjusted to each field. You can add absorbing clouds, when choosing this option. Other models of extinction are in progress.

Magnitude range for luminosity function

The luminosity functions used in the model cover the absolute V magnitude range:

-7 to 20 for the thin disc,

-2 to 13 for the thick disc,

-2 to 11 for the spheroid.

The thin disc luminosity function is computed from a stellar evolution model, the star formation rate and Initial Mass Function being fitted to available star counts (Haywood, 1994, Haywood et al., 1995). Stellar types for which stellar models are not available (T Tauri, AGB) are added on empirical bases. White dwarfs are computed from Wood et al. models.

The thick disc is supposed to be a 11 Gyr, one generation of stars with a power law IMF of slope x=-0.5, and a metallicity of -0.78. Isochrones are from Bergbush and vandenberg (1992).

The spheroid luminosity function is computed in the same conditions, with an age of 14 Gyr and a metallicity of -1.78.

More details are given in Robin et al. (2003) A&A 409, 523.

Spectral type range

You may select a range of spectral type by giving the earliest type and the latest type which define the interval. The spectral types are MK types, unless the type marked C which included both the oxygened and the carbon AGB stars, and DA white dwarfs.

In simulated catalogues you will find the spectral types encoded as follows:

The integer part represents the main MK type: 1=O, 2=B, etc... 7=M. The code 8 is used for oxygened and carbon AGB stars. The code 9 for white dwarfs.

The decimal part is the subtype. For example the encoded type 6.4 represents the MK type K4. For AGB 8.0 means carbon AGBs, 8.1 means oxygen-rich AGB.

Age/Populations

The age/population code is:

1 to 7 for the disc (corresponding to ages: 0 to 0.15 Gyr, 0.15 to 1 Gyr, 1 to 2 Gyr, 2 to 3 Gyr, 3 to 5 Gyr, 5 to 7 Gyr, 7 to 10 Gyr, respectively)
8 for the thick disc (also called intermediate population)
9 for the spheroid
10 for the bulge.

You may select stars by their age, or population.

Apparent magnitude

The apparent magnitude is to be chosen among U B V R I J H K L.

You are asked for the minimum and maximum values of apparent magnitude for which you want to keep the stars. The range should be given independently for each passband of interest. The simulation will keep the stars which satisfy all conditions.

Give an arbitrary large range for the band you are not interested in.

Warning: If you want cumulative counts and give a high value of the minimum magnitude, the resulting counts will be wrong by the number of bright stars not included in this range.

Colour indexes: Colour indexes may be computed from any combination of the UBVRIJHKL bands.

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Colour index and interval

You may choose up to 4 colour indexes, made from UBVRIJHKL bands.

UBV are in the Johnson's system while RI are in the Cousins, JHKL from Koornneef.

In the "full version" you may choose a restricted colour range for your simulation (Select for example stars with a B-V between 0.6 and 0.8).

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Photometric errors

Photometric errors are important to model, when you want a realistic simulation, specially for a catalogue.

The errors are either:

a second order polynomial function of the apparent magnitude m in each band:
err = A + B*m + C*m²
or an exponential function of the apparent magnitude m in each band:
err = A + exp(C*m-B)

When the simulation is made to be compared with true data, error coefficients should be determined from these data. You are asked to enter the values of the coefficients A, B and C as determined by you.

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Proper motions may be computed

in the equatorial coordinate system
$(\mu_alpha*cos(delta),\mu_delta)$ ,
or in the galactic coordinate system
$(\mu_l, \mu_b)$ .

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Errors on the proper motion generally grow with the apparent magnitude. The errors are supposed to vary with the apparent magnitude as the photometry. If you have chosen a polynomial dependency in the photometry, you will have the same for the proper motion and the radial velocities. Values of the coefficients A,B and C are to be given for proper motions in milli-arcsec/year.

When the simulation is made to compare with true data, error coefficients should be determined from these data.

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Errors on the radial velocities generally grow with the apparent magnitude. The errors are supposed to vary with the apparent magnitude as the photometry. If you have chosen a polynomial dependency in the photometry, you will have the same for the radial velocities. Values of the coefficients A,B and C are to be given for radial velocities in km/s.

When the simulation is made to compare with true data, error coefficients should be determined from these data.

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Tables and differential counts

Apart from the differential and cumulated counts in apparent magnitude, the resulting counts may be obtained in tables with double entry.

You may choose up to 8 diagrams. Enter the chosen abscissa, ordinate and check their steps.

The dimension of the tables is limited to 55. If the dimension is larger than 55, you should put a larger step.

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Magnitude intervals

You may want some of the above diagrams for a restricted apparent magnitude range.

For example, a Mv versus Age diagram for 14 < V < 15.
This option allows you to select such magnitude ranges.

Enter the minimum (included) and maximum (excluded) of apparent magnitude you desire for your diagrams (up to 8 different intervalles).

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e-mail

Be careful in providing a correct internet address here.
In case your address is wrong your simulation will be lost.

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