Synthetic CMD generator
This WEB tool can be used after requesting a user ID to the BaSTI-IAC team members. Here we provide some detailed information on the inputs and how the code works. Less detailed information is also provided by moving the mouse's cursor on each window used to insert the input selection.
How to specify the requested Star Formation History?
The input Star Formation History gives the relative weight of the star formation episodes
at various ages (Star Formation Rate -- SFR) in order of decreasing age, and the mean
metallicity ([Fe/H]) of the stars formed at that age (Age-Metallicity Relationship -- AMR)
plus a 1 sigma Gaussian spread around this mean [Fe/H]. The SFH is tabulated for N key
ages t_i (t=0 corresponds to stars that are forming now).
Regardless of how the relative weights are provided, before starting the numerical simulation
the individual values are normalized to the largest value provided as input.
After the SFH is read by the code, the following cycle starts, running along the SFH age index,
from i=0 to N. First, the age difference between two generic t_i and t_(i+1) values is
computed; then the number of stars formed in this age interval is obtained through the product of the scale
factor specified by the user times the value of the SFR at t_i.
This number can not exceed a specific value (to avoid exceedingly large computational
times) that for the moment has been fixed to 10 000 000.
For each star formed in this age interval, a random age t (t_i<= t < t_(i+1))
is drawn from a flat probability distribution, together with a mass selected according to
the specified IMF, and the value of [Fe/H] (Gaussian probability distribution with the input sigma) at time t_i from the AMR.
With these three specified values of t, M and [Fe/H] the program interpolates quadratically
in age, metallicity and mass among the isochrones in the grid, to calculate the star's photometric
properties. Once all stars formed between ages t_i and t_(i+1) are generated, the next time interval
is considered and the cycle repeats, ending when all stars in the final age bin between
t_(N-1) and t_N are generated. The value of the SFR and AMR at t_N are not considered and
can be set to any arbitrary value. To compute the synthetic CMD of a single-age stellar population, just one age value needs to be provided as input. Please, note that in all cases ages have to be specified in order of decreasing value.
In the following we provide some examples of SFHs and the corresponding web interface inputs:
- Single age (13 Gyr), single metallicity ([Fe/H]=-1.30), no metallicity dispersion:
Age: 13000
SFR: 1.00
Metallicities: -1.30
Sigma for metallicities: 0.00
- Two ages (3 and 12 Gyr), two metallicities ([Fe/H]=-1.30 and -0.55) with no dispersion,
star formation bursts with the same SFR strengths:
Age: 12000 12000 3000 3000
SFR: 1.00 0.00 1.00 0.00
Metallicities -1.30 -1.30 -0.55 -0.55
Sigma for metallicities: 0.00 0.00 0.00 0.00
- More complex SFH made of three different parts:
- ) A low strength old burst between 12.5Gyr and 11Gyr with metallicity -2.5 dex and 0.15 dex dispersion.
- ) A star formation episode between 8Gyr and 4Gyr with strengths is 5 times larger than the first burst.
Metallicity [Fe/H]=-1.00 with 0.2 dex dispersion.
- ) A continuous star formation between 4Gyr and 1Gyr, with [Fe/H]=-0.25 and a dispersion of 0.15 dex, and strength 2 times larger than the earlier episode.
Age: 12500 11000 8000 4000 1000
SFR: 0.10 0.00 0.50 1.00 0.00
Metallicities: -2.50 -2.50 -1.00 -0.25 -0.25
Sigma for metallicities: 0.15 0.15 0.20 0.15 0.00