en:praktikum:photometrie

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en:praktikum:photometrie [2021/04/06 16:30] – [1. Alternative] rhainichen:praktikum:photometrie [2023/09/19 06:52] (current) – rhainich
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-====== N2 - Photometry of open star clusters ======+====== N2 - Photometry of open star clusters (GDL) ======
  
 /* /*
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 ==== Preliminary color magnitude diagram (CMD) ==== ==== Preliminary color magnitude diagram (CMD) ====
  
-The next step is to write the dataset that will be used to plot the CMD, i.e. visual magnitude over color (blue minus visual magnitude). The CMD itself can be plotted with different tools, here the version with //gnuplot// is explained (please, don't use //Excel//), also //GDL// can create PostScript plots (see below). First write the data into a file named ''cmd.dat'': +The next step is to write the dataset that will be used to plot the CMD, i.e. visual magnitude over color (blue minus visual magnitude). The CMD itself can be plotted with different tools, here the version with //Python// is explained (please, don't use //Excel//), also //GDL// can create PostScript plots (see below). First write the data into a file named ''cmd.dat'': 
   openw, out, 'cmd.dat', /get_lun   openw, out, 'cmd.dat', /get_lun
   for i=0,num-1 do printf,out,i,(bmag[i]-vmag[i]),vmag[i]   for i=0,num-1 do printf,out,i,(bmag[i]-vmag[i]),vmag[i]
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 The script can be executed in the terminal with the following command     The script can be executed in the terminal with the following command    
  
-  ./plot_cmd.py+  python plot_cmd.py
  
 Afterwards, a figure, showing the CMD, can be found in the current directory. The file type of this figure can be set via the variable ''filetype''.  Afterwards, a figure, showing the CMD, can be found in the current directory. The file type of this figure can be set via the variable ''filetype''. 
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 Afterwards, recompile and run your //GDL// program once more. Then save and run the //Python// script from a console: Afterwards, recompile and run your //GDL// program once more. Then save and run the //Python// script from a console:
  
-  ./plot_stars.py+  python plot_stars.py
  
 An image "starmap.png" will be the result if all went well. It shows the image (e.g. ''vadd.fit'') as color-inverted background and all identified stars as overlays **(please include or attach this image to your report)**. This image can be compared with the calibration stars mentioned in the beginning. After one of the star's from the "starmap" has been matched to a calibration star, the number of the corresponding mark on the "starmap"  can be looked up in the file ''stars.dat'' to obtain the corresponding magnitude. The stars in the ''stars.dat'' file are ordered by increasing Y values. So you can search for the position of the star in your image which you used as background image for the star map and then you can search for the coordinates in the ''stars.dat'' file. The difference between this magnitude and the value given in //Simbad// is the calibration shift. Repeat this procedure for 5-6 stars and calculate the average of the calibration shifts. The variance should not be larger than 0.1 mag. An image "starmap.png" will be the result if all went well. It shows the image (e.g. ''vadd.fit'') as color-inverted background and all identified stars as overlays **(please include or attach this image to your report)**. This image can be compared with the calibration stars mentioned in the beginning. After one of the star's from the "starmap" has been matched to a calibration star, the number of the corresponding mark on the "starmap"  can be looked up in the file ''stars.dat'' to obtain the corresponding magnitude. The stars in the ''stars.dat'' file are ordered by increasing Y values. So you can search for the position of the star in your image which you used as background image for the star map and then you can search for the coordinates in the ''stars.dat'' file. The difference between this magnitude and the value given in //Simbad// is the calibration shift. Repeat this procedure for 5-6 stars and calculate the average of the calibration shifts. The variance should not be larger than 0.1 mag.
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   fluxb = fluxb / 440.   fluxb = fluxb / 440.
 After the conversion to magnitudes, the difference to the literature values can be calculated and as a correction applied to the observation of the star cluster. After the conversion to magnitudes, the difference to the literature values can be calculated and as a correction applied to the observation of the star cluster.
 +\\
 +\\
 +\\
  
 Either way, the B and V magnitudes need to be calibrated accordingly by the calculated shifts: Either way, the B and V magnitudes need to be calibrated accordingly by the calculated shifts:
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   magvcal = magv - vcal   magvcal = magv - vcal
  
-With these values the final CMD can be plotted. To ease the output, the color can be calculated already at this point:+With these values the calibrated CMD with apparent magnitudes can be plotted. For this we need the color, so we add the line:
  
   magbminusv = magbcal - magvcal   magbminusv = magbcal - magvcal
  
-In comparison with literature diagrams the main sequence is likely to be shifted. This occurs due to the interstellar medium which is spread between the stars of our Galaxy. Like all other baryonic matter, it can be excited by light. It will reemit  this energy usually at a longer wavelength. Therefore, this effect is called reddening (do not confuse it with redshift):+==== Reddening & absolute magnitudes ==== 
 + 
 +When comparing your apparent FHD to the literature, you will notice that the main sequence is likely to be shifted. This occurs due to the interstellar medium which is spread between the stars of our Galaxy. Like all other baryonic matter, it can be excited by light. It will reemit  this energy usually at a longer wavelength. Therefore, this effect is called reddening (do not confuse it with redshift):
  
 $(B-V)_{0} = (B-V) - E_{(B-V)}$ $(B-V)_{0} = (B-V) - E_{(B-V)}$
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 In the solar neighborhood $R_V$ usually is set to 3.1 ([[http://adsabs.harvard.edu/abs/1979MNRAS.187P..73S|Seaton 1979]]). Find an appropriate value for $E_{(B-V)}$ for the line of sight to the observed cluster, i.e. in //[[http://viz-old.u-strasbg.fr/viz-bin/VizieR-2|VizieR]]// or in //Simbad// by means of the papers that are associated with your object. In any case, refer to the used catalog or paper in your report! Apply this correction to your data and plot the CMD again.  In the solar neighborhood $R_V$ usually is set to 3.1 ([[http://adsabs.harvard.edu/abs/1979MNRAS.187P..73S|Seaton 1979]]). Find an appropriate value for $E_{(B-V)}$ for the line of sight to the observed cluster, i.e. in //[[http://viz-old.u-strasbg.fr/viz-bin/VizieR-2|VizieR]]// or in //Simbad// by means of the papers that are associated with your object. In any case, refer to the used catalog or paper in your report! Apply this correction to your data and plot the CMD again. 
  
-If the calibration was performed on apparent magnitudes, one need to account for dilution due to the distance of the cluster by applying the distance modulus.+Finally the apparent magnitudes should be converted into absolute magnitudes, so that later a comparison with isochrons is possible. For this, the corresponding distance modulus or the distance of the star cluster must be looked up in papers (publications) and the corresponding correction must be applied. 
  
-To plot the final CMD with the //gnuplot// script, write the corrected data again, e.g. overwrite the preliminary version of ''cmd.dat'':+To plot the final CMD with the //Python// script, write the corrected data again, e.g. overwrite the preliminary version of ''cmd.dat'':
  
   openw, cmd, 'cmd.dat',/get_lun   openw, cmd, 'cmd.dat',/get_lun
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 As previously described, the file ''cmd.dat'' can be plotted with the script ''plot_cmd.py'', which can be run with As previously described, the file ''cmd.dat'' can be plotted with the script ''plot_cmd.py'', which can be run with
  
-   ./plot_cmd.py+   python plot_cmd.py
  
 It also offers the possibility to include isochrones, which can be downloaded from the websites of various stellar evolution projects. The variables of the script ''plot_cmd.py'' need to be adjusted according to the requirements of the downloaded isochrone files. The script expects that the isochrones are given as individual files, which should be located in a single directory (''isoDir'').   It also offers the possibility to include isochrones, which can be downloaded from the websites of various stellar evolution projects. The variables of the script ''plot_cmd.py'' need to be adjusted according to the requirements of the downloaded isochrone files. The script expects that the isochrones are given as individual files, which should be located in a single directory (''isoDir'').  
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 ===== Report ===== ===== Report =====
 +
 +A usual report is to be handed in. See a general overview about the required structure and content [[https://polaris.astro.physik.uni-potsdam.de/wiki/doku.php?id=en:praktikum:protocol|here]].
 +
 +For this experiment, the theoretical overview in the report should describe open and globular cluster with emphasis on the observed kind, and their differences to other accumulations and groups of stars. Explain Hertzsprung-Russell diagrams (HRD) and the color-magnitude diagrams (CMD) and the difference between them. //Shortly// describe the evolution of stars of different masses in the context of a HRD. Explain the concepts of isochrones and the turn-off point and how one estimates the age of a cluster using them.
 +
 +In the methods section describe the observations and the data reduction, highlight points that deviate from general description in here and list all the parameters you set for the extraction. Further, include all the plots of the data reduction in the report (a few in the text, most of them in the appendix). Describe the calibration procedure and list the stars you use and the shifts you determine. Also include any parameters for reddening, extinction, and distance that you adopt from the literature.
 +
 +The results part presents the cluster CMDs and describes the observable features in it.
 +
 +The analysis of the CMDs contains the estimation of the cluster age based on the turn-off point and an isochrone fit.
 +
 +Finally, discuss your findings. Bring your results into a larger context and make a literature comparison when possible (i.e., for the cluster age). This also includes that you identify potential problems with the data, the data reduction, or the analysis (especially the isochrone fit) and possible solutions for them. Are their inconsistencies? Do you see specific and obvious features in the CMD you cannot explain, that do not match your expectations?
 +
 +//**Note:** The plots of the individual orders are in large files which generally do not fit into an email appendix. You may upload your report to the [[https://boxup.uni-potsdam.de/index.php/login|University cloud system (BoxUP)]] or send us the path to it on our lab course computer. //
 +
 +/*
 +**OLD BELOW**
  
 Describe the difference between globular clusters and open clusters, with special emphasis on the character of the observed object. Discus the differences of stellar clusters and general accumulations/groups of stars. Shortly describe the evolution of a star and how it manifests in the Hertzsprung-Russell diagram and the color-magnitude diagram, respectively. Describe the age estimation of stellar clusters by means of the analysis of the turn-off point and by means of isochrones. Use these methods to determine the age of the observed clusters.  Write a normal laboratory-course report. Discuss your results and compare them to the literature. Address shortcomings in your results and discuss possible causes. Describe the difference between globular clusters and open clusters, with special emphasis on the character of the observed object. Discus the differences of stellar clusters and general accumulations/groups of stars. Shortly describe the evolution of a star and how it manifests in the Hertzsprung-Russell diagram and the color-magnitude diagram, respectively. Describe the age estimation of stellar clusters by means of the analysis of the turn-off point and by means of isochrones. Use these methods to determine the age of the observed clusters.  Write a normal laboratory-course report. Discuss your results and compare them to the literature. Address shortcomings in your results and discuss possible causes.
 +*/
  
 [[en:praktikum:index|Overview: Laboratory Courses]] [[en:praktikum:index|Overview: Laboratory Courses]]
  
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  • Last modified: 2021/04/06 16:30
  • by rhainich