Title: The Collinder 69 open cluster (Bayo et al., 2008 A&A 492, 277)
- Step 4.- Upload the file in VOSA (“File to upload”). Give a description (free text) and do not forget to select “magnitudes” as file type. Then, click “Upload”. The message “your-file-name” has been succesfully uploaded! will appear. Click “Continue”.
- Step5.- Skip the tag “Objects”. With the next tag (“VO Phot.”) we can complement our “user photometry” with photometry found in VO services. For this use case, click “unmark All” and select only 2MASS, WISE and CMC-14. Then, click “Query selected services” at the bottom of the page. Once this is done, a summary table with the VO photometry (in flux units) will appear.
- Step6.- This tag gives us the possibility of visualising/modifying the SED before the model fitting. Bad photometric points / upper limits can be deleted / not included in the fitting by cliking on the corresponding check box and click on “Apply changes”. If VOSA detects an infrared excess, the photometric points are drawn in black and are not considered in the fitting process. The user can manually overrride it and specify a new limit in the “Excess” panel. Veiling can also be taking into account: Photometric points bluewards than the wavelength included in the “Apply UV/blue excess up to” box will not be included in the fit. For this use case, do not make any change.
Tag "Chi-2 Fit"
- Step7.- In the next tag (“Chi-2 Fit”), different grids of theoretical models covering different ranges of physical parameters are displayed. For this case, click “unmark All” and select only the “BT-Settl-CIFIST”. To save time, do not tick either “Include model spectrum in fit plots?” or “Estimate fit parameter uncertainties using an estatistical approach”. Finally, click on “Next:Select model params”.
- Step8.- In this window, we can refine the range of physical parameters that will be used for the fit. We will make the following assumption:
- Teff: 2000-5000K
- logg: 3.5-5.0
- Then, click on “Next: Make the fit”.
- Step9.- We can now see a summary table with the best fit results. Click on “Show graphs” to have a look at the graphics. The effective temperatures and logg obtained after the fitting are:
- LOri0005: Teff: 4000K logg: 4.0
- LOri0029: Teff: 3600K logg:4.0
- LOri0048: Teff: 3600K logg: 4.0
- LOri0158: Teff: 3300K logg:4.5
Tag "Bayes analysis"
- Step10.- Alternatively, you can perform a Bayesian fitting using the “Bayes analysis” tag. To do so, we select the same collection of models and range of physical parameters as in Step9. Then, click “Make the fit”. A summary table with information on the model with the highest probability is shown. For each object, the information is graphically displayed by clicking on the object name (top left panel).
- LOri0005: Teff: 4000K(46.78 %) logg:3.5 (90%)
- LOri0029: Teff:3600K (46.26 %) logg:4.0 (65.28%)
- LOri0048: Teff: 3600K (79.39 %) logg:4.0 (79.29%)
- LOri0158: Teff: 3300K (80.24 %) logg: 4.5 (50.72%)
Tag "HR Diag."
- Step11.- In order to estimate ages and masses for our objects we will make use of the “HR Diag.” tab. Click on “Make the HR diagram”. Ages derived for LOri0005, LOri0029 and LOri0048 are consistent with the age of the cluster (upper limit: 12-16 My).
Tag "Save results"
- Step12.- You can save different type of results (plots, VO photometry, Bayes fit, chi-2 fit,…) using the “Save Results” tag.
- Step13.- A summary of all steps executed to carry out the workflow can be found in the “Log” tag.
- Step14.- A detailed description of how VOSA works can be found in the “Help” tag.
The role of extinction on the physical parameters obtained from the SED fitting.
- Step 1.- Go to the “Files” tag. On the right hand side (section “Create a single object data file”, type HD302505 in the “Obj. Name” box. Include a description (e.g. “second VOSA case”. Including a description is not a compulsory step). Click “create”. The message “HD302505 has been succesfully uploaded” will appear. Click “Continue”.
- Step 2.- Go to the “Objects” tag. Click “Search for Obj. Coordinates”. Tick on “Sesame” coordinates. Click “Save Obj. Coordinates”.
- Step3.- With the tag “VO Phot” we look for photometry in VO services. To save time, we only select the following services (the rest of services do not provide results):
- Infrared: 2MASS, DENIS, WISE
- Optical: Tycho-2, Stromgren, UBV
- Click “Query selected services”. Once this is done, a summary table with the VO photometry (in flux units) will appear.
- Step4.- Go to the “Chi-2 Fit” tag. Select “Kurucz ODFNEW /NOVER models”. Click on “Next: Select model params”. Do not change the parameter range. Then click on “Next:Make the fit”.
- Step5.- We can now see a summary table with the best fit results. Click on “Show graphs” to have a look at the graphic. The effective temperatures obtained after the (very good) fitting is Teff: 6500K
- Step6.- Open SIMBAD (http://simbad.u-strasbg.fr/simbad/sim-fid). Type HD302505 in the “Identifier” box. Click “submit id”. This star has a B2 spectral type, which is inconsistent with the effective temperature derived from the SED fitting (Teff: 6500 K). What is the problem here?
- Step7.- Go back to the “Files” tag. Upload a VOSA input file with just a single line:
- HD302505 — — — — — — — — Av:0.0/3.0
- Include a description (e.g. “vosa_case2b”). Click “Upload”. It is irrelevant to select “Fluxes” or “Magnitudes”. Click “Continue”.
- Step8.- Repeat steps 2-4. Now we get Teff: 32000K with also a very good fit. What is causing the large differences in Teff if we compare these values with those calculated in Step5? The answer is extinction which has a strong impact on the SED shape. In the first case we were assuming no extinction but HD302505 is a B2 star (SIMBAD) with a E(B-V)=1.15 (Denoyelle 1977A&AS…27..343D).
To have an idea of the importance of extinction, see the figure given below. Red circles represent the observed SED of a star with E(B-V)=0.76. Blue squares represent the SED of the same star after the reddening correction. Forget about the green triangles.