Determination of the distance to Pleiades using TOPCAT

Determination of the distance to Pleiades using TOPCAT Original case by Mark Taylor Adapted by Francisco Jiménez-Esteban & Belén López Martí Update: Ricardo Rizzo (November 2023); Francisco Jiménez-Esteban (November 2025) We will explore the use of TOPCAT (Tool for Operations on Catalogues And Tables) in a real science case. We will therefore access catalogues, cross-match tables, filter sources, create subsets, and represent the results using different kinds of plots. This tutorial is based on TOPCAT v4.9, running on a linux PC with Debian distribution and also tested on a MacBook (chip M1) running on macOS Sonoma 14.1.1. TOPCAT can be downloaded from: hNp:// www.star.bristol.ac.uk/~mbt/topcat/#install SCIENCE CASE We will use data from the Tycho-Gaia Astrometric Solution (TGAS) catalogue to determine the mean parallax of the stars in the Pleiades open star cluster, thus obtaining its distance. SCIENTIFIC BACKGROUND Stars do not form isolated in space. They are born in large groups from the same natal interstellar cloud. We call them star clusters. Because all the stars in a given open cluster have the same origin, they share important properties, and have the same age, similar chemistry, and the same kinematics. The Pleiades (M45) is one of the star clusters nearest to Earth. It is visible by the naked eye from both hemispheres and contains thousands of stars. WORKFLOW Block 0: Launch TOPCAT Step 1: Open a terminal and go to the directory where topcat-full.jar was saved. Step 2: Type: java -jar topcat-full.jar. On a Mac, click on the TOPCAT icon. Under Windows, double-click the downloaded file. 2 Block 1: Data discovery Let’s begin by acquiring TGAS data in the Pleiades region. Step 3: In the TOPCAT main menu, select: VO → Cone Search Step 4: The Cone Search window opens. In the Available Cone Services panel: In the Keywords field, enter: TGAS Click Find Services. Step 5: When the list of services is displayed, select ARI-Gaia. The partial URL of the service appears in the Cone URL field in the Cone Parameters box. Step 6: In Cone Parameters panel: Enter Object Name: Pleiades Click Resolve to fill in sky position fields. Set Radius to 5 degrees. Click OK Step 7: A table with 3,100 entries called Pleiades-ARI-GAIA-5d is loaded in the TOPCAT main window. 3 Block 2: Select comoving sources Now let’s explore the proper motion diagram of this sky region to separate probable Pleiades members from the field stars. Step 8: In the TOPCAT main menu, select: Graphics → Plane Plot As an alternative, click on the Plane plotting window button of the top panel in the main window Step 9: In the Plane Plot window, go to the Position tag at the button of the window, and select the columns to be plotted: X: pmra Y: pmdec Step 10: A Plane Plot pops up. Note the overdensity around (20,−45). Use the mouse to navigate. The wheel may be used to zoom in both axes, or just in one of them if it is pressed outside the plot. Zoom is centred at the cursor position. 4 Step 11: Graphically select this comoving cluster as a new subset: In the Plane Plot window, follow: Subsets → Draw Blob Subset As an alternative, click on the Draw Freehand Region button on the top of the Plane Plot window. Drag the mouse around the cluster on the plot, and click same button again. The New Subset window pops up. This operation may be repeated several times if you want. In the New Subset window, enter the New Subset Name: comoving. Click on Add Subset. The size of the comoving subsample will depend on the area selected with the mouse. Therefore, in what follows, the graphs and the numbers in the manual may slightly differ from your own results. Note that the new subset comoving is now overplotted onto the All subset, probably in blue. Step 12: Go to the Subsets tab at the button of the Plane Plot window. Deselect “All” and “ Activated” click boxes. Select comoving click box. Now only the “comoving” subset will be displayed. Step 13: Click on the Rescale Plot button on top of the Plane Plot window to rescale the plot. Try different options to be familiar with them. 5 Block 3: Identification of Pleiades members We will now use the parallax to refine the selection and identify the Pleiades members. Step 14: In the TOPCAT main menu, follow: Graphics → Histogram Plot. As an alternative, click on the Histogram button of the upper panel in the main window Step 15: In the Histogram Plot window, go to the Position tab on the button right panel, and select the column to be plotted: X: parallax Step 16: In the Subsets tab, make sure that only the subset comoving (and not “All” or “Activated”) is plotted. Step 17: Rescale the plot by clicking on the Rescale Plot button on the top panel, or navigate with mouse. There are some outliers visible, probably not cluster members. We want to create a new subset excluding those parallax outliers, and limited to parallaxes between 6 and 9. Step 18: In the TOPCAT main menu, follow: Views → Row Subsets. As an alternative, click on the Display Row Subsets button of the top panel in the main window. Step 19: In the Row Subsets window, click on the New Subsets button. The Define New Subset window will pop up. As an alternatively, go to: Subsets → New Subset in the TOPCAT main window, 6 Step 20: In the Define Row Subset window, type: Subset Name: cluster Expression: comoving && parallax > 6 && parallax < 9 Click OK. Step 21: The new subset will be listed in the Row Subsets window. Step 22: Plot only the cluster subset using the Subsets tab in the Histogram Plot window. Rescale the histogram to display it conveniently. You can use the Rescale Plot button, the mouse wheel or the doublearrow button. 7 Block 4: Estimate mean parallax and mean distance Now we will use TOPCAT to do some statistics and find the mean parallax of the cluster members. This will provide us with the distance to the cluster. Step 23: In the TOPCAT main menu, follow: Views → Column Statistics As al alternative, click on the Display Statistics button of the upper panel in the main window. Step 24: In the Row Statistics window, select cluster as Subset for calculations. Step 25: Read off the Mean and SD (standard deviation) of the parallax column. How far away is the Pleiades open cluster? Distance in parsec is the reciprocal of parallax in arcsec. However, inverting parallax to get distance is problematic if parallax errors are large, let’s say, larger than 10%. Let's select only sources with good parallax. Step 26: Create a new subset as we did in steps 18-21, using: Subset Name: good distance Expression: cluster && parallax_error/parallax < 0.1 Step 27: Look at the histogram. Now the two subsets “cluster” and “good distance” are depicted. You may select only ”good distance” in the Subsets tab of the Histogram Plot window. 8 Step 28: Let’s do a Gaussian fit to the parallax distribution: In the Histogram Plot window, click on the Subsets tab. Select only “good distance” subset. Click on the Form tab. Click on the +Forms button and select Add Gaussian in the menu. Scroll down the button-right panel to find the Report box. Note the Mean and the Standard Deviation. They should be similar (but not equal) to the values obtained in Step 25. Step 29: In the TOPCAT main menu, follow: Views → Column Info. As alternative, click on the Display Column Metadata button of the upper panel in the main window. Step 30: In the Table Columns window, follow: Columns → New Synthetic Column. Optionally, click on the Add Column button of the upper panel in this window. Step 31: In the Define Synthetic Column window, type: Name: distance Expression: 1000/parallax Units: pc Click OK. 9 Step 32: Go back to the Row Statistics window (like in step 23): Set Subset for calculations: good distance. Click on the Recalculate button on the top panel of the window. Step 33: Read off the Mean and SD (standard deviation) of the distance column. How far away is the cluster? You should have got something close to 134 ± 7 pc. Step 34: In the TOPCAT main menu, follow: Graphics → Cube Plot. Optionally, click on the Cube Plot button of the upper panel in this window. Step 35: In the Cube Plot window, go to the Position tab and select the columns to be displayed: X: ra Y: dec Z: distance Step 36: In the Subsets tab of the Cube Plot window, make sure that only the subset good distance is plotted.