Using VizieR to Measure Neglected Double Stars

Transcription

1 Page 19 Richard Harshaw Cave Creek, Arizona Abstract: The VizierR service of the Centres de Donnes Astronomiques de Strasbourg (France) offers amateur astronomers a treasure trove of resources, including access to the most current version of the Washington Double Star Catalog (WDS) and links to tens of thousands of digitized sky survey plates. These plates allow the amateur to make accurate measurements of position angle and separation for many neglected pairs that fall within reasonable tolerances for the use of Aladin, the digitized sky survey add-in called by VizieR. This paper presents 221 measurements of 102 neglected pairs from the WDS. As I sit at my keyboard in Cave Creek, Arizona this July afternoon, it is the start of monsoon season in Arizona a time of frequent clouds, wind, and short but severe thunderstorms. When you combine the atmospheric conditions with the oppressive summer heat of the desert, you see why amateur astronomy in Arizona more or less goes on vacation during the summer! This is a good time of year, then, for me to do my astronomy with my computer. A tool I have found and grown quite fond of for double star research is the VizieR service of the Centres de Donnes Astronomiques de Strassbourg (France). This portal can take the amateur to over 8,000 on-line catalogs (some obsolete and of historical value only), and others as fresh as last week. Of particular interest to me are the daily-updated Washington Double Star Catalog (WDS) and its links to the Aladin applet within VizieR, an applet that calls any of dozens of different digitized sky survey plates of the coordinates in question in a variety of formats and wavelengths. As you will see in this paper, VizieR gives the amateur double star observer the power of a professional grade telescope and the tools to actually do serious and useful science. Navigating Your Way Around VizieR Let s begin with a brief tutorial on how to use the VizieR service. To access the VizieR site, use this URL: VizieR s home screen is shown in Figure 1. Note the Catalog Call Window highlighted in Figure 1. This is where you can call up any of 10,178 catalogs, ranging from deep sky objects to double stars to orbital catalogs to spectroscopy, and much, much more. To call up the WDS (which is updated nightly), enter this in the call window: b/wds/wds Then click Find. This will call the WDS query form. It is shown in Figure 2. Before you start asking for WDS data, you may want to scroll down the screen until you see the four lines for proper motion. (They are labeled pmra1, pmde1, pmra2 and pmde2.) I always check these so I can also see the proper motion on a pair (when that data is available - most of the time it is not). The proper motion data can help you find a pair that is moving rapidly across the field from plate epoch to plate epoch. The Value of Photography in Double Star Research William Hartkopf and Brian Mason, chief cura- (Continued on page 21)

2 Page 20 Figure 1: VizieR Home Page with Catalog call window highlighted Figure 2: The WDS input form showing the WDS identifier input window

3 Page 21 (Continued from page 19) tors of the WDS and eminent double star astronomers in their own right, have published a superb monograph on the web site for the 6 th Orbital Catalog (called at this URL: orb6.html). Hartkopf did extensive research on the quality of measurement methods as it is imperative that when one derives an orbit that the input data for the solution is carefully weighed for its reliability, and the method of making the measurements is almost as important as the skill of the astronomer making those measurements. In the section on grading orbits, Hartkopf writes ( orb6text.html#grading) that visual observers (those using such devices as micrometers or reticules like the Celestron MicroGuide or Meade Astrometric eyepiece) received a wide range in relative weight whereas Photographic observers were of fairly uniform quality. Hartkopf went on to say, Since photographic techniques are presumably somewhat more objective than visual measurement, this finding seems reasonable. The graphs he publishes on the web page clearly show the superiority of photographic measurements. The down side is that photographic measurements are difficult if not impossible for extremely close pairs or pairs with wide ranges in magnitude. Calling A Double Star Let me illustrate how to pull data for a double star by showing you the steps I would use to check on WDS (STI 567 in common parlance). I call this data by typing into the WDS identifier window and clicking Submit. This action calls the first data screen, see Figure 3. Depending on your screen resolution, this image may be a bit difficult to read, so Figure 4 is a blown up version of the vital data. I did not show the proper motion data as we only have data for the primary, and it is modest (12 milliarcseconds [mas] a year east in RA and 19 mas a year south in declination). Figure 3: The WDS data screen showing the link to the Digitized Sky Survey plate applet Figure 4: Enlargement of the WDS data

4 Page 22 We note that the last measure on file with the WDS was made in 1908 and it recorded a value of 156 for θ and for ρ. The stars, with magnitudes in the mid-12 s, should show up as nice discs on the digitized sky survey plate. We want to measure this pair for the most recent data available, and for that we will need to pull up the digitized sky survey (DSS) plate for this position. We do that by clicking the hyperlink Optical Image of this region with Aladin-Java. The result of clicking on the hyperlink is shown in Figure 5. There is a tremendous amount of work you can do with this amazing little applet. First, we want to know the epoch of the plate. To do that, I will click on the plate name ( DSS2.F.POSSII on the right side of the image pane). Once that tag is highlighted by bluegray shading, I can get the plate data by clicking on the prop icon, shown in Figure 6. When I do, this information window appears, Figure 7. Just below the Get original header button, I see the plate epoch ( ). To learn more about this plate and the telescope that made it, I click the Get original header button and see a long dialog box appear (you ll need to scroll it to see all of it). Here is the section where the telescope data can be found, Figure 8. Figure 7: The Properties dialog box Figure 5: The DSS image of the region showing the DSS plate used Figure 6: Click the "prop" icon after selecting the plate Figure 8: The properties dialog showing the telescope used for the plate

5 Page 23 I make a note of the epoch and telescope and close the dialog boxes, returning to the raw plate pane. As you can see from the plate, the images of the stars are pretty close together, so making an accurate measurement will be difficult unless I let Aladin do all the heavy lifting! On the right side of the image is a column of tool icons that you will find very helpful. One of them is the ZOOM icon, Figure 9. When I click the mouse, the shaded area expands to fill the plate pane as shown in Figure 10. This is better, but still not good enough. I want to Figure 9: After clicking the ZOOM tool, the zoom target selection window appears; center it on the star to measure and click the mouse Figure 10: The target after one zoom operation

6 Page 24 keep repeating the ZOOM steps until I get an image of the pair to measure that nearly fills the pane. After four zooms, I get the image shown in Figure 11. This is much better! But now I need to pinpoint the centers of the images, and as you can tell, the stars, as faint as they are, cover several pixels on the plate. Here is where the next handy tool comes into play. I use it by first clicking on the DSS2.F.POSSII label at the lower right of the window, then click the pixel tool. Figure 12 is a close-up. This action calls the Pixel dialog, and it is shown Figure 11: The target after 4 zoom operations. Figure 12: Selecting the PIXEL tool

7 Page 25 in Figure 13. You can use the mouse and drag a point along the diagonal line to change the pixilation of the image, but I prefer to use the Pow2 option (the last one of the radio buttons). Selecting Pow2 gives the result shown in Figure 14. Figure 13: The Pixel adjustment dialog box and tool This option takes most of the background noise out and leaves me with two images for which it will be easier to find the centers. Finding the center of a photographic image is not easy, but the enlargement and Pow2 tools make it easier. For dim stars, like these, you will have to select the center axes for both vertical and horizontal, and where they intersect should be the center (or at least within a pixel of it). For brighter pairs, the diffraction pattern induced by the Schmidt plate holder will result in four spikes radiating outward from the star image centers. It is easier in such a case to merely use the diffraction spikes as guides to the center. What I want to do is mark the centers so I can measure them repeatedly and improve accuracy. Here is where the next handy tool in Aladin comes into play: the draw tool. By clicking draw, you can use the mouse to find a point on the image then click it to anchor the line. Click the next point you want to include in the drawing, and Aladin will automatically draw a line from the starting point to your new point. Then move the mouse to the other axis and repeat the procedure. To keep the image cleaner, I usually break the draw command after marking one star (by clicking the select tool at the top of the pane) and then clicking draw again and drawing the other star s axis markers. Figure 15 is what my drawing produced. (I have Figure 14: The image after Pow2 is applied Figure 15: The star centers have been marked with the "draw" tool

8 Page 26 Figure 16: Final zoom and ready for measurement also used the properties tool for drawings, called by pressing ALT + ENTER at the same time, to make the drawing lines red instead of their default blue. The blue lines get hard to read on the dark backgrounds.) I next use the dist tool (fourth from the top) and measure the distance and separation between the marked star centers, but first, I want to ZOOM the image one more time (if the frame will contain it) to get the star centers as far apart as the frame will allow to maximize measurement accuracy. Figure 16 is the result, ready for measurement. It is a much easier image to work with than the original DSS plate image. I will then click the dist tool and place the mouse over the primary s center and click it, then drag the mouse to the secondary s center and click it again. When I do, Aladin measures the distance (in seconds) and the position angle for me automatically and displays the results in a small window at the bottom of the pane as shown in Figure 17. Figure 17: A measurement of high accuracy

9 Page 27 Note that Aladin displays for ρ and for θ. Not bad, and I did not even have to stay up late to get this result! I then record the values for ρ and θ, then use the mouse to drag the measurement arrow aside. I then click dist again and repeat the measurement, this time starting at the secondary and moving to the primary, and adding 180 to the measurement for θ. I again note the values for ρ and θ, then drag the arrow aside, and repeat this process six times, alternating the starting point from the primary to the secondary for each measurement. When finished, I will have six measurements and arrows that should be virtually parallel and of the same length as in Figure 18. This has already been a powerful tool and easy to use. With practice and skill, you can probably call a WDS pair and do a set of six measurements in fewer than 8 minutes. But wait - there s more! We have only measured one of about 15 DSS plates available to us for this region. To see what else we could call and measure, we need to ask Aladin to show us what it has up its sleeve. Figure 19 illustrates how that is done. By clicking the file folder icon at the upper left of the Aladin window, a dialog for plate selection appears, which is shown in Figure 20. The default is to use the Aladin images (the highlighted tab on the left). By clicking Submit, Aladin looks at its records and reports back to you what plates you can pull up to review as shown in Figure 21. Figure 18: Six measurements in the bag! Figure 19: Call other DSS plates with this icon

10 Page 28 Figure 20: Opening dialog to call other plates I have widened the results window so you can see the Date column for each plate. The trick is to select a plate from a different epoch (at least 10 years if possible) and of the same general scale. The POSS II plate we started with was about 11 minutes square, so use the Size column to find a plate of about the same dimensions. In this example, I find a POSS I plate that would do the fourth one in the list (POSS I O plate). By clicking on that line, I can then load that plate too and repeat the epoch discovery and make the measurements I did for the POSS II plate. And so on, and so on. But it gets even better! The Aladin pane has a command menu at the top, with a very important command option. It is the Image command and it has the menu shown in Figure 22. The blink comparator is a valuable tool in double star research, especially when you are trying to find a pair that has not been measured in over 100 years and an examination of the first plate called does not show anything obvious. Often, by calling earlier (and later) plates, and then setting up a blink sequence, the pair you seek becomes obvious as the companion moves relative to the primary and extrapolation of the motion back to the last measurement epoch shows that you have the right pair. Limitations of Aladin While Aladin is an awesome tool for the double star astronomer, it does have its limitations. First, a really bright primary (and here, anything brighter than 5 th magnitude is blinding) will produce a huge image on the plates really bright stars like Sirius burn out almost the entire 11 minute plate! Anything close by (like the Pup) will be totally invisi- Figure 21: A list of other DSS plates to view

11 Page 29 Figure 22: How to call up the blink comparator ble in the glare of the primary. So if you have a bright primary, you need a very wide separation to have any chance of pulling the companion out of the background overburn of the image. Second, pairs closer than 10 seconds will be difficult to measure accurately, even with high zoom factors and pixel adjustments. Third, some plates (especially the older ones) used older emulsions which required longer exposure times which resulted in larger star images and fuzzier pixilation. Try to use the cleanest plate(s) you can retrieve. Fourth, to take maximum advantage of VizieR for photographic analysis of doubles, you need as large a screen as you can afford for your computer (mine has a 68 cm diagonal) and a mouse with as high a resolution as you can afford. Small screens or low-resolution screens and mice will result in rather poor results. Criteria for Selecting the Pairs in This Paper I began by going to the WDS home page ( ad.usno.navy.mil/proj/wds/wds.html) and downloading the latest TXT version of the WDS; importing it into Excel, and after appropriate formatting, ran a query using the multiple filter option in Excel. The filter let pass only those stars that met the following criteria: No measurements made after 1990 No pairs closer than 10 as of the last measurement No more than 12 measurements on record The results of this query were exported to a new spreadsheet which was used for the research for this paper. A total of 4,642 pairs were thus chosen. Methodology I then began (on cloudy or super-hot nights) to use VizieR and check each pair on the list. In most cases, the Aladin image was not of good enough quality for a measurement, so the pair was removed from the list. In other cases, the pair could not be identified on any plate (this is often the case with the faint pairs found in the 1880 s-1920 s by such astronomers as Scheiner, Stein, Baillaud, and others). Where the Aladin plate showed a usable image, the pair was measured six times as described above and the results entered into the spreadsheet. The spreadsheet then computed the average values for r and q and compared them to the last measures on record. Yearly changes in separation (in mas per year) and position angle were then computed (but not shown in the table below). In general, if the yearly changes among the measures are on the same order of magnitude, the results are probably reliable. If they vary widely, there may be issues at play. For instance, operator error is one possibility; also, if a pair has passed periastron or apastron between two measurements, we would expect to see sign reversals in the change computations. Even if the stars have not yet reached periastron but are merely approaching it, rapid changes in the rate of change of the separation or position angle could be the result of the companion s acceleration as it approaches periastron. Where changes in the rate of change appear, I will call attention to that fact in the Notes column. Finally, to assist orbit computers of the future in grading the quality of each measurement, I added a 5- step subjective scale for the plate quality, ranging from Excellent (crisp images, little back ground noise) to Very Poor (fuzzy images, bloated stars, merging of star images, background noise, etc.). The Results The results of 102 pairs studied in the first six hour angles of right ascension are given in the table on the following pages. Codes are as follows: Telescope: 1 = 48-inch; 2 = 1.3 meter Series: 1 = POSS I; 2 = POSS II; 3 = 2 MASS; 4 = SERC; 5 = MAMA; 6 = SERC/MAMA Quality: 1 = Excellent; 2 = Good; 3 = Medium; 4 = Poor; 5 = Very Poor (Continued on page 36)

12 Page 30 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ STT 547 BP SLE Unk Series HO HO A 1514 AC STF 90 AD HJ 3417 AB-C HO J 926 AC Qual Notes BUP 14 AE STT 551 AF BUP 15 AB STI STT 30 AD PLQ 48 AB-C BU 82 AD STI STI STI BUP Unk BU 1363 AB Table continues on next page.

13 Page 31 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ Series STI HJ Unk STI Qual HJ Unk STI HJ ALI STI BAR 21 BC BUP 28 AC STI POU BU 513 AD J 1080 AC Notes Table continues on next page.

14 Page 32 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ Series HJ Unk BU 785 AC in STF 231 AC STI DOR 66 AB Qual STI BU 1371 AB BUP 30 AB AG 38 AC in WYN 1 AC in WYN 1 AD in WYN 1 AE WYN 1 AF WYN 1 AG Notes Table continues on next page.

15 Page 33 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ Series STF 270 AD STI STI STI A 2337 AD STI BUP 36 AB ES 2146 AC STF HO 219 AD STI POU BLL 11 AB BUP 40 AC Qual Notes Table continues on next page.

16 Page 34 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ Series STI HJ BU 1377 AC HL 7 AD BUP 48 AF BU HO 329 AB BUP 61 AB HJ 3679 AB HJ 3679 AC STI STI STI BU BUP STF 612 AC BUP 72 AC STI ES 57 AC BU 313 AD STI BU 1046 AE Qual Notes Table concludes on next page.

17 Page 35 WDS CODE ID Yr 2 Sep 2 PA 2 Epoch Tel Avg ρ Avg θ Series POU 562 AB POU STI SEI 113 AB ES 1231 AC SEI BU 888 AC STI POU POU ALI STI BUP 83 AD Unk POU BU 192 AB STI POU Qual Notes Notes: 1. BUP 14 AE: annual rate of change from 1907 to 1954 was mas r and -0.7 q; from 1954 to 1989 it was mas r and +5.4 q. Could this pair have had periastron between 1954 and 1989? 2. STT 551 AF: annual rate of change from 1854 to 1954 was mas r and -0.2 q; from 1954 to 1989, it was mas r and -0.3 q. Did this pair pass through periastron during the window? 3. J 1080 AC: the annual rate of change between 1950 and 1991 was +4.5 mas and 0.1, but between 1991 and 1998 it flipped to mas and 0.0. The 1998 data was from a 2 MASS plate; several anomalous pairs in this project showed that pattern with 2 MASS data. I believe there may be a systematic error with how Aladin orients 2 MASS plates with regards to celestial north and have sent a note to Centres de Donnes Astronomiques de Strassbourg, but have yet to receive a reply. 4. STI 1835: annual rate of change from 1954 to 1990 was mas / 0.0 ; from 1990 to 1999 (a 2 MASS plate again), it flipped to mas / was from a 2 MASS plate.

18 Page WYN 1 AD: The rate of change was mas / 0.0, but from 1989 to 1999 it flipped to mas / was from a 2 MASS plate. 6. STI 1903: A flip in the rate of change occurred between the era (+17.4 mas / 0.0 ) and the era (-25.7 mas / 0.0 ). 7. STI 1928: The rate of change from 1911 to 1957 was mas / 0.0 per year; between 1957 and 1993 it switched to -3.1 mas / BUP 36 AB: Between 1925 and 1954, rate of change was mas / 0.0 per year; from 1954 to 1993, it jumped to mas / STF 324: The rate of change from 1953 to 1989 was mas / 0.0 ; between 1989 and 1998, it was mas / Another 2 MASS case for the 1998 plate. 10. BU 1382: From 1913 to 1955, the rate of change was mas / +0.6 ; from 1955 to 1989, it dropped to mas / BU 1388: The rate of change averaged mas / -0.2 per year; from 1955 to 1996, it was mas / -0.1 per year. (Continued from page 29) Recommendations for Further Research Any amateur with a high resolution monitor and mouse and fairly fast internet download speeds should be able to use VizieR and Aladin to make measurements on neglected pairs all over the sky the researcher is not limited to studying just those stars that can be seen from his or her location on the earth! I urge amateurs with the proper computer equipment to try a few cases to see how easy this really is and to contribute hard scientific data to a growing database, the WDS. Acknowledgements The author wishes to recognize the following as sources for material for this paper: Hartkopf, William I. and Mason, Brian D., The Sixth Catalog of Orbits of Visual Binary Stars; ad.usno.navy.mil/wds/orb6.html; July The Washington Double Star Catalog (WDS), hosted at this exhaustive database is the authoritative source for all modern double star research and is maintained at the U.S. Naval Observatory. This research has also made use of the VizieR catalogue access tool, CDS, Strasbourg, France. The original description of the VizieR service was published in A&AS 143, 23. Richard Harshaw has been viewing the heavens for 50 years, the last 5 from his new home in Cave Creek, Arizona. He served on the board of the Astronomical Society of Kansas City while a resident of that city, and has served two terms as President of Phoenix s Saguaro Astronomy Club and is currently its Secretary and newsletter editor. Richard has logged over 26,000 double star observations, including over 1,400 measurements registered with the WDS. He is the author of several papers for astronomical journals and also one book, The Complete CD Atlas of the Universe (published by Springer Verlag).