T. V. Bryant III

Little Tycho Observatory

703 McNeill Road, Silver Spring, Md 20910
rkk_529@hotmail.com

Abstract

The advent of computers with large amounts of RAM memory and fast processors, as well as easy internet access to large online astronomical databases has made computer searches based on astrometric data practicable for most researchers. This paper describes one such search that has uncovered hitherto unrecognized double stars.

Recently, four previously unlisted double stars were added to the Washington Double Star Catalog (WDS)¹ as a result of this process:

These pairs were located by a computer search of the fourth USNO CCD Astrographic Catalog (UCAC4)². Using the CD provided by the United States Naval Observatory³ (USNO), a suite of programs written in Perl and C was used to search the UCAC4 for candidate binary pairs that were not in the WDS.

Pairs that passed this test were then placed into a list for manual verification, using the Aladin⁴ tool.

Using Aladin, the candidate pair was manually examined. Pairs that seemed to be associated with nearby clusters were eliminated, as were pairs enmeshed in nebulosity to the point that they could not be accurately measured with Aladin. Pairs that passed these tests were then measured for position angle and separation, using Aladin's UCAC4 template on the DSS plate, and placed in a final list of candidates.

The four entries from this final list of pairs were chosen based on their visibility in the summer sky from 39° north latitude, brightness, separation, and a large proper motion. These stars were observed from the Little Tycho Observatory, and then submitted to William Hartkopf⁵, of the USNO.

Search for unlisted double stars from the UCAC4 data.

Note:The instructions given below have been shown to work in a Linux development environment. While they can be replicated in other environments, e.g, an Apple Mac, or the Cygwin environment on a Windows PC, they have not been tested in these environments.

Obtain a copy of the UCAC4 data.

You'll need to send an email to brenda.hicks[at]navy.mil with "UCAC4" in subject and your full name and postal mailing address in the body of your e-mail.

A CD of the data will be sent to your address. When it arrives, mount the CD(s) in your computer's CD drive and download the 900 UCAC4 declination zones to a directory on your computer.

Obtain a copy of the WDS data.

On a computer equipped with the wget program and connected to the internet, the following command should do the trick:

   cd /DirectoryWhereYouWantToStoreTheData/
   wget -r -O wdsData http://ad.usno.navy.mil/wds/Webtextfiles/wdsweb_summ.txt

The file wdsData should now be downloaded to your current working directory, DirectoryWhereYouWantToStoreTheData.

Note: It is assumed that the reader is somewhat familiar with programming and the C and Perl languages in particular in this section.

Getting the code.

Code can be obtained from GitHub,

or the author's web site.

In downloading code from the author's web site, you can list the program by left clicking on the link below, and download it by right clicking (Ctl-click on a Mac)

The WDS is listed in order of its positional Ids, a low precision J2000 coordinate designation, as opposed to a pair's precise coordinates. A short Perl program has been written to extract the precise coordinates from the WDS:

The program is run from the directory containing a recent copy of the WDS catalog.

The resultant file, WDS.pre, is then sorted using the unix sort command:

   sort -n WDS.pre > wdsPrecisionCoordinates

As the UCAC4, as of this writing, consists of 113,780,093 stars, it was not efficient to search the entire declination range of the catalog for each of the candidates. MkUCAC4_Regions.c reads the catalog and then splits it into files, each of which cover approximately a square degree of sky. Stars that border (within 30") a square degree are also included in the file. In this way, when a candidate star is searched for possible companions, only a single square degree need be searched. The program also creates a list of possible primary candidate stars, all brighter than mvC (Default setting for this is 11000 = 11.0mv) that findUnlistedDoubles.c will use for the search.

Edit the program's holding directory location in lines 65 and 68 to match where you would like its square degree files to be written. The directory used to store the UCAC4 data must also be changed in line 104.

The program can be compiled using this command:

   gcc -std=gnu99 -O2 -o mkr -W mkUCAC4_Regions.c -lm

and run using this:

   mkr 

Find candidate binary pairs not in the WDS from UCAC4 data.

Now that we have the precision coordinates from the WDS and the UCAC4 data for the brighter stars stored in square degree files, we can search for pairs not in the WDS with findUnlistedDoubles.c.

Many of the search parameters set by both programs can be changed, the programs recompiled, and different results obtained. If a very loose set of parameters is used, many thousands of possible pairs can be generated. More restrictive parameters will result in fewer pairs. Once the user is familiar with editing and compiling the programs, many interesting lists can be generated. Each list takes only about 5 minutes to compile and create on the author's machine. It complies with a command similar to the one given above.

The program's output file is in HTML format. Read it in your browser by opening it as a file. A sample output can be seen in listing 1.

The program ships with these default parameters. Their default values are the numbers in parentheses following the parameter's name:

• Only stars brighter than mvC (11000) are considered as primary candidates. Generally, the brighter star, the more information about it is to be found in the literature, simplifying subsequent study. This can be changed by changing the value of mvC in mkUCAC4_Regions.c.
  
  
• Secondary stars must be brighter than mvS (12000) to be considered. This again can be changed by changing mvS in findUnlistedDoubles.c and mkUCAC4_Regions.c. Note that the UCAC4 does not list many stars fainter than 16.0mv (16000).
  
  
• The primary and secondary should be within dMv (1000) of each other. This is a fairly arbitrary restriction, and is used to simply cut down on the number of pairs found.
  
  
• XXX (30) is the size of the region of sky that will be searched, in arc seconds. A box of sky twice this distance on a side will be searched for possible companions to a given candidate star. This can be changed by changing XXX in both mkUCAC4_Regions.c and findUnlistedDoubles.c. Note: The programs have only been tested with XXX set to 30". Change this at your own risk!
  
  
• The minimum separation between the stars should be minSep (10) arc seconds or more. Pairs are manually verified by examining them on DSS plates. A pair of stars with less separation can be very difficult to differentiate from a single star.
  
  
• The larger the maximum separation, maxSep (30), the less likely a pair will be physical. Unless XXX is changed, 30 should be the largest separation used.
  
  
• The average proper motion of the two stars must be larger than minPM (50) milliarcseconds per year (mas/yr). This will help to eliminate slow moving pairs that are optical.
  
  
• The difference between the average proper motion of the candidate pair and their individual proper motions should be less than a given quantity pmR (5). This condition helps to eliminate possible optical pairs, and is defined as the differences between the primary and secondary proper motions. For example, if a pair had an average proper motion of 70 mas/yr but one component was moving north of the average proper motion at 30 mas/yr, the other south at 20 mas/yr, the pair would be rejected because of their individually disparate, thought large, proper motions.
  
  
• There must not be a binary listed by the WDS within XXX arc seconds of the candidate pair. Visual magnitudes were ignored for this condition, as many WDS listings show magnitudes different from the UCAC4 magnitudes.

Other notes:

• The program converts all of its angles to radian measure for calculation, and then back to degrees or hours, minutes and seconds for output.
  
  
• The output coordinates are in a format such that you can easily drag and drop them into DSS display tools such as Aladin, GoogleSky, or WikiSky.
  
  
• When a parameter is changed, the changed program must be recompiled before the changes will take effect.
  
  
• Directories that hold the UCAC4, WDS, and output data are as found on the author's computer. These will probably need to be changed to fit the user's computer.
  
  

1) The Washington Double Star Catalog Brian D. Mason, Gary L. Wycoff, William I. Hartkopf, Geoffrey G. Douglass, and Charles E. Worley, 2001.

2) The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4). Zacharias, et al, 2012.

3) United States Naval Observatory

4) Aladin web site

5) William Hartkopf, Astrometry Department, U.S. Naval Observatory 3450 Massachusetts Ave, NW, Washington, DC 20392