April 29th, 2006


Some of the planets that have been detected via the radial velocity technique have been announced in the refereed literature without the supporting evidence of a published table of radial velocities. For the planets that fall in this category, the end-user gets a star name, a list of orbital elements for the planet, and a graph showing a model velocity curve running through the data points. Occasionally, the data is folded, and only a .gif file of the phased radial velocity fit is published.

In a previous post, I wrote about why I can certainly appreciate the planet detection teams’ reasons for not wanting to divulge their radial velocity data when they announce a new planet. If a star has one detectable planet, then the odds are about 50-50 that another planet will be detected after several additional years of monitoring. For a variety of reasons, multiple-planet systems are scientifically more valuable than single-planet systems. In particular, a multiple-planet system (such as GJ 876) tells a fascinating dynamical story, which in turn yields valuable information about the formation and evolution of the planetary system. Obtaining radial velocities is hard, expensive work.

The unavailability of the radial velocity data sets for some of the planet-bearing stars has led to something of a gray market industry in which the radial velocity plots of the parent stars of interesting multiple-planet systems such as HD 82943 and HD 202206 are digitized, and the radial velocities are reconstructed from the graphs. For an example of this technique, see this preprint on astro-ph.

I bear some of the responsibility for the radial velocity .gif digitization industry. In 2001, a press release was sent out announcing the discovery of eleven new planets. This bumper crop included two particularly amazing systems, HD 80606, and HD 82943. HD 80606 harbors a massive planet on an extremely eccentric orbit, and I was very interested to fit the data myself in order to estimate the uncertainties in the transit windows.

The tabulated radial velocities on which the fits were based were not published, but postscript files showing plots of the radial velocities versus time were posted. I went into the files, and by placing commands to print characters in red, I was able to figure out how the plot was encoded. I was then able to extract the exact measured radial velocities for both HD 80606, and HD 82943 from the press conference postings. I didn’t try to publish the analysis that I did with this data, since the procedure seemed a little under-the-table. I did tell people what I was doing, however, and the radial velocity plots on the websites were soon changed from postscripts to .gif files, which are much harder to reverse-engineer.

One of our initial goals with the systemic collaboration is to provide the ability for anyone who is interested to perform a uniform analysis on all of the radial velocities underlying all of the published planets that make up the current galactic planetary census. In order to do this, we need a mechanism for accurately extracting the data from image files in .gif and .jpg format. Systemic team member Eugenio Rivera has been working on this, and has been getting good results with the Dexter Java Applet (available from ADS). The ADS information page gives the following overview:

Dexter is a tool to extract data from figures on scanned pages from our article service. In order to use it, you need a browser that can execute Java Applets and has that feature enabled. Netscape users can verify this by selecting “Edit” -> “Preferences” -> “Advanced” from the top-bar menu and making sure that the button “Enable Java” is checked.

Dexter can be quite useful in generating data points from published figures containing images, plots, graphs, and histograms, whenever the original datasets used by the authors to produce figures in the papers are not available electronically.

We’ll be posting velocity sets extracted from .gif files shortly, and Eugenio will post a detailed write-up of the technique and pitfalls of “observing” the observations.

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