The asteroid lightcurve database
Introduction
As electronic photometry, first photoelectric and then CCD, advanced so did the number of asteroid lightcurves produced, almost exponentially. For some time, there was no single compilation of results and so those wanting to do statistical work based on lightcurve periods, amplitudes, phase angles, and so on had either to get enough data on their own or cull through numerous paper journals. The first compilation from which the current database descends appeared in Harris and Burns (1979), which included 182 objects. By the time of Asteroids II, Lagkerkvist et al. (1989) had expanded the list to more than 600 objects, making way for sound statistical studies such as rotation rates versus size. The task of maintaining the lightcurve database, now with more than 3700 objects, resides in the hands of the authors. This requires going through numerous journals, abstracts, and meeting proceedings to gather whatever information is available. This process also includes assessing data from different sources for the same asteroid and assigning a quality rating (U) for the period and amplitude that takes into account the quality of each data set.
Initially, the lightcurve database (LCDB) was maintained by Harris and consisted of two ASCII text files. One file stored all lightcurve data while the other stored the references for each set of data. In that first file, the so-called “summary” line carried the values that Harris judged to be the most likely period solution. Below each summary line were one or more “detail” lines. Each detail line included a short reference and the pertinent results from that work for the asteroid in the summary line. Harris also created a quality code rating (U) that had four possibilities, ‘1’ through ‘4’, with ‘1’ indicating the reported period might be completely wrong, to ‘3’, which indicated a secure result. The ‘4’ rating was used to indicate that a pole solution had been found but it should be noted that the ‘4’ rating did not necessarily indicate the lightcurve itself warranted a high-quality rating. There were, and still are, instances where a pole solution was based on a less-than-perfect data set. For example, the lightcurve period solution itself may warrant a rating of only ‘2’.
Table 1 shows the current-day U code assignment criteria. The ‘4’ rating has been dropped since, as noted above, a pole solution did not necessarily reflect the quality of the available lightcurves. A separate quality rating is now given to the pole solution that parallels that for lightcurves. The new system also allows for ‘+’ and ‘−‘ subdivisions, e.g., 2+ or 3−, to refine the assessments even more. Those results with or better are judged to be of sufficient quality for statistical studies of rotation rates.
The rapid explosion of lightcurves produced in recent years (e.g., from ∼1600 asteroids with or better in 2004 to ∼3700 in late-2008) necessitated a change in the structure of the database from simple ASCII text files to a relational database with Structured Query Language (SQL) capabilities in order to facilitate data entry, allow additional data to be entered, and provide a consistent format for reports as well as the means to generate complex searches without having to write custom parsing programs. This resulted in the current version of the LCDB that now stores a wealth of new data beyond what was in the original files and allows complex searches of the more than 30 tables. In 2005, Warner began assisting Harris with the LCDB by gathering data from the Minor Planet Bulletin, the journal in which most “amateurs” publish their results and now publishes the majority of asteroid lightcurves. As an aside, we prefer the term “backyard astronomer” and “backyard observatory” over “amateur” or “amateur observatory” since this implies the size of telescopes used rather than the abilities or status of the observers.
Many database engines were available but we settled on dBASE IV-type files. These are not the most efficient for data storage but they are supported by a number of other engines, which means that the data can be easily converted to other formats such as Microsoft Access™ or MySQL©. Using some of the other engines could have involved proprietary formats, which we wanted to avoid for the most flexibility and compatibility. We will not describe the database structure and table schemes in detail here. Those interested in those details can view the README.TXT file that is part of the download from the CALL web site.1 At this time, the actual tables are not available as part of the download. What is included are several custom reports in simple ASCII text format, one of those being an expanded but well-formatted version of the original data file with summary and details lines. We also generate custom tables for the Minor Planet Center (MPC), the Russian Ephemerides of Minor Planets (EMP), and the Planetary Data System (PDS) for their use and publication.
We are looking at options that would make a subset of the LCDB available where researchers could do custom searches of the data files. The subset would exclude proprietary, unpublished results that researchers have not approved for public release. The options include providing a Windows-based program that includes the routines in the LCDB program that generate the public data files as well as SQL-based queries for custom reports. The full set of tables (with proprietary data excluded) would be updated from time-to-time and placed on the CALL web site or other easily-accessed location. This has the advantage of letting end-users convert the tables to a database engine of their choice and write custom applications. Another possibility is to work with the Strasbourg data center to place the files on their VizieR site (http://webviz.u-strasbg.fr/viz-bin/VizieR) where queries could be made on-line. This would require considerably more effort on our part but has some distinct advantages. For now, our concentration is to complete the transition from the original format to the new, which includes revisiting all entries to include previously unreported data and update existing data as needed.
Section snippets
Data sources
Some data in the LCBD are taken directly from the journals while others must be derived using accepted formulae or assumptions. At this time, it must be made clear that any data that are not actually measured, i.e., those that are derived or assumed, should be taken for informational purposes only and should not be used in critical analysis. The exception to this is the combination of albedo (), absolute magnitude (H), and diameter (D) of an asteroid. If two of the three values are known, the
Evaluation of lightcurve results
The U code provides our assessment of the quality of the period solution, not necessarily of the data per se. A U value is assigned to each detail record for a given asteroid as well to the summary record for that asteroid (with some exceptions; see below). The latter is our overall assessment of the detail line values and is most often that of the highest quality assignment among the details records. Table 1 gives the general criteria used in making the assessments.
Many factors come into play
Frequency–diameter distribution
The LCDB has been used most often for spin rate-size distribution analysis. As the number of objects available for analysis has grown, a number of significant trends have been revealed that have changed the thinking on how asteroids were created and evolved. (Pravec and Harris, 2007, Pravec et al., 2007). Fig. 1 is the plot of spin rate versus size for ∼3000 asteroids with , i.e., 2−, 2, 2+, 3−, and 3. In the figure, we indicate the “spin barrier”, first identified by Harris (1996), which
Summary
The Lightcurve Database (LCDB) has evolved considerably over the past 20 years, going from ∼180 objects in 1989 to more than 3700 objects as of late-2008. Its structure has been modified from two simple ASCII text files to an SQL-capable relational database with approximately 30 dBASE IV tables and custom report generating. The LCDB stores a number of parameters obtained mostly from photometric observations. These include rotation period, amplitude, parameters, albedo, diameter, color
Acknowledgments
Funding for Warner and Harris was provided by NASA Planetary Astronomy grant NNG 06GI32G and National Science Foundation grant AST-0607505. Funding for Pravec was provided by the Grant Agency of the Czech Republic, Grant 205/05/0604.
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