Pro-Am White Dwarf Monitoring (PAWM)
Bruce Gary, Hereford Arizona Observatory (HAO)
This web page describes a 1-month "pilot project" of observations of white dwarf (WD) stars in a search for exoplanet transits. The goal is to explore ways of using amateur and professional observers in a way that most efficiently produces an assessment of the fraction of white dwarfs exhibiting transits by Earth-size planets in the habitable zone. If no transits are found during the 1-month pilot study, allowing for an evaluation of an upper limit to their presence, such a result can be useful in planning a comprehensive professional search for WD transits. Amateur telescopes are suitable for this project so it is anticipated that most observations will come from the community of amateurs with experience observing exoplanet transits (of main sequence stars). Any WD exoplanet in the habitable zone will orbit with a very short period (4 to 30 hours), will have very short transit lengths (a few minutes) and will produce very deep transits (complete eclipse possible for central crossing). Such transits would be easy for amateurs to detect for stars as bright as typical known transiting exoplanet stars, V-mag 10 - 13, but only a few WDs are this bright; the faintness of WDs, and the short transit times of exoplanets that are of interest, means that only advanced amateurs with prior experience in observing exoplanet transits are being recruited for this pilot study. This web page will be the home site for an archive of light curve submissions, and links will be included for web pages devoted to specific WDs when they have light curves. The 1-month observing period is set for September, 2011.
    
Both images and the animation were created for PAWM by Manuel Mendez (Spain). 

Links Supporting PAWM

List of "hot" & "cold" WD targets ("hot" need observing, "cold" don't; includes RA/DE, mag's & links to observed targets)
List of observers & submission statistics (observer location, etc & total hrs and obs'g dates)
What's New (history of data submissions & web site updates)  
Additional Supporting Material ("catch-all")
   
Overview   

This proposed "pilot project" endeavors to demonstrate that advanced amateur astronomers are capable of detecting transits of exoplanets orbiting white dwarfs in sufficent numbers to provide a constraint on models for the presence of such exoplanets. If WD exoplanets in close-in orbits are common, then an important opportunity will exist for detecting Earth-sized exoplanets in habitable zones. Such exoplanets will be comparable in size to their WD parent star which means they can produce deep transits, possibly totally eclipsing the star for a central crossing. Deep transits help observability, all other things being equal, but two other things render the WD transit search difficult: 1) the population of WDs are faint due to their extremely low absolute brightness (~ 0.01% of main sequence stars), and 2) transit lengths are short due to the small size of the WD star and this means only a few minutes worth of observations can be used to establish the presence of a transit. The PAWM is therefore challenging for amateurs using modest aperture telescopes, but it is possible for those with experience observing exoplanet transits (of main sequence stars). There is sufficient scientific merit in the search for exoplanets in WD habitable zones that serious thought deserves to be made for the funding of a comprehensive survey with a network of professional telescopes that are optimum for the search. One of the main reservations for funding such a survey is the matter of whether or not planets exist in close-in orbits around WDs. They can either migrate inward after the expansion to the red giant phase and subsequent explosive shedding of an outer layer, or planets can form in close-in orbit after this cataclysmic event. Whether either of these processes occur constitutes the most important unknown in the argument for investing in a professional search for WD exoplanets. Amateurs can provide a constraint on this matter by observing the brightest WDs in search of at least one transit. A single transit, confirmed by subsequent observations of it, could not only establish the feasibility of a comprehensive professional search but it might possibly constitute the first actual detection of an Earth-sized exoplanet in a star's habitable zone!

The proposed pilot project may produce nothing, or it may produce an important discovery. The chance of "success" in PAWM detecting a transit is probably low, but the payoff for such a success is unquestionably high. Even if no transits are detected an important constraint will be made on the likely success of a professional search. A secondary goal of the PAWM and any follow-up professional survey is the possibility of discovering eclipsing binaries consisting of two white dwarfs. Any such discoveries could contribute to the mass-radius relation for white dwarfs as well as contributing to an understanding of how a WD binary might merge to produce a Type Ia supernova.

The pilot project of observations is scheduled to last one month, during September, 2011. Observers are encouraged to work in teams of at least two so that any transit feature seen by one observer can be corroborated. Because of the faintness of most WDs the observations will be challenging, and I think previous experience with observing known exoplanet transits is an essential requirement for attempting to participate in this project. Although I anticipate that most observers for PAWM will come from the 60 or so amateurs who have experience with exoplanet transit observing I also welcome participation by university students with access to larger apertures and better CCDs, perhaps under the supervision of a professional astronomer. I also welcome contributions, both observations and advice, by professional astronomers interested in this subject.

I will maintain an archive of WD observations submitted to me via e-mail attachments, similar to the way it was done for the Amateur Exoplanet Archive (AXA) project. I think it will be adequate for observers to submit only their light curve, but I will also accept a data file in a format described below.

Professional guidance for this pilot project will be provided by Prof. Eric Agol (University of Washington, Seattle). Any other astronomer with a suggestion for how PAWM should be conducted are welcome to contact me (e-mail at bottom). Background reading for this project can be found at New Scientist magazine (2011 July 2 issue, page 37) and an Astrophysical Journal Letters article by Eric Agol, that is also available as an arXiv document, link.

Some Basic Background Related to Observing Strategy

There are ~ 20,000 known white dwarfs (from the ~60% of sky that avoids low galactic latitudes). Of these only ~ 168 are brighter than V-mag = 14.0; another 305 are within the range V-mag = 14 to15, and 742 are between V-mag = 15.0 and 16.0. In other words, 473 WDs have V-mag < 15, and 1215 have V-mag < 16.0. These abundance statistics were provided by Eric Agol's amateur astronomer associate Howard Relles and are based on a catalog compiled by Rowell and Hambly (2011), link, as well as the VizieR database (mostly from McCook, 2008). The Villanova White Dwarf Catalog contains photometry and distance information, and it's user-friendly: link.

The next figure illustrates that transits of WDs are likely to be very deep. Note that an Earth-size planet with a central crossing would produce a 50% flux drop (depth of ~ 750 mmag).


Figure 1. Model transit light curves for a typical WD and 3 exoplanet sizes. (copied from Agol, 2011).

The following figure shows the kind of light curve (LC) that's possible with a 14-inch telescope observing unfiltered with 20-second exposures.    
Figure 2. Example of a light curve for a V-mag = 15.1 white dwarf using a 14-inch Meade telescope (M14) with a "clear with blue-blocking" filter (Cbb) at the Hereford Arizona Observatory (HAO) second dome. Individual image sxposure times were 30 seconds. The model includes an arbitrarily placed transit with 750 mmag depth and 4-minute length, to illustrate what an Earth-sized exoplanet could produce for a central crossing. The small "+" symbols correspond to individual images, and the red circles are 11-point averages. The individual images exhibit an RMS with respect to themselves of 29 mmag, and with respect to a straight line fit of 28 mmag. The most important source for scatter is "thermal noise" (associated with the CCD); Poisson noise and scintillation are unimportant.

Since we're looking for very deep transits, in the several hundred mmag range, this LC shows that a 14-inch telescope can produce useful data for WDs as faint as V-mag = 15. How many WDs this bright can be observed during the pilot project? There are ~ 450 WDs in the entire sky this bright, but fewer are observable during a 1-month observing interval from the northern mid-latitudes. I estimate that ~ 150 WDs brighter than V-mag 15.0 are observable using amateur hardware during this September's pilot project.

Target Selection and Observer Coordination

The best targets for mid-September observing have RA ~ 23:30. At mid-latitudes stars at this RA that pass overhead can be observed for ~10.0 hours. I propose to maintain a list of bright WDs near this RA and with DE (declination) values between +25 to +50 degrees. Observers are encouraged to select a target star in coordination with other observers. Observers with a small telescope aperture will have to settle for a bright WD, whereas observers with a large aperture are encouraged to select a faint WD. The ideal observing session is with 4 observers, 2 in Europe and 2 in USA, observing the same WD target. This will provide continuous converage for a light curve that could be 18 hours long (in September). This four-some strategy allows corroboration of any interesting features, provided all had clear skies. A two-some strategy is almost as valuable. It could consist of two observers at a similar longitude (allowing feature corroboration) or at widely spaced longitudes (providing long continuous coverage). Finally, a single-observer strategy can be employed; this may be best for an observer with an especially large telescope aperture for it will provide a sampling of the very faintest WDs.

I will suggest a short "hot target" list that everyone is invited to use for selecting a target. When a target has been observed sufficiently (e.g., 20 to 30 hours) I will drop it from the "hot" list. Professional astronomer Eric Agol will be consulted in creating the "hot" target list because his interests are focused on WDs that are more likely to harbor exoplanets in the habitable zone (medium to cool WDs).

Suggested Procedures for Observing, Image Processing and Data Submission

This material has been moved to another web page: http://brucegary.net/WDE/WDcatchall.html#Suggestions_for_Observing_Data_Analysis

Professional Follow-up and Publications

Prof. Eric Agol will provide overall scientific guidance to this project. He will take the lead in assessing the scientific value of  how this pilot project is conducted. Howard Relles, an amateur astronomer associate, will provide support for such an assessment. I expect that Prof. Agol will decide at the end of the observing month whether a scientific publication of results is warranted, and if he writes such a paper he will be lead author. Co-authorship will be based on total number of data submission hours as well as an estimate of data quality. Since Prof. Agol has expressed interest in seeking funding for a large observing project of professional telescopes I suspect that he will some day use the results of this pilot study to support any proposed follow-on project.

Expected Comprehensiveness of PAWM

One of the goals for PAWM is to establish a relationship between specific telescope systems and the faintest WDS for which useful monitoring can be achieved. Already with just three weeks of exploratory WD light curves in the PAWM archive it is clear that a 12-inch telescope can produce useful LCs for a star having V-mag ~ 16 (and probably fainter), provided the observer has experience with exoplanet transit observing. See for example the excellent LC of a 15.6 mag star by Gregor Srdoc at link. The following plots summarize what has been learned so far.

     
Figure. 3 Left: Preliminary result showing the faintest V-mag's for which useful LCs can be obtained versus telescope aperture.
Figure. 3 Right: The number of known WDs brighter than V-mag values.

For example, it appears that a 14-inch telescopes in use by experienced observers are capable of producing useful LCs for more than 2500 WDs.

Observer Sign-Ups

Here's a map of locations for observers who have signed-up for PAWM, so far.

                       
Figure 4. As of 2011.09.03.

A lisitng of the observer names, locations, hardware, etc can be found at http://brucegary.net/WDE/observers.htm.

Additional Supporting Material

Questions keep coming, and they deserve to be answered in a way that everyone interested in PAWM can access, and comment on as necessary. Most people won't want to be burdened by that material so I'm creating a "catch-all" web page for that material: Additional PAWM Supporting Material ("catch-all" web page).

Weekly Status Update E-mails  

I will send e-mails at weekly intervals (Fridays) to everyone who has signed-up to be an observer or "follower" of PAWM.

Apropos Quote
"When we are exploring the universe and looking for evidence of life, either we may look for things that are probable but hard to detect or we may look for things that are improbable but easy to detect. In deciding what to look for, detectability is at least as useful a criterion as probability. Primitive organisms such as bacteria and algae hidden underground may be more probable, but freeze-dried fish in orbit are more detectable. To have the best chance of success, we should keep our eyes open for all possibilities."  Freeman Dyson, Atlantic Monthly, 1997 November.
References

Agol, E., "Transit Surveys for Earths in the Habitable Zones of White Dwarfs," ApJL, 2011, arXiv link 
Rowell, N. and N. Hambly, 2011, arXiv link
Agol, E., "Finding Habitable Earths Around White Dwarfs with a Robotic Telescope Transit Survey," 2011, conference proceeding, PDF download link

Related Links
    AXA (Amateur Exoplanet Archive)
    Exoplanet Observing for Amateurs, Second Edition (book), by Bruce Gary (free PDF download)
    Villanova White Dwarf Catalog
    S&T web article about PAWM
    Interview (Sep 16) w/ Prof. Agol at Jackson Hole meeting (good summary of the reason for doing PAWM)
    Resume of Bruce Gary
    Resume of Eric Agol
 

   
 Created by Manuel Mendez for PAWM
 
WebMaster: B. Gary (B L G A R Y @ u m i c h . e d u).  This site opened:  2011.07.14.  Last Update:  2011.10.01