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
"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)
(history of data submissions & web site updates)
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
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
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
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.
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"
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"
"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.
Agol, E., "Transit Surveys for Earths in the
Habitable Zones of White Dwarfs," ApJL, 2011, arXiv
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
AXA (Amateur Exoplanet Archive)
Exoplanet Observing for Amateurs,
Second Edition (book), by Bruce Gary (free PDF download)
White Dwarf Catalog
web article about PAWM
(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
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