Steve Howell is an astronomer at the WIYN Observatory located on Kitt Peak near Tucson, Arizona.
Steve pursues research on interacting binaries, extra-solar planets, and observational techniques with CCDs.
Steve has
authored so many papers on cataclysmic variables that a Google search results in hundreds of hits. He is also author of
the 'Handbook of CCD Astronomy' (2nd Edition coming out this fall), and he's got his own space rock named after him,
asteroid 15091, Howell. By the way, this asteroid is eccentric (in its orbit), a fact unlikely to be random.
We're going
to discuss polars, magnetic fields and the nature of the secondary stars in these systems.
CVnet: Hi, Steve. To begin with, can you give us a working definition of a polar?
Steve:
A Polar is a binary star containing a white dwarf and a more-or-less normal late type star. The two stars are very close (a
solar diameter or less) and the white dwarf has a very strong magnetic field. The field strength on the white dwarf can be
10 up to 250 MegaGauss. The gravitational field of the white dwarf, a star of 1/2 to ~1 times the mass of the sun but of a
size about equal to the Earth, is so strong that material is pulled from the secondary star. This material falls onto the
white dwarf at one or both of its magnetic poles, releasing tremendous amounts of energy. The light from this energy release
outshines both the white dwarf and the secondary star in the optical and infrared.
These binaries are short orbital period systems, 80 minutes to about 8 hours, and tend to be discovered
via their copious x-ray emission or optical brightness. Oh, by the way, they are called polars as the first to be discovered,
AM Herculis, showed (as they all do) highly polarized optical light. This group of cataclysmic variable stars are also called
AM Her stars.
CVnet: Where does the intense magnetic field in the white dwarf come from?
Steve: The intense magnetic field of the white dwarf is believed to come from the original star which
produced it. A class of A stars with higher than normal magnetic fields are thought to be the progenitors of highly magnetic
white dwarfs. The connection is far from proven, but seems logical.
CVnet: Doesn't the presence
of a strong magnetic field complicate our understanding of these systems? What kind of computing power is necessary to model
polars?
Steve: YES!! True, three-dimensional, fully correct models are yet to be realized.
Jennifer Cash, an astronomer at the South Carolina State University, has about the most detailed models to date and runs these
smooth-particle hydrodynamics on supercomputers. Each "run", to model one part of one system, may take weeks to compute fully.
CVnet: When a polar is in its low state you are better able to study the secondary. Why is this, and
what have you learned about the nature of the secondaries in these interacting binaries?
Steve:
First off, lets talk about low states and the corresponding high states. Polars have times of high rates of mass accretion
(mass pulled from the lower mass secondary star to the white dwarf) and times of low to nearly zero rates of mass accretion.
These high and low states are apparently random and can cause the polar to change by 2 to 5 magnitudes in optical brightness.
The cause is thought to be related to stellar activity on the late type star (star spots etc.) but absolute proof of this
is hard to come by.
OK, low states... During these times, the mass transferred is often very low to nearly zero, and
the two component stars now become the dominant light sources in the optical (white dwarf) and infrared (secondary star).
Thus, during low states one can actually observe the stars themselves.
During the low states, study of the secondary stars has led to an amazing discovery. Theory
had predicted that if a secondary star lost mass to its white dwarf companion for a long time, maybe up to nearly the
age of the Galaxy, it would become a very low mass, non-normal star like object. The polar EF Eri has not had mass transfer
for over 9 years now and detailed study of its secondary reveals an object best described as a cross between a former
star, an odd brown dwarf, and an extra-solar planet.
The secondary has become a small (Jupiter size) low mass (about 40-50 Jupiter masses) very
cool (T=1100K) object. This former star is not like anything else known in the universe. Four other candidates are in
the works including the recent campaign on the polar VV Pup which included a large participation by the AAVSO. Initial
results are that the secondary in VV Pup is similar to that in EF Eri.
An additional interesting observational
fact about EF Eri is that at present, optical observations only detect the white dwarf and IR observations only
detect the secondary. It'd be very hard today to know this star is a polar let alone even that it is a binary star.
CVnet: Polars appear to be a small sub-set of cataclysmic variables.
Is this a reality, or due to some observational selection effect?
Steve: Single, highly
magnetic white dwarfs make up about 20-25% of all single white dwarfs. Polars make up about 20-25% of all cataclysmic variables.
So maybe the numbers are about correct.
However, every time a new X-ray mission or large area optical survey occurs,
more polars seem to be discovered.
CVnet: So far, we've talked about research done with
large telescopes, spectroscopes and super-computers. What can observers with modest telescopes contribute to researching polars?
Steve: Here is where we can get serious about observing. The AAVSO has a wonderful polar program they
are starting in connection with myself.
The idea is two-fold:
1) Look at polars as often as possible to try to get better
statistics on the times of high and low states and how often they occur. Do these measurements in colors to extract physics
as well.
2) Obtain time series observations, that is, stare at one polar as often as you can for as long as you can, and
collect a light curve. Again do this in color and do it at high states and , if you can, at low states.
Both of these
projects are not generally doable by professionals due to the amount of telescope time required. Check out the AAVSO web pages
to get the details of this program.
Although not comprehensive, this astrophysics pre-print search result
will bring up a list of papers authored or co-authored by Steve Howell.
