Research: Thesis
The bulk of my thesis is focused on doing phase-resolved cyclotron spectroscopy on a large sample of the polars which are known to show cyclotron harmonics in the infrared. For the purpose of my thesis we have broken this sample into three large groups.
1.) Polars in high states of emission
2.) Polars in low states which show strong secondary contamination
3.) Uncontaminated polars in low-states. (The easy ones to model)
So far our sample consists of 12 Polars: AM Her, AN UMa, EF Eri, EQ Cet, MR Ser, QQ Vul, VV Pup, ST LMi, HU Aqr, V1432 Aql, V388 Peg, and UZ For. Of these only EF Eri, EQ Cet, AN UMa, and VV Pup fit into group three which show the cyclotron harmonics without emission lines or secondary contamination. The modeling work on these systems has proceeded well, and come this fall we should have two papers to show for it. In paper I (see bottom of page), we discuss the method we will used to model cyclotron emission in polars and test it on the Polar EF Eridani. In Paper II, which is in prep. we imploy the procedure adopted in Paper I on AN UMa, EQ Cet, and VV Pup.
At this stage we have chosen to use so-called "Constant-Lambda (CL)" codes, to produce our models (see Chanmugam and Dulk, 1981 for formalism). CL codes are fast and relatively straight forward to set up, which is why we have chosen to implement them. However, CL codes suffer from a "crisis of reality" of sorts in that their input parameters are not strongly physical. They are: B- the magnetic field strength, T- the plasma temperature, Theta- the viewing angle to the accretion spot, and namesake Lambda, the so-called optical depth parameter, which is closely tied to the column density which is seen by the observer for a given line-of-sight. In CLmodels, each of these parameters is constant, despite the fact that accretion columns clearly have magnetic, temperature, and magnetic field structures. Furthermore, it is often difficult to disentangle the effects of each of the parameters from eachother, especially T, the temperature and Lambda, the size parameter. Thus, we are curently upgrading our modelling approach from a CL code to a Structured Shock (SS) model based on the work of Fischer & Beuermann (2001). The beauty of SS models is that it does the raditative transfer through a fintite plasma for which normalized profiles to Temperature and Density have been calculated for a given mass accretion rate and white dwarf mass. By stacking a series of these accretion regions side-by-side in a self-consistent manner we can mimic the arc shaped/ or even amorphous accretion regions probably displayed on these stars.
Results From Paper I (currently recommended for publication):
EF Eri was found to have results broadly in line with priors. We obtained phase resolved spectra using SPEX, a NIR spectragraph on the Infrared Telescope Facility (ITRF) a 3-m telescope in Hawaii, with a sampling rate of ~ 0.1 in phase. Harrison et al., 2003 have provided JHK photometry for this object. Suprisingly, they find that the J band to be anti-correlated with H and K. We began by populating a grid with "likely" cyclotron models, which were co-added to a blackbody to account for contamination from the WD and was normalized to the 1.0 Micron flux derived by Schwope et al. 2007. The models fit well near phase 0.00 (Bailey et al. 1981). But diverged significantly from our SPEX data in the J band near phase 0.50. Approximately, the parameters are as follows. B ~ 12. 6, T= 5.0 KeV, Lambda = 5.8. Theta, the viewing angle followed the expected curve for a system with an inclinatinon of 58 degrees, and a magnetic colatitude of 6 degrees, which peaks at phase 0.00.
The poor fit iin the J-Band and previous work incl. (Beuermann, Stella, and Patterson 1987) which studied the X-ray emission of EF Eri to show that the accretion region was structured in a "paisley-like" arc of emission (they call this their X-Ray Auroral Oval), convinced us that more meaningful results may be obtained if we had two cyclotron components for our fit. One, denser (Lambda =1e6) and hotter(T=6.0 KeV) may be the cyclotron signature of the core of this emission, the other tenuous(Lambda=1e5) and relatively cool (T=4.5 KeV) representing the extended tail. The fits were on average much tighter than in the one component case.
Finally, we extended our IR models into the Optical and UV and attempted to model the GALEX photometry of Szkody et al. 2006 as cyclotron in origin. Beuermann et al. 2007, have used Zeeman splitting to produce magnetic tomography of the surface WD. Indeed they show a global magnetic field
near to the canonical 13.0 MG, but intrestingly show a "high field" (~80-100 MG) spot at high southern declination. We find that a similar strength field, with theta, lambda, and temperatures similar to our IR models produces photometry in line with the Szkody results. While there is no reasons to a priori assume that the cyclotron parameters would be similar in the UV, our intent here is merely to proove that cyclotron emission should be considered as the source of emission.
Future Work:
My future work will (hopefully) proceed along this trajectory, although you never really know what direction a research project will take. In this sense, they are kind of like kids. You can guide them but
they have personalities all their own. I see three stages: near, intermediate, and far.
1) Near - Right now I am finishing up Paper II on AN UMa, EQ Cet, and VV Pup and will hopefully submit it later this month. Afterward, I will turn my attention to Paper III, which concerns modelling objects for which secondary subtraction is needed to effectively model the cyclotron harmonics. I have already obtained secondary spectra from late K out to T types stars, and built a feature into the code to handle their subtraction. Results to come... :)
2) Intermediate: Fully implement Fischer and Beuermann. For this purpose, we will remodel several of our objects which we studied in Papers 1-III. Most likely candidates are AM Her, VV Pup, EQ Cet, and EF Eri. The modeling work used there will be used as a baseline for setting up this more sophisticated set of models. Questions to address: How do the parameters derived by CL models and SS models compare? Because of the complex shapes of the accretion regions, is it possible to produce realistic constraints on the nature of the accretion region using these models? Are such complex models needed, or do CL models do a "good enough" job for the data on hand.
3) Far: I would like to begin to address how changing accretion states affect the cyclotron emission we see using my SS code. Paper II, provide the first glimpse in this direction as we modeled phase-resolved spectra for VV Pup for 4 different ephochs each of which as in a different state. The early results are difficult to interpret, but clearly the accretion state and temperature are anti-correlated with the effective magnetic field seen in these systems; indicating of course that the shock is higher when the mass accretion rate is tuned up. I would like to explore the implications of similar state changes to a large sample of polars, as well as seeing how (if at all), the shape of the accretion region alters.
Important Documents:
(In postscript format)
Paper I, "Phase-Resolved Cyclotron Spectroscopy of Polars I: EF Eridani"
MY CV
