THE ASTROPHYSICAL JOURNAL, 557:978È982, 2001 August 20 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. CHEMICAL ABUNDANCES OF THE ACCRETING MAGNETIC WHITE DWARF IN THE POLAR AM HERCULIS JOSEPH DEPASQUALE AND EDWARD M. SION Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085; joseph.depasquale=villanova.edu, edward.sion=villanova.edu Received 2001 March 1; accepted 2001 April 19 ABSTRACT We consider the presence of absorption lines due to accreted metals in the photosphere of the magnetic (28 MG) white dwarf in the prototype polar AM Herculis. We have applied the Massa-Fitzpatrick Ñux-calibration correction to the archival IUE NEWSIPS SWP spectra of AM Her obtained during the optical low states, when the accretion rate is very low and emission due to thermal bremsstrahlung and cyclotron processes is essentially absent. We have examined low-state spectra at UV maximum (phase D 0.6: main accreting pole most directly exposed to the observer) and UV minimum (phase D 0.1: line of sight perpendicular to the magnetic Ðeld). We have used the model atmosphere codes TLUSTY, SYNSPEC, and ROTIN to determine the surface temperature of the white dwarf at each UV phase during quiescence and the chemical abundance of detected metal species. We Ðnd that the abundances of metals in the photosphere of the accreting magnetic degenerate in AM Her range between 0.05 and 0.001 times solar. To our knowledge, these are the Ðrst photospheric metal abundances to be determined for any accreting magnetic white dwarf in a magnetic cataclysmic variable. Our preliminary results indicate that (1) there is no correlation between the time since the last high state and either the surface temperature or the chemical abundance and (2) the metal abundances do not appear to be signiðcantly different between spectra obtained at UV maximum and UV minimum. This implies that, based upon the limited sample of spectra presented here, the abundance of metals may be similar across the stellar surface. This would result if sideways di usion and spreading of material from the small polar-cap accretion regions had occurred. Subject headings: accretion, accretion disks È binaries: close È stars: abundances È stars: individual (AM Herculis) È white dwarfs 1. INTRODUCTION AM Herculis is the prototype system of a class of synchronously locked magnetic cataclysmic variables (polars) containing a very strongly magnetic white dwarf primary and a late-type, Roche lobeèðlling secondary star. Because of the intense magnetic Ðeld generated by the white dwarf, accreting material from the secondary does not form a disk. In fact, the Ðeld is so strong that material is threaded along the magnetic Ðeld lines of the white dwarf, forming accretion columns that fall onto the dwarf in one or two accretion spots. During its on state of high accretion, the far-uv spectrum is dominated by the radiation from the brilliant accretion column. However, when the system goes into its low-brightness state, the thermal bremsstrahlung and cyclotron radiation no longer contribute signiðcantly, line emission greatly weakens or disappears, and the magnetic white dwarf photosphere dominates the far-uv radiation. The seminal spectroscopic work on the system was carried out by Young, Schneider, & Shectman (1981). The surface temperature of the white dwarf in the low state was found to be 20,000 K (Heise & Verbunt 1988). Ga nsicke (1997) and Ga nsicke, Beuermann, & de Martino (1995) Ðrst analyzed the archival low-state spectra. They derived the e ective temperature and sizes of the accretion-heated polar caps in both states, as well the temperature and radius of the white dwarf itself. These three investigations also noted the presence of weak absorption features in the IUE spectrum. Ga nsicke et al. (1995) drew attention to the presence of Si II jj1260 and 1265 in the IUE spectra, which they compared with a Ðxed-abundance, solar-composition, white dwarf atmosphere. Our paper focuses on the nature of the interaction of the infalling material with the white dwarf 978 photosphere. Our objective is to determine the surface temperatures and chemical abundances of the white dwarf at various times during low states of little or no accretion. These abundances would be the Ðrst to be determined for an accreting magnetic degenerate in a cataclysmic variable. To accomplish our objective, we have analyzed IUE SWP low-dispersion spectra obtained during low states of AM Her. Knowing the orbital phase of the white dwarf, we can determine when the accreting spot is out of view. If there is evidence of heavy elements in the white dwarf atmosphere when the spot is out of view, then we assume that the material has probably spread over the entire surface of the dwarf, rather than being conðned to the spot. Also, we aim to use these abundances to shed new light on the so-called soft X-ray puzzle, in which the soft X-ray Ñux of some polars far exceeds the sum of their thermal bremsstrahlung and cyclotron emission. It is possible that, during accretion, temperatures in the infalling column of material can reach levels allowing thermonuclear reactions to take place on the surface of the white dwarf, thus accounting for the observed excess of soft X-rays. Chemical abundances could provide a test of whether there is any evidence of nucleosynthetic processing. We have selected archival IUE data from a number of observing runs during AM Her low states. The largeaperture, low-dispersion SWP spectra obtained while AM Her was in or near its low state are tabulated in Table 1. The table lists (1) the SWP number, (2) the date of observation, (3) the exposure time in seconds, (4) the maximum counts (DN) in the continuum, (5) the background counts (DN), and (6) the time elapsed, in days, since the end of the last high state.
MAGNETIC WHITE DWARF IN AM HERCULIS 979 TABLE 1 IUE OBSERVING LOG Exposure Time Maximum Counts Background Counts Time PostÈHigh State Frame Date (s) (DN) (DN) (days) (1) (2) (3) (4) (5) (6) SWP 09343... 1980 Jun 22 5399.6 141 25 20 SWP 09403... 1980 Jun 30 3599.8 0 21 20 SWP 09404... 1980 Jun 30 3599.8 75 22 20 SWP 09405... 1980 Jun 30 3599.8 45 22 20 SWP 09406... 1980 Jun 30 3599.8 63 32 20 SWP 10235... 1980 Sep 28 2399.7 0 19 108 SWP 10236... 1980 Sep 28 4799.4 78 26 108 SWP 21437... 1983 Nov 03 1919.7 37 19 35 SWP 21438... 1983 Nov 03 1799.7 35 18 35 SWP 21439... 1983 Nov 03 1799.7 36 19 35 SWP 21440... 1983 Nov 03 5999.7 78 28 35 SWP 21441... 1983 Nov 03 2339.5 38 20 35 SWP 39670... 1990 Sep 21 2099.5 58 18 100 SWP 39671... 1990 Sep 21 4199.5 73 21 100 SWP 39672... 1990 Sep 21 3599.8 70 20 100 SWP 44841... 1992 Jun 03 1619.8 37 18 300 SWP 44842... 1992 Jun 03 1619.8 37 18 300 SWP 44843... 1992 Jun 03 1619.8 46 20 300 SWP 44844... 1992 Jun 03 1619.8 46 23 300 SWP 44845... 1992 Jun 03 1739.8 55 31 300 SWP 44846... 1992 Jun 03 1619.8 64 38 300 SWP 44847... 1992 Jun 03 1739.8 73 39 300 SWP 44848... 1992 Jun 03 1739.8 46 24 300 SWP 44849... 1992 Jun 03 1619.8 39 19 300 SWP 44850... 1992 Jun 03 1499.8 44 18 300 The spectra are generally of good quality. We eliminated from further consideration those spectra Ñagged by Ga nsicke (1997) as being of poor quality. The spectra have a varied range of appearance, mostly depending on when the accretion spot is in view. This a ects the continuum greatly, and so we see large Ñux variations among data sets obtained within the same time period, depending on orbital phase. We noted a number of peculiarities that arise within each set and are common to most sets. For instance, there are sharp absorption lines at 1580 and 1600 A for almost every spectrum observed. There are interesting characteristics, speciðcally three absorption lines in a row, in each spectrum at the Lya turnover. The spectra seem to have some very odd characteristics, including what appear to be multicomponent absorptions resembling Zeeman subcomponents. However, it is unlikely that Zeeman components in the metal lines have been resolved with low-resolution IUE observations. Some spectra also show some emission lines as well. This is most likely the result of some residual accretion still taking place on the white dwarf. SWP 09343 seems to have been taken just at the tail end of a high state; it shows some emission and a steep continuum. The SWP 940X series of spectra all show some emission lines. A very interesting characteristic of the UV minimum spectra is that we still see signs of absorption, even though the accreting pole is out of view, giving some indication as to the dispersion of material on the white dwarf itself. Further discussion of absorption features is given in 2. Before we proceeded with the analysis, we applied a newly derived IUE Ñux-calibration correction. For lowdispersion IUE spectra from the Ðnal archive ÏÏ (NEWSIPS), Massa & Fitzpatrick (2000) have shown that the absolute Ñux-calibration of the NEWSIPS lowdispersion data was inconsistent with its reference model and subject to time-dependent systematic e ects; together, these distortions total as much as 10%È15% of the Ñux. Therefore, in order to correct the data and optimize the signal-to-noise ratio, we used the interactive data language programs that apply Massa-Fitzpatrick corrections to the low-dispersion IUE data. 2. SYNTHETIC SPECTRAL FITTING Theoretical spectra were computed using the model atmosphere code TLUSTY (Hubeny 1988) to calculate the atmospheric structure and SYNSPEC (Hubeny & Lanz 1995) to generate synthetic line proðles for a range of e ective temperatures, gravities, and abundances. We adopted the following procedure for the error bars. We took the error dispersion as p \ 0.1F ] (5 ] 10~15). (1) j j This error bar is taken to be 10% of the Ñux, but we added a small quantity of Ñux, 5 ] 10~15, to avoid assigning too much weight to the low Ñux levels. For our synthetic spectral Ðtting of the IUE spectra of AM Her, we constructed models convolved with the IUE low-dispersion spectral resolution (see Table 2). For our model Ðtting we used a s2 minimization routine that yields the scale factor and s2-value. In preparation for the spectral Ðtting, any signiðcant emission lines were masked out (e.g., geocoronal Lya and C IV j1550). The spectra were individually Ðtted with models at Ðxed gravity log g \ 8, covering a range of variation of T from 20,000 to 28,000 K in increments of 1000 K and variation of chemical abundances (relative to solar) of 0.001, 0.01, 0.05, 0.1, 0.2, 0.5, and 1.0.
980 DEPASQUALE & SION Vol. 557 TABLE 2 LOW-STATE T AND CHEMICAL ABUNDANCES T Abundances SWP Spectrum Phase (K) (] Solar) (1) (2) (3) (4) 09343... 0.69 25,000 0.01 09404... 0.39 22,000 0.01 09405... 0.03 21,000 0.001 10236... 0.45 23,000 0.05 21439... 0.94 20,000 0.001 21440... 0.46 23,000 0.01 39670... 0.61 24,000 0.001 39671... 0.12 21,000 0.001 44841... 0.97 20,000 0.001 44842... 0.31 21,000 0.01 44844... 0.87 21,000 0.001 44845... 0.15 20,000 0.001 44846... 0.43 21,000 0.001 44847... 0.71 22,000 0.001 44849... 0.27 20,000 0.001 44850... 0.54 23,000 0.01 The best-ðtting result for each SWP spectrum is given in Table 2, where column (1) lists the SWP spectrum number, column (2) the phase of accretion pole visibility, column (3) the derived best-ðt surface temperature, and column (4) the derived best-ðt metal abundance. As a check on the accuracy of the best-ðt T and metal- abundance results from our two-parameter (T, metal abundance Z) model grid, we performed error analyses of our Ðts to SWP 09343. First, we carried out an error analysis assuming a 10% error in the Ñuxes, as described above. Our estimates of the 3 p values of the two-parameter Ðtting of SWP 09343 are T \ 25,000 `600 K and a metal wd ~1000 abundance less than 0.3 times solar. The metal abundance is therefore signiðcantly less than 0.3 times solar at the 3 p conðdence level. Second, we carried out an error analysis using the errors associated with the IUE NEWSIPS Ñuxes. We determined that the 3 p values from our two-parameter Ðtting of SWP 09343 are T \ 24,000 `1100 K and abun- wd ~400 dances for all metals less than 0.07 times solar. Hence, the metal abundance is signiðcantly less than 7% of solar at the 3 p conðdence level. In Figures 1, 2, 3, 4, and 5, we display representative Ðts for spectra at di erent orbital phases, where UV minimum and maximum correspond to phase 0.0 (line of sight perpendicular to the Ðeld) and phase 0.68 (accretion pole facing the observer), respectively. Between these two extremes we see that the temperature varies between 20,000 K at phase 0.0 and 25,000 K at phase 0.65. This di erence of 5000 K is almost certainly due to the residual heat retained at the poles after the white dwarf polar regions cool rapidly following the onset of the low state. Note the observed weak metal absorptions due to the Si II doublet (jj1260, 1265), present in all of the spectra, C II j1335 appearing weakly in all Ðve displayed spectra, weak Si III j1300 absorption, present in all of the spectra except SWP 44850, where it appears to be replaced by weak emission, and persistent weak absorption in two regions, around 1320 A and between 1400 and 1420 A, in all Ðve of the spectra displayed. The latter two weak features cannot be identiðed with any certainty but could be contributed to by a blend of C II jj1323.906 and 1323.951 and an Si III transition at 1417 A. FIG. 1.ÈRepresentative IUE SWP spectrum, SWP 09404 (Ñux F vs. j wavelength A), obtained during the low state of AM Her about halfway between UV maximum (when the accretion spot is pointed at the observer) and UV minimum (when the spot is perpendicular to the observer). The best-ðt model atmosphere model is shown with T \ 22,000 K and metal abundance 0.01 times solar. The spectrum was obtained 20 days after the high state ended. Using our derived metal abundances for the magnetic white dwarf during the low state, an accretion rate can be estimated as follows. Because the rate of accretion is expected to be very low during the low state, an equilibrium would likely be established between accretion and di usion. This will occur if the replacement timescale of the accreted atmosphere is longer than the di usion timescale. For this equilibrium M0 (Z/H) \ Aov d (Z/H) ] M0 (Z/H), (2) where M0 is the accretion rate, A is the area of the magnetic accretion caps, o is the gas density at Rosseland mean optical depth of 2 is the di usion velocity, and is 3, v (Z/H) d FIG. 2.ÈSame as Fig. 1, but for SWP 9343, obtained at UV maximum. The best-ðt model atmosphere model is shown with T \ 25,000 K.
No. 2, 2001 MAGNETIC WHITE DWARF IN AM HERCULIS 981 FIG. 3.ÈSame as Fig. 1, but for SWP 10236, obtained more than halfway between the UV maximum and the UV minimum. The best-ðt model atmosphere is shown with T \ 23,000 K and metal abundance 0.05 times solar. The spectrum was obtained 108 days after the high state ended. FIG. 5.ÈSame as Fig. 1, but for SWP 44850, obtained near UV maximum. The best-ðt model atmosphere is shown with T \ 23,000 K and metal abundance 0.01 times solar. The spectrum was obtained 300 days after the high state ended. the assumed solar metal abundance of the accreting matter. Taking M \ 0.6 M, o \ 1.32 ] 10~7 gcm~3, a di u- wd sion velocity (for Si) v \ 1.58 ] 10~10 cm s~1, and assum- d ing the fractional areas of the accretion poles are D0.1, we Ðnd that M0 \ 2.2 ] 10~19 M yr~1. This extremely low rate of accretion indicates that, essentially, accretion has nearly shut o during the low state. The rate is many orders FIG. 4.ÈSame as Fig. 1, but for SWP 39671, obtained at UV minimum. The best-ðt model atmosphere is shown with T \ 21,000 K and metal abundance 0.001 times solar. The spectrum was obtained 100 days after the high state ended. of magnitude lower than the rate 10~11 M yr~1 implied by the integrated state X-ray Ñux. Either our assumption of accretion-di usion equilibrium in the above calculation is incorrect, or the low-state X-ray emission originates from a source other than accretion. 3. SUMMARY OF RESULTS We have analyzed all the archival IUE far-uv spectra of the magnetic cataclysmic variable AM Her during its lowbrightness states, when line emission has weakened or disappeared and thermal bremsstrahlung no longer contributes signiðcantly to the far-uv continuum. We have carried out a synthetic spectral analysis of the exposed magnetic white dwarf photosphere. We have determined the surface temperature and metal abundance of the white dwarf from 22 individual spectra, obtained at many di erent pole-visibility (orbital) phases and at varying times since the end of the last high state of the system. Our principal results are as follows: 1. The T of the white dwarf varies between 20,000 K at phase 0.0, when the accretion pole is perpendicular to the observer, and 25,000 K, when the accretion pole is facing the observer. This result implies an equator-to-pole meridional temperature gradient corresponding to roughly 5000 K during the low state. However, we regard this gradient as being only a lower limit to the actual pole-to-equator gradient, because we are deriving a temperature that is an average value over the entire stellar disk. 2. We Ðnd no correlation between the T and the time since the end of the last high state. This is likely due to the fact that the white dwarf accretion cap cools very quickly because of relatively shallow hard X-ray heating. The higher temperature of the accretion pole may be due primarily to compressional heating by the weight of the gas added at the polar regions. This would require a longer time to radiate away (Sion 1995). An anonymous referee noted a possible correlation involving a decline in the T of the accretor at 20, 100, and 300 days postèhigh state. However,
982 DEPASQUALE & SION we doubt the reality of this correlation because of uncertainty in our T -values and the fact that the T sampling involves three di erent preceding high states rather than the same high state. 3. The metal abundances of the white dwarf from spectrum to spectrum ranges from 0.05 to 0.001 times solar. We Ðnd no correlation between the metal abundance and the time since the onset of the low state. These abundances represent the Ðrst ever derived for an accreting magnetic white dwarf in a cataclysmic variable. 4. Using our derived metal abundances we have estimated the accretion rate during the low state. If the rate of accretion is low enough that an equilibrium is established between accretion and di usion, then we Ðnd the accretion rate to be 2 ] 10~19 M yr~1. These preliminary results indicate that there is no correlation between the time since the last high state and either the surface temperature or the chemical abundance and that the metal abundances do not appear to be signiðcantly di erent between spectra obtained at UV maximum and UV minimum. The range in temperatures is dependent on the phase at the time of observations. If the accreting pole is in view, the temperature will be slightly higher, because this is the UV maximum. It seems, from this limited sample of spectra, that the metal abundance of the white dwarf photosphere is almost uniform. This can be explained through a process of sideways di usion and spreading of infalling materials from the accreting polar cap to the rest of the stellar surface. We thank an anonymous referee for helpful comments. This research was supported in part by NSF grant 99-01195, by NASA ADP grant NAG 5-8388, and by summer research funding from the NASA-Delaware Space Grant Colleges Consortium. REFERENCES Ga nsicke, B. T. 1997, Ph.D. thesis, Univ. Go ttingen Massa, D., & Fitzpatrick, E. L. 2000, ApJS, 126, 517 Ga nsicke, B. T., Beuermann, K., & de Martino, D. 1995, A&A, 303, 127 Sion, E. M. 1999, PASP, 111, 532 Heise, J., & Verbunt, F. 1988, A&A, 189, 112 ÈÈÈ. 1995, ApJ, 438, 876 Hubeny, I. 1988, Comput. Phys. Commun., 52, 103 Young, P., Schneider, D. P., & Shectman, S. A. 1981, ApJ, 245, 1043 Hubeny, I., & Lanz, T. 1995, ApJ, 439, 875