Using precisely controlled laboratory conditions we have begun to establish a spectral catalogue of the intermediate ionization states of iron, Fe IX - Fe XXIV, in the extreme ultraviolet. The measurements are being performed in support of the development of reliable modeling codes for the analysis of data from the Extreme Ultraviolet Explorer and future space astrophysics missions sensitive to extreme ultraviolet radiation. They aim to resolve the controversies surrounding the short-wavelength spectra of stellar coronae. Preliminary measurements showing the wealth of iron lines in the 50-120 {Angstrom} region are presented

Beiersdorfer, P.

Brown, G. V.

Utter, S. B.

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May 13, 1998This is a preprint of a paper intended for publication in a journal or proceedings.  Sincechanges may be made before publication, this preprint is made available with theunderstanding that it will not be cited or reproduced without the permission of theauthor.LawrenceLivermoreNationalLaboratoryUCRL-JC-130832PREPRINTSpectral Catalogue of the Intermediate Ionization States ofIron in the Extreme UltravioletThis paper was prepared for submittal to theProceedings of the Laboratory Space Science WorkshopCambridge, MAApril 1-3, 1998P. Beiersdorfer, S. B. Utter, G. V. Brown, D. A. Liedahl, C. W. Mauche,S. M. Kahn, N. S. Brickhouse, A. K. Dupree, R. Mewe, J. S. KaastraDISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.Spectral Catalogue of the Intermediate Ionization States of Ironin the Extreme UltravioletP. Beiersdorfer, S. B. Utter, G. V. Brown, D. A. Liedahl, C. W. MaucheDepartment of Physics and Space TechnologyLawrence Livermore National Laboratory, Livermore, CA 94550, USAS. M. KahnDepartment of Physics, Columbia University, New York, NY 10027, USAN. S. Brickhouse, A. K. DupreeHarvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USAR. Mewe, J. S. KaastraSpace Research of the Netherlands, 3584 CA Utrecht, NetherlandsUsing precisely controlled laboratory conditions we have begun to establish a spectral cat-alogue of the intermediate ionization states of iron, Fe IX { Fe XXIV, in the extreme ul-traviolet. The measurements are being performed in support of the development of reliablemodeling codes for the analysis of data from the Extreme Ultraviolet Explorer and futurespace astrophysics missions sensitive to extreme ultraviolet radiation. They aim to resolvethe controversies surrounding the short-wavelength spectra of stellar coronae. Preliminarymeasurements showing the wealth of iron lines in the 50{120 A region are presented.1. IntroductionWith the launch of the Extreme Ultraviolet Explorer (EUVE) in 1992, the extremeultraviolet spectral region from 70 to 400 A has been opened to stellar astrophysicists forhigh-resolution exploration. EUVE observations have provided superb spectra of stellarcoronae. These are used to derive densities, abundances, and, very importantly, explicitinformation on the temperature structure of stellar coronae not provided by any other means(Dupree et al. 1993, Brown 1994, Vedder et al. 1994, Mewe et al. 1995, Schrijver et al. 1995).The tremendous diagnostic opportunities are the result of the wealth of emission lines thatmark the extreme ultraviolet spectral region. Apart from several important lines producedby carbon, nitrogen, oxygen, and neon, the extreme ultraviolet region contains rich lineemission from virtually all charge states of iron. In particular, the region contains the M-shell spectra, i.e., transitions of the type 3`-n`0, of ionization states Fe IX { Fe XVII, andthe L-shell spectra, i.e., transitions of the type 2s-2p, of the ionization states Fe XVIII { FeXXIV. These provide line diagnostics for a broad temperature range: the transition region(T  5 105 K), the \quiet" corona (T  1  2 106 K; e.g. Fe IX { Fe XII), active regions(T  2  5 106 K; e.g. Fe XIV { Fe XX), and ares (T  107 K; e.g. Fe XXI { Fe XXIV).The analysis of high-resolution spectra from EUVE is, however, not without problems.In fact, the spectra, especially those obtained in the short wavelength (SW) band, 70{180 A,cannot be tted properly without making controversial assumptions. The reason is that theintensity of the observed lines and that of the continuum level even of well behaved stars,such as the spectrum of the G-K binary  Cen cannot be tted in a consistent manner. Forexample, to obtain a t with the widely used spectral analysis computer code SPEX (Kaastraet al. 1996) required the unphysical assumption of a high-temperature tail on the dierentialemission measure (DEM). By making the assumptions that the metal abundances are wellbelow (solar) photospheric values this problem can be resolved. However, this would implya coronal abundance eect that is opposite to what is seen in the Sun. Another, yet morecontroversial resolution to the problem was oered by Schrijver, van den Oord, & Mewe(1994) and Mewe et al. (1995) who suggested that resonance scattering of the emissionlines may be the cause for the low line-to-continuum ratio. A nal scenario for tting thespectra was oered by Jordan (1996), Schmitt, Drake, & Stern (1996) and Drake, Laming,& Widing (1997), who argue that what appears to be a high continuum is the result of manylines missing in the spectral tting model. They argue that including such missing lines willlower the continuum emission necessary to t the SW spectrum and will provide a good twithout invoking opacity eects, high-temperature tails on the dierential emission measure,or metal abundance deciencies.To address the controversies surrounding the interpretation of the EUVE spectra wehave recently begun a systematic laboratory investigation to measure the emission lines fromthe intermediate charge states of iron between Fe IX and Fe XXIV in the extreme ultravioletregime. First results are presented below.2. Spectroscopic Facility and First ResultsThe measurements are being carried out using the spectroscopic capabilities at theLivermore Electron Beam Ion Trap facility. The EBIT is a modied electron beam ionsource built to study the interaction of highly charged ions with an electron beam by lookingdirectly into the trap. The device is described by Levine et al. (1988) and the spectroscopiccapabilities are described by Beiersdorfer et al. (1993,1994). The electron density for thepresent experiments is about 5 1011 cm 3, i.e., comparable to the density found in stellarcoronae. By setting the electron beam to a particular energy it is possible to select thecharge state of interest for spectroscopic study.Spectra are acquired with a at-eld spectrometer utilizing a variable line spacinggrating (average 1200 `/mm) with a focal distance of about 27 cm. The spectrometer utilizesa thinned, back-illuminated, LN2-cooled CCD camera (1024 x 1024 pixels) for the detectionof the dispersed photons. We use the CCD in the charge integration mode.We have begun to makemeasurements of iron emission lines by setting the spectrometerto the short wavelength portion of the spectral region of interest. A typical spectrum coveringthe region between 50 and 120 A is shown in Fig. 1. The spectrum was obtained by settingthe electron beam energy to 280 eV, i.e., an energy where iron charge states no higher thanFe XI are produced. For comparison, we show in Fig. 2 a spectrum recorded at a beamenergy of 330 eV. At this energy iron charge states no higher than Fe XIII are produced.The two spectra in Figs. 1 and 2 are clearly dierent. By systematically recording050010001500200050 60 70 80 90 100 110 120Intensity (Arb. U.)Wavelength (Å)Figure 1: Line emission spectrum of iron in the extreme ultraviolet measuredat the Livermore spectroscopy facility. The electron beam energy was 280 eV.Only charge states as high as Fe XI were produced and excited.04008001200160020002400280050 60 70 80 90 100 110 120Intensity (Arb. U.)Wavelength (Å)Figure 2: Line emission spectrum of iron in the extreme ultraviolet measuredat the Livermore spectroscopy facility. The electron beam energy was 330 eV.Only charge states as high as Fe XIII were produced and excited.spectra at dierent energies we are able to identify which lines are produced by which chargestate of iron. This procedure is necessary because essentially none of the lines shown in theFigs. 1 and 2 have been observed and identied in previous measurements.3. ConclusionEven though our measurements have just begun, they already show that the extremeultraviolet wavelength band indeed contains numerous unidentied emission lines. Figures1,2 show rich spectra comprised of many blended lines that almost look like continuumemission. This provides evidence for the contention that missing lines could be the reasonwhy it is so dicult to t EUVE SW spectra. Indeed, comparing these with an EUVESW spectrum from SS Cygni (Mauche, Raymond, & Mattei 1995) corrected for interstellarabsorption show striking similarities. This suggests that the laboratory measurements mighttruly play a key role in understanding the astrophysical data. Spectra in the 200-A regionimportant for cool stars will be studied soon as well.References:Beiersdorfer, P. et al. 1993, in UV and X-Ray Spectroscopy of Astrophysical and Laboratory Plas-mas, ed. by E. Silver & S. M. Kahn (Cambridge: Cambridge Univ. Press), 59-68Beiersdorfer, et al. 1994, in 9th APS Conference on Atomic Processes in Plasmas, AIP Conf. Proc.No. 322, ed. by W. L. Rowan (AIP, New York), 365.Brown, A. 1994, in Cool Stars, Stellar Systems, and the Sun, Eight Cambrigde Workshop, ed. byJ.-P. Caillault, ASP Conf. Series, 64, 23.Drake, J.J., Laming, J.M., & Widing, K.G. 1997, ApJ, 478, 403-416.Dupree, A.K., Brickhouse, N.S., Doschek, G.A., Green, J.C., & Raymond, J.C. 1993, ApJ (Letters),418, L41-L44.Jordan, C. 1996, in Astrophysics in the Extreme Ultraviolet, ed. by S. Bowyer and R. F. Malina(Kluwer, Dordrecht), 81.Kaastra, J.S., Mewe, R., & Nieuwenhuijzen, H. 1996, in UV and X-Ray Spectroscopy of Astro-physical and Laboratory Plasmas, ed. by K. Yamashita and T. Watanabe (Universal, Tokyo),411.Levine, M.A., Marrs, R.E., Henderson, J.R., Knapp, D.A., & Schneider, M.B. 1988, Phys. Scr.T22, 157.Mauche, C. W., Raymond, J. C. & Mattei, J. A. 1995, ApJ, 446, 842.Mewe, R., Kaastra, J.S., Schrijver, C.J., van den Oord, G.H.J., & Alkemade, F.J.M. 1995, A&AS,296, 477.Schmitt, J.H.M.M., Drake, J.J., & Stern, R.A. 1996, ApJ (Letters), 465, L51-L54Schrijver, C.J., Van den Oord, G.H.J., & Mewe, R. 1994, A&A, 289, L23.Schrijver, C.J., Van den Oord, G.H.J., Mewe, R., & Kaastra, J.S. 1995, A&A, 302, 438.Vedder, P. W.et al. 1994, in Cool Stars, Stellar Systems, and the Sun, Eight Cambrigde Workshop,ed. by J.-P. Caillault, ASP Conf. Series, 64, 13.Acknowledgments:This work was supported by the National Aeronautics and Space Administration Ultraviolet, Vis-ible, and Gravitational Astrophysics Research and Analysis Program under grant NAG5-6731 andperformed under the auspices of the Department of Energy by Lawrence Livermore National Lab-oratory under contract W-7405-ENG-48.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Spectral catalogue of the intermediate ionization states of iron in the extreme ultraviolet

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Spectral catalogue of the intermediate ionization states of iron in the extreme ultraviolet

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