University of Kentucky UKnowledge Physics and Astronomy Faculty Publications Physics and Astronomy 11-28-2015 Ultraviolet Emission Lines of Si II in Cool Star and Solar Spectra Sibasish Laha Queen’s University Belfast, UK Francis P. Keenan Queen’s University Belfast, UK Gary J. Ferland University of Kentucky, [email protected] Catherine A. Ramsbottom Queen’s University Belfast, UK Kanti M. Aggarwal Queen’s University Belfast, UK See next page for additional authors Right click to open a feedback form in a new tab to let us know how this document benefits oy u. Follow this and additional works at: https://uknowledge.uky.edu/physastron_facpub Part of the Astrophysics and Astronomy Commons, and the Physics Commons Repository Citation Laha, Sibasish; Keenan, Francis P.; Ferland, Gary J.; Ramsbottom, Catherine A.; Aggarwal, Kanti M.; Ayres, Thomas R.; Chatzikos, Marios; van Hoof, Peter A. M.; and Williams, Robin J. R., "Ultraviolet Emission Lines of Si II in Cool Star and Solar Spectra" (2015). Physics and Astronomy Faculty Publications. 458. https://uknowledge.uky.edu/physastron_facpub/458 This Article is brought to you for free and open access by the Physics and Astronomy at UKnowledge. It has been accepted for inclusion in Physics and Astronomy Faculty Publications by an authorized administrator of UKnowledge. For more information, please contact [email protected]. Authors Sibasish Laha, Francis P. Keenan, Gary J. Ferland, Catherine A. Ramsbottom, Kanti M. Aggarwal, Thomas R. Ayres, Marios Chatzikos, Peter A. M. van Hoof, and Robin J. R. Williams Ultraviolet Emission Lines of Si II in Cool Star and Solar Spectra Notes/Citation Information Published in Monthly Notices of the Royal Astronomical Society, v. 455, issue 3, p. 3405-3412. This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. The opc yright holders have granted the permission for posting the article here. Digital Object Identifier (DOI) https://doi.org/10.1093/mnras/stv2566 This article is available at UKnowledge: https://uknowledge.uky.edu/physastron_facpub/458 MNRAS 455, 3405–3412 (2016) doi:10.1093/mnras/stv2566 Ultraviolet emission lines of Si II in cool star and solar spectra Sibasish Laha,1‹ Francis P. Keenan,2‹ Gary J. Ferland,3 Catherine A. Ramsbottom,1 Kanti M. Aggarwal,2 Thomas R. Ayres,4 Marios Chatzikos,3 PeterA.M.vanHoof5 and Robin J. R. Williams6 1 Centre for Theoretical Atomic, Molecular and Optical Physics, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, UK 2Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, UK 3Department of Physics and Astronomy, The University of Kentucky, Lexington, KY 40506, USA 4Center for Astrophysics and Space Astronomy, 389 UCB, University of Colorado, Boulder, CO 80309, USA 5Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium 6AWE plc, Aldermaston, Reading RG7 4PR, UK Accepted 2015 October 29. Received 2015 October 27; in original form 2015 September 16 ABSTRACT Recent atomic physics calculations for Si II are employed within the CLOUDY modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, β Gemino- rum, α Centauri A and B, as well as previously published HST/GHRS observations of α Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepan- cies found previously between theory and observation for line intensity ratios involving the 2 2 24 3s 3p PJ–3s3p PJ intercombination multiplet of Si II at ∼ 2335 Å are significantly reduced, 2 2 22 as are those for ratios containing the 3s 3p PJ–3s3p DJ transitions at ∼1816 Å. This is primarily due to the effect of the new Si II transition probabilities. However, these atomic data are not only very different from previous calculations, but also show large disagreements with measurements, specifically those of Calamai et al. for the intercombination lines. New measurements of transition probabilities for Si II are hence urgently required to confirm (or otherwise) the accuracy of the recently calculated values. If the new calculations are con- firmed, then a long-standing discrepancy between theory and observation will have finally been resolved. However, if the older measurements are found to be correct, then the agreement between theory and observation is simply a coincidence and the existing discrepancies remain. Key words: atomic processes – Sun: UV radiation – stars: late-type. observations of several late-type stars. They noted that, in previous 1 INTRODUCTION studies, observed line ratios involving these lines lay well outside Emission lines involving transitions in Si II are important diagnos- the range of theoretical values, and indeed beyond the high-density tics for astrophysical plasmas, with several intensity ratios being limit. This limit depends on the transition probabilities rather than sensitive to temperature and density variations (Dufton & Kingston the collisional rates, and hence Dufton et al. (1991) suggested 1985; Judge, Carpenter & Harper 1991; Keenan et al. 1992). Over that the discrepancies must be due to errors in the former. the past 40 years there have been many calculations of Si II tran- Although the transition probabilities calculated by Dufton et al. sition probabilities and electron impact excitation rates by several improved agreement between theory and observation, significant authors (Nussbaumer 1977; Dufton & Kingston 1991; Nahar 1998; discrepancies remained. Tayal 2008), which have been subsequently employed in modelling Similarly, Judge et al. (1991) undertook an observational study codes to predict theoretical line intensities for different types of of Si II emission line intensity ratios in a range of astronomical plasma. However, discrepancies between observation and theory sources, including the Sun, several late-type stars and the slow nova found in early studies of the Sun and other late-type stars remain RR Telescopii. In particular, they analysed a high signal-to-noise, unresolved. For example, Dufton et al. (1991) investigated the 3s23p high-resolution spectrum of α Tau, obtained with the Goddard High 2 24 PJ–3s3p PJ intercombination multiplet at ∼2335 Å in the Sky- Resolution Spectrograph (GHRS) on the Hubble Space Telescope lab spectra of the Sun and International Ultraviolet Explorer (IUE) (HST). These authors once again found discrepancies between the- ory and observation for line ratios involving the 2335 Å multiplet, which they attributed in part to blending of the Si II lines with Ni II E-mail: [email protected] (SL); [email protected] (FPK) transitions. However, they also suggested that the discrepancies may C 2015 The Authors Downloaded fromPublished https://academic.oup.com/mnras/article-abstract/455/3/3405/2892765 by Oxford University Press on behalf of the Royal Astronomical Society by University of Kentucky Libraries user on 21 November 2017 3406 S. Laha et al. Table 1. Emission lines studied in this work.a expt N AK T AK Lower level Upper level Wavelength (Å) Aij Aij Aij ECSij ECSij −1 −1 −1 (i)(j) (NIST) (s )(s)(s)(Te = 7000 K) (Te = 7000 K) 2 2 2 2 3s 3p P1/2 3s 3d D3/2 1260.42 – 2.60E+9 2.01E+9 3.42 2.31 2 2 2 2 3s 3p P3/2 3s 3d D5/2 1264.73 – 3.04E+9 2.31E+9 6.76 4.71 2 2 2 2 3s 3p P3/2 3s 3d D3/2 1265.02 – 4.63E+8 5.23E+8 1.85 1.37 2 2 22 3s 3p P1/2 3s3p S1/2 1304.37 – 3.64E+8 3.60E+8 1.00 0.89 2 2 22 3s 3p P3/2 3s3p S1/2 1309.28 – 6.23E+8 6.60E+8 1.98 1.79 2 2 2 2 3s 3p P1/2 3s 4s S1/2 1526.70 – 3.81E+8 3.90E+8 1.06 1.21 2 2 2 2 3s 3p P3/2 3s 4s S1/2 1533.40 – 7.52E+8 7.90E+8 2.13 2.43 2 2 22 3s 3p P1/2 3s3p D3/2 1808.01 – 2.54E+6 1.00E+5 2.77 1.91 2 2 22 3s 3p P3/2 3s3p D5/2 1816.93 – 2.65E+6 2.00E+5 7.46 5.25 2 2 22 3s 3p P3/2 3s3p D3/2 1817.45 – 3.23E+5 5.30E+4 4.17 3.08 2 2 24 3s 3p P1/2 3s3p P3/2 2329.23 10 ± 50 23.5 11.1 0.80 0.75 2 2 24 3s 3p P1/2 3s3p P1/2 2335.12 5200 ± 19 5510 2296 0.51 0.47 2 2 24 3s 3p P3/2 3s3p P5/2 2335.32 2460 ± 8 2440 397 2.38 2.07 2 2 24 3s 3p P3/2 3s3p P3/2 2344.92 1220 ± 10 1310 157 1.05 1.04 2 2 24 3s 3p P3/2 3s3p P1/2 2350.89 4410 ± 21 4700 3078 0.31 0.40 Notes. aAexpt are the experimental A-values from Calamai et al. (1993); AN and ECST the A-values and ECS from Nahar (1998) and Tayal (2008), respectively; AAK the wavelength-corrected transition probabilities and ECSAK the ECS from Aggarwal & Keenan (2014). be partly due to errors in the adopted electron impact excitation We also study some previously published Si II measurements, rates, in contrast to the findings of Dufton et al. (1991). namely those for the quiet Sun obtained by the HRTS experiment In the present paper, we use recent calculations of atomic data for during a sounding rocket flight, which recorded the 1185–1730 Å Si II by Aggarwal & Keenan (2014) in the modelling code CLOUDY solar spectral region on photographic emulsion at a resolution of (Ferland et al.