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Figure 1. Visual brightness changes observed in ρ Cas between 1993 and 2003 (upper panel) are compared to changes (lower panel) observed from an unblended photospheric absorption line of Fe i λ5572. Fast atmospheric expansion in late 1999 and early 2000 precedes the deep visual outburst minimum of summer 2000. Continuous high- resolution spectroscopic observations are marked with short vertical lines in the lower panel. A prominent emission line spectrum is observed during phases of fast atmospheric expansion in Dec. ’93 and Oct. ’98 (labeled 1 and 2), and in Jan. ’02 and July ’04 (see Fig. 2). a P Cygni profile (Fig. 3). The P Cygni line shape of Hα is very remarkable because it was not clearly observed during the 2000 brightness minimum. It indicates a new strong expansion of the upper atmosphere. We currently measure expansion velocities up to 120 km s−1 for the Hα atmosphere. Our very recent high-resolution spectroscopic observations of April 2004 indicate that the P Cygni line shape further strengthens, resulting in a new and excep- tional variability phase of enhanced mass-loss. This spec- tral evolution is expected since we observe that the radial velocity curve of the absorption portion of Hα is at least twice longer than the (quasi-) period of photospheric ab- sorption lines with P ∼300-500 d (Lobel 2001b).

Figure 5 shows dynamic spectra of Hα and Fe i λ5572 observed between 1993 and late 2003. The white spots in Hα are emission above the stellar continuum level. The ra- Figure 2. Spectral changes observed in ρ Cas since mid dial velocity curves of the Hα absorption core and the pho- 2004 reveal prominent atomic emission lines in the opti- tospheric Fe i lines (white dashed lines) reveal a velocity cal and near-IR spectrum. The Ni i λ6108 line transforms stratified dynamic atmosphere. Notice the large blueshift from absorption into emission above the level of the stellar of the Fe i lines during the outburst of mid 2000. The continuum during phases of fast atmospheric expansion in outburst is preceded by enhanced emission in the short Fig. 1. Similar emission features appear in the central core wavelength wing of Hα, while the Fe i line shifts far red- of photospheric absorption lines with small lower excita- ward. A strong collapse of the upper Hα atmosphere and tion energy as Fe ii λ6113, causing recurrent double (or the lower photosphere precedes the outburst event during split) absorption line cores. the pre-outburst cycle of 1999 (Lobel et al. 2003c) (see Fig. 1). 3

Figure 3. Line profile changes of Hα in ρ Cas reveal that Figure 4. FUSE observations show that high-temperature following the outburst event of 2000 the upper atmosphere emission lines of O vi λ1032 & λ1037 are neither observed collapsed over a period of ∼2.5 years producing a strong in yellow hypergiant ρ Cas (upper panel), nor in red super- inverse P Cygni profile. Hα transformed into a P Cygni giant α Ori (middle panel). The lines are however clearly profile since mid 2003, presently showing highly supersonic observed in the smaller supergiant β Dor atmospheric expansion comparable to the 2000 outburst. (lower panel). The vertical line is drawn at the stellar rest velocity of −47 kms−1. panion star in ρ Cas, also ruled out by our long-term radial velocity and light curve monitoring. However, we 3. Far-UV, Optical, and Near-IR Emission Lines clearly observe these warm transition region plasma emis- sion lines in the classical Cepheid variable β Dor (F-G The optical and near-IR spectrum of ρ Cas (Ia0e) only Iab-Ia). Its smaller radius of R∗=64 R⊙, compared to ρ sporadically reveals prominent emission lines. The emis- Cas (R∗≃400-500 R⊙ with log(g)=0.5-1.0), signals that sion lines are due to neutral and singly ionized atoms. the larger average atmospheric gravity acceleration is an In a spectrum of ρ Cas we observed on 29 July 2004, important stellar parameter to sustain high-temperature many exceptionally strong emission lines with low exci- plasmas in the outer atmosphere of less luminous (pulsat- tation potentials have re-appeared. The lines have only ing) supergiants. This is supported by the observation that been observed in emission during previous phases of very transition region plamsa emission lines are also absent in fast atmospheric expansion in late ’93 (Lobel 1997), fall red supergiant α Ori with R∗=700 R⊙ and log(g)=−0.5. 1998, and early 2002. Figure 2 shows how the Ni i λ6108 line transformed from absorption into emission with the 4. Conclusions fast expansion of the upper atmosphere over the past half year. The line appears however in absorption during pulsa- We present new recent spectral observations of the yellow tion phases of photospheric collapse when Teff increases. hypergiant ρ Cas that reveal enhanced dynamic activity of The formation of these rare emission lines therefore ap- its pulsating atmosphere during the past three years after pears linked with an excitation phenomenon that can re- the outburst event of 2000. A strong inverse Balmer Hα P sult from supersonic wind expansion during exceptional Cygni profile that transformed into a P Cygni profile, cur- −1 variability phases. A possible interpretation is that the rently signals supersonic wind expansion up to 120 km s emission spectrum is excited by a propagating circumstel- of the upper atmosphere with enhanced mass-loss. During lar shock wave passing through the line formation region the fast atmospheric expansion we observe the return of a in the extended outer atmosphere (de Jager et al. 1997). prominent neutral and singly ionized atomic emission line The optical and near-IR emission line spectrum of ρ spectrum in the optical and near-IR. Very recent obser- Cas is currently under investigation together with the far- vations of the far-UV spectrum with FUSE demonstrate UV spectrum observed with FUSE on 27 July ’04. The the absence of high ion emission lines due to warm tran- FUSE spectrum does not reveal the high temperature sition region plasma in the extended dynamic atmosphere emission lines of O vi λ1032 & λ1037 (Fig 4), or the C iii of the hypergiant. λ977 line. Our far-UV observations also demonstrate that Acknowledgements there is no evidence of a faint hot (e.g. ) com- This research is based in part on data obtained by the NASA- 4

Figure 5. Dynamic spectra of Hα (panel left) and Fe i λ5572 (panel right). The line profiles are linearly interpolated between consecutive observation nights in the past decade, marked by the left-hand tickmarks. Notice the strong blue- shift of the Fe i line during the outburst of mid 2000. The outburst is preceded by very strong emission (white spots) in the short-wavelength wing of Hα, while the absorption core extends longward, and the photospheric Fe i line strongly red-shifts. A strong collapse of the upper and lower atmosphere precedes the outburst. Fast expansion of the upper Hα atmosphere is also observed over the past year.

CNES-CSA FUSE mission operated by the Johns Hopkins Lobel, A. 2001a, ApJ, 558, 780 University. Financial support has been provided by NASA FUSE Lobel, A. 2004, Mercury Magazine, ASP Publ., 33, 13 grants GI-D107 and GI-E068. Lobel, A., de Jager, C., Nieuwenhuijzen, H., Smolinski, J., & Gesicki, K. 1994, A&A, 291, 226 References Lobel, A., Dupree, A., Stefanik, R., Torres, G., Israelian, G., Morisson, N.R., de Jager , C., Nieuwenhuijzen, H., Ilyin, I., de Jager, C., Lobel, A., & Israelian, G. 1997, A&A, 325, 714 & Musaev, F. 2003a, ApJ, 583, 923 de Jager, C., Lobel, A., Nieuwenhuijzen, H., & Stothers, R. Lobel, A., Israelian, G., de Jager, C., Musaev, F., Parker, J. 2001, MNRAS, 327, 452 Wm., Mavrogiorgou, A. 1998, A&A, 330, 659 Lobel, A. 2001b, Festschrift, Astro-ph No. 0108358, Two Lobel, A., Dupree, A. K., Stefanik, R. P., Torres, Decades of Hypergiant Research G., Israelian, G., Morrison, N., Ilyin, I., de Jager, Lobel, A. 1997, Pulsation and Atmospherical Instability of Lu- C., & Nieuwenhuijzen, H. 2002, CfA Press Re- minous F- and G-type Stars; The Yellow Hypergiant ρ Cas- lease, No. 03-02, Hypergiant Erupts, online at siopeiae, Shaker Publ., Maastricht, The Netherlands, ISBN cfa-www.harvard.edu/∼alobel/pressrel/rhocas.html, 90-423-0014-0, Chapter 4, p. 43 based on BAAS 2002, 201, 4909 5

Lobel, A., Dupree, A. K., Stefanik, R. P., Torres, G., Israelian, G., Morrison, N., de Jager, C., Nieuwenhuijzen, H., Ilyin, I, & Musaev, F. 2003b, ASP Conf. Proc., IAU Symp. 210, F10, San Francisco Lobel, A., Stefanik, R. P., Torres, G., Davis, R. J., Ilyin, I., & Rosenbush, A. E. 2003c, Stars as : Activity, Evolu- tion and , Proc. IAU, Symp. No. 219, 218, Sydney, Australia