Mathematics – Logic
Scientific paper
Jan 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt.......116s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF TORONTO (CANADA), 1990.Source: Dissertation Abstracts International, Volume: 53-02, Section: B, pa
Mathematics
Logic
3
Helium I Lambda 10830 Line
Scientific paper
Helium is a key element to the understanding of many astrophysical and cosmological processes, yet, despite the large abundance of helium, its spectrum is not readily present in most stars. In stars cooler than spectral type B helium produces only chromospheric features in the near infrared (He I lambda 10830), the ultraviolet (He II lambda 1640), and the extreme ultraviolet, however usually the He I lambda 10830 line is the only one strong enough to detect or technically feasible to observe. In addition, the experience with the Sun indicates a complex mechanism of line formation, hence few, if any, detailed model calculations have been successful. The regularly pulsating atmospheres of classical Cepheids provide a unique laboratory to study the formation of the helium spectrum in cool stars. We present high resolution high S/N spectroscopy of seven Cepheids at 1.08 μm with the FTS at the Canada-France-Hawaii telescope. Three Cepheids, X Sgr, eta Aql, and zeta Gem, were observed with full phase coverage. We report the first certain detection of the He I lambda 10830 line in Cepheids. The He I lambda 10830 absorption is present at all times. Its velocity variation lags behind the photospheric lines and the cores of Ca II K and Hα . The strength of the He I lambda 10830 line is comparable to the strength of the He I lambda 10830 line in non-variable yellow supergiants on the average, and appears to depend on T_{rm eff}. We find that the He I lambda 10830 absorption is systematically blueshifted in at least four Cepheids, indicating a steady outflow in the upper chromosphere. In the analysis we apply an iterative procedure between a hydrodynamic code and a version of the non-LTE radiation transfer code MULTI. We use model atoms of H, Ca II, Mg II, and a specially constructed helium atom with 15 levels and both stages of ionization. The static model is very successful in reproducing the helium spectrum of the Sun and beta Gem. Our time-dependent modeling of pulsating Cepheid atmospheres offers additional physical insight into the results already present in the observational data. Concerning Cepheid chromospheres, we conclude that Cepheids should have chromospheres heated by acoustic or magnetic wave dissipation, in addition to the pulsation. The existence of hot coronae around Cepheids is unlikely, as judged by the height of formation of the He I lambda 10830 line in Cepheid atmospheres. Concerning the mechanism of formation of the He I lambda 10830 line, we conclude that the chromospheres of luminous stars favor collisional excitation over photoionization-recombination (due to X -ray back-radiation) to overpopulate the 2^3 S level, except for stars with very strong coronae. The flux from the wings of the Lyalpha He II lambda 304 line may play an important role for the He I lambda 10830 line in stars with less prominent coronae or none at all. Once the 2^3S level is overpopulated, the He I lambda 10830 line is formed by scattering photospheric photons (hence appears in absorption projected on the disk), a process so efficient that geometrical effects of atmospheric extension are needed to explain the He I lambda 10830 emission observed in zeta Gem.
Sasselov Dimitar Dimitrov
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