The Spectrum of υ Sagittarii.
Abstract
The ultraviolet region of the spectrum of v Sgr, from X 3317 to X 3975, has been investigated on plates taken at the McDonald Observatory. The measured wave lengths of 400 lines, together with intensities and identifications, are given in Table i. The lines of hydrogen are very weak, while the lines of all series of helium are strong. The singly ionized metals are dominant, but a few weak lines of Fe i, Mg i, and Cr i are present. The Fe n spectrum is particularly strong and includes many new pre- dicted lines from high-level transitions. Table 2 lists the elements identified. No indi- cation of effects arising from dilution of the radiation can be found. The star is fairly massive and has an absolute magnitude near -~. A comparison of the metaffic spectrum with that of a Cyg suggests that the level of ionization is slightly higher in v Sgr. The most striking difference between these two supergiants is the extreme weakness of the hydrogen lines in v Sgr and the almost complete disappearance of the jump in intensity at the Balmer limit. An attempt has been made to determine the relative abundance of helium, hydrogen, and iron, on the basis of approximate theoretical predictions of line intensities. The effects of stratifica- tion are discussed. The equivalent widths given in Table 9 are used to determine the relative abundances given in Table 12. Estimates of f-values, together with the rela- tive intensities of the Fe I, Fe ii, and Fe iii lines, fix the level of ionization. A model atmosphere with T = io,ooo~ and with an electron pressure near 100 bars gives in- tensities similar to the observed values. The abundance of helium is nearly one hun- dred times that of hydrogen; hydrogen is one hundred times as abundant as iron. The presence of lines of other elements of very high excitation potential, notably A ri ap- parently indicates a higher excitation temperature for the extreme ultraviolet (Table 13). The low abundance of hydrogen would still persist if such deviations were real. The contribution of helium to the continuous absorption coefficient is found to be greater than that of hydrogen and assists in reducing the jump at the Balmer absorption limit. An atmosphere in which helium is very abundant may exhibit absolute-magni- tude effects, i.e., low electron pressure, for higher values of the surface gravity
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 1940
- DOI:
- 10.1086/144185
- Bibcode:
- 1940ApJ....91..438G