BALMER JUMP

The 3647 Å Balmer Jump in the spectra of the hot star alpha Lyra (Vega), observed by Barbier and Chalonge (1939).
Sir William Huggins was the first to observe the Balmer jump at the turn of the century in the stellar spectra of Vega and similar stars. W.H. Wright observed the jump in 1918 and in 1926 his student Ch'ing Sung Yü studied it in detail. From lab studies of hydrogen it was known that the Balmer series limit was at 3647 Å, but Yü demonstrated that the coalescence of the higher lines of the series often shifts the apparent discontinuity to longer wavelengths. This effect is more pronounced at low resolution.

A decade later Barbier and Chalonge of the Institut d'Astrophysique de Paris measured the detailed properties of the Balmer jump. gradients on both sides of the jump and the logarithmic intensity drop. They found that alpha Lyra (Vega) had the largest discontinuity, and the jump decreases for hotter or cooler spectral types. They also found anomalous stars such as the shell star Gamma Cassiopeia with a negative Balmer jump (sometimes called the Balmer continuum in 'emission' because the spectra was brighter below the jump). They also noticed that line coalescence was weaker in supergiants whose jump occurs nearer to 3647 Å.

Along with L.Divan they were able to construct a 3 dimensional stellar classification scheme based on three parameters: The wavelength of the midpoint of the intensity decrease (lambda_1), the logarithmic intensity drop (D) and the blue spectral gradients between 3800 and 4800 Å (Phi_b). The highly quantitative nature of the BCD system (Barbier-Chalonge-Divan) is a more precise way of classifying early stars compared to the older two dimensional spectral MK scheme still in use today (type O, B, A, F..., class Ia, Ib, II, III,...).

COMMENTARY

In hot stars we obtain the equivalent of the Balmer jump for the helium atom, however there are now four levels from which photoionization can occur. These helium series limits are plotted in red in the energy level diagram below. They indicate the wavelength thresholds for photoionization by continuum radiation from the stellar photosphere. In shell stars the singlet and triplet 2s levels can become overpopulated due to their metastability, the continuous absorption coefficient of such a nLTE gas can produce discontinuities in the continuous spectral distribution from the star. This has been observed by Varshni (1985, 1988, 1989) in several quasars, further proof that they are stars within our galaxy.

Helium Jumps

REFERENCES

  1. Balmer series
  2. The BCD classification system (from the GCPD; Photometric Systems)
  3. Liu,X.-W., Brlow,M.J., Danziger,I.J., Storey.P.J.: 1995, ApJ., 450, L59. Balmer Discontinuity Temperatures in Orion Nebula
  4. Werner,K.: 1996, ApJ., 457, L39. Balmer Line Problem
  5. Huggins,Sir Wm.and Lady Huggins: 1899, An Atlas of Representative Stellar Spectra, William Wesley and Son, p.85.
  6. Wright,W.H.: 1918, Publ. Lick Observ. 13, 191.
  7. Yü,C.S.: 1926, Lick Observ. Bull. 12, (no.375) 104.
  8. Chalonge,D., Divan,L.: 1976, I.A.U. Symposium 72, 143.
  9. Varshni,Y.P.: 1989, Astrophys.Space Sci., 153, 153.
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