PLANETARY NEBULAE : Part 6
Hydrogen Beta (Hβ) and the
Line Emissions from Planetary Nebulae
It has long been known that most PNe can be calibrated by
measuring the single line strength of Hβ. This value is measured
photoelectrically, and is the amount of energy (flux) liberated from
the PNe structure or even the PNN itself. Hβ is expresses in mW.m-2 and
abbreviated F(Hβ) — the
absolute Hβ flux. It is not easy to
imagine this quantity. For example, the Sun produces energy that can
be openly felt on exposed skin to the warm Sun, whose energies are
roughly 1,368 Watts per square metre or 1.368mW.m-2. This
knowledge is quite useful, for example, in charging electric cells in
generating electrical power. Such energies are intense enough to
permanently damage the retina of the eye in less than 10,000th of a
second when using any optical aid! This abundant energy is across all
wavelengths but is much smaller for the Hβ output alone.
For all planetaries, and stars, these quantities are very very
tiny, mainly because of the tremendous distances of these objects.
Although the output of planetaries and stars is simply stupendous,
the energy per square metre of surface decreases by the square of the
distance. For an example of the PNe NGC 4071’s, the measured flux is minuscule at
2.239×10-12 mW.-2, some 1018
times fainter than observed solar output! To avoid having to write
down so many small numbers, values for the absolute fluxes are given
as the log of the measured flux. I.e. log
−11.66±0.02mW.m.-2. Typical values− for
these indices compared to all PNe may range anywhere from -9 to
−16 with the midrange of the distribution being about
−11.5.
Among the southern planetary, NGC 3242 has the highest Hβ flux of -9.789 or
1.626×10-10 mW.m-2 ’ seventy-two times more intense than the
surface area of NGC 4071. An example of low Hβ fluxes is planetary nebula He2-77
/ Sa3-16 / PK 298-0.1 / PN G298.1-07 (12091-6307) at log
−13.28mW.m-2. [He2-77 appears in south western Crux,
being 26 arcsec across and 17.6 magnitude. It remains impossible to
see in any amateur telescope.]
Calibration of the absolute Hβ
fluxes are made by the examination of several “standard stars”. I.e. β3 Orionis and α Lyrae (Vega), or the bright PNe. For
southern planetaries, NGC 3242 and IC 418 are used as Hβ ‘standard
candles’. In other terms, the
relative Hβ fluxes are used to
compare strengths of the emission lines of other PNe, allowing
differences between other PNe to be quickly examined. This is made by
giving the absolute Hβ flux the value
of exactly 100.0. From this, other prominent PNe lines can be
assessed. For example, the values for NGC 4071 (From the
Strasbourg-ESO Planetary Nebulae Catalogue) are;
Hβ=100, He(II)=63, O-III= -,
O-III=786, He I=-, Hα=294,
N(II)=259, S(II)=30 and S(II)=24 For bright PNe
NGC 3918, the values from the Strasbourg-ESO Planetary Nebulae
Catalogue are;
Hβ=100, He(II)=16, O-III= 15,
O-III=1667, He I=15, Hα=374,
N(II)=259, S(II)=6 and S(II)=10
Amateurs should be aware with the important ratios of
[O-III/Hβ], [O-III/Hα] and, as they tell much about of the
likely telescopic appearance of the PNe and the effectiveness of
employing [O-III] filters, both photographically and visually. The
experienced French amateur observer Yann Pothier has also used this
same ratio between the Hydrogen Beta (Hβ) with much success. By combining the two
lines from the two O-III emissions, this gives some rough indication
of the effectiveness of the [O-III] filter on the object. This is
calculated by;
n = [O-III] & 496nm+501nm] / [Hβ, 486nm]
Where ; n = PNe Effectiveness Ratio Example 1 : NGC 4071 O-III is (786 + 0), Hβ is 100. Therefore, N = 7.86, rounded to
8. (Note: O-III at 486nm was not measured accurately, but is
likely quite small in value.)
Example 2 : NGC 3918 O-III is [1667+15]/100 =
1682/100 = 16.8 or 17 (Typical with the moderately high
excitation PNe.)
Correlation with known PNe finds that the smaller this value the
lower the excitation of the PNe nebulosity. Values can range anywhere
from 0.1 to 180, with the skewed mean being about 13.0.
References and Abbreviation Used in Text
- Burnham, R., “Burnham’s
Celestial Handbook : Volume 1 ”
(BCH1)
- Burnham, R., “Burnham’s Celestial Handbook : Volume 2 ” (BCH2)
- Burnham, R., “Burnham’s Celestial Handbook : Volume 3 ” (BCH3)
- Dreyer, J.L.E., “Second Index
Catalogue of Nebulae and Clusters of Stars; Containing Objects Found
Between 1895 and 1907.”; Memoirs of
the R.A.S., Vol. LIX-Part II (1908) (IC)
- Hartung, E.J., “Astronomical
Objects for Southern Telescopes : 1 ” (1968) (AOST1)
- 6. Malin, D. and Frew, D., “Astronomical Objects for Southern
Telescopes : 2 ” (1994) (AOST2)
Last Update : 27th November 2012
Southern Astronomical Delights ©
(2012)
For any problems with this Website or Document please e-mail me.
|