Herbig Ae stars generally show emission in the
H line (outside of the
spectral range of the spectra used in this atlas), and quite often
emission in H
and even H
. Many
Ae stars are still contracting to the main sequence, and are
thus either still surrounded by remnants of their stellar cocoons, or have
developed massive stellar winds.
Recently, Gray & Corbally (1998 AJ 116, 2530) have devised an
extension of the MK Classification System to the Herbig Ae-type stars. This
system, in addition to the usual MK-type, utilizes indices to indicate emission
or stronger than normal absorption in lines of the Fe II (42) multiplet, the
strength of the Balmer decrement, and characteristics of the emission in
the H
line. The three stars illustrated here demonstrate that system. The extended
spectral type consists of a normal MK type along with an indication of the
nature of the Balmer line emission. An "e" indicates strong emission in the
H
line, (e)
indicates marginal or weak emission, and an "r" or "b" indicates
whether this emission is shifted to the red or blue of the photospheric
line. The
strength of the Balmer decrement is indicated by the symbols <,
, =,
, >
for weak, somewhat weak, normal, somewhat strong, and strong. The nature
of the emission and/or absorption, plus the strength relative to the normal
absorption strength of the relevant standard of the lines of the Fe II (42)
multiplet are indicated with the N index. Nem indicates these lines are in
emission, Nab that they are in stronger than normal absorption, and Npc and
Nipc indicate P Cygni and inverse P Cygni profiles in these lines.
The spectral types of these Herbig Ae stars can change quite dramatically on time scales of a few days. As a consequence, the spectral types of these stars should always be accompanied by a date.
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