S. Johansson, K. Davidson,
D. Ebbets, G. Weigelt, B.
Balick, A.Frank, F. Hamann,
R.M. Humphreys, J. Morse, R.L.
White
Department of Physics, University of Lund, Sölvegatan 14, S-223 62 Lund, Sweden Lund Observatory, University of Lund, Lund, Sweden University of Minnesota, Minneapolis, MN, USA Ball Aeroespace Corporation, Boulder, CO, USA Max Planck Institute for Radio Astronomy, Bonn, Germany University of Washington, Seattle, WA, USA University of California, San Diego, CA, USA University of Colorado, Boulder, CO, USA Space Telescope Science Institute, Baltimore, MD, USA
Carinae, act as a dichromatic UV laser. The proposal is
based on intensity anomalies in two pairs of Fe II lines around
2510Å observed in recent spectra obtained with the HST/GHRS. The
break-down of the branching ratios for spontaneous emission is explained as
contributions from stimulated emission. An inverted population of two
highly excited Fe II levels is due to selective photoexcitation by H I and
N I.
Pre-COSTAR observations in 1991 aimed at three settings with
the Faint Object Spectrograph (FOS) of the central core region
of 
Carinae, which has been shown to consist of four objects
(Weigelt & Ebergsberger 1986) separated by only a few tenths
of an arcsecond. The objects are labeled A, B, C and D, where
A is the central star and B, C, D most probably blobs of
ejected material (Davidson et al. 1995). The 1991 observations
gave spectra for two different settings (one failed) in the
region 1200--5500Å, but the spectrum of the blobs could not
be directly separated from the spectrum of the star. By making
some assumptions about the blobs, the observed spectra were
treated as linear combinations of the stellar spectrum (A) and
the combined blob spectrum (BCD). From these linear
combinations the stellar spectrum was separated from the blob
spectrum, and the resulting spectra showed remarkable
differences (see Figure 6 in Davidson et al. 1995). The stellar
spectrum (component A) contains emission lines due to allowed
Fe II transitions in the resonance region (2000--3000Å) and the
Balmer series of hydrogen. The blob spectrum (components BCD) shows strong
forbidden Fe II lines and a dominant emission feature at about 2508Å.
The latter was identified as a blend of two highly-excited Fe II
transitions, first explained in cool stars as
fluorescence lines pumped by H Ly
(Johansson & Jordan
1984). The same lines were discussed by Johansson & Hamann (1993) in
a recent paper on UV fluorescence observed in IUE spectra, and
the particular case of Eta Carinae was emphasized.
To further study the 2508Å feature, new observations were
needed at higher spatial and spectral resolution.
The central core region of 
Carinae was re-observed with HST in June
1995 using the Goddard High Resolution Spectrograph (GHRS) at medium
resolution (grating G270M). The six different spectrograph settings, 1--6,
of the four target objects ABCD are shown in Figure 1.
Figure: Positions of six 0.1" spaced GHRS 0.2" aperture observations of
the core region of 
Carinae, covering the targets ABCD.
Figure: Spectra around the 2507 and 2509Å lines obtained at positions
2 and 4. (The intensity scale of the top panel has been changed in order to
demonstrate the strength of the two lines in position 2.)
Three different wavelength regions about 40Å wide and centered at 2510, 2740 and 2845Å, respectively, were recorded. However, in this paper we discuss only the 2508Å feature. In Figure 2 is shown a comparison of the spectra around this feature recorded at target positions 2 (HRS-2) and 4 (HRS-4). GHRS resolves the feature observed in the FOS spectrum as two well-resolved, strong lines. The scale of the upper panel in Figure 2 has been changed in order to show the full intensity of the two lines.
A blow-up of the spectrum around 2508Å from positions 2
and 1 is shown in Figure 3. Besides the strong lines 
and
, we have also marked the positions of two transitions
and 
, which have connections to 
and
, as
Figure: Spectra around the 2507 and 2509Å lines obtained at positions
2 (solid line) and 1 (crosses). The four lines discussed in the paper are
marked
explained in the next section. The measured heliocentric vacuum
wavelengths for 
and 
on HRS-2 are 2508.719 and
2507.170. The laboratory wavelengths are 
and 
, corresponding to
a blueshift of 0.379 and 0.382Å (45
). The same
blueshift has been found for about 6--7 other high-excitation Fe
II lines in the same spectral region.
The lines 
and 
are identified as two Fe II transitions labeled
and 
, where 
and 
(Johansson 1978).
From LS selection rules the
intra-parental transition 
is supposed to give a strong
line, whereas 
is an extraordinary intercombination line
and an unlikely transition. However, the upper levels of 
and 
have about the same excitation energy, and are,
therefore, strongly mixed (Johansson 1978). The level mixing
results in two states having the same decay channels, for which
the relative intensities of lines to a common level is constant
and determined by the strength of the mixing. A similar case of
level mixing in Fe II has recently been studied (Johansson et
al. 1995).
The upper levels in the transitions 
and 
can also
decay to the J=9/2 level of 
through the transitions 
and
, as shown in Figure 4. These transitions
result in the satellite lines 
and 
, respectively
(see Figure 4). To get the predicted stellar wavelengths of 
and 
, we have corrected the laboratory wavelengths for a
Figure: Simplified level diagram of Fe II illustrating the transitions
and 
that correspond to the lines 
in Figure 3.
blueshift of 0.380Å and obtain 
Å
and 
. The intensity ratio of two lines
coming from the same upper level have been determined
from a laboratory emission spectrum of Fe II. We get
and 
. As the lines
are close in wavelength, these intensity ratios reflect the
ratios between the Einstein coefficients for spontaneous
radiation, A
. We should point out that 
is blended
in the laboratory spectrum. Due to the particular level mixing in this
case and the general complex structure of Fe II, theoretical transition
probabilities have large uncertainties and have not been used.
The satellite lines 
and 
are very faint in the
stellar spectrum, which result in much larger values of the intensity
ratios 
and 
. At target position HRS-2, 
and 
, and at HRS-1, 
. 
is
difficult to measure as 
is hidden in the noise.
There are two possibilities to get an increase in the intensity ratios:
A) The strong lines 
are enhanced or
B) the satellite lines 
are reduced in intensity.
A) The strong lines can be enhanced due to blends with other lines or
because of stimulated emission. Accidental blending of both 
and 
requires that other lines coincide in wavelength with 
and 
within a few mÅ. The blending lines also need to be
selectively excited like 
and 
. We have not found any candidates
for blending that fulfill these requirements. The stimulated emission
requires a combination of a strong radiation field
at these particular wavelengths as well as an inverted population.
B) The satellite lines can be reduced due to self absorption or
absorption by other transitions coincident in wavelength. The
two lower levels in the two pairs of transitions belong to the same
term, 
, and they have similar decay channels down to 
in the
infrared. Therefore, there is no build-up in the J=7/2 level that should
cause self-absorption. The 
line lies close to a broad absorption
line of Fe II at 2505.80Å. Considering the widths of and
the wavelength difference between the two lines, we estimate that the Fe
II absorption has a very little effect on 
. We find no
absorption feature in the spectrum in the vicinity of 
and no
potential candidate that could affect it by absorption.
From this discussion we draw the conclusion that 
and 
are
enhanced due to stimulated emission. The low probability for stimulated
emission at UV wavelengths is probably compensated by a strong inverted
population. The upper levels are close in energy and can be
populated through photoexcitation by the same pumping line.
Different possibilities were discussed by Johansson & Hamann (1993) for
different stars, but the photoexcitation by an accidental resonance
in wavelength between two Fe II lines and HLy
was supposed to
dominate and HLy
should be the major pump. However, the great
intensity of the two fluorescence lines in 
Carinae initiated
the search for another pump that could build up the population of the
upper levels. This pump has been found in NI lines around 1130Å.
A support for N I pumping is the presence of other strong nitrogen lines in
the spectrum of 
Car due to a high nitrogen abundance in the star. A
more detailed discussion of the pumping will be included in a
forthcoming paper on these two laser lines in Eta Carinae.
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