A total of ten stars were observed with cameras of the International Ultraviolet Explorer (IUE) in both high and low dispersion. One star, X Persei (HD 24534, 6.0 BE), was analyzed in detail. Ultraviolet observations of the column densities of CO match those derived from the radio to within a factor of 4, with the difference probably due to the larger beam size of the radio measurement and the assumption of a thermal population in the rotational levels of CO. Upper limits are given to the log column densities for OH, HCl, and CH2 of 14.0, 12.3 and 12.8. The carbon abundance was found to be about solar with a possible depletion of about a factor of 2. With precautions concerning both noise and correct background, the IUE can be used for studies of interstellar molecules

Buhl, D.

Crutcher, R. M.

Donn, B.

Lien, D.

Smith, A. M.

Snyder, L. E.

Stief, L. J.

English

NASA Technical Reports Server

A SEARCH FOR INTERSTELLAR MOLECULES IN THE
SPECTRA OF HIGHLY REDDENED STARS*
D. Lien, 1 D. Buhl, 2 R.M. Crutcher, 1 B. Donn, 2
1
A.M. Smith, 3 L.E. Snyder, L.J. Stief 2
ABSTRACT
The dark cloud in the line of sight towards X Persei has been searched
for the molecular species OH, CH2, HCI, CO and C2 us$og the IUE satellite._
We h_e detected CO, with a column density of 5 x 1015cm -2 and 1 x 1014cm -z
for CO and CO, respectively. We have placed upper limits on the log
of the column densities of OH, CH2 and HCf of 14.0, 12.8, and 12.3, respec-
tively. Radio observations toward X Persei give a factor of 4 greater col-
umn density in CO, and a velocity width in the cloud containlng the CO of
2 km/s, FWHM. Log N(OH) = 13.8 from radio data, which is consistent with our
upper limit. We also have apparently detected the Pl_ u - XIE_ transition of
C2 at 1341_. The equivalent widths of the C II lines are great enough to
place them on the damping portion of the curve of growth. The derived column
density of carbon implies a C/H ratio of 2/3 the solar value.
INTRODUCTION
Most molecules of astrophysical interest have electronic transitions in
the spectral region covered by the International Ultraviolet Explorer Satel-
lite (IUE). Because of this, we have undertaken the analysis of high disper-
sion spectra of ten stars embedded in or behind dark interstellar clouds
which are known to contain strong lines of CH and CH+ from optical studies
(ref i). These stars were observed in October, 1978, Janua_ry, 1979, and
March, 1980. The initial data reduction was done using the GSPC PDP 11/40
Forth Reduction System, set up by D. Klinglesmith. From this preliminary
overview of all the data, we decided to analyze in detail only one star,
and then return to the others. With this in mind, we spent the March 1980
observing run observing the chosen star in order to build up the total sig-
nal-Ko-noise ratio of the stacked data. This star, to which all further dis-
cussion is directed, is X Persei (HD 24534).
X Persei is a 6th magnitude O or B emission star. It has recently been
identified with the X-ray source 3U 0352 + 30 (ref 2), and has a Vsini of
about 500 km/s (ref 3). The broad stellar lines are easily distinguishable
from the narrow interstellar lines. There is no indication in any of our
data for a stellar wind or super-surface emission features.
* Supported in part by NASA under contract NSG 5278 to the University of
Illinois
i. Department of Astronomy, University of _lllnois
2. Laboratory for Extraterrestrial Physics, GSFC
3. Laboratory for Astronomy and Solar Physics, GSFC
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https://ntrs.nasa.gov/search.jsp?R=19810017420 2020-03-21T12:07:03+00:00Z
Copernicus observations (ref 2) of the atomic and diatomic hydrogen
lines give the total hydrogen column density toward X Persei as 2.4 x
1021cm -2, with a fractional abundance of H2 of 0.9, and an associated kine-
tic temperature derived from the first two rotational levels of H2 of 71°K.
DATA REDUCTION
There are two main problems associated with the identification and
subsequent analysis of weak interstellar lines: the noise and the back-
ground.
As the system stands, the ITF should remove all systematic tube de-
fects, and hence all remaining noise should be random. Due to Pixel-ITF
mismatching and possible time-dependent spots, this is not the case. In
order to detect such non-random noise, we have found that sequential obser-
vations of the same star in the same camera should be taken with the star
positioned at alternate ends of the large aperture for each exposure.
This places a given spectral feature at different physical locations on the
tube face for each exposure. The plotted results then show a wavelength
shift between exposures, whereas any non-random noise feature will appear
at the same wavelength. An example of this effect is shown in Figure i.
With this procedure, tube defects cannot be interpreted as weak lines.
When the gross spectrum is divided by the extracted background, a
noisy straight line results. This implies that there are data mixed in
with the background (e.g. order overlap). To correct for this problem, we
assume that the background is of the form BT = C + (B0 - C)f, where B0 is
the observed background at a given wavelength, BT is the "true" background
at that point, and C and f are empirically derived constants. An example
of this is shown in Figure 2, where B0, BT and C are shown, and f = 0.6.
We chose this value of C because at the edge of the order there are very
little data, hence that value of the background probably represents the
component due to thermal noise and any reading induced noise. A best-fit
value for f is found by varying the value of f and subtracting BT from the
gross spectrum until the equivalent widths of the same line in two different
orders are equal. Thus C is a constant for each order, and f is a constant
for the spectrum. We find that f = 0.6 reproduces equivalent widths between
orders within 5%, and between exposures within 10%.
After the proper background for each spectrum has been_subtracted from
the gross spectrum for a given order, spectra are stacked after shifting
each to the same radial velocity, at the wavelength of interest. All equi-
valent widths and line profiles are extracted from this stack. For X
Persei, there are 6 spectra, shifted and stacked as described above.
RESULTS
Radio observations of the CO I-0 rotational transition obtained by one Of
us (RMC) have been analyzed assuming LTE. We find T0_4= 5_K, a velocity width
of 2 km/s FWHM, and a I_CO column density of 4.2 x i0 cm- . Assuming a
12C0/13CO ratio of 40 gives the column density of 12C0 as 1.8 x 1016cm -2.
Subsequent analysis in this section will assume a velocity width of 2 km/s,
582
and a rotation temperature for CO of 5° K. This may not be a valid assump-
tion when comparisons between atomic and molecular column densities are
made; however, it is adequate as a first approximation.
We have observed the v" = 0 progression of the Fourth Positive system
of CO through v '.=12 for 12C0. Observations of the i-0 rotational transi-
tion and the 2-0 vibrational band of the A-X system are shown in figure 3
for 12CO and 13C0. We have done an initial analysis of the system assuming
that only the first four rotational levels are populated, at a rotation
temperature of 5°K. For each frequency over the band, we then calculate
the optical depth contributed at that frequency by each of the nine transitions
from the four levels, assuming that a Voigt profile describes the line. Under
these assumptions, we find the column density of 12CO to be about 5 x
lOl5cm -2, and the column density of 13C0 to be about i x 1014cm -2. These
values compare favorably with the column densities derived from radio
observations; reasons for the small differences may be due to the assump-
tion of a thermal population of the rotational levels (which strongly af-
fects the saturation of the UV CO lines), and the larger beamwidth of the
radio measurement. It seems reasonable that the cloud is completely in front
of the star.
We have made a detailed search for OH (1222_), HCL (1290_), and CH2
1416_), and report that none of these molecules were detected above the 5
level. This puts upper limits on the logarithm of the column densities
for these molecules at 14.0 (.004), 12.3 (.16), and 12.8 (.05), respective-
ly. The numbers in parentheses are the oscillator strengths (ref 5,6,7).
These limits are derived assuming that the molecular line is optically
thin.
One of us (RMC) has detected the 1667 MHz radio OH line toward X
Persei. He finds log N(OH)=I3.8, which is consistent with our upper limit.
i i+
We have apparently detected the F _u - X E system of C2 (ref 8), of
which we have observed the 0-0 vibrational band at 1341 _. The D-X system
in the ultraviolet has previously been reported (ref 9). Since C2 does
not have a permanent electric dipole moment, transitions between rotational
levels are forbidden; hence, there is expected to be a thermal population of
the ground state rotational levels. The resultant profile due to the
convolution of lines arising from these rotational levels with the response
of the IUE spectrograph should show the transition as a broad profile, with a
possible separation of the bands. This is about what we observe, as shown in
Figure 4. We are in the process of modeling the C2 line in much the same
way as described for the CO lines.
The C II doublet at 1334_, 1335A is important not only as an indicator
of the total hydrogen density (ref. 4), but also the abundance of this domi-
nant species can provide some information about the composition of the dust.
The equivalent widths of the two lines toward X Persei are 267 and 172 m_,
for the 1334A and 1335_ lines, respectively. For a velocity parameter, b,
of 5 km/s or less, this puts the 1334_ line on the damping part of the
curve of growth, thus making it independent of the exact velocity dispersion,
providing it is less than about 5 km/s. This implies a column density of C
(assuming most carbon is C II) of 5.6 x 1017cm -2. For a total hydrogen
583
1021column density of 2.4 x , this implies a C/H ratio of 2.3 x 10-4 or
about 2/3 the solar value. Formal errors have not been worked out, but
errors in the exact placement on the curve of growth do not change by more
than ± .15 in dex. The kinetic temperature based on the relative populatlon_
of the fine structure levels of C II is 55°K, which agrees favorably with
the kinetic temperature of 71°K derived from the relative populations oZ the
J = 0 and J = i levels of H2. This implies that collisions dominate the
de-excitatlon of the excited fine structure level of C II, and puts the
brunt of the cooling on H2 and other molecules.
SUMMARY
We have observed a total of ten stars with both the SWP and LWR cameras
of the IUE in both high and low dispersion. We have chosen one star, X
Persei (HD 24534, 6.0 BE), to analyze in detail.
Our ultraviolet observations of the column densities of CO match those
derived from the radio to within a factor of 4, with the difference probably
due to the larg&r beam size of the radio measurement and the assumption of
a thermal population in the rotational levels of CO,
We give upper limits to the log column densities for OH, HCf, and CH2
of 14.0, 12.3 and 12.8, respectively, and report the identification of the
FI_ u - xlz_ system of C2. We find the carbon abundance to be about solar,
with a possible depletion of about a factor of 2. We feel that with the
proper precautions concerning both noise and correct background, the IUE
can effectively be used for some studies of interstellar molecules.
REFERENCES
i. Cohen, J.G., Ap.J. 186, 149, 1973.
2. Mason, K.O., White, N.E., Sanford, P.W., Hawkins, F.K., Drake, J.F., and
York, D.G., MNRAS 176, 193, 1976.
3. Hutchings, J.B., Cowley, A.O., Crampton, D., and Redman, R.O., Ap.J.
191, LI01, 1974.
4. Morton, D.C., Ap.J. 197, 85, 1975.
5. Crutcher, R.M. and Watson, W.D., Ap.J. 203, L123, 1976.
6. Smith, P.L., Yoshino, K., and Parkinson, W.H., preprint_ 1979 .
7. Snow, T.P. Ap.J. Letters, 201, L21, 1975.
8. Ballik, B. and Ramsay, R., Ap.J. 137, 61, 1963.
9. Snow, T.P., Ap.J. Letters, 220, L93, 1978.
584
I I
I r nn7
Figure I j i
2590 _.(._) 2600
i. During sequential observations of the same star, if the star is shifted
in the large aperture, spectral lines shift, but systematic noise does
not; f indicates lines due to Fell, m indicates lines due to Mnll, and
n indicates systematic noise features on the tube.
585
Sx I05 -
1341.63
i,
Ixl(_ -
Figure4
1337 X (,_) 1343
2. An example of the gross spectrum and the background subtraction tech-
nique used; B0 is the IUE extracted spectrum, and BT is the "true"
background, as indicated by the equation. After the computation of
BT, it is smoothed bv a median smoothing routine.
586
/, ° , J
1
I A Figure 3
3. Radio and optical observations of CO. The radio observations are of
the 1-0 rotational transition, and the UV CO are due to the 2-0 vibra-
tional band of the Fourth Positive electronic system.
587
Figure 2
60000
BY=C+(B,- C)xf
I I
1335 1345
4. 'meFlwu - XIE + system of C2, showing the 0-0 vibrational band at
1341.63_ (ba_d origin). The strong feature at the left is the 1335_
line of CII which originates from the excited fine structure level of
the ion.
588


A search for interstellar molecules in the spectra of highly reddened stars

A search for interstellar molecules in the spectra of highly reddened stars

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