To elucidate the nature of the interstellar medium in the vicinity of the Vela supernova remnants (SNR) an extensive study with the International Ultraviolet Explorer of interstellar absorption lines toward 35 stars in the vicinity of the Vela SNR was undertaken. Observations of interstellar absorption, in particular of CI, towards one of these stars, HD 72350 (type B4 III), is of particular interest

Jenkins, E. B.

Leep, E. M.

Silk, J.

Wallerstein, G.

English

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A COMPRESSED CLOUD IN THE VELA SUPERNOVA REMNANT
Edward B. Jenkins
Princeton University Observatory
George Wallerstein and E. Myccky Leep
University of Washington
Joseph Silk
University of California, Berkeley
INTRODUCTION
Our previous interstellar line survey, carried out with the Copernicus
satellite, was limited to selected lines in only four bright stars (ref. I).
Nevertheless, our data revealed the presence of numerous absorbing, primarily
ionized gaseous sheets and filaments, that had evidently been shocked, com-
pressed, and accelerated by interaction with the SNR.
The significance of the interaction of supernovae with interstellar
clouds was first stressed by Oplk (ref. 2), who suggested t_at star formation
would be initiated. More recent studies have supported this viewpoint, al-
though the direct evidence for triggering of star formation by supernovae is
very tenuous. Herbst and Assousa (ref. 3,4) emphasized the location of young
stellarassociations at the edges of expanding shells of gas: such shells are
not necessarily old SNR, however. A more direct connection was sought by Woo-
tten (ref. 5) who found evidence for compression by a factor _i0, heating, and
enhanced line broadening in molecular clouds near the SNR W44 and W28.
Another connection between a supernova and star formation has been in-
ferred from the presence of excess 26Mg in certain inclusions in the Allende
meteorite, taken to indicate that the unstable isotope 26AI was injected into
the protosolar nebula within -106 yr of the nucleosynthesis of the 26AI in a
nearby supernova (ref. 6). Such circumstantial evidence suggests that inter-
action with the supernova may both have enriched and initiated the collapse of
the interstellar, cloud destined to form the sun and the solar system.
Finally, recent models of spiral structure have utilized supernova-lnduc-
ed star formatlon as a means of enabling star formation to be self-propagatlng
in the galactic disk (ref. 7,8). To justify this type of theoretical work,
one would llke to know whether there is any evidence for the extreme compres-
sion of ambient gas near a SNR that is required to initiate star formation.
In an attempt to shed light on these issues and to further elucidate the
nature of the interstellar medium in the vicinity of the Vela SNR, we have
undertaken an extensive study with the IUE of interstellar absorption lines
toward 35 stars in the vicinity of the Vela SNR. Observations of interstellar
absorption, in particular of CI, towards one of these stars, HD 72350 (type
B4 III), are of sufficient interest that we report here a preliminary analysis
of this data before the entire survey has been reduced.
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OBSERVATIONS
In June 1979 we obtained high resolution IUE spectra of 35 stars in the
field of the Vela Supernova Remnant. From the video displays of the short
wavelength echelle spectra it was clear that HD 72350 showed multiplet struc-.
tures in all of the C I transitions which were especially prominent, indicat-
ing that the excited fine-structure levels are heavily populated. Two addi-
tional short-wavelength exposures of this star were recorded in September 1979
to verify this result and improve the net signal-to-noise ratio.
Column densities of various species were derived using curves of growth
based on Voigt profiles. For unresolved blends of two or more transitions
from C I, special curves of growth were calculated to derive the total equiv-
alent width of the partially overlapping components. Table i lists the de-
rived column densities as a function of the velocity dispersion parameter b,
averaged from all but the weakest lines. We calculated the standard error of
the results for cases where we measured three or more lines, and these disper-
sions are shown in parentheses after the respective entries. For our study of
cloud compression, our primary objective is to derive column densities of neu-
tral carbon atoms, i.e. (N(CI), N(CI*) and N(CI**), in the three levels of fine
structure excitation, 3P0, JPI and 3P2, respectively. From the standard
errors in Table 2, we see that the best internal consistency for the column
densities occurs for b _ i0 km s-I. Within the range i0 < b < 20 km s-1, the
total column density of neutral carbon varies by a factor-of Three, but the
ratios of C I, C I* and C I** populations are relatively insensitive to the
choice of b. Our inability to detect the weaker transitions indicates that
the absorptions we have measured do not have heavily saturated cores. In par-
ticular, our detection limit for X1276.48 forces us to conclude that log
N(CI) < 14.7 for any b greater than 4 km s-1, if log f% for this transition is
about 0.8 (this line strength is based on a preliminary analysis of C I absorp-
tion data recorded by ref. i0; see also ref. ii). Hence, we are confident
that the conspicuousness of the excited lines is attributable to a strong
excitation of carbon rather than a large difference in line saturations.
If we assume log N(H) = 20.8 from the star's B-V color excess of 0.14
(ref. 12) and the general gas to reddening ratio of 4.8 x 1021 atoms cm-2
mag -I (ref. 13), we find that the relative abundances of N I and O I are con-
sistent with the cosmic abundance ratio if b ranges between I0 and 12 km s-I.
From earlier research works on ultraviolet interstellar data, we know that
velocity dispersions of atoms in a stage of ionization below the dominant one
for H I regions generally exhibit a lower b than those species in their domi-
nant stages (e.g. see Figure 2 of ref. 14), so our N I and O I b values should
exemplify only an upper limit for the b of C I. Doublet ratios for the Ca I]
and Na I absorptions in the visible yield b values of i0 and 5 km s-I, respec-
tively (ref. 9). The value for sodium appears to be too low perhaps because
the equivalent widths were not very accurate; 8 km s-I would be acceptable,
but a larger b value implies the D lines are virtually unsaturated which is
inconsistent with the observed line strength ratio. From the above consider-
ations, we feel that it is reasonable to assume that the most probable value
of b for C I is 12 km s-I.
590
TABLE I.-COLU_IN DENSITIES (LOG NL) FOR LIGHT ELEMENTS
IN THE H I REGION IN FRONT OF HD 72350
No. of log NL for various b-values
Element Lines 8 i0 12 15 20 Notes
Cl 3 15.3(0.4) 14.8(0.2) 14.6(0.2) 14.4(0.2) 14.4(0.2)
Cl* 9 15.5(0.5) 15.1(0.3) 14.8(0.2) 14.7(0.2) 14.6(0.2)
CI** 8 14.8(0.2) 14.6(0.1) 14.5(0.1) 14.4(0.1) 14.3(0.1)
Cl(total) 22 15.8 15.4 15.1 15.0 14.9 i
NI 3 17.6(0.3) 17.1(0.4) 16.3(0.4) 15.5(0.3) 15.0(0.2)
Ol i 18.0 17.7 17.2 16.2 15.4
01" I <13.9 <13.9 <13.8 <13.8 <13.8
Mg I 2 13.3 13.2 13.2 13.2 13.2
C0 5 14.2(0.1) 14.2(0.1) 14.2(0.1) 14.2(0.1) 14.2(0.1)
Na I 2 12.8 12.4 12.3 12.3 12.3 2
Ca II 2 13.2 12.9 12.8 12.7 12.7 2
Notes: 1-Includes two blends of CI* and CI** features.
2-From optical spectra (ref. 9).
THE NEUTRAL CARBON ABSORPTION LINE REGION
CAN THE CI ABSORPTION BE CIRCUMSTELLAR?
Prior to discussing the implications of the CI populations in Table 2 we
must demonstrate that the observed cloud is not a circumstellar feature whose
CI levels are pumped by radiation from HD 72350. Radiation from 34 III star
is capable of ionizing CI. We first have shown that there is a negligible
amount of CI recombination in the CII zone. Moreover, a sufficiently great
column density is needed to shield CI near HD 72350 from the carbon ionizing
photons that there can only be a negligible amount of CI at high pressure
(p/k >>103 cm"3 K) near the star. Finally, the stellar radiation field is not
sufficiently intense to radiatively pump the CI without requiring an excess-
ive large column density of shielding CII.
PROPERTIES OF THE C I CLOUD
Since we have shown that the cloud containing the CI is not circum-
stellar, we can adopt the perspective that it is normal interstellar material
subjected to unusual physical conditions. From the fine-structure population
ratios fl E Cl*/Cltotal and f2 E Cl**/Cltotai we may arrive at permitted com-
binations of pressure and temperature using the diagrams for eollisional equi-
libria in H I regions given by ref. 15. Figure i shows the combinations of
pressure and temperature which are consistent with our population ratios_
allowing for reasonable errors in column densities.
However, there are two additional constraints, shown in the figure, which
we may impose on the conditions. First, we can require that a solution for
the ionization equilibrium between CI and CII (ref. 15) not give a computed
591
value N(CI) less than the observed value assuming log N(H) = 20.8, a cosmic
abundance ratio for C/H, and an ionization rate rC = 2 x i0-IU s-I for the
general interstellar medium. (This argument is only a limiting case because
the computed value could greatly exceed the observed N(CI)/N(H) if much of the
reddening was not associated with the CI region.) This constraint may be even
stronger than shown here, since there is a good chance that FC in the Vela re-
gion is higher than the usual interstellar value and some of the free carbon
atoms are depleted onto grains.
The second constraint comes from our upper limit for N(01*). If we solw_
for the colllsional equilibrium for the 0 1 fine structurelevels, assume a
cosmic abundance ratio for C/O, and compute the total carbon density from the
ionization equilibrium (see above), we obtain log T < 2.1. This result is in-
dependent of log p/k because the computed ratios 01"/01 and C/CI both scale
approximately linearly with pressure.
DISCUSSION
From Fig. i we see that the tem[perature of the cloud lles between 25 and
IO0*K and P/K is greater than 104 , and could be 106 or larger. The minimum
density is about 250 hydrogen atoms/cm 3, and the density could easily be
3 x 103 or substantially higher. Such conditions could be realized by compres-
sion of pre-existing clouds by a supernova shock moving at about 400 km s-I
through the intercloud medium. Such a shock could compress the gas by a fac-
tor 300 so the shocked cloud could have had an initial density of order
i0 cm-3. The origin of shock clouds within the Vela B association poses an
interesting problem. They are likely to be material left over from the for-
marion of stars that have recently reached the maln-sequence of the associa-
tion.
REFERENCES
I. Jenkins, E. B., Silk, J., and Wallersteln, G. Ap. J. Suppl. 32, 681,
1976.
2. _pik, E. J. Irish Astron. J., 2, 219, 1953.
3. Herbst, W. and Assousa, G. E. Ap. J. 217, 473, 1977.
4. Herbst, W. and Assousa, G. E. In Protostars and Planets, ed. by
T. Gehrels (Tucson: University of Arizona Press), p. 368, 1978.
5. Wootten, H. A. Ap. J. 216, 440, 1977.
6. Lee, T., Papanastassion, D. A., and Wasserburg, G. W. G_ophys. Res.
Lett., 3, 41, 1976.
7. Mueller, M. W. and Arnett, W. D. Ap. J., 210, 670, 1976.
592
8. Gerola, H. and Seiden, P. E. Ap. J. 223, 129, 1978.
9. Wallerstein, G., Silk, J. and Jenkins, E. B. Ap. J. (in press) 15 Sept.
1980.
i0. Jenkins, E. B., Jura, M. and Lowenstein, M. (in preparation).
ii. de Boer, K. S. and Morton, D. C. Astron. and Astrophys. 71, 141, 1979.
12. Ferro, A. A. and Gerrison, R. F. Rev. Mex. de Astron. y Astrophys. 4,
351, 1979.
13. Bohlin, R. C., Savage, B. D. and Drake, J. F. Ap. J. 224, 132, 1978.
14. Spitzer, L. Jr. and Jenkins, Ann. Rev. of Astron. and Astrophys. 13,
133, 1975.
15. Jenkins, E. B. and Shaya, E. J. Ap. J. 231, 55, 1979.
593
(b)
41 I2 =
logT
Fig. i. Allowed pressures and temperatures for the CI cloud.
Excluded areas are the shaded side of the lines. Lines ma_ked
(a) are from CI fine-structure excitation. Line (b) is from
the relative abundance and ionization equilibrium of carbon.
Line (c) is from the absence of observed absorption from 0I*.
594


A compressed cloud in the Vela supernova remnant

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