An IUE survey of B supergiants has been conducted to study the persistence with spectral type of the ultraviolet resonance lines of N V, C IV and Si IV. N V is seen as late as B2.5Ia, C IV until B6Ia and Si IV throughout the range from B1.5 to B9. This is in fairly good agreement with the Auger ionization model of Cassinelli and Olson (1979). The terminal velocities are derived for the 20 stars in the sample and it is found that the ratio v(T)/v(esc) decreases monotonically with spectral type from the value of 3.0 that it has in the O spectral range to the value 1.0 at B9Ia

Abbott, D. C.

Cassinelli, J. P.

English

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ANOMALOUS IONIZATION SEEN IN THE SPECTRA OF B SUPERGIANTS
Joseuh P. Cassinelll and David C. Abbott
Washburn Observatory
Madison, Wl 53706
ABSTRACT
An IUE survey of B supergiants has been conducted to study the persis-
tence with spectral type of the ultraviolet resonance lines of N V, C IV,
and Si IV. N V is seen as late as B2.5Ia, C IV until B6Ia and Si IV through-
out the range from B1.5 to B9. This is in fairly good agreement with the
Auger ionization model of Cassinelli and Olson (1979). The terminal veloc-
ities are derived for the 20 stars in the sample and it is found that the
ratio VT/Ves c decreases monotonically with spectral type from the value of
3.0 that it has in the O spectral range to the value 1.0 at B9Ia.
INTRODUCTION
Unexpectedly high ionization stages such as 0 VI were seen in the
Copernicus spectra of 0 stars (Snow and Morton, 1976). Several possible
explanations were considered by Cassinelli, Lamers and Castor (1978). X-ray
emission has now been discovered from 0 stars by observations from the
Einstein satellite (Harnden et el. 1979, Seward et al. 1979, Long and White
1980), and it is now clear that there is some very hot gas in the outer at-
mospheres of the stars. The x-ray emission is sufficient to produce the
ionization anamolies in the cool outflow by the Auger mechanism as had been
proposed by Cassinelli and Olson (1979)o In this process, a total of two
electrons are ejected from C, N or O following the K shell absorption of an
x-ray photon. (Weisheit 1974). Thus if the dominant stage of ionization in
the outflow is O IV a trace amount (~ l0-_ ) of O VI is produced, which is
sufficient to explain the P Cygni lines seen in the UV spectra. As O IV
should be an abundant ion stage (>10%) for stars as late as B0.bIa (Teff =
29000) 0 VI should persist to that spectral type and no further. Morton
(1979) carried out a careful study of Copernicu s spectra of early B super-
giants and found that O VI does persist to B0.5 and is absent at Blla.
In a similar fashion N V should persist aslong as N III iS a dominant
ion stage and C IV as long as C II is _ dominant ion stage in the cool parts
Jf the wind. To give further test of the Augerionizatlon model we have
carried out a survey of 20 B supergiants in every spectral class from BI.5
to B91a.
OBSERVATIONAL RESULTS
Figure i shows the spectra at N V 1239, 1243_ for ii of the B super-
giants from BI.5 to B5. The llne is clearly present from BI.5 to B21a but
disappears at later spectral types. At the bottom of thearray of spectra is
shown the results of theoretical calculations of absorption llnestrengths
expected in a 20,000 K star at Ne = 1011 cm-3 • These were calculated using
127
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f values from Kurucz and Peytremann (1975) supplemented by more recent work
by Abbott (1978), Morton (1978) and others. LTE populations were used in tLe
calculation of the line strength,: and the plotted line segments are propor-
tional to the logarithm of the strength. After accounting for these back-
ground lines, we concluded that N V is present only as far as B2.51a where,
using the calibration of Underhill et_ial. 1979, Teff = 20400. As usual ter-
mlnal velocities of the winds, VT, can be derived for each llne from the
maximal shortward displacement of the absorption feature.
Figure 2 shows the spectra of al_ 20 supergiants in our survey in the
region of the C IV doublet 1548, 1551A. For the earlier stars the llne dis-
plays thebroad strong P Cygni profile familiar in O stellar spectra. The
llne becomes progressively weaker for the later spectral types. After con-
sideration of the calculated strengths of the photospheric lines, we concluded
that C IV in the wind could be traced as far as B61a at which the effective
temperature is about 12000_. In an earlier IUE survey Underhill (1979) noted
the presence of C IV as late as BSla. The theoretical photospheric llne
strengths at the bottom of the figure were calculated as before, but nowus±r_g
15000 K for the left panel and i0000 K for the right panel. Again terminal
velocities are derived from each llne.
Figure 3 shows the Si IV doublet 1394, 1403_ region of the spectrum.
The llne clearly persists beyond either N V or C IV and continues throughout
t_e sequence to Bgla. (If the wind were in radiative equilibrium and not sub-
ject tO x-radiatlon it would disappear at B31a.)
The spectrum of 0 Ara shows distinctive structures in the resonance
lines of N V, C IV and Si IV. There is an apparent contribution from the
lines at zero velocity displacement. This roughly symmetric contribution
could perhaps have been produced in a "transition region" between the photo-
sphere and x-ray forming region.
DISCUSSION
The run of "terminal" velocities, derived for the three ions, versus
spectral type is shown in Figure 4. For Si IV the velocity is determined
from the stronger component at 1393_. Indicated in this Figure is the re-
sult that NV persists to B2.51a and C IV to B61a. These are in rather good
agreement with what is expected if N V is produced from N III and C IV is
produced from C II by the Auger mechanism. (Casslnelli and Olson 1979). The
persistence of Si IV can also be explained by the Auger mechanism but as the
ion Si II has more than I0 electrons the analysis is more complicated
(Weisheit 1976). The ion could eject from 2 to 4 electrons following L or
K shell absorption of x-radiation. In any case it is not surprising that it
should persist to later spectral types than does C IV.
Also shown in Figure 4 is the escape speed at the photosphere versus
spectral type. These were derived from the data on cluster membership dis-
tances, effective temperatures (Barlow and Cohen 1977_ and masses derived
from theoretical evolutionary tracks (Chiosi et el. [978), in the manner de-
scribed in the terminal velocity survey of Abbott (1978). In that paper
Abbott derived the ratios of VT/Vesc for 36 stars, all but one of spectral
128
type B1 or earlier plus some Wolf Rayet stars. He found VT/Ves c = 3.0. IUE
spectra of other O-type stars also show a VT/Ves c = 3.0 scaling (Garmany ....
1980). This relation does not hold for B supergiants as is seen from Figure
4. The terminal velocity decreases monotonically from _ 2.7 Vesc at BI.bIa
to 1.0 Yes c at B9Ia. The A2Ia star, _ Cyg, also has VT = 1.0 Yes c.
Thus , there is a continuous relation between vT and Yes c extending from
the earliest 0 stars to the latest B supergiants. The reason for the de-
creasing value of VT/Ves c in the later type stars is not clear. The effec-
tive temperature, gravity and rotational velocity are all properties which
decreas_monotonically from early O-stars to late B supergiants.
The decrease in VT/ves c does not conflict with the llne driven wind
theory of Castor, Abbott and Klein (1975). As discussed by Abbott (1978),
that theory predict s vT = [_/(i-_)] I/2 Vesc where _ is a numerical constant
which is determined:by the mixture of optically thick and optically thin
lines. If all the lines are optically thick @=i, and if all the lin_s are
thin _=0.0. In this model _ = 0.9 gives VT/Ves c = 3.0, and e=0.5 gives VT/
Vesc = 1.0. The result of Figure 4 therefore may indicate that the ioniza-
tion balance changes in B supergiants from a radiation force dominated by a
few very strong lines to a force produced from lines which are more numerous
but weaker.
The x-ray emission of early type supergiants also changes in character
at around Bi.bIa. Vaiana et al. (1980) notes that essentially every O star
observed by Einstein is an x-ray source. However in the Einstein survey of
O and B supergiants Cassinelli et al. (1980) have found that there is a
change at BI.5. All objects at BI.5I and earlier were detected with Lx
10-7.:5 L_o _ whereas limits on x-ray luminosities beyond Bi.bIa were lower
than 10-_.u LBo I. Thus there is reason for pursuing further studies of the
anomalous ionization in B supergiants. It may be the best diagnostic for
studyingthe coronae or hot flow instabilities now available.
REFERENCES
Abbott, D.C. 1978a, J, Phys. B: Atom. Molec. Phys. ii, 3479.
Abbott, D.C. 1978b, Ap.J., 225, 893.
Barlow, M.J., Cohen, M. 1977, Ap.J., 213, 737
Cassinelli, J.P., Castor, J.I., Lamers, H.J.G.L.M. 1978, P.A_S.B.,: 90, 496.
Cassinelli, J.P., and Olson, G.L. 1979, Ap.J., 229, 304.
Cassinelli, J.P., Waldron, W.L., Vaiana, G., Rosner, R. 1979, in prep.
Castor, J.I., Abbott, D.C. and Klein, R.I. 1975, Ap.J., 195, 157.
Chiosi, C., Nasi, E. and Sreenivasan, S.R. 1978, Astr. Ap., 63, 103.
Garmany, K. 1980, private comm.
Harnden, F.R., Branduardl, G., Elvis, M., Gorensteln, P., Grindlay, J.,
Pye, J.P., Rosner, R., Topka, K., and Viand, G.S. 1979, Ap.J. (Letters),
234, L51.
Kurucz, R.L., Peytremann, E. 1975, SAO Sp. Npt. 362. Smithsonian Inst.
Washington, D.C.
Lamers, H.J.G.L.M., Snow, T.P. 1978, Ap.J., 219, 504.
Long, K.S. and White, R.L. 1980, Columbia Astro. Lab. Contribution No. 183.
Morton, D.C. 1978, Ap.J., 222, 863.
129
Morton, D.C. 1979, M.N.R.A.S., 189, 57.
Snow, T.P. and Morton, D.C. 1976, Ap.J. Suppl., 32, 429.
Seward, F.D., Forman, W.R., Giacconi, R., Griffiths, R.E., Harnden, F.R.,
Jones, C. and Pye, J.P. 1979, Ap.J., 234, LSI.
Underhill, A.B. 1980, Ap.J,, 235, 149.
Underhill, A.B., Divan, L., Prevot-Bmrnichan, M.L. and Doazan, V. 1979,
M.N.R.A.S., 189.
Vaiana, G. et al, 1980, submitted to Ap.J.
Welsheit, J.C. 1974, Ap.J., 190, 735.
130
N¥
B 1.51o
il1B2.01a
tB2.01b
B2.5 la
B2.SIb
B2.51b
55 CM
B3 lo
HO75149
B4 Is
' VAVEI..ENGTH
Fig. i: The spectral region near 1240_ in B supergiants from BI.5 tO
BS. The major photospheric and interstellar lines in the region are
identified at the bottpm. The predicted relative strength of these
spectral lines is indicated by the length of the llne segment. Lines
whose opacity was not _alculated (usually interstellar) are indicated
by dashed lines.
131
{, C_
B2.01o
l
B 2.0ZQ
r _ _ BIIb
B2.01b B51b
B2.01b B61o
B 2.51a B 61b
B2,5Ib HDgI_871o
B 8I°
83 Io BBIb
=4,° U _I_ "; .91°
i ,i,=, I I ,II il
_I I m I J s I ! i i I I I IIll,
_536 1540 1544 1548 #552 1556 _560 _564 _536 1540 #544 1548 _52 151 (I0 1564
WAVELENGTH WAVELENGTH
Fig. 2: The @pectral region near 15502 in B supergiants from BI.5 to B9
132
l
SiE/ i
I
f
HD I06343 , , _ )"'J"v, (A*,....4:.. _ ,,,.A_!/'V_.d., r.1 ,,./
X t Orl \ _/_j_ B5IaB2.0 Ia
.,_j ,
I0Per BSIbB2.01o
OAra BSlbB2.01b
9 Cep B61o
B 2.0Ib
HD92964 B6IbB2.5 Ia
.°.....116084 B 7 IoB 2.51b
3Gem B8Io
B2.5 Ib
B3 Io _ _ \ / ,,_f BsI_
",,'V"
B 4 1o , _v...Z,_
I,I, 1,1,1II,,'I I _ _ I I I I I I I I I I I I I I l I
1584 1388 1392 1396 1400 1404 1408 1412 1384 1388 1392 1396 1400 1404 1408 1412
WAVELENGTH WAVELENGTH
Fig. 3: The spectral region near 1400A for B supergiants from BI.5 to B9.
133
I / I I I I Ir_ I I I I I
1200 H N NV 1240
C C_' 1548
I000 1 _ • Si_' 1394
vT _
600- ..... -
li o
-_ Q • -
400 - Vesc
Q
I I I I I I I I I I.
BI.5 B2.0 B2.5 B3 B4 B5 B6 B7 B8 B9
F__ 4: Terminal velocities vT derived from the resonance lines of N V,
C IV and Si IV 1394. A box surrounds the measurements of one star in those
cases in which more than one of the lines appears in the spectrum. Also
shown in the run of escape speed for the supergiants of spectral type
BI.5 to B9.
134


Anomalous ionization seen in the spectra of B supergiants

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