The International Ultraviolet Explorer (IUE) spectra of alpha CMi (F5 IV-V), beta Cas (F2 IV), alpha Car (F0 Ib), and gamma Boo (A7 III) in the context of the question as to whether chromospheres disappear in the early F late A portions of the Hertzsprung-Russell diagram. Both alpha CMi (Procyon) and beta Cas show bright emission line spectra indicative of chromospheres and transition regions, but neither alpha Car (Canopus) nor gamma Boo show any evidence of emission in their SWP spectra or at the Mg II lines, despite very deep exposures. Alpha CMi has emission line fluxes roughly 6 times those of the quiet Sun, but the rapidly rotating delta Scuti type variable beta Cas has surface fluxes 10 to 50 times those of the quiet Sun. Upper limits on emission line fluxes for alpha Car are 4 to 20 and for gamma Boo 15 to 80 times the quiet Sun. It is concluded that the apparent absence of emission lines in the spectra of alpha Car and gamma Boo should not be interpreted as due to the absence of nonradiatively heated outer atmospheres in stars hotter than spectral type F0, but rather to the inability to see emission lines with IUE against a background of scattered light and a bright photospheric absorption line spectrum either in low or high dispersion

Linsky, J. L.

Marstad, N. C.

English

NASA Technical Reports Server

IUE SPECTRA OF F AND LATE A STARS 1
Jeffrey L. Linsky 2'3 and Norman C. Marstad
Joint Institute for Laboratory Astrophysics
National Bureau of Standards and University of Colorado
ABSTRACT
We report on IUE spectra of _ CMi (F5 IV-V), B Cas (F2 IV), _ Car
(F0 Ib), and y Boo (A7 III) in the context of the question as to whether
chromospheres disappear in the early F-late A portions of the HR diagram.
Both _ CMi (Procyon) and 8 Cas show bright emission line spectra indicative
of chromospheres and transition regions, but neither _ Car (Canopus) nor
y Boo show any evidence of emission in their SWP spectra or at the Mg II
lines, despite very deep exposures. These results are consistent with those
recently published by BShm-Vitense and Dettmann. We note that _ CMi has
emission line fluxes roughly 6 times those of the quiet Sun, but the rapidly
rotating _ Scuti-type variable _ Cas has surface fluxes 10-50 times those of
the quiet Sun. Upper limits on emission line fluxes for _ Car are 4-20
times those of the quiet Sun and for y Boo are 15-80 times the quiet Sun.
We conclude that the apparent absence of emission lines in the spectra of
Car and ¥ Boo should not be interpreted as due to the absence of nonradia-
tively heated outer atmospheres in stars hotter than spectral type F0, but
rather to our inability to see emission lines with IUEagainst a background
of scattered light and a bright photospheric absorption line spectrum either
in low or high dispersion.
INTRODUCTION
We consider here the important question of whether the outer atmospheres
of stars change in a fundamental way near spectral type F0 as convection
zones become thin and carry little flux with increasing stellar effective
temperature. The earliest-type stars with Ca II H and K line emission noted
by Wilson (1966) in his I0 A mm -I photographic survey are at spectral type
F5. These data, together with the empirical result that stellar rotational
velocities decrease rapidly in the middle F stars (e.g. Kraft 1967) and the
theoretical result that acoustic wave heating (the presumed chromospheric
heating source) should decrease rapidly in the early F-type stars, led many
authors to conclude that chromospheres disappear in the middle F stars.
Subsequently, using higher dispersion spectra, Warner (1966,1968) found weak
iThis work was supported by NASA through grants NAS5-23274 and NGL-06-003-057
to the University of Colorado.
2Staff Member, Quantum Physics Division, National Bureau of Standards.
3Guest Observer with the International Ultraviolet Explorer (IUE___)satellite.
287
https://ntrs.nasa.gov/search.jsp?R=19810017387 2020-03-21T12:27:28+00:00Z
Ca II emission in Canopus (F0 Ib) and y Vlr N (F0 V), and Le Contel et al.
(1970) reported occasional Ca II emission in the _-Scuti-type star y Boo
(A7 III). Frelre (1979) and Frelre et al. (1977,1978) have searched for
emission in the Ca II lines and ultraviolet C II and Si ll lines in Vega
(AO V) and two other early A-type stars without success.
BShm-Vitense and Dettmann (1980) have observed 21 F- and 13 A-type
stars with__IUE. They find that chromospheric emission lines begin to appear
at (B-V) _ 0.32 (about spectral type F0) on the main sequence, but at the
Cepheid instability strip for the more luminous stars. Our observations con-
firm their identification of an empirical dividing line for the appearance oE
chromospheric emission lines in the Hertzsprung-Russell (HR) diagram, but we
feel that the data cannot determine whether chromospheres disappear in this
region of the HR diagram.
OBSERVATIONS
We summarize the stellar parameters and our IUE observations in Tables [
and 2. Since our goal was to search for emission features superimposed on a
bright backgrouod consisting of the stellar ultraviolet absorption llne spec-
trum and scattered near ultraviolet light, we obtained several exposures in-
cluding very long exposures in which the long wavelength portions of the
spectra are heavily overexposed.
The composite low dispersion spectra (deleting saturated pixels) are
presented in Figure I in absolute flux units at Earth. The calibration fac-
tors used are those described by Turnrose et al. (1980) and Cassatella et al.
(1980), except that the small aperture e CMi spectra were calibrated using
the low dispersion fluxes of the strong C II %1335 and C IV %1550 doublets
cited by Brown and Jordan (1980). These data imply that the transmission
of the small aperture for our e CMi observations was 0.33. Since the _ CMI
spectra were obtained with the small aperture, we are able to measure fluxes
in the Si III %1206 and L_ features accurately.
Clearly both e CMi and 8 Cas exhibit bright emission lines due to chro-
mospheric ions (0 I, C I) and ions (C II, C III, C IV, Si IV, N V) formed at
temperatures of 20-200 x 103 K, perhaps in geometrically thin regions analo-
gous to the solar transition region. However, neither _ Car nor _ Boo show
any of these emission lines in the low dispersion data. A high dispersion
SWP spectrum of e Car also shows no evidence for any emission features.
High dispersion spectra of the Mg II features in 8 Cas, e Car, and ? Boo
are shown in Figure 2. These spectra show no clear evidence of Mg II emis-
sion, despite the very long exposure times to bring up the line cores to typi-
cally 150 DN. B_hm-Vitense and Dettmann (1980) call attention to a strong
correlation between M_ II emission and the appearance of chromospheric emis-
sion lines in the SWP images. We feel that the apparent absence of Mg II
emission in B Cas is probably due to rotational smearing of a weak emission
feature in the core of an absorption line. Emission features appear in the
1200-1600 region of this star despite rotation smearing, because the contrast
between the emission line flux and 6 A of continuum is large. We find no
288
evidence for Mg II emission in _ Car (see Fig. 2), although Evans et al.
(1975) identify emission features in their Copernicus V2 spectrum of the star.
We feel that our data have higher signal-to-noise and higher spectral resolu-
tion than their data. Thus we feel that the absence of Mg II emission in our
data is real. Also interstellar absorption features in the cores of the Mg II
lines can suggest double emission features which may not be present.
We list in Table 3 observed fluxes at Earth and probable identifica-
tions of emission features we consider to be real. Also given are flux
upper limits for _ Car and y Boo. These are estimated as roughly equal to
the flux in adjacent "emission" features with widths comparable to emission
lines in _ Cas and measured above a curved line drawn through the low points
in the spectra. These "emission" features are probably stretches of con-
tinuum between absorption lines. Thus the "noise" in the _ Car and y Boo
spectra is not true noise but rather the up and down character of absorption
line spectra. Our estimates of emission line flux upper limits thus refer
to the maximum emission line flux that could be confused with the absorption
line spectrum and thus not identified as an emission line.
The observed fluxes were then converted to stellar surface fluxes using
the Barnes-Evans relation for stellar angular diameters (cf. Linsky et al.
1979). The derived angular diameters and ratios of surface flux to flux ob-
served at Earth are given in Table i. Listed in Table 4 are quiet Sun sur-
face fluxes cited by Linsky et al. (1978) and the ratios of stellar surface
fluxes to quiet Sun surface fluxes.
DISCUSSION
The _ CMi surface fluxes are about aZfactor of 6 times larger than the
quiet Sun, whereas those of _ Cas are lypically 30 times larger than the
quiet Sun. Brown and Jordan (1980) estimate transition region pressures for
CMi consistent with pressures at the top of the chromospheric model de-
rived by Ayres et al. (1974). Since transition region surface fluxes in
solar plages are typically 10 times the quiet Sun, the nonradiative heating
rates in the outer atmosphere of B Cas exceed those of solar plages by a
factor of 3. We know that chromospheric nonradiative heating rates are well
correlated with stellar rotational velocities for stars of similar spectral
type (e.g. Kraft 1967, Skumanich 1972), and there is growing evidence of a
correlation between nonradiative heating rates in transition regions and
coronae as well (Ayres and Linsky 1980, Linsky 1980). The significant in-
crease in emission line surface fluxes from _ CMi to B Cas is consistent
with this picture. An alternative explanation for the large heating rate
in the outer atmosphere of B Cas is dissipation of shock waves excited by
the stellar oscillations.
We feel that the upper limits on the surface fluxes for emission lines
in _ Car and y Boo are significant. For example, if one added emission lines
with surface fluxes comparable to those of _ CMi to the observed spectrum of
Car, these emission lines would be extremely hard to detect against the
bright absorption line spectrum of _ Car. Similarly, emission lines with
surface brightnesses even as large as those of B Cas would be undetectable
289
Lagainst the bright absorption line spectrum of y Boo. Thus, with IUE we are
unable to determine whether or not chromospheres and transition regions cea_e
to exist as one proceeds from the early F to hotter stars. All we can say
is that our data and those of B_hm-Vitense and Dettmann (1980) show that
chromospheres and transition regions cease to appear spectroscopically in
the ultraviolet as observed with an instrument like IUE.
It is important to recognize the instrumental and astrophysical reason_
behind the latter statement. The instrumental limitations of IUE are its
limited signal-to-noise, sensitivity of the short wavelength cameras to
scattered long wavelength light, halation in the image converters, and the
potential for SECVidicon damage when oversaturating the long wavelength por-
tion of an image to bring up the relatively weak short wavelength portion of
the spectrum. As a result, We would caution future observers against taking
even deeper exposures of early F- and late A-type stars, and we expect that
deeper exposures would not be productive in any case. The High Resolution
Spectrograph (HRS) now being built for Space Telescope (Brandt et al. 1979)
will not have many of the limitations of IUE as it will include a solar
blind, photon-counting detector capable of very high signal-to-noise, large
dynamic range, and up to 1.2 x 105 resolution. With this instrument we ex-
pect that chromospheric emission lines will be detected in the late A-type
stars until at some spectral type we will run into the fundamental astro-
physical limitation of detecting weak emission features against a very
bright photospheric absorption line spectrum.
Finally, we expect that most stars hotter than spectral type F0 probably
contain nonradiatively heated outer atmospheres. This is based on the detec-
tion by Einstein of X-rays from many types of hot stars including O-type and
WR stars, the A-type dwarfs Sirius A and Vega, and _ Car (Vaiana et al. 198(!).
Also, Underhill (1980) has presented evidence for nonradiatively heated outer
atmospheres (mantles) for O, B, and A-type supergiants. Thus nonradiatively
heated outer atmospheres can be produced even in the absence of deep convec-
tion zones. Linsky (1980) and Underhill (1980) argue that magnetic fields,
both remnant and dynamo regenerated, are responsible for nonradiative
heating in most if not all stars.
We wish to thank Dr. A. Boggess and the staff of the IUE Observatory
for their assistance in the acquisition and reduction of these data. We
also wish to thank Dr. C. Jordan for permission to use her IUE fluxes of
CMi prior to publication.
REFERENCES
Ayres, T. R. and Linsky, J. L. 1980, Ap. J., in press.
Ayres, T. R., Linsky, J. L., and Shine, R. A. 1974, Ap. J., 192, 93.
B_hm-Vitense, E. and Dettmann, T. 1980, Ap. J., 236, 560.
Brandt, J. C. et al. 1979, SPI____E,172, 254.
Brown, A. and Jordan, C. 1980, in Proc. Second Year of IUE, Tubingen, in
press.
Cassatella, A., Holmj A., Ponz, D., and Schiller, F.H. 1980, NASA IUE
Newsletter, No. 8, p. I.
290
Evans, R. G., Jordan, C., and Wilson, R. 1975, M.N.R.A.S., 172, 585.
Freire, R. 1979, Astr. Ap., 78___,148.
Freire, R., Czarny, J., Felenbok, P., and Praderie, F. 1977, Astr. Ap.,
61____,785.
• 1978, Astr. Ap., 68, 89.
Johnson, H. L., Mitchell, R. I., Iriarte, B., and Wisniewski, W. Z. 1966,
Comm. Lunar Planetary Lab., 4, 99.
Kraft, R. P. 1967, Ap. J., __150,551.
Le Contel, J. M., Praderie, F., Bijaoui, A., Dantel, M., and Sareyan, J. P.
1970, Astr. Ap., _8, 159.
Linsky, J. L. 1980, Proc. WorkshOp on Cool Stars_ Stellar Systems and the
Sun, S.A.O. Special Report, in press.
Linsky, J. L. et al. 1978, Nature, 275, 389.
Linsky, J. L., Worden, S. P., McClintock, W., and Robertson, R.M. 1979,
Ap. J. Suppl., 41, 47.
Skumanich, A. l_72_---Ap.J., 171, 565.
Turnrose, B., Bohlin, R., and Harvel, C. 1980, NASA IUE Newsletter , No. 8,
p. 28.
Uesugi, A. and Fukuda, I. 1970, Cont. Inst. Ap. and Kwasan Obs., Unlv• of
Kyoto, No. 189•
Underhill, A. B. 1980, preprint.
Vaiana, G. et al. 1980, submitted to _.
Warner, B. 1966, Observatory, 86, 82.
• 1968, Observatory, 88, 217.
Wilson, O. C. 1966, Science, 151, 1487.
Table I
STELLAR PARAMETERS
Star HD Spectral Va V-R a Ang. Dia. F/fb v sin ic
Type (milliarcsec) (km s-I)
CMi 61421 F5 IV-V 0.37 0.42 5.94 4.82(15) 6
8 Cas 432 F2 IV 2.27 0.31 1.99 4.29(16) 72
Car 45348 F0 Ib -0.75 0.24 6.97 3.50(15) 0
y Boo 127762 A7 III 3.02 0.14 I.O1 1.67(17) 145
aFrom Johnson et al. (1966).
bRatio of surface flux to flux observed at Earth.
CRotational velocities from Uesugi and Fukuda (1970).
291
Table 2
SIIMMARY OF IUE OBSERVATIONS
Star Spectral IUE Exp. Time Aperture a Dlsperslon b
Type Image (Min. )
CMI F5 IV-V SWP 1306 30 S L
SWP 1317 30 S HI
SWP 1318 6 S L
SWP 1319 i0 S L
SWP 1320 20 • S L
Cas F2 IV SWP 2372 26 L L
SWP 2373 6.5 L L
LWR 2156 ii L HI
Car F0 Ib SWP 2302 30 L HI
SWP 5439 i L L
LWR 2083 0.67 L HI
LWR 4703 3 L HI
Boo A7 III SWP 2395 24 L L
LWR 2172 16 L HI
= small, L = large.
= low, HI = high.
Table 3
SUMMARY OF OBSERVED FLUXES a
Wavelength Ion _ CMi B Cas _ Car e y Boo c
1175 C III 1.1(-12)
1206 Si III 3.5(-12)
1216 H Ib 4.1(-11) 1.6(-11) <8(-12)
1239 N V 1.5(-12) 3.2(-13) <5(-12) <2(-13)
1253 S II? 3.9(-13)
1273 C I 2.8(-12) 4.4(-13):
1304 O I 3.1(-12) 2.3(-12) <5(-12) <4(-13)
1316 6.9(-13): 3.5(-13):
1335 C II 7.5(-12) 2.0(-12) <5(-12) <4(-13)
1354 O I 1.2(-12): 5.5(-13)
1371 O V 1.0(-12):
1394 Si IV 1.1(-12) 7.3(-13) <5(-12) <4(-13)
1403 Si IV+O IV 6.6(-13) 1.1(-12) <5(-12) <4(-13)
1440 2.0(-12) :
1466 C I 3.8(-12)
1549 C IV I.I(-II) 4.7(-12) <i(-I I) <8(-13)
aFluxes at Earth (ergs cm-2 s-i).
buncorrected for interstellar absorption.
CUpper limits.
292
Table 4
STELLAR SURFACE FLUXES AND STELLAR/QUIET SUN SURFACE FLUX RATIOS
CMi b 8 Cas _ Car c _ Booc
Feature Quiet a
Sun SF SF/QS SF SF/QS SF SF/QS SF SF/QS
N V 1239 8.6(2) 7.2(3) 8 1.4(4) 16 <1.8(4) <21 <3.3(4) <38
C IV 1550 5.8(3) 5.3(4) 9 1.8(5) 31 <3.5(4) <6 <1.3(5) <22
Si IV 1394 1.7(3) 5.3(3) 3 3.1(4) 18 <1.8(4) <ii <6.7(4) <39
Si IV+O IV 1403 9.4(2) 3.2(3) 3 4.6(4) 49 <1.8(4) <19 <6.7(4) <71
C III 1175 1.6(3) 4.5(4) 28
Si III 1206 3.4(3) 1.7(4) 5
C II 1335 4.6(3) 3.6(4) 8 8.6(4) 19 <1.8(4) <4 <6.7(4) <15
O 1 1304 4.0(3) 1.5(4) 4 1.4(5) 35 <1.8(4) <5 <6.7(4) <17
O 1 1355 3.2(2) 5.8(3): 18: 2.4(4) 75
H 1 1216 2.1(5) 2.0(5) d I 6.9(5) d 3 <2.8(4) d <0.I
aQulet Sun fluxes cited by Linsky et al. (1978). Units ergs cm-2 s-I.
bAbsolute flux calibration by comparison with C II %1335 and C IV %1549 low dispersion fluxes in
Brown and Jordan (1980).
Cupper limits estimated as equal to typical "noise" features in the spectra, which are probably
sections of continuum between absorption features.
duncorrected for interstellar absorption.
_ 0 .... I +"--r', l l .... I .... I -r'-. , , I .... I .... I .... I I_F'I
I
7" Boo (A7 TIT) Cl?
'+ t
-i
--- S
T 8 --.- cTr ¢ j
- .HI N3Z: OTII/)
(M 4' ..J ' i ^ r __ / .-/NJ _-
'E
U1 0 _l ..... , ..... , , , _ I , , , t I
11..
e Cot (Fo Ib)
I°"'1--r 200100 I ,, ,rr
0 , i , r I , , , I ...... i , I i ...... L I , , , I I
,,,F-
• < 12 /9 Cos (g2 ]2) w .
x _!
-J 8h
° fW 4
W
om I _ _J_ Iv
30
,s _' IL.
CM' (FS ]:2-V) t
0 J- J + J I , , , , I , , , , I , , , , I ...... _ , l i , i , I L K K , I , L ¢ L I_
1200 1300 1400 1500 1600
WAVELENGTH (,&)
Fig. 1. Composite low dispersion spectra in absolute flux units at Earth.
Vertical lines indicate the location of important emission lines
that are present in the spectra of B cas and _ CMi but absent in
the spectra of T Boo and _ Car. lhese spectra are saturated beyond
about 1600 A, and scattered light is probably a major contributor
to the apparent continua.
294
8 ' ' ' ' ' '"' ' ' 1 ....... " ' I ........ ' I ......... I
I
4
I
u'}
¢,,,j
,E_-o Y I -_, 7 Boo (A7 TTT)"- I
-,o_- ,
--_o- '1 _ "
m 20
!-- i
i
x
J
tu
- I a Car(Fo Ib)
a
W 0 ......... I ........ I, ,, ,_1,, ,, I,, , ...... I
uJ,_ __,
m 6
4. --
B Cas,(F2 ,]_,)
2" ....... ,,I, ....... ,I ......... ......
2790 2795 2800 2805 2810
WAVELENGTH (_)
Fig. 2. High dispersion spectra in absolute flux units at Earth, obtained
by merging the two echelle orders containing the Ng II lines. De-
spite the deep exposures used to bring up the cores of the Mg II
lines, there are no apparent emission features. The deep absorp-
tion features in the Mg II lines of _ Car are interstellar.
295


IUE spectra of F and late A stars

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