A survey of F supergiant stars to evaluate the extension of chromospheric and circumstellar characteristics commonly observed in the slightly cooler G, K, and M supergiant is discussed. An ultraviolet survey was elected since UV features of Mg II and Fe II might persist in revealing outer atmosphere phenomena even among F supergiants. The encompassed spectral types F0 to G0, and luminosity classes Ib, Ia, and Ia-0. In addition, the usefulness of the emission line width-to-luminosity correlation for the G-M stars in both the Ca II and Mg II lines is examined

Giampapa, M. S.

Stencel, R. E.

Worden, S. P.

English

NASA Technical Reports Server

UV CHROMOSPHERIC AND CIRCUMSTELLAR DIAGNOSTIC FEATURES
AMONG F SUPERGIANT STARS
Robert E. Stencel
Joint Institute for Laboratory Astrophysics
University of Colorado and National Bureau of Standards
Simon P. Worden
Sacramento Peak Observatory, AURA & S.A.M.S.O./U.S. Air Force
and
Mark S. Giampapa
Steward Observatory, University of Arizona
INTRODUCTION
We undertook a survey of F supergiant stars to evaluate the extension
of chromospheric and circumstellar (CS) characteristics commonly observe_ in
the slightly cooler G, K and M supergiant spectra. In the optical re-
gions, the usual diagnostic spectral features are swamped by the brighter
photospheric light in F stars. Therefore, an ultraviolet survey was elected
since UV features of Mg II and Fe II might persist in revealing outer atmos-
phere phenomena even among F superglants. Our survey encompassed spectral
types F0 to GO, and luminosity classes Ib, la and la-O.
PROFILETYPES
Four generic types emerged from our limited survey -- two indicative of
chromospheres, one strictly photospheric and one indicative of CS envelopes
(Figure I).
CHROMOSPHERICPROFILES
Optical and UV observations of the Sun and other G, K and M stars have
revealed that the emission cores of the Ca II "H&K" (3933,3968 _), and Mg II
"h&k" (2795,2802 _) resonance lines are valuable indicators of the existence
of and conditions in stellar chromospheres. Such lines are very optically
thick at line center and collisionally coupled to the excitation temperature
(Te) in the stellar atmosphere to heights above the temperature minimum
and into the chromospheric temperature rise [cf. Stencel et al. (i)].
Doubly reversed emission is seen in the F8 Ib and GO Ib stars surveyed,
including yCyg, G UMi and _ Aqr. However, the F8 profiles are strikingly
different from the GO type, despite a very small difference in Te. _ Aqr
shows a strong longward dominated emission with substantial CS absorption
superposed on the shortward emission component. In contrast, yCyg and
UMi show far weake_ doubly reversed central emission features. In YCyg
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it is difficult to be certain that the shortward emission component is not
affected by CS absorption. B_hm-Vitense and Parsons have described 7Cyg as
lacking in Mg II emission, suggesting to us that chromospheric or CS varla-
bility is at play, possibly as has been now seen for GAqr (G2 Ib).
Among more luminous late F supergiants we find another Mg II profile
type, characterizedby _ CMa (F8 la), which shows strong shortward emission
near llne center, but no longward emission. This profile type is reminis-
cent of an inverse P Cygnl profile and has been seen also in HD 96918 (GO
la-0) and pCas (F8p la), where it is even more pronounced. To avoid the
difficulties of invoking global chromospheric collapse (inverse of the
standard P Cygni explanation), we could explain the appearance of such
profiles by noting an excess longward absorption component, perhaps due to
large scale downflows along the edges of giant cells (as seen in the solar
network). This could obscure the longward emission and leave us with the
observed inverse P Cygnl profile. It is noteworthy that we find this chro-
mospheric profile type only for stars of the highest intrinsic luminosity.
PHOTOSPHERICPROFILES
As _e look to F stars of warmer Te, evidence in Mg II for chromo-
spheric emission vanishes, presumably due to the increasing degree of ioni-
zation. For spectral types F0 to F5 we find repeatedly a smooth broad
absorption feature with strongly damped line wings and a narrow Doppler core
of essentially zero residual intensity. Examples of this include GCar (F0
Ib), pAql (F2 IB) and G Per (F5 Ib), all objects of comparable Mv. Among
objects of greater intrinslc luminosity we find a hybrid profile -- strong
damping line wings with a boxy, non-Doppler core. This boxy dark core is
probably CS in orlgin and is seen in i_ Sco (F2 la), HD 74180 (F2 la) and
in 89 Her (F2 la), except that 89 Her also reveals a trace of longward
emission next to the CS absorption at llne center. Again we find a distinct
luminosity difference in the Mg II profiles.
CIRCUMSTELLARPROFILES
Finally we detected one object which may provide the llnk between
strong mass-losing stars like _Cyg (A2 la) and _ Aqr (GO Ib) in having
qualitatively similar Mg II profiles. GLep (F0 Ib) was found to have a
pair of strong CS components shifted by -1.24 and -1.73 _ from llne center.
No chromospheric emission is obvious, although strong interstellar absorp-
tion at line center could obscure this. This CS structure suggests a sub-
stantial envelope and extensive mass loss. Alternately, one might identify
the region between CS and line center absorptions as due to emission, but
rather arcane radiative transfer effects might be required to verify such a
picture. Success in CS profile modeling of GCYG by Kunasz should help to
clarify the meaning of the GLep profile.
THE OUTER ATMOSPHERESOF FSUPERGIANTS
With the IUE observations of our survey, it is possible to make some
general comments regarding the atmospheric structure of F superglant stars.
312
DETECTIONOF CHROMOSPHERES
We have obtained high dispersion Ca II K line spectra for many of the
stars in our UV study and find in general that outer atmospheric structure
is far more apparent in the Mg II k line than in Ca II K. Hence the neces-
sity for UV observations in this area is now clear. Deep exposures of the
Ca II K line center for _ CMa and _ Lep revealed only photospheric absorp-
tion without a hint of the striking profile structure seen in Mg II k. This
occurs because the abundances and ionizations are different, but the general
lack of significant chromospheric emission in Ca II K and Mg II k places
strong constraints on the geometric extent of chromospheres since radiative
losses in these lines (dominant sinks for G, K and M chromospheres) are
minimal. Either the losses are confined to resonance lines of higher ions
(hot chromospheres) or the chromospheres are "thin." Additional far UV and
EUV observations will clarify this matter.
FAILURE OF THE WILSON-BAPPUEFFECTS
One of the aims of our survey was to determine the usefulness of the
emission line wldth-to-luminosity correlation in use for the G-M stars in
both the Ca II and Mg II lines [Wilson and Bappu (2), Weiler and Oegerle
(3)]. In Table 1 we collect relevant information on our survey stars and
present measured widths and infer Mv where possible. Inspection reveals
that only the F8-G0 la-0 stars are even approximated in this way. We con-
clude that the Wilson-Bappu and Weiler-Oegerle correlations are probably not
suitable for stars earlier than GO, because of significant physical differ-
ences in emission core formation as compared to cooler stars. We have
already noted the striking difference between F8 and GO Ib stars despite the
small change in Te. Some combinations of rotation, pulsation, evolution
and magneto-acoustic effects makes the respective chromospheric signatures
quite different. Whereas the wing emission lines of Ca II [Stencel (4)] are
useful among F-M stars in evincing mass loss and indicating Mv, we found
no comparable features in the Mg II line wings among F supergiants.
VELOCITYFIELDS
Where Mg II emission cores are seen, asymmetries can be interpreted as
indicative of globally averaged motions. The near symmetric profiles of
Cyg and _UMi suggest that inflow and outflow roughly cancel -- as they
would in large scale convection-like motions. The inverse P Cygni profile
of the F8 la-0 stars is more difficult to understand, but could arise from
extensive network-like structures which are the sites of material downflow,
vis-a-vis the Sun. A height dependent ionization transition could help
accomplish this, i.e., hot (Mg III) matter wells up at the centers of large
cells and cools (Mg II) as it begins its descent along the cell boundaries
(the network). The alternative explanation for an inverse P Cygni profile,
that of a collapsing envelope, might have its place in the evolutionary
scenario for such volatile objects, but it is difficult to reconcile with
other indicators for substantial stellar winds and mass loss, especially for
p Cas.
3 13
Where CS core absorption is seen, assuming one can disentangle the in-
terstellar component, the width and/or displacements from llne center argue
for significant stellar winds, mass loss and CS envelopes. As has been
described by Lamers et al. (5) for GCyg, stars llke GLep may provide
additional evidence for the sporadic, non-continuous or "puffy" mass loss
since we clearly detect a pair of outward moving shells.
FUTURERESEARCH
With the limited time available for our survey, we didnot have the
opportunity to explore the SWP (1200-2000 _) region of F supergiant spectra,
nor the detailed comparison between Cepheld and non-Cepheld objects in this
region of the HRD. Because of increasing continuum brightness, it will
probably be difficult to clearly identify emission lines arising from any
analog of the solar 'transition region' (TR: 100,000 to 250,000 K), or from
cooler coronal hole outflow sites (CH: <25,000 K). Representative lines
would include: TR -- N V 1240 _, Si IV blend at 1400 _ and C IV 1550 _;
CH -- 0 1 1300 _. Again, B_hm-Vltense and Parsons have surveyed similar
stars and hopefully will describe their findings at this conference and
elsewhere. A comprehensive survey would clarify the distribution of
chromospheres, TR, coronae and CS material among the F supergiants.
We thank A. Boggess and his dedicated staff for assistance in obtaining
and analyzing data under program LFBSW, as well as for contractual support.
Mg II region spectral plots of stars mentioned herein will be available on
request for a limited time from the first author.
REFERENCES
I. Stencel, R., Mullan, D., Linsky, J., Basri, G., and Worden, S. "The
Outer Atmospheres of Cool Stars. VII. High Resolution, Absolute Flux
Profiles of the Mg II h & k Lines in Stars of Spectral Types F8 to M5,"
1980 Astrophys. J. Suppl., in press.
2. Wilson, O. and Bappu, V. "H and K Emission in Late-Type Stars: Depen-
dence of Line Width on Luminosity," 1957 Astrophys. J. 125, 661.
3. Weiler, E. and Oegerle, W. "A COPERNICUS Survey of Mg II Emission in
Late-Type Stars," 1979 Astrophys. J. Suppl. 39, 537.
4. Stencel, R. "Extensions of the Wilson-Bappu Effect Among Very Luminous
Stars," 1978 IAU Colloquium 80: The HertzsRrung-Russell Diagram_, ed. A.
Davis-Philip and D. Hayes (D. Reidel, Dordrecht), p. 59.
5. Lamers, H., Stalio, R., and Kondo, Y. "A Study of Mass Loss from the
Mid-UV Spectrum of _Cyg, _Ori and 6 Leo," 1978 Astrophys. J. 223, 949.
314
TABLE i. - Comparison of Mg II k and Ca II K in F Supergiants
Mg II k Ca IIK
Sp. UV Fe II
...... _a b c Profile Wo c >Iv(K)e Emission?Star Type .w_r Exp. Mv Profile Wo Mv(k)d
Car F0 Ib 4703 3 -4.8 No em. -- -- No obs. No
Lep F0 Ib 5021 16 -4.5 CS&IS 1.2CS -- No em.+CS 1.0a -- No
Aql F2 Ib 5015 70 -4.7 Wk.em? .... No era. 0.9CS? -- No
tI Sco F2 la 5046 25 -8.4 CS core 1.4CS -- No em. 1.1CS? -- No
!{D 74180 F2 Ia 5043 60 -g.4 CS core 1.0CS -- No. obs. No
p89 Her F2 Ia 5047 i00 -8.4 _S+em. 2. ICS -- CS+w.e.* 2.2CS -- Yes
Per F5 Ib 5018 14 -4.6 No era. -- -- Wk. era. 1.5e? -2.g No
_o 8 CMa F_q la 5020 20 -7.5 Inv. P Cyg 2.3_ -5.0 No era. 0.3core -- Weak?
UMi FS_Ib 5017 15 -4.6 DR era. 1.8 +0.3 No era. 0.6core -- No
y Cyg F8 Ib 5016 25 -4.6 DR em+CS? 2.3 -1.3 No em.+w.e. 0.6core -- No
p Cas F8p la 5014 447 -9: Inv. P Cyg 2.1HW -5.3 CS+w.e. 1.8CS -- Yes
HD 96918 GO laO 4702 75 -9: Inv. P Cyg 2.9FIW -7.4 No obs. Yes
6 Aqr GO Ib 4446 18 -4.5 DR em.+CS 3.8 -4.6 Wk.em. 2.0 -4.7 No
aLarge aperture, high dispersion; exposure time in minutes.
bData from Patterson and Neff, 1979 Astrophys. J. Suppl. 41, 215.
CEmission base width (AXk 1), half width (HW) or CS absorption width, in A.
dMv(k) = -15.15 log Wo(km/sec ) + 34_93.
eMv(K ) = -15.0 log W (km/sec) + 28_0.
Wing emission lines, cf. Stencel 1977 Astrophys. J. 215, 176.
Mg TT Profile Types in F Supergiant Stars
a) Chromospheric aUMi b) Photospheric a Per
J
- I I
I Xo ' Xo
x
u_ c) Inverse P Cygni _ CMo d) Circumstellor a LepI
k_
1 -k4
I
- LXo _ - LXo
-- Wavelength = ,
Figure i
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UV chromospheric and circumstellar diagnostic features among F supergiant stars

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