The central results of a survey of the Mg II resonance line emission in a sample of over 50 evolved late type stars, including spectral-luminosity type F8 to M5 and La to IV are presented. Observed and surface fluxes are derived and correlations noted. The major findings include: (1) Mg II k emission core asymmetry transition near K1 III, analogous to that known for Ca II K; (2) a small gravity and temperature dependence of the Mg II chromospheric radiative loss rate

Basri, G. S.

Linsky, J. L.

Mullan, D. J.

Stencel, R. E.

English

NASA Technical Reports Server

HIGH RESOLUTION ABSOLUTE FLUX PROFILES OF THE MG II h & k
LINES IN EVbLVED F8 TO M5 STARS
Robert E. Stencel 1
Joint Institute for Laboratory Astrophysics
University of Colorado and National Bureau of Standards
Dermott J. Mullan I
Bartol Research Foundation, University of Delaware
Gibor S. Basri
Department of Astronomy, University of California at Berkeley
Jeffrey L. Linsky 1'2
Joint Institute for Laboratory Astrophysics
University of Colorado and National Bureau of Standards
"One in Five Stars is Remarkable" -- O. Struve
INTRODUCTION
We present the central results of a survey of the Mg II resonance line
emission in a sample of over 50 evolved late-type stars, including spectral-
luminosity types F8-M5 and la-lV. Observed and surface fluxes have been
derived and correlations noted. The major findings include: a) Mg II k
emission core asymmetry transition near K1 III, analogous to that known for
Ca II K; b) a small gravity and temperature dependence of the _ II chromo-
spheric radiative loss rate. These results and others are fully discussed
in a report in press in the Astrophysical Journal Supplement Series [Stencel
et al. (I)], encompassing _!E second year programs CCBDM, CMBRS and LFBSW.
MESSAGES IN T]_ MG II PROFILES
We posit that asymmetry in the doubly-reversed emission core of colli-
sionally-dominated resonance lines (e.g. Mg II k, Ca II K) can be used to
infer chromospheric velocity fields, once the interstellar component is ac-
counted for. We recognize the uniqueness problems of inhomogeneous atmo-
spheres with arbitrary relative motions. However, the thrust of the observed
asymmetry transitions and Occam's Razor provide us with some degree of con-
fidence in the interpretation we choose.
A SHORT HISTORY LESSON
A fascinating observational connection between chromospheric velocity
fields, mass loss and the existence of stellar coronae has developed over the
past few years. The extensive high dispersion work on cool stars by Olin
IGuest Observer with the International Ultraviolet Explorer (IUE) satellite.
2Staff Member D Quantmn Physics Division, National Bureau of Standards.
317
https://ntrs.nasa.gov/search.jsp?R=19810017391 2020-03-21T12:27:39+00:00Z
Wilson has been re-surveyed by Reimers (2) and Stencel (3) who discussed
evidence for mass loss in Ca II K4 features and the occurrence of Ca II "wing
emission" features, respectively, in the Hertzsprung-Russell Diagram (HRD).
Both found evidence for mass loss above a locus in the HRD running between
mid-K giants and early-G supergiants. Stencel (4) also pointed out that the
Ca II K core emission asymmetry was K2R* dominated above a similar locus.
Contemporaneously, Mullan (5) described his seminal STL (supersonic transi-
tion locus) theory which predicts a significant increase in mass loss rate
for stars above a locus similar to that just described observationally. In
addition to velocity information, temperature information has been added by
Linsky and Haisch (6)and Vaiana et al. (7) who looked for high excitation
(i00,000 K) UV lines, and soft X-ray flux, respectively, and found that stars
cooler and more luminous than K1 III rarely exhibit evidence for outer atmo-
sphere material much hotter than about 20,000 K. The anti-correlation of
coronae and strong mass loss seemed established. The analogy with solar
coronal holes and active region loops was irresistible. But the simplicity
of the scenario may also prove to be its downfall, as we shall see shortly.
STUDY OF THE MG II EMISSION ASYMMETRY
We undertook a survey of cool stars to establish whether Mg II might
exhibit an asymmetry bifurcation in the HRD comparable to that known for
Ca II K. Our initial sample of over 40 late-type stars (G and K, luminosity
classes II, III and IV) showed that a rough segregation does exist along the
lines of that seen for Ca II [Stencel and Mullan (8)]. However, the asymme-
try locus was "fuzzy" with several discrepant stars on either side of the
dividing line. Initially we considered these discrepancies as due to small
number statistics, but careful analysis of the strength of interstellar (IS)
_ II absorption by B_hm-Vitense has clarified the situation. IS Mg II
column densities are much larger than Ca II and result in superposition of
non-negligible IS absorption features on the intrinsic chromospheric Mg II
emission profile, even for stars closer than I00 pc. When we screen our
asymmetry data to remove IS effects (accept only S/L > I for vr > 0, and
S/L < 1 for vr < 0) the resulting 19 stars show a very clear asymmetry seg-
regation as is seen for Ca II K, but now occurring among slightly warmer
stars (KI III rather than K3 III). See Figure la. We compare these transi-
tion loci in Figure ib and note a plausible scenario emerging vis-a-vis the
STL theory; as a star evolves from the main sequence, it retains a corona
until it reaches the early K giants where it crosses the temperature dividing
line (TDL) due to the encroachment of the sonic point of the stellar wind
into the upper chromosphere, disrupting the global corona. As stellar
evolution to the right in the HRD continues, the stellar wind appears first
as outflow in the Mg II core-forming regions, and then in the deeper Ca II
core-forming layers. Finally as the mass loss becomes substantial and sus-
tained, circumstellar (CS) features begin to appear in the cores of neutral
optical lines.
We adopt the conventions that K2R or R refers to the _ line long wavelength
peak and K2V or V refers to the K line short wavelength peak. The corre-
sponding features in the Mg IIk line are L and S, respectively.
318
CORONA - MASS LOSS ANTI-CORRELATION?
Despite the appeal of such scenarios, re-evaluation of theory and new
data give us pause. The foundations of the STL theory have been challenged
by Ilolzer (9). A larger sample of IS effect-free Mg II profiles would be
desirable. Recent correlation studies between Mg II and C IV 1550 A flux by
Ayres and Marstad (i0) -- see paper by Ayres in these proceedings -- reveal
a strong functional dependence between fluxes in these two lines. The sense
of the correlation is that for a ten-fold decrease in _ II surface flux for
a given Teff, a 30-40 fold decrease occurs in C IV surface flux in normal
cool stars. Since, as we will shortly discuss, Mg II surface flux ratioed to
bolometric flux decreases with decreasing Tef f across the TDL of Figure ib
(cf. Fig. 2), C IV flux would rapidly vary across a similar range in Tef f.
Given detection sensitivities, C IV might appear to be present on one side
of the TDL and absent on the other. In addition, hotter material might be
confined to magnetic flux tubes in the chromosphere and corona, while the
cooler outflowing matter might follow magnetic open regions between loop
structures (analogous to coronal holes). The point is that for different
Teff, log g combinations, as well as age, rotation, chemical composition,
pulsation and whatever, the filling factors for either temperature regime
could vary significantly, as is evidenced by the observations. Thus, cool
stars showing strong mass loss might also possess small patches of coronal
plasma. This surface detail study may prove to be the major use for UV
spectra of cool stars. Hartmann et al. (ii) argue that Alpha Aqr (G2 Ib).
may be a good example of this, as C IV is bright but obvious CS absorption
features are seen in Ca II K and Mg II k. The lack of soft X-rays in this
case could be due to self-absorption in the CS envelope (i0). Additional
"hybrid" stars like Alpha Aqr are also found among the K giants, and pos-
sibly all along the STL locus (third year IUE work in progress). If the
Mg II-C IV-soft X-ray correlations are valid, then all stars are "hybrid"
in possessing both coronal and mass loss features to varying degrees, and
the usefulness of the term "hybrid" becomes very limited.
TALES FROM THE SUPERGIANTS
Late-type supergiants are too often considered in aggregate as cool
mass losing stars with few other individualizing characteristics. Our Mg II
survey has revealed some differences in kind between stars with Tef f differ-
ences as small as a few hundred degrees K. Also, we find a 3 to 4 times
greater chromospheric radiative loss rate in Mg II emission for cool super-
giants than is typical for higher gravity cool giants of similar Tef f.
MG II PROFILE DIFFERENCES
At least four distinct profile types are seen among F8,M5 Ib superg i-
ants. F8 supergiants like Gamma Cyg and Alpha UMi show weak doubly-reversed
Mg II emission cores. G0-G5 supergiants (Beta Aqr, Alpha Aqr, 9 Peg) exhibit
very strong emission with S>>L and substantial CS absorption at -125 km/sec.
The G5-K5 supergiants also show strong emission with S>L but with more moder-
ate strength CS absorption shifted generally less than -50 km/sec (Xi Pup,
Epsilon Peg, Lambda Vel). Finally, the coolest supergiants we have surveyed
319
(M2,M5: Alpha Ori and Alpha Her) again show strong Mg II emission with a
strong CS or selective absorption (Fe, Mn?) feature superimposed on the k2
emission component. The _ II surface flux ratioed to bolometric flux
(oTeff) appears to peak at spectral type GO and fall off steeply toward
F8, and only gradually toward M5 (Fig. 2).
EVIDENCE FOR ACOUSTIC HEATING?
Following Linsky and Ayres (13), we define the chromospheric radiative
loss rate in Mg II h & k, Rhk , as the ratio between integrated surface flux
of the h & k emission, divided by bolometric flux of the star (see also Ayres,
proceedings of this conference). In Figure 2 we present derived Rhk values
for the sample of cool stars we obtained during second year IUE programs and
note that the supergiants tend to lie a factor 3 or 4 above giants and bright
giants with comparable Tef f. Proponents of acoustic wave heating of stellar-
chromospheres have computed several orders of magnitude difference in acous-
tic flux over a similar range of gravity and temperature, in contrast to our
observational findings. However, Ulmschneider (private communication) note_
that first-order corrections to the acoustic heating theory greatly reduce
this predicted _ravity dependence, toward the observed amount. This matter
deserves further attention. We were able to find the small gravity depen-
dence in Rhk because of the uniformity of the data used, in contrast to pre--
vious studies by Linsky and Ayres (13) and Basri and Linsky (14), which
relied on UV data of varying resolution and signal-to-noise.
THE PROBLEM WITH 56 PEG
Our survey has defined observationally a subgroup of stars falling be-
tween Mg II and Ca II asymmetry dividing lines that challenges our supposed
understanding of emission line formation and chromospheric velocity fields.
56 Peg-type stars have the unique defining property of discrepant asymme-
tries between Ca II K (V/R > I) and Mg II k (S/L < i). 56 Peg (K0 Ib-llp)
is the archetype, and Sigma Oph (K3 II), Alpha Boo (K2 III) and Gamma Aql
(K3 II) are examples. Stencel and _llan (8) first called attention to 56
Peg by virtue of the nearly 4 magnitude discrepancy between Mv(Ca K) = -1.6,
and Mv(Mg k) = -5.3. SWP spectra of some of these show substantial C IV
emission line flux, indicative of a hot outer atmosphere. A more complete
report on 56 Peg itself is in preparation [Basri et al. (15)], but we wished
to provide a status report at this time.
SEMI-EMPIRICAL MODELING
In terms of the STL theory, 56 Peg-type stars have outer atmospheres in
whlch the supersonic stellar wind penetrates only into the upper chromosphere
where Mg II is formed, but not as deep as where Ca II is formed. We have
begun to derive semi-empirlcal models to match the surface flux and asymme-
tries in various UV lines, using the computation methods of Basri and Linsky
(16) for Ca II and Mg II, and of Lites and Cook (17) for C II, III and IV.
To date, experiments with meso-scale waves [Basri and Linsky (16)] are prom-
ising, but far from adequate. To produce the asymmetries observed, we need
320
m
to increase the physical segregation in height of formation between t1_e I¢3
and K3 features. One way of accomplishing this spread in the respective op-
tical depth scales is to assume a Mg/Ca abundance ratio much different from
solar. Alternately, CS _ II could be more pronounced than for Ca II due to
STL effects in the upper chromosphere.
INTE RPRETAT IONS
Waves or global pulsations remain a possible mechanism for production
of the discrepant asymmetries in this portion of the HR_, since the Pop. II
Cepheid-like RV Tau irregular variables frequently are assigned spectral
types like early K, class II. For 56 Peg itself, profile variability has
not yet been observed in the Ca II K line over several years of monitoring,
hut additional _ IIk observations are being pursued. _otospheric analysis
does not support an unusual "%/Ca ratio. If the asymmetry loci are as sta-
tistically significant as we believe they are, then a non-negligible fraction
of the HRD, early to mid K giants and early K bright giants, exhibit an un-
usual atmospheric structure characterized by fine structure in chromospheric
velocity fields, and selective CS absorption. These might be expected to
show Ca II K variability and sporadic mass loss as reported by _liu et al.
(18) for Arcturus.
We thank A1 Boggess and his dedicated staff for assistance in obtaining
and analyzing the data discussed here, as well as for support under contracts
NAS5-23274 to the Univ. of Colo. and NAS5-25762 to Bartol Research Foundation.
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Atmospheres of Cool Stars. VII. High Resolution Absolute Flux Profiles
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of Line Widths on Luminosity" 1977 Astrophys. J. 215, 176.
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321
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H & K Lines in Stars of Spectral Type FO-M2" 1979 AStrophys. J. Suppl.
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1978 Astrophys. J. 220, 619.
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_lux Profiles and Chromospheric Radiative Loss Rates" 1979 Astrophys.
J. 234, i023.
15. Basri, G., Stencel, R. and Linsky, J. "Stellar Hodel Chromospheres.
XIII. The Peculiar Outer Atmosphere of 56 Peg (7(0 Ibp)" 1980 Astrophys.
J., in preparation.
16. Basri, G. and Linsky, J. "Outer Atmospheres of Cool Stars. VIII. Chro-
mospheric Models for the Supergiants Beta Dra, Epsilon Gem and Alpha
Ori" 1980, in preparation.
17. Lites, B. and Cook, J. "A Semi-Empirical Model of the Upper Flare Chro-
mosphere" 1979 Astrophys. J. 228, 598.
18. Chiu, H.-Y., Adams, P., Linsky, J., Basri, G., Maran, S. and Hobbs, R.
"Variable Mass Loss from Arcturus and the Hypothesis of Giant Convec-
tion Cells" 1977 Astrophys. J. 211, 453.
322
-5 _ •
.
• I
0 • • -
O LS. CORRECTED MG II ASYI_IETRY
.O
0 X \ 0 so_ s/L•z.o5
0 X 0.96< s/L • 1.o4\
O OUT_OW S/L< 0.95
O
+5 gG5 KO K2 K4 gMO -
I,,,=l,l,,111111Tilll=,,,I,,,,I,,,,
0.6 0.7 0.8 0.9 1.0 1.1 1.2
V'R
Fig. la. Mg II core emission asymmetry, k2-/k2+, or S/L, on a color-
magnitude diagram for late-type stars. This sample accounts for
the effect of over!ying interstellar Mg II.
.o-\\ \ \ -
M
I \
+5 gG5 K0 K2 K4 0M0 -
0.6 0.7 0.8 0,9 1.0 1.1 1.2
V--R
Fig. lb. Collected transition loci in the HRD. Cf. reference 8.
323
I l I I I I 'I I
• 41'74
7+_ +56 Peg
/ n + , ,8 Peg+ o
-5- _.1'1 i m""_ n_l:tn " +" _-- ,'_
n= m n_rr. !
f BASRI & LINSKY (19801 cLOri +
'1"
_6 _ u
+ [a, ]:b, ]:I:_ ct Her
m m, m-I_ _--ff--q
- J-
-7 -' I I I , I I I i N_J--
0.6 0.8 1.0 1.2 _15.
V-R
I i I , I
6000K 4900K 4500K 4000K 3500K 3000K
<Teff)
Fig. 2. Chromospheric radiative loss rate in Mg II, _k' which is the
ratio of integrated Mg II h and k surface flux to total bolometric
luminosity, as a function of V-R photometric color. See text for
discussion.
324


High resolution absolute flux profiles of the MC 2 h and k lines in evolved F8 to M5 stars

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