Strong high temperature emission lines in the EUVE spectra of binary stars containing cool components (Alpha Aur (Capella), 44 iota Boo, Lambda And, and VY Ari) provide the basis to define reliably the differential emission measure of hot plasma. The emission measure distributions for the short-period (P less than or equal to 13 d) binary systems show a high temperature enhancement over a relatively narrow temperature region similar to that originally found in Capella (Dupree et al. 1993). The emission measure distributions of rapidly rotating single stars 31 Com and AB Dor also contain a local enhancement of the emission measure although at different temperatures and width from Capella, suggesting that the enhancement in these objects may be characteristic of rapid rotation of a stellar corona. This feature might be identified with a (polar) active region, although its density and absolute size are unknown; in the binaries Capella and VY Ari, the feature is narrow and it may arise from an interaction region between the components

Brickhouse, Nancy S.

Dupree, A. K.

Hanson, G. J.

English

NASA Technical Reports Server

N96-14307
High Temperature Structure in Cool
Binary Stars
By A. K. Dupree1, N. S. Brickhouse1
AND G. J. Hanson1
1
 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA
Strong high temperature emission lines in the EUVE spectra of binary stars containing cool
components [Alpha Anr (Capella), 44* Boo, Lambda And, and VY Ari] provide the basis to
define reliably the differential emission measure of hot plasma. The emission measure distribu-
tions for the short-period (P< 13 d) binary systems show a high temperature enhancement over
a relatively narrow temperature region similar to that originally found in Capella (Dupree et al.
1993). The emission measure distributions of rapidly rotating single stars 31 Com and AB Dor
also contain a local enhancement of the emission measure although at different temperatures and
width from Capella, suggesting that the enhancement in these objects may be characteristic of
rapid rotation of a stellar corona. This feature might be identified with a (polar) active region,
although its density and absolute size are unknown; in the binaries Capella and VY Ari, the
feature is narrow and it may arise from an interaction region between the components.
1. Introduction
The emission measure defined by the well-exposed EUV spectra of the binary system
Capella (G8 III + GO III) surprisingly revealed a narrow high temperature "bump"
(Dupree et al. 1993) occurring at T = 6.3 x 106 K. The feature is continuously present
irrespective of the orbital phase of the system (Dupree et al. 1994; 1995), and may
arise from a continuously visible hot region on one component, or from an interacting
region between the stars. The distribution of the high temperature emission measure
in the Capella system differs from the broad emission measure distribution found in a
solar active region (see Brickhouse et al. 1995). We investigate whether this feature is
characteristic of binary systems and whether rotation controls its presence.
2. Spectra and the Emission Measure Distribution
EUVE spectra from our Guest Observer program and from the EUVE Science Data
Archives were obtained for 4 binary systems (VY Ari, Capella, 44t Boo, A And) and
two rapidly rotating single stars (AB Dor and 31 Com). EUV spectra were extracted
from the images and calibrated in a uniform manner. The reduced spectra are shown in
Fig. 1. In spite of the varying physical characteristics of the stars, there are similarities
among the spectra. The resonance line of He II (A304) dominates the MW spectra;
strong resonance lines of iron dominate the SW spectra. However, the relative strengths
of the lines differ among these objects, and so require detailed evaluation of the emission
measure. It is the strength of the Fe XVIII and Fe XIX transitions relative to Fe XX,
XXIII that controls the presence of the locally enhanced emission measure (the "bump")
in Capella.
Signal to noise ratios were evaluated at the EUVE spectral resolution, and only the
strongest features (S/N « 9 - 15) were selected to determine the emission measure.
Correction of the observed fluxes for interstellar absorption was made with Ng from
Table 1. The H/He abundance ratio was set to 11.6 (Kimble et al. 1993) and helium
was assumed to be neutral. Assumption of higher values for Ng would strengthen the
https://ntrs.nasa.gov/search.jsp?R=19960007142 2020-06-16T05:17:59+00:00Z
Dupree et al. : High Temperature Structure
X
1
X
X
1
XX
XX
1 /
.a
X X X
X X X X
1
X
X
1
_
c
x X
a>
X X
80 100 120 140 160 150 200 250 300 350
Wavelength (A)
FIGURE 1. EUVE spectra in the SW (left panel) and MW (right panel) band for the rapidly
rotating single stars AB Dor and 31 Com and the binaries 44 Boo, Capella, VY An, and A And.
Species of Fe are marked by ionization stage. Fe XXIII (A132.8) is blended with Fe XX.
Dupree et al. : High Temperature Structure
HD
17433
34029
36705
111812
133640
222107
Star
VYArif
a Aur$
AB Dorf
31 Com||
44t Booft
A Andt
Sp. Type/Lum.
K3-1V-IV
GO III/G8 III
Kl IIIp
GO III
GO V
G8IV-III
Por6«tal
(days)
13.208
104.0
<6
0.268
20.52
A phtm.
(days)
16.64
8/80
0.514
53.95
NH
(cm'2)
1.0 xlO18
1.8 xlO18
1.0 xlO18
1.0 xlO18
1.0 xlO18
4.0 xlO18
SWExp
W
168557
281571
166460
84503
104145
105701
TABLE 1. PARAMETERS OF TARGET STARS
t RS CVn-type binary.
+ RS CVn-type binary; EUVE spectra of Capella taken at several phases were averaged
together.
™ Rapidly rotating single star.
" 31 Com is a rapidly rotating single star thought to be a Hertzsprnng Gap giant. The
"orbital" period was evaluated from the t>sin t value of 77 km s~1, assuming a radius of 9JZ0.
it \\r UMa-type contact binary.
Fe XVI emission (A336) relative to the Fe XV emission (A284) where observed, but leave
the fluxes in the short wavelength region (A85-A170) essentially unchanged.
The emission measure distribution is determined iteratively, by fully integrating the
line emissivities through a trial emission measure distribution, and comparing the pre-
dicted and observed line fluxes. There is no restriction on the "smoothness" of the
emission measure distribution such as occurs with spectral fitting procedures; moreover
the fit is not degraded by bins containing noise or weak signals. Only strong lines are
considered in evaluating the emission measure. We require that these fluxes agree within
acceptable uncertainties which in this case is better than a factor of two. Detailed current
models for all iron ions (Brickhouse et al. 1995) were used. Results for A And are taken
from another publication (Hanson et al. 1995) where fluxes of ultraviolet lines measured
with IUE were included so that the emission measure below 2 x 10s K is well-defined.
3. Conclusions
All of the target stars exhibit a range of high ion stages indicating a generally contin-
uous distribution of temperature. The spectral resolution of EUVE demonstrates that
two-temperature fits which rely on energy band measures do not represent these stel-
lar atmospheres. For the weaker spectra, the strongest observable lines arise from high
stages of ionization (Fe XVIII... XXIII), the Fe XV (A284) and Fe XVI (A336 and A361)
emission is not prominent, and the emission measure can only be reliably defined in a
restricted temperature range.
Inspection of the emission measure distributions contained in Fig. 2 and 3 shows that
the Capella "bump" is not unique. The binary systems (VY Ari, 44t Boo) clearly show
the enhanced emission measure distribution similar to Capella. The appearance of the
emission measure enhancement is most pronounced in binary systems with short orbital
period (<13 days) and/or high rotational velocity. The orbital period of Capella is 104
days, however the hot component of Capella is rotating rapidly with a period of « 8
days (Fekel et al. 1986) so that the system may be classified as a rapid rotator for these
purposes. The strong lines of Fe XVIII (A93.9) and Fe XIX (A108.4) demand the presence
Dupree et al. : High Temperature Structure
53
E 52
o
3 51
<D
^50
49 _ Capella*AiAndl^
4.5 5.0 5.5 6.0 6.5 7.0 7.5
Log Te (K)
FIGURE 2. Emission measure distribution determined for 4 binary systems.
53
CM
<D
50
31 .Com____
AB Dor
Capella __-_!.
4.5 5.0 5.5 6.0 6.5 7.0 7.5
Log Te (K)
FIGURE 3. Emission measure distribution determined by the strongest lines for 2 rapidly
rotating single stars (31 Com and AB Dor) and the binary system Capella.
of an increased emission measure in Capella for their production. That 44t Boo would
have a similar feature is apparent from inspection of its SW spectrum; the enhancement
is broader in temperature than the Capella bump. The SW spectrum of VY Ari shows an
exceptionally strong feature of Fe XX, XXIII (A132.8) relative to the Fe XVIII and XIX
transitions; thus the local enhancement of the emission measure occurs at a temperature
higher by 0.2 dex, namely T(K) — 7.0 dex. The long period system, A And has a much
Dupree et al. : High Temperature Structure 5
less well defined emission measure enhancement (Hanson et al. 1995). The slopes of the
emission measures with temperature are consistent with models of magnetic loops or
arcades.
Two single stars can address the question of whether the "bump" is unique to binaries.
The rapidly rotating single giant 31 Com is of particular interest because it lies in the
Hertzsprung gap (Wallerstein et al. 1994). Since the hot rapidly rotating secondary
in the Capella system is believed (Pilachowski & Sowell 1992) to be passing through
the Hertzsprung gap too, 31 Com might prove helpful in disentangling the composite
EUV spectrum of Capella. The star 31 Com has a broad enhancement required by the
dominant Fe XXII and Fe XXIII transitions, at a temperature higher (7.0 dex) than
the Capella feature. Such a broad hot component is not found in the Capella emission
measure. AB Dor, another rapidly-rotating single giant shows a broad enhancement,
also different from Capella. Thus, rapidly rotating single stars have "bumps" but they
appear different from those in Capella and VY Ari.
Rotation appears to be a significant physical parameter in producing an enhanced emis-
sion measure feature in cool star atmospheres. This feature is generally present in those
systems with periods < 13 days. However the temperature of the maximum and the
width and strength of the enhancement differ from star to star. It is plausible to asso-
ciate this feature with magnetic structures in a rapidly-rotating corona, in which case it
could be dense and of small scale; however, in the binary systems Capella and VY Ari,
a narrow enhancement (of unknown density) is found. It may be that such a feature
results from interaction between the components. Binaries of longer period, represented
by A And and <r Gem (Hanson et al. 1995) do not show such well-defined enhancements
in the emission measure, although they possess equally hot plasma. However longer EUV
exposures are needed on many of these stars to detect weaker species and to define the
atmospheric structure over a wider temperature range.
This work is partly supported by NASA NAG5-2330 to the Smithsonian Institution.
REFERENCES
BRICKHOUSB, N. S., RAYMOND, J. C., & SMITH, B. W. 1995 New Model of Iron Spectra in the
Extreme Ultraviolet and Application to SERTS and EUVE Observations of a Solar Active
Region and Capella. Ap. J. Suppl., 97, 551-570.
DUPREE, A. K., BRICKHOUSE, N. S., DOSCHEK, G. A., GREEN, J. C., & RAYMOND, J. C.
1993 The Extreme Ultraviolet Spectrum of Alpha Avrigae (Capella). Ap. J., 418, L41-L44.
DUPREE, A. K., BRICKHOUSE, N. S., DOSCHEK, G. A., HANSON, G. J., & RAYMOND, J. C.
1994 EUVE Spectra of Alpha Aurigae (Capella) at Different Phases. B.A.A.S., 26, 864.
DUPREE, A. K., BRICKHOUSE, N. S., & HANSON, G. J. 1995, Inhomogeneity of Coronal
Structures in Capella, in preparation.
FEKBL, F. C., MOPPETT, T. J., & HENRY, G. W. 1986 A Survey of Chromospherically Active
Stars Ap. J. Suppl., 60, 551-576.
HANSON, G. J., BRICKHOUSE, N. S., & DUPREE, A. K. 1995 EUVE Spectra of Lambda An-
dromedae and Sigma Geminorum. Ap. J., submitted.
KlMBLE, R. PLUS 13 AUTHORS 1993 Extreme Ultraviolet Observations of G191-B2B and the
Local Interstellar Medium with the Hopkins Ultraviolet Telescope. Ap. J., 404, 663-672.
PHACHOWSKI, C. A., & SOWELL, J. R. 1992 The Lithium Abundance of the Capella Giants.
A. J., 103, 1668-1672.
WALLERSTEIN, G., BoHM-VrrENSE, E., VANTURE, A. D., & GONZALEZ, G. 1994 The Lithium
Content and Other Properties of F2-G5 Giants in the Hertzsprung Gap. A. J., 107, 2211-
2221.


High temperature structure in cool binary stars

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