The observed X-ray luminosities seen from an Einstein survey of nine Algol like systems are similar to those found by Pallavicini, et al. (1981) for single or widely separated rapidly rotating late stars, but fall an order of magnitude below those seen from RS CVn stars with similar orbital periods and spectral types. It is concluded that the X-ray emission is most probably associated with a hot coronae surrounding the secondary. Possible explanations for the lower luminosity of the Algol systems relative to the RS CVn systems are considered

Marshall, F. E.

White, N. E.

English

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https://ntrs.nasa.gov/search.jsp?R=19830013803 2020-03-21T03:49:35+00:00Z
(NASA-TH-84992) AN X-RAY SURVEY OF 9 ALGOL
	 N83-22074
SYSTEHS (NASA) 16 p HC A02/8F A01 CSCL 03A
Unclas
G3/89 09547
f
Technical Memorandum 84992
AN X-RAY SURVEY OF NINE
ALGOL SYSTEMS
N. E. White and F. E. Marshall
FEBRUARY 1983
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
_ s
1
ORIGINAL PAGE
p, POOR QUAI-rM
AN X-RAY SURVEY OF NINE ALGOL SYSTEMS
N.E. White l and F.E. Marshall
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
The observed X-ray luminosities seen from an Einstein survey of nine
Algol-like systems are similar to those found by Pallavicini et al. (1981) for
single or widely separated rapidly rotating late stars, but fall an order of
magnitude below those sQen from RS CVn stars with similar orbital periods and
spectral types. We conclude that the X-ray emission is most probably
associated with a hot coronae surrounding the secondary. Possible
explanations for the lower luminosity of the Algol systems relative to the RS
CVn systems are considered.
lAlso Dept. of Physics and Astronomy, Univ. of Maryland
2ORIGINAL PAGE IS
OF POOR QUALITY
I. INTRODUCTION
The correlation between coronal X-ray luminosity and rapid stellar
rotation in stars later than F5 is now well established, although the exact
form of the relationship remains controversial. Without any detailed physical
model for guidance, there is no obvious form to use in relating the X-ray
luminosity L x
 to observable stellar parameters. Walter and Bowyer (1981) in a
study of the rapidly rotating stars in RS CVn systems suggest that the ratio
of the X-ray luminosity to the bolometeric luminosity (L x/L* ) is inversely
proportional to the stellar rotation period, with the constant of
i
proportionality increasing for later spectral types (independent of luminosity
class). Walter (1981) and Walter (1982) extended this relation to other stars
and found evidence for a break in it at an orbital period of — 12 days.
Alternatively Pallavicini et al. (1981) examined mostly single late type stars
and found that for all luminosity classes and spectral types L x is
proportional to the square of the projected equatorial velocity of the star, v
sini. The two models show the same general trends although they in general
make different quantitative predictions.
Algols are a class of binaries that contain a rapidly rotating late star
and represent an independent sample of rapid rotation in late stars. They are
semi-detached binaries where a G or K subgiant fills its Roche lobe and spills
material onto a B or A main sequence companion. In fact Algol was one of the
first stellar X-ray sources to be discovered (Schnopper et al. 1975).
Observations by the Einstein Solid State Spectrometer (SSS) eliminated models
for the X-ray emission involving either the collision of the gas stream with
the primary (or a corona surrounding the primary) and by analogy with the RS
CVn systems it was proposed that the lobe filling normally unseen subgiant has
an act!ve corona (White et al. 1980; Paper 1). Gibson (1980, priv. comm.)
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ORIGINAL PAGC 13
	
3
OF POOR QUALITY
independently reached the same conclusion based on the similarity of the radio
outburst properties of RS CVn stars and Algol. The presence of the bright
early star in Algol systems masks the normal coronal activity diagnostics su 0
as Ca II K emission that are so distinctive ir, the RS CVn stars.
This suggests that Algols as a class could, like the RS CVn stars, be
among the most active coronal X-ray sources. In §II we present an Einstein
Imaging Proportional Counter (IPC) survey of some optically well studied
Algols and in §III we compare the observed X-ray luminosities and rotational
parameters with those of previous studies. We find that the Algols are
typically an order of a magnitude less luminous than RS CVn stars with the
same rotation period, but that they are consistent with an extrapolation of
the relation found for single or widely separated stars by Pallavicini et al
(1981). In §IV we discuss possible explanations for this unexpected result.
II. RESULTS
The Einstein IPC made a series of four or five - 2000s observations of
each of the Algol-like systems listed in Table 1, except for Algol itself
which was observed with the Einstein SSS (Paper 1). Six of the nine IPC
targets were detected. In the case of RY GEM the source was only seen once
out of four observations. This is illustrated in Figure 1 where the average
count rate for each observation is shown with a light curve of U CEP for
comparison. RY GEM is barely detected, if at all, on three occasions, but on
1980 day 107 a factor of - 20 flare occurred such that the IPC rate was twice
that from U CEP. There was no variability seen within the 2000s interval when
the source was bright. We note that in Paper 1 a factor of 3 flare that
lasted at least 12 hours was reported from Algol. No other flares or
variability was seen from the other five sources. We have also examined 2 to
60 keV HEAD-1 A2 scanning data of the same IPC targets for any significant.
ORIGINAL PAGE IS
	 4
OF POOR QUALITY
detections or flares. None of the sources were detected at a level greater
than	 2 x 10-11 erg s- 1 cm- 2 (2-10 keV).
Table 1 contains the relevant parameters for each system. Minimum and
maximum X-ray luminosities are given for the variable sources Algol and
RY GEM. The stellar parameters come from Popper (1980). Cester et al. (1978).
Cester et al. (1977) and Allen (1973). We were unable to find any orbital
solution for QS AQL and will not include it in any of the following
discussion. The distances assume that the primaries are main sequence.
III. A COMPARISON WITH OTHER SURVEYS
The non-flare X-ray luminosities vary from < 1 x 10 30 erg s- 1 up to 7 x
10 30 erg s- 1 , with a peck luminosity during the RY GEM flare of 2 x 10 31 erg
S- 1 (Table 1). These seem rather low when compared to the range of luminosity
from the RS CVn stars of -- 1 x 10 30 to 3 x 10 31 erg s- 1 (Walter and Bowyer
1981). In Figure 2 we show L x /L * versus Porb for all the Algols in our
sample. The dashed lines indicrte the range of variation seen during the
flares from RY GEM and Algol. Also given is the region of scatter occupied by
the RS CVn stars studied by Walter and Bowyer (1981). Two of the Algol
secondaries have earlier spectral types than is typical of the subgiant in RS
CVn stars and are given a different symbol in Figure 2 (because L x/L * depends
on spectral type). The spectral types of the secondaries in the remaining
seven are similar to the subgiants in the RS CVn systems. All the Algol-like
systems lie about one order of magnitude below the values of L x /L * seen from
the RS CVn stars.
In Figure 3 we show L x
 versus v rot for both our sample of Algols and also
for the RS CVn and related systems with Porb < 12 days studied by Walter and
Bowyer (1981) and Walter (1981). In both cases the relation of Lx
1027 v rot found by Pallavicini et al. (1981) is indicated. where vrot is
-90
ORIGINAL PAGE IS
OF POOR QUALITY 	 5
equatorial velocity assuming synchronized rotation. The quiescent Algol X-ray
luminosities are in excellent agreement with the Pallavicini relation; the RS
CVn stars lie systematically above it. Pallavicini et al. (1981) include in
their survey stars of all spectral types and find for stars earlier than A
that the X-ray luminosities do not show any rotational dependence, but that
there is a good correlation of Lx
 with L* . This predicts for the early
primaries of the Algol systems coronal X-ray luminosities of -- 1029 erg s-1,
far below that observed from many in our sample.
IV. DISCUSSION
By analogy with the solar corona, current models of late type stars
picture the X-ray emitting plasma confined in magnetic loops, with the coronal
activity increasing with stellar rotation through the action of an internal
dynamo. This model was initially applied to explain the early X-ray
detections of the RS CVn systems (e.g. Walter et al. 1980), but has since
provided a plausible qualitative description of the coronae of late type stars
in general. In Paper 1 we argued that this model is also appropriate for the
late type secondary of Algol. the results presented here show that this model
can be extended to Algols as a class. In particular the observed X-ray
luminosities are comparable to that seen from other late type stars.
The L x a v 2 relation proposed by Pallavicini et al. (1981) for mainly
single or widely seperated late stars gives a good estimate of the observed
X-ray luminosities of Algol systems. The Lx/L* a Po 1b relation found by
Walter (1981) for RS CVn stars overestimates the observed flux, in particular
at the longer orbital periods. This latter result is at first sight
surprising given that the late component in both the RS CVn and Algol systems
have roughly the same spectral type and class (-- KOIV) and are also rotating
at similar periods. However, during its evolution the subgiant in an Algol
ORIGINAL PAGE 1.,
OF POOR QUALITf
system has filled its Roche lobe while evolving off the main sequence and
spilled a large part of its original mass onto its companion. This mass loss
process is still procedding and may be disrupting the magnetic loops in the
vicinity of the inner Lagrangian point. Kondo, McCluskey and Harvel (1981)
have found that in U CEP material leaves from a significant fraction of the
surface of the secondary and this could significantly reduce the number of
closed loops. But the disruption should, for co-rotation, preferentially
occur on the side facing the primary. The lack of strong orbital modulation
(e.g. Fig. 1) tends to argue either that the mass flow does not disrupt the
corona or that the loop sizes are large. than a stellar radius. A similar
argument tends to discount any disruption of the loops by the stellar wind of
the primary.
The subgiant in an RS CVn system is probably also post-main sequence
(Popper and Ulrich 1977) but in this case there is no evidence that any
substantial mass transfer has yet occurred between the two components. It is
usually assumed that both components (the other is typically a G dwarf) are
evolving as single stars. In contrast the progenitor to an Algol subgiant
must originally have been much more massive and of an earlier spectral type.
As a consequence of loosing mass it is now substantially over luminous
compared to a "normal" subgiant in, for example, an RS CVn system (e.g.
Paczynski 1971). The reduced L.
	 of an Algol subgiant may be caused by this
difference in the mass to bolometric luminosity ratio.
That the Algols X-ray luminosities are in such good agreement with the
relation proposed by Pallavicini et al. (1981) for mainly single stars may
suggest that the coronal activity of the RS CVn systems is abnormal. There
has been the suggestion that a significant fraction of the coronal loops have
dimensions comparable to the binary orbit (e.g. Swank et al. 1981) and that in
6
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ORIGINAL PAGE 13
	
7
OF POOR QUALITY
RS CVn systems there may be "interactions" between the loops on the two stars
(Simon, linsky and Schiffer 1981). This could lead to an overall enhancement
in the coronal activity either by superposition of many small flares, or more
likely, by making available an increased magnetic volume to contain the X-ray
emitting plasma. Such interstar or magnetic loop interactions may not be
possible due to the earlier spectral type of the primary in Algol systems,
where the origin of the corona may not involve magnetic loops (cf. Lucy and
WiA to 1980). However we would then expect that the longer orbital period RS
CVn systems where the binary separation is larger than the loop lengths would
show a marked decrease in X-ray flux; no such cutoff is seen (Walter and
Bowyer 1981).
We conclude therefore that the reduced coronal activity of the Algol
secondaries relative to the RS CVn systems is somehow caused by the different
evolutionary histories of the two populations. The similarly low values of
Lx/L* vs. Porb found by Cruddace and Dupree (1983) for the W UMa contact
binaries may also reflect such effects. Without a better understanding of the
underlying physical processes that are responsible for the coronal activity it
is premature to speculate as to the cause of these effects. Nonetheless this
result does show that it is unrealistic to expect a single rotational relation
to be applicable to all stars irrespective of their history.
ACKNOWLEDGMENTS
We thank Fred Walter for some helpful comments on an earlier manuscript.
8I
ORIGINAL PAGE 13OF POOR QUALITY
TABLE 1. RESULTS
Source rSp. Type Radii
(days) o)
RY AQR 2.0 A5 1.5
Ki 1.7
U CEP 2.5 87 2.9
G8 1.4
QS AQL 2.5 B4 3.9
AS ERI 2.7 A3 1.8
KO 2.2
TW DRA 2.8 A3 2.4
KO 3.4
ALGOL 2.9 B8 3.1
G8 3.2
U SGE a 3.4 B8 4.1
G5 5.3
RS VUL 4.5 B5 4.1
GO 5.6
RY GEM 9.3 A2 3.3
K2 6
S CNC 9.5 AO 2.1
K2 5
log(lr/Lo) distance log(L,i b log(Lx/l.) VroL.
(pc) (erg s	 ) (km s)
0.9 2130 30.3 -3.44 43
0.2
2.1 220 30.9 -3.80 95
1.0
2.9 417 <30.4 ? ?
1.2 200 30.1 -3.89 41
0.4
1.4 190 30.8 -3.64 62
0.8
2.2 27 30.6 -3.43 55
0.8 31.0 -3.82
2.6 180 '30.7 -4.15 80
1.3
2.9 380 30.3 -4.80 63
1.5
1.8 263 31.3 -4.38 33
1.1 <30.0 -4.68
1.5 340 <30.2 -4.36 27
1.0
a Source was at the edge of the FOV
b Assumes 1 1PC ct s- 1 - 2 x 10- 11 erg cm-2s-I
9ONGINAL PAGE 18
OF POOR QUALITY
REFERENCES
Allen, C.W. 1973, Astrophysical quantities, 3rd ed: London:Althone.
Cester, B., Fedel, B., Giuricin, G., Mardirossan, F., Pucillo, M. 1977, Astr.
Ap. 61, 469.
Cester, E., r'edel, B., G i uricin, G., Mardirossan, F., Mezzetti, M. 1978, Astr.
Ap. 62, 291.
Cruddace, R., and Dupree, A. 1983, preprint.
Kondo, Y., McCluskey, Jr., G.E., and Harvel, C.A. 1981, Ap. J. 247, 202.
Lucy, L.B., and White, R.L. 1980, Ap. J. 241, 300.
Paczynski, B. 1971, Ann. Rev. Ast. Ap. 9, 183.
Pallavicini, R., Golub, L., Rosner, R., Vaiana, G.S., Ayres, T., and Linsky,
J.L. 1981, Ap. J. 248, 279.
Popper, D.M. 1980, Ann. Rev. Astr. Ap. 18, 115.
Popper, D.M., and Ulrich, R.K. 1977, Ap. J. 212, L131.
Schnopper, N.W. et al. 1976, Ap. J. 210, L75.
Simon, T., Linsky, J.L., and Schiffer, F.M. 1980, 239, 911.
Swank, J.N., White, N.E., Holt, S.S., and Becker, R.M. 1981, Ap. J. 246, 208.
Vaughan, A., and Preston, G. 1980, Pub. A.S.P. 92, 385.
Walter, F.M., Cash, W., Charles, P.A., and Bowyer, C.S. 1980, Ap. J. 236, 212.
Walter, F.M. 1981, Ap. J.
-
245, 677.
Walter, F.M. 1982, Ap. J.
-
253, 745.
Walter, F.M. and Bowyer, S. 1981, Ap. J. 245, 671.
White, N.E., Holt, S.S., Becker, R.H., Boldt, E.A., and Serlemitsos, P.J.
1980, Ap. J. 239, L69.
ORIGINAL PAGE IgOF POOR QUALnY
FIGURE CAPTIONS
Figure 1 - The background subtracted X-ray light curves as a function of
time for U CEP and RY GEM. Each 2500s observation has been sunned.
Figure 2 - LX/L* versus Porb for the Algols given in Table 1. The hatched
region shows the range of values seen from the RS CVn stars by Walter and
Bowyer (1981).
Figure 3 - LX
 vs Vrot for both the RS CVn and the Algol systems. The dashed
lines indicate the range of variability seen from those sources where
multiple observations have been made. Systems with Porb < 10 days taken
from Walter and Bowyer (1981) are shown. For the RS CVn stars we assumed
for simplicity an average radius of 3 R o and that both stars contribute
equally, which either overestimates Vrot or underestimates LX.
..et
10
Pab= 9.3 DAYS
N0
4
z0
V 2
Va.
N
0
RY GEM
ORIGINAL PAGE 18
OF POOR QUALrry
BINARY PHASE
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Pore= 2.5 DAYS	 U CEP
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80	 81
DAY OF 1980
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I	 I	 1	 1	 I	 1	 1	 I	 i	 1	 1	 1	 1
102	 104	 106	 108
	 110	 112
	
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DAY OF 1980
ORIGIPIAL, PAQR if
Of POOR QUALITY
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ADDRESS OF AUTHORS
F.E. MARSHALL, Code 661, Laboratory for High Energy Astrophysics, NASA/Goddard
Space Flight Center, Greenbelt, Maryland 20771
N.E. WHITE, Division S9, ESTEC, Postbus 299, 2200 AG Noordwijk Zh, THE
NETHERLANDS


An X-ray survey of 9 algol systems

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