Long period eclipsing binary stars, an important bridge between the close and distant binaries

Vl.ltasin Astronomy,

0083-6656/88 $0.00+ .50 Copyright © 1988 Science Press & Pergamon Journals Ltd.

Vol. 31, pp. 185-192, 1988

L O N G P E R I O D E C L I P S I N G B I N A R Y STARS, AN I M P O R T A N T B R I D G E B E T W E E N THE C L O S E AND D I S T A N T B I N A R I E S ll-Seong

Yonsei

University

Nha

Observatory,

Republic

of

Korea

SUMMARY

A statistical survery of long period, say P ~ I0 days, eclipsing binaries among all the known eclipsing binaries (over 3500 in number) yielded some 200 binaries (~ 6%). Among such long period binaries the number of stars with radial velocities available is only 57 (~ 21%), whose number is again reduced to half for those which have decent light curves. The overwhelming needs for intensive photometric observations to secure light curves for these long period systems which play an important role for understanding the nature of giants and supergiants led to the commencement in 1982 of a systematic long-term project of about I0 years at the Yonsei University Observatory. The results obtained in the last five years are presented.

I. INTRODUCTION

Due to the low frequency of discovery and the fact that many eclipses occur at times unfavorable for observation,

decent photometric data

are lacking for the majority of long-period eclipsing binaries, say P ~ 10 days, with the exception of some extreme cases of the Zeta Aurigae stars. Reliable statistics show that out of all the known eclipsing binaries in over 3500 systems only about 6% have P > i0 days, and among this number only a little over 20 of them have been well studied. Not only is the number of systems few but the light curves for almost all of these 20 systems are very complicated. The complexity of the light curves suggests that these systems contain a wealth of information which may open up our understanding of stars in the advanced stage of evolution whose (apparent) characteristics may be considered somewhat different from those of the abundant, short period binary systems. Furthermore,

it is hoped that spectro-

scopic and photometric detection techniques that can begin to bridge the gap between classical visual binaries and close binaries can be developed.

185

186

ll-Seong Nha An attempt to obtain full light curves for long period eclip-

sing binaries has been made by a group at the Yonsei University Observatory and UBV observations have been uninterruptedly continued for about 30 program stars since 1982. The preliminary results obtained so far for 16 stars are included in this discussion.

2. STATISTICAL SURVEY OF LONG PERIOD ECLIPSING BINARIES

Two major sources used in this survey are the Seventh Catalogue of the Orbital Elements of Spactroscopic Binary Systems (Batten

et al.

1978) and A Finding List for Observers of Interacting Binary Stars (Wood

et al. 1980). The present survey has been focused particular-

ly on the present status of the spectroscopic and photometric

inves-

tigations of the long period eclipsing binaries with orbital periods of IO days and longer. Table I presents the numbers of spectroscopic and eclipsing binaries according to the different categories under investigation. The various columns of this table are self-explanatory,

but they

essentially focus on the fact that the stars with sufficient spectroscopic and photometric

information are too few.

In Figure I, all together 70 binaries with P > I0 days are plotted according to their orbital period,

log P in days, and spec-

tral type. Symbols used in the figure indicate the five different luminosity classes of the brighter component of each binary system. Table i. Statistics of Spectroscopic and Eclipsing Binaries. Spectroscopic binaries (I)* No. of stars Stars of P > IO days Stars with luminosity class known Stars with decent light curve available

978 412 312

*Batten et al. (1978), ** Wood *** Estimates by the author.

Eclipsing binaries (II)** 3,546 212

(II) in (I) 275 57

56

45

20***

20***

et al. (1980),

Long Period Eclipsing Binary Stars

I

I

i

il

i

I

187

I

0 0

0 0 0

0

CO

0

0

0

8

0

0

1[ ~ A A 'i

•

0 0

o

Figure

Six stars

I llO

I ~

each

•

"- o i

,

I FO

t GO

&a, •

I KO

L rio

i. Bright components of 70 long period eclipsing binaries (Wood et al. 1980; Nha, this ~tudy). in the orbital

range,

and in the second range, giants.

,

Finally

4 > log P > 3, are all supergiants,

3 > log P > 2, 9 are supergiants

in the third range,

2 > log P > I, the numbers

luminosity class are quite homegeneously 6 supergiants, 6 subgiants,

The orbital

range,

6 bright giants,

in

distributed as below:

21 giants

and IO dwarfs.

4 > log P > 3, is the important

this range the visual

and 5

range because

in

binaries and the eclipsing binaries coexist,

with

the former being only dwarfs and the latter only supergiants.

This

implies

that we are

looking at completely different

The former belong to the solar neighborhood while

samples.

stars within 25 pc,

the latter are all further away and ought to be large in size

to have more chance of becoming an eclipsing binary.

188

I1-Seong

3.

Nha

YUO OBSERVATIONS AND THE RESULTS

The

instrumentation

the

observations

year

project

elsewhere

in

the

al.

the

Among t h e

gives

the

right

of

the

additional

program

Except

for

stars

progress

are

reason

orbital

all

of Table for

stars

neous

Observations

phase

coverage.

behaves

in a strange

tion

eclipse

of

SY And:

either

BH C a s :

very

AQ C a s :

season

in

primary

20 h o u r s

and

YUO t o

are

Eps Aur:

After

the

is

the or

a trend

(Fig. 4).

in of

the

V

last

(197.2 complete of

longer

recent

this

increasing

they clear

studies,

years

curve

in

analysis this

than

the

star

the

qualities

in

curve

amplitude

can

five stars.

a homoge-

phase

interval

U light

with

curve

no i n d i c a -

2). were

unsuccessful

eclipses. binary

light

requires

have been

is

(Nha a n d

way.

shows a t o t a l i t y (Fig.

previously

existed and d u r a t i o n

one

successfully

elsewhere

under

three

only

curve.

1982-1984,

following

is one

remaining

the

scatter

respectively those

table

available

and t h i s

the

eclipse,

days)

star

this

are

provide

two s e a s o n s

its

listed

as program s t a r s by YUO.

have been given

eclipse

light

As i s

three

secondary

of

i n g o f Eps Aur was made f o r distortions

last

i n V a n d B,

significatly

if

or

year

which are

on e a c h o f t h e

O~5 ( F i g .

results

eclipse

17 h o u r s

our five

column of

were selected

V light

and a more d e t a i l e d

The p r i m a r y

not existing

last

previous

secondary

long period

Preliminary

Chun 1 9 8 6 ) ,

tions

made

at

The

showing a large

The UBV o b s e r v a t i o n s

completed.

AY P e r :

the

minimum a r o u n d

in d e t e c t i n g

additional

the

a clear manner

Observations

This

in

While

O~3 - 0~7 d e m o n s t r a t e s

as o u r p r o -

(AZ Cas, RX Cas, a n d NY Cep) we

made

10

1980). These a r e m a i n l y

16 s t a r s

1).

to

described

were selected

determinations

poor

stars

2.

of the

are

totally

the

short n o t e s w i t h l i g h t curves i f n e c c e s s a r y

SX C a s :

binaries

are

for

applied

(1982-87)

so a d o p t e d ,

in Table

why t h e s e

three

period

curves

references

Table

techniques

(Wood e t . a l .

light

the

foot-noted

shown i n

half

eclipsing

List

ascension

quality

the

first

Finding

of either

of

observational

1 9 8 6 ) . A b o u t 40 s t a r s

made s i g n i f i c a n t

in order

(See

et

and t h e

long period

from the

known.

efforts

the

(Nha,

stars

poorly

made

for

gram stars those

used

of

3).

years.

in

These

dura-

reported.

continuous

every

about

The w a v e l i k e

year, the

monitor-

and

there

distortions

Long

Table

Period

2. P r o g r a m m

Name

Eclipsing

Stars

Cas And Cas Cas Cas Cas Per Aur Aur Aur Cnc Cnc Cnc Cep Cas Cas

36.5 34.9 197.2 11.7 3404. 32.3 11.7 9885. 972.1 10.6 25.5 96.7 70.7 15.2 11.1 13.4

* Koch

Zet Aur:

The

is h o p e d

to get

then

The

star

during

type

with

thus

, ** B a t t e n

eclipse

it a g a i n phase

et al.

in M a r c h

in N o v e m b e r

coverage

will

d e e d e d c d c d

(1978).

1985 and

Ds Esa Es Eos Do -

was

observed

December

be a c h i e v e d

this

in UBV. year,

by May,

It

and

1988

5).

C Q Aur:

0.5

primary

the w h o l e

(Fig.

(1970

Quality Ptm* Sp**

gA6e ÷ gG6 AO + KI A51ae B3 + B9 BO + M O l b g A 5 e + gG3 AO F2 1 K 4 1 b + B6V GO A81V-V K4 lllp F 8 1 V + G6111 BOIV ÷ BOIV B BO.511

9.6 + 1.4 9.7 + 1.5 9.8 + 0.I~ IO.O + 1.O li.O + O.8 8.5 + 0.8 9.8 + 0.8 3.6 + 0.8 5.0 + 0.6 9.6 + 1.0 9.4 + 1.5 8.7 + O.7 9.0 + 0.7 7.4 + O.I 9.9 + 0.5 6.2 + 0.1

et al.

189

(1982-1987)

Spectral type

+

Depth of p r i m a r y SX SY BM AQ AZ RX AY Eps Zet CQ ZZ UU TW NY V396 V373

at Y U O

Stars

(max) V

Period

Binary

two

photoelectric

seasons

a shallow

phase the

first

by a b o u t light

elliptical

elements

orbit

ZZ Cnc:

The

V

outside

eclipse.

(1985-86

secondary -O.1

(Kim

light

and

The

require

curve

light

curves

1986-87)

minimum

phase.

et al.

This

UBV

which

period

made

for

indicate

an A l g o l -

is d i s p l a c e d

has

been

a significant

this

from

the

lengthening,

refinement

for

and

the

1987).

shows

is u n u s u a l

almost for

negligible

a system

light

which

has

variation a giant

component. U U Cnc: binary are

The has

observation been

discussed

T W Cnc: effect

The

of this

successfully

by Lee

primary

is a p p r e c i a b l e

long

period

completed

and

(96.7 the

days)eclipsing

preliminary

results

(1987). eclipse

in V shows

in B or U.

an a s y m m e t r y ,

Outside

the

eclipse

while the

no

curve

such is

190

Ii-Seong Nha

flat indicating

negligible

interactions

between

the subgaint

com-

ponent~ (Fig. 6). V396 Cas: Observations

which covered the phase uniformly

failed to

detect neither the primary minimum 0.5 mag deep nor the secondary 0.3 mag deep (Fig.

7).

V373 Cas: Seasonal

light variations

among our program stars. by Chun

et al.

here are the most significant

Its light behavior

is discussed

elsewhere

(1987).

REFERENCES

Batten, A. H., Fletcher, J.M. and Mann, P. J. (1978) Publ. Dominion Ap. Obs., 15 (5). Chun, Y.-W., Kim, H.-I. and Nha, Y.-S. (1987) presented in this volume. Kim, H.-I. (1987) private communication. Kim, H.-I., Chun, Y.-W. and Lee, Y.-S. (1987) presented in this volume. Koch, R. H., Plavec, M. and Wood, F. B. (1970) Publ. Univ. Penna. Astr. Series II. Lee, Y.-S. (1987) presented in this volume. Nha, I.-S. and Chun, Y.-W. (1986) Critical Observations vs. Physical Models for Close Binary Systems, ed. K.-C. Leung (Gordon and Breach, N.Y.). Nha, I.-S., Lee, Y.-S., Chun, Y.-W., Kim, H.-I., and Kim, Y.-S. (1986) Korean J. Astr. Space Sci. 3, I. Wood, F. B. Oliver, J.P., Florkowski, D. R. and Koch, R. H. (1980) Publ. Dept. Astr., Univ. Florida, I.

Long

Period

0.5

Eclipsing

e~e

--

Stars

191

1.5

I

"--'-:-4...

Binary

I

I

::-f • • •

-

~u

~V

m

--

e,,._o.J

I

t

1.0

!

I

I

2.0 0.3

I 0.,4

I

I

0.5

0.6

" 0.7

FHASE Fig. 2. S e c o n d a r y and blue (right)-

minimum

light

curves

of

SX

Cas;

yellow

3.1 3.2 3.3

k

3.4 3.5 3.6 .00

I

I

.O5

.I0

Fig. 3. P r i m a r y m i d - e c l i p s e c u r v e s of AY Per.

V .1.5

JD2425983.0÷

2.8 _

I

I

I

I

J

I

w

~V ~...

3.0

--

.~

__

I.

:'~" "~t: ~i

m

t

~

'"

t"""

:+'""

' "+ -'°" " ""

;

:

...

~

; "

u

3.2

u

1984-5

3.4

1 JD2426000

1985-6

1986-7

I

I

I

I

6200

6400

6600

6800

Fig. 4. P o s t - e c l i p s e Aur (1984-87)

light

variations

I 7000

of

c

(left)

192

II-Seong

-1 .O

I

l

i

l

Nha

I

!

I

I

l ..

,-

°.9

• :.

•. . . . . . . . .

".7

• •

!

.~. •.

I

.

I

,8

- '•w . ' , & r -

I

-- , . r ' r ; ~ - : ' - t

I

.0

I

, -

~.

l

.2

- ~

I- !~

I

| . •' .

•

~,:.~-

I

.4

.6

PHASE

Fig.

5. N e a r l y c o m p l e t e d

12

-

curve of

~ Aur.

I

I --q 0:026 l~-f:--~,

•

•

light

LV 14 I

1.6

- -

I

I

8.i

0,0

0.9

PH~E

Fig.

6.

Shoulder

parts

1.6

I

of

the p r i m a r y

i

I

minimum

light

i

I

i

.~

:" . ~

;. 'r

T

I

l

i

LV 1.8

f

2,0

Fig.

$

i

•

~" ..

:

I

I

.8

,0

7. Y e l l o w

light

" :.

I

I ,2

curve

of V 3 9 6

,4

Cas

:

PHASE

(1983-87).

""

I ,6

of TW Cnc

I