ecture for summer studentsJ uly 1972.

:..r J l 19 '- RADIO STARS

R. M. Hj ellming

Since about 1890 people have been trying to detect radio emission

from stars, beginning with our own' Sun. However, it was not until the

1940's that Reber, Southworth, and Hey independently detected radio emission

from the Sun. Only during the last decade have any other radio stars been

discovered. Between the 1940's and the early 1960's, radio astronomers

had the habit of calling any new radio source a radio star. However, the

data eventually showed that in all cases these sources were not single

gravitationally bound bodies, as are all stars; by definition.

To appreciate the problem of detecting radio stars, let us consider

-26 -2 -1the simple relationship between flux density, Sv (in flux units, 10 Wm Hz )

and brightness temperature, T (in K) for a uniform source of solid angle

0 (= nO /4 where 0 is an equivalent disc diameter):S

T 2S =B

Sv 2 (1)

1970 ?

In equation (1) . is measured in arcseconds.

One of the most sensitive radio instruments presently available is the

NRAO interferometer for which the minimum detectable flux density is 0.005 flux

units for eight hours of observing. Therefore, for a source to be detectable

S > '0.005

and using equation (1)

2 2

TB 2 > 9.9X, (2)

2or T 2> 135 at 3.7 cm

2 (3)T 0 > 1210 at 11.1 cmB %

The optical sizes of the stars with the largest apparent diameters

(the red supergiants) are at the most 0."05, hence for these one needs

T > 53,000°K at 3.7 cm

T > 484,000 °K at 11.1 cm

Normal process on the surfaces of stars easily produce TB i10 6K, but

it is difficult to produce larger values of Tg.

Some processes with larger TB are known on the surface of the SunS - . ,.

cc

L.

V

0

C

to0

10.000 300(Frec. .ncy. Mcjs -

0 1 CO 300 Ito

10"12

1011

10 1oB78ursts Storms

component/ j '

10 (la rg e)/ 1

10 s

cm tsun 0

icm 3cm 10cm 30cm Im "3m 1 r

Wavelength- (b)

Fig. 1-1(b) The spectra of different components of solar radio emission, plottedin terms of brightness temperature. To calculate the brightness temperature,

the source area is assumed to be as follows: the optical disk for the quiet sun; themunspot area for the slowly varying component; and estimated mean areas basedupon available observations for the bursts and storms (After Smerd 1964a).

The above figure shows the brightness temperatures for some of the major

types of radio emission for the Sun.

--'

~.

'':I

.dil 'q IDm

'-3-

Quiet Sun - chromosphere and coronal thermal emission

Slowly varying component - radio emission associated with sunspots

or plages

Storms - non-thermal emission with time scales of hours to days. At

sunspot maximum, in progress about 10% -of the time

Radio bursts - short lived non-thermal events with time scales of a few

minutes to hours.

" Unfortunately, the quiet Sun at 10 cm could be seen only if closer than

0.07 pc, the slowly varying component could be seen only at less than 0.1 pc,

and very strong bursts could be seen only within 2.5 pc. .

The nearest star, ProximaCentauri, is only 1.3 pc away. You thus can

see that only unusual processes are likely to produce a radio star. The worst

problem with stars is that : is always rather small, hence it takes a fantastic

" TB to make up for this.

Despite all this, there are now five ,(or more) different types of

S radio.stars. *

FLARE STARS

Certain red dwarf stars are known to increase their brightness radically

for brief periods of time (typically a few hours). These objects are called

S flare stars, and the flares they exhibit could be more intense versions of the

well known solar flares. Since the solar flares are accompanied by intense

radio bursts, the possibility of detecting these flare stars led British and

Australian radio astronomers to monitor them extensively in the last decade.

During several thousand hours of observing time, these investigators have

found several cases of simultaneous radio and optical flares for a few stars,

notably UV CETI, YZ CANIS NINORIS, and V371 ORIONIS. These were the first

true radio stars.

-4-

One of the best examples is the flare of YZ CANIS MINORIS shown below.

The interpretation of radio flare stars is still quite uncertain. There

have been attempts to attribute the radio emission to shock waves moving through a

corona, or to non-thermal flare phenomena like the radio bursts on the Sun.

There is the difficulty, however, that the brightness temperatures are at least

1510 K in some cases. This is a few orders of magnitude greater than has

been observed on the Sun.

o 40 ... : . . : .,- .

01"00 02OO f03400' 0400" 0500 060OO'- .UT JANUARY 19.1969

NOVAE

These stars, called novae) were first detected as radio stars by myself and

..ss .s .how -f .ilnl -it .....tie .hc •yial ,.c 00

C. M. Wade in June 1970 with the Green Bank interferometer. Three novae:

Nova Delphini 1967, Nova Serpentis 1970, and Nova Scuti" 1970 have so far

been detected.

-5-

Data for about 1-1/2 years .of observations, of Nova Delphini 1967 and

Nova Serpentis 1970 are plotted as a function of t, the time since initial

outburst. Most of the measurements are at 3.7 and 11.1 cm, but several measure-

ments at 0.35, 0.95, and 1.95 cm were made at various times. The solid -

curves are the fits to a thermal model in which emission measure, E, and

solid angle, s are varying as functions of "time. The following are the

solutions for the parameters:

Nova Serpentis 1970:

1.2 1.2 0.2

S

262.1

6I)t -22 5 + 0.3 1

x *=S• (1.O2 + 02) t ( --

3.7

0.4

i.e.. . .a t1

S

E a t

0.4+ 0. .

3 .-

For fre ex = (6 + 1) t shl (

E 0" ""t-

" '• . °' , . -. f. l a t -,..: ""::: =(I0 +O2) - "6 : . ,.".....- ...."

for one-dimension expansion

C a ts

-6-

hence we see that the expansion of Nova Serpentis 1970 is slightly confined

in at least one dimension. However, Nova Delphini 1967 must be somewhat

Sconfined in its expansion for all directions.

Further analysis of this data is still underway. Recent data .indicate

both Novae may have become resolvable. If so, we will soon have distances

determinable by the radio data.

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RED SUPERGIANTS

This type of radio star must be placed in the maybe category. Kellerman

and Pauliny-toth have reported a possible flare in the red .supergiant called

Betelguese (a ORIONIS). On February 21, 1966, they found a flux of

(0.11 - 0.03) f.u. at a wavelength of 1.9 cm. No signal was found, however,

on eleven following nights.

In addition, Seaquist reported a possible detection-of R AURIGAE, another

. .

red supergiant. However, again the result- is not reproducible.

This non-reproducibility is the curse of this subject. We know now that

every known radio star is variable and some are erratically variable, hence it

may be.difficult to obtain a particular flare on a particular star.

ANTARES B

Extensi've observations of Antares, another red supergiant, have produced

an unexpected result and a new type of radio star. Observations at 11.1 cm

by myself and C. M. Wade on the NRAO interferometer have shown that in March,

June and November 1970, Antares had a flux density of 0.005 + 0.001 flux units.

In June the flux at 3;7 cm was less than 0.003 f.n. On June 1, 1971 the Antares

radio source was "flaring" at 3.7 cm to a level of 0.011 + .002 f.u. A month

later it was again less than 0.005 f.u. at this wave-length.

Antares A. the red supergiant, but rather.was Antares B, a B3V blue companion

3".2 from the supergiant. The following table and a 3.7 cm map of the source

proves this.

------ I- I -- ---- ; a 4 ~~ . -C-L-CIO- e~~Url

-10 -

TABLE 1

FLUX-DENSISTY MEASUREMENTS OF THE ANTARES RADIO SOURCE

Faux D.rnsrr (flux units),DArs o ANrt-NA SPACInGs

OasravAno 11.1 cm 3.7 cm (meters)

1970 March 25......... 0.003±0.004 * 900, 1800, 27001970 June 4........... 0.006±0.C002 <0.003 900, 1800, 27001970 November 12....... 0.000.001 * 900, 1800, 27001971 June ............. 0.008±0.002 0.0114±0.002 900, 1800, 27001971 July 8............. 0.007±0.002 <0.005 100, 1800, 1900

S ri-- r rrr .r-

SNot observed at 3.7 cm on these dates.

- • 2f" ." •Z.o'

RfstcnwE TO sPo lt .SOuRCE j.'

.. . . ..a. . ... ,T R ."

. " .

*2*19'30'

21 204 Is' " ;

. - . 16 26 m

FI. 1.--Aperture-synthesis mnp of the Antares radio source, derived from observations at 3.7 cm on.1970 June 1. The uncertainty of the placement of the radio contours is about 1" in each coordinate. Co.ordinates are for the epoch 1950.0.

Whereas a red supergiant is very large, a B3V star is very small.

Hence fantastic brightness temperatures are probably needed to produce the

radio emission.

The variable radio source may also be related to a long known optical

anomaly of the blue star: sharp forbidden emission lines of Fell and NiIl

with no other emission lines. This is physically very hard to explain

because anything that excites these lines should excite hydrogen

lines.

The answer to both anomalies seems to be unusual particle steaming

in the atmosphere of the B star.

-II-

Radio Binary Stars

The most important contribution Antares B has made to the radio star

business is the clue that matter streaming in binary systems might be

important. Following up this clue, the eclipsing binary 8 Persei (Algol)

was immediately detected in October 1971. Several days later another

famous binary star, . Lyrae, was found to be a radio star. The following

table shows the data obtained for these relatively weak radio stars.

Tablo 2 Times of Observation and Flux Densities

Flux densityStar Date UT (t0 -

2' W m -2 Hz-')2,695 MHz 8,065 MHz

. .Persci Oct.23, 1971 0543-1106 0.006±0.004 0.015-C..005Oct. 31 445-0930 0.0i14 0,0 0 3 0.0 19 - 0 .0 0 3

Nov. 6 0026-1044 <0.04 0.00410- 0.C033Nov. 7 0247-1041 0.011 - 0.003 0.01=- 0.003S Nov. 8 0244-1037 < 0.005 0..015 0.03Nov. 9 0239-0459 0.0112 0.005 0.006 0.005N . Nov. 10 0235-0756 <0.005 0.010= 0003

. Lyrae Nov. 4-5,. .1971 224 0241 O.6±O0.0C5 0.015 A0.004

Nov. 5-6 2239-011 <0.005 .. 0.020- 0.005Nov. 7 0018-0232 <0.005 - 0.015-0.005

. Nov. 7-8 . -23 ,28 <0.005 0.011 ±0.004-Nov.8-9 343- 04 <0.005 0.013 ± 0.0(0

Nov. 9-10 2125-0220 < 0005 0.015 : 0.005

An extensive study of Algol, S Lyrae, and about 50 other similar binar

systems in Jan.-Feb. 1972 produced the following results:

1. Algol was undergoing extensive flaring up to leves of 0.34 f.u.

2. 8 Lyrae weakened and went below the detection limit, thus proving

it is variable.

3. No other binary, even though many are similar in every way, was

detected.

The following two figures summarize the data for Jan.-Feb. 1972 and

compare it with the optical eclipse cycle. No obvious correlation can be

found.

- - -- - - - ~ ---

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12.0 13.0 14.0 .15.,0 16.0 17.0 18.0 19.0UNIVERSAL T

20.0IME

21.0.. 22.0 23.0 24.0 January

. . . . .( I'ft..~~ ~ ~ .. *.. .N* 9'f t 4.'.*~-~* .9. , . .*,.,* .-. 9-.-.... .. *.*9 Y* . .* .

LUX DENSITY (26 Wm H) AT 269 MHz

S"1 . , I . I i I ,I-2

FLUX DENSITY (126 Wm2 H) AT 808695 MHz

....

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5 .6 7 8 9 10 11 12UNIVERSAL TIME.

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FLUX DENSITY (10-6 Wm H z ) AT 8085 MHz

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CURVE.

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-14-

The most interesting property of Algol as a radio star is the characteristic

flare behavior, particularly in spectral index. The following two figures give

details on some of the stronger flares and a schematic representation of a

typical flare is shown.

The data can virtually all be fit by a thermal bremsstrahlung model. The

. frequent a = 0 behavior is easily explainable only by an optically thin model.

Further, during peak flares one can use a slightly optically thick model to

find that ..

T "'10 K.. • ... ... .. . .

D- 4 0.3JA.U.

10 -3. _ N 10 cm

e

Thus Algol should be a transient X-ray source.

20* The above parameters imply a mass loss rate of the order of 10 g/sec

27.5during peak flaring. With a duration of about a year, this means 110 g

33mass loss. Since the mass in stars is v 2 x 10 g, this is a few parts

in 106 of the whole mass of the binary. This should produce a comparable

change in period.

Interestingly, Algol is known to undergo period discontinuities of the

order of a few parts in 106 roughly once every 30 years. Ths last known

one was in 1942. Hence it was due this year.

1~ r*~ lr7~.yr~q?~l~~sz~ r~~.~~.~~?rra-~ - e~--~Cce -srt! n --- ' - - s ,

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ALGO

JAN. I1 122

-0.5I .

o02

0

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00 , ,t I t l l , I , I2200 0000 0200 0400 0600 0800 1000.

UT

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1800 2000 2200 0000 0200 0400 0600UT

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2000 2200 0000 0200 0400 0600UT

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1800 2000 2200 0000 0200 0400 0600

UT

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1800 200 220 0000 0200 0400

UT

2000 2200

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SPECTRAL INDEX

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-15-

.PULSARS.

Pulsars are true radio stars. Because they have been a subject of special

lectures this summer I will not discuss them.

X-RAY STARS

One of the most interesting developments of the last year is the

coupling between X-ray astronomy and radio astronomy because of the fact that

many X-ray sources are radio stars.- .- .. *.

It all began with Sco X-l. In 1968 Andrew and Purton found it was a

radio source and in 1969 Ables obtained data indicating it was a variable

TN 5910source. I and C. 1. Wade entered the subject-last-yer and have had a great

deal of fun since then. The first thing we found, by using an interferometer

which could accurately place radiation in the field of view, was that

Sco X-l is a triple radio source with the components arranged in a line..

The next page shows a map of the source. This is reminiscent of one of the

major unsolved phenomena in radio astronomy: why do double sources tend to

be arranged on opposite sides of interesting objects.

~--smre ~llppr.r~--~-~ ~I - -- -- -- --.

-16--

Sco X l

* . . . .

RCSPONSE TO AP OoiNT SOURCE

S .... 15*33':. .r

, • 04 2O

Frc. 2.-Map of the Sco S-1 tield, derived from observations during 1971 February, March, andApril. Position of the X-ray star is shown by an X. Diathal line through the component sources is atposition angle 290. The coordinate grid is for the epoch 19O.O.

* Indeed the central source sits exactly on the peculiar star identified

as the X-ray source. Furthermore, the central source varies in flux'by as

much as two orders of magnitude in a few hours. Nobody yet understands this

radio source, which appears to be non-thermal with a very variable spectral

rtdex.

Some pages of data on the time variations of this source follow.

- .. 1. . .M ! * - . . .. - .

CkI Cv N en4 C , eot v.- S. MARCH 26

. 0.08 -

0.06

0.04

0.02

0.00

0. 12

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0.08

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0.02 ff0.00 f

-. 12 14 16 18 20

JUNE 3

S3.7

Sl.

12 14 16 18 20

-SEPTEMBER 29-

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MARCH 24. 197.1

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,..--

-

.. ,,-.:.. . . " - ~C . ~: '~- --~~I~~?+r c7~wmr3p 4~ I--P'~SMP~nrr ~ ~ ~ ~.~.Y~V--'Xi,_

.. r. .. . rs

+" -20-

.The idea of hunting for other X-ray sources by looking for radio variables

.in the relatively large X-ray error boxes has lead to a number of successful

'detections and one important identification.

GX17+2: A lovely Sco X-l like flare was detected for this object

i May 27, 1971. The fact that it appeared and disappeared

is shown beautifully by, not only the map, but by the

amplitude and phase data for two different days.

.4

GXI7+2

14'02' Moy 27..1971

14*9O3 - .4 Oil'

, .... . .. -

16' 14s 12S 10' . 08' 06s 04s

18 13m

Fo. 1.--Map of the radio source appearing in the GX 17±2 field on 1971 May 27. The crcle repre-sents the error limit of the MIT X-ray position (with the 4' collimator); the polygon represents the,error limit of the Uhuru X-ray position.

"GXI7+ 2 My 27. 1971- 1 -GXt7 + 2 Moy 30. 1971003

. 02 S . .-

.200 • " " *

6 . 6 *SO- ...f 1. ! i + 4 ." .

" 0 ,* so0 - -," *

4 16 18 20 22 144 6 48 20 22 LST

0._ 8 t4 6o U

S6 18 20 22 14 .6 20 2 tsr

4 6 8 10 4 6 8 10 or

-21-

CYG X-l: The identification of a radio counterpart to CYG X-1 was made in-

dependently with the Green Bank interferometer and the Westerbork

array. The following is the GB 11.1 cm map of the source. The

accurate radio position led to an identification with HDE 226868

a spectroscopic binary with a BOIb star.and a nearly invisible

companion. Recent work has found a weak helium emission line

associated with the "other" object. If this data is confirmed,

the mass of the "other" star is. 15 solar masses, and would be

indeed a "black hole". .

*35* 06 esro

CYGX-I . o,

. . . .. Poi-t .ce

35*04

" 0 ° . ..02. - - "-°

ara. 3T- a fotho s it the B-. reseto pitso the ourr tce r40 35 30Is 25% 20' 1 to 05"

* tE,.e y r t

theurate rio posittion e iat h D

.. e ero .... o heIi:lu psii

SGX9+1: In Nov. 1971, a weak radio source appeared in the error box of this

X-ray source.

CYG X-3: This radio counterpart was first detected with the Westerbork array.

It is unusual because

1) It is the strongest of the radio stars

"2) Its spectrum is mostly thermal

3) Most of the time it looks just like 8 Persei (Algol). The

following two figures show this.

Most recently we have found CYG X-3 to undergo rapid flaring at mm

.: avelengths on time scales of minutes.

i

- ~

4

.

i . , _ ... rq ..

'

r+ i • • ,y ,

i (Ii, " .. o" .+, . •° ,. " . . ) ,+ ' + ,

.. orS . '

:i i b

CYG X-3MAY 1,1972 IA At.0

MIViT 4, ltw 4

a

0i4I.

Q0.3 -

S 2695 MHz) 0.2

0

0.A4 }

S (8085 MHz) 0.3

0.2

0.1[

ni

r"

00

wir rb

1 ' I ' I " I

.

! t I i " 1 t "

6 "8 10 12 14 16UT

0 0-

00.

. . .oo o *

1 I , .

.J , o" o " + +'+ ~ -

6 8

UT10 12

I ' I ' I ' I

* , .40-9 +i +

A ., , .4 04 *

*' t o I'Ir ~. o

6 8 10 12UTI

* 4

1 4 4

.. .

" "

• , . f

• +. + . . . " . . : .... . L .. .I.4 •. .. 4

, . .

:~ . +" Y

.. " , . f •! ,

'

" , i -

.

.. , .• • . :. .'

* 4

.'" "• '" " i + It +

• I

V . ' V. .

.+ - ,o . • . .$4"- , . • . , ' .. .

" - . " ..

• r . . . .4 4

4444 .p

r. ..

i '

" f .

v 1l '

so

',

B..

08;

oo o- ,00,o000,

. 1 . 1

- o oo a

MAY 3 1972

.. . o ... -. *•

- --- OWVW

1

I

i.

t..

i

" . ' :r .

s ; 1!

I •

I

S

,..

' R

. . "

"

" i

-r I ' I" ' I I ' i

r

' .''

I '

.. ,

". .

I41

ii

.v I'a.

i[ t 1 l --__ -s _ -'

p i- i . I + "1 P1 i i . * r i *SNJ 1r.. . , .. , n lr SI* LML){I )1.# L I C I * rC ' -. l*.a a. r.. . *i. temp. .. N:I"

h

I

ii

i I"4 O

I.0

0.0

1.0

0.3

0.2

061

. 0.01 *

r,

/: PERSEI

JAN. 14-15. 1972 JAN. 1I-17 19792

SPECTRAL INDEX : SPECTRAL INDEX

S-----* - 0. ,0 -

0.21- 1 , "". ..

22 0 2 4UNIVERSAL TIME

6

0.1 f

0.0

0.3

0.2

0.1

0.022 0 2 4

UNIVERSAL TIME6

JAN. 22-23, 1972

S SPECTRAL INDEX0.0[..e.. .-.-, -.--- -" * *

toS

0.

0.

0 (

0.3{-

0.2

0.1

... . .. ............ ......20 22 0 2

UNIVERSAL TIME4

a nI- -- -

S(2695 MHz)

'l ', I' + I' 'I '' l

21

s 2

S(2695 MHz)

O " . ,.

v i ! I • • r •

SS(8085 MHz)

ri r ~ ~ rr rr r~rr .~rr, , 'o ,

, 1t + + 4 +1p * I * p

S(8085 MHz)

. .

..

- 1- +

.i Ir • i v 1 r i ~ .w

S(8085 MHz)

t t t. t !.

. .,•

, , . . ..

li

*.,

i I,-I 1 ,I ~J, 1 - _ _ _ I - AA'

I r ' . ... " L __ "L ' __ '

' . I E • ! s l l .f ! .

.. +

. .

.~ .t /I

t

I .

. .

• ." . .,

iI

S..

1'

...

I .

3

,,""

...• . •

" . ...

..., ..

.. .

r

. i

f .

. . .

- .

...

695 MHz)

O

f:"