ecture for summer students VS/ R /July 1971

RADIO STARS

R. M. Hjellming

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, S (in flux units, 10 Wm Hz )

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

T 6

SB

V 2 ( 1)1970 A

In equation (1) 0 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 0.003 flux

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

S > 0.003

and using equation (1)

2 2TB > 62 ,(2)

or TB 6 > 81 at 3.7 cm

T 82 > 730 at 11.1 cm

M"

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 > 32,0000 K at 3.7 cm

T > 292,000K .at 11.1 cmti

Normal process on the surfaces of stars easily produce T < 10 60K, butB . . •

it is difficult to produce larger values of TB.

Some processes with larger TB are known on the surface of the SunB ,

1012

1011

" 1010

S109

C)n.E

108

4,

tCo -07

106S

.105

Fre,,. 'ncy, Mc/s10,000 3000 1. 000 300. 100

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; thesunspot 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.

Bursts .Storms

..(max) (large)

Slowly varying /component /

(large) /

1 cm 3rcm 10cr.m 3 nrm 1 m M ,10. VIf mJil V1f.1

rn

-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 8 is always rather small, hence it takes a fantastic

TB to make up for this.

Despite all this, there are now five (and possibly 6) different types of

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

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 MINORIS, 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

10 pK in some cases. This is a few orders of magnitude greater than has

been observed on the Sun,

.807060

'6 50

4 .

Elis 10.5

1512 d

S13'....-

4 ."

0 I ^"OO'm 0200 0300m 0400 0500 n 06OOmUT JANUARY 19, 1969

NOVAE

of their atmospheres. The mass which is ejected, perhaps 10-5 of their total

mass, is thrown of violently with velocities which typically reach 1000 km/sec.

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

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-

Nova Delphini 1967 Novo Serpentis 1970

' I " lSept.7-10J

0.1 July 25, 26

Oct. 27, 28

June 19, 23

I f(). 1 2(b)

0.00o11.0 10.0 100.0 1.0 10.0 100.0

v (GHz) (GHz)

Some of the data for two of the noevae are shown above. Serpentis 1970

has been. seen to increase in flux, level off, and start a decrease. Delphini

1967 has shown only a decrease in radio flux at 3.7 cm.

The promise of the nova data is that at first look it is incompatible

with all known theories. There is little doubt that the radiation comes from

thermal emission of the ionized matter cast from the star, but the evolution

of the radio flux shows interesting features.

.TA$9 . . ... . " •.. ' ' , --/. - •€ . .

' i

11 r

In particular, the rapid decrease expected when the density in the

expanding shell drops rapidly is not.seen. Probably either (1) re-ejection

of new matter or (2) something holding parts of the shell together will be

needed to fit the data.

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 ( 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, Seasquist reported a possible detection of H[ 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

Extensive 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.

The surprise inherent in this data was that the radio source was not

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

3".2 from the supergiant. The next page shows a 3.7 cm map of the source proving

this.

- 260 19'OO"

-.266 191 5t5 "

-26 ° 19'30"

-26 19'4 5"

K-1

-7-

16 26m

-8-

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 Felland NiI

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.

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

may X-ray sources are radio stars.

It all began with Sco X-1. In 1968 Andrew and Purton found it was a

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

source. I and C. M. Wade entered the subject last year 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-1 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

,.

-9-

be arranged on opposite sides of interesting objects.

~~F~ - -.- ,~

Sco X- 1

- 150,3O t

-15031

RESPONSE TO APOINT SOURCE

- 1 5°32't-

-1 5o33'k-

1O -. .. 4.. - .. - h • . . .".1 0 8' 6s : 4S 2 . Os

16h17m

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"f : -

1 I ii

I ~' ' c ' - I

t . . .I

-10-

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

index.

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

During the last month we have followed the clue of the variable radio

star to find other X-ray stars. We have succeeded in finding one object in

a beautiful Sco X-i-like.flare: GX17+2. The map of this source and the

proof of variation are shown in figures below.

A third X-ray source Ctg X-l, has been detected as a different type of

variable radio star. It was not detectable before April 28, 1971, but

Iwas a steady radio source for a space of three weeks in May. The map of this

source is also shown below.

okwr v-y4-uVgC

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0.00

12 14 16 18 20

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0.00

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NOVEMBER 6-

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NOVEMBER 20

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