Surface distribution of M stars with different IRAS colour

0 0 8 3 - 6 6 5 6 / 8 8 $0.00+ .50 Copyright © 1988 Science Press & Pergamon Journals Ltd.

Vl.Ttasin Astronomy, Vol. 31, pp. 441--444, 1988

SURFACE DISTRIBUTION O F M STARS WITH DIFFERENT IRAS COLOUR

Physics

Mazlan Othman National U n i v . of

Dept.,

Malaysia,

Keiichi Ishida Tokyo A s t r o n o m i c a l O b s e r v a t o r y ,

Malaysia

Japan

1.INTRODUCTION

The Two Micron Sky Survey catalogue 1969) represents statistical

(IRC, Neugebauer

a very good sample of late-type

study of the Galactic

Ishida and Mikami

structure

1982). The late-type

and Leighton

stars suitable

(Mikami and Ishida

and interstellar

extinction

of the visual colour band. Mikami and Ishida (1981) majority

1981,

stars are detected by a whole

sky survey at the two micron colour band where they are bright spectral energy distribution

for

in

is one-tenth

found that the

of the IRC stars consist of late giants and supergiants

a statistical method. part in a hundred

The probability

The visual by Bidelman

spectral

(1980)

to the present

(Stephenson

1984,

of the IRC stars identified

in the IRAS

is N stars (70%). The frequency distribution

of the infrared colour of these stars is bimodal of the two peaks is centered at (m12-m25) to the photospheric

The other at (m12-m25)=0.8

(Figure

I). One

= 0. I mag with FWHM 0.2mag

temperatures

of late-type

stars.

mag with FWHM 0.8 mag is broad and

to the temperatures

around late-type

1986; MacConnell

1987).

point source catalog

corresponds

sample stars.

in another catalog and are found in several

The major component

corresponding

brighter

types of the IRC stars have been compiled

recent papers in the literature 1983; Epchtein,

of being a dwarf star is one

for early M stars of apparent magnitude

than V =12 mag., which corresponds

by

of circumstellar

dustshells

(CS)

stars.

2. ANALYSIS The stars are divided

into two categories: HI

3tars without

circums-

442

M. Othman and K. Ishida

tellar dust shells (CS), i.e. stars with colours (m12-m25)<0.4,

and

stars with CS, i.e. those with colours redder than the former. The visual spectral component of each category is listed in Table Th~ Galactic latitudinal distribution

].

(Fig. 2) shows that the

early M stars (M5 and earlier), with CS have a greater preference for the

Galactic equator than those without CS. The late M stars

(M6 and later) without CS show only a weak preference Galactic equator while the lateM stars w i t h C S

for the

are however strongly

concentrated towards the Galactic equator. The Galactic longitudinal distribution for early M stars with or without CS (Fig. 3) confirms the result of Sanduleak's

(]957),

that the number of early M stars is nearly equal in the Galactic center and anticenter directions

(Alksne and Ikaunieks

1981).

Meanwhile, more late M stars with CS (62%) are found in the hemisphere

towards the Galactic center than towards the anticenter

(38%). Blanco (]965), Mavridis

(1967), and Stephenson (1986) found

that the late M stars were preferentially

found towards the

direction of the Galactic center but we conclude from our study that while this is true for late M stars with CS, which is a majority of the late M stars, this behaviour

is not exhibited by the late M

stars without CS.

3. SUMMARY Previous works on Galactic apparent distributions of M stars took no account of circumstellar dust shells (CS). We show in this study that the apparent latitudinal and longitudinal

distribution of M

stars are not determined by spectral subclasses alone but appear to be strongly dependent on presence or absence of CS. We also conclude that the study of Galactic structure using stellar distributions and apparent magnitudes

in the infrared

colour band will do well to concentrate on M stars with infrared colours (ml2-m25) redder than 0.4 mag as the luminosities of these stars are high and interstellar extinction is low enough in the two-micron colour band to allow us to overcome the selection effects that sometimes plague our interpretations.

Surface Distribution of M Stars

443

ACKNOWLEDGEMENT H.O. is greatly indebted to the Japan Society for the Promotion of Science for auspices and to all the staff members at the Kiso Observatory for their unfailing assistance and warm hospitality, especially Dr. S.Okamura who assisted with the computer programming. REFERENCES Alksne, Z.K. and Ikaunieks, Ya. Ya., (1981) Carbon Stars translated by J.H. Baument, (Tucson: Pachart Pub1". Hse.) p. 155. Bidelman, W.P., (1980) Publ. Warner Swasey Obs., 2, 185. Blanco, V.W., (1965) Astrophys. J., 266, LII. Ishida, K. and Mikami, T., (1982) __Publ" Astron. Soc..Japan, 34, 89. MacConnell, D.J., (1983) Contrib. Van Vleck Obs., No. i, 365. Havridis, L.N., (1967), in Colloquium on Late-Type Stars. ed. M. Hack: (Trieste),p. 420. Nikami, T. and Ishida, K 4 (1981) Publ. Astron. Soc. Japan, 33, 135. Neucebauer, G. and Leighton, R.B., (1969) Two-Nicron Sky Survey, NAS> SF-3047, Washington, D.C. (IRC). Sanduleak, N., (1957)Astron. J., 62, 150. Stephenson, C.B., (1984) Pub1. Warner Swasey Obse., 3, No. i. Stephenson, C.Bo, (1986) Astrophys. J., 301, 927. COLOR FREO 1200

1'112

-

M2S

,

r

0.0

I-0

[I'IAG] -

-

I

~6 rrL~J r~ ~E

400

z

M]2

Fig.

-

H2b

2.0

3.0

~.,c,

]. Frequency distribution (m12 - m25) colour of the IRC stars. Sparp peak Corresponds to photospheric temperature and broad peak at right hand to radiation of circumstellar dust shell (CS).

444

M. Othman

Table

I.

spectral

Spectral

and K.

composition

type

of stars without

total

13 18 1650 325 31 130

36 614 I 117 40 58

1354 2264 1442 71 188

3454

1865

53 19

H SlAt ~; WltHOIJT (.5

,sT6R3 ~ T H C$

300

//

200

and with CS.

number of stars without CS with CS

K and earlier early M (M5 & earlier) late M (M6 & later) S type stars carbon stars total 1RC stars identified with an 1RAS point source

14

Ishida

\\ .\

/ /

/"

O

-~o"

I\

*"x ,~ ~%x.,',,

•

ItJO

o,,.

)'---°2

o"

3o"

¢r

:p

~o"

• •"/e/"~/l~ii-':::,,.-.,

.........~,-~:~-@.:~

- 60"

-

O"

30"

• &LL F~ STAIIq;

30"

o Hcj 4M~ EAIILI61L

~.

4;o"

8 H( JMIDI.ATEI(

Fig.2 Galactic latitudinal distributions of the stars without ~nd with CS decomposed into the early (M5 and earlier) and late M (M6 and later) stars. H ~tAt~ ~ l t n ~ t

CS

N

"\

....,/\/\...~ .o. , • - / - \ 1 . . . . / " " o

~

.'/

i

I

_./%,. . i

•

i !

".- . . . . . , : . .

A .:

'1

:

"i ...... •..../

M ST&I~ ~i111H C ¢.

. /',/

/'\/

,.V,.

\.-./

/-:'"L

I

!.,. ,

,-......•-d-~,;•-•-. -, ..- v ._~."--. . . . . . . . . . . :I • ;-•~ or." I1~o"

Fig.

"0

• 50"

• L:.L:"~i

i :.~;:,p.',.~ 0

270"

'i" .... "'i .Q

Ito"

3. Galactic longitudinal distributions of the stars without and with CS decomposed into the early and late M stars• The dip between ] =240 ° -360 ° is due to the survey limit of the IRC stars to the sky of 6=-33 - +8J °