A study of the infrared spectrum of massive young stellar objects

CHINESE ASTRONOMY AND ASTROPHYSICS Chinese

PERGAMON

Astronomy

and Astrophysics

23 (1999) 51-58

A study of the infrared spectrum

of massive

young stellar objectA* ZHANG Yan-ping Department of Astronomy,

SUN Jin

Beijing Normal University, Beijing 100875

This paper studies a set of 12 massive young stellar objects, based on their infrared spectra from IRAS, infrared data from other observatories and our

Abstract

own JHK photometric

data.

Prom the low-resolution spectra between 7-23pm,

the dust particles are classified into three types, those showing clearly the 9.7 pm silicate feature, the 11.3pm PAH feature and others. Using a symmetric and isotropic model of radiative transfer that includes both absorption and scattering, we obtain the physical parameters of these YSOs. Prom the model fitting, we find (1) that in the majority of envelopes the dust temperature varies as the radius to the power -0.4, the same as the value expected if the absorption/emission ratio varies as uP and /3 = 1, and (2) that the dust density varies as the radius to a power between -2.0 and N -1.5, which indirectly reflects the transition the molecular envelope from isothermal collapse to the free fall regime. Key

words:

stellar formation-low-resolution

spectra-dust

of

model-infrared

spectrum

1. INTRODUCTION

Compared to small mass young stellar objects (YSOs), large mass YSOs have evolutionary timescales so short that they practically have not a clearly defined pre-main sequence stage, rather, they directly enter the main sequence. Meanwhile, because they are embedded inside thick dusty envelopes, it often happens that while the central star has entered the main sequence stage, its infrared spectrum shows little variation on the two main features ( near infrared-1OOpm) range and peaking around of a steep gradient in the JHK-IRAS 100pm. With the aim of studying the features throughout the entire evolutionary course of massive YSOs from their formation in the cores of molecular clouds to the main sequence, t Supported Received

by National Natural Science Foundation 1997-11-10;

*A translation

revised version 1997-12-25

of Acta Astrophys.

Sin. Vol. 18, No. 4, pp. 423-431

0275-1062/99/$ - see front matter 0 1999 Elsevier PII: sO275-1062(99)00026-O

Science

B.V. All rights reserved.

52

Zhang

Yan-ping,

Sun Jin

1 Chinese Astronomy and Astrophysics

23 (1999) 51-58

we have selected a set of massive YSOs with central star luminosities

L > lo3 L,,

masses

M > 7 m 8 Ma, either at or just after the formation stage for our analysisI’~21. The present work differs from previous ones in the following respects: (1) By means of a more thorough use of the IRAS low-resolution spectrum (LRS) data we first identify the possible types of the associated dust particles. (2) We then ascertain some of their basic physical parameters such as the absorbing and scattering cross sections. Then, through a detailed fitting of the model of radiative transfer we evaluate a series of physical parameters of the associated dusty envelopes (size, radial profiles of the dust temperature and density, the optical depth at the characteristic wavelength, etc.) and those of the central stars (effective temperature at the surface, luminosity, radius, etc.). (3) On the basis of the model fit, we search for various relations in the infrared spectrum. the dependence of the optical depths at the characteristic wavelengths and the total optical depth on the spectral gradient, and on the pre-main sequence evolution.

2. SOURCE

OF DATA.

THE

LRS SPECTRA

The sources for the data of our selected sample are the following:

the Catalogue of Ma.+

sive Young Stellar Objects131, the 1993 and 1996 NASA catalogues141, the results of our several JHK photometric observations using the Xinglong 1.26-m infrared telescope11y21, IRAS-PSC, IRAS-LRS, the IR.AS catalogue of 5000 sources151, the newly released LRS data (see Refs. [6,7] and part unpublished spectra), and the catalogue of high-velocity molecular outflowslsl. When selecting the observational data we used as standard the positions given by the IRAS Point Source Catalogue, rather than the positions of the associated stars. The majority of the our selected sources are massive YSOs with known distances. For individual sources with unknown distances, we find the distances by the kinematic method. The new LRS are morphologically classified161. We looked up all the LRS spectra of our selected sample, and merely from the LRS features we can classify them into three types as follows: (1) Type H (Old Classification

79-79):

containing the 9.1 pm silicate feature and a red

continuum. containing the 11.3 pm feature, a relatively red (2) Type P (Old Classification 8046): continuum, and a clear rise at the blue end. The main component of the dust particle is PAH (polycyclic aromatic hydrocarbons), attached to the surface of the cosmic dust, described “either as large molecules or as very small grains”-their properties will be specifically discussed below. (3) Others: with indistinct features or large noise.

3. PROPERTIES

OF THE

DUST

PARTICLES

When we discuss the properties of the particles we mainly want to clarify their contribution to the absorption and scattering of the radiation from the central star, that is, we wish to

Zhang

and Astrophysics

23 (1999) 51-58

and scattering

cross sections

for the different

particles, so as to have available the right absorption the equation of radiative transfer.

and scattering

determine

Yan-ping,

the models

Sun Jin / Chinese

of absorption

(1) Type

H Spectra.

provides

the cross section

adopt

this model

As the model models

Astronomy

of infrared for silicate

spectrum particles

coefficients

constructed with

types of

when solving

by Leung

the 9.7pm

53

feature,

et al.Ig~lol we shall

(see Fig. l), with some slight modification.

-14-

-16 -

-18

-20 h\ -16

. t

\

\

-2

I

2

0

4

log,0 h

Fig. 1

Fig. 2

The absorption and scattering cross

(2) Properties

of PAH.

The absorption cross section of the PAH particles in the infrared

sections of the silicate particles in the infrared

PAH were early discovered

in the 7Os, and further

experimental

and

observational studies were made in the 80s. Up to now, however, the relevant absorption cross section is still uncertain, only approximate results from experiments and theoretical analysis are available. LCger et al.llll and D&et et a1.1121gave an empirical PAH particle model,

which

divides

the infrared

range

into three

regions

according

to their

absorption

features: (a) Visible-Near Infrared. It is essentially a near-infrared extension of the electron Here, the formula for calculating the absorption cross continuum for the visible region. section of a carbon atom isl121,

where u, is the same as the aforementioned

C(y) =

QabSxu2, x = X-l pm-l,

fv(x)

= 1

fu(x)

= x2(3zr - 2%)/X? = (x - 5.9)2(O.1x + 0.41) = 0

?r-l arctan(103(y

- 1)3/y) + 0.5,

Pr = 4.0,&

2 2 21 = 4/pm x < 21 x 2 5.9/fim x < 5.9/pm

y = x/xc,

xc = 125(lnm/a)

= 1.1 and

54

Zhang Yan-ping, Sun Jin / Chinese Astronomy and Astrophysics 23 (1999) 51-58

(b) Near Infrared.

We adopt

the values

of c for different

structures

of PAH at the

characteristic wavelengths 3.3, 6.2, 7.7, 8.6, 11.3pm, given in 1989 by Lkger et al.I’rl. (c) Middle-Far Infrared (X > 10 pm). The formula here i&l21 u, = (A/~)e-‘x”/x’* where A = 3.3x 10m2’ cm2 pm/C!, in the range 0.4 approximately NC We took a = hydrogen atoms. the size of PAH is neglected.

X, = 10 pm. The radius of the PAH particle, a, is taken 1.2nm, the number of carbon atoms contained in one PAH particle is = 120(a/lnm)2, and the number of hydrogen atoms is NH = (6Nc)‘/“. lnm, so that each PAH particle contains 120 carbon atoms and 27 The above formulae then gave the cross section shown in Fig. 2. As very small, its scattering of the radiation from the central star can be

4. RESULTS 4.1

The Theoretical

OF MODEL

FITTING

AND

DISCUSSION

Model

In order to study the physical properties of the massive YSOs embedded in dusty envelopes and of the envelopes, we must make a rather precise fit with the theoretical model of radiative transfer. In this paper we adopt a spherical symmetric and isotropic model that takes into account simultaneously of both absorption and scattering. In the spherical geometry, the radiative intensity of the dust at frequency V, I”(T,~), satisfies the following equation of transfer:

where r is the distance of the current point in the envelope to the central star, p = cos$,@ being the angle between the radial direction and the line of sight, IC”,and K.; are the dust volume absorption

and scattering

coefficients, sca~u2),

P”(,u, p’) is the scattering and the total

absorption

probability. coefficient

/c”,(r) = is K~(T) =

n&)(QalBwa2), $5(r) = n&)(Q The scattering na(r)((Qabs’r~2> + (1 - (d,)(Qtmm2)~ h),,= 0 for isotropic scattering). At each T, the final temperature of the dust is coefficient is +(T) = nd(r)(Q &._?ra2),,&(r). determined by the equilibrium between absorption and scattering, that is, by the equation

I

[G,(T) - /c;(r) . J,(r)]dv

= 0

,

0

J”(r)

being the average intensity at r. The optical depth of the dust at wavelength

X is given by

rout T(X) =

Q(+a2w(+~

,

J Tin

the integration

is between

the inner and outer radii of the dusty

envelope.

Zhang

Yan-ping,

The radiative

Sun Jin / Chinese

Astronomy

and Astrophysics

-1 -

0

-8-

03236+5836

A

k A 0 2

l

h. I

-9

B

-

: A ,*

-lOi-

.

L

-8

-

@)

_-

\

0 (

i,? $1

\

q

\

, ::

4

.\ I

_g -

A;

\

l

t

‘0.0

’

.>“# - ,I

.

s’

55

flux of the dust at X is given by

(4

A

23 (1999) 51-D

““I.

0.5

1.0

““‘#

2.0

1.5

2.5

A’

. \ -10

3.0

’ 0.0

’

0.5

. ’

I

log,,

log,0 h

’

2.0

1.5

1.0

’

2.5

I

3.0

h

-1 -

’ -8-

72

0

2

.

-9 -10

-

:j

0.0

0.5

1.0

1.5 log,,

-I-6-

2.0

21381+5000

3.0

(0

20275+4001

-7 -

2.5

h

.-z -8

2 0 z

. -11

’ : 0.0

’

0.5

’

1.0

.

’

’

’

1.5 2.0 log,, a.

.

’

2.5

’

3.0

-9

-10 c

Fig. 3 Model fit of the infrared

“.l-n-‘.‘.“‘C

0.0

0.5

1.0

1.5 log,,

spectrum

2.0 h

of IRAS sources (Type H)

2.5

3.0

56

Zhang Yan-ping, Sun Jin / Chinese Astronomy and Astrophysics 23 (1999) 51-58

T, being the temperature

of the central star and R,, its radius.

For our numerical solution of the equation of radiative transfer we adopt the method of quasi-diffusion developed by Leung fg~lol. Having fixed the model of absorption and scattering and the density profile, we adopt Kevin Volk’s new code DUSTCD3NEW to carry out the numerical solution. L

02575+6017

0303S+S819

(a)

--..

-7

q

#‘‘au.

.-I -8

*I’

-

z

.

:

0 on- -9 -

P :*

0

. -lO-

‘, I

; :

\

,

, \

I.

I. 1.0

I.

I.

1.5 2.0 log,, h

-6 - 0426W3510

-1

2

_

0

I.

I. 3.0

2.5

-10.

‘..

‘0. \

:r

:” 1, *.

, ‘* ,:* . at

-9 -

0.5

;‘::

:, B . ’ .

: ,:

-8 -

”

\

‘::5 :*

:

0.0

.-;

\

0: *I

W --.

-

\

\ \

< I

=t

’ : ’ B ’ m ’ m ’ ’ ’ * ’ 2.0 2.5 1.0 1.5 0.0 0.5 3.0

log,0 h -7

w

-8

_

. ,o .E!

4’

-8 -

log,,

,

\

a.

-9 -

-j I

-101

:

c 0.6

* 0.5

n 1.0

’ 1.5

m I ’ 2.0 2.5

8 ’ 3.0

, ;; 0.0

, , , 0.5

1.0

-7 -7

E

-8

1<-8 k .-z 0

-9

$

F

,

,

,.

1.5

2.0

2.5

3.0

log,0 h

log,, a

z

,

23152+6034

(f)

-9

-10

, ‘? -11 0.0

0.5

1.0

1.5 2.0 log,0 h

2.5

3.0

-10

I

0.0

0.5

I

1.0

I

2.0 1.5 log,0 h

Fig. 4 Model fit of the infrared spectrum of IRAS sources (Type P)

2.5

3.0

Zhang Yan-ping, Sun Jin I Chinese Astronomy and Astrophysics

4.2

Results

of Model

23 (1999) 51-58

57

Fitting

In our fitting we used different cross sections for different type of dust. For the H type, we used the cross sections of silicate as input and the results are shown in Fig. 3 (a-f).

For the

P type, we used a mixed model of the absorption and scattering cross sections of silicate and the absorption cross section of PAH as input, and the results are shown in Fig. 4 (a-f), The triangles are our JHK photometric data obtained with the 1.26-m Xinglong telescope over the last few years, the squares are IRAS flux and the filled circles are data from other observatories. The dotted line is our result of model fitting. The data shown for IRAS 02575+6017 (Fig. 4(a)), 03236+5836 (Fig. 3(b)) and 06319+0415 (Fig. 3(d)) are revisions of perviously published results of fittingf21, e.g., for the first object, we calculated anew using the silicate+PAH model. For the last two, the fit was improved. Table 1 lists the model used (pure silicate or mixed silicate and PAH) and the results obtained for the best fit. The last four columns are the effective temperature of the central star, the total optical depth and the exponents in the density and temperature profiles. Table 1 The Physical Parameters at the Best Fit IRAS

associate

D

Sp

LRS

model

@PC)

L

T

(lo3 L,)

103K

I

0

a

02219$6152

W3-IRS5

2.3

H

Si

316.0

46.39

12

-1.5

0.42

02575-j-6017

A4029

2.2

Bl

80

Si+P

14.0

20.2

9.5

-1.5

0.42

03035$5819

A437

2.0

Bo

81

Si+P

32.0

28.29

8

-1.5

0.4

03236+5836

A490

0.9

73

Si

3.20

15.10

6.5

-1.5

0.40

04269+3510

LlcHa 101

0.8

Be

P

Si+P

21.99

26.0

7.1

-1.5

0.41

06053-0622

Mon R2

0.8

Bl

76

Si

44.92

25.0

12

-1.7

0.40

06058+2138

A5180

1.5

P

SitP

10.87

20.57

7.5

-1.5

0.40

06319+0415

A961

1.6

53

Si

7.93

12.04

8.2

-1.5

0.38

06384+0932

NGC 2264

0.76

P

Si+P

3.3

20.17

8.1

-1.5

0.42

20275+4OOl

A2591

1.2

08.5

38

Si

33.0

33.0

15

-1.5

0.42

21381+5OOO

v645 Cyg

5.6

07

32

Si

104.9

35.5

9

-1.5

0.43

2.5

Bo

72

SitP

8.5

30.0

9.2

-1.5

0.39

23152+6034

MWC

1080

P?

4.3

Discussion

(1) Since the H type infrared sources have clear 9.7pm absorption feature, the silicate absorption model generally gives a good fit. A large part of the dust particles associated

with massive YSOs belong to the H type, and the various parameters of this type of sources can thereby be obtained from the best model fit. (2) P type infrared sources account for a large fraction of massive YSOs. But if we merely use the absorption cross section of PAH as input, then the result of fitting will not be good, especially in the middle and far infrared. Considering that silicate particles are also present in the envelopes of P type sources, we therefore used a mixed model of both PAH and silicate absorption cross sections. As the PAH particles have a much grater number density than the silicates (by a factor of some 103), we increased the number density of PAH

Zhang

58

and through

Yan-ping,

adjustment

Sun Jin

/ Chinese Astronomy and Astrophysics

arrived

at the best-fit

proportion

values of the physical parameters. (3) The effective temperatures of the central stars agreement with the sources with known spectral types.

23 (1999) 51-58

and obtained

the corresponding

given by our model

fit are in fair

(4) The profile of the dust temperature given by the best fit is TV 0: T-‘.~*O.O~. Let Q(V) = u(v)/~FQ~ be the dust absorption/emission ratio, O(V) being the absorption cross section, and if we assume Q(V) = (V/VO)P , then for an optically thin cloud we shall have the equilibrium dust temperature Ta(r) 0: (~/~o)-~/(~+P)l~~l. Our result is thus the same as the value expected for p = 1; it is also close to the value obtained by Sun Jin and Wu Yue-fang11f21 using a simple model fit to the infrared spectrum. This result shows once again that the massive YSOs have not only similar features in their infrared spectra, their dust envelopes have also a similar thermal structure, whether they have already reached the main sequence or are still in the pre-main sequence stage. (5) The dust density

has a profile of the form 7rd(T) oc r-2.0N-1.5

.

If the gas/dust ratio is a constant, this will also be the profile of the gas density, that is, a power law with exponent -2.0 - -1.5. We know that, at the onset of gravitational collapse, so long as pressure gradient balances gravity, the molecular gas envelope will be proportional to rm2. But when supersonic inward flows are produced in the envelope, the gas density distribution will begin to assume the free-fall form, that is, the gas density will be oc ~-l.~. Hence, the result of this paper is a reflection of the transition of the molecular envelope from isothermal collapse to the free fall regime.

References 111 [21

Sun Jin and Wu Yuefang,

CAA 1191,15,375

Sun Jin, Tang Ge-shi and Zhang Yan-ping,

= AAuS 1991, 32,134 CAA 1998, 22, 179 = AAnS 1997, 38, 412

I31

Chau S. J., Henning T., Schreyer K. A&AS, 1996, 115, 285

141

Catalog

of Infrared Observations,

IRAS Science Team.

;:I

NASA, 1993, 1996

A&AS, 1986, 65, 607

Volk K., Cohen M., AJ, 1989,98,931 Volk K., Kwok S. et al., ApJS, 1991, 77, 607

[71 I31 [91 PO1 Pll WI

D6sert F.-X.,

[I31

Genzel Ft., In: The Galactic Interstellar

Huang W. Y., He J. M., A&AS, 1996,115, Leung C.M., ApJ, 1975,199, Leung C. M., ApJ,

283

340

1976, 209, 75

Ldger A., d’Hendecour Boulanger

L., Dhfourneau D., A&A, 1989, 216, 148 F., Puget J. L., A&A, 1990, 237, 215 Medium, Springer-%&g,

1992, 275