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Near infrared photometry and analysis of OH maser stars of different types
Chin.Astron.Astrophys.(l991)15/3,306-317 A translationof
Acta Astron.Sin.(1991)32/1,17-27
Q, Pergaon Press plc Printed in Great Britain 0275-1062/91$10.00+.00
NEARINFRAREDP HOTOMETRY ANDANALYSIS OFOHMASERSTARSOFDIFFERENTTYPES’ SUN Jin
FANG Geng
Department of Astronomy, Beijing Normal University Shanghai Natural Science Museum
WNI Han
Received
1990 May 28
ABSTRACT We give results of near infrared observations of 17 stars of different OH maser types including their J, H and K magnitudes and fluxes. Their IRAS conterparts are identified. From a siMult.aneous analysis of their near infrared, far infrared and OH radio data we derive their near and far infrared luminosities 4 and 4, infrared gradients a, characteristics so of the silicate 10~ feature and the mass loss rate of the central star M. We found that, for all OH maser types, 4, L2, & and so all tend to increase with increasing Mass loss rate. This shows that, for the red giants in the AGB sub-branch, there exists indeed a tightly connected evolutionary sequence from optical Mira stars to optically invisible OH/IR stars, from OH-1665/1667 masers to OH-1612 masers.
1. INTRODUCTION When red giants evolve and reach the asymptotic giant branch (AGB), they become OH maser stars. Over the last twenty years, following the publication of the data from the near infrared sky survey at 2.2 p (IRC, [l]), the AFGL survey at 10~ and 20~ [21, and particularly, the whole-sky infrared data at wavelengths as long as 100~ of the infrared astronomical satellite IRAS that went up in 1983, our understanding of the properties and evolution of the that many OH OH maser stars has greatly improved. It was discovered maser stars give very strong infrared radiation, yet do not have implying that they have a very large mass optical counterparts, loss rate and that the strong OH maser we observe is a natural of mass loss on the consequence of such mass loss. The effect evolution of the stars of this kind has been discussed from many directions [4]. From their observed optical and OH emission features, these stars can be divided into three groups: (1) Type I that is, optically visible Mira-1665/1667 MHz-OH OH-Mira stars, that is, optically visible maser stars; (2) Type II OH-Mira stars, Mira-1612NHs-OH Maser stars; (3) OH/IR stars, that is, optically invisible, 1612NHs-OH maser stars. We can be sure that tight relations exist among the infrared spectral features, the mass loss 1 Program supported by the National Natural Science Foundation
307
OH Maser Stars
and the OH uaser emission in the various types of maser stars. Much work has already been done on the near and far infrared observation and analysis involving the different types of OH maser stars [5-81; but simultaneous analyses of their infrared (near and far) and radio data are few. Especially muy optically invisible OH/IR stars lack complete near infrared observational data. Therefore, in this paper, we first use the 1.26 u infrared telescope of Beijing Observatory Xinglong Station to carry out J, H, K broad-band photometry of 17 OH maser stars of different types, including 2 for which the J magnitude and flux are given for the first time (marked with asterisks in TABLES 1 and 2. Then, according to the properties of the AGB stars, we anticipate that, aa the different types evolve on the AGB, the gradient between the far and near infrared and the far infrared luminosity will increase. Available observations and studies at present 191 have already shown that, as a star evolves from an optical Mira to an OH/IR star, the maas loss rate fi keeps on increasing. Therefore, we take #f as a clue in the present paper and discuss the dependence of H of the infrared gradient, luminosity and the silicate 10~ feature. The values of M are calculated from the radito observations of the OH maser.
2. OBSERVATION 1. Source of the Sample The observational sample used in htis paper comes entirely from Engels’ (1979, [lo]) catalogue of late type stars with OH, Hz0 or SiO maser emssions. Although this catalogue was published some years back, it is still a rather complete listing of the various kines of stellar maser. To reduce the effect of local variations in the atmospheric conditions, the sample was restricted to observations near the eenith of sources with right ascension between 24”-gh and declination greater than -20°. Accordingly, all the 7 OH maser stars in Engels’ catalogue with no optical identification were selected. There were 15 optically identified OH stars and we observed 11; of the sources not selected, some because of lack of OH flux data, some, unclear classification and one, U Lyn because our observation differed much from previous ones. The cause is to be investigated and it is not included in TABLE 1. 2.
Instrument
and Observation
On 1988 January 24, 25 and 26, we used the 1.26 n infrared telescope of Xinglong Station, Beijing Observatory and observed 17 OH maser stars in the wavebands J, H and K. For the combination of the telescope and the infrared photometer used, the characteristics of the wavebands are as follows: waveband central wavelength (p) effective bandwidth transparency
J 1.26 0.25 60%
H 1.68 0.40 71%
K 2.28 0.48 79%
SUN
308
TABLE 1
Jin et al.
JHK Photometry of OH Maser Stars
STAR NAME
IRC
J
s-l
K
NV AUR
50137
05073+5248
9.13
6.53
4.79
U OR1
20127
05528+2010
0.15
-0.57
-0.*9
R LEO
10215
09440+1139
0.06
-0.35
-1.18
R CM
50484
23558+SlO6
2.30
Y CM
6000,
00007+5s24
IK
TAU
AUR
RU
X TAU
IRAS
1.47
-1.12
1.17
-1.73
-1.98
10050
03507+1115
3.45
1.65
40135
05376+3736
2.79
2.07
1.55
10060
04255+1003
2.19
1.34
0.99
0.30
RX
l-AU
10066
04355+0814
1.72
1.89
1.46
PZ
CAS
60417
23416+6130
1.44
0.94
0.83
174.7+13.5
40156
06297+4045
7.56
5.62
4.03
154.37+21.75
60169
06300+6058
93.74
2.5,
03206+6521
l12.4,
*,2.57
'11.51 07.79
136.0f7.2 141.7+3.5
l.S7
03293+6010
'Il.37
*lo.42
07054-1039
4.07
3.61
2.84
235.3+18.1
08357-,013
8.85
6.90
5.65
,27.8+0.0
0,304+62,,
*I,.32
6.99
5.86
-10154
223.59-1.3
TABLE 2
JHK and IFlAS Fluxes
of OH Maser
Stars
STAR NV
NAM1 AUR
I
FJ(JY)
FK(JY) 2.495
0.355
8.006
F,,(JY)
F..(Jy)
FAJY)
*27.0
274.0
72.0
22.4
FAJY)
U
ORI
1387.8
1729.5
1494.6
682.0
259.0
38.3
13.8
R
LEO
1509.3
1408.7
1957.0
2160.0
653.0
LL4.0
38.4
R
CAS
1340.0
555.0
102.0
38.6
Y
CAS
97.6
46.9
7.33
500.83
4630.0
2380.0
332.0
IK RU R
192.94
264.66
4503.9
TAU
5025.1 223.40
66.80
AUR TAU
225.62 4063.L
122.54
151.38
158.4,
L54.0
81.0
213.72
296.74
266.68
69.5
46.2
7.68
70.7
29.9
4.57
373.0
398.0
RX
TAU
L30.34
178.88
172.767
PZ
CAS
423.19
430.93
308.22
174.7-kl3.5
1.511
154.37+21.7
951.09
5.763 IO,.39
16.096 155.75
11.5
96.1
IO3.0
94.4
20.5
296.0
213.0
45.4
,38.0+7.2
*0.017
'O.O,O
'0.262
95.8
134.0
37.2
,41.7+3.5
'0.045
l0.070
go.504
37.3
69.5
17.1
223.59-1.3
37.63
36.73
46.11
52.9
39.9
*.2,
235.3+,8.,
0.462
1.782
3.622
62.5
59.2
10.5
127.8+0.0
00.047
0.259
2.939
289.0
445.0
194.0
-
-
-
2.22 LO,., 4.29 2.65 2.09 38.6 5.56 14.9 9.63 5.35 <,2.6 2.56 49.3
The limiting magnitude in K is about 12.5 mag for a l-minute integration, cooled with liquid nitrogen. The measuring accuracy of the telescope is 0.04-0.05 msg. The intrumental details can be found in QIAK Thong-yu’s “Handbook of the 1.24 Metre Infrared Atmospheric extinction and magnitude reduction were Telescope”. made using standards 6 itmes on 24, 8 times on 25 and 9 times on 26, the standards being chosen from Ref. (111. Thus, we first found the observed magnitudes corrected for the atmospheric extinction. The next step was to correct for interstellar absorption. For this we use the Van Herk (1965) formula: Av = 0.14 csc Ibl [l - exp(-10
rsin
lb111
(1)
309
OH Maser Stars
where b is the galctic latitude of the star and r is its distance in kpc. We then use Rieke and Lebofsky's formulae [121 to get the extinctions in J, H and K: AJ = 0.282 Av, Au = 0.175 Av, Ax = 0.112 Av. The final data after the corrections for the 17 stars are given in TABLE 1. Column 2 is the serial number in the IRC catalogue and Column 3 the identified IRAS desigantion. For 4 of the stars listed, their near infrared photometry is given here for the first time. They are OH138.0t7.2, OH141.7t3.5, OH235.3t18.1 and 0H127.8t0.0. We found them to be indeed very weak sources in the near infrared. The conversion from magnitude into flux was made using the standard data of the NASA system. The J, H, K fluxes are given in TABLE 2, followed by their IRAS fluxes at 12~, 25/~,60~ and 100~.
3. DATA ANALYSIS AND CALCULATION OF THE PARAMETERS TABLE 3 gives the following data for 15 OH maser stars: the distance D in pc, velocity of expansion of the maser envelope I& in km/s, the fluxes at the two velocity peaks of the OH-1665/1667 MHz and 1662 MHz masers in Jy and the derived maser luminosity Iox in Jykpc'. Two observed sources, R Leo and RU Aur were not included because of lack of full data on the flux. According to the OH emission and infrared survey data of TABLE 3, we can divide the stars into four kinds: (1) Type-I OH-Mira Stars: R Cas, Y Cas, R Tau and RX Tau.
TABLE 3 STAR
NAME
NV
Distances, Fluxes and Luminosities of OH Maser Stars
-
_-
AUR
D(PC) 620
U ORI
7.80
IK
TAU
270
I’Z
CAS
2700
R CAS
359
Y
955
CAS
594
R TAU RX
7535
TAU
174.7+13.5
,400
,54.37+2,.75
7.760
1311.Of7.2
3370
,41.7+3.5
4940
223.59--1.3
3980
235.3flO.L
3000
117.*+0.0
-
6980
Vr(Km/s)
1665/1667(1,)
16.5
0.7
2.0
Z.0
17.0
2.5
I
6.0
_l-
1611(Jy) -___
26.5
4.0
IL.0
33.467
6.6
16.0
10.124
2.5
3.0
1.8
I.,
2.509 186.396
4.0
2.2
5.1
5.425
4.5
5.9
4.I
56.068
2.5
0.2
0.3
I.5
0.2
0.1
1.086 100.90
11.0
4.6
2.1
76.551
12.5
1.1
0.4
66.321
9.37
9.7
7.I
1184.362
12.5
5.0
1.0
685.727
13.0
0.4
0.2
56.302
I.4
2.3
202.946
21.5
LO.98
-
32
I4
12956.38
NOTES: The distances D for 0H138.0t7.2 and 127.8t0.0 are from 191; that for OH141.7t3.5 is a kinematic distance calculated by us; all other distances are from 1101.
310
SUN Jin et al.
NV Aur, U Ori, IK Tau and PZ Gas. For (2) Type-II OH-Mira Stars: the first three stars, their fluxes in the main 1665/1667 MBz line are either incomplete (one peak only) or weak compared to the auxilliary 1612 MHz line. PZ Gas is, in fact, a supergiant. (3) Optically Invisible I&C-1612MHt OH/M Stqars: 0H174.7t13.5, 0H154.37t21.75 and OH223.59-1.3. They were recognieed aa OH/IU stars in the*1969 near infrared 2p survey. (4) Optically Invisible IRAS-1612HII~ OH/IR Stars: OH138.0+7.2, OHl41.7+3.5, OH235.3t18.1 and OHl27.8+0,0. These differ from the last in that, before the IRAS, they were not seen in the infrared. Using the infrared and radio data given in TABLES 2 and 3 we calculated for each of the OH star, the mass ejection rate R, the infrared gradients S, and 4 between 25-2.28~ and 2.28-1.26p, the infrared luminosities Lr and & between 1-100~ and 40-120 @* The 10 p (strctly speaking 9.7 pf emission or aborption feature of silicate dust particales in the cirucmstellar envelopes were taken from Uef. 1141. For some individual.sources like RX Tau and OH223.59-1.3, we used directly their IRAS-LRS spectra 1151. We list (1)
the formulae
For the maas loss
used in the calculations: rate
of the central
stars
@:
where = 4x++, = (F&i)” , Va = (VH - &f/Z, Fl and Fz, and VH and VL being two velocity peaks. (2)
For the gradient
the fluxes
& between the far
and velocities
and near
at the
infrared: (3)
(3)
For the gradient
s*(4)
k
infrared
waves:
(+yk($jy).
For the infrared L, -
between two near
luminosity
(4) & over
the range 1-100~:
4rD’ ‘-* F(l)dl(crg/r). l#
The spectrum F(x) was obtained by least-squares fluxes in the near, middle and far infrared.
(5) fit
to the observed
OH Maser Stars
(5)
For the far Lz -
infrared
luminosity
311
over
1.26[FeAy, + F,,,&,~14nD’(ly
the range 40-120 cc:
- Kpc’).
(6)
where Av60
=
2.58 x 10”
Awoo
-
1.00
HE, Hs.
10 ”
X
According to the analysis in [171, & so defined of the luminosity over the specified range. (6)
For the silicate
10~
feature,
is a good measure
so:
( 1
s,. - 1OIa & . Ft.7
(7)
In the above equations, the numeral suffix in F indicates the wavelength in p and the suffix c, the continuun. &o is negative for an emission feature , and positive for an absorption feature. All 4.
the results
calculated
TABLE 4
STAR
NAME
NV AUR
according
to (2)-(7)
are given
Mass Loss Rates and Infrared Gradients Luminosities of OH Maser Stars
hi(M@yr-‘)
L,(ewl*)
0. L696.F -
4
L.475
5.254
&(JY
in TABLE
and
KPC’)
s..
+ 16
-3.5
+ 36
0.1414H + 15
-6.1
+ 37
0.1105E
+ 16
-5.7
0.4458
+ 38
0.3307E
+ I7
-9.8
0.3548
+ 31
0.61468
+ 15
-4.8
0.1528
+
0.125
0.7868
3.397
0.2608
37
0.1267B
U ORI
O.IL31E -
5
IK TAU
0.47858
-
5
0.651
PZ CAS
0.64298
-
4
0.107
R CAS
0.16568
-
5
0.376
Y CAS
0.6004E -
5
-1.865
0.717E!+37
0.30523
+ 15
-2.4
R TAU
0.463OR -
6
-0.732
0.373
0.75772 + 36
0.1255E
+ IS
-2.2
RX TAU
0.2677R -
5
-0.132
0.475
0.8968
+ 38
0.1248E
+ 17
174.7+13.5
0.1666E -
4
0.739
3.989
0.1878 + 31
0.1114E
+ 16
-6.4
154.38+21.75
0.16098
-
4
0.131
1.810
0.4348 + 3)
0.1592E
+ 17
-6.5
136.0+7.2
0.5730E -
4
2.60s
4.573
0.1008 + 38
0.1899R
+ 17
141.7+3.5
0.56178
-
4
1.057
4.066
0.1068
+ 38
0.19ilE
+ 17
223.59-1.3
0.1733R -
4
0.414
0.9958
+ 37
0.84738
+ 16
-8.8
235.3+11.1
0.54438
-
4
I.167
3.472
0.524R + 37
0.4221E
+ 16
-4.I
*27.9-0.0
0.212L.R -
3
2.096
6.963
0.192if+39
0.42411
+ 10
-0.732
-0.070
-0.534 0.264 -0.165
0.0
6.1 4.a
L4.4
4. DISCUSSION 1. Infrared
Spectrum of Different
Types of OH Stars
Figures l-4 give typical infrared spectra of the four kinds stars. The triangles are the J, H, K results of this paper, are from the four IBAS fluxes and squares are from other observations.
of OH crosses
312
SUN Jin et al.
Fig. 1 is for the Type-I OH/Mira star, R Cas. It is very flat and there is no infrared excess of far infrared over near infrared. Its LRS spectrum shows only a weak silicon emission (&o= -4.8). These observations show that the dusty envelope of R Cas is optically thin. The mss loss rate & of R Cas, based on radiation pressure driven wind is mall, from the two peaks of its 166511667 maser, we found 8 M 1.7 x 10e6 & yr-*, such a small loss rate is incapable of exciting a saturated 1612MHm maser 1181. Hence we only observe in the X = 18 cm main line. (N 0t e: U Ori is an exception). R Cm
I- lvllras
0.0
II- Mirar
2.0
I.0 km
b f
Fig. 1 The flux rpccrrum of typt I OH-Minx
IK Tau
R
IRC-OH/IR
Fig.
Gas
2
The
flux
OH-h&a:
OH 174.7f 13.5.
of
spectrum IK
Tm
IRC-OH/IR
type II
= OH 121.8+O.D
4.0 +j-=----
T,
t65K
* J.H.K
n - 2.0 0.0
1.0 IWAbl
2.0
Fig. 3 The flux ipccrrum of IIIC_OH/IR star: 0H174.7 -3. 13.5
Fig.
4
t I 0.0 The
+ IRAS
I , 1.0 b?Abl
, , 2.0
flux spectrum of IRAS-OI$/IR star? OH127.8 + 0.0
Fig. 2 is for the Type-II OH-Mira star, IK Tau. The relative flux in the far infrared, relative to the near infrared, is much stronger in IR Tau than in R Cas. Also, the 10~ feature is seen Hence the optical thickness of the envelope and here in esission. the mass loss rate are such greater in this case. From the OH data
OH Maser Stars
313
we obtain H -. 4.8 x lOWe’& yr_;‘. According to the calculation in [ 181, when NC>13 x 10e6 “0 Y’ OH-1612MHs can pump. , a saturated Fig. 3 is for the optically invisible IRC-OH/IR star, 0H174.7t13.5. The infrared gradient here is larger still, a greater reddening by dust and a larger A?.
implying
Fig. 4 is for the optically invisible (and also invisible in the previous near infrared IRC) OH/IR star, OH127.8t0.0. We observed this time very weak JHK fluxes (u0.05-2.9Jy). Such a drastic decrease in the near infrared is caused by absorption by an optically thick envelope. Its 10)~ feature can also been seen in the strong absorption, &o being as high as 14.4, and the calculated H is as high 2.2 x lo-‘MO yra-‘. Obviously, it is a star that has almost reached the end of the AGB. Figs. 2-4 also give blackbody fits to JHA and the IRAS fluxes. Details of the calculation can be found in [141. A Newtonian iteration with an iteration accuracy better than 0.1% was used. The colour temperature of the dusty shell or the effective blackbody temperature of the dust is indicated. Because, when fitting the blackbody temperature, the contribution from the central star was not deducted, the temperature found refers to the combined system, the dust plus the central star. Since we mainly use the temperature to compare the different types, we do not need to set up a radiation model for the central star. We see that both the cooler outer shell and the warmer inner shell are cooler in the optically (and IRC-) invisible OH/IR stars than in the optically visible OH stars.
InpMwf
Fig.
5 Infrared ye,.sus m.‘l maser
stars
2. Relation Different
0
lOtiM(M.
y-1 , *
lumina8ity L,(~-~OOP) loss r.tC ni of OH
Fig.
6
Far-infrared
t,(40-l20p) of OH
between the Infrared Luminsoity kinds of OH Maser Stars
versus
maser
v;’ I luminosity masr
log8 rate
ti
wrs
and Mass Loss rate
for
a. Between 4 (l-loop) and ff. This is given in Fig. 5. The triangles are Type-I Mira stars, plus signs are Type-II Mira stars and squares are optically invisible OH/IR stars. RX Tau excepted, the following statistical trend can be seen: both L and H increase
SUN Jin
314
et al.
as we go from the Type-I, through the Type-II, to the OR/IR stars. Within each type, both 4 and H have a considerable scatter, due to a dispersion in the maas of the central star. The distributions for the different types merge into one another and there is no clear cut lines of demarcation between them. The same picture will be seen in the several following figures. If we had a sufficiently large number of observed sources then it sight have been possible to find the relation between Lr and H for a fixed range of the central mass, which , to some extent, will reflect the relation between the total luminosity of the star f*; and H. From Reimers’ empirical relation we know that H increases with increasing La. b. Between the Far Infrared Luminosity 12 (40-120 p) and 8. The relation is given in Fig. 6, where the symbols have the same meaning as in Fig. 5. We see that the same statistical regularity as in Fig. 5 is seen here. Moreover, the increase in the luminosity with increasing H is more marked. This is a consequence of a greater amount of the energy of the central star being transferred to the envelope by a thicker and more absorbing shell, and more energy in the near infrared being transferred to the far infrared.
0 PI
D
t + I-Miras Ii-
Pip. 7 The rlope St of infrared spectrum “CtlUl In*‘* lorr rafC ni of OH rnlfCC star*
3
Relation Infrared
Miras
Fig. 8 The 2.28/12/25pmcolor-color diagram
between the Spectral Gradients and the Mass boss Rate
of OH tnascf
stsri
in the Near and Far
Fig. 7 gives the relation between the gradient .!& (25~-2.28~) and fi. The sys~bois have the same mesning as in Fig. 5. The variation of & with & confirms the increasing difference between the far and near infrared fluxes with increasing M, mentioned above. The two gradients for the far and near infrared are plotted against each other in the two-colour diagram of Fig. 8. On the average we have &(GR/IR) > &(II-Mira) > &(I-Mira), and also &(OR/IR) > Se(II-Mira) > &(I-Mira). This mesns that, as an OH maser star evolves along the AGR from an optically visible OH-1665/1667KRr-Mira star, through the OH-1612MRr-Mira stage, to emission gets an optically invisible OH/IR star , its infrared increasingly more reddened, the spectral peak gradually moves from the near, through the middle, to the far infrared and the IRAS
315
OH Maser Stars
colour temperature falls from 1OOOK to about 200K. As shown by the statistics given in [14), it reflects a certain special connection between the two kinds of maser and the infrared emission. 4. Relation Gradient
between the Silicate
lo-mu Feature
and the Infrared
Excess infrared emission at 10~ has been discovered in late-type stars and was identified by Wolf Rayet to come from silicate particles in dust. Infrared photometry shows the peak to be at 9.7~. In the IRAS and AFGL sky surveys this feature was often found to be in hte absorption of sources with no optical counterparts. With increasing optical thickness of the dusty envelope, this feature always changes from emission to self-absorption. Numerical data of the 10~ feature for different kinds of OH maser stars have been given in TABLE 4. We see that, for optically (or IRC-) visible stars, this feature is in emission and that, as we go from Type-I to Type-II, the emission increases in strength. When the evolution has reached the optically invisible stage, the 10~ feature reaches a maximumand mostly in, absorption, the strongest absorption was as high as 14 for a star that has reached to the top of the AGB, as mentioned earlier. Fig. 9 shows the relation between &Q and the gradient a for the Type-II OH maser stars, where the triangles are the optically visible, and squares, the optically invisible objects. For both kinds, we see that there is a clear tendency for &o to increase with increasing $. This means that the silicate 10~ feature depends not only on the extinction by the interstellar medium, it is also closed related with the absorption by the dust in the circumstellar shell. It is an interesting problem to separate the two contributions. Moreover, since & is correlated with R, &o is also correlated with fi.
5. CONCLUSIONS (1) Observations and analyses of 17 OH maser stars of different kinds tell us that red giant with maser emission have their own
SUN Jin et al.
316
properties in the infrared range, possibly reflecting the particular requirement on the infrared by the maser emissions, for example, the main 1665/1667 MHr line may come from pumping in the near infrared while the auxilliary 1662 MHz line, from the far infrared. (2) As judged from the mass loss rates calculated from the OH maser data, as the loss rate increases, the main OH maser first appears, and it is only when the rate reaches a certain level that the auxilliary line be ins to appear. The level is about (3-5) x lo-akIG pr- k. (3) Our analyses show that, for the various kinds of OH maser stars, there is a tendency for 4, 4, & and &J to increase monotonically with increasing R of the central star. This means that as the red giants evolve on the AGB, from the optical Mira stars to the optically invisible OH/IR stars, from the OH-1665/1667 MHx maser to the OH-1612 MHz maser, there is indeed a closely connected evolutionary series. In this series, the Mira-OH stars are in the earlier stage when !I is low and the velocity with which the gas is ejected is also low. As it evolves on the AGB, the expansion velocity gets increasingly greater until it stops at the A level of e 20 km/s [ 191, and H on the order of lo-’ Mg yr-‘. mass loss rate of such size is sufficient to complete the evoluiton to a planetary nebula within a short time. Also, as fl increase, the 10 p feature changes from emission to self-absorption, showing that the dusty shell is thicker and the temperature is lower, in the OH/IR stars than in the optical Mira stars, and the excess infrared emission we observe comes mainly from the dust emission of this shell.
ACKNOWLEDGEMENT We heartily thank Colleagues QIAN Zhong-yao and HU Jing-yao of Beijing astronomical Observatory for their support and help for this work.
REFERENCES
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Neugebauer, SP-3047
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255.
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[Ill
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[I21
Ricke,
G.
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Acta Astron.Sin.(1991)32/1,17-27
Q, Pergaon Press plc Printed in Great Britain 0275-1062/91$10.00+.00
NEARINFRAREDP HOTOMETRY ANDANALYSIS OFOHMASERSTARSOFDIFFERENTTYPES’ SUN Jin
FANG Geng
Department of Astronomy, Beijing Normal University Shanghai Natural Science Museum
WNI Han
Received
1990 May 28
ABSTRACT We give results of near infrared observations of 17 stars of different OH maser types including their J, H and K magnitudes and fluxes. Their IRAS conterparts are identified. From a siMult.aneous analysis of their near infrared, far infrared and OH radio data we derive their near and far infrared luminosities 4 and 4, infrared gradients a, characteristics so of the silicate 10~ feature and the mass loss rate of the central star M. We found that, for all OH maser types, 4, L2, & and so all tend to increase with increasing Mass loss rate. This shows that, for the red giants in the AGB sub-branch, there exists indeed a tightly connected evolutionary sequence from optical Mira stars to optically invisible OH/IR stars, from OH-1665/1667 masers to OH-1612 masers.
1. INTRODUCTION When red giants evolve and reach the asymptotic giant branch (AGB), they become OH maser stars. Over the last twenty years, following the publication of the data from the near infrared sky survey at 2.2 p (IRC, [l]), the AFGL survey at 10~ and 20~ [21, and particularly, the whole-sky infrared data at wavelengths as long as 100~ of the infrared astronomical satellite IRAS that went up in 1983, our understanding of the properties and evolution of the that many OH OH maser stars has greatly improved. It was discovered maser stars give very strong infrared radiation, yet do not have implying that they have a very large mass optical counterparts, loss rate and that the strong OH maser we observe is a natural of mass loss on the consequence of such mass loss. The effect evolution of the stars of this kind has been discussed from many directions [4]. From their observed optical and OH emission features, these stars can be divided into three groups: (1) Type I that is, optically visible Mira-1665/1667 MHz-OH OH-Mira stars, that is, optically visible maser stars; (2) Type II OH-Mira stars, Mira-1612NHs-OH Maser stars; (3) OH/IR stars, that is, optically invisible, 1612NHs-OH maser stars. We can be sure that tight relations exist among the infrared spectral features, the mass loss 1 Program supported by the National Natural Science Foundation
307
OH Maser Stars
and the OH uaser emission in the various types of maser stars. Much work has already been done on the near and far infrared observation and analysis involving the different types of OH maser stars [5-81; but simultaneous analyses of their infrared (near and far) and radio data are few. Especially muy optically invisible OH/IR stars lack complete near infrared observational data. Therefore, in this paper, we first use the 1.26 u infrared telescope of Beijing Observatory Xinglong Station to carry out J, H, K broad-band photometry of 17 OH maser stars of different types, including 2 for which the J magnitude and flux are given for the first time (marked with asterisks in TABLES 1 and 2. Then, according to the properties of the AGB stars, we anticipate that, aa the different types evolve on the AGB, the gradient between the far and near infrared and the far infrared luminosity will increase. Available observations and studies at present 191 have already shown that, as a star evolves from an optical Mira to an OH/IR star, the maas loss rate fi keeps on increasing. Therefore, we take #f as a clue in the present paper and discuss the dependence of H of the infrared gradient, luminosity and the silicate 10~ feature. The values of M are calculated from the radito observations of the OH maser.
2. OBSERVATION 1. Source of the Sample The observational sample used in htis paper comes entirely from Engels’ (1979, [lo]) catalogue of late type stars with OH, Hz0 or SiO maser emssions. Although this catalogue was published some years back, it is still a rather complete listing of the various kines of stellar maser. To reduce the effect of local variations in the atmospheric conditions, the sample was restricted to observations near the eenith of sources with right ascension between 24”-gh and declination greater than -20°. Accordingly, all the 7 OH maser stars in Engels’ catalogue with no optical identification were selected. There were 15 optically identified OH stars and we observed 11; of the sources not selected, some because of lack of OH flux data, some, unclear classification and one, U Lyn because our observation differed much from previous ones. The cause is to be investigated and it is not included in TABLE 1. 2.
Instrument
and Observation
On 1988 January 24, 25 and 26, we used the 1.26 n infrared telescope of Xinglong Station, Beijing Observatory and observed 17 OH maser stars in the wavebands J, H and K. For the combination of the telescope and the infrared photometer used, the characteristics of the wavebands are as follows: waveband central wavelength (p) effective bandwidth transparency
J 1.26 0.25 60%
H 1.68 0.40 71%
K 2.28 0.48 79%
SUN
308
TABLE 1
Jin et al.
JHK Photometry of OH Maser Stars
STAR NAME
IRC
J
s-l
K
NV AUR
50137
05073+5248
9.13
6.53
4.79
U OR1
20127
05528+2010
0.15
-0.57
-0.*9
R LEO
10215
09440+1139
0.06
-0.35
-1.18
R CM
50484
23558+SlO6
2.30
Y CM
6000,
00007+5s24
IK
TAU
AUR
RU
X TAU
IRAS
1.47
-1.12
1.17
-1.73
-1.98
10050
03507+1115
3.45
1.65
40135
05376+3736
2.79
2.07
1.55
10060
04255+1003
2.19
1.34
0.99
0.30
RX
l-AU
10066
04355+0814
1.72
1.89
1.46
PZ
CAS
60417
23416+6130
1.44
0.94
0.83
174.7+13.5
40156
06297+4045
7.56
5.62
4.03
154.37+21.75
60169
06300+6058
93.74
2.5,
03206+6521
l12.4,
*,2.57
'11.51 07.79
136.0f7.2 141.7+3.5
l.S7
03293+6010
'Il.37
*lo.42
07054-1039
4.07
3.61
2.84
235.3+18.1
08357-,013
8.85
6.90
5.65
,27.8+0.0
0,304+62,,
*I,.32
6.99
5.86
-10154
223.59-1.3
TABLE 2
JHK and IFlAS Fluxes
of OH Maser
Stars
STAR NV
NAM1 AUR
I
FJ(JY)
FK(JY) 2.495
0.355
8.006
F,,(JY)
F..(Jy)
FAJY)
*27.0
274.0
72.0
22.4
FAJY)
U
ORI
1387.8
1729.5
1494.6
682.0
259.0
38.3
13.8
R
LEO
1509.3
1408.7
1957.0
2160.0
653.0
LL4.0
38.4
R
CAS
1340.0
555.0
102.0
38.6
Y
CAS
97.6
46.9
7.33
500.83
4630.0
2380.0
332.0
IK RU R
192.94
264.66
4503.9
TAU
5025.1 223.40
66.80
AUR TAU
225.62 4063.L
122.54
151.38
158.4,
L54.0
81.0
213.72
296.74
266.68
69.5
46.2
7.68
70.7
29.9
4.57
373.0
398.0
RX
TAU
L30.34
178.88
172.767
PZ
CAS
423.19
430.93
308.22
174.7-kl3.5
1.511
154.37+21.7
951.09
5.763 IO,.39
16.096 155.75
11.5
96.1
IO3.0
94.4
20.5
296.0
213.0
45.4
,38.0+7.2
*0.017
'O.O,O
'0.262
95.8
134.0
37.2
,41.7+3.5
'0.045
l0.070
go.504
37.3
69.5
17.1
223.59-1.3
37.63
36.73
46.11
52.9
39.9
*.2,
235.3+,8.,
0.462
1.782
3.622
62.5
59.2
10.5
127.8+0.0
00.047
0.259
2.939
289.0
445.0
194.0
-
-
-
2.22 LO,., 4.29 2.65 2.09 38.6 5.56 14.9 9.63 5.35 <,2.6 2.56 49.3
The limiting magnitude in K is about 12.5 mag for a l-minute integration, cooled with liquid nitrogen. The measuring accuracy of the telescope is 0.04-0.05 msg. The intrumental details can be found in QIAK Thong-yu’s “Handbook of the 1.24 Metre Infrared Atmospheric extinction and magnitude reduction were Telescope”. made using standards 6 itmes on 24, 8 times on 25 and 9 times on 26, the standards being chosen from Ref. (111. Thus, we first found the observed magnitudes corrected for the atmospheric extinction. The next step was to correct for interstellar absorption. For this we use the Van Herk (1965) formula: Av = 0.14 csc Ibl [l - exp(-10
rsin
lb111
(1)
309
OH Maser Stars
where b is the galctic latitude of the star and r is its distance in kpc. We then use Rieke and Lebofsky's formulae [121 to get the extinctions in J, H and K: AJ = 0.282 Av, Au = 0.175 Av, Ax = 0.112 Av. The final data after the corrections for the 17 stars are given in TABLE 1. Column 2 is the serial number in the IRC catalogue and Column 3 the identified IRAS desigantion. For 4 of the stars listed, their near infrared photometry is given here for the first time. They are OH138.0t7.2, OH141.7t3.5, OH235.3t18.1 and 0H127.8t0.0. We found them to be indeed very weak sources in the near infrared. The conversion from magnitude into flux was made using the standard data of the NASA system. The J, H, K fluxes are given in TABLE 2, followed by their IRAS fluxes at 12~, 25/~,60~ and 100~.
3. DATA ANALYSIS AND CALCULATION OF THE PARAMETERS TABLE 3 gives the following data for 15 OH maser stars: the distance D in pc, velocity of expansion of the maser envelope I& in km/s, the fluxes at the two velocity peaks of the OH-1665/1667 MHz and 1662 MHz masers in Jy and the derived maser luminosity Iox in Jykpc'. Two observed sources, R Leo and RU Aur were not included because of lack of full data on the flux. According to the OH emission and infrared survey data of TABLE 3, we can divide the stars into four kinds: (1) Type-I OH-Mira Stars: R Cas, Y Cas, R Tau and RX Tau.
TABLE 3 STAR
NAME
NV
Distances, Fluxes and Luminosities of OH Maser Stars
-
_-
AUR
D(PC) 620
U ORI
7.80
IK
TAU
270
I’Z
CAS
2700
R CAS
359
Y
955
CAS
594
R TAU RX
7535
TAU
174.7+13.5
,400
,54.37+2,.75
7.760
1311.Of7.2
3370
,41.7+3.5
4940
223.59--1.3
3980
235.3flO.L
3000
117.*+0.0
-
6980
Vr(Km/s)
1665/1667(1,)
16.5
0.7
2.0
Z.0
17.0
2.5
I
6.0
_l-
1611(Jy) -___
26.5
4.0
IL.0
33.467
6.6
16.0
10.124
2.5
3.0
1.8
I.,
2.509 186.396
4.0
2.2
5.1
5.425
4.5
5.9
4.I
56.068
2.5
0.2
0.3
I.5
0.2
0.1
1.086 100.90
11.0
4.6
2.1
76.551
12.5
1.1
0.4
66.321
9.37
9.7
7.I
1184.362
12.5
5.0
1.0
685.727
13.0
0.4
0.2
56.302
I.4
2.3
202.946
21.5
LO.98
-
32
I4
12956.38
NOTES: The distances D for 0H138.0t7.2 and 127.8t0.0 are from 191; that for OH141.7t3.5 is a kinematic distance calculated by us; all other distances are from 1101.
310
SUN Jin et al.
NV Aur, U Ori, IK Tau and PZ Gas. For (2) Type-II OH-Mira Stars: the first three stars, their fluxes in the main 1665/1667 MBz line are either incomplete (one peak only) or weak compared to the auxilliary 1612 MHz line. PZ Gas is, in fact, a supergiant. (3) Optically Invisible I&C-1612MHt OH/M Stqars: 0H174.7t13.5, 0H154.37t21.75 and OH223.59-1.3. They were recognieed aa OH/IU stars in the*1969 near infrared 2p survey. (4) Optically Invisible IRAS-1612HII~ OH/IR Stars: OH138.0+7.2, OHl41.7+3.5, OH235.3t18.1 and OHl27.8+0,0. These differ from the last in that, before the IRAS, they were not seen in the infrared. Using the infrared and radio data given in TABLES 2 and 3 we calculated for each of the OH star, the mass ejection rate R, the infrared gradients S, and 4 between 25-2.28~ and 2.28-1.26p, the infrared luminosities Lr and & between 1-100~ and 40-120 @* The 10 p (strctly speaking 9.7 pf emission or aborption feature of silicate dust particales in the cirucmstellar envelopes were taken from Uef. 1141. For some individual.sources like RX Tau and OH223.59-1.3, we used directly their IRAS-LRS spectra 1151. We list (1)
the formulae
For the maas loss
used in the calculations: rate
of the central
stars
@:
where = 4x++, = (F&i)” , Va = (VH - &f/Z, Fl and Fz, and VH and VL being two velocity peaks. (2)
For the gradient
the fluxes
& between the far
and velocities
and near
at the
infrared: (3)
(3)
For the gradient
s*(4)
k
infrared
waves:
(+yk($jy).
For the infrared L, -
between two near
luminosity
(4) & over
the range 1-100~:
4rD’ ‘-* F(l)dl(crg/r). l#
The spectrum F(x) was obtained by least-squares fluxes in the near, middle and far infrared.
(5) fit
to the observed
OH Maser Stars
(5)
For the far Lz -
infrared
luminosity
311
over
1.26[FeAy, + F,,,&,~14nD’(ly
the range 40-120 cc:
- Kpc’).
(6)
where Av60
=
2.58 x 10”
Awoo
-
1.00
HE, Hs.
10 ”
X
According to the analysis in [171, & so defined of the luminosity over the specified range. (6)
For the silicate
10~
feature,
is a good measure
so:
( 1
s,. - 1OIa & . Ft.7
(7)
In the above equations, the numeral suffix in F indicates the wavelength in p and the suffix c, the continuun. &o is negative for an emission feature , and positive for an absorption feature. All 4.
the results
calculated
TABLE 4
STAR
NAME
NV AUR
according
to (2)-(7)
are given
Mass Loss Rates and Infrared Gradients Luminosities of OH Maser Stars
hi(M@yr-‘)
L,(ewl*)
0. L696.F -
4
L.475
5.254
&(JY
in TABLE
and
KPC’)
s..
+ 16
-3.5
+ 36
0.1414H + 15
-6.1
+ 37
0.1105E
+ 16
-5.7
0.4458
+ 38
0.3307E
+ I7
-9.8
0.3548
+ 31
0.61468
+ 15
-4.8
0.1528
+
0.125
0.7868
3.397
0.2608
37
0.1267B
U ORI
O.IL31E -
5
IK TAU
0.47858
-
5
0.651
PZ CAS
0.64298
-
4
0.107
R CAS
0.16568
-
5
0.376
Y CAS
0.6004E -
5
-1.865
0.717E!+37
0.30523
+ 15
-2.4
R TAU
0.463OR -
6
-0.732
0.373
0.75772 + 36
0.1255E
+ IS
-2.2
RX TAU
0.2677R -
5
-0.132
0.475
0.8968
+ 38
0.1248E
+ 17
174.7+13.5
0.1666E -
4
0.739
3.989
0.1878 + 31
0.1114E
+ 16
-6.4
154.38+21.75
0.16098
-
4
0.131
1.810
0.4348 + 3)
0.1592E
+ 17
-6.5
136.0+7.2
0.5730E -
4
2.60s
4.573
0.1008 + 38
0.1899R
+ 17
141.7+3.5
0.56178
-
4
1.057
4.066
0.1068
+ 38
0.19ilE
+ 17
223.59-1.3
0.1733R -
4
0.414
0.9958
+ 37
0.84738
+ 16
-8.8
235.3+11.1
0.54438
-
4
I.167
3.472
0.524R + 37
0.4221E
+ 16
-4.I
*27.9-0.0
0.212L.R -
3
2.096
6.963
0.192if+39
0.42411
+ 10
-0.732
-0.070
-0.534 0.264 -0.165
0.0
6.1 4.a
L4.4
4. DISCUSSION 1. Infrared
Spectrum of Different
Types of OH Stars
Figures l-4 give typical infrared spectra of the four kinds stars. The triangles are the J, H, K results of this paper, are from the four IBAS fluxes and squares are from other observations.
of OH crosses
312
SUN Jin et al.
Fig. 1 is for the Type-I OH/Mira star, R Cas. It is very flat and there is no infrared excess of far infrared over near infrared. Its LRS spectrum shows only a weak silicon emission (&o= -4.8). These observations show that the dusty envelope of R Cas is optically thin. The mss loss rate & of R Cas, based on radiation pressure driven wind is mall, from the two peaks of its 166511667 maser, we found 8 M 1.7 x 10e6 & yr-*, such a small loss rate is incapable of exciting a saturated 1612MHm maser 1181. Hence we only observe in the X = 18 cm main line. (N 0t e: U Ori is an exception). R Cm
I- lvllras
0.0
II- Mirar
2.0
I.0 km
b f
Fig. 1 The flux rpccrrum of typt I OH-Minx
IK Tau
R
IRC-OH/IR
Fig.
Gas
2
The
flux
OH-h&a:
OH 174.7f 13.5.
of
spectrum IK
Tm
IRC-OH/IR
type II
= OH 121.8+O.D
4.0 +j-=----
T,
t65K
* J.H.K
n - 2.0 0.0
1.0 IWAbl
2.0
Fig. 3 The flux ipccrrum of IIIC_OH/IR star: 0H174.7 -3. 13.5
Fig.
4
t I 0.0 The
+ IRAS
I , 1.0 b?Abl
, , 2.0
flux spectrum of IRAS-OI$/IR star? OH127.8 + 0.0
Fig. 2 is for the Type-II OH-Mira star, IK Tau. The relative flux in the far infrared, relative to the near infrared, is much stronger in IR Tau than in R Cas. Also, the 10~ feature is seen Hence the optical thickness of the envelope and here in esission. the mass loss rate are such greater in this case. From the OH data
OH Maser Stars
313
we obtain H -. 4.8 x lOWe’& yr_;‘. According to the calculation in [ 181, when NC>13 x 10e6 “0 Y’ OH-1612MHs can pump. , a saturated Fig. 3 is for the optically invisible IRC-OH/IR star, 0H174.7t13.5. The infrared gradient here is larger still, a greater reddening by dust and a larger A?.
implying
Fig. 4 is for the optically invisible (and also invisible in the previous near infrared IRC) OH/IR star, OH127.8t0.0. We observed this time very weak JHK fluxes (u0.05-2.9Jy). Such a drastic decrease in the near infrared is caused by absorption by an optically thick envelope. Its 10)~ feature can also been seen in the strong absorption, &o being as high as 14.4, and the calculated H is as high 2.2 x lo-‘MO yra-‘. Obviously, it is a star that has almost reached the end of the AGB. Figs. 2-4 also give blackbody fits to JHA and the IRAS fluxes. Details of the calculation can be found in [141. A Newtonian iteration with an iteration accuracy better than 0.1% was used. The colour temperature of the dusty shell or the effective blackbody temperature of the dust is indicated. Because, when fitting the blackbody temperature, the contribution from the central star was not deducted, the temperature found refers to the combined system, the dust plus the central star. Since we mainly use the temperature to compare the different types, we do not need to set up a radiation model for the central star. We see that both the cooler outer shell and the warmer inner shell are cooler in the optically (and IRC-) invisible OH/IR stars than in the optically visible OH stars.
InpMwf
Fig.
5 Infrared ye,.sus m.‘l maser
stars
2. Relation Different
0
lOtiM(M.
y-1 , *
lumina8ity L,(~-~OOP) loss r.tC ni of OH
Fig.
6
Far-infrared
t,(40-l20p) of OH
between the Infrared Luminsoity kinds of OH Maser Stars
versus
maser
v;’ I luminosity masr
log8 rate
ti
wrs
and Mass Loss rate
for
a. Between 4 (l-loop) and ff. This is given in Fig. 5. The triangles are Type-I Mira stars, plus signs are Type-II Mira stars and squares are optically invisible OH/IR stars. RX Tau excepted, the following statistical trend can be seen: both L and H increase
SUN Jin
314
et al.
as we go from the Type-I, through the Type-II, to the OR/IR stars. Within each type, both 4 and H have a considerable scatter, due to a dispersion in the maas of the central star. The distributions for the different types merge into one another and there is no clear cut lines of demarcation between them. The same picture will be seen in the several following figures. If we had a sufficiently large number of observed sources then it sight have been possible to find the relation between Lr and H for a fixed range of the central mass, which , to some extent, will reflect the relation between the total luminosity of the star f*; and H. From Reimers’ empirical relation we know that H increases with increasing La. b. Between the Far Infrared Luminosity 12 (40-120 p) and 8. The relation is given in Fig. 6, where the symbols have the same meaning as in Fig. 5. We see that the same statistical regularity as in Fig. 5 is seen here. Moreover, the increase in the luminosity with increasing H is more marked. This is a consequence of a greater amount of the energy of the central star being transferred to the envelope by a thicker and more absorbing shell, and more energy in the near infrared being transferred to the far infrared.
0 PI
D
t + I-Miras Ii-
Pip. 7 The rlope St of infrared spectrum “CtlUl In*‘* lorr rafC ni of OH rnlfCC star*
3
Relation Infrared
Miras
Fig. 8 The 2.28/12/25pmcolor-color diagram
between the Spectral Gradients and the Mass boss Rate
of OH tnascf
stsri
in the Near and Far
Fig. 7 gives the relation between the gradient .!& (25~-2.28~) and fi. The sys~bois have the same mesning as in Fig. 5. The variation of & with & confirms the increasing difference between the far and near infrared fluxes with increasing M, mentioned above. The two gradients for the far and near infrared are plotted against each other in the two-colour diagram of Fig. 8. On the average we have &(GR/IR) > &(II-Mira) > &(I-Mira), and also &(OR/IR) > Se(II-Mira) > &(I-Mira). This mesns that, as an OH maser star evolves along the AGR from an optically visible OH-1665/1667KRr-Mira star, through the OH-1612MRr-Mira stage, to emission gets an optically invisible OH/IR star , its infrared increasingly more reddened, the spectral peak gradually moves from the near, through the middle, to the far infrared and the IRAS
315
OH Maser Stars
colour temperature falls from 1OOOK to about 200K. As shown by the statistics given in [14), it reflects a certain special connection between the two kinds of maser and the infrared emission. 4. Relation Gradient
between the Silicate
lo-mu Feature
and the Infrared
Excess infrared emission at 10~ has been discovered in late-type stars and was identified by Wolf Rayet to come from silicate particles in dust. Infrared photometry shows the peak to be at 9.7~. In the IRAS and AFGL sky surveys this feature was often found to be in hte absorption of sources with no optical counterparts. With increasing optical thickness of the dusty envelope, this feature always changes from emission to self-absorption. Numerical data of the 10~ feature for different kinds of OH maser stars have been given in TABLE 4. We see that, for optically (or IRC-) visible stars, this feature is in emission and that, as we go from Type-I to Type-II, the emission increases in strength. When the evolution has reached the optically invisible stage, the 10~ feature reaches a maximumand mostly in, absorption, the strongest absorption was as high as 14 for a star that has reached to the top of the AGB, as mentioned earlier. Fig. 9 shows the relation between &Q and the gradient a for the Type-II OH maser stars, where the triangles are the optically visible, and squares, the optically invisible objects. For both kinds, we see that there is a clear tendency for &o to increase with increasing $. This means that the silicate 10~ feature depends not only on the extinction by the interstellar medium, it is also closed related with the absorption by the dust in the circumstellar shell. It is an interesting problem to separate the two contributions. Moreover, since & is correlated with R, &o is also correlated with fi.
5. CONCLUSIONS (1) Observations and analyses of 17 OH maser stars of different kinds tell us that red giant with maser emission have their own
SUN Jin et al.
316
properties in the infrared range, possibly reflecting the particular requirement on the infrared by the maser emissions, for example, the main 1665/1667 MHr line may come from pumping in the near infrared while the auxilliary 1662 MHz line, from the far infrared. (2) As judged from the mass loss rates calculated from the OH maser data, as the loss rate increases, the main OH maser first appears, and it is only when the rate reaches a certain level that the auxilliary line be ins to appear. The level is about (3-5) x lo-akIG pr- k. (3) Our analyses show that, for the various kinds of OH maser stars, there is a tendency for 4, 4, & and &J to increase monotonically with increasing R of the central star. This means that as the red giants evolve on the AGB, from the optical Mira stars to the optically invisible OH/IR stars, from the OH-1665/1667 MHx maser to the OH-1612 MHz maser, there is indeed a closely connected evolutionary series. In this series, the Mira-OH stars are in the earlier stage when !I is low and the velocity with which the gas is ejected is also low. As it evolves on the AGB, the expansion velocity gets increasingly greater until it stops at the A level of e 20 km/s [ 191, and H on the order of lo-’ Mg yr-‘. mass loss rate of such size is sufficient to complete the evoluiton to a planetary nebula within a short time. Also, as fl increase, the 10 p feature changes from emission to self-absorption, showing that the dusty shell is thicker and the temperature is lower, in the OH/IR stars than in the optical Mira stars, and the excess infrared emission we observe comes mainly from the dust emission of this shell.
ACKNOWLEDGEMENT We heartily thank Colleagues QIAN Zhong-yao and HU Jing-yao of Beijing astronomical Observatory for their support and help for this work.
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