IR stars with SiC circumstellar envelopes

CHINESE ASTRONOMY AND ASTROPHYSICS PERGAMON

Chinese Astronomy and Astrophysics 23 (1999) 475-433

OH/IR

stars with Sic circumstellar envelopes

CHEN Pei-sheng Yunnan

t

*

WANG Xun-hao

WANG Feng

Observatory, Chinese Academy of Sciences, Kunming

Joint Laboratory for Optical Astronomy, Chinese Academy of Sciences,

650011 Shanghai 200030

Abstract Prom the LRS spectra of almost 2000 IRAS sources showing the 1612 MHz OH emission we found 9 candidates for OH/IR stars with SIC features in their envelopes. Further study indicates that 6 of these are definitely or very probably carbon star with the 11.3 pm Sic emission feature in addition of being OH/IR stars showing the 1612 MHz OH maser emission. Key words:

OH/JR

star-circumstellar

envelope-carbon

star

1. INTRODUCTION The strong OH maser at 1612 MHz originates predominately in circumstellar shells around AGB stars. A circumstellar shell is created when there is a high mass loss from the star (up to 10s4 Ma/yr). The dust in the shell absorbs radiation from the star and re-emits in infrared wavelengths longer than 10 pm. This infrared radiation is believed to be responsible for the pumping of the OH maser emission originating in the shell. Therefore, those AGB stars with OH maser emission at 1612MHz have been given the name “OH/IR starsn11-31. Almost all OH/IR stars are highly evolved, low or intermediate mass stars with very dense circumstellar shells, and are likely to be important progenitors of planetary nebulae12*41. It is generally accepted that there are four different kinds of stars in the AGB phase: Mira variables, OH/IR stars, carbon stars and intrinsic S-type stars containing Tc15@l. In carbonrich ([C/O] > 1) stars, Sic particles are generally formed and these have a characteristic t Supported by National Natural Science Foundation (No. 19673017) aud CAS Fund for Basic Research (KJ1952-51-310) Received 1993-00-02; revised veraion 199349-24 * A trausIation of Acto As&on. Sin. Vol. 40, No. 2, pp. 174-182, 1999 0275-1062/99/$ - see front matter @ 1999 EIsevier Science B. V. All rights reserved. PII: SO275-1062(99)00080-6

476

GWEN Pei-sheng

et al. / Chinese

Astronomy

and Aatrophysica

23 (1999)

/75-483

emission feature at 11.3pm. In oxygen-rich stars including the Mira variables with M type spectra and all OH/IR stars,([C/O] < l), a 1most all the carbon elements are bounded in CC molecules and silicate grains are primarily formed which have their fingerprints at 9.7pm in emission or absorption and at 18pm in emission 11r21.Of course, there are some exceptions discovered recently in certain sources. For example, 22 carbon stars have been found to show circumstellar silicate features at 9.7pm and they are named “silicate carbon stars”[7~81. And more than 10 M-type stars have been found to have the Sic feature at 11.3~m[gl. Another important exception was pointed out by Jura et al., namely, IRAS 17482-2824, 18551+0323, 18585+0900, and 20491+4236, classified as carbon-rich stars, had OH maser emissions [lo]. Again, LewisI’ ‘1 gave 25 OH masers that have 4n in their IRAS LRS number classification. These sources found by Jura and Lewis are likely to be another important class of exception-OH/IR stars with Sic circumstellar envelopes. In this article we try to justify further the results of Ref. [lO,ll], to identify more such sources in the literature, and to discuss some of their evolutionary properties.

2. DATA

PROCESSING

AND

RESULTS

A complete data base for OH/IR stars with the 1612 MHz OH emission discovered before the launching of the IRAS was compiled by te Lintel Hekkert et al.Ir21. After the IRAS mission, many intensive searches for OH/IR stars with the 1612MHz OH emission have been made for IRAS objects selected on the basis of their infrared colors1131. By the end of 1997, nearly 2000 OH/IR stars with the 1612 MHz emission were discovered [14-351. These OH/IR stars form the basic working sample of this paper. We have extracted and examined the IRAS low-resolution spectra (LRS) of the sample from Refs. [36-381 and the IRAS Data Analysis Facility, University of Calgary, Canada. A final set of 9 OH/IR stars were found to have the LRS classification C, i.e., showing the Sic feature at 11.3pm[361. However, among the sources suggested by Jura et al.fr’l, only IRAS 18551+0323 belongs to this category. The rest, IRAS 17482-2824, 18585+0900 and 20491+4236 are classified as E, that is, showing the silicate emission at 9.7pm. Among Lewis’s sample, only IRAS 08304-4313, 17371-3021, and 18551+0323 belong to the C classification. Although IRAS 19029+0808 has the Sic feature at 11.3pm, it is an emitter of the main OH line at 1665MHz. The other 21 sources all belong to group E. Lloyd Evans and Kwok et al. pointed out that sometimes the mechanical method applied to the LRS Atlas may cause problem& 7*aq, therefore sources that have the number classification 4n, but have not the letter classification C, are not included in our sample. Nine CH/IR stars with type C in the LRS are listed in Table 1. Besides the IRAS names for these 9 sources, the names in the GCGCS14’l, IRCi4’l and RAFGL1411 and the classification in the LRS are also given. The Table gives also the following data: the 1950.0 positions in the IRAS Point Source Catalog (hereafter PSC)1421, the HST Guide Star Catalog (hereafter HST GSC) [431, the GCGCSiQ51 and the United States Naval Observatory Catalog (hereafter USN0)1441; the magnitudes given in the above catalogs; the IRAS fluxes at 12pm, 25pm and 60pm; and the average value of the two observed peak flux densities of the OH emission at 1612 MHz. It should be noted that the optical counterparts of the IRAS sources in these catalogs are all inside the positional error ellipses of the IRAS PSC.

CHEN Pci-sheng

et al. / Chinese

Because of possible procedure

was taken

mis-classification. (1) First, possible

Astronomy

source confusion

in our re-examination

and Astrophysics

23 (1999)

due to the large OH observing of the sources

IRAS sources are searched

for within

in Table

/‘Z-i83

477

beam, the following

1 in an effort to avoid

the observing

OH beam of each

source. (2) Because the 1612 MHz OH maser is pumped by the 35pm infrared photons in the circumstellar shell of the star, and theoretically, only when the pumping efficiency, defined as the OH/infrared photon ratio, is less than 0.25, i.e., there are at least 4 infrared photons to produce one OH photon, can the pumping process take place[46t4rI. In the absence of detailed models, a good estimate of the pumping efliciency is the flux density ratio between the OH emission and the infrared emission at 35,um. So we compared the mean flux density of the OH peak emissions with the infrared flux density at 35pm and examined whether the IRAS source within the observed OH beam is the pumping source of the OH maser. Because the IRAS observations were only made at 25pm and 60pm, the flux density at 35pm was found by linear interpolation. In this way, from the IRAS sources resulting from (l), we picked out the most likely pumping sources. (3) The position of the JRAS source from (2) is compared with the corresponding positions in the GCGCS, GSC and USNO, and those falling within the IRAS error ellipse are taken as optical counterpart candidates. (4) A comparison of magnitudes in these catalogs is further made in order to confirm their correspondence. The B and R magnitudes are taken from the USN0 observation. If B-R is larger than 2.5, the star is indeed in the late stage of evolution with a spectral type later than M014’l. F or a source that has no optical counterparts in these catalogs, the near infrared observation is examined for a reasonable explanation for the absence of such counterparts. Using the procedure above we were able to state that four of the sources are confirmed OH/IR stars with an envelope that shows the Sic feature (indicated by the letter ‘ ‘Y” in the last column of Table 1) and two are possible such objects (letter “P”). For the remaining three, indicated with “?“, we were not able to decide. More information for each individual source now follows: 2.1

IRAS 04X30+3918

The 1612 MHz emission was detected in IRAS 04130+3918 by the Parks radio telescope with a 12.‘5 beam size. The flux density is 0.79 Jy and shows the single peak pattern1231. Besides this source there are two other IRAS sources, IRAS 04126+3909 and IRAS 04132+3915, within the beam. However the interpolated flux densities at 35pm for these two sources are too low to pump the OH emission, so IRAS 04130+3918 is the only one responsible for the pumping. On the other hand, Jura et al.llOl suggested that it is a very dusty carbon star. Chan et al.ls21 identified it as GCGCS645. Recently Cohen et al.lszl made spectral observation and confirmed it to be a real carbon star. Besides, as shown in Table 1, GCGCS 645 is located in the error ellipse of IRAS 04130+3918, and there are also a GSC star and a corresponding USN0 star in the ellipse. Furthermore the B-R color for the USN0 star indicates that it is an evolved late-type star. Two other USN0 stars in the ellipse have been rejected because their B-R indicates that they are not at the late evolutionary stage.

NO.

C(43)

C(42)

/

1273

/

/

/

/

/

IRC - 10095

RAFGL 940

/

/

/

/

/

RAFGL 2287

05373-0810

06238+0904

08304-4313

16458-4425

17209-3318

17371-3021

17445-3128

18551+0323

C(42)

C(42)

C

C(42)

cc431

C(U)

cc431

645

RAFGL 6312s

LRS

04130+391s

GCGCS

N8llle

IRfis

Table

Candidates

(15.1)

14.3

R

16.2

21.6

20.9

20.1

13.3

19.5

11.3

10.6

( USNO)

B

76.56

21.14

114.7

56.76

8.632

24.78

37.15

31.99

53.59

Fl2(Jy)

Stars with Sic Feature

(GSC) (GCGCS)

V(B)

of OH/IR

RA. (195O)Dec. (IRA% (GSC) (GCGCS) ( USNO) 041301.7 391802 041301.32 391820.6 041301.6 391826 041301.40 391820.7 053718.4 -081042 053718.39 -081041.8 / / 053718.40 -081042.4 062353.0 090406 / / 062353.2 090406 / / 083027.7 -431329 / / / / / / 164550.0 -442507 / / / / 164547.35 -442455.9 172059.8 -331837 / / / / 172101.66 -331831.1 173706.8 -302126 / / / / / / 174435.9 -312824 / / / / / / 185506.0 032320 / / / / / /

1

81.07

15.92

95.62

43.08

7.548

18.52

19.20

8.813

16.57

F25(Ju)

21.52

9.139

23.42

10.56

27.651.

4.514

4.925

2.113

3.687

F~O(JY)

0.97

1.13

0.53

0.13

0.56

0.37

0.02

1.35

FOH(Jy)

Y

?

P

?

?

P

Y

Y

Note

CHEN Pei-sheng

Therefore,

et al. / Chinese

IRAS 04130+3918

with the OH emission

Astronomy

is a carbon

and Astrophysics

23 (1999)

star with the Sic feature

475-483

and also an OH/IR

479

star

at 1612MHz.

2.2 IRAS 05373-0810 IRAS 05373-0810 was also found to have the 1612MHz OH emission by the Parks radio telescope. The flux densities at the two peaks are 1.94 and 0.76 Jy 1231. Besides IRAS 05373-0810, there are two more IRAS sources within the beam, IRAS 04126+3909 and IRAS 04132+3915. However the interpolated flux densities at 35pm of these two sources are too low to pump the OH emission. On the other hand, Claussen et a1.154l pointed out is a carbon star. Boffin et a1.1s51have also found that that IRAS 05373-0810 (=IRC-10095) IRAS 05373-0810 is a carbon star containing the element lithium. Chanl52l identified it as GCGCS 1017. On the other hand, It is shown that there is a GSC star and a corresponding USN0 star in the error ellipse of IRAS 05373-0810, and the color in B-R from the USN0 indicates a late star feature, although GCGCS 1017 is not in the error ellipse. IRAS 05373-0810 is identified as IRC-10095 (R.A.=053719, Dec.= -081124,195O) and RAFGL 796 (R.A.=053718.5, Dec.=-081045). In the IRAS PSC, at the same time, Bidelman1561 classified IRC-10095 as having a spectrum of “later C”. It is considered that the observations in GCGCS were often made with objective prisms that sometimes have large positional errors1451. Therefore, IRAS 05373-0810 is a carbon star with the Sic emission feature, and also an OH/IR star with the OH emission at 1612MHz. 2.3 IRAS

06238+0904

IRAS 06238+0904 was found to have the 1612 MHz OH maser emission by the Aricibo radio telescope (beam size 3.2’). The flux density is 0.02 Jy and shows the single peak pattern12q. Within the beam there are no other IRAS sources. IRAS 06238+0904 is the only one responsible for pumping the OH emission. On the other hand, Chan et al. 15q suggested that it is an infrared carbon star. Epchtein et a1.15’l observed it in the near infrared and gave K = 4.38, J = 8.29. This implies a spectral type at least later than M7. Loup et a1.15’l a 1so confirmed its carbon-rich property. Egan et a1.16”l considered it to be a carbon star with infrared excess at lOOtim. Furthermore, Chan152l identified it as GCGCS 1273. It is seen that the position of GCGCS 1273 is located within the error ellipse of IRAS 06238+0904. There are no GSC stars and USN0 stars in the error ellipse; however, it should be noted that, since J - K = 3.91, the visual magnitude would probably be fainter than 2014’l. We conclude therefore that IRAS 06238+0904 is an infrared carbon star with the Sic emission, and also an OH/IR star with the 1612MHz OH emission. 2.4 IRAS

08304-4313

IRAS 08304-4313 was also found to have the 1612MHz OH emission by the Parks radio telescope. The flux density is 0.74 Jy and shows the single peak pattern123j241. Besides it, within the beam, there are 10 other IRAS PSC sources, among them IRAS 08303-4303 also has an interpolated flux density at 35pm sufficient for pumping the OH emission. IRAS 08304-4313 was considered to be an infrared carbon star by Chan et a1.15”l. Epchtein et a1.15’l observed it in the near infrared and gave K = 9.91, H = 12.5. These values indicate that it is indeed very red and very faint, and with a spectral type at least later than M714’l. There are no GSC and only one USN0 star within the error ellipse; however, this USN0

480

CHEN Pei-sheng

et al. / Chinese

Astronomy

and Astrophysics

23 (1999)

475-483

star has B = R = 16.1, so it is certainly not the optical counterpart of IRAS 08304-4313. So we conclude that IRAS 08304-4313 is a carbon star with the Sic feature, and possibly is also an OH/IR star with the OH emission at 1612 MHz. 2.5 IRAS

16458-4425

IRAS 16458-4425 was found to have the 1612 MHz OH emission by the ATCA telescope (beam size 29.7’). Its flux densities at 1612 MHz are 0.66 and 0.45 Jy at the two peaks [351. Because it is located in the galactic plane, there are no less than 54 other IRAS sources within the beam. Of these, 27 have sufficient interpolated flux densities at 35pm for pumping the OH emission. However the closest of these sources is at a distance of 4.5’ from the OH emission position while IRAS 16458-4425 is only 0.08’ from it. And the OH emission is inside the error ellipse of IRAS 16458-4425. Although only an upper limit for the flux density at 60pm was given in the IRAS PSC, the flux at 25pm indicates that there is enough flux at 35pm to pump the OH emission. Inside the error ellipse of the IRAS source, there are no GSC stars, only one USN0 star with B=21.6 and R=19.5, indicating that its spectral type may be earlier than MO. Besides its type C in the LRS classification, there is no observation to confirm its carbon-rich character. Furthermore, within the observing beam of the radio telescope there are many IRAS sources that may also be the source for the OH pumping. Therefore the status of IRAS 16458-4425 remains undecided. 2.6 IRAS 17209-3318 A single-peak OH emission of 0.26Jy was detected in IRAS 17209-3318 by the ATCA telescope1341. Because it is located in the galactic bulge, there are no less than 32 other IRAS sources with sufficient interpolated flux density at 35pm within the radio beam. However, the closest one among them is 5.44’ from the OH position whereas IRAS 17209-3318 is only 0.65’ from it. In addition, the OH emission position is located right on the edge of the error ellipse of IRAS 17209-3318. Epchtein et al. l’s1 observed it in the near infrared, and gave X = 5.48 and H = 7.84, so indicating a spectral type later than M014’l. Within the error ellipse of IRAS 17209-3318, there are no GSC stars, only one USN0 star with B - R = 2.9. This implies that IRAS 17209-3318 is in the late stage of evolution. Therefore, IRAS 17209 -0328 could be considered as a carbon star with the LRS type of C(43). However, we cannot be certain of its correlation with the OH emission. 2.7 IRAS IRAS

17371-3021 17371-3021

was found to have 1612MHz OH emission by the Nancay radio with peak Aux densities 0.45 and 0.65 J~l~~l. telescope (beam size 3.5’(R.A.) x lS’(Dec.)) Within the beam there are 9 other IRAS sources; among them IRA’S 17372-3005 also has a to sufficient interpolated flux density at 35pm. Jura et al. [loI considered IHAS 17371-3021 be a very dusty carbon-rich AGB star. Nguyen-Q-Rieu et a1.1611pointed out that it is a CO and HCN carbon star with Li,= 4x lo4 L,. Volk et a1.l62l considered it to be a candidate for an extreme carbon star while Le Squeren et a1.1z51considered it to be a genuine carbon-rich star. Epchtein et a~l’~l observed it in the near infrared and gave K = 9.3 and L = 4.41, thus indicating a late evolutionary stage. However there are no GSC and USN0 stars in the error ellipse of IRAS 17371-3021. Therefore IRAS 17371-3021 is a carbon star with the SiC feature, and possibly also an OH/IR star with the OH emission at 1612MHz.

CHEN Pei-sheng

et al. / Chinese

2.8 IRAS 17445-3128 IRAS 17445-3128 was measured

Astronomy

and Astrophysics

by the ATCA

telescope

23 (1999)

475-483

to have peak flux densities

481

of

1.08 and 1.18 ~~1~~1. It is located in the galactic bulge, and there are 9 other IRAS sources within the radio beam. These 9 sources have enough interpolated 35pm flux densities for pumping the OH emission, but the nearest one is at a distance of 18.0’ from the position of OH emission while IRAS 17445-3128 is only about 3.4’ from it. However, the position of the OH emission is outside the error ellipse of the IRAS source. In addition, there are no GSC and USN0 stars within the error ellipse. Therefore apart from its LRS type of C, there are observations to confirm its carbon star status. Furthermore, because there are many other IRAS sources inside the OH beam, it is impossible to say whether or not the OH emission is from IRAS 17445-3128.

2.9 IRAS 18551+0323 IRAS 18551+0323 was first measured by the Aricibo radio telescope with peak flux densities at 1612 MHz 0.89 and 1.05 Jy l141. It was also measured by the NanGay telescope with flux densities of 0.21 and 0.23 J~l~~l. Besides IRAS 18551+0323 there is another IRAS source, IRAS 18550+0326, within the Aricibo beam. However the latter has not a sufficient interpolated flux density at 35pm. Le Squeren et a1.125l emphasized that it is a genuine and gave J = 13.26 carbon-rich star. Guglielmo et al. 1~ made near infrared observations and K = 6.54. They considered it to be an infrared carbon star. Lawrence et a1.164l made photometric observation from about lprn to 18pm and found a peak emission around llpm. There are no GSC and USN0 stars within the error ellipse of IRAS 18551+0323. Therefore IRAS 18551+0323 is an infrared carbon star with the Sic emission feature and also an OH/IR star with the OH emission at 1612MHz.

3. DISCUSSION

AND

PRELIMINARY

CONCLUSION

Nine candidates for OH/IR stars with the Sic feature in their circumstellar envelopes are given in this paper. Further investigation shows that four of these, IRAS 04130+3918, 05373-0810 06238+0904 and 18551+0323, are indeed carbon stars containing the Sic feature and are also OH/IR stars with the OH emission at 1612MHz. Two other sources, IRAS 08304-4313 and 17371-3021, are possible sources of this kind. The situation for IRAS 164584425, 17209-3318 and 17445-3128 is not clear. It is seen from the results here that, unlike what was thought previously, not all OH/IR stars are oxygen-rich, and carbon-rich stars can also have OH maser emission. Take the evolutionary scenario from Ibenl65l and Lattanziolesl where all carbon stars are thought to have evolved from oxygen-rich stars with spectral type M, when shell helium flash triggers off convection that causes carbon enrichment of the photosphere. It then follows that if the original M type stars have OH emission in their circumstellar envelopes, the carbon stars observed shortly after the transition may also have carbon-rich inner envelopes with OH emission in the outer envelope. After further evolution, the outer oxygen-rich envelope is dispersed into the interstellar medium, and the number density of oxygen-rich particles will have fallen to a level no longer sufficient to operate the OH maser. The OH maser ceases and the star becomes a normal carbon star with a carbon-rich envelope. Following and extending

482

CHEN Pei-sheng

a notation

et al. / Chinese

in Chen et al.16q, we can represent

ME+OH + (CEtOH) where the main letter describing

--+ CC+OH

represents

short

duration,

rich, but the oxygen-rich

out, within the lifetime

OH emission observed

at about

these sources

Dr.

of a -

as

106yr

star and the suffix

to indicate stars

that it is one

has become

carbon-

cast away and the silicate feature

On the other hand, as Le Squeren AGB

star,

thus the transitional

recently

phase

lo2 yr. This appears

In any case, the properties

ejected

et

carbon-rich

of carbon

and evolutionary

We thank Dr. S. Kwok for his encouragement Facility

star with the

to be consistent

with the status

of

further.

in the University

for helpful discussion.

Foundation

AGB

the central

is still present.

deserve to be investigated

R. Szczerba

Science

namely,

of about

of such objects.

Data Analysis

schematically

10 km/s would take some lo2 yr to reach the OH masing region

has at most a lifetime

ACKNOWLEDGEMENT the IRAS

envelope

1016 cm from the star,

frequency

type of the central

envelope has not been completely

in the original

some

sequence

475-483

CC

as in FJF270,

OH emission

expanding

23 (1999)

The second stage is bracketed,

a11z51pointed located

+

and Astrophysics

the evolutionary

the spectral

the state of the envelope.

of extremely

matter

Astronomy

of Calgary

This research

of China and the Chinese

is supported

Academy

and for providing

for this work. We also thank by the National

Nature

of Sciences.

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