Masers in Massive Star-forming Regions Associated with the Brightest ...

THE ASTRONOMICAL JOURNAL, 116 : 1897È1905, 1998 October ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

MASERS IN MASSIVE STAR-FORMING REGIONS ASSOCIATED WITH THE BRIGHTEST STEEP-SPECTRUM IRAS POINT SOURCES GORDON C. MACLEOD Instituto Nacional de Astrof• sica, Optica y Electronica, Apdo. Postal 216 y 51, 72000 Puebla, Pue., Mexico ; gmacleod=inaoep.mx

EUGENIO SCALISE, JR. Departamento de Radioastronomia e Fisica Solar, Instituto Nacional de Pesquisas Espaciais, C.P. 515, 12200 Sa8 o Jose dos Campos, Sa8 o Paulo, Brazil

SHARON SAEDT Department of Physics, University of the Witswatersrand, Private Bag 3, Wits 2050, South Africa

JOHN A. GALT Dominion Radio Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada, P.O. Box 248, Penticton, BC V2A 6K3, Canada

AND MICHAEL J. GAYLARD Hartebeesthoek Radio Astronomy Observatory, P.O. Box 443, Krugersdorp 1740, Transvaal, South Africa Received 1998 February 11 ; revised 1998 June 22

ABSTRACT Twenty-four bright, steep-spectrum far-infrared IRAS point sources, which were originally discussed by Hughes & MacLeod, have been searched for hydroxyl, water, and methanol masers and for formaldehyde absorption features. Distances and spectral types for the stars exciting these masers were estimated using radio continuum, IRAS spectra, and maser velocity data. It is found that 21 of the 24 sources are regions of massive star formation containing stars earlier than type B2. We report the discovery of Ðve new OH masers, one new 6.7 GHz CH OH maser, and two new 12.2 GHz CH OH masers toward these 3 3 IRAS sources. By contrast, no 4.765 GHz excited-OH masers were found toward these sources. This sample contains only star-forming regions ; there are no planetary or reÑection nebulae or extragalactic sources present, as seen in samples with less extreme IRAS colors. Key words : H II regions È masers È stars : formation

1.

INTRODUCTION

[25[12]1 \ 1.86, [60[25] \ 1.20, [100[60] \ 0.20, and S \ 20,450 Jy. The speciÐc criteria used to select 24 100km similar to Cep A are deÐned in ° 2. MacLeod (1990) sources and HM93 showed that the exciting stars are late O to early B type, the median being type B0, using Ñux densities from the IPSC and 6 cm radio continuum maps made with the VLA for the sources in this sample at declinations greater than [40¡. However, the usefulness of these results is limited ; the distances were estimated by unreliable methods such as comparison of IRAS Ñuxes and colors. In the present paper, we obtain improved kinematic distance estimates to permit more accurate classiÐcation of the exciting stars. This sample of 24 sources, including Cep A, represent the brightest steep-spectrum sources in the IPSC (MacLeod 1990 ; HM93). As such, they may represent a class of object (Hughes & MacLeod 1994) di†erent from the bright IRAS objects studied by Wood & Churchwell (1989). They might, for example, represent a very early epoch of massive star formation. Their IRAS colors imply that the objects reside in dense, cool dust clouds. The steepness of the FIR spectra may be an artifact of the di†erent beam sizes employed by

Bright far-infrared (FIR) sources and ultracompact (UC) continuum radio sources are indicators of the presence of young massive stars (i.e., earlier than type B3) (Panagia 1973). Hence, the IRAS Point Source Catalog, Version 2 (1988, hereafter IPSC) has been used to select potential UC H II regions in searches for continuum radio emission (Chini et al. 1986 ; Hughes & MacLeod 1989 ; Kurtz, Churchwell, & Wood 1994). The interaction of the UC H II regions with their surrounding molecular cloud gives rise to hydroxyl, water, and methanol masers, so the IPSC has also been used to select potential maser sources (e.g., Cohen, Baart, & Jonas 1988 ; Braz et al. 1989 ; Galt, Kwok, & Frankow 1989 ; Palla et al. 1991, 1993 ; Schutte et al. 1993 ; van der Walt, Gaylard, & MacLeod 1995 ; Walsh et al. 1995, 1997 ; Lyder & Galt 1997). These searches using the IPSC have been very successful, as many star-forming regions have been identiÐed and studied. Cepheus A is a particularly well studied region in which massive stars have recently formed (Hughes 1988). MacLeod (1990) and Hughes & MacLeod (1993, hereafter HM93) attempted to Ðnd and study other objects similar to Cep A using its infrared properties as a basis for identifying such sources (Hughes & MacLeod 1989 ; MacLeod 1990). The IRAS colors and 100 km Ñux density for Cep A are

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Throughout this work we adopt the IRAS color conventions using the nonÈcolor-corrected Ñux densities S in the 12, 25, 60, and 100 km IRAS bands of [j [ j ] 4 log (S /S ). jWe also deÐne a steeply rising FIR j1 shorter to longer wavelengths. 1 spectrum as 2one rising sharplyj2from

1897

1898

MACLEOD ET AL.

IRAS and source size at di†erent wavelengths ; the beam sizes employed were 30A, 30A, 60A, and 100A for the 12, 25, 60, and 100 km observing bands (IRAS Catalogs and Atlases 1989). The [25[12] color is the best criterion for source selection, as it is the least a†ected by source size and Galactic background contamination, and the beam sizes employed at 12 and 25 km are the same. The IRAS objects studied by MacLeod (1990) and HM93 have the largest values of [25[12]. Even if the FIR sources are extended and their [60[25] colors are then corrected for the di†erence in beam sizes, they will still lie well away from other young star-forming objects, as shown in the IRAS twocolor diagrams of MacLeod et al. (1998). Similar corrections must also be applied to these other objects. The sample of 24 objects studied here constitute an extreme class of IRAS sources, and further investigations such as this will aid in deÐning aspects of the star formation process, such as the detection rates of masers, and relationships with IRAS Ñuxes and colors. As stated above, hydroxyl, water, and methanol masers are associated with UC H II regions. Although both hydroxyl and methanol masers are found to be associated with regions in which stars of type B2 or earlier are forming (van der Walt et al. 1995 ; Cohen 1989), OH masers can also be formed in the later stages of a starÏs life. Cohen (1989) noted that water masers can be found in a wide range of environments, including massive and low-mass starforming regions. Currently the only identiÐed sources of methanol masers are massive star-forming regions, and hence they are uniquely suited for detecting such regions. In this paper, we present the results of searches for 1.6 GHz OH, 4.7 and 6.0 GHz excited-OH, 22.2 GHz H O, and 6.7 and 12.2 GHz CH OH masers and 4.8 GHz 1 2 [ 1 3 10 11 HCHO absorption toward the 24 brightest steep-spectrum IRAS point sources. The preliminary results of the groundstate hydroxyl, water, and methanol maser searches were reported by MacLeod et al. (1992b) ; here we report the conÐrmed detections and show their spectra. 2.

SOURCE SELECTION

The criteria adopted to select IRAS sources similar to Cep A were : [25[12] º 1.3, [60[25] º 0.8, and S º 500 Jy. Infrared sources with upper limits to S 100 km that 12 kmIRAS otherwise met the criteria were included. Twenty-four sources met these criteria.

Vol. 116 3.

OBSERVATIONS

Observations were made using the Hartebeesthoek Radio Astronomy Observatory (Johannesburg, South Africa), the Dominion Radio Astrophysical Observatory (Penticton, Canada), and the Itapetinga Radio Observatory (Atibaia, Brazil). Both Hartebeesthoek and Penticton are equipped with 256 channel correlators and employ frequencyswitching observing techniques, while Itapetinga utilizes a 1000 channel acousto-optic spectrometer and employs beam-switching observing techniques. Table 1 summarizes the characteristics of each receiver system and observing mode used. 3.1. Observations at Hartebeesthoek All of the 22 sources that are south of ]42¡ were searched for maser emission or absorption features of CH OH at 3 6.7 and 12.2 GHz, HCHO at 4.8 GHz, and OH at 1.720, 1.667, 1.665, and 1.612 GHz using the 26 m paraboloid antenna of the Hartebeesthoek Radio Astronomy Observatory (HRAO). Eleven of the sources were searched for excited-OH masers at 4.765 GHz, and two at 6.035 GHz. The remainder had been covered in previous searches at these two transitions. To conÐrm new maser detections reported by MacLeod et al. (1992b), observations were made with higher velocity resolution, and position checking was carried out using observations made at the cardinal half-power points of the beam. 3.2. Observations at Penticton IRAS 22272]6358A was searched for masers at the four ground-state hydroxyl transitions and at the 6.7 GHz methanol transition using the 26 m paraboloid antenna of the Dominion Radio Astrophysical Observatory (DRAO). Position checks were carried out on detected masers to conÐrm their association with the IRAS source position. The calibration of the DRAO 6.7 GHz receiver was checked by comparing results for sources detected by Menten (1991) ; a similar calibration test was completed for the HRAO 6.7 GHz receiver. 3.3. Observations at Itapetinga All of the 24 sources, except IRAS 22272]6358A and 22543]6145, which are too far north, were searched for associated 6 È5 22.2 GHz H O masers using the 14 m 16 23 2

TABLE 1 SUMMARY OF THE RECEIVER SYSTEMS USED

Receiver (GHz) HRAO : 1.6 . . . . . . . 4.7 . . . . . . . 4.8 . . . . . . . 6.0 . . . . . . . 6.7 . . . . . . . 12.2 . . . . . . DRAO : 1.6 . . . . . . . 6.7 . . . . . . . IRO : 22.2 . . . . . .

Half-Power Beamwidth (arcmin)

System Temperature at Zenith (K)

Point-Source Sensitivity (Jy K~1)

Polarization

Velocity Resolution Used (km s~1)

3 p Detection Level (Jy)

30.0 10.0 10.0 8.0 7.0 3.5

45 75 60 130 130 130

5.1 ^ 0.1 11.6 ^ 0.6a D10 6.9 ^ 0.3 14.5 ^ 1.0a 28.0 ^ 1.0a

RC ] LC Linear Linear RC ] LC Linear Linear

0.11, 0.23, 0.45 0.16 0.62 0.06 0.25 0.12

D0.5È1.0 D4.0a D0.5 D0.5È1.0 D1.0a D2.0a

30.0 7.0

50 140

5.0 ^ 0.5 D39.0

RC ] LC RC ] LC

0.18, 0.35 0.09

D0.1 D2.0

3.7

D600

D40a

Linear

0.12

D20.0a

a For unpolarized radiation.

No. 4, 1998

MASERS IN MASSIVE STAR-FORMING REGIONS

antenna of the Itapetinga Radio Observatory (IRO). The search was carried out over Ðve epochs from 1991 July 20 to November 20. The measured Ñuxes of detected masers have been multiplied by an average correction factor of 1.5 to correct for atmospheric and radome attenuation. All observations were carried out at maximum elevation to minimize the atmospheric e†ects. Note that these e†ects are accounted for by the correction factor applied. A consistency test of this receiver against others was completed and demonstrated that these corrections gave similar values. However, comparisons were difficult because of the time elapsed between observations and the variability of the common sources. 4.

RESULTS

4.1. 4.8 GHz Formaldehyde Absorption Features In Table 2, we summarize the formaldehyde and maser velocities used to estimate the kinematic distances of the 24 IRAS point sources. Formaldehyde was detected in absorption in the direction of 12 sources. This traces the bulk molecular gas and is less likely to be a†ected by peculiar motions than the masing material, and hence should provide more reliable measures of the velocity of the ambient cloud in which the star formation is taking place. The maser and HCHO velocities lie within 5 km s~1 of each other in all cases ; the di†erences are less than the half-width of the HCHO absorption feature. 4.2. Distance Estimates Kinematic distances were estimated using the Galactic rotation model of Wouterloot & Brand (1989), for which

1899

the SunÏs galactocentric distance is 8.5 kpc. These distances are listed in Table 2. All sources in the Ðrst and fourth quadrants are assumed to lie at the near kinematic distance, unless there is evidence to the contrary. The latitudes ( o b o º 0¡.5) of seven of the 18 sources with ambiguous distances suggest that they are located at the near distance. Fourteen of the distances used in HM93 di†er by more than the estimated uncertainty from those in the present paper. The distances for nine of the 24 sources in HM93 were not determined using cloud or maser gas kinematics. The kinematic distance for IRAS 16445[4459 had been determined using the velocity of hydroxyl masers now known to be unassociated with the source. For IRAS 16594[4137 the far kinematic distance is adopted here, rather than the near distance used in HM93, so that the estimated spectral type of the exciting star is earlier than B2. This source has an associated methanol maser, and van der Walt et al. (1995) showed that such masers are associated with exciting stars no later than B2. We use the near kinematic distance for IRAS 20081]3122 rather than the far distance used in HM93, because the source lies more than 0¡.5 from the Galactic plane. Finally, for both IRAS 16547[4247 and 17233[3606 (HM93) the distances estimated by HM93 were based on maser velocities that were di†erent from the ambient velocity of the molecular gas cloud as determined from the formaldehyde observations in this paper. 4.3. Radio and Infrared Data Spectral types for the exciting stars can be estimated from the IRAS Ñux densities and the radio Ñux densities of the H II regions using the adopted kinematic distances in

TABLE 2 IRAS SOURCES AND DISTANCE ESTIMATES OBSERVED COORDINATES (deg)

FORMALDEHYDE

KINEMATIC DISTANCE (kpc)

NO.

IRAS NAME

l

b

T a (K)

v lsr (km s~1)

dv (km s~1)

Maser v lsr (km s~1)

1 ....... 2 ....... 3 ....... 4 ....... 5 ....... 6 ....... 7 ....... 8 ....... 9 ....... 10 . . . . . . 11 . . . . . . 12 . . . . . . 13 . . . . . . 14 . . . . . . 15 . . . . . . 16 . . . . . . 17 . . . . . . 18 . . . . . . 19 . . . . . . 20 . . . . . . 21 . . . . . . 22 . . . . . . 23 . . . . . . 24 . . . . . .

05480]2545 05553]1631 07427[2400 12091[6129 12326[6245 14498[5856 15360[5554 15520[5234 16060[5146 16445[4459 16547[4247 16594[4137 16596[4012 17016[4124 17233[3606 17352[3153 18265[1517 19035]0641 19088]0902 19095]0930 20081]3122 20126]4104 22272]6358A 22543]6145

183.348 192.161 240.315 298.262 301.134 318.047 324.922 328.808 330.949 340.253 343.126 344.583 345.717 345.001 351.776 356.664 16.868 40.621 43.303 43.793 69.540 78.122 108.186 109.874

[0.576 [3.815 ]0.071 ]0.740 [0.225 ]0.088 [0.564 ]0.632 [0.174 [0.051 [0.062 [0.025 ]0.817 [0.220 [0.543 [0.268 [2.158 [0.137 [0.207 [0.125 [0.976 ]3.633 ]5.519 ]2.115

[0.09 ^ 0.03 ¹0.02 ¹0.07 [0.05 ^ 0.03 ¹0.06 [0.10 ^ 0.05 ¹0.03 [0.14 ^ 0.03 [0.11 ^ 0.04 [0.16 ^ 0.06 [0.13 ^ 0.04 ¹0.03 [0.13 ^ 0.04 [0.30 ^ 0.03 [0.16 ^ 0.04 ¹0.04 [0.05 ^ 0.02 ¹0.05 [0.09 ^ 0.03 ¹0.04 [0.09 ^ 0.03 ¹0.04 ... ...

[9.2 ... ... [30.5 ... [50.7 ... [40.0 [90.0 [121.3 [28.0 ... [12.0 [27.5 [3.8 ... ]18.0 ... ]17.0 ... ]11.2 ... ... ...

1.0 ... ... 3.9 ... 3.0 ... 6.0 10.0 7.0 25.0 ... 2.0 5.0 3.0 ... 2.0 ... 1.0 ... 3.0 ... ... ...

[10.0a [14.0a ]63.5 [29.5 [39.0 [53.5 [78.0 [45.0 [85.0 [128.0 [30.5 [5.5 ... [26.5 ]0.0 [54.0 ]17.0 ]33.0 ]28.0 ]42.0 ]12.5 [6.5 [8.5 [9.0

D

near ... ... 6.4b 3.9 4.3 3.7 5.3 2.9 5.6 7.2 2.9 0.8 1.7 3.0 1.0 7.0 2.1 2.4 1.3 3.1 1.8 4.6b 1.0b 0.7b

D

far ... ... ... (4.1) 4.5 8.9 (8.6) (11.7) 9.2 8.8 13.4 15.6 (14.8) 13.4 (15.9) 9.9 (14.2) 10.5 11.1 9.1 (4.1) ... ... ...

NOTES.ÈDerived kinematic distances are based on the HCHO velocities if available, otherwise on the maser line centroids. The far kinematic distance is unlikely if the source is greater than 0¡.5 from the Galactic plane (denoted by parentheses). Limits and errors quoted for the antenna temperature are 3 p values. a No kinematic distance can be calculated at this velocity for circular galactic rotation. b There is no distance ambiguity.

1900

MACLEOD ET AL.

Vol. 116

TABLE 3 ESTIMATED SPECTRAL TYPES OF THE EXCITING STARS ESTIMATED SPECTRAL TYPE NO.a 1 ....... 2 ....... 3 ....... 4 ....... 5 ....... 6 ....... 7 ....... 8 ....... 9 ....... 10 . . . . . . 11 . . . . . . 12 . . . . . . 13 . . . . . . 14 . . . . . . 15 . . . . . . 16 . . . . . . 17 . . . . . . 18 . . . . . . 19 . . . . . . 20 . . . . . . 21 . . . . . . 22 . . . . . . 23 . . . . . . 24 . . . . . .

S 100 km (Jy)

D kin (kpc)

643 D3300c 525 744 795 9245 2209 1411 16390 19170 1977 6471 1126 525 7000 20410 748 1297 922 666 2744

2.1d 2.1d 1.2d 6.4 3.9 4.3 3.7 5.3 2.9 5.6 7.2 2.9 15.6 1.7 3.0 1.0 7.0 2.1 2.4 1.3 3.1 3.1 1.8 4.6 1.0 1.0 0.7e

3120 1947 702 D4400c 20450

log (L /L ) IR _ 3.6 4.3 3.1 4.7 4.3 5.5 4.6 4.7 5.3 5.9 5.1 4.8 5.6 3.2 4.9 4.4 4.7 3.8 3.9 3.2 4.6

IR B1.5 B0.1 B2.7 O8.5 B0.1 O5.7 O9.5 O8.5 O6.3 O4.7 O6.7 O8.0 O5.5 B2.5 O7.5 B0.0 O8.5 B0.8 B0.7 B2.5 O9.5

4.1 4.8 3.0 3.8 4.2

B0.4 O8.0 B3.0 B0.8 B0.3

1.3 cm

2 cm

6 cm

20 cm

B0.5f B2.3f B0.4f

¹B0.3f ¹O8.2f

¹B0.5f ¹B1.6f ¹B2.6f

O8.3g ¹O9.4g O8.3g O9.3g ¹O6.3g ¹O6.5g ¹O9.4g ¹O5.7g ¹B0.2g O9.5g B0.3f B0.3f B1.0f B1.0f

O9.8f

B0.2f

B0.3f

B0.3f ¹B1.0f ¹B2.8f

O9.7f B0.0f B0.4f B1.0f B1.6f

B1.4f

B1.4f

O9.6f ¹B0.5f ¹O9.6f B0.4f B0.4f B0.5f ¹B0.7f ¹B0.8f ¹B0.5f B0.6f DB0.8f B1.4f

log (L

/L ) Rad _ 4.0 4.0 3.4 4.1

4.8 ¹4.6 4.8 4.6 ¹5.3 ¹5.2 ¹4.6 ¹5.5 ¹4.3 4.6 4.2 4.2 3.7 3.7 ¹3.8 4.5 4.4 4.2 3.7 3.8 3.8 3.6

log (L /L ) IR Rad [0.4 ]0.3 [0.3 ]0.6 ]0.7 º0.0 [0.1 ]0.7 º]0.6 º[0.1 º]0.2 º]0.1 º[1.1 ]0.3 ]0.2 ]0.5 ]0.1 ]0.2 º[0.6 ]0.1 ]0.2 [0.1 ]1.1 [0.8 0.0 ]0.6

REFERENCEb 1 1 1 2 2 2 2 2 2, 4, 4, 2 2, 4, 1, 6, 5, 3 4, 1 9, 6, 6,

3 2, 3 2, 3 3 1, 3 5 1 1, 3 7, 8, 3 9, 1, 9 1 1, 10

6, 11, 11

NOTE.ÈThe adopted kinematic distances are determined from those in Table 2 or other sources. a Numbers correspond to those in Table 2. b The Ðrst reference corresponds to the Ðrst column in this table in which radio emission is reported. c Flux density estimated from the extended source. d Distance estimate from Snell, Dickman, & Huang 1989. e Distance estimate from Hughes & Wouterloot 1984. f Results obtained via interferometric observations. g Results obtained via single-dish observations. REFERENCES.È(1) HM93 ; (2) Haynes, Caswell, & Simons 1979 ; (3) Zoonematkermani et al. 1990 ; (4) Forster & Caswell 1989 ; (5) MacLeod 1990 ; (6) Wilking et al. 1989 ; (7) Kurtz et al. 1994 ; (8) Altenho† et al. 1979 ; (9) Zheng et al. 1985 ; (10) Condon & Broderick 1985 ; (11) Hughes & Wouterloot 1984.

Table 3. The total IR luminosity was estimated using the method described in Cataloged Galaxies in the IRAS Survey, Version 2 (1989). For a more thorough description refer to HM93. The spectral type of the exciting star of the infrared source was determined using the model of Panagia (1973), assuming that only a single ZAMS star is present. We calculated the excitation parameters U from the radio Ñux densities using the equations derivedj by Schraml & Mezger (1969). We assumed that the radio emission is optically thin at the observed radio frequency. From the excitation parameters we determined the spectral types of the exciting stars at the adopted kinematic distance (Table 3), again using the models of Panagia (1973) for ZAMS-type stars. The spectral type of the exciting star will be underestimated if the source is optically thick or partly resolved in interferometric observations. The assumption that the UC H II region is optically thin is likely incorrect at longer wavelengths ; for only one source, IRAS 19088]0902, was radio data at a wavelength longward of 6 cm used to estimate the spectral type. In Table 3 the radio data are identiÐed as being either single-dish or interferometric. Two of the sources, IRAS 05480]2545 and 22272]6358A, are extended FIR objects in the IRAS Small Scale Structure Catalog (1988). We have estimated the 100 km Ñux densities of these extended sources, and have included them in Table 3. Henceforth, we use only these

estimated Ñuxes to determine the IR spectral type of their exciting stars. The spectral types of these stars estimated from the corrected IR Ñux densities are then consistent with those estimated from the radio continuum data (see Table 3). All these sources appear to be earlier than B3 stars, from the infrared luminosities. This is consistent with the presence of a massive star, assuming that a single star dominates the FIR Ñux. Continuum radio emission has been detected toward 18 of these sources, conÐrming the presence of massive ionizing stars. The remaining six objects may contain nonionizing stars. Alternatively they may produce radio emission that is below the detection level of the searches, or UC H II regions that are optically thick and hence also undetectable. 4.4. 1.6 GHz Hydroxyl Masers Nineteen of the 24 sources have associated OH masers. Two 1.665 GHz, one 1.667 GHz, and two 1.720 GHz masers are new detections. The results are listed in Table 4 and the spectra are presented in Figure 1. The detection rates of 1.612, 1.667, and 1.720 GHz OH masers toward those sources with 1.665 GHz OH masers are similar to those rates obtained in surveys by Caswell, Haynes, & Goss (1980), Caswell & Haynes (1983a, 1987), and MacLeod (1997). Only surveys where all of the four

No. 4, 1998


1901

TABLE 4 NEW MASER DETECTIONS No.a

IRAS Name

1 .......

05480]2545

4 .......

12091[6129

5 .......

12326[6245

7 ....... 8 .......

15360[5554 15520[5234

10 . . . . . . 15 . . . . . .

16445[4459 17233[3606

17 . . . . . . 23 . . . . . .

18265[1517 22272]6358A

Frequency (GHz)

Polarization

6.7c 12.2b 22.2c 1.665b 6.7c 12.2b 22.2c 6.7cd 1.720b 1.720b 6.7c 1.720b 1.720b 6.7c 1.665b 1.665b 1.667b 1.667b 6.7b

Linear Linear Linear LC Linear Linear Linear Linear RC LC Linear RC LC Linear RC LC RC LC Linear

Velocity Range (km s~1) [15, [4

[44, [40 [49, [41 [49, [35 [134, [125 ]14, ]20 [12, [8 [19, ]2 [12, [8 [19, ]2 [12, [9

Peak Velocity (km s~1)

Peak Flux (Jy)

[14.8 [4.8 [4.8 [28.3 [29.9 [43.7 [40.7 [78.5 [43.1 [43.2 [126.4 ]4.9 ]0.2 ]14.9 [9.9 [12.0 [10.6 [12.0 [11.1

17.8 ^ 2.0 2.4 ^ 1.0 2.4 ^ 1.0 0.45 ^ 0.15 12.3 ^ 1.0 2.9 ^ 1.0 68 ^ 20 2.0 ^ 0.7 15.2 ^ 0.8 12.2 ^ 0.5 11.0 ^ 1.0 1.7 ^ 0.5 1.8 ^ 0.5 17.0 ^ 1.0 0.2 ^ 0.1 0.7 ^ 0.1 0.3 ^ 0.1 0.5 ^ 0.1 69.4 ^ 2.0

NOTES.ÈThis table includes the results Ðrst reported by MacLeod et al. 1992b. The uncertainties in the peak Ñuxes are 3 p. a Numbers correspond to those in Table 2. b Detected in this work. c Reported Ðrst by MacLeod et al. 1992b. d ConÐrmed by Walsh et al. 1997.

ground-state 18 cm OH transitions were observed and where reliable positions were determined are considered here. 4.5. 4.765 and 6.035 GHz Hydroxyl Masers No new 4.765 or 6.035 GHz excited-OH masers were detected toward these sources. Eight of the 23 sources that have been searched have associated 6.035 GHz excited-OH masers. No new 4.765 GHz masers were detected toward these sources, nor were any found in the literature. Only IRAS 22272]6358A has not been searched for these excited-OH masers. The detection rate of 6.035 GHz masers is comparable to that obtained by Caswell & Vaile (1995) (D35%). However, one would have expected to Ðnd two or three 4.765 GHz OH masers in this sample based on the results of Cohen, Masheder, & Walker (1991) and Cohen, Masheder, & Caswell (1995). This may be the result of small number statistics or poor sensitivity. 4.6. 6.7 GHz Methanol Masers Nineteen of the 24 sources have associated 6.7 GHz methanol masers ; one of these (IRAS 22272]6358A) is a new detection. This rate of detection is consistant with that found by van der Walt et al. (1995). The results are shown in Table 4 and the spectra in Figure 1. The results of Ðve others originally reported as new by MacLeod et al. (1992b) are also included in Table 4 ; their spectra are presented by MacLeod et al. (1992b) and Walsh et al. (1997). A Ðfth 6.7 GHz maser, toward IRAS 15360[5554, was listed as a tentative detection by MacLeod et al. (1992b) and conÐrmed by Walsh et al. (1997). 4.7. 12.2 GHz Methanol Masers Four of these sources were known to have 12.2 GHz masers. Two new detections were made toward IRAS 05480]2545 and 12326[6245. The results are listed in Table 4 and the spectra presented in Figure 1. IRAS 22272]6358A was not searched for 12.2 GHz masers.

For all known maser sources that have both 1.6 GHz OH and 6.7 GHz CH OH masers, the detection rate of 12.2 3 GHz methanol masers is 61 of 161, or about 40% (Menten 1991 ; MacLeod, Gaylard, & Nicolson 1992a ; MacLeod & Gaylard 1992 ; Gaylard & MacLeod 1993). The relatively low 12.2 GHz methanol detection rate obtained here (four of 23) may be sensitivity limited, as the 12.2 GHz masers are typically an order of magnitude weaker than their 6.7 GHz counterparts. 4.8. 22.2 GHz W ater Masers Sixteen of the 24 sources have associated water maser emission. This detection rate is similar to that determined by Palla et al. (1993) (D50%). Two new detections, toward IRAS 05480]2545 and 12326[6245, were reported in the preliminary search (MacLeod et al. 1992b). The spectra were not shown previously, hence, they are presented in Figure 1 and the data in Table 4. 5.

DISCUSSION

We summarize the results of the observations of these sources in Table 5, where an ellipsis represents a nondetection. For the detections, the numbers refer to the references in which the masers were Ðrst reported. The sources are sorted in descending order of the infrared color [60[25] ; all sources have reliable values for this index. 5.1. Nature of the IRAS Sources A revised plot of the IRAS luminosity versus radio ionizing Ñux, originally presented in HM93, is shown in Figure 2, using the results presented in Table 3. It can be seen in this Ðgure and in Table 3 that seven of the 24 sources have luminosities derived from the infrared that are at least 3 times greater than those estimated from their radio continuum Ñuxes. The seven sources are : IRAS 07427[2400, 12326[6245, 15520[5234, 16060[5146, 17352[3153, 20126]4104, and 22543]6145 (Cep A). The Ðrst and last

1902

MACLEOD ET AL.

FIG. 1.ÈSpectra of the detected masers. The source name is printed above each spectrum, and the line frequency is given for each maser. OH maser spectra with solid and dashed lines represent the RCP and LCP spectra, respectively.

three of these seven were mapped at 6 cm by HM93. Cep A and IRAS 17352[3153 have complex structure ; the other two are unresolved radio objects. The remainder of the sample of 24 have IR and radio luminosities within a factor of 2 of each other, with the exception of IRAS 16596[4012 and 19088]0902, where only lower limits were estimated. Using the classiÐcation scheme of McCutcheon et al. (1991), the seven sources with IR excesses would be deemed preÈ main-sequence stars ; the others are main-sequence stars. An alternative explanation proposed by HM93 for the infrared excess is that more than one exciting star is present (as in Cep A), or that the radio source is not optically thin as assumed here. The complex radio structure associated with IRAS 17352[3153 suggests that it too has multiple exciting

stars. Six of the 11 sources mapped in HM93 appear to be complex and may have more than one exciting source. It is also possible that nonionizing stars, later than type B3, contribute signiÐcantly to the FIR, and hence cause the infrared excess. Four of the these seven sources, IRAS 07427[2400, 12326[6245, 15520[5234, and 16060[5146, have been observed in the near-infrared (NIR) (Osterloh, Henning, & Launhart 1997). From Osterloh et al. (1997) we estimate that there are about 50 NIR objects for the Ðrst three sources and 100 for the last source in the 100 km IRAS beam ; it is assumed that all are associated with the FIR sources. If one estimates the number of B3 stars required to produce the infrared excess, it can be shown that only for IRAS 07427[2400 would it be equal to or less than the

16060[5146 16547[4247 17233[3606 20081]3122 12326[6245 15520[5234 16594[4137 22272]6358A 22543]6145 17016[4124 19095]0930 05480]2545 20126]4104 14498[5856 07427[2400 12091[6129 18265[1517 17352[3153 19035]0641 15360[5554 16596[4012 16445[4459 05553]1631 19088]0902

9............................. 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5............................. 8............................. 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 . . . . . . . . . . . . . . . . . . . . . . . . . . . 1............................. 22 . . . . . . . . . . . . . . . . . . . . . . . . . . . 6............................. 3............................. 4............................. 17 . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 . . . . . . . . . . . . . . . . . . . . . . . . . . . 7............................. 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . 2............................. 19 . . . . . . . . . . . . . . . . . . . . . . . . . . . No. Detected . . . . . . . . . . . . No. Searched . . . . . . . . . . . .

1.85 º1.38 1.70 1.72 1.60 1.54 1.46 º1.87 1.86 1.89 1.39 º2.07 º1.63 º1.34 1.49 1.55 º1.49 º1.33 1.47 1.41 º1.36 1.36 º1.70 1.51

[25[12] 1.70 1.63 1.58 1.39 1.39 1.30 1.30 1.29 1.20 1.18 1.13 1.11 1.10 1.10 1.01 0.99 0.98 0.98 0.96 0.94 0.94 0.89 0.82 0.82

[60[25] ... 1 2 ... ... 3 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 2 ... ... 4 24

1.612 GHz 3 1 1 4 5 3 1 6 7 1 8 ... 9 5 10 6 10 1 8 ... ... ... ... ... 18 24

1.665 GHz 3 1 1 4 5 3 1 6 7 1 8 ... ... 5 ... ... 10 1 8 ... ... ... ... ... 15 24

1.667 GHz

OH

... ... 6 ... ... 6 ... ... ... 1 ... ... ... ... ... ... ... ... ... ... ... ... ... ... 3 24

1.720 GHz 11 ... 12 11 ... 12 ... b ... 12 11 ... ... ... 11 ... ... ... 12 ... ... ... ... ... 8 23

6.035 GHz 13 ... 14 14 15 16 14 6 14 14 14 17 18 13 ... 17 17 13 14 17, 19 ... 17 ... ... 19 24

... ... 20 ... 6 21 ... b 22 21 ... 6 ... ... ... ... ... ... ... ... ... ... ... ... 6 23

CH OH 3 6.7 GHz 12.2 GHz

H O 22.2 GHz 2 23 24 25 26 17 ... 27 ... 28 ... 29 17 30 31 32 ... 33 [ 31 ... ... ... 30 30 16 24

NOTE.ÈDetections are indicated by the reference number ; nondetections by a minus sign. a Numbers correspond to those in Table 2. b Not observed. REFERENCES.È(1) Caswell & Haynes 1983a ; (2) Caswell et al. 1981 ; (3) Caswell et al. 1980 ; (4) Ellder, Ronnang, & Winnberg 1969 ; (5) Caswell & Haynes 1987 ; (6) this paper ; (7) Wouterloot, Habing, & Herman 1980 ; (8) Caswell & Haynes 1983b ; (9) Cohen et al. 1988 ; (10) MacLeod 1991 ; (11) Smits 1994 ; (12) Caswell & Vaile 1995 ; (13) Gaylard & MacLeod 1993 ; (14) Menten 1991 ; (15) Caswell et al. 1995 ; (16) MacLeod et al. 1992a ; (17) MacLeod et al. 1992b ; (18) MacLeod & Gaylard 1992 ; (19) Walsh et al. 1997 ; (20) MacLeod, Gaylard, & Kemball 1993 ; (21) Norris et al. 1987 ; (22) Koo et al. 1988 ; (23) Caswell et al. 1974 ; (24) Batchelor et al. 1980 ; (25) Braz & Scalise 1982 ; (26) Sullivan 1973 ; (27) Caswell et al. 1983 ; (28) Blitz & Lada 1979 ; (29) Genzel & Downes 1977 ; (30) Palla et al. 1991 ; (31) Scalise, Rodriguez, & Mendoza-Torres 1989 ; (32) Henning et al. 1992 ; (33) Codella, Felli, & Natale 1994.

IRAS NAME

NO.a

TABLE 5 SUMMARY OF THE MASER ASSOCIATIONS FOR EACH SOURCE

1904

MACLEOD ET AL.

FIG. 2.ÈPlot of log (infrared luminosity), as estimated from the IRAS data, versus log (ionizing Ñux), as estimated from all radio emission data found in the literature, for the 24 objects studied here. Both the infrared luminosity and the ionizing Ñux are in units of solar luminosity. The solid line represents the curve for ZAMS stars using the model of Panagia (1973). For comparison, some ZAMS stars are plotted along the log (ionizing Ñux)-axis. The sources are numbered and these numbers correspond to those in Table 2. For Sources 10 and 12, where a distance ambiguity exists, both near (n) and far (f) values are shown and connected by a dashed and a triple-dotÈdashed line, respectively. For Sources 1 and 23, both the estimated point source (p) and integrated (i) IRAS luminosity are plotted and connected by a long-dashed and dot-dashed line, respectively.

number of NIR objects in the Ðeld. For the remaining three sources, 150 to 600 B3 stars would be required ; many more if they are later type stars. Twenty-one of the 24 objects studied have either radio continuum emission and/or hydroxyl and/or methanol masers, and these are found only toward regions containing young stars earlier than type B2 (van der Walt et al. 1996). Are the three exceptions, IRAS 05553]1631, 16596[4012, and 19088]0902, regions of massive star formation ? IRAS 05553]1631 has radio and FIR luminosities between B2 and B3 type stars and supports water masers, but does not support hydroxyl or methanol masers. The remaining two have neither detected radio emission nor methanol or hydroxyl masers ; however, IRAS 19088]0902 has an associated water maser. These two objects probably only contain stars later than type B3. Van der Walt et al. (1996) suggested that searches for massive star-forming regions toward IRAS-selected samples of objects will be contaminated with nonionizing sources ; these last two objects appear to represent this contamination. This sample of IRAS objects has no contamination with other classes of infrared objects, such as planetary nebulae, reÑection nebulae, or extragalactic sources. IRAS point sources with less extreme colors can have a great deal of contamination with objects that are not star-forming regions (Hughes & MacLeod 1989).

Vol. 116

5.2. Detection Rates of V arious Maser Species It can be seen in Table 5 that there are few detections of OH, H O, and CH OH masers for [60[25] ¹ 0.96. This 2 resulted 3from the choice of sources with large may have values of [25[12]. MacLeod et al. (1998) suggest that in the area of the IRAS two-color diagram, where [25[12] º 1.3 and [60[25] ¹ 1.0, that the methanol maser detection rate decreases because of (1) increasing numbers of contaminating nonionizing objects, and (2) the evolution of the infrared sources where the conditions for masing are diminishing. A similar e†ect can be shown for hydroxyl (Cohen et al. 1988) and water (Palla et al. 1993 ; Palumbo et al. 1994) masers. In this sample, none of the 21 objects earlier than type B2 have completely evolved beyond the masing stage. We Ðnd little di†erence statistically between the detection rates of ground-state hydroxyl (18/24), 22.2 GHz water (16/24), and 6.7 GHz methanol masers (19/24) (see Table 5). As mentioned above, however, no 4.765 GHz excited-OH masers are found toward these sources. We can infer from MacLeod (1997), and references therein, that the absence of 4.765 GHz OH masers may be a result of the gas density and the level of turbulence in the region. Conversely, the nondetection of 4.765 GHz OH masers may simply be a result of small number statistics or inadequate sensitivity.

6.

CONCLUSIONS

The sample of 24 IRAS sources chosen by MacLeod (1990) and HM93 has a†orded the opportunity to extend the study of masing regions to the extremely steep spectrum IRAS objects. We have found Ðve new ground-state OH masers, one new 6.7 GHz CH OH maser, and two new 12.2 GHz CH OH masers toward 3this sample. The data present3 ed here demonstrates that there is a high detection rate for hydroxyl, water, and methanol masers toward steepspectrum ([25[12] º 1.3) infrared sources and that there is a high incidence (19/24 or D80%) of multiple transitions masing in these objects. No 4.765 GHz masers were detected toward this selected sample of IRAS sources. More reliable distances for these sources were determined using spectroscopic data. These enabled the intrinsic radio and infrared luminosities to be recalculated and compared. The radio continuum, infrared, and maser data for these sources indicate that 21 of 24 contain young stars earlier than type B2 ; one is between type B2 and B3, and two are probably later than type B3. This sample contains only star-forming regions ; there are no planetary or reÑection nebulae or extragalactic sources present, as seen in samples with less extreme IRAS colors.

We thank the sta†s of the Hartebeesthoek Radio Astronomy Observatory, the Itapetinga Radio Observatory, and the Dominion Radio Astrophysical Observatory for their time and support.

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