1 Introduction

Massive star-forming regions (MSFRs) are characterized by the presence of high mas protostellar objects (HMPOs) that are actively accreting material from their surrounding molecular gas clouds. The gas in these regions is highly turbulent and clumpy, and is subject to a range of physical processes such as shocks, outflows, and heating from the embedded protostars. Compact HII regions, molecular outflows and circumstellar disks are signs of the existence of massive protostars (e.g. Garay and Lizano [1], Churchwell [2]). Maser emission is also found to be associated with these objects (e.g. Garay and Lizano [1], Edris et al. [3], Darwish [4]) with OH one of most widespread types of maser associated with these regions. OH masers are associated with two different stages of star formation. The first one is associated with circumstellar disks and molecular outflows (Cohen et al. [5], Brebner [6], Cohen et al. [7], Edris et al. [8]). The other one is a relatively advanced star forming stage of the appearance of UCHII region (e.g. Garay and Lizano [1], Darwish et al. [9] and references therein). The mm emission either thermal or non-thermal, could trace different phenomena of star formation process. Thermal mm emission from dust and molecules helps trace the structure and temperature distribution of protostellar disks, molecular envelopes, and dense regions. On the other hand, non-thermal mm emission, such as synchrotron emission, can reveal the presence of energetic phenomena associated with outflows, shocks, and supernova remnants, shedding light on the energetic processes involved in massive star formation.(e.g. Zinnecker et al. [10], Adams et al. [11], Beuther et al [12], Tychoniec et al. [13] ). Therefore the mm emission should be associated with the first type of OH masers mentioned above. The association between the maser emission and mm and sub-mm have studied in case of methanol masers by Breen et al. [14] and water masers by Jenness et al. [15].

The primary objective of this study is to examine the correlation between OH masers and mm emission in a selection of high mass star forming regions (HMSFRs). To the best of our knowledge, no previous investigations have been conducted in this particular area. The simultaneous observation of OH masers and mm emission can yield valuable insights into the physical characteristics, dynamics, and evolutionary status of massive star-forming regions. For instance, the distribution and kinematics of OH masers can provide information about the shape and velocity structure of shock fronts and outflows, while the mm emission can trace the colder, denser regions where protostars actively accrete material. To address these objectives, a sample of high mass protostellar objects was examined. Section 2 provides a description of the sample, while Sect. 3 presents the observations of OH masers and the mm archival data. The results, findings, and discussions are presented in Sects. 4 and 5, respectively. Finally, Sect.6 contains the concluding remarks.

2 The sample

The sample of sources in this present paper is partially drawn from the surveys of Sridharan et al. [16] and Molinari et al. [17]. These two samples are originally drawn from the IRAS Point Source Catalog based on their colour selection criteria. They are believed to contain massive sources in a very early stage of evolution prior to the forming of UCHII regions. Molinari et al. [17] divided their sample into two types: High and Low sources. The Sridharan et al. [16] sample and the High sources of Molinari et al [17] have similar colour. They both satisfy the criteria of Wood and Churchwell [18] for a UCHII region colour. However the sources in these two samples (as well as the Molinari et al. [17] Low sources) are not known to be associated with detectable HII regions. Molinari et al. [17] suggest that their Low sources comprises objects which are in a different evolutionary stage from those in their High sources (more evolved sources), and therefore also from the other sample. Further details about these sources and their selection criteria can be found in Sridharan et al.[16] and Molinari et al. [17] and references therein as well as their follow up papers.

Edris et al. ([8], hereafter EFC07) surveyed these two samples for OH maser emission and detected the masers towards 26 % of the sources. One of these sources (IRAS \(20126+4104\)) have been imaged in high angular resolution by Edris et al. [3]. The OH masers are originate from the circumstellar disk rotating around the protostar. This source is closely associated with 1.3 and 3 mm emission [19, 20]. The 27 sources studied in this paper is the OH maser sources that have been found to have images at 1.1-mm emission in the BGPS survey. These sample contains 13 sources from Sridharan et al. [16] sample and 17 sources from Molinari et al. [17] with three sources common between the two sample. These 17 sources are 11 High type sources and 6 Low type sources.

3 Observations and archive data

3.1 OH masers observations

The Nançay radio telescope and the 100−m single dish Green Bank Telescope (GBT) have been used to observe the four OH transitions at 1665, 1667, 1612 and 1720 MHz in both left and right circular polarizations. After confirming the presence of an OH maser towards the IRAS position of a source, the GBT was used to map the sources with small 3 arcmin sampled maps, typically \(3\times 3\) pixels in size, to determine the position of the peak emission. Further details about these observations can be found in Edris et al. [8].

3.2 Archive data

The millimeter data are based on the Bolocam Galactic Plane Survey (BGPS). The The BGPSFootnote 1 is a 1.1 mm continuum survey of 170 deg2 of the Galactic Plane in the northern hemisphere with the Bolocam instrument [21, 22] employed on the Caltech Submillimeter Observatory (CSO) which observes in a band centered at 268 GHz (1.1-mm) and a width of 46 GHz. The bandpass is designed to reject emission from the CO (2,1) transition, which is the dominant line contributor at these wavelengths. The details of the survey methods and data reduction are described in Aguirre et al. [23], and the source extraction algorithm and catalog (v1.0 BGPS data) are described in Rosolowsky et al. [24]. The effective FWHM beam size of the BGPS is 33\(''\), corresponding to a solid angle of \(2.9 \times 10^{-8}\) steradians, which is equivalent to a top-hat function with a 40 \(''\) diameter (\(\Omega = 2.95 \times 10^{-8}\)). Thus, the BGPS catalog presents aperture flux densities within a 40” diameter aperture (\(S_{40''}\)), corresponding to the flux density within one beam. The BGPS catalog also provides an integrated flux density (\(S_{\text {int}}\)), which is the sum of all pixels within a radius (R also given in the catalog) of the BGPS source. The peak positions for the dust continuum emission are used in our analysis. According to Aguirre et al. [23] it is stated that the error in the peak of Gaussian components, which are fitted to sources, is generally around 1\(''\) (representing the relative position error). However, it is important to note that this error is likely dependent on the signal-to-noise ratio (S/N). Therefore, when considering the error in quadrature, the typical total position error amounts to approximately 2.2\(''\). It is worth mentioning that this value is comparable to the position error for IRAS, which is around 3 arcseconds.Footnote 2 Consequently, it appears reasonable to search for counterparts within a 3\(''\) radius. Table 1 provides a summary of the angular resolution achieved with each instrument utilized in this paper.

Table 1 Facilities used, observing band, and angular resolution
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4 Results

Among the 27 OH maser sources, 23 sources have been found to be associated with multiple peaks of millimeter emission within 30 \('\). The offsets of the closest mm peak have been calculated. The offset of \(< 0.5\) arcsec has been found in 6 sources. Among them, three sources have offsets \(\le 0.05\) arcsec: IRAS \(18089-1732\), IRAS \(19035+0641\), and IRAS \(19092+0841\). These three sources represent different samples of High Mass Protostellar Object (HMPO) candidates. IRAS 18089-1732 and IRAS \(19092+0841\) are from the High (H) and Low (L) type samples of Molinari et al. [17], respectively, while IRAS 19035 + 0641 is from the sample of Sridharan et al. [16] (S), which also has a similar color to the High sources. The millimeter peaks towards IRAS \(18089-1732\) and IRAS \(19035+0641\) are probably coincident with the OH maser emission as well as the IRAS position.

The offsets of the next seven sources range from 0.5 to 3 arcseconds. The remaining ten sources have offsets greater than or equal to 3 arcseconds (Fig. 1, upper). Among the four sources without any close mm emission, three belong to the High sources identified by Molinari et al. [17], and one belongs to the sample of Sridharan et al. [16]. Figure 1 (lower) illustrates the association between the OH maser and IRAS positions. It is evident that the majority of the OH emission is offset by more than 3 arcsec from the IRAS position, while two sources lie within the separation range of 0.5 to 3 arcsec. These results align with the offset between the closest mm sources to the OH masers and IRAS sources as shown in Fig. 2.

Fig. 1
figure 1

The offsets of the OH masers from both the closest mm peak (upper) and IRAS position (lower) in arcsec

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Fig. 2
figure 2

Offset in arcseconds between the IRAS sources and the closest mm peak position to the OH maser position

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Figure 3 presents the number of OH masers associated with mm sources for different types identified by Molinari et al. [17] and Sridharan et al. [16] as a function of separation in different groups. The figure demonstrates that the majority of the Low sample sources, characterized as younger than their High counterparts, exhibit the largest offset of over 3 arcseconds (Fig. 3).

positions maps for the 23 sources are shown in Figures 4, 5 and 6. Table 2 gives the parameters of the studied sample, namely the source name, the type, the position of the OH maser emission, the position of the closest 1.1-mm peak, the distances of the sources, the offset from the closet mm as well as IRAS in arcsec and the peak velocities of the OH sources. The OH masers positions are from the GBT observations (Edris et al. [8]; these positions have been drawn from the 1665-MHz line emission. Some sources which show different position for the 1667-MHz line, the position of the 1667-MHz line emission is mentioned as well. Their positions and rms errors have been calculated from 9 points maps. The uncertainty The distances as well as the reference velocities (Local Stander of Rest (LSR)) of the sources have been taken from Molinari et al. [17] and Sridharan et al. [16] except IRAS \(19118+0945\), since no distance was given by Molinari et al. [17].

Fig. 3
figure 3

Distribution of HMPSOs types within separation groups

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Fig. 4
figure 4

The position of the OH masers and all mm peaks found within \(\sim\) 30 arcmin as well as the IRAS position. The IRAS name of the source is indicated in the upper right corner of the first three plots. The x-axis represents the right ascension and the y-axis represents the declination, both in degrees of arc

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Fig. 5
figure 5

Same as Fig. 4

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Fig. 6
figure 6

Same as Fig. 4

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Table 2 The OH masers positions and the positions of the closest 1.1 mm emission
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4.1 Comment on individual sources

IRAS 06056 + 2131. The OH masers were detected towards this source at 1665-, 1667-, and 1720-MHz lines. The velocity of the 1720-MHz OH satellite line is more consistent with the NH\(_3\) gas velocity (Molinari et al. [17]) than the two OH main lines. High angular resolution of the OH maser at the aforementioned transitions conducted by Darwish et al. [9]. They reported the detection of the emission line at 1665-MHz with a peak velocity of \(\sim\) 10 km s\(^{-1}\). Based on Darwish et al. [9] measurements, the OH maser is likely to be closer to the mm sources with   8 arcsec than the IRAS source (\(\sim\) 3 arc-minute) (see their Fig. 4). The CO outflow map of the region presented by Zhang et al. [25] shows at least two outflows. The IRAS source is consistent with one of these outflows and the OH main lines may be consistent with the other. Several 1.1-mm peaks were detected towards this region mostly in the shape of bow shock (Fig. 4). The peak seems to be closer to the OH main lines is much weaker than the two peaks closer to the IRAS source. The velocity of the CH\(_3\)OH maser source detected by Szymczak et al. [26] is consistent with the velocities of the OH maser main lines (\(\sim\) 10 km s\(^{-1}\)). All these indicate that this region is complex and contain at least two protostars in different evolutionary stages. This is consistent with the near-IR K\(_s\) band observations of Faustini et al [27] which suggest a cluster of several members with a radius of 0.3 pc.

IRAS 17527-2439. The 1665-MHz OH maser detected towards this region at velocity of 11.5 km s\(^{-1}\)is more consistent with the NH\(_3\) gas velocity (13.2 kms\(^{-1}\)) than the H\(_2\)O maser detected at velocity of \(\sim\) − 2 km s\(^{-1}\)by Palla et al. [28]. The 1.1-mm plot (Fig. 4) show that one of the peaks is coincident with the IRAS position while the OH maser is offset by \(\sim\) 40 arcsec.

IRAS 18024-2119. The OH masers is offset by \(\sim\) 19 arcsec from the 1.1-mm peak which is more consistent with the IRAS position and the 850 \(\mu\)m continuum [29]. No cluster was detected by Faustini et al [27].

IRAS 18048-2019. The OH masers (detected at the 1665, 1667-MHz, and 1720-MHz) and IRAS source are associated with different 1.1-mm peaks and the one closer to the IRAS source is \(\sim\) 7 times stronger than the one closer to the OH maser. This source is also associated with much stronger 6.7-GHz CH\(_3\)OH and H\(_2\)O masers [30,28] peaks at different velocities but at similar velocity ranges. This source shows absorption feature at the 1667-MHz OH and a weak maser at the 1720-MHz satellite line which indicates that this source may be associated with SNR.

IRAS 18089-1732 (G12.89 + 0.49). The OH maser towards this source was firstly detected by Cohen et al. [31] at the 1665-MHz line. Argon et al. [32] mapped this line in arc-second resolution which show three different components. The component which associated with strongest emission (\(\sim\) 4 Jy) is consistent with the IRAS position and the others are offset by \(\sim\) 1 and 2.7 arcsec. EFC07 detected the 1665-MHz line as well, but with ten times stronger flux density (\(\sim\) 30 Jy). EFC07 also detected an emission at the 1667-MHz line. This means that this is a variable source which is consistent with the daily monitoring nine-year observations of Goedhart et al. [33] at the 6.7 and 12.2 GHz CH\(_3\)OH maser lines. They measure a period of variability as short as less than a month. The OH masers, 1.1-mm peak, and the IRAS position may be in coincident (Fig. 4). These source is associated with H\(_2\)O [28] and CH\(_3\)OH [26] masers and is also associated with very weak 3.6 cm continuum emission, 0.9 mJy [16]. Recent high angular resolution submillimeter observations in various spectral lines by Beuther et al. [34] detect a massive rotating structure perpendicular to an emanating outflow which is likely associated with the central accretion disk.

IRAS 18090-1832. The OH maser emission detected towards this source at the two main lines (RHC only) is much weaker (\(\sim\) 0.8 Jy) than a relatively strong, 77 Jy 6.7-GHz CH\(_3\)OH maser [26]. The peak velocity of the CH\(_3\)OH maser is more consistent with the 1667-MHz line than the 1665-MHz. One of the mm peaks is coincident with the IRAS source (  1.5 arcsec offset) while the OH maser is offset by \(\sim\) 19 arcsec.

IRAS 18102-1800. The OH maser detected towards this sources at the 1665-MHz (RHC) only. One of the mm peaks is more consistent with the IRAS source than the OH maser which is offset by \(\sim\) 3 arcmin. This source is associated with CH\(_3\)OH maser and a 44 mJy radio emission (Sridharan et al. [16] and reference therein).

IRAS 18144-1723. The OH maser detected towards this sources at the main lines is a relatively stronger (80 Jy) than the other types of masers, CH\(_3\)OH  \(\sim\) 33 Jy [26] and H2O, \(\sim\) 24 Jy [28]. There is a significant gap, 13 km s\(^{-1}\) between the central velocities of strongest components in the 1665 and 1667 MHz lines. The OH maser and the IRAS source are not closely associated and are not consistent with any of the mm peaks to within \(\sim\) 18 arcmin. The radio continuum emission detected towards this region at 2 and 6 cm is offset from the IRAS source by \(\sim\) 90 arcsec [35] and no outflow was detected by Zhang et al. [25].

IRAS 18182-1433. The velocity of OH maser detected towards this source at the two main lines is consistent with the CH\(_3\)OH and H\(_2\)O masers detected by Szymczak et al. [26] and Beuther et al. [36], respectively. But although these two later traces were mapped using the VLBI by Sanna et al. (2010) may be coincident with VLA 3.6 cm observations by Zapata el al. [37], the OH maser mapped by Forster & Caswell (1999) is offset by \(\sim\) 25 arcsec and the closest mm peak is offset by \(\sim\) 3 arcsec (Fig. 7). An outflows was detected by Beuther et al. [36].

IRAS 18566 + 0408. Towards this source one of the 1.1 mm peaks is consistent with the IRAS positions. This has also been found with 1.2 mm observations by Beuther et al. [36]. The OH masers have a wide velocity range (\(\sim\) 40 km s\(^{-1}\)) and the two positions detected by EFC07 at the two mainlines are located in the two opposite sides of the IRAS/mm source (Fig. 4) with offset around 1 and 4 arcsec. This mm peak has the strongest flux among all other peaks within 30 arcsec. This source is also associated with CH\(_3\)OH maser emission [16, 26]. The H\(_2\)O maser emission was detected by Sridharan et al. [16], while was not detected by Palla et al. [28]. An outflow was detected by Beuther et al. [36] and Sridharan et al. [16] place an upper limit of 1 mJy on the 3.6 cm radio continuum flux from any source in this region.

IRAS 19035 + 0841 (G40.622 − 0.137). This is the second source with the three tracers, OH, mm, and IRAS are in very good agreement to within 0.05 arcsec (Fig. 6). The position of the OH maser is taken from the arcsec resolution observations of Argon et al. (2000). It is the strongest OH maser source (\(\sim\) 300 Jy) within this sample. The mm peak consist with the OH maser has the strongest flux of the other peaks within 30 arcsec. An outflow and 1.2 mm continuum emission were detected by Beuther et al. [36].

Fig. 7
figure 7

The OH (from Foster and Caswell [42]), CH\(_3\)OH  and H\(_2\)O masers (from Zapata el al. [37]), cm and the closest mm peak in the region of IRAS 18182 − 1433. The x-axis represents the right ascension and the y-axis represents the declination, both in degrees of arc

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IRAS 19092 + 0841. The OH maser position measured by high resolution observations using MERLIN (see, Edris et al. [38]) is 0.02 arcsec offset from the mm peak. It seems to be more closer than the IRAS source (13 arcsec). A VLA observations of 44-GHz class I methanol masers [39] show a close association between this tracer and the OH masers. This source is also associated with class II CH\(_3\)OH and H\(_2\)O masers.

IRAS 19118 + 0945. The mm and IRAS are seemingly closer (to within 12 aresec) than the OH maser which is offset by \(\sim\) 17 arcsec. No H\(_2\)O or CH\(_3\)OH masers have been detected towards this source [28,26].

IRAS 19410 + 2336 (G59.78 + 0.06). The OH and 1.1-mm emission are closely associated within   0.2 arcsec and consistent with previous 1.2-mm continuum observations as well as CH\(_3\)OH maser (Minier et al. [40], while the IRAS source is offset by 14 and 16 arcsec from OH and mm peak position, respectively. This observations show another weaker mm clump with no mid-IR emission. It is associated with weak radio source of \(\sim\) 1 mJy [16]. The OH maser associated with this source at only 1665-MHz line has one feature at velocity of \(\sim\) 20.6 km s\(^{-1}\) which falling in the CH\(_3\)OH masers velocity range (14 to 28 km s\(^{-1}\)) with several features peak at 17 km s\(^{-1}\) [26, 41]. Two outflows were detected by Beuther et al. [36] to associate each of the 1.2 mm clumps.

5 Discussion

In this study, our objective was to investigate the correlation between OH masers and mm emission in a selection of High Mass Star Forming Regions (HMSFRs) and gain insights into their physical characteristics, dynamics, and evolutionary status. Our analysis of the data has yielded several noteworthy findings, which are discussed in detail below.

Among the 27 OH maser sources examined, we observed that 23 of them exhibited associations with multiple peaks of millimeter emission within a 30\('\) radius of their positions. Furthermore, 20 of these sources had offsets of less than 20\('\). The offsets between the closest mm peak to the OH maser emission and the IRAS position were calculated, revealing that six sources had offsets of less than 0.05 arcsec. Particularly intriguing are the cases of IRAS 18089-1732 and IRAS 19035 + 0641, which represent different samples (types) of High Mass Protostellar Object (HMPO) candidates. The observed consistency in these sources between the three tracers (IRAS, mm, and OH maser) suggests a shared origin, providing evidence for a very early evolutionary stage in which the OH masers likely originate from the external shell of the collapsing core.

Additionally, we found that the remaining sources exhibited offsets ranging from 0.5 to 3 arcseconds, indicating a moderate spatial separation between the OH masers and mm peaks. Notably, the majority of the OH emission was offset by more than 3 arcseconds from the IRAS position, indicating a larger spatial extent of these regions. These results align with the distribution of the closest mm sources to the OH masers and IRAS sources, as depicted in Fig. 2. The spatial offset between the OH masers and mm emission can be attributed to various factors, including complex gas dynamics, outflows, and shock fronts within the HMSFRs.

Despite the limitations imposed by the low angular resolution in our study, our conclusions are supported by interferometer observations conducted on a subset of these sources (e.g., Edris et al. [3], Edris et al. [38], Darwish et al. [9]). These higher-resolution interferometric investigations provide additional validation of our findings.

Figure 3 demonstrates that the majority of the associated OH-mm sources (17 in total) are located at an offset of approximately 20\(''\) and belong to the High and \(``S''\) sub-samples of Molinari et al. [17] and Sridharan et al. [16]. These results provide evidence for probing advanced evolutionary stages within these sources.

In comparison to OH masers, the association between maser emission and mm/sub-mm emission has also been observed in other maser tracers, such as methanol masers. For example, Breen et al. [14], Chen et al. [43], and Sun et al. [44] have reported associations between 95 GHz methanol masers and mm sources. Chen et al. [43] used observations from the Purple Mountain Observatory 13.7 m radio telescope and found a 26\(\%\) association rate. Similarly, Sun et al. [44] reported a similar ratio at 6.7 GHz maser using observations from the Effelsberg 100 m radio telescope.

Compared to OH masers, our study indicates that OH masers exhibit a lower level of consistency with mm sources. We observed a coincidence rate of 11\(\%\) between OH masers and mm sources at offsets of less than or equal to 0.5 arcseconds. However, at larger offsets of 20 arcseconds, the association rate increases significantly to 63\(\%\). This variation in offset could be explained by the proposal of Gray et al. [45] that OH masers from different layers of the circumstellar disk may be present in some sources.

6 Conclusions

Our study aimed to investigate the correlation between OH masers and mm emission in High Mass Star Forming Regions (HMSFRs) and understand their physical characteristics and evolutionary status. We found that 23 out of 27 OH maser sources were associated with multiple peaks of millimeter emission within a 30\('\) radius, indicating a shared origin. These associations were particularly prominent in two high mass protostellar object (HMPO) candidates. The OH maser emission is more consistent with the mm peaks than the IR peaks.Interferometer observations supported our findings. Compared to methanol masers, OH masers exhibited a lower consistency with mm sources, with a coincidence rate of 11\(\%\) at offsets of 0.5 arcseconds or less. However, at larger offsets of 20 arcseconds, the association rate increased to 63\(\%\), indicating a broader spatial distribution of OH masers in these regions. The relatively higher offset between OH and IRAS emission than OH and mm means that the OH maser is associated with the colder dust more than the hotter.The relationship we have demonstrated between the offset of sources and their type (H, L, S) suggests that the association between OH masers and 1.1 mm emission may serve as a tracer for advanced stages of High Mass Protostellar Objects (HMPSOs). However, to gain a deeper understanding of the association between maser and dust continuum emission, high angular resolution observations are recommended.