A study of three massive star-forming regions in CS lines

We have mapped three star-forming regions (G265.14+1.45, G269.16?1.14, G291.27?0.71) in the CS(3–2) and C³⁴S(2–1) lines using the 15 m SEST telescope (Chile), and analyzed the relative positions of methanol and H₂O masers, IRAS sources, and emission maxima in the CS lines. In most cases, the maser positions are close to those of the IRAS sources. We compared the radial velocities of the maser sources and high-density CS cores, and estimated the CS column densities assuming LTE. The sizes, densities, and masses of the dense core are estimated; the masses obtained in the LTE approximation agree with the virial masses.     

Class I methanol masers in low-mass star-forming regions

Results of observations of Class I methanol masers in regions of low-mass star formation (MMIL) are summarized and analyzed. Four masers were detected at 44, 84, and 95 GHz towards “chemically active” bipolar outflows in the low-mass star-forming regions NGC1333 I4A, NGC 1333 I2A, HH 25, and L1157. Another maser was found at 36 GHz towards a similar outflow in NGC 2023. Thus, all the detected MMILs are associated with chemically active outflows. The brightness temperatures of the strongest 44-GHz maser spots in NGC 1333 I4A, HH 25, and L1157 exceed 2000 K, whereas the brightness temperature in NGC 1333 I2A is only 176 K, although a rotational-diagram analysis shows that this last source is also amaser. The flux densities of the newly detectedmasers are no higher than 18 Jy, and are much lower than those of strong masers in regions of high-mass star formation (MMIH). The MMIL luminosities match the maser luminosity-protostar luminosity relation established earlier for MMIHs. No MMIL variability was detected in 2004–2011. The radial velocities of the newly detected masers are close to the systemic velocities of the associated regions, except for NGC 2023, where the maser radial velocity is lower than the systemic velocity by approximately 3.5 km/s. Thus, the main MMILproperties are similar to those of MMIHs. MMILs are likely to be an extension of the MMIH population toward lower luminosities of both the masers and the associated young stellar objects. The results of VLA observations of MMILs can be explained using a turbulent-cloud model, which predicts that compact maser spots can arise in extended sources because the coherence lengths along some directions randomly appear to be longer than the mean coherence length in a turbulent velocity field. However, one must assume that the column density of methanol towardM1, the strongest maser in L1157, is appreciably higher than the mean column density of the clump B0a where the maser arises. The shape of the maser lines in L1157, forming double profiles with a red asymmetry, may indicate that the masers arise in collapsing clumps. However, although this model may be correct for L1157, it is specific to this source, since none of the other masers observed exhibited a double profile.     

The methanol emission of isolated maser condensations: Statistical velocity distribution

The distribution of the radial velocities of class I methanol masers relative to the velocities of their parent molecular clouds is analyzed. This analysis is based on catalog data for methanol masers detected up to the present time in both the northern and southern hemispheres, together with catalog data for the CS(2-1) line, which traces dense, quiescent gas. Results for a large sample of sources show that, in contrast to class II methanol masers, which undergo Keplerian motions in protoplanetary disks, class I methanol masers retain their velocities in the local system of rest of the surrounding medium, and do not participate in the ejection of matter in bipolar outflows. They can be adequately described using a model in which matter ejected from active parts of the associated star-forming regions flows around isolated maser condensations. This compresses the maser clumps, enhancing the concentration of methanol and facilitating collisional pumping of the masers.     

Mapping of bipolar outflows and methanol masers in the CS(2-1)line

Eighteen regions (bipolar outflows and methanol masers) are mapped in the CS(2-1) line using the 20-m Onsala radio telescope. The coordinates of the CS emission peaks are refined. The sizes and masses of dense regions are estimated for 13 maps. Measurement of the angular sizes of regions of emission indicates that all the sources were resolved by the Onsala radio telescope. The lower limit for the linear dimensions of the CS condensations studied is 0.2–2.1 pc. The hydrogen densities and masses of the CS condensations are estimated to be n(H₂)=(0.3–13.1)×10⁴ cm^?3 and (M ≈ 7–2800M _⊙). Methanol masers are associated with denser and more massive regions, whether or not the maser condensation is connected with a bipolar outflow.     

VLA observations of class I methanol masers in the region of low-mass star formation L1157

We present the results of VLA observations of a maser candidate in the low-mass star formation region L1157 in the 7₀-6₁ A ⁺ transition at 44 GHz. The line is emitted by a compact, undoubtedly maser source associated with clump B0a, which is seen in maps of L1157 in thermal lines of methanol and other molecules. A much weaker compact source is associated with clump B1a, which is brighter than B0a in thermal methanol lines. The newly detected masers may form in thin layers of turbulent post-shock gas. In this case, the maser emission may be beamed, so that only an observer located in or near the planes of the layers can observe strong masers. On the other hand, the maser lines are double with a “red” asymmetry, indicating that the masers may form in collapsing clumps. A detailed analysis of collapsing-cloud maser models and their applicability to the masers in L1157 will be developed in subsequent papers.     

Observations of OH and H<Subscript>2</Subscript>O Maser Emission in the Star-Forming Region S128

Results of monitoring hydroxyl and water masers in the star-forming region S128 are presented. A large number of emission features in the 1665 MHz OH line have been detected in both circular polarizations. In spite of the strong variability of the flux density in the main 1665 MHz line, the radial velocities of the features remained constant. Zeeman splitting of the 1720MHz line equal to 0.86 km/s was detected, corresponding to a longitudinal magnetic field of 3.6 mG. The variability of the H²O emission has a cyclic character with a quasi-period of 4–14 yrs. The evolution of individual features confirms that the H²O sources A and B are associated with an ionization front between two colliding CO clouds, and shows that the activity was transferred from maser B to maser A in 1999–2001.     

A magnetized disk around an O star in W75N. A VLBI map of the OH maser

W75N is a star-forming region containing ultracompact H II regions as well as OH, H₂O, and methanol masers. The VLBA maps obtained show that the masers are located in a thin disk rotating around an O star, which is the exciting star for the ultracompact H II region VLA1. A separate group of maser spots is associated with the ultracompact H II region VLA2. The radial velocity of the maser spots varies across the disk from 3.7 to 10.9 km/s. The disk diameter is 4000 AU. The maser spots revolve in Keplerian orbits around the O9 star.     

The detection of new methanol masers in the 5−1–40 E line

Forty-eight objects were detected in the 5_?1–4₀ E methanol line at 84.5 GHz during a survey of Class I maser sources. Narrow maser features were found in 14 of these. Broad quasi-thermal lines were detected toward other sources. One of the objects with narrow features at 84.5 GHz, the young bipolar outflow L1157, was also observed in the 8₀–7₁ A ⁺ line at 95.2 GHz; a narrow line was detected at this frequency. Analysis showed that the broad lines are usually inverted. The quasi-thermal profiles imply that there are no more than a few line opacities. These results confirm the plausibility of models in which compact Class I masers appear in extended sources as a result of a preferential velocity field.     

Masses of protostars with methanol protoplanetary disks

The masses of 12 protostars assumed to be the central bodies in circumstellar protoplanetary disks are estimated based on analysis of their methanol maser spectra and fine spatial structure. The calculations are based on the hypothesis that the class II methanol maser lines are formed in an edge-on Keplerian disk, while the thermal methanol emission and CS lines are formed in a cocoon around the protostar. This provides information about the velocities of the protostar and the methanol maser condensations relative to the center. In most of the star-forming regions studied, the derived masses are within limits admissible for disks around massive OB stars. The masses are in good agreement with the calculations of other authors based on models of the velocity gradients of the maser features. It is suggested that the methanol spectra display a triplet structure in which the two lateral features are class II methanol lines and the central component is a class I methanol maser line or thermal methanol line. This is consistent with the fact that the correlation of regions of maser emission with regions of emission of dense molecular gas in the CS line is about twice as strong(about 100%) as the correlation with ultracompact HII regions (about 50%). This should be taken into account when modeling protoplanetary disks and star-forming regions.     

High resolution OH observations of Extended Green Objects

Themaser pumping schemes proposed for the various OH lines may not be as clear-cut as they once seemed. The main OH lines, at 1665 and 1667 MHz, are thought to be radiatively pumped, with the radiation typically coming from nearby ultracompact HII regions. Recently, a new class of main-line maser has been posited, collisionally pumped by shocks due to molecular outflows. The W3(OH)/W3(OH)-TW system is the archetype: traditional OH masers are excited by theW3(OH) ultracompact HII region, while collisionally pumped OH masers arise in the younger object W3(OH)-TW, which is driving an outflow. The 1720 MHz OH satellite line maser, typically found in SNR–cloud interaction regions, is thought to be collisionally pumped, as are class I methanol masers found in star formation regions. Thus it is plausible that these two masers arise in similar (shocked gas) circumstances. In this study we observe all four OH transitions in the direction of Extended Green Objects (EGOs) that trace shocked gas (possibly from outflows) in high-mass star formation regions. Previous studies have found a high incidence of class I methanol maser emission in these objects, suggesting that OH(1720) masers might also be abundant in this sample. Observations of 20 northern EGOs (δ > −17°) were carried out with the Jansky Very Large Array of all four ground state OH transitions, the HI line, and the 20 centimeter continuum. Positive detection of OH lines was obtained for 10 EGOs: OH lines at 1665 and 1667 MHz were detected toward 45% of the sample. The stellar OH line at 1612 MHz was detected toward 15% of the sample. The 1720 MHz emission line was detected in only one EGO source, G45.47+0.07, which is also presents the strongest main-line OH emission of our sample. We measure the projected separations between OH masers and GLIMPSE point sources associated with EGOs (median value 0.04 pc), betweenOH and class II methanol masers (median value 0.03 pc), and between OH and class I methanol masers (median value 0.14 pc), thus confirming previous findings that class I methanol masers are located further from exciting sources than areOH and class II methanol masers. Bearing in mind the theoretical incompatibility of class I and class II methanol maser pumping schemes, and the obtained separations between class I methanol masers and other masers in the EGOs, we conclude that class I methanol masers do not co-exist with GLIMPSE point sources, OH and class II methanol masers in one and the same core. Rather, we suggest that the class I masers arise in distinct but neighboring cores, about 1 pc distant, and in a different evolutionary state.

     

Methanol radio emission at millimeter wavelengths: New masers at 1.3 and 2.8 millimeters

The results of a search for maser emission in the methanol lines 8_?1-7₀ E at 229.8 GHz, 3_?2-4_?1 E at 230.0 GHz, 0₀-1_?1 E at 108.9 GHz, and in the J ₁-J ₀ E series near 165 GHz in star-forming regions are reported. At least two masers and two candidates have been detected at 229.8 GHz. Thus, methanol masers have been detected in the 1-mm band for the first time. At 108.9 GHz, masers have been detected toward G345.01+1.79 and possibly toward M8E as well. Thermal emission was found toward 28 objects. The 229.8-GHz sources are class I masers, whereas the 108.9-GHz sources are class II masers. An analysis using a large velocity-gradient method shows that the 229.8-GHz masers can appear at densities of about 3×10⁴ cm^?3. The ratios of the flux densities in different class I lines toward DR 21(OH) and DR 21 West can be approximated in models with gas kinetic temperatures of about 50 K. Detection of the 108.9 GHz masers toward G345.01+1.79 and M8E may provide information about the geometry of these objects.     

Masers in the cool molecular cloud L 379

We present the results of monitoring the H₂O masers in the IR sources IRAS 18265-1517 and IRAS 18277-1516 associated with the cool molecular cloud L 379, which contains high-velocity bipolar molecular jets. The sources were observed in the 1.35 cm H₂O line using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) during 1991–2004. We detected H₂O maser emission from IRAS 18265-1517 at radial velocities of 17.8 and 18.4 km/s, virtually coincident with the velocity of the molecular cloud derived from CO-line observations (18.4 km/s). The maser emission towards the other source, IRAS 18277-1516, was at higher velocities than the central velocity of the CO molecular cloud. The H₂O maser spots are most likely associated with a redshifted region of CO emission. Cyclic variability of the integrated H₂O maser emission that may be related to cyclic activity of the central star was detected for IRAS 18277-1516. The strongest and most long-lived component (V_LSR ≈ 20.6 km/s) displays a radial-velocity drift, which could be due to deceleration of a dense clump of matter (maser condensation) in the circumstellar medium during the descending branch of a strong flare. We found numerous emission features for both IRAS 18265-1517 and IRAS 18277-1516, providing evidence for fragmentation of the medium surrounding their central objects.     

Evolution of the H<Subscript>2</Subscript>O and OH Maser Emission in W75 N

The results of a study of H₂O and OH maser emission in the complex region of active star formation W75 N are presented. Observations were obtained using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) and the Nan3ay radio telescope (France). Flaring H₂O maser features may be identified with maser spots associated with the sources VLA 1 and VLA 2. Themain H₂O flares occurred in VLA 1. The flare emission was associated with either maser clusters having closely spaced radial velocities and sizes up to ~2 AU or individual features. The maser emission is generated in a medium where turbulence on various scales is present. Analysis of the line shapes during flare maxima does not indicate the presence of the simplest structures—homogeneous maser condensations. Strong variability of the OH maser emission was observed. Zeeman splitting of the 1665-MHz line was detected for several features of the same cluster at a radial velocity of +5.5 km/s. The mean line-of-sight magnetic field in this cluster is ~0.5 mG, directed away from the observer. Flares of the OH masers may be due to gas compression at a shock or MHD wave front.     

Search for main-line OH emission toward high-latitude IRAS sources in the southern hemisphere

We present the results of a search for the emission in the main OH lines toward high-latitude IRAS sources using the Australia Telescope Compact Array (ATCA) in Narrabri. A sample of 239 sources from the IRAS catalog selected with the color criterion for ultracompact HII regions and with no restriction on flux was studied. No maser or thermal emission or absorption was detected in these sources. The reason for this could be the low sensitivity of the ATCA to faint high-latitude objects. However, we have mapped the regions of eight known masers in star-forming regions in the plane of the Galaxy. The results are given in the form of spectra and maps.     

Observations of Extended Green Objects in the 1.35-cm H<Subscript>2</Subscript>O Line on the 22-m Pushchino Radio Telescope

Observations of H₂O maser sources at 1.35 cm associated with extended regions of 4.5-µm emission (indicated as “green” on Spitzer survey maps—so-called Extended Green Objects, EGOs) are reported. EGOs are considered as characteristic signposts of regions of formation of massive stars, which host high-velocity outflows, as well as methanol, water, and hydroxyl masers. The observations were carried out in January–May 2015 on the 22-meter radio telescope of the Pushchino Radio Astronomy Observatory. The sample studied includes 24 EGOs north of declination -29° taken from the Spitzer GLIMPSE survey, together with one of the brightest Class I methanol masers G6.05-1.45 (M8E) and the Class I methanol maser in an IRDC G359.94+0.17. H₂O maser emission was detected toward 11 of the EGOs: G11.94-0.62, G14.33-0.64, G16.59-0.06, G23.01-0.41, G24.943+0.074, G28.83-0.25, G34.3+0.2, G34.403+0.233, G35.20-0.74, G45.47+0.07, and G49.267-0.337. These including the well known H₂O maser in the W44 region, G34.3+0.2. H₂O emission from G28.83-0.25 was detected for the first time, at 77.6 km/s, with a flux density of 19 Jy in January and 16 Jy in February 2015. The source was probably caught at an early stage of the propagation of a shock wave. The Class I methanol masers G359.94+0.17 and G6.05-1.45 (M8E) and 13 of the EGOs were not detected in the H₂O line, with 3s upper limits of ~6-7 Jy. Spectra and maser-emission parameters are given for the detected H₂Omasers, for some of which strong variability of the H₂O maser emission was observed. The detected H₂Omasers, together with the Class I methanol masers and extended 4.5-µm emission, are associated with a very early stage in the development of young stellar objects in the regions of the EGOs. However, this sample of EGOs is not uniform. The presence of 44-GHz Class I methanol masers together with EGOs cannot be considered the only sign of early stages of star formation.     

Methanol emission in distant protoplanetary disks

Thirty four-frequency line profiles of Class II methanol masers have been analyzed to investigate carefully the coincidences of various spectral features. Data at 6.7, 12.2, 107, and 156.6 GHz have been analyzed. Two clusters of Class II methanol maser lines at 6.7 and 12.2 GHz are observed in the spectra of many sources. These maser-line clusters are located on either side of the thermal methanol lines at 107 and 156.6 GHz. To avoid the effect of amplification in these thermal methanol lines, a similar analysis was performed for 80 sources having both maser emission at 6.7 GHz and thermal CS emission. The relative distributions of the methanol maser lines and the thermal CS line confirm on the basis of richer statistics that the maser lines are located in two clusters on either side of the thermal feature. It is proposed that the two maser-line clusters correspond to two edges of a Keplerian disk. The thermal methanol and CS emission is formed in dense molecular cores, whose centers are coincident with the disk centers.     

Search for class I methanol maser emission in various types of objects in the interstellar medium

Observations of various types of objects in the northern sky were obtained at 44 GHz in the 7₀-6₁ A ⁺ methanol line on the 20-m Onsala radio telescope (Sweden), in order to search for Class I methanol maser emission in the interstellar medium: regions of formation of high-mass stars, dust rings around HII regions, and protostellar candidates associated with powerful molecular outflows and Galactic HII regions. Seven new Class Imethanolmasers have been discovered toward regions of formation of highmass stars, and the existence of two previously observed masers confirmed. The following conclusions are drawn: (1) neither the association of a bipolar outflow manifest in the wings of CO lines with a highmass protostellar object (HMPO) nor the presence of thermal emission in lines of complex molecules are sufficient conditions for the detection of Class I methanol emission; no association with HMPOs radiating at 44 GHz was found for EGOs (a new class of object tracing bipolar outflows); (2) the existence of H₂O masers and Class II methanol masers in the region of aHMPOenhances the probability of detecting Class I methanol emission toward the HMPO; Class II methanol masers with stronger line fluxes are associated with Class I methanol masers.     

Methanol proplyds and a double protoplanetary system in the constellation Norma

We have carried out detailed studies of a star-forming region containing two maser sources in the constellation Norma. The sources display a complex spectral distribution of the maser lines and spatial distribution of the maser condensations. The maser condensations may have formed around objects that are hidden by dense molecular cocoons; the velocities of the maser features may represent Keplerian orbital motions. The cocoons, which radiate in thermal methanol and CS lines, correspond to the centers of mass in the maser sources + dense molecular core systems. The velocities of the CS lines or thermal methanol lines can be used to identify the locations of the centers of mass of these systems. If the maser radiation is generated in the atmospheres of protoplanets, the Norma radio source may correspond to two protoplanetary disks, each with a protostar and protoplanetary system. In this case, the masses of the protostars are approximately 13 M_⊙ and 38 M_⊙.     

A general catalog of class I methanol masers

A catalog of class I methanol masers discovered so far in the Southern and Northern hemispheres is presented. The catalog contains 160 sources. A statistical analysis shows that, within 2’ of the telescope pointing (which corresponds approximately to the field of view of single antennas used in search surveys), 50% of class I methanol masers are associated with objects characteristic of active starforming regions: IRAS sources, ultracompact HII regions, and dense gas—dust clouds, as well as OH and H₂O interstellar masers. At the same time, bipolar outflows (which could play an active part in pumping the methanol masers) are associated with fewer than 25% of class I methanol masers. In 72% of cases, class I methanol masers are associated with class II methanol maser sources. These results suggest that methanol maser condensations are more appropriately classified by the transition type (that is, the pumping mechanism) than their association with other astronomical objects.     

Spectral scan of the star-forming region DR21(OH). Observations and LTE analysis

Seventy-eight molecules have been detected as a result of a spectral survey of the star-forming region DR21(OH) at 84–115 GHz. The abundances of most molecules are typical of those in the dense cores of molecular clouds. The rotational temperatures derived using the lines of most molecules fall in the range 9–56 K, which is also typical for dense cores. However, emission from high-lying levels of methanol and sulfur dioxide was detected; since the rotational temperatures for methanol and sulfur dioxide are 252 and 186 K, this indicates the presence of hot regions. Another fact indicating the existence of hot regions is the detection of CH₃OCHO, CH₃CH₂OH, and CH₃OCH₃, which have thus far been observed only in hot cores and shock-heated regions. An interesting result is the tentative detection of the J = 2 − 1, v = 1 SiO line, with the upper level energy of 1775 K. This is probably a maser line, similar to but weaker than the well-known SiO masers in the star-forming regions Orion-KL,W51(N), and Sgr B2(N).