The detection of class I methanol masers towards regions of low-mass star formation

Six young bipolar outflows in regions of low-intermediate-mass star formation were observed in the 7₀-6₁ A ⁺, 8₀-7₁ A ⁺, and 5₋₁-4₀ E methanol lines at 44, 95, and 84 GHz, respectively. Narrow features were detected towards NGC 1333-IRS4A, HH 25MMS, and L1157-B1. The flux densities of the detected lines are not higher than 11 Jy, which is much lower than the flux densities of strong maser lines in regions of high-mass star formation. Analysis shows that the narrow features are most likely masers. Published in Russian in Astronomicheskiĭ Zhurnal, 2006, Vol. 83, No. 4, pp. 327–336. This text was submitted by the authors in English.     

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.     

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.

     

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.     

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.     

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.     

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.     

Structure of the class I methanol masers OMC-2 and NGC 2264

Results of interferometric observations of the class I methanol masers OMC-2 and NGC 2264 in the 7₀-6₁ A ⁺ and 8₀-7₁ A ⁺ lines at 44 and 95 GHz, respectively, are presented. The maser spots are distributed along the arcs bent toward infrared sources, which are young stellar objects. The distributions of the maser spots at 44 and 95 GHz are virtually identical, and the fluxes from the brightest spots are similar. The measured sizes of the maser spots at 44 GHz are, on average, about 50 AU. The brightness temperature of the strongest components at 44 GHz is 1.7 × 10⁷ K and 3.9 × 10⁷ K for OMC-2 and NGC 2264, respectively. A simple model for the excitation of Class I methanol masers is proposed; it yields an estimate of the limiting brightness temperature of the emission. The model is based solely on the properties of the methanol molecule without invoking the physical parameters of the medium. Using it, we showed that the emission opening angles for NGC 2264 and OMC-2 do not exceed 3° and 4.5°, respectively. The depth of the masing region is about 1000 AU. The emission directivity is naturally realized in the model of of maser consisting of a thermalized core and a thin inverted envelope, probably, with an enhanced methanol abundance. The maser emission has the greatest intensity in the direction tangential to the envelope. The size of the masing envelope estimated from the measured depth and spot extens is ～2 × 10⁴ AU, or 0.15 pc. This size is close to the sizes of the dense molecular cores surrounding the young stellar objects IRS 4 in OMC-2 and IRS 1 in NGC 2264.     

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.     

A study of the star-forming group RNO 129

We have studied the compact star-forming group RNO 129 in the cloud L1228. New images and both slit and integrated spectroscopy for several nebulas and HH objects are presented. We have detected several new HH objects, including two blobs embedded in a bright reflection nebula. The central star of this nebula is binary and is ejecting at least one collimated jet. Some peculiarities in the spectrum of the nebula indicate its similarity with NGC 2261.     

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.     

Thermal lines of methanol towards bipolar outflows

Observations of 26 regions of low-mass star formation and 17 regions of massive star formation in the 5₋₁-4₀ E, 7₀-6₁ A ⁺, 8₀-7₁ A ⁺, and 2_K-1_K methanol lines at 44.1, 84.5, 95.2 GHz, and 96.7 GHz yielded detections of methanol emission in 11 low-mass and 12 high-mass regions. The strongest lines in the low-mass regions were found towards bipolar outflows driven by Class 0 protostars with luminosities higher than or of the order of 10 L _⊙. These lines usually consist of cores 1–2 km s⁻¹ in width, which are emitted by quiescent gas, and broader wings, emitted by gas accelerated by high-velocity jets. The temperature of the accelerated gas derived from rotational diagrams and statistical equilibrium calculations is roughly 20–50 K. This means that a significant fraction of the accelerated gas cools to such temperatures. The widths of the lines detected in the massive star-forming regions are 2–3 km s⁻¹ or higher. Weak, broad wings were found towards only two sources: L1287 and AFGL5142. For most sources, the statistical-equilibrium calculations yielded gas temperatures of about 20–30 K and densities of about 10⁴–10⁶ cm⁻³, which are typical for warm clouds. However, different transitions emit in regions with different physical conditions located within the main beam of the telescope. Most of the 96.7 GHz emission arises in warm gas with kinetic temperatures of about 30 K, while most of the 95.2 GHz emission may arise in hot regions around Young Stellar Objects and/or be related to the wings of bipolar outflows. Published in Russian in Astronomicheskiĭ Zhurnal, 2007, Vol. 84, No. 1, pp. 48–59. The article was translated by the author.     

Class I methanol maser emission in infrared clouds and the third version of the Astro Space Center MMI/SFR catalog

We have revised the Astro Space Center catalog of Class I methanol masers detected in star-forming regions (MMI/SFR), mainly at 44 GHz, and created a new electronic version of the catalog. Currently, the catalog contains 206 objects, selected from publications through 2011 inclusive. The data from the survey of Chen et al. (2011), performed specifically for objects EGO, which form a new specific catalog, are not included. The MMI/SFR objects were identified with emission and absorption objects in the near IR, detected during the MSX and Spitzer space missions. Seventy-one percent of Class I methanol masers that emit at 44 GHz and fall within the Galactic longitude range surveyed by Spitzer (GLIMPSE) are identified with Spitzer Dark Clouds (SDCs), and 42% with Extended Green Objects (EGOs). It is possible that Class I methanol masers arise in isolated, self-gravitating clumps, such as SDCs, at certain stages of their evolution. A sample of SDCs is proposed as a new target list for Class I methanol maser searches. A detailed statistical analysis was carried out, taking into account the characteristics of the regions of MMI/SFR formation presented in the catalog.     

Methanol maser groups and class I emission: Flux density distribution

A search for a relationship between class I and class II methanol maser flux densities has been carried out. A large sample of mixed-type sources has been studied, with each source in the sample radiating as a class I and class II maser simultaneously. In methanol maser groups for which the positions of prominent spectral features at different radial velocities coincide at different frequencies, the fluxes are anticorrelated, and are related as log S _6.7+12.2 = (?1.68 ± 0.38) × log S ₄₄ + (4.01 ± 0.60). For group I, which includes sources with preferred pumping for masers emitting at 6.7 GHz, the relationship between the 6.7 GHz masers and 44 GHz masers is less steep than for group II, which contains sources with normal pumping of class II masers. This implies that class I methanol masers that correspond to group I are suppressed more strongly.     

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.     

Methanol and other molecular tracers of outflows and dense gas in the molecular cloud G345.01+1.79

The results of SEST millimeter observations of the molecular cloud G345.01+1.79 are presented. Spectra of CH₃OH, SO₂, SiO, HCO⁺, C¹⁸O, C³³S, C³⁴S, HCN, and DCN lines have been obtained. Mapping of the cloud in CH₃OH, SiO, and C³⁴S lines indicates that the maximum integrated intensity in the SiO and C³⁴S lines and in low-excitation CH₃OH transitions coincide with the northern group of methanol masers, while the corresponding maximum for high-excitation CH₃OH transitions coincides with the southern methanol-maser group. The physical parameters are estimated from the quasi-thermal CH₃OH lines under the large-velocity-gradient approximation, and their distribution on the sky derived. The density and temperature are higher toward the southern group of methanol masers than in the northern group. This may indicate that star formation is in an earlier stage of evolution in the northern than toward the southern group. A maser component can be distinguished in 14 (of 71) CH₃OH lines. We have detected for the first time weak, probably maser, emission in the CH₃OH lines at 148.11, 231.28, 165.05, 165.06, and 165.07 GHz. A blue wing is clearly visible in the CH₃OH, SiO, C¹⁸O, and SO₂ lines. The emission in this wing is probably associated with a compact source whose velocity is characteristic of the CH₃OH maser emission in the southern group of masers.     

OH 1720-MHz lines as tracers of bipolar outflows in the vicinity of Class I methanol masers

A sample of Class I methanol masers (MMI) has been surveyed at 1720 MHz to search for possible associations between MMI and 1720-MHz OH masers, which should be formed by the same collisional pumping mechanism. If the model for methanol masers is correct, the sample should contain a statistically significant number of 1720-MHz OH masers at the positions of MMI. The observations were conducted on the 70-meter radio telescope of the National Academy of Sciences of Ukraine (NASU). The results show that ??50% of 72 MMI are associated with OH emission at 1720 MHz. In many sources, strong absorption lines are also observed. In most cases, the OH (1720) lines are narrow (linewidths of <2 km/s) suggesting they may be maser lines. The OH column densities obtained from Gaussian fitting of these narrow OH lines are, on average, 1.5 × 10¹⁷ cm^?2. TheH² density in the emitting medium reaches 10⁷ cm^?3 if the region of the OH (1720) emission has been subject to interaction with a bipolar-outflow front. This is sufficient to excite MMI, and the presence of narrow, possibly masing OH (1720) lines at the MMI velocities indicates the likely presence of shocks from bipolar outflows in the vicinity of the maser condensations, supporting models in which these molecules are collisionally pumped.     

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.