Methanol masers which are traditionally divided into two classes provide possibility to study important parts of the star forming regions: Class II masers trace vicinities of the massive YSOs while class I masers are likely to trace more distant parts of the outflows where newer stars can form.

There are many methanol transitions which produce observed masers. This allows to use pumping analysis for estimation of the physical parameters in the maser formation regions and its environment, for the study of their evolution. Extensive surveys in different masing transitions allow to conclude on the values of the temperatures, densities, dust properties, etc. in the bulk of masing regions. Variability of the brightest masers is monitored during several years. In some cases it is probably caused by the changes of the dust temperature which follow variations in the brightness of the central YSO reflecting the character of the accretion process.

A unified catalogue of the class II methanol masers consisting of more than 500 objects is compiled. Analysis of the data shows that: physical conditions within the usual maser source vary considerably; maser brightness is determined by parameters of some distinguished part of the object–maser formation region; class II methanol masers are formed not within the outflows but in the regions affected by their propagation.

It is shown that the “near” solutions for the kinematic distances to the sources can be used for statistical analysis. The luminosity function of the 6.7 GHz methanol masers is constructed. It is shown that improvement of the sensitivity of surveys can increase number of detected maser sources considerably.

The distribution of class II methanol masers in the Galaxy is constructed on the basis of estimated kinematic distances. It is shown that most of the sources are located in the Molecular Ring and that the dependence of the number of sources on the distance from the Galactic Center has significant peaks at the positions corresponding to the spiral arms.

A survey of CS(2-1) line emission tracing dense gas is performed at Mopra toward the positions of the brightest class II methanol masers. Velocity correlations between the maser and CS lines are analyzed. It is shown that the sources with l from 320 to 350 deg in which the masers are relatively blue-shifted, form a group which is located in the region of the Scutum-Centaurus spiral arm. This can reflect existence of a grand design, i.e., grouping of the sources with similar peculiarity of morphology or evolutionary stage of the massive star forming regions.

Our mid-infrared and near-infrared surveys over the last five years have helped to strengthen and clarify the relationships between water, methanol, and OH masers and the star formation process. Our surveys show that maser emission seems to be more closely associated with mid-infrared emission than cm radio continuum emission from UC HII regions. We find that masers of all molecular species surveyed trace a wide variety of phenomena and show a proclivity for linear distributions. The vast majority of these linear distributions can be explained by outflows or shocks, and in general do not appear to trace circumstellar disks as was previously thought. Some water and methanol masers that are not associated with radio continuum emission appear to trace infrared-bright hot cores, the earliest observable stage of massive stellar life before the onset of a UC HII region.

We present Very Long Baseline Array (VLBA) proper motion observations of water masers toward two young stellar objects (YSOs) of the W75N(B) high-mass star forming region. These observations (Torrelles et al. 2003) show two objects having a similar spectral type (early-B stars), separated in the sky by 0$0^''$7 (corresponding to 1400 AU at the source distance), sharing the same molecular gas environment, but presenting a strikingly different outflow ejection geometry. One of these YSOs, W75N(B)-VLA 1, has a jet-like outflow at 2000 AU scale, with the water masers moving at velocities of ${\simeq}$20 km s$^{-1}$ along the major axis of the thermal radio continuum jet, while the other YSO has a water maser shell outflow of 160 AU radius expanding at ${\simeq}$30 km s$^{-1}$ in multiple directions with respect to the central compact radio continuum source W75N(B)-VLA 2. Given the small dynamical time obtained for the water maser shell (${\simeq}$13 yr), we propose that in the very early stages of the star-formation process there may exist short lived, possibly repetitive, events associated with very poorly collimated outflows. All these results suggest that outflow collimation is not only a consequence of ambient conditions, but something intrinsic to the evolution of the individual stars, opening the important question of how and when these non-collimated wind ejection events occur in the evolution of YSOs. Although we argue in this paper that W75N(B)-VLA 2 could be in an earlier stage of evolution than W75N(B)-VLA 1, we think that observations at (sub)millimeter wavelengths with angular resolutions of ${\simeq}0^''1$ can provide the key for determining the relative stage of evolution of these two remarkable YSOs.

6.7 and 12.2 GHz CH$_{3}$OH (methanol) and 22.2 GHz H$_{2}$O masers are believed to be good tracers of the earliest phases of high-mass star formation. Interferometric and VLBI (Very Long Baseline Interferometry) observations have shown that water masers are predominantly associated with the innermost portions of the jets/outflows emerging from (proto-)stellar objects. On the other hand, the astrophysical environment traced by the 6.7 GHz (and the associated 12.2 GHz) CH$_{3}$OH masers is still to be more precisely determined. So far, most high-resolution studies have focused either on CH$_{3}$OH or on H$_{2}$O masers and little is known on their connection, wehereas it would be important to study both types of maser emission in the same object. The goal of our long-term project is to perform interferometric and VLBI observations of H$_{2}$O and CH$_3$OH masers towards a selected sample of high-mass YSOs where both maser types have been detected. This work presents preliminary results obtained for a few objects of our sample, and discusses possible implications.

Young stars on their way to the ZAMS evolve in significantly different ways depending on their mass. While the theoretical and observational properties of low- and intermediate-mass stars are rather well understood and/or empirically tested, the situation for massive stars ([gsim ]10–15 M$_\odot$) is, to say the least, still elusive. On theoretical grounds, the PMS evolution of these objects should be extremely short, or nonexistent at all. Observationally, despite a great deal of effort, the simple (or bold) predictions of simplified models of massive star formation/evolution have proved more difficult to be checked. After a brief review of the theoretical expectations, I will highlight some critical test on young stars of various masses.

2-D and 3-D radiation transfer models of forming stars generally produce bluer 1-10 μm colors than 1-D models of the same evolutionary state and envelope mass. Therefore, 1-D models of the shortwave radiation will generally estimate a lower envelope mass and later evolutionary state than multidimensional models. 1-D models are probably reasonable for very young sources, or longwave analysis ($\lambda >100 \mu$m). In our 3-D models of high-mass stars in clumpy molecular clouds, we find no correlation between the depth of the 10 μm silicate feature and the longwave ($> 100 \mu$m) SED (which sets the envelope mass), even when the average optical extinction of the envelope is ${> }100$ magnitudes. This is in agreement with the observations of Faison et al. (1998) of several UltraCompact HII (UCHII) regions, suggesting that many of these sources are more evolved than embedded protostars.

We have calculated a large grid of 2-D models and find substantial overlap between different evolutionary states in the mid-IR color-color diagrams. We have developed a model fitter to work in conjunction with the grid to analyze large datasets. This grid and fitter will be expanded and tested in 2005 and released to the public in 2006.

I review recent observational progress on the search for accretion signatures in massive young stellar objects (MYSOs). In this context, the primary tool will be 1–5 μm high resolution ($\lambda/\Delta\lambda \,{=}\,$50,000) spectroscopy. The observational regime is the phase between massive star formation (collapse, accretion, mergers, etc) and the fully revealed OB star photosphere, the phase for which the term “Massive Star Birth” was originally coined. In this phase, we seek both the signature of the earlier accretion process and the photospheric markers of the truly massive star which is on the main sequence. The sample of objects used in this endeavor are massive young stars located in nascent clusters powering Galactic giant H II regions. Emission in the 2.3 μm 2–0 vibrational–rotational bandhead of CO is observed as are the ionized lines of Hydrogen, Helium, and other abundant elements. High spectral resolution is key in giving geometrical clues to the circumstellar emission sources, both through individual line profiles (i.e. shapes) and positions.

Searching for objects in the earliest phases of star formation, e.g sources at the beginning of a gravitational collapse, are essential to our understanding of massive star formation. Today a number of precursors of ultra compact HII regions (PUCHs) have been found. Embedded in dense gas and dust, these PUCHs have a high bolometric luminosity but little or no 6 cm radio continuum emission (Molinari, et al. 2000; Beuther, et al. 2002). Evidence for Collapse was found in ultra compact (UC) HII regions and 12 water maser sources (Zhang, et al. 1998; Wu & Evans II 2003). This paper presents the identification of massive cores with no detectable infrared and radio sources. These kinds of cores usually have strong sub-mm emission. A special case is the SCUBA core JCMT 18354-0649S which has both infall and outflow motions as indicated by the profiles of high excitation molecular lines. This core is at a stage earlier than PUCHs. Blue profiles are also found in UC HII region, which indicates that material is still infalling in this phase. Our observations suggest that infall exists in different evolutionary stages for high mass star formation, similar to the low mass cases.

Stars with masses of $\gtsim 20$$M_{odot}$ have short Kelvin times that enable them to reach the main sequence while still accreting from their natal clouds. The resulting nuclear burning produces a huge luminosity and a correspondingly large radiation pressure force on dust grains in the accreting gas. This effect may limit the upper mass of stars that can form by accretion. Indeed, simulations and analytic calculations to date have been unable to resolve the mystery of how stars of 50 $M_{odot}$ and up form. We present two new ideas to solve the radiation pressure problem. First, we use three-dimensional radiation hydrodynamic adaptive mesh refinement simulations to study the collapse of massive cores. We find that in three dimensions a configuration in which radiation holds up an infalling envelope is Rayleigh-Taylor unstable, leading radiation driven bubbles to collapse and accretion to continue. We also present Monte Carlo radiative transfer calculations showing that the cavities created by protostellar winds provides a valve that allow radiation to escape the accreting envelope, further reducing the ability of radiation pressure to inhibit accretion.

Molecular outflows in the form of wide-angle winds and/or well-collimated jets are associated with young stellar objects of all luminosities. Independent studies have established that the mass outflow rate is proportional to L$_{bol}^{0.6}$ for L$_{bol} = 0.3$ to $10^5$ L$_{\odot}$, suggesting that there is a strong link between accretion and outflow for a wide range of source luminosity and there is reasonable evidence that accretion-related processes are responsible for generating massive molecular flows from protostars up to spectral type B0. Beyond L$_{bol} \sim 10^4$ L$_{\odot}$, O stars generate powerful wide-angle, ionized winds that can dramatically affect outflow morphology and even call into question the relationship between outflow and accretion.

Recently Beuther & Shepherd 2005 have proposed an evolutionary scenario in which massive protostellar flows (up to early B spectral type) begin collimated. Once the star reaches the Main Sequence, ionizing radiation may affect the balance between magnetic and plasma pressure, inducing changes in the flow morphology and energetics. Here I review the properties of outflows from young OB stars, discuss implications and observational tests of this proposed evolutionary scenario, and examine differences between low-mass and massive star formation.

I review selected results obtained using Chandra X-ray observations of high mass star forming regions. Discussed topics include diffuse X-ray emission; the mechanism of X-ray emission in stars from the highest to the lowest masses; the role of imaging X-ray observations in the identification of star forming region members and the implications for star formation studies. Special attention will be given to results recently obtained for the Orion Nebula Cluster by the Chandra Orion Ultradeep Project (COUP).

I report X-ray features of young stage of intermediate and high mass stars. The giant molecular cloud Sagittarius B2 (Sgr B2) exhibits more than dozen X-ray sources, two are associated with the ultra compact (UC) HII complex, Sgr B2 Main. The sources show large absorption of $\gg$1023 Hcm−2, the largest among any known stellar X-ray sources.

The Arches cluster exhibits 3 extremely bright (a few $\times10^{33}$ ergs s−1) X-ray sources associated with the infrared (IR) massive stars. The X-ray spectra have 1-3 keV temperature in thin thermal model. Elongated diffuse 6.4 keV line emission is found.

The Monoceros R2 cloud exhibits half dozen X-ray sources associated with young high-mass IR stars. They show rapid time variability and a thin thermal spectrum of ${\sim }2$ keV temperature. Among 28 ASC A pointing on intermediate-mass pre-main-sequence stars, or Herbig Ae/Be stars (HAeBes), eleven are found to be plausible X-ray sources. The general X-ray properties of these HAeBes are; (1) the plasma temperature and time variability are higher than those of high mass main-sequence stars, and more similar to low mass stars; (2) the X-ray luminosities come to the upper end of low mass pre-main sequence stars, or in some cases exceed that; (3) the X-ray activity of HAeBe decreases at the age of about a few×106 years.

Using above observational facts, I propose a unified picture of X-ray activity of young stars in the wide mass range.

I review the formation of massive stars in the context of a forming stellar cluster. High-mass stars form in the centre of stellar clusters and thus must be understood in the context of low-mass star formation. Furthermore, they are predominantly in binary systems making further constraints on the formation mechanism. The fragmentation of a turbulent molecular cloud produces a large number of stars with initial masses close to the Jeans mass of the cloud. These stars fall together to form small-N clusters that grow through the infall of gas and stars into the cluster's potential well. Competitive accretion in clusters produces high-mass stars in the cluster centre and a full initial mass function of lower-mass stars. Massive star formation is a process that occurs in the cores of stellar clusters and commonly produces close binary systems. Accretion also forces the cluster to contract, increasing the stellar densities to the point where stellar collisions may occur. Furthermore, accretion in clusters reproduces the high binary frequency of massive stars. Systems evolve from low-mass wide binaries to high-mass close binaries due to gas accretion. This evolution can produce very tight binaries that are expected to merge to form the most massive stars. Binary mergers require stellar densities of order 106 stars pc−3, 100 times smaller than is required for single-star collisions.

Massive protostars are generally enshrouded in dust, so that most of their radiation emerges in the far infrared. For protostars embedded in opaque, spherical cores, the spectral energy distribution (SED) is determined by two distance-independent parameters, the luminosity-to-mass ratio, $L/M_c$, and the surface density of the core, $\Sigma=M_c/(\pi R_c^2)$, where $R_c$ is the radius of the core. Chakrabarti & McKee (2005a) have derived an approximate analytic expression for the SED that agrees well with numerical results. It is generally not possible to infer the power-law of the density from the SED of a massive protostar. Masses and accretion rates are inferred for several well-studied sources.

Any predictive theory of star formation must explain observed variations (or lack thereof) in the initial mass function. Recent work suggests that we might expect quantitative variations in the IMF as a function of metallicity (Larson 2005) or magnetic field strength (Shu et al. 2004). We summarize results from several on-going studies attempting to constrain the ratio of high to low mass stars, as well as stars to sub- stellar objects, in a variety of different environments, all containing high mass stars.

First, we examine the ratio of stars to sub–stellar objects in the nearby Mon R2 region utilizing NICMOS/HST data. We compare our results to the IMF by Kroupa (2002) and to the observed ratios for IC 348 and Orion. Second, we present preliminary results for the ratio of high to low mass stars in W51, the most luminous HII region in the galaxy. Based on ground–based multi–colour images of the cluster obtained with the MMT adaptive optics system, we derive a lower limit to the ratio of high-mass to low-mass stars and compare it to the ratios for nearby clusters. Finally, we present the derived IMF for the R136 region in the LMC where the metallicity is 1/4 solar using HST/NICMOS data. We find that the IMF is consistent with that characterizing the field (Chabrier 2003), as well as nearby star–forming regions, down to 1.0 M$_\odot$ outside 2 pc. Whereas the results for both Mon R2 and R136 are consistent with the nearby clusters, the ratio of high to low mass stars in W51 tentatively indicates a lack of low–mass objects.

Sub-Arcsec images in the JHK$_s$, H$_2$ and Br$_\gamma$ of three areas (I(N), F and NGC 6334 IV (MM3)) of the giant molecular cloud NGC 6334 are presented. The preliminary results indicate the presence of a deeply embedded young stellar cluster in the northermost part of the cloud (I(N)). We have identified the exciting source of the cometary UCHII NGC 6334 F. This source has an infrared luminosity L$_{\rm IRacute;{=} $3 10$^3$ L$_{\odot}$ and a very steep infrared spectral index. Finally, a new center of massive star formation associated with the millimeter peak MM3 has been found east of the bipolar HII region NGC 6334 a.

The Chandra X-ray Observatory is providing remarkable new views of massive star-forming regions, revealing all stages in the life cycle of high-mass stars and their effects on their surroundings. We present a Chandra tour of several high-mass star-forming regions, highlighting physical processes that characterize the life of a cluster of high-mass stars, from deeply-embedded cores too young to have established an HII region to superbubbles so large that they shape our views of galaxies. Along the way we see that X-ray observations reveal hundreds of stellar sources powering great HII region complexes, suffused by both hard and soft diffuse X-ray structures caused by fast O-star winds thermalized in wind-wind collisions or by termination shocks against the surrounding media. Finally, we examine the effects of the deaths of high-mass stars that remained close to their birthplaces, exploding as supernovae within the superbubbles that these clusters created. We present new X-ray results on W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and the W3 HII region complexes W3 Main and W3(OH).

Star formation at very low metallicity is expected to produce only massive stars. This is a result of the low cooling rate. Hydrodynamical simulations of star formation from zero metallicity gas suggest that the first stars had masses in excess of about 102M[odot ]. These stars can not be observed because of their large redshift and their very short lifetime. However, a similar (but not the same) effect might be observable in regions of star formation close to very luminous radiation sources, where intense radiation may destroy dust and CO molecules in starforming clouds. Such clouds will be warmer than in normal metal rich star forming regions because of the reduced cooling, which is then predominantly due to H2 and atomic C and O. Under those conditions star formation may result in the formation of high mass stars without the normal large numbers of accompanying low mass stars. Observations show that this process may occur close to the centre of the interacting galaxy M51, where the intense radiation of the nuclear starburst and the small dust content destroyed molecular CO in star forming regions. HST observations of this region show the presence of about 30 massive stars, of $25\textlessM\textless 150 M\odot$, without the accompanying clusters of low mass stars.

The Spitzer Space Telescope has delivered impressive infrared images of numerous low-mass star forming regions at unprecedented sensitivity. In this paper we focus on their high-mass counterparts, the most massive HII regions in the Local Group: NGC3603 (MW), W49A (MW), 30 Doradus (LMC), NGC346 (SMC), and NGC604 (M33). The deep mid-IR images, taken with IRAC show the complex structure of the ISM, the interplay between ISM and stellar clusters, and reveal new sites of star formation. We compare the IRAC data with observations from Chandra and find good anti-correlations between the mid-IR and soft X-ray emission regions. We also investigate the commonalities and differences between the giant HII regions NGC604 and 30 Doradus.

I review some important questions in the field of massive star formation: What are the initial conditions for proto star clusters and how do they arise? What are the initial conditions for individual massive star formation within star clusters? How do massive protostars accumulate their mass? I compare the Turbulent Core Model (McKee & Tan 2003) to several nearby regions, including Orion KL. Here I also discuss the origin of BN's high proper motion.