W3 IRS Research Articles

Water in massive star-forming regions: HIFI observations of W3 IRS5

We present Herschel observations of the water molecule in the massive star-forming region W3 IRS5. The o-H17O 110-101, p-H18O 111-000, p-H2O 22 202-111, p-H2O 111-000, o-H2O 221-212, and o-H2O 212-101 lines, covering a frequency range from 552 up to 1669 GHz, have been detected at high spectral resolution with HIFI. The water lines in W3 IRS5 show well-defined high-velocity wings that indicate a clear contribution by outflows. Moreover, the systematically blue-shifted absorption in the H2O lines suggests expansion, presumably driven by the outflow. No infall signatures are detected. The p-H2O 111-000 and o-H2O 212-101 lines show absorption from the cold material (T ~ 10 K) in which the high-mass protostellar envelope is embedded. One-dimensional radiative transfer models are used to estimate water abundances and to further study the kinematics of the region. We show that the emission in the rare isotopologues comes directly from the inner parts of the envelope (T > 100 K) where water ices in the dust mantles evaporate and the gas-phase abundance increases. The resulting jump in the water abundance (with a constant inner abundance of 10^{-4}) is needed to reproduce the o-H17O 110-101 and p-H18O 111-000 spectra in our models. We estimate water abundances of 10^{-8} to 10^{-9} in the outer parts of the envelope (T < 100 K). The possibility of two protostellar objects contributing to the emission is discussed.

A RING/DISK/OUTFLOW SYSTEM ASSOCIATED WITH W51 NORTH: A VERY MASSIVE STAR IN THE MAKING

Sensitive and high angular resolution ($\sim$ 0.4\arcsec) SO$_2$[22$_{2,20}$ $\to$ 22$_{1,21}$] and SiO[5$\to$4] line and 1.3 and 7 mm continuum observations made with the Submillimeter Array (SMA) and the Very Large Array (VLA) towards the young massive cluster W51 IRS2 are presented. We report the presence of a large (of about 3000 AU) and massive (40 M$_\odot$) dusty circumstellar disk and a hot gas molecular ring around a high-mass protostar or a compact small stellar system associated with W51 North. The simultaneous observations of the silicon monoxide molecule, an outflow gas tracer, further revealed a massive (200 M$_\odot$) and collimated ($\sim14^\circ$) outflow nearly perpendicular to the dusty and molecular structures suggesting thus the presence of a single very massive protostar with a bolometric luminosity of more than 10$^5$ L$_\odot$. A molecular hybrid LTE model of a Keplerian and infalling ring with an inner cavity and a central stellar mass of more than 60 M$_\odot$ agrees well with the SO$_2$[22$_{2,20}$ $\to$ 22$_{1,21}$] line observations. Finally, these results suggest that mechanisms, such as mergers of low- and intermediate- mass stars, might be not necessary for forming very massive stars.

YOUNG BROWN DWARFS IN THE CORE OF THE W3 MAIN STAR-FORMING REGION

We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR observations with the Subaru telescope, to search for very low mass young stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 region and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color (CC) and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low luminosity YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. Based on the comparison between theoretical evolutionary models of very low-mass PMS objects with the observed CM diagram, we find there exists a substantial substellar population in the observed region. The mass function (MF) does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the MF slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.

Millimeter interferometry of W3 IRS5: a trapezium in the making

Context. Although most young massive stars appear to be part of multiple systems, it is poorly understood how this multiplicity influences the formation of massive stars. The high-mass star-forming region W3 IRS5 is a prime example of a young massive cluster where the cluster center is resolved into multiple subsources at cm and infrared wavelengths, a potential proto-Trapezium system.

High Spatial Resolution Spectroscopy of W51 IRS 2E and IRS 2W: Two Very Massive Young Stars in Early Formation Stages

We present K-band spectra of the near infrared counterparts to IRS 2E and IRS 2W which is associated with the ultracompact H II region W51d, both of them embedded sources in the Galactic compact H II region W51 IRS 2. The high spatial resolution observations were obtained with the laser guide star facility and Near-infrared Integral Field Spectrograph (NIFS) mounted at the Gemini-North observatory. The spectrum of the ionizing source of W51d shows the photospheric features N III (21155 A) in emission and He II (21897 A) in absorption which lead us to classify it as a young O3 type star. We detected CO overtone in emission at 23000 A in the spectrum of IRS 2E, suggesting that it is a massive young object still surrounded by an accretion disk, probably transitioning from the hot core phase to an ultracompact H II region.

Forming an early O-type star through gas accretion?

We present high angular resolution (∼3) and sensitive 1.3 mm continuum, cyanogen (CN) and vinyl cyanide (C 2 H 3 CN) line observations made with the Submillimeter Array (SMA) toward one of most highly obscured objects of the W51 IRS2 region, W51 North. We find that the CN line exhibits a pronounced inverse P-Cygni profile indicating that the molecular gas is falling into this object with a mass accretion rate between 4 and 7 x 10 -2 M ⊙ yr -1 . The C 2 H 3 CN traces an east-west rotating molecular envelope that surrounds either a single obscured (proto)star with a kinematic mass of 40 M ⊙ or a small central cluster of B-type stars and that is associated with a compact high velocity bipolar outflow traced by H 2 O masers and SiO molecular emission. We thus confirm that the W51 North region is part of the growing list of young massive star forming regions that have been associated with infalling motion and with high mass accretion rates (∼10 -2 -10 -4 M ⊙ yr -1 ), strengthening the evidence that massive stars can form with very high accretion rates sufficient to quench the formation of a UCHII region.

W51 IRS 2: A Massive Jet Emerging from a Molecular Cloud into an H ii Region

We have mapped [Ne II] (12.8um) and [S IV] (10.5um) emission from W51 IRS 2 with TEXES on Gemini North, and we compare these data to VLA free-free observations and VLT near-infrared images. With 0.5" spatial and 4 km/s spectral resolution we are able to separate the ionized gas into several components: an extended H II region on the front surface of the molecular cloud, several embedded compact H II regions, and a streamer of high velocity gas. We interpret the high velocity streamer as a precessing or fan-like jet, which has emerged from the molecular cloud into an OB star cluster where it is being ionized.

A Molecular Line Survey of W3(OH) and W3 IRS 5 from 84.7 to 115.6 GHz: Observational Data and Analyses

We have carried out observations toward the W3 complex and G34.3+0.15 using the TRAO 14 m radio telescope to examine in detail the chemical variations occurring while molecular clouds evolve from the prestellar to the H II region phase. Observations include spectral surveys of these objects between 84.7 and 115.6 GHz; mapping observations toward W3(OH) with the emissions of CS (2-1), HCN (1-0), HNC (1-0), and HCO+ (1-0); and mapping of CS (2-1) emission toward W3 IRS 5. Chemical model calculations are used to estimate the age of W3(OH) by comparing with the fractional abundances of detected molecules. We found that G34.3+0.15 and W3(OH) are at a similar evolutionary stage, although large differences in the fractional abundances are found in CH3CN and HC3N. Overall, the properties of the detected species and abundances in three regions support the view that chemistry varies as molecular clouds evolve from a cold, collapsing phase to a high-temperature phase, such as the hot core and H II phase. Chemical model calculations for W3(OH) indicate that the evolutionary age of the cloud is 104-105 yr with temperature in the range 10-60 K.

Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster

Observations at mid-infrared (4.8-17.65 μm) and radio (0.7-1.3 cm) wavelengths are used to constrain the structure of the high-mass star-forming region W3 IRS5 on 0.1 (200 AU) scales. Two bright mid-infrared sources are detected, as well as diffuse emission. The bright sources have associated compact radio emission and probably are young high-mass stars. The measured sizes and estimated temperatures indicate that these sources together can supply the observed far-infrared luminosity. However, an optically thick radio source with a possible mid-infrared counterpart may also contribute significant luminosity; if so, it must be extremely deeply embedded. The infrared colour temperatures of 350-390 K and low radio brightness suggest gravitational confinement of the H II regions and ongoing accretion at a rate of a few 10 -8 M ○. yr -1 or more. Variations in the accretion rate would explain the observed radio variability. The low estimated foreground extinction suggests the existence of a cavity around the central stars, perhaps blown by stellar winds. At least three radio sources without mid-infrared counterparts appear to show proper motions of ∼100 km s -1 , and may be deeply embedded young runaway OB stars, but more likely are clumps in the ambient material which are shock-ionized by the OB star winds.

The Kinematics, Physical Condition and Magnetic Field of the W3 Irs5 Region

The Kinematics, Physical Condition and Magnetic Field of the W3 Irs5 Region

The Methanol Maser Emission in W51

The European VLBI Network (EVN) has been used to make phase referenced, wide-field (several arcminute) spectral line observations of the 6.7-GHz methanol maser emission towards W51. In the W51main region, the bulk of the methanol is offset from an UCHII region. This probably indicates the methanol emission arises at the interface of the expanding UCHII region and not from an edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region, the methanol emission is associated with a very compact, extremely embedded source supporting the hypothesis that methanol masers trace the earliest stages of massive star formation. As well as these two previously well studied sites of star formation, many previously unknown regions star formation are detected, demonstrating that methanol masers are powerful means of detection young massive stars.

Linear Polarization Observations of Water Masers in W3 IRS 5

We present a magnetic field mapping of water maser clouds in the star-forming region W3 IRS 5, which has been made on the basis of the linear polarization VLBI observation. Using the Very Long Baseline Array at 22.2 GHz, 16 of 61 detected water masers were found to be linearly polarized with polarization degrees up to 13%. Although 10 polarized features were widely distributed in the whole W3 IRS 5 water maser region, they had position angles similar to the magnetic field vectors (~75° east from the north). The magnetic field vectors are roughly perpendicular to the spatial alignments of the maser features. They are consistent with the hourglass model of the magnetic field, which was previously proposed to explain the magnetic field in the whole W3 Main region (r ~ 0.1 pc). They are, on the other hand, not aligned to the directions of maser feature proper motions observed previously. This implies that the W3 IRS 5 magnetic field was controlled by a collapse of the W3 Main molecular cloud rather than by the outflow originated from W3 IRS 5.

Submillimeter emission from water in the W3 region

Using the Odin satellite, we have mapped the submillimeter emission from the transition of ortho-water in the W3 star-forming region. A map of the W3 IRS4 and W3 IRS5 region reveals strong water lines at half the positions in the map. The relative strength of the Odin lines compared to previous observations by SWAS suggests that we are seeing water emission from an extended region. Across much of the map the lines are double-peaked, with an absorption feature at –39 km s-1; however, some positions in the map show a single strong line at –43 km s-1. We interpret the double-peaked lines as arising from optically thick, self-absorbed water emission near the W3 IRS5, while the narrower blue-shifted lines originate in emission near W3 IRS4. In this model, the unusual appearance of the spectral lines across the map results from a coincidental agreement in velocity between the emission near W3 IRS4 and the blue peak of the more complex lines near W3 IRS5. The strength of the water lines near W3 IRS4 suggests we may be seeing water emission enhanced in a photon-dominated region.

J-type shock chemistry for the W3 IRS5 region

A chemical evolution of W3 IRS5 have been studied during J-type shock. We modified our code of for one-dimensional J-type shock to satisfy the physical conditions of W3 IRS5. The chemical rate equations and the hydrodynamics of shocks are integrated simultaneously. We find that, a shock of speed Vs = 10 km s-1 and magnetic induction B=4.1×10-6 Gauss, propagate into molecular gas density of n=104cm-3, and with initial temperature of 30 K (pre shock) satisfy the physical conditions of W3 IRS5. The results using the extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation.

Magnetic Fields in Shocked Regions: Very Large Array Observations of H2O Masers

We present VLA observations of the Zeeman effect in 22 GHz H2O masers in several high-mass star-forming regions. These masers are believed to arise from collisional pumping in postshock environments. Therefore, the Zeeman effect data provide the most direct measurements of magnetic field strengths in high-density (n 108 cm-3) postshock gas, where the field energy determines other physical conditions. Our observations yield significant magnetic field detections in W3 IRS 5, W3(OH), W49 N, and OH 43.8-0.1. In these sources, we detect line-of-sight field strengths ranging from 13 to 49 mG. For some regions, the detected fields provide a 2-3 point sampling of the magnetic field, indicating the nature of field variations on arcsecond scales. These field strengths are consistent with a shock-driven maser model having relatively low speed (20 km s-1), C-type shocks. We examine the balance between magnetic field energy and turbulent kinetic energy in the masing regions. These energies appear close to equilibrium.

Microstructure of Water Masers in W3 IRS 5

We report sub-milliarcsecond structures of water maser features in W3 IRS 5 observed with the Very Long Baseline Array (VLBA) at 22.2 GHz. A cluster of maser spots (emission spots in individual velocity channels) within a maser feature exhibits a simple velocity gradient but a complicated spatial alignment. Two-point spatial correlation functions for the spots can be fitted by power laws with an index of about 2 in both of the two ranges of 0.04-0.4 and 1-300 AU. This fact indicates a similar clustering of spots in the two very different ranges. The Doppler-velocity difference of the spots as a function of spot separation demonstrates a Kolmogorov-type turbulence in the range of 0.04-300 AU. Similar power laws in these functions have been reported for the W49N water masers. A unique difference of the W3 IRS 5 from the W49N masers is that on a scale above 1 AU, the Doppler-velocity difference of the spots in W3 IRS 5 (≤5 km s-1) is much smaller than that in W49N (≤150 km s-1). Very likely, this difference between the two sources arises because of different outflow velocities.

Dust Characteristics of Massive Star‐forming Sites in the Mid‐Infrared

Four massive star-forming regions were imaged in the mid-infrared with the MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtained high spatial resolution (~1'') images at several wavelengths from 7.8 to 13.2 μm with the circular variable filter, as well as narrow-band continuum images at 12.5 and 20.6 μm toward each region. In each massive star-forming region, one or more sources show deep silicate absorption. For at least two of the massive star-forming regions, W51 IRS 2 and Mon R2, the absorbing material is highly localized and may be circumstellar material in disks or shells. The silicate absorption occurs at least as often around massive young stars as around young stars of lower mass (which are more often observed). The estimated optical depths of the silicate features are consistent with those predicted by radiative transfer models toward ultracompact H II regions, but substantially higher than observed toward T Tauri stars and other low-mass young stellar objects. There is no consistent correspondence between silicate absorption and either the dust color temperature or the 12.5 μm opacity. In W51 IRS 2, the two previously known mid-infrared sources have been resolved into at least six subsources. Infrared counterparts are newly reported for two radio-continuum sources in S140. Also, new mid-infrared sources have been detected in both W51 IRS 2 and S140. We suggest that the infrared source in the southwest of DR 21 may not be self-luminous, but may instead be heated by the three nearby radio continuum sources. The gas density in the ring at Mon R2 supports the blister scenario for the IRS 1 H II region.

Very Long Baseline Array Observations of the Zeeman Effect in H[TINF]2[/TINF]O Masers in W3 IRS 5

We present Very Long Baseline Array (VLBA) observations of the Zeeman effect in H2O masers in W3 IRS 5. These observations have yielded the first ultrahigh spatial resolution maps of the magnetic field in a star-forming region measured via the Zeeman effect—masers for which we can independently measure the magnetic field are separated by ≤25 mas (60 AU). This is the first VLBA detection of the Zeeman effect in H2O masers in a star-forming region. These observations also remove the confusion in observed magnetic fields from closely spaced masers. We shall explicitly demonstrate that observations at lower resolution can yield lower magnetic fields than with the VLBA, even though there are no field reversals within the larger beam.

Chemical models for the contracting W3 IRS4

We have studied the chemical evolution in the central core of a contracting cloud representing W3 IRS4. We modified the equation of temperature to satisfy the physical conditions of W3 IRS4. The chemical rate equations and the hydrodynamics are integrated simultaneously. The contraction is followed from a very low density of n = 10 cm −3 to high core density of n = 10 6 cm −3. The chemical evolution is studied for various chemical conditions, including both the effects of varying cosmic ray ionization rate and the effect of ion–dipole molecule collisions. The main results by using the more extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation at intermediate and enhanced cosmic ray ionization rate at high depletion of elemental atoms.

The Initial Mass Function of a Massive Star‐forming Region W51

Near-infrared observations of the central part of W51 have been carried out. We obtained J, H, and K' broadband and Brγ narrowband imaging data over an area of 15 arcmin2, centered near W51 IRS 2, which covers the whole of an H II region complex G49.5-0.4. K-band spectroscopic observations were also carried out for selected objects in this area. More than 7000 point sources were detected from our imaging data, and about 160 sources were identified as physically associated with G49.5-0.4. We estimated absolute J magnitude for each source from our photometric data and inferred the ages of H II regions from the sizes of nebulosities on our Brγ image. Coupling information of the ages and the luminosities of exciting stars, we estimated their initial masses and the derived initial mass function (IMF) of massive stars in G49.5-0.4. In the mass range between 10 and 30 M☉, the IMF slope is consistent with the value 1.8, which is derived for field stars. However, there is an excess of stellar population in the range above 30 M☉; i.e., a top heavy-shaped IMF was derived. We defined four subgroups which are aligned parallel to the Galactic plane. The age of the oldest group was estimated to be 2.3 Myr and that of the youngest group to be younger than 1 Myr. The youngest group contains 22 O stars, and it seems that 17 out of 22 O stars were born in the last 0.8 Myr. The massive star formation activity of this group is more violent than any other visible Galactic H II region.