H. Beuther

High-Mass Protostellar Candidates. II. Density Structure from Dust Continuum and CS Emission

We present a detailed 1.2 mm continuum and CS spectral line study of a large sample of 69 massive star forming regions in very early stages of evolution, most of them prior to building up an ultracompact H II region. The continuum data show a zoo of different morphologies and give detailed information on the spatial distributions, masses, column densities, and average densities of the whole sample. Fitting the radial intensity profiles shows that three parameters are needed to describe the spatial distribution of the sources: constant emission from the center out to a few arcseconds radius followed by a first power-law intensity distribution, which steepens farther outside into a second power-law distribution. The inner flat region is possibly caused by fragmentation of the large-scale cores into smaller subsources, whereas the steeper outer power-law distributions indicate finite sizes of the cores. Separating the sources into subsamples suggests that in the earliest stages prior to the onset of massive star formation, the intensity radial distributions are rather flat, resembling the structure of intensity peaks in more quiescent molecular clouds. Then in the subsequent collapse and accretion phase the intensity distributions become centrally peaked, with steep power-law indices. In this evolutionary stage the sources show also the broadest C34S line width. During the following phase, when ultracompact H II regions evolve, the intensity power-law radial distributions flatten out again. This is probably caused by the ignited massive stars in the center which disrupt the surrounding cores. The mean inner power-law intensity index mi (I ~ r) is 1.2, corresponding to density indices p (n ~ r-p) of 1.6. In total, the density distributions of our massive star formation sites seem to be not too different from their low-mass counterparts, but we show that setting tight constrains on the density indices is very difficult and subject to many possible errors. The local densities we derive from CS calculations are higher (up to 1 order of magnitude) than the mean densities we find via the millimeter continuum. Such inhomogeneous density distribution reflects most likely the ubiquitous phenomenon of clumping and fragmentation in molecular clouds. Line width-mass relations show a departure from virial equilibrium in the stages of strongly collapsing cores.

ATLASGAL - The APEX telescope large area survey of the galaxy at 870 μm

Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explore s the southern sky at submillimeter wavelengths, both in continuum and in spectral line emission. Studying continuum emission from interstellar dust is essential to locate the highest densit y regions in the interstellar medium, and to derive their masses, column densities, density structures, and larger scale morpholog ies. In particular, the early stages of (massive) star forma tion are still quite mysterious: only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characteriz ed sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submillimeter wavelengths also represents a pioneering work in preparation for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 µm, with a beam of 19. ′′ 2. Taking advantage of its large field of view (11. ′ 4) and excellent sensitivity, we have started an unbiased survey of the whole Galactic Plane accessible to APEX, with a typical noise level of 50‐70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we have covered ∼95 deg 2 of the Galactic Plane. These data reveal∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the c ompact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact Hii regions or young embedded clusters, thus tracing more evolved stages after star formation has occurred. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M⊙. In this first introductory paper, we show preliminary resul ts from these ongoing observations, and discuss the mid- and long-term perspectives of the survey.

ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns

Context. Thanks to its excellent 5100 m high site in Chajnantor, the Atacama Pathfinder Experiment (APEX) systematically explore s the southern sky at submillimeter wavelengths, both in continuum and in spectral line emission. Studying continuum emission from interstellar dust is essential to locate the highest densit y regions in the interstellar medium, and to derive their masses, column densities, density structures, and larger scale morpholog ies. In particular, the early stages of (massive) star forma tion are still quite mysterious: only small samples of high-mass proto-stellar or young stellar objects have been studied in detail so far. Aims. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. Only a systematic survey of the Galactic Plane can provide the statistical basis for unbiased studies. A well characteriz ed sample of Galactic star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submillimeter wavelengths also represents a pioneering work in preparation for Herschel and ALMA. Methods. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 µm, with a beam of 19. ′′ 2. Taking advantage of its large field of view (11. ′ 4) and excellent sensitivity, we have started an unbiased survey of the whole Galactic Plane accessible to APEX, with a typical noise level of 50‐70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Results. As a first step, we have covered ∼95 deg 2 of the Galactic Plane. These data reveal∼6000 compact sources brighter than 0.25 Jy, or 63 sources per square degree, as well as extended structures, many of them filamentary. About two thirds of the c ompact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact Hii regions or young embedded clusters, thus tracing more evolved stages after star formation has occurred. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M⊙. In this first introductory paper, we show preliminary resul ts from these ongoing observations, and discuss the mid- and long-term perspectives of the survey.

High-Mass Protostellar Candidates. I. The Sample and Initial Results

We describe a systematic program aimed at identifying and characterizing candidate high-mass protostellar objects (HMPOs). Our candidate sample consists of 69 objects selected by criteria based on those established by Ramesh & Sridharan using far-infrared, radio continuum, and molecular line data. IRAS and Midcourse Space Experiment data were used to study the larger scale environments of the candidate sources and determine their total luminosities and dust temperatures. To derive the physical and chemical properties of our target regions, we observed continuum and spectral line radiation at millimeter and radio wavelengths. We imaged the free-free and dust continuum emission at wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H2O and CH3OH maser emission, and observed the CO J = 2 → 1 line and several NH3 lines toward all sources in our sample. Other molecular tracers were observed in a subsample. While dust continuum emission was detected in all sources, most of them show only weak or no emission at 3.6 cm. Where detected, the centimeter emission is frequently found to be offset from the millimeter emission, indicating that the free-free and dust emissions arise from different subsources possibly belonging to the same (proto)cluster. A comparison of the luminosities derived from the centimeter emission with bolometric luminosities calculated from the IRAS far-infrared fluxes shows that the centimeter emission very likely traces the most massive source, whereas the whole cluster contributes to the far-infrared luminosity. Estimates of the accretion luminosity indicate that a significant fraction of the bolometric luminosity is still due to accretion processes. The earliest stages of HMPO evolution we seek to identify are represented by dust cores without radio emission. Line wings due to outflow activity are nearly omnipresent in the CO observations, and the molecular line data indicate the presence of hot cores for several sources, where the abundances of various molecular species are elevated because of evaporation of icy grain mantles. Kinetic gas temperatures of 40 sources are derived from NH3 (1, 1) and (2, 2) data, and we compare the results with the dust temperatures obtained from the IRAS data. Comparing the amount of dust, and hence the gas, associated with the HMPOs and with ultracompact H II (UCH II) regions, we find that the two types of sources are clearly separated in mass-luminosity diagrams: for the same dust masses, the UCH II regions have higher bolometric luminosities than HMPOs. We suggest that this is an evolutionary trend, with the HMPOs being younger and reprocessing less (stellar) radiation in the IR than the more evolved UCH II regions. These results indicate that a substantial fraction of our sample harbors HMPOs in a pre-UCH II region phase, the earliest known stage in the high-mass star formation process.

Probing the evolution of molecular cloud structure: From quiescence to birth

Context. Probability distribution of densities is a fundamental measure of molecular cloud structure, containing information on how the material arranges itself in molecular clouds. Aims. We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloud complexes closer than d < 200 pc. For comparison, additional complexes at d ≈ 250−700 pc are included in the study. Methods. We derive near-infrared dust extinction maps for 23 molecular cloud complexes, using the nicest colour excess mapping technique and data from the 2MASS archive. The extinction maps are then used to examine the column density PDFs in the clouds. Results. The column density PDFs of most molecular clouds are well-fitted by log-normal functions at low column densities (0. 5m ag< AV < 3− 5m ag, or−0.5 < lnAV /AV < 1). But at higher column densities prominent power-law-like wings are common. In particular, we identify a trend among the PDFs: active star-forming clouds always have prominent non-log-normal wings. In contrast, clouds without active star formation resemble log-normals over the whole observed column density range or show only low excess of higher column densities. This trend is also reflected in the cumulative forms of the PDFs, showing that the fraction of high column density material is significantly larger in star-forming clouds. These observations agree with an evolutionary trend where turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by them quickly become dominated by gravity (and other mechanisms), which is in turn strongly reflected by the shape of the column density PDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onset of active star formation in the clouds.

Circumventing the Radiation Pressure Barrier in the Formation of Massive Stars via Disk Accretion

We present radiation hydrodynamic simulations of the collapse of massive pre-stellar cores. We treat frequency-dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, for the first time it is possible to simulate the whole accretion phase (up to the end of the accretion disk epoch) for a forming massive star and to perform a broad scan of the parameter space. Our simulation series evidently shows the necessity to incorporate the dust sublimation front to preserve the high shielding property of massive accretion disks. While confirming the upper mass limit of spherically symmetric accretion, our disk accretion models show a persistent high anisotropy of the corresponding thermal radiation field. This yields the growth of the highest-mass stars ever formed in multi-dimensional radiation hydrodynamic simulations, far beyond the upper mass limit of spherical accretion. Non-axially symmetric effects are not necessary to sustain accretion. The radiation pressure launches a stable bipolar outflow, which grows in angle with time, as presumed from observations. For an initial mass of the pre-stellar host core of 60, 120, 240, and 480 M ? the masses of the final stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2 M ?, respectively.

IRAS 05358+3543: Multiple outflows at the earliest stages of massive star formation

We present a high-angular-resolution molecular line and millimeter continuum study of the massive star formation site IRAS 05358+3543. Observations with the Plateau de Bure Interferometer in CO 1{0, SiO 2{1 and H 13 CO + 1{0 reveal at least three outflows which cannot be separated in single-dish data. Observations at millimeter and sub-millimeter wavelengths from the IRAM 30 m telescope and the CSO provide additional information on the region. The most remarkable feature is a highly collimated (collimation factor 10) and massive (>10 M) bipolar outflow of 1 pc length, which is part of a quadrupolar outflow system. The three observed molecular outflows forming the IRAS 05358+3543 outflow system resemble, in structure and collimation, those typical of low-mass star-forming regions. They might therefore, just like low-mass outflows, be explained by shock entrainment models of jets. We estimate a mass accretion rate of10 4 M/yr, sucient to overcome the radiative pressure of the central object and to build up a massive star, lending further support to the hypothesis that massive star formation occurs similarly to low-mass star formation, only with higher accretion rates and energetics. In the millimeter continuum, we nd three sources near the center of the quadrupolar outflow, each with a mass of 75{100 M. These cores are associated with a complex region of infrared reflection nebulosities and their embedded illuminating sources. The molecular line data show that SiO is found mostly in the outflows, whereas H 13 CO + traces core-like structures, though likely with varying relative abundances. Thermal CH3OH comprises both features and can be disentangled into a core-tracing component at the line center, and wing emission following the outflows. A CO line-ratio study (using data of the J = 1{0, 2{1 and 6{5 transitions) reveals local temperature gradients.

CH3OH and H2O masers in high-mass star-forming regions

We present a comparison of Class ii CH3OH (6.7 GHz) and H2O (22.2 GHz) masers at high spatial resolution in a sample of 29 massive star-forming regions. Absolute positions of both maser types are compared with mm dust continuum, cm continuum and mid-infrared sources. All maser features - regardless of the species - are associated with massive mm cores, but only 3 out of 18 CH3OH masers and 6 out of 22 H2O masers are associated with cm emission likely indicating the presence of a recently ignited massive star. These observations of a homogenous sample of massive, young star-forming regions confirm earlier results, obtained for each maser species separately, that both maser types are signposts of high-mass star formation in very early evolutionary stages. The data are consistent with models that explain CH3OH maser emission by radiative pumping in moderately hot cores, requiring the absence, or only weak, free-free cm continuum radiation due to recently ignited stars. Mid-infrared sources are associated with both maser types in approximately 60% of the observed fields. Thus, mid-infrared objects may power maser sites, but the detection of strong mid-infrared emission is not strictly necessary because it might be heavily extincted. A comparison of the spatial separations between the dierent observed quantities and other properties of the star-forming regions does not reveal any correlation. Our data suggest that CH3OH and H2O masers need a similar environment (dense and warm molecular gas), but that, due to the dierent excitation processes (radiative pumping for CH3OH and collisional pumping for H2O), no spatial correlations exist. Spatial associations are probably coincidences due to insucient angular resolution and projection eects. The kinematic structures we find in the dierent maser species show no recognizable pattern, and we cannot draw firm conclusions as to whether the features are produced in disks, outflows or expanding shock waves.

Three-dimensional simulation of massive star formation in the disk accretion scenario

The most massive stars can form via standard disk accretion—despite the radiation pressure generated—due to the fact that the massive accretion disk yields a strong anisotropy in the radiation field, releasing most of the radiation pressure perpendicular to the disk accretion flow. Here, we analyze the self-gravity of the forming circumstellar disk as the potential major driver of the angular momentum transport in the massive disks responsible for the high accretion rates needed for the formation of massive stars. For this purpose, we perform self-gravity radiation hydrodynamic simulations of the collapse of massive pre-stellar cores. The formation and evolution of the resulting circumstellar disk is investigated in (1) axially symmetric simulations using an α-shear-viscosity prescription and (2) a three-dimensional simulation in which the angular momentum transport is provided self-consistently by developing gravitational torques in the self-gravitating accretion disk. The simulation series of different strengths of the α viscosity shows that the accretion history of the forming star is mostly independent of the α-viscosity parameter. The accretion history of the three-dimensional run driven by self-gravity is more time dependent than the viscous disk evolution in axial symmetry. The mean accretion rate, i.e., the stellar mass growth rate, is nearly identical to the α-viscosity models. We conclude that the development of gravitational torques in self-gravitating disks around forming massive stars provides a self-consistent mechanism to efficiently transport angular momentum to outer disk radii. The formation of the most massive stars can therefore be understood in the standard accretion disk scenario.

Millimeter multiplicity in NGC 6334 I and I(N)

Using the Submillimeter Array (SMA), we have imaged the 1.3 mm continuum emission at the centers of the massive star-forming regions NGC 6334 I and I(N). In both regions, the SMA observations resolvethe emission into multiple millimeter sources, with most of the sources clustered into areas only 10,000 AU in diameter. Toward NGC 6334I, wefindfourcompactsources:thetwo brightest (I-SMA1andI-SMA2)areassociatedwithpreviouslyknown ammonia cores; I-SMA3 coincides with the peak of the compact H ii region (NGC 6334 F), and I-SMA4 is a newly discovered object. While I-SMA3 exhibits a mixture of free-free and dust emission, the rest of the objects are dust cores.TowardNGC6334I(N),sevencompactdustcoresarefound,oneofwhichisassociatedwithafaintcentimeter source. With the exception of I-SMA3, none of the millimeter sources have infrared counterparts in Spitzer Space Telescope3‐8� mimages.Usingasimplephysicalmodelforthedustcontinuumemission,we estimatethatthemass of the interstellar material toward each of these compact objects is in the range of 3‐66 M� . The total mass in the compact objects appears to be similar in I and I(N). The small size of these groups of sources suggest that these objects are proto-Trapezia forming in the centers of clusters of low- to intermediate-mass stars.