Onset Of Massive Star Formation Research Articles

Filamentary Accretion Flows in the Infrared Dark Cloud G14.225–0.506 Revealed by ALMA

Abstract Filaments are ubiquitous structures in molecular clouds and play an important role in the mass assembly of stars. We present results of dynamical stability analyses for filaments in the infrared dark cloud G14.225−0.506, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. Full-synthesis imaging is performed with the Atacama Large Millimeter/submillimeter Array to map the emission in two hub-filament systems with a spatial resolution of ∼0.034 pc. Kinematics are derived from a sophisticated spectral fitting algorithm that accounts for line blending, large optical depth, and multiple velocity components. We identify five velocity coherent filaments and derive their velocity gradients with principal component analysis. The mass accretion rates along the filaments are up to and are significant enough to affect the hub dynamics within one freefall time (∼105 yr). The filaments are in equilibrium with virial parameter α vir ∼ 1.2. We compare α vir measured in the filaments, filaments, 870 μm dense clumps, and 3 mm dense cores. The decreasing trend in α vir with decreasing spatial scales persists, suggesting an increasingly important role of gravity at small scales. Meanwhile, α vir also decreases with decreasing nonthermal motions. In combination with the absence of high-mass protostars and massive cores, our results are consistent with the global hierarchical collapse scenario.

THE ONSET OF MASSIVE STAR FORMATION: THE EVOLUTION OF TEMPERATURE AND DENSITY STRUCTURE IN AN INFRARED DARK CLOUD

We present new NH3 (1,1), (2,2), and (4,4) observations from the Karl G. Jansky Very Large Array (VLA) compiled with work in the literature to explore the range of conditions observed in young, massive star-forming regions. To sample the effects of evolution independent from those of distance/resolution, abundance, and large-scale environment, we compare clumps in different evolutionary stages within a single Infrared Dark Cloud (IRDC), G32.02+0.06. We find that the early stages of clustered star formation are characterized by dense, parsec-scale filamentary structures interspersed with complexes of dense cores (<0.1 pc cores clustered in complexes separated by ~1 pc) with masses from about 10 to 100 Msun. The most quiescent core is the most extended while the star-forming cores are denser and more compact, showing very similar column density structure before and shortly after the onset of massive star formation, with peak surface densities >~ 1 g cm^-2. Quiescent cores and filaments show smoothly varying temperatures from 10-20 K, rising to over 40 K in star-forming cores. We calculate the virial parameters for 16 cores and find that the level of support provided by turbulence is generally insufficient to support them against gravitational collapse (alpha_vir ~ 0.6). The star-forming filaments show smooth velocity fields, punctuated by discontinuities at the sites of active star formation. We discuss the Massive Molecular Filament (MMF; M > 10^5 Msun, l > 60 pc) hosting the IRDC, hypothesizing that it may have been shaped by previous generations of massive stars.

Initial phases of massive star formation in high infrared extinction clouds

The onset of massive star formation is not well understood because of observational and theoretical difficulties. To find the dense and cold clumps where massive star formation can take place, we compiled a sample of high infrared extinction clouds, which were observed previously by us in the 1.2 mm continuum emission and ammonia. We try to understand the star-formation stages of the clumps in these high extinction clouds by studying the infall and outflow properties, the presence of a young stellar object (YSO), and the level of the CO depletion through a molecular line survey with the IRAM 30m and APEX 12m telescopes. Moreover, we want to know if the cloud morphology, quantified through the column density contrast between the clump and the clouds, has an impact on the star formation occurring inside it. We find that the HCO+(1-0) line is the most sensitive for detecting infalling motions. SiO, an outflow tracer, was mostly detected toward sources with infall, indicating that infall is accompanied by collimated outflows. The presence of YSOs within a clump depends mostly on its column density; no signs of YSOs were found below 4E22 cm-2. Star formation is on the verge of beginning in clouds that have a low column density contrast; infall is not yet present in the majority of the clumps. The first signs of ongoing star formation are broadly observed in clouds where the column density contrast between the clump and the cloud is higher than two; most clumps show infall and outflow. Finally, the most evolved clumps are in clouds that have a column density contrast higher than three; almost all clumps have a YSO, and in many clumps, the infall has already halted. Hence, the cloud morphology, based on the column density contrast between the cloud and the clumps, seems to have a direct connection with the evolutionary stage of the objects forming inside.

GIANT MOLECULAR CLOUDS IN THE SPIRAL ARM OF IC 342

We present results of 12CO (1--0) and 13CO (1--0) observations of the northeastern spiral arm segment of IC 342 with a ~50pc resolution carried out with the Nobeyama Millimeter Array. Zero-spacing components were recovered by combining with the existing data taken with the Nobeyama 45m telescope. The objective of this study is to investigate the variation of cloud properties across the spiral arm with a resolution comparable to the size of giant molecular clouds (GMCs). The observations cover a 1 kpc times 1.5 kpc region located ~2 kpc away from the galactic center, where a giant molecular association is located at trailing side and associated star forming regions at leading side. The spiral arm segment was resolved into a number of clouds whose size, temperature and surface mass density are comparable to typical GMCs in the Galaxy. Twenty-six clouds were identified from the combined data cube and the identified clouds followed the line width-size relation of the Galactic GMCs. The identified GMCs were divided into two categories according to whether they are associated with star formation activity or not. Comparison between both categories indicated that the active GMCs are more massive, have smaller line width, and are closer to virial equilibrium compared to the quiescent GMCs. These variations of the GMC properties suggest that dissipation of excess kinetic energy of GMC is a required condition for the onset of massive star formation.

From high-mass starless cores to high-mass protostellar objects

Aims: Our aim is to understand the evolutionary sequence of high-mass star formation from the earliest evolutionary stage of high-mass starless cores, via high-mass cores with embedded low- to intermediate-mass objects, to finally high-mass protostellar objects. Methods: Herschel far-infrared PACS and SPIRE observations are combined with existing data at longer and shorter wavelengths to characterize the spectral and physical evolution of massive star-forming regions. Results: The new Herschel images spectacularly show the evolution of the youngest and cold high-mass star-forming regions from mid-infrared shadows on the Wien-side of the spectral energy distribution (SED), via structures almost lost in the background emission around 100mum, to strong emission sources at the Rayleigh-Jeans tail. Fits of the SEDs for four exemplary regions covering evolutionary stages from high-mass starless cores to high-mass protostellar objects reveal that the youngest regions can be fitted by single-component black-bodies with temperatures on the order of 17K. More evolved regions show mid-infrared excess emission from an additional warmer component, which however barely contributes to the total luminosities for the youngest regions. Exceptionally low values of the ratio between bolometric and submm luminosity additionally support the youth of the infrared-dark sources. Conclusions: The Herschel observations reveal the spectral and physical properties of young high-mass star-forming regions in detail. The data clearly outline the evolutionary sequence in the images and SEDs. Future work on larger samples as well as incorporating full radiative transfer calculations will characterize the physical nature at the onset of massive star formation in even more depth.

EVIDENCE FOR DELAYED MASSIVE STAR FORMATION IN THE M17 PROTO-OB ASSOCIATION

Through analysis of archival images and photometry from the Spitzer GLIMPSE and MIPSGAL surveys combined with 2MASS and MSX data, we have identified 488 candidate young stellar objects (YSOs) in the giant molecular cloud M17 SWex, which extends ~50 pc southwest from the prominent Galactic H II region M17. Our sample includes >200 YSOs with masses >3 Msun that will become B-type stars on the main sequence. Extrapolating over the stellar initial mass function (IMF), we find that M17 SWex contains >1.3x10^4 young stars, representing a proto-OB association. The YSO mass function is significantly steeper than the Salpeter IMF, and early O stars are conspicuously absent from M17 SWex. Assuming M17 SWex will form an OB association with a Salpeter IMF, these results reveal the combined effects of (1) more rapid circumstellar disk evolution in more massive YSOs and (2) delayed onset of massive star formation.

Rotational structure and outflow in the infrared dark cloud 18223-3

We examine an Infrared Dark Cloud (IRDC) at high spatial resolution as a means to study rotation, outflow, and infall at the onset of massive star formation. Submillimeter Array observations combined with IRAM 30 meter data in 12CO(2--1) reveal the outflow orientation in the IRDC 18223-3 region, and PdBI 3 mm observations confirm this orientation in other molecular species. The implication of the outflow's presence is that an accretion disk is feeding it, so using high density tracers such as C18O, N2H+, and CH3OH, we looked for indications of a velocity gradient perpendicular to the outflow direction. Surprisingly, this gradient turns out to be most apparent in CH3OH. The large size (28,000 AU) of the flattened rotating object detected indicates that this velocity gradient cannot be due solely to a disk, but rather from inward spiraling gas within which a Keplerian disk likely exists. From the outflow parameters, we derive properties of the source such as an outflow dynamical age of ~37,000 years, outflow mass of ~13 M_sun, and outflow energy of ~1.7 x 10^46 erg. While the outflow mass and energy are clearly consistent with a high-mass star forming region, the outflow dynamical age indicates a slightly more evolved evolutionary stage than previous spectral energy distribution (SED) modeling indicates. The calculated outflow properties reveal that this is truly a massive star in the making. We also present a model of the observed methanol velocity gradient. The rotational signatures can be modeled via rotationally infalling gas. These data present evidence for one of the youngest known outflow/infall/disk systems in massive star formation. A tentative evolutionary picture for massive disks is discussed.

Ethynyl (C 2 H) in Massive Star formation: Tracing the Initial Conditions?

APEX single-dish observations at submillimeter wavelengths toward a sample of massive star-forming regions reveal that C2H is almost omnipresent toward all covered evolutionary stages from infrared dark clouds via high-mass protostellar objects to ultracompact H II regions. High-resolution data from the Submillimeter Array toward one hot-core-like high-mass protostellar object show a shell-like distribution of C2H with a radius of ~9000 AU around the central submillimeter continuum peak position. These observed features are well reproduced by a 1D cloud model with power-law density and temperature distributions and a gas-grain chemical network. The reactive C2H radical (ethynyl) is abundant from the onset of massive star formation, but later it is rapidly transformed to other molecules in the core center. In the outer cloud regions the abundance of C2H remains high due to constant replenishment of elemental carbon from CO being dissociated by the interstellar UV photons. We suggest that C2H may be a molecule well suited to study the initial conditions of massive star formation.

Caught in the Act: The Onset of Massive Star Formation

Combining mid-infrared data from the Spitzer Space Telescope with cold gas and dust emission observations from the Plateau de Bure Interferometer, we characterize the infrared dark cloud IRDC 18223-3 at high spatial resolution. The millimeter continuum data reveal a massive ~184 M☉ gas core with a projected size of ~28,000 AU that has no associated protostellar mid-infrared counterpart. However, the detection of 4.5 μm emission at the edge of the core indicates early outflow activity, which is supported by broad CO and CS spectral line-wing emission. Moreover, systematically increasing N2H+(1-0) line width toward the millimeter core center can be interpreted as additional evidence for early star formation. Furthermore, the N2H+(1-0) line emission reveals a less massive secondary core that could be in an evolutionary stage prior to any star formation activity.

Giant Molecular Clouds in M33. I. BIMA All‐Disk Survey

We present the first interferometric CO(J=1->0) map of the entire H-alpha disk of M33. The 13 diameter synthesized beam corresponds to a linear resolution of 50 pc, sufficient to distinguish individual giant molecular clouds (GMCs). From these data we generated a catalog of 148 GMCs with an expectation that no more than 15 of the sources are spurious. The catalog is complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of 2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that our survey detects ~50% of the CO flux, hence that the total molecular mass of M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our catalog are confined largely to the central region (R < 4 kpc). They show a remarkable spatial and kinematic correlation with overdense HI filaments; the geometry suggests that the formation of GMCs follows that of the filaments. The GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper than that found in the Milky Way and in the LMC. Combined with the total mass, this steep function implies that the GMCs in M33 form with a characteristic mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions, implying that the GMCs have a short quiescent period. Our results suggest the rapid assembly of molecular clouds from atomic gas, with prompt onset of massive star formation.

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.

The effects of ionizing radiation on star formation in molecular clouds

The gravitational stability of molecular cloud clumps before and after the onset of massive star formation is discussed. We suggest that the most massive clumps are magnetically supercritical but gravitationally stabilized by the hydromagnetic turbulence caused by FUV photoionization-regulated low-mass star formation in their interiors. The ionizing radiation of an O star can trigger star formation in initially sub- and supercritical clumps.

The effects of ionizing radiation on star formation in molecular clouds

The gravitational stability of molecular cloud clumps before and after the onset of massive star formation is discussed. We suggest that the most massive clumps are magnetically supercritical but gravitationally stabilized by the hydromagnetic turbulence caused by FUV photoionization-regulated low-mass star formation in their interiors. The ionizing radiation of an O star can trigger star formation in initially sub- and supercritical clumps.