A MASSIVE PROTOSTAR FORMING BY ORDERED COLLAPSE OF A DENSE, MASSIVE CORE

High-mass Stars are cosmic engines known to dominate the energetics in the Milky Way and other galaxies. However, their formation is still not well understood. Massive, cold, dense clouds, often appearing as Infrared Dark Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic fields in IRDCs in a state prior to the onset of high-mass star formation (HMSF) have previously been available, and prevailing HMSF theories do not consider strong magnetic fields. Here, we report observations of magnetic fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong magnetic field is as important as turbulence and gravity for HMSF. The main dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the lower density filament merging onto the main filament is parallel to the magnetic field. The implied magnetic field is strong enough to suppress fragmentation sufficiently to allow HMSF. Other mechanisms reducing fragmentation, such as the entrapment of heating from young stars via high mass surface densities, are not required to facilitate HMSF.

Context. Infrared dark clouds (IRDCs) can be the birth sites of high-mass stars, and hence determining the physical properties of dense cores in IRDCs is useful to constrain the initial conditions and theoretical models of high-mass star formation. Aims. We aim to determine the physical properties of dense cores in the filamentary Seahorse IRDC G304.74+01.32. Methods. We used data from the Wide-field Infrared Survey Explorer (WISE), Infrared Astronomical Satellite (IRAS), and Herschel in conjuction with our previous 350 and 870 μm observations with the Submillimetre APEX Bolometer Camera (SABOCA) and Large APEX BOlometer CAmera, and constructed the far-IR to submillimetre spectral energy distributions (SEDs) of the cores. The SEDs were fitted using single or two-temperature modified blackbody emission curves to derive the dust temperatures, masses, and luminosities of the cores. Results. For the 12 analysed cores, which include two IR dark cores (no WISE counterpart), nine IR bright cores, and one H II region, the mean dust temperature of the cold (warm) component, the mass, luminosity, H2 number density, and surface density were derived to be 13.3 ± 1.4 K (47.0 ± 5.0 K), 113 ± 29 M⊙, 192 ± 94 L⊙, (4.3 ± 1.2) × 105 cm−3, and 0.77 ± 0.19 g cm−3, respectively. The H II region IRAS 13039-6108a was found to be the most luminous source in our sample ((1.1 ± 0.4) × 103 L⊙). All the cores were found to be gravitationally bound (i.e. the virial parameter αvir < 2). Two out of the nine analysed IR bright cores (22%) were found to follow an accretion luminosity track under the assumptions that the mass accretion rate is 10−5 M⊙ yr−1, the stellar mass is 10% of the parent core mass, and the radius of the central star is 5 R⊙. Most of the remaing ten cores were found to lie within 1 dex below this accretion luminosity track. Seven out of 12 of the analysed cores (58%) were found to lie above the mass-radius thresholds of high-mass star formation proposed in the literature. The surface densities of Σ > 0.4 g cm−3 derived for these seven cores also exceed the corresponding threshold for high-mass star formation. Five of the analysed cores (42%) show evidence of fragmentation into two components in the SABOCA 350 μm image. Conclusions. In addition to the H II region source IRAS 13039-6108a, some of the other cores in Seahorse also appear to be capable of giving birth to high-mass stars. The 22 μm dark core SMM 9 is likely to be the youngest source in our sample that has the potential to form a high-mass star (96 ± 23 M⊙ within a radius of ~0.1 pc). The dense core population in the Seahorse IRDC has comparable average properties to the cores in the well-studied Snake IRDC G11.11-0.12 (e.g. Tdust and L agree within a factor of ~1.8); furthermore, the Seahorse, which lies ~60 pc above the Galactic plane, appears to be a smaller (e.g. three times shorter in projection, ~100 times less massive) version of the Snake. The Seahorse core fragmentation mechanisms appear to be heterogenous, including cases of both thermal and non-thermal Jeans instability. High-resolution follow-up studies are required to address the fragmented cores’ genuine potential of forming high-mass stars.

Radiative transfer calculations of massive star formation are presented. These are based on the Turbulent Core Model of McKee & Tan and self-consistently included a hydrostatic core, an inside-out expansion wave, a zone of free-falling rotating collapse, wide-angle dust-free outflow cavities, an active accretion disk, and a massive protostar. For the first time for such models, an optically thick inner gas disk extends inside the dust destruction front. This is important to conserve the accretion energy naturally and for its shielding effect on the outer region of the disk and envelope. The simulation of radiation transfer is performed with the Monte Carlo code of Whitney, yielding spectral energy distributions (SEDs) for the model series, from the simplest spherical model to the fiducial one, with the above components each added step-by-step. Images are also presented in different wavebands of various telescope cameras, including Spitzer IRAC and MIPS, SOFIA FORCAST and Herschel PACS and SPIRE. The existence of the optically thick inner disk produces higher optical wavelength fluxes but reduces near- and mid-IR emission. The presence of outflow cavities, the inclination angle to the line of sight, and the thickness of the disk all affect the SEDs and images significantly. For the high mass surface density cores considered here, the mid-IR emission can be dominated by the outflow cavity walls, as has been suggested by De Buizer. The effect of varying the pressure of the environment bounding the surface of the massive core is also studied. With lower surface pressures, the core is larger, has lower extinction and accretion rates, and the observed mid-IR flux from the disk can then be relatively high even though the accretion luminosity is lower. In this case the silicate absorption feature becomes prominent, in contrast to higher density cores forming under higher pressures.

Identified as extinction features against the bright Galactic mid-infrared background, infrared dark clouds (IRDCs) are thought to harbor the very earliest stages of star and cluster formation. In order to better characterize the properties of their embedded cores, we have obtained new 24um, 60-100um, and sub-millimeter continuum data toward a sample of 38 IRDCs. The 24um Spitzer images reveal that while the IRDCs remain dark, many of the cores are associated with bright 24um emission sources, which suggests that they contain one or more embedded protostars. Combining the 24um, 60-100um, and sub-millimeter continuum data, we have constructed broadband spectral energy distributions (SEDs) for 157 of the cores within these IRDCs and, using simple gray-body fits to the SEDs, have estimated their dust temperatures, emissivities, opacities, bolometric luminosities, masses and densities. Based on their Spitzer/IRAC 3-8um colors and the presence of 24um point source emission, we have separated cores that harbor active, high-mass star formation from cores that are quiescent. The active `protostellar' cores typically have warmer dust temperatures and higher bolometric luminosities than the more quiescent, perhaps `pre-protostellar', cores. Because the mass distributions of the populations are similar, however, we speculate that the active and quiescent cores may represent different evolutionary stages of the same underlying population of cores. Although we cannot rule out low-mass star-formation in the quiescent cores, the most massive of them are excellent candidates for the `high-mass starless core' phase, the very earliest in the formation of a high-mass star.

It is generally accepted that high-mass stars form through a hierarchical, multi-scale fragmentation process that range from molecular clouds down to individual protostars, involving intermediate scales such as filaments. However, a comprehensive understanding of this process remains limited due to the lack of high-resolution, multi-scale observational studies that would simultaneously probe the physical conditions across the full hierarchy of star-forming structures. We aim to understand a coherent picture of the physical processes connecting filament formation, fragmentation, and dynamical scenario of high-mass star formation in the IRAS,19074+0752 (hereafter I19074) region. Primarily using new 1.3,mm continuum mosaicked observations, as part of the ALMA-INFANT survey, we analyzed the S-shaped filamentary cloud I19074 at a ∼6000,AU resolution. Leveraging the multi-scale information, we investigated the filament and clump fragmentation properties, such as core separations and masses. ALMA 1.3,mm dust continuum emission reveals that the S-shaped filament consists of two physically connected components: a southern (Fs) and a northern (Fn) segment. Fn is associated with an infrared (IR)-bright HII region, while Fs appears IR-dark. The total filament length is ∼ 2.8,pc, with Fn and Fs spanning ∼ 1.0,pc and ∼ 1.8,pc, respectively. Their masses are ∼ 250--910$,M_⊙$, while their line masses (∼250--360,M_⊙ pc ) exceed the critical value for turbulence support, indicating they are gravitationally bound. The S-shaped morphology likely results from the expansion of the HII region, which swept up and compressed the northern part of the pre-existing filament into an arc-like structure in Fn; meanwhile, Fs retained a more linear form due to its greater distance from the ionized gas. Accordingly, a hybrid scenario could be responsible for Fn formation, which would combine the compression of a pre-existing filament by the HII region with fresh gas accumulation into the shocked-compression layer. We extracted 26 dense cores from 1.3,mm emission with masses between 1.0 and 22.9,M_⊙, with most (92%) being gravitationally bound (α_ ̊m vir łeq 2). The core separations lack periodicity; instead, four core groups define four clumps (clumps,1–4) with masses of 110--620,M_⊙. In the Fs segment, clump,1 at its southern end could be a product of edge fragmentation, while Fn exhibits hierarchical fragmentation modes: the filamentary mode responsible for clump formation within Fn and the spherical Jeans-like mode for core formation within clumps. Hierarchical fragmentation mechanisms are identified as shocked turbulence-driven within Fn and gravity-driven inside the clumps. Most cores have high mass surface densities of Σ_ ̊m core ≥ 1 but with no robust identification of high-mass prestellar candidates. This favors dynamical clump-fed accretion-type over core-fed accretion-type models for high-mass star formation in I19074. ii region, with the shocked-shell fragmentation mechanism in Fn and edge fragmentation in Fs serving as pathways for producing massive, star-forming clumps. Both mechanisms contribute to high-mass star formation via a dynamical clump-fed accretion process within their respective filamentary segments.

Theoretical models of high-mass star formation lie between two extreme scenarios. At one extreme, all the mass comes from an initially gravitationally bound core. At the other extreme, the majority of the mass comes from cluster scale gas, which lies far outside the initial core boundary. One way to unambiguously show high-mass stars can assemble their gas through the former route would be to find a high-mass star forming in isolation. Making use of recently available CORNISH and ATLASGAL Galactic plane survey data, we develop sample selection criteria to try and find such an object. From an initial list of approximately 200 sources, we identify the high-mass star-forming region G13.384 + 0.064 as the most promising candidate. The region contains a strong radio continuum source, that is powered by an early B-type star. The bolometric luminosity, derived from infrared measurements, is consistent with this. However, sub-millimetre continuum emission, measured in ATLASGAL, as well as dense gas tracers, such as HCO+(3–2) and N2H+(3–2) indicate that there is less than ~ 100 M⊙ of material surrounding this star. We conclude that this region is indeed a promising candidate for a high-mass star forming in isolation.

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel/PACS and SPIRE 70−500 μm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc × 1 pc ridge and two 0.8 pc × 0.8 pc hubs, with volume-averaged densities of ~105 cm-3. Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M⊙ for MDCs in NGC 6334. MDCs have temperatures of 9.5−40 K, masses of 75−1000 M⊙, and densities of 1 × 105−7 × 107 cm-3. Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 μm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1−7 × 104 yr and at most 3 × 105 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC 6334, favoring a scenario presented here, which simultaneously forms clouds, ridges, MDCs, and high-mass protostars.

The probability distribution function (PDF) of the mass surface density is an essential characteristic of the structure of molecular clouds or the interstellar medium in general. Observations of the PDF of molecular clouds indicate a composition of a broad distribution around the maximum and a decreasing tail at high mass surface densities. The first component is attributed to the random distribution of gas which is modeled using a log-normal function while the second component is attributed to condensed structures modeled using a simple power-law. The aim of this paper is to provide an analytical model of the PDF of condensed structures which can be used by observers to extract information about the condensations. The condensed structures are considered to be either spheres or cylinders with a truncated radial density profile at cloud radius r_cl. The assumed profile is of the form rho(r)=rho_c/(1+(r/r_0)^2)^{n/2} for arbitrary power n where rho_c and r_0 are the central density and the inner radius, respectively. An implicit function is obtained which either truncates (sphere) or has a pole (cylinder) at maximal mass surface density. The PDF of spherical condensations and the asymptotic PDF of cylinders in the limit of infinite overdensity rho_c/rho(r_cl) flattens for steeper density profiles and has a power law asymptote at low and high mass surface densities and a well defined maximum. The power index of the asymptote Sigma^(-gamma) of the logarithmic PDF (Sigma x P(Sigma)) in the limit of high mass surface densities is given by gamma = (n+1)/(n-1)-1 (spheres) or by gamma=n/(n-1)-1 (cylinders in the limit of infinite overdensity).

Our present understanding of high-mass star formation still remains very schematic. In particular, it is not yet clear how much of the difference between low-mass and high-mass star formation occurs during the earliest star formation phases. The chemical characteristics of massive cold clumps, and the comparison with those of their low-mass counterparts, could provide crucial clues about the exact role that chemistry plays in differentiating the early phases of low-mass and high-mass star formation. Water, in particular, is a unique probe of physical and chemical conditions in star-forming regions. Using the HIFI instrument of Herschel we have observed the ortho-NH3 (1_0-0_0) (572GHz), ortho-H2O (1_10-1_01) (557GHz) and N2H+ (6-5) (559GHz) lines toward a sample of high-mass starless and proto-stellar clumps selected from the "Herschel} Infrared Galactic Plane Survey" (Hi-GAL). We compare our results to previous studies of low-mass and high-mass proto-stellar objects. At least one of the three molecular lines was detected in 4 (out of 35) and 7 (out of 17) objects in the l=59deg and l=30deg galactic regions, respectively. All detected sources are proto-stellar. The water spectra are complex and consist of several kinematic components, identified through a Gaussian decomposition, and in a few sources inverse and regular P-Cygni profiles have been detected. All water line profiles of the l=59deg region are dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows. No such broad emission is detected toward the l=30deg objects. The ammonia line in some cases also shows line wings and an inverse P-Cygni profile, thus confirming that NH3 rotational transitions can be used to probe the dynamics of high-mass star forming regions. Both bolometric and water line luminosity increase with the continuum temperature.

Far-infrared and submillimetre surveys as the Herschel Galactic Plane Infrared Survey (Hi-GAL) represent an irreplaceable knowledge base about early phases of star formation, permitting statistical analysis based on thousands of Galaxy-wide distributed sources. Those with a regular spectral energy distribution in the Herschel wavelength range 70-500 μm span a variety of evolutionary stages, from quiescent to star forming clumps and, within the latter class, from mid-infrared dark clumps to sources appearing very bright also at shorter wavelengths (e.g. Spitzer 24 μm). A fraction of these clumps hosts the formation of high mass stars, which are expected to reach the zero-age main sequence and to develop a HII region in their surroundings while they are still embedded in their parental large-scale dusty envelope. This paper aims at selecting and studying in detail a robust sample of Hi-GAL clumps supposed to be candidate to host a HII region in their interior. They are expected to be the most evolved sources in the Hi-GAL catalogue. The Galactic locations and the physical properties (temperature, mass, bolometric luminosity and temperature, and surface density) of these sources are discussed here. The large number (1199) of selected sources constitutes an important starting point for planning further interferometric programs, aimed at resolving possible cores hosting a young high-mass star.

Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species.

A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars, using the Young & Evans (2005) model as our starting point. We calculate the radiative transfer of the collapsing cores throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions. We find that scattering, 2-D geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. A cycle of episodic mass accretion, however, can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function (CMF) and stellar initial mass function (IMF). The combination of outflow cavities and episodic mass accretion reduce the connection between observational Class and physical Stage to the point where neither of the two common observational signatures (bolometric temperature and ratio of bolometric to submillimeter luminosity) can be considered reliable indicators of physical Stage.

We present Cygnus X in a new multi-wavelength perspective based on an unbiased BLAST survey at 250, 350, and 500 micron, combined with rich datasets for this well-studied region. Our primary goal is to investigate the early stages of high mass star formation. We have detected 184 compact sources in various stages of evolution across all three BLAST bands. From their well-constrained spectral energy distributions, we obtain the physical properties mass, surface density, bolometric luminosity, and dust temperature. Some of the bright sources reaching 40 K contain well-known compact H II regions. We relate these to other sources at earlier stages of evolution via the energetics as deduced from their position in the luminosity-mass (L-M) diagram. The BLAST spectral coverage, near the peak of the spectral energy distribution of the dust, reveals fainter sources too cool (~ 10 K) to be seen by earlier shorter-wavelength surveys like IRAS. We detect thermal emission from infrared dark clouds and investigate the phenomenon of cold ``starless cores" more generally. Spitzer images of these cold sources often show stellar nurseries, but these potential sites for massive star formation are ``starless" in the sense that to date there is no massive protostar in a vigorous accretion phase. We discuss evolution in the context of the L-M diagram. Theory raises some interesting possibilities: some cold massive compact sources might never form a cluster containing massive stars; and clusters with massive stars might not have an identifiable compact cold massive precursor.

A comparison of star formation within the galactic centre and galactic disc

In this thesis I utilize large-scale millimeter and mid- to far-infrared surveys to address a number of outstanding questions regarding the formation of low mass stars in molecular clouds. Continuum λ = 1.1 mm maps completed with Bolocam at a resolution of 31 cover the largest areas observed to date at millimeter or submillimeter wavelengths in three molecular clouds: 7.5 deg² in Perseus (140 pc² at the adopted distance of d = 250 pc), 10.8 deg² (50 pc² at d = 125 pc) in Ophiuchus, and 1.5 deg² (30 pc² at d = 125 pc) in Serpens. These surveys are sensitive to dense substructures with mean density n ≳ 2 - 3 x 10⁴ cm⁻³. A total of 122 cores are detected in Perseus, 44 in Ophiuchus, and 35 in Serpens above mass detection limits of 0.1 - 0.2 Msun. Combining with Spitzer mid- and far-infrared maps from the c2d Legacy program provides wavelength coverage from λ = 1.25-1100 μm, and enables the assembly of an unbiased, complete sample of the youngest star forming objects in three environments. This sample includes 108 prestellar cores, 43 Class 0 sources and 94 Class I sources. The approximately equal number of starless cores and embedded protostars in each cloud implies a starless core lifetime of 2 - 4 x 10⁵ yr, only a few free-fall timescales. This timescale, considerably shorter than the timescale predicted by the classic scenario of magnetic field support in which core evolution is moderated by ambipolar diffusion, suggests that turbulence is the dominant process controlling the formation and evolution of dense cores. However, dense cores in all three clouds are found only at high cloud column densities, where AV≳ 7 mag, and the fraction of cloud mass in these cores is less than 10%, indicating that magnetic fields must play some role as well. Measured angular deconvolved sizes of the majority of starless cores are consistent with relatively flattened radial density profiles, or with Bonnor-Ebert spheres. The prestellar core mass distribution (CMD) has a slope of α = -2.5 ± 0.2 for M > 0.8 Msun, remarkably similar to recent measurements of the slope of the stellar initial mass function: α = -2.3 to -2.8. While this result does not rule out the importance of feedback or competitive accretion, it provides support for the hypothesis that stellar masses are determined during the core formation process. The lifetime of the Class 0 phase is estimated to be 1 - 2 x 10⁵ yr in Perseus and Serpens, or approximately half that of the Class I phase, arguing against a very rapid early accretion phase. In Ophiuchus the fraction of Class 0 sources is much smaller, consistent with previous measurements of a short (~ 10⁴ yr) Class 0 phase in that cloud. A large population of low luminosity Class I sources that cannot be explained by constant or monotonically decreasing accretion rates is observed in each cloud. This result strongly suggest that accretion during the Class I phase is episodic, with sources spending approximately 25% of the Class I lifetime in a quiescent state. Finally, I investigate the environmental dependence of star formation by comparing the dense core populations of the three clouds. Cores are found at considerably higher cloud column densities in Ophiuchus than in Perseus or Serpens; more than 75% of cores occur at visual extinctions of AV≳ 8 mag in Perseus, AV≳ 15 mag in Serpens, and AV≳ 20 - 23 mag in Ophiuchus. Cloud CMDs are well characterized by power-law fits above their empirically derived 50% completeness limits, resulting in slopes of α = -2.1 ± 0.1 in Perseus, α = -2.1 ± 0.3 in Ophiuchus, and α = -1.6 ± 0.2 in Serpens. Measured slopes for Perseus and Ophiuchus broadly agree with turbulent fragmentation, but the relative shapes of the observed cloud CMDs are inconsistent with detailed simulations of the dependence of CMD shape on Mach number.