Context. The earliest phases of the formation of high-mass stars are not well known. It is unclear whether high-mass cores in monolithic collapse exist or not, and what the accretion process and ...origin of the material feeding the precursors of high-mass stars are. As outflows are natural consequences of the accretion process, they represent one of the few (indirect) tracers of accretion. Aims. We aim to search for individual outflows from high-mass cores in Cygnus X and to study the characteristics of the detected ejections. We compare these to what has been found for the low-mass protostars, to understand how ejection and accretion change and behave with final stellar mass. Methods. We used CO (2–1) PdBI observations towards six massive dense clumps, containing a total of 9 high-mass cores. We estimated the bolometric luminosities and masses of the 9 high-mass cores and measured the energetics of outflows. We compared our sample to low-mass objects studied in the literature and developed simple evolutionary models to reproduce the observables. Results. We find that 8 out of 9 high-mass cores are driving clear individual outflows. They are therefore true equivalents of Class 0 protostars in the high-mass regime. The remaining core, CygX-N53 MM2, has only a tentative outflow detection. It could be one of the first examples of a true individual high-mass prestellar core. We also find that the momentum flux of high-mass objects has a linear relation to the reservoir of mass in the envelope, as a scale up of the relations previously found for low-mass protostars. This suggests a fundamental proportionality between accretion rates and envelope masses. The linear dependency implies that the timescale for accretion is similar for high- and low-mass stars. Conclusions. The existence of strong outflows driven by high-mass cores in Cygnus X clearly indicates that high-mass Class 0 protostars exist. The collapsing envelopes of these Class 0 objects have similar sizes and a similar fragmentation scale to the low-mass equivalents, and have enough mass to directly form high-mass stars from a monolithic collapse. If the pre-collapse evolution is quasi-static, the fragmentation scale is expected to limit the size of the initial mass reservoirs for all masses leading to higher densities at birth and therefore shorter free-fall times for higher mass stars. However, we find the collapse timescales to be similar for both low- and high-mass objects. This implies that in a quasi-static view, we would require significant turbulent/magnetic support to slow down the collapse of the more massive envelopes. But with this support still to be discovered, and based on independent indications of large dynamics in pre-collapse gas for high-mass star formation, we propose that such an identical collapse timescale implies that the initial densities, which should set the duration of the collapse, should be similar for all masses. Since the fragmentation scale is identical for all masses, a lower initial density requires that the mass that incorporates massive stars has to have been accreted from larger scales than those of low-mass stars and in a dynamical way.
We provide a first look at the results of the Herschel Gould Belt survey toward the IC 5146 molecular cloud and present a preliminary analysis of the filamentary structure in this region. The column ...density map, derived from our 70–500 μm Herschel data, reveals a complex network of filaments and confirms that these filaments are the main birth sites of prestellar cores. We analyze the column density profiles of 27 filaments and show that the underlying radial density profiles fall off as r-1.5 to r-2.5 at large radii. Our main result is that the filaments seem to be characterized by a narrow distribution of widths with a median value of 0.10 ± 0.03 pc, which is in stark contrast to a much broader distribution of central Jeans lengths. This characteristic width of ~0.1 pc corresponds to within a factor of ~2 to the sonic scale below which interstellar turbulence becomes subsonic in diffuse gas, which supports the argument that the filaments may form as a result of the dissipation of large-scale turbulence.
The Cygnus-X star-forming complex is one of the most active regions of low- and high-mass star formation within 2 kpc of the Sun. Using mid-infrared photometry from the IRAC and MIPS Spitzer Cygnus-X ...Legacy Survey, we have identified over 1800 protostar candidates. We compare the protostellar luminosity functions of two regions within Cygnus-X: CygX-South and CygX-North. These two clouds show distinctly different morphologies suggestive of dissimilar star-forming environments. We find the luminosity functions of these two regions are statistically different. Furthermore, we compare the luminosity functions of protostars found in regions of high and low stellar density within Cygnus-X and find that the luminosity function in regions of high stellar density is biased to higher luminosities. In total, these observations provide further evidence that the luminosities of protostars depend on their natal environment. We discuss the implications this dependence has for the star formation process.
Context. Numerical simulations have explored the possibility of forming molecular clouds through either a quasi-static, self-gravitating mechanism or the collision of gas streams or lower density ...clouds. They also quantitatively predict the distribution of matter at the transition from atomic to molecular gases. Aims. We aim to observationally test these models by studying the environment of W43, a molecular cloud complex recently identified near the tip of the Galactic long bar. Methods. Using Galaxy-wide H i and 12CO 1–0 surveys, we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the H i and H2 mass surface densities to constrain the transition from atomic to molecular gas around and within W43. Results. We found three cloud ensembles within the position-velocity diagrams of 12CO and H i gases. They are separated by ~20 km s-1 along the line of sight and extend into the 13CO velocity structure of W43. Since their velocity gradients are consistent with free fall, they could be nearby clouds attracted by and streaming toward the W43 ~107 M⊙ potential well. We show that the H i surface density, ΣH i = 45−85 M⊙pc-2, does not reach any threshold level but increases when entering the 130 pc-wide molecular complex previously defined. This suggests that an equilibrium between H2 formation and photodissociation has not yet been reached. The H2-to-H i ratio measured over the W43 region and its surroundings, RH2 ~ 3.5±23, is high, indicating that most of the gas is already in molecular form in W43 and in structures several hundred parsecs downstream along the Scutum-Centaurus arm. Conclusions. The W43 molecular cloud complex may have formed and, in fact may still be accreting mass from the agglomeration of clouds. Already in the molecular-dominated regime, most of these clouds are streaming from the Scutum-Centaurus arm. This clearly disagrees with quasi-static and steady-state models of molecular cloud formation.
We summarize the first results from the Gould Belt Survey, obtained toward the Aquila rift and Polaris Flare regions during the science demonstration phase of Herschel. Our 70–500 μm images taken in ...parallel mode with the SPIRE and PACS cameras reveal a wealth of filamentary structure, as well as numerous dense cores embedded in the filaments. Between ~350 and 500 prestellar cores and ~45–60 Class 0 protostars can be identified in the Aquila field, while ~300 unbound starless cores and no protostars are observed in the Polaris field. The prestellar core mass function (CMF) derived for the Aquila region bears a strong resemblance to the stellar initial mass function (IMF), already confirming the close connection between the CMF and the IMF with much better statistics than earlier studies. Comparing and contrasting our Herschel results in Aquila and Polaris, we propose an observationally-driven scenario for core formation according to which complex networks of long, thin filaments form first within molecular clouds, and then the densest filaments fragment into a number of prestellar cores via gravitational instability.
Context. Stars are born deeply embedded in molecular clouds. In the earliest embedded phases, protostars emit the bulk of their radiation in the far-infrared wavelength range, where $Herschel$ is ...perfectly suited to probe at high angular resolution and dynamic range. In the high-mass regime, the birthplaces of protostars are thought to be in the high-density structures known as infrared-dark clouds (IRDCs). While massive IRDCs are believed to have the right conditions to give rise to massive stars and clusters, the evolutionary sequence of this process is not well-characterized.Aims: As part of the Earliest Phases of Star formation (EPoS) $Herschel$ guaranteed time key program, we isolate the embedded structures within IRDCs and other cold, massive molecular clouds. We present the full sample of 45 high-mass regions which were mapped at PACS 70, 100, and 160 μm and SPIRE 250, 350, and 500 μm. In the present paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host molecular cloud and investigate their evolutionary stage.Methods: We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 μm. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. Results: The core population has a median temperature of 20 K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 μm are warmer and bluer on average than cores without a 24 μm counterpart. We conclude that cores bright at 24 μm are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 μm-dark cores. The 24 μm emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given cloud's core population. We uncover over 300 further candidate cores which are dark until 100 μm. These are possibly starless objects, and further observations will help us determine the nature of these very cold cores.
We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 μm, spanning the peak of emission of ...cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the “Centre-Ridge” and “South-Nest” sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.
Aims. For many years feedback processes generated by OB-stars in molecular clouds, including expanding ionization fronts, stellar winds, or UV-radiation, have been proposed to trigger subsequent star ...formation. However, hydrodynamic models including radiation and gravity show that UV-illumination has little or no impact on the global dynamical evolution of the cloud. Instead, gravitational collapse of filaments and/or merging of filamentary structures can lead to building up dense high-mass star-forming clumps. However, the overall density structure of the cloud has a large influence on this process, and requires a better understanding. Methods. The Rosette molecular cloud, irradiated by the NGC 2244 cluster, is a template region for triggered star-formation, and we investigated its spatial and density structure by applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and probability density functions (PDFs) on Herschel column density maps, obtained within the HOBYS key program. Results. The analysis reveals not only the filamentary structure of the cloud but also that all known infrared clusters except one lie at junctions of filaments, as predicted by turbulence simulations. The PDFs of sub-regions in the cloud show systematic differences. The two UV-exposed regions have a double-peaked PDF we interprete as caused by shock compression, while the PDFs of the center and other cloud parts are more complex, partly with a power-law tail. A deviation of the log-normal PDF form occurs at AV ≈ 9m for the center, and around 4m for the other regions. Only the part of the cloud farthest from the Rosette nebula shows a log-normal PDF. Conclusions. The deviations of the PDF from the log-normal shape typically associated with low- and high-mass star-forming regions at AV ≈ 3–4m and 8–10m, respectively, are found here within the very same cloud. This shows that there is no fundamental difference in the density structure of low- and high-mass star-forming regions. We conclude that star-formation in Rosette – and probably in high-mass star-forming clouds in general – is not globally triggered by the impact of UV-radiation. Moreover, star formation takes place in filaments that arose from the primordial turbulent structure built up during the formation of the cloud. Clusters form at filament mergers, but star formation can be locally induced in the direct interaction zone between an expanding H II-region and the molecular cloud.
Context. Massive clumps associated with infrared dark clouds (IRDCs) are promising targets for studying the earliest stages of high-mass star and cluster formation. Aims. We aim to determine the ...degrees of CO depletion, deuterium fractionation, and ionisation in a sample of seven massive clumps associated with IRDCs. Methods. The APEX telescope was used to observe the C17O(2−1), H13CO+(3−2), DCO+(3−2), N2H+(3−2), and N2D+(3−2) transitions towards the clumps. The spectral line data were used in conjunction with the previously published and/or archival (sub)millimetre dust continuum observations of the sources. The data were used to derive the molecular column densities and fractional abundances for the analysis of deuterium fractionation and ionisation. Results. The CO molecules do not appear to be significantly depleted in the observed clumps. The DCO+/HCO+ and N2D+/N2H+ column density ratios are about 0.0002–0.014 and 0.002–0.028, respectively. The former ratio is found to decrease as a function of gas kinetic temperature. A simple chemical analysis suggests that the lower limit to the ionisation degree is in the range x(e) ~ 10-8 − 10-7, whereas the estimated upper limits range from a few 10-6 up to ~10-4. Lower limits to x(e) imply that the cosmic-ray ionisation rate of H2 lies between ζH2 ~ 10-17 − 10-15 s-1. These are the first estimates of x(e) and ζH2 towards massive IRDCs reported so far. Some additional molecular transitions, mostly around 216 and 231 GHz, were detected towards all sources. In particular, IRDC 18102-1800 MM1 and IRDC 18151-1208 MM2 show relatively line-rich spectra. Some of these transitions might be assigned to complex organic molecules, although the line blending hampers the identification. The C18O(2−1) transition is frequently seen in the image band. Conclusions. The finding that CO is not depleted in the observed sources conforms to the fact that they show evidence of star formation activity, which is believed to release CO from the icy grain mantles back into the gas phase. The observed degree of deuteration is lower than in low-mass starless cores and protostellar envelopes. Decreasing deuteration with increasing temperature is likely to reflect the clump evolution. On the other hand, the association with young high-mass stars could enhance ζH2 and x(e) above the levels usually found in low-mass star-forming regions. On the scale probed by our observations, ambipolar diffusion cannot be a main driver of clump evolution unless it occurs on timescales ≫ 106 yr.
The formation of high-mass stars is tightly linked to that of their parental clouds. Here, we focus on the high-density parts of W43, a molecular cloud undergoing an efficient event of star ...formation. Using a column density image derived from Herschel continuum maps, we identify two high-density filamentary clouds, called the W43-MM1 and W43-MM2 ridges. Both have gas masses of 2.1 x 10 super(4) M sub(middot in circle) and 3.5 x 10 super(4) M sub(middot in circle) above > 10 super(23) cm super(-2) and within areas of ~6 and ~14 pc super(2), respectively. The W43-MM1 and W43-MM2 ridges are structures that are coherent in velocity and gravitationally bound, despite their large velocity dispersion measured by the N sub(2)H (1-0) lines of the W43-HERO IRAM large program. Another intriguing result is that these ridges harbor widespread (~10 pc super(2)) bright SiO (2-1) emission, which we interpret to be the result of low-velocity shocks (< or =, slant10 km s super(-1)). We measure a significant relationship between the SiO (2-1) luminosity and velocity extent and show that it distinguishes our observations from the high-velocity shocks associated with outflows. We use state-of-the-art shock models to demonstrate that a small percentage (10%) of Si atoms in low-velocity shocks, observed initially in gas phase or in grain mantles, can explain the observed SiO column density in the W43 ridges. The spatial and velocity overlaps between the ridges of high-density gas and the shocked SiO gas suggest that ridges could be forming via colliding flows driven by gravity and accompanied by low-velocity shocks. This mechanism may be the initial conditions for the formation of young massive clusters.
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