14067221
Search for massive protostar candidates in the southern hemisphere - II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellar candidates we have observed in various tracers a sample of 235 sources selected from the IRAS Point Source Catalog, mostly with $\delta Low sources and 93 High , which are believed to be in different evolutionary stages. Both sub-samples have been studied in detail by comparing their physical properties and morphologies. Massive dust clumps have been detected in all but 8 regions, with usually more than one clump per region. The dust emission shows a variety of complex morphologies, sometimes with multiple clumps forming filaments or clusters. The mean clump has a linear size of ~$ 0.5$ pc, a mass of ~$ 320~M_\odot$ for a dust temperature $T_{\rm d} = 30$ K, an H 2 density of $9.5\times10^5$ cm -3 , and a surface density of 0.4 g cm -2 . The median values are 0.4 pc, $102~M_\odot$, $4\times10^4$ cm -3 , and 0.14 g cm -2 , respectively. The mean value of the luminosity-to-mass ratio, $L/M \simeq 99~L_\odot/M_\odot$, suggests that the sources are in a young, pre-ultracompact Hii  phase. We have compared the millimeter continuum maps with images of the mid-IR MSX emission, and have discovered 95 massive millimeter clumps non-MSX emitters, either diffuse or point-like, that are potential prestellar or precluster cores. The physical properties of these clumps are similar to those of the others, apart from the mass that is ~3 times lower than for clumps with MSX counterpart. Such a difference could be due to the potential prestellar clumps having a lower dust temperature. The mass spectrum of the clumps with masses above $M \sim 100~M_\odot$ is best fitted with a power-law d$N/{\rm d}M \propto M^{-\alpha}$ with $\alpha = 2.1$, consistent with the Salpeter (1955) stellar IMF, with $\alpha=2.35$. On the other hand, the mass function of clumps with masses $10~M_\odot\la M \la 120~M_\odot$ is better fitted with a power law of slope $\alpha = 1.5$, more consistent with the mass function of molecular clouds derived from gas observations.
54613246
Clumpy outer Galaxy molecular clouds and the steepening of the IMF
We report the results of high-resolution (0.2 pc) CO (1{0) and CS (2{1) observations of the central regions of three star-forming molecular clouds in the far-outer Galaxy (16 kpc from the Galactic Center): WB89 85 (Sh 2-127), WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m and NRAO 12-m observations. The GMC's in which the regions are embedded were studied by means of KOSMA 3-m CO (2{1) observations (here we also observed WB89 399). We compare the BIMA maps with optical, radio, and near-infrared observations. Using a clumpnd routine, structures found in the CO and CS emission are subdivided in clumps, the properties of which are analyzed and compared with newly derived results of previously published single-dish measurements of local clouds (OrionB South and Rosette). We nd that the slopes of the clump mass distributions ({1.28 and {1.49, for WB89 85 and WB89 380, respectively) are somewhat less steep than found for most local clouds, but similar to those of clouds which have been analyzed with the same clumpnd program. We investigate the clump stability by using the virial theorem, including all possible contributions (gravity, turbulence, magnetic elds, and pressure due to the interclump gas). It appears that under reasonable assumptions a combination of these forces would render most clumps stable. Comparing only gravity and turbulence, we nd that in the far-outer Galaxy clouds, these forces are in equilibium (virial parameter 1) for clumps down to the lowest masses found (a few M). For clumps in the local clouds 1 only for clumps with masses larger than a few tens of M. Thus it appears that in these outer Galaxy clumps gravity is the dominant force down to a much lower mass than in local clouds, implying that gravitational collapse and star formation may occur more readily even in the smallest clumps. Although there are some caveats, due to the inhomogeneity of the data used, this might explain the apparently steeper IMF found in the outer Galaxy.
17623644
A Steeper Stellar Mass Spectrum in the Outer Galaxy
We discuss the results of high-resolution (~0.1-0.2 pc) BIMA CO observations of the central regions of 3 molecular clouds in the far-outer Galaxy (FOG). We identify clumps and investigate their stability by using the virial theorem, including terms due to gravity, turbulence, magnetic field, and interclump gas pressure, and make a comparison with clumps in local clouds (RMC and Orion B South). While a reasonable combination of these forces can render most clumps stable, an interesting difference between FOG and local clumps emerges when comparing only gravity and turbulence. In the FOG these forces are in equilibrium (virial parameter alpha ~ 1) for clumps down to the lowest masses found (a few Msol), but for local clumps alpha ~ 1 only for clumps with masses larger than a few tens of Msol. Thus it appears that in the FOG gravity is the dominant force down to a much lower mass than in local clouds, implying that gravitational collapse and star formation may occur more readily even in the smallest clumps. This might explain the apparently steeper IMF found in the outer Galaxy.
17549379
Properties of stellar clusters around high-mass young stars
Context. Twenty-six high-luminosity IRAS sources believed to be collection of stars in the early phases of high-mass star formation have been observed in the near-IR ( J , H , K s ) to characterize the clustering properties of their young stellar population and compare them with those of more evolved objects (e.g., Herbig Ae/Be stars) of comparable mass. All the observed sources possess strong continuum and/or line emission in the millimeter, being therefore associated with gas and dust envelopes. Nine sources have far-IR colors characteristic of UCHII regions, while the other 17 are probably experiencing an evolutionary phase that precedes the hot-cores, as suggested by a variety of evidence collected in the past decade. Aims. We attempt to gain insight into the initial conditions of star formation in these clusters (initial mass function [IMF], star formation history [SFH]), and to determine mean cluster ages. Methods. For each cluster, we complete aperture photometry. We derive stellar density profiles, color-color and color-magnitude diagrams, and color (HKCF) and luminosity (KLF) functions. These two functions are compared with simulated KLFs and HKCFs from a model that generates populations of synthetic clusters starting from assumptions about the IMF, SFH, and Pre-MS evolution, and using the average properties of the observed clusters as boundary conditions (bolometric luminosity, dust distribution, infrared excess, extinction). Results. Twenty-two sources show evidence of clustering with a stellar richness indicator that varies from a few up to several tens of objects, and a median cluster radius of 0.7 pc. A considerable number of cluster members present an infrared excess characteristic of young pre-main-sequence objects. For a subset of 9 detected clusters, we could perform a statistically significant comparison of the observed KLFs with those resulting from synthetic cluster models; for these clusters, we find that the median stellar age ranges between $2.5\times10^5$ and $5\times10^6$ years, with evidence of an age spread of the same entity within each cluster. We also find evidence that older clusters tend to be smaller in size, in agreement with our clusters being on average larger than those around relatively older Herbig Ae/Be stars. Our models allow us to explore the relationship between the mass of the most massive star in the cluster and both the cluster richness and the total stellar mass. Although these relationships are predicted by several classes of cluster formation models, their detailed analysis suggests that the properties of our modeled clusters may not be consistent with them resulting from random sampling of the IMF. Conclusions. Our results are consistent with star formation having occurred continuously over a period of time longer than the typical crossing time.
119247622
High CO depletion in southern infrared dark clouds
Infrared dark high-mass clumps are among the most promising objects to study the initial conditions of the formation process of high-mass stars and rich stellar clusters. In this work, we have observed the (3–2) rotational transition of C18O with the Atacama Pathfinder EXperiment telescope and the (1,1) and (2,2) inversion transitions of NH3 with the Australia Telescope Compact Array in 21 infrared dark clouds already mapped in the 1.2-mm continuum, with the aim of measuring basic chemical and physical parameters such as the CO depletion factor (fD), the gas kinetic temperature and the gas mass. In particular, the C18O (3–2) line allows us to derive fD in gas at densities higher (and hence potentially more depleted) than that traced by the (1–0) and (2–1) lines, typically used in previous works. We have detected NH3 and C18O in all targets. The clumps have a median mass of ∼244 M⊙, are gravitationally bound, have an average kinetic temperature of 17 K and possess mass, H2 column and surface densities consistent with being potentially the birthplace of high-mass stars. We have measured fD in between 5 and 78, with a mean value of 32 and a median of 29. These values are, to our knowledge, larger than the typical CO depletion factors measured towards infrared dark clouds and high-mass dense cores, and are comparable to or larger than the values measured in low-mass pre-stellar cores close to the onset of the gravitational collapse. This result suggests that the earliest phases of the high-mass star and stellar cluster formation process are characterized by fD larger than in low-mass pre-stellar cores. On the other hand, fD does not seem to be correlated to any other physical parameter, except for a faint anticorrelation with the gas kinetic temperature. 13 out of 21 clumps are undetected in the 24-m Spitzer images, and have slightly lower kinetic temperatures, masses and H2 column densities with respect to the eight Spitzer-bright sources. This could indicate that the Spitzer-dark clumps are either less evolved or are going to form less massive objects.
10224110
IRAS 23385+6053: A Prototype Massive Class 0 Object
IRAS 23385+6053 is a young stellar object with a luminosity of ~1.6 × 10 -->4 L☉${s}$ -->$t SUBgt {s}t/SUBgt $ --> at a kinematic distance of 4.9 kpc. This candidate precursor of an ultracompact H II region is associated with a millimeter source detected at the James Clerk Maxwell Telescope but is undetected at centimeter wavelengths with the VLA. We observed this source with the Owens Valley Radio Observatory millimeter array at 3.4 mm in the continuum, HCO -->+ (1→0), H -->13CO -->+ (1→0), and SiO (v=0, 2→1) line emission and with CAM aboard the Infrared Space Observatory at 6.75 and 15 μm. The IRAS source is coincident with a 3.4 mm compact (rcore${r core}$ -->$t SUBgt {r core}t/SUBgt $ --> 0.048 pc) and massive (M 370 M☉${s}$ -->$t SUBgt {s}t/SUBgt $ -->) core, which is undetected at 15 μm to a 3 σ level of 6 mJy; this is compatible with the derived H -->2 column density of ~2 × 10 -->24 cm -->−2 and the estimated visual extinction A -->V ~2000 mag. We find Lsubmm${r submm}$ -->$t SUBgt {r submm}t/SUBgt $ -->/Lbol${r bol}$ -->$t SUBgt {r bol}t/SUBgt $ --> ~3 × 10 -->−3 and Menv${r env}$ -->$t SUBgt {r env}t/SUBgt $ -->/M${s}$ -->$t SUBgt {s}t/SUBgt $ -->1, which is typical of class 0 objects. The source is also associated with a compact outflow characterized by a size rcore, a dynamical timescale of 7 × 10 -->3 yr, and a mass-loss rate ${u{M}{"705F}}$ -->${u{M}{"705F}}$ --> 10 -->−3 M☉$$ -->$t SUBgt {s}t/SUBgt $ --> yr -->−1. The axis of the outflow is oriented nearly perpendicular to the plane of the sky, ruling out the possibility that the nondetection at 15 μm is the result of a geometric effect. All these properties suggest that IRAS 23385+6053 is the first example of a bona fide massive class 0 object.
120833712
Subterranean ground motion studies for the Einstein Telescope
Seismic motion limits the low-frequency sensitivity of ground-based gravitational wave detectors. A conceptual design study into the feasibility of a future-generation gravitational wave observatory, coined the Einstein Telescope, has been completed. As part of this design phase, we performed a ground motion study to determine the seismic noise characteristics at various sites across the globe. This investigation focused on underground sites and encompassed a variety of geologies, including clay, salt, and hard rock, at 15 locations in nine European countries, the USA, and Japan. In addition, we analyzed data from the Virtual European Broadband Seismograph Network to characterize European seismic motion. We show that, in the region of interest for future-generation gravitational wave detectors (1–10 Hz), seismic motion is dominated by activity from anthropogenic sources. A number of sites were found that exhibited a reduction in seismic power of several orders of magnitude with respect to current detector sites, thus making it possible to set requirements for the Einstein Telescope seismic noise environment.