arXiv subjects

N. Schneider

Publications and source records attributed to N. Schneider.

At least 127 records · Page 7Linked to original sources

arXiv2012

Recent star formation in the Lupus clouds as seen by Herschel

We present a study of the star formation histories of the Lupus I, III, and IV clouds using the Herschel 70-500 micron maps obtained by the Herschel Gould Belt Survey Key Project. By combining the new Herschel data with the existing Spitzer catalog we obtained an unprecedented census of prestellar sources and young stellar objects in the Lupus clouds, which allowed us to study the overall star formation rate (SFR) and efficiency (SFE). The high SFE of Lupus III, its decreasing SFR, and its large number of pre-main sequence stars with respect to proto- and prestellar sources, suggest that Lupus III is the most evolved cloud, and after having experienced a major star formation event in the past, is now approaching the end of its current star-forming cycle. Lupus I is currently undergoing a large star formation event, apparent by the increasing SFR, the large number of prestellar objects with respect to more evolved objects, and the high percentage of material at high extinction (e.g., above A_V=8 mag). Also Lupus IV has an increasing SFR; however, the relative number of prestellar sources is much lower, suggesting that its star formation has not yet reached its peak.

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arXiv2012

Resolving the Vela C ridge with P-ArTeMiS and Herschel

We present APEX/P-ArTéMiS 450μm continuum observations of RCW 36 and the adjacent ridge, a high-mass high-column density filamentary structure at the centre of the Vela C molecular cloud. These observations, at higher resolution than Herschel's SPIRE camera, reveal clear fragmentation of the central star-forming ridge. Combined with PACS far-infrared and SPIRE sub-millimetre observations from the Herschel HOBYS project we build a high resolution column density map of the region mapped with P-ArTéMiS. We extract the radial density profile of the Vela C ridge which with a ~ 0.1pc central width is consistent with that measured for low-mass star-forming filaments in the Herschel Gould Belt survey. Direct comparison with Serpens South, of the Gould Belt Aquila complex, reveals many similarities between the two regions. Despite likely different formation mechanisms and histories, the Vela C ridge and Serpens South filament share common characteristics, including their filament central widths.

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arXiv2012

Worldwide site comparison for submillimetre astronomy

The most important limitation for ground-based submillimetre (submm) astronomy is the broad-band absorption of the total water vapour in the atmosphere above an observation site, often expressed as the Precipitable Water Vapour (PWV). A long-term statistic on the PWV is thus mandatory to characterize the quality of an existing or potential site for observational submm-astronomy. In this study we present a three-year statistic (2008-2010) of the PWV for ground-based telescope sites all around the world and for stratospheric altitudes relevant for SOFIA (Stratospheric Observatory for Far-infrared astronomy). The submm-transmission is calculated for typical PWVs using an atmospheric model. We present the absolute PWV values for each site sorted by year and time percentage. The PWV corresponding to the first decile (10%) and the quartiles (25%, 50%, 75%) are calculated and transmission curves between 150 μm and 3 mm for these values are shown. The Antarctic and South-American sites present very good conditions for submillimetre astronomy. The 350 μm and 450 μm atmospheric windows are open all year long whereas the 200 μm atmospheric window opens reasonably for 25 % of the time in Antarctica and the extremely high-altitude sites in Chile. Potential interesting new facilities are Macon in Argentinia and Summit in Greenland that show similar conditions as for example Mauna Kea (Hawaii). For SOFIA, we present in more detail transmission curves for different altitudes (11 to 14 km), PWV values, and higher frequencies (up to 5 THz). Though the atmosphere at these altitude is generally very transparent, the absorption at very high frequencies becomes more important, partly caused by minor species. In conclusion, the method presented in this paper could identify sites on Earth with a great potential for submillimetre astronomy, and guide future site testing campaigns in situ.

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arXiv2012

Herschel observations of B1-bS and B1-bN: two first hydrostatic core candidates in the Perseus star-forming cloud

We report far-IR Herschel observations obtained between 70 $μ$m and 500 $μ$m of two star-forming dusty condensations, B1-bS and B1-bN, in the B1 region of the Perseus star-forming cloud. In the Western part of the Perseus cloud, B1-bS is the only source detected in all of the 6 PACS and SPIRE photometric bands without being visible in the Spitzer map at 24 $μ$m. B1-bN is clearly detected between 100 $μ$m and 250 $μ$m. We have fitted the spectral energy distributions of these sources to derive their physical properties, and find that a simple greybody model fails to reproduce the observed SEDs. At least a two-component model, consisting of a central source surrounded by a dusty envelope is required. The properties derived from the fit, however, suggest that the central source is not a Class 0 object. We then conclude that while B1-bS and B1-bN appear to be more evolved than a pre-stellar core, the best-fit models suggest that their central objects are younger than a Class 0 source. Hence, they may be good candidates to be examples of the first hydrostatic core phase. The projected distance between B1-bS and B1-bN is a few Jeans lengths. If their physical separation is close to this value, this pair would allow the mutual interactions between two forming stars at a very early stage of their evolution to be studied.

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arXiv2012

3D simulations of globules and pillars formation around HII regions: turbulence and shock curvature

We investigate the interplay between the ionization radiation from massive stars and the turbulence inside the surrounding molecular gas thanks to 3D numerical simulations. We used the 3D hydrodynamical code HERACLES to model an initial turbulent medium that is ionized and heated by an ionizing source. Three different simulations are performed with different mean Mach numbers (1, 2 and 4). A non-equilibrium model for the ionization and the associated thermal processes was used. This revealed to be crucial when turbulent ram pressure is of the same order as the ionized-gas pressure. The density structures initiated by the turbulence cause local curvatures of the dense shell formed by the ionization compression. When the curvature of the shell is sufficient, the shell collapse on itself to form a pillar while a smaller curvature leads to the formation of dense clumps that are accelerated with the shell and therefore remain in the shell during the simulation. When the turbulent ram pressure of the cold gas is sufficient to balance the ionized-gas pressure, some dense-gas bubbles have enough kinetic energy to penetrate inside the ionized medium, forming cometary globules. This suggests a direct relation in the observations between the presence of globules and the relative importance of the turbulence compared to the ionized-gas pressure. The probability density functions present a double peak structure when the turbulence is low relative to the ionized-gas pressure. This could be used in observations as an indication of the turbulence inside molecular clouds.

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arXiv2012

The spine of the swan: A Herschel study of the DR21 ridge and filaments in Cygnus X

In order to characterise the cloud structures responsible for the formation of high-mass stars, we present Herschel observations of the DR21 environment. Maps of the column density and dust temperature unveil the structure of the DR21 ridge and several connected filaments. The ridge has column densities larger than 1e23/cm^2 over a region of 2.3 pc^2. It shows substructured column density profiles and branching into two major filaments in the north. The masses in the studied filaments range between 130 and 1400 Msun whereas the mass in the ridge is 15000 Msun. The accretion of these filaments onto the DR21 ridge, suggested by a previous molecular line study, could provide a continuous mass inflow to the ridge. In contrast to the striations seen in e.g., the Taurus region, these filaments are gravitationally unstable and form cores and protostars. These cores formed in the filaments potentially fall into the ridge. Both inflow and collisions of cores could be important to drive the observed high-mass star formation. The evolutionary gradient of star formation running from DR21 in the south to the northern branching is traced by decreasing dust temperature. This evolution and the ridge structure can be explained by two main filamentary components of the ridge that merged first in the south.

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arXiv2012

The M16 molecular complex under the influence of NGC6611. Herschel's perspective of the heating effect on the Eagle Nebula

We present Herschel images from the HOBYS key program of the Eagle Nebula (M16) in the far-infrared and sub-millimetre, using the PACS and SPIRE cameras at 70μm, 160μm, 250μm, 350μm, 500μm. M16, home to the Pillars of Creation, is largely under the influence of the nearby NGC6611 high-mass star cluster. The Herschel images reveal a clear dust temperature gradient running away from the centre of the cavity carved by the OB cluster. We investigate the heating effect of NGC6611 on the entire M16 star-forming complex seen by Herschel including the diffuse cloud environment and the dense filamentary structures identified in this region. In addition, we interpret the three-dimensional geometry of M16 with respect to the nebula, its surrounding environment, and the NGC6611 cavity. The dust temperature and column density maps reveal a prominent eastern filament running north-south and away from the high-mass star-forming central region and the NGC6611 cluster, as well as a northern filament which extends around and away from the cluster. The dust temperature in each of these filaments decreases with increasing distance from the NGC6611 cluster, indicating a heating penetration depth of \sim 10 pc in each direction in 3 - 6 \times 10^{22} cm-2 column density filaments. We show that in high-mass star-forming regions OB clusters impact the temperature of future star-forming sites, modifying the initial conditions for collapse and effecting the evolutionary criteria of protostars developed from spectral energy distributions. Possible scenarios for the origin of the morphology seen in this region are discussed, including a western equivalent to the eastern filament, which was destroyed by the creation of the OB cluster and its subsequent winds and radiation.

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arXiv2012

Herschel Observations of a Potential Core Forming Clump: Perseus B1-E

We present continuum observations of the Perseus B1-E region from the Herschel Gould Belt Survey. These Herschel data reveal a loose grouping of substructures at 160 - 500 micron not seen in previous submillimetre observations. We measure temperature and column density from these data and select the nine densest and coolest substructures for follow-up spectral line observations with the Green Bank Telescope. We find that the B1-E clump has a mass of ~ 100 solar masses and appears to be gravitationally bound. Furthermore, of the nine substructures examined here, one substructure (B1-E2) appears to be itself bound. The substructures are typically less than a Jeans length from their nearest neighbour and thus, may interact on a timescale of ~ 1 Myr. We propose that B1-E may be forming a first generation of dense cores, which could provide important constraints on the initial conditions of prestellar core formation. Our results suggest that B1-E may be influenced by a strong, localized magnetic field, but further observations are still required.

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arXiv2012

A multi-scale, multi-wavelength source extraction method: getsources

We present a multi-scale, multi-wavelength source extraction algorithm called getsources. Although it has been designed primarily for use in the far-infrared surveys of Galactic star-forming regions with Herschel, the method can be applied to many other astronomical images. Instead of the traditional approach of extracting sources in the observed images, the new method analyzes fine spatial decompositions of original images across a wide range of scales and across all wavebands. It cleans those single-scale images of noise and background, and constructs wavelength-independent single-scale detection images that preserve information in both spatial and wavelength dimensions. Sources are detected in the combined detection images by following the evolution of their segmentation masks across all spatial scales. Measurements of the source properties are done in the original background-subtracted images at each wavelength; the background is estimated by interpolation under the source footprints and overlapping sources are deblended in an iterative procedure. In addition to the main catalog of sources, various catalogs and images are produced that aid scientific exploitation of the extraction results. We illustrate the performance of getsources on Herschel images by extracting sources in sub-fields of the Aquila and Rosette star-forming regions. The source extraction code and validation images with a reference extraction catalog are freely available.

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arXiv2012

Globules and pillars seen in the [CII] 158 micron line with SOFIA

Molecular globules and pillars are spectacular features, found only in the interface region between a molecular cloud and an HII-region. Impacting Far-ultraviolet (FUV) radiation creates photon dominated regions (PDRs) on their surfaces that can be traced by typical cooling lines. With the GREAT receiver onboard SOFIA we mapped and spectrally resolved the [CII] 158 micron atomic fine-structure line and the highly excited 12CO J=11-10 molecular line from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source). We focus here on the globule and compare our data with existing Spitzer data and recent Herschel Open-Time PACS data. Extended [CII] emission and more compact CO-emission was found in the globule. We ascribe this emission mainly to an internal PDR, created by a possibly embedded star-cluster with at least one early B-star. However, external PDR emission caused by the excitation by the Cyg OB2 association cannot be fully excluded. The velocity-resolved [CII] emission traces the emission of PDR surfaces, possible rotation of the globule, and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km/s with respect to the expanding HII-region, which can be understood as the residual turbulence of the molecular cloud from which the globule arose. This scenario is compatible with recent numerical simulations that emphazise the effect of turbulence. It is remarkable that an isolated globule shows these strong dynamical features traced by the [CII]-line, but it demands more observational studies to verify if there is indeed an embedded cluster of B-stars.

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arXiv2012

Cluster-formation in the Rosette molecular cloud at the junctions of filaments

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. The Rosette molecular cloud, irradiated by the NGC2244 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. 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. 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 HII--region and the molecular cloud.

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arXiv2012

The Pipe Nebula as seen with Herschel: Formation of filamentary structures by large-scale compression ?

A growing body of evidence indicates that the formation of filaments in interstellar clouds is a key component of the star formation process. In this paper, we present new Herschel PACS and SPIRE observations of the B59 and Stem regions in the Pipe Nebula complex, revealing a rich, organized network of filaments. The asymmetric column density profiles observed for several filaments, along with the bow-like edge of B59, indicates that the Pipe Nebula is being compressed from its western side, most likely by the winds from the nearby Sco OB2 association. We suggest that this compressive flow has contributed to the formation of some of the observed filamentary structures. In B59, the only region of the entire Pipe complex showing star formation activity, the same compressive flow has likely enhanced the initial column density of the clump, allowing it to become globally gravitationally unstable. Although more speculative, we propose that gravity has also been responsible for shaping the converging filamentary pattern observed in B59. While the question of the relative impact of large-scale compression and gravity remains open in B59, large-scale compression appears to be a plausible mechanism for the initial formation of filamentary structures in the rest of the complex

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arXiv2012

The Herschel view of the on-going star formation in the Vela-C molecular cloud

As part of the Herschel guaranteed time key program 'HOBYS', we present the photometric survey of the star forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Vela-C has been observed with PACS and SPIRE in parallel mode between 70 um and 500 um over an area of about 3 square degrees. A photometric catalogue has been extracted from the detections in each band, using a threshold of 5 sigma over the local background. Out of this catalogue we have selected a robust sub-sample of 268 sources, of which 75% are cloud clumps and 25% are cores. Their Spectral Energy Distributions (SEDs) have been fitted with a modified black body function. We classify 48 sources as protostellar and 218 as starless. For two further sources, we do not provide a secure classification, but suggest they are Class 0 protostars. From SED fitting we have derived key physical parameters. Protostellar sources are in general warmer and more compact than starless sources. Both these evidences can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further. A virial analysis of the starless sources gives an upper limit of 90% for the sources gravitationally bound and therefore prestellar. We fit a power law N(logM) prop M^-1.1 to the linear portion of the mass distribution of prestellar sources. This is in between that typical of CO clumps and those of cores in nearby star-forming regions. We interpret this as a result of the inhomogeneity of our sample, which is composed of comparable fractions of clumps and cores.

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arXiv2011

3D simulations of pillars formation around HII regions: the importance of shock curvature

Radiative feedback from massive stars is a key process to understand how HII regions may enhance or inhibit star formation in pillars and globules at the interface with molecular clouds. We aim to contribute to model the interactions between ionization and gas clouds to better understand the processes at work. We study in detail the impact of modulations on the cloud-HII region interface and density modulations inside the cloud. We run three-dimensional hydrodynamical simulations based on Euler equations coupled with gravity using the HERACLES code. We implement a method to solve ionization/recombination equations and we take into account typical heating and cooling processes at work in the interstellar medium and due to ionization/recombination physics. UV radiation creates a dense shell compressed between an ionization front and a shock ahead. Interface modulations produce a curved shock that collapses on itself leading to stable growing pillar-like structures. The narrower the initial interface modulation, the longer the resulting pillar. We interpret pillars resulting from density modulations in terms of the ability of these density modula- tions to curve the shock ahead the ionization front. The shock curvature is a key process to understand the formation of structures at the edge of HII regions. Interface and density modulations at the edge of the cloud have a direct impact on the morphology of the dense shell during its formation. Deeper in the cloud, structures have less influence due to the high densities reached by the shell during its expansion.

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arXiv2011

Site testing for submillimetre astronomy at Dome C, Antarctica

Over the past few years a major effort has been put into the exploration of potential sites for the deployment of submillimetre astronomical facilities. Amongst the most important sites are Dome C and Dome A on the Antarctic Plateau, and the Chajnantor area in Chile. In this context, we report on measurements of the sky opacity at 200 um over a period of three years at the French-Italian station, Concordia, at Dome C, Antarctica. We also present some solutions to the challenges of operating in the harsh polar environ- ment. Dome C offers exceptional conditions in terms of absolute atmospheric transmission and stability for submillimetre astron- omy. Over the austral winter the PWV exhibits long periods during which it is stable and at a very low level (0.1 to 0.3 mm). Higher values (0.2 to 0.8 mm) of PWV are observed during the short summer period. Based on observations over three years, a transmission of around 50% at 350 um is achieved for 75% of the time. The 200-um window opens with a typical transmission of 10% to 15% for 25% of the time. Dome C is one of the best accessible sites on Earth for submillimetre astronomy. Observations at 350 or 450 μm are possible all year round, and the 200-um window opens long enough and with a sufficient transparency to be useful. Although the polar environment severely constrains hardware design, a permanent observatory with appropriate technical capabilities is feasible. Because of the very good astronomical conditions, high angular resolution and time series (multi-year) observations at Dome C with a medium size single dish telescope would enable unique studies to be conducted, some of which are not otherwise feasible even from space.

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arXiv2011

The Herschel view of massive star formation in G035.39--00.33: Dense and cold filament of W48 undergoing a mini-starburst

The filament IRDC G035.39--00.33 in the W48 molecular complex is one of the darkest infrared clouds observed by \textit{Spitzer}. It has been observed by the PACS (70 and 160\,$\micron$) and SPIRE (250, 350, and 500\,$\micron$) cameras of the \textit{Herschel} Space Observatory as part of the W48 molecular cloud complex in the framework of the HOBYS key programme. The observations reveal a sample of 28 compact sources (deconvolved FWHM sizes $<$0.3 pc) complete down to $\sim$$5 \msun$ in G035.39--00.33 and its surroundings. Among them, 13 compact sources are massive dense cores with masses $>$$20 \msun$. The cloud characteristics we derive from the analysis of their spectral energy distributions are masses of $20-50 \msun$, sizes of 0.1--0.2 pc, and average densities of $2-20 \times 10^{5} \cmc$, which make these massive dense cores excellent candidates to form intermediate- to high-mass stars. Most of the massive dense cores are located inside the G035.39--00.33 ridge and host IR-quiet high-mass protostars. The large number of protostars found in this filament suggests that we are witnessing a mini-burst of star formation with an efficiency of $\sim$15% and a rate density of $\sim$$40 \msun\,$yr$^{-1}\,$kpc$^{-2}$ within $\sim$8 pc$^2$, a large area covering the full ridge. Part of the extended SiO emission observed towards G035.39--00.33 is not associated with obvious protostars and may originate from low-velocity shocks within converging flows, as advocated by previous studies.

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arXiv2011

Convergent flows and low-velocity shocks in DR21(OH)

DR21(OH) is a pc-scale massive, 7000 Msun clump hosting three massive dense cores (MDCs) at an early stage of their evolution. We present a high angular-resolution mosaic, covering 70" by 100", with the IRAM PdBI at 3 mm to trace the dust continuum emission and the N2H+ (J=1-0) and CH3CN (J=5-4) molecular emission. The cold, dense gas traced by the compact emission in N2H+ is associated with the three MDCs and shows several velocity components towards each MDC. These velocity components reveal local shears in the velocity fields which are best interpreted as convergent flows. Moreover, we report the detection of weak extended emission from CH3CN at the position of the N2H+ velocity shears. We propose that this extended CH3CN emission is tracing warm gas associated with the low-velocity shocks expected at the location of convergence of the flows where velocity shears are observed. This is the first detection of low-velocity shocks associated with small (sub-parsec) scale convergent flows which are proposed to be at the origin of the densest structures and of the formation of (high-mass) stars. In addition, we propose that MDCs may be active sites of star-formation for more than a crossing time as they continuously receive material from larger scale flows as suggested by the global picture of dynamical, gravity driven evolution of massive clumps which is favored by the present observations.

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arXiv2011

Characterizing interstellar filaments with Herschel in IC5146

We provide a first look at the results of the Herschel Gould Belt survey toward the IC5146 molecular cloud and present a preliminary analysis of the filamentary structure in this region. The column density map, derived from our 70-500 micron 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 having 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, supporting the argument that the filaments may form as a result of the dissipation of large-scale turbulence.

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