arXiv subjects

N. Schneider

Publications and source records attributed to N. Schneider.

155 records · Page 9Linked to original sources

arXiv2009

Fragmentation and mass segregation in the massive dense cores of Cygnus X

We present Plateau de Bure interferometer observations obtained in continuum at 1.3 and 3.5 mm towards the six most massive and young (IR-quiet) dense cores in Cygnus X. Located at only 1.7 kpc, the Cygnus X region offers the opportunity of reaching small enough scales (of the order of 1700 AU at 1.3 mm) to separate individual collapsing objects. The cores are sub-fragmented with a total of 23 fragments inside 5 cores. Only the most compact core, CygX-N63, could actually be a single massive protostar with an envelope mass as large as 60 Msun. The fragments in the other cores have sizes and separations similar to low-mass pre-stellar and proto-stellar condensations in nearby protoclusters, and are probably of the same nature. A total of 9 out of these 23 protostellar objects are found to be probable precursors of OB stars with envelope masses ranging from 6 to 23 Msun. The level of fragmentation is globally higher than in the turbulence regulated, monolithic collapse scenario, but is not as high as expected in a pure gravo-turbulent scenario where the distribution of mass is dominated by low-mass protostars/stars. Here, the fractions of the total core masses in the high-mass fragments are reaching values as high as 28, 44, and 100 % in CygX-N12, CygX-N53, and CygX-N63, respectively, much higher than what an IMF-like mass distribution would predict. The increase of the fragmentation efficiency as a function of density in the cores is proposed to be due to the increasing importance of self-gravity leading to gravitational collapse at the scale of the dense cores. At the same time, the cores tend to fragment into a few massive protostars within their central regions. We are therefore probably witnessing here the primordial mass segregation of clusters in formation.

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arXiv2009

ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns

(Abridged) Studying continuum emission from interstellar dust is essential to locating and characterizing the highest density regions in the interstellar medium. In particular, the early stages of massive star formation remain poorly understood. Our goal is to produce a large-scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, to understand how and under what conditions star formation takes place. A well characterized sample of star-forming sites will deliver an evolutionary sequence and a mass function of high-mass, star-forming clumps. This systematic survey at submm wavelengths also represents a preparatory work for Herschel and ALMA. The APEX telescope is ideally located to observe the inner Milky Way. The Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 microns, with a beam size of 19". Taking advantage of its large field of view (11.4') and excellent sensitivity, we started an unbiased survey of the Galactic Plane, with a noise level of 50-70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). As a first step, we covered 95 sq. deg. These data reveal 6000 compact sources brighter than 0.25 Jy, as well as extended structures, many of them filamentary. About two thirds of the compact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact HII regions or young embedded clusters. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M_sun. In this introductory paper, we show preliminary results from these ongoing observations, and discuss the perspectives of the survey.

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arXiv2008

First 450-micron dust continuum mapping of the massive star-forming region NGC 3576 with the P-ArTeMiS bolometer camera

Context: The earliest phases of massive star formation are currently much debated. Aims. In an effort to make progress, we took a census of Class0-like protostellar dense cores in the NGC 3576 region, one of the nearest and most luminous embedded sites of high-mass star formation in the Galaxy. Methods: We used the P-ArTeMiS bolometer camera on the APEX telescope to produce the first 450-micron dust continuum map of the filamentary dense clump associated with NGC 3576. Results: Combining our 450-micron observations with existing data at other wavelengths, we have identified seven massive protostellar sources along the NGC 3576 filament and placed them in the M_env - L_bol evolutionary diagram for protostars. Conclusions: Comparison with theoretical evolutionary tracks suggests that these seven protostellar sources will evolve into massive stars with masses M* ~ 15-50 Msun. Four sources are classified as candidate high-mass Class 0 objects, two sources as massive Class I objects, and one source appears to be at an intermediate stage.

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arXiv2008

Submm/FIR astronomy in Antarctica: Potential for a large telescope facility

Preliminary site testing datasets suggest that Dome C in Antarctica is one of the best sites on Earth for astronomical observations in the 200 to 500 micron regime, i.e. for far-infrared (FIR) and submillimetre (submm) astronomy. We present an overview of potential science cases that could be addressed with a large telescope facility at Dome C. This paper also includes a presentation of the current knowledge about the site characterics in terms of atmospheric transmission, stability, sky noise and polar constraints on telescopes. Current and future site testing campaigns are finally described.

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arXiv2008

Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation

As Pr. Th. Henning said at the conference, cold precursors of high-mass stars are now "hot topics". We here propose some observational criteria to identify massive infrared-quiet dense cores which can host the high-mass analogs of Class 0 protostars and pre-stellar condensations. We also show how far-infrared to millimeter imaging surveys of entire complexes forming OB stars are starting to unveil the initial conditions of high-mass star formation.

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arXiv2007

The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X

We have made an extensive 1.2mm continuum mosaicing study of the Cygnus X molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. We then compared our mm maps with mid-IR images, and have made SiO(2-1) follow-up observations of the best candidate progenitors of high-mass stars. Our complete study of Cygnus X provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores, among which 42 are probable precursors of high-mass stars. Our study qualifies 17 cores as good candidates for hosting massive IR-quiet protostars, while up to 25 cores potentially host high-luminosity IR protostars. We fail to discover the high-mass analogs of pre-stellar dense cores in CygnusX, but find several massive starless clumps that might be gravitationally bound. Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the IR-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity IR phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (~ 3 x 10^4 yr and < 10^3 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.

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arXiv2006

The cooling of atomic and molecular gas in DR21

We present an overview of a high-mass star formation region through the major (sub-)mm, and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud. We used the KOSMA 3m telescope to map the core ($10'\times 14'$) of the Galactic star forming region DR 21/DR 21 (OH) in the Cygnus X region in the two fine structure lines of atomic carbon CI and four mid-$J$ transitions of CO and $^{13}$CO, and CS $J=7\TO6$. These observations have been combined with FCRAO $J=1\TO0$ observations of $^{13}$CO and C$^{18}$O. Five positions, including DR21, DR21 (OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the \OI 63 and 145 $μ$m lines, the \CII 158 $μ$m line, and four high-$J$ CO lines. We discuss the intensities and line ratios at these positions and apply Local Thermal Equilibrium (LTE) and non-LTE analysis methods in order to derive physical parameters such as masses, densities and temperatures. The CO line emission has been modeled up to J=20. From non-LTE modeling of the low- to high-$J$ CO lines we identify two gas components, a cold one at temperatures of T$_\RM{kin}\sim 30-40$ K, and one with T$_\RM{kin}\sim 80-150$ K at a local clump density of about n(H$_2$)$\sim 10^4-10^6$ cm$^{-3}$. While the cold quiescent component is massive containing typically more than 94 % of the mass, the warm, dense, and turbulent gas is dominated by mid- and high-$J$ CO line emission and its large line widths. The medium must be clumpy with a volume-filling of a few percent. The CO lines are found to be important for the cooling of the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the UV-heated source DR21, the gas cooling is dominated by line emission of atomic oxygen and of CO.

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arXiv2004

Star formation in RCW 108: triggered or spontaneous?

We present visible, near IR, and mm-wave observations of RCW 108, a molecular cloud complex in the AraOB1 association that is being eroded by the energetic radiation of two O-type stars in the nearby cluster NGC 6193. The western part of the RCW108 cloud contains an embedded compact HII region, IRAS 16362-4845, ionized by an aggregate of early-type stars. We notice a lack of stars later than A0 in the aggregate and speculate that this might be a consequence of its extreme youth. We examine the distribution of stars displaying IR excesses projected across the molecular cloud. While many of them are located in the densest area of the molecular cloud near IRAS16362-4845, we also find a group concentrating towards the edge of the cloud that faces NGC 6193, as well as some other stars beyond the edge of the molecular cloud. The intense ionizing radiation field by the O stars in NGC6193 is a clear candidate trigger of star formation in the molecular cloud, and we suggest that the existence and arrangement of stars in this region of the cloud supports a scenario in which their formation may be a consequence of this. However, IR excess stars are also present in some areas of the opposite side of the cloud, where no obvious candidate external trigger is identified. The existence of such tracers of recent star formation scattered across the more massive molecular cloud associated with IRAS 16362-4845, and the low star formation efficiency that we derive, indicate that it is in a state to still form stars. This is in contrast to the less massive cloud close to NGC 6193, which seems to be more evolved and mostly already recycled into stars, and whose internal kinematics show hints of having been perturbed by the presence of the massive stars formed out of it.

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arXiv2004

Emission of CO, CI, and CII in W3Main

We used the KOSMA 3m telescope to map the core 7'x5' of the Galactic massive star forming region W3Main in the two fine structure lines of atomic carbon and four mid-J transitions of CO and 13CO. The maps are centered on the luminous infrared source IRS5 for which we obtained ISO/LWS data comprising four high-J CO transitions, CII, and OI at 63 and 145 micron. In combination with a KAO map of integrated line intensities of CII (Howe et al. 1991), this data set allows to study the physical structure of the molecular cloud interface regions where the occurence of carbon is believed to change from C+ to C0, and to CO. The molecular gas in W3Main is warmed by the far ultraviolet (FUV) field created by more than a dozen OB stars. Detailed modelling shows that most of the observed line intensity ratios and absolute intensities are consistent with a clumpy photon dominated region (PDR) of a few hundred unresolved clumps per 0.84pc beam, filling between 3 and 9% of the volume, with a typical clump radius of 0.025pc (2.2"), and typical mass of 0.44Msun. The high-excitation lines of CO stem from a 100-200K layer, as also the CI lines. The bulk of the gas mass is however at lower temperatures.

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arXiv2003

CII, CI, and CO in the massive star forming region W3 Main

We have used the KOSMA 3m telescope to map the core 7'x5' of the Galactic massive star forming region W3Main in the two fine structure lines of atomic carbon and four mid-J transitions of CO and 13CO. In combination with a map of singly ionized carbon (Howe et al. 1991), and FIR fine structure line data observed by ISO/LWS at the center position, these data sets allow to study in detail the physical structure of the photon dominated cloud interface regions (PDRs) where the occurance of carbon changes from CII to CI, and to CO.

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arXiv2003

The Carbon content in the Galactic CygnusX/DR21 star forming region

Observations of Carbon bearing species are among the most important diagnostic probes of ongoing star formation. CO is a surrogate for H$_2$ and is found in the vicinity of star formation sites. There, [CI] emission is thought to outline the dense molecular cores and extend into the lower density regions, where the impinging interstellar UV radiation field plays a critical role for the dissociation and ionization processes. Emission of ionized carbon ([CII]) is found to be even more extended than [CI] and is linking up with the ionized medium. These different tracers emphasize the importance of multi-wavelength studies to draw a coherent picture of the processes driving and driven by high mass star formation. Until now, large scale surveys were only done with low resolution, such as the COBE full sky survey, or were biased to a few selected bright sources (e.g. Yamamoto et al. 2001, Schneider et al. 2003). A broader basis of unbiased, high-resolution observations of [CI], CO, and [CII] may play a key role to probe the material processed by UV radiation.

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