659 research outputs found
The molecular distribution of the IRDC G351.77-0.51
No full textContext. Infrared dark clouds are massive, dense clouds seen in extinction against the IR Galactic background. Many of these objects appear to be on the verge of star and star cluster formation.
Aims: Our aim is to understand the physical properties of IRDCs in very early evolutionary phases. We selected the filamentary IRDC G351.77-0.51, which is remarkably IR quiet at 8 μm.
Methods: As a first step, we observed mm dust continuum emission and rotational lines of moderate and dense gas tracers to characterise different condensations along the IRDC and study the velocity field of the filament.
Results: Our initial study confirms coherent velocity distribution along the infrared dark cloud ruling out any coincidental projection effects. Excellent correlation between MIR extinction, mm continumm emission and gas distribution is found. Large-scale turbulence and line profiles throughout the filament is indicative of a shock in this cloud. Excellent correlation between line width and MIR brightness indicates turbulence driven by local star formation
A gallery of bubbles. The nature of the bubbles observed by Spitzer and what ATLASGAL tells us about the surrounding neutral material
No full textContext. This study deals with infrared bubbles, the H ii regions they enclose, and triggered massive-star formation on their borders.
Aims: We attempt to determine the nature of the bubbles observed by Spitzer in the Galactic plane, mainly to establish if possible their association with massive stars. We take advantage of the very simple morphology of these objects to search for star formation triggered by H ii regions, and to estimate the importance of this mode of star formation.
Methods: We consider a sample of 102 bubbles detected by Spitzer-GLIMPSE, and catalogued by Churchwell et al. (2006; hereafter CH06). We use mid-infrared and radio-continuum public data (respectively the Spitzer-GLIMPSE and -MIPSGAL surveys and the MAGPIS and VGPS surveys) to discuss their nature. We use the ATLASGAL survey at 870 μm to search for dense neutral material collected on their borders. The 870 μm data traces the distribution of cold dust, thus of the dense neutral material where stars may form.
Results: We find that 86% of the bubbles contain ionized gas detected by means of its radio-continuum emission at 20-cm. Thus, most of the bubbles observed at 8.0 μm enclose H ii regions ionized by O-B2 stars. This finding differs from the earlier CH06 results (~25% of the bubbles enclosing H ii regions). Ninety-eight percent of the bubbles exhibit 24 μm emission in their central regions. The ionized regions at the center of the 8.0 μm bubbles seem to be devoid of PAHs but contain hot dust. PAH emission at 8.0 μm is observed in the direction of the photodissociation regions surrounding the ionized gas. Among the 65 regions for which the angular resolution of the observations is high enough to resolve the spatial distribution of cold dust at 870 μm, we find that 40% are surrounded by cold dust, and that another 28% contain interacting condensations. The former are good candidates for the collect and collapse process, as they display an accumulation of dense material at their borders. The latter are good candidates for the compression of pre-existing condensations by the ionized gas. Thirteen bubbles exhibit associated ultracompact H ii regions in the direction of dust condensations adjacent to their ionization fronts. Another five show methanol masers in similar condensations.
Conclusions: Our results suggest that more than a quarter of the bubbles may have triggered the formation of massive objects. Therefore, star formation triggered by H ii regions may be an important process, especially for massive-star formation
Search for starless clumps in the ATLASGAL survey
No full textContext. Understanding massive star formation requires comprehensive knowledge about the initial conditions of this process. The cradles of massive stars are believed to be located in dense and massive molecular clumps.
Aims: In this study, we present an unbiased sample of the earliest stages of massive star formation across 20 deg2 of the sky.
Methods: Within the region 10° < l < 20° and |b| < 1°, we search the ATLASGAL survey at 870 μm for dense gas condensations. These clumps are carefully examined for indications of ongoing star formation using YSOs from the GLIMPSE source catalog as well as sources in the 24 μm MIPSGAL images, to search for starless clumps. We calculate the column densities as well as the kinematic distances and masses for sources where the vlsr is known from spectroscopic observations.
Results: Within the given region, we identify 210 starless clumps with peak column densities >1 × 1023 cm-2. In particular, we identify potential starless clumps on the other side of the Galaxy. The sizes of the clumps range between 0.1 pc and 3 pc with masses between a few tens of M⊙ up to several ten thousands of M⊙. Most of them may form massive stars, but in the 20 deg2 area we only find 14 regions massive enough to form stars more massive than 20 M⊙ and 3 regions with the potential to form stars more massive than 40 M⊙. The slope of the high-mass tail of the clump mass function for clumps on the near side of the Galaxy is α = 2.2 and, therefore, Salpeter-like. We estimate the lifetime of the most massive starless clumps to be (6 ± 5) × 104 yr.
Conclusions: The sample offers a uniform selection of starless clumps. In the large area surveyed, we only find a few potential precursors of stars in the excess of 40 M⊙. It appears that the lifetime of these clumps is somewhat shorter than their free-fall times, although both values agree within the errors. In addition, these are ideal objects for detailed studies and follow-up observations
Chemical analysis of prestellar cores in Ophiuchus yields short timescales and rapid collapse
No full textSun-like stars form from the contraction of cold and dense interstellar clouds. How the collapse proceeds and what the main physical processes are driving it, however, is still under debate and a final consensus on the timescale of the process has not been reached. If the contraction proceeds slowly, supported by strong magnetic fields and mediated by ambipolar diffusion, or is driven by fast collapse with gravity dominating the entire process is still an open question. One way to answer this question is to measure the age of prestellar cores through statistical methods based on observations or via reliable chemical chronometers, which should better reflect the physical conditions of the cores. Here we report Atacama Pathfinder EXperiment observations of ortho-H2D+ and para-D2H+ for six cores in the Ophiuchus complex, and we combined them with detailed three-dimensional magneto-hydrodynamical simulations including chemistry, providing a range of ages for the observed cores of up to 200 kyr. The outcome of our simulations and subsequent analysis provides a good matching with the observational results in terms of physical parameters (core masses and volume densities) and dynamical parameters such as the Mach number and the virial parameter. We show that models of fast collapse successfully reproduce the observed range of chemical abundance ratios since the timescales to reach the observed stages is comparable to the dynamical time of the cores (i.e. the free-fall time) and much shorter than the ambipolar diffusion time, measured from the electron fraction in the simulations. To confirm that this ratio can be used to distinguish between different star-formation scenarios, a larger (statistically relevant) sample of star-forming cores should be explored
Survey of ortho-H2D+in high-mass star-forming regions
No full textContext. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H3+ isotopologues (e.g. ortho-H2D+) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive. Aims. We present a large sample of o-H2D+ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. Methods. APEX observations of the ground-state transition of o-H2D+ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-Averaged abundances of o-H2D+ with respect to H2, X(o-H2D+), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. Results. We detect 16 sources in o-H2D+ and find clear correlations between X(o-H2D+) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-To-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H3+ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H2D+) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H13CO+, DCO+, and C17O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Conclusions. Our study presents the largest sample of o-H2D+ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H2D+ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence
Establishing the evolutionary timescales of the massive star formation process through chemistry
No full textContext. Understanding the details of the formation process of massive (i.e. M greater than or similar to 8-10 M-circle dot) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging. Aims. We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process. Methods. We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH3CCH), acetonitrile (CH3CN), formaldehyde (H2CO), and methanol (CH3OH). Results. We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 10(3) to 10(5) AU during the warm-up phase. We succeed in reproducing the observed column densities of CH3CCH and CH3CN, but H2CO and CH3OH agree less with the observed values. The total (massive) star formation time is found to be similar to 5.2 x 10(5) yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: similar to 5 x 10(4) yr for 70-mu m weak, similar to 1.2 x 10(5) yr for mid-IR weak, similar to 2.4 x 10(5) yr for mid-IR bright, and similar to 1.1 x 10(5) yr for HII-region phases. Conclusions. With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales
A Survey of Large Molecules toward the Protoplanetary Nebula CRL 61 8
No full textWe present the results of our survey toward the protoplanetary nebula CRL 618 for several large, highly saturated, oxygen bearing organic molecules of biological importance including acetaldehyde (CH3CHO), acetic acid (CH3OOH), dimethyl ether (CH3OCH3), ethanol (CH3CH2OH), formic acid (HCOOH) and methyl formate (HCOOCH3); large carbon chain molecules including methyl cyanide (CH3CN) , methylcyanoacetylene (CH3C3N), cyanoacetylene (HC3N), cyanodiacetylene (HC5N), and C6H; and finally smaller molecules including SO-34, SO2, O(C-34)S and MgNC. No biologically important organic molecules were detected. However, we report the first interferometric detections of CH3CN and vibrationally excited HC3N and HC5N toward this source. The temperature and distribution of CH3CN toward CRL 618 indicates it is formed in the outer envelope surrounding the UC HII region. Furthermore, the P-Cygni line profile and corresponding channel maps of vibrationally excited HC5N supports its distribution in the extended envelope expanding radially from the central star. The detection of vibrationally excited HC3N confirmed the temperature structure and column density of HC3N in the inner envelope found by Wyrowski and colleagues (2003). Finally, our observations clearly indicate that CRL 618 is a good source of large carbon chain species but is a very poor source to detect or produce organic species of biological importance
Dense cores and star formation in the giant molecular cloud Vela C
No full textContext. The Vela Molecular Ridge is one of the nearest (700 pc) giant molecular cloud (GMC) complexes hosting intermediate-mass (up to early B, late O stars) star formation, and is located in the outer Galaxy, inside the Galactic plane. Vela C is one of the GMCs making up the Vela Molecular Ridge, and exhibits both sub-regions of robust and sub-regions of more quiescent star formation activity, with both low- and intermediate(high)-mass star formation in progress. Aims. We aim to study the individual and global properties of dense dust cores in Vela C, and aim to search for spatial variations in these properties which could be related to different environmental properties and/or evolutionary stages in the various sub-regions of Vela C. Methods. We mapped the submillimetre (345 GHz) emission from vela C with LABOCA (beam size ~19′′2, spatial resolution ~0.07 pc at 700 pc) at the APEX telescope. We used the clump-finding algorithm CuTEx to identify the compact submillimetre sources. We also used SIMBA (250 GHz) observations, and Herschel and WISE ancillary data. The association with WISE red sources allowed the protostellar and starless cores to be separated, whereas the Herschel dataset allowed the dust temperature to be derived for a fraction of cores. The protostellar and starless core mass functions (CMFs) were constructed following two different approaches, achieving a mass completeness limit of 3.7 M. Results. We retrieved 549 submillimetre cores, 316 of which are starless and mostly gravitationally bound (therefore prestellar in nature). Both the protostellar and the starless CMFs are consistent with the shape of a Salpeter initial mass function in the high-mass part of the distribution. Clustering of cores at scales of 1-6 pc is also found, hinting at fractionation of magnetised, turbulent gas
A timeline for massive star-forming regions via deuterium chemistry
No full textChemistry is an extremely powerful tool to estimate the duration of the prestellar phase; it can provide key tools to distinguish between a slow or a fast path towards the formation of stars. The most promising tracers of the quiescent phase are the light, depletion-resistant H_{2}{D}^{+} and D_{2}{H}^{+}. Our observational effort has led to the first detections of both ortho- and para-H_{2}{D}^{+} in massive clumps using APEX, ALMA and SOFIA. We confirm that the anticorrelation among the abundance of o-H_{2}{D}^{+} and N_{2}{D}^{+}, a species that can be relatively easily observed, is real and that their relative abundance strongly decreases with evolution in the very first stages of the star formation process. The behaviour of these species can be explained with simple considerations on the chemical formation paths, depletion of heavy elements, and evaporation from the dust grain mantles, and can be used as a powerful evolutionary indicator. Our unique 3D MHD simulations, coupled with chemistry, take us one step further than a simple relative timeline, allowing to follow abundance variations with time. Combining these pieces of the puzzle with the first measurement of the ortho-to-para ratio of H_{2}{D}^{+} in a massive clump, we will have the opportunity to investigate the duration of the quiescent phase in different mass regimes...
α-bandlimited diffuser in fractional Fourier optics
No full textWe propose a method for calculating appropriate α-band limited diffusers using the fractional Fourier transform. In order to do this, we implement a method for performing a numerical interpolation in the fractional Fourier domain. Such diffusers with compact support in the Fresnel regime may be used in fractional Fourier optical systems where the use of diffusers produce speckles, e.g. digital holography or optical encryption. Numerical simulations are presented. © 2016 SPIE.Brussels Photonics Team (B-PHOT);Research Foundation Flanders;The Society of Photo-Optical Instrumentation Engineers (SPIE);Visit Brussel
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