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Search Results: 1 - 10 of 462097 matches for " A. Bacmann "
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Direct evidence of dust growth in L183 from MIR light scattering
J. Steinacker,L. Pagani,A. Bacmann,S. Guieu
Physics , 2009, DOI: 10.1051/0004-6361/200912835
Abstract: Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission). We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree agree with the AV map and compare them to the Spitzer data. The Spitzer data of L183 show emission in the 3.6 and 4.5 micron bands, while the 5.8 micron band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 micron, which is not seen. Hence, we interpret this emission to be MIR cloudshine. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 micron.
Detecting scattered light from low-mass molecular cores at 3.6 $μ$m - Impact of global effects on the observation of coreshine
J. Steinacker,M. Andersen,W. -F. Thi,A. Bacmann
Physics , 2014, DOI: 10.1051/0004-6361/201323219
Abstract: Recently discovered scattered light at 3-5 $\mu$m from low-mass cores (so-called "coreshine") reveals the presence of grains around 1 $\mu$m, which is larger than the grains found in the low-density interstellar medium. But only about half of the 100+ cores investigated so far show the effect. This prompts further studies on the origin of this detection rate. From the 3D continuum radiative transfer equation, we derive the expected scattered light intensity from a core placed in an arbitrary direction seen from Earth. We use the approximation of single scattering, consider extinction up to 2nd-order Taylor approximation, and neglect spatial gradients in the dust size distribution. The impact of the directional characteristics of the scattering on the detection of scattered light from cores is calculated for a given grain size distribution, and local effects like additional radiation field components are discussed. The surface brightness profiles of a core with a 1D density profile are calculated for various Galactic locations, and the results are compared to the approximate detection limits. We find that for optically thin radiation and a constant size distribution, a simple limit for detecting scattered light from a low-mass core can be derived that holds for grains with sizes smaller than 0.5 $\mu$m. The extinction by the core prohibits detection in bright parts of the Galactic plane, especially near the Galactic center. For scattered light received from low-mass cores with grain sizes beyond 0.5 $\mu$m, the directional characteristics of the scattering favors the detection of scattered light above and below the Galactic center, and to some extent near the Galactic anti-center. We identify the local incident radiation field as the major unknown causing deviations from this simple scheme.
Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?
J. Steinacker,C. W. Ormel,M. Andersen,A. Bacmann
Physics , 2014, DOI: 10.1051/0004-6361/201322117
Abstract: The recently discovered coreshine effect can aid in exploring the core properties and in probing the large grain population of the ISM. We discuss the implications of the coreshine detected from the molecular cloud core L1506C in the Taurus filament for the history of the core and the existence of a primitive ISM component of large grains becoming visible in cores. The coreshine surface brightness of L1506C is determined from IRAC Spitzer images at 3.6 micron. We perform grain growth calculations to estimate the grain size distribution in model cores similar in gas density, radius, and turbulent velocity to L1506C. Scattered light intensities at 3.6 micron are calculated for a variety of MRN and grain growth distributions to compare with the observed coreshine. For a core with the overall physical properties of L1506C, no detectable coreshine is predicted for an MRN size distribution. Extending the distribution to grain radii of about 0.65 $\mu$m allows to reproduce the observed surface brightness level in scattered light. Assuming the properties of L1506C to be preserved, models for the growth of grains in cores do not yield sufficient scattered light to account for the coreshine within the lifetime of the Taurus complex. Only increasing the core density and the turbulence amplifies the scattered light intensity to a level consistent with the observed coreshine brightness. The grains could be part of primitive omni-present large grain population becoming visible in the densest part of the ISM, could grow under the turbulent dense conditions of former cores, or in L1506C itself. In the later case, L1506C must have passed through a period of larger density and stronger turbulence. This would be consistent with the surprisingly strong depletion usually attributed to high column densities, and with the large-scale outward motion of the core envelope observed today.
Mid-infrared observations of the SGR 1900+14 error box
S. Klose,B. Stecklum,D. H. Hartmann,F. J. Vrba,A. A. Henden,A. Bacmann
Physics , 2002, DOI: 10.1063/1.1579435
Abstract: We report on mid-infrared observations of the compact stellar cluster located in the proximity of SGR 1900+14, and the radio/X-ray position of this soft-gamma repeater. Observations were performed in May and June of 2001 when the bursting source was in an active state. At the known radio and X-ray position of the SGR we did not detect transient mid-IR activity, although the observations were performed only hours before and after an outburst in the high-energy band.
Abundant H2D+ in the pre-stellar core L1544
P. Caselli,F. F. S. van der Tak,C. Ceccarelli,A. Bacmann
Physics , 2003, DOI: 10.1051/0004-6361:20030526
Abstract: We have detected the 372 GHz line of ortho-H2D+ towards the pre-stellar core L1544. The strongest emission (T_mb ~ 1 K) occurs at the peak of the millimeter continuum emission, while measurements at offset positions indicate that H2D+ is confined within ~ 20 arcsec, where CO is highly depleted. The derived H2D+ abundance of ~ 10^{-9} is comparable with previous estimates of the electron abundance in the core, which suggests that H2D+ is the main molecular ion in the central 20 arcsec (2800 AU) of L1544. This confirms the expectations that H2D+ is dramatically enhanced in gas depleted of molecules other than H2. The measured abundance even exceeds the present model predictions by about a factor ten. One possibility is that all CNO-bearing neutral species, including atomic oxygen, are almost completely (> 98%) frozen within a radius of ~2800 AU.
Scattering from dust in molecular clouds: Constraining the dust grain size distribution through near-infrared cloudshine and infrared coreshine
M. Andersen,J. Steinacker,W-F. Thi,L. Pagani,A. Bacmann,R. Paladini
Physics , 2013, DOI: 10.1051/0004-6361/201322102
Abstract: Context. The largest grains (0.5-1 micron) in the interstellar size distribution are efficient in scattering near- and mid-infrared radiation. These wavelengths are therefore particularly well suited to probe the still uncertain high-end of the size distribution. Aims. We investigate the change in appearance of a typical low-mass molecular core from the Ks (2.2 micron) band to the Spitzer IRAC 3.6 and 8 micron bands, and compare with model calculations, which include variations of the grain size distribution. Methods. We combine Spitzer IRAC and ground-based near-infrared observations to characterize the scattered light observed at the near- and mid-infrared wavelengths from the core L260. Using a spherical symmetric model core, we perform radiative transfer calculations to study the impact of various dust size distributions on the intensity profiles across the core. Results. The observed scattered light patterns in the Ks and 3.6 micron bands are found to be similar. By comparison with radiative transfer models the two profiles place constraints on the relative abundance of small and large (more than 0.25 micron) dust grains. The scattered light profiles are found to be inconsistent with an interstellar silicate grain distribution extending only to 0.25 micron and large grains are needed to reach the observed fluxes and the flux ratios. The shape of the Ks band surface brightness profile limits the largest grains to 1-1.5 micron. Conclusions. In addition to observing coreshine in the Spitzer IRAC channels, the combination with ground-based near-infrared observations are suited to constrain the properties of large grains in cores.
Upper limit for the D2H+ ortho-to-para ratio in the prestellar core 16293E (CHESS)
C. Vastel,P. Caselli,C. Ceccarelli,A. Bacmann,D. C. Lis,E. Caux,C. Codella,J. A. Beckwith,T. Ridley
Physics , 2012, DOI: 10.1051/0004-6361/201219616
Abstract: The H3+ ion plays a key role in the chemistry of dense interstellar gas clouds where stars and planets are forming. The low temperatures and high extinctions of such clouds make direct observations of H3+ impossible, but lead to large abundances of H2D+ and D2H+, which are very useful probes of the early stages of star and planet formation. The ground-state rotational ortho-D2H+ 111-000 transition at 1476.6 GHz in the prestellar core 16293E has been searched for with the Herschel/HIFI instrument, within the CHESS (Chemical HErschel Surveys of Star forming regions) Key Program. The line has not been detected at the 21 mK km/s level (3 sigma integrated line intensity). We used the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in this source to determine an upper limit on the ortho-to-para D2H+ ratio as well as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison between our chemical modeling and the observations suggests that the CO depletion must be high (larger than 100), with a density between 5e5 and 1e6 cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times higher than the value estimated from the chemical modeling, making it impossible to detect this high frequency transition with the present state of the art receivers.
Stratified NH and ND emission in the prestellar core 16293E in L1689N
A. Bacmann,F. Daniel,P. Caselli,C. Ceccarelli,D. Lis,C. Vastel,F. Dumouchel,F. Lique,E. Caux
Physics , 2015,
Abstract: High degrees of deuterium fractionation are commonly found in cold prestellar cores and in the envelopes around young protostars. As it brings strong constraints to chemical models, deuterium chemistry is often used to infer core history or molecule formation pathways. Whereas a large number of observations is available regarding interstellar deuterated stable molecules, relatively little is known about the deuteration of hydride radicals, as their fundamental rotational transitions are at high frequencies where the atmosphere is mostly opaque. Nitrogen hydride radicals are important species in nitrogen chemistry, as they are thought to be related to ammonia formation. Observations have shown that ammonia is strongly deuterated, with [NH2D]/[NH3] ~ 10%. Models predict similarly high [ND]/[NH] ratios, but so far only one observational determination of this ratio is available, towards the envelope of the protostar IRAS16293-2422. In order to test model predictions, we aim here at determining [ND]/[NH] in a dense, starless core. We observed NH and ND in 16293E with the HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS guaranteed time key programme, and derived the abundances of these two species using a non-LTE non-local radiative transfer model. Both NH and ND are detected in the source, with ND in emission and NH in absorption against the continuum arising from the cold dust emission. Our model shows however that the ND emission and the NH absorption originate from different layers in the cloud, as further evidenced by their different velocities. In the central region of the core, we can set a lower limit to the [ND]/[NH] ratio of ~2%. This estimate is consistent with recent pure gas-phase models of nitrogen chemistry
Grain size limits derived from 3.6 μm and 4.5 μm coreshine
J. Steinacker,M. Andersen,W. -F. Thi,R. Paladini,M. Juvela,A. Bacmann,V. -M. Pelkonen,L. Pagani,C. Lefèvre,Th. Henning,A. Noriega-Crespo
Physics , 2015, DOI: 10.1051/0004-6361/201425434
Abstract: Recently discovered scattered light from molecular cloud cores in the wavelength range 3-5 {\mu}m (called "coreshine") seems to indicate the presence of grains with sizes above 0.5 {\mu}m. We aim to analyze 3.6 and 4.5 {\mu}m coreshine from molecular cloud cores to probe the largest grains in the size distribution. We analyzed dedicated deep Cycle 9 Spitzer IRAC observations in the 3.6 and 4.5 {\mu}m bands for a sample of 10 low-mass cores. We used a new modeling approach based on a combination of ratios of the two background- and foreground-subtracted surface brightnesses and observed limits of the optical depth. The dust grains were modeled as ice-coated silicate and carbonaceous spheres. We discuss the impact of local radiation fields with a spectral slope differing from what is seen in the DIRBE allsky maps. For the cores L260, ecc806, L1262, L1517A, L1512, and L1544, the model reproduces the data with maximum grain sizes around 0.9, 0.5, 0.65, 1.5, 0.6, and > 1.5 {\mu}m, respectively. The maximum coreshine intensities of L1506C, L1439, and L1498 in the individual bands require smaller maximum grain sizes than derived from the observed distribution of band ratios. Additional isotropic local radiation fields with a spectral shape differing from the DIRBE map shape do not remove this discrepancy. In the case of Rho Oph 9, we were unable to reliably disentangle the coreshine emission from background variations and the strong local PAH emission. Considering surface brightness ratios in the 3.6 and 4.5 {\mu}m bands across a molecular cloud core is an effective method of disentangling the complex interplay of structure and opacities when used in combination with observed limits of the optical depth.
The Structure of Prestellar Cores as derived from ISO Observations
Aurore Bacmann,Philippe Andre,Derek Ward-Thompson
Physics , 2001,
Abstract: Observations of dark cloud cores have been carried out in the mid-infrared using ISOCAM and in the far-infrared using ISOPHOT, both aboard the Infrared Space Observatory. The cores are in most cases detected in emission at 200 and 170 micron, remain undetected at 90 micron and are seen in absorption against the diffuse mid-infrared background at 7 micron. The observations are consistent with the cores being pre-stellar and not having a central heating source, and yield core temperatures of ~ 11-13 K. We were able to determine the density structure of the cores up to radii that extend beyond the sensitivity limit of previous submillimetre studies. The column density profiles of the cores studied here flatten out in the centre, as shown by previous submillimetre continuum results, and interestingly, in a few cases, the profiles steepen with radius beyond ~ 5000-10000 AU and present sharp edges. This could indicate that these cores are decoupled from their parent molecular cloud and represent finite reservoirs of mass for subsequent accretion.
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