Absence of coreshine in the Gum/Vela region

Pagani, L.; Lefèvre, Charlène; Bacmann, A.; Steinacker, J.

doi:10.1051/0004-6361/201219053

Cited by 9 publications

(10 citation statements)

References 42 publications

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“…For a special region, namely the Gum/Vela region, Pagani et al (2012) found a drastically reduced occurrence of scattered light (3 detections plus 3 uncertain cases out of 24 objects). They did not attribute this absence to systematic detection difficulties but to the action of a supernova remnant blast wave which has affected the grain size population in this region.…”

Section: Introductionmentioning

confidence: 92%

Detecting scattered light from low-mass molecular cores at 3.6μm

Steinacker

Andersen

Thi

et al. 2014

A&A

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Context. Recently discovered scattered light at 3−5 μm from low-mass cores (so-called "coreshine") reveals the presence of grains around 1 μm, 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. Aims. We aim to supply criteria for detecting scattered light at 3.6 μm from molecular cloud cores. Methods. 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. We analyze how scattered light can outshine the absorbing effect of extinction in front of background radiation by the core for given grain properties, anisotropic interstellar radiation field and background field. 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. Results. 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 μm. 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 μm, 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. Conclusions. The detection of coreshine at a wavelength of 3.6 μm is a complex interplay of the incident radiation, the extinction of the background radiation, the grain properties, and the core properties like sky position and mass.

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Section: Introductionmentioning

confidence: 92%

Detecting scattered light from low-mass molecular cores at 3.6μm

Steinacker

Andersen

Thi

et al. 2014

A&A

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“…The only known objects without observable coreshine, i.e. with no indications for large grains, are in Gum/Vela region, where Pagani et al (2012) suspect a nearby supernova to be responsible for large dust destruction.…”

Section: Introductionmentioning

confidence: 99%

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Dzyurkevich

Commerçon

Lesaffre

et al. 2017

A&A

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Context. Both theory and observations of star-forming clouds require the simulations which combine the co-evolving chemistry, magneto-hydrodynamics and radiative transfer in protostellar collapse simulation. A detailed knowledge of self-consistent chemical evolution for the main charge carriers (both gas species and dust grains) allows to correctly estimate the rate and nature of magnetic dissipation in the collapsing core. Last is of crucial importance for answering the grand question of star and planet formation: the magnitude and spatial distribution of magnetic flux as the initial condition to protoplanetary disk evolution. Aims. We use a chemo-dynamical version of RAMSES, described in a companion publication, to follow the chemo-dynamical evolution of collapsing dense cores with the various dust properties and interpret the occuring differences in the magnetic diffusivity terms. Later are of crucial importance for the circumstellar disk formation. Methods. We perform 3D chemo-dynamical simulations of 1 M⊙ isolated dense core collapse for a range in the dust size assumptions. The number density of dust and it's mean size are affecting the efficiency of charge capturing and the formation of ices. The radiative hydrodynamics and dynamical evolution of chemical abundances are used to reconstruct the magnetic diffusivity terms for clouds with various magnetisation. Results. The simulations are performed for a mean dust size ranging from 0.017µm to 1µm, and we adopt both a fixed dust size and a dust size distribution. The chemical abundances for this range of dust sizes are produced by RAMSES and serve as an input to calculations of Ohmic, ambipolar and Hall diffusivity terms. Ohmic resistivity only play a role at the late stage of the collapse, in the innermost region of the cloud where gas density exceeds few times 10 13 cm −3 . Ambipolar diffusion is a dominant magnetic diffusivity term in cases where mean dust size is a typical ISM value or larger. We demonstrate that the assumption of a fixed 'dominant ion' mass can lead to one order of magnitude mismatch in the ambipolar diffusion magnitude. 'Negative' Hall effect is dominant during the collapse in case of small dust, i.e. for the mean dust size of 0.02 µm and smaller, the effect which we connect to the dominance of negatively charged grains. We find that the Hall effect reverses its sign for mean dust size of 0.1µm and smaller. The phenomenon of the sign reversal is strongly depending on the number of negatively charged dust relative to the ions, and quality of coupling of the last to the magnetic fields. We have adopted different strength of magnetic fields, β = Pgas/Pmag = 2, 5, 25. We observe that the variation on the field strength only shifts the Hall effect reversal along the radius of the collapsing cloud, but do not prevent it. Conclusions. The dust grain mean size appears to be the parameter with the strongest impact on the magnitude of the magnetic diffusivity, dividing the collapsing clouds in Hall-dominated and ambipolar-dominated cloud...

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“…These big grains have built up from interstellar standard grains, and by applying a growth model such as those of Ormel et al (2009), it should be possible to determine the age of the cloud. As a first example, we made a simple application of the coreshine effect to study the age of clouds in the Gum/Vela region (Pagani et al 2012b). The other one is linked to chemistry but in a particular case where the chemistry is simple and the initial conditions known or of limited influence.…”

Section: Introductionmentioning

confidence: 99%

Ortho-H₂and the age of prestellar cores

Pagani

Lesaffre

Jorfi

et al. 2013

A&A

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Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based on the regulation of the deuteration by the abundance of ortho-H 2 that slowly decays away from the ortho-para statistical ratio of 3 down to or less than 0.001. We use a chemical network fully coupled to a hydrodynamical model that follows the contraction of a cloud, starting from uniform density, and reaches a density profile typical of a prestellar core. We compute the N 2 D + /N 2 H + ratio along the density profile. The disappearance of ortho-H 2 is tied to the duration of the contraction and the N 2 D + /N 2 H + ratio increases in the wake of the ortho-H 2 abundance decrease. By adjusting the time of contraction, we obtain different deuteration profiles that we can compare to the observations. Our model can test fast contractions (from 10 4 to 10 6 cm −3 in ∼0.5 My) and slow contractions (from 10 4 to 10 6 cm −3 in ∼5 My). We have tested the sensitivity of the models to various initial conditions. The slowcontraction deuteration profile is approximately insensitive to these variations, while the fast-contraction deuteration profile shows significant variations. We found that, in all cases, the deuteration profile remains clearly distinguishable whether it comes from the fast collapse or the slow collapse. We also study the para-D 2 H + /ortho-H 2 D + ratio and find that its variation is not monotonic, so it does not discriminate between models. Applying this model to L183 (=L134N), we find that the N 2 D + /N 2 H + ratio would be higher than unity for evolutionary timescales of a few megayears independently of other parameters, such as cosmic ray ionization rate or grain size (within reasonable ranges). A good fit to the observations is only obtained for fast contraction (≤0.7 My from the beginning of the contraction and ≤4 My from the birth of the molecular cloud based on the need to keep a high ortho-H 2 abundance when the contraction starts -ortho-H 2 /para-H 2 ≥ 0.2 -to match the observations). This chemical clock therefore rules out slow contraction in L183 and steady-state chemical models, since steady state is clearly not reached here. This clock should be applied to other cores to help distinguish slow and fast contraction theories over a large sample of cases.

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Absence of coreshine in the Gum/Vela region

Cited by 9 publications

References 42 publications

Detecting scattered light from low-mass molecular cores at 3.6μm

Detecting scattered light from low-mass molecular cores at 3.6μm

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Ortho-H₂and the age of prestellar cores

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Absence of coreshine in the Gum/Vela region

Cited by 9 publications

References 42 publications

Detecting scattered light from low-mass molecular cores at 3.6μm

Detecting scattered light from low-mass molecular cores at 3.6μm

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

Ortho-H2and the age of prestellar cores

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Ortho-H₂and the age of prestellar cores