Context.
The latest evolutionary phases of low- and intermediate-mass stars are characterized by complex physical processes like turbulence, convection, stellar pulsations, magnetic fields, ...condensation of solid particles, and the formation of massive outflows that inject freshly produced heavy elements and dust particles into the interstellar medium.
Aims.
By investigating individual objects in detail, we wish to analyze and disentangle the effects of the interrelated physical processes on the structure of the wind-forming regions around them.
Methods.
We use the Northern Extended Millimeter Array to obtain spatially and spectrally resolved observations of the semi-regular asymptotic giant branch (AGB) star RS Cancri and apply detailed 3D reconstruction modeling and local thermodynamic equilibrium radiative transfer calculations in order to shed light on the morpho-kinematic structure of its inner, wind-forming environment.
Results.
We detect 32 lines of 13 molecules and isotopologs (CO, SiO, SO, SO
2
, H
2
O, HCN, PN), including several transitions from vibrationally excited states. HCN, H
13
CN, and millimeter vibrationally excited H
2
O, SO,
34
SO, SO
2
, and PN are detected for the first time in RS Cnc. Evidence for rotation is seen in HCN, SO, SO
2
, and SiO(
v
= 1). From CO and SiO channel maps, we find an inner, equatorial density enhancement, and a bipolar outflow structure with a mass-loss rate of 1 × 10
−7
M
⊙
yr
−1
for the equatorial region and of 2 × 10
−7
M
⊙
yr
−1
for the polar outflows. The
12
CO/
13
CO ratio is measured to be ~20 on average, 24 ± 2 in the polar outflows and 19 ± 3 in the equatorial region. We do not find direct evidence of a companion that might explain this kind of kinematic structure, and explore the possibility that a magnetic field might be the cause of it. The innermost molecular gas is influenced by stellar pulsation and possibly by convective cells that leave their imprint on broad wings of certain molecular lines, such as SiO and SO.
Conclusions.
RS Cnc is one of the few nearby, low-mass-loss-rate, oxygen-rich AGB stars with a wind displaying both an equatorial disk and bipolar outflows. Its orientation with respect to the line of sight is particularly favorable for a reliable study of its morpho-kinematics. Nevertheless, the mechanism causing early spherical symmetry breaking remains uncertain, calling for additional high spatial- and spectral-resolution observations of the emission of different molecules in different transitions, along with more thorough investigation of the coupling among the different physical processes at play.
We propose a new statistical model that can reproduce the hierarchical nature of the ubiquitous filamentary structures of molecular clouds. This model is based on the multiplicative random cascade, ...which is designed to replicate the multifractal nature of intermittency in developed turbulence. We present a modified version of the multiplicative process where the spatial fluctuations as a function of scales are produced with the wavelet transforms of a fractional Brownian motion realisation. This simple approach produces naturally a log-normal distribution function and hierarchical coherent structures. Despite the highly contrasted aspect of these coherent structures against a smoother background, their Fourier power spectrum can be fitted by a single power law. As reported in previous works using the multiscale non-Gaussian segmentation (MnGSeg) technique, it is proven that the fit of a single power law reflects the inability of the Fourier power spectrum to detect the progressive non-Gaussian contributions that are at the origin of these structures across the inertial range of the power spectrum. The mutifractal nature of these coherent structures is discussed, and an extension of the MnGSeg technique is proposed to calculate the multifractal spectrum that is associated with them. Using directional wavelets, we show that filamentary structures can easily be produced without changing the general shape of the power spectrum. The cumulative effect of random multiplicative sequences succeeds in producing the general aspect of filamentary structures similar to those associated with star-forming regions. The filamentary structures are formed through the product of a large number of random-phase linear waves at different spatial wavelengths. Dynamically, this effect might be associated with the collection of compressive processes that occur in the interstellar medium.
Stokes drift dynamos HERREMAN, W.; LESAFFRE, P.
Journal of fluid mechanics,
07/2011, Letnik:
679
Journal Article
Recenzirano
Fluid particles can have a mean motion in time, even when the Eulerian mean flow disappears everywhere in space. In the present article, we demonstrate that this phenomenon, known as the Stokes ...drift, plays an essential role in the problem of magnetic field generation by fluctuation flows (kinematic dynamo) at high Rm. At leading order, the dynamo is generated by the Stokes drift that acts as if it were a mean flow. This result is derived from a mean-field dynamo theory in terms of time averages, which reveals how classical expressions for alpha and beta tensors actually recombine into a single Stokes drift contribution. In a test case, we find fluctuation flows that have a G. O. Roberts flow as Stokes drift and this allows to confront our model to direct integration of the induction equation. We find an excellent quantitative agreement between the prediction of our model and the results of our simulations. We finally apply our Stokes drift model to prove that a broad class of inertial waves in rapidly rotating flows cannot drive a dynamo.
Context. It is widely accepted that the onset of the explosive carbon burning in the core of a carbon-oxygen white dwarf (CO WD) triggers the ignition of a type Ia supernova (SN Ia). The features of ...the ignition are among the few free parameters of the SN Ia explosion theory. Aims. We explore the role for the ignition process of two different issues: firstly, the ignition is studied in WD models coming from different accretion histories. Secondly, we estimate how a different reaction rate for C-burning can affect the ignition. Methods. Two-dimensional hydrodynamical simulations of temperature perturbations in the WD core (“bubbles”) are performed with the FLASH code. In order to evaluate the impact of the C-burning reaction rate on the WD model, the evolution code FLASH_THE_TORTOISE from Lesaffre et al. (2006, MNRAS, 368, 187) is used. Results. In different WD models a key role is played by the different gravitational acceleration in the progenitor's core. As a consequence, the ignition is disfavored at a large distance from the WD center in models with a larger central density, resulting from the evolution of initially more massive progenitors. Changes in the C reaction rate at $T \la 5 \times 10^8~{\rm K}$ slightly influence the ignition density in the WD core, while the ignition temperature is almost unaffected. Recent measurements of new resonances in the C-burning reaction rate (Spillane et al. 2007, Phys. Rev. Lett., 98, 122501) do not affect the core conditions of the WD significantly. Conclusions. This simple analysis, performed on the features of the temperature perturbations in the WD core, should be extended in the framework of the state-of-the-art numerical tools for studying the turbulent convection and ignition in the WD core. Future measurements of the C-burning reactions cross section at low energy, though certainly useful, are not expected to affect our current understanding of the ignition process dramatically.
With an aim of probing the physical conditions and excitation mechanisms of warm molecular gas in individual star-forming regions, we performed Herschel SPIRE Fourier Transform Spectrometer (FTS) ...observations of 30 Doradus in the Large Magellanic Cloud. In our FTS observations, important far-infrared (FIR) cooling lines in the interstellar medium, including CO J = 4–3 to J = 13–12, C I 370 μm, and N II 205 μm, were clearly detected. In combination with ground-based CO J = 1–0 and J = 3–2 data, we then constructed CO spectral line energy distributions (SLEDs) on ~10 pc scales over a ~60 pc × 60 pc area and found that the shape of the observed CO SLEDs considerably changes across 30 Doradus. For example, the peak transition Jp varies from J = 6–5 to J = 10–9, while the slope characterized by the high-to-intermediate J ratio α ranges from ~0.4 to ~1.8. To examine the source(s) of these variations in CO transitions, we analyzed the CO observations, along with C II 158 μm, C I 370 μm, O I 145 μm, H2 0–0 S(3), and FIR luminosity data, using state-of-the-art models of photodissociation regions and shocks. Our detailed modeling showed that the observed CO emission likely originates from highly compressed (thermal pressure P∕kB ~ 107–109 K cm−3) clumps on ~0.7–2 pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field GUV ~ 103–105 Mathis fields) or low-velocity C-type shocks (pre-shock medium density npre ~ 104–106 cm−3 and shock velocity vs ~ 5–10 km s−1). Considering the stellar content in 30 Doradus, however, we tentatively excluded the stellar origin of CO excitation and concluded that low-velocity shocks driven by kiloparsec-scale processes (e.g., interaction between the Milky Way and the Magellanic Clouds) are likely the dominant source of heating for CO. The shocked CO-bright medium was then found to be warm (temperature T ~ 100–500 K) and surrounded by a UV-regulated low-pressure component (P∕kB ~ a few (104 –105) K cm−3) that is bright in C II 158 μm, C I 370 μm, O I 145 μm, and FIR dust continuum emission.
Aims. Previous literature suggests that the densest structures in the interstellar medium form through colliding flows, but patent evidence of this process is still missing. Recent literature ...proposes using SiO line emission to trace low-velocity shocks associated with cloud formation through collision. In this paper we investigate the bright and extended SiO(2−1) emission observed along the ~5 pc-long W43-MM1 ridge to determine its origin. Methods. We used high angular resolution images of the SiO(2−1) and HCN(1−0) emission lines obtained with the IRAM plateau de Bure (PdBI) interferometer and combined with data from the IRAM 30 m radiotelescope. These data were complemented by a Herschel column density map of the region. We performed spectral analysis of SiO and HCN emission line profiles to identify protostellar outflows and spatially disentangle two velocity components associated with low- and high-velocity shocks. Then, we compared the low-velocity shock component to a dedicated grid of one-dimensional (1D) radiative shock models. Results. We find that the SiO emission originates from a mixture of high-velocity shocks caused by bipolar outflows and low-velocity shocks. Using SiO and HCN emission lines, we extract seven bipolar outflows associated with massive dense cores previously identified within the W43-MM1 mini-starburst cluster. Comparing observations with dedicated Paris-Durham shock models constrains the velocity of the low-velocity shock component from 7 to 12 km s-1. Conclusions. The SiO arising from low-velocity shocks spreads along the complete length of the ridge. Its contribution represents at least 45% and up to 100% of the total SiO emission depending on the area considered. The low-velocity component of SiO is most likely associated with the ridge formation through colliding flows or cloud-cloud collision.
H2, H3+ and the age of molecular clouds and prestellar cores Pagani, L.; Lesaffre, P.; Roueff, E. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2012, Letnik:
370, Številka:
1978
Journal Article
Recenzirano
Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in ...understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H3+ into H2D+ to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H2 that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H2 phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO+ and N2D+ argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H2 remains comparable to para-H2 in abundance outside the dense cores.
Aims. We probe the effect of turbulent diffusion on the chemistry at the interface between a cold neutral medium (CNM) cloudlet and the warm neutral medium (WNM). Methods. We perform moving grid, ...multifluid, 1D, hydrodynamical simulations with chemistry including thermal and chemical diffusion. The diffusion coefficients are enhanced to account for turbulent diffusion. We post-process the steady-states of our simulations with a crude model of radiative transfer to compute line profiles. Results. Turbulent diffusion spreads out the transition region between the CNM and the WNM. We find that the CNM slightly expands and heats up: its CH and H sub(2) content decreases due to the lower density. The change of physical conditions and diffusive transport increase the H super(+) content in the CNM which results in increased OH and H sub(2) O. Diffusion transports some CO out of the CNM. It also brings H sub(2) into contact with the warm gas with enhanced production of CH super(+), H sub(3) super(+), OH and H sub(2) O at the interface. O lines are sensitive to the spread of the thermal profile in the intermediate region between the CNM and the WNM. Enhanced molecular content at the interface of the cloud broadens the molecular line profiles and helps exciting transitions of intermediate energy. The relative molecular yields are found higher for bigger clouds. Conclusions. Turbulent diffusion can be the source of additional molecular production and should be included in chemical models of the interstellar medium (ISM). It also is a good candidate for the interpretation of observational problems such as warm H sub(2), CH super(+) formation and presence of H sub(3) super(+).
The double pulsar J0737-3039 has become an important astrophysical laboratory for testing fundamental physics. Here we demonstrate that the low measured mass of Pulsar B can be used to constrain the ...equation of state of neutron star matter under the assumption that it formed in an electron-capture supernova. We show that the observed orbital parameters as well as the likely evolutionary history of the system support such a hypothesis, and discuss future refinements that will improve the constraints this test may provide.
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