We report the first detection of the ground-state rotational transition of the methylidyne cation CH+ towards the massive star-forming region DR 21 with the HIFI instrument onboard the Herschel ...satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR 21 molecular ridge and foreground gas. These observations allow us to determine a 12CH+J = 1–0 line frequency of ν = 835 137 ± 3 MHz, in good agreement with a recent experimental determination. We estimate the CH+ column density to be a few 1013 cm-2 in the gas seen in emission, and >1014 cm-2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance CH+ /H > 1.2 × 10-8. We show that the CH+ column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines.
Thanks to a stellar evolution code that is able to compute through the C flash we link the binary population synthesis of single degenerate progenitors of Type Ia supernovae (SNe Ia) to their ...physical condition at the time of ignition. We show that there is a large range of possible ignition densities and we detail how their probability distribution depends on the accretion properties. The low-density peak of this distribution qualitatively reminds of the clustering of the luminosities of Branch-normal SNe Ia. We tighten the possible range of initial physical conditions for explosion models: they form a one-parameter family, independent of the metallicity. We discuss how these results may be modified if we were to relax our hypothesis of a permanent Hachisu wind or if we were to include electron captures.
We present time-dependent 1D simulations of multifluid magnetic shocks with chemistry resolved down to the mean free path. They are obtained with an adaptive moving grid implemented with an implicit ...scheme. We examine a broad range of parameters relevant to conditions in dense molecular clouds, with preshock densities 10 super(3) cm super(-3) < n < 10 super(5) cm super(-3), velocities 10 km s super(-1) < u < 40 km s super(-1), and three different scalings for the transverse magnetic field: B = 0, 0.1, 1 mu Gx square root n/cm super(-3). We first use this study to validate the results of Chieze et al. (1998, MNRAS, 295, 672), in particular the long delays necessary to obtain steady C-type shocks, and we provide evolutionary time-scales for a much greater range of parameters. We also present the first time-dependent models of dissociative shocks with a magnetic precursor, including the first models of stationary CJ shocks in molecular conditions. We find that the maximum speed for steady C-type shocks is reached before the occurrence of a sonic point in the neutral fluid, unlike previously thought. As a result, the maximum speed for C-shocks is lower than previously believed. Finally, we find a large amplitude bouncing instability in J-type fronts near the H sub(2) dissociation limit (u 25-30 km s super(-1)), driven by H sub(2) dissociation/reformation. At higher speeds, we find an oscillatory behaviour of short period and small amplitude linked to collisional ionisation of H. Both instabilities are suppressed after some time when a magnetic field is present. In a companion paper, we use the present simulations to validate a new semi-analytical construction method for young low-velocity magnetic shocks based on truncated steady-state models.
Doubly-degenerate binary systems consisting of two white dwarfs (WDs) both composed of carbon and oxygen and close enough that mass is transferred from the less massive to the more massive are ...possible progenitors of Type Ia supernovae. If the mass-transfer rate is slow enough that the accreting WD can reach a mass of 1.38 M⊙, then it can ignite carbon degenerately at its centre. This can lead to a thermonuclear runaway and hence a supernova explosion. However, if the accretion rate is too high the outer layers of the WD heat up too much and carbon ignites there non-degenerately. A series of mild carbon flashes can then propagate inwards and convert the carbon to neon relatively gently. There is no thermonuclear runaway and no supernova. We examine the critical accretion rate at which ignition switches from the centre to the surface for a variety of WDs and find it to be about two-fifths of the Eddington rate. In a real binary star, the mass-transfer rate falls off as mass transfer proceeds and the system widens. Even if the initial transfer rate is high enough for carbon to ignite at the outer edge, if such rapid accretion were to persist, we find that it can extinguish if the rate drops sufficiently quickly. The interior of the WD remains carbon rich and, if sufficient mass can still be transferred from the companion, it can eventually ignite degenerately at the centre. The primary WD must be about 1.1 M⊙ or above and the companion about 0.3 M⊙. Though WDs of such low mass are expected to be pure helium, we note that a star of initial mass 2.5 M⊙ has a CO core of about 0.3 M⊙ when it begins to ascend the asymptotic giant branch. Alternatively, if the accretion rate can be limited to a maximum of 0.46 of the Eddington rate, then a 1.1-M⊙ WD accretes sufficiently slowly to explode from a companion WD of any large enough mass.
We derive a new formalism for convective motions involving two radial flows. This formalism provides a framework for convective models that guarantees consistency for the chemistry and the energy ...budget in the flows, allows time dependence and accounts for the interaction of the convective motions with the global contraction or expansion of the star. In the one-stream limit, the formalism reproduces several existing convective models and allows them to treat the chemistry in the flows. We suggest a version of the formalism that can be implemented easily in a stellar evolution code. We then apply the formalism to convective Urca cores in Chandrasekhar-mass white dwarfs and compare it to previous studies. We demonstrate that, in degenerate matter, nuclear reactions which change the number of electrons strongly influence the convective velocities, and we show that the net energy budget is sensitive to the mixing. We illustrate our model by computing stationary convective cores with Urca nuclei. Even a very small mass fraction of Urca nuclei (as little as 10−8) strongly influences the convective velocities. We conclude that the proper modelling of the Urca process is essential for determining the ignition conditions for the thermonuclear runaway in Chandrasekhar-mass white dwarfs.
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.
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
μ
m, and N
II
205
μ
m, 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
J
p
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
μ
m, C
I
370
μ
m, O
I
145
μ
m, H
2
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
∕
k
B
~ 10
7
–10
9
K cm
−3
) clumps on ~0.7–2 pc scales, which could be produced by either ultraviolet (UV) photons (UV radiation field
G
UV
~ 10
3
–10
5
Mathis fields) or low-velocity C-type shocks (pre-shock medium density
n
pre
~ 10
4
–10
6
cm
−3
and shock velocity
v
s
~ 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
∕
k
B
~ a few (10
4
–10
5
) K cm
−3
) that is bright in C
II
158
μ
m, C
I
370
μ
m, O
I
145
μ
m, and FIR dust continuum emission.
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–H2 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 N2D+/N2H+ ratio along the density profile. The disappearance of ortho-H2 is tied to the duration of the contraction and the N2D+/N2H+ ratio increases in the wake of the ortho-H2 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 104 to 106 cm-3 in ~0.5 My) and slow contractions (from 104 to 106 cm-3 in ~5 My). We have tested the sensitivity of the models to various initial conditions. The slow-contraction 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-D2H+/ortho-H2D+ 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 N2D+/N2H+ 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-H2 abundance when the contraction starts – ortho-H2/para-H2 ≥ 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.
We compute numerical simulations of spherical collapse triggered by a slow increase in external pressure. We compare isothermal models to models including cooling with a simple but self-consistent ...treatment of the coupling between gas, grains and radiation field temperatures. The hydrostatic equilibrium appears to hold past the marginally stable state, until the collapse proceeds. The last hydrostatic state before collapse has a lower central gas temperature in the centre due to the enhanced coupling between gas, grains and radiation field. This results in slightly lower pressure gradients in the bulk of the envelope which is hence slightly more extended than in the isothermal case. Due to the sensitivity of the collapse on these initial conditions, protostellar infall velocities in the envelope turn out to be much slower in the case with cooling. Our models also compute the radiative transfer and a rather large chemical network coupled to gas dynamics. However, we note that the steady-state chemisorption of CO is sufficient to provide an accurate cooling function of the gas. This justifies the use of post-processing techniques to account for the abundance of observed molecules. Existing observations of infall signatures put very stringent constraints on the kinematics and temperature profile of the class 0 protostar IRAM 04191+1522. We show that isothermal models fail to account for the innermost slow infall motions observed, even with the most hydrostatic initial conditions. In contrast, models with cooling reproduce the general shape of the temperature profile inferred from observations and are in much better agreement with the infall signatures in the inner 3000 AU.
In the first paper of this series (Paper I) we computed time dependent simulations of multifluid shocks with chemistry and a transverse magnetic field frozen in the ions, using an adaptive moving ...grid. In this paper, we present new analytical results on steady-state molecular shocks. Relationships between density and pressure in the neutral fluid are derived for the cold magnetic precursor, hot magnetic precursor, adiabatic shock front, and the following cooling layer. The compression ratio and temperature behind a fully dissociative adiabatic shock is also derived. To prove that these results may even hold for intermediate ages, we design a test to locally characterise the validity of the steady state equations in a time-dependent shock simulation. Applying this tool to the results of Paper I, we show that most of these shocks (all the stable ones) are indeed in a quasi-steady state at all times, i.e.: a given snapshot is composed of one or more truncated steady shock. Finally, we use this property to produce a construction method of any intermediate time of low velocity shocks (u < 20 km s super(-1)) with only a steady-state code. In particular, this method allows one to predict the occurrence of steady CJ-type shocks more accurately than previously proposed criteria.
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