Shocks In Outflows Research Articles

Adiabatic–radiative shock systems in YSO jets and novae outflows

Context. The termination regions of non-relativistic jets in protostars and supersonic outflows in classical novae are non-thermal emitters. This has been confirmed by radio and gamma-ray detection, respectively. A two-shock system is expected to be formed in the termination region where the jet, or the outflow material, and the ambient medium impact. Radiative shocks are expected to form in these systems given their high densities. However, in the presence of high velocities, the formation of adiabatic shocks is also possible. A case of interest is when the two types of shocks occur simultaneously. Adiabatic shocks are more efficient at particle acceleration while radiative shocks strongly compress the gas. Furthermore, a combined adiabatic–radiative shock system is very prone to developing instabilities in the contact discontinuity, leading to mixing, turbulence, and density enhancement. Additionally, these dense non-relativistic jets and outflows are excellent candidates for laboratory experiments as demonstrated by magnetohydrodynamics scaling. Aims. We aim to study the combination of adiabatic and radiative shocks in protostellar jets and novae outflows. We focus on determining the conditions under which this combination is feasible together with its physical implications. Methods. We performed an analytical study of the shocks in both types of sources for a set of parameters by comparing cooling times and propagation velocities. We also estimated the timescales for the growth of instabilities in the contact discontinuity separating both shocks. We studied the hydrodynamical evolution of a jet colliding with an ambient medium with 2D numerical simulations, confirming our initial theoretical estimates. Results. We show that for a wide set of observationally constrained parameters, the combination of an adiabatic and a radiative shock is possible at the working surface of the termination region in jets from young stars and novae outflows. We find that instabilities are developed at the contact discontinuity, mixing the shocked materials. Additionally, we explore the magnetohydrodynamic parameter scaling required for studying protostellar jets and novae outflows using laboratory experiments on laser facilities. Conclusions. The coexistence of an adiabatic and a radiative shock is expected at the termination region of protostellar jets and novae outflows. This scenario is very promising for particle acceleration and gamma-ray emission. The parameters for scaled laboratory experiments are very much in line with plasma conditions achievable in currently operating high-power laser facilities. This provides a new means for studying novae outflows that has never been considered before.

Atomic Shocks in the Outflow of L1551 IRS 5 Identified with SOFIA-upGREAT Observations of [O i

Abstract We present velocity-resolved Stratospheric Observatory for Infrared Astronomy (SOFIA)/upgrade German REceiver for Astronomy at Terahertz Frequencies observations of [O i] and [C ii] lines toward a Class I protostar, L1551 IRS 5, and its outflows. The SOFIA observations detect [O i] emission toward only the protostar and [C ii] emission toward the protostar and the redshifted outflow. The [O i] emission has a width of ∼100 km s−1 only in the blueshifted velocity, suggesting an origin in shocked gas. The [C ii] lines are narrow, consistent with an origin in a photodissociation region. Differential dust extinction from the envelope due to the inclination of the outflows is the most likely cause of the missing redshifted [O i] emission. Fitting the [O i] line profile with two Gaussian components, we find one component at the source velocity with a width of ∼20 km s−1 and another extremely broad component at −30 km s−1 with a width of 87.5 km s−1, the latter of which has not been seen in L1551 IRS 5. The kinematics of these two components resemble cavity shocks in molecular outflows and spot shocks in jets. Radiative transfer calculations of the [O i], high-J CO, and H2O lines in the cavity shocks indicate that [O i] dominates the oxygen budget, making up more than 70% of the total gaseous oxygen abundance and suggesting [O]/[H] of ∼1.5 × 10−4. Attributing the extremely broad [O i] component to atomic winds, we estimate an intrinsic mass-loss rate of (1.3 ± 0.8) × 10−6 M ⊙ yr−1. The intrinsic mass-loss rates derived from low-J CO, [O i], and H i are similar, supporting the model of momentum-conserving outflows, where the atomic wind carries most momentum and drives the molecular outflows.

FOREIGN DIRECT INVESMENT OUTFLOWS AND ENVIRONMENTAL POLLUTION: HIDDEN COINTEGRATION ANALYSIS FOR TURKEY

The relationship between foreign direct investment, which is a type of cross-border and long-term investment, and environmental quality is a current issue that is heavily debated. Foreign direct invesments can ensure economic growth and development of countries, while also causing a change in environmental quality. In the research conducted, it is seen that changes in carbon dioxide emissions with foreign direct capital inflows are mainly investigated from the point of view of the host countries. However, foreign direct invesment outflows may have an impact on the environmental quality of the home country. Because foreign direct invesment outflows can enable the transfer of more environmentally friendly techonogies to the country and strengthen management skills. The impact of foreign direct investment outflows on the home country's environmental pollution is shaped by many factors (scale, technique, and composition effects). In addition to these effects, it is necessary to pay attention to the regional and sectoral distribution of capital outflows. The main aim of this study is to examine the links between Turkey's foreign direct invesment outflows and carbon dioxide emissions for the period 1990-2018. For this reason, a unit root test was applied to variables whose natural logarithm was taken. Tests showed that all series are stable of the same degree. Engle&Granger(1987) and Granger&Yoon(2002) tests were used to determine the cointegration relationship between variables. The crouching error correction model(CECM) was applied to determine the causality relationship. According to the results of the analysis; i) In terms of the Engle&Granger(1987) test, there was no long-term relationship between variables. ii) According to the Granger&Yoon(2002) test, it was determined that there is a bidirectional hidden cointegration relationship between the positive shocks of carbon dioxide emissions and negative shocks of foreign direct invesment outflows. iii) There is a bidirectional asymmetric causality relationship between the positive shocks of carbon dioxide emissions and the negative shocks of foreign direct invesment outflows. iv) It is observed that 1% negative shocks in foreign direct invesment outflows reduce positive shocks in carbon dioxide emissions by 0,26%. As a result, since negative situations in foreign direct invesment outflows have an effect on improving the quality of the environment, the environmental dimension should be taken into account in the policies to be made.

Reverse shocks in the relativistic outflows of gravitational wave-detected neutron star binary mergers

ABSTRACT The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission from shocks generated as an ultrarelativistic outflow decelerates. A forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave-detected neutron star mergers the reverse shock has been neglected. Here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. The off-axis reverse shock will appear as a brightening feature in the rising afterglow at radio frequencies. For bursts at ∼100 Mpc, the system should be inclined ≲20° for the reverse shock to be observable at ∼0.1–10 d post-merger. For structured outflows, enhancement of the reverse shock emission by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. Early radio photometry of the afterglow could reveal the presence of a strong magnetic field associated with the central engine.

Sub-photospheric Shocks in Relativistic Explosions

Abstract This paper examines the mechanism of internal shocks in opaque relativistic outflows, in particular in cosmological gamma-ray bursts. The shocks produce neutrino emission and affect the observed photospheric radiation from the explosion. They develop from internal compressive waves and can be of different types depending on the composition of the outflow. (1) Shocks in “photon gas,” with negligible plasma inertia, have a unique structure determined by the force-free condition—zero radiation flux in the plasma rest frame. Radiation dominance over plasma inertia suppresses the formation of collisionless shocks mediated by collective electromagnetic fields. (2) If the outflow is sufficiently magnetized, a strong collisionless subshock develops, which is embedded in a thicker radiation-mediated structure. (3) Waves in outflows with a free neutron component lead to dissipation through nuclear collisions. At large optical depths, shocks have a thickness comparable to the neutron free path, with embedded radiation-mediated and collisionless subshocks. The paper also presents first-principles simulations of magnetized flows filled with photons, demonstrating the formation of shocks and their structure. Simple estimates show that magnetized sub-photospheric shocks are efficient producers of photons and have a great impact on the observed photospheric radiation. The shock structure changes as the outflow expands toward its photosphere. The dissipation is accompanied by strong pair creation, and the -dressed shock carries the photosphere with it up to two decades in radius, emitting a strong pulse of nonthermal radiation.

H2S in the L1157-B1 bow shock

Sulfur-bearing molecules are highly reactive in the gas phase of the ISM. However, the form in which most of the sulfur is locked onto interstellar dust grains is unknown. By taking advantage of the short time-scales of shocks in young molecular outflows, one could track back the main form of sulfur in the ices. In this paper, six transitions of H$_2$S and its isotopologues in the L1157-B1 bowshock have been detected using data from the Herschel-CHESS survey and the IRAM-30m ASAI large program. These detections are used to calculate the properties of H$_2$S gas in L1157-B1 through use of a rotation diagram and to explore the possible carriers of sulfur on the grains. The isotopologue detections allow the first calculation of the H$_2$S deuteration fraction in an outflow from a low mass protostar. The fractional abundance of H$_2$S in the region is found to be 6.0$\times$10$^{-7}$ and the deuteration fraction is 2$\times$10$^{-2}$. In order to investigate the form of sulfur on the grains, a chemical model is run with four different networks, each with different branching ratios for the freeze out of sulfur bearing species into molecules such as OCS and H$_2$S. It is found that the model best fits the data when at least half of each sulfur bearing species hydrogenates when freezing. We therefore conclude that a significant fraction of sulfur in L1157-B1 is likely to be locked in H$_2$S on the grains.

Linking low- to high-mass young stellar objects withHerschel-HIFI observations of water

Water probes the dynamics in young stellar objects (YSOs) effectively, especially shocks in molecular outflows. It is a key molecule for exploring whether the physical properties of low-mass protostars can be extrapolated to massive YSOs. As part of the WISH key programme, we investigate the dynamics and the excitation conditions of shocks along the outflow cavity wall as function of source luminosity. Velocity-resolved Herschel-HIFI spectra of the H2O 988, 752, 1097 GHz and 12CO J=10-9, 16-15 lines were analysed for 52 YSOs with bolometric luminosities (L_bol) ranging from <1 to >10^5 L_sun. The profiles of the H2O lines are similar, indicating that they probe the same gas. We see two main Gaussian emission components in all YSOs: a broad component associated with non-dissociative shocks in the outflow cavity wall (cavity shocks) and a narrow component associated with quiescent envelope material. More than 60% of the total integrated intensity of the H2O lines (L_H2O) comes from the cavity shock component. The H2O line widths are similar for all YSOs, whereas those of 12CO 10-9 increase slightly with L_bol. The excitation analysis of the cavity shock component, performed with the non-LTE radiative transfer code RADEX, shows stronger 752 GHz emission for high-mass YSOs, likely due to pumping by an infrared radiation field. As previously found for CO, a strong correlation with slope unity is measured between log(L_H2O) and log(L_bol), which can be extrapolated to extragalactic sources. We conclude that the broad component of H2O and high-J CO lines originate in shocks in the outflow cavity walls for all YSOs, whereas lower-J CO transitions mostly trace entrained outflow gas. The higher UV field and turbulent motions in high-mass objects compared to their low-mass counterparts may explain the slightly different kinematical properties of 12CO 10-9 and H2O lines from low- to high-mass YSOs.

Recollimation shocks in the relativistic outflows of active galactic nuclei: Doctoral Thesis Award Lecture 2014

AbstractWe analysed the single‐dish radio light curves of the blazar CTA 102 during its major flare around April 2006. The modelling of these data revealed a possible travelling shock–recollimation shock interaction during the flare. To verify this hypothesis, we used multi‐epoch and multi‐frequency very‐long baseline interferometry (VLBI) observations and performed a detailed kinematic and spectral analysis. The results confirmed the hypothesis of a shock‐shock interaction causing the 2006 radio flare and provided indications for additional recollimation shocks farther downstream. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Life in the fast lane: H2O reveals the universal nature of shocks in outflows

Water is an unique tracer of motion in protostars. It has broad and complex line profiles, dominated by the jet/outflow system. We summarise the results of spectrally resolved H 2 O line profiles observed with Herschel HIFI from the “Water in star-forming regions with Herschel” (WISH, van Dishoeck et al. 2011), “William Herschel Line Legacy” (WILL, Mottram et al. in prep.) and Cygnus-X surveys. In total, more than 100 low-, intermediate and high-mass (LM, IM & HM) protostars have been observed with L bol from 0.1 to 10 5 L ⊙ . On Galactic scales, water traces star formation.

Shocks in nova outflows – I. Thermal emission

Growing evidence for shocks in nova outflows includes (1) multiple velocity components in the optical spectra; (2) hard X-ray emission starting weeks to months after the outburst; (3) an early radio flare on time-scales of months, in excess of that predicted from the freely expanding photoionized gas; and, perhaps most dramatically, (4) ∼ GeV gamma-ray emission. We present a one-dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated thermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass-loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the early radio maximum assuming that the DES possesses a characteristic velocity ∼103 km s−1 and mass ∼ few 10−4 M⊙; the former is consistent with the velocities of narrow-line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferred independently by modelling the late radio peak as uniformly expanding photoionized gas. Importantly, however, our thermal model can only explain the peak radio fluxes if line cooling of the post-shock gas at temperatures ∼106 K is suppressed below its collisional ionization equilibrium value for solar abundances due to photoionization; if this condition is not satisfied, this strongly suggests that the early radio peak is instead non-thermal (e.g. synchrotron) in origin. Rapid evolution of the early radio light curves requires the DES to possess a steep outer density profile, which may indicate that the onset of mass-loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Re-processed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission.

The search for the magnetic precursor of C-type shocks in young molecular outflows

The detection of very narrow SiO emission and the higher degree of excitation of the ion fluid compared to the neutral one towards the protostar L1448-mm have been interpreted as signatures of the magnetic precursor of C-shocks. Since this is the only source where these features have been observed together, we carried out a survey to map the J = 2→ 1 and 8→ 7 lines of SiO, and the J = 1→ 0 and 4→ 3 lines of HCO (as the representative ion) and HNC (as the representative neutral species) towards four class 0/I low-mass protostars known to have young outflows (L1448-IRS3, NGC 1333-IRAS 4A/B and SSV 13, VLA 1623 and IRAS 20353). As previously reported, towards L1448-IRS3 and NGC 1333 strong, narrow SiO 2→ 1 is detected, and we also find a velocity component towards each outflow which shows high a HCO 4→ 3/1→ 0 integrated intensity ratio compared to that of HNC. Towards VLA 1623, we find very weak, narrow SiO, and a component that is visible in HCO but not at all in HNC. For IRAS 20353, we find neither narrow SiO nor high HCO 4→ 3/1→ 0 flux ratios. Previous work explained the higher degree of excitation of H13CO+ by the selective excitation of ions by electrons, whose abundance is expected to increase at the precursor stage. Here, however, by considering a range of kinetic temperatures for the gas, we find that it is possible to fit the observations with a single temperature and density, without an increased electron abundance, despite the large differences in 4→ 3/1→ 0 ratio between the two molecules. The high H13CO+ 4→ 3/1→ 0 flux ratios may simply be due to protostellar heating. However, the hypothesis that narrow SiO is formed at the precursor stage of shocks remains plausible, since other explanations require either the presence of shocks with larger dynamical timescales, or predict the narrow SiO to have a larger spatial extent than it appears to have towards the sources L1448-IRS3 and VLA 1623. Larger scale maps of the narrow SiO emission are needed to confirm that it is confined to the regions around protostars (as would be expected in the precursor scenario), and mapping of different ion and neutral species is necessary to confirm whether there are indeed precursor regions surrounding young outflows where the ion and neutral species behave as separate fluids.

Detection of warm water vapour in Taurus protoplanetary discs byHerschel

Line spectra of 68 Taurus T Tauri stars were obtained with the Herschel-PACS (Photodetector Array Camera & Spectrometer) instrument as part of the GASPS (Gas Evolution in Protoplanetary Systems) survey of protoplanetary discs. A careful examination of the line scans centred on the [OI] 63.18 microns fine-structure line unveiled a line at 63.32 micron in some of these spectra. We identify this line with a transition of ortho-water. It is detected confidently (i.e., >3 sigma) in eight sources, i.e., 24% of the sub-sample with gas-rich discs. Several statistical tests were used to search for correlations with other disc and stellar parameters such as line fluxes of [OI] 6300 Armstrong and 63.18 microns; X-ray luminosity and continuum levels at 63 microns and 850 microns. Correlations are found between the water line fluxes and the [OI] 63.18 microns line luminosity, the dust continuum, and possibly with the stellar X-ray luminosity. This is the first time that this line of warm water vapour has been detected in protoplanetary discs. We discuss its origins, in particular whether it comes from the inner disc and/or disc surface or from shocks in outflows and jets. Our analysis favours a disc origin, with the observed water vapour line produced within 2-3AU from the central stars, where the gas temperature is of the order of 500-600 K.

EFFICIENCY OF INTERNAL SHOCKS IN MAGNETIZED RELATIVISTIC JETS

We study the dynamic and radiative efficiency of conversion of kinetic-to-thermal/magnetic energy by internal shocks in relativistic magnetized outflows. A parameter study of a large number of collisions of cylindrical shells is performed. We explore how, while keeping the total flow luminosity constant, the variable fluid magnetization influences the efficiency and find that the interaction of shells in a mildly magnetized jet yields higher dynamic, but lower radiative efficiency than in a non-magnetized flow. A multi-wavelength radiative signature of different shell magnetizations is computed assuming that relativistic particles are accelerated at internal shocks.

Simulations reveal fast mode shocks in magnetic reconnection outflows

Magnetic reconnection is commonly perceived to drive flow and particle acceleration in flares of solar, stellar, and astrophysical disk coronae but the relative roles of different acceleration mechanisms in a given reconnection environment are not well understood. While outflow fast mode shocks have been predicted analytically, we show for the first time via direct numerical simulations that such shocks do indeed occur in the outflows of fast reconnection when an obstacle is present. These shocks are distinct from the slow mode Petschek inflow shocks. If Fermi acceleration of electrons operates in the weak fast shocks, the associated compression ratios will induce a Fermi acceleration particle spectrum that is significantly steeper than strong fast shocks commonly studied, but consistent with the demands of solar flares. While this is not the only acceleration mechanism operating in a reconnection environment, it is plausibly a ubiquitous one.

Hot core chemistry in young stellar objects: protoplanetary disks and outflows

Hot core chemistry, characterized by the evaporation of icy mantle molecules from dust grains and subsequent gas-phase chemistry, seems ubiquitous in hot, dense clouds. Hot core molecules have been observed, not only in high-mass star-forming regions, but also in protoplanetary disks, shocks in young stellar outflows, and so on. Here, we study grain-surface chemistry and chemistry in hot gas in disks and outflows. Our results suggest that observations of molecular lines by the forthcoming ALMA and other facilities will give us information on, for example, grain-surface chemistry in the outer disk, turbulent mixing in the inner disk, and the physical and chemical conditions at the launching points of outflows.

Water cooling of shocks in protostellar outflows

In the framework of the Water in Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H2O 179 um transition obtained toward the young outflow L1157. The 179 um map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the water abundance and total cooling. Strong H2O peaks are localized on both shocked emission knots and the central source position. The H2O 179 um emission is spatially correlated with emission from H2 rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH3. A strong H2O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 um intensity and its intensity ratio to the H2O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H2O abundance of the order of 10^-4. This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total water cooling is ~10^-1 Lo, about 40% of the cooling due to H2 and 23% of the total energy released in shocks along the L1157 outflow.

X-ray emission characteristics of two Wolf-Rayet binaries: V444 Cyg and CD Cru

We present X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru using the data from observations with XMM-Newton. The X-ray light curves show the phase locked variability in both binaries, where the flux increased by a factor of $\sim 2$ in the case of V444 Cyg and $\sim 1.5$ in the case of CD Cru from minimum to maximum. The maximum luminosities in the 0.3--7.5 keV energy band were found to be $5.8\times10^{32}$ and $2.8\times10^{32}$ erg s$^{-1}$ for V444 Cyg and CD Cru, respectively. X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe, and are found to be consistent with a two-temperature plasma model. The cooler plasma at a temperature of $\sim$ 0.6 keV was found to be constant at all phases of both binaries, and could be due to a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with orbital phase and found to be maximum during primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components.

On the dynamic efficiency of internal shocks in magnetized relativistic outflows

We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic energy of internal shocks in relativistic magnetized outflows. We model internal shocks as being caused by collisions of shells of plasma with the same energy flux and a non-zero relative velocity. The contact surface, where the interaction between the shells takes place, can break up either into two oppositely moving shocks (in the frame where the contact surface is at rest), or into a reverse shock and a forward rarefaction. We find that for moderately magnetized shocks (magnetization $\sigma\simeq 0.1$), the dynamic efficiency in a single two-shell interaction can be as large as 40%. Thus, the dynamic efficiency of moderately magnetized shocks is larger than in the corresponding unmagnetized two-shell interaction. If the slower shell propagates with a sufficiently large velocity, the efficiency is only weakly dependent on its Lorentz factor. Consequently, the dynamic efficiency of shell interactions in the magnetized flow of blazars and gamma-ray bursts is effectively the same. These results are quantitatively rather independent on the equation of state of the plasma. The radiative efficiency of the process is expected to be a fraction $f_r<1$ of the estimated dynamic one, the exact value of $f_r$ depending on the particularities of the emission processes which radiate away the thermal or magnetic energy of the shocked states.

Internal shocks in relativistic outflows: collisions of magnetized shells

(Abridged): We study the collision of magnetized irregularities (shells) in relativistic outflows in order to explain the origin of the generic phenomenology observed in the non-thermal emission of both blazars and gamma-ray bursts. We focus on the influence of the magnetic field on the collision dynamics, and we further investigate how the properties of the observed radiation depend on the strength of the initial magnetic field and on the initial internal energy density of the flow. The collisions of magnetized shells and the radiation resulting from these collisions are calculated using the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the shells with the external medium prior to their collision is also determined using an exact solver for the corresponding 1D relativistic magnetohydrodynamic Riemann problem. Our simulations show that two magnetization parameters - the ratio of magnetic energy density and thermal energy density, \alpha_B, and the ratio of magnetic energy density and mass-energy density, \sigma - play an important role in the pre-collision phase, while the dynamics of the collision and the properties of the light curves depend mostly on the magnetization parameter \sigma. The interaction of the shells with the external medium changes the flow properties at their edges prior to the collision. For sufficiently dense shells moving at large Lorentz factors (\simgt 25) these properties depend only on the magnetization parameter \sigma. Internal shocks in GRBs may reach maximum efficiencies of conversion of kinetic into thermal energy between 6% and 10%, while in case of blazars, the maximum efficiencies are \sim 2%.

A ‘dry’ condensation origin for circumstellar carbonates

The signature of carbonate minerals has long been suspected in the mid-infrared spectra of various astrophysical environments such as protostars. Abiogenic carbonates are considered as indicators of aqueous mineral alteration in the presence of CO2-rich liquid water. The recent claimed detection of calcite associated with amorphous silicates in two planetary nebulae and protostars devoid of planetary bodies questions the relevance of this indicator; but in the absence of an alternative mode of formation under circumstellar conditions, this detection remains controversial. The main dust component observed in circumstellar envelopes is amorphous silicates, which are thought to have formed by non-equilibrium condensation. Here we report experiments demonstrating that carbonates can be formed with amorphous silicates during the non-equilibrium condensation of a silicate gas in a H2O-CO2-rich vapour. We propose that the observed astrophysical carbonates have condensed in H2O(g)-CO2(g)-rich, high-temperature and high-density regions such as evolved stellar winds, or those induced by grain sputtering upon shocks in protostellar outflows.