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Context. The process of mass accretion in the earliest phases of star formation is still not fully understood: does the accretion rate smoothly decline with the age of the protostar or are there short, intermittent accretion bursts? The latter option would also yield the possibility for very low-luminosity objects (VeLLOs) to be precursors of solar-type stars, even though they do not seem to have sufficiently high accretion rates to reach stellar masses during their protostellar lifetime. Nevertheless, probing such intermittent events in the deeply embedded phase is not easy. Chemical signatures in the protostellar envelope can trace a past accretion burst. Aims. We aim to explore whether or not the observed C18O and N2H+ emission pattern towards the VeLLO IRAM 04191+1522 can be understood in the framework of a scenario where the emission is chemically tracing a past accretion burst. Methods. We used high-angular-resolution Plateau de Bure Interferometer (PdBI) observations of C18O and N2H+ towards IRAM 04191+1522 that were obtained as part of the CALYPSO IRAM Large Program (Continuum And Lines in Young ProtoStellar Objects). We model these observations using a chemical code with a time-dependent physical structure coupled with a radiative transfer module, where we allow for variations in the source luminosity. Results. We find that the N2H+ line emission shows a central hole, with the N2H+ emission peaking at a radius of about 10′′ (1400 au) from the source, while the C18O emission is compact (1.3′′ FWHM, corresponding to 182 au). The morphology of these two lines cannot be reproduced with a constant luminosity model based on the present-day internal luminosity (0.08 L⊙). However, the N2H+ peaks are consistent with a constant-luminosity model of 12 L⊙. Using a model with time-dependent temperature and density profiles, we show that the observed N2H+ peak emission could indeed be caused by a past accretion burst with a luminosity 150 times higher than the present-day luminosity. Such a burst should have occurred a couple of hundred years ago. Conclusions. We suggest that an accretion burst occurred in IRAM 04191+1522 in the recent past. If such bursts are common and sufficiently long in VeLLOs, they could lead to higher accretion onto the central object than their luminosity suggests. For IRAM 04191 in particular, our results yield an estimated final mass of 0.2–0.25 M⊙ by the end of the Class 0 phase, which would make this object a low-mass star rather than a brown dwarf. More generally, our analysis demonstrates that the combination of observations of N2H+ and C18O is a more reliable diagnostic of past outburst activity than C18O or N2H+ emission alone. [ABSTRACT FROM AUTHOR]

Context. Understanding the formation mechanisms of protoplanetary disks and multiple systems and also their pristine properties are key questions for modern astrophysics. The properties of the youngest disks, embedded in rotating infalling protostellar envelopes, have largely remained unconstrained up to now. Aims. We aim to observe the youngest protostars with a spatial resolution that is high enough to resolve and characterize the progenitors of protoplanetary disks. This can only be achieved using submillimeter and millimeter interferometric facilities. In the framework of the IRAM Plateau de Bure Interferometer survey CALYPSO, we have obtained subarcsecond observations of the dust continuum emission at 231 and 94 GHz for a sample of 16 solar-type Class 0 protostars. Methods. In an attempt to identify disk-like structures embedded at small scales in the protostellar envelopes, we modeled the dust continuum emission visibility profiles using Plummer-like envelope models and envelope models that include additional Gaussian disk-like components. Results. Our analysis shows that in the CALYPSO sample, 11 of the 16 Class 0 protostars are better reproduced by models including a disk-like dust continuum component contributing to the flux at small scales, but less than 25% of these candidate protostellar disks are resolved at radii >60 au. Including all available literature constraints on Class 0 disks at subarcsecond scales, we show that our results are representative: most (>72% in a sample of 26 protostars) Class 0 protostellar disks are small and emerge only at radii <60 au. We find a multiplicity fraction of the CALYPSO protostars ≲57% ± 10% at the scales 100–5000 au, which generally agrees with the multiplicity properties of Class I protostars at similar scales. Conclusions. We compare our observational constraints on the disk size distribution in Class 0 protostars to the typical disk properties from protostellar formation models. If Class 0 protostars contain similar rotational energy as is currently estimated for prestellar cores, then hydrodynamical models of protostellar collapse systematically predict a high occurrence of large disks. Our observations suggest that these are rarely observed, however. Because they reduce the centrifugal radius and produce a disk size distribution that peaks at radii <100 au during the main accretion phase, magnetized models of rotating protostellar collapse are favored by our observations. [ABSTRACT FROM AUTHOR]

Aims. We aim to clarify the origin of the multiple jet features emanating from the binary protostar SVS 13A (=VLA4A/VLA4B). Methods. We used the Plateau de Bure Interferometer to map at 0.3–0.800 (∼70–190 au) dust emission at 1.4 mm, CO(2–1), SiO(5–4), SO(65–54). Revised proper motions for VLA4A/4B and jet wiggling models are computed to clarify their respective contribution. Results. VLA4A shows compact dust emission suggestive of a disk <50 au, and is the hot corino source, while CO/SiO/SO counterparts to the small-scale H2 jet originate from VLA4B and reveal the jet variable velocity structure. This jet exhibits ' 3 00 wiggling consistent with orbital motion around a yet undetected '20–30 au companion to VLA4B, or jet precession. Jet wiggling combined with velocity variability can explain the large apparent angular momentum in CO bullets. We also uncover a synchronicity between CO jet bullets and knots in the HH7–11 chain demonstrating that they trace two distinct jets. Their '300 yr twin outburst period may be triggered by close perihelion approach of VLA4A in an eccentric orbit around VLA4B. A third jet is tentatively seen at PA ' 0 ◦ . Conclusions. SVS13 A harbors at least 2 and possibly 3 distinct jet sources. The CO and HH7-11 jets are launched from quasicoplanar disks, separated by 20–70 au. Their synchronous major events every 300 yr favor external triggering by close binary interactions, a scenario also invoked for FU Or outbursts. [ABSTRACT FROM AUTHOR]

Context: Protostellar jets and outflows are key features of the star-formation process, and primary processes of the feedback of young stars on the interstellar medium. Understanding the underlying shocks is necessary to explain how jet and outflow systems are launched, and to quantify their chemical and energetic impacts on the surrounding medium. Aims: We performed a high-spectral resolution study of the [OI]63μm emission in the outflow of the intermediate-mass Class 0 protostar Cep E-mm. The goal is to determine the structure of the outflow, to constrain the chemical conditions in the various components, and to understand the nature of the underlying shocks, thus probing the origin of the mass-loss phenomenon. Methods: We present observations of the OI ³P1 → ³P2, OH between ²Π1/2J = 3/2 and J = 1/2 at 1837.8 GHz, and CO (16-15) lines with the GREAT receiver onboard SOFIA towards three positions in the Cep E protostellar outflow: Cep E-mm (the driving protostar), Cep E-BI (in the southern lobe), and Cep E-BII (the terminal position in the southern lobe). Results: The CO (16-15) line is detected at all three positions. The [OI]63μm line is detected in Cep E-BI and BII, whereas the OH line is not detected. In Cep E-BII, we identify three kinematical components in OI and CO. These were already detected in CO transitions and relate to spatial components: the jet, the HH377 terminal bow-shock, and the outflow cavity. We measure line temperature and line integrated intensity ratios for all components. The OI column density is higher in the outflow cavity than in the jet, which itself is higher than in the terminal shock. The terminal shock is the region where the abundance ratio of OI to CO is the lowest (about 0.2), whereas the jet component is atomic (N(OI)/N(CO)~2.7). In the jet, we compare the [OI]63μm observations with shock models that successfully fit the integrated intensity of 10 CO lines. We find that these models most likely do not fit the [OI]63μm data. Conclusions: The high intensity of OI emission points towards the propagation of additional dissociative or alternative FUV-irradiated shocks, where the illumination comes from the shock itself. A picture emerges from the sample of low-to-high mass protostellar outflows, where similar observations have been performed, with the effects of illumination increasing with the mass of the protostar. These findings need confirmation with more observational constraints and a larger sample. [ABSTRACT FROM AUTHOR]

Context. So-called snow lines, indicating regions where abundant volatiles freeze out onto the surface of dust grains, play an important role for planet growth and bulk composition in protoplanetary disks. They can already be observed in the envelopes of the much younger, low-mass Class 0 protostars, which are still in their early phase of heavy accretion. Aims. We aim to use the information on the sublimation regions of different kinds of ices to understand the chemistry of the envelope, its temperature and density structure, and the history of the accretion process. This information is crucial to get the full picture of the early protostellar collapse and the subsequent evolution of young protostars. Methods. As part of the CALYPSO IRAM Large Program, we have obtained observations of C18O, N2H+, and CH3OH towards nearby Class 0 protostars with the IRAM Plateau de Bure interferometer at sub-arcsecond resolution. For four of these sources, we have modeled the emission using a chemical code coupled with a radiative transfer module. Results. We observe an anti-correlation of C18O and N2H+ in NGC 1333-IRAS4A, NGC 1333-IRAS4B, L1157, and L1448C, with N2H+ forming a ring (perturbed by the outflow) around the centrally peaked C18O emission. This emission morphology, which is due to N2H+ being chemically destroyed by CO, reveals the CO and N2 ice sublimation regions in these protostellar envelopes with unprecedented resolution. We also observe compact methanol emission towards three of the sources. Based on our chemical model and assuming temperature and density profiles from the literature, we find that for all four sources the CO snow line appears further inwards than expected from the binding energy of pure CO ices (~855 K). The emission regions of models and observations match for a higher value of the CO binding energy of 1200 K, corresponding to a dust temperature of ~24 K at the CO snow line. The binding energy for N2 ices is modeled at 1000 K, also higher than for pure N2 ices. Furthermore, we find very low CO abundances inside the snow lines in our sources, about an order of magnitude lower than the total CO abundance observed in the gas on large scales in molecular clouds before depletion sets in. Conclusions. The high CO binding energy may hint at CO being frozen out in a polar ice environment like amorphous water ice or in non-polar CO2-rich ice. The low CO abundances are comparable to values found in protoplanetary disks, which may indicate an evolutionary scenario where these low values are already established in the protostellar phase. [ABSTRACT FROM AUTHOR]

Molecular outflows are a direct consequence of accretion, and therefore they represent one of the best tracers of accretion processes in the still poorly understood early phases of high-mass star formation. Previous studies suggested that the SiO abundance decreases with the evolution of a massive young stellar object probably because of a decay of jet activity, as witnessed in low-mass star-forming regions. We investigate the SiO excitation conditions and its abundance in outflows from a sample of massive young stellar objects through observations of the SiO(8-7) and CO(4-3) lines with the APEX telescope. Through a non-local thermodynamic equilibrium analysis, we find that the excitation conditions of SiO increase with the velocity of the emitting gas. We also compute the SiO abundance through the SiO and CO integrated intensities at high velocities. For the sources in our sample we find no significant variation of the SiO abundance with evolution for a bolometric luminosity-to-mass ratio of between 4 and 50 Lϴ/Mϴ. We also find a weak increase of the SiO(8-7) luminosity with the bolometric luminosity-to-mass ratio. We speculate that this might be explained with an increase of density in the gas traced by SiO. We find that the densities constrained by the SiO observations require the use of shock models that include grain-grain processing. For the first time, such models are compared and found to be compatible with SiO observations. A pre-shock density of 105 cm-3 is globally inferred from these comparisons. Shocks with a velocity higher than 25 km s-1 are invoked for the objects in our sample where the SiO is observed with a corresponding velocity dispersion. Our comparison of shock models with observations suggests that sputtering of silicon-bearing material (corresponding to less than 10% of the total silicon abundance) from the grain mantles is occurring. [ABSTRACT FROM AUTHOR]

Context. When supernova blast waves interact with nearby molecular clouds, they send slower shocks into these clouds. The resulting interaction regions provide excellent environments for the use of MHD shock models to constrain the physical and chemical conditions in these regions. Aims. The interaction of supernova remnants (SNRs) with molecular clouds gives rise to strong molecular emission in the far-IR and sub-mm wavelength regimes. The application of MHD shock models in the interpretation of this line emission can yield valuable information on the energetic and chemical impact of SNRs. Methods. New mapping observations with the APEX telescope in 12CO (3-2), (4-3), (6-5), (7-6), and 13CO (3-2) towards two regions in the SNR W44 are presented. Integrated intensities are extracted on five di erent positions, corresponding to local maxima of CO emission. The integrated intensities are compared to the outputs of a grid of models, which combine an MHD shock code with a radiative transfer module based on the large velocity gradient approximation. Results. All extracted spectra show ambient and line-of-sight components as well as blue- and red-shifted wings indicating the presence of shocked gas. Basing the shock model fits only on the highest-lying transitions that unambiguously trace the shock-heated gas, we find that the observed CO line emission is compatible with non-stationary shocks and a pre-shock density of 104 cm-3. The ages of the modelled shocks scatter between values of ∼1000 and ∼3000 years. The shock velocities in W44F are found to lie between 20 km s-1 and 25 km s-1, while in W44E fast shocks (30-35 km s-1) as well as slower shocks (∼20 km s-1) are compatible with the observed spectral line energy diagrams. The pre-shock magnetic field strength components perpendicular to the line of sight in both regions have values between 100 μG and 200 μG. Our best-fitting models allow us to predict the full ladder of CO transitions, the shocked gas mass in one beam as well as the momentum and energy injection. [ABSTRACT FROM AUTHOR]

Context. Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high density environments. For densities higher than ∼105 cm-3, shattering becomes a self-enhanced process that imposes severe chemical and dynamical consequences on the shock characteristics. Shattering is accompanied by the vaporization of grains, which can, in addition to sputtering, directly release SiO to the gas phase. Given that SiO rotational line radiation is used as a major tracer of shocks in dense clouds, it is crucial to understand the influence of vaporization on SiO line emission. Aims. We extend our study of the impact of grain-grain processing on C-type shocks in dense clouds. Various values of the magnetic field are explored. We study the corresponding consequences for molecular line emission and, in particular, investigate the influence of shattering and related vaporization on the rotational line emission of SiO. Methods. We have developed a recipe for implementing the effects of shattering and vaporization into a 2-fluid shock model, resulting in a reduction of computation time by a factor ∼100 compared to a multi-fluid modelling approach. This implementation was combined with an LVG-based modelling of molecular line radiation transport. Using this combined model we calculated grids of shock models to explore the consequences of different dust-processing scenarios. Results. Grain-grain processing is shown to have a strong influence on C-type shocks for a broad range of magnetic fields: the shocks become hotter and thinner. The reduction in column density of shocked gas lowers the intensity of molecular lines, at the same time as higher peak temperatures increase the intensity of highly excited transitions compared to shocks without grain-grain processing. For OH the net effect is an increase in line intensities, while for CO and H2O it is the contrary. The intensity of H2 emission is decreased in low transitions and increased for highly excited lines. For all molecules, the highly excited lines become sensitive to the value of the magnetic field. Although vaporization increases the intensity of SiO rotational lines, this effect is weakened by the reduced shock width. The release of SiO early in the hot shock changes the excitation characteristics of SiO radiation, although it does not yield an increase in width for the line profiles. To significantly increase the intensity and width of SiO rotational lines, SiO needs to be present in grain mantles. [ABSTRACT FROM AUTHOR]

In the framework of the open-time key program "Herschel M33 extended survey (HerM33es)", we study the far-infrared emission from the nearby spiral galaxyM33 in order to investigate the physical properties of the dust such as its temperature and luminosity density across the galaxy. Taking advantage of the unique wavelength coverage (100, 160, 250, 350, and 500 μm) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 μm and Spitzer-MIPS 24 and 70 μm data, we construct temperature and luminosity density maps by fitting two modified blackbodies of a fixed emissivity index of 1.5. We find that the "cool" dust grains are heated to temperatures of between 11 K and 28 K, with the lowest temperatures being found in the outskirts of the galaxy and the highest ones both at the center and in the bright HII regions. The infrared/submillimeter total luminosity (5-1000 μm) is estimated to be 1.9 × 109 +4.0×108 -4.4×108 L☉. Fifty-nine percent of the total infrared/submillimeter luminosity of the galaxy is produced by the "cool" dust grains (∼15K), while the remaining 41% is produced by "warm" dust grains (∼55K). The ratio of the cool-to-warm dust luminosity is close to unity (within the computed uncertainties), throughout the galaxy, with the luminosity of the cool dust being slightly higher at the center than the outer parts of the galaxy. Decomposing the emission of the dust into two components (one emitted by the diffuse disk of the galaxy and one emitted by the spiral arms), we find that the fraction of the emission from the disk in the mid-infrared (24 μm) is 21%, while it gradually rises up to 57% in the submillimeter (500 μm).We find that the bulk of the luminosity comes from the spiral arm network that produces 70% of the total luminosity of the galaxy with the rest coming from the diffuse dust disk. The "cool" dust inside the disk is heated to temperatures in a narrow range between 18 K and 15 K (going from the center to the outer parts of the galaxy). [ABSTRACT FROM AUTHOR]

Context. Observing supernova remnants (SNRs) and modelling the shocks they are associated with is the best way to quantify the energy SNRs re-distribute back into the interstellar medium (ISM). Aims. We present comparisons of shock models with CO observations in the F knot of the W28 supernova remnant. These comparisons constitute a valuable tool to constrain both the shock characteristics and pre-shock conditions. Methods. New CO observations from the shocked regions with the APEX and SOFIA telescopes are presented and combined. The integrated intensities are compared to the outputs of a grid of models, which were combined from an MHO shock code that calculates the dynamical and chemical structure of these regions and a radiative transfer module based on the large velocity gradient (LVG) approximation. Results. We base our modelling method on the higher J CO transitions, which unambiguously trace the passage of a shock wave. We provide fits for the blue- and red-lobe components of the observed shocks. We find that only stationary, C-type shock models can reproduce the observed levels of CO emission. Our best models are found for a pre-shock density of 104 cm-3, with the magnetic field strength varying between 45 and 100 μG, and a slightly higher shock velocity for the so-called blue-shock (∼ 25 km s-1) than for the red one (∼20 km-1). Our models also satisfactorily account for the pure rotational H2 emission that is observed with Spitzer. [ABSTRACT FROM AUTHOR]