Добірка наукової літератури з теми "Protostellar outflows"

Abstract Observed protostellar outflows exhibit a variety of asymmetrical features, including remarkable unipolar outflows and bending outflows. Revealing the formation and early evolution of such asymmetrical protostellar outflows, especially the unipolar outflows, is essential for a better understanding of the star and planet formation because they can dramatically change the mass accretion and angular momentum transport to the protostars and protoplanetary disks. Here we perform three-dimensional nonideal magnetohydrodynamics simulations to investigate the formation and early evolution of the asymmetrical protostellar outflows in magnetized turbulent isolated molecular cloud cores. We find, for the first time to our knowledge, that the unipolar outflow forms even in the single low-mass protostellar system. The results show that the unipolar outflow is driven in the weakly magnetized cloud cores with the dimensionless mass-to-flux ratios of μ = 8 and 16. Furthermore, we find the protostellar rocket effect of the unipolar outflow, which is similar to the launch and propulsion of a rocket. The unipolar outflow ejects the protostellar system from the central dense region to the outer region of the parent cloud core, and the ram pressure caused by its ejection suppresses the driving of additional new outflows. In contrast, the bending bipolar outflow is driven in the moderately magnetized cloud core with μ = 4. The ratio of the magnetic to turbulent energies of a parent cloud core may play a key role in the formation of asymmetrical protostellar outflows.

AbstractWe perform 3D MHD simulations of cluster formation in turbulent magnetized dense molecular clumps, taking into account the effect of protostellar outflows. Our simulation shows that initial interstellar turbulence decays quickly as several authors already pointed out. When stars form, protostellar outflows generate and maintain supersonic turbulence that have a power-law energy spectrum of Ek ~ k−2, which is somewhat steeper than those of driven MHD turbulence simulations. Protostellar outflows suppress global star formation, although they can sometimes trigger local star formation by dynamical compression of pre-existing cores. Magnetic field retards star formation by slowing down overall contraction. Interplay of protostellar outflows and magnetic field generates large-amplitude Alfven and MHD waves that transform outflow motions into turbulent motions efficiently. Cluster forming clumps tend to be in dynamical equilibrium mainly due to dynamical support by protostellar outflow-driven turbulence (hereafter, protostellar turbulence).

Context. Molecular outflows are commonly detected originating from both protostellar and extragalactic sources. Separate studies of low-mass, isolated high-mass, and extragalactic sources reveal scaling relations connecting the force carried by an outflow and the properties of the source that drives it, as for example the mass and luminosity. Aims. The aim of this work is twofold: first, to examine the effects, if any, of clustered star formation on the protostellar outflows and their scaling relations and, second, to explore the possibility that outflows varying in scale and energetics by many orders of magnitude are consistent with being launched by the same physical processes. Methods. To that end, high-angular resolution CO J = 3–2 observations were used of ten high-mass protostars in the Cygnus-X molecular cloud, obtained at the SubMilliMeter Array as part of the Protostellar Interferometric Line Survey of Cygnus-X (PILS-Cygnus). From these data, the outflow force, that is the momentum ejection rate, was measured. In addition, an extended sample of protostellar and extragalactic outflow-force measurements was assembled from existing literature to allow for a direct comparison of the scaling relations of the two types of outflows. Results. Molecular outflows were detected originating from all ten sources of the PILS-Cygnus survey, and their outflow forces are found to be in close agreement with measurements from the literature. In addition, the comparison of the protostellar and extragalactic sources reveals, with 95% confidence, that Class 0 protostars and extragalactic sources follow the same outflow force–bolometric luminosity correlation. Conclusions. The close agreement between the Cygnus-X sources and sources of similar envelope mass and bolometric luminosity suggests that clustered star formation has no significant effect on protostellar outflows. We find a strong indication that protostellar and extragalactic outflows are consistent with having a similar launch mechanism. The existence of such a mechanism would enable the development of a single universal outflow launch model, although more observations are required in order to verify this connection.

The formation of massive stars differs from low-mass stars due to their rapid evolution, relatively low abundance, and their burial within molecular clouds, making observations more challenging. Moreover, the mechanisms give rise to the formation of O-type and B-type stars, in particular, differ from those responsible for the formation of low-mass stars. The paper presents an overview of the two main theories that have been proposed to explain the formation of massive stars: accretion and collision theories. The paper also investigates several mainstream theories of protostellar outflows during the massive star formation phase, and describe the model and their simulation of these theories. These results may provide useful references and guidance for future studies on the formation of massive stars.

Context. OMC-2/3 is one of the nearest embedded cluster-forming regions that includes intermediate-mass protostars at early stages of evolution. A previous CO (3–2) mapping survey towards this region revealed outflow activity related to sources at different evolutionary phases. Aims. The present work presents a study of the warm gas in the high-velocity emission from several outflows found in CO (3–2) emission by previous observations, determines their physical conditions, and makes a comparison with previous results in low-mass star-forming regions. Methods. We used the CHAMP+ heterodyne array on the APEX telescope to map the CO (6–5) and CO (7–6) emission in the OMC-2 FIR 6 and OMC-3 MMS 1-6 regions, and to observe 13CO (6–5) at selected positions. We analyzed these data together with previous CO (3–2) observations. In addition, we mapped the SiO (5–4) emission in OMC-2 FIR 6. Results. The CO (6–5) emission was detected in most of the outflow lobes in the mapped regions, while the CO (7–6) was found mostly in the OMC-3 outflows. In the OMC-3 MMS 5 outflow, a previously undetected extremely high-velocity gas was found in CO (6–5). This extremely high-velocity emission arises from the regions close to the central object MMS 5. Radiative transfer models revealed that the high-velocity gas from MMS 5 outflow consists of gas with nH2 = 104–105 cm−3 and T > 200 K, similar to what is observed in young Class 0 low-mass protostars. For the other outflows, values of nH2 > 104 cm−3 were found. Conclusions. The physical conditions and kinematic properties of the young intermediate-mass outflows presented here are similar to those found in outflows from Class 0 low-mass objects. Due to their excitation requirements, mid − J CO lines are good tracers of extremely high-velocity gas in young outflows likely related to jets.

Context. The accretion history of protostars remains widely mysterious, even though it represents one of the best ways to understand the protostellar collapse that leads to the formation of stars. Aims. Molecular outflows, which are easier to detect than the direct accretion onto the prostellar embryo, are here used to characterize the protostellar accretion phase in W43-MM1. Methods. The W43-MM1 protocluster hosts a sufficient number of protostars to statistically investigate molecular outflows in a single, homogeneous region. We used the CO(2–1) and SiO(5–4) line datacubes, taken as part of an ALMA mosaic with a 2000 AU resolution, to search for protostellar outflows, evaluate the influence that the environment has on these outflows’ characteristics and put constraints on outflow variability in W43-MM1. Results. We discovered a rich cluster of 46 outflow lobes, driven by 27 protostars with masses of 1−100 M⊙. The complex environment inside which these outflow lobes develop has a definite influence on their length, limiting the validity of using outflows’ dynamical timescale as a proxy of the ejection timescale in clouds with high dynamics and varying conditions. We performed a detailed study of Position–Velocity diagrams of outflows that revealed clear events of episodic ejection. The time variability of W43-MM1 outflows is a general trend and is more generally observed than in nearby, low- to intermediate-mass star-forming regions. The typical timescale found between two ejecta, ~500 yr, is consistent with that found in nearby protostars. Conclusions. If ejection episodicity reflects variability in the accretion process, either protostellar accretion is more variable, or episodicity is easier to detect in high-mass star-forming regions than in nearby clouds. The timescale found between accretion events could result from instabilities associated with bursts of inflowing gas arising from the close dynamical environment of high-mass star-forming cores.

Abstract Protostellar outflows are one of the most outstanding features of star formation. Observational studies over the last several decades have successfully demonstrated that outflows are ubiquitously associated with low- and high-mass protostars in solar-metallicity Galactic conditions. However, the environmental dependence of protostellar outflow properties is still poorly understood, particularly in the low-metallicity regime. Here we report the first detection of a molecular outflow in the Small Magellanic Cloud with 0.2 Z ⊙, using Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of 0.1 pc toward the massive protostar Y246. The bipolar outflow is nicely illustrated by high-velocity wings of CO(3–2) emission at ≳15 km s−1. The evaluated properties of the outflow (momentum, mechanical force, etc.) are consistent with those of the Galactic counterparts. Our results suggest that the molecular outflows, i.e., the guidepost of the disk accretion at the small scale, might be universally associated with protostars across the metallicity range of ∼0.2–1 Z ⊙.

ABSTRACT During the early phases of low-mass star formation, episodic accretion causes the ejection of high-velocity outflow bullets, which carry a fossil record of the driving protostar’s accretion history. We present 44 SPH simulations of $1\, {{\mathrm{M}}}_{\odot }$ cores, covering a wide range of initial conditions, and follow the cores for five free-fall times. Individual protostars are represented by sink particles, and the sink particles launch episodic outflows using a sub-grid model. The Optics algorithm is used to identify individual episodic bullets within the outflows. The parameters of the overall outflow and the individual bullets are then used to estimate the age and energetics of the outflow, and the accretion events that triggered it, and to evaluate how reliable these estimates are, if observational uncertainties and selection effects (like inclination) are neglected. Of the commonly used methods for estimating outflow ages, it appears that those based on the length and speed of advance of the lobe are the most reliable in the early phases of evolution, and those based on the width of the outflow cavity and the speed of advance are most reliable during the later phases. We describe a new method that is almost as accurate as these methods, and reliable throughout the evolution. In addition, we show how the accretion history of the protostar can be accurately reconstructed from the dynamics of the bullets if each lobe contains at least two bullets. The outflows entrain about 10 times more mass than originally ejected by the protostar.

Abstract The opening angles of some protostellar outflows appear too narrow to match the expected core–star mass efficiency (SFE) = 0.3–0.5, if the outflow cavity volume traces outflow mass, with a conical shape and a maximum opening angle near 90°. However, outflow cavities with a paraboloidal shape and wider angles are more consistent with observed estimates of the SFE. This paper presents a model of infall and outflow evolution based on these properties. The initial state is a truncated singular isothermal sphere which has mass ≈ 1 M ⊙, freefall time ≈ 80 kyr, and small fractions of magnetic, rotational, and turbulent energy. The core collapses pressure free as its protostar and disk launch a paraboloidal wide-angle wind. The cavity walls expand radially and entrain envelope gas into the outflow. The model matches the SFE values when the outflow mass increases faster than the protostar mass by a factor 1–2, yielding protostar masses typical of the IMF. It matches the observed outflow angles if the outflow mass increases at nearly the same rate as the cavity volume. The predicted outflow angles are then typically ∼50° as they increase rapidly through the stage 0 duration of ∼40 kyr. They increase more slowly up to ∼110° during their stage I duration of ∼70 kyr. With these outflow rates and shapes, the model predictions appear consistent with observational estimates of the typical stellar masses, SFEs, stage durations, and outflow angles, with no need for external mechanisms of core dispersal.

We present new spectroscopy and Hubble Space Telescope imaging of protostellar jets discovered in an Hαsurvey of the Carina Nebula. Near-IR [Fe II] emission from these jets traces dense gas that is self-shielded from Lyman continuum photons from nearby O-type stars, but is excited by non-ionizing FUV photons that penetrate the ionization front within the jet. New near-IR [Fe II] images reveal a substantial mass of dense, neutral gas that is not seen in Hαemission from these jets. In some cases, [Fe II] emission traces the jet inside its natal dust pillar, connecting the larger Hαoutflow to the embedded IR source that drives it. New proper motion measurements reveal tangential velocities similar to those typically measured in lower-luminosity sources (100−200 km/s⁻¹). Combining high jet densities and fast outflow speeds leads to mass-loss rate estimates an order of magnitude higher than those derived from the Hαemission measure alone. Higher jet mass-loss rates require higher accretion rates, implying that these jets are driven by intermediate-mass (~ 2−8 M⊙) protostars. For some sources, the mid-IR luminosities of the driving sources are clearly consistent with intermediate-mass protostars; others remain deeply embedded and require long-wavelength, high-resolution images to confirm their luminosity. These outflows are all highly collimated, with opening angles of only a few degrees. With this new view of collimated jets from intermediate-mass protostars, we argue that these jets reflect essentially the same outflow phenomenon seen in low-mass protostars, but that the collimated atomic jet core and the material it sweeps up are irradiated and rendered observable. Thus, the jets in Carina offer strong additional evidence that stars up to ~ 8 M⊙ form by the same accretion mechanisms as low-mass stars.

Gasausströmungen, oft in der Form hoch kollimierter Jets, sind ein allgegenwärtiges Phänomen bei der Geburt neuer Sterne. Emission von stossangeregtem molekularem Wasserstoff bei Wellenlängen im nahen Infrarotbereich ist ein Merkmal ihrer Existenz und auch in eingebetteten, im Optischen obskurierten Ausströmungen generell gut zu beobachten. In dieser Arbeit werden die Resultate einer von Auswahleffekten freien, empfindlichen, grossflächigen Suche nach solchen Ausströmungen von Protosternen in der v=1-0 S(1) Linie molekularen Wasserstoffs bei einer Wellenlänge von 2.12 µm vorgestellt. Die Durchmusterung umfasst eine Fläche von etwa einem Quadratgrad in der Orion A Riesenmolekülwolke. Weitere Daten aus einem grossen Wellenlängenbereich werden benutzt, um die Quellen der Ausströmungen zu identifizieren. Das Ziel dieser Arbeit ist es, eine Stichprobe von Ausströmungen zu bekommen, die so weit wie möglich frei von Auswahleffekten ist, um die typischen Eigenschaften protostellarer Ausströmungen und deren Entwicklung festzustellen, sowie um die Rückwirkung der Ausströmungen auf die umgebende Wolke zu untersuchen.<br /> Das erste Ergebnis ist, dass Ausströmungen in Sternentstehungsgebieten tatsächlich sehr häufig sind: mehr als 70 Jet-Kandidaten werden identifiziert. Die meisten zeigen eine sehr irreguläre Morphologie anstelle regulärer oder symmetrischer Strukturen. Dies ist auf das turbulente, klumpige Medium zurückzuführen, in das sich die Jets hineinbewegen. Die Ausrichtung der Jets ist zufällig verteilt. Insbesondere gibt es keine bevorzugte Ausrichtung der Jets parallel zum grossräumigen Magnetfeld in der Wolke. Das legt nahe, dass die Rotations- und Symmetrieachse in einem protostellaren System durch zufällige, turbulente Bewegung in der Wolke bestimmt wird. <br /> <br /> Mögliche Ausströmungsquellen werden für 49 Jets identifiziert; für diese wird der Entwicklungsstand und die bolometrische Leuchtkraft abgeschätzt. Die Jetlänge und die H2 Leuchtkraft entwickeln sich gemeinsam mit der Ausströmungsquelle. Von null startend, dehnen sich die Jets schnell bis auf eine Länge von einigen Parsec aus und werden dann langsam wieder kürzer. Sie sind zuerst sehr leuchtkräftig, die H2 Helligkeit nimmt aber im Lauf der protostellaren Entwicklung ab. Die Längen- und H2 Leuchtkraftentwicklung lässt sich im Wesentlichen durch eine zuerst sehr hohe, dann niedriger werdende Massenausflussrate erklären, die auf eine zuerst sehr hohe, dann niedriger werdende Gasakkretionsrate auf den Protostern schliessen lässt (Akkretion und Ejektion sind eng verknüpft!). Die Längenabnahme der Jets erfordert eine ständig wirkende Abbremsung der Jets. Ein einfaches Modell einer simultanen Entwicklung eines Protosterns, seiner zirkumstellaren Umgebung und seiner Ausströmung (Smith 2000) kann die gemessenen H2- und bolometrischen Leuchtkräfte der Jets und ihrer Quellen reproduzieren, unter der Annahme, dass die starke Akkretionsaktivität zu Beginn der protostellaren Entwicklung mit einer überproportional hohen Massenausflussrate verbunden ist.<br /> <br /> Im Durchmusterungsgebiet sind 125 dichte Molekülwolkenkerne bekannt (Tatematsu et al. 1993). Jets (bzw. Sterne) entstehen in ruhigen Wolkenkernen, d.h. solchen mit einem niedrigen Verhältnis von interner kinetischer Energie zu gravitativer potentieller Energie; dies sind die Wolkenkerne höherer Masse. Die Wolkenkerne mit Jets haben im Mittel grössere Linienbreiten als die ohne Jets. Dies ist darauf zurückzuführen, dass sie bevorzugt in den massereicheren Wolkenkernen zu finden sind, welche generell eine grössere Linienbreite haben. Es gibt keinen Hinweis auf stärkere interne Bewegungen in Wolkenkernen mit Jets, die durch eine Wechselwirkung der Jets mit den Wolkenkernen erzeugt sein könnte. Es gibt, wie von der Theorie vorausgesagt, eine Beziehung zwischen der Linienbreite der Wolkenkerne und der H2 Leuchtkraft der Jets, wenn Jets von Klasse 0 und Klasse I Protosternen separat betrachtet werden; dabei sind Klasse 0 Jets leuchtkräftiger als Klasse I Jets, was ebenfalls auf eine zeitabhängige Akkretionsrate mit einer frühzeitigen Spitze und einem darauffolgenden Abklingen hinweist.<br /> <br /> Schliesslich wird die Rückwirkung der Jetpopulation auf eine Molekülwolke unter der Annahme strikter Vorwärtsimpulserhaltung betrachtet. Die Jets können auf der Skala einer ganzen Riesenmolekülwolke und auf den Skalen von Molekülwolkenkernen nicht genügend Impuls liefern, um die abklingende Turbulenz wieder anzuregen. Auf der mittleren Skala von molekularen Klumpen, mit einer Grösse von einigen parsec und Massen von einigen hundert Sonnenmassen liefern die Jets jedoch genügend Impuls in hinreichend kurzer Zeit, um die Turbulenz “am Leben zu erhalten” und können damit helfen, einen Klumpen gegen seinen Kollaps zu stabilisieren.<br>The presence of outflows, often in the form of well-collimated jets, is a phenomenon commonly associated with the birth of young stars. Emission from shock-excited molecular hydrogen at near-infrared wavelengths is one of the signposts of the presence of such an outflow, and generally can be observed even if the flow is obscured at optical wavelengths. In this thesis, I present the results of an unbiased, sensitive, wide-field search for flows from protostellar objects in the H2 v=1-0 S(1) line at a wavelength of 2.12 µm, covering a 1 square degree area of the Orion A giant molecular cloud. Further data covering a wide wavelength range are used to search for the driving sources of the flows. The aim of this work is to obtain a sample of outflows which is free from biases as far as possible, to derive the typical properties of the outflows, to search for evolutionary trends, and to examine the impact of outflows on the ambient cloud.<br /> The first result from this survey is that outflows are indeed common in star forming regions: more than 70 candidate jets are identified. Most of them have a fairly ill-defined morphology rather than a regular or symmetric structure, which is interpreted to be due to the turbulent, clumpy ambient medium into which the jets are propagating. The jets are randomly oriented. In particular, no alignment of the jets with the large scale ambient magnetic field is found, suggesting that the spin and symmetry axis in a protostellar object is determined by random, turbulent motions in the cloud. <br /> <br /> Candidate driving sources are identified for 49 jets, and their evolutionary stage and bolometric luminosity is estimated. The jet lengths and H2 luminosities evolve as a function of the age of the driving source: the jets grow quickly from zero length to a size of a few parsec and then slowly shorten again. The jets are very luminous early on and fade during the protostellar evolution. The evolution in length and H2 luminosity is attributed to an early phase of strong accretion, which subsequently decreases. The shortening of the jets with time requires the presence of a continuous deceleration of the jets. A simple model of the simultaneous evolution of a protostar, its circumstellar environment, and its outflow (Smith 2000) can reproduce the measured values of H2 luminosity and driving source luminosity under the assumption of a strong accretion plus high ejection efficiency phase early in the protostellar evolution.<br /> <br /> Tatematsu et al. (1993) found 125 dense cloud cores in the survey area. The jet driving sources are found to have formed predominantly in quiet cores with a low ratio of internal kinetic energy to gravitational potential energy; these are the cores with higher masses. The cores which are associated with jets have on average larger linewidths than cores without jets. This is due to the preferred presence of jets in more massive cores, which generally have larger linewidths. There is no evidence for additional internal motions excited by the interaction of the jets with the cores. The jet H2 luminosity and the core linewidth (as predicted by theory) are related, if Class 0 and Class I jets are considered separately; the relation lies at higher values of the H2 luminosity for the Class 0 jets than for Class I jets. This also suggests a time evolution of the accretion rate, with a strong peak early on and a subsequent decay.<br /> <br /> Finally, the impact of a protostellar jet population on a molecular cloud is considered. Under the conservative assumption of strict forward momentum conservation, the jets appear to fail to provide sufficient momentum to replenish decaying turbulence on the scales of a giant molecular cloud and on the scales of molecular cloud cores. At the intermediate scales of molecular clumps with sizes of a few parsec and masses of a few hundred solar masses, the jets provide enough momentum in a short enough time to potentially replenish turbulence and thus might help to stabilize the clump against further collapse.

L’efficacité observée dans la formation des étoiles jusqu’au parsec ne dépasse pas quelques pourcents, sans oublier le décalage de la fonction initiale de masse (IMF) à seulement ∼30% de la masse du cœur pré-stellaire initial. Comprendre en détail le processus derrière d’aussi faibles efficacités reste encore à ce jour une question ouverte. En outre, les simulations numériques les plus récentes ont démontré que la turbulence et les champs magnétiques à eux seuls ne peuvent suffire à reproduire de telles valeurs. Elles montrent que la rétroaction des flots protostellaires joue un rôle crucial en perturbant les écoulements d’accrétion, en évacuant la matière des cœurs, et/ou en maintenant la turbulence. Malheureusement, que ce soit en termes de volume de nuage affecté, d’impulsion injectée, de masse entraînée, ou d’impact sur le disque et l’enveloppe en effondrement : l’importance de cette rétroaction dépend fortement de la géométrie sous-jacente du vent protostellaire. Cette dernière reste encore débattue : "vent X grand angle" rapide, vent de disque MHD plus lent, ou jet collimaté ? De toute évidence, afin d’évaluer fiablement l’impact de la rétroaction des flots sur la formation stellaire, il est d’une importance cruciale de déterminer la géométrie de vent la plus réaliste (et/ou les géométries que nous pouvons exclure). Pour apporter une nouvelle contribution quant à cette question, nous présentons des simulations numériques de flots poussés par un jet pulsé collimaté, lancé à travers un cœur pré-stellaire stratifié. Nous comparons nos simulations avec les observations ALMA récentes, ainsi qu’avec les prédictions analogues pour un vent X grand angle. Nos simulations sont les premières à combiner sur une échelle de 0.1 pc la variabilité du jet, la stratification en densité de l’enveloppe et des échelles de temps de 10 000 ans comparables aux flots jeunes observés. Les prédictions de nos simulations en termes de largeur de flot, de diagrammes position-vitesse, et de distribution masse-vitesse, montrent une ressemblance frappante avec les observations ALMA de flots CO tels que HH46/47 et CARMA-7. L'accord est même plus prometteur qu'avec les modèles de flots poussés par un "vent X grand angle". Ces résultats pourraient avoir une implication majeure sur le rôle des flots dans la régulation de la formation stellaire<br>A long-standing open question in star formation is the process responsible for its low efficiency on parsec scales (a few %), and for shifting down the Initial Mass Function (IMF) to only ∼30% of the prestellar core mass distribution. The most recent numerical simulations show that neither turbulence nor magnetic fields can, alone, reproduce these low efficiencies, and that feedback by protostellar outflows must play a crucial role by disrupting accretion streams, expelling material from cores, and/or sustaining turbulence. Unfortunately, the magnitude of outflow feedback (affected cloud volume, injected momentum, entrained mass, impact on the disk and infalling envelope) depends strongly on the underlying protostellar wind geometry, which remains uncertain and heavily debated: a fast wide-angle "X-wind”, a slower MHD disk wind, a narrow jet ? Clearly, if we want to reliably assess the role of outflow feedback in star formation, it is of utmost importance to determine which wind geometry is the most realistic (and/or which one can be excluded). As a new contribution towards this goal, we present, for the first time, numerical predictions for outflows driven by a narrow pulsed jet in a stratified prestellar core. We compare our simulations against recent ALMA observations and analogous predictions for a wide-angle X-wind. Our simulations are the first to combine jet variability, ambient density-stratification, and long timescales up to 10 000 yrs (typical of young outflows) on scales up to 0.1 pc. We find that the predicted widths, position-velocity diagrams, and mass-velocity distribution, show striking resemblance with ALMA observations of CO outflows such as HH46/47 and CARMA-7, and in closer agreement than models based on a wide-angle "X-wind". The results obtained in this work could have major implications for the feedback of protostellar outflows on star formation

L’étude des phases précoces de la formation des étoiles massives (M&gt;8Msol) est un sujet majeur de l’astrophysique encore mal compris. Pour élucider les processus précis de formation en action, il est vital d'identifier et d’étudier les plus phases les plus précoces de la formation, avant même la naissance d’une proto-étoile. Ces objets dits cœurs préstellaires permettent d’observer directement les processus à l’œuvre. Grâce au relevé ALMA-IMF, nous avons pu pour la première fois construire un échantillon de cœurs préstellaires massifs découverts de façon homogène dans un seul relevé. Cela nous permet d’étudier cet échantillon statistique et de proposer des pistes fortes de processus dominant la formation des étoiles massives.Pour cela, nous avons d’abord développé une nouvelle méthode systématique de détections d’éjections protostellaires dans le but de pouvoir différencier les cœurs préstellaires des protostellaires. Les cœurs ne présentant pas d’éjection sont d’excellents candidats pour être préstellaires. La méthode compare le spectre vers la source (On) avec celui moyenné dans son environnement proche (Off) et détermine si la source présente un excès d’émission à grande vitesse révélatrice d’un flot moléculaire dans le cœur lui-même. Les transitions moléculaires CO(2-1) et SiO(5-4) sont utilisées afin d’identifier ces flots moléculaires. Nous construisons et utilisons également des cartes d’émissions moléculaires à hautes vitesses afin de compléter et d’accélérer la méthode.Grâce à cette méthode, nous avons identifié 42 cœurs préstellaires parmi les 141 cœurs avec une masse supérieure à 8 Msol, dont 12 sont plus massifs que 16 Msol et sont les meilleurs candidats pour être des précurseurs d’étoiles massives. Ces candidats de cœurs préstellaires massifs couvrent des masses jusqu’à 55 Msol et apparaissent situés dans les régions les plus centrales et denses des régions de formation, où la confusion avec les flots moléculaires émanant des protoétoiles massives proches est importante. La statistique importante de cœurs pré et protostellaires a permis de calculer les meilleurs temps de vie statistiques de la phase préstellaire massive jamais obtenus. En adoptant deux hypothèses d’évolution protostellaire différentes, nous obtenons des temps de vie entre 120.000 et 240.000 ans pour des masses comprises entre 8 et 16 Msol, et entre 50.000 et 100.000 ans pour des masses entre 30 et 55 Msol. Ces temps de vie sont plus courts que ceux observés pour la phase préstellaire de faible masse (10^6 ans), mais beaucoup plus longs (10 à 30 fois) que les temps de chute libre des cœurs, suggérant un effondrement ralenti par le champ magnétique, la rotation ou la turbulence.Une analyse des propriétés de l'émission moléculaire des 12 candidats les plus massifs a montré que la turbulence, tracée par la dispersion de vitesse dans les raies moléculaires, n'est pas suffisante pour contrer la gravité pour 80 % des candidats préstellaires. Une analyse du Viriel montre qu’un champ magnétique de l’ordre de 10 mG serait nécessaire afin d’expliquer la stabilité de ces cœurs. Une telle intensité du champ magnétique n’est pas exclue pour ces nouveaux cœurs et demandera à être mesurée grâce à la polarisation de l’émission des poussières. Alternativement, il est possible que la rotation forte attendue de ces cœurs pourrait à petites échelles empêcher une formation rapide d’une protoétoile accrétant à haut taux. Cette seconde hypothèse pourra être testée par des observations moléculaires à plus haute résolution spatiale avec ALMA.Il ressort donc que (1) les cœurs préstellaires massifs existent et peuvent être révélés par une recherche systématique de flots moléculaires dans les régions les plus denses et confuses de formation d’étoiles massives ; (2) leurs temps de vie suggèrent un soutien au-delà de la turbulence, probablement magnétique ou rotationnel, retardant l'effondrement immédiat<br>The study of the early phases of high-mass star formation (M&gt;8Msol) is a major yet poorly understood topic in astrophysics. To elucidate which precise processes of formation are in action, it is vital to identify and study the earliest phases of formation, even before the birth of a protostar. These objects, called prestellar cores, allow us to observe the processes in action. Thanks to the ALMA-IMF survey, we have been able to construct for the first time a sample of high-mass prestellar cores discovered homogeneously in a single survey. This allows us to study this statistical sample and propose strong clues about the dominant processes in high-mass star formation.To do this, we first developed a new systematic method for detecting protostellar outflows to differentiate prestellar from protostellar cores. Cores without significant outflows are considered as excellent candidates to be prestellar cores. The method compares the spectrum towards the source (On) with the spectrum averaged in its nearby environment (Off) and determines if the source shows an excess of high-velocity emission indicative of a molecular outflow within the core itself. The molecular transitions CO(2-1) and SiO(5-4) are used to identify these molecular outflows. We also construct and use high-velocity molecular emission maps around each core to complete and accelerate the method.Thanks to this method, we identified 42 prestellar cores among the 141 cores with a mass greater than 8 Msol, 12 of which are more massive than 16 Msol and are the best candidates for being precursors of high-mass stars. These high-mass prestellar core candidates range in mass up to 55 Msol and appear located in the most central and dense regions of star-forming regions, where confusion with molecular outflows from nearby high-mass protostars is significant. The significant statistics of prestellar and protostellar cores allowed us to calculate the best statistical lifetimes of the massive prestellar phase ever obtained. By adopting two different protostellar evolution assumptions, we obtain lifetimes between 120,000 and 240,000 yr for masses between 8 and 16 Msol, and between 50,000 and 100,000 yr for masses between 30 and 55 Msol. These lifetimes are shorter than those observed for the low-mass prestellar phase (10^6 yr) but much longer (10 to 30 times) than the free-fall times of the cores, suggesting a collapse slowed by magnetic fields, rotation, or turbulence.An analysis of the molecular emission properties of the 12 most massive candidates showed that turbulence, traced by velocity dispersion in molecular lines, is insufficient to counter gravity for 80% of the prestellar candidates. A virial analysis shows that a magnetic field of about 10 mG would be necessary to explain the stability of these cores. Such a magnetic field intensity is not excluded for these new cores and will need to be measured through dust emission polarization. Alternatively, strong rotation expected in these cores could, on small scales, prevent rapid protostar formation with high accretion rates. This second hypothesis can be tested with higher spatial resolution molecular observations with ALMA.In conclusion, (1) high-mass prestellar cores exist and can be revealed by a systematic search for molecular outflows in the densest and most confused regions of high-mass star formation; (2) their lifetimes suggest support beyond turbulence, likely magnetic or rotational, delaying immediate collapse

In this project I have worked with high spatial and spectral resolution data on the protostellar binary system IRAS 16293-2422 obtained with Atacama Large Millimeter-submillimeter Array (ALMA). I focused in particular on one of the two sources, IRAS 16293A, which is a Class 0 protostar. IRAS 16293A has a much more complicated structure than its companion, it has indeed two outflows, along NE-SW and E-W directions, and the continuum emission appears to be one at sub-mm wavelengths but split into two at mm and cm wavelengths. Line emission toward this source is also characterized by FWHMs much higher than those observed toward IRAS 16293B, causing some lines to blend. Thus, in order to give a contribution toward the clarification of how these dynamics work, I investigated how different species are affected through studying their distribution and velocity. To do this I mapped integrated emission (Moment 0 maps) and velocity distribution (Moment 1 maps) for 33 different transitions from various species. It turned out that all species peak close to the continuum source (which represents the accretion disk) but most of the peaks present an offset towards South West, probably because of the influence of NE-SW outflow. Moreover, using rotational diagrams, I have been able to estimate methanol temperature and column density in every pixel around Source A. The temperature map doesn’t show increasing temperatures towards the center, as one would expect getting closer to the protostar, but warmer zones in the eastern and in the western part of methanol emitting region, where all methanol lines show kinetic deformation. Furthermore, methanol transitions Moment 0 maps are characterized by the presence of a horizontally elongated structure, in the lower part of methanol emitting region. I discuss how both these methanol emission features are compatible with the action of shocks generated by E-W outflow.

This review describes bipolar protostellar highly collimated fast ionized jets and slower molecular outflows. Jets and outflows play a crucial role in the process of star formation, carrying away excess angular momentum from the system, without which star formation would simply be impossible. The main observational characteristics of these phenomena are given and the main models of their formation are described.

The paper presents the results of processing observations of the formation region of massive stars S255IR. We identified the molecular lines SO, 33SO, SO2, SiO, OO, 13OO, O13O, OH3OH, OH3ON, OH3OOH 3, OH3OHO, H2OS, HOOOH, HO13N, HO3N, OOS, NS, 00H, HOOO, HNOO. These molecules are indicators of hot nuclei and outflows, from which we infer high temperatures and densities in this region.