39 research outputs found
Dusty spirals triggered by shadows in transition discs
Context. Despite the recent discovery of spiral-shaped features in
protoplanetary discs in the near-infrared and millimetric wavelengths, there is
still an active discussion to understand how they formed. In fact, the spiral
waves observed in discs around young stars can be due to different physical
mechanisms: planet/companion torques, gravitational perturbations or
illumination effects. Aims. We study the spirals formed in the gaseous phase
due to two diametrically opposed shadows cast at fixed disc locations. The
shadows are created by an inclined non-precessing disc inside the cavity, which
is assumed to be optically thick. In particular, we analyse the effect of these
spirals on the dynamics of the dust particles and discuss their detectability
in transition discs. Methods. We perform gaseous hydrodynamical simulations
with shadows, then we compute the dust evolution on top of the gaseous
distribution, and finally we produce synthetic ALMA observations of the dust
emission based on radiative transfer calculations. Results. Our main finding is
that mm- to cm-sized dust particles are efficiently trapped inside the
shadow-triggered spirals. We also observe that particles of various sizes
starting at different stellocentric distances are well mixed inside these
pressure maxima. This dynamical effect would favour grain growth and affect the
resulting composition of planetesimals in the disc. In addition, our radiative
transfer calculations show spiral patterns in the disc at 1.6 {\mu}m and 1.3
mm. Due to their faint thermal emission (compared to the bright inner regions
of the disc) the spirals cannot be detected with ALMA. Our synthetic
observations prove however that shadows are observable as dips in the thermal
emission.Comment: 15 pages, 11 figures, accepted for publication in A&
Multiple shells driven by disk winds: ALMA observations in the HH 30 outflow
We present archive Atacama Large Millimeter/Submillimeter Array (ALMA) Band 6
observations of the CO (J=2-1) and CO (J=2-1) molecular line
emission of the protostellar system associated with HH 30. The CO
molecular line shows the accretion disk while the molecular outflow is traced
by the emission of the CO molecular line. We estimated a dynamical mass
for the central object of M, and a mass for the molecular
outflow of M. The molecular outflow presents
an internal cavity as well as multiple outflowing shell structures. We
distinguish three different shells with constant expansion ( km
s) and possible rotation signatures ( km s). We find
that the shells can be explained by magnetocentrifugal disk winds with
launching radii au and a small magnetic lever arm
. The multiple shell structure may be the result of
episodic ejections of the material from the accretion disk associated with
three different epochs with dynamical ages of yr, yr, and
yr for the first, second, and third shells, respectively. The
outermost shell was ejected yr before the medium shell, while the
medium shell was launched yr before the innermost shell. Our
estimations of the linear and angular momentum rates of the outflow as well as
the accretion luminosity are consistent with the expected values if the outflow
of HH 30 is produced by a wide-angle disk wind
Star cluster progenitors are dynamically decoupled from their parent molecular clouds
The formation of stellar clusters dictates the pace at which galaxies evolve, and solving the question of their formation will undoubtedly lead to a better understanding of the Universe as a whole. While it is well known that star clusters form within parsec-scale overdensities of interstellar molecular gas called clumps, it is, however, unclear whether these clumps represent the high-density tip of a continuous gaseous flow that gradually leads towards the formation of stars, or a transition within the gas physical properties. Here, we present a unique analysis of a sample of 27 infrared dark clouds embedded within 24 individual molecular clouds that combine a large set of observations, allowing us to compute the mass and velocity dispersion profiles of each, from the scale of tens of parsecs down to the scale of tenths of a parsec. These profiles reveal that the vast majority of the clouds, if not all, are consistent with being self-gravitating on all scales, and that the clumps, on parsec-scale, are often dynamically decoupled from their surrounding molecular clouds, exhibiting steeper density profiles (ρ∝r−2) and flat velocity dispersion profiles (σ∝r0), clearly departing from Larson’s relations. These findings suggest that the formation of star clusters correspond to a transition regime within the properties of the self-gravitating molecular gas. We propose that this transition regime is one that corresponds to the gravitational collapse of parsec-scale clumps within otherwise stable molecular clouds
Characterizing Magnetic Field Morphologies in Three Serpens Protostellar Cores with ALMA
With the aim of characterizing the dynamical processes involved in the formation of young protostars, we present high-angular-resolution ALMA dust polarization observations of the Class 0 protostellar cores Serpens SMM1, Emb 8(N), and Emb 8. With spatial resolutions ranging from 150 to 40 au at 870 μm, we find unexpectedly high values of the polarization fraction along the outflow cavity walls in Serpens Emb 8(N). We use 3 mm and 1 mm molecular tracers to investigate outflow and dense-gas properties and their correlation with the polarization. These observations allow us to investigate the physical processes involved in the radiative alignment torques (RATs) acting on dust grains along the outflow cavity walls, which experience irradiation from accretion processes and outflow shocks. The inner core of SMM1-a presents a polarization pattern with a poloidal magnetic field at the bases of the two lobes of the bipolar outflow. To the south of SMM1-a we see two polarized filaments, one of which seems to trace the redshifted outflow cavity wall. The other may be an accretion streamer of material infalling onto the central protostar. We propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present at <500 au scales in Class 0 envelopes have grown larger than the 0.1 μm size of dust grains in the interstellar medium. Our observations allow us to constrain the magnetic field morphologies of star-forming sources within the central cores, along the outflow cavity walls, and in possible accretion streamers
The Explosion in Orion-KL as Seen by Mosaicking the Magnetic Field with ALMA
We present the first linear-polarization mosaicked observations performed by
the Atacama Large Millimeter/submillimeter Array (ALMA). We mapped the
Orion-KLeinmann-Low (Orion-KL) nebula using super-sampled mosaics at 3.1 and
1.3 mm as part of the ALMA Extension and Optimization of Capabilities (EOC)
program. We derive the magnetic field morphology in the plane of the sky by
assuming that dust grains are aligned with respect to the ambient magnetic
field. At the center of the nebula, we find a quasi-radial magnetic field
pattern that is aligned with the explosive CO outflow up to a radius of
approximately 12 arc-seconds (~ 5000 au), beyond which the pattern smoothly
transitions into a quasi-hourglass shape resembling the morphology seen in
larger-scale observations by the James-Clerk-Maxwell Telescope (JCMT). We
estimate an average magnetic field strength mG and a
total magnetic energy of 2 x 10^45 ergs, which is three orders of magnitude
less than the energy in the explosive CO outflow. We conclude that the field
has been overwhelmed by the outflow and that a shock is propagating from the
center of the nebula, where the shock front is seen in the magnetic field lines
at a distance of ~ 5000 au from the explosion center.Comment: Accepted for publication in Ap
ALMA-IMF VIII -- Combination of Interferometric Continuum Images with Single-Dish Surveys and Structural Analysis of Six Protoclusters
We present the combination of ALMA-IMF and single-dish continuum images from
the Mustang-2 Galactic Plane Survey (MGPS90) at 3 millimeters and the Bolocam
Galactic Plane Survey (BGPS) at 1 millimeter. Six and ten out of the fiffteen
ALMA-IMF fields are combined with MGPS90 and BGPS, respectively. The
combination is made via the feathering technique. We used the dendrogram
algorithm throughout the combined images, and performed further analysis in the
six fields with combination in both bands (G012.80, W43-MM1, W43-MM2, W43-MM3,
W51-E, W51-IRS2). In these fields, we calculated spectral index maps and used
them to separate regions dominated by dust or free-free emission, and then
performed further structural analysis. We report the basic physical parameters
of the dust-dominated (column densities, masses) and ionized (emission
measures, hydrogen ionization photon rates) structures. We also searched for
multi-scale relations in the dust-dominated structures across the analyzed
fields, finding that the fraction of mass in dendrogram leaves (which we label
as "Leaf Mass Eficiency", LME) as a function of molecular gas column density
follows a similar trend: a rapid, exponential-like growth, with maximum values
approaching 100% in most cases. The observed behaviour of the LME with gas
column is tentatively interpreted as an indicator of large star formation
activity within the ALMA-IMF protoclusters. W51-E and G012.80 stand out as
cases with comparatively large and reduced potential for further star
formation, respectively.Comment: Accepted to The Astrophysical Journal Supplemen
Hypermassive cloud, shock and stellar formation efficiency
Les étoiles massives, de type O ou B, sont d'une importance capitale pour le budget énergétique des galaxies et l'enrichissement du milieu interstellaire. Néanmoins, leur formation, contrairement à celle des étoiles de type solaire reste sujet à débats, sinon une énigme. Les toutes premières étapes de la formation des étoiles massives ainsi que la formation de leur nuage parent sont des thèmes qui stimulent une grande activité sur les plans théorique et observationnel depuis une décennie. Il semble maintenant acquis que les étoiles massives naissent dans des cœurs denses massifs, qui se forment au travers de processus dynamiques, tels que les flots de gaz collisionnels. Au cours de ma thèse, j'ai mené une étude approfondie de la formation des cœurs denses et des étoiles massives au sein de la structure hypermassive W43-MM1, localisée à 6~kpc du soleil. Dans un premier temps, j'ai montré une corrélation directe entre l'efficacité à former des étoiles et la densité volumique des nuages moléculaires, en décalage avec un certain nombre d'études précédentes. En effet, la distribution spatiale et de masse des cœurs denses massifs en formation au sein de W43-MM1 suggère que ce filament hypermassif est en phase de flambée de formation d'étoiles, flambée d'autant plus grande que l'on se rapproche de son cœur. J'ai comparé ces résultats observationnels aux modèles numériques et analytiques d'efficacité de formation stellaire les plus récents. Cette confrontation permet non seulement d'apporter de nouvelles contraintes sur la formation des filaments hypermassifs, mais suggère aussi que la compréhension de la formation d'étoiles dans les nuages hypermassifs nécessite une description fine de la structure de ces objets exceptionnels. En second lieu, ayant montré que la formation des étoiles massives est fortement dépendante des propriétés des filaments qui les forment, je me suis naturellement intéressé aux processus de formation de ces filaments, grâce à une étude de leur dynamique globale. Plus précisément, j'ai utilisé un traceur de chocs (la molécule de SiO) pour discerner les chocs dûs aux processus locaux de formation des étoiles (jets et flots bipolaires), des chocs dûs aux processus permettant la formation du nuage. J'ai ainsi pu, via une étude sans précédent alliant observations et modélisation de chocs dans une région formant de nombreuses étoiles, montrer l'existence de chocs à basse vitesse, première signature directe de la formation du nuage moléculaire dans lequel les étoiles massives se forment. Ces résultats constituent une étape importante reliant, via des processus dynamiques, la formation des nuages moléculaires à la formation des étoiles massives.O and B types stars are of paramount importance in the energy budget of galaxies and play a crucial role enriching the interstellar medium. However, their formation, unlike that of solar-type stars, is still subject to debate, if not an enigma. The earliest stages of massive star formation and the formation of their parent cloud are still crucial astrophysical questions that drew a lot of attention in the community, both from the theoretical and observational perspective, during the last decade. It has been proposed that massive stars are born in massive dense cores that form through very dynamic processes, such as converging flows of gas. During my PhD, I conducted a thorough study of the formation of dense cores and massive stars in the W43-MM1 supermassive structure, located at ~ 6 kpc from the sun. At first, I showed a direct correlation between the star formation efficiency and the volume gas density of molecular clouds, in contrast with scenarii suggested by previous studies. Indeed, the spatial distribution and mass function of the massive dense cores currently forming in W43-MM1 suggests that this supermassive filament is undergoing a star formation burst, increasing as one approaches its center. I compared these observational results with the most recent numerical and analytical models of star formation. This comparison not only provides new constraints on the formation of supermassive filaments, but also suggests that understanding star formation in high density, extreme ridges requires a detailed portrait of the structure of these exceptional objects. Second, having shown that the formation of massive stars depends strongly on the properties of the ridges where they form, I studied the formation processes of these filaments, thanks of the characterization of their global dynamics. Specifically, I used a tracer of shocks (SiO molecule) to disentangle the feedback of local star formation processes (bipolar jets and outflows) from shocks tracing the pristine formation processes of the W43-MM1 cloud. I was able, via an unprecedented study combining observations and modeling of shocks in a starbust region, to show the existence of widespread low velocity shocks, that are the first direct signature of the formation of the massive molecular cloud from which massive stars form.These results are an important step connecting, via dynamical processes, the formation of molecular clouds to the formation of massive stars
Nuage hypermassif, chocs et efficacité de formation stellaire
O and B types stars are of paramount importance in the energy budget of galaxies and play a crucial role enriching the interstellar medium. However, their formation, unlike that of solar-type stars, is still subject to debate, if not an enigma. The earliest stages of massive star formation and the formation of their parent cloud are still crucial astrophysical questions that drew a lot of attention in the community, both from the theoretical and observational perspective, during the last decade. It has been proposed that massive stars are born in massive dense cores that form through very dynamic processes, such as converging flows of gas. During my PhD, I conducted a thorough study of the formation of dense cores and massive stars in the W43-MM1 supermassive structure, located at ~ 6 kpc from the sun. At first, I showed a direct correlation between the star formation efficiency and the volume gas density of molecular clouds, in contrast with scenarii suggested by previous studies. Indeed, the spatial distribution and mass function of the massive dense cores currently forming in W43-MM1 suggests that this supermassive filament is undergoing a star formation burst, increasing as one approaches its center. I compared these observational results with the most recent numerical and analytical models of star formation. This comparison not only provides new constraints on the formation of supermassive filaments, but also suggests that understanding star formation in high density, extreme ridges requires a detailed portrait of the structure of these exceptional objects. Second, having shown that the formation of massive stars depends strongly on the properties of the ridges where they form, I studied the formation processes of these filaments, thanks of the characterization of their global dynamics. Specifically, I used a tracer of shocks (SiO molecule) to disentangle the feedback of local star formation processes (bipolar jets and outflows) from shocks tracing the pristine formation processes of the W43-MM1 cloud. I was able, via an unprecedented study combining observations and modeling of shocks in a starbust region, to show the existence of widespread low velocity shocks, that are the first direct signature of the formation of the massive molecular cloud from which massive stars form.These results are an important step connecting, via dynamical processes, the formation of molecular clouds to the formation of massive stars.Les étoiles massives, de type O ou B, sont d'une importance capitale pour le budget énergétique des galaxies et l'enrichissement du milieu interstellaire. Néanmoins, leur formation, contrairement à celle des étoiles de type solaire reste sujet à débats, sinon une énigme. Les toutes premières étapes de la formation des étoiles massives ainsi que la formation de leur nuage parent sont des thèmes qui stimulent une grande activité sur les plans théorique et observationnel depuis une décennie. Il semble maintenant acquis que les étoiles massives naissent dans des cœurs denses massifs, qui se forment au travers de processus dynamiques, tels que les flots de gaz collisionnels. Au cours de ma thèse, j'ai mené une étude approfondie de la formation des cœurs denses et des étoiles massives au sein de la structure hypermassive W43-MM1, localisée à 6~kpc du soleil. Dans un premier temps, j'ai montré une corrélation directe entre l'efficacité à former des étoiles et la densité volumique des nuages moléculaires, en décalage avec un certain nombre d'études précédentes. En effet, la distribution spatiale et de masse des cœurs denses massifs en formation au sein de W43-MM1 suggère que ce filament hypermassif est en phase de flambée de formation d'étoiles, flambée d'autant plus grande que l'on se rapproche de son cœur. J'ai comparé ces résultats observationnels aux modèles numériques et analytiques d'efficacité de formation stellaire les plus récents. Cette confrontation permet non seulement d'apporter de nouvelles contraintes sur la formation des filaments hypermassifs, mais suggère aussi que la compréhension de la formation d'étoiles dans les nuages hypermassifs nécessite une description fine de la structure de ces objets exceptionnels. En second lieu, ayant montré que la formation des étoiles massives est fortement dépendante des propriétés des filaments qui les forment, je me suis naturellement intéressé aux processus de formation de ces filaments, grâce à une étude de leur dynamique globale. Plus précisément, j'ai utilisé un traceur de chocs (la molécule de SiO) pour discerner les chocs dûs aux processus locaux de formation des étoiles (jets et flots bipolaires), des chocs dûs aux processus permettant la formation du nuage. J'ai ainsi pu, via une étude sans précédent alliant observations et modélisation de chocs dans une région formant de nombreuses étoiles, montrer l'existence de chocs à basse vitesse, première signature directe de la formation du nuage moléculaire dans lequel les étoiles massives se forment. Ces résultats constituent une étape importante reliant, via des processus dynamiques, la formation des nuages moléculaires à la formation des étoiles massives
High-Mass Star and Massive Cluster Formation in the Milky Way
International audienceThis review examines the state-of-the-art knowledge of high-mass star and massive cluster formation, gained from ambitious observational surveys, which acknowledge the multi-scale characteristics of these processes. After a brief overview of theoretical models and main open issues, we present observational searches for the evolutionary phases of high-mass star formation, first among high-luminosity sources and more recently among young massive protostars and the elusive high-mass prestellar cores. We then introduce the most likely evolutionary scenario for high-mass star formation, which emphasizes the link of high-mass star formation to massive cloud and cluster formation. Finally, we introduce the first attempts to search for variations of the star formation activity and cluster formation in molecular cloud complexes, in the most extreme star-forming sites, and across the Milky Way. The combination of Galactic plane surveys and high-angular resolution images with submillimeter facilities such as Atacama Large Millimeter Array (ALMA) are prerequisites to make significant progresses in the forthcoming decade