We present Atacama Large Millimeter Array (ALMA) Band 6 observations at 14−20 au spatial resolution of the disk and CO(2-1) outflow around the Class I protostar DG Tau B in Taurus. The disk is very ...large, both in dust continuum (
R
eff, 95%
= 174 au) and CO (
R
CO
= 700 au). It shows Keplerian rotation around a 1.1 ± 0.2
M
⊙
central star and two dust emission bumps at
r
= 62 and 135 au. These results confirm that large structured disks can form at an early stage where residual infall is still ongoing. The redshifted CO outflow at high velocity shows a striking hollow cone morphology out to 3000 au with a shear-like velocity structure within the cone walls. These walls coincide with the scattered light cavity, and they appear to be rooted within < 60 au in the disk. We confirm their global average rotation in the same sense as the disk, with a specific angular momentum ≃65 au km s
−1
. The mass-flux rate of 1.7−2.9 × 10
−7
M
⊙
yr
−1
is 35 ± 10 times that in the atomic jet. We also detect a wider and slower outflow component surrounding this inner conical flow, which also rotates in the same direction as the disk. Our ALMA observations therefore demonstrate that the inner cone walls, and the associated scattered light cavity, do not trace the interface with infalling material, which is shown to be confined to much wider angles (> 70°). The properties of the conical walls are suggestive of the interaction between an episodic inner jet or wind with an outer disk wind, or of a massive disk wind originating from 2 to 5 au. However, further modeling is required to establish their origin. In either case, such massive outflow may significantly affect the disk structure and evolution.
The angular resolution of a telescope is the primary observational parameter, along with the detector sensitivity in defining the quality of the observed images and of the subsequent scientific ...exploitation of the data. During the last decade in star formation research, many studies have targeted low- and high-mass star formation regions located at different distances, with different telescopes having specific angular resolution capabilities. However, no dedicated studies of the spatial resolution effects on the derived sizes and masses of the sources extracted from the observed images have been published. We present a systematic investigation of the angular resolution effects, with special attention being paid to the derived masses of sources as well as the shape of the resulting source mass functions (SMFs) and to their comparison with the initial stellar mass function. For our study, we chose two star-forming regions observed with
Herschel
, NGC 6334 and Aquila distant of 1750 and 460 pc respectively, and three (magneto)-hydrodynamical simulations, virtually positioned at the same distances as the observed regions. We built surface density maps with different angular resolutions by convolving the surface density images of the five regions to a set of four resolutions differing by a factor of two (9, 18, 36, and 72′′), which allowed us to cover spatial resolutions from 0.6 down to 0.02 pc. Then we detected and measured sources in each of the images at each resolution using getsf and we analysed the derived masses and sizes of the extracted sources. We find that the number of sources does not converge from 0.6 to ≳0.05 pc. It increases by about two when the angular resolution increases with a similar factor, which confirms that these large sources are cluster-forming clumps. Below 0.05 pc, the number of source still increases by about 1.3 when the angular resolution increases by two, suggesting that we are close to, but not yet at, convergence. In this regime of physical scales, we find that the measured sizes and masses of sources linearly depend on the angular resolution with no sign of convergence to a resolution-independent value, implying that these sources cannot be assimilated to isolated prestellar cores. The corresponding SMF peak also shifts with angular resolution, while the slope of the high-mass tail of the SMFs remains almost invariant. We propose that these angular resolution effects could be caused by the underestimated background of the unresolved sources observed against the sloping, hill-like backgrounds of the molecular clouds. If prestellar cores physically distinct from their background exist in cluster-forming molecular clouds, we conclude that their mass must be lower than reported so far in the literature. We discuss various implications for the studies of star formation: the problem of determining the mass reservoirs involved in the star-formation process; the inapplicability of the Gaussian beam deconvolution to infer source sizes; and the impossibility to determine the efficiency of the mass conversion from the cores to the stars. Our approach constitutes a simple convergence test to determine whether an observation is affected by angular resolution.
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.
Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, ...which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel/PACS and SPIRE 70−500 μm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc × 1 pc ridge and two 0.8 pc × 0.8 pc hubs, with volume-averaged densities of ~105 cm-3. Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M⊙ for MDCs in NGC 6334. MDCs have temperatures of 9.5−40 K, masses of 75−1000 M⊙, and densities of 1 × 105−7 × 107 cm-3. Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 μm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1−7 × 104 yr and at most 3 × 105 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC 6334, favoring a scenario presented here, which simultaneously forms clouds, ridges, MDCs, and high-mass protostars.
Context. Numerical simulations have explored the possibility of forming molecular clouds through either a quasi-static, self-gravitating mechanism or the collision of gas streams or lower density ...clouds. They also quantitatively predict the distribution of matter at the transition from atomic to molecular gases. Aims. We aim to observationally test these models by studying the environment of W43, a molecular cloud complex recently identified near the tip of the Galactic long bar. Methods. Using Galaxy-wide H i and 12CO 1–0 surveys, we searched for gas flowing toward the W43 molecular cloud complex. We also estimated the H i and H2 mass surface densities to constrain the transition from atomic to molecular gas around and within W43. Results. We found three cloud ensembles within the position-velocity diagrams of 12CO and H i gases. They are separated by ~20 km s-1 along the line of sight and extend into the 13CO velocity structure of W43. Since their velocity gradients are consistent with free fall, they could be nearby clouds attracted by and streaming toward the W43 ~107 M⊙ potential well. We show that the H i surface density, ΣH i = 45−85 M⊙pc-2, does not reach any threshold level but increases when entering the 130 pc-wide molecular complex previously defined. This suggests that an equilibrium between H2 formation and photodissociation has not yet been reached. The H2-to-H i ratio measured over the W43 region and its surroundings, RH2 ~ 3.5±23, is high, indicating that most of the gas is already in molecular form in W43 and in structures several hundred parsecs downstream along the Scutum-Centaurus arm. Conclusions. The W43 molecular cloud complex may have formed and, in fact may still be accreting mass from the agglomeration of clouds. Already in the molecular-dominated regime, most of these clouds are streaming from the Scutum-Centaurus arm. This clearly disagrees with quasi-static and steady-state models of molecular cloud formation.
Context.
The mass segregation of stellar clusters could be primordial rather than dynamical. Despite the abundance of studies of mass segregation for stellar clusters, those for stellar progenitors ...are still scarce, so the question concerning the origin and evolution of mass segregation is still open.
Aims.
Our goal is to characterize the structure of the NGC 2264 molecular cloud and compare the populations of clumps and young stellar objects (YSOs) in this region whose rich YSO population has shown evidence of sequential star formation.
Methods.
We separated the
Herschel
column density map of NGC 2264 into three subregions and compared their cloud power spectra using a multiscale segmentation technique. We extracted compact cloud fragments from the column density image, measured their basic properties, and studied their spatial and mass distributions.
Results.
In the whole NGC 2264 cloud, we identified a population of 256 clumps with typical sizes of ~0.1 pc and masses ranging from 0.08
M
⊙
to 53
M
⊙
. Although clumps have been detected all over the cloud, most of the massive, bound clumps are concentrated in the central subregion of NGC 2264. The local surface density and the mass segregation ratio indicate a strong degree of mass segregation for the 15 most massive clumps, with a median Σ
6
three times that of the whole clumps population and Λ
MSR
≃ 8. We show that this cluster of massive clumps is forming within a high-density cloud ridge, which is formed and probably still fed by the high concentration of gas observed on larger scales in the central subregion. The time sequence obtained from the combined study of the clump and YSO populations in NGC 2264 suggests that the star formation started in the northern subregion, that it is now actively developing at the center, and will soon start in the southern subregion.
Conclusions.
Taken together, the cloud structure and the clump and YSO populations in NGC 2264 argue for a dynamical scenario of star formation. The cloud could first undergo global collapse, driving most clumps to centrally concentrated ridges. After their main accretion phase, some YSOs, and probably the most massive, would stay clustered while others would be dispersed from their birth sites. We propose that the mass segregation observed in some star clusters is inherited from that of clumps, originating from the mass assembly phase of molecular clouds.
Context.
Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores.
Aims.
In an effort to ...improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars and carried out a comprehensive statistical analysis of dust polarization quantities.
Methods.
We analyze the statistical properties of the polarization fraction
P
frac
and the dispersion of polarization position angles
S
. More specifically, we investigate the relationship between
S
and
P
frac
as well as the evolution of the product
S
×
P
frac
as a function of the column density of the gas in the protostellar envelopes. We compare the observed trends with those found in polarization observations of dust in the interstellar medium and in synthetic observations of non-ideal magneto-hydrodynamic (MHD) simulations of protostellar cores.
Results.
We find a significant
S
∝
P
frac
−0.79
correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index significantly differs from the one observed by
Planck
in star-forming clouds. The product
S
×
P
frac
, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density (from
N
H
2
= 10
22
cm
−2
to
N
H
2
= 10
25
cm
−2
), with a mean value of 0.36
−0.17
+0.10
. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not systematically depend on the local conditions such as the local gas density. However, in the lowest-luminosity sources in our sample, we find a hint of less efficient dust grain alignment with increasing column density. Our observations and their comparison with synthetic observations of MHD models suggest that the total intensity versus the polarized dust are distributed at different intrinsic spatial scales, which can affect the statistics from the ALMA observations, for example, by producing artificially high
P
frac
. Finally, synthetic observations of MHD models implementing radiative alignment torques (RATs) show that the statistical estimator
S
×
P
frac
is sensitive to the strength of the radiation field in the core. Moreover, we find that the simulations with a uniform perfect alignment (PA) of dust grains yield, on average, much higher
S
×
P
frac
values than those implementing RATs; the ALMA values lie among those predicted by PA, and they are significantly higher than the ones obtained with RATs, especially at large column densities.
Conclusions.
Ultimately, our results suggest that dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further studies will be needed to understand how more efficient grain alignment via, for example, different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations.
Context. The formation of high-mass stars remains unknown in many aspects. There are two competing families of models to explain the formation of high-mass stars. On the one hand, quasi-static models ...predict the existence of high-mass pre-stellar cores sustained by a high degree of turbulence. On the other hand, competitive accretion models predict that high-mass proto-stellar cores evolve from low or intermediate mass proto-stellar cores in dynamic environments. Aims. The aim of the present work is to bring observational constraints at the scale of high-mass cores (~0.03 pc). Methods. We targeted with ALMA and MOPRA a sample of nine starless massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study. Their mass and size (~110 M⊙ and r = 0.1 pc, respectively) are similar to the initial conditions used in the quasi-static family of models explaining for the formation of high-mass stars. We present ALMA 1.4 mm continuum observations that resolve the Jeans length (λJeans ~ 0.03 pc) and that are sensitive to the Jeans mass (MJeans ~ 0.65 M⊙) in the nine starless MDCs, together with ALMA-12CO(2–1) emission line observations. We also present HCO+(1–0), H13CO+(1–0) and N2H+(1–0) molecular lines from the MOPRA telescope for eight of the nine MDCs. Results. The nine starless MDCs have the mass reservoir to form high-mass stars according to the criteria by Baldeschi et al. (2017). Three of the starless MDCs are subvirialized with αvir ~ 0.35, and four MDCs show sign of collapse from their molecular emission lines. ALMA observations show very little fragmentation within the MDCs. Only two of the starless MDCs host compact continuum sources, whose fluxes correspond to <3 M⊙ fragments. Therefore, the mass reservoir of the MDCs has not yet been accreted onto compact objects, and most of the emission is filtered out by the interferometer. Conclusions. These observations do not support the quasi-static models for high-mass star formation since no high-mass pre-stellar core is found in NGC 6334. The competitive accretion models, on the other hand, predict a level of fragmentation much higher than what we observe.
Aims.
It has been proposed that the magnetic field, which is pervasive in the interstellar medium, plays an important role in the process of massive star formation. To better understand the impact of ...the magnetic field at the pre- and protostellar stages, high-angular resolution observations of polarized dust emission toward a large sample of massive dense cores are needed. We aim to reveal any correlation between the magnetic field orientation and the orientation of the cores and outflows in a sample of protostellar dense cores in the W43-MM1 high-mass star-forming region.
Methods.
We used the Atacama Large Millimeter Array in Band 6 (1.3 mm) in full polarization mode to map the polarized emission from dust grains at a physical scale of ~2700 au. We used these data to measure the orientation of the magnetic field at the core scale. Then, we examined the relative orientations of the core-scale magnetic field, of the protostellar outflows, and of the major axis of the dense cores determined from a 2D Gaussian fit in the continuum emission.
Results.
We find that the orientation of the dense cores is not random with respect to the magnetic field. Instead, the dense cores are compatible with being oriented 20–50° with respect to the magnetic field. As for the outflows, they could be oriented 50–70° with respect to the magnetic field, or randomly oriented with respect to the magnetic field, which is similar to current results in low-mass star-forming regions.
Conclusions.
The observed alignment of the position angle of the cores with respect to the magnetic field lines shows that the magnetic field is well coupled with the dense material; however, the 20–50° preferential orientation contradicts the predictions of the magnetically-controlled core-collapse models. The potential correlation of the outflow directions with respect to the magnetic field suggests that, in some cases, the magnetic field is strong enough to control the angular momentum distribution from the core scale down to the inner part of the circumstellar disks where outflows are triggered.
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