Context.
Understanding the initial properties of star forming material and how they affect the star formation process is a key question. The infalling gas must redistribute most of its initial ...angular momentum inherited from prestellar cores before reaching the central stellar embryo. Disk formation has been naturally considered as a possible solution to this “angular momentum problem”. However, how the initial angular momentum of protostellar cores is distributed and evolves during the main accretion phase and the beginning of disk formation has largely remained unconstrained up to now.
Aims.
In the framework of the IRAM CALYPSO survey, we obtained observations of the dense gas kinematics that we used to quantify the amount and distribution of specific angular momentum at all scales in collapsing-rotating Class 0 protostellar envelopes.
Methods.
We used the high dynamic range C
18
O (2−1) and N
2
H
+
(1−0) datasets to produce centroid velocity maps and probe the rotational motions in the sample of 12 envelopes from scales ~50 to ~5000 au.
Results.
We identify differential rotation motions at scales ≲1600 au in 11 out of the 12 protostellar envelopes of our sample by measuring the velocity gradient along the equatorial axis, which we fit with a power-law model v ∝
r
α
. This suggests that coherent motions dominate the kinematics in the inner protostellar envelopes. The radial distributions of specific angular momentum in the CALYPSO sample suggest the following two distinct regimes within protostellar envelopes: the specific angular momentum decreases as
j
∝
r
1.6±0.2
down to ~1600 au and then tends to become relatively constant around ~6 × 10
−4
km s
−1
pc down to ~50 au.
Conclusions.
The values of specific angular momentum measured in the inner Class 0 envelopes suggest that material directly involved in the star formation process (<1600 au) has a specific angular momentum on the same order of magnitude as what is inferred in small T-Tauri disks. Thus, disk formation appears to be a direct consequence of angular momentum conservation during the collapse. Our analysis reveals a dispersion of the directions of velocity gradients at envelope scales >1600 au, suggesting that these gradients may not be directly related to rotational motions of the envelopes. We conclude that the specific angular momentum observed at these scales could find its origin in other mechanisms, such as core-forming motions (infall, turbulence), or trace an imprint of the initial conditions for the formation of protostellar cores.
We present the most sensitive 3 mm-survey to date of protoplanetary disks carried out in the Taurus-Auriga star forming region (average rms of about 0.3 mJy), using the IRAM PdBI. With our high ...detection rate of 17/19 we provide the first detections at wavelengths longer than about 1 mm for 12 sources. This enables us to statistically study the mm SED slopes and dust properties of faint disks and compare them to brighter disks using a uniform analysis method. With these new data and literature measurements at sub-millimeter and millimeter wavelengths, we analyze the dust properties of a sample of 21 isolated disks around T Tauri stars in the Taurus-Auriga star forming region. Together with the information about the disks' spatial extension from sub/mm-mm interferometric studies, we derive from the observed sub-mm/mm spectral energy distribution constraints on the dust opacity law at these wavelengths, using two-layer flared disk models and a self-consistent dust model that properly takes into account the variation of the dust opacity with grain growth. We find evidence for the presence of dust particles in the disk midplane that have grown to sizes as large as at least 1 millimeter in all the disks of our sample, confirming what was previously observed on smaller brighter objects. This indicates that the dust coagulation from ISM dust to mm-sized grains is a very fast process in protoplanetary disks, which appears to occur before a young stellar object enters the class II evolutionary stage. The amount of these large grains in the disk outer regions is also found to be stationary throughout the whole class II evolutionary stage, which indicates that mechanisms slowing down the dust inward migration play an important role in the Taurus-Auriga protoplanetary disks. Another result is that the spectral index between 1 and 3 mm for the six faintest disks in our sample is on average smaller than for the brighter disks, indicating either that these fainter yet unmapped disks are spatially much less extended than the brighter spatially resolved disks, or that fainter disks have typically larger dust grains in their outer regions. Considering that these fainter disks are more representative of the bulk of the disk population than the brighter ones, this may have important consequences for the theories of planetesimal formation and disk formation and evolution. Finally, we investigate the relations between the derived dust properties, namely dust mass and grain growth, and the properties of the central star, like its mass, age and mass accretion rate.
Abstract
Evidence is mounting that the small bodies of our Solar system, such as comets and asteroids, have at least partially inherited their chemical composition from the first phases of the Solar ...system formation. It then appears that the molecular complexity of these small bodies is most likely related to the earliest stages of star formation. It is therefore important to characterize and to understand how the chemical evolution changes with solar-type protostellar evolution. We present here the Large Program ‘Astrochemical Surveys At IRAM’ (ASAI). Its goal is to carry out unbiased millimetre line surveys between 80 and 272 GHz of a sample of 10 template sources, which fully cover the first stages of the formation process of solar-type stars, from pre-stellar cores to the late protostellar phase. In this paper, we present an overview of the surveys and results obtained from the analysis of the 3 mm band observations. The number of detected main isotopic species barely varies with the evolutionary stage and is found to be very similar to that of massive star-forming regions. The molecular content in O- and C-bearing species allows us to define two chemical classes of envelopes, whose composition is dominated by either (a) a rich content in O-rich complex organic molecules, associated with hot corino sources, or (b) a rich content in hydrocarbons, typical of warm carbon-chain chemistry sources. Overall, a high chemical richness is found to be present already in the initial phases of solar-type star formation.
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.
Context.
Complex organic molecules (COMs) have been detected in a few Class 0 protostars but their origin is not well understood. While the usual picture of a hot corino explains their presence as ...resulting from the heating of the inner envelope by the nascent protostar, shocks in the outflow, disk wind, the presence of a flared disk, or the interaction region between envelope and disk at the centrifugal barrier have also been claimed to enhance the abundance of COMs.
Aims.
Going beyond studies of individual objects, we want to investigate the origin of COMs in young protostars on a statistical basis.
Methods.
We use the CALYPSO survey performed with the Plateau de Bure Interferometer of the Institut de Radioastronomie Millimétrique to search for COMs at high angular resolution in a sample of 26 solar-type protostars, including 22 Class 0 and four Class I objects. We derive the column densities of the detected molecules under the local thermodynamic equilibrium approximation and search for correlations between their abundances and with various source properties.
Results.
Methanol is detected in 12 sources and tentatively in one source, which represents half of the sample. Eight sources (30%) have detections of at least three COMs. We find a strong chemical differentiation in multiple systems with five systems having one component with at least three COMs detected but the other component devoid of COM emission. All sources with a luminosity higher than 4
L
⊙
have at least one detected COM whereas no COM emission is detected in sources with internal luminosity lower than 2
L
⊙
, likely because of a lack of sensitivity. Internal luminosity is found to be the source parameter impacting the COM chemical composition of the sources the most, while there is no obvious correlation between the detection of COM emission and that of a disk-like structure. A canonical hot-corino origin may explain the COM emission in four sources, an accretion-shock origin in two or possibly three sources, and an outflow origin in three sources. The CALYPSO sources with COM detections can be classified into three groups on the basis of the abundances of oxygen-bearing molecules, cyanides, and CHO-bearing molecules. These chemical groups correlate neither with the COM origin scenarios, nor with the evolutionary status of the sources if we take the ratio of envelope mass to internal luminosity as an evolutionary tracer. We find strong correlations between molecules that are a priori not related chemically (for instance methanol and methyl cyanide), implying that the existence of a correlation does not imply a chemical link.
Conclusions.
The CALYPSO survey has revealed a chemical differentiation in multiple systems that is markedly different from the case of the prototypical binary IRAS 16293-2422. This raises the question of whether all low-mass protostars go through a phase showing COM emission. A larger sample of young protostars and a more accurate determination of their internal luminosity will be necessary to make further progress. Searching for correlations between the COM emission and the jet/outflow properties of the sources may also be promising.
Aims.We examine whether ejection phenomena from accreting T Tauri stars can be described by only one type of self-collimated jet model. Methods.We present analytical kinematic predictions valid soon ...after the Alfvén surface for all types of steady magnetically self-confined jets. Results.We show that extended disc winds, X-winds, and stellar winds occupy distinct regions in the poloidal speed vs. specific angular momentum plane. Comparisons with current observations of T Tauri jets yield quantitative constraints on the range of launching radii, magnetic lever arms, and specific energy input in disc and stellar winds. Implications on the origin of jet asymmetries and disc magnetic fields are outlined. Conclusions.We argue that ejection phenomena from accreting T Tauri stars most likely include three dynamical components: (1) an outer self-collimated steady disc wind carrying most of the mass-flux in the optical jet (when present); confining (2) a pressure-driven coronal stellar wind; and (3) a hot inner flow made of blobs sporadically ejected from the magnetopause. If the stellar magnetic moment is parallel to the disc magnetic field, then the highly variable inner flow resembles a “Reconnection X-wind”, that has been proven to efficiently brake down an accreting and contracting young star. If the magnetic moment is anti-parallel, then larger versions of the solar coronal mass ejections are likely to occur. The relative importance of these three components in the observed outflows and the range of radii involved in the disc wind are expected to vary with time, from the stage of embedded source to the optically revealed T Tauri star phase.
Methanol is a key species in astrochemistry because it is the most abundant organic molecule in the interstellar medium and is thought to be the mother molecule of many complex organic species. ...Estimating the deuteration of methanol around young protostars is of crucial importance because it highly depends on its formation mechanisms and the physical conditions during its moment of formation. We analyse several dozen transitions from deuterated methanol isotopologues coming from various existing observational datasets obtained with the IRAM-PdBI and ALMA sub-millimeter interferometers to estimate the methanol deuteration surrounding three low-mass protostars on Solar System scales. A population diagram analysis allows us to derive a CH2DOH/CH3OH abundance ratio of 3–6% and a CH3OD/CH3OH ratio of 0.4–1.6% in the warm inner (≤100–200 AU) protostellar regions. These values are typically ten times lower than those derived with previous single-dish observations towards these sources, but they are one to two orders of magnitude higher than the methanol deuteration measured in massive hot cores. Dust temperature maps obtained from Herschel and Planck observations show that massive hot cores are located in warmer molecular clouds than low-mass sources, with temperature differences of ~10 K. The comparison of our measured values with the predictions of the gas-grain astrochemical model GRAINOBLE shows that such a temperature difference is sufficient to explain the different deuteration observed in low- to high-mass sources. This suggests that the physical conditions of the molecular cloud at the origin of the protostars mostly govern the present-day observed deuteration of methanol and therefore of more complex organic molecules. Finally, the methanol deuteration measured towards young solar-type protostars on Solar System scales seems to be higher by a factor of ~5 than the upper limit in methanol deuteration estimated in comet Hale-Bopp. If this result is confirmed by subsequent observations of other comets, it would imply that an important reprocessing of the organic material likely occurred in the solar nebula during the formation of the Solar System.
Context. Fast jets are thought to be a crucial ingredient of star formation because they might extract angular momentum from the disk and thus allow mass accretion onto the star. However, it is ...unclear whether jets are ubiquitous, and likewise, their contribution to mass and angular momentum extraction during protostar formation remains an open question. Aims. Our aim is to investigate the ejection process in the low-mass Class 0 protostar L1157. This source is associated with a spectacular bipolar outflow, and the recent detection of high-velocity SiO suggests the occurrence of a jet. Methods. Observations of CO 2 −1 and SiO 5 − 4 at ~ \hbox{$0\farcs8$} 0 .̋ 8 resolution were obtained with the IRAM Plateau de Bure Interferometer (PdBI) as part of the CALYPSO large program. The jet and outflow structure were fit with a precession model. We derived the column density of CO and SiO, as well as the jet mass-loss rate and mechanical luminosity. Results. High-velocity CO and SiO emission resolve for the first time the first 200 au of the outflow-driving molecular jet. The jet is strongly asymmetric, with the blue lobe ~0.65 times slower than the red lobe. This suggests that the large-scale asymmetry of the outflow is directly linked to the jet velocity and that the asymmetry in the launching mechanism has been at work for the past 1800 yr. Velocity asymmetries are common in T Tauri stars, which suggests that the jet formation mechanism from Class 0 to Class II stages might be similar. Our model simultaneously fits the properties of the inner jet and of the clumpy 0.2 pc scale outflow by assuming that the jet precesses counter-clockwise on a cone inclined by 73° to the line of sight with an opening angle of 8° on a period of ~1640 yr. The estimated jet mass flux and mechanical luminosity are Ṁjet ~ 7.7 × 10-7M⊙ yr-1 and Ljet ~ 0.9L⊙, indicating that the jet could extract at least 25% of the gravitational energy released by the forming star.
Context.
The role of bipolar jets in the formation of stars, and in particular how they are launched, is still not well understood.
Aims.
We probe the protostellar jet launching mechanism using ...high-resolution observations of the near-infrared (IR) Fe
II
λ
1.53,1.64
μ
m emission lines.
Methods.
We consider the case of the bipolar jet from Classical T Tauri star, DO Tau, and investigate the jet morphology and kinematics close to the star (within 140 au) using AO-assisted IFU observations from GEMINI/NIFS.
Results.
We find that the brighter, blueshifted jet is collimated very quickly after it is launched. This early collimation requires the presence of magnetic fields. We confirm velocity asymmetries between the two lobes of the bipolar jet, and also confirm no time variability in the asymmetry over a 20-year interval. This sustained asymmetry is in accordance with recent simulations of magnetised disc winds. We examine the data for signatures of jet rotation. We report an upper limit on differences in radial velocity of 6.3 and 8.7 km s
−1
for the blue- and redshifted jets, respectively. Interpreting this as an upper limit on jet rotation implies that any steady, axisymmetric magneto-centrifugal model of jet launching is constrained to a launch radius in the disc plane of
r
0
< 0.5 and 0.3 au for the blue- and redshifted jets, respectively. This supports an X-wind or narrow disc-wind model. However, the result pertains only to the observed high-velocity Fe
II
emission, and does not rule out a wider flow launched from a wider radius. We report the detection of small-amplitude jet axis wiggling in both lobes. We rule out orbital motion of the jet source as the cause. Precession can better account for the observations but requires double the precession angle, and a different phase for the counter-jet. Such non-solid body precession could arise from an inclined massive Jupiter companion, or a warping instability induced by launching a magnetic disc wind.
Conclusions.
Overall, our observations are consistent with an origin of the DO Tau jets from the inner regions of the disc.
Context.
On the basis of its low luminosity, its chemical composition, and the absence of a large-scale outflow, the dense core Cha-MMS1 located in the Chamaeleon I molecular cloud, was proposed a ...decade ago as a candidate for a first hydrostatic core (FHSC).
Aims.
Our goal is to test this hypothesis by searching for a slow, compact outflow driven by Cha-MMS1 that would match the predictions of magnetohydrodynamic simulations for this short phase of star formation.
Methods.
We used the Atacama Large Millimetre/submillimetre Array to map Cha-MMS1 at high angular resolution in CO 3–2 and
13
CO 3–2 as well as in continuum emission.
Results.
We report the detection of a bipolar outflow emanating from the central core, along a (projected) direction roughly parallel to the filament in which Cha-MMS1 is embedded and perpendicular to the large-scale magnetic field. The morphology of the outflow indicates that its axis lies close to the plane of the sky. We measure velocities corrected for inclination of more than 90 km s
−1
, which is clearly incompatible with the expected properties of an FHSC outflow. Several properties of the outflow are determined and compared to previous studies of Class 0 and Class I protostars. The outflow of Cha-MMS1 has a much smaller momentum force than the outflows of other Class 0 protostars. In addition, we find a dynamical age of 200–3000 yr indicating that Cha-MMS1 might be one of the youngest ever observed Class 0 protostars. While the existence of the outflow suggests the presence of a disk, no disk is detected in continuum emission and we derive an upper limit of 55 au to its radius.
Conclusions.
We conclude that Cha-MMS1 has already gone through the FHSC phase and is a young Class 0 protostar, but it has not yet brought its outflow to full power.
PDF
