Takeshi SAKAI (酒井 剛)

Context. The (sub-)millimetre dust opacity spectral index (β) is a critical observable for constraining dust properties, such as the maximum grain size of an observed dust population. It has been widely measured at Galactic scales and down to protoplanetary disks. Because of observational and analytical challenges, however, quite a gap exists in following the evolution of dust in the interstellar medium (ISM): we lack measures of the dust properties in the envelopes that feed newborn protostars and their disks.

Aims. To fill this gap, we used sensitive dust continuum emission data at 1.2 and 3.1 mm from the ALMA FAUST Large Program and constrained the spectral index of the submillimetre dust opacity for a sample of protostars.

Methods. Our high-resolution data, along with a method that was more refined than the methods in previous efforts, allowed us to distinguish the contributions from the disk and envelope in the uv-plane, and thus, to measure spectral indices for the envelopes that are not contaminated by the optically thick emission of the inner disk regions.

Results. The FAUST sources (n = 13) include a variety of morphologies in continuum emission: compact young disks, extended collapsing envelopes, and dusty outflow cavity walls. Firstly, we found that the young disks in our sample are small (down to < 9 au) and optically thick. Secondly, we measured the dust opacity spectral index β at envelope scales for n = 11 sources: The β of n = 9 of these sources were not constrained before. We effectively doubled the number of sources for which the dust opacity spectral index β has been measured at these scales. Thirdly, by combining the available literature measurements with our own (a total n = 18), we showed the distribution of the envelope spectral indices between ISM-like and disk-like values. This bridges the gap in the inferred dust evolution. Finally, we statistically confirmed a significant correlation between β and the mass of protostellar envelopes, as previously suggested in the literature.

Conclusions. Our findings indicate that the optical dust properties smoothly vary from the ISM (≫ 0.1 parsec) through envelopes (∼ 500–2000 au) to protoplanetary disks (< 200 au). Multi-wavelength surveys including longer wavelengths and in controlled starforming regions are needed to further this study and make more general claims about the dust evolution in its pathway from the cloud to disks.

Context. During the early stages of star formation, accretion processes such as infall from the envelope and molecular streamers and ejection of matter through winds and jets take place simultaneously and distribute the angular momentum of the parent molecular cloud. The Class 0/I binary [BHB2007] 11 shows evidence for accretion and ejection at the scales of the circumbinary disk and the inner close binary. Recent observations of H₂CO, however, have shown two elongated structures with indications of outflowing motion almost perpendicular to the main CO outflow, which is launched from the circumbinary disk.

Aims. We study the kinematics of the molecular gas at intermediate scales of ~50–3000 au around [BHB2007] 11 to verify the nature of these elongated structures.

Methods. We analyzed the line emission of H¹³CO⁺, CCH, c-C₃H₂, and SiO observed with the Atacama Large Millimeter/submillimeter Array (ALMA) within the large program called Fifty AU STudy of the chemistry in the disc/envelope system of Solar-like protostars (FAUST). These molecules trace the material that moves at velocities close to that of the ambient cloud, which could not be probed in previous observations of the self-absorbed emission of CO.

Results. The images of H¹³CO⁺, CCH, and c-C₃H₂ show clear elongated structures similar to those previously detected in H₂CO, whose gas kinematics are consistent with outflowing motions and with rotation in the opposite sense to the main CO outflow. The derived mass-loss rate from these large-scale structures is (1.7 ± 0.5) × 10⁻⁶ M_⊙ yr⁻¹, which agrees with the rates measured in outflows driven by Class 0/I protostars. The SiO image reveals compact emission close to the binary system, with a slight elongation that is aligned with the larger-scale structures. This suggests that SiO is released from the sputtering of dust grains in the shocked material at the base of the potential new outflow, with a relative abundance of ≥(0.11–2.0) × 10⁻⁹. However, higher angular and spectral resolution observations are needed to accurately estimate the outflow-launching radius and its powering source. Based on the location and the abundance of the SiO emission, we propose that the second outflow may be launched from inside the circumbinary disk, likely by the less massive companion that actively accretes material from its surroundings.

Context. Planet formation around young stars requires the growth of interstellar dust grains from micron-sized (μm-sized) particles to kilometre-sized (km-sized) planetesimals. Numerical simulations have shown that large (mm-sized) grains found in the inner envelope of young protostars could be lifted from the disc via winds. However, we are still lacking unambiguous evidence for large grains in protostellar winds and outflows.

Aims. We investigated dust continuum emission in the envelope of the Class I binary L1551 IRS5 in the Taurus molecular cloud, aiming to identify observational signatures of grain growth, such as variations in the dust emissivity index (β_mm).

Methods. In this context, we present new, high-angular resolution (50 au) observations of thermal dust continuum emission at 1.3 mm and 3 mm in the envelope (∼3000 au) of L1551 IRS5, obtained as part of the ALMA-FAUST Large Program.

Results. We analysed dust emission along the cavity walls of the CO outflow, extended up to ∼1800 au. We found an H₂ volume density > 2 × 10⁵ cm⁻³, a dust mass of ∼58 M_⊕, and β_mm ≲ 1, implying the presence of grains ∼10³ times larger than typical sizes for the interstellar medium (ISM).

Conclusions. We present the first spatially resolved observational evidence of large grains within an outflow cavity wall. Our results suggest that these grains have been transported from the inner disc to the envelope by protostellar winds and may subsequently fall back into the outer disc by gravity and/or via accretion streamers. This cycle provides longer time for grains to grow, demonstrating their crucial role in the formation of planetesimals.

Context. While protostellar outflows are important in terms of mass accretion and angular momentum transport in star formation processes, high-resolution observations of outflows in protobinary systems are still sparse.

Aims. We aim to reveal outflow structures traced by millimeter SiO emission in a low-mass protobinary system, L483.

Methods. We observed the SiO (J = 5−4) line in L483 with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the large program FAUST (Fifty AU STudy of the chemistry in the disk/envelope systems of Solar-like protostars). The spatial and spectral resolutions were 0.39^′′×0.30^′′ (780 au×60 au) and 122 kHz (0.17 km s⁻¹ at 217 GHz), respectively. The spectral lines of SO, CS, and C¹⁸O were also used to study the physical and dynamical properties of the SiO emitting regions.

Results. Two SiO emission peaks are identified in the central part of L483, which have offsets of 100 au and 200 au toward the northeast (SiO-peak) and north (SiO-N), respectively, from the continuum peak. The SiO-peak shows only blueshifted emission with a broad linewidth of 5 km s⁻¹, while that of SiO-N corresponds to the systemic velocity. Furthermore, weak and compact SiO emission components are distributed up to 2400 au away from the continuum position. They have narrow linewidths of ∼1 km s⁻¹. One of these components is a blueshifted isolated emission feature, 2400 au northeast of the continuum peak, NE-cloud, located outside the east-west outflow lobes. The SiO abundances relative to H₂ are 10⁻¹⁰−10⁻⁹ and 10⁻¹⁰ in the central part and more widely distributed components, respectively. These are intermediate values between those of strongly shocked regions caused by high-velocity outflows and quiescent molecular clouds.

Conclusions. The central SiO emission could be interpreted as either two different outflows driven by both protostars or as an outflow ejected from one of the circumstellar disks in the binary system. The NE-cloud region is most likely explained as a remnant of an old shock produced by past outflow activity, as has been proposed for the low-mass protostar IRAS 15398–3359. The complex structures of the outflows traced by the SiO line could reflect dynamical processes of the newly formed protobinary system in L483.

Abstract

We have observed the late Class I protostellar source Elias 29 at a spatial resolution of 70 au with the Atacama Large Millimeter/submillimeter Array as part of the FAUST Large Program. We focus on the line emission of SO, while that of ³⁴SO, C¹⁸O, CS, SiO, H¹³CO⁺, and DCO⁺ are used supplementarily. The spatial distribution of the SO rotational temperature (T _rot(SO)) is evaluated by using the intensity ratio of its two rotational excitation lines. Besides in the vicinity of the protostar, two hot spots are found at a distance of 500 au from the protostar; T _rot(SO) locally rises to 53 K at the interaction point of the outflow and the southern ridge, and 72 K within the southeastern outflow probably due to a jet-driven bow shock. However, the SiO emission is not detected at these hot spots. It is likely that active gas accretion through the disk-like structure and onto the protostar still continues even at this evolved protostellar stage, at least sporadically, considering the outflow/jet activities and the possible infall motion previously reported. Interestingly, T _rot(SO) is as high as 20–30 K even within the quiescent part of the southern ridge apart from the protostar by 500–1000 au without clear kinematic indication of current outflow/jet interactions. Such a warm condition is also supported by the low deuterium fractionation ratio of HCO⁺ estimated by using the H¹³CO⁺ and DCO⁺ lines. The B-type star HD147889 ∼0.5 pc away from Elias 29, previously suggested as a heating source for this region, is likely responsible for the warm condition of Elias 29.

Abstract

Methanol is one of the most abundant complex organic molecules in interstellar environments. Molecular lines of its rare isotopologues ¹²CH₃ ¹⁷OH and ¹²CH₃ ¹⁸OH therefore play a crucial role in examining the column density of ¹²CH₃ ¹⁶OH, which serves as a reference for organic molecular chemistry in interstellar clouds. In this study, we have recorded the spectroscopic emission spectrum of ¹²CH₃ ¹⁷OH in the frequency range between 216 and 264 GHz by making use of an emission-type millimeter and submillimeter spectrometer. We have specifically paid attention to the Q-branch transitions, which are the strongest line series in this frequency region. Among the stable oxygen isotopes, ¹⁶O, ¹⁷O, and ¹⁸O of methanol, only ¹²CH₃ ¹⁷OH obviously shows line profiles having double and/or triple peaks in low-J transitions, due to the nuclear quadrupole interaction. The newly obtained ¹²CH₃ ¹⁷OH data will play an important role in facilitating a deeper understanding of the organic chemistry related to star and planet formation. The ¹²CH₃ ¹⁷OH line data allow us to trace and constrain the isotopic ratio ¹⁷O/¹⁸O in methanol, which is efficient to investigate the galactic-scale evolution of elements. In addition, we also assigned some transitions of ¹³CH₃ ¹⁷OH in the recorded spectrum.