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We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC5N (J = 35–34) line has been detected from three MYSOs, where nitrogen (N)-bearing COMs (CH2CHCN and CH3CH2CN) have been detected. The HC5N spatial distributions show compact features and match with a methanol (CH3OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC5N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC5N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs.

Prestellar cores represent the initial conditions of star formation, but heavy molecules such as CO are strongly depleted in their cold, dense interiors, limiting the ability to probe core centers. Deuterated molecular ions therefore emerge as key tracers because deuterium fractionation is enhanced at low temperatures. We present the first direct observation of ortho-H2D+ depletion in the prestellar core G205.46−14.56 M3 using ALMA 820 μm continuum and ortho-H2D+ (110–111) data at ∼300 au resolution. We confirm the previously reported two substructures, B1 and B2, and identify a central ortho-H2D+ depletion zone toward B1 with ∼6σ contrast and an inferred diameter ≲600 au, together with a peak x(N2D+)/x(N2H+) = 1.03−0.56+0.07 . The observationally inferred profiles of x(ortho-H2D+) and x(N2D+)/x(N2H+) are reproduced by a deuteration-focused chemodynamical model; however, the central o-H2D+ depletion is only marginally matched within the 2σ upper limit, likely suggesting additional deuteration in the depletion zone. From these models we infer a core age of ∼0.42 Ma, comparable to the freefall time, suggesting that the substructures formed via rapid, turbulence-dominated fragmentation rather than slow, quasistatic contraction. Our observations also reveal that ortho-H2D+ velocity dispersions are largely subsonic in the core and nearly thermal between B1 and B2, consistent with turbulence dissipating within a few freefall times. These results highlight the critical role of deuterated ions for both chemical evolution and dynamics in dense cores.

Complex organic molecules (COMs) in young stellar objects have attracted significant attention in recent years due to their potential connection to prebiotic chemistry and their utility as tracers of warm or shocked gas components. Proto-binary and multiple systems with close separations are particularly valuable targets for investigating chemical inheritance and reaction, as their members are expected to form from similar material in their parental cloud. We present Atacama Large Millimeter/submillimeter Array observations of the hierarchical proto-triple system HOPS-288, focusing on the physical structure, kinematics, and COM compositions. The system is treated as a proto-binary system consisting of HOPS-288-A and HOPS-288-B due to the limited spatial resolutions, with a separation of 200 au. Three COM-rich features are revealed: two hot corinos associated with the two members, rich in a variety of COMs, and an intervening component between the two members traced by CH3OH and tentatively by CH3CHO. The hot corino in HOPS-288-A exhibits rotational features and might trace a disk. The hot corino in HOPS-288-B is also possibly exhibiting rotational motion. The intervening component could possibly trace a shocked region in the circumbinary disk or a bridge between the two members. The column densities of COMs, including 13CH3OH, CH2DOH, CH3CHO, HCOOCH3, C2H5OH, 13CH3CN, and NH2CHO, are broadly similar between the two sources, possibly suggesting the complex organic similarities among proto-binary/multiple systems. Given the complexity of the studied physical structures, further detailed investigations will be essential to confirm this result.

Abridged Context: Snowlines during star and disk formation are responsible for a range of effects during the evolution of protostars, such as setting the chemical composition of the envelope and disk. This in turn influences the formation of planets by changing the elemental compositions of solids and affecting the collisional properties and outcomes of dust grains. Snowlines can also reveal accretion bursts, providing insight into the formation process of stars. Methods: A numerical chemical network coupled with a grid of cylindrical-symmetric physical models was used to identify what physical parameters alter the CO and H\\(_2\\)O snowline locations. The investigated parameters are the initial molecular abundances, binding energies of CO and H\\(_2\\)O, heating source, cloud core density, outflow cavity opening angle, and disk geometry. Simulated molecular line emission maps were used to quantify the change in the snowline location with each parameter. Conclusions: The models presented in this work show that the CO and H\\(_2\\)O snowline locations do not occur at a single, well-defined temperature as is commonly assumed. Instead, the snowline position depends on luminosity, cloud core density, and whether a disk is present or not. Inclination and spatial resolution affect the observability and successful measurement of snowline locations. We note that N\\(_2\\)H\\(^+\\) and HCO\\(^+\\) emission serve as good observational tracers of CO and H\\(_2\\)O snowline locations. However, constraints on whether or not a disk is present, the observation of additional molecular tracers, and estimating envelope density will help in accurately determining the cause of the observed snowline position. Plots of the N\\(_2\\)H\\(^+\\) and HCO\\(^+\\) peak emission radius versus luminosity are provided to compare the models with observations of deeply embedded protostars aiming to measure the CO and H\\(_2\\)O snowline locations.

L1448C(N) is a young protostar in Perseus, driving an outflow and an extremely high-velocity (EHV) molecular jet. We present multi-epoch observations of SiO \\(J = 8-7\\), CO \\(J = 3-2\\) lines, and 345 GHz dust continuum toward L1448C(N) in 2006, 2010, and 2017 with the Submillimeter Array. The knots traced by the SiO line show the averaged proper motion is \\(0''.06~ yr^-1\\) and \\(0''.04~ yr^-1\\) for the blue- and red-shifted jet, respectively. The corresponding transverse velocities are \\(78~ km s^-1\\) (blueshifted) and \\(52~ km s^-1\\) (redshifted). Together with the radial velocity, we found the inclination angle of the jets from the plane of the sky to be \\(34\\)\\( \\) for the blueshifted jet and \\(46\\)\\( \\) for the redfshifted jet. Given the new inclination angles, the mass-loss rate and mechanical power were refined to be \\(1.8 10^-6~M_\\) and \\(1.3~L_\\), respectively. In the epoch of 2017, a new knot is detected at the base of the redshifted jet. We found that the mass-loss rate of the new knot is three times higher than the averaged mass-loss rate of the redshifted jet. Besides, continuum flux has enhanced by \\(37\\%\\) between 2010 and 2017. These imply that the variation of the mass-accretion rate by a factor of \\(3\\) has occurred in a short timescale of \\(10-20\\) yr. In addition, a knot in the downstream of the redshifted jet is found to be dimming over the three epochs.

We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC\\(_5\\)N (\\(J=35-34\\)) line has been detected from three MYSOs, where nitrogen(N)-bearing COMs (CH\\(_2\\)CHCN and CH\\(_3\\)CH\\(_2\\)CN) have been detected. The HC\\(_5\\)N spatial distributions show compact features and match with a methanol (CH\\(_3\\)OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC\\(_5\\)N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC\\(_5\\)N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs.

A classical paradox in high-mass star formation is that powerful radiation pressure can halt accretion, preventing further growth of a central star. Disk accretion has been proposed to solve this problem, but the disks and the accretion process in high-mass star formation are poorly understood. We executed high-resolution (\\(R\\)=35,000-70,000) iSHELL spectroscopy in \\(K\\)-band for eleven high-mass protostars. Br-\\(\\) emission was observed toward eight sources, and the line profiles for most of these sources are similar to those of low-mass PMS stars. Using an empirical relationship between the Br-\\(\\) and accretion luminosities, we tentatively estimate disk accretion rates ranging from \\(\\)10\\(^-8\\) and \\(\\)10\\(^-4\\) \\(M_\\) yr\\(^-1\\). These low-mass-accretion rates suggest that high-mass protostars gain more mass via episodic accretion as proposed for low-mass protostars. Given the detection limits, CO overtone emission (\\(v\\)=2-0 and 3-1), likely associated with the inner disk region (\\(r 100\\) au), was found towards two sources. This low-detection rate compared with Br-\\(\\) emission is consistent with previous observations. Ten out of the eleven sources show absorption at the \\(v\\)=0-2 \\( R(7)-R(14)\\) CO R-branch. Most of them are either blueshifted or redshifted, indicating that the absorption is associated with an outflow or an inflow with a velocity of up to \\(50\\) km s\\(^-1\\). Our analysis indicates that the absorption layer is well thermalized (and therefore \\(n_ H_2 10^6\\) cm\\(^-3\\)) at a single temperature of typically 100-200 K, and located within 200-600 au of the star.

The water snowline location in protostellar envelopes provides crucial information about the thermal structure and the mass accretion process as it can inform about the occurrence of recent (≲1000 yr) accretion bursts. In addition, the ability to image water emission makes these sources excellent laboratories to test indirect snowline tracers such as H13CO+. We study the water snowline in five protostellar envelopes in Perseus using a suite of molecular-line observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) at ∼0.″2−0.″7 (60–210 au) resolution. B1-c provides a textbook example of compact H218O (31,3−22,0) and HDO (31,2−22,1) emission surrounded by a ring of H13CO+ (J = 2−1) and HC18O+ (J = 3−2). Compact HDO surrounded by H13CO+ is also detected toward B1-bS. The optically thick main isotopologue HCO+ is not suited to trace the snowline, and HC18O+ is a better tracer than H13CO+ due to a lower contribution from the outer envelope. However, because a detailed analysis is needed to derive a snowline location from H13CO+ or HC18O+ emission, their true value as a snowline tracer will lie in the application in sources where water cannot be readily detected. For protostellar envelopes, the most straightforward way to locate the water snowline is through observations of H218O or HDO. Including all subarcsecond-resolution water observations from the literature, we derive an average burst interval of ∼10,000 yr, but high-resolution water observations of a larger number of protostars are required to better constrain the burst frequency.

In recent years evidence has been building that planet formation starts early, in the first \\(\\) 0.5 Myr. Studying the dust masses available in young disks enables understanding the origin of planetary systems since mature disks are lacking the solid material necessary to reproduce the observed exoplanetary systems, especially the massive ones. We aim to determine if disks in the embedded stage of star formation contain enough dust to explain the solid content of the most massive exoplanets. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of embedded disks in the Perseus star-forming region together with Very Large Array (VLA) Ka-band (9 mm) data to provide a robust estimate of dust disk masses from the flux densities. Using the DIANA opacity model including large grains, with a dust opacity value of \\(_ 9\\ mm\\) = 0.28 cm\\(^2\\) g\\(^-1\\), the median dust masses of the embedded disks in Perseus are 158 M\\(_\\) for Class 0 and 52 M\\(_\\) for Class I from the VLA fluxes. The lower limits on the median masses from ALMA fluxes are 47 M\\(_\\) and 12 M\\(_\\) for Class 0 and Class I, respectively, obtained using the maximum dust opacity value \\(_ 1.3mm\\) = 2.3 cm\\(^2\\) g\\(^-1\\). The dust masses of young Class 0 and I disks are larger by at least a factor of 10 and 3, respectively, compared with dust masses inferred for Class II disks in Lupus and other regions. The dust masses of Class 0 and I disks in Perseus derived from the VLA data are high enough to produce the observed exoplanet systems with efficiencies acceptable by planet formation models: the solid content in observed giant exoplanets can be explained if planet formation starts in Class 0 phase with an efficiency of \\(\\) 15%. Higher efficiency of \\(\\) 30% is necessary if the planet formation is set to start in Class I disks.

We present an outflow survey toward 20 Low Luminosity Objects (LLOs), namely protostars with an internal luminosity lower than 0.2 Lsun. Although a number of studies have reported the properties of individual LLOs, the reasons for their low luminosity remain uncertain. To answer this question, we need to know the evolutionary status of LLOs. Protostellar outflows are found to widen as their parent cores evolve, and therefore, the outflow opening angle could be used as an evolutionary indicator. The infrared scattered light escapes out through the outflow cavity and highlights the cavity wall, giving us the opportunity to measure the outflow opening angle. Using the Canada-France-Hawaii Telescope, we detected outflows toward eight LLOs out of 20 at Ks band, and based on archival Spitzer IRAC1 images, we added four outflow-driving sources from the remaining 12 sources. By fitting these images with radiative transfer models, we derive the outflow opening angles and inclination angles. To study the widening of outflow cavities, we compare our sample with the young stellar objects from Arce & Sargent 2006 and Velusamy et al. 2014 in the plot of opening angle versus bolometric temperature taken as an evolutionary indicator.Our LLO targets match well the trend of increasing opening angle with bolometric temperature reported by Arce & Sargent and are broadly consistent with that reported by Velusamy et al., suggesting that the opening angle could be a good evolutionary indicator for LLOs. Accordingly, we conclude that at least 40% of the outflow-driving LLOs in our sample are young Class 0 objects.