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We present the highest angular resolution (20x15mas - 44x33au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16km probes scales 150K, this does not preclude that the substructures formed earlier through disc fragmentation., 9 Total, inc references, appendix, long author list, 4 figures (inc appendix)

Recent measurements carried out at comet 67P/C-G with the ${\it Rosetta}$ probe revealed that molecular oxygen, O$_2$, is the fourth most abundant molecule in comets. Models show that O$_2$ is likely of primordial nature, coming from the interstellar cloud from which our Solar System was formed. However, gaseous O$_2$ is an elusive molecule in the interstellar medium with only one detection towards quiescent molecular clouds, in the $\rho$ Oph A core. We perform a deep search for molecular oxygen, through the $2_1 - 0_1$ rotational transition at 234 GHz of its $^{16}$O$^{18}$O isotopologue, towards the warm compact gas surrounding the nearby Class 0 protostar IRAS 16293--2422 B with the ALMA interferometer. The targeted $^{16}$O$^{18}$O transition is surrounded by two brighter transitions at $\pm 1$ km s$^{-1}$ relative to the expected $^{16}$O$^{18}$O transition frequency. After subtraction of these two transitions, residual emission at a 3$\sigma$ level remains, but with a velocity offset of $0.3 - 0.5$ km s$^{-1}$ relative to the source velocity, rendering the detection "tentative". We derive the O$_2$ column density for two excitation temperatures $T_{\rm ex}$ of 125 and 300 K, as indicated by other molecules, in order to compare the O$_2$ abundance between IRAS16293 and comet 67P/C-G. Assuming that $^{16}$O$^{18}$O is not detected and using methanol CH$_3$OH as a reference species, we obtain a [O$_2$]/[CH$_3$OH] abundance ratio lower than $2-5$, depending on the assumed $T_{\rm ex}$, a three to four times lower abundance than the [O$_2$]/[CH$_3$OH] ratio of $5-15$ found in comet 67P/C-G. Such a low O$_2$ abundance could be explained by the lower temperature of the dense cloud precursor of IRAS16293 with respect to the one at the origin of our Solar System that prevented an efficient formation of O$_2$ in interstellar ices., Comment: 12 pages, 3 figures, accepted for publication in Astronomy & Astrophysics

The deuterium fractionation of water can serve as a tracer for the chemical and physical evolution of water during star formation and can constrain the origin of water in Solar System bodies. We determine the HDO/H$_2$O ratio in the inner warm gas toward three low-mass Class 0 protostars selected to be in isolated cores, i.e., not associated with any cloud complexes. Previous sources for which the HDO/H$_2$O ratio have been established were all part of larger star-forming complexes. Targeting these isolated protostars allows comparison of the water chemistry in isolated and clustered regions to determine the influence of local cloud environment. We present ALMA observations of the HDO $3_{1,2}$-$2_{2,1}$ and $2_{1,1}$-$2_{1,2}$ transitions at 225.897 GHz and 241.562 GHz along with the H$_2^{18}$O $3_{1,3}$-$2_{2,0}$ transition at 203.407 GHz. The high angular resolution (0\farcs3-1\farcs3) allow the study of the inner warm envelope gas. Model-independent estimates for the HDO/H$_2$O ratios are obtained and compared with previous determinations in the warm gas toward low-mass protostars. We detect the targeted water transitions toward the three sources with S/N > 5. We determine the HDO/H$_2$O ratio toward L483, B335 and BHR71-IRS1 to be ($2.2\pm0.4$)$\times 10^{-3}$, ($1.7\pm0.3$)$\times 10^{-3}$, and ($1.8\pm0.4$)$\times 10^{-3}$, respectively, assuming $T_\mathrm{ex} = 124$ K. The degree of water deuteration of these isolated protostars are a factor of 2-4 higher relative to Class 0 protostars that are members of known nearby clustered star-forming regions. The results indicate that the water deuterium fractionation is influenced by the local cloud environment. This effect can be explained by variations in either collapse timescales or temperatures, which depends on local cloud dynamics and could provide a new method to decipher the history of young stars., 13 pages, 6 figures. Accepted for publication in A&A (18/09/2019)

Much attention has been placed on the dust distribution in protostellar envelopes, but there are still many unanswered questions regarding the structure of the gas. We aim to start identifying the factors that determine the chemical structure of protostellar regions, by studying and comparing low-mass embedded systems in key molecular tracers. The cold and warm chemical structures of two embedded Class 0 systems, IRAS16293 and VLA1623 are characterized through interferometric observations. DCO+, N2H+ and N2D+ are used to trace the spatial distribution and physics of the cold regions of the envelope, while c-C3H2 and C2H from models of the chemistry are expected to trace the warm (UV-irradiated) regions. Both sources show a number of striking similarities and differences. DCO+ consistently traces the cold material at the disk-envelope interface, where gas and dust temperatures are lowered due to disk shadowing. N2H+ and N2D+, also tracing cold gas, show low abundances towards VLA1623, but for IRAS16293, the distribution of N2D+ is consistent with the same chemical models that reproduce DCO+. c-C3H2 and C2H show different spatial distributions for the two systems. For IRAS16293, c-C3H2 traces the outflow cavity wall, while C2H is found in the envelope material but not the outflow cavity wall. In contrast, toward VLA1623 both molecules trace the outflow cavity wall. Finally, hot core molecules are abundantly observed toward IRAS16293 but not toward VLA1623. We identify temperature as one of the key factors in determining the chemical structure of protostars as seen in gaseous molecules. More luminous protostars, such as IRAS16293, will have chemical complexity out to larger distances than colder protostars, such as VLA1623. Additionally, disks in the embedded phase have a crucial role in controlling both the gas and dust temperature of the envelope, and consequently the chemical structure., 24 pages, 15 figures, accepted to A&A

A wide variety of ring-like dust structures has been detected in protoplanetary disks, but their origin and frequency are still unclear. We characterize the structure of an extended, multi-ringed disk discovered serendipitously in the ALMA Lupus disk survey and put it in the context of the Lupus disk population. ALMA observations in Band 6 at 234 GHz and Band 7 at 328 GHz at 0.3" resolution toward the K6 star V1094 Sco in Lupus III are presented, and its disk structure is analyzed. The spectral index $\alpha_{mm}$ is determined in the inner 150 AU of the disk. The ALMA continuum data show a very extended disk with two gap/ring pairs. The gaps are located at 100 AU and 170 AU, the bright rings at 130 AU and 220 AU. Continuum emission is detected out to a 300 AU distance, similar to IM Lup but a factor of 5 larger than typically found for Lupus disks at this sensitivity and resolution. The bright central region of the disk (within 35 AU) is possibly optically thick at 1 mm wavelengths, and has a brightness temperature of only 13 K. The spectral index increases between the inner disk and the first ring, at the location of the first gap. Due to the low temperature of the disk midplane, snow lines can be excluded as the drivers behind the ring and gap formation in this disk. Disks the size of V1094 Sco are rare, and only 2.1+-1.5% of disks in Lupus show continuum emission beyond 200 AU. Possible connections between the large primordial disk population, transition disks, and exoplanets are discussed., Comment: 12 pages, 7 figures. Accepted for publication in Astronomy & Astrophysics

Our ALMA Cycle 0 and 2 observations in Band 6 provide tracers of the disk (C18 O, 13 CO), the envelope (13 CO, DCO+ ) and the outflow and its cavity (12 CO, 13 CO, c-C3 H2 ) towards VLA1623, a triple non-coeval system in ρ Ophiuchus (d∼120 pc). The observations are combined with simple chemical and physical models. We find differing circumstellar envelope and outflows. VLA1623 appears to not be as chemically rich as other deeply embedded sources.

(Abridged) Star and planet formation theories predict an evolution in the density, temperature, and velocity structure as the envelope collapses and forms an accretion disk. The aim of this work is to model the evolution of the molecular excitation, line profiles, and related observables during low-mass star formation. Specifically, the signatures of disks during the deeply embedded stage are investigated. Semi-analytic 2D axisymmetric models have been used to describe the evolution of the density, stellar mass, and luminosity from the pre-stellar to the T-Tauri phase. A full radiative transfer calculation is carried out to accurately determine the time-dependent dust temperatures and CO abundance structure. We present non-LTE near-IR, FIR, and submm lines of CO have been simulated at a number of time steps. In contrast to the dust temperature, the CO excitation temperature derived from submm/FIR lines does not vary during the protostellar evolution, consistent with C18O observations obtained with Herschel and from ground-based telescopes. The near-IR spectra provide complementary information to the submm lines by probing not only the cold outer envelope but also the warm inner region. The near-IR high-J (>8) absorption lines are particularly sensitive to the physical structure of the inner few AU, which does show evolution. High signal-to-noise ratio subarcsec resolution data with ALMA are needed to detect the presence of small rotationally supported disks during the Stage 0 phase and various diagnostics are discussed., Accepted for publication in A&A, 17 pages with appendix; 14 figures

(Abridged) Water is a key tracer of dynamics and chemistry in low-mass protostars, but spectrally resolved observations have so far been limited in sensitivity and angular resolution. In this first systematic survey of spectrally resolved water emission in low-mass protostellar objects, H2O was observed in the ground-state transition at 557 GHz with HIFI on Herschel in 29 embedded Class 0 and I protostars. Complementary far-IR and sub-mm continuum data (including PACS data from our program) are used to constrain the spectral energy distribution of each source. H2O intensities are compared to inferred envelope and outflow properties and CO 3-2 emission. H2O emission is detected in all objects except one. The line profiles are complex and consist of several kinematic components. The profiles are typically dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows, not the quiescent envelope. Several sources show multiple shock components in either emission or absorption, thus constraining the internal geometry of the system. Furthermore, the components include inverse P-Cygni profiles in 7 sources (6 Class 0, 1 Class I) indicative of infalling envelopes, and regular P-Cygni profiles in 4 sources (3 Class I, 1 Class 0) indicative of expanding envelopes. "Bullets" moving at >50 km/s are seen in 4 Class 0 sources; 3 of these are new detections. In the outflow, the H2O/CO abundance ratio as a function of velocity is nearly the same for all sources, increasing from 10^-3 at 10^-1 at >10 km/s. The H2O abundance in the outer envelope is low, ~10^-10. The different H2O profile components show a clear evolutionary trend: in the Class 0 sources, emission is dominated by outflow components originating inside an infalling envelope. When the infall diminishes during the Class I phase, the outflow weakens and H2O emission disappears., Accepted for publication in A&A

Aims. Young stars interact vigorously with their surroundings, as evident from the highly rotationally excited CO (up to Eup=4000 K) and H2O emission (up to 600 K) detected by the Herschel Space Observatory in embedded low-mass protostars. Our aim is to construct a model that reproduces the observations quantitatively, to investigate the origin of the emission, and to use the lines as probes of the various heating mechanisms. Methods. The model consists of a spherical envelope with a bipolar outflow cavity. Three heating mechanisms are considered: passive heating by the protostellar luminosity, UV irradiation of the outflow cavity walls, and C-type shocks along the cavity walls. Line fluxes are calculated for CO and H2O and compared to Herschel data and complementary ground-based data for the protostars NGC1333 IRAS2A, HH 46 and DK Cha. The three sources are selected to span a range of evolutionary phases and physical characteristics. Results. The passively heated gas in the envelope accounts for 3-10% of the CO luminosity summed over all rotational lines up to J=40-39; it is best probed by low-J CO isotopologue lines such as C18O 2-1 and 3-2. The UV-heated gas and the C-type shocks, probed by 12CO 10-9 and higher-J lines, contribute 20-80% each. The model fits show a tentative evolutionary trend: the CO emission is dominated by shocks in the youngest source and by UV-heated gas in the oldest one. This trend is mainly driven by the lower envelope density in more evolved sources. The total H2O line luminosity in all cases is dominated by shocks (>99%). The exact percentages for both species are uncertain by at least a factor of 2 due to uncertainties in the gas temperature as function of the incident UV flux. However, on a qualitative level, both UV-heated gas and C-type shocks are needed to reproduce the emission in far-infrared rotational lines of CO and H2O., 15 pages (+4 pages appendix), 20 figures, accepted by A&A