Context. Models of the young solar nebula assume a hot initial disk in which most volatiles are in the gas phase. Water emission arising from within 50 AU radius has been detected around low-mass ...embedded young stellar objects. The question remains whether an actively accreting disk is warm enough to have gas-phase water up to 50 AU radius. No detailed studies have yet been performed on the extent of snowlines in an accreting disk embedded in a dense envelope (stage 0). Aims. We aim to quantify the location of gas-phase volatiles in the inner envelope and disk system for an actively accreting embedded disk. Methods. Two-dimensional physical and radiative transfer models were used to calculate the temperature structure of embedded protostellar systems. Heating due to viscous accretion was added through the diffusion approximation. Gas and ice abundances of H2O, CO2, and CO were calculated using the density-dependent thermal desorption formulation. Results. The midplane water snowline increases from 3 to ~55 AU for accretion rates through the disk onto the star between 10-9–10-4M⊙ yr-1. CO2 can remain in the solid phase within the disk for Ṁ ≤ 10-5M⊙ yr-1 down to ~20 AU. Most of the CO is in the gas phase within an actively accreting disk independent of disk properties and accretion rate. The predicted optically thin water isotopolog emission is consistent with the detected H218O emission toward the stage 0 embedded young stellar objects, originating from both the disk and the warm inner envelope (hot core). An accreting embedded disk can only account for water emission arising from R< 50 AU, however, and the extent rapidly decreases for Ṁ ≤ 10-5M⊙ yr-1. Thus, the radial extent of the emission can be measured with future ALMA observations and compared to this 50 AU limit. Conclusions. Volatiles such as H2O, CO2, CO, and the associated complex organics sublimate out to 50 AU in the midplane of young disks and, thus, can reset the chemical content inherited from the envelope in periods of high accretion rates (>10-5M⊙ yr-1). A hot young solar nebula out to 30 AU can only have occurred during the deeply embedded stage 0, not during the T Tauri phase of our early solar system.
Context. Disks are observed around pre-main sequence stars, but how and when they form is still heavily debated. While disks around young stellar objects have been identified through thermal dust ...emission, spatially and spectrally resolved molecular line observations are needed to determine their nature. Only a handful of embedded rotationally supported disks have been identified to date. Aims. We identify and characterize rotationally supported disks near the end of the main accretion phase of low-mass protostars by comparing their gas and dust structures. Methods. Subarcsecond observations of dust and gas toward four Class I low-mass young stellar objects in Taurus are presented at significantly higher sensitivity than previous studies. The 13CO and C18O J = 2–1 transitions at 220 GHz were observed with the Plateau de Bure Interferometer at a spatial resolution of ≤0.8″ (56 AU radius at 140 pc) and analyzed using uv-space position velocity diagrams to determine the nature of their observed velocity gradient. Results. Rotationally supported disks (RSDs) are detected around 3 of the 4 Class I sources studied. The derived masses identify them as Stage I objects; i.e., their stellar mass is higher than their envelope and disk masses. The outer radii of the Keplerian disks toward our sample of Class I sources are ≤100 AU. The lack of on-source C18O emission for TMR1 puts an upper limit of 50 AU on its size. Flattened structures at radii >100 AU around these sources are dominated by infalling motion (υ ∝ r-1). A large-scale envelope model is required to estimate the basic parameters of the flattened structure from spatially resolved continuum data. Similarities and differences between the gas and dust disk are discussed. Combined with literature data, the sizes of the RSDs around Class I objects are best described with evolutionary models with an initial rotation of Ω = 10-14 Hz and slow sound speeds. Based on the comparison of gas and dust disk masses, little CO is frozen out within 100 AU in these disks. Conclusions. Rotationally supported disks with radii up to 100 AU are present around Class I embedded objects. Larger surveys of both Class 0 and I objects are needed to determine whether most disks form late or early in the embedded phase.
Context.
Establishing the origin of the water D/H ratio in the Solar System is central to our understanding of the chemical trail of water during the star and planet formation process. Recent ...modeling suggests that comparisons of the D
2
O/HDO and HDO/H
2
O ratios are a powerful way to trace the chemical evolution of water and, in particular, determine whether the D/H ratio is inherited from the molecular cloud or established locally.
Aims.
We seek to determine the D
2
O column density and derive the D
2
O/HDO ratios in the warm region toward the low-mass Class 0 sources B335 and L483. The results are compared with astrochemical models and previous observations to determine their implications for the chemical evolution of water.
Methods.
We present ALMA observations of the D
2
O 1
1,0
–1
0,1
transition at 316.8 GHz toward B335 and L483 at ≲0.′′5 (≲100 au) resolution, probing the inner warm envelope gas. The column densities of D
2
O, HDO, and H
2
18
O are determined by synthetic spectrum modeling and direct Gaussian fitting, under the assumption of a single excitation temperature and similar spatial extent for the three water isotopologs.
Results.
D
2
O is detected toward both sources in the inner warm envelope. The derived D
2
O/HDO ratio is (1.0 ± 0.2) × 10
−2
for L483 and (1.4 ± 0.1) × 10
−2
for B335. These values indicate that the D
2
O/HDO ratio is higher than the HDO/H
2
O ratios by a factor of ≳2 toward both sources.
Conclusions.
The high D
2
O/HDO ratios are a strong indication of chemical inheritance of water from the prestellar phase down to the inner warm envelope. This implies that the local cloud conditions in the prestellar phase, such as temperatures and timescales, determine the water chemistry at later stages and could provide a source of chemical differentiation in young systems. In addition, the observed D
2
O/H
2
O ratios support an observed dichotomy in the deuterium fractionation of water toward isolated and clustered protostars, namely, a higher D/H ratio toward isolated sources.
Context. Much attention has been placed on the dust distribution in protostellar envelopes, but there are still many unanswered questions regarding the physico-chemical structure of the gas. Aims. ...Our aim is 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. Methods. The cold and warm chemical structures of two embedded Class 0 systems, IRAS 16293−2422 and VLA 1623−2417 were characterized through interferometric observations. DCO+, N2H+, and N2D+ were 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. Results. The two 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 toward VLA 1623−2417, but for IRAS 16293−2422, the distribution of N2D+ is consistent with the same chemical models that reproduce DCO+. The two systems show different spatial distributions c-C3H2 and C2H. For IRAS 16293−2422, c-C3H2 traces the outflow cavity wall, while C2H is found in the envelope material but not the outflow cavity wall. In contrast, toward VLA 1623−2417 both molecules trace the outflow cavity wall. Finally, hot core molecules are abundantly observed toward IRAS 16293−2422 but not toward VLA 1623−2417. Conclusions. 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 IRAS 16293−2422, will have chemical complexity out to larger distances than colder protostars, such as VLA 1623−2417. 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.
Context. How water is delivered to planetary systems is a central question in astrochemistry. 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. Aims. The aim is to determine the HDO/H2O 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/H2O ratio have been established were all part of larger star-forming complexes. Determining the HDO/H2O ratio toward three isolated protostars allows comparison of the water chemistry in isolated and clustered regions to determine the influence of local cloud environment. Methods. We present ALMA Band 6 observations of the HDO 31,2–22,1 and 21,1–21,2 transitions at 225.897 GHz and 241.562 GHz along with the first ALMA Band 5 observations of the H 218 $_2^{18}$ 218 O 31,3–22,0 transition at 203.407 GHz. The high angular resolution observations (0′′.3–1′′.3) allow the study of the inner warm envelope gas. Model-independent estimates for the HDO/H2O ratios are obtained and compared with previous determinations of the HDO/H2O ratio in the warm gas toward low-mass protostars. Results. We successfully detect the targeted water transitions toward the three sources with signal-to-noise ratio (S/N) > 5. We determine the HDO/H2O ratio toward L483, B335 and BHR71–IRS1 to be (2.2 ± 0.4) × 10−3, (1.7 ± 0.3) × 10−3, and (1.8 ± 0.4) × 10−3, respectively, assuming Tex = 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. Conclusions. 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.
Context. The chemical evolution of water through the star formation process directly affects the initial conditions of planet formation. The water deuterium fractionation (HDO/H2O abundance ratio) ...has traditionally been used to infer the amount of water brought to Earth by comets. Measuring this ratio in deeply-embedded low-mass protostars makes it possible to probe the critical stage when water is transported from clouds to disks in which icy bodies are formed. Aims. We aim to determine the HDO/H2O abundance ratio in the warm gas in the inner 150 AU for three deeply-embedded low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B through high-resolution interferometric observations of isotopologues of water. Methods. We present sub-arcsecond resolution observations of the 31,2−22,1 transition of HDO at 225.89672 GHz in combination with previous observations of the 31,3−22,0 transition of H218O at 203.40752 GHz from the Plateau de Bure Interferometer toward three low-mass protostars. The observations have similar angular resolution (0.̋7–1.̋3), probing scales R ≲ 150 AU. In addition, observations of the 21,1−21,2 transition of HDO at 241.561 GHz toward IRAS 2A are presented to constrain the excitation temperature. A direct and model independent HDO/H2O abundance ratio is determined for each source and compared with HDO/H2O ratios derived from spherically symmetric full radiative transfer models for two sources. Results. From the two HDO lines observed toward IRAS 2A, the excitation temperature is found to be Tex = 124 ± 60 K. Assuming a similar excitation temperature for H218O and all sources, the HDO/H2O ratio is 7.4 ± 2.1 × 10-4 for IRAS 2A, 19.1 ± 5.4 × 10-4 for IRAS 4A-NW, and 5.9 ± 1.7 × 10-4 for IRAS 4B. The abundance ratios show only a weak dependence on the adopted excitation temperature. The abundances derived from the radiative transfer models agree with the direct determination of the HDO/H2O abundance ratio for IRAS 16293-2422 within a factor of 2–3, and for IRAS 2A within a factor of 4; the difference is mainly due to optical depth effects in the HDO line. Conclusions. Our HDO/H2O ratios for the inner regions (where T > 100 K) of four young protostars are only a factor of 2 higher than those found for pristine, solar system comets. These small differences suggest that little processing of water occurs between the deeply embedded stage and the formation of planetesimals and comets.
Context.
Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. Recent studies allude to an early start to ...planet formation already during the formation of a disk. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed.
Aims.
We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation.
Methods.
We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, HCO
+
, HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in local thermodynamical equilibrium (LTE) as well as through more detailed non-LTE radiative transfer calculations.
Results.
Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO
+
, HCN, and SO are also detected from the inner 100 au region. We further report on upper limits to vibrational HCN
υ
2
= 1, DCN, and N
2
D
+
lines. The HCO
+
emission appears to trace both the Keplerian disk and the surrounding infalling rotating envelope. HCN emission peaks toward the outflow cavity region connected with the CO disk wind and toward the red-shifted part of the Keplerian disk. From the derived HCO
+
abundance, we estimate the ionization fraction of the disk surface, and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk’s molecular abundances relative to Solar System objects.
Conclusions.
Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and H
2
O molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation.
Context.
Gas accretion flows transport material from the cloud core onto the protostar. In multiple protostellar systems, it is not clear if the delivery mechanism is preferential or more evenly ...distributed among the components.
Aims.
The distribution of gas accretion flows within the cloud core of the deeply embedded, chemically rich, low-mass multiple protostellar system IRAS 16293−2422 is explored out to 6000 AU.
Methods.
Atacama Large Millimeter/submillimeter Array Band 3 observations of low-
J
transitions of various molecules, such as HNC, cyanopolyynes (HC
3
N, HC
5
N), and N
2
H
+
, are used to probe the cloud core structure of IRAS 16293−2422 at ~100 AU resolution. Additional Band 3 archival data provide low-
J
HCN and SiO lines. These data are compared with the corresponding higher-
J
lines from the PILS Band 7 data for excitation analysis. The HNC/HCN ratio is used as a temperature tracer.
Results.
The low-
J
transitions of HC
3
N, HC
5
N, HNC, and N
2
H
+
trace extended and elongated structures from 6000 AU down to ~100 AU, without any accompanying dust continuum emission. Two structures are identified: one traces a flow that is likely accreting toward the most luminous component of the IRAS 16293−2422 A system. Temperatures inferred from the HCN/HNC ratio suggest that the gas in this flow is cold, between 10 and 30 K. The other structure is part of an
uv
-irradiated cavity wall entrained by one of the outflows driven by the source. The two outflows driven by IRAS 16293−2422 A present different molecular gas distributions.
Conclusions.
Accretion of cold gas is seen from 6000 AU scales onto IRAS 16293−2422 A but not onto source B, indicating that cloud core material accretion is competitive due to feedback onto a dominant component in an embedded multiple protostellar system. The preferential delivery of material could explain the higher luminosity and multiplicity of source A compared to source B. The results of this work demonstrate that several different molecular species, and multiple transitions of each species, are needed to confirm and characterize accretion flows in protostellar cloud cores.
The impact of stellar multiplicity on the evolution of planet-forming disks is still the subject of debate. Here we present and analyze disk structures around ten multiple stellar systems that were ...included in an unbiased, high spatial resolution survey performed with ALMA of 32 protoplanetary disks in the Taurus star-forming region. At the unprecedented spatial resolution of ~0.12′′ we detect and spatially resolve the disks around all primary stars, and those around eight secondary and one tertiary star. The dust radii of disks around multiple stellar systems are smaller than those around single stars in the same stellar mass range and in the same region. The disks in multiple stellar systems also show a steeper decay of the millimeter continuum emission at the outer radius than disks around single stars, suggestive of the impact of tidal truncation on the shape of the disks in multiple systems. However, the observed ratio between the dust disk radii and the observed separation of the stars in the multiple systems is consistent with analytic predictions of the effect of tidal truncation only if the eccentricities of the binaries are rather high (typically >0.5) or if the observed dust radii are a factor of two smaller than the gas radii, as is typical for isolated systems. Similar high-resolution studies targeting the gaseous emission from disks in multiple stellar systems are required to resolve this question.
Context. The physical structure of deeply embedded low-mass protostars (Class 0) on scales of less than 300 AU is still poorly constrained. While molecular line observations demonstrate the presence ...of disks with Keplerian rotation toward a handful of sources, others show no hint of rotation. Determining the structure on small scales (a few 100 AU) is crucial for understanding the physical and chemical evolution from cores to disks. Aims. We determine the presence and characteristics of compact, disk-like structures in deeply embedded low-mass protostars. A related goal is investigating how the derived structure affects the determination of gas-phase molecular abundances on hot-core scales. Methods. Two models of the emission, a Gaussian disk intensity distribution and a parametrized power-law disk model, are fitted to subarcsecond resolution interferometric continuum observations of five Class 0 sources, including one source with a confirmed Keplerian disk. Prior to fitting the models to the de-projected real visibilities, the estimated envelope from an independent model and any companion sources are subtracted. For reference, a spherically symmetric single power-law envelope is fitted to the larger scale emission (~1000 AU) and investigated further for one of the sources on smaller scales. Results. The radii of the fitted disk-like structures range from ~90−170 AU, and the derived masses depend on the method. Using the Gaussian disk model results in masses of 54−556 × 10-3 M⊙, and using the power-law disk model gives 9−140 × 10-3 M⊙. While the disk radii agree with previous estimates the masses are different for some of the sources studied. Assuming a typical temperature distribution (r-0.5), the fractional amount of mass in the disk above 100 K varies from 7% to 30%. Conclusions. A thin disk model can approximate the emission and physical structure in the inner few 100 AU scales of the studied deeply embedded low-mass protostars and paves the way for analysis of a larger sample with ALMA. Kinematic data are needed to determine the presence of any Keplerian disk. Using previous observations of p-H218O, we estimate the relative gas phase water abundances relative to total warm H2 to be 6.2 × 10-5 (IRAS 2A), 0.33 × 10-5 (IRAS 4A-NW), 1.8 × 10-7 (IRAS 4B), and < 2 × 10-7 (IRAS 4A-SE), roughly an order of magnitude higher than previously inferred when both warm and cold H2 were used as reference. A spherically symmetric single power-law envelope model fails to simultaneously reproduce both the small- and large-scale emission.
PDF
