Stages Of Low-mass Star Formation Research Articles

Shedding Light on the Ejection History of Molecular Outflows: Multiple Velocity Modes and Precession

Variable accretion has been well studied in the evolved stages of low-mass star formation. However, the accretion history in the initial phases of star formation is still a seldom studied topic. The outflows and jets emerging from protostellar objects could shed some light on their accretion history. We consider the recently studied case of W43-MM1, a protocluster containing 46 outflows driven by 27 protostellar cores. The outflow kinematics of the individual cores and associated knots in W43-MM1 indicate episodic protostellar ejection. We take the observed parameters of an individual core system (core #8) and perform 3D hydrodynamic simulations of such a system, including episodic changes in the velocity of the outflow. The simulations consist of a collimated jet emerging from a core, taking into account one- and two-velocity modes in the variation of the ejection velocity of the jet. In addition, we investigated the effect of including the precession of the jet in the one- and two-velocity-mode models. From the simulations, we constructed position–velocity diagrams and compared them with the observations. We find that including a second mode in the ejection velocity, as well as the precession, are required to explain the positions of the outflow knots and other position–velocity features observed in core #8 in W43-MM1.

First BISTRO Observations of the Dark Cloud Taurus L1495A-B10: The Role of the Magnetic Field in the Earliest Stages of Low-mass Star Formation

We present BISTRO Survey 850 μm dust emission polarization observations of the L1495A-B10 region of the Taurus molecular cloud, taken at the James Clerk Maxwell Telescope (JCMT). We observe a roughly triangular network of dense filaments. We detect nine of the dense starless cores embedded within these filaments in polarization, finding that the plane-of-sky orientation of the core-scale magnetic field lies roughly perpendicular to the filaments in almost all cases. We also find that the large-scale magnetic field orientation measured by Planck is not correlated with any of the core or filament structures, except in the case of the lowest-density core. We propose a scenario for early prestellar evolution that is both an extension to, and consistent with, previous models, introducing an additional evolutionary transitional stage between field-dominated and matter-dominated evolution, observed here for the first time. In this scenario, the cloud collapses first to a sheet-like structure. Uniquely, we appear to be seeing this sheet almost face on. The sheet fragments into filaments, which in turn form cores. However, the material must reach a certain critical density before the evolution changes from being field dominated to being matter dominated. We measure the sheet surface density and the magnetic field strength at that transition for the first time and show consistency with an analytical prediction that had previously gone untested for over 50 yr.

SOLIS

Context. Recent results in astrochemistry have revealed that some molecules, such as interstellar complex organic species and deuterated species, can serve as valuable tools in the investigation of star-forming regions. Sulphuretted species can also be used to follow the chemical evolution of the early stages of a Sun-like star formation process. Aims. The goal is to obtain a census of S-bearing species using interferometric images towards SVS13-A, a Class I object associated with a hot corino that is rich in interstellar complex organic molecules. Methods. To this end, we used the NGC 1333 SVS13-A data at 3 mm and 1.4 mm obtained with the IRAM-NOEMA interferometer in the framework of the SOLIS (Seeds of Life in Space) Large Program. The line emission of S-bearing species was imaged and analyzed using local thermodynamic equilibrium (LTE) and large velocity gradient (LVG) approaches. Results. We imaged the spatial distribution on ≤300 au scale of the line emission of 32SO, 34SO, C32S, C34S, C33S, OCS, H2C32S, H2C34S, and NS. The low excitation (9 K) 32SO line traces: (i) the low-velocity SVS13-A outflow and (ii) the fast (up to 100 km s−1 away from the systemic velocity) collimated jet driven by the nearby SVS13-B Class 0 object. Conversely, the rest of the lines are confined in the inner SVS13-A region, where complex organics were previously imaged. More specifically, the non-LTE LVG analysis of SO, SO2, and H2CS indicates a hot corino origin (size in the 60–120 au range). Temperatures between 50 K and 300 K, as well as volume densities larger than 105 cm−3 have been derived. The abundances of the sulphuretted are in the following ranges: 0.3–6 × 10−6 (CS), 7 × 10−9–1 × 10−7 (SO), 1–10 × 10−7 (SO2), a few 10−10 (H2CS and OCS), and 10−10–10−9 (NS). The N(NS)/N(NS+) ratio is larger than 10, supporting the assessment that the NS+ ion is mainly formed in the extended envelope. Conclusions. The [H2CS]/[H2CO] ratio, once measured at high-spatial resolutions, increases with time (from Class 0 to Class II objects) by more than one order of magnitude (from ≤10−2 to a few 10−1). This suggests that [S]/[O] changes along the process of Sun-like star formation. Finally, the estimate of the [S]/[H] budget in SVS13-A is 2–17% of the Solar System value (1.8 × 10−5), which is consistent with what was previously measured towards Class 0 objects (1–8%). This finding supports the notion that the enrichment of the sulphuretted species with respect to dark clouds remains constant from the Class 0 to the Class I stages of low-mass star formation. The present findings stress the importance of investigating the chemistry of star-forming regions using large observational surveys as well as sampling regions on the scale of the Solar System.

Detection of complex organic molecules in young starless core L1521E

ABSTRACT Determining the level of chemical complexity within dense starless and gravitationally bound pre-stellar cores is crucial for constructing chemical models, which subsequently constrain the initial chemical conditions of star formation. We have searched for complex organic molecules (COMs) in the young starless core L1521E, and report the first clear detection of dimethyl ether (CH3OCH3), methyl formate (HCOOCH3), and vinyl cyanide (CH2CHCN). Eight transitions of acetaldehyde (CH3CHO) were also detected, five of which (A states) were used to determine an excitation temperature to then calculate column densities for the other oxygen-bearing COMs. If source size was not taken into account (i.e. if filling fraction was assumed to be one), column density was underestimated, and thus we stress the need for higher resolution mapping data. We calculated L1521E COM abundances and compared them to other stages of low-mass star formation, also finding similarities to other starless/pre-stellar cores, suggesting related chemical evolution. The scenario that assumes formation of COMs in gas-phase reactions between precursors formed on grains and then ejected to the cold gas via reactive desorption was tested and was unable to reproduce observed COM abundances, with the exception of CH3CHO. These results suggest that COMs observed in cold gas are formed not by gas-phase reactions alone, but also through surface reactions on interstellar grains. Our observations present a new, unique challenge for existing theoretical astrochemical models.

ALMA observations of the young protostellar system Barnard 1b: signatures of an incipient hot corino in B1b-S.

The Barnard 1b core shows signatures of being at the earliest stages of low-mass star formation, with two extremely young and deeply embedded protostellar objects. Hence, this core is an ideal target to study the structure and chemistry of the first objects formed in the collapse of prestellar cores. We present ALMA Band 6 spectral line observations at ~0.6″ of angular resolution towards Barnard 1b. We have extracted the spectra towards both protostars, and used a Local Thermodynamic Equilibrium (LTE) model to reproduce the observed line profiles. B1b-S shows rich and complex spectra, with emission from high energy transitions of complex molecules, such as CH3OCOH and CH3CHO, including vibrational level transitions. We have tentatively detected for the first time in this source emission from NH2CN, NH2CHO, CH3CH2OH, CH2OHCHO, CH3CH2OCOH and both aGg' and gGg' conformers of (CH2OH)2. This is the first detection of ethyl formate (CH3CH2OCOH) towards a low-mass star forming region. On the other hand, the spectra of the FHSC candidate B1b-N are free of COMs emission. In order to fit the observed line profiles in B1b-S, we used a source model with two components: an inner hot and compact component (200 K, 0.35″) and an outer and colder one (60 K, 0.6″). The resulting COM abundances in B1b-S range from 10-13 for NH2CN and NH2CHO, up to 10-9 for CH3OCOH. Our ALMA Band 6 observations reveal the presence of a compact and hot component in B1b-S, with moderate abundances of complex organics. These results indicate that a hot corino is being formed in this very young Class 0 source.

The onset of energetic particle irradiation in Class 0 protostars

The early stages of low-mass star formation are likely to be subject to intense ionization by protostellar energetic MeV particles. As a result, the surrounding gas is enriched in molecular ions, such as HCO$^{+}$ and N$_{2}$H$^{+}$. Nonetheless, this phenomenon remains poorly understood for Class 0 objects. Recently, based on Herschel observations taken as part of the key program Chemical HErschel Surveys of Star forming regions (CHESS), a very low HCO$^{+}$/N$_{2}$H$^{+}$ abundance ratio of about 3-4, has been reported toward the protocluster OMC-2 FIR4. This finding suggests a cosmic-ray ionization rate in excess of 10$^{-14}$ s$^{-1}$, much higher than the canonical value of $\zeta$ = 3$\times$10$^{-17}$ s$^{-1}$ (value expected in quiescent dense clouds). To assess the specificity of OMC-2 FIR4, we have extended this study to a sample of sources in low- and intermediate mass. More specifically, we seek to measure the HCO$^{+}$/N$_2$H$^{+}$ abundance ratio from high energy lines (J $\ge$ 6) toward this source sample in order to infer the flux of energetic particles in the warm and dense gas surrounding the protostars. We use observations performed with the Heterodyne Instrument for the FarInfrared spectrometer on board the Herschel Space Observatory toward a sample of 9 protostars. We report HCO$^{+}$/N$_2$H$^{+}$ abundance ratios in the range of 5 up to 73 toward our source sample. The large error bars do not allow us to conclude whether OMC-2~FIR4 is a peculiar source. Nonetheless, an important result is that the measured HCO$^{+}$/N$_2$H$^{+}$ ratio does not vary with the source luminosity. At the present time, OMC-2 FIR4 remains the only source where a high flux of energetic particles is clearly evident. More sensitive and higher angular resolution observations are required to further investigate this process.

The JCMT Transient Survey: Identifying Submillimeter Continuum Variability over Several Year Timescales Using Archival JCMT Gould Belt Survey Observations

Abstract Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterized across the observable SED. In this study, we develop methods to robustly analyze repeated observations of an area of the sky for submillimeter variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare 850 μm JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2–4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find seven variable candidates, five of which we classify as Strong, and the remaining two we classify as Extended to indicate that the latter are associated with larger-scale structure. For the Strong variable candidates, we find an average fractional peak brightness change per year of ∣ 4.0 ∣ % yr − 1 , with a standard deviation of 2.7 % yr − 1 . In total, 7% of the protostars associated with 850 μm emission in our sample show signs of variability. Four of the five Strong sources are associated with a known protostar. The remaining source is a good follow-up target for an object that is anticipated to contain an enshrouded, deeply embedded protostar. In addition, we estimate the 850 μm periodicity of the submillimeter variable source, EC 53, to be 567 ± 32 days, based on the archival Gould Belt Survey data.

Evidence for disks at an early stage in class 0 protostars?

The formation epoch of protostellar disks is debated because of the competing roles of rotation, turbulence, and magnetic fields in the early stages of low-mass star formation. Magnetohydrodynamics simulations of collapsing cores predict that rotationally supported disks may form in strongly magnetized cores through ambipolar diffusion or misalignment between the rotation axis and the magnetic field orientation. Detailed studies of individual sources are needed to cross check the theoretical predictions. We present 0.06 - 0.1 ″ resolution images at 350 GHz toward B1b-N and B1b-S, which are young class 0 protostars, possibly first hydrostatic cores. The images have been obtained with ALMA, and we compare these data with magnetohydrodynamics simulations of a collapsing turbulent and magnetized core. The submillimeter continuum emission is spatially resolved by ALMA. Compact structures with optically thick 350 GHz emission are detected toward both B1b-N and B1b-S, with 0.2 and 0.35″ radii (46 and 80 au at the Perseus distance of 230 pc), within a more extended envelope. The flux ratio between the compact structure and the envelope is lower in B1b-N than in B1b-S, in agreement with its earlier evolutionary status. The size and orientation of the compact structure are consistent with 0.2″ resolution 32 GHz observations obtained with the Very Large Array as a part of the VANDAM survey, suggesting that grains have grown through coagulation. The morphology, temperature, and densities of the compact structures are consistent with those of disks formed in numerical simulations of collapsing cores. Moreover, the properties of B1b-N are consistent with those of a very young protostar, possibly a first hydrostatic core. These observations provide support for the early formation of disks around low-mass protostars.

The JCMT Transient Survey: Data Reduction and Calibration Methods

Abstract Though there has been a significant amount of work investigating the early stages of low-mass star formation in recent years, the evolution of the mass assembly rate onto the central protostar remains largely unconstrained. Examining in depth the variation in this rate is critical to understanding the physics of star formation. Instabilities in the outer and inner circumstellar disk can lead to episodic outbursts. Observing these brightness variations at infrared or submillimeter wavelengths constrains the current accretion models. The JCMT Transient Survey is a three-year project dedicated to studying the continuum variability of deeply embedded protostars in eight nearby star-forming regions at a one-month cadence. We use the SCUBA-2 instrument to simultaneously observe these regions at wavelengths of 450 and 850 μm. In this paper, we present the data reduction techniques, image alignment procedures, and relative flux calibration methods for 850 μm data. We compare the properties and locations of bright, compact emission sources fitted with Gaussians over time. Doing so, we achieve a spatial alignment of better than 1″ between the repeated observations and an uncertainty of 2%–3% in the relative peak brightness of significant, localized emission. This combination of imaging performance is unprecedented in ground-based, single-dish submillimeter observations. Finally, we identify a few sources that show possible and confirmed brightness variations. These sources will be closely monitored and presented in further detail in additional studies throughout the duration of the survey.

Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores

Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.

The ALMA view of the protostellar system HH212

Context: Because it is viewed simply edge-on, the HH212 protostellar system is an ideal laboratory for studying the interplay of infall, outflow, and rotation in the earliest stages of low-mass star formation. Aims: We wish to exploit the unmatched combination of high angular resolution, high sensitivity, high-imaging fidelity, and spectral coverage provided by ALMA to shed light on the complex kinematics of the innermost central regions of HH212. Methods: We mapped the inner 10" (4500 AU) of the HH212 system at about 0.5 arcsec resolution in several molecular tracers and in the 850 $\mu$m dust continuum using the ALMA interferometer in band 7 in the extended configuration of the Early Science Cycle 0 operations. Results: Within a single ALMA spectral set-up, we simultaneously identify all the crucial ingredients known to be involved in the star formation recipe namely: (i) the fast, collimated bipolar SiO jet driven by the protostar, (ii) the large-scale swept-up CO outflow, (iii) the flattened rotating and infalling envelope, with bipolar cavities carved by the outflow (in C$^{17}$O(3--2)), and (iv) a rotating wide-angle flow that fills the cavities and surrounding the axial jet (in C$^{34}$S(7--6)). In addition, the compact high-velocity C$^{17}$O emission ($\pm$ 1.9--3.5 km s$^{-1}$ from systemic) shows a velocity gradient along the equatorial plane consistent with a rotating disk of about 0.2 arcsec = 90 AU around a $\simeq 0.3 \pm 0.1 M_{\rm \odot}$ source. The rotating disk is possibly Keplerian. Conclusions: HH212 is the third Class 0 protostar with possible signatures of a Keplerian disk of radius $\geq 30 AU$. The warped geometry in our CS data suggests that this large keplerian disk might result from misaligned magnetic and rotation axes during the collapse phase. The wide-angle CS flow suggests that disk winds may be present in this source.

COMPLEX ORGANIC MOLECULES DURING LOW-MASS STAR FORMATION: PILOT SURVEY RESULTS

Complex organic molecules (COMs) are known to be abundant toward some low-mass young stellar objects (YSOs), but how these detections relate to typical COM abundance are not yet understood. We aim to constrain the frequency distribution of COMs during low-mass star formation, beginning with this pilot survey of COM lines toward six embedded YSOs using the IRAM 30m telescope. The sample was selected from the Spitzer c2d ice sample and covers a range of ice abundances. We detect multiple COMs, including CH3CN, toward two of the YSOs, and tentatively toward a third. Abundances with respect to CH3OH vary between 0.7 and 10%. This sample is combined with previous COM observations and upper limits to obtain a frequency distributions of CH3CN, HCOOCH3, CH3OCH3 and CH3CHO. We find that for all molecules more than 50% of the sample have detections or upper limits of 1-10% with respect to CH3OH. Moderate abundances of COMs thus appear common during the early stages of low-mass star formation. A larger sample is required, however, to quantify the COM distributions, as well as to constrain the origins of observed variations across the sample.

The effect of episodic accretion on the phase transition of CO and CO2in low-mass star formation

We study the evaporation and condensation of CO and CO_2 during the embedded stages of low-mass star formation by using numerical simulations. We focus on the effect of luminosity bursts, similar in magnitude to FUors and EXors, on the gas-phase abundance of CO and CO_2 in the protostellar disk and infalling envelope. The evolution of a young protostar and its environment is followed based on hydrodynamical models using the thin-disk approximation, coupled with a stellar evolution code and phase transformations of CO and CO_2. The accretion and associated luminosity bursts in our model are caused by disk gravitational fragmentation followed by quick migration of the fragments onto the forming protostar. We found that bursts with luminosity on the order of 100-200 L_sun can evaporate CO ices in part of the envelope. The typical freeze-out time of the gas-phase CO onto dust grains in the envelope (a few kyr) is much longer than the burst duration (100-200 yr). This results in an increased abundance of the gas-phase CO in the envelope long after the system has returned into a quiescent stage. In contrast, luminosity bursts can evaporate CO_2 ices only in the disk, where the freeze-out time of the gas-phase CO_2 is comparable to the burst duration. We thus confirm that luminosity bursts can leave long-lasting traces in the abundance of gas-phase CO in the infalling envelope, enabling the detection of recent bursts as suggested by previous semi-analytical studies.

Circularly Polarized Radio Emission from an X-Ray Protostar

IRS 5, an embedded young stellar object in the nearby Corona Australis molecular cloud, was recently detected as an X-ray source with the ASCA and ROSAT satellites. We report here the detection of circularly polarized continuum emission from IRS 5 at centimeter wavelengths. Already known to be a highly variable radio source, the polarization fraction is seen to range from V/I10% to 37% on a day timescale. This demonstrates that radio emission from protostars, previously attributed in most cases to ionized thermal outflows, can sometimes arise instead from nonthermal processes, i.e., gyrosynchrotron emission from particles accelerated in situ by magnetic reconnection flares. Together with the X-ray data and indications of MeV particles in the solar nebula obtained from meteoritic materials, it contributes to the growing evidence for high-energy processes during the earliest stages of low-mass star formation.