arXiv subjects

Hauyu Baobab Liu

Publications and source records attributed to Hauyu Baobab Liu.

At least 109 records · Page 6Linked to original sources

arXiv2021

Sub-arcsecond Imaging of the Complex Organic Chemistry in Massive Star-forming Region G10.6-0.4

Massive star-forming regions exhibit an extremely rich and diverse chemistry, which in principle provides a wealth of molecular probes, as well as laboratories for interstellar prebiotic chemistry. Since the chemical structure of these sources displays substantial spatial variation among species on small scales (${\lesssim}10^4$ au), high angular resolution observations are needed to connect chemical structures to local environments and inform astrochemical models of massive star formation. To address this, we present ALMA 1.3 mm observations toward OB cluster-forming region G10.6-0.4 (hereafter "G10.6") at a resolution of 0.14$^{\prime\prime}$ (700 au). We find highly-structured emission from complex organic molecules (COMs) throughout the central 20,000 au, including two hot molecular cores and several shells or filaments. We present spatially-resolved rotational temperature and column density maps for a large sample of COMs and warm gas tracers. These maps reveal a range of gas substructure in both O- and N-bearing species. We identify several spatial correlations that can be explained by existing models of COM formation, including NH$_2$CHO/HNCO and CH$_3$OCHO/CH$_3$OCH$_3$, but also observe unexpected distributions and correlations which suggest that our current understanding of COM formation is far from complete. Importantly, complex chemistry is observed throughout G10.6, rather than being confined to hot cores. The COM composition appears to be different in the cores compared to the more extended structures, which illustrates the importance of high spatial resolution observations of molecular gas in elucidating the physical and chemical processes associated with massive star formation.

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arXiv2021

Distinguishing between different mechanisms of FU-Orionis-type luminosity outbursts

Aims. Accretion and luminosity bursts triggered by three distinct mechanisms: the magnetorotational instability in the inner disk regions, clump infall in gravitationally fragmented disks and close encounters with an intruder star, were studied to determine the disk kinematic characteristics that can help to distinguish between these burst mechanisms. Methods. Numerical hydrodynamics simulations in the thin-disk limit were employed to model the bursts in disk environments that are expected for each burst mechanism. Results. We found that the circumstellar disks featuring accretion bursts can bear kinematic features that are distinct for different burst mechanisms, which can be useful when identifying the burst origin. The disks in the stellar encounter and clump-infall models are characterized by tens of percent deviations from the Keplerian rotation, whie the disks in the MRI models are characterized only a few percent deviation, which is mostly caused by the gravitational instability that fuels the MRI bursts. Velocity channel maps also show distinct kinks and wiggles, which are caused by gas disk flows that are peculiar to each considered burst mechanism. The deviations of velocity channels in the burst-hosting disks from a symmetric pattern typical of Keplerian disks are strongest for the clump-infall and collision models, and carry individual features that may be useful for the identification of the corresponding burst mechanism. The considered burst mechanisms produce a variety of light curves with the burst amplitudes varying in the Δm=2.5-3.7 limits, except for the clump-infall model where Δm can reach 5.4, although the derived numbers may be affected by a small sample and boundary conditions. Conclusions. Burst triggering mechanisms are associated with distinct kinematic features in the burst-hosting disks that may be used for their identification. Abridged.

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arXiv2020

Ring formation by coagulation of dust aggregates in early phase of disk evolution around a protostar

Ring structures are observed by (sub-)millimeter dust continuum emission in various circumstellar disks from early stages of Class 0 and I to late stage of Class II young stellar objects (YSOs). In this paper, we study one of the possible scenarios of such ring formation in early stage, which is coagulation of dust aggregates. The dust grains grow in an inside-out manner because the growth timescale is roughly proportional to the orbital period. The boundary of the dust evolution can be regarded as the growth front, where the growth time is comparable to the disk age. With radiative transfer calculations based on the dust coagulation model, we find that the growth front can be observed as a ring structure because dust surface density is sharply changed at this position. Furthermore, we confirm that the observed ring positions in the YSOs with an age of $\lesssim1$ Myr are consistent with the growth front. The growth front could be important to create the ring structure in particular for early stage of the disk evolution such as Class 0 and I sources.

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arXiv2020

Formation of the Hub-Filament System G33.92+0.11: Local Interplay between Gravity, Velocity, and Magnetic Field

The formation of filaments in molecular clouds is an important process in star formation. Hub-filament systems (HFSs) are a transition stage connecting parsec-scale filaments and proto-clusters. Understanding the origin of HFSs is crucial to reveal how star formation proceeds from clouds to cores. Here, we report JCMT POL-2 850 $μ$m polarization and IRAM 30-m C$^{18}$O (2-1) line observations toward the massive HFS G33.92+0.11. The 850 $μ$m continuum map reveals four major filaments converging to the center of G33.92+0.11 with numerous short filaments connecting to the major filaments at local intensity peaks. We estimate the local orientations of filaments, magnetic field, gravity, and velocity gradients from observations, and we examine their correlations based on their local properties. In the high-density areas, our analysis shows that the filaments tend to align with the magnetic field and local gravity. In the low-density areas, we find that the local velocity gradients tend to be perpendicular to both the magnetic field and local gravity, although the filaments still tend to align with local gravity. A global virial analysis suggests that the gravitational energy overall dominates the magnetic and kinematic energy. Combining local and global aspects, we conclude that the formation of G33.92+0.11 is predominantly driven by gravity, dragging and aligning the major filaments and magnetic field on the way to the inner dense center. Traced by local velocity gradients in the outer diffuse areas, ambient gas might be accreted onto the major filaments directly or via the short filaments.

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arXiv2020

Magnetically regulated disk-formation in the inner 100 au region of the Class 0 young stellar object OMC-3/MMS~6 resolved by JVLA and ALMA

We have carried out polarization calibration for archival JVLA ($\sim$9 mm) full polarization observations towards the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared with the archival ALMA 1.2 mm observations. We found that the innermost $\sim$100 au region of OMC-3/MMS 6 is likely very optically thick (e.g., $τ\gg$1) at $\sim$1 mm wavelength such that the dominant polarization mechanism is dichroic extinction. It is marginally optically thin (e.g., $τ\lesssim$1) at $\sim$9 mm wavelength such that the JVLA observations can directly probe the linearly polarized emission from non-spherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape, while this "hourglass" appears $\sim$40$^{\circ}$ inclined with respect to the previously reported outflow axis. The geometry of this system is consistent with a magnetically regulated dense (pseudo-)disk. Based on the observed 29.45 GHz flux density and assuming a dust absorption opacity $κ^{abs}_{29.45\,GHz}=$0.0096 cm$^{2} $g$^{-1}$, the derived overall dust mass within a $\sim$43 au radius is $\sim$14000 $M_{\oplus}$. From this case study, it appears to us that some previous 9 mm surveys towards Class 0/I YSOs might have systematically underestimated dust masses by one order of magnitude, owing to that they assumed the too high dust absorption opacity ($\sim$0.1 cm$^{2}$ g$^{-1}$) for $\sim$9 mm wavelengths but without self-consistently considering the dust scattering opacity.

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arXiv2020

The effect of luminosity outbursts on the protoplanetary disk dynamics

Aims: Response of a protoplanetary disk to luminosity bursts of various duration is studied with the purpose to determine the effect of the bursts on the strength and sustainability of gravitational instability in the disk. A special emphasis is paid to the spatial distribution of gas and grown dust (from 1 mm to a few cm) during and after the burst. Methods: Numerical hydrodynamics simulations using the FEOSAD code were employed to study the dynamics of gas and dust in the thin-disk limit. Dust-to-gas friction including back reaction and dust growth were also considered. Bursts of various duration (from 100 to 500 yr) were initiated in accordance with a thermally ignited magnetorotational instability. Luminosity curves for constant- and declining-magnitude bursts were adopted to represent two typical limiting cases for FU-Orionis-type eruptions. Results: The short-term effect of the burst is to reduce the strength of gravitational instability by heating and expanding the disk. The longest bursts with duration comparable to the revolution period of the spiral can completely dissolve the original two-armed spiral pattern in the gas disk by the end of the burst, while the shortest bursts only weaken the spiral pattern. The reaction of grown dust to the burst is somewhat different. The spiral-like initial distribution with deep cavities in the inter-armed regions transforms into a ring-like distribution with deep gaps. This transformation is most expressed for the longest-duration bursts. The long-term effect of the burst depends on the initial disk conditions at the onset of the burst. In some cases, vigorous disk fragmentation sets in several thousand years after the burst, which was absent in the model without the bursts (abridged).

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arXiv2020

Possible Time Correlation Between Jet Ejection and Mass Accretion for RW Aur A

For the active T-Taur star RW Aur A we have performed long-term (~10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644-micron emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 A and O I 7772 A line profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past ~15 years with an irregular interval of 2-6 years. The time scale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (Delta_V < -1 mag.) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r <<0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few au of the star, and therefore possible ongoing planet formation at these radii.

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arXiv2020

Ionized Gas in the NGC 5253 Supernebula:High Spatial and Spectral Resolution Observations with the JVLA and TEXES

The youngest, closest and most compact embedded massive star cluster known excites the supernebula in the nearby dwarf galaxy NGC 5253. It is a crucial target and test case for studying the birth and evolution of the most massive star clusters. We present observations of the ionized gas in this source with high spatial and spectral resolution. The data includes continuum images of free-free emission with ~0.15'' resolution made with the JVLA at 15, 22 and 33 GHz, and a full data cube of the [SIV]10.5 micron fine-structure emission line with ~4.5 km/s velocity resolution and 0.3'' beam, obtained with TEXES on Gemini North. We find that 1) the ionized gas extends out from the cluster in arms or jets, and 2) the ionized gas comprises two components offset both spatially and in velocity. We discuss mechanisms that may have created the observed velocity field; possibilities include large-scale jets or a subcluster falling onto the main source.

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arXiv2020

Hot Corinos Chemical Diversity: Myth or Reality?

After almost 20 years of hunting, only about a dozen hot corinos, hot regions enriched in interstellar complex organic molecules (iCOMs), are known. Of them, many are binary systems with the two components showing drastically different molecular spectra. Two obvious questions arise. Why are hot corinos so difficult to find and why do their binary components seem chemically different? The answer to both questions could be a high dust opacity that would hide the molecular lines. To test this hypothesis, we observed methanol lines at centimeter wavelengths, where dust opacity is negligible, using the Very Large Array interferometer. We targeted the NGC 1333 IRAS 4A binary system, for which one of the two components, 4A1, has a spectrum deprived of iCOMs lines when observed at millimeter wavelengths, while the other component, 4A2, is very rich in iCOMs. We found that centimeter methanol lines are similarly bright toward 4A1 and 4A2. Their non-LTE analysis indicates gas density and temperature ($\geq2\times10^6$ cm$^{-3}$ and 100--190 K), methanol column density ($\sim10^{19}$ cm$^{-2}$) and extent ($\sim$35 au in radius) similar in 4A1 and 4A2, proving that both are hot corinos. Furthermore, the comparison with previous methanol line millimeter observations allows us to estimate the optical depth of the dust in front of 4A1 and 4A2, respectively. The obtained values explain the absence of iCOMs line emission toward 4A1 at millimeter wavelengths and indicate that the abundances toward 4A2 are underestimated by $\sim$30\%. Therefore, centimeter observations are crucial for the correct study of hot corinos, their census, and their molecular abundances.

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arXiv2020

Substructure Formation in a Protostellar Disk of L1527 IRS

We analyze multi-frequency, high-resolution continuum data obtained by ALMA and JVLA to study detailed structure of the dust distribution in the infant disk of a Class~0/I source, L1527 IRS. We find three clumps aligning in the north-south direction in the $7 {\rm \,mm}$ radio continuum image. The three clumps remain even after subtracting free-free contamination, which is estimated from the $1.3{\rm \,cm}$ continuum observations. The northern and southern clumps are located at a distance of $\sim 15{\rm \,au}$ from the central clump and are likely optically thick at $7{\rm \,mm}$ wavelength. The clumps have similar integrated intensities. The symmetric physical properties could be realized when a dust ring or spiral arms around the central protostar is projected to the plane of the sky. We demonstrates for the first time that such substructure may form even in the disk-forming stage, where the surrounding materials actively accrete toward a disk-protostar system.

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arXiv2020

GW Ori: Interactions Between a Triple-star System and its Circumtriple Disk in Action

GW Ori is a hierarchical triple system which has a rare circumtriple disk. We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 1.3 mm dust continuum and 12CO J=2-1 molecular gas emission of the disk. For the first time, we identify three dust rings in the disk at ~46, 188, and 338 AU, with estimated dust mass of ~70-250 Earth masses, respectively. To our knowledge, the outer ring in GW Ori is the largest dust ring ever found in protoplanetary disks. We use visibility modelling of dust continuum to show that the disk has misaligned parts and the innermost dust ring is eccentric. The disk misalignment is also suggested by the CO kinematics modelling. We interpret these substructures as evidence of ongoing dynamical interactions between the triple stars and the circumtriple disk.

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arXiv2020

The MUSTANG-2 Galactic Plane Survey (MGPS90) pilot

We report the results of a pilot program for a Green Bank Telescope (GBT) MUSTANG Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a typical $1σ$ depth of $1-2$ mJy beam$^{-1}$ with a 9" beam. We describe the survey parameters, quality assessment process, cataloging, and comparison with other data sets. We have identified 709 sources over seven observed fields selecting some of the most prominent millimeter-bright regions between $0°< \ell < 50°$ (total area $\approx 7.5 °^2$). The majority of these sources have counterparts at other wavelengths. By applying flux selection criteria to these sources, we successfully recovered several known hypercompact HII (HCHII) regions, but did not confirm any new ones. We identify 126 sources that have mm-wavelength counterparts but do not have cm-wavelength counterparts and are therefore candidate HCHII regions; of these, 10 are morphologically compact and are strong candidates for new HCHII regions. Given the limited number of candidates in the extended area in this survey compared to the relatively large numbers seen in protoclusters W51 and W49, it appears that most HCHII regions exist within dense protoclusters. Comparing the counts of HCHII to ultracompact HII (UCHII) regions, we infer the HCHII region lifetime is 16-46% that of the UCHII region lifetime. We additionally separated the 3 mm emission into dust and free-free emission by comparing with archival 870 $μ$m and 20 cm data. In the selected pilot fields, most ($\gtrsim80$%) of the 3 mm emission comes from plasma, either through free-free or synchrotron emission.

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arXiv2020

ALMA observations of NGC 6334S $-$ I: Forming massive stars and cluster in subsonic and transonic filamentary clouds

We present Atacama Large Millimeter/submillimeter Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) observations of the massive infrared dark cloud NGC 6334S (also known as IRDC G350.56+0.44), located at the southwestern end of the NGC 6334 molecular cloud complex. The H$^{13}$CO$^{+}$ and the NH$_{2}$D lines covered by the ALMA observations at a $\sim$3$^{\prime\prime}$ angular resolution ($\sim$0.02 pc) reveal that the spatially unresolved non-thermal motions are predominantly subsonic and transonic, a condition analogous to that found in low-mass star-forming molecular clouds. The observed supersonic non-thermal velocity dispersions in massive star forming regions, often reported in the literature, might be significantly biased by poor spatial resolutions that broaden the observed line widths due to unresolved motions within the telescope beam. Our 3~mm continuum image resolves 49 dense cores, whose masses range from 0.17 to 14 $M_{\odot}$. The majority of them are resolved with multiple velocity components. Our analyses of these gas velocity components find an anti-correlation between the gas mass and the virial parameter. This implies that the more massive structures tend to be more gravitationally unstable. Finally, we find that the external pressure in the NGC 6334S cloud is important in confining these dense structures, and may play a role in the formation of dense cores, and subsequently, the embedded young stars.

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arXiv2019

The detection of dust gap-ring structure in the outer region of the CR Cha protoplanetary disk

We observe the dust continuum at 225 GHz and CO isotopologue (12CO, 13CO, and C18O) J=2-1 emission lines toward the CR Cha protoplanetary disk using the Atacama Large Millimeter/Submillimeter Array (ALMA). The dust continuum image shows a dust gap-ring structure in the outer region of the dust disk. A faint dust ring is also detected around 120 au beyond the dust gap. The CO isotopologue lines indicate that the gas disk is more extended than the dust disk. The peak brightness temperature of the 13CO line shows a small bump around 130 au while 12CO and C18O lines do not show. We investigate two possible mechanisms for reproducing the observed dust gap-ring structure and a gas temperature bump. First, the observed gap structure can be opened by a Jupiter mass planet using the relation between the planet mass and the gap depth and width. Meanwhile, the radiative transfer calculations based on the observed dust surface density profile show that the observed dust ring could be formed by dust accumulation at the gas temperature bump, that is, the gas pressure bump produced beyond the outer edge of the dust disk.

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arXiv2019

Physical Properties of the star-forming clusters in NGC 6334

We have studied the high-mass star-forming complex NGC 6334 with ALMA in the continuum emission at a frequency of 87.6 GHz, achieving a spatial resolution of 1300 au. Detecting 142 compact sources distributed over the whole observed area, we then used machine learning algorithms to group the compact cores in different clusters. A total of four main clusters were identified: NGC 6334-E, NGC 6334-I, NGC 6334-I(N) and NGC 6334-I(NW). The typical separations between cluster members (4000-12000 au) together with the core masses (0.2-100 $M_{\odot}$) are in agreement with turbulent fragmentation at scales of 0.1 pc. We find that the CMFs (core mass functions) show an excess of massive cores compared to the IMF. Typical uncertainties in temperature or unresolved multiplicity may mimic the observed excess of massive cores. Evidence of mass segregation in NGC 6334-I and NGC 6334-I(N), with the most massive cores located closer to the center, was found. Correlations between the physical properties of the clusters and their evolutionary stage were searched for, and found to show a larger separation between cores in the more evolved clusters, favouring the role of gas expulsion and stellar ejection over time. From these results, we suggest that NGC 6334-I(N) was primordially segregated, while NGC 6334-I may have become mass-segregated over time due to dynamical effects. Finally, the lack of massive cores in the most evolved cluster suggests that the gas reservoir may be already exhausted, while the less evolved clusters still have access to a larger gas mass reservoir. In general, the fragmentation in NGC 6334 at large scales (about 1 pc) is governed by turbulent pressure, while at smaller scales (a few hundred au), thermal pressure regulates the fragmentation process.

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arXiv2019

Resolving the FU Ori System with ALMA: Interacting Twin Disks?

FU Orionis objects are low-mass pre-main sequence stars characterized by dramatic outbursts of several magnitudes in brightness. These outbursts are linked to episodic accretion events in which stars gain a significant portion of their mass. The physical processes behind these accretion events are not yet well understood. The archetypical FU Ori system, FU Orionis, is composed of two young stars with detected gas and dust emission. The continuum emitting regions have not been resolved until now. Here, we present 1.3 mm observations of the FU Ori binary system with ALMA. The disks are resolved at 40 mas resolution. Radiative transfer modeling shows that the emission from FU Ori north (primary) is consistent with a dust disk with a characteristic radius of $\sim$11 au. The ratio between major and minor axes shows that the inclination of the disk is $\sim$37 deg. FU Ori south is consistent with a dust disk of similar inclination and size. Assuming the binary orbit shares the same inclination angle as the disks, the deprojected distance between north and south components is 0.6'', i.e. $\sim$250 au. Maps of $^{12}$CO emission show a complex kinematic environment with signatures disk rotation at the location of the northern component, and also (to a lesser extent) for FU Ori south. The revised disk geometry allows us to update FU Ori accretion models (Zhu et al.), yielding a stellar mass and mass accretion rate of FU Ori north of 0.6 M$_{\odot}$ and 3.8$\times10^{-5}$ M$_{\odot}$ yr$^{-1}$, respectively.

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arXiv2019

Resolving Linear Polarization due to Emission and Extinction of Aligned Dust Grains on NGC1333 IRAS4A with JVLA and ALMA

We report high angular resolution observations of linearly polarized dust emission towards the Class 0 young stellar object (YSO) NGC1333 IRAS4A (hereafter, IRAS4A) using the Karl G. Jansky Very Large Array (JVLA) at K (11.5-16.7 mm), Ka (8.1-10.3 mm), and Q bands (6.3-7.9 mm), and using the Atacama Large Millimeter Array (ALMA) at Band 6 (1.2 mm) and Band 7 (0.85-0.89 mm). On 100-1000 AU scales, all of these observations consistently trace the hourglass shaped magnetic field topology as shown in the previous studies. In the innermost 100 AU region of IRAS4A1, the polarization position angles (E-field) detected at 6.3-16.7 mm are consistent, however, are nearly 90 degrees offset from those detected at 1.2 mm and 0.85-0.89 mm. Such a 90 degree offset may be explained by that the inner $\sim$100 AU area is optically thick at wavelengths shorter than $\sim$1.5 mm, whereby the observations probe the absorption of aligned dust against the weakly or unpolarized warm dust emission from the innermost region. This can also consistently explain why the highest angular resolution ALMA images at Band 7 show that the polarization percentage increases with dust brightness temperature in the inner $\sim$100 AU region of IRAS4A1. Following this interpretation and assuming that the dust grains are aligned with the magnetic fields, the inferred magnetic field position angle based on the 90$^{\circ}$ rotated at 6.3-7.9 mm in the central peak of IRAS4A1 is $\sim-22^{\circ}$, which is approximately consistent with the outflow direction $\sim-9^{\circ}$.

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arXiv2019

An ALMA Study of the FU-Ori Type Object V900 Mon: Implications for the Progenitor

We present ALMA observations of 12CO, 13CO, and C18O J=2--1 lines and the 230 GHz continuum for the FU Ori-type object (FUor) V900 Mon (d~1.5 kpc), for which the accretion burst was triggered between 1953 and 2009. We identified CO emission associated with a molecular bipolar outflow extending up to a ~10^4 au scale and a rotating molecular envelope extending over >10^4 au. The interaction with the hot energetic FUor wind, which was observed using optical spectroscopy, appears limited to a region within ~400 au of the star. The envelope mass and the collimation of the extended CO outflow suggest that the progenitor of this FUor is a low-mass Class I young stellar object (YSO). These parameters for V900 Mon, another FUor, and a few FUor-like stars are consistent with the idea that FUor outbursts are associated with normal YSOs. The continuum emission is marginally resolved in our observations with a 0."2x0."15 (~300x225 au) beam, and a Gaussian model provides a deconvolved FWHM of ~90 au. The emission is presumably associated with a dusty circumstellar disk, plus a possible contribution from a wind or a wind cavity close to the star. The warm compact nature of the disk continuum emission could be explained with viscous heating of the disk, while gravitational fragmentation in the outer disk and/or a combination of grain growth and their inward drift may also contribute to its compact nature.

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