arXiv subjects

Z. -Y. Li

Publications and source records attributed to Z. -Y. Li.

13 recordsLinked to original sources

arXiv2025

Mid-infrared extinction curve for protostellar envelopes from JWST-detected embedded jet emission: the case of TMC1A

Context: Dust grains are key components of the interstellar medium and play a central role in star formation, acting as catalysts for chemical reactions and as building blocks of planets. Extinction curves are essential for characterizing dust properties, but mid-infrared (MIR) extinction remains less constrained in protostellar environments. Gas-phase line ratios from embedded protostellar jets provide a spatially resolved method for probing extinction through protostellar envelopes, complementing background starlight approaches. Aims: We aim to derive MIR extinction curves along sightlines toward a protostellar jet embedded in an envelope and assess whether they differ from those in dense molecular clouds. Methods: We analyze JWST NIRSpec IFU and MIRI MRS observations of four positions along the blue-shifted TMC1A jet. We extract observed [Fe II] line intensities and model intrinsic ratios using the Cloudy spectral synthesis code across a range of electron densities and temperatures. By comparing observed near-IR (NIR) and MIR line ratios to Cloudy predictions, we infer the relative extinction between NIR and MIR wavelengths. Results: Electron densities (ne) derived from NIR [Fe II] lines range from ~5 x 10^4 to ~5 x 10^3 cm^-3 at scales <~350 AU. MIR extinction values show stronger reddening than the empirical dark cloud curve from McClure (2009) at similar ne and temperatures (~10^3 to 10^4 K). If MIR emission arises from cooler, lower-density gas, extinction curves remain consistent with background starlight measurements. Conclusions: This method enables spatially resolved MIR extinction curves in embedded protostellar systems. Results suggest either a change in dust size distribution (e.g., from grain growth) or that MIR emission originates from cooler, less dense regions than NIR emission. (Abstract shortened for arXiv. See PDF for full version.)

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arXiv2024

The protostars in Orion: Characterizing the properties of their magnetized envelopes

We present a study connecting the physical properties of protostellar envelopes to the morphology of the envelope-scale magnetic field. We used the ALMA polarization observations of 61 young prtostars at 0.87 mm on $\sim400-3000$ au scales from the {\em B}-field Orion Protostellar Survey to infer the envelope-scale magnetic field, and used the dust emission to measure the envelope properties on comparable scales. We find that protostars showing standard-hourglass-field morphology tend to have larger masses and lower velocity dispersions in their envelopes, whereas systems with spiral-field morphologies have higher velocity dispersion. Combining with the disk properties taken from the Orion VLA/ALMA Nascent Disk and Multiplicity survey, we connect envelope properties to fragmentation. Our results show that the fragmentation level is positively correlated with the angle dispersion of the magnetic field, suggesting that the envelope fragmentation tends to be suppressed by the magnetic field. We also find that protostars exhibiting standard hourglass magnetic field structure tend to have a smaller disk and smaller angle dispersion of the magnetic field than other field configurations, specially the rotated hourglass, but also the spiral and others, suggesting a more effective magnetic braking in the standard hourglass morphology of magnetic fields. Nevertheless, significant misalignment between the magnetic field and outflow axes tends to reduce magnetic braking, leading to the formation of larger disks.

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arXiv2019

Probing the cold magnetized Universe with SPICA-POL (B-BOP)

SPICA, the cryogenic infrared space telescope recently pre-selected for a `Phase A' concept study as one of the three remaining candidates for ESA's fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager (SPICA-POL, now called B-BOP), which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetized Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100-350 micron images of linearly polarized dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 micron images will also have a factor ~30 higher resolution than Planck polarization data. This will make B-BOP a unique tool for characterizing the statistical properties of the magnetized interstellar medium and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars.

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arXiv2018

Resolving the polarized dust emission of the disk around the massive star powering the HH~80-81 radio jet

Here we present deep (16 mumJy), very high (40 mas) angular resolution 1.14 mm, polarimetric, Atacama Large Millimeter/submillimeter Array (ALMA) observations towards the massive protostar driving the HH 80-81 radio jet. The observations clearly resolve the disk oriented perpendicular to the radio jet, with a radius of ~0.171 arcsec (~291 au at 1.7 kpc distance). The continuum brightness temperature, the intensity profile, and the polarization properties clearly indicate that the disk is optically thick for a radius of R<170 au. The linear polarization of the dust emission is detected almost all along the disk and its properties suggest that dust polarization is produced mainly by self-scattering. However, the polarization pattern presents a clear differentiation between the inner (optically thick) part of the disk and the outer (optically thin) region of the disk, with a sharp transition that occurs at a radius of 0.1 arcsec (~170 au). The polarization characteristics of the inner disk suggest that dust settling has not occurred yet with a maximum dust grain size between 50 and 500 mum. The outer part of the disk has a clear azimuthal pattern but with a significantly higher polarization fraction compared to the inner disk. This pattern is broadly consistent with self-scattering of a radiation field that is beamed radially outward, as expected in the optically thin outer region, although contribution from non-spherical grains aligned with respect to the radiative flux cannot be excluded.

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arXiv2017

On the relation between the column density structures and the magnetic field orientation in the Vela C molecular complex

We statistically evaluate the relative orientation between gas column density structures, inferred from Herschel submillimetre observations, and the magnetic field projected on the plane of sky, inferred from polarized thermal emission of Galactic dust observed by BLASTPol at 250, 350, and 500 micron, towards the Vela C molecular complex. First, we find very good agreement between the polarization orientations in the three wavelength-bands, suggesting that, at the considered common angular resolution of 3.0 arcminutes that corresponds to a physical scale of approximately 0.61 pc, the inferred magnetic field orientation is not significantly affected by temperature or dust grain alignment effects. Second, we find that the relative orientation between gas column density structures and the magnetic field changes progressively with increasing gas column density, from mostly parallel or having no preferred orientation at low column densities to mostly perpendicular at the highest column densities. This observation is in agreement with previous studies by the Planck collaboration towards more nearby molecular clouds. Finally, we find a correspondence between the trends in relative orientation and the shape of the column density probability distribution functions. In the sub-regions of Vela C dominated by one clear filamentary structure, or "ridges", we find a sharp transition from preferentially parallel or having no preferred relative orientation at low column densities to preferentially perpendicular at highest column densities. In the sub-regions of Vela C dominated by several filamentary structures with multiple orientations, or "nests", such a transition is also present, but it is clearly less sharp than in the ridge-like sub-regions. Both of these results suggest that the magnetic field is dynamically important for the formation of density structures in this region.

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arXiv2014

Testing Magnetic Field Models for the Class 0 Protostar L1527

For the Class 0 protostar, L1527, we compare 131 polarization vectors from SCUPOL/JCMT, SHARP/CSO and TADPOL/CARMA observations with the corresponding model polarization vectors of four ideal-MHD, non-turbulent, cloud core collapse models. These four models differ by their initial magnetic fields before collapse; two initially have aligned fields (strong and weak) and two initially have orthogonal fields (strong and weak) with respect to the rotation axis of the L1527 core. Only the initial weak orthogonal field model produces the observed circumstellar disk within L1527. This is a characteristic of nearly all ideal-MHD, non-turbulent, core collapse models. In this paper we test whether this weak orthogonal model also has the best agreement between its magnetic field structure and that inferred from the polarimetry observations of L1527. We found that this is not the case; based on the polarimetry observations the most favored model of the four is the weak aligned model. However, this model does not produce a circumstellar disk, so our result implies that a non-turbulent, ideal-MHD global collapse model probably does not represent the core collapse that has occurred in L1527. Our study also illustrates the importance of using polarization vectors covering a large area of a cloud core to determine the initial magnetic field orientation before collapse; the inner core magnetic field structure can be highly altered by a collapse and so measurements from this region alone can give unreliable estimates of the initial field configuration before collapse.

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arXiv2013

The evolution of the jet from Herbig Ae star HD 163296 from 1999 to 2011

Young A and B stars, the so-called Herbig Ae/Be stars (HAeBe), are surrounded by an active accretion disk and drive outflows. We study the jet HH 409, which is launched from the HAeBe star HD 163296, using new and archival observations from Chandra and HST/STIS. In X-rays we can show that the central source is not significantly extended. The approaching jet, but not the counter-jet, is detected in Ly alpha. In addition, there is red-shifted Ly alpha emission extended in the same direction as the jet, that is also absent in the counter-jet. We can rule out an accretion or disk-wind origin for this feature. In the optical we find the knots B and B2 in the counter-jet. Knot B has been observed previously, so we can derive its proper motion of 0.37+-0.01 arcsec/yr. Its electron density is 3000/cm^3, thus the cooling time scale is a few months only, so the knot needs to be reheated continuously. The shock speed derived from models of H alpha and forbidden emission lines (FELs) decreased from 50 km/s in 1999 to 30 km/s in 2011 because the shock front loses energy as it travels along the jet. Knot B2 is observed at a similar position in 2011 as knot B was in 1999, but shows a lower ionization fraction and higher mass loss rate, proving variations in the jet launching conditions.

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arXiv2010

The disk-bearing young star IM Lup

Classical T Tauri stars (CTTS) differ in their X-ray signatures from older pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess and deviations from the low-density coronal limit in the He-like triplets. We test whether these features correlate with accretion or the presence of a disk by observing IM Lup, a disk-bearing object apparently in transition between CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum and additional XMM-Newton data of IM Lup and accompanying optical spectra, some of them taken simultaneously to the X-ray observations. We fit the X-ray emission lines and decompose the Ha emission line in different components. In X-rays IM Lup has a bright and hot active corona, where elements of low first-ionisation potential are depleted. The He-like Ne IX triplet is in the low-density state, but due to the small number of counts a high-density scenario cannot be excluded on the 90% confidence level. In all X-ray properties IM Lup resembles a main-sequence star, but it is also compatible with CTTS signatures on the 90% confidence level, thus we cannot decide if the soft excess and deviations from the low-density coronal limit in the He-like triplets in CTTS require accretion or only the presence of a disk. IM Lup is chromospherically active, which explains most of the emission in Ha. Despite its low equivalent width, the complexity of the Ha line profile is reminiscent of CTTS. We present an estimate for the mass accretion rate of 10e-11 Msun/yr.

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arXiv2009

Probing the role of protostellar feedback in clustered star formation. Mapping outflows in the collapsing protocluster NGC 2264-C

The role played by protostellar feedback in clustered star formation is still a matter of debate. In particular, protostellar outflows have been proposed as a source of turbulence in cluster-forming clumps, which may provide support against global collapse for several free-fall times. Here, we seek to test the above hypothesis in the case of the well-documented NGC 2264-C protocluster, byquantifying the amount of turbulence and support injected in the surrounding medium by protostellar outflows. Using the HERA heterodyne array on the IRAM 30m telescope, we carried out an extensive mapping of NGC 2264-C in the three molecular line transitions 12CO(2-1), 13CO(2-1), and C18O(2-1). We found widespread high-velocity 12CO emission, testifying to the presence of eleven outflow lobes, closely linked to the compact millimeter continuum sources previously detected in the protocluster. We carried out a detailed analysis of the dynamical parameters of these outflows, including a quantitative evaluation of the overall momentum flux injected in the cluster-forming clump. These dynamical parameters were compared to the gravitational and turbulent properties of the clump. We show that the population of protostellar outflows identified in NGC 2264-C are likely to contribute a significant fraction of the observed turbulence but cannot efficiently support the protocluster against global collapse. Gravity appears to largely dominate the dynamics of the NGC 2264-C clump at the present time. It is however possible that an increase in the star formation rate during the further evolution of the protocluster will trigger sufficient outflows to finally halt the contraction of the cloud.

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arXiv2008

Revealing the fastest component of the DG Tau outflow through X-rays

Some T Tauri stars show a peculiar X-ray spectrum that can be modelled by two components with different absorbing column densities. We seek to explain the soft X-ray component in DG Tau, the best studied of these sources, with an outflow model, taking observations at other wavelengths into consideration. We constrain the outflow properties through spectral fitting and employ simple semi-analytical formulae to describe properties of a shock wave that heats up the X-ray emitting region. The X-ray emission is consistent with its arising from the fastest and innermost component of the optically detected outflow. Only a small fraction of the total mass loss is required for this X-ray emitting component. Our favoured model requires shock velocities between 400 and 500 km/s. For a density >10^5 /cm^3 all dimensions of the shock cooling zone are only a few AU, so even in optical observations this cannot be resolved. This X-ray emission mechanism in outflows may also operate in other, less absorbed T Tauri stars, in addition to corona and accretion spots.

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arXiv1999

Magneto-Centrifugal Launching of Jets from Accretion Disks. I: Cold Axisymmetric Flows

The magneto-centrifugal model for jet formation is studied by time-dependent simulations reaching steady state in a cold gas with negligible fluid pressure, in an axisymmetric geometry, using a modification of the Zeus3D code adapted to parallel computers. The number of boundary conditions imposed at the coronal base takes into account the existence of the fast and Alfvenic critical surfaces, avoiding over-determination of the flow. The size and shape of the computational box is chosen to include these critical surfaces, reducing the influence of the outer boundary conditions. As there is a region, near the origin, where the inclination of field lines to the axis is too small to drive a centrifugal wind, we inject a thin, axial jet, expected to form electromagnetically near black holes. Acceleration and collimation appear for wide generic conditions. A reference run is shown in detail, with a wind leaving the computational volume in the axial direction with a poloidal velocity equal to 4 times the poloidal Alfven speed, collimated inside 11 degrees. Finally, the critical surfaces, fieldlines, thrust, energy, torque and mass discharge of the outgoing wind are shown for simulations with various profiles of mass and magnetic flux at the base of the corona.

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arXiv1999

Gamma-Ray Burst Environments and Progenitors

Likely progenitors for the GRBs (gamma-ray bursts) are the mergers of compact objects or the explosions of massive stars. These two cases have distinctive environments for the GRB afterglow: the compact object explosions occur in the ISM (interstellar medium) and those of massive stars occur in the preburst stellar wind. We calculate the expected afterglow for a burst in a Wolf-Rayet star wind and compare the results to those for constant, interstellar density. The optical afterglow for the wind case is generally expected to decline more steeply than in the constant density case, but this effect may be masked by variations in electron spectral index, and the two cases have the same evolution in the cooling regime. Observations of the concurrent radio and optical/X-ray evolution are especially useful for distinguishing between the two cases. The different rates of decline of the optical and X-ray afterglows of GRB 990123 suggest constant density interaction for this case. We have previously found strong evidence for wind interaction in SN 1998bw/GRB 980425 and here present a wind model for GRB 980519. We thus suggest that there are both wind type GRB afterglows with massive star progenitors and ISM type afterglows with compact binary star progenitors. The wind type bursts are likely to be accompanied by a supernova, but not the ISM type.

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arXiv1999

Scale-free Equilibria of Magnetized Polytropic Clouds

We investigate the equilibrium properties of self-gravitating magnetized clouds with polytropic equations of state with negative index n. In particular, we consider scale-free isopedic configurations that have constant dimensionless spherical mass-to-flux ratio lambda_r and that may constitute ``pivotal'' states for subsequent dynamical collapse to form groups or clusters of stars. For given Gamma = 1 + 1/n, equilibria with smaller values of lambda_r are more flattened, ranging from spherical configurations with lambda_r = infinity to completely flattened states for lambda_r=1. For a given amount of support provided by the magnetic field as measured by the dimensionless parameter H_0, equilibria with smaller values of Gamma are more flattened. However, logatropic (defined by Gamma = 0) disks do not exist. The only possible scale-free isopedic equilibria with logatropic equation of state are spherical uniformly magnetized clouds.

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