arXiv subjects

Zu-Jia Lu

Publications and source records attributed to Zu-Jia Lu.

13 recordsLinked to original sources

arXiv2026

Beyond Idealized PAHs: Infrared Signatures of Carbon-Chain Defects from Shock Synthesis

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized as carriers of the aromatic infrared bands (AIBs). However, most spectral models rely on idealized structures that fail to capture the energetic environments of interstellar PAH formation. This work investigates the infrared (IR) signatures of PAHs formed under shock conditions and explores whether produced defective structures can explain observational features unpredicted by standard, idealized models. We combine two-stage reactive molecular dynamics simulations of PAH formation via condensation and shock processing with density functional theory spectral calculations, and compare our theoretical results with James Webb Space Telescope (JWST) observations of NGC 7023 and MRK 1066. Shock processing produces PAHs featuring fullerene-like carbon skeletons and linear carbon-chain attachments. These structural defects yield distinct IR signatures, including prominent carbon-chain stretching features at 4.6-5.5 micron that is absent in ideal PAHs, and significantly enhanced out-of-plane skeletal modes in the 14.5-20 micron regime. Our findings attribute the observed 5.2 micron band in NGC 7023 and MRK 1066 to carbon-chain vibrations and the 15-18 micron emission to curved skeletal modes, providing observational support for the prevalence of defective, shock-formed PAHs in the interstellar medium.

astro-ph.GA↗

arXiv2026

Tracing expansion-driven star formation in the Perseus molecular cloud using young stellar objects

$Context$. Recent studies have linked star formation in the solar neighbourhood to the expansion of the Local Bubble. The Per-Tau Shell (PTS), a vast 3D dust structure hosting the Perseus and Taurus clouds on its surface, is a candidate for such triggered formation. $Aims$. We aim to reconstruct the star formation history of the Perseus molecular cloud and investigate its potential link to the historical expansion of the PTS. $Methods$. We performed a 6D phase-space analysis of 406 young stellar objects (YSOs) of Perseus. Our approach integrates Gaia DR3 kinematics with Bayesian age inference and a heteroscedastic likelihood framework to resolve radial gradients in YSO ages and intrinsic tangential velocity dispersions relative to the PTS centre. We constructed a unified catalogue by integrating existing infrared-based classifications to examine the projected spatial segregation of different evolutionary stages and compared stellar 3D kinematics with $^{12}$CO and $^{13}$CO gas velocity fields to assess kinematic coherence. $Results$. The YSOs exhibit coherent outward motions relative to the centre of the PTS, a negative age-distance gradient with younger populations at larger radii, and a radial decline in tangential velocity dispersions. Younger YSO classes also show a stronger directional concentration towards the PTS centre. Both the coherent proper motions and the line-of-sight velocity gradients of these YSOs align with the gas velocity field, indicating large-scale kinematic coherence. $Conclusions$. The Perseus YSOs exhibit concordant kinematic and temporal signatures consistent with an expansion-driven origin. These results provide robust evidence for a layered, inside-out pattern of star formation, suggesting that the historical expansion of the PTS triggered a sequential progression of star birth and effectively shaped the star formation history of the Perseus complex.

astro-ph.SR↗

arXiv2026

Fragmentation of massive clumps: constant mass fraction and growing mass inequality of cores

The fragmentation of massive molecular clumps into dense cores sets the initial conditions for stellar clusters, yet how core mass distributions evolve remains unclear. Using the ALMAGAL survey (1,013 clumps and 6,348 cores), we analyze the Gini coefficient of core mass distributions for the 514 clumps containing at least four cores ($N_{\rm frag} \ge 4$), which host 5,728 cores in total. We find that the most massive core consistently accounts for $\sim 38\%$ of the total core mass ($f_{\rm core,max} \approx 0.38$), with no systematic trend with the total mass of the core per clump $M_{\rm core,tot}$. The Gini coefficient increases with total core mass ($M_{\rm core,tot}$, $r=0.55$) and evolutionary stage ($L/M$, $r=0.38$). Clumps with $L/M > 6.0$ have significantly higher Gini values (median $0.62$) than those with $L/M \le 6.0$ (median $0.49$; $p = 4.23\times10^{-15}$). These results demonstrate that while the mass fraction of the most massive core is constant, overall mass inequality among cores grows systematically as clumps evolve.

astro-ph.GA↗

arXiv2026

From filaments to clumps: filament properties with synthetic Herschel observations

Systematic surveys of filaments have been conducted to study their properties and their relationship to the process of star formation. In this paper, we use synthetic Herschel observations derived from 3D numerical simulations to compute column density maps, then use the \texttt{FILFINDER} algorithm to identify filaments. We obtain a large sample of 8,832 filaments that we further decompose into 110,193 branches. We characterize the physical properties of these filamentary structures and explore their correlations with embedded clumps. Furthermore, we directly compare our synthetic results with an observational catalogue of 32,059 filaments from the Herschel Infrared Galactic Plane Survey (Hi-GAL). Our results show that filaments are central to the star formation process, hosting $94\%$ of clumps from synthetic observations and $93\%$ of stars from our 3D numerical simulation. Filaments that host clumps have higher median column densities ($1.1\times10^{21}\,\rm{cm}^{-2}$) than those without ($3.8\times10^{20}\,\rm{cm}^{-2}$). We find power-law distributions for our synthetic filament masses and lengths, with power-law indexes of $α_{\rm M}=-0.86$ and $α_{\rm L} = -1.71$, respectively. We also find that the relation between the density of filaments and the background density is $N_{\rm{fs}} \propto N_{\rm{bs}}^{0.78}$. The measured properties of the filaments from the 2D synthetic observations are qualitatively consistent with those of the filaments from the Hi-GAL survey.

astro-ph.GA↗

arXiv2026

B-field Orion Protostellar Survey (BOPS). IV: The Relative Orientation Between Magnetic Fields and Density Structures in Young Protostellar Envelopes

We investigate the relative alignment between density structures and magnetic fields in eight young protostars from the ALMA B-field Orion Protostellar Survey. Column density maps are derived from 870 $μ$m dust continuum emission, and the Histogram of Relative Orientations (HRO) method is applied to quantify the correlation between magnetic field orientations and density structures on envelope scales ($\sim$10$^{3}$~au). We find that the relative alignment shows overall weak evidence of systematic evolution with column density, suggesting that column density alone does not fully determine the alignment. The magnetization level also plays a crucial role, with weakly magnetized envelopes exhibiting predominantly parallel or random alignment, whereas strongly magnetized ones show perpendicular configurations even at moderate densities. These results reveal that density and magnetization jointly shape the morphology of protostellar envelopes and the coupling between gravity and magnetic fields during early stages of star formation.

astro-ph.GA↗

arXiv2024

Kinematics of Young Stellar Objects Under Various Stellar Feedback

Based on the Gaia Data Release 3 and APOGEE datasets, we investigate the kinematic differences between young stellar objects (YSOs) and their parent clouds in five nearby star-forming regions. Overall, the 1D velocity differences between Class II YSOs and their parent molecular cloud range from [0, 1.4] km/s. In feedback environments dominated by outflows, massive stars, and supernova feedback, the corresponding velocity differences range from [0, 1.4] km/s, [0.1, 0.4] km/s, and [0.1, 1] km/s, respectively. These results indicate that YSO kinematics are not significantly affected by these different types of feedback environment. Additionally, compared to the Class II YSOs, Class III YSOs have slightly larger velocities and dispersions.

astro-ph.GA↗

arXiv2024

Updated kinematics of the Radcliffe Wave: non-synchronous, dipole-like vertical oscillations

The kinematic information of the Radcliffe Wave (RW) is essential for determining its existence and gaining insights into its origin and evolution. In this work, we present an accurate measurement of the vertical velocity ($V_Z$) of RW by incorporating the radial velocity (RV) measures through two methods, which is crucial but was neglected previously. First, the velocities are measured towards young stars, using their RV measurements from APOGEE-2 and proper motion measurements from Gaia DR3. Second, we combine RV measurements toward clouds with proper motion measurements of associated Young Stellar Objects (YSOs) to determine the vertical velocities of the clouds. The results reveal that the oscillations in $V_Z$ are not synchronous with the vertical coordinate $Z$, which differs from the conclusions of previous studies. Instead, we find a 5 km$\cdot$s$^{-1}$$\cdot$kpc$^{-1}$ gradient in $V_Z$ along the RW, exhibiting a dipole-like pattern. Consequently, the kinematic arrangement does not show a corresponding coherence with the spatial arrangement, bringing the Radcliffe Wave model into question.

astro-ph.GA↗

arXiv2021

The Dynamical State of Massive Clumps

The dynamical state of massive clumps is key to our understanding of the formation of massive stars. In this work, we study the kinematic properties of massive clumps using synthetic observations. We have previously compiled a very large catalog of synthetic dust-continuum compact sources from our 250 pc, SN-driven, star formation simulation. Here, we compute synthetic $\rm N_{2}H^{+}$ line profiles for a subsample of those sources and compare their properties with the observations and with those of the corresponding three-dimensional (3D) clumps in the simulation. We find that the velocity dispersion of the sources estimated from the $\rm N_{2}H^{+}$ line is a good estimate of that of the 3D clumps, although its correlation with the source size is weaker than the velocity-size correlation of the 3D clumps. The relation between the mass of the 3D clumps, $M_{\rm main}$, and that of the corresponding synthetic sources, $M_{\rm SED}$, has a large scatter and a slope of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to uncertainties arising from the observational band-merging procedure and from projection effects along the line of sight. As a result, the virial parameters of the 3D clumps are not correlated with the clump masses, even if a negative correlation is found for the compact sources, and the virial parameter of the most massive sources may significantly underestimate that of the associated clumps.

astro-ph.GA↗

arXiv2021

Physical Properties and Real Nature of Massive Clumps in the Galaxy

Systematic surveys of massive clumps have been carried out to study the conditions leading to the formation of massive stars. These clumps are typically at large distances and unresolved, so their physical properties cannot be reliably derived from the observations alone. Numerical simulations are needed to interpret the observations. To this end, we generate synthetic Herschel observations using our large-scale star-formation simulation, where massive stars explode as supernovae driving the interstellar-medium turbulence. From the synthetic observations, we compile a catalog of compact sources following the exact same procedure as for the Hi-GAL compact source catalog. We show that the sources from the simulation have observational properties with statistical distributions consistent with the observations. By relating the compact sources from the synthetic observations to their three-dimensional counterparts in the simulation, we find that the synthetic observations overestimate the clump masses by about an order of magnitude on average due to line-of-sight projection, and projection effects are likely to be even worse for Hi-GAL Inner Galaxy sources. We also find that a large fraction of sources classified as protostellar are likely to be starless, and propose a new method to partially discriminate between true and false protostellar sources.

astro-ph.GA↗

arXiv2020

The Effect of Supernovae on the Turbulence and Dispersal of Molecular Clouds

While the importance of supernova feedback in galaxies is well established, its role on the scale of molecular clouds is still debated. In this work, we focus on the impact of supernovae on individual clouds, using a high-resolution magneto-hydrodynamic simulation of a region of 250 pc where we resolve the formation of individual massive stars. The supernova feedback is implemented with real supernovae that are the natural evolution of the resolved massive stars, so their position and timing are self-consistent. We select a large sample of molecular clouds from the simulation to investigate the supernova energy injection and the resulting properties of molecular clouds. We find that molecular clouds have a lifetime of a few dynamical times, less then half of them contract to the point of becoming gravitationally bound, and the dispersal time of bound clouds, of order one dynamical time, is a factor of two shorter than that of unbound clouds. We stress the importance of internal supernovae, that is massive stars that explode inside their parent cloud, in setting the cloud dispersal time, and their huge overdensity compared to models where the supernovae are randomly distributed. We also quantify the energy injection efficiency of supernovae as a function of supernova distance to the clouds. We conclude that intermittent driving by supernovae can maintain molecular-cloud turbulence and may be the main process of cloud dispersal. The role of supernovae in the evolution of molecular clouds cannot be fully accounted for without a self-consistent implementation of their feedback.

astro-ph.GA↗

arXiv2016

Variabilities of Gamma-ray Bursts from Black Hole Hyper-accretion Disks

The emission from black hole binaries (BHBs) and active galactic nuclei (AGNs) displays significant aperiodic variabilities. The most promising explanation for these variabilities is the propagating fluctuations in the accretion flow. It is natural to expect that the mechanism driving variabilities in BHBs and AGNs may operate in a black hole hyper-accretion disk, which is believed to power gamma-ray bursts (GRBs). We study the variabilities of jet power in GRBs based on the model of propagating fluctuations. It is found that the variabilities of jet power and the temporal profile of erratic spikes in this scenario are similar to those in observed light curves of prompt gamma-ray emission of GRBs. Our results show that the mechanism driving X-ray variabilities in BHBs and AGNs may operate in the central engine to drive the variabilities of GRBs.

astro-ph.HE↗

arXiv2016

On the Late-Time Spectral Softening Found in X-ray Afterglows of Gamma-Ray Bursts

Strong spectral softening has been revealed in the late X-ray afterglows of some gamma-ray bursts (GRBs). The scenario of X-ray scattering around circum-burst dusty medium has been supported by previous works due to its overall successful prediction of both the temporal and spectral evolution of some X-ray afterglows. To further investigate the observed feature of spectral softening, we now systematically search the X-ray afterglows detected by X-Ray Telescope (XRT) of Swift and collect twelve GRBs with significant late-time spectral softening. We find that dust scattering could be the dominant radiative mechanism for these X-ray afterglows regarding their temporal and spectral features. For some well observed bursts with high-quality data, their time-resolved spectra could be well produced within the scattering scenario by taking into account the X-ray absorption from circum-burst medium. We also find that during spectral softening the power-law index in the high energy end of the spectra does not vary much. The spectral softening is mainly manifested by the spectral peak energy continually moving to the soft end.

astro-ph.HE↗

arXiv2015

Correlations of Disk and Jet Emission Deviating from the Fundamental Plane

The variability of accretion rate, which is believed to induce the aperiodic variability of X-ray emission from disk, may affect the energy injection into the jet. In this spirit, a correlation between disk emission and jet emission can be formed even if the mean luminosity of disk emission keeps constant. In this work, these correlations are found in the situation that the luminosity of disk emission is variable and kept with a constant mean value. The obtained correlations may be shallower than that of the fundamental plane of black hole activity. In addition, the slope of correlation may increase with increasing observed frequency of jet emission. For the luminosities spacing with three days, the slope of correlation decreases with increasing black hole mass. The deviation of our found correlations from that of the fundamental plane is related to the suppression of variability in the jet emission in comparison with that in the disk emission. This mechanism may work in some sources in which shallower correlations have been reported. Moreover, it implies that luminosities used to estimate the relation of fundamental plane should cover an appropriate timescale, in which the variability of jet emission is not significantly suppressed.

astro-ph.HE↗