arXiv subjects

H. Dong

Publications and source records attributed to H. Dong.

At least 19 recordsLinked to original sources

arXiv2025

A New cw-NMR Q-meter for Dynamically Polarized Targets for Particle Physics

Polarized solid targets produced via Dynamic Nuclear Polarization rely on Continuous-Wave Nuclear Magnetism Resonance measurements to accurately determine the degree of polarization of bulk samples polarized to nearly 100%. Since the late 1970's phase sensitive detection methods have been utilized to observe the magnetization of a sample as a small change in inductance under RF excitation near the Larmor frequency of the nuclear species of interest, using a device known as a Q-meter. Liverpool Q-meters, produced in the UK in the 80's and 90's, have been the workhorse devices for these targets for decades, however their age and scarcity has meant new systems are needed. We describe a Q-meter system designed and built at Jefferson Lab in the Liverpool style to have comparable electronic performance with several improvements to update and adapt the devices for modern use.

physics.ins-det↗

arXiv2023

Global well-posedness for the one-phase Muskat problem in 3D

This paper is concerned with the long time dynamics of the free boundary of a Darcy fluid in three space dimensions, also known as the one-phase Muskat problem. The dynamics of the free boundary is governed by a nonlocal fully nonlinear parabolic partial differential equation. It is proven that for any periodic Lipschitz graph given as initial data, the problem has a unique global-in-time solution which satisfies the equation in the strong sense. Moreover, all H\"older norms of the solution decay exponentially in time. These results have been previously established in two space dimensions. This paper addresses new challenges to extend the results to the more difficult three dimensional setting. The approach developed is critical in three space dimensions and crucially relies on Dahlberg-Kenig's $W^{1, 2+\varepsilon}$ optimal regularity for layer potentials together with delicate structures of the Dirichlet-to-Neumann operator and layer potentials in Lipschitz domains of $\mathbb{T}^2\times \mathbb{R}$.

math.AP↗

arXiv2021

Early formation and recent starburst activity in the nuclear disc of the Milky Way

The nuclear disc is a dense stellar structure at the centre of the Milky Way, with a radius of $\sim$150 pc. It has been a place of intense star formation in the past several tens of millions of years but its overall formation history has remained unknown up to now. Here we report the first detailed star formation history of this region. The bulk of its stars formed at least eight billion years ago. This initial activity was followed by a long period of quiescence that was ended by an outstanding event about 1 Gyr ago, during which roughly 5% of its mass formed in a time window $\sim$100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few percent of the stellar mass in the past $\sim$500 Myr, with an increased rate up to $\sim$30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the centre of the Milky Way. The long quiescent phase agrees with the overall quiescent history of the Milky Way and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life, and that the central black hole may have acquired most of its mass already in the early days of the Milky Way.

astro-ph.GA↗

arXiv2021

GALACTICNUCLEUS: A high angular resolution $JHK_s$ imaging survey of the Galactic Centre. I. Methodology, performance, and near-infrared extinction towards the Galactic Centre

The Galactic Centre is of fundamental astrophysical interest, but existing near-infrared surveys fall short to cover it adequately. We introduce the GALACTICNUCLEUS survey, a $JHK_s$ imaging survey of the centre of the Milky Way with a $0.2"$ angular resolution. We present the observations of Field 1 of our survey, centred approximately on SgrA* with an approximate size of $7.95'\times3.43'$. We describe the observational set-up and data reduction pipeline and discuss the quality of the data. Finally, we present some preliminary analysis of the data. The data were acquired with the near-infrared camera HAWK-I at the ESO VLT. Short readout times in combination with the speckle holography algorithm allowed us to produce final images with a stable, Gaussian PSF of $0.2"$ FWHM. Astrometric calibration is achieved via the VVV survey and photometric calibration is based on the SIRIUS/IRSF survey. The quality of the data is assessed by comparison between observations of the same field with different detectors of HAWK-I and at different times. We reach 5 sigma detection limits of approximately $J=22$, $H=21$, and $K_s=20$. The photometric uncertainties are less than 0.05 at $J<20$, $H<17$ and $K_s<16$. We distinguish five stellar populations in the colour-magnitude diagrams; three of them appear to belong to foreground spiral arms, and the other two correspond to a high- and a low-extinction star groups at the Galactic Centre. We use our data to analyse the near-infrared extinction curve and conclude that it can be described very well by a power-law with an index of $JHK_s=2.30\pm0.08$. We do not find any evidence that this index depends on the position along the line-of-sight, or on the absolute value of the extinction. We produce extinction maps that show the clumpiness of the ISM at the Galactic Centre. Finally, we estimate that the majority of the stars have solar or super-solar metallicity.

astro-ph.GA↗

arXiv2021

Star formation history and metallicity in the Galactic inner bulge revealed by the red giant branch bump

The study of the inner region of the Milky Way's bulge is hampered by high interstellar extinction and extreme source crowding. Sensitive high angular resolution near-infrared imaging is needed to study stellar populations in such a complex environment. We use the 0.2$''$ angular resolution $JHK_s$ data from the GALACTICNUCLEUS survey to study the stellar population within two $8.0'\times 3.4'$ fields, about 0.6$^\circ$ and 0.4$^\circ$ to the Galactic north of the Milky Way's centre and to compare it with one in the immediate surroundings of Sagittarius A*. We also characterise the extinction curve of the two fields. The average interstellar extinction to the outer and the inner field is $A_{K_s} \sim 1.20 \pm 0.08$ mag and $\sim 1.48 \pm 0.10$ mag, respectively. We present $K_{s}$ luminosity functions that are complete down to at least 2 mag below the red clump (RC). We detect a feature in the luminosity functions that is fainter than the RC by $0.80\pm0.03$ and $0.79\pm0.02$ mag, respectively, in the $K_s$-band. It runs parallel to the reddening vector. We identify the feature as the red giant branch bump. Fitting $α$-enhanced BaSTI luminosity functions to our data, we find that a single old stellar population of $\sim12.8 \pm 0.6$ Gyr and $Z = 0.040 \pm 0.003$ provides the best fit. We obtain that the stellar population in the innermost bulge is old, similar to the one at larger distances from the Galactic plane, and that its metallicity increases down to distances as short as about 60 pc from the centre of the Milky Way. Comparing it with previous known values at larger latitudes ($|b|>2^\circ$), our results favour a flattening of the gradient at $|b|<2^\circ$. As a secondary result we obtain that the extinction index in the studied regions agrees the value of $α= 2.30\pm0.08$, derived in Nogueras-Lara et al. 2018 for the very Galactic centre.

astro-ph.GA↗

arXiv2021

Radio observations of massive stars in the Galactic centre: The Arches Cluster

We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and at 6 and 10 GHz with the Karl G. Jansky Very Large Array (JVLA). The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary (CWB), and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function.

astro-ph.SR↗

arXiv2020

New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging

The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now. Aims. We aim to study the size and spatial structure of the MWNSC. Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use $0.2''$ angular resolution $K_{s}$ data to create a stellar density map in the central $86.4$pc x $21$pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use IRAC/Spitzer mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry. Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of $q=0.71 \pm0.10$, an effective radius of $R_{e}=(5.1 \pm1.0)$ pc, and a Sérsic index of $n=2.2 \pm0.7$. Its major axis may be tilted out of the Galactic plane by up to $-10$ degrees. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of $γ=1.38\pm0.06_{sys}\pm0.01_{stat}$. Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.

astro-ph.GA↗

arXiv2019

First results from a large-scale proper motion study of the Galactic Centre

Proper motion studies of stars in the centre of the Milky Way have been typically limited to the Arches and Quintuplet clusters and to the central parsec. Here, we present the first results of a large-scale proper motion study of stars within several tens of parsecs of Sagittarius A* based on our $0.2''$ angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with NICMOS/HST data from the Paschen-$α$ survey (epoch 2008). This study will be the first extensive proper motion study of the central $\sim 36' \times 16'$ of the Galaxy, which is not covered adequately by any of the existing astronomical surveys such as Gaia because of its extreme interstellar extinction ($A_{V} \gtrsim 30$ mag). Proper motions can help us to disentangle the different stellar populations along the line-of-sight and interpret their properties in combination with multi-wavelength photometry from GALACTICNUCLEUS and other sources. It also allows us to infer the dynamics and interrelationship between the different stellar components of the Galactic Centre (GC). In particular, we use proper motions to detect co-moving groups of stars which can trace low mass or partially dissolved young clusters in the GC that can hardly be discovered by any other means. Our pilot study in this work is on a field in the nuclear bulge associated by HII regions that show the presence of young stars. We detect the first group of co-moving stars coincident with an HII region. Using colour-magnitude diagrams, we infer that the co-moving stars are consistent with being the post-main sequence stars with ages of few Myrs. Simulations show that this group of stars is a real group that can indicate the existence of a dissolving or low to intermediate mass young cluster. A census of these undiscovered clusters will ultimately help us to constrain star formation at the GC in the past few ten Myrs.

astro-ph.GA↗

arXiv2018

An updated stellar census of the Quintuplet cluster

The Quintuplet is one of the most massive galactic clusters known, but appears to host a diverse stellar population that is difficult to reconcile with an instantaneous formation event. We present HST photometry and VLT spectroscopy in order to improve observational constraints, finding the Quintuplet to be far more homogeneous than previously thought. O7-8 Ia and O9-B0 Ia supergiants form a smooth morphological sequence with a cohort of seven early-B hypergiants and six luminous blue variables and WN9-11h stars, which together comprise the richest population of such stars of any known stellar aggregate. No further H-free Wolf-Rayets were found, resulting in a 13:1 ratio for WC/WN stars. However a small population of late-O hypergiants and WN8-9ha stars was identified, while a subset of the supergiants are unexpectedly faint, suggesting they are both less massive and older than the greater cluster population. We find an impressive coincidence between cluster members preceding the H-free WR phase and the evolutionary predictions for a 60Msun star, suggesting an age of ~3.0-3.6Myr for the Quintuplet. Neither the late-O hypergiants nor the low luminosity supergiants are predicted; we suggest that the former either result from rapid rotators or are the products of binary driven mass-stripping, while the latter may be interlopers. The H-free WRs must have had initial masses >60Msun, but it is difficult to reconcile their observational properties with theoretical expectations. Since the WRs represent an evolutionary phase directly preceding core-collapse, they are are crucial to understanding both this process and the nature of the resultant relativistic remnants, providing unique constraints on the evolution and death of the most massive stars forming in the local, high metallicity Universe (Abridged).

astro-ph.SR↗

arXiv2018

Mixed-norm $L_p$-estimates for non-stationary Stokes systems with singular VMO coefficients and applications

We prove the mixed-norm Sobolev estimates for solutions to both divergence and non-divergence form time-dependent Stokes systems with unbounded measurable coefficients having small mean oscillations with respect to the spatial variable in small cylinders. As a special case, our results imply Caccioppoli's type estimates for the Stokes systems with variable coefficients. A new $ε$-regularity criterion for Leray-Hopf weak solutions of Navier-Stokes equations is also obtained as a consequence of our regularity results, which in turn implies some borderline cases of the well-known Serrin's regularity criterion.

math.AP↗

arXiv2018

The Arches cluster revisited: I. Data presentation and stellar census

Located within the central region of the Galaxy, the Arches cluster appears to be one of the youngest, densest and most massive stellar aggregates within the Milky Way. As such it has the potential to be a uniquely instructive laboratory for the study of star formation in extreme environments and the physics of very massive stars. In order to determine the fundamental physical properties of both cluster and constituent stars, we provide and analyse new HST+VLT near-IR datasets. Stacking multiple epochs of spectroscopy results in the deepest view of the cluster ever obtained, allowing us to to identify candidate giant and main sequence stars for the first time. All cluster members are found to be WNLh or O stars, with the smooth and continuous progression in spectral morphologies from O super-/hypergiants through to the WNLh cohort implying a direct evolutionary connection. Importantly no H-free Wolf-Rayets are found, while no products of binary interaction/mass-transfer may be unambiguously identified, despite the presence of massive binaries within the Arches. We infer a main sequence turn-off around O4-5V, corresponding to ~30-38Msun, while the eclipsing binary F2 implies current masses of ~80Msun and ~60Msun for the WNLh and O hypergiant cohorts, respectively. A cluster age of ~2-3Myr is suggested by the location of the main-sequence turn-off. While the absence of H-free Wolf-Rayets argues against the prior occurrence of SNe, such an age does accommodate such events for exceptionally massive stars. Future progress requires quantitative analysis of cluster members combined with additional spectroscopic observations to better constrain the binary population; nevertheless it is already abundantly clear that the Arches offers an unprecedented insight into the formation, evolution and death of the most massive stars Nature allows to form in the local universe (Abridged).

astro-ph.SR↗

arXiv2017

The distribution of old stars around the Milky Way's central black hole I: Star counts

(abridged) In this paper we revisit the problem of inferring the innermost structure of the Milky Way's nuclear star cluster via star counts, to clarify whether it displays a core or a cusp around the central black hole. Through image stacking and improved PSF fitting we push the completeness limit about one magnitude deeper than in previous, comparable work. Contrary to previous work, we analyse the stellar density in well-defined magnitude ranges in order to be able to constrain stellar masses and ages. The RC and brighter giant stars display a core-like surface density profile within a projected radius R<0.3 pc of the central black hole, in agreement with previous studies, but show a cusp-like surface density distribution at larger R. The surface density of the fainter stars can be described well by a single power-law at R<2 pc. The cusp-like profile of the faint stars persists even if we take into account the possible contamination of stars in this brightness range by young pre-main sequence stars. The data are inconsistent with a core-profile for the faint stars.Finally, we show that a 3D Nuker law provides a very good description of the cluster structure. We conclude that the observed stellar density at the Galactic Centre, as it can be inferred with current instruments, is consistent with the existence of a stellar cusp around the Milky Way's central black hole, Sgr A*. This cusp is well developed inside the influence radius of about 3 pc of Sgr A* and can be described by a single three-dimensional power-law with an exponent gamma=1.23+-0.05. The apparent lack of RC stars and brighter giants at projected distances of R < 0.3 pc (R<8") of the massive black hole may indicate that some mechanism has altered their distribution or intrinsic luminosity.

astro-ph.GA↗

arXiv2017

The distribution of stars around the Milky Way's central black hole II: Diffuse light from sub-giants and dwarfs

This is the second of three papers that search for the predicted stellar cusp around the Milky Way's central black hole, Sagittarius A*, with new data and methods. We aim to infer the distribution of the faintest stellar population currently accessible through observations around Sagittarius A*. We use adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through optimised PSF fitting we remove the light from all detected stars above a given magnitude limit. Subsequently we analyse the remaining, diffuse light density. The analysed diffuse light arises from sub-giant and main-sequence stars with KS ~ 19 - 20 with masses of 1 - 2 Msol . These stars can be old enough to be dynamically relaxed. The observed power-law profile and its slope are consistent with the existence of a relaxed stellar cusp around the Milky Way's central black hole. We find that a Nuker law provides an adequate description of the nuclear cluster's intrinsic shape (assuming spherical symmetry). The 3D power-law slope near Sgr A* is γ= 1.23 +- 0.05. At a distance of 0.01 pc from the black hole, we estimate a stellar mass density of 2.3 +- 0.3 x 10^7 Msol pc^-3 and a total enclosed stellar mass of 180 +- 20 Msol. These estimates assume a constant mass-to-light ratio and do not take stellar remnants into account. The fact that no cusp is observed for bright (Ks 16) giant stars at projected distances of roughly 0.1-0.3 pc implies that some mechanism has altered their appearance or distribution.

astro-ph.GA↗

arXiv2017

Novel magnesium borides and their superconductivity

With the motivation of searching for new superconductors in the Mg-B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0-200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB$_3$ and Mg$_3$B$_{10}$. Experimentally known MgB$_2$ is stable in the entire pressure range 0-200 GPa, while MgB$_7$ and MgB$_{12}$ are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB$_4$, which becomes unstable against decomposition into MgB$_2$ and MgB$_7$ at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB$_2$-type structure. However, with the exception of MgB$_2$, all other magnesium borides have low electron-phonon coupling constants $λ$ of 0.32 to 0.39 and are predicted to have T$_c$ below 3 K.

cond-mat.supr-con↗

arXiv2016

Production of highly-polarized positrons using polarized electrons at MeV energies

The Polarized Electrons for Polarized Positrons experiment at the injector of the Continuous Electron Beam Accelerator Facility has demonstrated for the first time the efficient transfer of polarization from electrons to positrons produced by the polarized bremsstrahlung radiation induced by a polarized electron beam in a high-$Z$ target. Positron polarization up to 82\% have been measured for an initial electron beam momentum of 8.19~MeV/$c$, limited only by the electron beam polarization. This technique extends polarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positron beam physics to a wide community.

physics.acc-ph↗

arXiv2012

A Multiwavelength Study of Evolved Massive Stars in the Galactic Center

The central region of the Milky Way provides a unique laboratory for a systematic, spatially-resolved population study of evolved massive stars of various types in a relatively high metallicity environment. We have conducted a multi-wavelength data analysis of 180 such stars or candidates, most of which were drawn from a recent large-scale HST/NICMOS narrow-band Pa-a survey, plus additional 14 Wolf-Rayet stars identified in earlier ground-based spectroscopic observations of the same field. The multi-wavelength data include broad-band IR photometry measurements from HST/NICMOS, SIRIUS, 2MASS, Spitzer/IRAC, and Chandra X-ray observations. We correct for extinctions toward individual stars, improve the Pa-a line equivalent width measurements, quantify the substantial mid-IR dust emission associated with WC stars, and find X-ray counterparts. In the process, we identify 10 foreground sources, some of which may be nearby cataclysmic variables. The WN stars in the Arches and Central clusters show correlations between the Pa-a equivalent width and the adjacent continuum emission. However, the WN stars in the latter cluster are systematically dimmer than those in the Arches cluster, presumably due to the different ages of the two clusters. In the EW-magnitude plot, WNL stars, WC stars and OB supergiants roughly fall into three distinct regions. We estimate that the dust mass associated with individual WC stars in the Quintuplet cluster can reach 1e-5 M, or more than one order of magnitude larger than previous estimates. Thus WC stars could be a significant source of dust in the galaxies of the early universe. Nearly half of the evolved massive stars in the GC are located outside the three known massive stellar clusters. The ionization of several compact HII regions can be accounted for by their enclosed individual evolved massive stars, which thus likely formed in isolation or in small groups.

astro-ph.GA↗

arXiv2012

The Panchromatic Hubble Andromeda Treasury

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31's disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged)

astro-ph.CO↗

arXiv2012

A quantum solution to Gibbs Paradox with few particles

We present a fully quantum solution to the Gibbs paradox (GP) with an illustration based on a gedanken experiment with two particles trapped in an infinite potential well. The well is divided into two cells by a solid wall, which could be removed for mixing the particles. For the initial thermal state with correct two-particle wavefunction according to their quantum statistics, the exact calculations shows the entropy changes are the same for boson, fermion and non-identical particles. With the observation that the initial unmixed state of identical particles in the conventional presentations actually is not of a thermal equilibrium, our analysis reveals the quantum origin of the paradox, and confirm the E. J. Jaynes' observation that entropy increase in Gibbs mixing is only due to the including more observables measuring the entropy. To further show up the subtle role of the quantum mechanism in the GP, we study the different finite size effect on the entropy change and shows the works performed in the mixing process are different for various types of particle.

quant-ph↗