arXiv subjects

Rui Xue

Publications and source records attributed to Rui Xue.

97 records · Page 6Linked to original sources

arXiv2011

Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data

We have determined column densities of H I and/or H_2 for sight lines in the Magellanic Clouds from archival HST and FUSE spectra of H I Lyman-alpha and H_2 Lyman-band absorption. Together with some similar data from the literature, we now have absorption-based N(H I) and/or N(H_2) for 285 LMC and SMC sight lines (114 with a detection or limit for both species) -- enabling more extensive, direct, and accurate determinations of molecular fractions, gas-to-dust ratios, and elemental depletions in these two nearby, low-metallicity galaxies. For sight lines where the N(H I) estimated from 21 cm emission is significantly higher than the value derived from Lyman-alpha absorption (presumably due to emission from gas beyond the target stars), integration of the 21 cm profile only over the velocity range seen in Na I or H_2 absorption generally yields much better agreement. Conversely, N(21 cm) can be lower than N(Ly-alpha) by factors of 2--3 in some LMC sight lines -- suggestive of small-scale structure within the 21 cm beam(s) and/or some saturation in the emission. The mean gas-to-dust ratios obtained from N(H_tot)/E(B-V) are larger than in our Galaxy, by factors of 2.8--2.9 in the LMC and 4.1--5.2 in the SMC -- i.e., factors similar to the differences in metallicity. The N(H_2)/E(B-V) ratios are more similar in the three galaxies, but with considerable scatter within each galaxy. These data may be used to test models of the atomic-to-molecular transition at low metallicities and predictions of N(H_2) based on comparisons of 21 cm emission and the IR emission from dust.

astro-ph.GA↗

arXiv2011

CARMA Survey Toward Infrared-bright Nearby Galaxies (STING) II: Molecular Gas Star Formation Law and Depletion Time Across the Blue Sequence

We present an analysis of the relationship between molecular gas and current star formation rate surface density at sub-kpc and kpc scales in a sample of 14 nearby star-forming galaxies. Measuring the relationship in the bright, high molecular gas surface density ($\Shtwo\gtrsim$20 \msunpc) regions of the disks to minimize the contribution from diffuse extended emission, we find an approximately linear relation between molecular gas and star formation rate surface density, $\nmol\sim0.96\pm0.16$, with a molecular gas depletion time $\tdep\sim2.30\pm1.32$ Gyr. We show that, in the molecular regions of our galaxies there are no clear correlations between \tdep\ and the free-fall and effective Jeans dynamical times throughout the sample. We do not find strong trends in the power-law index of the spatially resolved molecular gas star formation law or the molecular gas depletion time across the range of galactic stellar masses sampled (\mstar $\sim$$10^{9.7}-10^{11.5}$ \msun). There is a trend, however, in global measurements that is particularly marked for low mass galaxies. We suggest this trend is probably due to the low surface brightness CO, and it is likely associated with changes in CO-to-H2 conversion factor.

astro-ph.CO↗

arXiv2011

CARMA Survey Toward Infrared-bright Nearby Galaxies (STING): Molecular Gas Star Formation Law in NGC4254

This study explores the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law. Using four star formation rate (SFR) tracers (Hαwith azimuthally averaged extinction correction, mid-infrared 24 micron, combined Hαand mid-infrared 24 micron, and combined far-ultraviolet and mid-infrared 24 micron), several fitting procedures, and different sampling strategies we probe the relation between SFR and molecular gas at various spatial resolutions and surface densities within the central 6.5 kpc in the disk of NGC4254. We find that in the high surface brightness regions of NGC4254 the form of the molecular gas star formation law is robustly determined and approximately linear and independent of the assumed fraction of diffuse emission and the SFR tracer employed. When the low surface brightness regions are included, the slope of the star formation law depends primarily on the assumed fraction of diffuse emission. In such case, results range from linear when the fraction of diffuse emission in the SFR tracer is ~30% or less (or when diffuse emission is removed in both the star formation and the molecular gas tracer), to super-linear when the diffuse fraction is ~50% and above. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 micron SFR tracer by itself shows the tightest correlation with the molecular gas surface density, whereas the Hαcorrected for extinction using an azimuthally-averaged correction shows the highest dispersion. We find that for R<0.5R_25 the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time ~2 Gyr.

astro-ph.CO↗

arXiv2009

Exact nonlinear Bloch-state solutions for Bose-Einstein condensates in a periodic array of quantum wells

A set of exact closed-form Bloch-state solutions to the stationary Gross-Pitaevskii equation are obtained for a Bose-Einstein condensate in a one-dimensional periodic array of quantum wells, i.e. a square-well periodic potential. We use these exact solutions to comprehensively study the Bloch band, the compressibility, effective mass and the speed of sound as functions of both the potential depth and interatomic interaction. According to our study, a periodic array of quantum wells is more analytically tractable than the sinusoidal potential and allows an easier experimental realization than the Krönig-Penney potential, therefore providing a useful theoretical model for understanding Bose-Einstein condensates in a periodic potential.

cond-mat.quant-gas↗

arXiv2008

High-Resolution Observations of Molecular Lines toward the Hot Core G28.20-0.04N

We present the results from arcsecond resolution observations of various line transitions at 1.3 mm toward hypercompact HII region G28.20-0.04N. With the SMA data, we have detected and mapped the transitions in the CH$_{3}$CN, CO, $^{13}$CO, SO$_{2}$, OCS, and CH$_{3}$OH molecular lines as well as the radio recombination line H30$α$. The observations and analysis indicate a hot core associated with G28.20-0.04N. The outflow and possible rotation are detected in this region.

astro-ph↗

arXiv2008

A Multi-wavelength Study of the Massive Star-forming Region S87

This article presents a multi-wavelength study towards S87, based on a dataset of submillimeter/far-/mid-infrared (sub-mm/FIR/MIR) images and molecular line maps. The sub-mm continuum emission measured with JCMT/SCUBA reveals three individual clumps, namely, SMM1, SMM2, and SMM3. The MIR/FIR images obtained by the Spitzer Space Telescope indicate that both SMM1 and SMM3 harbor point sources. The J=1-0 transitions of CO, 13CO, C18O, and HCO+, measured with the 13.7m telescope of the Purple Mountain Observatory, exhibit asymmetric line profiles. Our analysis of spectral energy distributions (SEDs) shows that all of the three sub-mm clumps are massive (110--210 $M_{\odot}$), with average dust temperatures in the range ~20--40K. A multi-wavelength comparison convinces us that the asymmetric profiles of molecular lines should result from two clouds at slightly different velocities, and it further confirms that the star-forming activity in SMM1 is stimulated by a cloud-cloud collision. The stellar contents and SEDs suggest that SMM1 and SMM3 are high-mass and intermediate-mass star-forming sites respectively. However, SMM2 has no counterpart downwards 70 micron, which is likely to be a cold high-mass starless core. These results, as mentioned above, expose multiple phases of star formation in S87.

astro-ph↗

arXiv2007

Signatures of inflow motion in cores of massive star formation: Potential collapse candidates

Using the IRAM 30 m telescope, a mapping survey in optically thick and thin lines was performed towards 46 high mass star-forming regions. The sample includes UC H{\sc ii} precursors and UC H{\sc ii} regions. Seventeen sources are found to show "blue profiles", the expected signature of collapsing cores. The excess of sources with blue over red profiles ([$N_{\rm blue}$ -- $N_{\rm red}$]/$N_{\rm total}$) is 29% in the HCO$^+$ $J$=1--0 line, with a probability of 0.6% that this is caused by random fluctuations. UC H{\sc ii} regions show a higher excess (58%) than UC H{\sc ii} precursors (17%), indicating that material is still accreted after the onset of the UC H{\sc ii} phase. Similar differences in the excess of blue profiles as a function of evolutionary state are not observed in low mass star-forming regions. Thus, if confirmed for high mass star-forming sites, this would point at a fundamental difference between low- and high-mass star formation. Possible explanations are inadequate thermalization, stronger influence of outflows in massive early cores, larger gas reserves around massive stellar objects or different trigger mechanisms between low- and high- mass star formation.

astro-ph↗