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X. D. Tang

Publications and source records attributed to X. D. Tang.

At least 37 records · Page 2Linked to original sources

Searching for further evidence for cloud-cloud collisions in L1188

In order to search for further observational evidence of cloud-cloud collisions in one of the promising candidates, L1188, we carried out observations of multiple molecular lines toward the intersection region of the two nearly orthogonal filamentary molecular clouds in L1188. Based on these observations, we find two parallel filamentary structures, both of which have at least two velocity components being connected with broad bridging features. We also found a spatially complementary distribution between the two molecular clouds, as well as enhanced $^{13}$CO emission and $^{12}$CO self-absorption toward their abutting regions. At the most blueshifted velocities, we unveil a 1~pc-long arc ubiquitously showing $^{12}$CO line wings. We discover two 22 GHz water masers, which are the first maser detections in L1188. An analysis of line ratios at a linear resolution of 0.2 pc suggests that L1188 is characterised by kinetic temperatures of 13--23~K and H$_{2}$ number densities of 10$^{3}$--10$^{3.6}$ cm$^{-3}$. On the basis of previous theoretical predictions and simulations, we suggest that these observational features can be naturally explained by the scenario of a cloud-cloud collision in L1188, although an additional contribution of stellar feedback from low-mass young stellar objects cannot be ruled out.

astro-ph.GA↗

Constraining the $^{12}$C+$^{12}$C astrophysical S-factors with the $^{12}$C+$^{13}$C measurements at very low energies

We use an underground counting lab with an extremely low background to perform an activity measurement for the $^{12}$C+$^{13}$C system with energies down to $E\rm_{c.m.}$=2.323 MeV, at which the $^{12}$C($^{13}$C,$p$)$^{24}$Na cross section is found to be 0.22(7) nb. The $^{12}$C+$^{13}$C fusion cross section is derived with a statistical model calibrated using experimental data. Our new result of the $^{12}$C+$^{13}$C fusion cross section is the first decisive evidence in the carbon isotope systems which rules out the existence of the astrophysical S-factor maximum predicted by the phenomenological hindrance model, while confirming the rising trend of the S-factor towards lower energies predicted by other models, such as CC-M3Y+Rep, DC-TDHF, KNS, SPP and ESW. After normalizing the model predictions with our data, a more reliable upper limit is established for the $^{12}$C+$^{12}$C fusion cross sections at stellar energies.

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ALMA view of the $^{12}$C/$^{13}$C isotopic ratio in starburst galaxies

We derive molecular-gas-phase $^{12}$C/$^{13}$C isotope ratios for the central few 100 pc of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of the $λ$ $\sim$ 3 mm $^{12}$CN and $^{13}$CN $N$ = 1--0 lines in the ALMA Band 3. The $^{12}$C/$^{13}$C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 $\pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $\pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $\pm$ 0.3 in NGC 4945. The highest $^{12}$C/$^{13}$C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. In case of NGC 1068, the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar. Low $^{12}$C/$^{13}$C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material.

astro-ph.GA↗

A systematic TMRT observational study of Galactic $^{12}$C/$^{13}$C ratios from Formaldehyde

We present observations of the C-band $1_{10}-1_{11}$ (4.8 GHz) and Ku-band $2_{11}-2_{12}$ (14.5 GHz) K-doublet lines of H$_2$CO and the C-band $1_{10}-1_{11}$ (4.6 GHz) line of H$_2$$^{13}$CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT). Our sample with 112 sources includes strong H$_2$CO sources from the TMRT molecular line survey at C-band and other known H$_2$CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34\%. Complementary observations of their continuum emission at both C- and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H$_2$$^{12}$CO/H$_2$$^{13}$CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue's rotational mm-wave lines connecting our measured K-doublets, and to obtain $^{12}$C/$^{13}$C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from \citet{Wiesenfeld2013}. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of $^{12}$C/$^{13}$C = (5.08$\pm$1.10)D$_{GC}$ + (11.86$\pm$6.60), with a correlation coefficient of 0.58. D$_{GC}$ refers to Galactocentric distances. Our $^{12}$C/$^{13}$C ratios agree very well with the ones deduced from CN and C$^{18}$O but are lower than those previously reported on the basis of H$_2$CO, tending to suggest that the bulk of the H$_2$CO in our sources was formed on dust grain mantles and not in the gas phase.

astro-ph.GA↗

Mass Measurements of Neutron-Deficient Y, Zr, and Nb Isotopes and Their Impact on $rp$ and $νp$ Nucleosynthesis Processes

Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $α$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $νp$-process simulations.

nucl-ex↗

Molecular line emission in NGC 4945, imaged with ALMA

NGC 4945 is one of the nearest (~3.8 Mpc; 1" ~ 19 pc) starburst galaxies. ALMA band 3 (3--4\,mm) observations of HCN, HCO+, CS, C3H2, SiO, HCO, and CH3C2H were carried out with ~2" resolution. The lines reveal a rotating nuclear disk of projected size 10" x 2" with position angle ~45 deg, inclination ~75 deg and an unresolved bright central core of size <2.5". The continuum source (mostly free-free radiation) is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0.39" +_ 0.14" northeast of the nuclear accretion disk defined by H2O maser emission. Outside the nuclear disk, both HCN and CS delineate molecular arms on opposite sides of the dynamical center. These are connected by a (deprojected) 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside <100 pc) with tilted rings indicates a coplanar outflow reaching a characteristic deprojectd velocity of ~50 km/s. All our molecular lines, with the notable exception of CH3C2H, show significant absorption near the systemic velocity (~571 km/s), within a range of ~500-660 km/s. Apparently, only molecular transitions with low critical H2-density do not show absorption. The velocity field of the nuclear disk, derived from CH3C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M = (2.1+_0.2) x 10^8 Mo inside a galactocentric radius of 2.45", with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to 1.5" southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk.

astro-ph.GA↗

ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde

We aim to directly determine the kinetic temperature and spatial density with formaldehyde for the $\sim$100 brightest ATLASGAL-selected clumps at 870 $μ$m representing various evolutionary stages of high-mass star formation. Ten transitions ($J$ = 3-2 and 4-3) of ortho- and para-H$_2$CO near 211, 218, 225, and 291 GHz were observed with the APEX 12 m telescope. Using non-LTE models with RADEX, we derive the gas kinetic temperature and spatial density using the measured p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$, 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$, and 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ ratios. The gas kinetic temperatures derived from the p-H$_2$CO 3$_{21}$-2$_{20}$/3$_{03}$-2$_{02}$ and 4$_{22}$-3$_{21}$/4$_{04}$-3$_{03}$ line ratios are high, ranging from 43 to $>$300 K with an unweighted average of 91 $\pm$ 4 K. Deduced $T_{\rm kin}$ values from the $J$ = 3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the p-H$_2$CO 4$_{04}$-3$_{03}$/3$_{03}$-2$_{02}$ line ratios yield 0.6-8.3 $\times$ 10$^6$ cm$^{-3}$ with an unweighted average of 1.5 ($\pm$0.1) $\times$ 10$^6$ cm$^{-3}$. A comparison of kinetic temperatures derived from p-H$_2$CO, NH$_3$, and the dust emission indicates that p-H$_2$CO traces a distinctly higher temperature than the NH$_3$ (2,2)/(1,1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H$_2$CO linewidths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H$_2$CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H$_2$CO increase with time through the evolution of the clumps.

astro-ph.GA↗

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde. III. The Orion Molecular Cloud 1

We mapped the kinetic temperature structure of the Orion molecular cloud 1 with para-H2CO(303-202, 322-221, and 321-220) using the APEX 12m telescope. This is compared with the temperatures derived from the ratio of the NH3(2,2)/(1,1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H2CO 322-221/303-202 and 321-220/303-202. The gas kinetic temperatures derived from the para-H2CO line ratios are warm, ranging from 30 to >200 K with an average of 62 K at a spatial density of 10$^5$ cm$^{-3}$. These temperatures are higher than those obtained from NH3(2,2)/(1,1) and CH3CCH(6-5) in the OMC-1 region. The gas kinetic temperatures derived from para-H2CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H2CO(3-2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH3(2,2)/(1,1) and the CH3CCH(6-5) line series. With dust at MIR wavelengths and para-H2CO(3-2) on one side and dust at FIR wavelengths, NH3(2,2)/(1,1), and CH3CCH(6-5) on the other, dust and gas temperatures appear to be equivalent in the dense gas of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H2CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number >2.5) of the OMC-1, implying that the higher temperature traced by para-H2CO is related to turbulence on a 0.06 pc scale. Combining the temperature measurements with para-H2CO and NH3(2,2)/(1,1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s$^{-1}$ filament heated by radiation from the central Orion nebula.

astro-ph.GA↗

Kinetic temperature of massive star forming molecular clumps measured with formaldehyde. II. The Large Magellanic Cloud

The Large Magellanic Cloud (LMC), the closest star forming galaxy with low metallicity, provides an ideal laboratory to study star formation in such an environment. The classical dense molecular gas thermometer NH3 is rarely available in a low metallicity environment because of photoionization and a lack of nitrogen atoms. Our goal is to directly measure the gas kinetic temperature with formaldehyde toward six star-forming regions in the LMC. Three rotational transitions of para-H2CO near 218 GHz were observed with the APEX 12m telescope toward six star forming regions in the LMC. Those data are complemented by C18O 2-1 spectra. Using non-LTE modeling with RADEX, we derive the gas kinetic temperature and spatial density, using as constraints the measured para-H2CO 321-220/303-202 and para-H2CO 303-202/C18O 2-1 ratios. Excluding the quiescent cloud N159S, where only one para-H2CO line could be detected, the gas kinetic temperatures derived from the preferred para-H2CO 321-220/303-202 line ratios range from 35 to 63 K with an average of 47 K. Spatial densities of the gas derived from the paraH2CO 303-202/C18O 2-1 line ratios yield 0.4-2.9x10^5 cm^-3 with an average of 1.5x10^5 cm^-3. Temperatures derived from the para-H2CO line ratio are similar to those obtained with the same method from Galactic star forming regions and agree with results derived from CO in the dense regions of the LMC. A comparison of kinetic temperatures derived from para-H2CO with those from the dust also shows good agreement. This suggests that the dust and para-H2CO are well mixed in the studied star forming regions. A correlation between the gas kinetic temperatures derived from para-H2CO and infrared luminosity, represented by the 250um flux, suggests that the kinetic temperatures traced by para-H2CO are correlated with the ongoing massive star formation in the LMC.

astro-ph.GA↗

Kinetic temperature of massive star forming molecular clumps measured with formaldehyde

For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are strongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature.We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps.Three 218 GHz transitions (JKAKC=303-202, 322-221, and 321-220) of para-H2CO were observed with the 15m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H2CO 322-221/303-202and 321-220/303-202 ratios. The gas kinetic temperatures derived from the para-H2CO (321-220/303-202) line ratios range from 30 to 61 K with an average of 46 K. A comparison of kinetic temperature derived from para-H2CO, NH3, and the dust emission indicates that in many cases para-H2CO traces a similar kinetic temperature to the NH3 (2,2)/(1,1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H2CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H2CO trace turbulence to a higher degree than NH3 (2,2)/(1,1) in the massive clumps. The non-thermal velocity dispersions of para-H2CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.

astro-ph.GA↗

The first direct measurement of 12C(12C,n)23Mg at stellar energies

Neutrons produced by the carbon fusion reaction 12C(12C,n)23Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction 12C(12C,p)23Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that 12C(12C,n)23Mg is crucial to the production of Na and Al in Pop-III Pair Instability Supernovae. It also plays a non-negligible role in the production of weak s-process elements as well as in the production of the important galactic gamma-ray emitter 60Fe.

nucl-ex↗

Measurement of the $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb reactions for the nucleosynthesis of elements around A=90

Cross section measurements of the reactions $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb were performed using the NSCL SuN detector at the University of Notre Dame. These reactions are part of the nuclear reaction flow for the synthesis of the light p nuclei. For the $^{90}$Zr(p,$γ$)$^{91}$Nb reaction the new measurement resolves the disagreement between previous results. For the $^{92}$Zr(p,$γ$)$^{93}$Nb reaction the present work reports the first measurement of this reaction cross section. Both reaction cross sections are compared to theoretical calculations and a very good agreement with the standard NON-SMOKER model is observed.

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Systematic study of (p,γ) reactions on Ni isotopes

A systematic study of the radiative proton capture reaction for all stable nickel isotopes is presented. The results were obtained using 2.0 - 6.0 MeV protons from the 11 MV tandem Van de Graaff accelerator at the University of Notre Dame. The γ-rays were detected by the NSCL SuN detector utilising the γ-summing technique. The results are compared to a compilation of earlier measurements and discrepancies between the previous data are resolved. The experimental results are also compared to the theoretical predictions obtained using the NON-SMOKER and SMARAGD codes. Based on these comparisons an improved set of astrophysical reaction rates is proposed for the (p,γ) reactions on the stable nickel isotopes as well as for the 56Ni(p,γ)57Cu reaction.

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Suppression of the centrifugal barrier effects in the off-energy-shell neutron+$^{17}$O interaction

The reaction $^{17}$O($n,α$)$^{14}$C was studied at energies from $E_{cm}=0$ to $E_{cm}=350$ keV using the quasi-free deuteron break-up in the three body reaction $^{17}$O$+d \rightarrow α+ ^{14}$C$+p$, extending the Trojan Horse indirect method (THM) to neutron-induced reactions. It is found that the $^{18}$O excited state at $E^*=8.125 \pm 0.002$ MeV observed in THM experiments is absent in the direct measurement because of its high centrifugal barrier. The angular distributions of the populated resonances have been measured for the first time. The results unambiguously indicate the ability of the THM to overcome the centrifugal barrier suppression effect and to pick out the contribution of the bare nuclear interaction.

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Compensation effect and magnetostriction in CoCr2-xFexO4

The magnetic compensation and magnetostriction properties in Fe doped CoCr2O4 samples have been investigated. Structural and magnetic measurements imply that the doped Fe3+ ions initially occupy the B1(Cr) sites when x<0.1, and then mainly take the A(Co) sites. This behaviour results in a role conversion of magnetic contributors and a composition compensation between two competitively magnetic sublattices at x=0.1. Temperature dependence compensation has also been found in the samples with x=0.1~0.22, with the compensation temperature in the range of 40~104 K. The Fe3+ doping also modulates the exchange interaction of the system and prevents the formation of long range conical order of spins. The magnetoelectric transition temperature at 23K in CoCr2O4 is shifted to lower temperature by increasing the dopants. The magnetostriction effect in this system has been observed for the first time. The strain has a maximum value of about 280ppm at x=0.4. The magnetostriction is in consistent with the behaviour of the two magnetic compensations.

cond-mat.mtrl-sci↗

The infrared dust bubble N22: an expanding HII region and the star formation around it

Aims. To increase the observational samples of star formation around expanding Hii regions, we analyzed the interstellar medium and star formation around N22. Methods. We used data extracted from the seven large-scale surveys from infrared to radio wavelengths. In addition we used the JCMT observations of the J = 3-2 line of 12CO emission data released on CADC and the 12CO J = 2-1 and J =3-2 lines observed by the KOSMA 3 m telescope. We performed a multiwavelength study of bubble N22. Results. A molecular shell composed of several clumps agrees very well with the border of N22, suggesting that its expansion is collecting the surrounding material. The high integrated 12CO line intensity ratio (ranging from 0.7 to 1.14) implies that shocks have driven into the molecular clouds. We identify eleven possible O-type stars inside the Hii region, five of which are located in projection inside the cavity of the 20 cm radio continuum emission and are probably the exciting-star candidates of N22. Twenty-nine YSOs (young stellar objects) are distributed close to the dense cores of N22. We conclude that star formation is indeed active around N22; the formation of most of YSOs may have been triggered by the expanding of the Hii region. After comparing the dynamical age of N22 and the fragmentation time of the molecular shell, we suggest that radiation-driven compression of pre-existing dense clumps may be ongoing.

astro-ph.GA↗

A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System

The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its alpha-decay daughter 142Nd. Based on analyses of 146Sm/147Sm alpha-activity and atom ratios, we determined the half-life of 146Sm to be 68 \pm 7 (1sigma) million years (Ma), which is shorter than the currently used value of 103 \pm 5 Ma. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm_0 = 0.0094\pm0.0005 (2sigma), than previously estimated. Terrestrial, Lunar and Martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)_0 values.

astro-ph.EP↗

Study of the Hindrance Effect in Sub-barrier Fusion Reactions

We have measured the fusion cross sections of the 12C(13C, p)24Na reaction through off-line measurement of the beta-decay of 24Na using the beta-gamma coincidence method. Our new measurements in the energy range of Ec.m. = 2.6-3.0 MeV do not show an obvious S-factor maximum but a plateau. Comparison between this work and various models is presented.

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