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Register a new userA systematic TMRT observational study of Galactic $^{12}$C/$^{13}$C ratios from Formaldehyde
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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 isotopologues 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.
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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 $lambda$ $sim$ 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.
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 $Erm_{c.m.}$=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.
We present Green Bank Telescope (GBT) observations of the 3(12)-3(13) (29 GHz) and 4(13)-4(14) (48 GHz) transitions of the H2CO molecule toward a sample of 23 well-studied star-forming regions. Analysis of the relative intensities of these transitions can be used to reliably measure the densities of molecular cores. Adopting kinetic temperatures from the literature, we have employed a Large Velocity Gradient (LVG) model to derive the average hydrogen number density [n(H2)] within a 16 arcsecond beam toward each source. Densities in the range of 10^{5.5}--10^{6.5} cm^{-3} and ortho-formaldehyde column densities per unit line width between 10^{13.5} and 10^{14.5} cm^{-2} (km s^{-1})^{-1} are found for most objects, in general agreement with existing measurements. A detailed analysis of the advantages and limitations to this densitometry technique is also presented. We find that H2CO 3(12)-3(13)/4(13)-4(14) densitometry proves to be best suited to objects with T_K >~ 100 K, above which the H2CO LVG models become relatively independent of kinetic temperature. This study represents the first detection of these H2CO K-doublet transitions in all but one object in our sample. The ease with which these transitions were detected, coupled with their unique sensitivity to spatial density, make them excellent monitors of density in molecular clouds for future experiments. We also report the detection of the 9_2--8_1 A^- (29 GHz) transition of CH3OH toward 6 sources.
The neutron yield in $^{12}$C(d,n)$^{13}$N and the proton yield in $^{12}C(d,p)^{13}$C have been measured by deuteron beam from 0.6 MeV to 3 MeV which is delivered from a 4-MeV electro static accelerator bombarding on the thick carbon target. The neutrons are detected at $0degree$, $24degree$, $48degree$ and the protons at $135degree$ in the lab frame. The ratios of the neutron yield to the proton one have been calculated and can be used as an effective probe to pin down the resonances. The resonances are found at 1.4 MeV, 1.7 MeV, 2.5 MeV in $^{12}C(d,p)^{13}$C and at 1.6 MeV, 2.7 MeV in $^{12}$C(d,n)$^{13}$N. This method provides a way to reduce the systematic uncertainty and helps to confirm more resonances in compound nuclei.
The lithium abundances in a few percent of giants exceed the value predicted by the standard stellar evolution models, and the mechanisms of Li enhancement are still under debate. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey has obtained over six million spectra in the past five years, and thus provides a great opportunity to search these rare objects and to more clearly understand the mechanisms of Li enhancement. Based on the high-resolution spectrum we obtained the stellar parameters ($T_mathrm{eff}$, $log g$, [Fe/H]), and determined the elemental abundances of Li, C, N, $alpha$, Fe-peak, r-process, s-process elements, and the projected rotational velocity. For a better understanding of the effect of mixing processes, we also derived the $^{12}rm{C}$ to $^{13}rm{C}$ ratio, and constrained the evolutionary status of TYC,3251-581-1 based on the BaSTI stellar isochrones. The super Li-rich giant TYC,3251-581-1 has $rm{A(Li)} = 3.51$, the average abundance of two lithium lines at $lambda = 6708$ AA and 6104 AA based on the non-local thermodynamic equilibrium (NLTE) analysis. The atmospheric parameters show that our target locates on the luminosity function bump. The low carbon isotopic ratio ($^{12}rm{C}/^{13}rm{C} = 9.0 $), a slow rotational velocity $vsin i = 2.2 rm{km,s}^{-1}$, and no sign of IR excess suggest that additional mixing after first dredge up (FDU) should occur to bring internal synthesized Li to the surface. The low carbon ($[rm{C}/rm{Fe}] sim -0.34$ ) and enhanced nitrogen ($[rm{N}/rm{Fe}] sim 0.33$) are also consistent with the sign of mixing. Given the evolutionary stage of TYC,3251-581-1 with the relatively low $^{12}rm{C}/^{13}rm{C}$, the internal production which replenishes Li in the outer layer is the most likely origin of Li enhancement for this star.
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