A linelist of vibration-rotation transitions for ^13^C substituted HCN is presented. The line list is constructed using known experimental levels where available, calculated levels and ab initio line intensities originally calculated for the major isotopologue. Synthetic spectra are generated and compared with observations for cool carbon star WZ Cas. It is suggested that high resolution HCN spectra recorded near 14um should be particularly sensitive to the ^13^C-^12^C ratio.
We present a study of cyanoacetylene (HC_3_N) and cyanodiacetylene (HC_5_N) in Orion KL, using observations from two line surveys performed with the IRAM 30-m telescope and the HIFI instrument onboard the Herschel telescope. The frequency ranges covered are 80-280GHz and 480-1906GHz. This study (divided by families of molecules) is part of a global analysis of the physical conditions of Orion KL and the molecular abundances in the different components of this cloud. We modeled the observed lines of HC_3_N, HC_5_N, their isotopologues (including DC_3_N), and vibrational modes using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code. In addition, to investigate the chemical origin of HC_3_N and DC_3_N in Orion KL, we used a time-dependent chemical model.
Fifty-four ultracompact (UC) HII regions in the GLIMPSE survey region (|b|<1{deg} and 30{deg}<l<70{deg}) were observed in H_2_CO and H110{alpha} using the 305 m Arecibo telescope. By analyzing H_2_CO absorption against the UC H II region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H II regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H II regions in this survey is consistent with a ``local spur'', the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations.
We report observations of the H110{alpha} radio recombination line and H_2_CO (1_10_-1_11_) toward 21 ultracompact H II regions with the Arecibo 305m radio telescope. We detect the H110{alpha} line in 20 sources, and for each of these we also detect a H_2_CO absorption feature at nearly the same velocity, demonstrating the association between molecular and ionized gas. We determine kinematic distances and resolve the distance ambiguity for all observed HII regions, as well as for 19 intervening molecular clouds. A plot of the Galactic distribution of these objects traces part of the spiral structure in the first Galactic quadrant. We compare flux densities and velocities as measured with the Arecibo Telescope with interferometric measurements of our sample of ultracompact HII regions. In general, the single-dish fluxes exceed the interferometric values, consistent with an extended component of radio continuum emission.
HCO+ and HCN obs. toward Planck Galactic Cold Clumps
Short Name:
J/ApJ/820/37
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the first survey of dense gas toward Planck Galactic Cold Clumps (PGCCs). Observations in the J=1-0 transitions of HCO^+^ and HCN toward 621 molecular cores associated with PGCCs were performed using the Purple Mountain Observatory's 13.7m telescope. Among them, 250 sources were detected, including 230 cores detected in HCO^+^ and 158 in HCN. Spectra of the J=1-0 transitions from ^12^CO, ^13^CO, and C^18^O at the centers of the 250 cores were extracted from previous mapping observations to construct a multi-line data set. The significantly low detection rate of asymmetric double-peaked profiles, together with the good consistency among central velocities of CO, HCO^+^, and HCN spectra, suggests that the CO-selected Planck cores are more quiescent than classical star-forming regions. The small difference between line widths of C^18^O and HCN indicates that the inner regions of CO-selected Planck cores are no more turbulent than the exterior. The velocity-integrated intensities and abundances of HCO^+^ are positively correlated with those of HCN, suggesting that these two species are well coupled and chemically connected. The detected abundances of both HCO^+^ and HCN are significantly lower than values in other low- to high-mass star-forming regions. The low abundances may be due to beam dilution. On the basis of an inspection of the parameters given in the PGCC catalog, we suggest that there may be about 1000 PGCC objects that have a sufficient reservoir of dense gas to form stars.
We report the results of an HCO^+^ (3-2) and N_2_D^+^ (3-2) molecular line survey performed toward 91 dense cores in the Perseus molecular cloud using the James Clerk Maxwell Telescope, to identify the fraction of starless and protostellar cores with systematic radial motions. We quantify the HCO^+^ asymmetry using a dimensionless asymmetry parameter {delta}_v_, and identify 20 cores with significant blue or red line asymmetries in optically thick emission indicative of collapsing or expanding motions, respectively. We separately fit the HCO^+^ profiles with an analytic collapse model and determine contraction (expansion) speeds toward 22 cores. Comparing the {delta}_v_ and collapse model results, we find that {delta}_v_ is a good tracer of core contraction if the optically thin emission is aligned with the model-derived systemic velocity. The contraction speeds range from subsonic (0.03km/s) to supersonic (0.40km/s), where the supersonic contraction speeds may trace global rather than local core contraction. Most cores have contraction speeds significantly less than their free-fall speeds. Only 7 of 28 starless cores have spectra well-fit by the collapse model, which more than doubles (15 of 28) for protostellar cores. Starless cores with masses greater than the Jeans mass (M/M_J_>1) are somewhat more likely to show contraction motions. We find no trend of optically thin non-thermal line width with M/M_J_, suggesting that any undetected contraction motions are small and subsonic. Most starless cores in Perseus are either not in a state of collapse or expansion, or are in a very early stage of collapse.
The formaldehyde H_2_CO(1_10_-1_11_) absorption line and H110{alpha} radio recombination line have been observed toward the Aquila Molecular Cloud using the Nanshan 25m telescope operated by the Xinjiang Astronomical Observatory CAS. These first observations of the H_2_CO (1_10_-1_11_) absorption line determine the extent of the molecular regions that are affected by the ongoing star formation in the Aquila molecular complex and show some of the dynamic properties. The distribution of the excitation temperature Tex for H_2_CO identifies the two known star formation regions W40 and Serpens South as well as a smaller new region Serpens 3. The intensity and velocity distributions of H_2_CO and ^13^CO(1-0) do not agree well with each other, which confirms that the H_2_CO absorption structure is mostly determined by the excitation of the molecules resulting from the star formation rather than by the availability of molecular material as represented by the distribution. Some velocity-coherent linear ^13^CO(1-0) structures have been identified in velocity channel maps of H2CO and it is found that the three star formation regions lie on the intersect points of filaments. The H110{alpha} emission is found only at the location of the W40 HII region and spectral profile indicates a redshifted spherical outflow structure in the outskirts of the HII region. Sensitive mapping of H_2_CO absorption of the Aquila Complex has correctly identified the locations of star formation activity in complex molecular clouds and the spectral profiles reveal the dominant velocity components and may identify the presence of outflows.
The Orion Kleinmann-Low nebula (Orion-KL) is a complex region of star formation. Whereas its proximity allows studies on a scale of a few hundred AU, spectral confusion makes it difficult to identify molecules with low abundances. We studied an important oxygenated molecule, HCOOCH3, to characterize the physical conditions, temperature, and density of the different molecular source components.
An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory. We present the experimental study and its theoretical analysis for two ^13^C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130/cm (200K) in Orion-KL. New spectra of HCO-O-^13^CH_3_ (^13^C_2_) methyl formate were recorded with the mm- and submm-wave spectrometer in Lille from 50 to 940GHz. A global fit for v_t_=0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy. We analysed 5728 and 2881 new lines for v_t_=0 and 1 for HCOO^13^CH_3_. These new lines were globally fitted with 846 previously published lines for v_t_=0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V_3_=369.93168(395)/cm) was improved. We report the detection of the first excited torsional states (v_t_=1) in Orion-KL for the ^13^C_2_ and ^13^C_1_ methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of ^13^C-methyl formate isotopologues in Orion-KL in the 80-280GHz frequency range, without missing lines.
We present observations of the C-band 1_10_-1_11_ (4.8GHz) and Ku-band 2_11_-2_12_ (14.5GHz) K-doublet lines of H_2_CO and the C-band 1_10_-1_11_ (4.6GHz) line of H_2_^13^CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65m 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 Wiesenfeld & Faure. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of ^12^C/^13^C=(5.08+/-1.10)D_GC_+(11.86+/-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.
