- ID:
- ivo://CDS.VizieR/J/A+A/609/A129
- Title:
- HCN, HNC and DNC spectra of 27 sources
- Short Name:
- J/A+A/609/A129
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The ratio between the two stable isotopes of nitrogen, ^14^N and ^15^N, is well measured in the terrestrial atmosphere (~272), and for the pre-solar nebula (~441, deduced from the solar wind). Interestingly, some pristine solar system materials show enrichments in 15N with respect to the pre-solar nebula value. However, it is not yet clear if and how these enrichments are linked to the past chemical history because we have only a limited number of measurements in dense star-forming regions. In this respect, dense cores, which are believed to be the precursors of clusters and also contain intermediate- and high-mass stars, are important targets because the solar system was probably born within a rich stellar cluster, and such clusters are formed in high-mass star-forming regions. The number of observations in such high-mass dense cores has remained limited so far. In this work, we show the results of IRAM-30m observations of the J=1-0 rotational transition of the molecules HCN and HNC and their 15N-bearing counterparts towards 27 intermediate- and high-mass dense cores that are divided almost equally into three evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact Hii regions. We have also observed the DNC(2-1) rotational transition in order to search for a relation between the isotopic ratios D/H and ^14^N/^15^N. We derive average ^14^N/^15^N ratios of 359+/-16 in HCN and of 438+/-21 in HNC, with a dispersion of about 150-200. We find no trend of the ^14^ N/^15^ N ratio with evolutionary stage. This result agrees with what has been found for N_2_H^+^ and its isotopologues in the same sources, although the ^14^N/^15^N ratios from N2H+ show a higher dispersion than in HCN/HNC, and on average, their uncertainties are larger as well. Moreover, we have found no correlation between D/H and ^14^N/^15^N in HNC. These findings indicate that (1) the chemical evolution does not seem to play a role in the fractionation of nitrogen, and that (2) the fractionation of hydrogen and nitrogen in these objects is not related.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/903/111
- Title:
- HCN, HNC, H13CN & HC3N of Galactic Center map. II.
- Short Name:
- J/ApJ/903/111
- Date:
- 15 Mar 2022
- Publisher:
- CDS
- Description:
- We report a statistical analysis exploring the origin of the overall low star formation efficiency (SFE) of the Galactic central molecular zone (CMZ) and the SFE diversity among the CMZ clouds using a wide-field HCN J=4-3 map, whose optically thin critical density (~10^7^/cm^3^) is the highest among the tracers ever used in CMZ surveys. Logistic regression is performed to empirically formulate star formation probability of 195 HCN clumps, 13 of which contain star formation signatures. The explanatory parameters in the best-fit model are reduced into the virial parameter {alpha}_vir_ without significant contribution from other parameters, whereas the performance of the model without {alpha}_vir_ is no better than that using randomly generated data. The threshold {alpha}_vir_ is 6, which translates into a volume density (n_H_2__) of 10^4.6^/cm^3^ with the n_H_2__-{alpha}_vir_ correlation. The scarcity of the low-{alpha}_vir_ clumps, whose fraction to all HCN clumps is 0.1, can be considered as one of the immediate causes of the suppressed SFE. No correlation between the clump size or mass and star formation probability is found, implying that HCN J= 4-3 does not immediately trace the mass of star-forming gas above a threshold density. Meanwhile, star-forming and non-star-forming clouds are degenerate in the physical parameters of the CS J=1-0 clouds, highlighting the efficacy of the HCN J=4-3 line to probe star-forming regions in the CMZ. The timescale of the high-{alpha}_vir_ to low-{alpha}_vir_ transition is <~2Myr, which is consistent with the tidal compression and X1/X2 orbit transition models but possibly does not fit the cloud-cloud collision picture.
- ID:
- ivo://CDS.VizieR/J/A+A/635/A4
- Title:
- HCN-to-HNC intensity ratio
- Short Name:
- J/A+A/635/A4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The gas kinetic temperature (TK) determines the physical and chemical evolution of the interstellar medium (ISM). However, obtaining reliable TK estimates usually requires expensive observations including the combination of multi-line analysis and dedicated radiative transfer calculations. This work explores the use of HCN and HNC observations, and particularly the I(HCN)-to-I(HNC) intensity ratio (I(HCN)/I(HNC)) of their J=1-0 lines, as direct probe of the gas kinetic temperature in the molecular ISM. We obtained a new set of large-scale observations of the HCN and HNC (1-0) lines throughout the Integral Shape Filament (ISF) in Orion. In combination with ancillary gas and dust temperature measurements, we find a systematic temperature dependence of the observed I(HCN)-to-I(HNC) intensity ratio throughout our maps. Additional comparisons with chemical models demonstrate that these observed I(HCN)/I(HNC) variations are driven by the effective destruction and isomerization mechanisms of HNC under low-energy barriers. The observed variations of I(HCN)/I(HNC) with TK can be described with a two-part linear function. This empirical calibration is then used to create a temperature map of the entire ISF. Comparisons with similar dust temperature measurements in this cloud, as well as in other regions and galactic surveys, validate this simple technique for obtaining direct estimates of the gas kinetic temperature in a wide range of physical conditions and scales with an optimal working range between 15K<TK<=40K. Both observations and models demonstrate the strong sensitivity of the I(HCN)/I(HNC) ratio to the gas kinetic temperature. Since these lines are easily obtained in observations of local and extragalactic sources, our results highlight the potential use of this observable as new chemical thermometer for the ISM.
- ID:
- ivo://CDS.VizieR/J/ApJ/652/1230
- Title:
- H_2_CO absorption toward Galactic anticenter
- Short Name:
- J/ApJ/652/1230
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have carried out a blind search in the general direction of the Galactic anticenter for absorption of the cosmic microwave background (CMB) radiation near 4.83GHz by molecular clouds containing gaseous ortho-formaldehyde (H_2_CO). The observations were done using the 25m radio telescope at Onsala in Sweden and covered strips in Galactic latitude, 1{deg}<=b<=+1{deg}, at several longitudes in the region 170{deg}<=l<=190{deg}. Spectra were obtained in these strips with a grid spacing corresponding to the telescope resolution of 10'. We have detected H_2_CO CMB absorption at 10% of the survey pointings.
- ID:
- ivo://CDS.VizieR/J/A+A/587/A130
- Title:
- HCO and CH_3_O in prestellar cores
- Short Name:
- J/A+A/587/A130
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The recent unexpected detection of terrestrial complex organic molecules in the cold (~10K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH_3_O, as well as the stable species formaldehyde H_2_CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces. We present new observations of the HCO and CH_3_O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H_2_CO, CH_3_O, and CH_3_OH, which represent the various stages of grain- surface hydrogenation of CO to CH_3_OH. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H_2_CO:CH_3_O:CH_3_OH~10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH_3_O are gas-phase products of the precursors H_2_CO and CH_3_OH, respectively. Various gas-phase pathways are considered, including neutral-neutral and ion-molecule reactions, and simple estimates of HCO and CH_3_O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified.
- ID:
- ivo://CDS.VizieR/J/A+A/551/A28
- Title:
- H2CO and CO in 4 molecular clouds
- Short Name:
- J/A+A/551/A28
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We seek to understand how the 4.8GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H_2_CO, ^12^CO, and ^13^CO spectral lines. The four regions of MON R2 (60'x90'), S156 (50'x70'), DR17/L906 (40'x60'), and M17/M18 (70'x80') were observed for H_2_CO (beam 10'), H110{alpha} recombination (beam 10'), 6 cm continuum (beam 10'), ^12^CO (beam 1'), and ^13^CO (beam 1'). We compared the H_2_CO, ^12^CO, ^13^CO, and continuum distributions, and also the spectra line parameters of H_2_CO, ^12^CO, and ^13^CO. Column densities of H_2_CO, ^13^CO, and H_2_ were also estimated. We found out that the H_2_CO distribution is similar to the ^12^CO and the ^13^CO distributions on a large scale. The correlation between the ^13^CO and the H_2_CO distributions is better than between the ^12^CO and H_2_CO distributions. The H_2_CO and the ^13^CO tracers systematically provide consistent views of the dense regions. Their maps have similar shapes, sizes, peak positions, and molecular spectra and present similar central velocities and line widths. Such good agreement indicates that the H_2_CO and the ^13^CO arise from similar regions.
- ID:
- ivo://CDS.VizieR/J/ApJS/138/63
- Title:
- H_2_CO and H{alpha} observations of UC HII
- Short Name:
- J/ApJS/138/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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.
- ID:
- ivo://CDS.VizieR/J/ApJ/587/714
- Title:
- H_2_CO and H{alpha} observations of UC HII
- Short Name:
- J/ApJ/587/714
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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.
- ID:
- ivo://CDS.VizieR/J/ApJ/820/37
- Title:
- 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.
- ID:
- ivo://CDS.VizieR/J/ApJ/819/143
- Title:
- HCO+ and N2D+ dense cores in Perseus
- Short Name:
- J/ApJ/819/143
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- 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.