ABSTRACT We report the first detection in the interstellar medium (ISM) of the Z-isomer of cyanomethanimine (HNCHCN), an HCN dimer proposed as precursor of adenine. We identified six transitions of ...Z-cyanomethanimine, along with five transitions of E-cyanomethanimine, using IRAM 30m observations towards the Galactic Centre quiescent molecular cloud G + 0.693. The Z-isomer has a column density of (2.0 ± 0.6) × 1014 cm−2 and an abundance of 1.5 × 10−9. The relative abundance ratio between the isomers is Z/E∼6. This value cannot be explained by the two chemical formation routes previously proposed (gas phase and grain surface), which predicts abundances ratios between 0.9 and 1.5. The observed Z/E ratio is in good agreement with thermodynamic equilibrium at the gas kinetic temperature (130−210 K). Since isomerization is not possible in the ISM, the two species may be formed at high temperature. New chemical models, including surface chemistry on dust grains and gas-phase reactions, should be explored to explain our findings. Whatever the formation mechanism, the high abundance of Z-HNCHCN shows that precursors of adenine are efficiently formed in the ISM.
Aims. The circumnuclear disk (CND) of the Galactic center is exposed to many energetic phenomena coming from the supermassive black hole Sgr A* and from stellar activities. These energetic activities ...can affect the chemical composition in the CND through interaction with UV photons, cosmic rays, X-rays, and shock waves. We aim to constrain the physical conditions present in the CND through chemical modeling of observed molecular species detected toward it. Methods. We analyzed a selected set of molecular line data taken toward a position in the southwest lobe of the CND with the IRAM 30m and APEX 12-m telescopes and derived the column density of each molecule via a large velocity gradient (LVG) analysis. The determined chemical composition is compared with a time-dependent, gas-grain chemical model based on the UCL_CHEM code,which includes the effects of shock waves with varying physical parameters. Results. We detect molecules, such as CO, HCN, HCO+, HNC, CS, SO, SiO, NO, CN, H2CO, HC3N, N2H+, and H3O+, and obtain their column densities. Total hydrogen densities obtained from LVG analysis range between 2 × 104 and 1 × 106cm-3 and most species indicate values around several × 105cm-3. These values are lower than those corresponding to the Roche limit, which shows that the CND is tidally unstable. The chemical models show good agreement with the observations in cases where the density is ~104cm-3, the cosmic-ray ionization rate is high, > 10-15s-1, or shocks with velocities > 40 km s-1 have occurred. Conclusions. Comparison of models and observations favors a scenario where the cosmic-ray ionization rate in the CND is high, but precise effects of other factors, such as shocks, density structures, UV photons, and X-rays from the Sgr A*, must be examined with higher spatial resolution data.
Context. Observations of chemical species can provide insights into the physical conditions of the emitting gas however it is important to understand how their abundances and excitation vary within ...different heating environments. C2H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments. Aims. As part of the ALCHEMI ALMA large program, we map the emission of C2H in the central molecular zone of the nearby starburst galaxy NGC 253 at 1.6″ (28 pc) resolution and characterize it to understand its chemical origins. Methods. We used spectral modeling of the N = 1−0 through N = 4−3 rotational transitions of C2H to derive the C2H column densities towards the dense clouds in NGC 253. We then use chemical modeling, including photodissociation region (PDR), dense cloud, and shock models to investigate the chemical processes and physical conditions that are producing the molecular emission. Results. We find high C2H column densities of ∼1015 cm−2 detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced if it is assumed that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C2H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.
Abstract
We present C
ii
158
μ
m and O
i
63
μ
m observations of the bipolar H
ii
region RCW 36 in the Vela C molecular cloud, obtained within the SOFIA legacy project FEEDBACK, which is ...complemented with APEX
12/13
CO (3–2) and Chandra X-ray (0.5–7 keV) data. This shows that the molecular ring, forming the waist of the bipolar nebula, expands with a velocity of 1–1.9 km s
−1
. We also observe an increased line width in the ring, indicating that turbulence is driven by energy injection from the stellar feedback. The bipolar cavity hosts blueshifted expanding C
ii
shells at 5.2 ± 0.5 ± 0.5 km s
−1
(statistical and systematic uncertainty), which indicates that expansion out of the dense gas happens nonuniformly and that the observed bipolar phase might be relatively short (∼0.2 Myr). The X-ray observations show diffuse emission that traces a hot plasma, created by stellar winds, in and around RCW 36. At least 50% of the stellar wind energy is missing in RCW 36. This is likely due to leakage that is clearing even larger cavities around the bipolar RCW 36 region. Lastly, the cavities host high-velocity wings in C
ii
, which indicates relatively high mass ejection rates (∼5 × 10
−4
M
⊙
yr
−1
). This could be driven by stellar winds and/or radiation but remains difficult to constrain. This local mass ejection, which can remove all mass within 1 pc of RCW 36 in 1–2 Myr, and the large-scale clearing of ambient gas in the Vela C cloud indicate that stellar feedback plays a significant role in suppressing the star formation efficiency.
Phosphorus-bearing molecules in the Galactic Center Rivilla, V M; Jiménez-Serra, I; Zeng, S ...
Monthly notices of the Royal Astronomical Society. Letters,
03/2018, Letnik:
475, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Phosphorus (P) is one of the essential elements for life due to its central role in biochemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are ...present in star-forming regions, although their formation routes remain poorly understood. In this letter, we report observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterized by different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detections correspond to clouds exposed to intense UV/X-rays/cosmic ray (CR) radiation. PO is detected only towards the cloud G+0.693−0.03, with a PO/PN abundance ratio of ∼1.5. We conclude that P-bearing molecules likely form in shocked gas as a result of dust grain sputtering, while are destroyed by intense UV/X-ray/CR radiation.
ABSTRACT
We use both new and archival ALMA data of three energy lines each of CN and HCN to explore intensity ratios in dense gas in NGC 3256, NGC 7469, and IRAS 13120-5453. The HCN (3–2)/HCN (1–0) ...intensity ratio varies in NGC 3256 and NGC 7469, with superlinear trends of 1.53 ± 0.07 and 1.55 ± 0.05, respectively. We find an offset to higher HCN (3–2)/HCN (1–0) intensity ratios (∼0.8) in IRAS 13120-5453 compared to NGC 3256 (∼0.3−0.4) and NGC 7469 (∼0.3−0.5). The HCN (4–3)/HCN (3–2) intensity ratio in NGC 7469 has a slope of 1.34 ± 0.05. We attribute the variation within NGC 3256 to excitation associated with the northern and southern nuclei. In NGC 7469, the variations are localized to the region surrounding the active galactic nucleus. At our resolution (∼700 pc), IRAS 13120-5453 shows little variation in the HCN intensity ratios. Individual galaxies show nearly constant CN (2–1)/CN (1–0) intensity ratios. We find an offset to lower CN (2–1)/CN (1–0) intensity ratios (∼0.5) in NGC 3256 compared to the other two galaxies (∼0.8). For the CN (3–2)/CN (2–1) intensity ratio, NGC 7469 has a superlinear trend of 1.55 ± 0.04, with the peak localized towards the active galactic nucleus. We find high (∼1.7) CN (1–0)/HCN (1–0) intensity ratios in IRAS 13120-5453 and in the northern nucleus of NGC 3256, compared to a more constant ratio (∼1.1) in NGC 7469 and non-starbursting regions of NGC 3256.
Context. One of the most important aspects of modern astrophysics is related to our understanding of the origin of elements and chemical evolution in the large variety of astronomical sources. ...Nuclear regions of galaxies undergo heavy processing of matter, and are therefore ideal targets to investigate matter cycles via determination of elemental and isotopic abundances. Aims. To trace chemical evolution in a prototypical starburst environment, we spatially resolve carbon and oxygen isotope ratios across the central molecular zone (CMZ; full size ∼600 pc) in the nearby starburst galaxy NGC 253. Methods. We imaged the emission of the optically thin isotopologues 13CO, C18O, C17O, 13C18O at a spatial resolution ∼50 pc, comparable to the typical size of giant molecular associations. Optical depth effects and contamination of 13C18O by C4H are discussed and accounted for to derive column densities. Results. This is the first extragalactic detection of the double isotopologue 13C18O. Derived isotopic ratios 12C/13C ∼ 21 ± 6, 16O/18O ∼ 130 ± 40, and 18O/17O ∼ 4.5 ± 0.8 differ from the generally adopted values in the nuclei of galaxies. Conclusions. The molecular clouds in the central region of NGC 253 show similar rare isotope enrichment to those within the CMZ of the Milky way. This enrichment is attributed to stellar nucleosynthesis. Measured isotopic ratios suggest an enhancement of 18O as compared to our Galactic centre, which we attribute to an extra 18O injection from massive stars. Our observations show evidence for mixing of distinct gas components with different degrees of processing. We observe an extra molecular component of highly processed gas on top of the already proposed less processed gas being transported to the central region of NGC 253. Such a multicomponent nature and optical depth effects may hinder the use of isotopic ratios based on a spatially unresolved line to infer the star formation history and/or initial stellar mass function properties galaxy nuclei.
Aims. We aim to better understand the imprints that the nuclear activity in galaxies leaves in the molecular gas. Methods. We used the IRAM 30 m telescope to observe the frequency range ~86−116 GHz ...towards the central regions of the starburst galaxies M 83, M 82, and NGC 253, the galaxies hosting an active galactic nucleus (AGN) M 51, NGC 1068, and NGC 7469, and the ultra-luminous infrared galaxies (ULIRGs) Arp 220 and Mrk 231. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of 27 molecules and 10 isotopologues (or their upper limits in case of non-detections). Results. Among others, we report the first tentative detections of CH3CHO, HNCO, and NS in M 82 and, for the first time in the extragalactic medium, HC5N in NGC 253. Hα recombination lines were only found in M 82 and NGC 253. Vibrationally excited lines of HC3N were only detected in Arp 220. CH3CCH emission is only seen in the starburst-dominated galaxies. By comparison of the fractional abundances among the galaxies, we looked for the molecules that are best suited to characterise the chemistry of each group of galaxies (starbursts, AGNs and ULIRGs), as well as the differences among galaxies within the same group. Conclusions. Suitable species for characterising and comparing starburst galaxies are CH3OH and HNCO as tracers of large-scale shocks, which dominate early to intermediate starburst stages, and CH3CCH, c-C3H2, and HCO as tracers of UV fields, which control the intermediate-to-old or post starburst phases. M 83 shows signs of a shock-dominated environment. NGC 253 is characterised by both strong shocks and some UV fields. M 82 stands out for its bright photo-dissociated region tracers, which indicate an UV field-dominated environment. Regarding AGNs, the abundances of HCN and CN (previously claimed as enhanced in AGNs) in M 51 are similar to those in starburst galaxies, while the HCN/HCO+ ratio is high in M 51 and NGC 1068, but not in NGC 7469. We did not find a correlation between the HCN/CS ratio (recently claimed as a possible starburst/AGN discriminator) and the AGN activity. However, a high enough spatial resolution to separate their circumnuclear disks from the surrounding star-forming regions is needed to find molecular abundance trends in AGNs. High abundances of H13CN and HC3N, as well as a similarity between the column densities of 13CO and C18O, are representative of the molecular interstellar medium in the ULIRGs. Furthermore, the chemistry of Arp 220 points towards a more starburst-dominated environment, while that of Mrk 231 more resembles the AGNs of our sample.
Carbon and oxygen isotope ratios are excellent measures of nuclear processing, but few such data have been taken toward extragalactic targets so far. Therefore, using the IRAM 30-m telescope, CN and ...CO isotopologues have been measured toward the nearby starburst galaxy NGC 253 and the prototypical ultraluminous infrared galaxy Mrk 231. Toward the center of NGC 253, the CN and 13CN N = 1 → 0 lines indicate no significant deviations from expected local thermodynamical equilibrium after accounting for moderate saturation effects (10 and 25%) in the two detected spectral components of the main species. Including calibration uncertainties, which dominate the error budget, the 12C/13C ratio becomes 40 ± 10. This is larger than the ratio in the central molecular zone of the Galaxy, suggesting a higher infall rate of poorly processed gas toward the central region. Assuming that the ratio also holds for the CO emitting gas, this yields 16O/18O = 145 ± 36 and 16O/17O = 1290 ± 365 and a 32S/34S ratio close to the one measured for the local interstellar medium (20–25). No indication of vibrationally excited CN is found in the lower frequency fine structure components of the N = 1 → 0 and 2 → 1 transitions at rms noise levels of 3 and 4 mK (15 and 20 mJy) in 8.5 km s-1 wide channels. Peak line intensity ratios between NGC 253 and Mrk 231 are ~100 for 12C16O and 12C18O J = 1 → 0, while the ratio for 13C16O J = 1 → 0 is ~250. This and similar 13CO and C18O line intensities in the J = 1 → 0 and 2 → 1 transitions of Mrk 231 suggest 12C/13C ~ 100 and 16O/18O ~ 100, in agreement with values obtained for the less evolved ultraluminous merger Arp 220. Also, when accounting for other (scarcely available) extragalactic data, 12C/13C ratios appear to vary over a full order of magnitude, from >100 in ultraluminous high redshift galaxies to ~100 in more local such galaxies to ~40 in weaker starbursts that are not undergoing a large scale merger to 25 in the central molecular zone of the Milky Way. With 12C being predominantly synthesized in massive stars, while 13C is mostly ejected by longer lived lower mass stars at later times, this is qualitatively consistent with our results of decreasing carbon isotope ratios with time and rising metallicity. It is emphasized, however, that both infall of poorly processed material, initiating a nuclear starburst, and the ejecta from newly formed massive stars (in particular in the case of a top-heavy stellar initial mass function) can raise the carbon isotope ratio for a limited amount of time.
PDF
