arXiv subjects

J. R. Rizzo

Publications and source records attributed to J. R. Rizzo.

45 records · Page 3Linked to original sources

arXiv2006

Detection of CO+ in the nucleus of M82

We present the detection of the reactive ion CO+ towards the prototypical starburst galaxy M82. This is the first secure detection of this short-lived ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+] ratio had only been previously measured towards the atomic peak in the reflection nebula NGC7023. This detection corroborates that the molecular gas reservoir in the M82 disk is heavily affected by the UV radiation from the recently formed stars. Comparing the column densities measured in M82 with those found in prototypical Galactic photon-dominated regions (PDRs), we need \~20 clouds along the line of sight to explain our observations. We have completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR chemical model successfully explains the [CO+]/[HCO+] ratios measured in the M~82 nucleus but fails by one order of magnitude to explain the large measured CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to reconcile the chemical model and the observations.

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arXiv2005

Insights into the Carbon chemistry of Mon R2

Aiming to learn about the chemistry of the dense PDR around the ultracompact (UC) HII region in Mon R2, we have observed a series of mm-wavelength transitions of C3H2 and C2H. In addition, we have traced the distribution of other molecules, such as H13CO+, SiO, HCO, and HC3N. These data, together with the reactive ions recently detected, have been considered to determine the physical conditions and to model the PDR chemistry. We then identified two kind of molecules. The first group, formed by the reactive ions (CO+, HOC+) and small hydrocarbons (C2H, C3H2), traces the surface layers of the PDR and is presumably exposed to a high UV field (hence we called it as "high UV", or HUV). HUV species is expected to dominate for visual absorptions 2 < Av < 5 mag. A second group (less exposed to the UV field, and hence called "low UV", or LUV) includes HCO and SiO, and is mainly present at the edges of the PDR (Av > 5 mag). While the abundances of the HUV molecules can be explained by gas phase models, this is not the case for the studied LUV ones. Although some efficient gas-phase reactions might be lacking, grain chemistry sounds like a probable mechanism able to explain the observed enhancement of HCO and SiO. Within this scenario, the interaction of UV photons with grains produces an important effect on the molecular gas chemistry and constitutes the first evidence of an ionization front created by the UC HII region carving its host molecular cloud. The physical conditions and kinematics of the gas layer which surrounds the UC HII region were derived from the HUV molecules. Molecular hydrogen densities > 4 10^6 cm^(-3) are required to reproduce the observations. Such high densities suggest that the HII region could be pressure-confined by the surrounding high density molecular gas.

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arXiv2004

Photon-Dominated Chemistry in the Nucleus of M82: Widespread HOC+ emission in the inner 650 pc disk

The nucleus of M82 has been mapped in several 3mm and 1mm lines of CN, HCN, C2H, c-C3H2, CH3C2H, HC3N and HOC+ using the IRAM 30m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured [CN]/[HCN] ~ 5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC+ 1--0 line with an intensity similar to that of the H13CO+ 1--0 line. This implies a [HCO+]/[HOC+] ratio of ~40. These results corroborate the existence of a giant photo-dissociation region (PDR) in the nucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained if the nucleus of M82 is formed by small (r<0.02-0.2 pc) and dense (n ~ a few 10^4--10^5 cm^{-3}) clouds immersed in an intense UV field (G_0 ~ 10^4 in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.

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arXiv2004

Chemical evolution in the environment of intermediate mass young stellar objects: NGC7129--FIRS2 and LkH$α$234

We have carried out a molecular survey of the Class 0 IM protostar NGC 7129 -- FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkH$α$ 234 with the aim of studying the chemical evolution of the envelopes of intermediate-mass (IM) young stellar objects (YSOs). Both objects have similar luminosities (~500 Lsun) and are located in the same molecular cloud which minimizes the chemical differences due to different stellar masses or initial cloud conditions. Moreover, since they are located at the same distance, we have the same spatial resolution in both objects. A total of 17 molecular species (including rarer isotopes) have been observed in both objects and the structure of their envelopes and outflows is determined with unprecedent detail. Our results show that the protostellar envelopes are dispersed and warmed up during the evolution to become a pre-main sequence star. In fact, the envelope mass decreases by a factor >5 from FIRS 2 to LkH$α$234, while the kinetic temperature increases from ~13K to 28K. On the other hand, there is no molecular outflow associated with LkH$α$234. The molecular outflow seems to stop before the star becomes visible. These physical changes strongly affect the chemistry of their envelopes. Based on our results in FIRS2 and LkH$α$ 234, we propose some abundance ratios that can be used as chemical clocks for the envelopes of IM YSOs. The SiO/CS, CN/N2H+, HCN/N2H+, DCO+/HCO+ and D2CO/DCO+ ratios are good diagnostics of the protostellar evolutionary stage.

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arXiv2003

Detection of reactive ions in the ultracompact HII regions Mon R2 and G29.96-0.02

We report the first detection of the reactive ions CO+ and HOC+ towards ultracompact (UC) HII regions, particularly in Mon R2 and G29.96-0.02. We have observed two positions in Mon R2, namely the peak of the UC HII region and a position in the high density molecular cloud which bounds it. CO+ and HOC+ were detected at the UC HII region but not at the molecular cloud, as expected if the CO$^+$ and HOC$^+$ emissions arise in the PDR surrounding the \uch. The measured CO$^+$ and HOC$^+$ column densities are of the order of 10$^{11}$ cm$^{-2}$ in both sources, which yields a strikingly low [HCO$^+$]/[HOC$^+$] abundance ratio of 460 in Mon R2. These values are similar to those found in some other well-known PDRs, like NGC 7023 and the Orion Bar. We briefly discuss the chemical implications of these results.

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arXiv2003

Shocked gas layers surrounding the WR nebula NGC 2359

NGC 2359 is a Wolf-Rayet (W-R) nebula partially bound by a rather dense and warm molecular cloud. We present the results derived from CO and $^{13}$CO fully sampled maps of the molecular material with angular resolutions up to 12". We have detected three different velocity components, and determined their spatial distribution and physical properties. The kinematics, morphology, mass and density are clearly stratified with respect to the W-R star. These features allow us to learn about the recent evolutionary history of HD 56925, because the multiple layers could be associated to several energetic events which have acted upon the surrounding circumstellar medium. Hence, a careful study of the different shockfronts contain clues in determining the present and past interaction of this evolved massive star with its surroundings. From the analysis of the mass-loss history in massive stars like HD 56925, we suggest that the multiple layers of shocked molecular gas are likely to be produced during the earlier LBV phase and/or the actual W-R stage of HD 56925.

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arXiv2001

Shocked ammonia in the WR nebula NGC 2359

We report the detection of the (1,1) and (2,2) metastable lines of ammonia (NH_3) in the molecular cloud associated with the Wolf--Rayet (WR) nebula NGC 2359. Besides the CO and H_2, this is the first molecule detected in the environs of a WR star. Width (3 km/s) and radial velocity (54 km/s) indicate that the NH$_3$ lines arises from the molecular cloud which is interacting with the WR star. The rotational temperature derived from the (1,1) and (2,2) line intensity ratios is about 30 K, significantly larger than the typical kinetic temperature of the ambient gas of 10 K. The derived NH_3 abundance is 10^{-8}. Linewidth, abundance and kinetic temperature can be explained if NH_3 is released from dust grain mantles to the gas phase by shocks produced by the expansion of the bubble created by the WR stellar wind. We briefly discuss the implications of the detection of warm NH_3 associated with a WR star in connection to the hot NH_3 emission detected in the Galactic Center and in the nuclei of external galaxies.

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arXiv2000

Large scale grain mantle disruption in the Galactic Center

We present observations of C2H5OH toward molecular clouds in Sgr A, Sgr B2 and associated with thermal and non-thermal features in the Galactic center. C2H5OH emission in Sgr A and Sgr B2 is widespread, but not uniform. C2H5OH emission is much weaker or it is not detected in some molecular clouds in both complexes, in particular those with radial velocities between 70 and 120 km/s. While most of the clouds associated with the thermal features do not show C2H5OH emission, that associated with the Non-Thermal Radio Arc shows emission. The fractional abundance of C2H5OH in most of the clouds with radial velocities between 0 and 70 km/s in Sgr A and Sgr B2 is relatively high, of few 1e-8. The C2H5OH abundance decreases by more than one order of magnitude (aprox. 1e-9) in the clouds associated with the thermal features. The large abundance of C2H5OH in the gas-phase indicates that C2H5OH has formed in grains and released to gas-phase by shocks in the last aprox. 1e5 years.

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arXiv2000

On the history of the interplay between HD 56925 and NGC 2359

NGC 2359 is an optical nebula excited by the powerful wind and the radiation of the Wolf-Rayet star HD 56925. We have investigated the interaction between this massive star and the surrounding neutral gas by analyzing the large-scale 21cm-HI emission and by mapping the nebula in the J = 1-0 and the J = 2-1 lines of CO. We found a conspicuous (70 x 37 pc) HI shell, expanding at 12 km/s, likely produced during the main-sequence phase of the star. The molecular gas towards NGC 2359 shows three velocity components. Two of these components, A1 and A2, have narrow linewidths (1-2 km/s) and radial velocities of 35-38 and 64-68 km/s, respectively. The third component is detected at radial velocities between 50 and 58 km/s and has a broader profile (up to 5.5 km/s). Furthermore, this component is morphologicaly related with the nebula and has a velocity gradient of a few km/s. We have also estimated the physical parameters of the molecular gas by means of a LVG modelling of the CO emission. The gas projected onto the southern HII region of the nebula has low CO column density and is rather hot, probably up to 80 K. Several profiles of the 13CO J = 1-0 line near the peak of the emission, together with a weak emission bridge between the broad and one of the narrow components (component A2), suggest the presence of a shock front acting in the southern part of the nebula. This shock was likely produced in a previous RSG/LBV phase of HD 56925.

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