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Pilleri P.,CSIC - National Institute of Aerospace Technology | Fuente A.,Observatorio Astronomico Nacional | Cernicharo J.,CSIC - National Institute of Aerospace Technology | Ossenkopf V.,University of Cologne | And 14 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Mon R2, at a distance of 830 pc, is the only ultracompact H≠ii region (UCHII) where the associated photon-dominated region (PDR) can be resolved with Herschel. Owing to its brightness and proximity, it is one of the best-suited sources for investigating the chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to estimate the abundance of H2O and NH3 in this region and investigate their origin. Methods. We present new observations ([C II], 12CO, 13CO, C18O, o-H 2O, p-H2O, o-H2 18O and o-NH 3) obtained with the HIFI instrument onboard Herschel and the IRAM-30≠m telescope. We investigated the physical conditions in which these lines arise by analyzing their velocity structure and spatial variations. Using a large velocity gradient approach, we modeled the line intensities and derived an average abundance of H2O and NH 3 across the region. Finally, we modeled the line profiles with a non-local radiative transfer model and compared these results with the abundance predicted by the Meudon PDR code. Results. The variations of the line profiles and intensities indicate complex geometrical and kinematical patterns. In several tracers ([CII], CO 9 8 and H2O) the line profiles vary significantly with position and have broader line widths toward the H II region. The H2O lines present strong self-absorption at the ambient velocity and emission in high-velocity wings toward the H II region. The emission in the o-H2 18O ground state line reaches its maximum value around the HII region, has smaller linewidths and peaks at the velocity of the ambient cloud. Its spatial distribution shows that the o-H2 18O emission arises in the PDR surrounding the HII region. By modeling the o-H2 18O emission and assuming the standard [16O] [ 18O] = 500, we derive a mean abundance of o-H2O of ∼10-8 relative to H2. The ortho-H2O abundance, however, is larger (∼1 × 10-7) in the high-velocity wings detected toward the HII region. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope of the core with an average abundance of ∼2 × 10-9 relative to H2. Conclusions. The Meudon PDR code, which includes only gas-phase chemical networks, can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a ≲1 mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the water and ammonia abundances in the rest of the cloud, the molecular freeze out and grain surface chemistry would need to be included. © 2012 ESO.

Cernicharo J.,CSIC - National Institute of Aerospace Technology | Cernicharo J.,CSIC - Institute of Materials Science | Bailleux S.,CNRS Atomic and Molecular Physics Laboratory | Alekseev E.,Ukrainian Academy of Sciences | And 10 more authors.
Astrophysical Journal | Year: 2014

We report the tentative detection in space of the nitrosylium ion, NO+. The observations were performed toward the cold dense core Barnard 1-b. The identification of the NO+ J = 2-1 line is supported by new laboratory measurements of NO+ rotational lines up to the J = 8-7 transition (953207.189 MHz), which leads to an improved set of molecular constants: B 0 = 59597.1379(62) MHz, D 0 = 169.428(65) kHz, and eQq 0(N) = -6.72(15) MHz. The profile of the feature assigned to NO+ exhibits two velocity components at 6.5 and 7.5 km s-1, with column densities of 1.5 × 1012 and 6.5 × 1011 cm-2, respectively. New observations of NO and HNO, also reported here, allow us to estimate the following abundance ratios: X(NO)/X(NO+) ≃ 511, and X(HNO)/X(NO+) ≃ 1. This latter value provides important constraints on the formation and destruction processes of HNO. The chemistry of NO+ and other related nitrogen-bearing species is investigated by the means of a time-dependent gas phase model which includes an updated chemical network according to recent experimental studies. The predicted abundance for NO+ and NO is found to be consistent with the observations. However, that of HNO relative to NO is too high. No satisfactory chemical paths have been found to explain the observed low abundance of HNO. HSCN and HNCS are also reported here with an abundance ratio of ≃ 1. Finally, we have searched for NNO, NO2, HNNO+, and NNOH+, but only upper limits have been obtained for their column density, except for the latter for which we report a tentative 3σ detection. © 2014. The American Astronomical Society. All rights reserved..

Pilleri P.,Los Alamos National Laboratory | Pilleri P.,CSIC - National Institute of Aerospace Technology | Fuente A.,Observatorio Astronomico Nacional | Gerin M.,Paris Observatory | And 13 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only ultra-compact H ii region (UC H ii) where its associated photon-dominated region (PDR) can be resolved with the Herschel Space Observatory. Aims. Our aim is to investigate observationally the kinematical patterns in the interface regions (i.e., the transition from atomic to molecular gas) associated with Mon R2. Methods. We used the HIFI instrument on board Herschel to observe the line profiles of the reactive ions CH+, OH+, and H 2O+ toward different positions in Mon R2. We derive the column density of these molecules and compare them with gas-phase chemistry models. Results. The reactive ion CH+ is detected both in emission (at central and red-shifted velocities) and in absorption (at blue-shifted velocities). The OH+ ion is detected in absorption at both blue-and red-shifted velocities, with similar column densities; H2O + is not detected at any of the positions, down to a rms of 40 mK toward the molecular peak. At this position, we find that the OH+ absorption originates in a mainly atomic medium, and therefore is associated with the most exposed layers of the PDR. These results are consistent with the predictions from photo-chemical models. The line profiles are consistent with the atomic gas being entrained in the ionized gas flow along the walls of the cavity of the H ii region. Based on this evidence, we are able to propose a new geometrical model for this region. Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate the existence of a layer of mainly atomic gas for which we have derived, for the first time, some physical parameters and its dynamics. © 2014 ESO.

Pilleri P.,CSIC - National Institute of Aerospace Technology | Pilleri P.,Los Alamos National Laboratory | Trevino-Morales S.,Institute Radio Astronomia Milimetrica IRAM | Fuente A.,Observatorio Astronomico Nacional | And 16 more authors.
Astronomy and Astrophysics | Year: 2013

Context. We study the chemistry of small hydrocarbons in the photon-dominated regions (PDRs) associated with the ultra-compact H ii region (UCH ii) Mon R2. Aims. Our goal is to determine the variations in the abundance of small hydrocarbons in a high-UV irradiated PDR and investigate the chemistry of these species. Methods. We present an observational study of the small hydrocarbons CH, CCH, and c-C3H2 in Mon R2 that combines spectral mapping data obtained with the IRAM-30 m telescope and the Herschel space observatory. We determine the column densities of these species, and compare their spatial distributions with that of polycyclic aromatic hydrocarbon (PAH), which trace the PDR. We compare the observational results with different chemical models to explore the relative importance of gas-phase, grain-surface, and time-dependent chemistry in these environments. Results. The emission of the small hydrocarbons show different spatial patterns. The CCH emission is extended, while CH and c-C3H2 are concentrated towards the more illuminated layers of the PDR. The ratio of the column densities of c-C3H2 and CCH shows spatial variations up to a factor of a few, increasing from N(c-C3H2)/N(CCH) ≈ 0.004 in the envelope to a maximum of ≈0.015-0.029 towards the 8 μm emission peak. Comparing these results with other galactic PDRs, we find that the abundance of CCH is quite constant over a wide range of G0, whereas the abundance of c-C3H2 is higher in low-UV PDRs, with the N(c-C 3H2)/N(CCH) ratio ranging ≈0.008-0.08 from high to low UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account relatively well for the abundances of CH and CCH in the most exposed layers of the PDR, but falls short by a factor of 10 of reproducing c-C3H2. In the low-density molecular envelope, time-dependent effects and grain surface chemistry play dominant roles in determining the hydrocarbon abundances. Conclusions. Our study shows that the small hydrocarbons CCH and c-C 3H2 present a complex chemistry in which UV photons, grain-surface chemistry, and time dependent effects contribute to determining their abundances. Each of these effects may be dominant depending on the local physical conditions, and the superposition of different regions along the line of sight leads to the variety of measured abundances. © 2013 ESO.

Quintana-Lacaci G.,CSIC - National Institute of Aerospace Technology | Quintana-Lacaci G.,Institute Radio Astronomia Milimetrica IRAM | Agundez M.,University of Bordeaux 1 | Agundez M.,French National Center for Scientific Research | And 5 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. During a full line survey towards IRC +10420 in the 3 and 1 mm bands, we detected the emission of circumstellar nitric oxide for the first time. We aim to study the formation of NO and to confirm the enrichment of nitrogen expected for the most massive, evolved stars predicted by the hot bottom burning process. Methods. We counted on a detailed model of the structure and kinematics of the molecular gas around IRC+̇10420. In addition, we used a chemical model to derive the NO abundance profile. We modified the initial nitrogen abundance in order to fit the observed NO profiles. These synthetic profiles were obtained using an LVG radiative transfer code. Results. We have detected NO in a circumstellar envelope for the first time, along with a variety of N-rich molecules, which in turn shows that IRC +10420 presents a N-rich chemistry. Furthermore, we have found that to reproduce the observed NO line profiles, the initial abundance of nitrogen in the chemical model has to be increased by a factor 20 with respect to the values of the standard O-rich stars. © ESO, 2013.

Ginard D.,Observatorio Astronomico Nacional OAN | Gonzalez-Garcia M.,Institute Radio Astronomia Milimetrica IRAM | Fuente A.,Observatorio Astronomico Nacional OAN | Cernicharo J.,CSIC - National Institute of Aerospace Technology | And 14 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Ultracompact (UC) Hii regions constitute one of the earliest phases in the formation of a massive star and are characterized by extreme physical conditions (G0 > 105 Habing field and n > 106 cm-3). The UC Hii Mon R2 is the closest example and an excellent target to study the chemistry in these complex regions. Aims. Our goal is to investigate the chemistry of the molecular gas around UC Hii Mon R2 and the variations caused by the different local physical conditions. Methods. We carried out 3 mm and 1 mm spectral surveys using the IRAM 30-m telescope towards three positions that represent different physical environments in Mon R2: (i) the ionization front (IF) at (0″, 0″), and two peaks in the molecular cloud; (ii) molecular Peak 1 (hereafter MP1) at the offset (+15″,-15″); and (iii) molecular Peak 2 (hereafter MP2) at the farther offset (0″, 40″). In addition, we carried out extensive modeling to explain the chemical differences between the three observed regions. Results. We detected more than 30 different species (including isotopologues and deuterated compounds). In particular, we detected SO+ and C 4H confirming that ultraviolet (UV) radiation plays an important role in the molecular chemistry of this region. In agreement with this interpretation, we detected the typical photo-dissociation region (PDR) molecules CN, HCN, HCO, C2H, and c-C3H2. There are chemical differences between the observed positions. While the IF and the MP1 have a chemistry similar to that found in high UV field and dense PDRs such as the Orion Bar, the MP2 is similar to lower UV/density PDRs such as the Horsehead nebula. Our chemical modeling supports this interpretation. In addition to the PDR-like species, we detected complex molecules such as CH 3CN, H2CO, HC3N, CH3OH, and CH 3C2H that are not usually found in PDRs. The sulfur compounds CS, HCS+, C2S, H2CS, SO, and SO 2 and the deuterated species DCN and C2D were also identified. The origin of these complex species requires further study. The observed deuteration fractionations, [DCN]/[HCN] ∼ 0.03 and [C 2D]/[C2H] ∼ 0.05, are among the highest in warm regions. Conclusions. Our results show that the high UV/dense PDRs have a different chemistry from the low UV case. Some abundance ratios such as [CO +]/[HCO+] or [HCO]/[HCO+] are good diagnostics for differentiating between them. In Mon R2, we have the two classes of PDRs, a high UV PDR towards the IF and the adjacent molecular bar, and a low-UV PDR, which extends towards the north-west following the border of the cloud. © 2012 ESO.

Falgarone E.,Ecole Normale Superieure de Paris | Ossenkopf V.,University of Cologne | Ossenkopf V.,SRON Netherlands Institute for Space Research | Gerin M.,Ecole Normale Superieure de Paris | And 31 more authors.
Astronomy and Astrophysics | Year: 2010

We report the first detection of the ground-state rotational transition of the methylidyne cation CH+ towards the massive star-forming region DR 21 with the HIFI instrument onboard the Herschel satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR 21 molecular ridge and foreground gas. These observations allow us to determine a 12CH+J = 1-0 line frequency of ν = 835 137 ± 3 MHz, in good agreement with a recent experimental determination. We estimate the CH+ column density to be a few 1013 cm-2 in the gas seen in emission, and >1014 cm-2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance [CH+] /[H] > 1.2 × 10-8. We show that the CH + column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines. © 2010 ESO.

Mookerjea B.,Tata Institute of Fundamental Research | Ossenkopf V.,University of Cologne | Ricken O.,University of Cologne | Gusten R.,Max Planck Institute for Radio Astronomy | And 5 more authors.
Astronomy and Astrophysics | Year: 2012

By observing radiation-affected gas in the Cepheus B molecular cloud, we probe whether the sequential star formation in this source is triggered by the radiation from newly formed stars. We used the dual band receiver GREAT onboard SOFIA to map [C II] and CO 13-12 and 11-10 in Cep B and compared the spatial distribution and the spectral profiles with complementary ground-based data of low-J transitions of CO isotopes, atomic carbon, and the radio continuum. The interaction of the radiation from the neighboring OB association creates a large photon-dominated region (PDR) at the surface of the molecular cloud traced through the photoevaporation of C +. Bright internal PDRs of hot gas are created around the embedded young stars, where we detect evidence of the compression of material and local velocity changes; however, on the global scale we find no indications that the dense molecular material is dynamically affected. © 2012 ESO.

Baez-Rubio A.,CSIC - National Institute of Aerospace Technology | Martin-Pintado J.,CSIC - National Institute of Aerospace Technology | Thum C.,Institute Radio Astronomia Milimetrica IRAM | Planesas P.,Observatorio Astronomico Nacional IGN | Torres-Redondo J.,CSIC - National Institute of Aerospace Technology
Astronomy and Astrophysics | Year: 2014

The UC-HII region of MWC 349A is the prototype of an ionized wind driven by a massive star surrounded by a disk. Recent high angular resolution observations of the millimeter recombination lines have shown that the disk rotates with a Keplerian law in its outer parts. However, the kinematics of innermost regions in the UC-HII region of MWC 349A is still unknown, in particular, the radius where the wind is launched from the disk. We performed hydrogen recombination line observations with the Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory to study the kinematics of its innermost regions by studying their spectral features. In addition to the two laser peaks, we report the first detection of two new components that are blueshifted with respect to the laser peaks for all the recombination lines with principal quantum number inline-formula specific-use=simple-mathitalic = 21. These new spectral features originate from the region where the wind is ejected from the disk. We used our 3D non-LTE radiative transfer model for recombination lines MORELI to show that these features are consistent with the wind being ejected at a radius of 24 AU from the star, which supports magnetohydrodynamic wind models.

Baez-Rubio A.,CSIC - National Institute of Aerospace Technology | Martin-Pintado J.,CSIC - National Institute of Aerospace Technology | Thum C.,Institute Radio Astronomia Milimetrica IRAM | Planesas P.,Observatorio Astronomico Nacional IGN
Astronomy and Astrophysics | Year: 2013

Context. The best example of a massive star with an ionized outflow launched from its photoevaporating disk is MWC349A. The large amount of reported radio-continuum and radio-recombination line (RRL) observations toward this galactic UC-HII region offers a unique possibility to build a model of the ionized envelope of this source. Aims. To understand the physical conditions and kinematics of the ionized region of the circumstellar disk and also of the outflow of MWC349A. Methods. We compared the bulk of radio-continuum maps, RRL profiles, and the H30α centroid map published to date with the predictions of our non-LTE 3D radiative transfer model, MOdel for REcombination LInes (MORELI), which we describe here in detail. Results. Our non-LTE 3D radiative transfer model provides new evidence that the UC-HII region of MWC349A is composed of an ionized circumstellar disk rotating in Keplerian fashion around a star of 38 M™, and an ionized outflow expanding with a terminal velocity of 60 km s-1 and rotating in the same sense as the disk. The model shows that while maser amplification is the dominant process involved for Hnα RRL emission with quantum numbers n < 41, stimulated emission is relevant for the emission of RRLs with n > 41 up at least the H76α line. Conclusions. For the first time, we present a model of MWC349A which satisfactorily explains the vast amount of reported observational data for a very wide range of frequencies and angular resolutions. © 2013 ESO.

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