arXiv subjects

J. R. Goicoechea

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

At least 145 records · Page 8Linked to original sources

arXiv2010

Detection of anhydrous hydrochloric acid, HCl, in IRC+10216 with the Herschel SPIRE and PACS spectrometers

We report on the detection of anhydrous hydrochloric acid (hydrogen chlorine, HCl) in the carbon-rich star IRC+10216 using the spectroscopic facilities onboard the Herschel satellite. Lines from J=1-0 up to J=7-6 have been detected. From the observed intensities, we conclude that HCl is produced in the innermost layers of the circumstellar envelope with an abundance relative to H2 of 5x10^-8 and extends until the molecules reach its photodissociation zone. Upper limits to the column densities of AlH, MgH, CaH, CuH, KH, NaH, FeH, and other diatomic hydrides have also been obtained.

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arXiv2010

Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)

We report the detection of strong absorption by interstellar hydrogen fluoride along the sight-line to the submillimeter continuum source G10.6-0.4 (W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz J=1-0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 at LSR velocities in the range -10 to -3 km/s, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km/s. The spectrum is similar to that of the 1113.3430 GHz 1(11)-0(00) transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H2O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6E+14 cm-2 on the HF column density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance, 6E-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ~ 30 and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that - because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen - HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions.

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arXiv2010

The 35Cl/37Cl isotopic ratio in dense molecular clouds: HIFI observations of hydrogen chloride towards W3A

We report on the detection with the HIFI instrument on board the Herschel satellite of the two hydrogen chloride isotopologues, H35Cl and H37Cl, towards the massive star-forming region W3A. The J=1-0 line of both species was observed with receiver 1b of the HIFI instrument at 625.9 and 624.9 GHz. The different hyperfine components were resolved. The observations were modeled with a non-local, non-LTE radiative transfer model that includes hyperfine line overlap and radiative pumping by dust. Both effects are found to play an important role in the emerging intensity from the different hyperfine components. The inferred H35Cl column density (a few times 1e14 cm^-2), and fractional abundance relative to H nuclei (~7.5e^-10), supports an upper limit to the gas phase chlorine depletion of ~200. Our best-fit model estimate of the H35Cl/H37Cl abundance ratio is ~2.1+/-0.5, slightly lower, but still compatible with the solar isotopic abundance ratio (~3.1). Since both species were observed simultaneously, this is the first accurate estimation of the [35Cl]/[37Cl] isotopic ratio in molecular clouds. Our models indicate that even for large line opacities and possible hyperfine intensity anomalies, the H35Cl and H37Cl J=1-0 integrated line-intensity ratio provides a good estimate of the 35Cl/37Cl isotopic abundance ratio.

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arXiv2010

Detection of interstellar oxidaniumyl: abundant H2O+ towards the star-forming regions DR21, Sgr B2, and NGC6334

We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin. The characteristic hyperfine pattern of the H2O+ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H2O+ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation. In DR21, the velocity distribution of H2O+ matches that of the [CII] line at 158μ\m and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the HII-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H2O+ column density of 7.2e12 cm^-2 in NGC 6334, 2.3e13 cm^-2 in DR21, and 1.1e15 cm^-2 in Sgr B2.

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arXiv2010

HIFI observations of warm gas in DR21: Shock versus radiative heating

The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. HIFI observations of mid-J transitions of CO and HCO+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. In spite of the known shock-excitation of H2 and the clearly visible strong outflow, we find that the emission of all lines up to > 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.

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arXiv2010

Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

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arXiv2010

A line confusion limited millimeter survey of Orion KL (I): sulfur carbon chains

We perform a sensitive (line confusion limited), single-side band spectral survey towards Orion KL with the IRAM 30m telescope, covering the following frequency ranges: 80-115.5 GHz, 130-178 GHz, and 197-281 GHz. We detect more than 14 400 spectral features of which 10 040 have been identified up to date and attributed to 43 different molecules, including 148 isotopologues and lines from vibrationally excited states. In this paper, we focus on the study of OCS, HCS+, H2CS, CS, CCS, C3S, and their isotopologues. In addition, we map the OCS J=18-17 line and complete complementary observations of several OCS lines at selected positions around Orion IRc2 (the position selected for the survey). We report the first detection of OCS v2 = 1 and v3 = 1 vibrationally excited states in space and the first detection of C3S in warm clouds. Most of CCS, and almost all C3S, line emission arises from the hot core indicating an enhancement of their abundances in warm and dense gas. Column densities and isotopic ratios have been calculated using a large velocity gradient (LVG) excitation and radiative transfer code (for the low density gas components) and a local thermal equilibrium (LTE) code (appropriate for the warm and dense hot core component), which takes into account the different cloud components known to exist towards Orion KL, the extended ridge, compact ridge, plateau, and hot core. The vibrational temperature derived from OCS v2 = 1 and v3 = 1 levels is about 210 K, similar to the gas kinetic temperature in the hot core. These OCS high energy levels are probably pumped by absorption of IR dust photons. We derive an upper limit to the OC3S, H2CCS, HNCS, HOCS+, and NCS column densities. Finally, we discuss the D/H abundance ratio and infer the following isotopic abundances: 12C/13C=45+-20, 32S/34S=20+-6, 32S/33S=75+-29, and 16O/18O=250+-135.

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arXiv2010

The chemistry of vibrationally excited H2 in the interstellar medium

The internal energy available in vibrationally excited H2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this article we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H2 with C+, He+, O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H2 (v>0) and C+ has a profound impact on the abundances of some molecules, especially CH+, which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H2, relative to ground state H2, in excess of 10^(-6), independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds zeta Oph and HD 34078, the dense PDR Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH+ is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10^(13) cm^(-2) are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH+ with a column density close to the values derived from astronomical observations.

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arXiv2010

Exoplanetary Systems with SAFARI: A Far Infrared Imaging Spectrometer for SPICA

The far-infrared (far-IR) spectral window plays host to a wide range of spectroscopic diagnostics with which to study planetary disk systems and exoplanets at wavelengths completely blocked by the Earth atmosphere. These include the thermal emission of dusty belts in debris disks, the water ice features in the "snow lines" of protoplanetary disks, as well as many key chemical species (O, OH, H2O, NH3, HD, etc). These tracers play a critical diagnostic role in a number of key areas including the early stages of planet formation and potentially, exoplanets. The proposed Japanese-led IR space telescope SPICA, with its 3m-class cooled mirror (~5 K) will be the next step in sensitivity after ESA's Herschel Space Observatory (successfully launched in May 2009). SPICA is a candidate "M-mission" in ESA's Cosmic Vision~2015-2025 process. We summarize the science possibilities of SAFARI: a far-IR imaging-spectrometer (covering the ~34-210um band) that is one of a suite of instruments for SPICA.

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arXiv2009

Polycyclic aromatic hydrocarbons with SPICA

Thanks to high sensitivity and angular resolution and broad spectral coverage, SPICA will offer a unique opportunity to better characterize the nature of polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs), to better use them as probes of astrophysical environments. The angular resolution will enable to probe the chemical frontiers in the evolution process from VSGs to neutral PAHs, to ionized PAHs and to "Grand-PAHs" in photodissotiation regions and HII regions, as a function of G$_0$/n (UV radiation field / density). High sensitivity will favor the detection of the far-IR skeletal emission bands of PAHs, which provide specific fingerprints and could lead to the identification of individual PAHs. This overall characterization will allow to use PAH and VSG populations as tracers of physical conditions in spatially resolved protoplanetary disks and nearby galaxies (using mid-IR instruments), and in high redshift galaxies (using the far-IR instrument), thanks to the broad spectral coverage SPICA provides. Based on our previous experience with ISO and Spitzer we discuss how these goals can be reached.

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arXiv2009

Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs

Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and molecular hydrogen emission are a potentially powerful tool to derive physical properties of dense environments irradiated by intense UV fields. We present new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high UV-field and dense photodissocation region (PDR) around Monoceros R2, the closest ultracompact \hII region, revealing the spatial structure of ionized gas, PAHs and H$_2$ emissions. Using a PDR model and PAH emission feature fitting algorithm, we build a comprehensive picture of the physical conditions prevailing in the region. We show that the combination of the measurement of PAH ionization fraction and of the ratio between the H$_2$ 0-0 S(3) and S(2) line intensities, respectively at 9.7 and 12.3 $μ$m, allows to derive the fundamental parameters driving the PDR: temperature, density and UV radiation field when they fall in the ranges $T = 250-1500 $K, $n_H=10^4-10^6$cm$^{-3}$, $G_0=10^3-10^5$ respectively. These mid-IR spectral tracers thus provide a tool to probe the similar but unresolved UV-illuminated surface of protoplanetary disks or the nuclei of starburst galaxies.

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arXiv2009

Simulated CII observations for SPICA/SAFARI

We investigate the case of CII 158 micron observations for SPICA/SAFARI using a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in length. The interplay of thermal instability, magnetic field and self-gravity leads to the formation of cold, dense clumps within a warm, turbulent interclump medium. We sample several clumps along a line of sight through the simulated cube and use them as input density profiles in the Meudon PDR code. This allows us to derive intensity predictions for the CII 158 micron line and provide time estimates for the mapping of a given sky area.

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arXiv2009

The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds

The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13C fractionation reactions and HCO+/HOC+ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. The detection of HOC+ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few times ~10(-9) in the core. PAH^- anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] >10(-8)).

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arXiv2009

Exoplanet research with SAFARI: A far-IR imaging spectrometer for SPICA

The far-IR spectral window plays host to a wide range of both spectroscopic and photometric diagnostics with which to open the protoplanetary disks and exoplanet research to wavelengths completely blocked by the Earth's atmosphere. These include some of the key atomic (e.g., oxygen) and molecular (e.g., water) cooling lines, the dust thermal emission, the water ice features as well as many other key chemical tracers. Most of these features can not be observed from ground-based telescopes but play a critical diagnostic in a number of key areas including the early stages of planet formation and potentially exoplanets. The proposed Japanese-led IR space telescope SPICA, with its 3.5m cooled mirror will be the next step in sensitivity after Herschel. SPICA has been recently selected to go to the next stage of the ESA's Cosmic Vision 2015-2025 process. This contribution summarizes the design concept behind SAFARI: an imaging far-IR spectrometer covering the ~30-210um waveband that is one of a suite of instruments for SPICA; it also highlights some of the science questions that it will be possible to address in the field of exoplanets and planet formation.

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arXiv2008

Using SPICA Space Telescope to characterize Exoplanets

We present the 3.5m SPICA space telescope, a proposed Japanese-led JAXA-ESA mission scheduled for launch around 2017. The actively cooled (<5 K), single aperture telescope and monolithic mirror will operate from ~3.5 to ~210 um and will provide superb sensitivity in the mid- and far-IR spectral domain (better than JWST at lambda > 18 um). SPICA is one of the few space missions selected to go to the next stage of ESA's Cosmic Vision 2015-2025 selection process. In this White Paper we present the main specifications of the three instruments currently baselined for SPICA: a mid-infrared (MIR) coronagraph (~3.5 to ~27 um) with photometric and spectral capabilities (R~200), a MIR wide-field camera and high resolution spectrometer (R~30,000), and a far-infrared (FIR ~30 to ~210 um) imaging spectrometer - SAFARI - led by a European consortium. We discuss their capabilities in the context of MIR direct observations of exo-planets (EPs) and multiband photometry/high resolution spectroscopy observations of transiting exo-planets. We conclude that SPICA will be able to characterize the atmospheres of transiting exo-planets down to the super-Earth size previously detected by ground- or space-based observatories. It will also directly detect and characterize Jupiter/Neptune-size planets orbiting at larger separation from their parent star (>5-10 AU), by performing quantitative atmospheric spectroscopy and studying proto-planetary and debris disks. In addition, SPICA will be a scientific and technological precursor for future, more ambitious, IR space missions for exo-planet direct detection as it will, for example, quantify the prevalence exo-zodiacal clouds in planetary systems and test coronographic techniques, cryogenic systems and lightweight, high quality telescopes. (abridged)

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arXiv2008

Physical parameters for Orion KL from modelling its ISO high resolution far-IR CO line spectrum

As part of the first high resolution far-IR spectral survey of the Orion KL region (Lerate et al. 2006), we observed 20 CO emission lines with Jup=16 to Jup=39 (upper levels from approx 752 K to 4294 K above the ground state). Observations were taken using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO), in its high resolution Fabry-Perot (FP) mode (approx 33 km s$^{-1}$). We present here an analysis of the final calibrated CO data, performed with a more sophisticated modelling technique than hitherto, including a detailed analysis of the chemistry, and discuss similarities and differences with previous results. The inclusion of chemical modelling implies that atomic and molecular abundances are time-predicted by the chemistry. This provides one of the main differences with previous studies in which chemical abundances needed to be assumed as initial condition. The chemistry of the region is studied by simulating the conditions of the different known components of the KL region: chemical models for a hot core, a plateau and a ridge are coupled with an accelerated Lambda-iteration (ALI)radiative transfer model to predict line fluxes and profiles. We conclude that the CO transitions with 18<Jup<25 mainly arise from a hot core of diameter 0.02 pc and a density of 10$^{7}$ cm$^{-3}$ rather from the plateau as previous studies had indicated.

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arXiv2007

Sulfur chemistry in the Horsehead. An interferometric view of the Horsehead PDR

Sulfur is an abundant element which remains undepleted in diffuse interstellar gas (Av<1) but it is traditionally assumed to deplete on dust grains at higher densities and larger Av. Photodissociation regions (PDRs) are an interesting intermediate medium between translucent and dark clouds where the chemistry and energetics are dominated by the illuminating FUV radiation field. Thus they can provide new insights about the sulfur depletion problem. However, physical and chemical gradients in PDRs take place at small angular scales (~1 to 10''). Aperture synthesis observations are therefore required to resolve such gradients. Besides, a complete understanding of molecular excitation is needed to correctly determine molecular abundances but also the preavailing physical conditions. Hence, multi-J observations at increasing frequencies are also required. Such high angular resolution and broad frequency coverage observations will be provided by ALMA in the near future. In this work we present IRAM-PdBI observations of the CS J=2-1 line toward the Horsehead PDR complemented with IRAM-30m observations of several rotational lines of different sulfur bearing molecules (CS, HCS+, SO, H2S, etc.). Photochemical and nonlocal, non-LTE radiative transfer models adapted to the Horsehead geometry have been developed. The gas phase sulfur abundance has been inferred in the PDR.

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arXiv2007

Molecular survey of CRL618

We present the complete data set, model and line identification of a survey of the emission from the C-rich protoplanetary nebula CRL 618 performed with the IRAM-30m telescope in the following frequency ranges: 80.25-115.75 GHz, 131.25-179.25 GHz, and 204.25-275.250 GHz. A selection of lines from different species has been used in previous works to derive the structure of the source, its physical conditions and the chemical abundances in the different gas regions. In this work, we have used this information to run a global simulation of the spectrum in order to check the consistency of the model and to ease the task of line identification. The total number of lines that have a correspondence in both data and model is ~3100, although quite often in this object many lines blend into complex features so that the model, that takes into account line blending, is a key tool at this stage of the analysis. Of all the lines that we have been able to label, ~55% of them belong to the different forms of HC3N, and ~18% to those of HC5N. The density of remaining unidentified features above the 3sigma limit is only one per ~2.1 GHz (74 features), which is unprecedented in the analysis of this type of large millimeter-wave line surveys.

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