arXiv subjects

A. Fuente

Publications and source records attributed to A. Fuente.

At least 181 records · Page 10Linked to original sources

arXiv2003

XDR Chemistry in the Circumnuclear Disk of NGC 1068

We have studied the feedback influence that the central engine of the Seyfert 2 galaxy NGC 1068 may have on the chemistry of the 200 pc circumnuclear gas disk (CND). With this purpose, we have conducted a multi-species/multi-transition survey of molecular gas in the CND of NGC 1068 using the IRAM 30m telescope. Abundances of several molecular species have been estimated, including HCN, CN, CS, HCO+, SiO and HOC+. We report on the detection of significant SiO emission in this galaxy, as well as on the first extragalactic detection of the active radical HOC+. We conclude that the chemistry of the molecular gas reservoir in the CND can be best explained in the framework of X-rays Dominated Regions (XDR) models.

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arXiv2003

Molecular Gas Chemistry in Starbursts and AGN

We present the main results of an extragalactic survey aiming to study the chemistry of molecular gas in a limited sample of starburst galaxies (SB) and AGN hosts. Observations have been carried out with the IRAM 30m telescope and the Plateau de Bure Interferometer (PdBI). The high resolution/sensitivity of the PdBI has made possible to obtain high quality images of the galaxies using specific molecular gas tracers of Shock Chemistry, Photon Dominated Regions (PDR) and X-ray Dominated Regions (XDR). The occurrence of large-scale shocks and the propagation of PDR chemistry in starbursts can be studied. We also discuss the onset of XDR chemistry in AGN.

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arXiv2003

First evidence for dusty disks around Herbig Be stars

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.4mm and 2.7mm using the IRAM interferometer at the Plateau de Bure (PdBI) . In this letter, we report data on three well studied B0 stars, MWC 1080, MWC 137 and R Mon. The two latter have also been observed in the continuum at 0.7 cm and 1.3 cm using the NRAO Very Large Array (VLA) . We report the detection of circumstellar disks around MWC 1080 and R Mon with masses of Md ~ 0.003 and 0.01 Msun, respectively, while for MWC 137 we estimate a disk mass upper limit of 0.007 Msun. Our results show that the ratio Md/M* is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.

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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

Warm molecular gas, dust and ionized gas in the 500 central pc of the Galaxy

We present infrared and millimeter observations of molecular gas, dust and ionized gas towards a sample of clouds distributed along the 500 central pc of the Galaxy. The clouds were selected to investigate the physical state, in particular the high gas temperatures, of the Galactic center region (GCr) clouds located far from far-infrared of thermal radio continuum sources. We have found that there is ionized gas associated with the molecular gas. The ionizing radiation is hard (~35000 K) but diluted due to the inhomogeneity of the medium. We estimate that ~30 % of the warm molecular gas observed in the GCr clouds is heated by ultra-violet radiation in photo-dissociation regions.

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arXiv2002

Widespread HCO emission in the M82's nuclear starburst

We present a high-resolution (~ 5'') image of the nucleus of M82 showing the presence of widespread emission of the formyl radical (HCO). The HCO map, the first obtained in an external galaxy, reveals the existence of a structured disk of ~ 650 pc full diameter. The HCO distribution in the plane mimics the ring morphology displayed by other molecular/ionized gas tracers in M82. More precisely, rings traced by HCO, CO and HII regions are nested, with the HCO ring lying in the outer edge of the molecular torus. Observations of HCO in galactic clouds indicate that the abundance of HCO is strongly enhanced in the interfaces between the ionized and molecular gas. The surprisingly high overall abundance of HCO measured in M82 (X(HCO) ~ 4x10^{-10}) indicates that its nuclear disk can be viewed as a giant Photon Dominated Region (PDR) of ~ 650 pc size. The existence of various nested gas rings, with the highest HCO abundance occurring at the outer ring (X(HCO) ~ 0.8x10^{-9}), suggests that PDR chemistry is propagating in the disk. We discuss the inferred large abundances of HCO in M82 in the context of a starburst evolutionary scenario, picturing the M82 nucleus as an evolved starburst.

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arXiv2002

Warm molecular gas in the Galactic center region

We present some recent studies on the physical conditions and the chemistry of the interstellar medium (ISM) in the Galactic center region (GCR). Large scale shocks in the context of a bar potential can play a role in explaining the observed properties, at least in some particular regions. Alternatively, we propose that the observed properties (inhomogeneity, high temperatures of the molecular gas, rich chemistry,...) of the GCR ISM can be the consequence of the recent past ($10^6-10^7$ years ago) of massive star formation in the GCR.

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arXiv2002

SiO chimneys and supershells in M82

In this Letter we present the first images of the emission of SiO and H13CO+ in the nucleus of the starburst galaxy M82. Contrary to other molecular species that mainly trace the distribution of the star-forming molecular gas within the disk, the SiO emission extends noticeably out of the galaxy plane. The bulk of the SiO emission is restricted to two major features. The first feature, referred to as the SiO supershell, is an open shell of 150 pc diameter, located 120 pc west from the galaxy center. The SiO supershell represents the inner front of a molecular shell expanding at 40 km/s, produced by mass ejection around a supercluster of young stars containing supernova remnant SNR 41.95+57.5. The second feature is a vertical filament, referred to as the SiO chimney, emanating from the disk at 200 pc east from the galaxy center. The SiO chimney reaches a 500 pc vertical height, and it is associated with the most prominent chimney identified in radio continuum maps. The kinematics, morphology, and fractional abundances of the SiO gas features in M82 can be explained in the framework of shocked chemistry driven by local episodes of gas ejection from the starburst disk.

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arXiv2001

Cometary molecular clouds around RNO6. On-going star formation near the double cluster h and chi Persei

We present molecular line observations of the star-forming cloud around RNO6 along with a newly discovered nearby molecular cloud that we name RNO6NW. Both clouds display striking similarities in their cometary structures and overall kinematics. By using 13CO line observations, we estimate that these clouds have similar sizes (~4.5 pc) and masses (~200 solar masses). Both molecular clouds RNO6 and RNO6NW are active in star formation. From new high resolution near-IR narrowband images, we confirm that RNO6 hosts an embedded IR cluster that includes a Herbig Be star. A conspicuous H2 filament is found to delineate the dense cometary head of the globule. RNO6NW hosts at least two IR sources and a bipolar molecular outflow of ~0.9 pc of length and ~0.5 solar masses. We show that the cometary structure of both clouds has been created by the UV radiation from numerous OB stars lying ~1.5 degree to the north. Such OB stars are associated with the double cluster h and chi Persei, and are probably members of the PerOB1 association. Thus star formation inside these clouds has been very likely triggered by the Radiation Driven Implosion (RDI) mechanism. From comparison to RDI theoretical models, we find that the similar kinematics and morphology of both clouds is well explained if they are at a re-expansion phase. Triggered sequential star formation also explains the observed spatial distribution of the members of the near-IR cluster inside the RNO6 cloud, and the morphology of the H2 filament. We conclude that the RNO6 and RNO6NW clouds are high-mass counterparts to the cometary globules of smaller masses which have been studied up to now. Thus our observations demonstrate that the RDI mechanism can produce, not only low mass stars in small globules, but also intermediate mass stars and clusters in massive clouds.

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arXiv2001

A High Density Thin layer confining the HII region M42. HHT measurements

We present HHT observations in the N=3-2 rotational transition of the CN radical toward selected positions of the Trapezium region and of the molecular Ridge in the Orion molecular cloud. Two of the positions in the Ridge were also observed in the N=2-1 line of CN and 13CN. The N=3-2 CN lines have been combined with observations of the N=2-1 and N=1-0 transitions of CN, and of the N=2-1 of 13CN to estimate the physical conditions and CN abundances in the molecular gas. We analyze in detail the excitation of the CN lines and find that the hyperfine ratios of the N=3-2 line are always close to the Local Thermodynamic Equilibrium (LTE) optically thin values even in the case of optically thick emission. This is due to different excitation temperatures for the different hyperfine lines. From the line intensity ratios between the different CN transitions we derive H_2 densities of ~10^5cm^-3 for the molecular Ridge and of ~3x10^6cm^-3 for the Trapezium region. The CN column densities are one order of magnitude larger in the Ridge than in the Trapezium region, but the CN to H_2 ratio is similar both in the Trapezium and in the Ridge. The combination of the low CN column densities, high H_2 densities and relatively high CN abundances toward the Trapezium region requires that the CN emission arises from a thin layer with a depth along the line of sight of only \~5x10^15cm. This high density thin layer of molecular gas seems to be related with material that confines the rear side of the HII region Orion A. However the molecular layer is not moving as expected from the expansion of the HII region, but it is ``static'' with respect to the gas in the molecular cloud. We discuss the implication of a high density ``static'' layer in the evolution of an HII region.

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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

Warm H2 in the Galactic center region

We present ISO observations of several H2 pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ~ 500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1 observations of these sources made with the IRAM-30m telescope. With the CO data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the H2 emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is predominantly gas with T=150 K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm H2 column densities represent a fraction of ~ 30 % of the gas traced by the CO isotopes emission. The cooling by H2 in the warm component is comparable to that by CO. Comparing our H2 and CO data with available ammonia NH3 observations from literature one obtains relatively high NH3 abundances of a few 10e(-7) in both the warm and the cold gas. A single shock or Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines. Alternatives for the heating mechanisms are discussed.

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arXiv2000

A correlation between the SiO and the Fe 6.4 keV line emission from the Galactic center

In this letter we present a map of the GC in the 1-0 line of SiO covering the same region than the ASCA observations of the Fe 6.4 keV line. The SiO emission is restricted to molecular clouds with radial velocity between 10 and 60 km/s in the Sgr A and Sgr B complexes. We find a correlation between the SiO morphology and the spatial distribution of the Fe 6.4 keV line, on the large scale and also within Sgr A and Sgr B. The SiO abundance increases by a factor of 20 in the regions with strong Fe 6.4 keV line. This indicates that the Fe 6.4 keV line mainly arises from molecular clouds with large gas phase abundance of refractory elements. We discuss the implications of the correlation on the origin of the hard X-rays, and the heating and the chemistry of the molecular clouds in the GC

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arXiv2000

Non-equilibrium H2 ortho-to-para ratio in two molecular clouds of the Galactic Center

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of 13CO and C18O and J=2-1 of C18O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of 15-20 magnitudes toward these GC clouds. After correcting the H2 data for extinction, the gas temperatures are \~250 K and the column densities of warm gas are ~2e21 cm-2. This is the first direct measure of the H2 column densities of the warm component; with this, we estimate an NH3 abundance in the warm gas of ~ 2e-7. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is ~ 1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250 K of ~3. Low velocity shocks (~ 10 km/s) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H2 OTPR.

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arXiv2000

Large-scale shocks in the starburst galaxy NGC253. Interferometer mapping of a 600 pc SiO/H13CO+ circumnuclear disk

This paper presents the first high-resolution SiO map made in an external galaxy. The nucleus of the nearby barred spiral NGC253 has been observed simultaneously in the v=0, J=2-1 line of SiO and in the J=1-0 line of H13CO+ with the IRAM interferometer, with a resolution of 7.5"x2.6". Emission from SiO and H13CO+ is extended in the nucleus of NGC253. The bulk of the SiO/H13CO+ emission arises from a 600pcx250pc circumnuclear disk (CND) with a double ringed structure. The inner ring, of radius r=60pc, viewed edge-on along PA=51deg, hosts the nuclear starburst; the outer pseudo-ring opens out as a spiral-like arc up to r=300pc. The kinematics of the gaseous disk, characterized by strong non-circular motions, is interpreted in terms of the resonant response of the gas to the barred potential. The inner ring would correspond to the inner Inner Lindblad Resonance(iILR), whereas the outer region is linked to the onset of a trailing spiral wave across the outer ILR (oILR). Most notably, we report the detection of a molecular gas counterpart of the giant outflow of hot gas, previously seen in X-ray and optical lines, and tentatively identified as a dust chimney in the the 450mu continuum band.The SiO shows a high average fractional abundance in the CND of X(SiO)=1.5x10E-10. This is more than an order of magnitude above the predicted value of a PDR. Moreover, X(SiO) varies at least by an order of magnitude between the inner starburst region, which dominates the global emission, where we derive X(SiO)=1--2x10E-10, and the outer region, where X(SiO) reaches a few 10E-9. SiO abundance is also significantly enhanced in the outflow (X(SiO)=3--5x10E-10). Different mechanisms are explored to explain the unlike chemical processing of molecular gas within the nucleus.

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arXiv2000

ISO SWS-LWS observations of the prototypical reflection nebula NGC 7023

We present SWS and LWS ISO observations towards a strip across the photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and LWS01 spectra have been taken towards the star and the brightest infrared filaments located NW and SW from the star (hereafter NW and SW PDRs). In addition, SWS02 spectra have been taken towards two intermediate positions. While all the PDR tracers (the [CII] 157.7$\mic$, [OI] 63.2 and 145.6$\mic$, [HI] 21cm and the H$_2$ rotational lines) present a ring-like morphology with the peaks toward the NW and SW PDRs and a minimum around the star, the SiII emission is filling the hole of this ring with the peak towards the star. This morphology can only be explained if the SiII emission arise in the lowest extinction layers of the PDR (A$_v$ $<$ 2 mag) and the HII region. At least 20% -- 30% of the Si must be in gas phase in these layers. For A$_v$ $\geq$ 2 mag, the Si is mainly in solid form ($δ$ Si = -1.3). In both, the NW and SW PDRs, the intensities of the H$_2$ rotational lines can only be fitted by assuming an ortho-to-para-H$_2$ ratio lower than 3 in gas with rotation temperatures from 400 to 700 K. Therefore, there is a non-equilibrium OTP ratio in the region. The comparison between the OTP ratio derived from H$_2$ vibrational lines and the pure H$_2$ rotational lines, shows that the OTP ratio increases from $\sim$ 1.5 to 3 across the photodissociation region with larger values in the less shielded gas (A$_v$$<$ 0.7 mag). This behavior is interpreted as a consequence of an advancing photodissociation front.

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arXiv1999

ISO observations toward the reflection nebula NGC 7023: A nonequilibrium ortho- to para-H2 ratio

We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical PDR is discussed as the most likely explanation for this low OTP ratio.

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arXiv1997

SiO emission from the Galactic Center Molecular Clouds

We have mapped the J=1-0 line of SiO in a 1 degree x 12 arcmin. (lxb) region around the Galactic center (GC) with an angular resolution of 2 arcmin. (approx. 4 pc). In contrast to the spatial distribution of other high dipole moment molecules like CS, whose emission is nearly uniform, the SiO emission is very fragmented and it is only associated with some molecular clouds. In particular, it is remarkable that the SiO emission closely follows the non-thermal radio arc in the GC. The SiO clouds are more extended than the beam with typical sizes between 4 and 20 pc. High angular resolution (26 arcsec.) mapping in the J=1-0 line of SiO toward the molecular clouds in Sgr B2 and Sgr A shows that the SiO emission is relatively smooth with structures of typically 2 pc. From the line intensities of the J=1-0, J=3-2 and J=5-4 transitions of SiO we derive H2 densities for these clouds of a few 1e4 cm-3. The SiO fractional abundances are approx. 1e-9 for the SiO clouds and 1e-10 for the other molecular clouds in the GC. The characteristics (size and H2 densities) of the SiO emission in the GC are completely different from those observed in the Galactic disk, where the SiO emission arises from much smaller regions with larger H2 densities. We briefly discuss the implications of the SiO emission in the molecular clouds of the GC. We conclude that the particular chemistry in these clouds is probably related to large scale fast shocks occurring in the Galactic center region.

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