arXiv subjects

Gerardo Ramos-Larios

Publications and source records attributed to Gerardo Ramos-Larios.

29 records · Page 2Linked to original sources

arXiv2016

Spitzer mid-infrared spectroscopic observations of planetary nebulae

We present Spitzer Space Telescope archival mid-infrared (mid-IR) spectroscopy of a sample of eleven planetary nebulae (PNe). The observations, acquired with the Spitzer Infrared Spectrograph (IRS), cover the spectral range 5.2-14.5 μm that includes the H2 0-0 S(2) to S(7) rotational emission lines. This wavelength coverage has allowed us to derive the Boltzmann distribution and calculate the H2 rotational excitation temperature (Tex). The derived excitation temperatures have consistent values ~900+/-70 K for different sources despite their different structural components. We also report the detection of mid-IR ionic lines of [Ar III], [S IV], and [Ne II] in most objects, and polycyclic aromatic hydrocarbon (PAH) features in a few cases. The decline of the [Ar III]/[Ne II] line ratio with the stellar effective temperature can be explained either by a true neon enrichment or by high density circumstellar regions of PNe that presumably descend from higher mass progenitor stars.

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arXiv2015

The excitation mechanism of H2 in bipolar planetary nebulae

We present near-IR K-band intermediate-dispersion spatially-resolved spectroscopic observations of a limited sample of bipolar planetary nebulae (PNe). The spectra have been used to determine the excitation mechanism of the H2 molecule using standard line ratios diagnostics. The H2 molecule is predominantly shock-excited in bipolar PNe with broad equatorial rings, whereas bipolar PNe with narrow equatorial waists present either UV excitation at their cores (e.g., Hb 12) or shock-excitation at their bipolar lobes (e.g., M1-92). The shock-excitation among bipolar PNe with ring is found to be correlated with emission in the H2 1-0 S(1) line brighter than Brγ. We have extended this investigation to other PNe with available near-IR spectroscopic observations. This confirms that bipolar PNe with equatorial rings are in average brighter in H2 than in Brγ and show dominant shock excitation.

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arXiv2015

Time-variable non-thermal emission in the planetary nebula IRAS 15103-5754

The beginning of photoionization marks the transition between the post-Asymptotic Giant Branch (post-AGB) and planetary nebula (PN) phases of stars with masses < 8 M_sun. This critical phase is difficult to observe, as it lasts only a few decades. The combination of jets and magnetic fields, the key agents of PNe shaping, could give rise to synchrotron emission, but this has never been observed before in any PNe, since free-free emission from the ionized gas is expected to dominate its radio spectrum. In this paper we report radio continuum observations taken with the Australia Telescope Compact Array between 1 and 46 GHz of the young PN IRAS 15103-5754. Our observations in 2010-2011 show non-thermal emission compatible with synchrotron emission from electrons accelerated at a shock with spectral index $α\simeq -0.54$. However, in 2012, the spectral index $α\simeq -0.28$ is no longer compatible with synchrotron emission in these types of processes. Several hypothesis are discussed to explain this change. The more plausible ones are related to the presence of the newly photoionized region in this young PN: either energy loss of electrons due to Coulomb collisions with the plasma, or selective suppression of synchrotron radiation due to the Razin effect. We postulate that the observed flattening of non-thermal radio spectra could be a hallmark identifying the beginning of the PN phase.

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arXiv2014

NGC 6309, a Planetary Nebula that Shifted from Round to Multipolar

We present new narrow-band Ha, [N II], and [O III] high-resolution images of the quadrupolar planetary nebula (PN) NGC 6309 that show in great detail its bipolar lobes and reveal new morphological features. New high- and low-dispersion long-slit spectra have been obtained to help in the investigation of the new nebular components. The images and spectra unveil two diffuse blobs, one of them located at 55 arcsec from the central star along the NE direction (PA= +71) and the other at 78 arcsec in the SW direction (PA= -151). Therefore, these structures do not share the symmetry axes of the inner bipolar outflows. Their radial velocities relative to the system are quite low: +3 and -4 km/s, respectively. Spectroscopic data confirm a high [O III] to Ha ratio, indicating that the blobs are being excited by the UV flux from the central star. Our images convincingly show a spherical halo 60 arcsec in diameter encircling the quadrupolar nebula. The expansion velocity of this shell is low, 66 km/s. The software SHAPE has been used to construct a morpho-kinematic model for the ring and the bipolar flows that implies an age of 4,000 yrs, the expansion of the halo sets a lower limit for its age 46,000 yrs, and the very low expansion of the blobs suggests they are part of a large structure corresponding to a mass ejection that took place 150,000 yrs ago. In NGC 6309 we have direct evidence of a change in the geometry of mass-loss, from spherical in the halo to axially-symmetric in the two pairs of bipolar lobes.

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arXiv2013

Weak and Extended H2 Emission in NGC 6369

NGC 6369 is a double-shell PN with a filamentary outer shell or envelope and faint bipolar extensions. We have used ground- and space-based narrowband optical and near-IR images, broadband mid-IR images, optical long-slit echelle spectra, and mid-IR spectra to investigate its physical structure. The observations confirm a bipolar structure for the inner shell of NGC 6369, but they also reveal evidence for H2 and strong polycyclic aromatic hydrocarbons (PAHs) emission from a photo-dissociation region (PDR) with molecular inclusions located outside the bright inner shell.

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arXiv2013

Kn 26, a New Quadrupolar Planetary Nebula

Once classified as an emission line source, the planetary nebula (PN) nature of the source Kn 26 has been only recently recognized in digital sky surveys. To investigate the spectral properties and spatio-kinematical structure of Kn 26, we have obtained high spatial-resolution optical and near-IR narrow-band images, high-dispersion long-slit echelle spectra, and intermediate-resolution spectroscopic observations. The new data reveal an hourglass morphology typical of bipolar PNe. A detailed analysis of its morphology and kinematics discloses the presence of a second pair of bipolar lobes, making Kn 26 a new member of the subclass of quadrupolar PNe. The time-lap between the ejection of the two pairs of bipolar lobes is much smaller than their dynamical ages, implying a rapid change of the preferential direction of the central engine. The chemical composition of Kn 26 is particularly unusual among PNe, with a low N/O ratio (as of type II PNe) and a high helium abundance (as of type I PNe), although not atypical among symbiotic stars. Such an anomalous chemical composition may have resulted from the curtail of the time in the Asymptotic Giant Branch by the evolution of the progenitor star through a common envelope phase.

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arXiv2012

Discovery of Multiple Shells Around the Planetary Nebula IC 418

We have analysed optical, near-, and mid-IR images of the bright planetary nebula (PN) IC 418. These images probe unambiguously for the first time a number of low surface brightness structures and shells around the bright main nebula, including radial filaments or rays, a system of three concentric rings, and two detached haloes with sizes ~ 150" and 220"\times250", respectively. The main nebula is slightly off-centered with respect to the elliptical outer halo. The time-lapse between the two haloes is 10,000-50,000 yr, whereas the time-lapse between the three concentric rings is ~ 630 yr. We emphasize the advantages of near- and mid-IR imaging for the detection of faint structures and shells around optically bright nebulae.

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arXiv2011

Optical and Infrared Imaging and Spectroscopy of the Multiple-Shell Planetary Nebula NGC 6369

NGC 6369 is a double-shell planetary nebula (PN) consisting of a bright annular inner shell with faint bipolar extensions and a filamentary envelope. We have used ground- and space-based narrow-band optical and near-IR images, broad-band mid-IR images, optical long-slit echelle spectra, and mid-IR spectra to investigate its physical structure. These observations indicate that the inner shell of NGC 6369 can be described as a barrel-like structure shape with polar bubble-like protrusions, and reveal evidence for H2 and strong polycyclic aromatic hydrocarbons (PAHs) emission from a photo-dissociative region (PDR) with molecular inclusions located outside the bright inner shell. High-resolution HST narrow-band images reveal an intricate excitation structure of the inner shell and a system of "cometary" knots. The knotty appearance of the envelope, the lack of kinematical evidence for shell expansion and the apparent presence of emission from ionized material outside the PDR makes us suggest that the envelope of NGC 6369 is not a real shell, but a flattened structure at its equatorial regions. We report the discovery of irregular knots and blobs of diffuse emission in low-excitation and molecular line emission that are located up to 80" from the central star, well outside the main nebular shells. We also show that the filaments associated to the polar protrusions have spatial extents consistent with post-shock cooling regimes, and likely represent regions of interaction of these structures with surrounding material.

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arXiv2011

Optical and Mid-Infrared Observations of the Planetary Nebula NGC 6781

Although the planetary nebula NGC 6781 appears to possess an elliptical morphology, its kinematic and emission characteristics are in many ways unusual, and it is possible that it may represent a bipolar source oriented close to the line of sight. We shall present deep imaging of this nebula in [O III], Ha and [N II], and using broad-band (F555W and F814W) filters. These were taken with the 2.56-m Nordic Optical Telescope and Hubble Space Telescope. This is combined with mid-infrared (MIR) imaging and spectroscopy acquired with the Spitzer Space Telescope (Spitzer), and near-infrared spectroscopy deriving from the Infrared Space Observatory (ISO). These reveal details of the complex [N II] structure associated with extended shell emission, perhaps associated with highly inclined bipolar lobes. We also note the presence of narrow absorbing filaments and clumps projected against the surface of the envelope, components which may be responsible for much of the molecular emission. We point out that such clumps may be responsible for complex source structure in the MIR, and give rise to asymmetries in emission along the major axis of the source.

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arXiv2011

IRAS 18113-2503: the water fountain with the fastest jet?

We present Expanded Very Large Array (EVLA) water maser observations at 22 GHz toward the source IRAS 18113-2503. Maser components span over a very high velocity range of ~500 km/s, the second largest found in a Galactic maser, only surpassed by the high-mass star forming region W49N. Maser components are grouped into a blue and a redshifted cluster, separated by 0.12". Further mid-IR and radio data suggest that IRAS 18113-2503 is a post-AGB star, thus a new bona fide member of the rare class of "water fountains". It is the evolved object with the largest total velocity spread in its water masers, and with the highest velocity dispersion within its red- and blue-shifted lobes (~170 km/s). The large total velocity range of emission probably indicates that IRAS 18113-2503 has the fastest jet among the known water fountain stars. On the other hand, the remarkably high velocity dispersion within each lobe may be interpreted in terms of shocks produced by an episode of mass ejection whose velocity increased up to very high values or, alternatively, by projection effects in a jet with a large opening angle and/or precessing motions.

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arXiv2009

A FUSE View of the Stellar Winds of Planetary Nebula Central Stars

Since the IUE satellite produced a vast collection of high-resolution UV spectra of central stars of planetary nebulae (CSPNe), there has not been any further systematic study of the stellar winds of these stars. The high spectral resolution, sensitivity and large number of archival observations in the FUSE archive allow the study of the stellar winds of CSPNe in the far UV domain where lines of species spanning a wide excitation range can be observed. We present here a preliminary analysis of the P Cygni profiles of a sample of 60 CSPNe observed by FUSE. P Cygni profiles evidencing fast stellar winds with velocities between 200 and 4,300 km/s have been found in 40 CSPNe. In many cases, this is the first time that fast stellar winds have been reported for these PNe. A detailed study of these far-UV spectra is on-going.

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