Radio Line Observations Research Articles

Sampling molecular gas in the Helix planetary nebula: Variation in HNC/HCN with UV flux

Context. Observations of molecular clouds, prestellar cores, and protoplanetary disks have established that the HNC/HCN ratio may be a potent diagnostic of molecular gas physical conditions. The processes that govern the relative abundances of these molecules nevertheless remain poorly understood. Aims. We seek to exploit the wide range of UV irradiation strengths within the ∼pc diameter Helix planetary nebula to explore the potential role of UV radiation in driving HNC/HCN. Methods. We performed IRAM 30 m and APEX 12 m radio line observations across six positions within the Helix Nebula, making use of radiative transfer and photodissociation modeling codes to interpret the results for line intensities and line ratios in terms of the molecular gas properties. Results. We have obtained the first detections of the plasma-embedded Helix molecular knots (globules) in HCN, HNC, HCO+, and other trace molecules. Analysis of the HNC/HCN integrated line intensity ratio reveals an increase with radial distance from the Helix central star. In the context of molecular line ratios of other planetary nebulae from the literature, the HNC/HCN ratio appears to be anticorrelated with UV emission over four orders of magnitude in incident flux. Models of the photodissociation regions within the Helix using the RADEX and Meudon codes reveal strong constraints on the column density (1.5–2.5 × 1012 cm−2) of the molecular gas, as well as pressure and temperature. Analysis of the molecular ion HCO+ across the Helix indicates that X-ray irradiation is likely driving HCO+ production in the outer regions of planetary nebulae, where photodissociation is limited but cold gas and ionized molecules are abundant. Conclusions. Although the observational results clearly indicate that UV irradiation is important in determining the HNC/HCN ratio, our photodissociation region modeling indicates that the UV flux gradient alone cannot reproduce the observed variation in HNC/HCN across the Helix Nebula. Instead, HNC/HCN appears to be dependent on both UV irradiation and gas pressure and density.

HCL1 and HCL2 - low mass star formation in violent and quiet environments

AbstractRecently a large number of Galactic cold clumps were located with the Planck all-sky survey. Our radio line observations have revealed the distribution and physical properties of the interstellar medium in dozens of PGCC sources. Clumps can be affected by many external effects. HCL1 (a.k.a. L1251) and HCL2 (which contains also TMC-1) are examples of low mass star forming clouds in violent and quiet environments.

De-projection of radio observations of axisymmetric expanding circumstellar envelopes

The problem of de-projection of radio line observations of axi-symmetric expanding circumstellar envelopes is studied with the aim of easing their analysis in terms of physics models. The arguments developed rest on the remark that, in principle, when the wind velocity distribution is known, the effective emissivity can be calculated at any point in space. The paper provides a detailed study of how much this is true in practice. The wind velocity distribution assumed to be axi-symmetric and in expansion, is described by four parameters: the angles defining the orientation of the symmetry axis, an overall velocity scale and a parameter measuring the elongation (prolateness) of the distribution. Tools are developed that allow for measuring, or at least constraining, each of the four parameters. The use of effective emissivity as relevant quantity, rather than temperature and density being considered separately, implies important assumptions and simplifications meaning that the approach being considered here is only a preliminary to, and by no means a replacement for, a physics analysis accounting for radiative transfer and hydrodynamics arguments. While most considerations are developed using simulated observations as examples, two case studies (EP Aqr, observed with ALMA, and RS Cnc, with NOEMA) are presented that illustrate their usefulness in practical cases.

Circumstellar molecular line emission from S-type AGB stars: mass-loss rates and SiO abundances

The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40 S-type AGB stars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGB stars and carbon stars to look for trends with chemical type. The circumstellar envelopes are assumed to be spherically symmetric and formed by a constant mass-loss rate. The mass-loss rates are estimated from fitting the CO observations using a non-local, non-LTE radiative transfer code. Once the physical properties of the circumstellar envelopes are determined, the same radiative transfer code is used to model the observed SiO lines in order to derive circumstellar abundances and the sizes of the SiO line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGB stars and carbon stars. The estimated mass-loss rates also correlate well with the corresponding expansion velocity. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGB stars. In addition, we have estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of the sample S-type AGB stars. The derived SiO abundances are, on average, about an order of magnitude higher than predicted by stellar atmosphere thermal equilibrium chemistry, indicating that non-equilibrium chemical processes determines the abundance of SiO in the circumstellar envelope. Moreover, a comparison with the results for M-type AGB stars and carbon stars show that for a certain mass-loss rate, the circumstellar SiO abundance seems independent (although with a large scatter) of the C/O-ratio.

Imaging of detached shells around the carbon stars R Scl and U Ant through scattered stellar light

We present the first optical images of scattered light from large, detached gas/dust shells around two carbon stars, R Scl and U Ant, obtained in narrow band filters centred on the resonance lines of neutral K and Na, and in a Str. b filter (only U Ant). They confirm results obtained in CO radio line observations, but also reveal new and interesting structures. Towards R Scl the scattering appears optically thick in both the K and Na filters, and both images outline almost perfectly circular disks with essentially uniform intensity out to a sharp outer radius of 21. These disks are larger -- by about a factor of two -- than the radius of the detached shell which has been marginally resolved in CO radio line data. In U Ant the scattering in the K filter appears to be, at least partially, optically thin, and the image is consistent with scattering in a geometrically thin (3) shell (radius 43) with an overall spherical symmetry. The size of this shell agrees very well with that of the detached shell seen in CO radio line emission. The scattering in the Na filter appears more optically thick, and the image suggests the presence of at least one, possibly two, shells inside the 43 shell. There is no evidence for such a multiple-shell structure in the CO data, but this can be due to considerably lower masses for these inner shells. Weak scattering appears also in a shell which is located outside the 43 shell. The present data do not allow us to conclusively identify the scattering agent, but we argue that most of the emission in the K and Na filter images is to due to resonance line scattering, and that there is also a weaker contribution from dust scattering in the U Ant data. Awaiting new observational data, our interpretation must be regarded as tentative.

The Neutral Envelopes around AGB and Post-AGB Objects

This review discusses the large-scale geometry and kinematics of envelopes around evolved stars, as inferred from radio line observations of circumstellar molecules, as a function of the evolutionary stage of the central object. In particular, the drastic change in morphology from largely spherical envelopes around asymptotic giant branch stars to distinctly non-spherical neutral envelopes around planetary nebulae is addressed. In addition, the small-scale structure of the envelopes, i.e., whether the circumstellar medium is smooth or clumpy, is discussed. Finally, a scenario for the morphological evolution, based on the assumption of a highly clumped medium, is presented.

Evidence for Infall toward Z Canis Majoris from Radio and Near‐Infrared Spectroscopy

We report radio line observations of CO(1-0),13CO(1-0), CS(5-4), CS(2-1), C34S(2-1), H13CO+(1-0), HCS+(2-1), and N2H+(1-0) and near-infrared spectroscopy (in the K, L, and M bands) of Z CMa and its surroundings. Our results show that the CS(2-1) cloud around Z CMa is in approximate virial equilibrium and has a mass of some 42 M☉. The CS(5-4), C34S(2-1), and H13CO+(1-0) data reveal a centrally condensed and flattened inner cloud core structure perpendicular to the CO(1-0) outflow, which has a dynamical timescale of some (2 × 103)-(1 × 104) yr. The mass of the CS(5-4) core is 8.0-15 M☉, which is close to the magnetic critical mass. Along the major axis of the CS(5-4) core there is a velocity gradient, which can be interpreted as a superposition of initial cloud rotation and infall. Evidence for an infalling inner cloud core with a temperature gradient, an r-1.5 density law, and an r-0.5 velocity law is provided by the redshifted self-absorption feature in the H13CO+ profile, present in a very compact region oriented perpendicularly to the CO(1-0) outflow of Z CMa. Motivated by these signs of infall in the inner cloud core, we probe with our observational data the inside-out collapse model of Shu and the predictions of Galli & Shu for the collapse of a magnetized cloud core. Our medium-resolution K-band spectrum shows besides the redshifted, very marginal Brγ line four vibrationally excited first-overtone CO band heads in absorption. The slopes seen in the K-band spectrum are intrinsic to the FU Orionis-type disk of Z CMa and suggest the presence of the vibration-rotation bands of water vapor, implying a temperature around 2000 K and a hydrogen nuclei density on the order of 1012 cm-3 at 2.2 μm. The likely cause of these water-band wings is the strong heating produced by the mass accretion through the inner disk onto the star. Our L band does not show the 3.08 μm water ice feature. Instead, our spectrum suggests the presence of a 2.9 μm feature, probably due to stretching vibrations of OH, or a shift of the water-ice band to 2.9 μm, caused by large grains when scattering dominates.

Radio line observations of comet 109P/Swift-Tuttle at IRAM

Observations are presented of comet 109P/Swift-Tuttle (1992t and 1992 XXVIII in the old style designations) obtained at the IRAM 30 m millimetre radio telescope both before (Nov. 1992) and after perihelion (Jan. 1993), when r h was ∼ 1 AU and Δ between 1 and 2 AU. The molecules HCN, H 2S, H 2CO and CH 2OH were detected, with good signal-to-noise ratios (up to 30). The line profiles are strongly asymmetric with a cusp at negative velocities; this leads to an important shift (−0.45 km s −1) of the mean gas velocity with respect to the nucleus. This profile is most probably linked to the jets seen at visual wavelengths. From methanol rotation diagrams, average rotational temperatures of 70 K for November 21 and 45 K for January are estimated. An isotropic distribution of the molecules is assumed, and Q(H 2O) ∼ 4.0 and 3.5 × 10 29 molec. s −1 for November 21 and January, respectively. Relative production rates Q Q (H 2O) of 0.1, 0.4, 0.5 and 4–7% on November 21, 1992 for HCN, H 2S, H 2CO and CH 3OH, respectively, and 0.05, 0.2 and 2% on January 6–7, 1993, for HCN, H 2CO and CH 3OH, respectively, are derived. The effect of coma anisotropy on the derivation of these rates is briefly discussed. The decrease of non-water parent molecules from November to January, to be confirmed, raises questions about the nucleus homogeneity or sublimation process.

The Local LY alpha Forest: Association of Clouds with Superclusters and Voids

The Goddard High Resolution Spectrograph aboard the Hubble Space Telescope was used with the G160M grating to obtain high-resolution (6.2 A) spectra of three very bright active galactic nuclei located behind voids in the nearby distribution of bright galaxies (i.e., CfA and Arecibo redshift survey regions). A total of eight definite (greater than or equal to 4 sigma) Ly(alpha) absorption lines were discovered ranging in equivalent width from 26 to 240 mA at Galactocentric velocities 1740-7740 km/s. Of these eight systems, we locate seven in supercluster structures and one, in the sight line of Mrk 501 at 7740 km/s, in a void. In addition, one of two tentative (3-4 sigma) Ly(alpha) absorption lines are found in voids. Thus, the voids are not entirely devoid of matter, and not all Ly(alpha) clouds are associated with galaxies. Also, since the path lengths through voids and superclusters probed by our observations thus far are nearly equal, there is some statistical evidence that the Ly(alpha) clouds avoid the voids. The nearest galaxy neighbors to these absorbing clouds are 0.45-5.9 Mpc away, too far to be physically associated by most models. The lower equivalent width absorption lines (W(sub lambda) less than or equal to 100 mA) are consistent with random locations with respect to galaxies and may be truly intergalactic, similar to the bulk of the Ly(alpha) forest seen at high z. These results on local Ly(alpha) clouds are in full agreement with those found by Morris et al. (1993) for the 3C 273 sight line but are different from the results for higher equivalent width systems where closer cloud-galaxy associations were found by Lanzetta et al. (1994). Pencil-beam optical and 21 cm radio line observations of the area of sky surrounding Mrk 501 fail to find faint galaxies near the velocities of the Ly(alpha) clouds in that sight line. Specifically, for the 'void absorption' system at 7740 km/s, we find no galaxy at comparable redshift to the absorber within 100 h(sub 75)(sup -1) kpc (H(sub 0) = 75 h(sub 75) km/s Mpc(sup -1)) with an absolute magnitude of B less than or equal to - 16 and no object with H I mass greater than or equal to 7 x 10(exp 8) h(sub 75)(sup -2) M(solar) within 500 h(sub 75)(sup -1) kpc. Thus, neither a faint optical galaxy nor a gas-rich, optically dim or low surface brightness galaxy is present close to this absorber.

The Kinematics of the Ionized Filaments in Sgr A West

The ionized gas within a few parsecs of the center of our galaxy is contained primarily in a number of thin elongated filaments (Ekers, this volume). Infrared and radio line observations have succeeded in determining the gas velocities along the majority of these filaments. A comparison of the line emission velocities with general kinematic models indicates that the motion of the filaments between 0.3 and 2 pc from the center is consistent with predominantly orbital motion.

Molecular Radio Line Observations of Mass Loss From Red Giants

AbstractThe number of molecules detected at radio wavelengths in envelopes around red giants stands presently at 36. Among these OH and CO have proven to be the most useful for the study of the physical characteristics of a circumstellar envelope. The mass loss rate of the central star can be relatively accurately estimated and it appears possible to trace its evolution with time. Also fascinating objects in transition from the red giant phase to the planetary nebula phase are becoming observationally accessible.