- ID:
- ivo://CDS.VizieR/J/A+A/641/L4
- Title:
- Radio source J1402+5347 1.4GHz light curves
- Short Name:
- J/A+A/641/L4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Propagation of the radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy leads to twinkling of these sources, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. intensity modulation of a few per cent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the Milky Way's interstellar plasma. So far, only three sources were known that show more extreme variations, with modulations at the level of tens of per cent on timescales less than an hour. This requires propagation through nearby (d<~10pc) anomalously dense (n_e_>10^2^cm^3^) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (~50%) and rapid variations on a timescale of just 6.5 minutes in the decimetre band (1.4GHz). The spatial scintillation pattern is highly anisotropic with a semi-minor axis of about 20,000 km. Canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sight-line to J1402+5347 however passes unusually close to the B3 star Alkaid (eta UMa) at a distance of 32pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor-axis of the scintels must be smaller than 10 microarcseconds yielding an apparent brightness temperature for an isotropic source of >10^14^K.
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- ID:
- ivo://CDS.VizieR/J/A+A/602/A61
- Title:
- Radio spectra of globulettes in Carina nebula
- Short Name:
- J/A+A/602/A61
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Carina nebula hosts a large number of globulettes. An optical study of these tiny molecular clouds shows that the majority are of planetary mass, but there are also those with masses of several tens up to a few hundred Jupiter masses. We seek to search for, and hopefully detect, molecular line emission from some of the more massive objects; in case of successful detection we aim to map their motion in the Carina nebula complex and derive certain physical properties. Methods.We carried out radio observations of molecular line emission in ^12^CO and ^13^CO (2-1) and (3-2) of 12 globulettes in addition to positions in adjacent shell structures using APEX. All selected objects were detected with radial velocities shifted relative to the emission from related shell structures and background molecular clouds. Globulettes along the western part of an extended dust shell show a small spread in velocity with small velocity shifts relative to the shell. This system of globulettes and shell structures in the foreground of the bright nebulosity surrounding the cluster Trumpler 14 is expanding with a few km/s relative to the cluster. A couple of isolated globulettes in the area move at similar speed. Compared to similar studies of the molecular line emission from globulettes in the Rosette nebula, we find that the integrated line intensity ratios and line widths are very different. The results show that the Carina objects have a different density/temperature structure than those in the Rosette nebula. In comparison the apparent size of the Carina globulettes is smaller, owing to the larger distance, and the corresponding beam filling factors are small. For this reason we were unable to carry out a more detailed modelling of the structure of the Carina objects in the way as performed for the Rosette objects. The Carina globulettes observed are compact and denser than objects of similar mass in the Rosette nebula. The distribution and velocities of these globulettes suggest that they have originated from eroding shells and elephant trunks. Some globulettes in the Trumpler 14 region are quite isolated and located far from any shell structures. These objects move at a similar speed as the globulettes along the shell, suggesting that they once formed from cloud fragments related to the same foreground shell.
- ID:
- ivo://CDS.VizieR/J/ApJ/872/56
- Title:
- R-band linear polarization of Gaia stars
- Short Name:
- J/ApJ/872/56
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The availability of large data sets with stellar distance and polarization information will enable a tomographic reconstruction of the (plane-of-the-sky-projected) interstellar magnetic field in the near future. We demonstrate the feasibility of such a decomposition within a small region of the diffuse interstellar medium (ISM). We combine measurements of starlight (R-band) linear polarization obtained using the RoboPol polarimeter with stellar distances from the second Gaia data release. The stellar sample is brighter than 17mag in the R-band and reaches out to several kiloparsecs from the Sun. HI emission spectra reveal the existence of two distinct clouds along the line of sight. We decompose the line-of-sight-integrated stellar polarizations to obtain the mean polarization properties of the two clouds. The two clouds exhibit significant differences in terms of column density and polarization properties. Their mean plane-of-the-sky magnetic field orientation differs by 60{deg}. We show how our tomographic decomposition can be used to constrain our estimates of the polarizing efficiency of the clouds as well as the frequency dependence of the polarization angle of polarized dust emission. We also demonstrate a new method to constrain cloud distances based on this decomposition. Our results represent a preview of the wealth of information that can be obtained from a tomographic map of the ISM magnetic field.
- ID:
- ivo://CDS.VizieR/J/ApJ/748/99
- Title:
- Reactions of acetic acid with hydroxylamine
- Short Name:
- J/ApJ/748/99
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A computational study of the reactions of hydroxylamine and its ionized and protonated derivatives with acetic acid is provided. The reaction of neutral hydroxylamine with acetic acid, despite being clearly exothermic, involves a very large energy barrier. The reaction of ionized hydroxylamine with acetic acid is also clearly exothermic, but again a significant energy barrier is found (around 24 kcal/mol at the CCSD(T) level). The reaction of the most stable protonated isomer of hydroxylamine, NH_3_OH^+^, with acetic acid also involves a high barrier (more than 27 kcal/mol at the CCSD(T) level). Only the higher energy isomer, NH_2_OH^+^_2_, leads to a sensibly lower energy barrier (about 2.3 kcal/mol at the CCSD(T) level). Nevertheless, an estimate of the reaction coefficient at low temperatures such as those reigning in the interstellar medium gives very low values. Therefore, it seems that precursors of interstellar glycine could not be efficiently produced from the reactions of hydroxylamine-derived ions with acetic acid.
- ID:
- ivo://CDS.VizieR/J/ApJ/623/897
- Title:
- Reddening and extinction at high galactic latitude
- Short Name:
- J/ApJ/623/897
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present near-infrared (JHKL) photometry of 103 southern stars located behind translucent interstellar clouds at high Galactic latitude. Our data are combined with visual photometry and spectral type information from the literature in a detailed analysis of the wavelength dependence of interstellar extinction by dust in these high-latitude clouds. We investigate the shape of the near-infrared extinction curve and evaluate the total line-of-sight extinction (A_V_) and ratio of total-to-selective extinction (R_V_) in each line of sight.
- ID:
- ivo://CDS.VizieR/J/A+A/592/A142
- Title:
- Reduced high-resolution HI and CO data cubes
- Short Name:
- J/A+A/592/A142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Intermediate-velocity clouds (IVCs) are HI halo clouds that are likely related to a Galactic fountain process. In-falling IVCs are candidates for the re-accretion of matter onto the Milky Way. Aims. We study the evolution of IVCs at the disk-halo interface, focussing on the transition from atomic to molecular IVCs. We compare an atomic IVC to a molecular IVC and characterise their structural differences in order to investigate how molecular IVCs form high above the Galactic plane. With high-resolution HI observations of the Westerbork Synthesis Radio Telescope and ^12^CO(1-0) and ^13^CO(1-0) observations with the IRAM 30m telescope, we analyse the small-scale structures within the two clouds. By correlating HI and far-infrared (FIR) dust continuum emission from the Planck satellite, the distribution of molecular hydrogen (H_2_) is estimated. We conduct a detailed comparison of the HI, FIR, and CO data and study variations of the XCO conversion factor. The atomic IVC does not disclose detectable CO emission. The atomic small-scale structure, as revealed by the high-resolution HI data, shows low peak HI column densities and low HI fluxes as compared to the molecular IVC. The molecular IVC exhibits a rich molecular structure and most of the CO emission is observed at the eastern edge of the cloud. There is observational evidence that the molecular IVC is in a transient and, thus, non- equilibrium phase. The average XCO factor is close to the canonical value of the Milky Way disk. We propose that the two IVCs represent different states in a gradual transition from atomic to molecular clouds. The molecular IVC appears to be more condensed allowing the formation of H_2_ and CO in shielded regions all over the cloud. Ram pressure may accumulate gas and thus facilitate the formation of H_2_. We show evidence that the atomic IVC will evolve also into a molecular IVC in a few Myr.
- ID:
- ivo://CDS.VizieR/J/A+A/540/A2
- Title:
- Refractive index of hydrocarbon solids
- Short Name:
- J/A+A/540/A2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The properties of hydrogenated amorphous carbon (a-C:H) dust are known to evolve in response to the local conditions. We present an adaptable model for the determination of the optical properties of low-temperature, interstellar a-C:H grains that is based on the fundamental physics of their composition.
- ID:
- ivo://CDS.VizieR/J/ApJS/210/21
- Title:
- Refractive indices of grain materials
- Short Name:
- J/ApJS/210/21
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As small solid grains grow into larger ones in protoplanetary nebulae, or in the cloudy atmospheres of exoplanets, they generally form porous aggregates rather than solid spheres. A number of previous studies have used highly sophisticated schemes to calculate opacity models for irregular, porous particles with sizes much smaller than a wavelength. However, mere growth itself can affect the opacity of the medium in far more significant ways than the detailed compositional and/or structural differences between grain constituents once aggregate particle sizes exceed the relevant wavelengths. This physics is not new; our goal here is to provide a model that provides physical insight and is simple to use in the increasing number of protoplanetary nebula evolution and exoplanet atmosphere models appearing in recent years, yet quantitatively captures the main radiative properties of mixtures of particles of arbitrary size, porosity, and composition. The model is a simple combination of effective medium theory with small-particle closed-form expressions, combined with suitably chosen transitions to geometric optics behavior. Calculations of wavelength-dependent emission and Rosseland mean opacity are shown and compared with Mie theory. The model's fidelity is very good in all comparisons we have made except in cases involving pure metal particles or monochromatic opacities for solid particles with sizes comparable to the wavelength.
- ID:
- ivo://CDS.VizieR/J/A+A/555/A85
- Title:
- Resolved IR spectra of pure CO2 ice (15-75K)
- Short Name:
- J/A+A/555/A85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The v2 bending mode of pure CO_2_ ice around 15.2um exhibits a fine double-peak structure that offers a sensitive probe to study the physical and chemical properties of solid CO_2_ in space. Current laboratory spectra do not fully resolve the CO_2_ ice features. To improve the fitting of the observed CO_2_ features, high-resolution solid-state infrared spectra of pure CO_2_ ice are recorded in the laboratory for a series of astronomically relevant temperatures and at an unprecedented level of detail. The infrared spectra of pure CO_2_ ice were recorded in the 4000 to 400cm^-1^ (2.5-25um) region at a resolution of 0.1cm^-1^ using Fourier transform infrared spectroscopy. Accurate band positions and band widths (FWHM) of pure CO_2_ ice are presented for temperatures of 15, 30, 45, 60, and 75K. The focus of this spectroscopic work is on the CO_2_ (v2) bending mode, but more accurate data are also reported for the ^12^CO_2_ and ^13^CO_2_ (v3) stretching mode, and CO_2_ (v1+v3) and (2v2+v3) combination bands.
- ID:
- ivo://CDS.VizieR/J/A+A/455/1161
- Title:
- Rest frequencies of sub-mm spectrum of SiN
- Short Name:
- J/A+A/455/1161
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The submillimetre-wave spectrum of the SiN radical has been investigated in the laboratory using a source-modulation microwave spectrometer equipped with a negative glow discharge cell. SiN was produced in a SiCl_4_/N_2_ discharge plasma. Twenty-one new fine and hyperfine components up to N=17-16 were observed reaching a frequency as high as 740GHz. The new laboratory measurements provide much improved rest frequencies in the submillimetre spectral region useful for the identification of SiN lines in hot core sources and circumstellar shells.