Netherlands Institute for Space Research

About: Netherlands Institute for Space Research is a facility organization based out in Utrecht, Netherlands. It is known for research contribution in the topics: Galaxy & Neutron star. The organization has 737 authors who have published 3026 publications receiving 106632 citations. The organization is also known as: SRON & Space Research Organisation Netherlands.

Herschel/HIFI observations of [C II] and [13C II] in photon-dominated regions

Abstract: Context. Chemical fractionation reactions in the interstellar medium can result in molecular isotopologue abundance ratios that differ by many orders of magnitude from the isotopic abundance ratios. Understanding variations in the molecular abundance ratios through astronomical observations provides a new tool to sensitively probe the underlying physical conditions. Aims. Recently, we have introduced detailed isotopic chemistry into the KOSMA-τ model for photon-dominated regions (PDRs), which allows calculating abundances of carbon isotopologues as a function of PDR parameters. Radiative transfer computations then allow to predict the observed [C II]/[^(13)C II] line intensity ratio for specific geometries. Here, we compare these model predictions with new Herschel observations. Methods. We performed Herschel/HIFI observations of the [C II] 158 μm line in a number of PDRs. In all sources, we observed at least two hyperfine components of the [^(13)C II] transition, allowing determination of the [C II]/[^(13)C II] intensity ratio, using revised intrinsic hyperfine ratios. Comparing the observed line ratios with the predictions from the updated KOSMA-τ model, we identify conditions under which the chemical fractionation effects are important, and not masked by the high optical depth of the main isotopic line. Results. An observable enhancement of the [C II]/[^(13)C II] intensity ratio due to chemical fractionation depends mostly on the source geometry and velocity structure, and to a lesser extent on the gas density and radiation field strength. The enhancement is expected to be largest for PDR layers that are somewhat shielded from UV radiation, but not completely hidden behind a surface layer of optically thick [C II]. In our observations the [C II]/[^(13)C II] integrated line intensity ratio is always dominated by the optical depth of the main isotopic line. However, an enhanced intensity ratio is found for particular velocity components in several sources: in the red-shifted material in the ultracompact H II region Mon R2, in the wings of the turbulent line profile in the Orion Bar, and possibly in the blue wing in NGC 7023. Mapping of the [^(13)C II] lines in the Orion Bar gives a C^+ column density map, which confirms the temperature stratification of the C^+ layer, in agreement with the PDR models of this region. Conclusions. Carbon fractionation can be significant even in relatively warm PDRs, but a resulting enhanced [C II]/[^(13)C II] intensity ratio is only observable for special configurations. In most cases, a reduced [C II]/[^(13)C II] intensity ratio can be used instead to derive the [C II] optical depth, leading to reliable column density estimates that can be compared with PDR model predictions. The C^+ column densities show that, for all sources, at the position of the [C II] peak emission, the dominant fraction of the gas-phase carbon is in the form of C^+.

The IRAM M 33 CO(2–1) survey - A complete census of molecular gas out to 7 kpc

Abstract: To study the interstellar medium and the interplay between the atomic and molecular components in a low-metallicity environment, we present a complete high angular and spectral resolution map and position-position-velocity data cube of the 12CO(J = 2-1) emission from the Local Group galaxy Messier 33. Its metallicity is roughly half-solar, such that we can compare its interstellar medium with that of the Milky Way with the main changes being the metallicity and the gas mass fraction. The data have a 12″ angular resolution (~50 pc) with a spectral resolution of 2.6 km s-1 and a mean and median noise level of 20 mK per channel in antenna temperature. A radial cut along the major axis was also observed in the 12CO(J = 1-0) line. The CO data cube and integrated intensity map are optimal when using H i data to define the baseline window and the velocities over which the CO emission is integrated. Great care was taken when building these maps, testing different windowing and baseline options, and investigating the effect of error beam pickup. The total CO(2-1) luminosity is 2.8 × 107 K km s-1 pc2, following the spiral arms in the inner disk, with an average decrease in intensity approximately following an exponential disk with a scale length of 2.1 kpc. There is no clear variation in the CO(2-1/1-0) intensity ratio with radius and the average value is roughly 0.8. The total molecular gas mass is estimated, using a N(H2) /ICO(1 - 0) = 4 × 1020cm-2/(K km s-1) conversion factor, to be 3.1 × 108 M⊙, including helium. The CO spectra in the cube were shifted to zero velocity by subtracting the velocity of the H i peak from the CO spectra. Stacking these spectra over the whole disk yields a CO line with a half-power width of 12.4 km s-1. As a result, the velocity dispersion between the atomic and molecular components is extremely low, independently justifying the use of the H i line in building our maps. Stacking the spectra in concentric rings shows that the CO linewidth and possibly the CO-H i velocity dispersion decrease in the outer disk. The error beam pickup could produce the weak CO emission apparently from regions in which the H i line peak does not reach 10 K, such that no CO is actually detected in these regions. Using the CO(2-1) emission to trace the molecular gas, the probability distribution function of the H2 column density shows an excess at high column density above a log-normal distribution.

“TNOs are Cool”: A survey of the trans-Neptunian region - VI. Herschel/PACS observations and thermal modeling of 19 classical Kuiper belt objects

Abstract: Trans-Neptunian objects (TNO) represent the leftovers of the formation of the Solar System. Their physical properties provide constraints to the models of formation and evolution of the various dynamical classes of objects in the outer Solar System. Based on a sample of 19 classical TNOs we determine radiometric sizes, geometric albedos and beaming parameters. Our sample is composed of both dynamically hot and cold classicals. We study the correlations of diameter and albedo of these two subsamples with each other and with orbital parameters, spectral slopes and colors. We have done three-band photometric observations with Herschel/PACS and we use a consistent method for data reduction and aperture photometry of this sample to obtain monochromatic flux densities at 70.0, 100.0 and 160.0 \mu m. Additionally, we use Spitzer/MIPS flux densities at 23.68 and 71.42 \mu m when available, and we present new Spitzer flux densities of eight targets. We derive diameters and albedos with the near-Earth asteroid thermal model (NEATM). As auxiliary data we use reexamined absolute visual magnitudes from the literature and data bases, part of which have been obtained by ground based programs in support of our Herschel key program. We have determined for the first time radiometric sizes and albedos of eight classical TNOs, and refined previous size and albedo estimates or limits of 11 other classicals. The new size estimates of 2002 MS4 and 120347 Salacia indicate that they are among the 10 largest TNOs known. Our new results confirm the recent findings that there are very diverse albedos among the classical TNOs and that cold classicals possess a high average albedo (0.17 +/- 0.04). Diameters of classical TNOs strongly correlate with orbital inclination in our sample. We also determine the bulk densities of six binary TNOs.

Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

Abstract: We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) AT 2018fyk/ASASSN-18ul discovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering a plateau after $\sim$40 days of decline from peak. After 80 days the UV/optical lightcurve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high ionisation (O III) and low ionisation (Fe II) lines, which are visible for $\sim$45 days. We similarly identify Fe II lines in optical spectra of ASASSN-15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe II lines can be associated with the formation of a compact accretion disk, as the emergence of low ionisation emission lines requires optically thick, high density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a two-component lightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, and is a natural consequence of a highly relativistic orbital pericenter.