Using Methanol Beacons to Find Water in the Dark

Interstellar methanol is only formed efficiently from hydrogenation of CO molecules accreted onto grains, and icy grain mantles are observed to consist of 1-30% methanol relative to water. In regions of both low and high mass star formation gas-phase methanol abundances are consistent with partial or complete removal of the ices, either by thermal evaporation or by shock-induced sputtering in outflows. However, the widespread presence of gas-phase methanol in molecular clouds attests to some non-thermal desorption process at work. In particular, distinct peaks of methanol emission at positions significantly offset from protostellar activity implies a transient desorption process, such as clump-clump collisions, rather than a continuous one like photodesorption. Such processes are likely to disrupt a major part of the ice mantles and lead to high gas-phase water abundances clearly distinguishable from what is expected from photodesorption or steady-state gas-phase chemistry. We will report on the first detection of gas-phase water in a cold dark cloud - well offset from protostellar activity - resulting from a small scale survey with Herschel HIFI towards methanol peaks. Physical properties of the sources as well as implications for mantle desorption mechanisms and chemistry in dark clouds will be discussed and compared to those of active star formation

The EDIBLES survey:VI. Searching for time variations of interstellar absorption features

Context. Interstellar absorption observed toward stellar targets changes slowly over long timescales, mainly due to the proper motion of the background target relative to the intervening clouds, such that over time, different parts of the intervening cloud are probed. On longer timescales, the slowly changing physical and chemical conditions in the cloud can also cause variation. Detecting such time variations thus provides an opportunity to study cloud structure.Aims. We searched for systematic variations in the absorption profiles of the diffuse interstellar bands (DIBs) and interstellar atomic and molecular lines by comparing the high-quality data set from the recent ESO diffuse interstellar bands large exploration survey (EDIBLES) to older archival observations, bridging typical timescales of ~10 yr with a maximum timescale of 22 yr.Methods. For 64 EDIBLES targets, we found adequate archival observations. We selected 31 strong DIBs, seven atomic lines, and five molecular lines to focus our search on. We carefully considered various systematic effects and used a robust Bayesian quantitative test to establish which of these absorption features could display significant variations.Results. While systematic effects greatly complicate our search, we find evidence for variations in the profiles of the λλ4727 and 5780 DIBs in a few sightlines. Toward HD 167264, we find a new Ca I cloud component that appears and becomes stronger after 2008. The same sightline furthermore displays marginal, but systematic changes in the column densities of the atomic lines originating from the main cloud component in the sightline. Similar variations are seen toward HD 147933.Conclusions. Our high-quality spectroscopic observations in combination with archival data show that it is possible to probe interstellar time variations on time scales of typically a decade. Despite the fact that systematic uncertainties as well as the generally somewhat lower quality of older data complicate matters, we can conclude that time variations can be made visible, both in atomic lines and DIB profiles for a few targets, but that generally, these features are stable along many lines of sight. We present this study as an archival baseline for future comparisons, bridging longer periods.<br/

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Depletion Cores - the O2 hideout?

Molecular oxygen has proven to be the most elusive molecule in the interstellar medium. Despite the fact that it in theory forms easily in both warm and cold dense gas, extensive searches with SWAS, Odin and Herschel have only resulted in detections in a handful of sources. In addition, upper limits in various astronomical environments are at levels of 1000 times less abundant than predicted by chemical models. This situation requires either for atomic carbon to be abundant enough to suppress the O2 by CO formation, or for atomic oxygen to accrete onto grains and remain bound there. However, the binding energies of atoms to grains are highly uncertain and high abundances of OI in depleted gas have both been directly observed and inferred from observations of other molecules. A possible explanation is that OI is bound to grains by fixing (get hydrogenated to form ices) rather than sticking (van der Waals bonding to the surface potential), which will become less efficient in high density gas. We will present a stochastic gas-grain model including the kinetics of OI fixing - demonstrating the possibility of elevated O2 abundances at times when CO is significantly depleted - as well as results from searches for O2 emission in a small sample of starless depletion cores using Herschel HIFI

Depletion Cores - the O2 hideout?

Molecular oxygen has proven to be the most elusive molecule in theinterstellar medium. Despite the fact that it in theory forms easily in both warm and cold dense gas, extensive searches with SWAS, Odin and Herschel have only resulted in detections in a handful of sources. In addition, upper limits in various astronomical environments are at levels of 1000 times less abundant than predicted by chemical models. This situation requires either for atomic carbon to be abundant enough to suppress the O2 by CO formation, or for atomic oxygen to accrete onto grains and remain bound there. However, the binding energies of atoms to grains are highly uncertain and high abundances of OI in depleted gas have both been directly observed and inferred from observations of other molecules. A possible explanation is that OI is bound to grains by fixing (get hydrogenated to form ices) rather than sticking (van der Waals bonding to the surface potential), which will become less efficient in high density gas. We will present a stochastic gas-grain model including the kinetics of OI fixing - demonstrating the possibility of elevated O2 abundances at times when CO is significantly depleted - as well as results from searches for O2 emission in a small sample of starless depletion cores using Herschel HIFI

Using Methanol Beacons to Find Water in the Dark

Interstellar methanol is only formed efficiently from hydrogenation of COmolecules accreted onto grains, and icy grain mantles are observed to consist of 1-30% methanol relative to water. In regions of both low and high mass star formation gas-phase methanol abundances are consistent with partial or complete removal of the ices, either by thermal evaporation or by shock-induced sputtering in outflows. However, the widespread presence of gas-phase methanol in molecular clouds attests to some non-thermal desorption process at work. In particular, distinct peaks of methanol emission at positions significantly offset from protostellar activity implies a transient desorption process, such as clump-clump collisions, rather than a continuous one like photodesorption. Such processes are likely to disrupt a major part of the ice mantles and lead to high gas-phase water abundances clearly distinguishable from what is expected from photodesorption or steady-state gas-phase chemistry.We will report on the first detection of gas-phase water in a cold dark cloud - well offset from protostellar activity - resulting from a small scale survey with Herschel HIFI towards methanol peaks. Physical properties of the sources as well as implications for mantle desorption mechanisms and chemistry in dark clouds will be discussed and compared to those of active star formation

The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions

The Large Magellanic Cloud (LMC) offers a unique laboratory to study the diffuse interstellar bands (DIBs) under conditions that are profoundly different from those in the Galaxy. DIB carrier abundances depend on several environmental factors, in particular the local UV radiation field. In this paper we present measurements of twelve DIBs in five lines of sight to early-type stars in the LMC, including the 30 Doradus region. From the high resolution spectra obtained with VLT/UVES we also derive environmental parameters that characterise the local interstellar medium (ISM) in the probed LMC clouds. These include the column density components (including total column density) for the atomic resonance lines of Na I, Ca II, Ti II, K I. In addition, we derive the H I column density from 21 cm line profiles, the total-to-selective visual extinction RV and the gas-to-dust ratio N(H I)/A_V. Furthermore, from atomic line ratios we derive the ionisation balance and relative UV field strength in these environments. We discuss the properties of the LMC ISM in the context of DIB carrier formation. The behaviour of DIBs in the LMC is compared to that of DIBs in different local environmental conditions in the Milky Way. A key result is that in most cases the diffuse band strengths are weak (up to factor 5) with respect to Galactic lines of sight of comparable reddening, EB-V. In the line of sight towards Sk -69 223 the 5780 and 5797 �DIBs are very similar in strength and profile to those observed towards HD 144217, which is typical of an environment exposed to a stron

Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering

International audienceUnidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon (PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules. © 2021 American Association for the Advancement of Science. All rights reserved