arXiv subjects

Dieter Engels

Publications and source records attributed to Dieter Engels.

10 recordsLinked to original sources

arXiv2026

Cosmic Rulers: Masers as Tools for Probing Structure in the Galaxy and Beyond, from AU to kpc

Maser emission provides an unique window into astronomical sources across vast spatial scales - from tens to hundreds of astronomical units around protostars and evolved stars up to kiloparsecs in distant galaxies. These natural microwave amplifiers penetrate dust shells in star-forming regions, revealing the dynamics of accretion disks and outflows, trace envelopes and winds of evolved stars, map Galactic structure, while also allowing us to follow the evolution of all these systems. Owing to their compactness and brightness masers provide precise astrometry as cosmic rulers: measuring positions, structures and kinematics in dense regions, not easily accessible at other wavelengths. SKA-Mid will observe hydroxyl, methanol, and formaldehyde masers using bands 2, 5a and 5b, and later methylidyne radical masers in band 4. SKA-Mid's sensitivity and broad frequency coverage will support discoveries of new maser types and allow for simultaneous multi-transition and multi-species maser observations. In addition, bright masers can serve in the science verification of the SKA-Mid array during the deployment phase.

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arXiv2025

Water vapour masers in long-period variable stars IV. Mira variables RCas, oCet and RLeo

Between 1987 and 2023 we carried out single-dish monitoring campaigns of water-maser emission at 22 GHz in the circumstellar envelopes (CSEs) of four stars: RCas, oCet, RLeo and ChiCyg. The variability in integrated flux in RCas and oCet followed that in the optical with the same period, but with a lag of about one-third in phase. RLeo was too often below our sensitivity threshold for us to determine a radio period. No maser was detected in ChiCyg. The variability in the masers in RCas has a distinctive pattern that confirms the existence of a zone in the CSE with favourable conditions for maser excitation. The total flux, modulated by the pulsations of the star, gradually increases to a maximum, which is followed by a similar decrease. This takes about 20 years. The pattern is repeated after an interval of quiescence of several years. During its decline, the variation in the flux resembles a damped harmonic oscillator. There are two dominant emission components that move almost tangentially on either side of the star with respect to the observer. The redshifted one is consistent with an origin in a single cloud that lived for at least about eight years, falling back towards the star. The blue component, moving in the CSE hemisphere nearest to us, has no drift in the line-of-sight velocity and appears to originate in a time series of short-living clouds with line-of-sight velocities within ca. 1 km/s of each other. No systematic drifts are found in RLeo and oCet. A few bursts of emission were detected in RCas and RLeo that lasted about a year and caused an increase in the flux density by 1-2 orders of magnitude. The velocity range of the maser emission is < 10 km/s and is clearly correlated with the bolometric luminosity of the stars.

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arXiv2025

The Gaia Catalogue of Galactic AGB Stars

This paper describes the identification of Gaia counterparts of a sample of oxygen-rich AGB stars with OH maser emission as a first step towards the compilation of a general Gaia Catalogue of Galactic AGB stars. With this catalogue, tests of evolutionary models for the AGB star population in the solar neighbourhood will become feasible. We cross-matched AGB star candidates showing OH maser emission with Gaia DR3 using a cross-match with AllWISE and 2MASS as intermediate steps to avoid ambiguities. With the help of the Virtual Observatory, we retrieved photometric data from the near-ultraviolet to the far-infrared and built spectral energy distributions (SEDs) of the sources. The SEDs were fitted with theoretical models. The fit results, together with information from the literature, allowed us to clean the sample from non-AGB stars and to obtain bolometric fluxes for the AGB stars. Distances based on Gaia parallaxes were used to derive the stellar luminosities. We identified unique Gaia counterparts for 1487 OH masers. Of these, 1172 had an unambiguous classification as AGB stars; they make up the Gaia OH/IR star sample. Parallaxes with relative errors $<$20\% and astrometric excess noise $<$1.5 mas were available for 222 OH/IR stars. We conclude that the study of the AGB population in the solar neighbourhood is limited by the obscuration by circumstellar dust, as \G\ DR3 only provides parallaxes for a few of our candidates. The location of the OH/IR stars matches that of LPV discovered by \G\ in the (BP$-$RP; G$_{abs}$) diagram, but the OH/IR star sample is biased towards redder colours (BP$-$RP$>$4) mag and larger amplitudes ($>$1 mag in the G-band), which are typical for periodic large-amplitude Mira variables.

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arXiv2024

Patterns in water maser emission of evolved stars on the timescale of decades

We present our past and current long-term monitoring program of water masers in the circumstellar envelopes of evolved stars, augmented by occasional interferometric observations. Using as example the Mira-variable U Her, we identify three types of variability: periodic (following the optical variation), long-term (years-decades) and short-term irregular (weeks-months). We show there are regions in the maser shell where excitation conditions are favourable, which remain stable for many years. Lifetimes of maser clouds in the wind-acceleration zone are of the order of up to a few years. Much longer lifetimes are found for the peculiar case of a maser cloud outside that zone (as in RT Vir), or in some cases where the motion of spectral features can be followed for the entire 2 decade monitoring period (as in red supergiant VX Sgr).

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arXiv2018

The loss of large amplitude pulsations at the end of AGB evolution

Since 2013 we are performing with the Nancay Radio Telescope (NRT) a monitoring program of >100 Galactic disk OH/IR stars, having bright 1612-MHz OH maser emission. The variations of the maser emission are used to probe the underlying stellar variability. We wish to understand how the large-amplitude variations are lost during the AGB - post-AGB transition. The fading out of pulsations with steadily declining amplitudes seems to be a viable process.

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arXiv2017

Distances of Stars by mean of the Phase-lag Method

Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.

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arXiv2015

OH masers in the Milky Way and Local Group galaxies in the SKA era

The intense line emission of OH masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. From SKA deep surveys at 18 cm, where the maser lines from the ground-state of the OH molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the Galaxy. The study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from VLBI astrometry. A new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. For the first time, larger numbers of OH masers will be detected in Local Group galaxies. New insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. With the full capabilities of SKA, further maser transitions such as from excited OH and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular.

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arXiv2006

Near-IR/optical monitoring programme of the 'Arecibo sample of OH/IR stars'

We performed a near-IR/optical monitoring programme from 1999 to 2005 in order to study the variability properties of the 'Arecibo sample of OH/IR stars' (periods, amplitudes, and colour variations). Here we describe this multi-wavelength long-term monitoring programme. Data analysis is still in process. Our ultimate goal is to study in particular the oxygen-rich AGB stars with masses larger than 2 Solar Mass, which are probably rare among AGB stars discovered optically and/or in the near-IR, but are common in samples discovered in the mid- to far-IR.

astro-ph↗

arXiv2005

AGB and post-AGB stars

Intermediate mass stars (1-8 solar masses) evolve along the Asymptotic Giant Branch after completion of hydrogen and helium core burning. At the tip they lose for several ten to hundred thousand years copious amounts of mass and exhibit various forms of variability, as for example large-amplitude variations with periods of one to five years. In the oxygen-rich circumstellar envelopes powerful OH, H2O and SiO masers may operate. Part of the AGB population is converted to carbon stars. During the latest phases of AGB evolution the mass loss rates approach several 10e-5 solar masses/year, so that the stars become invisible in the optical and in part in the near infrared. On departure from the AGB a fundamental transition in the mass loss process is taking place changing from a spherically symmetric outflow on the AGB to axi-symmetric or point-symmetric geometries. While the mass loss rates decrease to ~10e-8 solar masses/year, the velocities are strongly increasing. Stars are now in their "post-AGB" phase. Mounting evidence is gathered that during the very latest phase of AGB evolution and during the post-AGB phase, evolution proceeds on very short timescales, which in extreme cases is comparable to the working life of an astronomer.

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arXiv2000

BLR velocities in optically and X-ray selected AGN samples

We have analyzed optical spectra of 473 X-ray and 235 optically selected AGNs, to study their emission line properties. We present results of an analysis of the H-beta linewidths. We find that the linewidth distribution of quasars is shifted towards higher velocities ( =4300 km s-1) compared to the distribution of Sy 1s ( =3000 km s-1). There are no Narrow Line Quasars, i.e. there are no AGNs with quasar luminosities and FWHM(H-beta) < 2000 km s-1. NLSy1s comprise 20-30 per cent of the AGN population at faint absolute magnitudes (MB>-22), irrespective of the selection method. In the RASS sample we find Gamma [0.1-2.4 keV] < 3.3. The Gamma vs. FWHM(H-beta) distribution for Sy 1 galaxies is consistent with previous work. For QSOs the spectral index also flattens with increasing FWHM(H-beta), but they have larger linewidths than Seyfert 1s.

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