Water vapour masers in long-period variable stars

Context. Water masers emitting at a radiofrequency of 22 GHz are often found in the circumstellar envelopes of evolved stars. We monitored the H2O maser emission of a larger sample of evolved stars of different types to study the maser properties as a function of stellar type. Aims. We wish to understand the origin and evolution of the H2O masers in circumstellar envelopes. In this paper, we take a closer look at R Crt and RT Vir, two nearby (<250 pc) semi-regular variable stars. The findings complement our monitoring results for RX Boo and SV Peg, two other semi-regular variable stars that we have discussed in a previous paper. Methods. Within the framework of the Medicina/Effelsberg H2O maser monitoring programme, we observed the maser emission of R Crt and RT Vir for more than two decades with single-dish telescopes. To get insights into the distribution of maser spots in the circumstellar envelopes at different times, to get an idea of their longevity, and, where possible, to be able to link the phenomena seen in our observations to maser locations within the envelopes, we collected interferometric data for these stars, taken within the same period, from the literature. Results. The H2O masers in R Crt and RT Vir exhibit brightness variations on a variety of timescales. We confirm short-time variations of individual features on timescales of months to up to 1.5 yr, as seen by previous monitoring programmes. Also decade-long variations of the general brightness level, independent from individual features, were seen in both stars. These long-term variations are attributed to brightness variations occurring independently from each other in selected velocity ranges and they are independent of the optical light curve of the stars. Expected drifts in velocity of individual features are usually masked by the blending of other features with similar velocities. However, in RT Vir, we found the exceptional case of a single feature with a constant velocity over 7.5 yr (<0.06 km s−1 yr−1). Conclusions. We attribute the long-term brightness variations to the presence of regions with higher-than-average density in the stellar wind and hosting several clouds which emit maser radiation on short timescales. These regions typically need ~20 yr to cross the H2O maser shell, where the right conditions for exciting H2O masers are present. Different clouds contained in such a region all move within a narrow range of velocities, and so does their maser emission. This sometimes gives the impression of longer-living features in single-dish spectra, in spite of the short lifetimes of the individual components that lie at their origin, thus, naturally explaining the longer timescales observed. The constant velocity feature (11 km s−1) is likely to come from a single maser cloud, which moved through about half of RT Vir’s H2O maser shell without changing its velocity. From this, we infer that its path was located in the outer part of the H2O maser shell, where RT Vir’s stellar wind has, apparently, already reached its terminal outflow velocity. This conclusion is independently corroborated by the observation that the highest H2O maser outflow velocity in RT Vir approaches the terminal outflow velocity, as given by OH and CO observations. This is generally not observed in other semi-regular variable stars. All four stars in our study are of optical variability type SRb, indicating the absence of periodic large-amplitude variations. Therefore, any likely responses of the maser brightness to variations of the optical emission are masked by the strong short-term maser fluctuations.

The path to Z And-type outbursts: The case of V426 Sagittae (HBHA 1704-05)

Context. The star V426 Sge (HBHA 1704-05), originally classified as an emission-line object and a semi-regular variable, brightened at the beginning of August 2018, showing signatures of a symbiotic star outburst. Aims. We aim to confirm the nature of V426 Sge as a classical symbiotic star, determine the photometric ephemeris of the light minima, and suggest the path from its 1968 symbiotic nova outburst to the following 2018 Z And-type outburst. Methods. We re-constructed an historical light curve (LC) of V426 Sge from approximately the year 1900, and used original low- (R ∼ 500–1500; 330–880 nm) and high-resolution (R ∼ 11 000–34 000; 360–760 nm) spectroscopy complemented with Swift-XRT and UVOT, optical UBVRCIC and near-infrared JHKL photometry obtained during the 2018 outburst and the following quiescence. Results. The historical LC reveals no symbiotic-like activity from ∼1900 to 1967. In 1968, V426 Sge experienced a symbiotic nova outburst that ceased around 1990. From approximately 1972, a wave-like orbitally related variation with a period of 493.4 ± 0.7 days developed in the LC. This was interrupted by a Z And-type outburst from the beginning of August 2018 to the middle of February 2019. At the maximum of the 2018 outburst, the burning white dwarf (WD) increased its temperature to ≳2 × 105 K, generated a luminosity of ∼7 × 1037 (d/3.3 kpc)2 erg s−1 and blew a wind at the rate of ∼3 × 10−6 M⊙ yr−1. Our spectral energy distribution models from the current quiescent phase reveal that the donor is a normal M4-5 III giant characterised with Teff ∼ 3400 K, RG ∼ 106 (d/3.3 kpc) R⊙ and LG ∼ 1350 (d/3.3 kpc)2 L⊙ and the accretor is a low-mass ∼0.5 M⊙ WD. Conclusions. During the transition from the symbiotic nova outburst to the quiescent phase, a pronounced sinusoidal variation along the orbit develops in the LC of most symbiotic novae. The following eventual outburst is of Z And-type, when the accretion by the WD temporarily exceeds the upper limit of the stable burning. At this point the system becomes a classical symbiotic star.

Overall variation of the H2O masers around W Hydrae in 28 years

In this paper, we present the distribution of H$_2$O masers associated with the semi-regular variable star W Hydrae (W Hya). We have collected the radio interferometric data of the maser distribution taken with the Very Large Array (VLA), the Kashima–Nobeyama InterFErometer (KNIFE), the Multi-Element Radio Link Network (MERLIN), the VLBI Exploration of Radio Astrometry (VERA), and the combined array of the Korean VLBI Network (KVN) and VERA (KaVA) in order to trace the maser distribution variation in two decades. Even though differences in the sensitivities and angular resolutions of the interferometric observations should be taken into account, we attempt to find possible correlation of the maser distribution with the stellar light curve. Our failure in the measurement of the annual parallax of the masers with VERA is likely caused by the properties of the maser features, which have been spatially resolved by the synthesized beam and survived for only half a year or less. No dependence of the maser spot flux density on its size is found in the KNIFE data, suggesting that maser spot size is determined by the physical boundary, as is expected for a clump affected by outward propagation of a stellar pulsation shock wave, rather than the (spherical) geometry of maser beaming in the maser gas clump.

Gaia Data Release 2

Context. Gaia Data Release 2 (DR2) provides a unique all-sky catalogue of 550 737 variable stars, of which 151 761 are long-period variable (LPV) candidates with G variability amplitudes larger than 0.2 mag (5–95% quantile range). About one-fifth of the LPV candidates are Mira candidates, the majority of the rest are semi-regular variable candidates. For each source, G, GBP, and GRP photometric time-series are published, together with some LPV-specific attributes for the subset of 89 617 candidates with periods in G longer than 60 days. Aims. We describe this first Gaia catalogue of LPV candidates, give an overview of its content, and present various validation checks. Methods. Various samples of LPVs were used to validate the catalogue: a sample of well-studied very bright LPVs with light curves from the American Association of Variable Star Observers that are partly contemporaneous with Gaia light curves, a sample of Gaia LPV candidates with good parallaxes, the All-Sky Automated Survey for Supernovae catalogue of LPVs, and the Optical Gravitational Lensing Experiment (OGLE) catalogues of LPVs towards the Magellanic Clouds and the Galactic bulge. Results. The analyses of these samples show a good agreement between Gaia DR2 and literature periods. The same is globally true for bolometric corrections of M-type stars. The main contaminant of our DR2 catalogue comes from young stellar objects (YSOs) in the solar vicinity (within ~1 kpc), although their number in the whole catalogue is only at the percent level. A cautionary note is provided about parallax-dependent LPV attributes published in the catalogue. Conclusions. This first Gaia catalogue of LPVs approximately doubles the number of known LPVs with amplitudes larger than 0.2 mag, despite the conservative candidate selection criteria that prioritise low contamination over high completeness, and despite the limited DR2 time coverage compared to the long periods characteristic of LPVs. It also contains a small set of YSO candidates, which offers the serendipitous opportunity to study these objects at an early stage of the Gaia data releases.

Portable snapshot spectral imaging for agriculture

High-resolution proximal and remote sensing applications can consolidate sustainable, prevention- and precision-oriented crop management strategies to decrease production risks. This paper shows significant perspectives, own developments and technical aspects of high resolution remote sensing in the context of field applications. Moreover, we provide an overview of snapshot hyperspectral imaging and potential field video sensors to identify areas of interest for their future development. One of the main conclusions of our paper is that non-scanning snapshot hyperspectral imaging technology may enable researchers to overcome the gap in the “point to image” transition of field sensing, while providing a flexible solution for regular variable-rate applications.

Added Powering Measurements of KCS Maneuvering in Regular Variable Heading Waves

To know more detail of added powering and propeller load fluctuations in regular waves during free-maneuvering, free-running tests of KRISO Container Ship model (KCS) are conducted. KCS 2.7 m model that was used in the previous surge-free added resistance experiments is modified and new free-running system with compact dynamometer is installed. Free-running tests in calm water and in regular variable heading waves are performed at IIHR 40 m × 20 m × 3 m wave basin to obtain 6DOF motions with thrust/torque data. Propeller open water tests were performed at Osaka University towing tank (OU). To evaluate facility bias and scale effects, trajectories, motions and maneuvering characteristic parameters are compared with those of different size model taken at other facilities. Free-running course keeping tests in regular variable heading waves are performed as same conditions with Tokyo 2015 A Workshop on CFD in Ship Hydrodynamics (T2015) case 2.10 and case 2.11. Those results are compared with the data taken at FORCE with 6 m free-running model and OU with 3.2 m model by surge-free mount. In head waves, trends of RAO for heave and pitch are the same under surge-free and free-running. Added thrust/torque and propeller open water efficiency reduction of IIHR and OU become maximum at λ/ L=1.15 where the added resistance was maximum under surge-free condition. In oblique waves, added thrust and torque become larger where the wave encounter angle is from 0° to 45° and both trends agree with other type of container ship. Thrust and torque fluctuations of KCS become larger in beam and following waves. Variation of self-propulsion factors due to wave encounter angles are small in oblique waves.