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This paper describes an overview of the FLASHING (Finest Legacy Acquisitions of SiO-/H2O-maser Ignitions by the Nobeyama Generation) project promoted using the 45 m telescope of Nobeyama Radio Observatory, which aims to intensively monitor H2O (at 22 GHz) and SiO (at 43 GHz) masers associated with so-called “water fountain” sources. Here we show scientific results based on the data taken in the first five seasons of FLASHING, from 2018 December to 2023 April. We show the evolution of the H2O maser spectra, discovering new spectral components that break the record of the jet’s top speed and/or systematic velocity drifts in the spectrum indicating acceleration or deceleration of the maser gas clumps. For the 43 GHz SiO maser emission, we newly detect it in one source and find that it is no longer present in another source. This implies that the jets from these water fountains can be accelerated or decelerated, and show how circumstellar envelopes are destroyed.

PeVatrons are the most powerful naturally occurring particle accelerators in the Universe. The identification of counterparts associated to astrophysical objects such as dying massive stars, molecular gas, star-forming regions, and star clusters is essential to clarify the underlying nature of the PeV emission, i.e., hadronic or leptonic. We present 12,13CO (J= 2→1) observations made with the 1.85 m radio-telescope of the Osaka Prefecture University toward the Cygnus OB7 molecular cloud, which contains the PeVatron candidate LHAASO J2108+5157. We investigate the nature of the sub-PeV (gamma-ray) emission by studying the nucleon density determined from the content of H iand H2, derived from the CO observations. In addition to MML[2017]4607, detected via the observations of the optically thick 12CO (J= 1→0) emission, we infer the presence of an optically thin molecular cloud, named [FKT-MC]2022, whose angular size is 1${_{.}^{\circ}}$1 ± 0${_{.}^{\circ}}$2. We propose this cloud as a new candidate to produce the sub-PeV emission observed in LHAASO J2108+5157. Considering a distance of 1.7 kpc, we estimate a nucleon (H i+ H2) density of 37 ± 14 cm−3, and a total nucleon mass(H i+ H2) of 1.5 ± 0.6 × 104M⊙. On the other hand, we confirm that Kronberger 82 is a molecular clump with an angular size of 0${_{.}^{\circ}}$1, a nucleon density ∼103cm−3, and a mass ∼103M⊙. Although Kronberger 82 hosts the physical conditions to produce the observed emission of LHAASO J2108+5157, [FKT-MC]2022 is located closer to it, suggesting that the latter could be the one associated to the sub-PeV emission. Under this scenario, our results favour a hadronic origin for the emission.

We studied the central region of bipolar nebula M 2-9 using radio-continuum observations obtained from the Jansky Very Large Array (JVLA) and the Atacama Large Millimeter Array (ALMA) interferometers. This work presents new images at ∼43 GHz (∼7.0 mm) and ∼345 GHz (∼0.9 mm) with angular resolutions of $\sim {0{^{\prime \prime}_{.}}047}$and ${0{^{\prime \prime}_{.}}09}$, respectively. The continuum emission at ∼43 GHz shows an elongated jet-like structure perpendicular to the ∼345 GHz observation. We conclude that both emissions could correspond to tracing an isothermal collimated fast wind with constant expansion velocity and being driven by the circumstellar ring traced by ALMA. Although this configuration has been discussed within the scope of planetary nebulae models, there is a remarkable fact: the collimated fast wind shows morphological spatial variability. This supports the idea of a symbiotic binary system within the innermost part of M 2-9, which would be composed of a white dwarf and an AGB star. The latter could explain the mirror symmetry observed at larger scales due to their orbital motion.

We present the results of astrometic observations of H$_{2}$O masers associated with the star-forming region G 192.16$-$3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H$_{2}$O masers seem to be associated with two young stellar objects (YSOs) separated by $\sim\ $1200 AU, as reported in previous observations. In the present observations, we successfully detected an annual parallax of $\pi$$=$0.66$\ \pm\ $0.04 mas for the H$_{2}$O masers, which corresponds to a distance to G 192.16$-$3.84 of $D$$=$1.52$\ \pm\ $0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern YSO to be 10.0$^{+4.3}_{-3.6}\ M_{\odot}$. We also estimate the galactocentric distance and the peculiar motion of G 192.16$-$3.84, relative to a circular galactic rotation, to be $R_{\ast}$$=$9.99$\ \pm\ $0.08 kpc and $Z_{\ast}$$=$$-$0.10$\ \pm\ $0.01 kpc, and ($U_{\ast}$, $V_{\ast}$, $W_{\ast}$) $=$($-$2.8$\ \pm\ $1.0, $-$10.5$\ \pm\ $0.3, 4.9$\ \pm\ $2.7) [km s$^{-1}$], respectively. The peculiar motion of G 192.16$-$3.84 is not beyond the range of recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H$_{2}$O maser features indicate the existence of a bipolar outflow with a major axis along the northeast–southwest direction.

We report on results of astrometric observations of H$_{2}$O masers in the pre-planetary nebula candidate source IRAS 19312$+$1950 with the VLBI Exploration of Radio Astrometry (VERA), yielding an annual parallax, $\pi$$=$263$\ \pm\ $47$\ \mu$as, and a secular motion, ($\mu_{\alpha}$, $\mu_{\delta}$) $=$($-$2.61$\ \pm\ $0.47, $-$6.73$\ \pm\ $0.14) [mas yr$^{-1}$], for IRAS 19312$+$1950. We then derived a heliocentric distance of $D$$=$3.80$^{+0.83}_{\-0.58}\ $kpc, a location in the Galaxy ($R$, $z$) $=$(7.07$\ \pm\ $0.12 kpc, 28$\ \pm\ $3 pc), and a three-dimensional velocity vector ($V_{R}$, $V_{\theta}$, $V_{z}$) $=$(33$\ \pm\ $28, 214$\ \pm\ $4, $-$14$\ \pm\ $8) [km s$^{-1}$] in galactic cylindrical coordinates. Based on these parameters, we discuss the intrinsic properties of IRAS 19312$+$1950. We estimated the mass of the central star to be $\sim\ $8$\ M_{\odot}$and the deviation of the source motion from the circular galactic rotation to be $\sim\ $40 km s$^{-1}$. These are consistent with this source being an intermediate-mass late AGB or early post-AGB star.

In this paper we present the results of very long baseline interferometry (VLBI) observations carried out with the VLBI Exploration of Radio Astrometry (VERA) array and the Very Long Baseline Array (VLBA) toward H$_{2}$O masers in a young planetary nebula K 3$-$35. From the VERA observations we measured the annual parallax and proper motion of a bright water maser spot in K 3$-$35. The resulting distance is $D$$=$3.9$^{+0.7}_{\hskip1.11pt-0.5}$kpc. This is the first time that the parallax of a planetary nebula is obtained by observations of its maser emission. On the other hand, the proper motion of K 3$-$35 as a whole was estimated to be $\mu_{\alpha}$$=$$-$3.34 $\pm$0.10 mas yr$^{-1}$, $\mu_{\delta}$$=$$-$5.93 $\pm$0.07 mas yr$^{-1}$. From these results we determined the position and velocity of K 3$-$35 in Galactic cylindrical coordinates: ($R,\theta,\textsl{z}$) $=$(7.11$^{+0.08}_{\hskip1.11pt-0.06}$kpc, 27$^{\circ}$$\pm$5$^{\circ}$, 140$^{+25}_{\hskip1.11pt-18}$pc) and ($V_{R}, V_{\theta}, V_{{\sl z}}$) $=$(33 $\pm$16, 233 $\pm$11, 11 $\pm$2) km s$^{-1}$. Additionally, from our VLBA observations we measured the relative proper motions among the water maser spots located in the central region of the nebula, which have been proposed to be tracing a toroidal structure. The distribution and relative proper motions of the masers, compared with previous reported observed epochs, suggest that such structure could be totally destroyed within a few years, due to the action of high-velocity winds and the expansion of the ionization front in the nebula.

We present new sensitive, high angular resolution 1.3, 2, and 6 cm observations of the continuum emission from the peculiar emission-line star MWC 349A made with the Very Large Array. This radio emission is believed to originate in an ionized flow produced by the photoevaporation of a disk that surrounds the star. We determine for the first time the proper motion of this source, which is consistent with that expected for the location of the source in the Galaxy. Our analysis of the images, that include the new observations as well as archive data covering a time interval of more than 20 years, indicates that the appearance of MWC 349A has been systematically changing over time. The well-defined "hourglass" shape that characterized the 2 and 1.3 cm appearance of the source in the early 1980s has disappeared, to be replaced by a more "square" shape. We discuss if these changes can be accounted for by precession of the MWC 349A disk or by intrinsic changes in the parameters of the disk, but could not reach a satisfactory explanation.

We discovered new high-velocity components of H2O maser emission in one of the “water fountain” sources, IRAS 18286−0959, which has been monitored using the Nobeyama 45 m telescope in the FLASHING (Finest Legacy Acquisitions of SiO- and H2O-maser Ignitions by Nobeyama Generation) project since 2018 December. The maser spectra show new components with extremely high expansion velocities (>200 km s−1projected in the line of sight), some of which are located symmetrically in the spectrum with respect to the systemic velocity. They were also mapped with KaVA (KVN and VERA Combined Array) in 2019 March. We located some of these maser components closer to the central stellar system than other high-velocity components (50–200 km s−1) that have been confirmed to be associated with the known bipolar outflow. The new components would have flashed in the fast collimated jet at a speed of over 300 km s−1(soon) after 2011 when they had not been detected. The fastest of the new components seem to indicate rapid deceleration in these spectra; however, our present monitoring is still too sparse to unambiguously confirm it (up to 50 km s−1yr−1) and too short to reveal their terminal expansion velocity, which will be equal to the expansion velocity that has been observed ($v$exp∼ 120 km s−1). Future occurrences of such extreme-velocity components may provide a good opportunity to investigate possible recurrent outflow ignitions. Thus, the sculpture of the parental envelope will be traced by the dense gas that is entrained by the fast jet and exhibits spectacular distributions of the relatively stable maser features.

The infrared dark cloud G359.94$+$0.17 is a conspicuous, opaque cloud, which is seen in silhouette against the Galactic center. We found unexpectedly strong ($\sim\ $50 Jy) maser emission of CH$_3$OH at 44 GHz with additional weak 22 GHz H$_2$O maser and 43 GHz SiO thermal emissions toward this cloud. Detections of these molecular lines indicate that strong star-forming activities are proceeding in this cloud, which was not reported previously despite of numerous studies toward the Galactic center. The line profiles of the NH$_3$inversion lines at 23 GHz indicate that G359.94$+$0.17 is composed of mainly two clouds with $V_{\rm lsr}$$=$0, and 15 km s$^{-1}$overlapped on the line of sight. The maser emission is associated with the 15 km s$^{-1}$cloud, suggesting that it is located at the Norma spiral arm.

We present distance measurements of the semiregular variable RX Bootis (RX Boo) with its annual parallax. Using the unique dual-beam system of the VLBI Exploration of Radio Astrometry (VERA) telescope, we conducted astrometric observations of a water maser spot accompanying RX Boo, referred to the quasar J1419$+$2706 separated by 1$^\circ\!\!\!.$69 from RX Boo. We measured the annual parallax of RX Boo to be 7.31$\ \pm\ $0.50 mas, corresponding to a distance of 136$^{+10}_{-9}\ $pc, from one-year monitoring observation data of one maser spot at $V_{{\rm LSR}}$$=$3.2 km s$^{-1}$. The distance, itself, is consistent with that obtained with Hipparcos. The distance uncertainty was reduced by a factor of two, allowing us to determine the stellar properties more accurately. Using our distance, we discuss the location of RX Boo in various sequences of Period–Luminosity (PL) relation. We found that RX Boo is located in the Mira sequence of the PL relation. In addition, we calculated the radius of the photosphere and the mass limitation of RX Boo, and evaluated its evolutionary status.