arXiv subjects

Jun-ichi Nakashima

Publications and source records attributed to Jun-ichi Nakashima.

At least 19 recordsLinked to original sources

arXiv2025

Infrared SED Modeling of Velocity-Excess Maser Sources: Identifying Incipient Water-Fountain Candidates

We investigated whether "velocity excess" in circumstellar maser lines can diagnose the earliest evolutionary phases of Water Fountains (WFs). Here we define "velocity excess" as maser emission (e.g., H$_2$O 22.235 GHz or OH 1665/1667 MHz) detected at velocities outside the velocity range of the OH 1612 MHz line, which traces the terminal expansion velocity of a spherical circumstellar envelope (CSE). Such excess velocities serve as an indicator of gas motions deviating from spherical expansion and may signify the onset of asymmetric outflows. Based on recent studies (Fan et al.2024; Xie et al.2025), we analyzed 17 sources showing velocity excess and fitted their infrared spectral energy distributions (SEDs) with the one-dimensional radiative transfer code DUSTY. Seven sources are well reproduced, implying outer CSEs that remain nearly spherical despite inner asymmetries. Among these, five exhibit single-peaked, AGB-like SEDs and two show double-peaked, post-AGB-like profiles. IRAS variability indices and NEOWISE-R light curves reveal pulsations (~600-1000 days) in three sources, supporting their AGB classification. Considering the magnitude of the velocity excess, two objects-IRAS 19229+1708 and IRAS 19052+0922-may represent the earliest or incipient WF phase, in which asymmetric outflows are beginning to emerge within otherwise spherical envelopes. These results support a morphological sequence in which bipolar jets and tori arise first in the central regions while the outer CSE remains spherical, and they show that selecting WF candidates via velocity excess effectively identifies objects at the onset of jet formation and early morphological transformation.

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arXiv2025

Gas-phase Molecules in Protoplanetary Nebulae with the 21 μm Emission Feature II. Carbon monosulfide

The carrier of the 21 $μ$m emission feature discovered in a handful of protoplanetary nebulae (PPNe) is one of the most intriguing enigmas in circumstellar chemistry. Investigating the gas-phase molecules in PPNe could yield important hints for understanding the 21 $μ$m feature. In this paper, we report observations of the CS $J = 5 \to 4$ line at 245 GHz and the CO $J = 1 \to 0$ line at 115 GHz toward seven PPNe exhibiting the 21 $μ$m feature. We find that CS is extremely scarce in these PPNe and the CS line is only detected in one source, IRAS Z02229+6208. Based on the assumption of local thermal equilibrium and negligible optical depth, we derive that the CS column densities and fractional abundances relative to H$_{2}$ are $N$(CS) < 9.1 ${\times}$ 10$^{13}$cm$^{-2}$ and $f$(CS) < 8.1 ${\times}$ 10$^{-7}$. A comparison of the CS abundances across different circumstellar envelopes reveals that the variations in CS abundance are complex, depending not only on the evolutionary stages but also on the properties of individual objects.

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arXiv2025

FAST search for circumstellar atomic hydrogen. III. Planetary nebulae at Galactic latitudes |b|>10degree

The detection of circumstellar atomic hydrogen (\ion{H}{1}) via the 21\,cm line remains a persistent challenge in planetary nebula (PN) studies, primarily due to contamination from ubiquitous interstellar \ion{H}{1} 21\,cm emission. In this paper, we report the results of a \ion{H}{1} survey of 12 high surface brightness PNe located at Galactic latitudes $|b|\geq10\arcdeg$, performed with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), which is currently the most sensitive telescope in the $L$ band. Although the contamination from interstellar emission is still severe, we detect or tentatively detect circumstellar \ion{H}{1} 21\,cm absorption associated with two PNe: NGC\,6905 and NGC\,7662. \ion{H}{1} exhibits a comparatively high detection frequency in bipolar PNe. We estimate the \ion{H}{1} masses of the two PNe to range from 0.01 to 0.14 $M_{\sun}$, resulting in atomic-to-ionized hydrogen ratios of 0.02--0.3. The \ion{H}{1} shells have dynamical ages of 2100--2400 years. Our measurements confirm previous findings that the optical depth of \ion{H}{1} decreases with increasing linear radius of the nebula. The mass loss rates traced by the \ion{H}{1} absorption are larger than 10$^{-5}$\,M$_\sun$ yr$^{-1}$, indicating that they originate from the superwind phase at the tip of the asymptotic giant branch.

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arXiv2025

Considerations on the Origin of IRAS 19312+1950 Based on Long-Term Maser Observations

IRAS source 19312+1950 (hereafter I19312) is an infrared point source with maser emissions of SiO, H$_2$O, and OH molecules. Although initial observations suggested that I19312 might be an evolved star, its characteristics are not fully consistent with this classification. This study aims to further investigate the nature of I19312 by conducting long-term monitoring of its maser emissions and comparing the results with other known astrophysical objects. We conducted long-term monitoring of SiO, H$_2$O, and OH maser emissions using single-dish radio telescopes. The results were then compared with historical maser data and the characteristics of similar objects to infer the possible origin of I19312. The SiO maser emissions from I19312 were detected over a wide velocity range and exhibited significant time variability. The OH maser lines suggest characteristics of an evolved star, while the H$_2$O maser lines indicate molecular outflows. These features suggest that I19312 could be a candidate for a Water Fountain (WF) star, though there are inconsistencies, such as the large molecular gas mass, that challenge this hypothesis. The possibility of I19312 being a Red Nova Remnant (RNR) is also considered, but this remains speculative due to the lack of direct evidence. The evolutionary stage of I19312 remains unclear, but it shares multiple characteristics with both evolved stars with peculiar properties and RNRs. Further long-term monitoring and high-resolution interferometric observations are required to better constrain the nature of this object.

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arXiv2024

A Search for Asymmetric Kinematic Components in Circumstellar Envelopes Using OH Main Line Masers

Circumstellar OH maser lines are useful for studying the dynamics of the circumstellar envelope (CSE) around evolved stars. This study aims to identify CSEs around cold stars, which exhibit deviations from the spherical expansion, by comparing the velocity ranges of the OH main lines (1665/1667 MHz) with those of the satellite line (1612 MHz), using a database of circumstellar OH maser sources. We performed this comparison for 377 circumstellar OH maser sources. In addition, using infrared two-color diagrams, we examined the evolutionary stages and infrared properties of objects showing velocity excess (velocity excess means the detection of the main lines outside the velocity range of the satellite line). A periodicity analysis of the WISE light curves was also carried out. As a result of the velocity range comparison, eight circumstellar OH maser sources were found to exhibit velocity excess. The infrared colors of these objects match those of post-AGB stars. Periodic variations were observed in the WISE light curves of five of these eight objects. The results suggest that examining velocity excess of the main lines relative to the satellite line is scientifically significant because mainline masers probe the CSE dynamics over a broader range of evolutionary stages compared to the 22.235 GHz H${_2}$O maser line. Additionally, during the post-AGB phase, the emission regions of the mainline and 22.235 GHz H${_2}$O masers may overlap in a CSE, whereas they originate from different regions during the AGB phase.

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arXiv2024

FAST detection of OH emission in the carbon-rich planetary nebula NGC 7027

We present the first detection of the ground-state OH emission line at 1612 MHz toward the prototypical carbon-rich planetary nebula (PN) NGC 7027, utilizing the newly installed ultra-wideband (UWB) receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This emission is likely to originate from the interface of the neutral shell and the ionized region. The other three ground-state OH lines at 1665, 1667, and 1721 MHz are observed in absorption and have velocities well matched with that of HCO$^+$ absorption. We infer that the OH absorption is from the outer shell of NGC 7027, although the possibility that they are associated with a foreground cloud cannot be completely ruled out. All the OH lines exhibit a single blue-shifted component with respect to the central star. The formation of OH in carbon-rich environments might be via photodissociation-induced chemical processes. Our observations offer significant constraints for chemical simulations, and they underscore the potent capability of the UWB receiver of FAST to search for nascent PNe.

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arXiv2024

Excited-State OH Masers in the Water Fountain Source IRAS 18460-0151

Water fountain objects are generally defined as "evolved stars with low to intermediate initial mass accompanied by high-velocity molecular jets detectable in the 22.235 GHz H$_2$O maser line". They are the key objects of understanding the morphological transitions of circumstellar envelopes during the post asymptotic giant branch phase. Masers are useful tools to trace the kinematic environments of the circumstellar envelopes. In this letter we report the discovery of exceptionally uncommon excited-state hydroxyl (ex-OH) masers at 4660 and 6031 MHz toward the water fountain source IRAS 18460-0151. These are the brightest ex-OH masers discovered in late-type objects to date. To the best of our knowledge, prior to the current work, no evolved stellar object has been observed in the 4660 MHz ex-OH maser line. The ground-state hydroxyl (g-OH) masers at 1612 and 1665 MHz are also observed. The velocity components of the 4660 MHz ex-OH maser line and the much weaker 1665 MHz g-OH maser line all can be seen in the 1612 MHz g-OH maser line profile. The blue-shifted components of the three masers are more intense than the red-shifted ones, in contrast to the ex-OH maser line at 6031 MHz. The relevance of the behaviors of the ex-OH masers to the circumstellar environments is unclear.

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arXiv2023

Gas-phase molecules in protoplanetary nebulae with the 21 μm emission feature

It has been more than 30 years since the enigmatic 21 μm emission feature was first discovered in protoplanetary nebulae (PPNs). Although dozens of different dust carrier candidates have been proposed, there is as yet no widely accepted one. We present the results of molecular observations toward 21μm objects using the 10m Submillimeter Telescope of Arizona Radio Observatory at the 1.3 mm band and the 13.7 m telescope of Purple Mountain Observatory at the 3mm band, aiming to investigate whether the gas-phase environments of these unusual sources have some peculiarities compared to normal PPNs. We detect 31 emission lines belonging to seven different molecular species, most of which are the first detection in 21 μm PPNs. The observations provide clues on the identification of the 21 μm feature. We report a correlation study between the fractional abundance of gas-phase molecules and the strengths of the 21 μm emission. Our study shows that given the small sample size, the 21 μm feature has weak or no correlations with the gas-phase molecules. Future radio observations of high spatial and spectral resolution toward a large sample are desirable to elucidate the 21 μm emission phenomena.

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arXiv2023

Systematic Search for Water Fountain Candidates using the Databases of Circumstellar Maser Sources

Water fountains (WFs) are thought to be objects in the morphological evolution of the circumstellar envelopes of low- and intermediate-mass evolved stars, transitioning from spherically symmetric to asymmetric shapes. We used databases of circumstellar 1612 MHz OH and 22.235 GHz H$_2$O maser sources to search for new WF candidates using the criterion of a larger velocity range of the H$_2$O maser emission compared to that of the OH maser emission. Thus, it is in principle possible to identify WFs with an H$_2$O velocity ranges smaller than those for the previously known WFs. For the OH maser line, we analyzed database entries of 8,474 observations from 2,195 sources, and 6,085 observations from 3,642 sources for H$_2$O maser line. After a close examination of the velocity ranges and line profiles, we identified 11 sources that meet the criterion mentioned above. We examined the IRAS colors of the selected sources and found that two of them (IRAS 19069+0916 and IRAS 19319+2214) are in the color region for post-AGB stars. We find that the maser velocity criterion can discover other astrophysically interesting objects than just WFs. Such objects may include peculiar planetary nebulae with maser emissions and stellar merger remnants.

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arXiv2023

FAST search for circumstellar atomic hydrogen. II. Is BD+303639 an interacting planetary nebula?

The young, compact, very high surface brightness but low excitation planetary nebula (PN) BD+303639 is one of the very few PNe that have been reported to exhibit the 21cm HI emission line. As part of a long-term programme to search for circumstellar atomic hydrogen, we observed the 21cm feature toward BD+303639 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Assuming a direct association between the PN and the detected HI emission, these new observations show that this surrounding emission is significantly more spatially extended than indicated by previous interferometric observations, and can be resolved into two velocity components. The estimated HI mass is larger than 100M_sun, invalidating an origin from the host star itself or its ejecta for the emitting material. We discuss the possibility that the extended HI emission stems from the interstellar medium (ISM) swept out over time by the stellar wind. Moreover, we report tentative detections of HI absorption features lying near and blueward of the systemic velocity of this PN, which are probably from a stalled asterosphere at the outer boundary of the expanding ionized region. The mass of the gas producing the HI absorption is insufficient to solve the so-called `PN missing mass problem'. We demonstrate the capability of FAST to investigate the interaction process between a PN and the surrounding ISM.

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arXiv2022

Molecules in the peculiar age-defying source IRAS 19312+1950

Context. IRAS 19312+1950 is an isolated infrared source that exhibits a characteristic quasi-point-symmetric morphology in the near- and mid-infrared images and is also very bright in molecular radio lines. Because of its unique observational characteristics, various observational studies have been conducted and several hypotheses have been proposed regarding its origin, which is still unclear. So far, it has been suggested that it could be a peculiar evolved star, a young stellar object, or even a red nova remnant. Regardless of which type of object it is ultimately classified as, IRAS 19312+1950 is exceptionally bright in the infrared and molecular radio lines and therefore will undoubtedly be crucial as a prototype of this kind of object having a peculiar nature or unusual evolutionary phase. Aims. This study aims to reveal the molecular composition of the central part of IRAS 19312+1950 by performing an unbiased molecular radio line survey and discussing the origin of the object from a molecular chemical point of view. Methods. We carried out a spectral line survey with the IRAM 30 m telescope towards the center of IRAS 19312+1950 in the 3 and 1.3 mm windows. Results. In total, 28 transition lines of 22 molecular species and those isotopologues are detected towards IRAS 19312+1950, some of which exhibit a broad and a narrow components. Seventeen thermal lines and 1 maser line are newly detected. The molecular species of C$^{17}$O, $^{30}$SiO, HN$^{13}$C, HC$^{18}$O$^{+}$, H$_{2}$CO, and $c$-C$_{3}$H$_{2}$ are detected for the first time in this object. Conclusions. Our results, in combination with previous studies, favor the hypothesis that IRAS 19312+1950 might be a red nova remnant, in which the progenitors that merged to become a red nova may have contained at least two evolved stars with oxygen-rich and carbon-rich chemistry, respectively.

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arXiv2022

The Astrometric Animation of Water Masers towards the Mira Variable BX Cam

We report VLBI monitoring observations of the 22 GHz water (H$_{2}$O) masers around the Mira variable BX Cam, which were carried out as a part of the EAVN Synthesis of Stellar Maser Animations (ESTEMA) project. Data of 37 epochs in total were obtained from 2018 May to 2021 June with a time interval of 3-4 weeks, spanning approximately three stellar pulsation periods ($P= \sim$440 d). In particular, the dual-beam system equipped on the VERA stations was used to measure the kinematics and parallaxes of the H$_{2}$O maser features. The measured parallax, $π=1.79\pm 0.08$ mas, is consistent with $Gaia$ EDR3 and previously measured VLBI parallaxes within a 1-$σ$ error level. The position of the central star was estimated, based on both the $Gaia$ EDR3 data and the center position of the ring-like 43 GHz silicon-monoxide (SiO) maser distribution imaged with the KVN. The three-dimensional H$_{2}$O maser kinematics indicates that the circumstellar envelope is expanding at a velocity of $13\pm4$ km s$^{-1}$, while there are asymmetries in both the spatial and velocity distributions of the maser features. Furthermore, the H$_{2}$O maser animation achieved by our dense monitoring program manifests the propagation of shock waves in the circumstellar envelope of BX Cam.

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arXiv2022

FAST search for circumstellar atomic hydrogen--I: the young planetary nebula IC 4997

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Guizhou, China, we detect the 21cm neutral atomic hydrogen absorption in the young planetary nebula IC 4997. The absorption arises from a shell also associated with Na I D lines. The H I shell has a mass of $1.46\times10^{-2}$ M$_\odot$ and a dynamic age of 990yr. The column density of H I is estimated to be $7.1\times10^{20}$ cm$^{-2}$, which can be well explained in terms of a photodissociation region around the ionized nebula, limited by self shielding of H$_2$. We find that the atomic-to-ionized hydrogen ratio is 0.6, suggesting that H I substantially contributes to overall nebular mass.

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arXiv2022

Molecules in the carbon-rich protoplanetary nebula CRL 2688

We present observations of the carbon-rich protoplanetary nebula (PPN) CRL 2688 made with the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescope in the 3mm and 2mm bands. In total, 196 transition lines belonging to 38 molecular species and isotopologues are detected, among which, to our best knowledge, 153 transition lines and 13 species are the first report for this object. Additionally, in order to contribute to future research, we have collected observational data on the molecular lines of CRL 2688 from the literature and compiled them into a single unified catalog. We find that the molecular abundance of CRL 2688 cannot be explained by the standard model of a circumstellar envelope. The implications of metal-bearing molecules on circumstellar chemistry are discussed.

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arXiv2020

The spectra of evolved stars at 20--25\,GHz: tracing circumstellar chemistry during the asymptotic giant branch to planetary nebula transition

We report an unbiased radio line survey towards the circumstellar envelopes of evolved stars at the frequency range from 20 to 25 GHz, aiming to obtain a more complete unbiased picture of the chemical evolution in the final stages of stellar evolution. The observation sample includes the asymptotic giant branch (AGB) star IRC+10216, the proto-planetary nebulae (PPNs) CRL\,2688 and CRL\,618, and the young planetary nebula (PN) NGC\,7027, representing an evolutionary sequence spanning about 10,000 years. Rotational transitions from cyanopolyyne chains and inversion lines from ammonia are detected in the AGB star and PPNs, while the PN displays several recombination lines. The different spectral behaviors of these evolved stars clearly reflect the evolution of circumstellar chemistry during the AGB-PPN-PN transitions.

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arXiv2020

Shaping the envelope of the asymptotic giant branch star W43A with a collimated fast jet

One of the major puzzles in the study of stellar evolution is the formation process of bipolar and multi-polar planetary nebulae. There is growing consensus that collimated jets create cavities with dense walls in the slowly-expanding (10--20 ~km~s$^{-1}$) envelope ejected in previous evolutionary phases, leading to the observed morphologies. However, the launching of the jet and the way it interacts with the circumstellar material to create such asymmetric morphologies have remained poorly known. Here we present for the first time CO emission from the asymptotic giant branch star W43A that traces the whole stream of a jet, from the vicinity of its driving stellar system out to the regions where it shapes the circumstellar envelope. We found that the jet has a launch velocity of 175~km~s$^{-1}$ and decelerates to a velocity of 130~km~s$^{-1}$ as it interacts with circumstellar material. The continuum emission reveals a bipolar shell with a compact bright dot in the centre that pinpoints the location of the driving source of the jet. The kinematical ages of the jet and the bipolar shell are equal, $τ$$\sim$60~years, indicating that they were created simultaneously, probably by a common underlying mechanism, and in an extremely short time. These results provide key initial conditions for the theoretical models that aim to explain the formation of bipolar morphologies in the circumstellar envelopes of low and intermediate mass stars.

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arXiv2016

Do Water Fountain Jets Really Indicate the Onset of the Morphological Metamorphosis of Circumstellar Envelopes?

The small-scale bipolar jets having short dynamical ages from "water fountain (WF)" sources are regarded as an indication of the onset of circumstellar envelope morphological metamorphosis of intermediate-mass stars. Such process usually happens at the end of the asymptotic giant branch (AGB) phase. However, recent studies found that WFs could be AGB stars or even early planetary nebulae. This fact prompted the idea that WFs may not necessarily be objects at the beginning of the morphological transition process. In the present work, we show that WFs could have different envelope morphologies by studying their spectral energy distribution profiles. Some WFs have spherical envelopes that resembles usual AGB stars, while others have aspherical envelopes which are more common to post-AGB stars. The results imply that WFs may not represent the earliest stage of the morphological metamorphosis. We further argue that the dynamical age of a WF jet, which can be calculated from maser proper motions, may not be the real age of the jet. The dynamical age cannot be used to justify the moment when the envelope begins to become aspherical, nor to tell the concrete evolutionary status of the object. A WF jet could be the innermost part of a larger well-developed jet, which is not necessarily a young jet.

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arXiv2016

Wide Field CO Mapping in the Region of IRAS 19312+1950

We report the results of a wide field CO mapping in the region of IRAS 19312+1950. This IRAS object exhibits SiO/H$_2$O/OH maser emission, and is embedded in a chemically-rich molecular component, of which the origin is still unknown. In order to reveal the entire structure and gas mass of the surrounding molecular component for the first time, we have mapped a wide region around IRAS 19312+1950 in the $^{12}$CO $J=1-0$, $^{13}$CO $J=1-0$ and C$^{18}$O $J=1-0$ lines using the Nobeyama 45m telescope. In conjunction with the archival CO maps, we investigated a region with a size up to $20' \times 20'$ around this IRAS object. We calculated CO gas mass assuming the LTE condition, a stellar velocity against to the interstellar medium assuming an analytic model of a bow shock, and absolute luminosity using the latest archival data and trigonometric parallax distance. The derived gas-mass (225 M$_{\odot}$ $-$ 478 M$_{\odot}$) of the molecular component and the relatively large luminosity ($2.63\times10^{4}$ L$_{\odot}$) suggest that the central SiO/H$_2$O/OH maser source seems to be a red supergiant (RSG) rather than an asymptotic giant branch (AGB) star or post-AGB star.

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