Database of Molecular Masers and Variable Stars

We present the database of maser sources in H2O, OH and SiO lines that can be used to identify and study variable stars at evolved stages. Detecting the maser emission in H2O, OH and SiO molecules toward infrared-excess objects is one of the methods of identification long-period variables (LPVs, including Miras and Semi-Regular), because these stars exhibit maser activity in their circumstellar shells. Our sample contains 1803 known LPV objects. 46% of these stars (832 objects) manifest maser emission in the line of at least one molecule: H2O, OH or SiO. We use the database of circumstellar masers in order to search for long-periodic variables which are not included in the General Catalogue of Variable Stars (GCVS). Our database contains 4806 objects (3866 objects without associations in GCVS catalog) with maser detection in at least one molecule. Therefore it is possible to use the database in order to locate and study the large sample of long-period variable stars. Entry to the database at http://maserdb.netComment: Accepted for publication in RA

SiO Maser Survey of the Large-Amplitude Variables in the Galactic Center

We have surveyed ~400 known large-amplitude variables within 15' of the galactic center in the SiO J=1--0 v=1 and 2 maser lines at 43 GHz, resulting in 179 detections. SiO lines were also detected from 16 other resulting in 180 detections. SiO lines were also detected from 16 other sources, which are located within 20" (the telescope half beamwidth) of the program objects. The detection rate of 48 percent is comparable to that obtained in Bulge IRAS source surveys. Among the SiO detections, five stars have radial velocities greater than 200 km/s. The SiO detection rate increases steeply with the period of light variation, particularly for stars with P>500 d, where it exceeds 80%. We found that, at a given period, the SiO detection rate is approximately three times that for OH. These facts suggest that the large-amplitude variables in the Nuclear Disk region are AGB stars similar in their overall properties to the inner and outer Bulge IRAS/SiO sources. From the set of radial velocity data, the mass distribution within 30 pc of the galactic center is derived by a new method which is based on the collisionless Boltzmann equation integrated along the line of sight. The mass within 30 pc is about 6.4 [\pm 0.7] \times 10^7 M_{\odot} and the mass of the central black hole is 2.7 [\pm 1.3] \times 10^6 M_{\odot}. Consideration of the line-of-sight velocity of each star and its potential energy leads to the conclusion that the five high-velocity stars come from galactocentric distances as high as 300 pc. The high-velocity subsample of stars with negative radial velocities exhibits a tendency to have brighter K magnitudes than the subsample of stars with positive velocities. The origin of these high-velocity stars is discussed.Comment: Hires. figures are available as No.604 of NRO report at http://www.nro.nao.ac.jp/library/report/list.html . PASJ 56 (april 28 issue) in pres

Detections of SiO Masers from the Large-Amplitude Variables in the Galactic Nuclear Disk

We have surveyed known large-amplitude variables within 15' of the Galactic center in the SiO J=1-0 v=1 and 2 maser lines at 43 GHz, resulting in 79 detections and 58 non-detections. The detection rate of 58 percent is comparable to that obtained in Bulge IRAS source surveys. SiO lines were also detected from four other sources near the program objects. The SiO detection rate increases steeply with the period, particularly for stars with P>500 d, where it exceeds 80%. We found at a given period that the SiO detection rate is approximately double that for OH. These facts suggest that the large-amplitude variables in the Nuclear Disk region are AGB stars similar in their overall properties to the inner and outer bulge IRAS/SiO sources.Comment: 5 pages, 2 figures, 1 Table. PASJ 54, No 2 April 25 issue in pres

SiO Maser Survey of IRAS Sources in the Inner Galactic Disk

We have surveyed 401 color selected IRAS sources in the Galactic disk in the SiO J=1--0 v= 1 and 2 maser lines at 43 GHz, resulting in 254 (239 new) detections. The observed sources lie mostly in a strip of the inner Galactic disk with boundaries -10<l<40 deg and |b|<3 deg. This survey provides radial velocities of inner-disk stars for which optical measurements cannot be made due to interstellar extinction. The SiO $l$--$v$ diagram in the area $-10<l<40 deg exhibits fewer objects coincident with the molecular ring feature than the OH 1612 MHz source$l$--$v$ map does, indicating a slight difference of stellar type between SiO and OH emitting stars. After identifying all of the SiO detected sources in the 2MASS near-infrared catalog, we computed their luminosity distances based on the infrared fluxes. We then mapped these objects onto the first quadrant of the Galactic plane. Combining the distances with the SiO radial-velocities, we obtained a pattern speed for SiO maser sources, Omega_P=21 (+- 13) km s^{-1} kpc^{-1}, between the distances 1 and 5.5 kpc, without the use of any dynamical models. The increase of the pattern speed toward the Galactic center (up to 60 km s^{-1} kpc^{-1} between the distances, 5.5 and 7 kpc) suggests the presence of two pattern speeds in the Galaxy.Comment: 38 page 9 figures, high res. eps files are available as NRO report No. 608 (http://www.nro.nao.ac.jp/library/report/list.html). PASJ 56 No. 4 in pres

Do water fountain jets really indicate the onset of the morphological metamorphosis of circumstellar envelopes?

Small-scale bipolar jets with 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 a 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. SomeWFs have spherical envelopes that resemble 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 morphological metamorphosis. We argue further 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. © 2016 The Authors

Identification of oxygen-rich late/post-asymptotic giant branch stars and water fountains via maser and infrared criteria

The transitional phase between the asymptotic giant branch (AGB) and post-AGB phases holds the key to our understanding of the late-stage metamorphosis of intermediate-mass stars. In particular, high velocity jets forming during this phase are suggested to contribute significantly to the shaping of planetary nebulae. For oxygen-rich stars, the rare "water fountains (WFs)" have been regarded as representative objects in this phase, and it is important to identify more of them for further studies. Here we briefly report the results of our latest OH and H2O maser surveys in which a new WF candidate (IRAS 19356+0754) was found. We also performed radiative transfer modelling on the spectral energy distributions (SEDs) of all known WFs. It is concluded that WFs might in fact not be the transitional objects, as opposed to previous belief. WFs could be AGB or post-AGB stars with no obvious similarities amongst their SEDs. Further efforts are still needed to improve the identification criteria.This work is supported by a grant awarded to JN from the Research Grants Council of Hong Kong (project code: HKU 703308P; HKU 704209P; HKU 704710P)