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Register a new userVLA survey of the 22 GHz H2O masers toward 10 silicate carbon stars
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Despite their carbon-rich photospheres, silicate carbon stars show evidence of oxygen-rich circumstellar material, which is considered to exist in disks. Silicate carbon stars represent interesting cases that allow us to study the possible effects of binarity on stellar evolution and the mass loss accompanied by the formation of disks. We present a small survey of the 22 GHz H2O masers toward 10 silicate carbon stars with much better sensitivity than the previous observations. We observed our sample with the Karl G. Jansky Very Large Array (VLA) using the most expanded configuration (A-configuration) with a maximum baseline of 36 km. For some of our program stars with noisy IRAS Low Resolution Spectra (LRS), we present new mid-IR spectra obtained with the Very Large Telescope Interferometer and the Spitzer Space Telescope. We detected the H2O masers toward 5 out of 10 silicate carbon stars (EU And, V778 Cyg, IRAS06017+1011, V1415 Cyg, and NC83=V1945 Cyg), with NC83 being new detection. No H2O masers were detected toward BM Gem, IRAS07221-0431, IRAS08002-3803, IRAS18006-3213, and HD189605. The velocity separation between the most blue- and red-shifted maser features is 10--14 km/s. If we assume that the masers originate in circum-companion disks, the measured velocity separations translate into a lower limit of the rotational velocity of 5--7 km/s, and the upper limit of the radius of the maser emitting region is estimated to be 10--68 AU for a companion mass of 0.5--1.7 Msun. The new mid-IR spectra of NC83, IRAS06017+1011, and HD189605 confirm the 10 micron silicate emission. The latter two stars show a bump at ~11.5 micron, which is presumably due to SiC originating in the ongoing mass loss from the carbon-rich primary star, not due to crystalline silicate. We also report on the detection of the UV flux at 2271 A toward HD189605.
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We wish to perform a statistical study of the location and motion of individual 22 GHz water maser cloudlets, characterized by sizes that are within a few au, with respect to the radio thermal emission from young stellar objects (YSO). For this purpose, we have been carrying out the Protostellar Outflows at the EarliesT Stages (POETS) survey of a sample (38) of high-mass YSOs. The water maser positions and three-dimensional (3D) velocities were determined through Very Long Baseline Array observations with accuracies of a few milliarcsec (mas) and a few km/s, respectively. The position of the ionized core of the protostellar wind, marking the YSO, was determined through sensitive continuum Jansky Very Large Array observations with a typical error of 20 mas. The statistic of the separation of the water masers from the radio continuum shows that 84% of the masers are found within 1000 au from the YSO and 45% of them are within 200 au. Therefore, we can conclude that the 22 GHz water masers are a reliable proxy for the YSO position. The distribution of maser luminosity is strongly peaked towards low values, indicating that about half of the maser population is still undetected with the current Very Long Baseline Interferometry detection thresholds of 50-100 mJy/beam. Next-generation, sensitive radio interferometers will exploit these weak masers for an improved sampling of the velocity and magnetic fields around the YSOs. The average direction of the water maser proper motions provides a statistically-significant estimate for the orientation of the jet emitted by the YSO: 55% of the maser proper motions are directed on the sky within an angle of 30 deg from the jet axis. Finally, we show that our measurements of 3D maser velocities statistically support models in which water maser emission arises from planar shocks with propagation direction close to the plane of the sky.
We obtained, for the first time, astrometrically registered maps of the 22.2 GHz H2O and 42.8, 43.1, and 86.2 GHz SiO maser emission toward the semiregular b-type variable (SRb) R Crateris, at three epochs (2015 May 21, and 2016 January 7 and 26) using the Korean Very-long-baseline Interferometry Network. The SiO masers show a ring-like spatial structure, while the H2O maser shows a very asymmetric one-side outflow structure, which is located at the southern part of the ring-like SiO maser feature. We also found that the 86.2 GHz SiO maser spots are distributed in an inner region, compared to those of the 43.1 GHz SiO maser, which is different from all previously known distributions of the 86.2 GHz SiO masers in variable stars. The different distribution of the 86.2 GHz SiO maser seems to be related to the complex dynamics caused by the overtone pulsation mode of the SRb R Crateris. Furthermore, we estimated the position of the central star based on the ring fitting of the SiO masers, which is essential for interpreting the morphology and kinematics of a circumstellar envelope. The estimated stellar coordinate corresponds well to the position measured by Gaia.
We present $sim1-4$ resolution Very Large Array (VLA) observations of four CH$_3$OH $J_2-J_1$-$E$ 25~GHz transitions ($J$=3, 5, 8, 10) along with 1.3~cm continuum toward 20 regions of active massive star formation containing Extended Green Objects (EGOs), 14 of which we have previously studied with the VLA in the Class~I 44~GHz and Class~II 6.7~GHz maser lines (Cyganowski et al. 2009). Sixteen regions are detected in at least one 25~GHz line ($J$=5), with 13 of 16 exhibiting maser emission. In total, we report 34 new sites of CH$_3$OH maser emission and ten new sites of thermal CH$_3$OH emission, significantly increasing the number of 25~GHz Class I CH$_3$OH masers observed at high angular resolution. We identify probable or likely maser counterparts at 44~GHz for all 15 of the 25~GHz masers for which we have complementary data, providing further evidence that these masers trace similar physical conditions despite uncorrelated flux densities. The sites of thermal and maser emission of CH$_3$OH are both predominantly associated with the 4.5 $mu$m emission from the EGO, and the presence of thermal CH$_3$OH emission is accompanied by 1.3~cm continuum emission in 9 out of 10 cases. Of the 19 regions that exhibit 1.3~cm continuum emission, it is associated with the EGO in 16 cases (out of a total of 20 sites), 13 of which are new detections at 1.3~cm. Twelve of the 1.3~cm continuum sources are associated with 6.7~GHz maser emission and likely trace deeply-embedded massive protostars.
Silicate carbon stars show the 10 micron silicate emission, despite their carbon-rich photospheres. They are considered to have circumbinary or circum-companion disks, which serve as a reservoir of oxygen-rich material shed by mass loss in the past. We present N-band spectro-interferometric observations of the silicate carbon star BM Gem using MIDI at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution of oxygen-rich dust with high spatial resolution. BM Gem was observed with VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline configurations. The N-band visibilities observed for BM Gem show a steep decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of ~44--46 m show significant non-zero values (~ -70 degrees) in the central part of the silicate emission feature between ~9 and 11 micron, revealing a photocenter shift and the asymmetric nature of the silicate emitting region. The observed N-band visibilities and differential phases can be fairly explained by a simple geometrical model in which the unresolved star is surrounded by a ring with azimuthal brightness modulation. The best-fit model is characterized by a broad ring (~70 mas across at 10 micron) with a bright region which is offset from the unresolved star by ~20 mas at a position angle of ~280 degrees. This model can be interpreted as a system with a circum-companion disk and is consistent with the spectroscopic signatures of an accretion disk around an unseen companion recently discovered in the violet spectrum of BM Gem.
22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments they occur in. The aim was to obtain accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission recently mapped with Very Long Baseline Interferometry. A sample of 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously. Water maser clusters were detected towards 27 sites finding 15 new sources. The detection rate of water maser emission associated with methanol sources was as high as 71%. In a large number of objects (18/21) the structure of water maser is well aligned with that of the extended emission at 4.5 $mu$m confirming the origin of water emission from outflows. The sources with methanol emission with ring-like morphologies, which likely trace a circumstellar disk/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario that 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appears to trace outflows. The two types of associations might be related to different evolutionary phases.
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