Stellar Masers Research Articles

Galactic Astrometry with VLBI

AbstractAstrometric very long baseline interferometry (VLBI) observations of stellar masers are an excellent method to determine distances and proper motions in our Galaxy. Large maser astrometry surveys, the Bar and Spiral Structure Legacy survey and the VLBI Exploration for Radio Astrometry, allowed astronomers to determine fundamental Galactic parameters, such as the rotation curve and the distance to the Galactic centre, as well as to trace the spiral arms. In this review, the results of these surveys will be summarised and compared with astrometric measurements using other methods.

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

We report very long baseline interferometry (VLBI) monitoring observations of the 22 GHz water (H2O) masers around the Mira variable BX Cam, which were carried out as a part of the EAVN Synthesis of Stellar Maser Animations 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 = ∼440 days). In particular, the dual-beam system equipped on the VERA stations was used to measure the kinematics and parallaxes of the H2O maser features. The measured parallax, π = 1.79 ± 0.08 mas, is consistent with Gaia EDR3 and previously measured VLBI parallaxes within a 1σ 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 Korean VLBI Network. The three-dimensional H2O maser kinematics indicates that the circumstellar envelope is expanding at a velocity of 13 ± 4 km s−1, while there are asymmetries in both the spatial and velocity distributions of the maser features. Furthermore, the H2O maser animation achieved by our dense monitoring program manifests the propagation of shock waves in the circumstellar envelope of BX Cam.

Results of KVN Key Science Program for evolved stars

AbstractWe present the results of KVN Key Science Program (KSP) for evolved stars, which was launched in 2014. The first phase of KSP ended in June 2020 and the second phase started in October 2020. The goal of KSP is to study the physical characteristics of the evolved stars by observing the spatial distribution and temporal variability of the stellar masers at four frequency-bands (K, Q, W and D bands). The 22 GHz H2O maser is usually observed from the outer part of circumstellar envelopes compared to the 43, 86, 129 GHz SiO masers, thus the kinematic links between these regions can be studied by the multi-frequency simultaneous observations of KSP along the stellar pulsation cycles. This eventually enable us to study the enormous mass-loss rate of evolved stars, and the accumulated results from KSP are expected to shed light on the study of the late stage of the stellar evolution.

3D Kinematics of Stellar SiO Masers in the Galactic Center

Abstract We present 3D velocity measurements and acceleration limits for stars within a few parsec of the Galactic Center (GC) black hole, Sgr A*, based on observations of 43 and 86 GHz circumstellar maser emission. Observations were taken with the Very Large Array in 2013, 2014, and 2020 and with the Atacama Large Millimeter/submillimeter Array in 2015 and 2017. We detect 28 masers in total, of which four are new detections. Combining these data with extant maser astrometry, we calculate stellar proper motions and accelerations with uncertainties as low as ∼10 μas yr−1 and 0.5 μas yr−2, respectively, corresponding to approximately 0.5 km s−1 and 0.04 km s−1 yr−1 at a distance of 8 kpc. We measure radial velocities from maser spectra with ∼0.5 km s−1 uncertainties, though the precision and accuracy of such measurements for deducing the underlying stellar velocities are limited by the complex spectral profiles of some masers. We therefore measure radial acceleration limits with typical uncertainties of ∼0.1 km s−1 yr−1. We analyze the resulting 3D velocities and accelerations with respect to expected motions resulting from models of the mass distribution in the GC.

Stellar SiO masers in the Galaxy: The Bulge Asymmetries and Dynamic Evolution (BAaDE) survey

AbstractCircumstellar SiO masers can be observed in red giant evolved stars throughout the Galaxy. Since stellar masers are not affected by non-gravitational forces, they serve as point-mass probes of the gravitational potential and form an excellent sample for studies of the Galactic structure and dynamics. Compared to optical studies, the non-obscured masers are in particular valuable when observed close to the highly obscured Galactic Bulge and Plane. Their line-of-sight velocities can easily be obtained with high accuracy, proper motions can be measured and distances can be estimated. Furthermore, when different mass and metallicity effects can be accounted for, such a large sample will highlight asymmetries and evolutionary traces in the sample. In our Bulge Asymmetries and Dynamic Evolution (BAaDE) survey we have searched 20,000 infrared selected evolved stars for 43 GHz SiO masers with the VLA in the northern Bulge and Plane and are in the process of observing another 10,000 stars for 86 GHz SiO masers with ALMA in the southern Bulge. Our instantaneous detection rate in the Bulge is close to 70%, both at 43 and 86 GHz, with occasionally up to 7 simultaneous SiO transitions observed in a single star. Here we will outline the BAaDE survey, its first results and some of the peculiar maser features we have observed. Furthermore we will discuss the prospects for obtaining proper motions and parallaxes for individual maser stars to reconstruct individual stellar orbits.

The comparison between H2CO and OH in the Milky Way

A catalog of 4231 Galactic H2CO absorption lines ( $\sim 2241$ sources) at 6 cm has been collected from the literature published up to 2014. The distribution of these H2CO absorption lines has been compared with the OH masers at 18 cm in the Milky Way. Most of the OH masers are distributed in the inner Galaxy ( $-75^{\circ} < l < 75^{\circ}$ ) with a higher Galactic latitude than the H2CO sources ( $-5^{\circ} < b < 5^{\circ}$ ). The longitude-velocity distributions of H2CO and OH show that the majority of the objects are located in the molecular ring following the CO molecular gas distribution. The range of velocities of the OH masers is larger and distribution more diffuse than the H2CO sources in the Galactic longitude range $30^{\circ} < l < 90^{\circ}$ . The H2CO sources have been matched with the four ground state transitions of OH and a close associations have been found with H2CO clouds with small velocity differences ( $<5~\mbox{km}\,\mbox{s}^{-1}$ ). The H2CO sources observed with a higher resolution telescope have a better correlation with the OH maser, which may imply a common excitation process and may be attributed to shock excitation. The correlation index of the 1720 MHz OH for the relations $L_{\mathrm{OH}}$ – $L_{\mathrm{H}_{2}\mathrm{CO}}^{\alpha}$ and $L_{\mathrm{OH}}$ – $L_{\mathrm{IR}}^{\alpha}$ is much weaker as compared with 1612, 1665, and 1667 MHz OH transitions. The correlation between H2CO and the interstellar OH maser is much stronger than between H2CO and the stellar OH maser. Similarly the correlation is stronger for the presence than for the absence of far-infrared radiation (FIR) sources. This may indicate that both the FIR and the radio continuum connects the OH with H2CO characteristics.

KaVA ESTEMA project

The ESTEMA (Expanded Study on Stellar Masers) project is one of three Large Programs of the KaVA (the combined array of the Korean VLBI Network and Japanese VLBI Exploration of Radio Astrometry), and conducted in 2015-2016. It aims to publish a database of the largest sample of VLBI images of circumstellar water (H2O) and silicon-monoxide (SiO) maser sources towards circumstellar envelopes (CSEs) of 80 evolved stars in late AGB to early post-AGB phase. Here we present the specifications of the ESTEMA observations and the planned scientific goals in order to share the basic information of the ESTEMA with astronomical community and encourage future collaborations with the ESTEMA and future follow-up observations for the targeted stars.

Annual Parallax of the K-Type Star System IRAS 22480+6002 Measured with VERA

Abstract We present astrometric VLBI observations of H$_2$ O masers associated with IRAS 22480$+$ 6002 ($=$ IRC$+$ 60370, hereafter I22480) with the VLBI Exploration of Radio Astrometry (VERA). The stellar type of I22480 looks unusual in stellar maser sources, and has been debated since the 1970's. We successfully determined that an annual parallax of a group of H$_2$ O maser spots is $\pi$$=$ 0.400 $\pm$ 0.025 mas, corresponding to a distance to I22480 of $D$$=$ 2.50$^{+0.17}_{-0.15}$ kpc. This suggests that an estimated bolometric luminosity of I22480 should be revised to 35000 $L_{\odot}$ , preferring a K-type supergiant to an RV Tau-type variable star, previously suggested. Although its spectral type is unusual in stellar maser sources, internal motions of H$_2$ O maser features suggest that the H$_2$ O masers are associated with the circumstellar envelope of this star. We derived a secular proper motion of I22480, ($\mu_{\alpha}$ , $\mu_{\delta}$ ) $=$ ($-$ 2.58 $\pm$ 0.33, $-$ 1.91 $\pm$ 0.17) [mas yr$^{-1}$ ], from a possible stellar motion relative to the maser feature motion. The derived motion of I22480 in the Milky Way has a deviation of $\sim-$ 30 km s$^{-1}$ in the Galactic azimuthal direction from the circular motion, estimated from a galactocentric distance to I22480 and an assumption of a flat Galactic rotation curve. This peculiar motion is still comparable to those typically seen in H$_2$ O maser sources located in the Perseus Arm. Taking into account its peculiar motion and proximity to the Galactic midplane (z$\simeq$ 60 pc), I22480 may be a member of the Galactic thin disk.

Infrared characteristics of sources associated with OH, H2O, SiO and CH3OH masers

AbstractWe collect all published OH, H2O, SiO and CH3OH masers in the literature. The associated infrared sources of these four masers were identified with MSX PSC catalogues. We look for common infrared properties among the sources associated with four masers and make a statistical study. The MSX sources associated with stellar OH, stellar H2O and SiO masers concentrated in a small regions and the MSX sources associated with interstellar OH, interstellar H2O and CH3OH masers also concentrated in a small regions in an [A]-[D].vs.[A][-[E] diagram. These results give us new criterion to search for coexisting stellar maser samples for OH, H2O and SiO masers and interstellar maser samples for OH, H2O and CH3OH masers.

Characteristics of infrared point sources associated with OH masers

We collect 3249 OH maser sources from the literature published up to April 2007, and compile a new catalog of OH masers. We look for the exciting sources of these masers and their infrared properties from IRAS and MSX data, and make a statistical study. MSX sources associated with stellar 1612 MHz OH masers are located mainly above the blackbody line; this is caused by the dust absorption of stellar envelopes, especially in the MSX_A band. The mid-IR sources associated with stellar OH masers are concentrated in a small region in an [A]–[D] vs. [A]–[E] diagram with a small fraction of contamination; this gives us a new criterion to search for new stellar OH masers and distinguish stellar masers from unknown types of OH masers. IR sources associated with 1612 MHz stellar OH masers show an expected result: the average flux of sources with F60 > F25 increases with increasing wavelength, while those with F60 < F25 vary little with wavelength, because the sources with F60 < F25 are much hotter than those with F60 > F25.

Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic center

We present lunar occultation observations obtained in August 2006 with the recently demonstrated burst mode of the ISAAC instrument at the ESO VLT. The results presented here follow the previously reported observations carried out in March 2006 on a similar but unrelated set of sources. Interstellar extinction in the inner regions of the galactic bulge amounts to tens of magnitudes at visual wavelengths. As a consequence, the majority of sources in that area are poorly studied and large numbers of potentially interesting sources such as late-type giants with circumstellar shells, stellar masers, infrared stars, remain excluded from the typical investigations which are carried out in less problematic regions. Also undetected are a large numbers of binaries. By observing LO events in this region, we gain the means to investigate at least a selected number of sources with an unprecedented combination of sensitivity and angular resolution. The LO technique permits to achieve mas resolution with a sensitivity of K~12mag at a very large telescope. We have used the opportunity of a favorable passage of the Moon over a crowded region in the general direction of the Galactic Center to observe 78 LO events of heavily reddened stellar sources. We have detected six new binary and one triple star, with typical projected separation of ~0.01. We have also detected the compact circumstellar emission around one maser and one central star of a planetary nebula. Additionally we have measured the diameter and/or circumstellar shell of two carbon stars and other IR sources. In agreement with our previous result, we conclude that lunar occultations in fast read-out mode on a detector subwindow at an 8m-class telescope can achieve an angular resolution close to 0.001 with a sensitivity K~12mag.

Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic center

Context. We present lunar occultation (LO) observations obtained in August 2006 with the recently demonstrated burst mode of the ISAAC instrument at the ESO VLT. The results presented here follow the previously reported observations carried out in March 2006 on a similar but unrelated set of sources. Aims. Interstellar extinction in the inner regions of the galacti c bulge amounts to tens of magnitudes at visual wavelengths. As a consequence, the majority of sources in that area are poorly studied and large numbers of potentially interesting sources such as latetype giants with circumstellar shells, stellar masers, inf rared stars, remain excluded from the typical investigatio ns which are carried out in less problematic regions. Also undetected are a large numbers of binaries. By observing LO events in this region, we gain the means to investigate at least a selected number of sources with an unprecedented combination of sensitivity and angular resolution. Methods. The LO technique permits to achieve milliarcsecond resolution with a sensitivity of K≈12 mag at a very large telescope. We have used the opportunity of a favorable passage of the Moon over a crowded region in the general direction of the Galactic Center to observe 78 LO events of heavily reddened stellar sources. Results. We have detected six new binary and one triple star, with typical projected separation of≈ 0. 01. We have also detected the compact circumstellar emission around one maser and one central star of a planetary nebula. Additionally we have measured the diameter and/or circumstellar shell of two carbon stars and other IR sources. Conclusions. We have used the upper limits on the size of about 60 unresolved or marginally resolved sources to establish the performance of the method. In agreement with our previous result, we conclude that lunar occultations in fast read-out m ode on a detector subwindow at an 8 m-class telescope can achieve an angular resolution close to 0. ′′ 001 with a sensitivity K≈ 12 mag.

JVN Observations of H2O Masers around the Evolved Star IRAS 22480+6002

Abstract We report on the H$_2$O maser distributions around IRAS 22480$+$6002 ($=$ IRC$+$60370), observed with the Japanese VLBI Network (JVN) at three epochs spanning 2 months. This object was identified as a K-type supergiant in the 1970s, which was unusual as a stellar maser source. The spectrum of H$_2$O masers consists of 5peaks separated roughly equally by a fewkms$^{-1}$ each. The H$_2$O masers were spatially resolved into more than 15features, which spread about 50mas along the east–west direction. However, no correlation was found between the proper-motion vectors and their spatial positions; the velocity field of the envelope seems to be random. A statistical parallax method applied to the observed proper-motion data set gives a distance of $1.0\pm 0.4$kpc for this object, which is considerably smaller than previously thought. The distance indicates that this is an evolved star with $L\sim$5800$L_{\odot}$. This star shows radio, infrared, and optical characteristics that are quite similar to those of population II post-AGB stars, such as RV Tau variables.

Stellar masers, circumstellar envelopes and supernova remnants

AbstractThis paper reviews recent advances in the study or circumstellar masers and masers found toward supernova remnants. The review is organized by science focus area, including the astrophysics of extended stellar atmospheres, stellar mass-loss processes and outflows, late-type evolved stellar evolution, stellar maser excitation and chemistry, and the use of stellar masers as independent distance estimators. Masers toward supernova remnants are covered separately. Recent advances and open future questions in this field are explored.

The hypergiant masers: episodic mass loss, convection and magnetic fields

AbstractThe cool hypergiants are the most luminous known stars in the upper HR Diagram in the apparent temperature range represented by spectral types A to M. Most of the stars in this regime are unstable as evidenced by their high mass loss rates, variability, and in some cases large IR excesses and circumstellar ejecta. We have obtained high resolution multi-wavelength images with HST/WFPC2 of several of the most known evolved cool stars including several well known stellar masers. VX Sgr and S Per were marginally resolved, while NML Cyg has a peculiar asymmetric envelope that has been shaped by its environment. The powerful maser sources IRC+10420 and VY CMa have extensive and complex circumstellar ejecta due to high mass loss episodes apparently driven by large-scale convective activity.

Twenty-six-year monitoring of water masers

AbstractSince 1980 variability of a sample of H2O maser sources has been monitored on the 22-metre radio telescope in Pushchino, Russia. The interval between successive observational sessions is 1–2 months. The sample includes 125 maser sources in star-forming regions (SFR) and late-type variable stars. Twenty-six-year time series of H2O line profiles have detected flares and velocity drift of spectral features. Very fast variations in the H2O maser flux (Δt ≲ 1 hour) have been detected in several SFR sources, in particular, W33B. Variations of circumstellar H2O masers in late-type stars correlate with visual light curves with a time lag of 0.3-0.4P (P is the star's period). Exceptionally strong H2O maser flares were recorded in SFR sources (Sgr B2 and others) and in the stars W Hya, R Cas and U Ori. Models for H2O maser variability are reviewed. For stellar masers shock-wave excitation of H2O line variability is discussed.

Evolution of Maser/IR Objects with Very Thick Dust Envelopes

Some stellar maser sources at preplanetary stage have very thick circumstellar envelopes, for which no near-infrared identifications have been made. We investigated such stars at radio/NIR/MIR wavelengths using the NRO 45-m, ANU 2.2-m, UH 2.2-m, and SUBARU 8.2-m telescopes. Furthermore, using the Spitzer/Glimpse survey of the Galactic plane, we found counterparts in the 3.6 micron band for all of the OH/IR objects without previous NIR identification. One of the most interesting objects among these is IRAS 18450–0148 (W43A) with collimated outflows. Their spectra indicate that they have massive thick disks.

Observations of Stellar Maser Sources with No IRAS Counterpart

Abstract We investigated stellar maser sources with no IRAS counterpart at the radio, middle-infrared, and near-infrared wavelengths. A 43 GHz SiO maser search for 120 2MASS/MSX objects, and 10 OH 1612 MHz sources with no or a very faint MSX counterpart, resulted in 43 SiO detections: one OH 1612 MHz source, 2 near-infrared stars, and 40 MSX sources. Additional near-infrared $J$-, $H$-, and $K$-band observations of the OH 1612 MHz sources detected 5 near-infrared counterparts. Furthermore, middle-infrared imaging observations at 8.8, 9.7, 12.4, and 24.5$\, {\mu \mathrm {m}}$ with the Subaru 8.2-m telescope found counterparts for 2 near-infrared stars with SiO masers, and counterparts for 6 OH 1612 MHz sources. However, 4 OH 1612 MHz sources were not detected in the sensitive near- and middle-infrared searches; three of these are relatively strong OH maser sources for which the positions were known accurately. We conclude that one of these (OH 028.286$-$01.801) must be a young object in a star-forming region.

Infrared Characteristics of Associated Sources of Water Masers

We present an analysis of the infrared properties of 1417 water masers collected from the literature published by December 2004. The associated infrared sources of the water masers were identified with IRAS and MSX (Midcourse Space Experiment) catalogues. There are 1252 water masers associated with IRAS sources within 1', which include 700 interstellar and 552 stellar sources. For 382 sources, the IRAS counterpart identification and the maser classification are new. We found the colors of the interstellar maser sources are much redder than those of the stellar ones at IRAS wavelength bands; 99% of the interstellar maser sources are above black body line, while 95% of the stellar masers are below. The distribution difference of the two kinds of masers shown in the color-color diagram is due to their different optical depths and temperature distributions of dust regions. There are 743 water masers with MSX counterparts, of which 552 are interstellar masers and 191 are stellar masers. MSX colors of the associated sources of water masers are here analyzed for the first time. The color differences among the MSX bands are small and the interstellar masers are redder than the stellar masers. There is a correlation between the intensity of the stellar water maser emission and that of the 12 μm and 25 μm emissions, while there is no correlation between the water maser emission and the 8 μm emission. The infrared intensity increases with increasing wavelength for the interstellar masers, while it is the opposite for stellar masers. These results may provide clues for the pumping of water maser and for the properties of the two kinds of maser emission regions.

AGB maser stars as tracers of stellar populations

AbstractMasers are known to occur in circumstellar envelopes of AGB stars that are long-period variables and have entered the TP-AGB phase. The maser radiation is intrinsically weak and detection limits prevent us from seeing maser stars much beyond the Galactic Center. Very deep searches have led to the detection of a handful of maser stars in the Large Magellanic Cloud. Some 1000 stellar masers are known. They trace a population of stars with main sequence masses between 1 and 6 M⊙ and ages between 2 and 8 Gyr, the same population that is traced by Cepheids. Their distribution in our Galaxy is the same as that of the red giants, and therefore the stars trace the galactic bar. An outstanding feature is a small disk of radius 150 pc around the galactic center. The disk is in rapid, solid body rotation and leads to a good measurement of the mass density within that distance for Sgr A.