RT Vir Research Articles

RT VIR – A SEMIREGULAR STAR WITH MASER EMISSION: MATHEMATICAL MODELLING OF OPTICAL VARIABILITY

The historical light curve of the semiregular pulsating star RT Vir (SRb, Sp M8III) with a ma-
 ser emission is analyzed based on the AAVSO database. The star is extensively studied in the IR, radio and the optical ranges. The value of the period from the CCVS (155 d ) conflicts with some observational data. Signs of a systematic variability were also detected in the radio frequency range with a period that was close to 200 days. Studies of the period showed that the period of this star varies with time and the latest value was found to be roughly 136 days. Period analysis result of the visual data from 1904 May to 2016 August provided by AAVSO for RT Vir. Published databases of "virtual observatories" - AAVSOnet, Hipparcos, ASAS-SN, KWS were used. Periodogram analysis revealed that the star still has several periods (320, 280, 169 and 153 days). In fact, the variations are not truly periodic, there are large shifts in time, changes in amplitude, and these smaller periods occur as smaller-amplitude peaks in-between the main ones with 320 period. Two-color diagrams of gloss in I and color index (V-I) from V were constructed. Approximation of “Running parabola” was carried out, average values <V>=8.457, <I>=3.658, <B-V>=+1.8 were determined.

Patterns in water maser emission of evolved stars on the timescale of decades

AbstractWe present our past and current long-term monitoring program of water masers in the circumstellar envelopes of evolved stars, augmented by occasional interferometric observations. Using as example the Mira-variable U Her, we identify three types of variability: periodic (following the optical variation), long-term (years-decades) and short-term irregular (weeks-months). We show there are regions in the maser shell where excitation conditions are favourable, which remain stable for many years. Lifetimes of maser clouds in the wind-acceleration zone are of the order of up to a few years. Much longer lifetimes are found for the peculiar case of a maser cloud outside that zone (as in RT Vir), or in some cases where the motion of spectral features can be followed for the entire 2 decade monitoring period (as in red supergiant VX Sgr).

Water vapour masers in long-period variable stars

Context. Water masers emitting at a radiofrequency of 22 GHz are often found in the circumstellar envelopes of evolved stars. We monitored the H2O maser emission of a larger sample of evolved stars of different types to study the maser properties as a function of stellar type. Aims. We wish to understand the origin and evolution of the H2O masers in circumstellar envelopes. In this paper, we take a closer look at R Crt and RT Vir, two nearby (<250 pc) semi-regular variable stars. The findings complement our monitoring results for RX Boo and SV Peg, two other semi-regular variable stars that we have discussed in a previous paper. Methods. Within the framework of the Medicina/Effelsberg H2O maser monitoring programme, we observed the maser emission of R Crt and RT Vir for more than two decades with single-dish telescopes. To get insights into the distribution of maser spots in the circumstellar envelopes at different times, to get an idea of their longevity, and, where possible, to be able to link the phenomena seen in our observations to maser locations within the envelopes, we collected interferometric data for these stars, taken within the same period, from the literature. Results. The H2O masers in R Crt and RT Vir exhibit brightness variations on a variety of timescales. We confirm short-time variations of individual features on timescales of months to up to 1.5 yr, as seen by previous monitoring programmes. Also decade-long variations of the general brightness level, independent from individual features, were seen in both stars. These long-term variations are attributed to brightness variations occurring independently from each other in selected velocity ranges and they are independent of the optical light curve of the stars. Expected drifts in velocity of individual features are usually masked by the blending of other features with similar velocities. However, in RT Vir, we found the exceptional case of a single feature with a constant velocity over 7.5 yr (<0.06 km s−1 yr−1). Conclusions. We attribute the long-term brightness variations to the presence of regions with higher-than-average density in the stellar wind and hosting several clouds which emit maser radiation on short timescales. These regions typically need ~20 yr to cross the H2O maser shell, where the right conditions for exciting H2O masers are present. Different clouds contained in such a region all move within a narrow range of velocities, and so does their maser emission. This sometimes gives the impression of longer-living features in single-dish spectra, in spite of the short lifetimes of the individual components that lie at their origin, thus, naturally explaining the longer timescales observed. The constant velocity feature (11 km s−1) is likely to come from a single maser cloud, which moved through about half of RT Vir’s H2O maser shell without changing its velocity. From this, we infer that its path was located in the outer part of the H2O maser shell, where RT Vir’s stellar wind has, apparently, already reached its terminal outflow velocity. This conclusion is independently corroborated by the observation that the highest H2O maser outflow velocity in RT Vir approaches the terminal outflow velocity, as given by OH and CO observations. This is generally not observed in other semi-regular variable stars. All four stars in our study are of optical variability type SRb, indicating the absence of periodic large-amplitude variations. Therefore, any likely responses of the maser brightness to variations of the optical emission are masked by the strong short-term maser fluctuations.

Very fast variations of SiO maser emission in evolved stars

Context.Stars on the asymptotic giant branch (AGB) are long-period variables that present strong flux variations at almost all wavelengths, including the SiO maser lines. The periods of these variations are of 300–500 days in Mira-type stars and somewhat shorter in semi-regular variables. The variability of the SiO lines on short timescales has been investigated, but the data are inconclusive.Aims.We aim to study the time evolution of the SiO maser lines in Mira-type and semi-regular variables at short timescales. We also discuss the origin of the observed fast variations.Methods.We observed the SiO maser lines at 7 mm (28SiOv= 1,2J= 1–0) and 3 mm (28SiOv= 1J= 2–1) using the 40 m Yebes antenna and the 30 m IRAM telescope, respectively, with a minimum spacing of 1 day. We studied the semi-regular variables RX Boo and RT Vir and the Mira-type variables U Her, R LMi, R Leo, andχCyg. We performed a detailed statistical analysis of the variations on different timescales.Results.RX Boo shows strong and fast variations in the intensity of the different spectral features of the SiO lines at 7 and 3 mm. On a timescale of one day, we find variations of ≳10% in 25% of the cases. Variations of greater than ~50% are often found when the observations are separated by 2 or 3 days. A similar variation rate of the SiO lines at 7 mm is found for RT Vir, but the observations of this object are less complete. On the contrary, the variations of the SiO maser line intensity in the Mira-type variables are moderate, with typical variation rates around ≲10% in 7 days. This phenomenon can be explained by the presence of particularly small maser-emitting clumps in semi-regular variables, which would lead to a strong dependence of the intensity on the density variations due to the passage of shocks.

VLBA Trigonometric Parallax Measurement of the Semi-regular Variable RT Vir

Abstract We report a trigonometric parallax measurement of 22 GHz H2O masers toward the semi-regular variable RT Vir from multi-epoch Very Long Baseline Array observations. The parallax of 4.417 ± 0.134 mas, corresponding to a distance of 226 ± 7 pc, is significantly different from the Hipparcos parallax, but is consistent with a distance derived from the period–luminosity relation (PLR). This suggests that the Hipparcos parallax suffers from systematic bias, possibly owing to a variable photo-center for this giant star. As such, VLBI parallax measurements will serve as an important cross-check for Gaia. Combing our RT Vir parallax with other long period variable parallaxes, we find a Galactic PLR at an infrared K band of , assuming that the slope of the Large Magellanic Cloud relation is the same to that of our Galaxy.

Magnetic fields around evolved stars: further observations of H2O maser polarization

We aim to detect the magnetic field and infer its properties around four AGB stars using H$_2$O maser observations. The sample we observed consists of the following sources: the semi-regular variable RT Vir and the Mira variables AP Lyn, IK Tau, and IRC+60370. We observed the 6$_{1,6}-5_{2,3}$ H$_2$O maser rotational transition, in full-polarization mode, to determine its linear and circular polarization. Based on the Zeeman effect, one can infer the properties of the magnetic field from the maser polarization analysis. We detected a total of 238 maser features, in three of the four observed sources. No masers were found toward AP Lyn. The observed masers are all located between 2.4 and 53.0 AU from the stars. Linear and circular polarization was found in 18 and 11 maser features, respectively. We more than doubled the number of AGB stars in which magnetic field has been detected from H$_2$O maser polarization, as our results confirm the presence of fields around IK Tau, RT Vir and IRC+60370. The strength of the field along the line of sight is found to be between 47 and 331 mG in the H$_2$O maser region. Extrapolating this result to the surface of the stars, assuming a toroidal field ($\propto$ r$^{-1}$), we find magnetic fields of 0.3-6.9 G on the stellar surfaces. If, instead of a toroidal field, we assume a poloidal field ($\propto$ r$^{-2}$), then the extrapolated magnetic field strength on the stellar surfaces are in the range between 2.2 and $\sim$115 G. Finally, if a dipole field ($\propto$ r$^{-3}$) is assumed, the field strength on the surface of the star is found to be between 15.8 and $\sim$1945 G. The magnetic energy of our sources is higher than the thermal and kinetic energy in the H$_2$O maser region of this class of objects. This leads us to conclude that, indeed, magnetic fields probably play an important role in shaping the outflows of evolved stars. (abridged)

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (< ~5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 micron. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 micron visibility measurements for all baselines. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 micron spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

Water Maser Emission Around Low/Intermediate Mass Evolved Stars

AbstractWe present results of Very Long Baseline Array (VLBA) polarimetric 22 GHz H2O maser observations of a number of low/intermediate mass evolved stars. We observed 3 Miras (Ap Lyn, IK Tau and IRC+60370), 1 semi-regular variable (RT Vir) and 1 pPN (OH231.8+4.2). Circular polarization is detected in the H2O maser region of OH231.8+4.2 and we infer a magnetic field of |B||| = ~45 mG. This implies an extrapolated magnetic field of ~2.5 G on the surface of the central star. The preliminary results on RT Vir and IRC+60370 also indicate the first detection of weak H2O maser linear polarization.

Observational evidence for the shrinking of bright maser spots

The nature of maser emission means that the apparent angular size of an individual maser spot is determined by the amplification process as well as by the instrinsic size of the emitting cloud. Highly sensitive MERLIN radio interferometry images spatially and spectrally resolve water maser clouds around evolved stars. We measured the properties of clouds around the red supergiant S Per and the AGB stars IK Tau, RT Vir, U Her and U Ori, to test maser beaming theory. Spherical clouds are expected to produce an inverse relationship between maser intensity and apparent size, which would not be seen from cylindrical or slab-like regions. We analysed the maser properties, in order to estimate the saturation state, and investigated the variation of observed spot size with intensity and across the spectral line profiles. Circumstellar masers emanate from discrete clouds from about one to 20 AU in diameter depending on the star. Most of the maser features have negative excitation temperatures close to zero and modest optical depths, showing that they are mainly unsaturated. Around S Per and (at most epochs) RT Vir and IK Tau, the maser component size shrinks with increasing intensity. In contrast, the masers around U Ori and U Her tend to increase in size, with a larger scatter. The water masers from S Per, RT Vir and IK Tau are mainly beamed into spots with an observed angular size much smaller than the emitting clouds and smallest of all at the line peaks. This suggests that the masers are amplification-bounded, emanating from approximately spherical clouds. Many of the masers around U Her and U Ori have apparent sizes which are more similar to the emitting clouds and have less or no dependence on intensity, suggesting that these masers are matter-bounded. This is consistent with an origin in flattened clouds and these two stars have shown other behaviour indicating the presence of shocks.

The Three‐dimensional Kinematics of Water Masers around the Semiregular Variable RT Virginis

We report observations of water masers around the semiregular variable RT Virginis, which have been made with the Very Long Baseline Array of the National Radio Astronomy Observatory at five epochs, each separated by 3 weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The three-dimensional maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of 8 km s-1. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but they were less significant than those found in other semiregular variables. Systematic radial velocity drifts of individual maser features were found with amplitudes of ≤2 km s-1 yr-1. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km s-1 yr-1, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

Detection of 1612 MHz OH emission in the semiregular variable stars RT Vir, R Crt and W Hya

We present evidence of 1612 MHz emission in SR variable stars. The two SRb, RT Vir and R Crt, as well as the SRa W Hya have been monitored with the upgraded Nancay radio telescope since February 2001. All three objects have shown a weak 1612 MHz emission occuring in the velocity range of the strongest emission observed in the main-lines. Such a detection is the second observational evidence for emission in the 1612 MHz OH maser satellite line from SRb stars. It also confirms the presence of 1612 MHz emission in the SRa W Hya discovered by Etoka et al. (2001). Such a finding strongly suggests that the shell properties of those three objects are quite similar to those of the Mira stars with similar IR characteristics.

Sub-au imaging of water vapour clouds around four asymptotic giant branch stars

We present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with an angular resolution of ∼15 milliarcsec and a velocity resolution of 0.1 km s −1 . The H2O masers are found in thick expanding shells with inner radii ∼6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5‐0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2‐4 au and filling factors of only ∼0.01 with respect to the whole H2O maser shells. The circumstellar envelope velocity structure gives additional evidence the maser clouds are densitybounded. Masing clouds can be identified over a similar time-scale to their sound crossing time (∼2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than for those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.

OH masers in semiregular variables: Insights from long-term monitoring

Drifts in radial velocities of OH maser features of the three semiregular variable stars R Crt, RT Vir and W Hya have been analyzed. The radial velocities of the extreme blue- and red-shifted features drifted by about 0.2 kms−1 to 1 km s−1 on time-scales of few months to several years. In the case of W Hya a blending effect of spectral features can account for those drifts. In the two other sources the velocity drifts are likely to be intrinsic to the clumps for which the velocities of peak emission vary due to appearance or disappearance of maser emission in the regions where circumstellar matter is still accelerated.

The H2O maser proper motions of RT Vir and VX Sgr

We have measured significant proper motions using the water maser clouds in the outflows from RT Vir (6 epochs) and VX Sgr (2 epochs). In both cases proper motions are measured in the bright emission found perpendicular to the OH emission. This strongly suggests latitude dependent mass-loss with the water masers tracing a dense equatorial outflow and the OH emission tracing a less dense polar outflow. In both cases the rotational velocities are &lt; 1 km s−1 thus ruling out a strongly rotating circumstellar envelope. This suggests the outflow is not shaped by an orbiting companion. In the case of VX Sgr the proper motions are contained in a wedge perpendicular to the measured magnetic field axis, thus strongly suggesting that the magnetic field is helping to shape the mass outflow.The proper motion results have thrown up a puzzle. For VX Sgr the material is being radially accelerated and material enters the water maser zone at 4 km s−1. This is consistent with the mass loss model of Bowen (1988). However the RT Vir proper motion results show no such radial acceleration. The masers have already received their acceleration before they reach the water maser zone at 3-AU (our numerical models seem to support the small radius). This was probably done in the pulsation zone. The acceleration mechanism is unclear for this source.

Variations of polarisation properties of OH maser emission from three semiregular variables

We present an analysis of polarimetric observations of the three semiregular variables RT Vir, R Crt and W Hya obtained in the 1665 and 1667 MHz OH maser lines. Circular polarisation data were taken at intervals over ap eriod of 10 14 years. During the two last years of the monitoring program the targets were observed at intervals of 3 4 weeks in circular and linear polarisation. Circular polarisation was dominant, whereas linear polarisation, if detected, was weak. The degree of circular polarisation varied considerably across the maser proles. It decreased at velocities where strong OH total flux density was observed, most likely due to blending eects. Individual circularly polarised features exhibited various types of changes; some features were transient or showed signicant variations on timescales of a few weeks to few months. Other features varied slowly over the period of observations of about 5000 days and polarisation reversal occurred. Net circular polarised emission was detected in the all three stars. Variations of net circular polarisation and the degree of circular polarisation of the maser features of RT Vir implies a well-aligned circumstellar magnetic eld. Linearly polarised features preferentially appeared at blue- shifted velocities where they are not suppressed by Faraday rotation. In the blue-shifted 1667 MHz features of RT Vir we observed a systematic increase of degree of linear polarisation associated with a gradual decrease in the degree of circular polarisation. This behaviour was possibly linked to variations in the electron density or propagation eect in the maser regions. It is suggested that changes in the observational characteristics of the polarised maser emission of the studied stars can be caused by turbulence eects in the circumstellar magnetic eld and by global magnetic eld reversal.

Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir

We present and interpret the results of a long-term OH variability study of three semiregular stars, one type a (SRa), W Hya, and two type b (SRb), R Crt and RT Vir. The 1665 and 1667 MHz OH masers of the three semiregulars were observed at intervals in the period 1982 January{1995 December using the Nan cay radio telescope, and we searched for 1612 MHz emission. The OH maser proles of the studied stars signicantly deviated from a standard double-peaked prole. The timescale of prole changes in the two SRb stars R Crt and RT Vir was as short as a month. The OH proles of the SRa star W Hya were much more stable but since November 1986 a very blue-shifted feature appeared at 1667 MHz. Our phase-lag measurements suggest that this feature comes from a detached OH shell of radius 3 10 16 cm. Faint 1612 MHz emission was found in W Hya only. Weak emission at velocities very close to the systemic velocity usually appeared during some intervals of high maser activity in R Crt and RT Vir and was almost always present in W Hya. For R Crt we estimated that this tangential emission disappeared when the kinetic temperature in the OH maser regions dropped below 150 200 K. For a few features, line narrowing and re-broadening were observed on timescales of 90 200 days. The linewidth was inversely proportional to the peak flux density, suggesting unsaturated amplication. Cyclic variations in the integrated flux density were observed in all the three stars. The OH variability curves were generally characterised by large amplitude (4 6 m ) variations over 400 800 days superimposed with 100 300-day variations of 0:2 2 m . Only the measured OH period of W Hya, of 362 7 days, was in agreement with the optical period. The two SRb stars exhibited multi-periodic OH variability including with two statistically signicant periods. The behaviour of their red- and blue-shifted emission was less correlated than in W Hya. The ratios of the flux densities at 1667 MHz to that at 1665 MHz in all the three stars were about 2 at epochs of high OH activity and usually increased during weak maser emission. Long term behaviour of the OH masers from W Hya resembled that of standard OH Miras, while that of R Crt and RT Vir suggested thin and clumpy envelopes where unsaturated emission was sustained in some clouds.

A MERLIN movie of mass-loss from RT Vir

We used MERLIN to observe RT Vir at 22 GHz at six epochs during 10 weeks. The water maser emission comes from a thick expanding shell with an elliptical velocity field. MERLIN has a velocity resolution of 0.1 km s−1 and milli-arcsecond angular resolution, revealing details within the individual maser clouds, typically 12 mas in diameter spanning 15 velocity channels. The brightest peak doubles in intensity to 800 Jy/beam. Features at velocities close to the stellar velocity show the largest proper motions of ∼ 3 mas away from the centre of emission. Some features are seen near the outer limits to the maser shell at early epochs only, but new masers appear close to the inner rim. The variability of individual maser features is not a simple function of the stellar luminosity.

Evolution of H2O maser emission in the direction of the semiregular variable RT Virginis during 1985-1996

We present a catalog of the 22 GHz H2 Om aser proles of the semiregular variable RT Vir. The observa- tions were made from 1985 to 1996 at the RT-22 radio telescope of the Radio Astronomical Station at Pushchino, Russia. Results obtained after the analysis of the inte- grated fluxes, the average spectra, the centroid velocity and the superposition of the spectra are reported. Anticorrelation of the integrated fluxes of two spectral intervals (VLSR 21 km s 1 ) was found. The characteristic time variation of the maser emission at each of these groups was about 5-6 years. This phe- nomenon may be explained by a variation of the outflow of the matter from the star. The outflow is weakly bipolar. We can suppose that in such a model the enhancement of the outflow occured alternately with a period of about 5-6 years. The anticorrelation of the integrated fluxes of two groups of maser features, located symmetrically relative to the star, may be a result of this alternation.

Positions and distributions of water masers around late-type stars

High angular resolution (0.07″) observations of the water masers in the circumstellar shells of the supergiant stars VY CMa and VX Sgr and four M giant stars (IK Tau, RT Vir, W Hya, and RX Boo) are presented. For all these sources, the standard spherically symmetric expanding shell model fails to fit the water maser data as well as it fits 1612 MHz OH maser data. A component of radial outflow appears to be present in the H 2 O shells of three stars (RT Vir, IK Tau, VX Sgr)

Long-term starspot activity of short-period RS Canum Venaticorum stars. IV - WY CANCRI

The photometric distortion waves in the short-period RS CVn system WY CnC are parametrized by a dark, circular spot model for the active regions on the primary star. From recent observations and archival sources, the changes in positions and areas of the active regions are inferred. For light curves with simultaneous visual and red or infrared data, the average temperatures of the spotted regions relative to the photosphere are evaluated. In 1989, it is found that T(spot) = 3970 + or - 300 K for a primary star temperature of 5520 K. Some indication is found of a nonuniform temperature distribution in longitude across the spotted area at this time. The active regions tend to occur at low latitudes, in contrast to the high latitudes previously found for SV Cam, RT And, and BH Vir. It is found again that the active regions fall into preferred longitudes near 90 and 270 deg. 27 refs.