Supergiant VX Sgr Research Articles

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).

Precision: a fast python pipeline for high-contrast imaging – application to SPHERE observations of the red supergiant VX Sagitariae

ABSTRACT The search for extrasolar planets has driven rapid advances in instrumentation, resulting in cameras such as SPHERE at the VLT, GPI at Gemini South and SCExAO at Subaru, capable of achieving very high contrast (∼106) around bright stars with small inner working angles (${\sim}0.1\,{\rm arcsec}$). The optimal exploitation of data from these instruments depends on the availability of easy-to-use software to process and analyse their data products. We present a pure-python pipeline, precision, which provides fast, memory-efficient reduction of data from the SPHERE/IRDIS near-infrared imager, and can be readily extended to other instruments. We apply precision to observations of the extreme red supergiant VX Sgr, the inner outflow of which is revealed to host complex, asymmetric structure in the near-IR. In addition, optical polarimetric imaging reveals clear extended polarized emission on ∼0.5 arcsec scales that varies significantly with azimuth, confirming the asymmetry. While not conclusive, this could suggest that the ejecta are confined to a disc or torus, which we are viewing nearly face on, although other non-spherical or clumpy configurations remain possible. VX Sgr has no known companions, making such a geometry difficult to explain, as there is no obvious source of angular momentum in the system.

MAGNETIC FIELDS IN EVOLVED STARS: IMAGING THE POLARIZED EMISSION OF HIGH-FREQUENCY SiO MASERS

We present Submillimeter Array observations of high frequency SiO masers around the supergiant VX Sgr and the semi-regular variable star W Hya. The J=5-4, v=1 28SiO and v=0 29SiO masers of VX Sgr are shown to be highly linearly polarized with a polarization from ~5-60%. Assuming the continuum emission peaks at the stellar position, the masers are found within ~60 mas of the star, corresponding to ~100 AU at a distance of 1.57 kpc. The linear polarization vectors are consistent with a large scale magnetic field, with position and inclination angles similar to that of the dipole magnetic field inferred in the H2O and OH maser regions at much larger distances from the star. We thus show for the first time that the magnetic field structure in a circumstellar envelope can remain stable from a few stellar radii out to ~1400 AU. This provides further evidence supporting the existence of large scale and dynamically important magnetic fields around evolved stars. Due to a lack of parallactic angle coverage, the linear polarization of masers around W Hya could not be determined. For both stars we observed the 28SiO and 29SiO isotopologues and find that they have a markedly different distribution and that they appear to avoid each other. Additionally, emission from the SO 5_5-4_4 line was imaged for both sources. Around W Hya we find a clear offset between the red- and blue-shifted SO emission. This indicates that W Hya is likely host to a slow bipolar outflow or a rotating disk-like structure.

The magnetic field around late-type stars revealed by the circumstellar H2O masers

Through polarization observations, circumstellar masers are excellent probes of the magnetic field in the envelopes of late-type stars. Whereas observations of the polarization of the SiO masers close to the star and on the OH masers much further out were fairly commonplace, observations of the magnetic field strength in the intermediate density and temperature region where the 22 GHz water masers occur have only recently become possible. Here we present the analysis of the circular polarization, due to Zeeman splitting, of the water masers around the Mira variable stars U Her and U Ori and the supergiant VX Sgr. We present an upper limit of the field around U Her that is lower but consistent with previous measurements, reflecting possible changes in the circumstellar envelope. The field strengths around U Ori and VX Sgr are shown to be of the order of several Gauss. Moreover, we show for the first time that large scale magnetic fields permeate the circumstellar envelopes of an evolved star; the polarization of the water masers around VX Sgr reveals a dipole field structure. We discuss the coupling of the magnetic field with the stellar outflow, as such fields could possibly be the cause of distinctly aspherical mass-loss.

The radially expanding molecular outflow of VX Sagittarii

We used MERLIN to observe 22-GHz H2O maser emission from the red supergiant VX Sgr in 1994 and 1999. The masers lie in a region 300-400 mas in diameter. The angular size and the velocity distribution of the masers are similar at both epochs, although the total flux density in 1999 is only half of that seen in 1994.The maser emission is resolved into clouds of average diameter (11.5 +/- 4.0) mas which we infer are 300 times denser than the surrounding wind. The cloud radius is comparable to the stellar radius. We have previously found an order of magnitude variation in the size of H2O clouds around other evolved stars of masses 1-20 M-., but the average size is consistently proportional to stellar size.43 out of 92 distinct maser clouds observed in 1994 were matched with clouds seen in 1999. The mean change in position is 10.7 mas. The radial component of proper motion is directed away from the centre of expansion and increases with increasing angular separation. There is no significant rotational proper motion. The expansion proper motions are consistent with the maser Doppler velocities if VX Sgr is at (1.8 +/- 0.5) kpc. The maser distribution and kinematics suggest a spheroidal thick shell in which the stellar wind is undergoing radial acceleration, from 10 km s(-1) at the inner edge of the maser shell to 20 km s(-1) at the outer edge. We suggest a model for the asymmetric appearance of the maser shell and compare this with the magnetic field direction deduced from OH 1612-MHz maser observations.The maser expansion velocities have doubled between 1983 and 1994, suggesting the entire H2O maser shell has become more effectively accelerated, on a timescale much longer than the light-crossing time for the maser shell and much shorter than the wind-crossing time.

H2O maser emission of the M-type supergiant VX Sgr

Abstract We present the results of observations of the M-type semiregular supergiant VX Sgr in the water-vapour line at λ = 1.35 cm. The observations were carried out on the 22-metre radio telescope RT-22 in Pushchino in 1981–1996. The profiles of the H2O line have a complex structure typical for H2O masers associated with late-type supergiants. We reveal some features persistent during the last 15 years; we suggest that they arise from masering H2O molecules located in the circumstellar disk and the bipolar outflows from the disk poles. The parameters of the outflow are estimated.

The Spatial Distribution of Circularly Polarized 1612 MHz OH Maser Emission From VX SGR

view Abstract Citations (12) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Spatial Distribution of Circularly Polarized 1612 MHz OH Maser Emission From VX SGR Zell, Philip J. ; Fix, John D. Abstract We have used the VLA to observe the 1612 MHz OH emission from the M supergiant VX Sgr in both right and left circular polarization. The maps of polarized emission from VX Sgr provide the first evidence of spatial segregation between oppositely polarized emitting regions within the circumstellar shell of a cool star and suggest that the circumstellar magnetic field of VX Sgr is globally organized. We present a simple model that can account for the spatial segregation of the polarization maps of VX Sgr. Publication: The Astronomical Journal Pub Date: July 1996 DOI: 10.1086/118007 Bibcode: 1996AJ....112..252Z Keywords: MASERS; STARS: VX SGR; RADIO LINES: STARS; STARS: GIANT full text sources ADS | data products SIMBAD (1)

VLBI Observations of the V=1 and V=2 SiO Masers in W Hydra and VX Sagittarius

Our previous observations established the small angular size and high brightness temperature of emission from the v=1, J=1-0 transition of SiO from the circumstellar envelopes of the supergiant VX Sgr and the Mira variable R Cas. We performed a second VLBI experiment on the SiO masers in several late type stars on 31 Oct.-2 Nov. 1978 to compare the physical characteristics of the SiO masers in the v=1 and v=2 states. With an energy separation of 1258 cm-1 (an equivalent temperature of 1753 K) between the two vibrational states, differences in excitation and pumping of the maser states may lead to different maser properties.

An alternative mechanism for production of emission features in some infrared objects

Two dust-envelope models of the M supergiant VX Sgr, which exhibits a prominent emission feature at 10 microns, are presented. The models indicate that, for certain envelope sizes, the presence of the observed emission feature does not necessarily indicate that the emitting grains possess a similar feature in their emissivity profile. The mechanism which may in some cases be producing the observed emission feature is discussed.