J/ApJ/728/149 Polarized SiO masers of VX Sgr (Vlemmings+, 2011)
Magnetic fields in evolved stars: imaging the polarized emission of
high-frequency SiO masers.
Vlemmings W.H.T., Humphreys E.M.L., Franco-Hernandez R.
<Astrophys. J., 728, 149 (2011)>
=2011ApJ...728..149V 2011ApJ...728..149V
ADC_Keywords: Masers ; Polarization ; Stars, late-type ; Radio lines
Keywords: masers - polarization - stars: individual (VX Sgr, W Hya) -
stars: late-type
Abstract:
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=128SiO and v=029SiO masers of VX Sgr are shown
to be highly linearly polarized with a polarization from ∼5% to 60%.
Assuming the continuum emission peaks at the stellar position, the
masers are found within ∼60mas of the star, corresponding to ∼100AU at
a distance of 1.57kpc. 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 ∼1400AU. 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 distributions and that they appear to
avoid each other. Additionally, emission from the SO 55-44 line
was imaged for both sources. Around W Hya, we find a clear offset
between the red- and blueshifted SO emission. This indicates that W
Hya is likely host to a slow bipolar outflow or a rotating disk-like
structure.
Description:
The SiO masers of VX Sgr and W Hya were observed on UTC 2008 July 20
with the Submillimeter Array (SMA) in extended configuration. The
resulting beam size was ∼1.5x1.0 arcsec. The observations were
centered at 214.8GHz (λ=1.4mm) to allow simultaneous
observation of both the v=1 and v=2 28SiO maser lines at 215.5959
and 214.0885GHz, respectively, in one 2GHz band. This additionally
covered the frequencies of the 29SiO v=0 maser and the SO 55-44
line. The frequency resolution was 400kHz, corresponding to ∼0.56km/s.
Objects:
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RA (2000) DE Designation(s)
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18 08 04.05 -22 13 26.6 V* VX Sgr = HD 165674
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 73 77 Polarized SiO masers of VX Sgr
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See also:
V/68 : Stellar Maser Observations (Benson+ 1990)
V/18 : Catalogue of Late-type Stars with Maser Emission (Engels, 1979)
J/A+A/524/A99 : Polarisation of S Per and VX Sgr in OH (Szymczak+, 2010)
J/ApJ/640/982 : 43GHz SiO maser features around VX Sgr (Chen+, 2006)
J/A+A/434/201 : The 35um absorption line towards 1612MHz masers (He+, 2005)
J/AJ/123/2772 : OH-selected AGB and post-AGB objects. I. (Sevenster+, 2002)
J/PASJ/53/517 : Water maser survey of late-type stars (Takaba+, 2001)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1 I1 --- Obs [1/2] SiO maser observation type (1)
3- 6 F4.1 km/s VLSR Local Standard of Rest velocity
8- 12 F5.1 mas oRA Offset in Right Ascension (2)
14- 17 F4.1 mas e_oRA Uncertainty in oRA
19- 23 F5.1 mas oDE Offset in Declination (2)
25- 28 F4.1 mas e_oDE Uncertainty in oDE
30- 33 F4.2 Jy S0 Average brightness; Jy/beam
35- 38 F4.2 Jy E_S0 Upper limit uncertainty in S0
40- 43 F4.2 Jy e_S0 Lower limit uncertainty in S0
45- 48 F4.2 Jy Sp Polarized brightness; Jy/beam
50- 53 F4.2 Jy E_Sp Upper limit uncertainty in Sp
55- 58 F4.2 Jy e_Sp Limit limit uncertainty in Sp
60- 62 I3 deg chi Electric-vector position angle
64- 66 I3 deg E_chi Upper limit uncertainty in chi
68- 70 I3 deg e_chi Lower limit uncertainty in chi
72- 73 I2 % Pl Fractional linear polarization
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Note (1): Observation code as follows:
1 = v=1, 28SiO masers (215.5959GHz)
2 = v=0, 29SiO masers (214.0885GHz)
Note (2): Positions relative to α=18:08:04.068±0.014 and
δ=-22:13:28.416±0.200 (J2000).
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 07-Sep-2012