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Polarisation Observations of VY Canis Majoris H2O 532–441 620.701 Ghz UvA-DARE (Digital Academic Repository) Polarisation observations of VY Canis Majoris H2O 5(32)-4(41) 620.701 GHz maser emission with HIFI Harwit, M.; Houde, M.; Sonnentrucker, P.; Boogert, A.C.A.; Cernicharo, J.; De Beck, E.; Decin, L.; Henkel, C.; Higgins, R.D.; Jellema, W.; Kraus, A.; McCoey, C.; Melnick, G.J.; Menten, K.M.; Risacher, C.; Teyssier, D.; Vaillancourt, J.E.; Alcolea, J.; Bujarrabal, V.; Dominik, C.; Justtanont, K.; de Koter, A.; Marston, A.P.; Olofsson, H.; Planesas, P.; Schmidt, M.; Schöier, F.L.; Szczerba, R.; Waters, L.B.F.M. DOI 10.1051/0004-6361/201015042 Publication date 2010 Document Version Final published version Published in Astronomy & Astrophysics Link to publication Citation for published version (APA): Harwit, M., Houde, M., Sonnentrucker, P., Boogert, A. C. A., Cernicharo, J., De Beck, E., Decin, L., Henkel, C., Higgins, R. D., Jellema, W., Kraus, A., McCoey, C., Melnick, G. J., Menten, K. M., Risacher, C., Teyssier, D., Vaillancourt, J. E., Alcolea, J., Bujarrabal, V., ... Waters, L. B. F. M. (2010). Polarisation observations of VY Canis Majoris H2O 5(32)-4(41) 620.701 GHz maser emission with HIFI. Astronomy & Astrophysics, 521, L51. https://doi.org/10.1051/0004-6361/201015042 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) A&A 521, L51 (2010) Astronomy DOI: 10.1051/0004-6361/201015042 & c ESO 2010 Astrophysics Polarisation observations of VY Canis Majoris H2O532–441 620.701 GHz maser emission with HIFI, M. Harwit1, M. Houde2, P. Sonnentrucker3 , A. C. A. Boogert4, J. Cernicharo5, E. de Beck6, L. Decin6,16,C.Henkel7, R. D. Higgins8, W. Jellema9,A.Kraus7,C.McCoey10,2,G.J.Melnick11,K.M.Menten7, C. Risacher9, D. Teyssier12, J. E. Vaillancourt13,J.Alcolea14, V. Bujarrabal15,C.Dominik16,17, K. Justtanont18,A.deKoter16,19,A.P.Marston12, H. Olofsson18,20, P. Planesas15,21,M.Schmidt22,F.L.Schöier18, R. Szczerba22, and L. B. F. M. Waters6,16 (Affiliations are available on page 5 of the online edition) Received 26 May 2010 / Accepted 3 September 2010 ABSTRACT Context. Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight. Aims. We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H2O. Methods. In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 km s−1, which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck- Institut für Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission. Results. We report the first astronomical detection to date of H2O maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings. Key words. stars: AGB and post-AGB – stars: winds, outflows – supergiants – circumstellar matter – masers – reference systems 1. Introduction Although a theoretical model of Neufeld & Melnick (1991)pre- dicted that the 22.235 GHz transition should be far more lumi- VY Canis Majoris is a highly luminous, strongly obscured, vari- ∼ nous than these submillimetre transitions, Menten et al. (2008) able supergiant with a high infrared excess. Only 1% of the find them to be comparable in flux density, or at most a factor total luminosity is observed at optical wavelengths. The star’s ∼ = . of 6 lower. This suggests a need for further observations that distance has been measured to be D 1 1 kpc (Choi et al. 2008) might explain the discrepancies. implying a luminosity L = 3 × 105 L (Menten et al. 2008). One of strongest submillimetre masers predicted by Neufeld Three thermally emitted mid-infrared water vapour emission − lines seen in spectra obtained with the short wavelength spec- & Melnick (1991) is due to the 532 441 620.701 GHz transi- trometer (SWS) on ISO indicate a mean radial velocity of order tion of ortho water, whose photon luminosity was expected to be −1 −1 roughly 16% that of the 22.235 GHz maser. Given the multiple 20 ± 2kms and a 25 km s H2O outflow velocity (Neufeld et al. 1999), significantly lower than the 32 km s−1 velocities that masers observed at 22.235 GHz we expected the 620.701 GHz maser of VY CMa to exhibit similar multiple peaks at photon Reid & Muhleman (1978) reported for 1612 MHz OH maser out- ∼ flow. densities 16% of those exhibited at 22.235 GHz. The effective temperature of the central star, T∗ = 2800 K (Monnier et al. 1999), combined with distance and inferred lumi- 2. HIFI observations at 620.701 GHz nosity fixes the star’s radius at R∗ ∼ 10 AU. Working at 11 μm, The Herschel Heterodyne Instrument for the Far Infrared Danchi et al. (1994) reported a variable photospheric radius (HIFI) covers seven frequency bands, ranging from 488.1 to ranging from 9.5 to 11 mas, where 10 mas corresponds to a stel- 1901.8 GHz (de Graauw et al. 2010). The 620.701 GHz radiation lar radius of 10 AU. of the 5 −4 maser of ortho H O is observed in HIFI’s Band VY CMa is a strong source of 6 −5 22.235 GHz 32 41 2 16 23 1B, which nominally covers the range from 562.6 to 628.4 GHz. water vapour maser emission. Recent observations at the Two channels cover each of the frequency bands, one sensitive Atacama Pathfinder EXperiment (APEX) telescope have re- to linearly polarised radiation roughly parallel to the spacecraft vealed H O maser emission also at eight submillimetre fre- 2 horizontal (H) direction, the other roughly parallel to the verti- quencies, ranging from 321 to 475 GHz (Menten et al. 2008). cal direction (V). For Band 1B the H direction of polarisation ◦ is at an angle of 82.5 relative to the spacecraft V axis, while Herschel is an ESA space observatory with science instruments ◦ provided by European-led Principal Investigator consortia and with im- the V direction of polarisation is at an angle of –7.5 to that axis. portant participation from NASA. Our data were obtained in the HIFI high-resolution spectroscopy Appendix (page 5) is only available in electronic form at (HRS) mode with spectral resolution 0.125 MHz, or line-of-sight http://www.aanda.org velocity resolution 0.06 km s−1. Article published by EDP Sciences Page 1 of 5 A&A 521, L51 (2010) The H and V beams on HIFI are not fully coincident. In where the matrix A and vector s are given by Band 1B they are separated by ∼19% of the ∼34.4 full-width- ⎛ ⎞ ( ϕ )/σ ( ϕ )/σ half-power beam diameter; the offset between the two beams is cos 2 1 1 sin 2 1 1 ⎜ . ⎟ ∼6.6 . VY CMa is a spatially unresolved source at these fre- A = ⎝ . ⎠ (8) quencies (Decin et al. 2006). In our observations the star was cos(2ϕ )/σ sin(2ϕ )/σ positioned half-way between beam centres, i.e., displaced ∼3.3 N N N N relative to the centre of each beam, although a random ∼1.8 Q s = . (9) pointing error can slightly increase alignment uncertainty. U In order to determine the orientation on the sky of any ob- served linear polarisation, a source has to be viewed at least The vector s, and therefore the Stokes parameters, are easily ob- at two different rotation angles relative to the telescope. The tained by inverting Eq. (7) using the pseudo-inverse matrix of A. Herschel Space Observatory (Pilbratt et al. 2010), however, can- We then find not be rotated without producing undesirable thermal drifts. A = T −1 T · . rotation is best achieved by observing a target at two epochs sep- s A A A d (10) arated by a number of weeks. The further the target lies above −1 the ecliptic plane, the faster is the rotation produced. In our ob- The elements of the covariance matrix C = AT A can be servations, an interval of three weeks between two sightings of shown to yield the corresponding uncertainties on the estimated VY CMa resulted in a rotation of the telescope of ∼16◦ about Stokes parameters with the line of sight to the star. σ 2 σ 2 We first observed the star on March 21, 2010 in two con- = QQ QU , C σ 2 σ 2 (11) tiguous segments, lasting 10 139 s apiece, for a total observing QU UU time of 5.633 h.
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