A ‘water spout’ maser jet in S235AB-MIR

Monthly Notices of the Royal Astronomical Society, Oct 2015

We report on annual parallax and proper motion observations of H2O masers in S235AB-MIR, which is a massive young stellar object in the Perseus Arm. Using multi-epoch VLBI (very long baseline interferometry) astrometry we measured a parallax of π = 0.63 ± 0.03 mas, corresponding to a trigonometric distance of $D= 1.56^{+0.09}_{-0.08}$ kpc, and source proper motion of (μαcos δ, μδ) = (0.79 ± 0.12, −2.41 ± 0.14) mas yr−1. Water masers trace a jet of diameter 15 au which exhibits a definite radial velocity gradient perpendicular to its axis. 3D maser kinematics were well modelled by a rotating cylinder with physical parameters: vout = 45 ± 2 km s−1, vrot = 22 ± 3 km s−1, i = 12° ± 2°, which are the outflow velocity, tangential rotation velocity and line-of-sight inclination, respectively. One maser feature exhibited steady acceleration which may be related to the jet rotation. During our 15-month VLBI programme there were three ‘maser burst’ events caught ‘in the act’ which were caused by the overlapping of masers along the line of sight.

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A ‘water spout’ maser jet in S235AB-MIR

R. A. Burns 1 ‹ H. Imai 1 T. Handa [email protected] 1 ‹ T. Omodaka 1 A. Nakagawa 1 T. Nagayama 0 Y. Ueno 0 0 Mizusawa VLBI Observatory, National Astronomical Observatory of Japan , 2-12 Hoshigaoka-cho, Mizusawa, Iwate 023-0861 , Japan 1 Graduate School of Science and Engineering, Kagoshima University , 1-21-35 Koˆrimoto, Kagoshima 890-0065 , Japan A B S T R A C T We report on annual parallax and proper motion observations of H2O masers in S235ABMIR, which is a massive young stellar object in the Perseus Arm. Using multi-epoch VLBI (very long baseline interferometry) astrometry we measured a parallax of π = 0.63 ± 0.03 mas, corresponding to a trigonometric distance of D = 1.56+−00..0098 kpc, and source proper motion of (μα cos δ, μδ ) = (0.79 ± 0.12, −2.41 ± 0.14) mas yr−1. Water masers trace a jet of diameter 15 au which exhibits a definite radial velocity gradient perpendicular to its axis. 3D maser kinematics were well modelled by a rotating cylinder with physical parameters: vout = 45 ± 2 km s−1, vrot = 22 ± 3 km s−1, i = 12◦ ± 2◦, which are the outflow velocity, tangential rotation velocity and line-of-sight inclination, respectively. One maser feature exhibited steady acceleration which may be related to the jet rotation. During our 15-month VLBI programme there were three 'maser burst' events caught 'in the act' which were caused by the overlapping of masers along the line of sight. Masers - stars; individual; S235AB-MIR - ISM; jets and outflows 1 I N T R O D U C T I O N The formation of massive stars has been one of the most exciting astronomical problems of the recent decade. Forming stars above 8 M cannot be explained by simply upscaling low-mass star for mation theories since radiation pressure limits further accretion (Palla & Stahler 1993). Of the mainstream theories it is emerging that disc aided accretion is a strong candidate for explaining the formation of stars up to at least B type, as observational evidence of discs associated with such accreting stars is found (Beltra´n et al. 2004; Hirota et al. 2014). Such protostellar discs are thought to be able to shield accreting material from stellar radiation and transport it down to radii of a few ∼100 au from the central object (Cesaroni et al. 2007; Zinnecker & Yorke 2007). However, one necessity of disc accretion theory which has yet to be properly confronted with observations concerns how angular momentum is carried away from the protostellar disc to enable accretion. Proposed methods of removing angular momentum include Alfve´n waves in the disc material, magnetic breaking in the ionic component of the rotating disc, and by physically carrying rotating material away from the disc vertically via magnetocentrifugally driven rotating jets (see review by Konigl & Pudritz 2000). In addition to disc observations it is therefore also appealing to look for evidence of rotating jets near massive young stellar objects (MYSOs) in order to uncover their role in the angular momentum budget of accreting stars. A demonstration of such an investigation is the Ori-S6 MYSO (Zapata et al. 2010). This kind of work poses an observational challenge since MYSOs are deeply embedded and rarely have observable collimated jets (Navarete et al. 2015). We therefore turn to masers in our investigation of rotation in MYSO jets. Collisionally excited H2O masers are associated with a diverse range of kinematic phenomena in the young stellar objects (YSOs) of star-forming regions (SFRs), including jets, wide-angle outflows and expanding shells (see for example Torrelles et al. 2012). The bright and compact masers are ideal for very long baseline interferometry (VLBI) observations, which can penetrate the obscuring parent cores in which massive stars form and reveal the motions of young jets (Furuya et al. 2000; Moscadelli et al. 2011). Multi-epoch observations allow determination of the internal 3D kinematics at shocked regions via combination of line-of-sight and proper motion measurement. Furthermore the annual parallax of a maser source provides a precise estimate of its distance, which is crucial for evaluating its physical properties. As such VLBI is an indispensable technique for studying the formation of stars and provides a mechanism for the search for rotating MYSO jets. VLBI observations in this paper were carried out using VERA (VLBI exploration of radio astrometry) (Kobayashi et al. 2003) which is a Japanese VLBI network dedicated to maser astrometry of primarily Galactic sources. The bright H2O maser in S235AB was chosen as a VERA target for its interesting ‘maser burst’ behaviour (up to 120 Jy; Felli et al. 2007) which has a dominant blueshift maser (DBSM) spectrum. Its annual parallax was also sought for its contribution to the mapping of the Perseus Arm. In addition to H2O masers, the S235AB region also exhibits class I methanol masers (Kurtz, Hofner & A´lvarez 2004; Pratap et al. 2008) and two bipolar outflows of HCO+, driven from a dusty (...truncated)


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R. A. Burns, H. Imai, T. Handa, T. Omodaka, A. Nakagawa, T. Nagayama, Y. Ueno. A ‘water spout’ maser jet in S235AB-MIR, Monthly Notices of the Royal Astronomical Society, 2015, pp. 3163-3173, 453/3, DOI: 10.1093/mnras/stv1836