H₂O masers in a jet-driven bow shock: episodic ejection from a massive young stellar object

Abstract

We report the results of VERA multi-epoch VLBI 22 GHz water maser observations of S255IR-SMA1, a massive young stellar object located in the S255 star forming region. By annual parallax the source distance was measured as D = 1.78 +0.12 −0.11 kpc and the source systemic motion was (µ α cos δ, µ δ ) = (−0.13 ± 0.20, −0.06 ± 0.27) mas yr −1 . Masers appear to trace a U-shaped bow shock whose morphology and proper motions are well reproduced by a jet-driven outflow model with a jet radius of about 6 AU. The maser data, in the context of other works in the literature, reveal ejections from S255IR-SMA1 to be episodic, operating on timescales of ∼ 1000 years.

Radio outburst from a massive (proto)star

“…This might indicate that the source underwent other outburst episodes in the past, consistent with a scenario where accretion occurs mainly in an irregular way. A similar conclusion was attained by Burns et al (2016), who identify the signposts of three distinct ejection events from S255 NIRS 3. Figure 3 illustrates the evolution of the flux density of NIRS 3 from March to December 2016.…”

“…However, evidence of such a burst was reported by Tapia et al 2015 in V723 Car, a YSO with a relatively high luminosity (∼4 × 10 3 L ⊙ ). More recently, Fujisawa et al (2015) detected a powerful methanol maser flare at 6 GHz towards the 20 M ⊙ YSO S255 NIRS 3, located at a distance of 1.78 +0.12 −0.11 kpc (Burns et al 2016). Since CH 3 OH class II masers are believed to be radiatively pumped, follow-up observations at IR wavelengths were performed, which indeed revealed a dramatic brightening of the source of 2.9 mag and 3.5 mag at K and H bands, respectively (Stecklum et al 2016), and a corresponding increase in bolometric luminosity of ∼1.3 × 10 5 L ⊙ (Caratti o Garatti et al 2017).…”

Disk-mediated accretion burst in a high-mass young stellar object

“…2, left Line luminosity. The line luminosities in the redshifted lobe were inferred from the dereddened line fluxes using A V = 28 ± 9 mag and assuming a distance to the object of 1.8 ± 0.1 kpc 15 . mass is inferred assuming that the stellar radius is R * = 10 R and using E = GM * M acc /R * , where G is the gravitational constant, M * is the mass of the star and M acc is the accreted mass.…”

“…Moreover, the released energy and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk-accretion as the common mechanism of star formation across the entire stellar mass spectrum.S255IR NIRS 3 (aka S255IR-SMA1) is a well-studied ∼20 M (L bol ∼ 2.4×10 4 L ) high-mass young stellar object (HMYSO) 13,14 in the S255IR massive star-forming region 13 , located at a distance of ∼1.8 kpc 15 . It exhibits a disk-like rotating structure 13 , very likely an accretion disk, viewed nearly edge-on 16 (inclination angle ∼80 • ).…”

“…This would also explain the LETTERS observation of several knots in jets from HMYSOs 28 , assuming major outflows events can be linked to major accretion events. Indeed, the morphology of different gas tracers along the outflow axis of NIRS 3, which shows a discrete number of knots, suggests that the source experienced multiple bursts within the past few thousand years [13][14][15] . Our burst detection proves the erratic behaviour of the accretion process in HMYSOs.…”