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

Abstract

Solar-mass stars form via disk-mediated accretion. Recent findings indicate that this process is probably episodic in the form of accretion bursts 1 , possibly caused by disk fragmentation 2-4 . Although it cannot be ruled out that high-mass young stellar objects arise from the coalescence of their low-mass brethren 5 , the latest results suggest that they more likely form via disks 6-9 . It follows that disk-mediated accretion bursts should occur 10,11 . Here we report on the discovery of the first disk-mediated accretion burst from a roughly twenty-solar-mass high-mass young stellar object 12 . Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical for accretion bursts in low-mass protostars, but orders of magnitude more luminous. 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 • ).A molecular outflow has been detected 13 (blueshifted lobe position angle (P.A.) ∼247 • ) perpendicular to the disk. Two bipolar lobes (cavities), cleared by the outflow, are illuminated by the central source and show up as reflection nebulae towards the southwest (blueshifted lobe) and northeast (redshifted lobe, see Fig.

Radio outburst from a massive (proto)star

“…1 of Caratti o Garatti et al (2017). This figure shows that scattered IR emission is clearly detected from both lobes, whereas for i close to 0 only one lobe should be seen.…”

“…Finally, by comparison between the accretion rate computed from the IR outburst (Caratti o Garatti et al 2017) and the ionized mass loss rate of the jet, we estimate for the first time the ionization fraction in the jet and find a conservative upper limit of 14%. This result strongly indicates that the radio emission from thermal jets very likely traces only a negligible fraction of the jet mass, which is largely neutral.…”

“…Our VLA monitoring started on March 11, 2016, about 9 months after the estimated onset of the outburst (mid-June 2015; see Caratti o Garatti et al 2017). As illustrated in Fig.…”

“…Caratti o Garatti et al (2017) have estimated an accretion rate of at least ∼ 5 × 10 −3 M ⊙ yr −1 during the outburst. It is reasonable that at least 10% of the infalling material will be redirected into the outflow, so we can assumeṀ > 10 Table 3) and 0 ≃ 900 km s −1 , we obtain x 0Ṁ < 1.4 × 10 −5 M ⊙ yr −1 and x 0 < 0.14.…”

“…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). Our study has shown that such an increase is likely driven by an accretion burst of ∼5 × 10 −3 M ⊙ yr −1 that occurred around mid-June 2015.…”