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.

We present an in-depth near-IR analysis of a sample of H 2 outflows from young embedded sources to compare the physical properties and cooling mechanisms of the different flows. The sample comprises 23 outflows driven by Class 0 and I sources having low-intermediate luminosity.We have obtained narrow band images in H 2 2.12 µm and [Fe ii] 1.64 µm and spectroscopic observations in the range 1−2.5 µm. From [Fe ii] images we detected spots of ionized gas in ∼74% of the outflows which in some cases indicate the presence of embedded HH-like objects. H 2 line ratios have been used to estimate the visual extinction and average temperature of the molecular gas. A v values range from ∼2 to ∼15 mag; average temperatures range between ∼2000 and ∼4000 K. In several knots, however, a stratification of temperatures is found with maximum values up to 5000 K. Such a stratification is more commonly observed in those knots which also show [Fe ii] emission, while a thermalized gas at a single temperature is generally found in knots emitting only in molecular lines. Combining narrow band imaging (H 2 , 2.12 µm and [Fe ii], 1.64 µm) with the parameters derived from the spectroscopic analysis, we are able to measure the total luminosity of the 2 . We find that ∼83% of the sources have a L H 2 /L bol ratio ∼0.04, irrespective of the Class of the driving source, while a smaller group of sources (mostly Class I) have L H 2 /L bol an order of magnitude smaller. Such a separation reveals the non-homogeneous behaviour of Class I, where sources with very different outflow activity can be found. This is consistent with other studies showing that among Class I one can find objects with different accretion properties, and it demonstrates that the H 2 power in the jet can be a powerful tool to identify the most active sources among the objects of this class.

Abstract. As part of a 1-2.5 µm spectroscopic survey of jets and molecular outflows, we present the spectra of three Herbig Haro chains (HH111, HH240/241, HH120) characterized by strong emission from several Fe  transitions originating from the first 13 fine structure levels. Such emission is correlated with optical S  emission and appears to decrease moving away from the driving source. From the analysis of the Fe  lines we have derived electron densities values in the range 3 × 10 3 -2 × 10 4 cm −3 , which are systematically larger than those inferred from optical S  line ratios. We suggest that Fe  lines, having critical densities higher than the optical S  transitions, trace either regions of the post-shock cooling layers with higher compression, or a section of the jet axis at a higher degree of ionization. Strong H 2 emission lines are also detected along the three flows and their analysis indicates that a combination of different shocks can be responsible for their excitation in the different objects. Consequently the Fe  line emission, which requires the presence of fast dissociative shocks, is completely independent from the excitation mechanism giving rise to the molecular emission. In addition to the Fe  and H 2 lines, emission from other species such as C , S , N  as well as recombination lines from the Paschen series are detected and have been used as a reference to infer the gas-phase iron abundance in the observed HH objects. We estimate a grain destruction efficiency of about 30-60%: the highest value is found for HH240A, which also shows the highest degree of excitation among the observed objects.