A search for infrared ring nebulae associated with regions of ionized hydrogen has been carried out. The New GPS Very Large Array survey at 20 cm forms the basis of the search, together with observations obtained with the Spitzer Space Telescope at 8 and 24 m and the Herschel Space Telescope at 70 m. Objects having ringlike morphologies at 8 m and displaying extended emission at 20 cm were selected visually. Emission at 24 m having the form of an inner ring or central peak is also observed in the selected objects. A catalog of 99 ring nebulae whose shapes at 8 and 70 m are well approximated by ellipses has been compiled. The catalog contains 32 objects whose shapes are close to circular (eccentricities of the fitted ellipses at 8 m no greater than 0.6, angular radius exceeding 20). These objects are promising for comparisons with the results of one-dimensional hydrodynamical simulations of expanding regions of ionized hydrogen.2 more than 5000 objects [4]. These objects are sometimes referred to as IR ring nebulae [5], without any interpretation of this morphology. On the other hand, it was already proposed in [2] that ring nebulae are actually projections of three-dimensional shells onto the plane of the sky, leading to the use of the alternate term "bubble", implying that these are threedimensional objects. Analyses of images of ring nebulae at 20 cm have shown that most of them are probably associated with regions of ionized hydrogen created by one or more O or B stars. In particular, Deharveng et al.. [6] concluded that 86% of the objects in the catalog [2] correspond to H II regions formed around massive, hot O-B2 stars. The basis of the catalog [2] is images at 8 m. However, specific morphologies are also characteristic of IR ring nebulae at other IR wavelengths. Emission at 24 m is observed inside nearly all rings or arcs of emission at 8 m (see, e.g., [7]). It is usually believed that the 8 m emission is mainly a manifestation of polycyclic aromatic hydrocarbons (PAHs) [8]. The absence of 8 m emission or presence of only weak 8 m emission inside ring nebulae could indicate that any PAHs inside the H II region have been partially or completely destroyed by UV radiation from the central star [9]. On the other hand, since the IR emission of PAHs is excited by the absorption of UV photons, the farther from the star the PAHs (or other small aromatic particles) are located, the weaker the intensity of their emission. Thus, the 8 m ring apparently is located between the zone where PAHs are destroyed and where they are visible.A less bright ring at 24 m is also coincident with the outer ring of 8 m emission, but most of the IR emission at 24 m comes from the inner region. This appears as a central peak or region of extended emission, fairly often resembling a ring or arc with a smaller diameter. The emission at 70, 100, and 160 m, which corresponds to larger dust grains, also appears as an outer ring surrounding the region of ionization. Here and below, we will refer to rings of emission with various diameters ro...

Aims. Luminosity bursts in young FU Orionis-type stars warm up the surrounding disks of gas and dust, thus inflicting changes on their morphological and chemical composition. In this work, we aim at studying the effects that such bursts may have on the spatial distribution of dust grain sizes and the corresponding spectral index in protoplanetary disks. Methods. We use the numerical hydrodynamics code FEOSAD, which simulates the co-evolution of gas, dust, and volatiles in a protoplanetary disk, taking dust growth and back reaction on gas into account. The dependence of the maximum dust size on the water ice mantles is explicitly considered. The burst is initialized by increasing the luminosity of the central star to 100–300 L⊙ for a time period of 100 yr. Results. The water snowline shifts during the burst to a larger distance, resulting in the drop of the maximum dust size interior to the snowline position because of more efficient fragmentation of bare grains. After the burst, the water snowline shifts quickly back to its preburst location followed by renewed dust growth. The timescale of dust regrowth after the burst depends on the radial distance so that the dust grains at smaller distances reach the preburst values faster than the dust grains at larger distances. As a result, a broad peak in the radial distribution of the spectral index in the millimeter dust emission develops at ≈10 au, which shifts further out as the disk evolves and dust grains regrow to preburst values at progressively larger distances. This feature is most pronounced in evolved axisymmetric disks rather than in young gravitationally unstable counterparts, although young disks may still be good candidates if gravitational instability is suppressed. We confirmed our earlier conclusion that spiral arms do not act as strong dust accumulators because of the Stokes number dropping below 0.01 within the arms, but this trend may change in low-turbulence disks. Conclusions. We argue that, depending on the burst strength and disk conditions, a broad peak in the radial distribution of the spectral index can last for up to several thousand years after the burst has ended and can be used to infer past bursts in otherwise quiescent protostars. The detection of a similar peak in the disk around V883 Ori, an FU Orionis-type star with an unknown eruption date, suggests that such features may be common in the post-outburst objects.

Total infrared fluxes are estimated for 99 HII regions around massive stars. The following wavebands have been used for the analysis: 8 and 24 µm, based on data from Spitzer space telescope (IRAC and MIPS, respectively); 70, 160, 250, 350, and 500 µm, based on data from Herschel Space Observatory (PACS and SPIRE). The estimated fluxes are used to evaluate the mass fraction of polycyclic aromatic hydrocarbons (q PAH ) and the intensity of the ultraviolet emission in the studied objects. It is shown that the PAH mass fraction, q PAH , is much lower in these objects than the average Galactic value, implying effective destruction of aromatic particles in HII regions. Estimated radiation field intensities (U ) are close to those derived for extragalactic HII complexes. Color indices [F 24 /F 8 ], [F 70 /F 24 ], [F 160 /F 24 ], [F 160 /F 70 ] are compared to criteria proposed to distinguish between regions of ionized hydrogen and planetary nebulae. Also, we relate our results to analogous color indices for extragalactic complexes of ionized hydrogen.