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- Gahm, Gösta, et al.
(författare)
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Mass and motion of globulettes in the Rosette Nebula
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 555, s. A57-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Tiny molecular clumps are abundant in many H II regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark patches against the background of bright nebulosity. The majority of the globulettes were found to be of planetary mass in a previous optical investigation, while the largest objects may contain more than half a solar mass. Aims. We aim to clarify the physical nature of globulettes by deriving densities and masses, and to determine their velocities as a function of position over the nebula. This information will provide clues to the question of origins, evolution, and fate of globulettes. The Rosette Nebula is relatively rich in globulettes, and we selected a sample of well-confined objects of different sizes for the present investigation. Methods. Radio observations were made of molecular line emission from 16 globulettes combined with near-infrared (NIR) broad-band JHKs and narrow-band Paschen beta and H-2 imaging. Ten objects, for which we collected information from several transitions in (CO)-C-12 and (CO)-C-13, were modelled using a spherically symmetric model. Results. Practically all globulettes were detected in our CO survey. The observed (CO)-C-12 (3-2) and (2-1) line temperatures range from 0.6 K to 6 K, the (CO)-C-13 being a third of this. As a rule, the lines are narrow, similar to 1.0 km s(-1). The best fit to observed line ratios and intensities was obtained by assuming a model composed of a cool and dense centre and warm and dense surface layer. This model provides estimates of maximum and minimum mass; the average masses range from about 50 to 500 Jupiter masses, which is similar to earlier estimates based on extinction measures. The selected globulettes are dense, n(H) similar to 10(4) cm(-3), with very thin layers of fluorescent H-2 emission, showing that the gas is in molecular form just below the surface. The NIR data show that several globulettes are very opaque and contain dense cores. No infrared-excess stars in the fields are associated with globulettes. Internal gas motions are weak, but some larger objects show velocity-shifted components associated with tails. However, most globulettes show no signs of tails or pronounced bright rims in contradiction to current numerical simulations of clumps exposed to intense stellar radiation. Because of the high density encountered already at the surface, the rims become thin, as evidenced by our P beta images, which also show extended emission that most likely comes from the backside of the globulettes. We conclude that the entire complex of shells, elephant trunks, and globulettes in the northern part of the nebula is expanding with nearly the same velocity of similar to 22 km s(-1), and with a very small spread in velocity among the globulettes. We note that the velocities observed for background shells do not fit into a spherically expanding nebular complex. Conclusions. Some globulettes are in the process of detaching from elephant trunks and shells, while other more isolated objects must have detached long ago and are lagging behind in the general expansion of the molecular shell. Several globulettes are presently subject to heavy erosion from the intense radiation field from the central stars and eject gas streams (tails), while other quite isolated objects lack such signatures. We envision that after detachment, the objects erode to isolated and dense clumps. The suggestion that some globulettes might collapse to form planetary-mass objects or brown dwarfs is strengthened by our finding of dense cores in several objects. Such free-floating low-mass objects would move at high speed from the start and escape from the region.
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| 2. |
- Makela, M. M., et al.
(författare)
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Rosette globulettes and shells in the infrared
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567, s. A108-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Giant galactic H II regions surrounding central young clusters show compressed molecular shells, which have broken up into clumps, filaments, and elephant trunks interacting with UV light from central OB stars. Tiny, dense clumps of subsolar mass, called globulettes, form in this environment. Aims. We observe and explore the nature and origin of the infrared emission and extinction in these cool, dusty shell features and globulettes in one H II region, the Rosette nebula, and search for associated newborn stars. Methods. We imaged the northwestern quadrant of the Rosette nebula in the near-infrared (NIR) through wideband JHKs filters and narrowband H-2 1-0 S(1) and P beta plus continuum filters using the Son of Isaac (SOFI) instrument at the New Technology Telescope (NTT) at European Southern Observatory (ESO). We used the NIR images to study the surface brightness of the globulettes and associated bright rims. We used the NIR JHKs photometry to create a visual extinction map and to search for objects with NIR excess emission. In addition, archival images from Spitzer Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) 24 mu m and Herschel Photoconductor Array Camera and Spectrometer (PACS) observations, covering several bands in the mid-infrared and far-infrared, were used to further analyze the stellar population, to examine the structure of the trunks and other shell structures and to study this Rosette nebula photon-dominated region in more detail. Results. The globulettes and elephant trunks have bright rims in the Ks band, which are unresolved in our images, on the sides facing the central cluster. An analysis of 21 globulettes, where surface brightness in the H-2 1-0 S(1) line at 2.12 mu m is detected, shows that approximately a third of the surface brightness observed in the Ks filter is due to this line: the observed average of the H-2/Ks surface brightness is 0.26 +/- 0.02 in the globulettes' cores and 0.30 +/- 0.01 in the rims. The estimated H-2 1-0 S(1) surface brightness of the rims is -3-8 x 10(-8) Wm(-2) sr(-1) mu m(-1). The ratio of the surface brightnesses support fluorescence instead of shocks as the H-2 excitation mechanism. The globulettes have number densities of n(H-2) similar to 10(-4) cm(-3) or higher. We estimated masses of individual globulettes and compared them to the results from previous optical and radio molecular line surveys. We confirm that the larger globulettes contain very dense cores, that the density is also high farther out from the core, and that their mass is subsolar. Two NIR protostellar objects were found in an elephant trunk and one was found in the most massive globulette in our study.
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